Was the Earth's sea-level significantly lower in ancient times?

Was the Earth's sea-level significantly lower in ancient times?

A lot of times, ancient artifacts or even whole ancient civilizations are found buried very deep into the ground surface of the earth.

Entire civilizations that had high buildings have been found many meters below the earth.

Does that mean the earth's sea-level was much lower than it is now? If this is true, then are we experiencing a lower level of gravity?

How can just sedimentation of sand, bury big building to such depths?

If lack of habitation by humans eventually results in burying of things, then what about the places like Chernobyl, which has not been inhabited for a long time?

I am not a physics grad or anything, just a curious guy.

Any help is much appreciated.


The beginnings of human Civilization by-and-large are coincident with the start of our current interglacial period, known as the Holocene at roughly 10,000BC. At the start of it, worldwide sea levels were about 60m lower than today (and that was down from more than 120 at the glacial maximum 10,000 years prior). They rose quite rapidly after that, to nearly present levels by 6,000 BC.

However, many areas over 40 degrees of latitude were artificially depressed due to the weight of all that ice by about 190m lower than today's levels (yes, it weighed that much). Land moves much slower than water, so these areas are still slowing rising today. So at the higher latitudes you generally see the opposite effect of areas that were previously underwater rising. This caused particularly interesting history for the Baltic, which went through several periods of being a lake, then a sea, then a lake, then a sea, as both water and land levels rose.


I'm not sure of the premise of your opening statement:

Most of the times, ancient artifacts or even whole ancient civilizations are found buried very deep into the ground surface of the earth.

In fact, most artifacts are quite close to the surface (less than a few metres) - that's why many of them turn up in ploughed fields or when digging shallow trenches.

It is true that ruins appear to be deeper in major cities (London and Paris), but that's because the old buildings were simply built over. Even then, the Roman walls of London are still only a couple of metres below the surface, and are openly visible to the public from ground floor galleries in some buildings (Greater London Authority building, for one).

There are some odd circumstances - such as the pyramids and sphinx being buried under desert sands. However, desert sands are very mobile and will pile up against things like the pyramids without continuing care.

As for sea level - the coast is constantly changing. There are ancient ports on the English east coast that are now miles inland. There was also once a land bridge, called Doggerland, connecting Britain to Europe tens of thousands of years ago. Sea levels actually rose considerably after the end of the last major ice age, and the polar caps released large volumes of water.


It's easier to break-down the answers to your questions into parts:

A lot of times, ancient artifacts or even whole ancient civilizations are found buried very deep into the ground surface of the earth.

This may happen occasionally, but I can't personally think of any instance where evidence of a whole civilization, or any significant human artifacts are found buried very deep into the ground (say, below 5m or so) where they have not been deliberately buried, or subject to an explainable event such as flooding or cave-in.

Artifacts from Human history are usually found at the surface, or near to the surface. When human artifacts are buried, it is usually as a result of other human activities such as agriculture, over-building or deliberate burial. Some human artifacts can be covered by flooding laying down sediments, volcanic eruptions or desertification (for example, shifting sand-dunes).

Older artifacts, such as fossils, are often found deeper because their age means that they have been subject to other geological processes that move them around much more within the structure of the Earth.

Entire civilizations that had high buildings have been found many meters below the earth.

See above

Does that mean the earth's sea-level was much lower than it is now? If this is true, then are we experiencing a lower level of gravity?

The sea level on Earth has varied since the last ice-age, but 'sea level' is a difficult context when experienced over millions of years, as over those timescales other geological processes such as tectonic movement (movement of continental plates) uplift and erosion mean that sea-level becomes a 'relative' measure.

Sea level is not related to gravity. The gravity on Earth has always been the same (at least, since animals evolved) because the gravity of the Earth relates to it's mass (how much it weighs).

Sea level is only related to the amount of liquid water on the earth and how that water accumulates across the surface of the Earth from the lowest point up. Sea level does vary very, very slightly due to the effects of different levels of gravity around the world, but these relative differences in gravity are not large and only make a very small difference. The action of the moon in creating tides has a much larger effect.

How can just sedimentation of sand, bury big building to such depths?

Sedimentation of sand tends not to do this, sedements are generally carried by rivers and any large structure under water is generally eroded by the water that brings sediments. On land, there are cases of sand dunes, which are moved by the action of wind, burying structures to some depths. Volcanic ash can also do this, as in the case of Pompeii in Italy.

If lack of habitation by humans eventually results in burying of things, then what about the places like Chernobyl, which has not been inhabited for a long time?

A lack of habitation is not strongly connected with the burying of structures and objects, although the growth of plants around buildings may lead to an increase in biomatter (dead plant material) which will break down into soil and may build up around buildings. Likewise, the erosion of the buildings themselves through the action of rain, wind and ice may cause them to break-up and effectively 'bury themselves' but this would not to be any great depth.

I am not a physics grad or anything, just a curious guy.

Any help is much appreciated.


The Dead Sea in the Bible – Biblical History and Significance of the Lowest Point on Earth

Have you ever been to the Dead Sea? If not, put it on your wish list – it is well worth the travel. The Dead Sea, lying 1,300 feet below sea level, is the lowest and most mineral-rich body of water in the world. I ts 34.2% salinity makes it one of the world’s saltiest bodies of water. The Dead Sea depth is 304 meters (997 feet), which makes it the deepest hyper-saline lake in the world. A significant landmark, too salty to sustain any marine life, the Dead Sea is famous for the water’s mysteriously buoyant qualities which allow people to float across the top of the water, without needing to swim. The special features of the Dead Sea don’t end there: the lake’s waters are a light turquoise-blue color and it is surrounded by golden brown hills here and there, bright white salt crystals jut out of the water. The mineral-rich water and mud of the Dead Sea are believed to have numerous benefits for the body, especially for skin, respiratory and arthritic conditions. For this reason, many people visit the Dead Sea every year to get special treatments at the spas surrounding it they are joined by tourists who visit the area for its beauty, uniqueness, and luxurious spa resorts.


History of Sea Level Rise

Almost all of the water on Earth is stored in two places: in the oceans (currently 97 percent of all water) and in glaciers (currently about 2.7 percent). How much water is in the oceans—and thus how high sea level is—largely depends on how much water is trapped in glacial ice.

Throughout our planet's history, sea level has risen and fallen dramatically. At times, there was no ice at the poles and the ocean was hundreds of feet higher than it is now at other times, ice covered the planet and sea level was hundreds of feet lower. These changes are part of Earth's natural glacial cycles and have occurred over millions of years. Scientists use sediment and ice cores to learn more about sea level before the advent of tide gauges and satellites.

Last Glacial Period

This map depicts the Earth during the last ice age, specifically the Late Glacial Maximum (roughly 14,000 BCE) when the climate began to warm substantially. With so much of the planet's water tied up in ice, global sea level was more than 400 feet lower than it is today. The artist worked with climatologists and glaciologists to make the map as accurate as possible. (© Martin Vargic)

Earth's most recent glacial period peaked about 26,500 years ago. At that time, around 10 million square miles (26 million square kilometers) of ice covered the Earth. The Laurentide ice sheet covered Canada and the American Midwest, stretching over Minnesota and Wisconsin south to New York and the Rocky Mountains. Across the Atlantic, ice blanketed Iceland and stretched down over the British Isles and northern Europe, including Germany and Poland. The Patagonian ice sheet crept north from Antarctica to cover parts of Chile and Argentina. The climate was colder and drier globally rain was scarce, but pockets of rainforest survived in the tropics. With so much of the planet's water tied up in ice, global sea level was more than 400 feet lower than it is today.

Low sea level meant that some land masses that are currently submerged were accessible to people. One of the best known is the Bering Land Bridge, which connected Alaska to Siberia. The first people to reach the Americas migrated across the land bridge and settled here. Land animals also made the journey over the bridge in both directions to colonize new continents. As the world's glaciers and ice sheets melted during the following millennia, the Bering Land Bridge was flooded and disappeared beneath the ocean's surface, cutting off the migration route.

Sea Level on the Rise

The amount of carbon dioxide (CO2) in the atmosphere has been measured at Mauna Loa Observatory on Hawaii since the 1950's. There has been a steady rise in carbon dioxide since the measurements began, and you can see the rise and fall on a yearly basis due to plants growing and absorbing CO2 every spring and summer. In 2015 the annual growth rate jumped by 3.05 parts per million, the largest year-to-year increase in their 56 years of measurements. (Scripps Institution of Oceanography & NOAA)

Over the past 20,000 years or so, sea level has climbed some 400 feet (120 meters). As the climate warmed as part of a natural cycle, ice melted and glaciers retreated until ice sheets remained only at the poles and at the peaks of mountains. Early on, the sea rose rapidly, sometimes at rates greater than 10 feet (3 meters) per century, and then continued to grow in spurts of rapid sea level rise until about 7,000 years ago. Then, the climate stabilized and sea level rise slowed, holding largely steady for most of the last 2,000 years, based on records from corals and sediment cores. Now, however, sea level is on the rise again, rising faster now than it has in the past 6,000 years. The oldest tide gauges and coastal sediment preserved beneath swamps and marshes show that sea level began to rise around 1850, which is right around the time people started burning coal to propel steam engine trains, and it hasn't stopped since. The climate likely started warming as a part of a natural cycle, but the accelerated warming in the last two hundred years or so is due to a rise in atmospheric carbon dioxide. The resulting rise in sea level is likely twice what we would have seen without the increase in greenhouse gasses due to human activities.

Today, global sea level is 5-8 inches (13-20 cm) higher on average than it was in 1900. Between 1900 and 2000, global sea level rose between 0.05 inches (1.2 millimeters) and 0.07 inches (1.7 millimeters) per year on average. In the 1990s, that rate jumped to around 3.2 millimeters per year. In 2016 the rate was estimated to be 3.4 millimeters per year, and it is expected to jump higher by the end of the century. Scientists with the Intergovernmental Project on Climate Change predict that global sea level will rise between 0.3 and 1 meter by 2100. Eventually, sea level is expected to rise around 2.3 meters for every degree (°C) that climate change warms the planet, and Earth has warmed by 1°C already. What scientists don't know is how long it will take for sea level to catch up to the temperature increase. Whether it takes another 200 or 2000 years largely depends on how quickly the ice sheets melt. Even if global warming were to stop today, sea level would continue to rise.


What Earth's Climate Was Like Last Time CO2 Was Over 400ppm (Like Now)

This is what Earth's climate was like the last time global carbon dioxide levels were consistently at or above 400 parts per million.

“What was the climate and sea level like at times in Earth’s history when carbon dioxide in the atmosphere was at 400ppm?”

The last time global carbon dioxide levels were consistently at or above 400 parts per million (ppm) was around four million years ago during a geological period known as the Pliocene Era (between 5.3 million and 2.6 million years ago). The world was about 3℃ warmer and sea levels were higher than today.

We know how much carbon dioxide the atmosphere contained in the past by studying ice cores from Greenland and Antarctica. As compacted snow gradually changes to ice, it traps air in bubbles that contain samples of the atmosphere at the time . We can sample ice cores to reconstruct past concentrations of carbon dioxide, but this record only takes us back about a million years.

Beyond a million years, we don’t have any direct measurements of the composition of ancient atmospheres, but we can use several methods to estimate past levels of carbon dioxide. One method uses the relationship between plant pores, known as stomata, that regulate gas exchange in and out of the plant. The density of these stomata is related to atmospheric carbon dioxide , and fossil plants are a good indicator of concentrations in the past.

Another technique is to examine sediment cores from the ocean floor. The sediments build up year after year as the bodies and shells of dead plankton and other organisms rain down on the seafloor. We can use isotopes (chemically identical atoms that differ only in atomic weight) of boron taken from the shells of the dead plankton to reconstruct changes in the acidity of seawater. From this we can work out the level of carbon dioxide in the ocean.

The data from four-million-year-old sediments suggest that carbon dioxide was at 400ppm back then .

Sea levels and changes in Antarctica

During colder periods in Earth’s history, ice caps and glaciers grow and sea levels drop. In the recent geological past, during the most recent ice age about 20,000 years ago, sea levels were at least 120 metres lower than they are today.

Sea-level changes are calculated from changes in isotopes of oxygen in the shells of marine organisms. For the Pliocene Era, research shows the sea-level change between cooler and warmer periods was around 30-40 metres and sea level was higher than today. Also during the Pliocene, we know the West Antarctic Ice Sheet was significantly smaller and global average temperatures were about 3℃ warmer than today. Summer temperatures in high northern latitudes were up to 14℃ warmer.

This may seem like a lot but modern observations show strong polar amplification of warming: a 1℃ increase at the equator may raise temperatures at the poles by 6-7℃. It is one of the reasons why Arctic sea ice is disappearing.

Impacts in New Zealand and Australasia

In the Australasian region, there was no Great Barrier Reef, but there may have been smaller reefs along the northeast coast of Australia . For New Zealand, the partial melting of the West Antarctic Ice Sheet is probably the most critical point.

One of the key features of New Zealand’s current climate is that Antarctica is cut off from global circulation during the winter because of the big temperature contrast between Antarctica and the Southern Ocean. When it comes back into circulation in springtime, New Zealand gets strong storms. Stormier winters and significantly warmer summers were likely in the mid-Pliocene because of a weaker polar vortex and a warmer Antarctica.

It will take more than a few years or decades of carbon dioxide concentrations at 400ppm to trigger a significant shrinking of the West Antarctic Ice Sheet. But recent studies show that West Antarctica is already melting .

Sea-level rise from a partial melting of West Antarctica could easily exceed a metre or more by 2100. In fact, if the whole of the West Antarctic melted it could raise sea levels by about 3.5 metres . Even smaller increases raise the risk of flooding in low-lying cities including Auckland, Christchurch and Wellington.

James Shulmeister is a Professor at the University of Canterbury.

This article is republished from The Conversation under a Creative Commons license. Read the original article .


Contents

The Precambrian includes approximately 90% of geologic time. It extends from 4.6 billion years ago to the beginning of the Cambrian Period (about 541 Ma). It includes three eons, the Hadean, Archean, and Proterozoic.

Major volcanic events altering the Earth's environment and causing extinctions may have occurred 10 times in the past 3 billion years. [4]

Hadean Eon Edit

During Hadean time (4.6–4 Ga), the Solar System was forming, probably within a large cloud of gas and dust around the sun, called an accretion disc from which Earth formed 4,500 million years ago . [5] The Hadean Eon is not formally recognized, but it essentially marks the era before we have adequate record of significant solid rocks. The oldest dated zircons date from about 4,400 million years ago . [6] [7] [8]

Earth was initially molten due to extreme volcanism and frequent collisions with other bodies. Eventually, the outer layer of the planet cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as a result of the impact of a large planetoid with the Earth. [9] [10] Some of this object's mass merged with the Earth, significantly altering its internal composition, and a portion was ejected into space. Some of the material survived to form an orbiting moon. More recent potassium isotopic studies suggest that the Moon was formed by a smaller, high-energy, high-angular-momentum giant impact cleaving off a significant portion of the Earth. [11] Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered from comets, produced the oceans. [12] However, more recently, in August 2020, researchers reported that sufficient water to fill the oceans may have always been on the Earth since the beginning of the planet's formation. [1] [2] [3]

During the Hadean the Late Heavy Bombardment occurred (approximately 4,100 to 3,800 million years ago ) during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus and Mars as well.

Archean Eon Edit

The Earth of the early Archean ( 4,000 to 2,500 million years ago ) may have had a different tectonic style. During this time, the Earth's crust cooled enough that rocks and continental plates began to form. Some scientists think because the Earth was hotter, that plate tectonic activity was more vigorous than it is today, resulting in a much greater rate of recycling of crustal material. This may have prevented cratonisation and continent formation until the mantle cooled and convection slowed down. Others argue that the subcontinental lithospheric mantle is too buoyant to subduct and that the lack of Archean rocks is a function of erosion and subsequent tectonic events. Some geologists view the sudden increase of aluminum content in zircons as indicator of the beginning of plate tectonics. [13]

In contrast to the Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments and banded iron formations. Greenstone belts are typical Archean formations, consisting of alternating high- and low-grade metamorphic rocks. The high-grade rocks were derived from volcanic island arcs, while the low-grade metamorphic rocks represent deep-sea sediments eroded from the neighboring island rocks and deposited in a forearc basin. In short, greenstone belts represent sutured protocontinents. [14]

The Earth's magnetic field was established 3.5 billion years ago. The solar wind flux was about 100 times the value of the modern Sun, so the presence of the magnetic field helped prevent the planet's atmosphere from being stripped away, which is what probably happened to the atmosphere of Mars. However, the field strength was lower than at present and the magnetosphere was about half the modern radius. [15]

Proterozoic Eon Edit

The geologic record of the Proterozoic ( 2,500 to 541 million years ago ) is more complete than that for the preceding Archean. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas furthermore, many of these rocks are less metamorphosed than Archean-age ones, and plenty are unaltered. [16] Study of these rocks show that the eon featured massive, rapid continental accretion (unique to the Proterozoic), supercontinent cycles, and wholly modern orogenic activity. [17] Roughly 750 million years ago , [18] the earliest-known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 Ma. [7] [19]

The first-known glaciations occurred during the Proterozoic, one began shortly after the beginning of the eon, while there were at least four during the Neoproterozoic, climaxing with the Snowball Earth of the Varangian glaciation. [20]

The Phanerozoic Eon is the current eon in the geologic timescale. It covers roughly 541 million years. During this period continents drifted about, eventually collected into a single landmass known as Pangea and then split up into the current continental landmasses.

The Phanerozoic is divided into three eras – the Paleozoic, the Mesozoic and the Cenozoic.

Most of the evolution of multicellular life occurred during this time period.

Paleozoic Era Edit

The Paleozoic spanned from roughly 542 to 251 million years ago (Ma) [7] and is subdivided into six geologic periods from oldest to youngest they are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian. Geologically, the Paleozoic starts shortly after the breakup of a supercontinent called Pannotia and at the end of a global ice age. Throughout the early Paleozoic, the Earth's landmass was broken up into a substantial number of relatively small continents. Toward the end of the era the continents gathered together into a supercontinent called Pangaea, which included most of the Earth's land area.

Cambrian Period Edit

The Cambrian is a major division of the geologic timescale that begins about 541.0 ± 1.0 Ma. [7] Cambrian continents are thought to have resulted from the breakup of a Neoproterozoic supercontinent called Pannotia. The waters of the Cambrian period appear to have been widespread and shallow. Continental drift rates may have been anomalously high. Laurentia, Baltica and Siberia remained independent continents following the break-up of the supercontinent of Pannotia. Gondwana started to drift toward the South Pole. Panthalassa covered most of the southern hemisphere, and minor oceans included the Proto-Tethys Ocean, Iapetus Ocean and Khanty Ocean.

Ordovician period Edit

The Ordovician period started at a major extinction event called the Cambrian–Ordovician extinction event some time about 485.4 ± 1.9 Ma. [7] During the Ordovician the southern continents were collected into a single continent called Gondwana. Gondwana started the period in the equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician the continents Laurentia, Siberia and Baltica were still independent continents (since the break-up of the supercontinent Pannotia earlier), but Baltica began to move toward Laurentia later in the period, causing the Iapetus Ocean to shrink between them. Also, Avalonia broke free from Gondwana and began to head north toward Laurentia. The Rheic Ocean was formed as a result of this. By the end of the period, Gondwana had neared or approached the pole and was largely glaciated.

The Ordovician came to a close in a series of extinction events that, taken together, comprise the second-largest of the five major extinction events in Earth's history in terms of percentage of genera that became extinct. The only larger one was the Permian-Triassic extinction event. The extinctions occurred approximately 447 to 444 million years ago [7] and mark the boundary between the Ordovician and the following Silurian Period.

The most-commonly accepted theory is that these events were triggered by the onset of an ice age, in the Hirnantian faunal stage that ended the long, stable greenhouse conditions typical of the Ordovician. The ice age was probably not as long-lasting as once thought study of oxygen isotopes in fossil brachiopods shows that it was probably no longer than 0.5 to 1.5 million years. [21] The event was preceded by a fall in atmospheric carbon dioxide (from 7000ppm to 4400ppm) which selectively affected the shallow seas where most organisms lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it. Evidence of these ice caps have been detected in Upper Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time.

Silurian Period Edit

The Silurian is a major division of the geologic timescale that started about 443.8 ± 1.5 Ma. [7] During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian ice caps were less extensive than those of the late Ordovician glaciation. The melting of ice caps and glaciers contributed to a rise in sea levels, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity. Other cratons and continent fragments drifted together near the equator, starting the formation of a second supercontinent known as Euramerica. The vast ocean of Panthalassa covered most of the northern hemisphere. Other minor oceans include Proto-Tethys, Paleo-Tethys, Rheic Ocean, a seaway of Iapetus Ocean (now in between Avalonia and Laurentia), and newly formed Ural Ocean.

Devonian Period Edit

The Devonian spanned roughly from 419 to 359 Ma. [7] The period was a time of great tectonic activity, as Laurasia and Gondwana drew closer together. The continent Euramerica (or Laurussia) was created in the early Devonian by the collision of Laurentia and Baltica, which rotated into the natural dry zone along the Tropic of Capricorn. In these near-deserts, the Old Red Sandstone sedimentary beds formed, made red by the oxidized iron (hematite) characteristic of drought conditions. Near the equator Pangaea began to consolidate from the plates containing North America and Europe, further raising the northern Appalachian Mountains and forming the Caledonian Mountains in Great Britain and Scandinavia. The southern continents remained tied together in the supercontinent of Gondwana. The remainder of modern Eurasia lay in the Northern Hemisphere. Sea levels were high worldwide, and much of the land lay submerged under shallow seas. The deep, enormous Panthalassa (the "universal ocean") covered the rest of the planet. Other minor oceans were Paleo-Tethys, Proto-Tethys, Rheic Ocean and Ural Ocean (which was closed during the collision with Siberia and Baltica).

Carboniferous Period Edit

The Carboniferous extends from about 358.9 ± 0.4 to about 298.9 ± 0.15 Ma. [7]

A global drop in sea level at the end of the Devonian reversed early in the Carboniferous this created the widespread epicontinental seas and carbonate deposition of the Mississippian. There was also a drop in south polar temperatures southern Gondwana was glaciated throughout the period, though it is uncertain if the ice sheets were a holdover from the Devonian or not. These conditions apparently had little effect in the deep tropics, where lush coal swamps flourished within 30 degrees of the northernmost glaciers. A mid-Carboniferous drop in sea-level precipitated a major marine extinction, one that hit crinoids and ammonites especially hard. This sea-level drop and the associated unconformity in North America separate the Mississippian Period from the Pennsylvanian period. [22]

The Carboniferous was a time of active mountain building, as the supercontinent Pangea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America-Europe (Laurussia) along the present line of eastern North America. This continental collision resulted in the Hercynian orogeny in Europe, and the Alleghenian orogeny in North America it also extended the newly uplifted Appalachians southwestward as the Ouachita Mountains. [23] In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural mountains. There were two major oceans in the Carboniferous the Panthalassa and Paleo-Tethys. Other minor oceans were shrinking and eventually closed the Rheic Ocean (closed by the assembly of South and North America), the small, shallow Ural Ocean (which was closed by the collision of Baltica, and Siberia continents, creating the Ural Mountains) and Proto-Tethys Ocean.

Permian Period Edit

The Permian extends from about 298.9 ± 0.15 to 252.17 ± 0.06 Ma. [7]

During the Permian all the Earth's major land masses, except portions of East Asia, were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean (Panthalassa, the universal sea), and the Paleo-Tethys Ocean, a large ocean that was between Asia and Gondwana. The Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic Era. Large continental landmasses create climates with extreme variations of heat and cold ("continental climate") and monsoon conditions with highly seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea.

Mesozoic Era Edit

The Mesozoic extended roughly from 252 to 66 million years ago . [7]

After the vigorous convergent plate mountain-building of the late Paleozoic, Mesozoic tectonic deformation was comparatively mild. Nevertheless, the era featured the dramatic rifting of the supercontinent Pangaea. Pangaea gradually split into a northern continent, Laurasia, and a southern continent, Gondwana. This created the passive continental margin that characterizes most of the Atlantic coastline (such as along the U.S. East Coast) today.

Triassic Period Edit

The Triassic Period extends from about 252.17 ± 0.06 to 201.3 ± 0.2 Ma. [7] During the Triassic, almost all the Earth's land mass was concentrated into a single supercontinent centered more or less on the equator, called Pangaea ("all the land"). This took the form of a giant "Pac-Man" with an east-facing "mouth" constituting the Tethys sea, a vast gulf that opened farther westward in the mid-Triassic, at the expense of the shrinking Paleo-Tethys Ocean, an ocean that existed during the Paleozoic.

The remainder was the world-ocean known as Panthalassa ("all the sea"). All the deep-ocean sediments laid down during the Triassic have disappeared through subduction of oceanic plates thus, very little is known of the Triassic open ocean. The supercontinent Pangaea was rifting during the Triassic—especially late in the period—but had not yet separated. The first nonmarine sediments in the rift that marks the initial break-up of Pangea—which separated New Jersey from Morocco—are of Late Triassic age in the U.S., these thick sediments comprise the Newark Supergroup. [24] Because of the limited shoreline of one super-continental mass, Triassic marine deposits are globally relatively rare despite their prominence in Western Europe, where the Triassic was first studied. In North America, for example, marine deposits are limited to a few exposures in the west. Thus Triassic stratigraphy is mostly based on organisms living in lagoons and hypersaline environments, such as Estheria crustaceans and terrestrial vertebrates. [25]

Jurassic Period Edit

The Jurassic Period extends from about 201.3 ± 0.2 to 145.0 Ma. [7] During the early Jurassic, the supercontinent Pangaea broke up into the northern supercontinent Laurasia and the southern supercontinent Gondwana the Gulf of Mexico opened in the new rift between North America and what is now Mexico's Yucatan Peninsula. The Jurassic North Atlantic Ocean was relatively narrow, while the South Atlantic did not open until the following Cretaceous Period, when Gondwana itself rifted apart. [26] The Tethys Sea closed, and the Neotethys basin appeared. Climates were warm, with no evidence of glaciation. As in the Triassic, there was apparently no land near either pole, and no extensive ice caps existed. The Jurassic geological record is good in western Europe, where extensive marine sequences indicate a time when much of the continent was submerged under shallow tropical seas famous locales include the Jurassic Coast World Heritage Site and the renowned late Jurassic lagerstätten of Holzmaden and Solnhofen. [27] In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface. [28] Though the epicontinental Sundance Sea left marine deposits in parts of the northern plains of the United States and Canada during the late Jurassic, most exposed sediments from this period are continental, such as the alluvial deposits of the Morrison Formation. The first of several massive batholiths were emplaced in the northern Cordillera beginning in the mid-Jurassic, marking the Nevadan orogeny. [29] Important Jurassic exposures are also found in Russia, India, South America, Japan, Australasia and the United Kingdom.

Cretaceous Period Edit

The Cretaceous Period extends from circa 145 million years ago to 66 million years ago . [7]

During the Cretaceous, the late Paleozoic-early Mesozoic supercontinent of Pangaea completed its breakup into present day continents, although their positions were substantially different at the time. As the Atlantic Ocean widened, the convergent-margin orogenies that had begun during the Jurassic continued in the North American Cordillera, as the Nevadan orogeny was followed by the Sevier and Laramide orogenies. Though Gondwana was still intact in the beginning of the Cretaceous, Gondwana itself broke up as South America, Antarctica and Australia rifted away from Africa (though India and Madagascar remained attached to each other) thus, the South Atlantic and Indian Oceans were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising eustatic sea levels worldwide.

To the north of Africa the Tethys Sea continued to narrow. Broad shallow seas advanced across central North America (the Western Interior Seaway) and Europe, then receded late in the period, leaving thick marine deposits sandwiched between coal beds. At the peak of the Cretaceous transgression, one-third of Earth's present land area was submerged. [30] The Cretaceous is justly famous for its chalk indeed, more chalk formed in the Cretaceous than in any other period in the Phanerozoic. [31] Mid-ocean ridge activity—or rather, the circulation of seawater through the enlarged ridges—enriched the oceans in calcium this made the oceans more saturated, as well as increased the bioavailability of the element for calcareous nanoplankton. [32] These widespread carbonates and other sedimentary deposits make the Cretaceous rock record especially fine. Famous formations from North America include the rich marine fossils of Kansas's Smoky Hill Chalk Member and the terrestrial fauna of the late Cretaceous Hell Creek Formation. Other important Cretaceous exposures occur in Europe and China. In the area that is now India, massive lava beds called the Deccan Traps were laid down in the very late Cretaceous and early Paleocene.

Cenozoic Era Edit

The Cenozoic Era covers the 66 million years since the Cretaceous–Paleogene extinction event up to and including the present day. By the end of the Mesozoic era, the continents had rifted into nearly their present form. Laurasia became North America and Eurasia, while Gondwana split into South America, Africa, Australia, Antarctica and the Indian subcontinent, which collided with the Asian plate. This impact gave rise to the Himalayas. The Tethys Sea, which had separated the northern continents from Africa and India, began to close up, forming the Mediterranean Sea.

Paleogene Period Edit

The Paleogene (alternatively Palaeogene) Period is a unit of geologic time that began 66 and ended 23.03 Ma [7] and comprises the first part of the Cenozoic Era. This period consists of the Paleocene, Eocene and Oligocene Epochs.

Paleocene Epoch Edit

The Paleocene, lasted from 66 million years ago to 56 million years ago . [7]

In many ways, the Paleocene continued processes that had begun during the late Cretaceous Period. During the Paleocene, the continents continued to drift toward their present positions. Supercontinent Laurasia had not yet separated into three continents. Europe and Greenland were still connected. North America and Asia were still intermittently joined by a land bridge, while Greenland and North America were beginning to separate. [33] The Laramide orogeny of the late Cretaceous continued to uplift the Rocky Mountains in the American west, which ended in the succeeding epoch. South and North America remained separated by equatorial seas (they joined during the Neogene) the components of the former southern supercontinent Gondwana continued to split apart, with Africa, South America, Antarctica and Australia pulling away from each other. Africa was heading north toward Europe, slowly closing the Tethys Ocean, and India began its migration to Asia that would lead to a tectonic collision and the formation of the Himalayas.

Eocene Epoch Edit

During the Eocene ( 56 million years ago - 33.9 million years ago ), [7] the continents continued to drift toward their present positions. At the beginning of the period, Australia and Antarctica remained connected, and warm equatorial currents mixed with colder Antarctic waters, distributing the heat around the world and keeping global temperatures high. But when Australia split from the southern continent around 45 Ma, the warm equatorial currents were deflected away from Antarctica, and an isolated cold water channel developed between the two continents. The Antarctic region cooled down, and the ocean surrounding Antarctica began to freeze, sending cold water and ice floes north, reinforcing the cooling. The present pattern of ice ages began about 40 million years ago . [ citation needed ]

The northern supercontinent of Laurasia began to break up, as Europe, Greenland and North America drifted apart. In western North America, mountain building started in the Eocene, and huge lakes formed in the high flat basins among uplifts. In Europe, the Tethys Sea finally vanished, while the uplift of the Alps isolated its final remnant, the Mediterranean, and created another shallow sea with island archipelagos to the north. Though the North Atlantic was opening, a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar. India continued its journey away from Africa and began its collision with Asia, creating the Himalayan orogeny.

Oligocene Epoch Edit

The Oligocene Epoch extends from about 34 million years ago to 23 million years ago . [7] During the Oligocene the continents continued to drift toward their present positions.

Antarctica continued to become more isolated and finally developed a permanent ice cap. Mountain building in western North America continued, and the Alps started to rise in Europe as the African plate continued to push north into the Eurasian plate, isolating the remnants of Tethys Sea. A brief marine incursion marks the early Oligocene in Europe. There appears to have been a land bridge in the early Oligocene between North America and Europe since the faunas of the two regions are very similar. During the Oligocene, South America was finally detached from Antarctica and drifted north toward North America. It also allowed the Antarctic Circumpolar Current to flow, rapidly cooling the continent.

Neogene Period Edit

The Neogene Period is a unit of geologic time starting 23.03 Ma. [7] and ends at 2.588 Ma. The Neogene Period follows the Paleogene Period. The Neogene consists of the Miocene and Pliocene and is followed by the Quaternary Period.

Miocene Epoch Edit

The Miocene extends from about 23.03 to 5.333 Ma. [7]

During the Miocene continents continued to drift toward their present positions. Of the modern geologic features, only the land bridge between South America and North America was absent, the subduction zone along the Pacific Ocean margin of South America caused the rise of the Andes and the southward extension of the Meso-American peninsula. India continued to collide with Asia. The Tethys Seaway continued to shrink and then disappeared as Africa collided with Eurasia in the Turkish-Arabian region between 19 and 12 Ma (ICS 2004). Subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea resulting in the Messinian salinity crisis near the end of the Miocene.

Pliocene Epoch Edit

The Pliocene extends from 5.333 million years ago to 2.588 million years ago . [7] During the Pliocene continents continued to drift toward their present positions, moving from positions possibly as far as 250 kilometres (155 mi) from their present locations to positions only 70 km from their current locations.

South America became linked to North America through the Isthmus of Panama during the Pliocene, bringing a nearly complete end to South America's distinctive marsupial faunas. The formation of the Isthmus had major consequences on global temperatures, since warm equatorial ocean currents were cut off and an Atlantic cooling cycle began, with cold Arctic and Antarctic waters dropping temperatures in the now-isolated Atlantic Ocean. Africa's collision with Europe formed the Mediterranean Sea, cutting off the remnants of the Tethys Ocean. Sea level changes exposed the land-bridge between Alaska and Asia. Near the end of the Pliocene, about 2.58 million years ago (the start of the Quaternary Period), the current ice age began. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40,000–100,000 years.

Quaternary Period Edit

Pleistocene Epoch Edit

The Pleistocene extends from 2.588 million years ago to 11,700 years before present. [7] The modern continents were essentially at their present positions during the Pleistocene, the plates upon which they sit probably having moved no more than 100 kilometres (62 mi) relative to each other since the beginning of the period.

Holocene Epoch Edit

The Holocene Epoch began approximately 11,700 calendar years before present [7] and continues to the present. During the Holocene, continental motions have been less than a kilometer.

The last glacial period of the current ice age ended about 10,000 years ago. [34] Ice melt caused world sea levels to rise about 35 metres (115 ft) in the early part of the Holocene. In addition, many areas above about 40 degrees north latitude had been depressed by the weight of the Pleistocene glaciers and rose as much as 180 metres (591 ft) over the late Pleistocene and Holocene, and are still rising today. The sea level rise and temporary land depression allowed temporary marine incursions into areas that are now far from the sea. Holocene marine fossils are known from Vermont, Quebec, Ontario and Michigan. Other than higher latitude temporary marine incursions associated with glacial depression, Holocene fossils are found primarily in lakebed, floodplain and cave deposits. Holocene marine deposits along low-latitude coastlines are rare because the rise in sea levels during the period exceeds any likely upthrusting of non-glacial origin. Post-glacial rebound in Scandinavia resulted in the emergence of coastal areas around the Baltic Sea, including much of Finland. The region continues to rise, still causing weak earthquakes across Northern Europe. The equivalent event in North America was the rebound of Hudson Bay, as it shrank from its larger, immediate post-glacial Tyrrell Sea phase, to near its present boundaries.


Engaging Your Core

To find evidence of a human presence, researchers begin not by looking for evidence of people, but by reconstructing the environment these early explorers would have encountered.

“We’re not on a treasure hunt,” says Todd Braje, an archaeologist at the California Academy of Sciences who is working with Gusick, Erlandson and colleagues on a project in the Channel Islands off Southern California. “We are mapping and sampling paleolandscapes. . Once we’re able to predict landforms, the soil, the ecology, we’ll start to have success identifying potential archaeological sites.”

A paleocoastline research project often starts by developing a customizable digital map from existing seafloor maps. The new maps can highlight data specific to the project’s focus, such as identifying sediment buried below the seafloor that could indicate the course of a long-extinct river.

Researchers use these maps to zero in on areas of interest. They then use different imaging tools to show both the seafloor and what’s beneath it on an ever-finer scale. Depending on depth and conditions, the team might also deploy remote sensing equipment or divers to refine their understanding of the specific location.

Coring is often the next step, when researchers sample layers of ancient soils, or paleosols, that were subsequently buried by marine sediments as the sea levels rose.

Paleosols are typically full of pollen and microfossils of simple organisms, such as diatoms, a kind of algae that can indicate climate conditions. The paleosols may even preserve sedimentary DNA shed from ancient organisms. Identifying what flora and fauna were present at the site can help researchers reconstruct the environment and determine whether it might have been attractive to human hunter-gatherers on the move.


Was the Earth's sea-level significantly lower in ancient times? - History

Cobscook Bay State Park, Maine. Photo: W. Menke

Last month I gave a public lecture entitled, “When Maine was California,” to an audience in a small town in Maine. It drew parallels between California, today, and Maine, 400 million years ago, when similar geologic processes were occurring. Afterward, a member of the audience asked me what geology had to say about global warming. The following is an expanded version of my answer. Note that I use the word geology to mean any element of the earth sciences that is focused on earth history, and do not distinguish the many sub-disciplines about which a specialist would be familiar.

Geologists think of the last 50 million years as the recent past, both because they represents only about one percent of the age of the earth, and because plate tectonics, the geologic process that controls conditions within the solid part of the earth, has operated without major change during that time period. This is the time period that is most relevant to gaining insights about earth’s climate that can be applied to the present-day global warming debate.

The geological record of ancient climate is excellent. Ancient temperatures can be determined very precisely, because the composition of the shells of corals and other marine organisms varies measurably with it. Furthermore, the plants and animals that lived during a given time and are now preserved as fossils indicate whether the climate was wet or dry. The overall climatic trend has been cooling, from an unusually warm period, called the Eocene Optimum, 55-45 million years ago, to an unusually cool period, colloquially called the Ice Age, which ended just 20,000 years ago. The overall range in temperature was enormous, about 35°F. The earth was so warm during the Eocene Optimum that Antarctica was ice-free ice caps did not start to form there until about 35 million years ago. Palm trees grew at high latitudes and cold-blooded animals, such as crocodiles, lived in the Arctic.

Lesson 1. The earth’s climate (including its average temperature) is highly variable.

Notwithstanding very divergent conditions, life flourished both during the Eocene Optimum and the Ice Age, though in both cases life was more abundant in some parts of the world than in others. The fossil record indicates that forests were common during the Eocence Optimum, yet some areas were sparsely vegetated steppes and deserts. While the great glaciers of the Ice Age were lifeless, extremely large mammals such as Woolly Mammoth and Giant Ground Sloth inhabited lower latitudes. The changing climate produced both winners and losers. Some species adapted others went extinct.

Lesson 2. Life flourished during both warm and cold periods changes in climate produced both winners and losers.

Roque Bluffs State Park, Maine. Photo: W. Menke

An important issue is whether climate variability is due to processes occurring on the earth, or to changes in the intensity of sunlight – for it’s the sun that keeps our planet warm. The geological evidence, though subtle, strongly supports earthly, and not solar, causes. This evidence is drawn from the study of the many shorter period climate fluctuations, some which last millions of years and other just thousands, which are superimposed on the long-term cooling trend.

Climate during the Ice Age (the last 4 million years) has been particularly unstable, with many swings of more than 10°F. These fluctuations are recorded in the annual layers of snow preserved in glaciers and in marine sediments, whose properties track the temperature at which they were formed. The timing of these swings closely follows regular fluctuations in the tilt of the earth’s axis and the shape of its orbit around the sun. Called Milankovitch cycles, they are due to the gravitational influence of the moon and planets. Their magnitude can be reliably calculated, since they are due to fluctuations of the position and orientation of the earth relative to the sun, and not to any change in the sun’s brightness. Surprisingly, they are too small to account for the large swings in temperature, unless the earth’s climate system is acting to amplify them. Here’s the subtle part of the argument: This mismatch between the feeble amplitude of the Milankovitch cycles and the large swings in climate is strong evidence that internal processes can cause strong climate variability.

Lesson 3. Variations in climate are mainly due to processes occurring on the earth, as contrasted to in the sun.

Ice Age carbon dioxide levels are well known, because bubbles of Ice Age air are preserved within the Antarctic and Greenland glaciers. More ancient carbon dioxide levels are difficult to measure, since no samples of older air have been preserved. Several indirect methods are in use, one based on the effect of ocean carbon dioxide levels on the composition of marine sediments, and another on its effect on now-fossil plant leaves. These measurements show fairly convincingly that the long-term cooling trend over the last 50 million years is associated with a gradual decrease in carbon dioxide levels, from 2000-3000 parts per million during the Eocene Optimum to 200 p.p.m. during the Ice Age. The cause of this decrease is not fully understood, but seems to indicate that the total amount of carbon that can influence climate (carbon in the atmosphere, biosphere and ocean) is slowly decreasing, possibly because an increasing amount of carbon is being tied up in sedimentary rocks such as limestone.

Lesson 4. Atmospheric carbon dioxide levels are highly variable, with the highest levels being associated with warm periods and the lowest levels associated with cold periods.

The correlation of atmospheric temperature with carbon dioxide reflects the latter’s role as a greenhouse gas. By absorbing heat radiated from the earth’s surface and re-radiating it back downward, it causes the earth’s surface to be warmer than it otherwise would be. The earth would be uninhabitable without the greenhouse effect, as can be seen by comparing the earth’s average temperature of about 60°F to the minus 100°F average temperature of the moon, which receives exactly the same amount of sunlight. An important question is whether the high carbon dioxide level at the time of the Eocene Optimum was the cause of the high temperatures that occurred during that time period.

Ascribing causes to fluctuation in climate is a tricky business, because atmospheric carbon dioxide level is only one factor among several that determine earth’s climate. Other important factors include: the amount of water vapor (another greenhouse gas) in the atmosphere the percentage of the sky covered by clouds, which reflect sunlight back into space the percent of land covered with ice and snow, which are also very reflective and the percentage covered by oceans and and forests, which are very absorbing. All factors act together to maintain a given temperature yet they feed back upon one another in complicated ways. Thus, for instance, had the Antarctic been glaciated during the Eocene Optimum (and the geological evidence is that it was ice-free), the world would have been somewhat cooler due to the high reflectivity of the ice. On the other hand, glaciers were absent precisely because the world was so warm. Geologic evidence alone cannot prove that the high levels of atmospheric carbon dioxide during the Eocene Optimum caused the high temperatures then, since the contribution of other factors, such as clouds and water vapor are unknown. Nevertheless, global climate models seem to indicate that such a high temperature only can be maintained in a world with high carbon dioxide no other combination of factors can explain it.

Changing global temperatures induce changes in patterns of rainfall, winds and ocean currents, all of which can have a profound effect on the ecosystem of a given region. A large decrease in rainfall will, of course, turn rainforest into a desert. However, geology has few specifics to offer on the subject of how any particular region will be affected. The factors that cause climate change at a given geographical location are too varied to allow convincing geological analogues. However, geology shows that variability is the norm. Some of today’s deserts were forested a few million years ago, and some of today’s forests were formerly deserts. From the human perspective, climate change has the potential of causing some areas to become less agriculturally productive (and therefore less inhabitable), and other to become more so.

Lesson 5. Local climates are very variable, changing dramatically over periods of thousands to millions of years.

Wolfe Neck Woods State Park, Maine. Photo: W. Menke

Changing global temperature can cause a rise or fall in sea level due to the accumulation or melting of glacial ice. This effect is global in extent and one that can have an extremely deleterious effect on us human beings, since so many of us live near the coast. The geological evidence is very strong that sea level was higher by about 200 feet at times, such as during the Eocene Optimum, when Antarctica was ice-free, and was about 400 feet lower during the height of the Ice Age. The range is enormous the world’s coastlines are radically altered by such changes. The continental shelves were substantially exposed during the low stands, and many low-lying coastal areas were underwater during the high stands. Woolly Mammoths roamed hundreds of miles offshore of Virginia during the Ice Age. Beach sand deposits in inland North Carolina indicate that the shoreline was far inland during the Eocene Optimum.

Lesson 6. Sea level has fluctuated as the world’s glaciers grow or recede, and was about 200 feet higher at times when Antarctica was ice-free.

Carbon dioxide levels have risen since the end of the Ice Age, first to a natural level of about 280 p.p.m. just before the start of the Industrial Era, and then to 400 p.p.m. as people burned coal and petroleum in large quantities. Carbon dioxide is currently increasing at a rate of about 2.6 p.p.m. per year.

A critical question is the level of atmospheric carbon dioxide 35 million years ago, when glaciers began to form in Antarctica, for it serves as a rough estimate of the concentration needed to melt present-day Antarctica. It’s a rough estimate only, for geological conditions were not exactly the same now and then. In particular, strong ocean currents that today keep warmer waters away from Antarctica were not present 35 million years ago, owing to the somewhat different configuration of tectonic plates. Unfortunately, the best currently-available estimates of atmospheric carbon dioxide during this critical time period have large uncertainties. Carbon dioxide decreased from 600-1400 p.p.m. at the start of the glaciations to 400-700 p.p.m. several million years later. These measurements are consistent with modeling results, which give a threshold of about 780 p.p.m. for the formation of a continental-scale ice cap on Antarctica. This value will be reached by the year 2150 at the present growth rate of atmospheric carbon dioxide – or sooner if emission rates continue to soar – suggesting that Antarctica will be at risk of melting at that time.

Antarctic ice will not melt overnight even should the threshold be reached. The deglaciation at the end of the Ice Age provides a useful example. The rate of sea level rise was initially low, just one-tenth of an inch per year. It then gradually increased, peaking at about 3 inches per year about 14,000 years ago, which was about 5,000 years after the start of the deglaciation. This rate persisted for 1,600 years, during which time sea level rose a total of 60 feet. The average rate of sea level rise was slower, about a half-inch per year.

Lesson 7. Sea level rise as fast as a few inches per year can persist over thousands of years.

The most extreme scenario for future carbon dioxide levels considered by the Intergovernmental Panel on Climate Change (IPCC) predicts about 0.4 inches per year of sea level rise over the next century. This rate is less than, but similar in magnitude, to the average rate during the Ice Age deglaciation, but considerably smaller than its peak. Because of its focus on the current century, a reader of the IPCC report might be left with the sense that sea level rise will be over by 2100. Precisely the opposite is true! Geology demonstrates that melting accelerates with time and can last for several thousand years.

The most important lessons drawn from geology are that the earth’s climate can change radically and that the pace of change can be rapid. Geology also supports the theory that past periods of especially warm temperature were caused by high atmospheric carbon dioxide level. Of the many effects of global warming, geology is currently most relevant to sea level rise caused by melting glaciers. The precision of the measurement is currently too poor to give an exact answer to a critical question, At what carbon dioxide level are we in danger of melting Antarctica? However, while crude, these estimates suggest that this threshold will be reached in 150-300 years, if carbon dioxide levels continue to rise at the current rate.

William Menke of the Lamont-Doherty Earth Observatory is a professor of earth and environmental sciences.


Earth's Heat Keeps America Afloat

Heat from the Earth’s deep interior helps keep much of North America afloat by warming the continental crust and making it buoyant, scientists say.

If not for this effect, many American coastal cities would lie beneath the sea.

“We have shown for the first time that temperature differences within the Earth’s crust and upper mantle explain about half of the elevation of any given place in North America,” said study team member David Chapman of the University of Utah. Rock composition differences can explain the other half, he added.

Using previously published data of how rock density varies with depth in North America’s crust, the researchers created a hypothetical continental crust with a uniform thickness and composition.

“Once we’ve done that, we can see the thermal effect,” Chapman explained. The researchers could then calculate how much heat flow contributes to elevation in each of the 36 tectonic provinces, or “mini-plates,” of North America.

The findings are detailed in two studies published in the Journal of Geophysical-Solid Earth, a publication of the American Geophysical Union.

Cities beneath the sea

The findings show that if North America had a uniform crust, many American cities would be underwater. New York City, for example, would be dunked 1,427 feet beneath the Atlantic. Boston would be 1,823 below sea level, and Los Angeles would be 3,756 beneath the Pacific.

Other cities would soar to new heights. Seattle, for instance, would reach an elevation of 5,949 feet, up from its current elevation of about 500 feet above sea level. The rock beneath America’s Emerald City is cooler than average for North America removing the temperature difference would cause the rock to expand and become more buoyant, so Seattle would rise.

Some regions would remain at the same elevation. “If you subtracted the heat that keeps North American elevations high, most of the continent would be below sea level, except the high Rocky Mountains, the Sierra Nevada and the Pacific Northwest of the Cascade Range,” says study team member Derrick Hasterok of the University of Utah.

No immediate threat

According to Chapman, scientists have largely overlooked differences in rock temperature as an explanation of elevations on continents. Instead, they usually attribute the buoyancy and elevation of various continental areas to variations in the thickness and mineral composition of crustal rocks.

As an example of how rock temperature affects elevation, the researchers point to the Colorado Plateau, which sits 6,000 feet above sea level, and the Great Plains, which is only 1,000 feet above sea level, despite having the same rock composition.

“We propose this is because, at the base of the crust, the Colorado Plateau is significantly warmer [1,200 degrees Fahrenheit] than the Great Plains [930 degrees Fahrenheit,” Hasterok said.

American cities are in no danger of being submerged any time soon, however, as it will take billions of years for North American rock to cool and become dense enough that it sinks, Chapman said.

If anything, coastal cities face flooding much sooner from sea level rise due to global warming, he added.

Here are other locations, their elevations and how they would sink if the crust had a uniform temperature:

--Atlanta, 1,000 feet above sea level, 1,416 feet below sea level. -- Dallas, 430 feet above sea level, 1,986 feet below sea level. -- Chicago, 586 feet above sea level, 2,229 feet below sea level. -- St. Louis, 465 feet above sea level, 1,499 feet below sea level. -- Las Vegas, 2,001 feet above sea level, 3,512 feet below sea level. -- Phoenix, 1,086 feet above sea level, 4,345 feet below sea level. -- Albuquerque, 5,312 feet above sea level, 48 feet above sea level. -- Mount Whitney, Calif., tallest point in the lower 48 states, 14,496 feet above sea level, 11,877 feet above sea level.


The Last Time CO2 Was This High, Humans Didn’t Exist

The last time there was this much carbon dioxide (CO2) in the Earth's atmosphere, modern humans didn't exist. Megatoothed sharks prowled the oceans, the world's seas were up to 100 feet higher than they are today, and the global average surface temperature was up to 11°F warmer than it is now.

As we near the record for the highest CO2 concentration in human history &mdash 400 parts per million &mdash climate scientists worry about where we were then, and where we're rapidly headed now.

According to data gathered at the Mauna Loa Observatory in Hawaii, the 400 ppm mark may briefly be exceeded this month, when CO2 typically hits a seasonal peak in the Northern Hemisphere, although it is more likely to take a couple more years until it stays above that threshold, according to Ralph Keeling, a researcher at the Scripps Institute of Oceanography.

CO2 levels are far higher now than they have been for anytime during the past 800,000 years.
Click image to enlarge. Credit: Scripps Institution of Oceanography.

Keeling is the son of Charles David Keeling, who began the CO2 observations at Mauna Loa in 1958 and for whom the iconic &ldquoKeeling Curve&rdquo is named.

Carbon dioxide is the most important long-lived global warming gas, and once it is emitted by burning fossil fuels such as coal and oil, a single CO2 molecule can remain in the atmosphere for hundreds of years. Global CO2 emissions reached a record high of 35.6 billion tonnes in 2012, up 2.6 percent from 2011. Carbon dioxide and other greenhouse gases warm the planet by absorbing the sun&rsquos energy and preventing heat from escaping back into space.

The news that CO2 is near 400 ppm for the first time highlights a question that scientists have been investigating using a variety of methods: when was the last time that CO2 levels were this high, and what was the climate like back then?

There is no single, agreed-upon answer to those questions as studies show a wide date range from between 800,000 to 15 million years ago. The most direct evidence comes from tiny bubbles of ancient air trapped in the vast ice sheets of Antarctica. By drilling for ice cores and analyzing the air bubbles, scientists have found that, at no point during at least the past 800,000 years have atmospheric CO2 levels been as high as they are now.

That means that in the entire history of human civilization, CO2 levels have never been this high.

The Keeling Curve, showing CO2 concentrations increasing to near 400 ppm in 2013.
Credit: NOAA.

Other research, though, shows that you have to go back much farther in time, well beyond 800,000 years ago, to find an instance where CO2 was sustained at 400 ppm or greater.

For a 2009 study, published in the journal Science, scientists analyzed shells in deep sea sediments to estimate past CO2 levels, and found that CO2 levels have not been as high as they are now for at least the past 10 to 15 million years, during the Miocene epoch.

&ldquoThis was a time when global temperatures were substantially warmer than today, and there was very little ice around anywhere on the planet. And so sea level was considerably higher &mdash around 100 feet higher &mdash than it is today,&rdquo said Pennsylvania State University climate scientist Michael Mann, in an email conversation. &ldquoIt is for this reason that some climate scientists, like James Hansen, have argued that even current-day CO2 levels are too high. There is the possibility that we&rsquove already breached the threshold of truly dangerous human influence on our climate and planet.”

Sea levels are increasing today in response to the warming climate, as ice sheets melt and seas expand due to rising temperatures. Scientists are projecting up to 3 feet or more of global sea level rise by 2100, which would put some coastal cities in peril.

While there have been past periods in Earth's history when temperatures were warmer than they are now, the rate of change that is currently taking place is faster than most of the climate shifts that have occurred in the past, and therefore it will likely be more difficult to adapt to.

A 2011 study in the journal Paleoceanography found that atmospheric CO2 levels may have been comparable to today&rsquos as recently as sometime between 2 and 4.6 million years ago, during the Pliocene epoch, which saw the arrival of Homo habilis , a possible ancestor of modern homo sapiens, and when herds of giant, elephant-like Mastadons roamed North America. Modern human civilization didn&rsquot arrive on the scene until the Holocene Epoch, which began 12,000 years ago.

Regardless of which estimate is correct, it is clear that CO2 levels are now higher than they have ever been in mankind&rsquos history. With global CO2 emissions continuing on an upward trajectory that is likely to put CO2 concentrations above 450 ppm or higher, it is extremely unlikely that the steadily rising shape of the Keeling Curve is going to change anytime soon.

“There's an esthetic to the curve that's beautiful science and troubling reality,&rdquo Keeling said. &ldquoI'd very much like to see the curve change from going steadily upward to flattening out.”


References

Most of the references and quotations in the Chronology have been been taken from the Catastrophism by Richard Huggett. This work is a synoptic view of changing perspectives both of change in the inorganic and organic world. Dalrymple's Age of the Earth is a standard source for understanding how the age of the Earth is determined.

Russell, H.N., 1921. A superior limit to the age of the Earth's crust in Proceedings of the Royal Society of London , series A, vol. 99, pp. 84-86.

Dalrymple, G. Brent, 1991. The Age of the Earth . California: Stanford University Press, ISBN 0-8047-1569-6.

Richard Huggett, Catastrophism , 1997, Verso, ISBN 1-85984-129-5.

Hugh Miller, The Testimony of the Rocks , 1857, Gould and Lincoln: Boston

Patterson, C.C., 1953. "The isotopic composition of meteoritic, basaltic and oceanic leads, and the age of the Earth" in Proceedings of the Conference on Nuclear Processes in Geologic Settings , Williams Bay, Wisconsin, September 21-23, 1953. pp. 36-40.

Patterson, Clair C., 1997. Duck Soup and Lead in Engineering & Science (Caltech Alumni Magazine) volume LX, number 1, pp. 21-31.

Russell, H.N., 1921. A superior limit to the age of the Earth's crust in Proceedings of the Royal Society of London , series A, vol. 99, pp. 84-86.