Pliocene climate

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Pliocene sst anomaly.png
Mid-Pliocene reconstructed annual sea surface temperature anomaly
Pliocene benthic carbonate 18O.png
δ18O Benthic foraminifera 0–7 Ma

During the Pliocene epoch (5.3 to 2.6 million years ago (Ma)), the Earth's climate became cooler and drier, as well as more seasonal, marking a transition between the relatively warm Miocene to the cooler Pleistocene. [1]

Contents

However, the global average temperature in the mid-Pliocene (3.3 Ma–3 Ma) was 2–3 °C higher than today, [2] global sea level 25 meters higher, [3] and the northern hemisphere ice sheet was ephemeral before the onset of extensive glaciation over Greenland that occurred in the late Pliocene around 3 Ma. [4] The formation of an Arctic ice cap is signaled by an abrupt shift in oxygen isotope ratios, [5] the appearance of ice-rafted cobbles in the North Atlantic and North Pacific ocean beds, [6] and high-resolution rock magnetic records indicating iceberg surges. [5] Mid-latitude glaciation was probably underway before the end of the epoch. The global cooling that occurred during the Pliocene may have spurred on the disappearance of forests and the spread of grasslands and savannas. [7]

During the Pliocene the earth climate system response shifted from a period of high frequency-low amplitude oscillation dominated by the 41,000-year period of Earth's obliquity to one of low-frequency, high-amplitude oscillation dominated by the 100,000-year period of the orbital eccentricity characteristic of the Pleistocene glacial-interglacial cycles. [8]

The equatorial Pacific Ocean sea surface temperature gradient was considerably lower than it is today. Mean sea surface temperatures in the east were substantially warmer than today but similar in the west. This condition has been described as a permanent El Niño state, or “El Padre.” [9] Several mechanisms have been proposed for this pattern, including increased tropical cyclone activity. [10]

Setting

During the late Pliocene and early Pleistocene Series of the Cenozoic Era, 3.6 to 2.2 Ma (million years ago), the Arctic was much warmer than it is at the present day (with summer temperatures some 8 °C warmer than today). That is a key finding of research into a lake-sediment core obtained in Eastern Siberia, which is of exceptional importance because it has provided the longest continuous late Cenozoic land-based sedimentary record thus far. [11]

Central Asia became more seasonal during the Pliocene, with colder, drier winters and wetter summers, which contributed to an increase in the abundance of C4 plants across the region. [12] In the Loess Plateau, δ13C values of occluded organic matter increased by 2.5% while those of pedogenic carbonate increased by 5% over the course of the Late Miocene and Pliocene, indicating increased aridification. [13] Further aridification of Central Asia was caused by the development of Northern Hemisphere glaciation during the Late Pliocene. [14] A sediment core from the northern South China Sea shows an increase in dust storm activity during the middle Pliocene. [15]

In the south-central Andes, an arid period occurred from 6.1 to 5.2 Ma, with another occurring from 3.6 to 3.3 Ma. These arid periods are coincident with global cold periods, during which the position of the Southern Hemisphere westerlies shifted northward and disrupted the South American Low Level Jet, which brings moisture to southeastern South America. [16]

In northwestern Africa, tropical forests extended up to Cape Blanc during the Zanclean until around 3.5 Ma. During the Piacenzian, from about 3.5 to 2.6 Ma, the region was forested at irregular intervals and contained a significant Saharan palaeoriver until 3.35 Ma, when trade winds began to dominate over fluvial transport of pollen. Around 3.26 Ma, a strong aridification event that was followed by a return to more humid conditions, which was itself followed by another aridification around 2.7 Ma. From 2.6 to 2.4 Ma, vegetation zones began repeatedly shifting latitudinally in response to glacial-interglacial cycles. [17]

The climate of eastern Africa was very similar to what it is today. Unexpectedly, the expansion of grasslands in eastern Africa during this epoch appears to have been decoupled from aridification and not caused by it, as evidenced by their asynchrony. [18]

Southwestern Australia hosted heathlands, shrublands, and woodlands with a greater species diversity compared to today during the Middle and Late Pliocene. Three different aridification events occurred around 2.90, 2.59, and 2.56 Ma, and may have been linked to the onset of continental glaciation in the Arctic, suggesting that vegetation changes in Australia during the Pliocene behaved similarly to during the Late Pleistocene and were likely characterised by comparable cycles of aridity and humidity. [19]

Onset of glaciation

Greenland ice sheet

Several mechanisms have been proposed to explain global cooling after 3 Ma and the onset of extensive northern hemisphere glaciation.

Panama seaway closure

The closure of the Panama seaway (13 Ma–2.5 Ma) increased the salinity contrast between Pacific and Atlantic Ocean and the northward oceanic heat transport. Warmer water increased snowfall and possibly Greenland ice sheet volume. [20] However, model simulations suggest reduced ice volume due to increased ablation at the edge of the ice sheet under warmer conditions. [21]

Southward shift of the North Atlantic Current

A dinoflagellate cyst turnover in the eastern North Atlantic approximately ~2.60 Ma, during MIS 104, has been cited as evidence that the North Atlantic Current (NAC) shifted significantly to the south at this time, causing an abrupt cooling of the North Sea and northwestern Europe. This reduction in heat transport to high latitude waters of the North Atlantic may have triggered the rapid cooling of the Northern Hemisphere and the development of major glaciation. [22]

Collapse of permanent El Niño

A permanent El Niño state existed in the early-mid-Pliocene. Warmer temperature in the eastern equatorial Pacific caused an increased water vapor greenhouse effect and reduced the area covered by highly reflective stratus clouds, thus decreasing the albedo of the planet. Propagation of the El Niño effect through planetary waves may have warmed the polar region and delayed the onset of glaciation in the northern hemisphere. Therefore, the appearance of cold surface water in the east equatorial Pacific around 3 million years ago may have contributed to global cooling and modified the global climate’s response to Milankovitch cycles. [23]

Uplift of the Rocky Mountains and Greenland’s west coast

Uplift of the Rocky Mountains and Greenland’s west coast may have cooled the climate due to jet stream deflection and increased snowfall due to higher surface elevation. [21]

Carbon dioxide

Carbon dioxide concentration during the Middle Pliocene has been estimated at around 400 ppmv from 13C/12C ratio in organic marine matter. [24] and stomatal density of fossilised leaves. [25] Decreasing carbon dioxide levels during the late Pliocene may have contributed substantially to global cooling and the onset of northern hemisphere glaciation. [26] [21] This decrease in atmospheric carbon dioxide concentrations may have come about by way of the decreasing ventilation of deep water in the Southern Ocean. [27]

West Antarctic ice sheet

Mid-Pliocene reconstructed terrain and ice sheet elevation Pliocene topography ice.png
Mid-Pliocene reconstructed terrain and ice sheet elevation

The extent of the West Antarctic Ice Sheet oscillated at the 40 kyr period of Earth's obliquity. Ice sheet collapse occurred when the global average temperature was 3 °C warmer than today and carbon dioxide concentration was at 400 ppmv. This resulted in open waters in the Ross Sea. [28] Global sea-level fluctuation associated with ice-sheet collapse was probably up to 7 meters for the west Antarctic and 3 meters for the east Antarctic.

Model simulations are consistent with reconstructed ice-sheet oscillations and suggest a progression from a smaller to a larger west Antarctic ice sheet in the last 5 million years. Intervals of ice sheet collapse were much more common in the early-mid Pliocene (5 Ma – 3 Ma), after three-million-year intervals with modern or glacial ice volume became longer and collapse occurs only at times when warmer global temperature coincide with strong austral summer insolation anomalies. [29]

Mid-Pliocene and future climate

Pliocene biomes. Pliocene megabiome.png
Pliocene biomes.

The mid-Pliocene warm period, also known as the mid-Piacenzian Warm Period (mPWP), [30] is considered a potential analogue of future climate. The intensity of the sunlight reaching the Earth, the global geography, and carbon dioxide concentrations were similar to present. Furthermore, many mid-Pliocene species are extant, helping calibrate paleotemperature proxies. Model simulations of mid-Pliocene climate produce warmer conditions at middle and high latitudes, as much as 10–20 °C warmer than today above 70°N. They also indicate little temperature variation in the tropics. Model-based biomes are generally consistent with Pliocene palaeobotanical data indicating a northward shift of the tundra and taiga and an expansion of savanna and warm-temperate forest in Africa and Australia. [31]

See also

Related Research Articles

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The Eocene Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name Eocene comes from the Ancient Greek ἠώς and καινός and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.

<span class="mw-page-title-main">Ice age</span> Period of long-term reduction in temperature of Earths surface and atmosphere

An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages, and greenhouse periods during which there are no glaciers on the planet. Earth is currently in the ice age called Quaternary glaciation. Individual pulses of cold climate within an ice age are termed glacial periods, and intermittent warm periods within an ice age are called interglacials or interstadials.

The Miocene is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words μείων and καινός and means "less recent" because it has 18% fewer modern marine invertebrates than the Pliocene has. The Miocene is preceded by the Oligocene and is followed by the Pliocene.

<span class="mw-page-title-main">Neogene</span> Second geologic period in the Cenozoic Era 23–2.6 million years ago

The Neogene is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period 23.03 million years ago (Mya) to the beginning of the present Quaternary Period 2.58 million years ago. The Neogene is sub-divided into two epochs, the earlier Miocene and the later Pliocene. Some geologists assert that the Neogene cannot be clearly delineated from the modern geological period, the Quaternary. The term "Neogene" was coined in 1853 by the Austrian palaeontologist Moritz Hörnes (1815–1868). The earlier term Tertiary Period was used to define the span of time now covered by Paleogene and Neogene and, despite no longer being recognized as a formal stratigraphic term, "Tertiary" still sometimes remains in informal use.

The Pliocene is the epoch in the geologic time scale that extends from 5.333 million to 2.58 million years ago. It is the second and most recent epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch. Prior to the 2009 revision of the geologic time scale, which placed the four most recent major glaciations entirely within the Pleistocene, the Pliocene also included the Gelasian Stage, which lasted from 2.588 to 1.806 million years ago, and is now included in the Pleistocene.

<span class="mw-page-title-main">Pleistocene</span> First epoch of the Quaternary Period

The Pleistocene is the geological epoch that lasted from c. 2.58 million to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in 2009 by the International Union of Geological Sciences, the cutoff of the Pleistocene and the preceding Pliocene was regarded as being 1.806 million years Before Present (BP). Publications from earlier years may use either definition of the period. The end of the Pleistocene corresponds with the end of the last glacial period and also with the end of the Paleolithic age used in archaeology. The name is a combination of Ancient Greek πλεῖστος (pleîstos), meaning "most", and καινός, meaning "new".

The Younger Dryas, which occurred circa 12,900 to 11,700 years BP, was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum, which lasted from circa 27,000 to 20,000 years BP. The Younger Dryas was the last stage of the Pleistocene epoch that spanned from 2,580,000 to 11,700 years BP and it preceded the current, warmer Holocene epoch. The Younger Dryas was the most severe and longest lasting of several interruptions to the warming of the Earth's climate, and it was preceded by the Late Glacial Interstadial, an interval of relative warmth that lasted from 14,670 to 12,900 BP.

<span class="mw-page-title-main">Dansgaard–Oeschger event</span> Rapid climate fluctuation in the last glacial period

Dansgaard–Oeschger events, named after palaeoclimatologists Willi Dansgaard and Hans Oeschger, are rapid climate fluctuations that occurred 25 times during the last glacial period. Some scientists say that the events occur quasi-periodically with a recurrence time being a multiple of 1,470 years, but this is debated. The comparable climate cyclicity during the Holocene is referred to as Bond events.

<span class="mw-page-title-main">Timeline of glaciation</span> Chronology of the major ice ages of the Earth

There have been five or six major ice ages in the history of Earth over the past 3 billion years. The Late Cenozoic Ice Age began 34 million years ago, its latest phase being the Quaternary glaciation, in progress since 2.58 million years ago.

<span class="mw-page-title-main">Global temperature record</span> Fluctuations of the Earths temperature over time

The global temperature record shows the fluctuations of the temperature of the atmosphere and the oceans through various spans of time. There are numerous estimates of temperatures since the end of the Pleistocene glaciation, particularly during the current Holocene epoch. Some temperature information is available through geologic evidence, going back millions of years. More recently, information from ice cores covers the period from 800,000 years before the present time until now. A study of the paleoclimate covers the time period from 12,000 years ago to the present. Tree rings and measurements from ice cores can give evidence about the global temperature from 1,000-2,000 years before the present until now. The most detailed information exists since 1850, when methodical thermometer-based records began. Modifications on the Stevenson-type screen were made for uniform instrument measurements around 1880.

The geologic temperature record are changes in Earth's environment as determined from geologic evidence on multi-million to billion (109) year time scales. The study of past temperatures provides an important paleoenvironmental insight because it is a component of the climate and oceanography of the time.

The Holocene Climate Optimum (HCO) was a warm period in the first half of the Holocene epoch, that occurred in the interval roughly 9,500 to 5,500 years BP, with a thermal maximum around 8000 years BP. It has also been known by many other names, such as Altithermal, Climatic Optimum, Holocene Megathermal, Holocene Optimum, Holocene Thermal Maximum, Hypsithermal, and Mid-Holocene Warm Period.

<span class="mw-page-title-main">Heinrich event</span> Large groups of icebergs traverse the North Atlantic.

A Heinrich event is a natural phenomenon in which large groups of icebergs break off from the Laurentide Ice Sheet and traverse the Hudson Strait into the North Atlantic. First described by marine geologist Hartmut Heinrich, they occurred during five of the last seven glacial periods over the past 640,000 years. Heinrich events are particularly well documented for the last glacial period but notably absent from the penultimate glaciation. The icebergs contained rock mass that had been eroded by the glaciers, and as they melted, this material was dropped to the sea floor as ice rafted debris forming deposits called Heinrich layers.

<span class="mw-page-title-main">Quaternary glaciation</span> Series of alternating glacial and interglacial periods

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<span class="mw-page-title-main">100,000-year problem</span> Discrepancy between past temperatures and the amount of incoming solar radiation

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<span class="mw-page-title-main">Maureen Raymo</span> American paleoclimatologist and marine geologist

Maureen E. "Mo" Raymo is an American paleoclimatologist and marine geologist. She is the Co-Founding Dean of the Columbia Climate School, Director of the Lamont–Doherty Earth Observatory of Columbia University, the G. Unger Vetlesen Professor of Earth & Environmental Sciences, and Director of the Lamont–Doherty Core Repository at the Lamont–Doherty Earth Observatory of Columbia University. She is the first female climate scientist and first female scientist to head the institution.

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<span class="mw-page-title-main">Amelia E. Shevenell</span> American marine geologist

Amelia E. Shevenell is an American marine geologist who specializes in high-latitude paleoclimatology and paleoceanography. She is currently a Professor in the College of Marine Science at the University of South Florida. She has made notable contributions to understanding the history of the Antarctic ice sheets and published in high-impact journals and, as a result, was awarded full membership of Sigma Xi. She has a long record of participation in international ocean drilling programs and has served in leadership positions of these organizations. Shevenell served as the elected Geological Oceanography Council Member for The Oceanography Society (2019-2021).

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