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Greenhouse to Icehouse: Earth’s Tumultuous Climate During the Eocene-Oligocene Boundary

The Cenozoic period in Earth’s history began around 65 Ma (million years) ago. Earth’s climate was beginning to evolve from the Cretaceous greenhouse where in which the dinosaurs flourished, to a cooler climate that lead to their demise, and the incidence of massive ice sheet or glaciers at the poles. The term icehouse refers to times of massive ice sheets on our planet, specifically at this time; during the Eocene-Oligocene boundary (33.79 Ma) there was extensive ice sheet growth in the southern polar region. A cooling spike in Earth’s climate marks the Eocene-Oligocene boundary. Many have postulated how the Earth went from a warm climate with little to no ice during the Paleocene-Eocene Thermal Maximum (PETM), to a significantly cooler climate at the Eocene-Oligocene boundary. Some of these causes are the paleogeography and plate tectonic movement of the continents to their more present day locations, very large igneous deposits from massive lava flows out of massive volcanic eruptions, impacts from extraterrestrial debris, possible Milankovich 400 Ka (thousand year) eccentricity cycle and the circulation, along with sea-level change in the major oceans will be addressed as possible triggers for Earth’s decent into an icehouse climate.

Plate tectonics
The continued breakup of the supercontinent Gondwanaland is looked at as a possible cause for climate change. Since the position of land on the planet can dictate surface weather, wind directions, precipitation, carbon burial, and isolation of radiant heat from the sun it’s a vital factor in the investigation into Earth’s past climates. As landmasses broke apart, oceans spread, and some land masses collided, Earth saw its climate begin to shift. Kevin Pickering in his chapter on the Cenozoic world (Insert further citation when available) claims that these tectonic movements are a large reason for the deterioration of Earth’s climate. Tectonic plate movement also involved ocean floor spreading along with movement of landmasses. One of the more prominent changes in land and ocean geography is the separation of Australia from Antarctica opening up for southern circumpolar circulation of cold water. Along with the critical separation of South America from Antarctica forming the Drake Passage. The Atlantic Ocean floor spread and continues to spread today, widening the gap between North America and Europe and Africa. The position of the continents changed the circulation of the ocean waters. Along with ocean spreading, there was the collision of India with Asia and the major uplift of the Tibetan Plateau. This massive mountain building may be responsible for taking atmospheric carbon out of the atmosphere and burying it within the sediment during the increased weathering that took place .. The rising mountains may have also altered weather patterns directing precipitation and increasing or decreasing weathering of silicates. A closer look at the uplift of the Tibetan Plateau also may involve the monsoonal flow of India. It is considered the most prominent factor in directing precipitation. The monsoonal flow increased weathering in this region and thus decreased atmospheric carbon, a greenhouse gas, ultimately causing global cooling. The other major tectonic trigger for climate change was the opening of the Drake Passage between South America and Antarctica and the drift of Australia away from Antarctica. These two movements isolated Antarctica in the South Pole creating circumpolar ocean circulation at the South Pole. This ocean circulation essentially cut off the heat transfer to Antarctica increasing its isolation from warmer water and forcing any precipitation to be in the form of snow and ice. It is here in the South Pole that we see Earth’s first ice sheet growth at the beginning of the Oligocene.

Igneous deposits/Volcanic eruptions
Large igneous deposits are also a possible cause for climate change during the late Eocene and early Oligocene. How do igneous deposits occur? Volcanic eruptions spew many things out into the atmosphere including dust and aerosols that can shield the sun’s warming radiation from the surface, reflecting it back out into space. The ultimate global affect of such eruptions is not significant enough to cause cooling but it’s not the only affect volcanic activity has on climate. Along with the stuff projected from the volcano, lava flows on the ground. It deposits and cools becoming what are known as an igneous deposit. These flood basalts from eruptions accumulated over long periods of time becoming a sink for carbon due to weathering of this newly formed earth. Pickering cites the Deccan Plateau as a large igneous deposit that occurred around 65 Ma. The regular eruption yielded about 2-8 km3 annually of igneous deposition. Although Pickering does go on to say that long term eruptions and large scale eruptions could cause greenhouse warming with the aerosols it releases, however, the effects were more likely short term. Pickering brings a parallel with perhaps the largest eruption happening further ahead in time from the Eocene-Oligocene boundary in the Quaternary, the Toba eruption. Its effects are seen in the isotopic record, which corresponds to a global drop in temperature of about 3-5 degrees Celsius. It’s estimated that this eruption ejected so much ash that it increased atmospheric turbidity, increasing ice formation, lowering sea levels and perpetuating the negative feedback cooling the Earth. Of course this even took place much later in time from the scope of this paper but it can infer that an eruption of this size may have had the same impact on climate during the Eocene-Oligocene boundary.

Impact theories
K.A. Farley’s paper, “Late Eocene and late Miocene cosmic dust events: Comet showers, asteroid collisions, or lunar impacts?” looks at 3He proxy data in order to designate when these events may have occurred in Earth’s history and to what extent their existence had on climate. This data differs slightly from the large impact that occurred during the Cretaceous-Tertiary boundary that may have been the demise of the dinosaurs. Impacts that cause impact craters do not have this 3He signature. This is demonstrated nicely with the impact crater that was created around 65 Ma at the K-T boundary. Farley notes that 3He proxy data does not indicated the amount of dust produced in any one event, just that a dust event took place. Dust can enter Earth’s atmosphere through comet collisions and also as solar debris passes between the Earth and the sun, shielding the solar radiation from the Earth. The most detailed record of a 3He spike around 35.8 Ma is in the Tethyan limestone exposed at Massignano, Italy. This peak corresponds with one of the two largest impact craters during the Cenozoic. Comet shower events are more intense during perihelion passages, and were once used as proxy data to predict the probability of Earth being hit during a comet shower. The late Eocene 3He data suggests that the peak was caused by a comet shower, however it was challenged by Tagle and Claeys (2005) where in which they conclude that the impact crater shows chemically distinct markers that differ from carbonaceous chondrites and therefore not a comet but an asteroid. If it were a large body impact then 3He would not be a significant marker, Fritz, et.al (2007) came along and suggested that an asteroid impact with the moon could have caused the increase in 3He rich dust. However, the debate is ongoing what caused the increase in 3He dust for the late Eocene and how they are related to the two large impacts that occurred just prior.

Milankovich
Rachel E. Brown et. al analyzed four climate proxies from the Massignano, Italy Eocene-Oligocene boundary of global stratotype section and point (GSSP) that indicates the bedded sedimentary seen here is controlled by Milankovich cycles. The late Eocene is a time of marked global cooling. The appearance of ice sheets in the southern pole, and a decrease in global temperature can been seen in the marine isotopic record. The cooling trend is complicated by the extraterrestrial collision evidence, which may have had a significant impact on global cooling. The data collected showed that δ18O plot was statistically high frequency with precessional peaks and the δ13C followed the eccentricity cycle. The reason for the opposition in cycles may be attributed to the cycle feedback responses. Marine carbon isotopic proxies are tied up in the carbon cycle, which turns over more slowly versus the relatively quicker response in the hydrological cycle and δ18O proxy is located. A question posed by Brown et.al was whether impact data in the sediment record could shadow the Milankovich cycle evidence. The Popigai impact event in Siberia, Russia struck land about 35.7 Ma and contributed to global cooling. Other impact events like the Chesapeake Bay impact struck ocean crust and show a trend to increased greenhouse gases, increasing global warming. However, based on stable isotope variations, the pulses of warm trends mark the end of a general cooling trend through the Eocene-Oligocene and these terrestrial impacts accelerated the cooling trend, triggering possibly the ice-albedo affect keeping the climate cool. And as we’ve seen, ice-albedo feedback is regulated by Milankovich cycles .

Circulation/sea-levels
Perhaps the most compelling and confident data for the formation of ice at the Eocene-Oligocene boundary is held in the oceans. Kenneth G. Miller et.al studied the relationship between ice volume and δ18O and sequences. Their hypothesis was that the ice event that marks the beginning of the Oligocene (Oi-1) was responsible for a drop in sea level. An increase in δ18O in benthic foraminifera in the oceans signifies an isotopically heavier ocean. The isotope δ16O is preferentially taken up by the formation of ice because it’s isotopically lighter, leaving behind its heavier counterpart to be taken up by the microscopic organisms that live in the oceans. Miller et.al noted a general increase in δ18O of 1.0 o/00 to 1.5 o/00 in the Indian, Pacific, Atlantic and Southern Oceans. Most studies will agree that the beginning of the Oligocene is marked by the onset of polar ice sheets in Antarctica. But δ18O values aren’t without their controversy in what then tell about the climate conditions of Earth’s past. Early studies concluded that the increase in δ18O values were due to deep water cooling and in effect surface water temperature decrease at high latitudes. If δ18O values in the deep Pacific are related to ice sheet growth some assumptions were made in order to correlate the two, 1) that ice sheet growth was 1.5 times faster than present day, 2) there were ice sheets in the North Atlantic polar region as well as Antarctica 3) a global sea level fall of about 150m . 	There is a typical 1.0 o/00 increase in benthic foraminifera in the deep Atlantic, however, the 1.5 o/00 at the Ocean Drilling Program site 1218 has implication that indicate there was a 2°C drop in temperature in the deep Pacific . The shifts in δ18O values are correlated with ice sheet growth and a drop in global sea level .  In the end, the exact relationship between δ18O values and glaciations of Antarctica are controversial.	Miller et. al conducted research on cores from New Jersey and the St. Stephens Quarry off the coast of Alabama.  These cores were similar in their sequences during the same time in Earth’s history. The similarities in the sequences indicated that there was a global δ18O increase, along with glacial growth and sea level drop as the main causes for the stratigraphy sequences in the cores that are geographically separated. Circulation of deep ocean water may have also played a crucial role in the cooling of the southern polar region causing increased precipitation in the form of snow and ice creating ice sheet growth at the Eocene-Oligocene boundary. Antarctica is isolated from north-south heat exchange by the way of ocean circulation by way of the Antarctic Circumpolar Current (ACC). This was made possibly by plate tectonic drifting, opening the waterway between Australia and Antarctica and South America and Antarctica. Helen A. Pfuhl et. al in a paper titled “Changes in Southern Ocean Circulation in Late Oligocene to Early Miocene” looked at how sediments from the Tasmanian Gateway between Australia and Antarctica and the Drake Passage between South America and Antarctica detail the evolution of the ACC and a possible explanation for the change in climate that ultimately caused ice sheet formation and the decent into an icehouse climate. The sediment core samples from the Tasmanian Gateway provide excellent data to point to the evolution of the ACC because at the time of the Oligocene in to the Miocene the waterway was relatively narrow casing a channeling effect and slower sedimentation rates. The data collected from four sites in the Ocean Drilling Program (ODP) Lag 189 showed stable isotopic deposition of δ18O and δ13C consistent with temperature changes, however the δ18O values are subject to further scrutiny since their interpretation is not clear. More sites at similar latitudes need to be studied and correlated in order to give high-resolution results. The major results from their research were that the tectonic drifting of the continents culminated in the opening of these waterways essentially cutting off Antarctica from the Oceanic circulation at lower latitudes and essentially keeping the continent in a state of perpetual cool weather affecting the waxing and waning of ice sheets.

Discussion
There is no doubt that there was a climate shift as the Earth left the Cretaceous and entered the Cenozoic. During this time the Earth evolved from a world with no ice to one with ice, as we know it today, although the glaciers of today are smaller than they were at this time in Earth’s history. But what caused the decent into an icehouse Earth? The continents were breaking apart from Gondwanaland, which opened and in some cases closed waterways, affecting the circulation of ocean currents changing ocean temperatures. Some theorize that the impacts from solar debris caused atmospheric changes that blocked solar radiation along with massive volcanic eruptions ejecting aerosols into the air further changing the quality of the atmosphere, lowering the global temperature. Even Earth’s position in it’s orbit around the sun and it’s tilt have some correlation to the change in seasonality from moderate to extreme essentially causing climate change. All these factors are related to what global data suggests was an overall cooling trend on the planet. Further study of high latitude sediment samples are needed to validate the δ18O values cited as being conclusive to ice-volume changes to sea level. Researchers cannot deny that these changes are evident, but what triggered these changes is still up for debate.

Figure References

 * Jovane, Luigi et. al. (2009). "The late Eocene greenhouse-icehouse transition: Observations from the Massignano global stratotype section and point (GSSP)," ' 'The Geological Society of America.' ' Fig 2 p. 153.
 * McGowran, Brian. (2009). "The Australo-Antarctic Gilf and the Auversian facies shift," ' 'The Geological Society of America.' ' Fig 4 p.220.