Antarctic Weathering and Carbonate Compensation at the Transition from the Eocene to the Oligocene

Australia: The Land Where Time Began

Antarctic Weathering and Carbonate Compensation at the Transition from the Eocene to the Oligocene

A permanent cover of ice began to form over Antarctica at the close of the Eocene as the cooling of the Earth from the high temperatures of the greenhouse conditions got under way. Weathering rate spikes are found in sediment cores offshore from the coast of Antarctica from the time at the onset of the ice growth that possibly indicates atmospheric carbon dioxide consumption.

Dramatic environmental changes occurred about 56-35 Ma during the Eocene. At about 49 Ma (Zachos et al., 2001), after the extreme greenhouse conditions of the Palaeocene-Eocene Thermal Maximum (PETM), the intense warming that continued in the Early Eocene peaked. Atmospheric CO₂ levels were higher than 750 ppmv (Beerling & Royer, 2011), compared to the present level of 390 ppmv [see Atmospheric Carbon Dioxide Levels Approach Record High 400 ppm], with a tropical and temperate biota ranging from pole to pole (Eberle & Greenwood, 2011). At this time the Earth was a hot greenhouse planet, then towards the end of the Eocene atmospheric CO₂ levels fell (Beerling & Royer, 2011) precipitously over a relatively short period of less than 500,000 years and ice accumulation began forming on Antarctica (4), remaining ice-covered to the present. It has been suggested that the drop in atmospheric CO₂ and glaciation both resulted from the high rate of continental weathering, though it has been difficult to test this hypothesis. A study reported in Nature Geoscience (Basak & Martin, 2013) has used lead isotope ratios from sediments off the coast of Antarctica of Eocene age to identify pulses of continental weathering that coincided with the beginning of the ice accumulation on Antarctica.

Other mechanisms have been proposed as explanations for the transition from greenhouse to icehouse conditions on Antarctica, such as tectonic movements as the southern continents broke from Antarctica and moved north allowed the formation of the deep Antarctic Circumpolar Current that isolated Antarctica and promoted the growth of ice. According to the author¹ it has been difficult to reconcile these slow plate movements with the apparently rapid onset of ice sheet growth on Antarctica. As well as ice growth and dropping atmospheric CO₂ levels (Beerling & Royer, 2011) other environmental changes were taking place at this time, sea levels also dropped and the ocean became less corrosive, as evidenced by the deepening of the carbonate compensation depth (Palike et al., 2011), the depth below which carbonate is undersaturated, which leads to the dissolution of carbonate minerals that sink from the upper levels of the ocean. The beginning of a stepwise transition from the greenhouse conditions of the previous age to the icehouse world of the present, these global changes being collectively known as the Eocene-Oligocene transition, to conditions that have characterised the past 40 My (Zachos et al., 2001) to the present. A mechanism has been searched for that could explain all of the symptoms of the Eocene-Oligocene transition.

A mechanism has been proposed is that the warmth and high atmospheric CO₂ concentrations that were typical of the Early Eocene led to continental weathering (DeConto & Pollard, 2003) of continental rock, a process which draws down CO₂ and moves carbonate to the oceans. The reduction in greenhouse forcing that results could, according to the author¹ explain the growth of ice and the simultaneous increase of the flux of alkalinity to the oceans, which would help to account for the lowering of the carbonate compensation depth. According to the author¹ evidence supporting this proposal has been elusive, as records of weathering from 34 Ma are difficult to find.

A clue has been found (Basak & Martin, 2013) by studying the lead isotope signatures of seawater, as measured from minerals that were deposited on the seafloor, and detrital grains eroded from Antarctica that were found in 2 off-shore sediment cores. When Basak & Martin compared lead isotope ratios they isolated a signal that was indicative of increased erosion of silicates and carbonates on the Antarctic continent during the Eocene, that appears to have occurred in 2 pulses that were coincident with 2 steps in the growth of ice on Antarctica. Based on this Basak & Martin argued that the growth of ice could possibly have provided a positive feedback, with the triggering of more weathering of Antarctica that lead to increased CO₂ drawdown. It is implied by their observations that the process of weathering may have played a central role in causing the largest environmental change in the history of the Earth.

While this is a robust advance in understanding of the transition many questions remain about the transition, with rate and timing possibly being the most fundamental of the questions. One question is, if higher rates of weathering provide the drawdown of atmospheric CO₂ that was a critical requirement for the initiation of ice accumulation then why did the process become climatically manifest only in the Late Eocene? Atmospheric CO₂ concentrations were equally as high in the millions of years that led up to the Eocene-Oligocene transition. Another question is, as with tectonic forcing and climatic forcing, weathering forcing also occurs over extended time periods, and is this in some way consistent with the 2 pulses observed by Basak & Martin? The author¹ suggests this could be the case as, unlike the case with tectonics, a mechanism of climate forcing affecting atmospheric CO₂ concentrations may exhibit nonlinear behaviour, such as triggering rapid changes once a threshold has been passed.

Atmospheric CO₂ concentration is a small part of the complex carbon cycle of the Earth, though it is climatically forceful, the carbon cycle involving biota on the land and in the oceans, the storage and return of deep ocean carbonate pools that are buffered by dissolution and subject to ocean circulation changes, as well as interactions with elements that are more stochastic, such as methane clathrate dissolution. As with weathering, the carbon pools in the oceans and atmosphere are affected by these latter aspects of the carbon and carbonate systems. The author¹ suggests these additional components might also be argued to operate on time scales that are more consistent with changes that have been observed in connection with the Eocene-Oligocene transition. The author¹ suggests it is evident that an environmental steady state, that had broadly persisted since before the Cainozoic, was lost during the Eocene-Oligocene transition, and climate feedbacks that had been maintaining the warm conditions were altered in such a way that they Earth began to cool.

Evidence has been presented (Basak & Martin, 2013) for a causal link between weathering and the abrupt initiation of these transformations, that are similar to what is expected for a threshold being crossed. The author¹ suggests that considering the present increasing atmospheric CO₂ concentrations, ice loss from both poles and the increasing corrosiveness of the ocean (Feely et al., 2004; Shepherd et al., 2012), the most important question may now be whether the environment of the present is undergoing a similar transition, though in the reverse direction, from icehouse to greenhouse.

Sources & Further reading

Haley, Brian A. "Palaeoclimate: Weathering Away Warmth." Nature Geosci 6, no. 2 (02//print 2013): 86-87.
Basak, Chandranath, and Ellen E. Martin. "Antarctic Weathering and Carbonate Compensation at the Eocene-Oligocene Transition." Nature Geosci 6, no. 2 (02//print 2013): 121-24.

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