Eocene/Oligocene ocean de-acidification linked to Antarctic glaciation by sea-level fall

Abstract

One of the most dramatic perturbations to the Earth system during the past 100 million years was the rapid onset of Antarctic glaciation near the Eocene/Oligocene epoch boundary (approximately 34 million years ago). This climate transition was accompanied by a deepening of the calcite compensation depth--the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution. Changes in the global carbon cycle, rather than changes in continental configuration, have recently been proposed as the most likely root cause of Antarctic glaciation, but the mechanism linking glaciation to the deepening of calcite compensation depth remains unclear. Here we use a global biogeochemical box model to test competing hypotheses put forward to explain the Eocene/Oligocene transition. We find that, of the candidate hypotheses, only shelf to deep sea carbonate partitioning is capable of explaining the observed changes in both carbon isotope composition and calcium carbonate accumulation at the sea floor. In our simulations, glacioeustatic sea-level fall associated with the growth of Antarctic ice sheets permanently reduces global calcium carbonate accumulation on the continental shelves, leading to an increase in pelagic burial via permanent deepening of the calcite compensation depth. At the same time, fresh limestones are exposed to erosion, thus temporarily increasing global river inputs of dissolved carbonate and increasing seawater delta13C. Our work sheds new light on the mechanisms linking glaciation and ocean acidity change across arguably the most important climate transition of the Cenozoic era.

Major shifts in calcareous phytoplankton assemblages through the Eocene‐Oligocene transition of Tanzania and their implications for low‐latitude primary production

“…[30] Contrary to previous opinion [Diester-Haass and Zachos, 2003;Salamy and Zachos, 1999;Zachos and Kump, 2005], recent carbon cycle modeling has suggested that organic carbon burial associated with increased productivity across the EOT may have played little or no role in driving either the global positive shift in d 13 C or the rapid deepening of the CCD, which are instead attributed to a reduction in shelf carbonate production and the weathering of exposed and relatively isotopically heavy neritic carbonates [Merico et al, 2008]. However, the calcareous nannofossil record presented here strongly indicates an increased availability of nutrients in the low-latitude surface ocean, adding to the growing number of studies that document an increase in surface ocean productivity directly coincident with the EOT [Anderson and Delaney, 2005;Diester-Haass et al, 1996;Diester-Haass and Zachos, 2003;Latimer and Filippelli, 2002;Ravizza and Paquay, 2008;Salamy and Zachos, 1999].…”

“…It is feasible that increased mixing in the Southern Ocean, coincident with Antarctic cooling and glaciation, enhanced the formation of SAMW which effectively connected southern high-latitude cooling with the tropical/subtropical oceans. New records are required to ascertain the extent of low-latitude organic carbon burial associated with these productivity increases [Ravizza and Paquay, 2008], which appears to be largely absent from the Southern Ocean [Rea and Lyle, 2005], and so test recent assertions that organic carbon burial did not play a significant role in the positive d 13 C excursion and deepening of the CCD [Merico et al, 2008].…”

“…The major ocean-wide increase in d 13 C, which appears to lag the shift in d 18 O by $10 ka [Coxall et al, 2005], is compelling evidence for a significant perturbation in the global carbon cycle through the EOT [Merico et al, 2008;Salamy and Zachos, 1999;Zachos and Kump, 2005;Zachos et al, 1996]. This positive shift in d 13 C is indicative of either an increase in the removal rate of light carbon from the ocean-atmosphere system, which has been attributed to the enhanced burial of organic carbon in marine sediments coupled to increased surface water productivity [Diester-Haass and Zachos, 2003;Salamy and Zachos, 1999;Zachos and Kump, 2005] or the input of isotopically heavy carbon via the increased weathering of neritic carbonates during a major sea level fall [Merico et al, 2008]. Both of these hypotheses are associated with a drawdown of atmospheric carbon dioxide that may be the principal positive feedback mechanism that drove the climate system into a strongly glaciated state during the EOGM [Merico et al, 2008;Zachos and Kump, 2005].…”

Carbonate compensation dynamics

“…The depths z sat and z cc are often equated in modeling and in reconstructions of past excursions of the carbonate compensation horizon [e.g. Merico et al, 2008;Roberts and Tripati, 2009], while our formulas predict that they are at least $0.7 km apart. At steady state, obtained on a 10 5 y time scale [Archer et al, 1998], z cc and z cc sed should be equal, and this then implies a separation between z sat and z cc sed of between $0.7 to $2 km (Figure 2), much greater than the $250 m uncertainty in global reconstructions of the z cc sed record [Roberts and Tripati, 2009].…”

“…In past models, z sat has been calculated implicitly [e.g., Sundquist, 1990;Merico et al, 2008;Tyrell, 2008]. Calculation of z sat is also possible if [CO 3 ] D is a function of depth, but that dependence must be known and a more complex equation will result; furthermore, while our emphasis here is on a mean ocean, one could also generate a form of equation (2) for a specific basin by employing the local temperature profile and an appropriate [CO 3 ] D depth-function.…”