Foraminiferal calcium isotope response to carbon cycle perturbations: comparison of the Paleocene-Eocene Thermal Maximum to Ocean Anoxic Event 2

Abstract:

Large and rapid injections of CO₂ to the Earth’s surface disrupt marine carbonate equilibrium and cause ocean acidification (OA). The calcium isotope composition (δ^44/40Ca) of primary carbonate producers may be sensitive to OA, as Ca isotope fractionation between seawater and calcite depends on precipitation rate. Sedimentological indicators, such as shoaling of the calcite compensation depth and boron isotope data, support OA during the Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma). Similar sedimentological evidence suggests that OA may have occurred just prior to Ocean Anoxic Event 2 (OAE2; ~94 Ma). Additionally, the planktic foraminifera Rotalipora cushmani, from the classic Bottaccione section in Gubbio, Italy shows morphological signs of environmental stress leading up to the Bonarelli Level that marks OAE2. Foraminiferal δ^44/40Ca (OSIL seawater scale) records spanning the PETM show an increase from −1.33‰ to −1.55‰ at ~170 kyr before the carbon isotope excursion that signifies the onset of the event. The PETM δ^44/40Ca records show no correspondence with fragmentation indices, sedimentation rate, carbonate contents, or other diagenetic indicators; therefore, the results suggest decreased Ca isotope fractionation due to reduced calcite precipitation rates. R. cushmani δ^44/40Ca values prior to the Bonarelli Level also increase from −1.55‰ to −1.40‰ and correspond with decreases in median test diameter. A comparison of geochemical and morphological indicators suggests R. cushmani experienced calcification stress prior to OAE2. These new results provide more evidence that OA might have occurred as a precursor to OAE2, likely as a result of increased CO₂ emissions from eruption of the Caribbean Large Igneous Province (and/or High Arctic LIP).