Oceanic calcium changes from enhanced weathering during the Paleocene‐Eocene thermal maximum: No effect on calcium‐based proxies

Abstract

[1] During the Paleocene-Eocene thermal maximum (PETM ∼55 Myr ago), prominent climatic and biogeochemical changes took place in the atmosphere, ocean, and on land. For example, deep-sea temperatures rose by 5°C to 6°C, while sea surface temperatures at high latitudes increased by up to 9°C. In the sedimentary record, the onset of the PETM is marked by widespread dissolution of calcium carbonate on the seafloor. In addition, there is evidence for globally higher humidity, precipitation and increased weathering during the PETM. Both calcium carbonate dissolution and enhanced weathering probably affected the seawater calcium concentration. Here we investigate implications that possible changes in the ocean's calcium inventory may have had on boron/calcium (B/Ca) and magnesium/calcium (Mg/Ca) ratios, which are used as proxies for deep water carbonate chemistry and temperature, respectively. We also examine effects on d 44 Ca of seawater, which is used as an indicator for variations in the marine calcium cycle. We focus on the magnitude of change in the ocean's calcium ion concentration as a result of the carbon perturbation, which resulted in increased weathering fluxes and the dissolution of calcite on the ocean floor during the PETM. Different ranges of carbon input scenarios and their effect on ocean chemistry were examined using the Long-term Ocean-atmosphere-Sediment CArbon cycle Reservoir (LOSCAR) model. We found that under the most plausible scenario, the calcium ion concentration change (D[Ca 2+ ]) was less than 0.7% and around 2% in the most extreme scenario. Our results show that B/Ca and Mg/Ca proxies were not affected within analytical precision by changes in oceanic calcium due to weathering and carbonate dissolution during the PETM.

Calibration of the B/Ca proxy in the planktic foraminifer Orbulina universa to Paleocene seawater conditions

“…First principles suggest that during the PETM, pH should have decreased and total DIC increased due to the combined effects of CO 2 injection, seafloor CaCO 3 dissolution, and on >100 kyr timescales, increased riverine input by enhanced continental weathering [ Zeebe et al , , Komar and Zeebe , , Penman , ]. Our Paleocene calibrations now allow us to combine published δ 11 B‐derived pH estimates [ Penman et al , ] with various scenarios for DIC change to evaluate the conditions necessary to explain the observed B/Ca excursion [ Penman et al , ].…”

A continental shelf perspective of ocean acidification and temperature evolution during the Paleocene-Eocene Thermal Maximum

“…Sediment and culture studies indicate that B/Ca ratios in planktonic foraminifera, based on the model of borate ion [B(OH) -4 ] incorporation into calcite, are governed by carbonate chemistry variables (Allen et al, 2012). Boron and calcium oceanic residence times are considerably longer than the duration of the PETM and are assumed to be constant through that interval (Komar and Zeebe, 2011). We apply the basic mechanism controlling boron partitioning in modern foraminifera to the extinct PETM taxa.…”

LOSCAR: Long-term Ocean-atmosphere-Sediment CArbon cycle Reservoir Model v2.0.4

“…The user is welcome to change and vary these parameters values (cf. Uchikawa and Zeebe, 2008;Komar and Zeebe, 2011). As mentioned above, in steady state, the silicate weathering flux balances the CO 2 degassing flux from volcanism:…”

“…We have previously published several applications of LOSCAR dealing, for instance, with future projections of ocean chemistry and weathering, pCO 2 sensitivity to carbon cycle perturbations throughout the Cenozoic, and carbon/calcium cycling during the Paleocene-Eocene Thermal Maximum (PETM) Zachos et al, 2008;Zeebe et al, 2009;Uchikawa and Zeebe, 2008;Stuecker and Zeebe, 2010;Uchikawa and Zeebe, 2010;Komar and Zeebe, 2011;Zeebe and Ridgwell, 2011;Zeebe, 2012). The subject of the present contribution is the detailed description of the model including numerical architecture, processes and parameterizations, tuning, and examples of input and output.…”