[1] The late Paleocene to the early Eocene ( 58-52 Ma) was marked by significant changes in global climate and carbon cycling. The evidence for these changes includes stable isotope records that reveal prominent decreases in ı 18 O and ı 13 C, suggesting a rise in Earth's surface temperature ( 4 ı C) and a drop in net carbon output from the ocean and atmosphere. Concurrently, deep-sea carbonate records at several sites indicate a deepening of the calcite compensation depth (CCD). Here we investigate possible causes (e.g., increased volcanic degassing or decreased net organic burial) for these observations, but from a new perspective. The basic model employed is a modified version of GEOCARB III. However, we have coupled this well-known geochemical model to LOSCAR (Long-term Ocean-atmosphere Sediment CArbon cycle Reservoir model), which enables simulation of seawater carbonate chemistry, the CCD, and ocean ı 13 C. We have also added a capacitor, in this case represented by gas hydrates, that can store and release 13 C-depleted carbon to and from the shallow geosphere over millions of years. We further consider accurate input data (e.g., ı 13 C of carbonate) on a currently accepted timescale that spans an interval much longer than the perturbation. Several different scenarios are investigated with the goal of consistency amongst inferred changes in temperature, the CCD, and surface ocean and deep ocean ı 13 C. The results strongly suggest that a decrease in net organic carbon burial drove carbon cycle changes during the late Paleocene and early Eocene, although an increase in volcanic activity might have contributed. Importantly, a drop in net organic carbon burial may represent increased oxidation of previously deposited organic carbon, such as stored in peat or gas hydrates. The model successfully recreates trends in Earth surface warming, as inferred from ı 18 O records, the CCD, and ı 13 C. At the moment, however, our coupled modeling effort cannot reproduce the magnitude of change in all these records collectively. Similar problems have arisen in simulations of short-term hyperthermal events during the early Paleogene (Paleocene-Eocene Thermal Maximum), suggesting one or more basic issues with data interpretation or geochemical modeling remain.

Groundwater-derived solute fluxes to the ocean have long been assumed static and subordinate to riverine fluxes, if not neglected entirely, in marine isotope budgets. Here we present concentration and isotope data for Li, Mg, Ca, Sr, and Ba in coastal groundwaters to constrain the importance of groundwater discharge in mediating the magnitude and isotopic composition of terrestrially derived solute fluxes to the ocean. Data were extrapolated globally using three independent volumetric estimates of groundwater discharge to coastal waters, from which we estimate that groundwater-derived solute fluxes represent, at a minimum, 5% of riverine fluxes for Li, Mg, Ca, Sr, and Ba. The isotopic compositions of the groundwater-derived Mg, Ca, and Sr fluxes are distinct from global riverine averages, while Li and Ba fluxes are isotopically indistinguishable from rivers. These differences reflect a strong dependence on coastal lithology that should be considered a priority for parameterization in Earth-system models.

No abstract

Negative carbon and calcium isotope excursions, as well as climate shifts, took place during the most severe mass extinction event in Earth's history, the end-Permian (∼252 Ma). Investigating the connection between carbon and calcium cycles during transient carbon cycle perturbation events, such as the end-Permian, may help resolve the intricacies between the coupled calcium-carbon cycles, as well as provide a tool for constraining the causes of mass extinction. Here we identify the deficiencies of a simplified calcium model employed in several previous studies, and we demonstrate the importance of a fully coupled carbon cycle model when investigating the dynamics of carbon and calcium cycling. Simulations with a modified version of the Long-term Ocean-atmosphere-Sediment CArbon cycle Reservoir model, which includes a fully coupled carbon-calcium cycle, indicate that increased weathering rates and ocean acidification (potentially caused by Siberian Trap volcanism) are not capable of producing trends observed in the record, as previously claimed. Our model results suggest that combined effects of carbon input via Siberian Trap volcanism (12,000 Pg C), the cessation of biological carbon export, and variable calcium isotope fractionation (due to a change in the seawater carbonate ion concentration) represents a more plausible scenario. This scenario successfully reconciles 13 C and 44 Ca trends observed in the sediment record, as well as the proposed warming of >6 ∘ C.

[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.