Abstract
Arguably among the most globally impactful climate changes in Earth’s past million years are the glacial terminations that punctuated the Pleistocene epoch. With the acquisition and analysis of marine and continental records, including ice cores, it is now clear that the Earth’s climate was responding profoundly to changes in greenhouse gases that accompanied those glacial terminations. But the ultimate forcing responsible for the greenhouse gas variability remains elusive. The oceans must play a central role in any hypothesis that attempt to explain the systematic variations in pCO2 because the Ocean is a giant carbon capacitor, regulating carbon entering and leaving the atmosphere. For a long time, geological processes that regulate fluxes of carbon to and from the oceans were thought to operate too slowly to account for any of the systematic variations in atmospheric pCO2 that accompanied glacial cycles during the Pleistocene. Here we investigate the role that Earth’s hydrothermal systems had in affecting the flux of carbon to the ocean and ultimately, the atmosphere during the last glacial termination. We document late glacial and deglacial intervals of anomalously old 14C reservoir ages, large benthic-planktic foraminifera 14C age differences, and increased deposition of hydrothermal metals in marine sediments from the eastern equatorial Pacific (EEP) that indicate a significant release of hydrothermal fluids entered the ocean at the last glacial termination. The large 14C anomaly was accompanied by a ∼4-fold increase in Zn/Ca in both benthic and planktic foraminifera that reflects an increase in dissolved [Zn] throughout the water column. Foraminiferal B/Ca and Li/Ca results from these sites document deglacial declines in [] throughout the water column; these were accompanied by carbonate dissolution at water depths that today lie well above the calcite lysocline. Taken together, these results are strong evidence for an increased flux of hydrothermally-derived carbon through the EEP upwelling system at the last glacial termination that would have exchanged with the atmosphere and affected both Δ14C and pCO2. These data do not quantify the amount of carbon released to the atmosphere through the EEP upwelling system but indicate that geologic forcing must be incorporated into models that attempt to simulate the cyclic nature of glacial/interglacial climate variability. Importantly, these results underscore the need to put better constraints on the flux of carbon from geologic reservoirs that affect the global carbon budget.
Highlights
E greenhouse gas variations, the answer remains elusive
If a substantial release of geologic carbon to the upper ocean occurred in the eastern equatorial Pacific (EEP) from one or more of these volcanic sources, it has important implications for reconciling why there was a large decrease in atmospheric Δ14C during the glacial termination when other cosmogenic radionuclides did not (Muscheler et al 2004, Petrenko et al 2016) because the EEP is a primary conduit for exchange of carbon from the ocean to the atmosphere (Takahashi et al 2009)
The results presented here point to a large amount of hydrothermal carbon released at times during the last glacial termination and that carbon upwelled to the surface, but the changes in carbonate chemistry were likely responding to changes in the upwelling intensity (Koutavas et al 2002, Koutavas and LynchStieglitz 2003, Koutavas and Sachs 2008) that indicates stronger upwelling
Summary
E greenhouse gas variations, the answer remains elusive. This likely reflects the fact that no single mechanism acting alone appears capable of explaining all aspects of carbon cycle behavior that accompanied the glacial cycles (Fischer et al 2010). Even if only a small percentage of the unsampled hydrothermal systems contain separate gas or liquid CO2 phases it could change the global marine carbon budget substantially (Burton et al 2013) Where these reservoirs have been found at shallowintermediate depths, the flux from the sediment and rock reservoirs is modulated by a density-stratified boundary layer at the sediment/water interface where a thin layer of CO2 saturated-seawater and CO2 hydrate forms (Inagaki et al 2006, Lupton et al 2006). The primary intent here is to further evaluate the suggestion by Stott and Timmermann (2011) that hydrothermal carbon was released to the ocean in the EEP during at the last glacial termination and hydrothermal carbon contributed to the very large Δ14C anomalies observed in marine sediments. An evaluation of those predictions here is one step in an effort to evaluate if and how geologic sources of carbon contributed significantly to the glacial/ interglacial carbon cycle variability
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