Authigenic Uranium Evidence for Deep-Ocean Respired Carbon Storage in the Eastern Equatorial Pacific Ocean over the Past 180 kyr

Deep ocean sequestration of carbon likely played a pivotal role in the lowering of atmospheric CO2 during the last ice age. The exact cause and location of the sequestered carbon in the deep ocean is critical to our understanding of the global biogeochemical cycle of carbon. Regions in which iron is a limiting micronutrient, such as the Southern Ocean, likely played a prominent role in drawing down atmospheric CO2 on glacial-interglacial timescales. Today, although the eastern equatorial Pacific (EEP) provides a significant flux of CO2 to the atmosphere, its behavior as a source or sink over time is debated. For example, recent work in the EEP provides evidence of a dampening effect on increasing global atmospheric CO2 levels in the early deglacial (consistent with the timing of Heinrich Stadial event 1) potentially due to an increase in productivity (Lovely et al, 2017).

There is the need to further understand the relationship between deep-ocean respired carbon storage and changes in atmospheric circulation and upwelling in the EEP. Here we present measurements of authigenic uranium (a proxy for export production or bottom-water oxygenation), 230Th-derived 232Th fluxes (a proxy for dust flux) and 230Th-derived non-lithogenic barium fluxes (xsBa flux, a proxy for export production) in a high-sedimentation-rate core from the Panama Basin of the EEP (MV1014-8JC, 6° 14.0' N, 86° 02.6' W; 1993 m water depth). Our record spans the past 180 kyr beginning in the penultimate glacial period (marine oxygen isotope stage, MIS, 6). The authigenic uranium content of ocean sediment is directly related to bottom-water redox conditions, which may be controlled by the oceanic rain of organic matter from surface to bottom waters, and/or bottom-water oxygen levels of a given water mass. In our core, there is little variability in xsBa fluxes, and no relationship between these values and authigenic uranium concentrations. Thus, authigenic uranium concentrations are likely controlled by changing bottom-water oxygen levels, and peak during glacial stages (MIS 2, 4, 6) when deep-ocean respired carbon storage is thought to have been high and atmospheric CO2 was low. We propose that the authigenic uranium signal in the EEP, like that in cores from the Southern Ocean, records the ebb and flow of the deep-ocean respired carbon pool.