INCREASED DEGLACIAL SURFACE PRODUCTIVITY IN THE WESTERN EQUATORIAL PACIFIC INFERRED FROM THE BA/CA RATIOS OF DEEPER DWELLING FORAMINIFERA

A key driver of glacial-interglacial carbonate cycles and deep ocean carbon storage is the production and export of organic matter from the surface ocean into sediment. Changes in surface ocean productivity over major climate transitions, such as the deglaciation, are not yet well understood. This is in part because common productivity proxies (e.g. opal, excess Ba, organic C fluxes) are subject to alterations in the water column and on the seafloor and are therefore linked to multiple processes. Elemental signals bound within the calcite shells of planktic foraminifera offer an advantage over these proxies because foraminifera are generally less prone to redeposition and shells can be carefully assessed for signs of dissolution and diagenesis.

Here, we test the utility of Ba/Ca ratios in deeper dwelling foraminifera as a proxy for surface ocean productivity. Ba enrichment in deeper dwelling foraminifera shells is hypothesized to be linked to the degradation of organic matter within the particulate microhabitat of some deeper dwelling species (Fehrenbacher et al., 2018). During periods of higher productivity, we hypothesize that Ba/Ca ratios would be higher and more variable among a population of specimens. In contrast, during periods of low productivity, Ba/Ca ratios would be low and population variability would decrease. Thus, Ba/Ca ratios would reflect surface ocean organic matter production at the time of shell calcification. We reconstruct Ba/Ca ratios since the Last Glacial Maximum using two species, Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, sampled from the Ontong Java Plateau in the western equatorial Pacific. We find that Ba/Ca ratios are highest during the deglacial period and attribute this to an increase in surface ocean productivity. Furthermore, careful examinations of individual shells reveal that we recover a primary signal even in specimens that have undergone dissolution. A coherent pattern identified in our data and consistent with other well-preserved proxies of deglacial productivity is a sharp increase in productivity onset at ~17 ka and sustained for ~6 kyrs. The Ba/Ca proxy supports low productivity during the glacial period in agreement with other regional proxies and an abrupt and sustained increase in productivity in the deglacial period.