Tracking Fluctuations of Oxygen Minimum Zones: A High-Resolution Study of δ15Nsed and Biogenic Silica in Laminated Sediments from the Gulf of California and the California Borderland

Abstract:

Oxygen Minimum Zones (OMZs) are sites of high primary productivity which contribute to the deposition of C_org-rich sediments. Unique nutrient and trace element cycling occurs within an OMZ. The extent and severity of OMZs is changing globally, making it critical to understand these fluctuations and their important implications for biogeochemical cycling in both the water column and sediments. The Eastern Tropical North Pacific OMZ impinges on basin slopes where laminated, hemipelagic sediments record useful proxies of OMZ fluctuations. In an OMZ, low O₂ content promotes water column denitrification; this enriches the residual nitrate pool in ¹⁵N. This ¹⁵N-enriched nitrate assimilated by primary producers imparts its isotopic signature on Particulate Organic Nitrogen (PON) exported from euphotic zone and buried in sediments. The intensity of anoxia is reflected in the degree of ¹⁵N enrichment in the sedimentary PON. We will present high-resolution δ¹⁵N records obtained from laminated sediment cores from the Pescadero Slope (Gulf of California) and the Santa Monica Basin (California Borderland) that show distinct temporal fluctuations in the δ¹⁵-PN with a decadal to multi-decadal frequency. The Pescadero gravity core has age control provided by 24 14C dates. Comparisons between δ¹⁵N_sed and percent biogenic silica in sediments help elucidate the factors driving the changes in the intensity of the OMZ. Preliminary data show enriched δ¹⁵N_sed values correspond to increased percentages of biogenic silica in sediments, indicating a connection between increased bSi productivity and the intensity of the OMZ in the ETNP.