A High-Resolution Study Of δ15Nsed And Biogenic Silica In Laminated Fine-Grained Sediments In The Gulf Of California And The California Borderland

Abstract

The extent and severity of Oxygen Minimum Zones (OMZs) is changing globally, making it critical to understand the mechanisms driving this change. Many important key microbially-mediated nutrient cycling processes occur in OMZs and thus changes in the severity of OMZs have important implications for biogeochemical cycling in the water column and sediments. The Eastern Tropical North Pacific OMZ, responsible for a third of global water column denitrification, impinges on basin slopes depositing hemi-pelagic, laminated sediments that can record useful proxies of OMZ fluctuations. In an OMZ, low O2 content promotes water column denitrification, which 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 fine-grained sediments. The intensity of anoxia and associated denitrification is reflected in the degree of 15N enrichment in the sedimentary PON. Our high-resolution d15N records obtained from laminated sediment cores from the Pescadero Slope (Gulf of California) and the Santa Monica Basin (California Borderland) show distinct temporal fluctuations in the δ15-PN with a ∼decadal frequency, likely driven by variability of water column O2 content, providing insight into oscillations in ocean circulation, or possibility changes in productivity. Comparisons between δ15Nsed and salinity, for sediments for which water column conditions were known, and δ15Nsed and percent biogenic silica in older sediments help elucidate the factors driving the changes in the intensity of the OMZ in the ETNP. The long-term goal of this work is to apply the methods used in modern sediment cores to the rock record, specifically the Monterey Formation.

AAPG Datapages/Search and Discovery © 2014 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Bakersfield, California, April 27-30, 2014