NITROGEN ISOTOPIC ANALYSIS OF MODERN AND ANCIENT BIVALVES FROM THE GULF OF MEXICO

The northern Gulf of Mexico hosts the second largest area of oxygen-depleted marine waters in the world. The extent of hypoxia is influenced by nutrient outflow from the Mississippi River. Hypoxia can be detrimental to organisms' health, reproduction, and growth, and revealing the recent history of hypoxia is critical for establishing future targets for mitigation. Nitrogen isotopes can help fingerprint the sources of modern N loading, but time averaging and the dynamics of a shallow coastal environment have made exploring the record of hypoxia into the recent past difficult. Bivalve mollusks from the Gulf of Mexico can help determine nitrogen sources and may elucidate changes in the N cycle through time by using the organic matter preserved within their shells. We will report N-concentrations and δ¹⁵N values from ¹⁴C-dated samples collected from 9 localities on the continental shelf (-20m isobath) distributed on a primary productivity gradient extending from Louisiana to Florida. Initial analyses of shells of the bivalve Liphora obliterata used in this study contain very low N-concentrations, less than 0.01 weight percent. It is currently unclear whether low N-content is primary or due to degradation over time, and whether N-loss has affected shell-associated δ¹⁵N values. We will assess how N-content varies with shell preservation and radiocarbon age. Due to the low N content, conventional elemental-analysis isotope ratio mass spectrometry (EA-IRMS) was not possible. We used a nano-EA-IRMS to examine the N content and δ¹⁵N within the sample suite. Initial shell δ¹⁵N ranges from +7 to +8‰, values that are ¹⁵N-enriched relative to known N baselines of nitrate in the Gulf of Mexico (4-5‰), consistent with a primary consumer trophic position. We will report nitrogen analyses from a suite of sub-fossils that span the recent geologic history of the northern Gulf, and test how nitrogen isotope signals are preserved in ancient shells. This analysis will give us perspective on how N preserves in subfossil shells and is a first step in reconstructing how the coastal nitrogen cycle in the northern Gulf of Mexico has evolved over the last few thousand years.