Stable Carbon Isotopes as a Proxy for Bottom Water Oxygenation in the Labrador Sea

• Evan Borodin, Environmental Science, University of Delaware

• Chandranath Basak, Earth Sciences, University of Delaware

Analyzing stable carbon and oxygen isotopes of sediment porewater and overlying seawater offers valuable insights into historic biogeochemical cycling within marine environments. Previous studies have demonstrated a relationship between the oxygenation of bottom water masses and the magnitude of the Δδ13C (carbon isotopic gradient) of underlying porewater, as measured between the sediment-water interface and oxic-anoxic boundary; greater bottom water oxygenation enhances microbial aerobic respiration processes, resulting in isotopically lightened dissolved inorganic carbon (DIC) within sediment porewater relative to seawater. Building on this fundamental relationship, my research employs two complementary methods to establish a calibration curve linking the carbon isotopic gradient and bottom water oxygenation within the Labrador Sea region. The first measures the change in δ13C between porewater at the oxic-anoxic boundary and overlying bottom water; the second applies a foraminifera-based paired-species approach recording the Δδ13C between epifaunal and infaunal foraminifera tests. Establishing this calibration curve enables the analysis and reconstruction of past changes in bottom water oxygenation levels throughout the Labrador Sea, an area critical to deep water formation and the Atlantic Meridional Overturning Circulation (AMOC). Bottom water, porewater, and foraminifera samples were collected during the SEALS (Sediment Exchange along the Labrador Sea) expedition aboard the R/V Roger Revelle between June-July of 2025 using multicoring and CTD instrumentation. Porewater was extracted by dividing sediment samples into centimeter sections, centrifuging at 4500 rpm for 20 minutes, and filtering with a 0.22 micron filter. The remaining sediment from each station and depth was refrigerated to preserve foraminifera tests. Bottom water, porewater, and foraminifera assemblages will be analyzed via isotope ratio mass spectrometry, with the resulting calibration curve enabling new insights into carbon cycling and oxygenation throughout the Labrador Sea region.