Methane Seep Contribution to Sedimentary Organic Matter in Puget Sound, Washington

Abstract

Marine sediments are a critical carbon reservoir, locking both inorganic and organic carbon in sediments for thousands to millions of years. This carbon is released through the production of methane ( ) at methane seeps, places where methane gas or dissolved methane is emitted 𝐶𝐻 from the seafloor. This 4 study quantified the amount of organic matter (OM) stored in Puget Sound sediments at a methane seep site and future publications will characterize source differences of OM and the amount of inorganic carbon stored in sediments. A multicorer collected sediment samples from the R/V Rachel Carson at two sites in December of 2023: the Alki Point methane vent field and a control site within the Main Basin of Puget Sound. Onboard, sediment cores were subsampled in 2-cm sections downcore and stored for later analysis. Analyses included physical parameters (grain size, percent loss on ignition (% LOI), and 210Pb sediment dating) and geochemical parameters (Total Organic Carbon (TOC), Total Carbon (TC), δ13𝐶𝑇𝑂𝐶 , δ13𝐶 ). Presently, analyses for 210Pb, TOC, TC, , and are still in the processing phase 𝑇𝐶 δ13𝐶𝑇𝑂𝐶 δ13𝐶𝑇𝐶 and data has not been received yet. Both sites were silt-dominated (mean grain size: Alki = 37 microns, Control = 30 microns), with a well-mixed layer of 15 cm thickness. Sediments had an average of 7.5% OM at the methane seep and an average of 6.9% at the control site, measured using the % LOI method. The methane seep core had both a larger mean grain size and a higher % OM above 15 cm, which is inconsistent with established literature that has proven that sediments with larger mean grain sizes typically have a lower amount of organic matter. Further analyses, including the analyses listed above, are needed to determine the cause of this anomaly. Given the amount of carbon they store and the potential for these reservoirs to be disturbed by bottom trawling, deep sea mining, and other invasive human activities, understanding how carbon is cycled through marine sediments is critical for preserving these reservoirs and adequately factoring their role in the carbon cycle into global climate models.