Organic matter degradation in sediments of the York River estuary: Effects of biological vs. physical mixing

Abstract

We investigated the influence of biological and physical seabed disturbance on the degradation of bulk organic matter and source specific lipid biomarker compounds by measuring downcore changes in bulk elemental composition, bulk stable isotopic (δ 13C and δ 15N) signatures, and lipid biomarker compounds in sediment cores collected from two sites in the York River, a subestuary of the Chesapeake Bay, USA. One site (LY) is influenced by biological mixing (bioturbation), restricted to the upper 15–20 cm, while the other site (POD) experiences intense, episodic physical mixing events that penetrate 50–100 cm into the sediment. We utilized a suite of auxiliary measurements to constrain the sources of organic matter, depositional environments, and general ages of the cores. Diagenetic modeling of total organic carbon and total nitrogen in sediments yielded higher apparent rate constants for POD than LY suggesting that the physical mixing regime promotes enhanced degradation of bulk organic matter. Apparent rate constants for select lipids representing distinct sources of organic matter were also higher at POD than LY for all but the most labile (i.e., diatom-derived fatty acids) biomarkers. Differences in stanol/stenol ratios also supported enhanced diagenesis of stenols at POD. The source-specific biomarkers, while useful in qualitatively identifying the sources of sedimentary organic matter, likely do not represent the full spectrum of its reactivity. However, based on our results, we hypothesize that the intense sediment disturbance at POD promotes degradation of more recalcitrant organic material, due to prolonged exposure to oxygen and other electron acceptors (e.g., NO 3 −, Mn and Fe oxides). In contrast, the degradation of more labile constituents is equally facilitated by biological and physical disturbance.