Fate of organic compounds associated with extractable and bound phases of estuarine sediments deposited under varying depositional regimes.

摘要

Surficial sediments and sediment cores were collected from two distinct depositional regimes of the York River subestuary of Chesapeake Bay to document seasonal inputs, spatial variability, and longer-term (>40 years) fate of total organic carbon (TOC), lipid biomarker compounds and polycyclic aromatic hydrocarbons (PAHs). These regimes included biological mixing in the lower York and episodic mixing at the mid river site. Compounds were selected to represent a range of chemical reactivities, biological and anthropogenic sources, and modes of entry to the environment. The depositional environments were characterized with a suite of analytical tools: x-radiographs, Eh, 210Pb and 137Cs, total organic carbon, total nitrogen, and their stable isotopes.; Each compound class was quantified in extractable and “bound” phases. Episodic mixing at the mid-river site resulted in stronger oxidizing conditions that promoted enhanced degradation of labile organic matter (e.g. diatoms) vs. refractory material (e.g. higher plants) in extractable sedimentary phases from sediments 5 yrs old. However, while apparent rate constants for bulk organic matter and total lipid were higher in older sediments (40 years) under physically mixed conditions, degradation rates of fatty acid and sterol biomarkers were similar at both study sites.; PAHs and lipid biomarkers isolated from “bound” phases were better preserved over time than corresponding “extractable” compounds. However, stabilization in the bound phase was not the same among compound classes. Differences in compound class fate were a function of inherent compound class reactivity (fatty acids > sterols and PAHs) rather than source or depositional regime. While compounds in bound phases may be formed over time during organic matter diagenesis, organic compounds did not increase in bound residues over time regardless of depositional regime, suggesting that bound phase compounds are formed within the source organism or very rapidly upon cell death and/or deposition to the sediments.; The fate of organic carbon in coastal sediments is dependent upon the source and reactivity of organic carbon, the depositional regime, and its association with the underlying sediment/macromolecular matrix. Models of coastal carbon dynamics that consider these parameters and how they change will yield more accurate forecasts of coastal biogeochemical cycling.