A geochemical record of eutrophication and anoxia in Chesapeake Bay sediments: anthropogenic influence on organic matter composition

Abstract

Abstract Organic and inorganic geochemical indicators were examined in a 3-m core collected from the mesohaline region of Chesapeake Bay (CB) to determine how sources of organic matter (OM) have changed during the preceding three centuries of increasing anthropogenic influence in this region. This study also establishes the history of eutrophication and anoxia/hypoxia and relates these processes to changes in OM deposition and preservation and to historical events within the Bay's watershed. The sediment record shows that a marked increase in organic carbon (35%–50%), biogenic silica (18%) and total sulfur (42%) occurs between 1934 and 1948. This transition is likely due to increasing anoxic/hypoxic bottom water conditions as indicated by an abrupt change in sulfur speciation. Lipid biomarker distributions indicate that a substantial change in the sources of OM deposited since 1934 has also occurred. Biomarker compounds derived from phytoplankton and microbial sources show a 2- to 4-fold increase in their abundance relative to total organic carbon (TOC) between 1948 and 1975. Using both diagenetic models and information on lipid reactivity, an effort is made to distinguish compositional changes due to changes in OM delivery (both quantity and quality) from changes that may be due to OM degradation. It appears that enhanced OM production in the mesohaline region of CB has contributed to the observed changes in quantity and character of OM preserved in sediments from this site. Increased inorganic fertilizer application and human population growth in the watershed are coincident with the onset of eutrophic and hypoxic conditions in CB, suggesting that anthropogenic activities within estuarine watersheds may exert a substantial influence on carbon cycling processes in estuaries and potentially the coastal ocean.