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Early diagenesis of chloropigment biomarkers in the lower Mississippi River and Louisiana shelf: implications for carbon cycling in a river-dominated margin
Institution:1. State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China;2. State Environmental Protection Key Laboratory For Lake Pollution Control, Research Center of Lake Environment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China;3. Yunnan Key Laboratory of Pollution Process and Management of Plateau Lake-Watershed, Kunming, 650000, China;4. Dongting Lake Ecological Observation and Research Station, Yueyang, 414000, China
Abstract:Sediment core samples were collected from two sites in the lower Mississippi River, an oxic shelf site and a hypoxic shelf site (in September 1998 and July 1999), and from a cross-shelf transect (in April 2000), to examine the differential effects of redox and sedimentation rate on carbon decay dynamics in a river-dominated margin. Downcore distribution of pigments, bulk organic carbon and nitrogen, and radioactive isotopes (210Pb, 7Be) were used to evaluate the decomposition and preservation of pigments and bulk organic carbon. The distinctly different sedimentary regimes and dynamic nature of the LA shelf limit the application of diagenetic models. Sedimentation processes in the lower Mississippi River and oxic shelf sites were significantly impacted by the river discharge. In areas with low sedimentation, the depth of the surface mixed layer fluctuated with seasonal variation of weather forcing. It was observed that pigment decay rate constants in the mixed layer (7.52 year?1 for chlorophyll-a) were greater than those in the accumulation layer (0.14–0.22 year?1 for chlorophyll-a) by 1–2 orders of magnitude. This suggests that enhanced decomposition of reactive organic carbon occurred in the mixed layer at locations with low sedimentation rates—due to higher decay rates. Conversely, at locations with high sedimentation rates (>10 cm year?1), the reactive carbon pool was rapidly buried below the mixed layer. The surface mixed layer likely worked as a biogeochemical reactor receiving high inputs of phytodetritus, supported by an active microbial community. We propose that despite the frequency of occurrence of bottom water hypoxia on the Louisiana shelf, sedimentation rate and lability of organic matter are more important in controlling the preservation of organic carbon.
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