Sources of organic carbon to the Louisiana shelf: Spatial and benthic-pelagic coupling dynamics
Description
Because of high sedimentation rates in the coastal ocean, particularly along river-dominated ocean margins, estuarine and coastal sediments can be important repositories for organic carbon derived from both allochthonous and autochthonous sources. The overall objective of this research was to investigate the sources of organic carbon and the coupling between organic carbon inputs and deposition on the Mississippi river-dominated Louisiana shelf using pigment and lignin biomarker analyses. During the course of these investigations a further goal was to conduct a comparison of two common methods of lignin analyses -- CuO oxidation and TMAH thermochemolysis Direct comparisons between data yielded from each method were determined to be not comparable. While conventional TMAH analyses can be employed more quickly and efficiently than the CuO method, CuO products appears more consistent and accurate in identifying bulk lignin sources within sediments, and were determined to be more appropriate in systems with low terrestrial organic matter such as the Louisiana shelf The spatial distribution of phytoplankton pigments in surface waters varied by season and appeared strongly influenced by river discharge. Total chlorophyll concentrations ranged up to 31nM in surface waters during the period of high river discharge, and up to 1.71 mg chla 100 mg OC-1 in the sediments during low river discharge. Diatoms made up the most abundant phytoplankton taxa in this system, contributing a substantial amount of the marine-derived organic carbon to surface waters and sediments on the shelf Concentrations of terrigenous organic matter were spatially variable across the shelf, and linked to distinct regions of sediment accumulation. The highest lignin concentrations (up to 4.7 mg lignin 100 mg OC-1 ) were found closest to the river mouth and in the shallow near shore regions of the study area. While there were no discernible seasonal differences in contributions of C3 and C4 terrestrial material to the sediments, the proportions of riverine-derived, marsh-derived, and phytoplankton-derived organic carbon varied according to season and location across the shelf