High sedimentation rates along river-dominated margins make these systems important repositories for organic carbon derived from both allochthonous and autochthonous sources. Using elemental carbon/nitrogen ratios, molecular biomarker (lignin phenol), and stable carbon isotopic (bulk and compound-specific) analyses, this study examined the sources of organic carbon to the Louisiana shelf within one of the primary dispersive pathways of the Mississippi River. Surface sediment samples were collected from stations across the inner, mid, and outer Louisiana shelf, within the Mississippi River plume region, during two cruises in the spring and fall of 2000. Lignin biomarker data showed spatial patterns in terrestrial source plant materials within the river plume, such that sediments near the mouth of the Mississippi River were comparatively less degraded and richer in C4 plant carbon than those found at mid-depth regions of the shelf. A molecular and stable isotope-based mixing model defining riverine, marsh, and marine organic carbon suggested that the highest organic carbon inputs to the shelf in spring were from marine sources (55-61% marine organic carbon), while riverine organic carbon was the highest (63%) in fall, likely due to lower inputs of marine organic carbon at this time compared with the spring season. This model also indicated that marsh inputs, ranging from 19 to 34% and 3-15% of the organic carbon in spring and fall, respectively, were significantly more important sources of organic carbon on the inner Louisiana shelf than previously suggested. Finally, we propose that the decomposition of terrestrial-derived organic carbon (from the river and local wetlands sources) in mobile muds may serve as a largely unexplored additional source of oxygen-consuming organic carbon in hypoxic bottom waters of the Louisiana shelf.
Bibliographical noteFunding Information:
Acknowledgments The authors would like to thank the following people for their assistance in the field and laboratory: Jeff Corbino and Brent Dalzell, the captain and crew of the R/V Pelican, and David Gamblin and Dr. Sergey Oleynik at Purdue University. This work was supported by a grant from the Office of Naval Research to Thomas S. Bianchi, and by a fellowship from the Louisiana Board of Regents and a grant in-aid-of-research from Sigma-Xi to Laura Wysocki. The authors would like to extend special thanks to Purdue University for valuable housing and research assistance to Laura Wysocki, making it possible to continue research after the disruption caused by Hurricane Katrina.
- Chemical biomarkers
- Compound-specific isotopic analyses
- Louisiana shelf
- Mobile muds
- Sediment carbon cycling