Sources of terrestrially-derived organic carbon in lower Mississippi River and Louisiana shelf sediments: implications for differential sedimentation and transport at the coastal margin

摘要

In this study, we examined the temporal and spatial variability of terrestrial organic carbon sources in lower Mississippi River and Louisiana shelf sediments (during 11 cruises over a 22-month period) to further understand the sorting dynamics and selective transport of vascular plant materials within the primary dispersal system of the river. Bulk δ13C values in lower river sediments ranged from −21.90%o −24.64%o (mean = −23.2 ± 1.09%o), these values were generally more depleted than those found in shelf sediments (−22.5%o to −21.2%o). The Λ8 (Λ8 = sum of vanillyl, syringyl and cinnamyl phenols produced from the oxidation of 100 mg of organic carbon) values in the lower river ranged from 0.71 to 3.74 (mean = 1.78±0.23). While there was no significant relationship between Λ8 and river discharge ( p u3e 0.05), the highest value occurred during peak discharge in April 1999—which corresponded to the highest observed C/N value of 17.41. The Λ8 values on the shelf ranged from 0.68 to 1.36 (mean = 0.54±0.30) and were significantly lower ( p u3c 0.05) than the average value for lower river sediments. The range of S/V (syringyl/vanillyl) and C/V (cinnamyl/vanillyl) ratios on the shelf, 0.11 to 0.95 and 0.01 to 0.08, respectively, were similar to that found in the lower river. These low C/V ratios are indicative a mixture of woody and non-woody carbon sources. Recent work by Goni et al. [Nature 389 (1997) 275; Geochim. Cosmochim. Acta 62 (1998) 3055], which did not include sampling transects within the primary dispersal system of the Mississippi River, showed a non-woody vascular plant signature on the Louisiana shelf. This suggests that riverine-derived woody tissues preferentially settle out of the water column, in the lower river and inner shelf, prior to the selective dispersal of C3 versus C4 non-woody materials in other regions the shelf and slope. This works further demonstrates the importance of differential settlement of particles, sampling location within the dispersal system, and river discharge, when examining biogeochemical cycles in river-dominated margins.