Organic matter distribution and accumulation on the inner Louisiana shelf west of the Atchafalaya River

Surficial sediment samples from seven shallow cross-shelf transects west of the Atchafalaya River in the northern Gulf of Mexico were analyzed during three sampling periods to determine the distribution of organic matter alon g the shelf and to evaluate the temporal variability of its deposition. Dow ncore sediment profiles from four sites, which represent 50-200yr of deposi tion, were also examined to assess long-term changes in organic matter accu mulation in this region. The Atchafalaya "mud stream," which transports fin e sediment westward parallel to the coastline, appears to play an important role in the transport of river-derived organic matter. In general, sedimen tary organic carbon (%OC) and total nitrogen (%TN) decrease seaward within each transect and westward along the shelf. Atomic organic carbon: nitrogen (C/N) ratios indicative of a terrestrial source (> 20) are observed near t he mouth of the river during each sampling period, but values along the rem ainder of the shelf fall within a narrow range (9-11) with no apparent offs hore trends. Depleted stable carbon isotope (delta C-13) values typical of C3 plant debris (-27 parts per thousand) are found near the river mouth and become more enriched (-22 to -21 parts per thousand) offshore. Organic mat ter distribution throughout much of the study area is similar during each s ampling period, with significant seasonal differences close to the river mo uth. Sediment, particulate organic carbon (POC), and particulate organic nitroge n (PON) budgets constructed for the study area reveal that 31 % of the sedi ment exported by the Atchafalaya River is presently deposited within the st udy area annually, while the organic matter burial rates in the same region represent only 21% and 22% of the riverine POC and PON inputs, respectivel y. The POC and PON budgets also suggest that the organic matter remineraliz ed in the water column is of algal origin (C/ N = 7.2), whereas riverine or ganic matter (C/N = 10) appears to be respired within the sediments. The 22 .7 g POC m(-2) yr(-1) and 2.7 g PON m(-2) yr(-1) buried in the study area a ccount for similar to 5 % of the combined riverine and autochthonous OC and ON inputs to this region of the Louisiana shelf. (C) 2001 Elsevier Science Ltd. All rights reserved.