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

Abstract

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 along the shelf and to evaluate the temporal variability of its deposition. Downcore sediment profiles from four sites, which represent 50–200 yr of deposition, were also examined to assess long-term changes in organic matter accumulation in this region. The Atchafalaya “mud stream,” which transports fine sediment westward parallel to the coastline, appears to play an important role in the transport of river-derived organic matter. In general, sedimentary 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 the mouth of the river during each sampling period, but values along the remainder of the shelf fall within a narrow range (9–11) with no apparent offshore trends. Depleted stable carbon isotope ( δ 13C) values typical of C3 plant debris (−27‰) are found near the river mouth and become more enriched (−22 to −21‰) offshore. Organic matter distribution throughout much of the study area is similar during each sampling period, with significant seasonal differences close to the river mouth. Sediment, particulate organic carbon (POC), and particulate organic nitrogen (PON) budgets constructed for the study area reveal that 31% of the sediment exported by the Atchafalaya River is presently deposited within the study area annually, while the organic matter burial rates in the same region represent only 21% and 22% of the riverine POC and PON inputs, respectively. The POC and PON budgets also suggest that the organic matter remineralized in the water column is of algal origin (C/N=7.2), whereas riverine organic 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 account for ∼5% of the combined riverine and autochthonous OC and ON inputs to this region of the Louisiana shelf.