Distribution of total organic carbon and total nitrogen in deep-sea sediments from the southwestern Gulf of Mexico

Abstract

The temporal and spatial variations of total organic carbon (TOC) and nitrogen (TN) contents, and the carbon-nitrogen ratio in superficial sediments collected at 115 locations at depths between 1,025 and 3,795 m in the Gulf of Mexico during the SIGSBEE, PROMEBIO and DGoMB cruises are herein reported. The average TOC concentration was 0.9 ± 0.3%, the TN 0.12 ± 0.03% and the C/N ratio 8.0 ± 1.7. None of the typically hemipelagic sediment samples recorded any sulfur. In contrast, chemosynthetic sediment samples from the immediate area of the Chapopote asphalt volcano in the Campeche Bay (3,300 m deep) were significantly higher both in their TOC and TN values than hemipelagic samples, and were characterized by high sulfur contents with local variations associated with bathymetric features in the lower continental slope and the abyssal plain. This study shows that the elemental composition of the abyssal sediments is controlled by depth and follows a parabolic pattern for both TOC and TN. Average values for the continental slope were TOC = 0.8 ± 0.2%, TN = 0.11 ± 0.0 4% (n=39), and for the abyssal plain TOC = 1.0 ± 0.3%, TN = 0.12 ± 0.03% (n=44). The C/N ratio suggests that the organic matter in the superficial sediments has a mixed hemipelagic-terrigenous origin, and it depends on the distance from the coast. This is a major contrast to open-coast gradients where C/N ratios remain constant, and thus it is characteristic of a marginal sea. The bathymetry affects TOC and TN, causing significant differences among the physiographic regions. TOC and TN recorded in a long-term time series station in the abyssal plain displayed changes with time, showing alternating periods of high terrigenous and photoautotrophic input. This study provides evidence of the inter-yearly variability in the origin of the organic matter in the abyssal plain. TOC and TN decrease with depth in the sediment due to the changing sedimentation conditions.