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

Resumen

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.