Stable isotopic characterization of organic carbon accumulation on the Amazon continental shelf

TLDR: The stable carbon isotopic ratio (13C:12C) of organic matter in the water column and sediments has been used to examine the distribution and accumulation of organic carbon on the Amazon continental shelf as discussed by the authors.

Abstract: The stable carbon isotopic ratio (13C:12C) of organic matter in the water column and sediments has been used to examine the distribution and accumulation of organic carbon on the Amazon continental shelf. Near the river mouth, in 0 × 10−3 salinity surface waters, particulate organic carbon (POC) is isotopically light (−27.3 per mil). Isotopic values of POC in continental shelf waters north and east of the river mouth range from −19.5 to −25.7 per mil. The isotopic variations in water column POC can be related to productivity, turbidity, and water density. The isotopic character of water column POC is controlled, therefore, by the dynamic mixing and northwestward migration of riverine and marine shelf waters. Terrestrial organic carbon dominates the isotopic signal in surface sediments (upper 10 cm) from the river mouth to areas 400 km to the northwest. Only on the outermost and northern parts of the shelf is marine organic carbon dominant in surface sediments. A sharp boundary between terrestrial and marine isotopic values is present in continental shelf sediments at the outer edge of the prograding mud delta. This boundary is associated with an abrupt decrease in sediment accumulation rate and a change in stratigraphic setting from topset and foreset regions to bottomset regions. The average TOC content of Amazon continental shelf sediments is 0.66 ± 0.20%. Based on the average TOC of Amazon shelf sediments and current estimates of sediment accumulation rates, approximately 4.5 × 1012 g-TOC y−1 is accumulating in Amazon shelf sediments. Using δ13C-POC values at salinities of 0 and 35 × 10−3 as end-members to isotopically resolve the contribution of terrestrial and marine carbon sources, approximately 3.1 × 1012 g-TOC y−1 or 69% of the organic carbon accumulating in Amazon shelf surface sediments is from terrestrial sources. This represents 6% of current estimates for Amazon riverine carbon discharge.