Sources, vertical fluxes, and accumulation of aliphatic hydrocarbons in coastal sediments of the Rio de la Plata Estuary, Argentina

Settling particles and underlying sediments collected at 1, 2.5 and 4 km off the metropolitan Buenos Aires coast were analyzed to evaluate the sources and accumulation of resolved (RES), unresolved (UCM), and biomarker aliphatic hydrocarbons. Sedimentary aliphatic concentrations (RES 0.11-14,mu g(.)g(-1); UCM 0.1-800,mu g(.)g(-1)) included variability associated with north-south gradients and an exponential offshore reduction. Highest concentrations were registered close to Buenos Aires port and sewer, compared to cleaner northern stations and southward sites affected by a seaward residue transport. Sediment traps deployed in the sewer area revealed large hydrocarbon (38 and 319 Mg(.)M(-2.)day(-1), RES and UCM) and total organic carbon fluxes (29 +/- 26 g(.)M(-2.)day(-1)). The composition of RES and hopanes evaluated by principal component analysis indicated a consistent offshore gradient-defined by the relative contribution of lower vs higher molecular weight components. Distant sites showed decreasing proportions of petrogenic n-C17-26 alkanes, isoprenoids, and C20-27 terpanes and relative enrichment of n-C-27,C-29,C-31,C-33 terrestrial plant alkanes and C31-33 homohopanes. Sediment hydrocarbon profiles showed an average 2-fold reduction down to 20 cm depth with preferential removal of lower molecular weight components and enrichment of n-C23-35 alkanes and hopanes. Sediment inventories and trap depositional fluxes indicate the accumulation of 58009700 tons of aliphatic hydrocarbons in the top 0-5 cm sediments with a strong interfacial alteration and selective preservation of refractory components: n-C13-22 (1.0%) < isoprenoids (3.2%) < n-C23-35 (6.1%) < hopanes (47%) similar to UCM (50%), compared to intermediate stability of organic carbon (12%) and quantitative preservation of polychlorinated biphenyls (PCBs) (91%).