Benthic hydrocarbons of Rhode Island Sound

Document Type


Date of Original Version



The hydrocarbon contents of surface sediments, sediment cores and ocean quahogs (Arctica islandica) from Rhode Island Sound have been determined. Hydrocarbon concentrations in surface sediments normally range from 1.0 to 56.1 μg/g, largely dependent on sediment type and sedimentation rates. However, concentrations up to 301 μg/g are observed in surface samples from a dredge spoil deposit located in the study area. Based on (1) qualitative and quantitative hydrocarbon distributions in the sediments, (2) the hydrocarbon to organic carbon ratio, and (3) the ratio of the concentration of a prominent cycloalkene compound to organic carbon, the normal hydrocarbon geochemistry of the region is defined. Using these criteria, the effect of the dredge spoil deposit (containing 5 to 20 × 103 metric tons of hydrocarbons) is seen to be insignificant beyond 2 km from the disposal site. Hydrocarbon contents of the ocean quahog do not reflect the sediment distributions qualitatively or quantitatively. Throughout the study area the clams' hydrocarbon contents vary by a factor of 2.5 (2.6-6.4 μg/g wet) while the sediment concentrations vary by two orders of magnitude. The hydrocarbon assemblage in the clams exhibits a lower boiling point distribution than that in the sediments. Key components of the surface sediments are two cycloalkene compounds of molecular weight 344 and 348. Their concentration covaries very significantly with the organic carbon content of the sediment. A major component of Arctica is another related cycloalkene of molecular weight 342. This compound is not present in the sediment. A sediment core from the area shows a decreasing concentration of hydrocarbons and a decreasing percentage of unresolved components (UCM) with increasing depth. It is proposed that the rapid increase in the quantity of the UCM observed at a certain depth within the sediment, can serve as a chemical marker in the recent sedimentary record. This marker corresponds to the increased usage of petroleum products at the end of the 19th century. © 1978.

Publication Title, e.g., Journal

Estuarine and Coastal Marine Science