Origin and Distribution of Total Organic Matter and Aliphatic Hydrocarbons in Sedimentary Fractions of a Highly Urbanized, Tropical Estuary

This study investigated the distribution and origin of organic carbon (OC) and aliphatic hydrocarbons (AHs) in sand (≥ 63 μm) and mud (< 63 μm) fractions of the Pina Sound (NE Brazil). Sedimentary OC and AHs were characterized using elemental [(C/N)a ratio], isotopic (δ13C), and molecular (AHs diagnostic ratio) proxies. On a dry weight basis, the average proportions of OC and AH contents in the mud fraction were 88 and 96%, respectively. The distribution of OC in the mud fraction was regulated by inputs from aquatic sources, while in the sand fraction, the distribution was associated with sedimentary mud contents. This reflects the hydrodynamic control on the deposition of low-density suspended particles. The mud fraction contained OC derived from aquatic producers, bacteria, and sewage while plant fragments were prevalent in the sand fraction. Sewage is a major source of oil-derived AHs in the Pina Sound. The mud fraction exhibited a high (> 100 μg/g dw) concentration of the unresolved complex mixture and a high proportion (31%) of petrogenic n-alkanes. Mud-associated pollutants can be exported to adjacent continental shelf. Geochemical analyses in size-fractionated sediments of the Pina Sound proved to be a useful procedure for better understanding estuarine OC dynamics.