Anthropogenic and natural variability in the composition of sedimentary organic matter of the urbanised coastal zone of Montevideo (Río de la Plata)

Highlights: • Historical and surface sediment were analysed for δ¹³C, δ¹⁵N, and C/N. • Historic changes after 1950 CE in δ¹³C and C/N is related to climatic variability. • Climatic variability exerted a major influence on the organic matter composition. • Surface organic matter composition showed spatial variability rather than temporal. • New insights on the dynamics affecting organic matter sources were provided. - Abstract: This study is aimed to identify the different sources of sedimentary organic matter (SOM) within Montevideo coastal zone (MCZ). To this end δ¹³C, δ¹⁵N and C/N ratio were analysed in surface sediments and a sediment core. Sediment core analysis showed that until ~ 1950 CE SOM was mainly marine, observing a shift towards lower δ¹³C in recent sediments, evidencing an estuarine composition. This trend was associated to the climatic variability, which exerted a major influence on the SOM composition, leading to an increased input of terrigenous material and associated anthropogenic contaminants. Surface sediments collected during different El Niño South Oscillation (ENSO) phases did not show inter-annual variability in SOM composition, which was mainly marine in both eastern and western region of MCZ and estuarine in Montevideo Bay. This spatial pattern provides new insights on the dynamics and factors affecting organic matter sources available for primary consumers along the study region.