Variability of sedimentary organic matter in subtropical estuarine systems due to anthropogenic and climatic events

Estuaries are ecosystems that have been changed by climatic and anthropogenic events, and subtropical estuaries located in the Southern Hemisphere are important examples of historical human occupation, land use and recent degradation. This study aims to assess whether climatic and anthropogenic events promoted the variability of sedimentary organic matter (OM) in two estuarine systems, the Paranagua Estuarine System and Guaratuba Bay, Brazil, using geochemical multiproxies. In this approach, bulk elementary and isotopic properties were integrated with specific molecular biomarkers (n-alkanes, n-alkanol and sterols) detected in the sedimentary OM from sediment cores. Our results showed a predominance of terrigenous OM in both estuarine systems over the last century. However, sterols proved to be a more robust proxy for indicating changes in the autochthonous input of sedimentary OM, especially in the core top sections. Based on the molecular biomarkers and total annual precipitation data, the climatic effects of periodic events (e.g., El Nino-Southern Oscillation) that occurred during the last century did not seem to have imposed noticeable changes in the sedimentary OM over the period covered by the cores (e.g., between 1912 and 2010 for the Paranagua Estuarine System). Instead, OM variations may respond to changes in the drainage basin or to specific local human activities. Plant extraction in the drainage basin was recorded in Guaratuba Bay before the 1960s, and the intensification of human occupation after the 1950s was recorded in the Paranagua Estuarine System. The multiproxy approach demonstrated that local environmental changes related to regional anthropogenic events in the adjacent drainage basin of estuaries may be considered for identifying human impacts in coastal zones.