Indicadores paleoambientais do estuário do rio Jaguaribe

Abstract

The Jaguaribe River is essential for Ceará state, providing water resources for economic and social activities, while its historical and environmental variations reveal its sensitivity to climate, highlighting the need for broader and more precise studies to understand its changes over time. This study aims to identify paleoenvironmental proxies in lacustrine sediments in the estuary of the Jaguaribe River, associated with natural and/or anthropogenic environmental changes, spanning the Late Holocene and the Anthropocene, linked to SDG 13 on climate change. The methodology began with the sampling of sediment cores from two lakes with distinct characteristics, one being a paleochannel (LRJ1) and the other with irregular fluvial input (LRJ2). Analyses included radiochemical determination of 210Pb, organic matter content, concentrations of metals (Al, Ba, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sr, V, and Zn), as well as the elemental and isotopic composition of organic matter. Dating indicates sediments deposited from 1970 to the present. Proxies showed the same deposition behavior among themselves and a positive correlation with Al, indicating that detrital input is the main carrier to the depositional environment. The multi-proxy approach proved essential to avoid misinterpretations through proxy correlations. Detrital input indicators (Al and Fe) and salinity (Sr/Ba) for paleohydrology and paleoclimate revealed wet and dry periods, and a positive and negative water balance. Both cores showed the same start point of decrease in detrital input from 2003, the same year as the inauguration of the Castanhão Dam, suggesting a reduction in sediment input from reduced river flow due to the dam. Paleoredox indicators (Mn/Fe, V/Ni, and V/Cr) showed differences in their application between the lakes. Among them, V/Ni proved to be the most reliable paleoredox indicator. In core LRJ1, Mn/Fe also proved useful for lake level variations. Paleoproductivity was interpreted through metal normalization (Ba/Al, Cu/Al, Ni/Al, Zn/Al) and organic matter components (MO, CT, NT, C/N ratio, δ 13C, and δ 15N). However, indicators may be influenced by other factors, such as carbonates and anthropogenic sources. Correlation among paleoproductivity indicators in core LRJ1 showed a significant relationship, but anthropogenic interferences, such as sewage and shrimp farming, may cause errors in interpretation, affecting natural concentrations of Cu, Ni, NT, and δ 15N, as showed in core LRJ2.