Sedimentary organic and inorganic biomarkers reveal historical environmental changes at the Lower Rhine River, Germany

Silted-up oxbow lakes along major river systems are valuable but under-explored archives of long-term socio-environmental changes. This study was conducted on a 5.7-m-long sediment core from the Kalkar oxbow (Lower Rhine River, Germany). To decipher such sensitive oxbow lake archives, we employ a multi-proxy approach based on gas chromatography - mass spectrometry, total organic carbon, element and grain size analyses. By integrating organic geochemical markers such as n -alkanes, fatty acid methyl esters, fecal steroids, linear alkyl benzenes (LABs), xenobiotics (1,2-diphenoxyethane, DDT metabolites) and polycyclic aromatic hydrocarbons (PAHs), we provide new insights into historical land-use practices, human settlement patterns, and riverine ecosystem responses to long-term environmental pressures from the Medieval period to the present. The core stratigraphy documents the oxbow's evolution from an active river channel to a disconnected lacustrine environment and its final terrestrialization into a floodplain soil. However, this natural fluvial alteration is superimposed by distinct anthropogenic imprints and floods. A pronounced peak in fecal steroid concentrations directly correlates with the demographic and economic summit of the city of Kalkar in the 15th and 16th centuries. Sharp spikes in pyrogenic PAHs, with clear biomass burning signatures, align with historically documented city fires. The abrupt appearance of LABs and xenobiotics and a shift to fossil fuel-derived PAHs in the upper core section, mark the onset of modern, post-war industrial pollution. This study demonstrates that oxbow sediments provide high-resolution archives capable of preserving natural alternation, specific local and regional historical events and long-term trends, revealing environmental information for understanding the cumulative impacts of human activities on riverine landscapes. • Multi-proxy approach to decipher oxbow's environmental and settlement history. • Sedimentological and geochemical reconstruction of six oxbow phases. • Human settlement indicators correlate with Kalkar's historic economic boom. • Combustion markers track fires and environmental proxies record land-use changes.