Elemental and isotopic constraints on organic matter source shifts in a tropical urbanized estuary-creek system

Investigating the organic matter (OM) provenance in urban estuarine systems is crucial for understanding their carbon cycling, and sequestration potential. Elemental and isotopic constraints were analyzed in sediment cores (C1-C4) in Ulhas-Thane-Estuary-Creek-Systems (UTECS), an urbanized marine ecosystem along western India. Results revealed, noticeable elemental and isotopic composition changes in sediment organic carbon (OC: 0.1 %-4.8 %; δ¹³C: -25.6 ‰ to -20.1 ‰), total nitrogen (TN: 0.05 %-4.95 %; δ¹⁵N: 0.3 ‰ to 13.2 ‰), and in the phosphorus contents (TP: 0.03 %-0.09 %), with varying physical properties (D₅₀: 4.5 μm-68.3 μm; porosity: 17.7 %-53.8 % and bulk density: 0.6 g/cm³-2.3 g/cm³) over space and depth. Depleted δ¹³C, lower δ¹⁵N averages, with elevated carbon to nitrogen ratio in C1 indicated dominant terrestrial OM input, than in C2-C4, accumulating OM sourced from marine biological production and anthropogenic detritus. Low-energy conditions were conducive to P accumulation, found significant at C1. Results from a MixSIAR model highlighted OM source shift from terrestrial (70 %-80 %) in C1 to phytoplankton and benthic algae (∼60 %) in C4. A first-order OC burial rate estimation highlighted maximum up to 4.9 Mg/ha/yr in the deeper sediments from the estuarine core (C4), while the average OC_burial remained similar across UTECS. Human-induced land-use change, and hydrological change have dampened control on the OM inputs in UTECS, while the hydrodynamic controlled distribution and modification through anthropogenic detritus, fertilization have significantly controlled the OM mixture and preservation across the river-estuary continuum, provides important insights into our understanding of carbon cycling complexities in urbanized marginal ecosystems.