Abstract:

Iron (Fe)-mediated preservation of organic carbon (OC) is important for carbon burial in estuarine and coastal sediments, yet its molecular selectivity across OC sources remains poorly constrained. This study investigated source-specific Fe-mediated OC preservation in surface sediments from the Pearl River Estuary (China) and adjacent coastal region using sequential water and hydrochloric acid (HCl) extraction, bulk OC source apportionment, and molecular characterization. HCl extraction targeted organic matter (OM) released during dissolution of acid-soluble reactive Fe phases, recovering more OC (6 ± 1% versus 5 ± 1% of sedimentary OC) and Fe (101 ± 35 versus 0.17 ± 0.16 μmol g-1) than water extraction. HCl-extractable OM was enriched in higher-mass, aromatic, unsaturated, and oxidized formulas. Within this fraction, OC/Fe ratios correlated negatively with aromatic, unsaturated, and oxidized formulas relevant to terrestrial sources and riverine primary production, but positively with hydrogen- and heteroatom-rich marine-relevant formulas. Comparison with a regional sedimentary OM molecular-fingerprint reference database supported the retention of these source-dependent signatures across the studied coastal sediments. These findings reveal molecularly selective, source-dependent Fe-mediated OC preservation and extend understanding beyond bulk mineral associations to the differential retention of OM during transport along the land–ocean aquatic continuum.