Abstract
<jats:p>Globally, riverine inputs deliver more terrestrial mercury (Hg) to the coastal ocean than direct atmospheric deposition. Once in coastal environments, Hg can sorb to particles, become buried in sediments, and undergo methylation, facilitating its entry into food webs. However, at regional scales, Hg sources and transport pathways remain poorly constrained due to limited observations and uncertainties in source apportionment. Here, we combine Hg concentrations with Hg and organic carbon stable isotopic signatures in surface sediments to trace Hg sources along an estuarine-to-marine gradient in the southern Bay of Biscay (Atlantic Ocean).</jats:p> <jats:p> In-situ transformation processes had a negligible impact on Hg isotopic composition, supporting the use of Hg isotopes as a tracer for Hg source apportionment. Hg isotopic composition (δ <jats:sup>202</jats:sup> Hg) was significantly correlated to carbon (δ <jats:sup>13</jats:sup> C), revealing distinct terrestrial and marine endmembers, with an enrichment in lighter isotopes in estuarine sediments (δ <jats:sup>202</jats:sup> Hg -0.83 ± 0.15‰, δ <jats:sup>13</jats:sup> C -27.3 ± 0.50‰) compared to shelf and canyon marine sediments (δ <jats:sup>202</jats:sup> Hg -0.54 ± 0.16‰, δ <jats:sup>13</jats:sup> C -25.2 ± 0.80‰). A binary-mixing model constrained by the Hg-C isotope relationship suggests a progressive transition from a predominantly terrestrial Hg pool in estuarine samples (79%) towards decreasing terrestrial contributions in offshore marine samples (15%). Despite their lower terrestrial fraction, offshore sediments contained the largest terrestrial Hg stock, reflecting efficient terrestrial Hg export across the estuarine-to-marine boundary. </jats:p> <jats:p>Our results demonstrate that combined Hg and C stable isotope observations provide a powerful framework for Hg source apportionment, offering the potential to directly constrain the land-to-ocean transfer of Hg into coastal sediments.</jats:p>