The competition between coastal trace metal fluxes and oceanic mixing from the 10Be/9Be ratio: Implications for sedimentary records

Abstract

At an ocean margin site 37°S offshore Chile, we use the meteoric cosmogenic 10Be/9Be ratio to trace changes in terrestrial particulate composition due to exchange with seawater. We analyzed the marine authigenic phase in surface sediments along a coast-perpendicular transect and compared to samples from their riverine source. We find evidence for growth of authigenic rims through coprecipitation, not via reversible adsorption, that incorporate an open ocean 10Be/9Be signature from a deep water source only 30 km from the coast, overprinting terrestrial 10Be/9Be signatures. Together with increasing 10Be/9Be ratios, particulate-bound Fe concentrations increase, which we attribute to release of Fe-rich pore waters during boundary exchange in the sediment. The implications for the use of 10Be/9Be in sedimentary records for paleodenudation flux reconstructions are the following: in coast-proximal sites the authigenic record will likely preserve local riverine ratios unaffected by exchange with seawater, whereas sites beneath well-mixed seawater will preserve global flux signatures.