Iron isotope constraints on source and transport of sediment-derived dissolved Fe in the Weddell Sea region

Abstract

In the Weddell Sea (WS), low iron (Fe) concentration limits primary productivity, making Fe supply and cycling crucial for air-sea exchange of CO2. This study investigates dissolved Fe concentrations ([dFe]) and isotope compositions (δ56Fe) in the western WS (south of 60°S), identifying Fe sources and tracing sediment-derived Fe transport along a transect from the Antarctic Peninsula to the central WS. Reductive dissolution within WS shelf sediments supplies isotopically light Fe to overlying shelf waters. Nearly identical δ56Fe depth profiles (–1.25 to –1.38‰) with varying [dFe] (1.20 to 7.14 nmol L−1) over the continental shelf suggest a consistent sedimentary δ56Fe signature with minimal fractionation during Fe loss. This isotope signal appears to advect from the shelf out into the central WS, as indicated by a persistent δ56Fe minimum (–0.66 to –0.34‰) at a depth of ∼200–300 m across the transect, following an isopycnal (28.1 to 28.2 kg m−3). Besides lateral transport, dense shelf water extensively mixes with Warm Deep Water and Weddell Sea Deep Water while descending along the slope, acting to prevent sediment-derived Fe from reaching the deep basin, instead diluting it into shallower waters where it is scavenged or upwelled, supporting phytoplankton growth. This study underscores the dominance of WS shelf sediment-derived Fe and its long-range transport within the WS, which potentially has having far-reaching impact via the connection with the Southern Ocean and beyond. The study also provides evidence for the use of relatively conserved δ56Fe signatures as a tracer of sedimentary Fe supply to the global ocean.