Radium-228 as a tracer of dissolved trace element inputs from the Peruvian continental margin

Abstract

Abstract Continental margins play a central role in the composition of seawater by being an important source of trace element essentials to the functioning of the ocean ecosystems. Here, we measured long-lived radium isotopes ( 226 Ra, 228 Ra) along a zonal transect at 12°S (US GEOTRACES GP16) in the eastern tropical South Pacific Ocean. We used 228 Ra to quantify the trace element and isotope (TEI) fluxes (DMn, DFe, and DCo) delivered from the Peruvian continental i) shelf and ii) slope. First, elevated 228 Ra activities were measured in surface water over the entire transect (~ 8500 km), evidence that the continental shelf is an important source of sediment-derived TEIs not only to coastal areas, but to central Pacific Ocean waters. Modeled 228 Ra shelf fluxes combined with water column dissolved TEI/ 228 Ra ratios were used to quantify the shelf-ocean input rates (normalized to shelf-area) for DMn (3.3 × 10 3 μmol m − 2 y − 1 ), DFe (1.5 × 10 3 μmol m − 2 y − 1 ), and DCo (1.0 × 10 2 μmol m − 2 y − 1 ). Second, co-occurring plumes of 228 Ra, DFe, and DMn extended over 1800 km from the margin at 1000–2500 m depth, indicative of a continental slope sediment TEI input to the intermediate water column. The 228 Ra gradient allowed us to derive an effective horizontal eddy diffusion coefficient (K h ) of 46 m 2 s − 1 , which in turn permitted the calculation of slope sediment DMn (6.4 μmol m − 2 y − 1 ) and DFe (5.9 × 10 2 μmol m − 2 y − 1 ) fluxes based on their offshore concentration gradients. On the scale of the South Pacific continental margin between 0–20°S, the DMn shelf flux is approximately 2–3 orders of magnitude higher than the slope flux, while the DFe shelf/slope flux is ~ 3:1. Both shelf and slope sediment derived DMn was transported over a significant distance towards the ocean interior, while DFe concentration gradients were steep, consistent with longer water column residence time for DMn as compared to DFe in marine systems. These findings highlight the importance of considering the continental slope-ocean boundary in the oceanic budgets of biologically-important trace elements.