Biogeochemical cycling of Fe and Fe stable isotopes in the Eastern Tropical South Pacific

Abstract

The basin-scale distributions of iron (Fe) and Fe isotopes provide important insights into the biogeochemical cycling of this growth-limiting micronutrient in the ocean. Here we present new observations of dissolved Fe concentrations and stable isotope ratios (δ56Fe) from the US GEOTRACES Eastern Pacific Zonal Transect GP16. The western portion of the transect is characterized by low dissolved Fe concentrations with a heavy δ56Fe signature of + 0.4 to + 0.6‰, similar to the dust-influenced North Atlantic deep waters. This is punctuated by Fe inputs from hydrothermal vents along the East Pacific Rise, with a δ56Fe of − 0.3‰. One striking feature of the transect is a large plume of high dissolved Fe and low δ56Fe (0 to − 0.5‰) in the east, near the Peru margin. Here, maximum dissolved Fe occurs between 1000 and 3000 m and the elevated concentrations persist over 1000 km from the margin. The region of markedly lower δ56Fe extends even further, to roughly 4000 km offshore. The mid-slope depth at which this plume occurs (1000–3000 m) is at odds with current conceptual and numerical models of Fe inputs along continental margins, which predict a shallower and more restricted dissolved Fe maximum (upper-slope; ~ 100–1000 m). Here, we explore four possible explanations for the mid-slope Fe plume: (1) Fe fluxes are actually higher from mid-slope sediments; (2) the mid-slope plume is transported from a remote region (3) mid-slope Fe originates from resuspended sediments in a very persistent form, which remains in the dissolved phase for longer than Fe released from the upper-slope; (4) Fe is supplied from upper-slope sediments, and then transferred to greater depth by reversible scavenging onto sinking particles. Simple modeling is used to show that both input of persistent Fe from mid-slope sediments and reversible scavenging could explain the data. Flux of a more persistent chemical form of Fe from the mid-slope would be consistent with other tracers such as particle composition and 228Ra, which suggest that lithogenic sediments are preferentially resuspended at this depth, but may be at odds with the low δ56Fe signature. Reversible scavenging is consistent with both Fe concentrations and δ56Fe. Whatever its provenance, the plume observed near the Peru margin impacts Fe concentrations and δ56Fe throughout the entire eastern South Pacific region, suggesting that the roles of persistent Fe input and reversible scavenging should be appraised in fully coupled iron-carbon cycle models in order to better understand the global cycling of Fe and δ56Fe.