Iron isotope insights into equatorial Pacific biogeochemistry

Abstract. The EUCFe cruise (RV Kilo Moana, 2006) was designed to characterize sources of Fe to the western equatorial Pacific and its transport by the Equatorial Undercurrent (EUC), a narrow and fast eastward current flowing along the equator, to the eastern equatorial Pacific High Nutrient Low Chlorophyll (HNLC) region. This study presents seawater dissolved (DFe) and particulate (PFe) iron concentrations and isotopic compositions (δ⁵⁶DFe and δ⁵⁶PFe) from 15 stations in the equatorial band (2° N–2° S) between Papua New Guinea and 140° W, over more than 8,500 km along the equator and in the upper 1,000 m of the water column.

δ⁵⁶DFe and δ⁵⁶PFe ranged from -0.22 to +0.79 ± 0.07 ‰ and from -0.52 to +0.43 ± 0.07 ‰, respectively (relative to IRMM-14, 95 % confidence interval). Source signatures, biogeochemical processes and transport all contribute to these observations. Two distinct areas, one under continental influence (the western equatorial Pacific) and an open ocean region (the central equatorial Pacific), emerged from the data. In the area under continental influence, high PFe concentrations along with δ⁵⁶DFe values systematically heavier than that of δ⁵⁶PFe indicated a permanent and reversible dissolved-particulate exchange. This exchange occurs through non-reductive processes, as previously proposed from three of the eight stations of this area (Labatut et al., 2014). In the open ocean area, preservation of a DFe isotopic signature of ~+0.36 ‰ within the EUC, from Papua New Guinea to the central equatorial Pacific (7,800 km), confirmed the origin of the DFe carried within this current toward the HNCL region. At the same depth, bordering the EUC at 2° N and 2° S at 140° W, light isotopic signatures suggested that was iron originating from the eastern Pacific oxygen minimum zones. These light signatures were also observed in deeper central waters, between 200 and 500 m. Our data did not allow conclusions about fractionation during uptake by phytoplankton, but indicated that this fractionation must be if any, is small, no larger than a few tenths of a per mil.