BGC-09 Biogeochemistry in Subtropical Oceans
Distribution of Dissolved Iron in the North Pacific Subtropical Gyre
Chengwang Wang* , College of Ocean and Earth Sciences
Yaojin Chen, State Key Laboratory of Marine Environmental Science
Nan Zhang, State Key Laboratory of Marine Environmental Science
Yihua Cai, State Key Laboratory of Marine Environmental Science
Zhimian Cao, State Key Laboratory of Marine Environmental Science
Minhan Dai, State Key Laboratory of Marine Environmental Science

Iron (Fe) is an essential micronutrient that plays a central role in regulating many biochemical processes such as nitrogen fixation in the ocean. The North Pacific Subtropical Gyre (NPSG) is one of the largest oligotrophic regimes in the world ocean where nitrogen fixation is considered as a major source of new nutrients to support new production in the euphotic zone (Ez). Though Fe is hypothesized as a primary factor limiting nitrogen fixation in the NPSG, Fe data remains very scarce, which seriously hidners our understanding to the biogeochemical cycle of Fe and its role in controling the bioloigcal productivity in the NPSG. Here we present the distribution of dissolved iron (dFe, < 0.4 μm) based on a large dataset composed of 56 surface samples and 246 depth profile samples collected during a cruise (KK2007) to the west part of NPSG (12-33 °N, 126-155 °E) in January- February, 2021. Surface dFe concentrations range 0.20-0.31 nmol/L with an average of 0.24 ± 0.03 nmol/L (1SD, n = 52). Higher concentrations are observed in the northeastern part (25-33 °N, 150-55 °E, 0.25-0.35 nmol/L) and in areas affected by the Kuroshio (15-20 °N, 122-128 °E, 0.25-0.30 nmol/L). In contrast, the stations located in the south area (11-15 °N, 140-155 °E) have lower dFe concentrations of < 0.25 nmol/L. The profiles of dFe shows minimum values at a depth of 80-120 m and gradually increases with depth to 0.50-0.85 nmol/L with a maximum at a depth of 600-2000 m. Below the maximum, dFe decreases with depth in deep waters (> 4000 m). At intermediate waters (600-1000 m) at stations K13a (11 °N, 130 °E) and K12a (13 °N, 135 °E) dFe concentrations are remarkably higher (0.98 and 0.80 nmol/L, respectively) than other stations. In combination with other parameters including Apparent Oxygen Utilization(AOU), Fe* (= [dFe] – RFe:P [PO43-]) and vertical diffusitive fluxes, we also discuss the underlying processes governing dFe distribution in the NPSG.