Dissolved Iron Concentration and the Solubility Inferred by Humic‐Like Fluorescent Dissolved Organic Matter in the Intermediate Water in the North Pacific Including the Marginal Seas. Issue 3 (27th February 2023)

• Record Type: Journal Article
• Authors: Yamashita, Youhei; Nishioka, Jun
• Is Part Of: Journal of geophysical research. Volume 128:Issue 3(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-02-27
• Subjects: Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1029/2022JG007159
• Languages: English
• ISSNs: 2169-8953

Abstract

Abstract: The marginal seas have been found to be important external sources of dissolved iron (Fe) in the North Pacific through circulation of intermediate water. Here, we show comprehensive spatial distributions of dissolved Fe concentrations and Fe(III) solubilities inferred from humic‐like fluorescent dissolved organic matter (FDOMH ) over the North Pacific, including the marginal seas, the Sea of Okhotsk, and the Bering Sea. FDOMH was used as a proxy of chemical speciation of dissolved Fe in the intermediate and deep waters where liner relationships were previously observed between FDOMH and Fe(III) solubility. When dissolved Fe concentration exceeds Fe(III) solubility, Fe(III) solubility is assumed to be equivalent to concentration of FDOMH ‐Fe complexes, and excess dissolved Fe concentration above the Fe(III) solubility is assumed to be colloidal Fe which is not complexed with FDOMH . In the intermediate water, the dissolved Fe concentration exceeded the Fe(III) solubility in the marginal seas, while excess Fe(III) solubility was evident downstream of the intermediate water circulation, suggesting that the major dissolved Fe chemical form derived from shelf and slope sediments in the marginal seas changed from colloidal Fe to FDOMH ‐Fe complexes. With the previous findings, namely the dominance of labile particulate Fe in total Fe in the intermediate water of the Sea of Okhotsk and the Bering Sea, we hypothesized that the average size of sediment‐derived Fe decreases during transportation by intermediate water, most likely due to reversible scavenging with the highest removal rate for labile particulate Fe and the lowest removal rate for FDOMH ‐Fe complexes. Plain Language Summary: Iron (Fe) supply from the exterior of the ocean is important in sustaining global primary productivity. However, the distribution/strength of external sources and the mechanism for long‐distance transport of dissolved Fe have not been well documented. We suggest that the major chemical form of marginal sea sediment‐derived dissolved Fe changed from Fe colloids to soluble Fe during long‐distance transport by the intermediate water of the North Pacific. The soluble Fe is likely dissolvable in seawater through forming complex with humic substances which are microbiologically and/or abiotically altered biogenic organic matter. This finding may help marine biogeochemical models better reproduce the Fe cycle. Key Points: Fe(III) solubility was inferred from humic‐like fluorescent dissolved organic matter Dissolved Fe concentration exceeded Fe(III) solubility in the intermediate water in the marginal seas Fe(III) solubility exceeded dissolved Fe concentration in the intermediate water away from the marginal seas