Iron, nutrients, and humic‐type fluorescent dissolved organic matter in the northern Bering Sea shelf, Bering Strait, and Chukchi Sea

Abstract

The concentrations of dissolved Fe ([DFe]), total dissolvable Fe, humic‐type fluorescence intensity (humic F intensity) as humic‐type fluorescent dissolved organic matter (humic‐type FDOM), and nutrients were determined vertically in the northern Bering Sea shelf (Yukon River estuary region and St. Lawrence Island polynya region), the eastern Chukchi Sea shelf (Chukchi shelf polynya region), and the central Bering Strait during 30 July to 12 August 2007 and 30 June to 12 July 2008. In early July 2008, with a nearly peak river water discharge period, remarkably high humic F intensity and high [DFe] were found with a decrease in salinity (S = 26–30) in the surface water of the Yukon River estuarine region. These results suggest that riverine humic‐type FDOM is responsible for [DFe] in coastal waters, with natural organic ligand complexation of Fe(III) apparently playing an important role in iron transport to the northern Bering Sea shelf. In the St. Lawrence Island and Chukchi shelf polynya regions, cold dense waters (T = −1.5 to −1.7°C and σθ = 26.2 to 26.7) were found in deep and bottom waters. The cold waters are characterized by high nutrient and iron concentrations and high humic F intensity, probably resulting from two main processes: brine rejection during sea ice formation and transport across the sediment‐water interface during early diagenesis. In the central Bering Strait, the waters are vertically uniform with high salinity (S ≥ 32.5) and high nutrient and iron concentrations. These waters appear to originate from the outer shelf of the Bering Sea and are not affected by denitrification occurring in bottom sediments of the Bering shelf.

Laterally spreading iron, humic-like dissolved organic matter and nutrients in cold, dense subsurface water of the Arctic Ocean

“…Here, N* is the deviation of the nitrogen concentration from the stoichiometric relationship between nitrogenous nutrients and phosphate, and large negative values indicate the occurrence of significant denitrification within the shelf sediments1920. In contrast, the high Fe is probably attributable to high Fe inputs from rivers21 and melting ice into nutrient-depleted surface waters. Denitrification, in which nitrate instead of oxygen is consumed during the bacterial decomposition of organic matter in low-oxygen waters in sediment pores, causes N* to decrease independently of the phosphate level.…”

“…Above the continental shelves of the Bering and Chukchi seas, cold, dense deep and bottom waters are characterized by high levels of nutrients, humic F-intensity, and Fe, and by denitrification/anammox, because of the transport of chemical species across the sediment–water interface during early diagenesis262123 (Fig. 7g).…”

“…Dissolved Fe and T-Fe were measured by an automated Fe analyzer (Kimoto Electric Co. Ltd.) using a combination of chelating resin concentration and luminol-hydrogen peroxide chemiluminescence (CL) detection in a closed flow-through system as described previously218213048 and verified against seawater reference materials. Briefly, acidified iron samples were buffered at pH 3.2 with an 8.15 M buffer solution of quartz-distilled formic acid and a 4.54 M ultrapure grade ammonium solution (0.8 ml per 100-ml sample solution) in a class 100 clean-air bench in the laboratory on shore.…”

“…Humic-like fluorescent DOM was quantified by the humic-like fluorescence intensity (humic F-intensity) by a method reported previously22130. The frozen 0.22-µm filtered samples in acrylic tubes were thawed and warmed overnight to room temperature in the dark, and then the humic F-intensity was measured in a 1-cm quartz cell with a Hitachi F-2000 fluorescence spectrophotometer at 320 nm excitation (Ex) and 420 nm emission (Em) wavelengths with a 10-nm bandwidth4647.…”

Impact of sea ice on the marine iron cycle and phytoplankton productivity

“…In the Arctic region, in situ studies have reported high dissolved iron concentrations in the Laptev Sea, the Fram Strait, the Bering Strait, and the Chukchi Sea: values are > 10 nM ), 5.7-23.1 nM (Tovar-Sanchez et al, 2010, 5-10 nM (Nishimura et al, 2012), and 2.1-16.3 nM (Nishimura et al, 2012), respectively. Lower concentrations are observed in other Arctic regions.…”