Dissolved Fe across the Weddell Sea and Drake Passage: impact of DFe on nutrient uptake

Abstract

Abstract. This manuscript reports the first full depth distributions of dissolved iron (DFe) over a high-resolution Weddell Sea and Drake Passage transect. Very low dissolved DFe concentrations (0.01–0.1 nM range) were observed in the surface waters of the Weddell Sea, and within the Drake Passage polar regime. Locally, enrichment in surface DFe was observed, likely due to recent ice melt (Weddell Sea) or dust deposition (Drake Passage). As expected, in low DFe regions, usually a small silicate drawdown compared to the nitrate drawdown was observed. However, the difference in drawdown between these nutrients appeared not related to DFe availability in the western Weddell Sea. In this region with relatively small diatoms, no relationship between N : P and N : Si removal ratios and DFe was observed. In comparison, along the Greenwich Meridian (Klunder et al., 2011a), where diatoms are significantly larger, the N : P and N : Si removal ratios did increase with increasing DFe. These findings confirm the important role of DFe in biologically mediated nutrient cycles in the Southern Ocean and imply DFe availability might play a role in shaping phytoplankton communities and constraining cell sizes. Over the shelf around the Antarctic Peninsula, higher DFe concentrations (>1.5 nM) were observed. These elevated concentrations of Fe were transported into Drake Passage along isopycnal surfaces. Near the South American continent, high (>2 nM) DFe concentrations were caused by fluvial/glacial input of DFe. On the Weddell Sea side of the Peninsula region, formation of deep water (by downslope convection) caused relatively high Fe (0.6–0.8 nM) concentrations in the bottom waters relative to the water masses at mid-depth (0.2–0.4 nM). During transit of Weddell Sea Bottom Water to the Drake Passage, through the Scotia Sea, additional DFe is taken up from seafloor sources, resulting in highest bottom water concentrations in the southernmost part of the Drake Passage in excess of 1 nM. The Weddell Sea Deep Water concentrations (∼0.32 nM) were consistent with the lowest DFe concentrations observed in Antarctic bottom water in the Atlantic Ocean.

Dynamics of dissolved iron and other bioactive trace metals (Mn, Ni, Cu, Zn) in the Amundsen Sea Polynya, Antarctica

“…The T-S properties (blue line in Figure 2B) of this "endmember", Station 4, show that the warmest water at this station, found at 550-650 m, was about 1.0°C (blue line in Figure 2B), indicating that water invading Dotson Trough at the shelf break was not pure CDW as would be found well off the shelf break (> 2.0°C; Walker et al, 2013), but was already substantially modified by turbulent mixing at the shelf break, and thus is termed mCDW. Nevertheless, at these near-bottom depths of Station 4, dFe was 0.37 nmol kg −1 , in good agreement with dFe concentrations in CDW of the open ACC south of the Polar Front, measured north of the Ross Sea (Sedwick et al, 2008), in the Bellingshausen Sea just to the east of the Amundsen Sea (de Baar et al, 1999), and near the Drake Passage (Klunder et al, 2014). There was a small local maximum of 0.475 nmol kg −1 in the dFe profile at 360 m at Station 4 ( Figure 3A), suggesting a modest local input, and possibly reflecting a sedimentary contribution advected laterally from the bathymetric highs that border the trough at the shelf edge.…”

“…At the same time, euphotic zone dFe at many polynya stations was 0.06-0.12 nmol kg −1 (Figure 3E), similar to dFe concentrations found in the Fe-limited regions of the Pacific sector of Southern Ocean (de Baar et al, 1999;Sedwick et al, 2008;Klunder et al 2014). Many ASP stations had a small dFe minimum near 20 m depth ( Figure 3E), corresponding at most central ASP stations to the mid-to lower euphotic zone .…”

“…With the exception of an apparent single point maximum at 10 m ( Figure 3A, E), dFe was very close to 0.1 nmol kg −1 throughout the upper 50 m, similar to or lower than concentrations found in ACC surface waters south of the polar front, in regions where Fe supply limits primary productivity (de Baar et al, 1999;Sedwick et al, 2008;Klunder et al, 2014). A recent global compilation of dissolved Fe data reported a mean of 0.15 ± 0.08 nmol kg −1 in the upper 100 m of Pacific Antarctic waters (Tagliabue et al, 2012).…”

Impact of icebergs on net primary productivity in the Southern Ocean

“…Klunder et al (2011) reported that within the upper surface mixed layer of the Atlantic sector of the SO, the concentrations of dissolved Fe vary between 0.1 and 0.3 nM. Klunder et al (2014) also observed very low dissolved Fe concentrations (0.01-0.1 nM range) in the surface waters of the Weddell Sea and within the Drake Passage polar regime. This low level of Fe is often insufficient to sustain the planktonic growth and hence limits the primary productivity of the ocean (Price and Morel, 1998).…”

Nitrous oxide variability at sub-kilometre resolution in the Atlantic sector of the Southern Ocean

“…4a and b). Iron from the sediments, as well as land run-off could have stimulated productivity in these areas (Klunder et al, 2014;Sañudo-Wilhelmy et al, 2002). In addition, the sampling region was more sheltered from the circumpolar winds by the Antarctic Peninsula, stabilising the water column over the shallow 30 bathymetry.…”