Search

Dissolved iron (dFe) availability limits the uptake of atmospheric CO 2 by the Southern Ocean (SO) biological pump. Hence, any change in bioavailable dFe in this region can directly influence climate. On the basis of Fe uptake experiments with Phaeocystis antarctica , we show that the range of dFe bioavailability in natural samples is wider (<1 to ~200% compared to free inorganic Fe') than previously thought, with higher bioavailability found near glacial sources. The degree of bioavailability varied regardless of in situ dFe concentration and depth, challenging the consensus that sole dFe concentrations can be used to predict Fe uptake in modeling studies. Further, our data suggest a disproportionately major role of biologically mediated ligands and encourage revisiting the role of humic substances in influencing marine Fe biogeochemical cycling in the SO. Last, we describe a linkage between in situ dFe bioavailability and isotopic signatures that, we anticipate, will stimulate future research.

Glacier meltwater supplies silicon (Si) and iron (Fe) sourced from weathered bedrock to downstream ecosystems. However, the extent to which these nutrients reach the ocean is regulated by the nature of the benthic cycling of dissolved Si and Fe within fjord systems, given the rapid deposition of reactive particulate fractions at fjord heads. Here, we examine the benthic cycling of the two nutrients at four Patagonian fjord heads through geochemical analyses of sediment pore waters, including Si and Fe isotopes (δ30Si and δ56Fe), and reaction‐transport modeling for Si. A high diffusive flux of dissolved Fe from the fjord sediments (up to 0.02 mmol m−2 day−1) compared to open ocean sediments (typically <0.001 mmol m−2 day−1) is supported by both reductive and non‐reductive dissolution of glacially‐sourced reactive Fe phases, as reflected by the range of pore water δ56Fe (−2.7 to +0.8‰). In contrast, the diffusive flux of dissolved Si from the fjord sediments (0.02–0.05 mmol m−2 day−1) is relatively low (typical ocean values are >0.1 mmol m−2 day−1). High pore water δ30Si (up to +3.3‰) observed near the Fe(II)‐Fe(III) redox boundary is likely associated with the removal of dissolved Si by Fe(III) mineral phases, which, together with high sedimentation rates, contribute to the low diffusive flux of Si at the sampled sites. Our results suggest that early diagenesis promotes the release of dissolved Fe, yet suppresses the release of dissolved Si at glaciated fjord heads, which has significant implications for understanding the downstream transport of these nutrients along fjord systems. Plain Language Summary: Through physical grinding and chemical reactions, glaciers are capable of releasing nutrients such as silicon and iron from the underlying rocks. In particular, there is a growing interest in the supply of these glacier‐sourced nutrients to sustain the growth of marine life in high‐latitude estuaries (fjords) and the adjacent coastal ocean. However, a substantial portion of the glacier‐sourced nutrients may be removed by rapid settling of debris (sediments) at the heads of these fjords, before they can be used by marine life. This study analyzes the chemistry of fluids within sediments collected from Patagonian fjord near river and glacial inputs to investigate the fate of glacier‐sourced nutrients (silicon and iron) that have been transferred to the fjord floor. Our laboratory analysis and calculations reveal that biological, chemical and geological processes in these sediments release significant amounts of iron back into the overlying fjord waters, but these high levels of iron chemically bind silicon, restricting its release into fjord waters. The disproportionate release of the two nutrients from fjord sediments found in this study advances our understanding of the modes of supply of glacier‐sourced nutrients to fjords and the coastal ocean. [ABSTRACT FROM AUTHOR]

The Southern Ocean is the largest high-nutrient low-chlorophyll environment in the global ocean, and represents an important source of intermediate and deep waters to lower latitudes. Constraining Southern Ocean trace metal biogeochemical cycling is therefore important not just for understanding biological productivity and carbon cycling regionally, but also for understanding trace metal distributions throughout the lower latitude oceans. We present dissolved Fe, Ni, Cu, Zn, Cd, Pb and macronutrient concentrations in the Indian and Pacific sectors of the Southern Ocean from the Antarctic Circumnavigation Expedition (austral summer 2016-17), which included the first opportunities to study trace metal cycling at the Mertz Glacier Polynya and the Balleny Islands, as well as two meridional cross-frontal transects. Dissolved Ni, Cu, Zn, Cd and macronutrient concentrations show similar or greater variability latitudinally within surface waters than vertically through the water column, reflecting the combined influence of circulation and biological drawdown in shaping the distributions of nutrient-type elements in the Southern Ocean. Slopes of Cu-Si(OH) 4 and Cd-PO 4 increase from the Polar Frontal Zone to south of the Southern ACC Boundary (Cu-Si(OH) 4) and from the Subantarctic Zone to the Antarctic Zone (Cd-PO 4). Latitudinal differences are also observed for Ni-Si(OH) 4 and Zn-PO 4 , with distinct Subantarctic Zone trends relative to those south of the Polar Front. Similarities between our Zn-Si(OH) 4 and Cd-PO 4 correlations and global compilations reflect the importance of exported Southern Ocean waters in setting these metal-macronutrient couples globally. Distinct Ni-macronutrient correlations are observed in this dataset relative to the global ocean, which supports a distinct cycling of Ni in the Southern Ocean compared to other basins. Concentrations of Pb are among the lowest observed in the global ocean; however, a local maximum is seen along the density level corresponding with Antarctic Intermediate Water. Concentrations within this isopycnal decrease with increasing latitude, which can be explained by decreasing atmospheric Pb input to more recently subducted waters. Substantial biological uptake of metals and macronutrients is observed at the Mertz Glacier Polynya. Here, inferred metal:macronutrient uptake ratios are comparable to those found in the Amundsen Sea Polynya, in Southern Ocean phytoplankton, and to metal-macronutrient correlations in our data set as a whole, highlighting the potential of Southern Ocean polynyas as natural systems for trace metal uptake and export studies. The Balleny Islands are a source of Fe to surface waters and the islands also appear to influence distributions of Zn, Cu and macronutrients, which may reflect the combined impact of Fe supply on biological uptake, mixing, and scavenging in deeper waters. The Kerguelen Plateau is also a source of Fe, as previously identified. Throughout our dataset, the ferricline is found deeper than the nitricline, in agreement with existing data and indicating that Fe is less easily entrained into the surface ocean than NO 3. Additionally, Fe:NO 3 ratios in most samples throughout the water column are Fe-limiting (<0.01 mmol mol−1). Therefore deep mixing, identified previously as the main Fe source to much of the Southern Ocean, would ultimately act to maintain Fe limitation. • Substantial biological trace metal uptake is observed at the Mertz Glacier Polynya. • The Balleny & Kerguelen Islands and the Antarctic Peninsula are local Fe sources. • Deep mixing supplies NO 3 more readily than Fe, and at Fe-limiting Fe:NO 3 ratios. • Ni-Si(OH) 4 , Cu-Si(OH) 4 , Zn-PO 4 and Cd-PO 4 relationships vary latitudinally. • Pb decreases from north to south along isopycnals south of Australia. [ABSTRACT FROM AUTHOR]