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Sea ice, an intrinsic component of the Arctic system, is undergoing rapid change. Sea ice dynamics play a role in the distributions of trace metals in Arctic waters through the incorporation, trans...

Atmospheric deposition is an important source of bioactive trace elements to the open ocean, but observations of this flux are sparse. Atmospheric deposition of aerosol iron is of particular interest as it can play an important role in supporting primary production in the global ocean, yet it represents a key uncertainty that hampers accurate numerical modeling of the marine iron cycle. We report concentrations of atmospheric trace elements from samples collected as part of the 2013 US GEOTRACES GP16 zonal transect of the eastern Pacific Ocean. The cruise transected a relatively dusty region which coincided with the Peruvian upwelling zone before entering the much less dusty region of the subtropical gyre. The aerosol chemical composition and elemental ratios indicate crustal sources for Al, Ti, V, Mn, and Fe while the analyses suggest that Cu, Cd, and Pb originate from anthropogenic emissions. Dry deposition fluxes were calculated by applying characteristic deposition velocities based on the expected particle size associated with each element. Bulk deposition, which includes wet and dry deposition, was calculated using the inventory of 7Be in the upper water column. Soluble aerosol iron flux estimates were compared with vertical iron fluxes within the water column to assess the relative importance of atmospheric deposition to the marine iron cycle in the region. Atmospheric deposition was insignificant relative to the upwelling input of iron in the areas near the continental sources but increased in relative importance seaward of the coastal upwelling zone even as the magnitude of deposition decreased away from the coast. This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. Gonzalez.

Melt ponds are a prominent feature of Arctic sea ice during the summer and play a role in the complex interface between the atmosphere, cryosphere and surface ocean. During melt pond formation and development, micronutrient and contaminant trace elements (TEs) from seasonally accumulated atmospheric deposition are mixed with entrained sedimentary and marine-derived material before being released to the surface ocean during sea ice melting. Here we present particulate and size-fractionated dissolved (truly soluble and colloidal) TE data from five melt ponds sampled in late summer 2015, during the US Arctic GEOTRACES (GN01) cruise. Analyses of salinity, δ18O, and 7Be indicate variable contributions to the melt ponds from snowmelt, melting sea ice, and surface seawater. Our data highlight the complex TE biogeochemistry of late summer Arctic melt ponds and the variable importance of different sources for specific TEs. Dissolved TE concentrations indicate a strong influence from seawater intrusion for V, Ni, Cu, Cd, and Ba. Ultrafiltration methods reveal dissolved Fe, Zn, and Pb to be mostly colloidal (0.003–0.2 μm), while Mn, Co, Ni, Cu, and Cd are dominated by a truly soluble (

The Gulf of Alaska (GoA) shelf is a highly productive regime bordering the nitrate-rich, iron (Fe)-limited waters of the central GoA. The exchange between nitrate-limited, Fe-replete coastal waters and nitrate-rich, Fe-deplete offshore waters, amplified by mesoscale eddies, is key to the productivity of the region. Previous summer field studies have observed the partitioning of Fe in the coastal GoA as being heavily dominated by the particulate phase due to the high suspended particulate loads carried by glacial rivers into these coastal waters. Here we present new physico-chemical iron data and nutrient data from the continental shelf of the GoA during spring and late summer 2011. The late summer data along the Seward Line showed variable surface dissolved iron (DFe) concentrations (0.052 nM offshore to 4.87 nM inshore), within the range of previous observations. Relative to available surface nitrate, DFe was in excess (at Fe:C=50 μmol:mol) inshore, and deficient (at Fe:C=20 μmol:mol) offshore. Summer surface total dissolvable iron (TDFe, acidified unfiltered samples) was dominated by the acid-labile particulate fraction over the shelf (with a median contribution of only 3% by DFe), supporting previously observed Fe partitioning in the GoA. In contrast, our spring data from southeast GoA showed TDFe differently partitioned, with surface DFe (0.28–4.91 nM) accounting on average for a much higher fraction (~25%) of the TDFe pool. Spring surface DFe was insufficient relative to available nitrate over much of the surveyed region (at Fe:C=50 μmol:mol). Organic Fe-binding ligand data reveal excess concentrations of ligands in both spring and summer, indicating incomplete titration by Fe. Excess concentrations of an especially strong-binding ligand class in spring surface waters may reflect in-situ ligand production. Due to anomalous spring conditions in 2011, river flow and phytoplankton biomass during our spring sampling were lower than the expected May average. We argue our samples are likely more representative of early spring pre-bloom conditions, providing an idea of the possible physico-chemical partitioning of Fe in coastal GoA waters relevant to initial spring bloom dynamics.

Trace element (TE) fluxes and their residence times (Fe, Mn, Cu, Pb, Cd, and V) within the surface ocean were determined along the GEOTRACES East Pacific Zonal Transect (GP16/EPZT) and found to reflect the diverse physical and geochemical conditions encountered across the track. The TE flux from atmospheric deposition, vertical mixing, and upwelling into the mixed layer and into the particle production zone (PPZ) along the GEOTRACES EPZT transect were evaluated with 7Be-based methods developed in earlier works. A horizontal input flux is driven from east to west by the South Equatorial Current (SEC), and estimated advection velocities were applied to horizontal gradients in the distributions of several TEs to approximate this term. There is a minimum in atmospheric deposition in the central gyre, with higher fluxes to the east due to large near-shore aerosol TE loadings, and higher to the west due to greater precipitation-driven deposition velocities (Vb). The 7Be-derived vertical diffusion (Kz) values range from 2.5 to 39 m2/d (0.29 × 10−4 to 4.5 × 10−4 m2/s) with higher values generally within the nearshore upwelling region and the lowest values within the stratified central gyre. Manganese displayed a well-defined gradient extending from the nearshore stations into the central gyre such that the advective term is a major component of the total input flux, particularly within the central gyre. Relative to other inputs the atmospheric input of soluble Mn is only of minor importance. Unlike Mn, there is no discernable horizontal gradient in the dissolved Fe data and therefore, there is no horizontal component of flux. Nearshore removal processes are more intense for dissolved Fe than for dissolved Mn and as a result, dissolved Mn remains elevated much farther offshore than does dissolved Fe. For the stratified mid-ocean gyre stations, the total input of Fe from all sources is relatively small compared to the inshore stations, and atmospheric deposition becomes the dominant mode of input. Aerosol Fe solubility determined by a 25% acetic acid leach with hydroxylamine hydrochloride was much greater than that derived from a leach using ultra-pure deionized water. This led to significant differences in the residence time of Fe calculated for the mid-ocean gyre using these different solubilities. Generally, each element displays relatively short (weeks–months) residence times within the nearshore region of robust upwelling, reflecting large input terms and rapid removal. Moving offshore, total input fluxes decrease and the residence times of the TEs increase markedly until the western edge of the transect. There, relaxation of ocean stratification permits greater upward turbulent flux and greater rainfall leads to greater atmospheric input of TEs.

The concentration of dissolved iron (DFe) and suspended leachable particulate iron (LPFe) in the water column of the western Beaufort Sea were investigated during the late summer of 2010. Elevated concentrations of surface DFe (0.49–1.42 nM) were similar to those reported in resent studies, likely reflecting input from melting sea ice and river discharge. The rapid decrease in DFe (5.20–0.48 nM) and LPFe (88.2–1.83 nM) values observed from inshore to offshore in Pacific influenced waters, suggest scavenging processes limit the input of DFe from the shelf to the deep basin. However, frequent eddies found in this region are likely important in promoting lateral advection, as suggested by higher surface DFe concentrations at an offshore station in the vicinity of a warm-core eddy. Within the Atlantic layer, relatively homogeneous DFe (0.69–0.80 nM) and LPFe (1.18–2.13 nM) concentrations were observed at all the stations, reflecting a balance in the interplay between input and removal processes within this watermass. An input of DFe east of the Lomonosov Ridge was inferred by comparing DFe values within the core of Atlantic water between the Eastern and Western Arctic.

[1] The Ross Sea polynya is among the most productive regions in the Southern Ocean and may constitute a significant oceanic CO2 sink. Based on results from several field studies, this region has been considered seasonally iron limited, whereby a “winter reserve” of dissolved iron (dFe) is progressively depleted during the growing season to low concentrations (∼0.1 nM) that limit phytoplankton growth in the austral summer (December–February). Here we report new iron data for the Ross Sea polynya during austral summer 2005–2006 (27 December–22 January) and the following austral spring 2006 (16 November–3 December). The summer 2005–2006 data show generally low dFe concentrations in polynya surface waters (0.10 ± 0.05 nM in upper 40 m, n = 175), consistent with previous observations. Surprisingly, our spring 2006 data reveal similar low surface dFe concentrations in the polynya (0.06 ± 0.04 nM in upper 40 m, n = 69), in association with relatively high rates of primary production (∼170–260 mmol C m−2 d−1). These results indicate that the winter reserve dFe may be consumed relatively early in the growing season, such that polynya surface waters can become “iron limited” as early as November; i.e., the seasonal depletion of dFe is not necessarily gradual. Satellite observations reveal significant biomass accumulation in the polynya during summer 2006–2007, implying significant sources of “new” dFe to surface waters during this period. Possible sources of this new dFe include episodic vertical exchange, lateral advection, aerosol input, and reductive dissolution of particulate iron.

[1] Concentrations of soluble ( 0.4 μm) and colloidal (0.025–0.4 μm) Fe size fractions were removed from surface waters within the inner and mid shelf. As a result the contribution of soluble Fe to the total Fe concentration increased from the inner shelf to the shelf break/slope waters. Surface water dissolved Fe concentrations on the northern GoA continental slope were higher than those previously observed in the central GoA gyre. Variations in surface water Fe concentrations from spring through summer appear to result from the changes in freshwater discharge and physical processes on the shelf.

[1] Factors influencing concentrations of nitrate, silicic acid, dissolved Fe, and dissolved Mn in the near-field Columbia River plume were examined during late spring and summer from 2004 to 2006 as part of the River Influences on Shelf Ecosystems program. Under upwelling-active phases, cold, high-nitrate coastal seawater was entrained in the plume, and nitrate concentrations of 16–19 μM were observed with as much as 90% from a coastal seawater origin. Under downwelling-relaxation phases, warm, nutrient-depleted coastal seawater was entrained forming a near-field plume with nitrate concentrations of 2.5–6 μM, with the river as the only source. Elevated silicic acid in the river is the dominant source, with concentrations of 60–80 μM in the near-field plume. During upwelling-active phases, high concentrations of dissolved Fe (as high as 40 nM) in the cold, low-oxygen, nutrient-rich coastal seawater were entrained to form a near-field plume with 15–20 nM dissolved Fe. During downwelling-relaxation phases, dissolved Fe in the intruding underlying warm coastal seawater was 1–3 nM, producing plume concentrations of 2–13 nM, with higher concentrations during the high river flow of May 2006. Dissolved Mn in the near-field plume covaried markedly as a function of increased tidal flushing in the estuary. The use of CORIE (pilot environmental observation and forecasting system for the Columbia River) time series conductivity-temperature-depth data within the estuary, along with data presented in this study, allows extrapolation of the near-field plume chemistry throughout the spring and summer seasons to provide insight into this important source of nutrients to the coastal waters in this region.

[1] Observational and modeling studies in the Bering Sea indicate that changes in the seasonal ice cover and time of ice retreat influence open-water productivity. In particular, the timing of the spring bloom and its phytoplankton community composition are affected. Dissolved iron (DFe) data in the water column and ice cores collected during the 2007- Bering Ecosystem Study (BEST) cruise indicate that the melting ice provided substantial DFe to the water column. The additional DFe input from melting sea ice could be biologically important along the outer shelf and shelf break where in ice-free areas insufficient DFe ( 20 μM). Variability in sea ice dynamics are likely to result in a varying supply of DFe to the outer shelf and shelf break in early spring, and to contribute to the observed changes in the timing and community composition of the spring phytoplankton bloom.