The Hawaii Aerosol Time-Series (HATS) was a coordinated effort to simultaneously monitor atmospheric and water column dust dynamics in the North Pacific Subtropical Gyre. Throughout 2022 and 2023, HATS measured the composition of water column particles and made complementary measurements of aerosol chemistry and deposition fluxes. Here we report the inventories, elemental composition, dust-relative residence times, and internal dynamics of size-fractionated marine particles at Station ALOHA from four expeditions between September 2022 and August 2023. Mixed layer inventories of lithogenic particulate elements (Al, Fe, and Ti) varied by factors of 9 and 5 in small (0.2-51 mu m) and large (>51 mu m) particles, respectively, and by factors of 3 and 2 through 300 m depth; much less than the factor of 25-42 variations of these elements observed in aerosols. Mixed layer residence times of small lithogenic particles, relative to dust fluxes, ranged from <1 day to 12 days and from 1 week to 6 months through 300 m, demonstrating persistent rapid packaging into large particles near the surface. Fractional lability of Fe and Al in marine particles ranged from 6% to 89% and 22%-80%, respectively, in the upper 200 m, with Fe showing greater variability across size fractions. Scavenged Fe was more abundant below 200 m than at the surface, coincident with longer particulate residence times and larger lithogenic inventories in the upper mesopelagic. Ti-normalized lithogenic ratios of marine particles were mostly lower than the aerosol time-series, suggesting loss of Fe and Al from minerals post-deposition and/or unquantified lateral inputs of Ti-rich material.

The chromium (Cr) stable isotope compositions of sedimentary rocks have been used to track the evolution of oxygen levels in Earth's ocean-atmosphere system. However, the Cr isotope fractionation during transfer of dissolved Cr to sediments is still unclear. The Chesapeake Bay is a seasonally redox-stratified estuary (oxic surface water overlying euxinic, or sulfide-bearing, deep water) and thus provides a good opportunity to investigate Cr isotope fractionation in euxinic systems. Here we report total dissolved Cr concentrations and stable isotope compositions (S53Cr) of a water column in this estuary. As salinity increases from 7.51 in the oxic surface water to 19.22 in the euxinic deep water, total dissolved Cr concentration decreases from 1.21 nmol kg- 1 to 0.51 nmol kg-1, while S53Cr decreases from 0.60 %o to -0.41 %o. Explaining these data requires partial reduction of Cr(VI) to Cr(III) followed by incomplete scavenging of Cr(III) from solution, which leads to isotope fractionation during removal of Cr from seawater to anoxic sediments. An extensive compilation of redoxstratified systems suggests that Cr isotope fractionation during removal from anoxic water column is a common phenomenon. This has major implications for using the S53Cr of anoxic sediments to reconstruct paleoseawater S53Cr values. Correcting for such fractionation will require better understanding of its governing factors, which would need more data for sediment trap, porewater, and redox-specific water column samples from anoxic settings.

The biological pump, a fundamental process governing atmospheric CO 2 , rapidly transfers particulate inorganic and organic carbon (PIC and POC) from surface waters to the deep sea but is inherently highly variable in space and time, and thus poorly observed. Here we synthesize PIC and POC data from satellites, CTD‐profiled optical sensors (birefringence and transmissometer), and from in situ pumps samples from GEOTRACES transects spanning 20,000 km from the North Pacific to Southern Ocean. High resolution profile data from PIC sensors revealed strong subsurface maxima in the deepest euphotic zone waters of oligotrophic gyres; furthermore, data showed high concentrations of PIC penetrating to >500 m south of the Subarctic Front (45°N–35°N), at the equator, and north of the Antarctic Polar Front (45°S–55°S) indicating high carbon export in these regions. We developed a new temporal/spatial interpolation scheme for satellite data that improved matchups with ship observations. North of the Antarctic Polar Front (APF), PIC sensor data was generally well aligned with sample PIC; however, a positive bias of satellite PIC was found in poor retrieval regions. South of the APF, both satellite and birefringence sensor greatly overestimated PIC by factors of >25 and 12, respectively, compared to sample PIC which averaged 15 nM. The unanticipated discovery of a non‐carbonate particle birefringence source coupled with a microscopic investigation of pump samples leads us to hypothesize that internal reflection within bubbles and/or cellular structures of heavily silicified colony‐forming diatoms ( Fragilariopsis and Pseudo‐nitzschia ) is the cause for anomalous birefringence and adds to backscattered satellite radiances.

The oceanic biogeochemical cycling of iron is globally important yet difficult to fully understand due to the many chemical processes involved. There is potential to use scandium, which has a similar ionic size and charge density to trivalent iron but lacks redox cycling, as a simpler analog for specific parts of the iron cycle, if we can sufficiently develop our understanding of scandium's reactivity. Here we move closer to this understanding. We look at particle reactivity and solubility through a 24‐hr incubation experiment: 5 nmol/kg of dissolved scandium and/or iron were added to filtered and unfiltered California Current System water. Particulate scandium formed only in the unfiltered treatments, at a quantity unlikely to have been taken up biologically. This is the first direct observation of scavenging of scandium, an attribute shared with iron. Our results also serve as the first test of scandium solubility in seawater: 1.9 nmol/kg of dissolved scandium was stable in the filtered treatment, 50 times more than the highest natural concentrations so far observed. This indicates that, in contrast to iron, scandium's oceanic cycling is unlikely to be influenced by solubility limits. We also compare particulate depth profiles: labile particulate iron was disproportionally higher than that of scandium in shelf‐influenced samples, likely due to iron reductively dissolving in the sediments, which scandium cannot do, and then precipitating in oxic seawater. Due to this combination of behaviors, our results suggest that paired observations of scandium and iron may help distinguish between iron sourced from sediment resuspension and reductive dissolution.

The transport and delivery of low-abundance, bioactive trace elements to the surface ocean by aerosol mineral dust is a major planetary control over marine primary production and hence the global carbon cycle. Variations in the concentration of atmospheric dust have established links to global climate over geologic timescales and to regional biogeographic shifts over seasonal timescales. Constraining atmospheric dust variability is thus of high value to understanding oceanographic systems, especially vast, constitutively low-nutrient subtropical gyre ecosystems and high-nutrient/low-chlorophyll ecosystems where availability of the trace element iron is a dominant ecological control. Here we leverage the MERRA-2 reanalysis product to examine over four decades of surface-level atmospheric mineral dust concentrations in a domain of the subtropical North Pacific centered at Ocean Station ALOHA. This study region has been sampled regularly since the mid-1980s and was the site of the Hawaii Aerosol Time-Series (HATS) project in 2022-2023. Two unequal semi-annual periods of elevated dust evident in the long-term results are described and constrained. We look for evidence of shifts in total and seasonal atmospheric dust abundances or in the timing of the onset of the dominant spring/summer dusty period, finding year-to-year variations but little evidence for long-term trends. We observe significant but complex relationships between the Pacific Decadal Oscillation (PDO) index and both dust and precipitation. The 2022 calendar year was among the dustiest years for the study domain in the preceding two decades and, by contrast, 2023 exhibited a significant early spring lull in dust.

Constraining the role of dust deposition in regulating the concentration of the essential micronutrient iron in surface ocean waters requires knowledge of the flux of seawater-soluble iron in aerosols and the replacement time of dissolved iron (DFe) in the euphotic zone. Here we estimate these quantities using seasonally resolved DFe data from the Bermuda Atlantic Time-series Study region and weekly-scale measurements of iron in aerosols and rain from Bermuda during 2019. In response to seasonal changes in vertical mixing, primary production and dust deposition, surface DFe concentrations vary from similar to 0.2 nM in early spring to >1 nM in late summer, with DFe inventories ranging from similar to 30 to similar to 80 mu mol/m(2), respectively, over the upper 200 m. Assuming the upper ocean approximates steady state for DFe on an annual basis, our aerosol and rainwater data require a mean euphotic-zone residence time of similar to 0.8-1.9 years for DFe with respect to aeolian input.

Particulate phases transport trace metals (TM) and thereby exert a major control on TM distribution in the ocean. Particulate TMs can be classified by their origin as lithogenic (crustal material), biogenic (cellular), or authigenic (formed in situ), but distinguishing these fractions analytically in field samples is a challenge often addressed using operational definitions and assumptions. These different phases require accurate characterization because they have distinct roles in the biogeochemical iron cycle. Particles collected from the upper 2,000 m of the northwest subtropical Atlantic Ocean over four seasonal cruises throughout 2019 were digested with a chemical leach to operationally distinguish labile particulate material from refractory lithogenics. Direct measurements of cellular iron (Fe) were used to calculate the biogenic contribution to the labile Fe fraction, and any remaining labile material was defined as authigenic. Total particulate Fe (PFe) inventories varied <15% between seasons despite strong seasonality in dust inputs. Across seasons, the total PFe inventory (+/- 1SD) was composed of 73 +/- 13% lithogenic, 18 +/- 7% authigenic, and 10 +/- 8% biogenic Fe above the deep chlorophyll maximum (DCM), and 69 +/- 8% lithogenic, 30 +/- 8% authigenic, and 1.1 +/- 0.5% biogenic Fe below the DCM. Data from three other ocean regions further reveal the importance of the authigenic fraction across broad productivity and Fe gradients, comprising ca. 20%-27% of total PFe.

Output associated with Tagliabue et al. Authigenic mineral phases as a driver of the upper ocean iron cycle

Processes controlling dissolved barium (dBa) were investigated along the GEOTRACES GA03 North Atlantic and GP16 Eastern Tropical Pacific transects, which traversed similar physical and biogeochemical provinces. Dissolved Ba concentrations are lowest in surface waters (∼35–50 nmol kg −1 ) and increase to 70–80 and 140–150 nmol kg −1 in deep waters of the Atlantic and Pacific transects, respectively. Using water mass mixing models, we estimate conservative mixing that accounts for most of dBa variability in both transects. To examine nonconservative processes, particulate excess Ba (pBa xs ) formation and dissolution rates were tracked by normalizing particulate excess 230 Th activities. Th‐normalized pBa xs fluxes, with barite as the likely phase, have subsurface maxima in the top 1,000 m (∼100–200 μmol m −2 year −1 average) in both basins. Barite precipitation depletes dBa within oxygen minimum zones from concentrations predicted by water mass mixing, whereas inputs from continental margins, particle dissolution in the water column, and benthic diffusive flux raise dBa above predications. Average pBa xs burial efficiencies along GA03 and GP16 are ∼37% and 17%–100%, respectively, and do not seem to be predicated on barite saturation indices in the overlying water column. Using published values, we reevaluate the global freshwater dBa river input as 6.6 ± 3.9 Gmol year −1 . Estuarine mixing processes may add another 3–13 Gmol year −1 . Dissolved Ba inputs from broad shallow continental margins, previously unaccounted for in global marine summaries, are substantial (∼17 Gmol year −1 ), exceeding terrestrial freshwater inputs. Revising river and shelf dBa inputs may help bring the marine Ba isotope budget more into balance.

Nano- and picophytoplankton are a major component of open-ocean ecosystems and one of the main plankton functional types in biogeochemical models, yet little is known about their trace metal contents. In cultures of the picoeukaryote Ostreococcus lucimarinus, iron limitation reduced iron quotas by 68%, a fraction of the plasticity known in diatoms. In contrast, a commonly co-occurring cyanobacterium, Prochlorococcus, showed variable iron contents with iron availability in culture. Synchrotron X-ray fluorescence was used to measure single-cell metal (Mn, Fe, Co, Ni, Zn) quotas of autotrophic flagellates (1.4-16.8-mu m diameter) collected from four ocean regions. Iron quotas were tightly constrained and showed little response to iron availability, similar to cultured Ostreococcus. Zinc quotas also did not vary with zinc availability but appeared to vary with phosphorus availability. These results suggest that macronutrient and metal availability may be equally important for controlling metal contents of small eukaryotic open-ocean phytoplankton.

Phytoplankton iron contents (i.e., quotas) directly link biogeochemical cycles of iron and carbon and drive patterns of nutrient limitation, recycling, and export. Ocean biogeochemical models typically assume that iron quotas are either static or controlled by dissolved iron availability. We measured iron quotas in phytoplankton communities across nutrient gradients in the Pacific Ocean and found that quotas diverged significantly in taxon-specific ways from laboratory-derived predictions. Iron quotas varied 40-fold across nutrient gradients, and nitrogen-limitation allowed diatoms to accumulate fivefold more iron than co-occurring flagellates even under low iron availability. Modeling indicates such "luxury" uptake is common in large regions of the low-iron Pacific Ocean. Among diatoms, both pennate and centric genera accumulated luxury iron, but the cosmopolitan pennate genus Pseudo-nitzschia maintained iron quotas 10-fold higher than co-occurring centric diatoms, likely due to enhanced iron storage. Biogeochemical models should account for taxonomic and macronutrient controls on phytoplankton iron quotas.

Chromium (Cr) has shown promise as a paleoceanographic proxy due to the redox-driven control of dissolved Cr concentrations ([Cr]) and stable isotope composition (delta Cr-53). To improve the mechanistic understanding of Cr cycling in the modern ocean and strengthen its potential proxy applications, we present new data from regeneration incubations, bottom and sediment pore waters, and a compilation of intermediate and deep water data. While Cr removal and biological export from the surface ocean is associated with organic carbon export, the deep water release of dissolved Cr from sinking particles is not directly dependent on organic carbon respiration, as indicated by differing trends between Cr, oxygen utilization and the regeneration of organic-associated macronutrients (e.g. N, P). Pore water and bottom water data demonstrate that benthic Cr fluxes are locally important and may be significant globally. The pore water dissolved Cr flux at our CaCO3-rich site is likely driven by the re-release of Cr scavenged from the water column by sinking particles, with minor contributions from lithogenic phases. We argue this is consistent with the highest open ocean [Cr] to date being found in the water column below oxygen minimum zones, likely reflecting the release of scavenged Cr in deep waters or surface sediments. Chromium released from suspended particles and surface sediments follows the global delta Cr-53-[Cr] array, supporting the proposed role of biological export and regeneration in shaping global Cr and delta Cr-53 distributions. Global intermediate and deep water [Cr], delta Cr-53 and Cr:macronutrient relationships are thus shaped by a synergy of circulation patterns, water mass mixing, a deep Cr regeneration cycle, and benthic Cr sources. A biogenic control on global Cr distributions indicates that sedimentary Cr records may reflect biogenic as well as O-2-dependent processes, while more research is needed to assess sediment Cr record fidelity based on an active diagenetic cycle. (c) 2021 The Author(s). Published by Elsevier B.V.