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 are evident in the long-term data are described and constrained. We look for evidence of shifts in total and seasonal atmospheric dust abundances and 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 data show that 2022 was among the dustiest years for the study domain in the preceding two decades and, by contrast, that 2023 exhibited a significant early-spring lull in dust.

The 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 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 variability 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 nearly semi-annual dust pulses evident in the long-term data are described and constrained. We look for evidence of shifts in total and seasonal atmospheric dust abundances and in the onset timing of the dominant spring/summer pulse, finding year-to-year variations but little evidence for long-term trends. We observe significant and complex monthly relationships between the Pacific Decadal Oscillation (PDO) index and both dust and precipitation, offering new insights into the role of timing for increased or diminished dust delivery. We observe that 2022 was among the dustiest years for the study domain in the preceding two decades and, by contrast, that 2023 exhibited a significant early-spring lull in dust.

The biological pump, a fundamental process governing atmospheric CO2, 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 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 15nM. The unanticipated discovery of a strong non-carbonate particle birefringence source coupled with a microscopic investigation of pump samples leads us hypothesize that internal reflection within bubbles and/or body structures of heavily silicified colony-forming diatoms (Fragilariopsis and Pseudo-nitzschia) is the cause for both anomalous birefringence and backscattered satellite radiances.

Chromium (Cr) has shown promise as a paleoceanographic proxy due to the redox-driven control of dissolved Cr concentrations ([Cr]) and stable isotope composition (δ53Cr). However, substantial uncertainties in the biogeochemical Cr cycle have limited its paleoproxy application to date. 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 δ53Cr–[Cr] array, supporting the proposed role of biological export and regeneration in shaping global Cr and δ53Cr distributions. Global intermediate and deep water [Cr], δ53Cr 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 O2-dependent processes, while more research is needed to assess sediment Cr record fidelity based on an active diagenetic cycle.