The North Atlantic subpolar gyre experienced strong freshening in recent years starting around 2012. Here, we investigate the imprint of this freshwater anomaly on the water column hydrography and transport variability of the Irminger Current (IC). The IC transports warm and saline waters northward along the western flank of the Reykjanes Ridge as part of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC). To investigate if the salinity anomaly spread and propagated downward, we used high-resolution mooring data from the IC covering the period 2014 – 2022 combined with hydrographic sections from the Irminger Sea and Iceland Basin. We found that the IC experienced a strong freshening starting in summer 2016. By 2018, this salinity anomaly covers the whole water column down to 1500 m depth and freshened the IC until 2022. In 2022, the IC was at its freshest state observed since the early 1990’s. Hydrographic sections across the adjacent basins showed that the recent freshening spread across the Irminger Sea and was also comparable to its fresh state in the early 1990’s. The salinity anomaly increased the freshwater transport of the IC by a factor of three from 2014-2015 to 2021-2022 and caused a decrease in density over much of the water column. This resulted in an increase in the transport of waters lighter than 27.55 kg m-3, potentially strengthening the upper limb of the AMOC.

The subpolar North Atlantic plays an outsized role in the atmosphere-to-ocean carbon sink. The central Irminger Sea is home to well-documented deep winter convection and high phytoplankton production, which drive strong seasonal and interannual variability in regional carbon cycling. We use observational data from moored carbonate system chemistry sensors and annual turn-around cruise samples at the Ocean Observatories Initiative’s Global Irminger Sea Array to construct a near-continuous time series of mixed layer dissolved inorganic carbon (DIC), pCO2, and total alkalinity from summer 2015 to summer 2022. We use these carbonate system chemistry time series to deconvolve the physical and biological drivers of surface ocean carbon cycling in this region on seasonal, annual, and interannual time scales. We find high annual net community production within the seasonally-varying mixed layer, averaging 9.7±1.7 mol m-2 yr-1 with high interannual variability (range of 6.0 to 13.7 mol m-2 yr-1). The highest daily net community production rates occur during the late winter and early spring, prior to the observed high chlorophyll concentrations associated with the spring phytoplankton bloom. As a result, the winter and early spring play a much larger role in biological carbon export from the mixed layer than traditionally thought.

The subarctic Pacific Ocean and Bering Sea comprise the second-largest high nitrate, low chlorophyll region in the world, where primary production is limited by the availability of iron (Fe). To estimate the potential impact of different terrestrial aerosol Fe sources on marine ecosystems, we performed a suite of laboratory assessments following established protocols, including: 1) leaching with Milli-Q water, 2) sequential extractions, 3) complete acid digestions, 4) X-ray diffraction, and 5) grain size analysis. Measurements were performed on 20 fine-grained (<5 μm) glacier-derived sediments from Alaska and the Yukon, 2 fresh, never-wetted volcanic ashes (Redoubt 2009 and Pavlof 2016), and 6 weathered ashes (Redoubt and Augustine) which span the past ~8700 years. We compared results to published data on Asian desert-derived sediments, finding that the glacier-derived sediments have five times higher easily-reducible Fe (median 2.3 ± 0.6 wt. %) than desert-derived samples (0.49 ± 0.1 wt. %) and fourteen times higher easily-reducible Fe than fresh ash (0.16 ± 0.1 wt. %). In addition, fractional Fe solubility was higher in glacial sediment (median cumulative 0.31 ± 0.11% FeS) than volcanic ash (0.04 ± 0.02 % FeS). Glacial sediments also contained higher concentrations of bioactive metals including Co, Ni, Cu, Zn, Mo, Cd, and Pb. Inferred Fe availability increased with sample age, pointing to the ability of environmental weathering processes to change Fe geochemistry. Together these results suggest that glacier-derived dust may provide the subarctic Pacific with more bioavailable iron per unit mass than either volcanic ash or desert-derived dust.