Meg F. Yoder

Year‐round, full water‐column hydrodynamic and biogeochemical observations at the Ocean Observatories Initiative's Irminger Sea Array provide a unique opportunity to examine the drivers of ocean carbon sequestration via the biological carbon pump in a region with deep winter convection. We use daily resolution oxygen and optical backscatter profiles to quantify carbon remineralization and large particles sinking throughout the water column from 2015 to 2022, during which deep winter convection ranged from 440 to 1,430 dbar. We use calibrated oxygen time series to determine depth‐resolved remineralization rates, finding 6.4 ± 2.0 mol C m ⁻² yr ⁻¹ were exported seasonally from the surface and remineralized at depth. However, 65%–100% of this remineralized carbon was re‐entrained into the mixed layer in the subsequent winter, such that only 0.8 ± 1.2 mol C m ⁻² yr ⁻¹ was sequestered annually below the winter mixing depth. The remineralized carbon sequestered each year depends on winter mixing and carbon transfer efficiency through the thermocline, quantified as remineralization length scale (). Interannual variability in is correlated with the maximum mixing depth of the previous winter, suggesting detrainment of biomass during spring shoaling is an important pathway for export to the mesopelagic. Backscatter profiles reveal a carbon pool of large particles that is efficiently transferred to depth, with sinking particle pulses sequestering ∼0.6–1.5 mol C m ⁻² yr ⁻¹ of carbon that is not captured in our oxygen‐based estimates. These results underscore the importance of sustained multi‐parameter biogeochemical time series data to unravel the intertwined biological and physical drivers of carbon cycling in this highly dynamic region.

Year-round, full water-column hydrodynamic and biogeochemical observations at the Ocean Observatories Initiative’s Irminger Sea Array provide a unique opportunity to examine drivers of ocean carbon sequestration via the biological carbon pump in a region with deep winter convection. We use daily-resolution oxygen and optical backscatter profiles to quantify carbon remineralization and large particles sinking throughout the water column from 2015 to 2022, during which deep winter convection ranged from 440-1430 dbar. We use calibrated oxygen time series to determine depth-resolved remineralization rates, finding 6.4 ± 2.0 mol C m-2 yr-1 were exported seasonally from the surface and remineralized at depth. However, 65-100% of this remineralized carbon was re-entrained into the mixed layer the subsequent winter, such that only 0.8 ± 1.2 mol C m-2 yr-1 was sequestered annually below the winter mixing depth. The remineralized carbon sequestered each year depends on winter mixing and carbon transfer efficiency through the thermocline, quantified as remineralization length scale (). Interannual variability in is correlated with the maximum mixing depth of the previous winter, suggesting detrainment of biomass during spring shoaling is an important pathway for export to the mesopelagic. Backscatter profiles reveal a carbon pool of large particles that is efficiently transferred to depth, with sinking particle pulses sequestering ~0.6–1.5 mol C m-2 yr-1 of carbon that is not captured in our oxygen-based estimates. These results underscore the importance of sustained multi-parameter biogeochemical time series data to unravel the intertwined biological and physical drivers of carbon cycling in this highly dynamic region.

In deep convective regions of the subpolar North Atlantic, large quantities of carbon are taken up from the atmosphere each year; however, the relative impacts of biological and physical drivers cannot be disentangled using observations alone. Using an atmospherically forced one-dimensional model with biogeochemical tracers, we systematically assess the role of both atmospherically driven physical dynamics and the timing and magnitude of biological production in driving air-sea carbon dioxide flux. We find that stronger atmospheric forcing results in lower annual carbon uptake, with seasonal distribution of atmospheric forcing playing a key role. Persistent winter mixing and entrainment events during the spring bloom can reduce annual carbon uptake by half, even with equal euphotic zone annual net community production. Changes in the timing of atmospheric forcing events due to our shifting climate therefore have the potential to significantly alter annual carbon uptake in deep convection regions.

Starting in 2012, the eastern subpolar North Atlantic experienced the strongest surface freshening in the past 120 years. It is yet unknown whether this salinity anomaly propagated downward into the water column and affected the properties of the boundary currents of the subpolar gyre, which could slow down the overturning. Here, we investigate the imprint of this salinity anomaly on the warm and saline Irminger Current (IC) in the decade thereafter. Using daily mooring data from the IC covering the period 2014–2022 combined with hydrographic sections across the adjacent basins from 1990, the evolving signal of the salinity anomaly over the water column and its imprint on the transport variability is studied. We find that due to the salinity anomaly, the northward freshwater transport of the IC increased by 10 mSv in summer 2016 compared to summer 2015. In 2018, the salinity anomaly covered the water column down to 1,500 m depth. Hydrographic sections across the basin showed that this recent freshening signal spread across the Irminger Sea. Overall, the freshwater transport of the IC increased by a factor of three between 2014–2015 and 2021–2022. The associated density decrease over the upper 1,500 m of the water column resulted in an increase in the northward transport of waters lighter than σ₀ = 27.55 kg m⁻³ from 1.7 to 4.2 Sv. This change in northward IC transport by density class may impact the characteristics of the overturning in the Northeastern Atlantic, its strength and the density at which it peaks.

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 chemistry system sensors and annual turn‐around cruise samples at the Ocean Observatories Initiative's Irminger Sea Array to construct a near‐continuous time series of mixed layer total dissolved inorganic carbon (DIC), pCO₂, and total alkalinity from summer 2015 to summer 2022. We use these carbonate chemistry system 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 9.8 ± 1.6 mol m⁻² yr⁻¹ with high interannual variability (range of 6.0–13.9 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 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 (S) than volcanic ash (0.04 ± 0.02% Fe_S). Glacial sediments contained higher concentrations of other bioactive metals including Co, Ni, Cu, Zn, Mo, Cd, and Pb. Results suggest that glacier‐derived dust may provide the subarctic Pacific with more bioavailable iron per unit mass than either volcanic ash or Asian desert‐derived dust.

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 (S ) than volcanic ash (0.04 ± 0.02 % Fe _S ). 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.