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The lowest latitude sea ice in the world (excluding coastal freezing) is in the southern Sea of Okhotsk (south of 46°N), where it has significant impacts on freshwater input and primary production. This region is subject to climate change, and accordingly the monitoring of sea ice conditions is important. However, the interannual variability of the region’s sea ice is poorly understood due to its logistical challenges. Sea ice observations have been conducted in this region every winter for the period 1996-2020. The interannual variability of the ice conditions and the likely factors responsible for it were investigated using visual observations following the international ASPeCt protocol, combined with satellite SSM/I-SSMIS ice concentration data (1988-2020). AMSR-derived ice drift data sets and ERA5 meteorological reanalysis data sets were also analyzed to examine the effects of dynamic and thermodynamic processes. Our analysis revealed that 1) sea ice area in this region varies differently from that in the central and northern Sea of Okhotsk, where decreasing trends are reported, 2) sea ice volume has remarkable interannual variation and the peaks appeared much to more affected by dynamically deformed ice than freezing conditions, and 3) prominently deformed ice can be explained by taking shear components into account based on sea ice rheology. These results suggest the importance of including the proper sea ice rheology in numerical sea ice models to reproduce the realistic sea ice volume and deformation processes, and thereby the realistic primary production for all seasonal ice zones., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

One of the most important breakthroughs in oceanography in the last 30 years was the discovery that iron (Fe) controls biological production as a micronutrient, and our understanding of Fe and nutrient biogeochemical dynamics in the ocean has significantly advanced. In this review, we looked back both previous and updated knowledge of the natural Fe supply processes and nutrient dynamics in the subarctic Pacific and its impact on biological production. Although atmospheric dust has been considered to be the most important source of Fe affecting biological production in the subarctic Pacific, other oceanic sources of Fe have been discovered. We propose a coherent explanation for the biological response in subarctic Pacific high nutrient low chlorophyll (HNLC) waters that incorporates knowledge of both the atmospheric Fe supplies and the oceanic Fe supplies. Finally, we extract future directions for Fe oceanographic research in the subarctic Pacific and summarize the uncertain issues identified thus far.

Vertical mixing in the ocean is an important physical process that brings deep nutrients to the upper layer. The Tokara Strait, where the Kuroshio flows out to the North Pacific from the East China Sea, is known for a mixing hotspot, but the associated vertical nutrient transport has not been well quantified yet, especially in the intermediate layer below 200 m depth. This study involved flow, turbulence, and nitrate observations to quantify the vertical turbulent nitrate flux in the Tokara Strait and discuss its impact on the Kuroshio nutrient transport. Temporal variation of current velocity, derived from a 3.5-day mooring observation, captured semi-diurnal internal waves propagating vertically at depths between 200 and 400 m. Vertical shear associated with the semi-diurnal flow was elevated at this depth range. Vertical shear due to the mean and the diurnal flow was also elevated at about 220 m and 400 m depths, respectively. Mean vertical eddy diffusivity was elevated to O(10–3–10–2 m2 s−1) at the density layers of 25.2–26.4 σθ (200–450 m depth). Associated vertical turbulent nitrate flux reached O(10 mmol m−2 day−1), especially at around 25.6 σθ (250 m depth) and 26.2–26.4 σθ (400–450 m depth). The enhanced vertical turbulent nitrate flux suggests a large amount of nitrate is injected vertically to the nutrient stream from the layer just above the North Pacific Intermediate Water and is transported throughout the core of the Kuroshio nutrient stream situated at 25.0–26.2 σθ.

We conducted a ship observation of two anticyclonic mesoscale eddies (Eddies A and B) between June and July 2016 in the offshore region of the western subarctic North Pacific to clarify the impact of anticyclonic eddies on biological production. Analysis of satellite data reveals that Eddy A formed south of the Alaskan Peninsula on May 2011 while Eddy B formed in the observation area on November 2015. Iron concentration was low both inside and outside Eddies A and B, and there were no large differences between chlorophyll a (chl-a) concentration inside and outside eddies. Satellite-derived chl-a concentration inside Eddy A had been high compared with that in the surrounding area when the eddy was located along the Aleutian Islands and just after it detached from the islands, suggesting that iron originating from the coastal region was consumed during the two-year journey from the coastal region to the observation area. Field measurements also reveal that vertical diffusivity was low both inside and outside Eddies A and B, which could be another factor underlying low iron both inside and outside Eddies A and B and small differences between chl-a concentration inside and outside the eddies.

Linking the amount of organic matter (OM) in sea spray aerosols (SSAs) to biological processes in ocean surface is essential for understanding marine aerosol formation and their potential to affect cloud formation. To date, chlorophyll (Chl) a concentration has been widely used as a surrogate for surface phytoplankton biomass or productivity to predict the relative abundance of OM in SSAs (OMSSA). Here we show a new index to present OMSSA using concentrations of Chl a and chlorophyllide (Chllide) a, which is a breakdown product of Chl a and has been used as a biomarker of senescent algal cells. The index was compared with submicrometer OMSSA, based on surface seawater and aerosol samples obtained during the pre-bloom in the western subarctic Pacific. Our results showed that the OMSSA was highly correlated with this unique index, suggesting that the OMSSA was closely linked with senescent algal cells and/or cell lysis. Furthermore, the hygroscopicity parameters κ derived from water-extracted SSA samples implied a reduction in the SSA hygroscopicity with increasing senescent status of phytoplankton. The index can represent OMSSA on a timescale of a day during the pre-bloom period, which should be further examined over different oceanic regions.

Significance A correct understanding of the iron and macronutrient dynamics at the termination of the global ocean conveyor belt circulation is critical for understanding the global carbon cycle and its changes in geological timescale. Newly obtained and compiled datasets of iron and macronutrients with the vertical mixing magnitude in the subarctic Pacific and marginal seas indicate the processes that determine the nutritional status of intermediate waters and the mechanisms by which subpolar marginal seas fuel the North Pacific Ocean through the intermediate water. The intermediate water formation processes play a major role in the connection of nutrients between the deep water and the surface water above it, and sustain biological production, at the termination of the global nutrient circulation., The mechanism by which nutrients in the deep ocean are uplifted to maintain nutrient-rich surface waters in the subarctic Pacific has not been properly described. The iron (Fe) supply processes that control biological production in the nutrient-rich waters are also still under debate. Here, we report the processes that determine the chemical properties of intermediate water and the uplift of Fe and nutrients to the main thermocline, which eventually maintains surface biological productivity. Extremely nutrient-rich water is pooled in intermediate water (26.8 to 27.6 σθ) in the western subarctic area, especially in the Bering Sea basin. Increases of two to four orders in the upward turbulent fluxes of nutrients were observed around the marginal sea island chains, indicating that nutrients are uplifted to the surface and are returned to the subarctic intermediate nutrient pool as sinking particles through the biological production and microbial degradation of organic substances. This nutrient circulation coupled with the dissolved Fe in upper-intermediate water (26.6 to 27.0 σθ) derived from the Okhotsk Sea evidently constructs an area that has one of the largest biological CO2 drawdowns in the world ocean. These results highlight the pivotal roles of the marginal seas and the formation of intermediate water at the end of the ocean conveyor belt.

An increasing body of work has shown the potential impacts of subglacial discharge from marine-terminating glaciers on the marine environment around Greenland. Upwelling of nutrients associated with rising buoyant plumes near the front of marine-terminating glaciers plays a key role in maintaining the high productivity of connected fjords. The response of protist communities to subglacial discharges into fjords nevertheless remains poorly understood. Here we show data of water properties, nutrients, and protist communities during two summers in 2018 and 2019 in a Greenlandic fjord system fed by marine-terminating glaciers. This study included the period of intense summer melting of the Greenland Ice Sheet in 2019. The data revealed high nutrient concentrations in 2019 that were attributed to intensified upwelling of nutrients and dissolved iron into the subsurface layer. The source of the iron and the nutrients was subglacial discharge and deep fjord water, respectively. Intense glacial discharges in 2019 mitigated nitrate and phosphate limitations of phytoplankton in the fjord and resulted in an increase of chlorophyll a in the subsurface layer and growth of large diatoms. Heterotrophic protists such as dinoflagellates, tintinnids, and nanoflagellates were more abundant in the summer of 2019. We concluded that nutrient upwelling by subglacial discharges was the major driver of the structure of lower trophic levels in fjord ecosystems. We hypothesize that the more intense melting of glaciers and related increase in subglacial discharge will enhance nutrient upwelling, and increased summer productivity in fjords until the glaciers retreat and terminate on land.

Linking the amount of organic matter (OM) in sea spray aerosols (SSAs) to biological processes in ocean surface is essential for understanding marine aerosol formation and their potential to affect cloud formation. To date, chlorophyll (Chl) a concentration has been widely used as a surrogate for surface phytoplankton biomass or productivity to predict the relative abundance of OM in SSAs (OM

We hypothesized that the characteristics of Coastal Oyashio water in the North Pacific Ocean during winter–spring have a major influence on the generation of massive spring diatom blooms throughout the Oyashio area. To test this hypothesis, we analyzed in situ data from May during 1990–2016 along a transect known as the “A-line” to monitor conditions in the Oyashio off the southeast coast of Hokkaido, Japan. Chlorophyll-a concentrations were high (≥5 mg m−3) on the shelf and in the offshore Oyashio areas around the First and Second Oyashio Intrusions. About 80% of the high chlorophyll-a concentrations in the Oyashio area were linked with low-salinity (≤33.0) water that was associated with the Coastal Oyashio water and its modified waters. For the remaining Oyashio area, the so-called “Oyashio water” with a high salinity of 33.0–33.7 was near the sea surface and associated with low chlorophyll-a concentrations with a frequency distribution mode at 1–2 mg m−3. Diatom communities with high abundance and diversity were also connected (disconnected) between the shelf and the offshore Oyashio areas with low (high) salinity and high (low) chlorophyll-a concentrations. Analysis of historical temperature–salinity data near the sea surface revealed that the Coastal Oyashio water and its modified waters appear most frequently around the shelf along Hokkaido and the Kuril Islands, and that the frequency of appearance decreases with increasing distance offshore. We suggest that the transport and pathway around the islands of the Coastal Oyashio water and its modified waters, which contain high nutrient concentrations and the seeds for massive spring blooms, are the primary factors regulating the generation and spatial extent of the massive spring blooms over the Oyashio area.

Biogeochemical cycling of trace metals in sea ice is important to the productivity of the Arctic Ocean. Unfortunately, the processes by which trace metals accumulate into sea ice are poorly understood. To gain a clearer understanding of the mechanisms behind trace metal accumulation, dissolved (D, < 0.2 mu m), and labile particulate (LP = Total Dissolvable Dissolved) iron (Fe), manganese (Mn), and cadmium (Cd) concentrations were compared to the structure observed in sea ice. Samples were pre-concentrated via solid-phase extraction on NOBIAS Chelate PA-1 resin and analyzed on a Graphite Furnace Atomic Absorption Spectrometer. Using photographic analysis for the percentage of pore microstructure and delta O-18 analysis, sea ice structure was determined to be snow ice, granular ice (frazil ice), mixed ice (granular and columnar ice) and columnar ice. Salinity and nutrients were low, indicating brine drainage and multi-year ice. High trace metal concentrations in snow ice indicated meteoric snow was a source of trace metals to sea ice. High concentrations of LPFe in granular ice indicated entrainment of suspended particulate trace metals by frazil ice during the formation of the granular ice structure. Whereas the high concentrations of DFe and DMn in granular ice may have been due to reduction from LPFe and LPMn after particle entrainment, indicating chemical transformation processes. Low dissolved and labile particulate trace metal concentrations in mixed and columnar ice indicated a release due to brine drainage. Our study clearly indicates that the differences observed in trace metals among sea ice structures, showed that sea ice formation, chemical reduction and brine release were the processes driving trace metal accumulation and release in the Arctic sea ice.

The first direct measurements of vertical turbulent nitrate flux, FluxNO3, were conducted in the Kuroshio across the Izu Ridge and revealed large FluxNO3 near the sill crest at the 1% light depth. This large FluxNO3 possibly enhances downstream phytoplankton growth. Extremely large FluxNO3 was also observed at 26.0–26.6 σθ between the core of the Kuroshio nutrient stream and the North Pacific Intermediate Water. We hypothesize that strong vertical mixing in the Izu Ridge draws sufficient nitrate upward from the North Pacific Intermediate Water to the nutrient stream, which eventually impacts the downstream biogeochemistry in the western North Pacific.

The 11th Symposium on Polar Science/Special session: [S] Accelerating Arctic research: Recent progress and future prospect of Arctic research, Tue. 1 Dec.

The 11th Symposium on Polar Science/Special session: [S] Accelerating Arctic research: Recent progress and future prospect of Arctic research, Tue. 1 Dec.

Determination of neodymium (Nd) isotopic composition in seawater is useful for tracing water masses and geochemical cycles for lithogenic elements in the ocean. A new separation procedure for determination of the Nd isotopic composition of in seawater samples was developed that offers enhanced sample throughput and improved measurement reliability. The procedure consists of conventional Fe hydroxide coprecipitation, solid phase extraction using DGA chelating resin column chromatography, and Ln Resin column chromatography to preconcentrate samples. High selectivity in HNO3 medium and elution by low concentration HCl medium for Nd are characteristics of extraction using DGA Resin®, and they allowed an evaporation step to be omitted between the chromatographic steps. These chromatographic steps, using DGA Resin to separate REEs and Ln Resin® to remove Sm, were refined from a previous study. The procedural blank value of Nd was obtained as 2 pg (n = 6) from 3 L of water samples. Chemical yield of Nd from 3 L of seawater ranged within 90–95%. The developed procedure was combined with multiple collector-ICP-MS and applied to analysis of vertical seawater samples obtained from the western subarctic gyre of the North Pacific Ocean, where eNd ranged from −1.29 ± 0.42 at the surface to −3.80 ± 0.41 at 4000 m depth. These results were validated by comparing them with results obtained by the conventional method verified in the GEOTRACES inter-calibration program.

The western subarctic Pacific (WSP) is known as one of the most productive regions among the world’s oceans in spring. However, its oceanic waters are also known as a High Nutrient, Low Chlorophyll (HNLC) region during summer due to low iron (Fe) availability in seawater. Indeed, recent studies have demonstrated that the distribution of Fe in the WSP is complex and heterogeneous. This study thus investigated the effects of Fe availability on the community composition and photophysiology of surface phytoplankton from coastal to offshore waters in the WSP in the summer of 2014. Although relatively high concentrations (>2 mg m(−3)) of chlorophyll (chl) a were found in the Sea of Okhotsk and some coastal waters, low chl a concentrations (

Iron (Fe) is well known as a limiting factor to control primary productivity especially in high-nutrient and low chlorophyll area such as the subarctic Pacific. The solubility of Fe is believed to be controlled by its complexation with natural organic ligands, while the distribution of organic ligands is poorly understood. Here, we report that dissolved (

Glacial fjords in Greenland show high productivity owing to the runoff of meltwater from the glaciers. Macronutrient dynamics (of nitrate, phosphate, and silicate) associated with subglacial discharge plumes in front of marine-terminating glaciers are widely cited as important drivers of summer phytoplankton blooms in the fjords. However, the dynamics of iron (Fe), an essential micronutrient for primary production, remain largely unstudied in glacial fjords. To investigate the role of subglacial discharge plumes in Fe supply processes in glacial fjords, a comprehensive survey of Bowdoin Fjord, adjacent to the marine-terminating Bowdoin Glacier in northwestern Greenland, was conducted. The subglacial discharge of Fe-rich meltwater induces a buoyancy-driven upwelling plume in front of the glacier that entrains nutrient-rich Arctic- and Atlantic-origin waters. The plume water potentially carried 4.5-8.7 x 10(7) g day(-1) of total dissolvable Fe out of Bowdoin Fjord in summer. The concentration of dissolved Fe (dFe) in the plume water (similar to 15.6 nmol kg(-1)) was 4 times higher than that in the water in the outer part of the fjord (similar to 3.8 nmol kg(-1)). The dFe:nitrate + nitrite ratio (mmol mol(-1)) in the plume water varied between 0.58 and 3.2, several orders of magnitude higher than the phytoplankton cellular Fe:nitrate ratio estimated using the hypothetical Fe:C ratio and observed particulate organic carbon:nitrate ratio of the fjord. Hence, the plume water is replete with Fe with respect to phytoplankton demands. Subglacial discharge drives the upwelling of Fe and macronutrients toward the euphotic zone, which is vital for the generation of summer phytoplankton growth in glacial fjords.

The availability of dissolved iron (dFe) exerts an important control on primary production. Recent ocean observation programs have provided information on dFe in many parts of the ocean, but knowledge is still limited concerning the rates of processes that control the concentrations and cycling of dFe in the ocean and hence the role of dFe as a determinant of global primary production. We constructed a three-dimensional gridded dataset of oceanic dFe concentrations by using both observations and a simple model of the iron cycle, and estimated the difference of processes among the ocean basins in controlling the dFe distributions. A Green's function approach was used to integrate the observations and the model. The reproduced three-dimensional dFe distribution indicated that iron influx from aeolian dust and from shelf sediment were 7.6 Gmol yr and 4.4 Gmol yr in the Atlantic Ocean and 0.4 Gmol yr and 4.1 Gmol yr in the Pacific Ocean. The residence times were estimated to be 12.2 years in the Atlantic and 80.4 years in the Pacific. These estimates imply large differences in the cycling of dFe between the two ocean basins that would need to be taken into consideration when projecting future iron biogeochemical cycling under different climate change scenarios. Although there is some uncertainty in our estimates, global estimates of iron cycle characteristics based on this approach can be expected to enhance our understanding of the material cycle and hence of the current and future rates of marine primary production.

Sea ice is important for the health of the polar oceans yet its role in the biogeochemical cycling of trace metals is not so clear. To understand the geochemical behaviour of trace metals and their accumulation into sea ice, dissolved (D, < 0.2 mu m), and labile particulate (LP, Total Dissolvable - Dissolved) Fe, Mn, and Cd were examined in sea ice and seawater collected from the Chukchi Sea in the Arctic Ocean. Samples were pre-concentrated utilizing the solid-phase extraction NOBIAS Chelate PA-1 resin (Hitachi High-Technologies Corporation) and analyzed on a Graphite Furnace Atomic Absorption Spectrometer. Chukchi seawater showed high percentage for DMn (71.5%) and DCd (66.3%) with a high percentage of LPFe (94.1%). In seawater, DCd was the only metal to correlate with phosphate (R-2 = 0.78) indicating a biogeochemical cycling source. Chukchi seawater concentrations of Fe and Mn may have been controlled through external sources such as sediments (shelf or river) and/or sediment reductive processes. Trace metal concentrations in Chukchi sea ice were heterogeneous. Sea ice showed high percentages for the LP fraction (99.2% Fe, 63.6% Mn and 71.2% Cd). This data indicated that, regardless of the trace metal behaviour in Chukchi seawater, Chukchi sea ice was observed to have a preference for the LP trace metal fraction.

The Tenth Symposium on Polar Science/Ordinary sessions: [OM] Polar Meteorology and Glaciology, Wed. 4 Dec. / Entrance Hall (1st floor) , National Institute of Polar Research