Seasonal Ice Research Articles

Disentangling Dynamic from Thermodynamic Summer Ice Area Loss from Observations (1979–2021): A Potential Mechanism for a “First-Time” Ice-Free Arctic

Abstract In recent decades, the Arctic minimum sea ice extent has transitioned from a predominantly thick multiyear ice cover to a thinner seasonal ice cover. We partition the total (observed) Arctic summer area loss into thermodynamic and dynamic (convergence, ridging, and export) sea ice area loss during the satellite era from 1979 to 2021 using a Lagrangian sea ice tracking model driven by satellite-derived sea ice velocities. Results show that the thermodynamic signal dominates the total summer ice area loss and the dynamic signal remains small (∼20%) even in 2007 when dynamic loss was largest. Sea ice loss by compaction (within pack ice convergence) dominates the dynamic area loss, even in years when the export is largest. Results from a simple (Ekman) free-drift sea ice model, supported by results from the Lagrangian model, suggest that nonlinear effects between dynamic and thermodynamic area loss can be important for large negative anomalies in sea ice extent, in accord with previous modeling studies. A detailed analysis of two all-time record minimum years (2007 and 2012)—one with a semipermanent high in the southern Beaufort Sea and the other with a short-lived but extreme storm in the Pacific sector of the Arctic in late summer—shows that compaction by Ekman convergence together with large thermodynamic melt in the marginal ice zone dominated the sea ice area loss in 2007 whereas, in 2012, it was dominated by Ekman divergence amplified by sea–ice albedo feedback—together with an early melt onset. We argue that Ekman divergence from more intense summer storms when the sun is high above the horizon is a more likely mechanism for a “first-time” ice-free Arctic.

Revelations of interannual dune evolution from the swiftest aeolian system on Mars by MRO/HiRISE long-term monitoring

The north polar region of Mars, with its greater atmospheric pressure and vast inventory of ever-changing volatiles (e.g., CO2, H2O), hosts arguably the most active and diverse aeolian bedform systems on the planet. Here, we explore how these dune fields evolve spatiotemporally using up to 8 Mars years (16 Earth years) of MRO HiRISE observations to test the impact of various boundary conditions on annual mobility. A high degree of sand flux heterogeneity was observed for some dunes, whereas other sites displayed steady-state migration relative to long-term rates. These large changes in annual migration are attributed to the variable length of the frost-free seasons, sediment availability in relation to the timing of peak katabatic winds, and the influence of global dust storms on seasonal ice thickness. Consistent with our previous work, we continue to observe extremely high transport rates at Olympia Cavi. All stages of aeolian system evolution are observable (sand patch > protodune > dune), along with additional phenomena not previously observed outside of terrestrial settings (e.g., dune calving and collisions, remote transfer). Transitory protodunes may evolve from modest sand mounds to prominent barchans with slipfaces several meters tall within 3–5 Mars years, while adjacent duneforms may suffer slipfaces collapse as they lose sand supply. These Martian protodunes, which appear to be larger than terrestrial equivalents, and mature dunes found downwind are among the swiftest yet reported on Mars. These rapidly evolving cryo-aeolian systems provide a window into longer-term landscape evolution of non-polar dune fields.

Sea ice concentration decline in an important Adélie penguin molt area.

Unlike in many polar regions, the spatial extent and duration of the sea ice season have increased in the Ross Sea sector of the Southern Ocean during the satellite era. Simultaneously, populations of Adélie penguins, a sea ice obligate, have been stable or increasing in the region. Relationships between Adélie penguin population growth and sea ice concentration (SIC) are complex, with sea ice driving different, sometimes contrasting, demographic patterns. Adélie penguins undergo a complete molt annually, replacing all their feathers while fasting shortly after the breeding season. Unlike most penguin species, a majority of Adélies are thought to molt on sea ice, away from the breeding colonies, which makes this period particularly difficult to study. Here, we evaluate the hypothesis that persistent areas of high SIC provide an important molting habitat for Adélie penguins. We analyzed data from geolocating dive recorders deployed year-round on 195 adult penguins at two colonies in the Ross Sea from 2017 to 2019. We identified molt by detecting extended gaps in postbreeding diving activity and used associated locations to define two key molting areas. Remotely sensed data indicated that SIC during molt was anomalously low during the study and has declined in the primary molt area since 1980. Further, annual return rates of penguins to breeding colonies were positively correlated with SIC in the molt areas over 20 y. Together these results suggest that sea ice conditions during Adélie penguin molt may represent a previously underappreciated annual bottleneck for adult survival.

Machine learning based classification of lake ice and open water from Sentinel-3 SAR altimetry waveforms

The aim of the study was to evaluate, for the first time, the capability of different machine learning (ML) algorithms in classifying along-track lake surface conditions (open water and ice types) across ice seasons (freeze-up, ice growth and break-up periods) from Sentinel-3 A/B synthetic aperture radar altimeter (SRAL) data. To achieve this goal, over 107,500 radar waveforms extracted from 11 large lakes across the Northern Hemisphere and three ice seasons (2018–2021) were manually labelled using complementary satellite data (Sentinel-1 imaging Synthetic Aperture Radar (SAR), Sentinel-2 Multispectral Instrument (MSI) Level 1C, and MODIS Aqua/Terra data) for the training and testing of the ML algorithms in discriminating between open water, young (thin) ice, growing ice and melting ice. The four ML algorithms tested include Random Forest (RF), Gradient Boosting Trees (GBT), K Nearest Neighbor (KNN) and Support Vector Machine (SVM). To characterize the waveforms, seven waveform parameters were derived: Leading Edge Width (LEW), Offset Center of Gravity (OCOG) Width, Pulse Peakiness (PP), backscatter coefficient (Sigma0), late tail to peak power (LTPP), early tail to peak power (ETPP) and the maximum value of the echo power (Max). Accuracies >95% were achieved across all classifiers using a 4-parameter combination (Sigma0, PP, OCOG Width, and LEW). Among all waveform parameters, Sigma0, OCOG width and PP were found to be the most important parameters for discriminating between lake ice types and open water. Despite showing comparable classification performances in the overall classification, RF and KNN are found to be a better fit for global lake ice mapping as both are less sensitive to their internal hyperparameters. Additionally, consistent results (>93.7% accuracy in all classifiers) achieved on the accuracy assessment carried out for each lake (out-of-sample testing) revealed the strength of the classifiers for spatial transferability. Implementation of RF and KNN could be valuable in a pre-or post-processing step for identifying lake surface conditions under which the retrieval of water level and ice thickness may be limited or not possible and, therefore, inform algorithms currently used for the generation of operational or research products. While the research focused on 11 of the largest lakes of the Northern Hemisphere, the classification approach presented herein has potential for application on smaller lakes too since data in SAR mode (∼300 m along-track resolution) are used.

Diffusive and advective cross-frontal fluxes of inorganic nutrients and dissolved inorganic carbon in the Barents Sea in autumn

The Atlantic Water, entering the Arctic through the Barents Sea and Fram Strait, is the main source of nutrients in the Arctic Ocean. The Barents Sea is divided by the Polar Front into an Atlantic-dominated domain in the south, and an Arctic-dominated domain in the north. The Polar Front is a thermohaline structure, which is topographically-steered at sub-surface, and influenced by the seasonal sea ice edge near the surface. Exchanges of nutrients between the inflowing Atlantic Water and the surrounding waters are key for the primary production in the Barents Sea. In October 2020, we measured nutrients (nitrate, phosphate and silicic acid), dissolved inorganic carbon (DIC), ocean stratification, currents and turbulence in the vicinity of the Polar Front in the Barents Sea within the framework of the Nansen Legacy project, allowing estimates of horizontal and vertical advective fluxes and turbulent fluxes of nitrate and DIC. We studied the autumn situation when primary production was declining. We found a substantial transfer of nitrate and DIC across the Polar Front from the Atlantic domain to the Arctic domain. Up to one quarter of the replenishment of the nitrate in the mixed layer during winter could be attributed to vertical mixing during wind events, shared approximately equally between advective and turbulent fluxes. The vertical turbulent fluxes bring nutrients from the subsurface Atlantic Water to the surface. We also identified an export of nitrate and DIC from the Barents Sea to the Nordic Seas occurring along the eastern shelf of Svalbard. Our study shows the role of vertical fluxes in fall and winter to precondition for the following spring bloom.

Variability of Arctic Sea ice age and its relationship with atmospheric circulation patterns

Since at least the 1990s, global warming has caused the decline of multi-year ice (MYI) in the Arctic, which has made the Arctic Ocean more susceptible to the effects of climate change and weather processes. In this study, an analysis of the variations in Arctic sea ice age was carried out based on the results of an Empirical Orthogonal Function (EOF) analysis. Variations in three time periods were investigated. The first period (1984-1995) is characterized by high sea ice age, with antiphase variation processes in the eastern and western parts of the Arctic Ocean. The second period (1996-2012) represents a rapid reduction in ice age, characterized by a gradual replacement of MYI in the Arctic Ocean with seasonal sea ice, resulting in a decreased extent of MYI. The third period (2013-2022) is characterized by low sea ice age, with ice age being in a significantly negative phase. Furthermore, trend-like changes were weak during this period. During the first period, the spatial distribution and significant variations in ice age were driven by cyclonic wind anomalies associated with the Arctic Oscillation (AO) atmospheric pressure modes. The cyclonic wind fields in winter and anticyclonic wind fields in summer collectively influenced the sea ice. This led to a decrease/increase in ice age in the eastern/western Arctic, and antiphase variations were apparent. During the second study period, winter ice age variations were driven by cyclonic wind anomalies associated with the AO atmospheric pressure modes, leading to ice divergence and a decrease in ice age. In summer, both thermodynamic (Arctic warming) and dynamic (DA) mechanisms play a role in modulating the changes of sea ice age, while Arctic warming is the primary driver. The DA generated anomalous wind patterns, characterized by the Beaufort Gyre and Transpolar Drift, which resulted in the outflow of MYI from the Arctic under the influence of meridional winds. During the third study period, the influence of AO/DA and air temperature on sea ice age changes is greatly weakened.

Spatial and Temporal Variation of Mars South Polar Ice Composition From Spectral Endmember Classification of CRISM Mapping Data

AbstractMultispectral mapping data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) provide a unique opportunity to characterize south polar ice deposits at higher spectral sampling, spatial resolution, or spatiotemporal coverage than previous work. This new perspective can help constrain the nature and distribution of different mixtures of CO2 ice, H2O ice, and dust that influence the formation, evolution, and preservation of Mars climate records. We processed 1,103 CRISM observations spanning the southern summer of six Mars Years (MYs) through a combination of k‐means clustering and random forest classification. Using a set of 12 spectral endmembers directly tied to previous work with high‐resolution CRISM targeted data, we made a series of temporally restricted mosaics showing surface spectral variation over time. The mosaics show the effects of the MY 28 dust storm on the removal of the seasonal CO2 ice cap that year and reveal how this process differed from the years that followed. A mosaic showing residual ice surfaces displays broad agreement with previous compositional maps while resolving new details in the distribution of H2O ice‐rich material around the periphery of the bright CO2 ice cap. By showing how surface composition varies across a broad swath of the south polar region through time, the endmember set and classified mosaics produced in this work can provide critical context for future studies of the dynamic processes that shape south polar ice deposits.

Effect of climate on the optimal sizing and operation of seasonal ice storage systems

Seasonal thermal storage systems can reduce the temporal mismatch between renewable energy availability and energy demand. Ice storage systems exhibit a non-linear behaviour in the heat exchange and storage processes, complicating the formulation of optimal design and operation problems. In this work, we propose a mixed-integer quadratically-constrained programming formulation, which minimizes the Levelized Cost of Energy for space heating and cooling, including sizing of a supporting PV array. The optimization was repeated for different storage volumes, finding the system optimal operation in each case –and thereby the optimal system sizing. The heating and cooling demands were computed from an archetypal office building, placed in three reference locations with cold and semi-arid, warm and humid continental, and temperate and humid continental climates. Results show that the optimal PV size decreases with growing ice storage volume, and an ice storage works best in a temperate continental climate, covering up to 47% of the cooling demand with a 250 m 3 storage.

Increasing ice production speed of a seasonal ice storage device based on experimental test and numerical simulation

Increasing ice production speed of a seasonal ice storage device based on experimental test and numerical simulation

МОДЕЛИРОВАНИЕ ЛЕДЯНОГО ПОКРОВА СЕВЕРНОГО КАСПИЯ В ЭКСТРЕМАЛЬНЫЕ СЕЗОНЫ

Based on the CICE model, the characteristics of the ice cover in the Northern Caspian Sea were simulated during the extreme ice seasons of 2011/2012 (long and cold) and 2015/2016 (short and warm). The characteristics of these seasons (length, frost degree-days, sea ice extent, maximum ice thickness) are given. Composite anomalies of surface air pressure and air temperature were constructed for the Northern Caspian region. Based on the JRA-55 reanalysis data, the concentration and thickness of sea ice were calculated. A comparison of the simulation results with observational data showed that the main differences in ice conditions in these seasons are reproduced satisfactorily.

GEOCHEMICAL CONDITIONS OF SULLAR SPRING: FORMATION OF AUTHIGENIC MINERALS AND METHANE FLUXES

Link for citation: Soldatova E.A., Sidkina E.S., Kiriukhin B.A., Maximov P.N., Krivenok L.A., Ivanov V., Tananaev N.I. Geochemical conditions of Sullar spring: formation of authigenic minerals and methane fluxes. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 10, рр. 16-33. In Rus. Relevance and the research object. Interpermafrost aquifers and areas of their discharge with seasonal ice covers are special permafrost-hydrogeological and hydrochemical systems that are not typical for the thick continuous permafrost zone of Central Yakutia. Differences in chlorofluorocarbon content indicate the anaerobic conditions of some springs suggesting microbial degradation of these compounds by methanogenic bacteria. In the discharge area of such springs, a sharp change in the geochemical conditions takes place leading to a transformation in the water chemical composition and boosting the processes of authigenic mineral formation in this buffer area. In current work we considered the geochemical system of the Sullar spring – the most northern and the least studied among interpermafrost groundwater discharge zones of the Lena River right bank. Intensive accumulation of authigenic minerals was found in the Sullar discharge area during field research. It was Fe compounds with bacterial mats as an opalescent film. Some researchers state the connection between the transformation of Fe compounds during permafrost melting and the migration and transformation of organic matter, including its consumption by microorganisms with further methane emission. We assumed that there is a relationship between the authigenic mineral formation and the content and emission of methane in the discharge area of the spring. This research is the first comprehensive description of the hydrochemistry of the interpermafrost water of the Sullar spring. The aim of the research was to explore influence of geochemical conditions and microbial communities on formation of authigenic minerals and methane fluxes, in particular the effect of the change of geochemical conditions due to groundwater discharge on Fe precipitation and to analyze the relation of that process with methane fluxes. For this purpose, we study water chemical composition of the Sullar spring, measure methane fluxes from water surface and analyze mineral and microbial composition of secondary mineral crusts and bottom eluvium collected near water sampling points. Methods. The main element content was determined by ion chromatography. The concentrations of the bicarbonate ion and dissolved carbon dioxide were calculated by the equilibrium modeling method based on the pH and Eh values of the system. The content of trace elements was measured by mass spectrometry with inductively coupled plasma. To identify seasonal and annual variability, a retrospective analysis of the water chemical composition of the Sullar spring for the period from 1962 to 2020 was carried out. Methane fluxes were measured using the chamber method. The methane concentration was determined by gas chromatography with a flame ionization detector. The features of the secondary mineral crusts and bottom eluvium were determined using a petrographic microscope. The chemical composition of individual mineral phases was evaluated using a scanning electron microscope equipped with a detector for energy dispersive X-ray spectral microanalysis. The DNA concentration was measured on a fluorimeter. Amplicon libraries were generated by polymer chain reaction with universal primers for the V4 region. Equilibrium modeling was carried out using the HCh software.

Majority of Southern Ocean Seasonal Sea Ice Zone Bloom Net Community Production Precedes Total Ice Retreat

AbstractThe Southern Ocean seasonal sea ice zone (SIZ) spring is characterized by sea ice retreat and the development of phytoplankton blooms. Until recently, assessing SIZ blooms and associated net community production (bNCP) has been limited by a lack of under‐ice observations. We relate the timing of phytoplankton growth to the drawdown of surface nitrate and sea ice cover and estimate bNCP from biogeochemical profiling float observations. The onset of biological production closely follows initial sea ice breakup and, on average, 64% of bNCP occurs before total sea ice retreat. This indicates that satellite‐derived estimates largely miss under‐ice production and underestimate SIZ bNCP. Float bNCP estimates range from <1 to >4 mol C m−2 bloom−1, with higher bNCP when sea ice breakup occurs early in the year, and the highest bNCP near topographic features that may increase micronutrient supply. Our results suggest changes in Southern Ocean sea ice will influence future bNCP.

Optimal sizing and operation of seasonal ice thermal storage systems

Ice storage systems can be used as an efficient cooling source during summer, as well as a heat source for heat pumps during winter. The non-linear behavior of the heat exchange process in storage makes formulations for optimizing the design and operation of these technologies complex. In this work, we propose a quadratically-constrained mixed-integer programming formulation, that can capture the latent and sensible behavior of the storage and its impact on the delivery of heating and cooling. A building demonstrator integrating an ice storage device was used as a case study. Monitoring data were used to validate the simplified ice storage model employed in the optimization. Results showed that the most common optimal storage cycle requires freezing the water during late winter and when the air temperature falls below 0∘C. Increasing the storage volume increases both storage efficiency and the amount of free cooling available during summer. For these reasons, economies of scale can make larger systems more competitive than smaller ones. Storage size and thermal insulation level affect the duration of the charging and discharging phases. Thermal insulation improves seasonal efficiency and free cooling significantly. A higher CO2 emissions price does not yield significant benefits in terms of emissions reductions. High investment costs and the seasonal variation in CO2 intensity of electricity reduce the economic and environmental competitiveness of long-term ice storage systems, respectively.

Observations of Ocean Surface Wave Attenuation in Sea Ice Using Seafloor Cables

AbstractThe attenuation of ocean surface waves during seasonal ice cover is an important control on the evolution of Arctic coastlines. The spatial and temporal variations in this process have been challenging to resolve with conventional sampling using sparse arrays of moorings or buoys. We demonstrate a novel method for persistent observation of wave‐ice interactions using distributed acoustic sensing (DAS) along existing seafloor fiber optic telecommunications cables. DAS measurements span a 36‐km cross‐shore cable on the Beaufort Shelf from Oliktok Point, Alaska. DAS optical sensing of fiber strain‐rate provides a proxy for seafloor pressure, which we calibrate with wave buoy measurements during the ice‐free season (August 2022). We apply this calibration during the ice formation season (November 2021) to obtain unprecedented resolution of variable wave attenuation rates in new, partial ice cover. The location and strength of wave attenuation serve as proxies for ice coverage and thickness, especially during rapidly evolving events.

Drivers of Antarctic sea ice advance

Antarctic sea ice is mostly seasonal. While changes in sea ice seasonality have been observed in recent decades, the lack of process understanding remains a key challenge to interpret these changes. To address this knowledge gap, we investigate the processes driving the ice season onset, known as sea ice advance, using remote sensing and in situ observations. Here, we find that seawater freezing predominantly drives advance in the inner seasonal ice zone. By contrast, in an outer band a few degrees wide, advance is due to the import of drifting ice into warmer waters. We show that advance dates are strongly related to the heat stored in the summer ocean mixed layer. This heat is controlled by the timing of sea ice retreat, explaining the tight link between retreat and advance dates. Such a thermodynamic linkage strongly constrains the climatology and interannual variations, albeit with less influence on the latter.

The seasonal Antarctic sea ice concentration anomalies related to the Atlantic Niño index

Antarctic sea ice concentration anomalies (ASICA) have been found to be linked to sea surface temperature anomalies in tropical oceans. However, it is not clear whether and how ASICA is linked to the Atlantic Niño mode (ANM). This study demonstrates a significant relationship between ASICA and ANM. The relationships vary by season, with a peak in austral winter and a secondary one in spring. Significant sea ice anomalies associated with a positive phase of ANM are mostly negative in austral winter and spring, and mostly positive in austral summer and autumn. This teleconnection is established by atmospheric wavetrains that are excited over the tropical southwestern Pacific and Indian Oceans and the southern Atlantic Oceans and propagate over the Southern Ocean. These wavetrains induce anomalous near-surface circulations, which generate dynamic and thermodynamic forcing on sea ice, resulting in the observed ASICA patterns. The absence of El Niño Southern Oscillation weakens the connection.

Insights into the interaction between defrosting seasonal ices and gully activity from CaSSIS and HiRISE observations in Sisyphi Cavi, Mars

Martian gullies are surface features, typically composed of an alcove, a channel and a depositional apron. They have been extensively studied since their first observations in the 2000s and were initially attributed to the action of liquid water. Later studies highlighted that their activity is spatially and temporally correlated with the seasonal presence of surface CO2 ice, suggesting a link between seasonal frost and gully activity. However, the exact mechanisms leading to gully formation are still under debate. Establishing whether or not gullies are formed by liquid water has important implications for Mars recent climate and habitability. Here, we study the evolution of seasonal frost and its connection to gully activity in Sisyphi Cavi, located in the southern circumpolar region (68°S - 74°S; 345°E − 5°E). The high latitude of this site allows for detailed temporal monitoring, owing to the high frequency imaging by spacecraft in polar orbits, largely unavailable for other martian gullies, hence this site has the potential to reveal new insights on gullies in general. In particular, we used repeat images from MRO HiRISE (Mars Reconnaissance Orbiter, High Resolution Imaging Science Experiment) and TGO CaSSIS (Trace Gas Orbiter, Colour and Stereo Surface Imaging System). Our findings show that the general timing and characteristics of the defrosting patterns in Sisyphi Cavi follow seasonal patterns that are consistent from year to year. The timing of the start of defrosting and its temporal evolution are controlled by the overall orientation of the host hillslope, with equator-facing slopes defrosting before pole-facing ones and the steepest pole-facing slopes defrosting last. Gully alcoves defrost before channels and aprons located on the same hillslope independent of the orientation of the hillslope likely because the aprons have a lower thermal inertia than the alcoves that counteracts the orientation effect, which on pole-facing slopes would mean the alcoves should defrost last. We observe the presence of seasonal and ephemeral dark spots and flows which are interpreted to be a result of sediment deposition on top of the seasonal ice deposits, triggered by basal sublimation. Our observations suggest that the appearance of dark spots and dark flows can be spatially correlated with surface roughness, including presence of boulders, contrasts in material types, irregular lobes/levees and braiding caused by gully activity, and therefore could be used to help detect recently active gullies on Mars in areas with seasonal frost without the need of repeat imaging. Finally, we propose that presently observed gully activity in Sisyphi Cavi is driven by defrosted material flowing on to a frosted apron. We infer that the presence of a frosted apron could be common precursor for this type gully activity. We note that this activity only involves the mobilisation of loose materials and we have not observed any erosion of the wall rock present in Sisyphi Cavi.

The Stepwise Reduction of Multiyear Sea Ice Area in the Arctic Ocean Since 1980

AbstractThe loss of multiyear sea ice (MYI) in the Arctic Ocean is a significant change that affects all facets of the Arctic environment. Using a Lagrangian ice age product, we examine MYI loss and quantify the annual MYI area budget from 1980 to 2021 as the balance of export, melt, and replenishment. Overall, MYI area declined at 72,500 km2/yr; however, a majority of the loss occurred during two stepwise reductions that interrupt an otherwise balanced budget and resulted in the northward contraction of the MYI pack. First, in 1989, a change in atmospheric forcing led to a +56% anomaly in MYI export through Fram Strait. The second occurred from 2006 to 2008 with anomalously high melt (+25%) and export (+23%) coupled with low replenishment (−8%). In terms of trends, melt has increased since 1989, particularly in the Beaufort Sea, export has decreased since 2008 due to reduced MYI coverage north of Fram Strait, and replenishment has increased over the full time series due to a negative feedback that promotes seasonal ice survival at higher latitudes exposed by MYI loss. However, retention of older MYI has significantly declined, transitioning the MYI pack toward younger MYI that is less resilient than previously anticipated and could soon elicit another stepwise reduction. We speculate that future MYI loss will be driven by increased melt and reduced replenishment, both of which are enhanced with continued warming and will one day render the Arctic Ocean free of MYI, a change that will coincide with a seasonally ice‐free Arctic Ocean.

Fecal Pellet‐Like Gyrodinium Species in Sinking Particles: Newly Found Potential Contributors for Carbon Export in the Antarctic Seasonal Ice Zone

AbstractFecal pellets (FPs) are generated by various species and have gained attention as contributors to the biological carbon pump. Metazoans and protozoans are known as FP and minipellet‐producers, respectively. Herein, we discovered fecal pellet‐like dinoflagellates (FLDs) in the seasonal sea ice zone in the Southern Ocean. The size and form of these FLDs were similar to those of zooplankton oval FPs. However, due to their appearance, they have been misclassified as FPs rather than dinoflagellates, leading to potential oversight of their role in the carbon cycle. Thus, we aimed to identify FLD cells at the species level and examine the impact of FLDs on flux estimation of particulate organic carbon (POC). Our findings are as follows: first, FLD cells were identified as Gyrodinium rubrum and Gyrodinium heterogrammum through 18S rRNA gene sequencing. Second, FLDs can potentially excrete larger FPs than minipellets. Third, the sinking rate of FLDs is higher than that of other protozoa and dinoflagellate cysts. Finally, a maximum of 12 mgC m−2 day−1 of the POC flux can be attributed to FLDs (representing 32% of POC flux). These results suggest that FLDs are important drivers not only for the microbial loop but also for the biological carbon pump. In future projections of carbon sequestration, the contribution of metazoans to carbon export must be considered, but not that of FLDs. Their unknown physiological and ecological characteristics, especially including the responses to climate changes, must be urgently investigated for future projections of carbon sequestration in the Southern Ocean.

Marine diatoms record Late Holocene regime shifts in the Pikialasorsuaq ecosystem.

The Pikialasorsuaq (North Water polynya) is an area of local and global cultural and ecological significance. However, over the last decades, the region has been subject to rapid warming, and in some recent years, the seasonal ice arch that has historically defined the polynya's northern boundary has failed to form. Both factors are deemed to alter the polynya's ecosystem functioning. To understand how climate-induced changes to the Pikialasorsuaq impact the basis of the marine food web, we explored diatom community-level responses to changing conditions, from a sediment core spanning the last 3800 years. Four metrics were used: total diatom concentrations, taxonomic composition, mean size, and diversity. Generalized additive model statistics highlight significant changes at ca. 2400, 2050, 1550, 1200, and 130 cal years BP, all coeval with known transitions between colder and warmer intervals of the Late Holocene, and regime shifts in the Pikialasorsuaq. Notably, a weaker/contracted polynya during the Roman Warm Period and Medieval Climate Anomaly caused the diatom community to reorganize via shifts in species composition, with the presence of larger taxa but lower diversity, and significantly reduced export production. This study underlines the high sensitivity of primary producers to changes in the polynya dynamics and illustrates that the strong pulse of early spring cryopelagic diatoms that makes the Pikialasorsuaq exceptionally productive may be jeopardized by rapid warming and associated Nares Strait ice arch destabilization. Future alterations to the phenology of primary producers may disproportionately impact higher trophic levels and keystone species in this region, with implications for Indigenous Peoples and global diversity.