Western Arctic Ocean Research Articles

Metalloenzyme signatures in authigenic carbonates from the Chukchi Borderlands in the western Arctic Ocean

Migration of methane-rich fluids at submarine cold seeps drives intense microbial activity and precipitation of authigenic carbonates. In this study, we analyzed microbially derived authigenic carbonate samples recently recovered from active gas hydrate mounds on the southwestern slope of the Chukchi Borderlands (CB), western Arctic Ocean. Our main aim was to characterize the distribution patterns of trace elements in carbonate-hosted lipid fractions to assess metalloenzyme requirements of microbes involved in anaerobic oxidation of methane (AOM). We measured stable isotopes, trace elements, lipid biomarkers, and genomic DNA, and results indicate the dominance of AOM-related lipid biomarkers in studied carbonate samples, as well as a predominant occurrence of the anaerobic methanotrophic archaea (ANME)-1. We also report evidence for significant preferential enrichments of various trace elements (Li, Ni, Co, Cu, Zn, and Mo) in the total lipid fractions of CB carbonates, relative to elemental compositions determined for corresponding carbonate fractions, which differ from those previously reported for other seep sites. We hypothesize that trace element enrichments in carbonate-hosted lipid fractions could vary depending on the type of AOM microbial assemblage. Additional work is required to further investigate the mechanisms of lipid-bound trace elements in cold seep carbonates as potential metalloenzymes in AOM.

Investigating Eddies From Coincident Seismic and Hydrographic Measurements in the Chukchi Borderlands, the Western Arctic Ocean

AbstractHalocline eddies transport mass and energy across the Arctic Ocean. Seismic oceanography uses multichannel seismic reflection (MCS) data to create high resolution images of the water column, revealing oceanic fine structures. In this paper, we present water column images processed from MCS data acquired during cruise MGL1112 on the Chukchi Borderlands in the western Arctic Ocean. Combined with along‐track images of current velocities measured during MCS acquisition by a hull‐mounted acoustic Doppler current profilers, a total of 23 mesoscale eddies were detected, of which 19 are anticyclonic, and 4 are cyclonic. They correspond to the lentoid and mounded reflections on the seismic images, respectively. These shallow eddies are constrained by the halocline and occur in regions with rugged seafloor. The geometric parameters of these eddies were estimated from the underway data collected during MGL1112. These parameters could be valuable for modeling 3D eddy structures and validating high‐resolution climate projection models. Expendable Bathythermography (XBT) profiles of water temperature and sound speed versus depth were collected during this ruise. Three of the 24 XBT stations sampled eddies opportunely, we found one was warm‐core and two were cold‐core anticyclonic eddies combined with historical conductivity‐temperature‐depth data. The cores of these eddies might be made up of Pacific water. We synthesized the MCS records with coincident and historical hydrographic data which permitted distinctions between the seismic responses of the eddies. Distinctive reflection structures are observed around the eddy core (e.g., chaotic, imbricate, layered, and listric reflections), which could constrain stirring and mixing process of the eddies. These results are useful for better understanding the structure and evolution of the eddies on the Chukchi Borderlands and better understanding regional mass and energy transport processes in the western Arctic Ocean.

North American Glaciations and Pacific Inputs in the Nd and Sr Isotope Pleistocene Record From the Western Arctic Ocean

Enduring questions remain regarding the transition from relatively warm and stable pre- and early-Pleistocene climate to that of the high amplitude glacial-interglacial cycles later in the Quaternary. The main shift in glacial intensity and periodicity around 1 Ma is known as the Mid-Pleistocene Transition (MPT). Here we analyze detrital strontium (Sr) and neodymium (Nd) isotopes in a western Arctic sediment core P23 previously investigated using several litho/biostratigraphic proxies. Based on an improved age framework combining lithostratigraphic cyclicity and Sr isotope stratigraphy, the P23 record extends to ∼3.3 Ma, thus providing a rare insight into the Quaternary Arctic climate change. The distinct pre-MPT P23 record is dominated by Pacific-sourced sediment inputs, with little to no intra-Arctic glacial inputs, except for a sandy interval around ∼2.5 Ma. A consistent decrease of Nd isotopic values toward North American continental signatures started in both the Arctic and Bering Sea at ∼1.5 Ma and led to a major threshold shift in P23 proxies at ∼0.9 Ma. We argue that this threshold was associated with the first prolonged closure of the Bering Strait for an entire obliquity cycle. This shift marked the expansion of the North American ice sheets to the Arctic margin, with dramatic impacts on depositional and hydrographic environments in the Arctic Ocean. These impacts intensified in the subsequent glacial intervals indicating further ice-sheet growth, probably fed back by continuing prolonged Bering Strait closures.

Increased terrigenous input from North America to the northern Mendeleev Ridge (western Arctic Ocean) since the mid-Brunhes Event

Mid-Brunhes Event (MBE) occurred at approximately 420 ka between Marine Isotope Stage 11 and 12, and is considered the most pronounced climatic shift during the last ~ 800 kyrs. On the other hand, it is unclear if the MBE was global, despite being observed in the high-latitude Northern Hemispheric cryosphere in terms of climate systems. A 5.35-m long gravity core ARC5-MA01 was obtained from the northern Mendeleev Ridge in the western Arctic Ocean to track the paleoenvironmental changes in terms of the terrigenous sedimentation in response to the glacial-interglacial climate changes across the MBE. Geochemical proxies (biogenic opal, total organic carbon, C/N ratio, carbon isotope of organic matter, and calcium carbonate) of MA01 suggest that the terrigenous input was generally higher during the interglacial periods. Based on a mineralogical examination, most of the terrigenous input was attributed to the abundance of dolomite and the increased kaolinite content from North America. In particular, most paleoceanographic proxies showed that the terrigenous input from North America was enhanced distinctly during the post-MBE interglacial periods. These results suggest that the MBE in the western Arctic Ocean was a global climatic shift closely linked to cryospheric development in North America during the middle Pleistocene.

Inter-species comparison of the copepodite stage morphology, vertical distribution, and seasonal population structure of five sympatric mesopelagic aetideid copepods in the western Arctic Ocean

Aetideidae is a calanoid copepod family dominant in the mesopelagic layer of the Arctic Ocean for which little ecological information is available because species identification, especially of early copepodite stages, is difficult. In this study, we developed a species identification flow for the whole copepodite stages of five sympatric aetideid copepods (Chiridius obtusifrons, Gaetanus tenuispinus, G. brevispinus, Aetideopsis minor, and A. rostrata). Vertical distributions and seasonal population structures of these species were evaluated using a year-round sample time-series collected at the drifting ice station (SHEBA) in the western Arctic Ocean. Combinations of morphological characteristics (prosome length, cephalosome, and prosome widths) were used to identify the early copepodite stages to species. Aetideopsis rostrata was distributed in deep waters (1,032–1,065 m) throughout the year. The other species all were found at 600–700 m during the midnight sun. However, during the polar night, the vertical distributions of each species were distinct, resulting from ascent, descent, or depth maintenance, indicating seasonal vertical migration which may function to reduce inter-specific competition during the polar night when food resources are scarce. Reproduction timing varied among four aetideid copepods: C.obtusifrons and G. tenuispinus showed polar night ascent and reproduction at the end of the polar night, whereas G. brevispinus and A. minor showed descent or depth maintenance during the polar night and reproduction at the beginning of the polar night. There was not sufficient data to examine reproduction timing of A. rostrata. Common for all aetideid species, δ15N values of the adult females indicate more carnivorous feeding modes during the polar night than those in the midnight sun. Such vertical distribution and timing of reproduction variation among these five aetideid copepods may function to reduce species competition in the mesopelagic layer of the Arctic Ocean.

Environmental heterogeneity mediated prokaryotic community variations in marine sediments

Marine ecosystems have been heavily affected by environmental changes, which led to shift of prokaryotic diversity and distribution, especially in the Arctic Ocean. However, how environmental changes affect prokaryotic community still unclear. Here, prokaryotic diversity, community composition and spatial distribution patterns in marine sediments from the Bering Sea to the western Arctic Ocean were surveyed. The results indicated that the lowest bacterial and archaeal diversities were observed in the western Arctic Ocean. Moreover, the prokaryotic richness and diversity showed strong correlation (R2=0.31∼0.83, P<0.05) with heavy metals (such as Cu, Zn, Ni and As). The contents of Ni and As represented a range with in which biological effects occur occasionally in the Bering Strait, but concentrations of Cu, Zn, Ni and As in the western Arctic Ocean showed a probable effect range with in which adverse biological effects occasionally or frequently occur. Furthermore, as geographical distance increased archaeal community similarity decreased faster than bacteria. The abundances of Gammaproteobacteria, Firmicutes and Thaumarchaeota significantly increased from the Bering Sea to the western Arctic Ocean. Conversely, much lower abundances of Alphaproteobacteria, Bacteroidetes and Crenarchaeota were observed in the western Arctic Ocean. In addition, all environmental could explain 73.8% and 85.1% bacterial and archaeal variations, respectively. And heavy metals could independently explain 40.3% and 30.4% community variation for archaea and bacteria, respectively. This study emphasized effects of heavy metals on prokaryotic diversities and distribution patterns in marine sediments, which were helpful to deep understand microbial variation under environmental changes.

Early Wintertime CO2 Uptake in the Western Arctic Ocean

AbstractTo investigate CO2 uptake in the western Arctic Ocean (north of 65°N), we conducted underway, ship‐based observations of the partial pressures of CO2 (pCO2) and total dissolved inorganic carbon (TCO2) in surface seawater in early winter (November 2018). From these two properties of the seawater inorganic carbon system, we calculated total alkalinity (TA). In the early winter, surface seawater pCO2 in most places was lower than atmospheric pCO2. The weighted mean of the air‐sea influxes of CO2 were calculated to be 7.5 ± 1.6 mmol m−2 d−1. The calculated influxes imply that the area acted as a moderate sink for atmospheric CO2 in early winter, and its rate of CO2 uptake was comparable to that (8.0 ± 1.7 mmol m−2 d−1) in summer (late August−September 2017). Spatial variations of surface seawater pCO2 in the early winter could be attributed mostly to conservative mixing changes of TCO2 and TA, which together accounted for more than 70% of the pCO2 variations. In the marginal ice zone, however, there was a decrease of surface seawater pCO2 by 70–90 μatm because of horizontal advection of water with an anomalously high temperature from the Pacific Ocean and its subsequent cooling. We concluded that mixing of water masses was as important as biological processes in causing spatial variations of pCO2 and CO2 uptake in the western Arctic Ocean, especially during seasons and in areas associated with little biological activity.

Spatial Distributions of Riverine and Marine Dissolved Organic Carbon in the Western Arctic Ocean: Results From the 2018 Korean Expedition

AbstractSeasonal primary production and river discharge increases in the Arctic Ocean exert a significant influence on the dissolved organic carbon (DOC) cycle. To improve our knowledge of the spatial heterogeneity of DOC source and concentration in the rapidly changing Arctic Ocean, we investigated the distributions of riverine and marine DOC in the western Arctic Ocean during the summer of 2018. Although the surface bulk DOC concentration indicated no clear distinction in its distribution between the Chukchi Borderland (CBL)/northern Chukchi Sea (NCS) and East Siberian Sea (ESS)/Mendeleyev Ridge (MR) regions, the estimated riverine DOC concentration (28 ± 4.2 μM C) and its contribution (40 ± 5.7%) in the surface layer of the CBL/NCS region were higher than those (19 ± 5.6 μM C and 26 ± 8.5%) in the ESS/MR region, which was attributed to the accumulation of freshwater, strong stratification, and a longer residence time in the CBL/NCS region. In contrast, although marine DOC was the dominant DOC component in both the CBL/NCS and ESS/MR regions, the higher marine DOC concentration (54 ± 8.1 μM C) and its contribution (73 ± 8.2%) in the East Siberian shelf/slope region were consistent with high bacterial abundance, which was associated with extremely high surface phytoplankton blooms sustained by nutrient supply from the deep layer, suggesting that the supply of bioavailable DOC resulted in active bacterial activities. Overall, the spatial differences in water properties between the two regions had large influences on the regional distributions of riverine and marine DOC.

Extreme Nitrate Deficits in the Western Arctic Ocean: Origin, Decadal Changes, and Implications for Denitrification on a Polar Marginal Shelf

AbstractThe western Arctic Ocean is known to be nitrate deficient relative to phosphate but the decadal trend and the processes contributing to the deficit are not clear. To investigate changes in this extreme nitrate deficit of over 10 μmol/kg and its causal mechanisms, nutrient concentrations were examined along a transect spanning the Bering Basin, the Bering–Chukchi Shelf, and the western Arctic Ocean Basin over the last two decades (1994–2018). The results show that the extreme nitrate deficit has extended to greater depths and further north during the past two decades, which coincided with the expansion of Pacific water in the western Arctic Ocean. Subsurface nutrient stocks in the basin areas appear to have increased, but are accompanied by a larger nitrate deficit, which may be due to stronger shelf denitrification. This nitrate loss (∆N) caused by shelf denitrification was estimated to be 7.3 ± 0.1 μmol/kg during the interval 2012–2018, which was ∼10% higher than that in 1994. This suggests an intensification of denitrification on this marginal shelf under climatic and environmental change in the Arctic Ocean.

Synchronous Variation Patterns of Monthly Sea Ice Anomalies at the Arctic and Antarctic

Abstract Sea ice variability in the opposite polar regions is examined holistically by applying the self-organizing map (SOM) method to global monthly sea ice concentration data over two periods. The results show that the variability modes of sea ice decrease in the Arctic correspond to an overall sea ice increase in the Antarctic, and vice versa. In particular, the monthly sea ice anomaly patterns are dominated by in-phase variability across the Arctic that is stronger in the marginal seas particularly the Barents Sea than the central Arctic Ocean. The corresponding Antarctic sea ice variability is characterized by a zonal wavenumber-3 structure or a dipole pattern of out-of-phase variability between the Bellingshausen/Amundsen Seas and the rest of the Southern Ocean. The frequency of occurrence of these dominant patterns exhibits pronounced seasonal as well as decadal variability and the latter is closely related to the Pacific decadal oscillation and Atlantic multidecadal oscillation. Other less frequent patterns seem to be associated with the central Pacific El Niño and spatially heterogeneous interannual variability of sea surface temperature (SST) in the Indian and the Atlantic Oceans. The dominant modes explain 57% of the four-decade domain-averaged trends in the annual polar sea ice concentration, with more explained in the eastern than western Arctic Ocean and in the Weddell Sea and the Amundsen Sea in the Antarctic. The spatial patterns of the leading modes can be largely explained by the dynamic (sea ice drift) and thermodynamic (sea ice melt) effects of the anomalous atmospheric circulations associated with SST and sea level pressure anomalies. Significance Statement The purpose of this study is to extract the main modes of monthly global sea ice concentration variability in the past four decades, explain the mechanisms behind the occurrences of these modes, and examine the contributions of these modes to the trend in annual global sea ice concentration. Sea ice extent in the past four decades has shown a significant declining trend in the Arctic and a slight, but significant increasing trend in the Antarctic. By jointly analyzing the sea ice variability and trends in the two polar regions, the results here provide a reference for what might have contributed to the opposite sea ice trends in Arctic and Antarctic and highlight the important influence of large-scale sea surface temperature anomalies on the trends in the two polar regions.

Spinning ice floes reveal intensification of mesoscale eddies in the western Arctic Ocean

Under-ice eddies are prevalent in the major circulation system in the western Arctic Ocean, the Beaufort Gyre. Theoretical studies hypothesize that the eddy-driven overturning and the ice-ocean drag are crucial mechanisms of the gyre equilibration in response to atmospheric winds. However, due to severe weather conditions and limitations of remote sensing instruments, there are only sparse eddy observations in the ice-covered Arctic Ocean. Hence, the evolution of the under-ice eddy field, its impact on the gyre variability, and their mutual response to the ongoing Arctic warming remain uncertain. Here, we infer the characteristics of the under-ice eddy field by establishing its tight connection to the angular velocities of isolated spinning sea ice floes in marginal ice zones. Using over two decades of satellite observations of marginal ice zones in the western Arctic Ocean, we identified and tracked thousands of floes and used idealized eddy modeling to infer the interannual evolution of the eddy energetics underneath the ice. We find that the eddy field is strongly correlated to the strength of the Beaufort Gyre on interannual timescales, which provides the major observational evidence consistent with the hypothesis of the gyre equilibration by eddies. The inferred trends over the past two decades signify that the gyre and its eddy field have been intensifying as the sea ice cover has been declining. Our results imply that with continuing sea ice decline, the eddy field and the Beaufort Gyre will keep intensifying and leading to enhanced transport of freshwater and biogeochemical tracers.

Decadal Observations of Internal Wave Energy, Shear, and Mixing in the Western Arctic Ocean

AbstractAs Arctic sea ice declines, wind energy has increasing access to the upper ocean, with potential consequences for ocean mixing, stratification, and turbulent heat fluxes. Here, we investigate the relationships between internal wave energy, turbulent dissipation, and ice concentration and draft using mooring data collected in the Beaufort Sea during 2003–2018. We focus on the 50–300 m depth range, using velocity and CTD records to estimate near‐inertial shear and energy, a finescale parameterization to infer turbulent dissipation rates, and ice draft observations to characterize the ice cover. All quantities varied widely on monthly and interannual timescales. Seasonally, near‐inertial energy increased when ice concentration and ice draft were low, but shear and dissipation did not. We show that this apparent contradiction occurred due to the vertical scales of internal wave energy, with open water associated with larger vertical scales. These larger vertical scale motions are associated with less shear, and tend to result in less dissipation. This relationship led to a seasonality in the correlation between shear and energy. This correlation was largest in the spring beneath full ice cover and smallest in the summer and fall when the ice had deteriorated. When considering interannually averaged properties, the year‐to‐year variability and the short ice‐free season currently obscure any potential trend. Implications for the future seasonal and interannual evolution of the Arctic Ocean and sea ice cover are discussed.

Comparison of population structure, vertical distribution and growth of sympatric, carnivorous, mesopelagic copepods,Paraeuchaeta glacialisandHeterorhabdus norvegicus, in the western Arctic Ocean

AbstractIn the Arctic Ocean, the life cycles of interzonal omnivorous copepods have been studied, whereas little information is available on the life cycles of mesopelagic carnivorous species. Here, the life cycles of two mesopelagic carnivorous copepods (Paraeuchaeta glacialis and Heterorhabdus norvegicus) are described from vertically stratified samples collected at an annual ice-station (SHEBA) in the western Arctic Ocean during 1996–1997. Reproduction estimated to occur between January and March for both species. Vertical distributions and population growth varied between the species. Early copepodite stages and adult males of P. glacialis were distributed in deeper layers, whereas late copepodite stages and adult females were observed in shallower layers. The skewed sex ratio towards females was observed for adults, which may be related to the cease feeding and short longevity for adult males. In contrast, all the H. norvegicus life stages were distributed in the mesopelagic layer. Clear peaks of the young stages and their sequence suggest that H. norvegicus may complete its life cycle within 1 year. A specialized feeding mode (incorporating a venomous injection spine and large beak) could be a key trait facilitating the achievement of rapid growth in H. norvegicus in the food-limited mesopelagic layer.

Source analysis of dissolved methane in Chukchi Sea and Bering Strait during summer–autumn of 2012 and 2013

Rapid sea-ice retreat in the Arctic Ocean can promote the release of methane (CH4) from microbe and from sediment on the continental shelf. The released methane can be the cause or effect of strong climate feedback. Nevertheless, few observations have been reported of temporal changes of dissolved CH4 in the Arctic Ocean, especially in western coastal shelf areas (Chukchi Sea and Bering Strait). This study therefore describes the spatial and temporal distributions of the concentrations and stable carbon isotope ratios (δ13C value) of CH4 in the western Arctic Ocean during August–October in 2012 and 2013. In both years, surface water was supersaturated with CH4 with respect to the atmosphere, but concentrations in 2013 were 2–3 times higher than in 2012. In 2012, the concentrations in the bottom layer were higher (up to 55.9 nmol kg−1), whereas δ13C values were lower (down to −63.8‰) than in the surface layer, indicating that CH4 was produced mainly by organic matter degradation in seafloor sediment via methanogens. In 2013, however, because of stronger vertical mixing than in 2012, vertical gradients of CH4 concentration and δ13C value were not prominent. Short-term (19 days) time series observations were also conducted at a fixed station in 2013. An episodic high CH4 concentration in the surface layer (17.2 nmol kg−1 with δ13C = −52.9‰) observed after a strong wind event suggests that CH4 production is accelerated because nutrients are supplied from the bottom of the mixed layer increase plankton activity. These findings suggest that the dynamic variation of dissolved CH4 in the Chukchi Sea in autumn 2012 and 2013 depended mainly on the strength of stratification by sea-ice melt water. Strong winds increase biological activity in the surface layer, consequently accelerating CH4 production.

The Beaufort Sea acoustic duct's variability and its impact on acoustic propagation using the mode interaction parameter

The Beaufort duct is a subsurface sound channel formed by cold Pacific Winter Water sandwiched between warmer Pacific Summer Water and Atlantic Water in the Western Arctic Ocean. This duct traps sound waves and allows them to travel long distances without losing energy to lossy interactions with sea ice and surface waves. This study quantifies Beaufort duct variability based on Canada Basin Acoustic Propagation Experiment (CANAPE) and Coordinated Arctic Acoustic Thermometry Experiment (CAATEX) oceanographic observations. Deterministic ocean features induce coupling between acoustic modes confined to the Beaufort duct and non-ducted modes by weakening the duct or causing it to take on an asymmetric form. A non-dimensional mode interaction parameter (MIP) can be defined using the acoustic frequency and the vertical and horizontal scales of sound speed perturbation to characterize coupling strength. It identifies three important wave propagation regimes: sudden approximation, adiabatic approximation, and maximum interaction regime (Colosi and Zinicola-Lapin, 2021). When the MIP between ducted and lossy modes is more and less than 1, strong and weak acoustic variability is predicted, respectively. Variability is high when the MIP between two ducted modes surpasses 1, but modei–mode interference patterns grow more complicated. Acoustic numerical simulations are used to demonstrate various effects.

Estimating Mass Flux From Size‐Fractionated Filtered Particles: Insights Into Controls on Sinking Velocities and Mass Fluxes in Recent U.S. GEOTRACES Cruises

AbstractWe compile full ocean‐depth size‐fractionated (1–51 and &gt;51 μm) particle concentration and composition of suspended particulate matter from three recent U.S. GEOTRACES cruises, and exploit detailed information of particle characteristics measured to give insights into controls on sinking velocity and mass flux. Our model integrates the concept of fractal scaling into Stokes' Law by incorporating one of two porosity‐size power law relationships that result in fractal dimensions of 1.4 and 2.1. The medians of pump‐derived total (&gt;1 μm) mass flux in the upper 100 m of gyre stations are 285.1, 609.2, and 99.3 mg/m2/d in the North Atlantic, Eastern Tropical South Pacific, and Western Arctic Ocean cruises, respectively. In this data set, variations in particle concentration were generally more important than sinking velocity in controlling variations in mass flux. We examine different terms in a Stokes' Law model to explore how variations in particle and water column characteristics from these three cruises affect mass flux. The decomposition of different aspects of the Stokes' relationship sheds light on the lowest total mass flux of the three cruises in the Western Arctic, which could be explained by the Arctic having the lowest particle concentrations as well as the lowest sinking velocities due to having the smallest particle sizes and the most viscous water. This work shows the importance of both particle characteristics and size distribution for mass fluxes, and similar methods can be applied to existing and future size‐fractionated filtered particulate measurements to improve our understanding of the biological pump elsewhere.

Water Mass Controlled Vertical Stratification of Bacterial and Archaeal Communities in the Western Arctic Ocean During Summer Sea-Ice Melting.

The environmental variations and their interactions with the biosphere are vital in the Arctic Ocean during the summer sea-ice melting period in the current scenario of climate change. Hence, we analysed the vertical distribution of bacterial and archaeal communities in the western Arctic Ocean from sea surface melt-ponds to deep water up to a 3040 m depth. The distribution of microbial communities showed a clear stratification with significant differences among different water depths, and the water masses in the Arctic Ocean - surface mixed layer, Atlantic water mass and deep Arctic water - appeared as a major factor explaining their distribution in the water column. A total of 34 bacterial phyla were detected in the seawater and 10 bacterial phyla in melt-ponds. Proteobacteria was the dominant phyla in the seawater irrespective of depth, whereas Bacteroidota was the dominant phyla in the melt-ponds. A fast expectation-maximization microbial source tracking analysis revealed that only limited dispersion of the bacterial community was possible across the stratified water column. The surface water mass contributed 21% of the microbial community to the deep chlorophyll maximum (DCM), while the DCM waters contributed only 3% of the microbial communities to the deeper water masses. Atlantic water mass contributed 37% to the microbial community of the deep Arctic water. Oligotrophic heterotrophic bacteria were dominant in the melt-ponds and surface waters, whereas chemoautotrophic and mixotrophic bacterial and archaeal communities were abundant in deeper waters. Chlorophyll and ammonium were the major environmental factors that determined the surface microbial communities, whereas inorganic nutrient concentrations controlled the deep-water communities.

Spatial Distribution of Colored Dissolved Organic Matter in the Western Arctic Ocean

Optical properties of colored dissolved organic matter (CDOM) were investigated along a latitudinal transect (67°–77° N) in upper water (&lt;80 m depth) of the western Arctic Ocean. The absorption coefficient at 280 nm was 0.48–1.25 m−1, with the average for the oligotrophic basin area (1.04 ± 0.08 m−1) being slightly higher than that of the productive shelf area (0.95 ± 0.16 m−1), implying a decoupling effect between CDOM concentration and biological productivity in the western Arctic Ocean. The spectral slope coefficient S270–350 was negatively correlated with salinity, indicating that DOM molecular weight increases with salinity, and may be affected by melt-water input. Four fluorescent components were identified by excitation emission matrices elaborated by parallel factor analysis, including three humic-like (C1, C3, and C4) components and one protein-like (C2) component. Significant increases in concentrations of terrestrially derived humic-like C3 and C4 components with salinity were observed in the basin, mainly controlled by the physical mixing of surface fresh water and subsurface inflowing Pacific Ocean water. Terrestrial material carried by Pacific inflow is thus an important factor affecting the distribution of CDOM fluorescence components. The C3 and C4 fluorescence components may be useful as tracers of Pacific water in the western Arctic Ocean.

Spatial and Interannual Patterns of Epipelagic Summer Mesozooplankton Community Structures in the Western Arctic Ocean in 2016–2020

AbstractMesozooplankton play a crucial role as primary or secondary consumers in Arctic ecosystems and are sensitive indicators of environmental changes. This research is the first comprehensive Arctic zooplankton study covering the area ranging from the southern Chukchi Sea (SCS) and the northern Chukchi Sea (NCS) to the East Siberian Sea (ESS). Mesozooplankton samples were collected at 151 stations in the western Arctic Ocean each August from 2016 to 2020. The mesozooplankton abundance of the study area ranged from 9 to 6,172 ind. m−3, and the predominant group was copepods at 7–3,866 ind. m−3, of which Pseudocalanus spp. and Calanus glacialis were the most abundant copepods. In the SCS, small copepods and meroplankton, such as Pseudocalanus spp., Cirripedia larvae, Echinodermata larvae, and Centropages abdominalis were the predominant taxa. Especially in 2019, C. abdominalis dominated over meroplankton when water temperatures were high (maximum 12.5°C; sea surface temperature, SST). In the NCS and ESS, C. glacialis, Pseudocalanus spp., Metridia longa, Oithona similis, Parasagitta elegans, and Calanus hyperboreus were abundant. The distributions and structures of mesozooplankton communities indicated variability over large spatial scales in the western Arctic waters because of variations in multiple factors, such as water temperature, salinity, and sea ice; however, geographical effects cannot be ignored even during alterations in the physical properties. Our results suggest that these variable patterns of mesozooplankton communities fluctuate horizontally from south to north as warming progresses on a regional bathymetric basis, and can be used to infer the fate of mesozooplankton communities in the study area.

The Potential of 233U/236U as a Water Mass Tracer in the Arctic Ocean

AbstractThis study explores for the first time the possibilities that the 233U/236U atom ratio offers to distinguish waters of Atlantic or Pacific origin in the Arctic Ocean. Atlantic waters entering the Arctic Ocean often carry an isotopic signature dominantly originating from European reprocessing facilities with some smaller contribution from global fallout nuclides, whereas northern Pacific waters are labeled with nuclides released during the atmospheric nuclear testing period only. In the Arctic Ocean, 233U originates from global fallout while 236U carries both, a global fallout and a prominent nuclear reprocessing signal. Thus, the 233U/236U ratio provides a tool to identify water masses with distinct U sources. In this work, 233U and 236U were analyzed in samples from the GN01 GEOTRACES expedition to the western Arctic Ocean in 2015. The study of depth profiles and surface seawater samples shows that: (a) Pacific and Atlantic waters show enhanced signals of both radionuclides, which can be unraveled based on their 233U/236U signature; and (b) Deep and Bottom Waters show extremely low 233U and 236U concentrations close to or below analytical detection limits with isotopic ratios distinct from known anthropogenic U sources. The comparably high 233U/236U ratios are interpreted as a relative increase of naturally occurring 233U and 236U and thus for gradually reaching natural 233U/236U levels in the deep Arctic Ocean. Our results set the basis for future studies using the 233U/236U ratio to distinguish anthropogenic and pre‐anthropogenic U in the Arctic Ocean and beyond.