Amundsen Basin Research Articles

Re-Os geochronology highlights widespread latest Mesoproterozoic (ca. 1090–1050 Ma) cratonic basin development on northern Laurentia

Abstract The terminal Mesoproterozoic was a period of widespread tectonic convergence globally, culminating in the amalgamation of the Rodinia supercontinent. However, in Laurentia, long-lived orogenesis on its eastern margin was punctuated by short-lived extension that generated the Midcontinent Rift ca. 1110–1090 Ma. Whereas this cratonic rift basin is typically considered an isolated occurrence, a series of new depositional ages demonstrate that multiple cratonic basins in northern Laurentia originated around this time. We present a Re-Os isochron date of 1087.1 ± 5.9 Ma from organic-rich shales of the Agu Bay Formation of the Fury and Hecla Basin, which is one of four closely spaced cratonic basins spanning from northeastern Canada to northwestern Greenland known as the Bylot basins. This age is identical, within uncertainty, to ages from the Midcontinent Rift and the Amundsen Basin in northwestern Canada. These ages imply that the late Mesoproterozoic extensional episode in Laurentia was widespread and likely linked to a common origin. We propose that significant thermal anomalies and mantle upwelling related to supercontinent assembly centered around the Midcontinent Rift influenced the reactivation of crustal weaknesses in Arctic Laurentia beginning ca. 1090 Ma, triggering the formation of a series of cratonic basins.

Circulation timescales of Atlantic Water in the Arctic Ocean determined from anthropogenic radionuclides

Abstract. The inflow of Atlantic Water to the Arctic Ocean is a crucial determinant for the future trajectory of this ocean basin with regard to warming, loss of sea ice, and ocean acidification. Yet many details of the fate and circulation of these waters within the Arctic remain unclear. Here, we use the two long-lived anthropogenic radionuclides 129I and 236U together with two age models to constrain the pathways and circulation times of Atlantic Water in the surface (10–35 m depth) and in the mid-depth Atlantic layer (250–800 m depth). We thereby benefit from the unique time-dependent tagging of Atlantic Water by these two isotopes. In the surface layer, a binary mixing model yields tracer ages of Atlantic Water between 9–16 years in the Amundsen Basin, 12–17 years in the Fram Strait (East Greenland Current), and up to 20 years in the Canada Basin, reflecting the pathways of Atlantic Water through the Arctic and their exiting through the Fram Strait. In the mid-depth Atlantic layer (250–800 m), the transit time distribution (TTD) model yields mean ages in the central Arctic ranging between 15 and 55 years, while the mode ages representing the most probable ages of the TTD range between 3 and 30 years. The estimated mean ages are overall in good agreement with previous studies using artificial radionuclides or ventilation tracers. Although we find the overall flow to be dominated by advection, the shift in the mode age towards a younger age compared to the mean age also reflects the presence of a substantial amount of lateral mixing. For applications interested in how fast signals are transported into the Arctic's interior, the mode age appears to be a suitable measure. The short mode ages obtained in this study suggest that changes in the properties of Atlantic Water will quickly spread through the Arctic Ocean and can lead to relatively rapid changes throughout the upper water column in future years.

О тектоно-геодинамических взаимоотношениях Евразийского бассейна и хребта Ломоносова с континентальной окраиной Сибири по новым сейсмическим данным

Basing on the results of the interpretation of new seismic materials, the authors consider the structural features of the southern segments of the Eurasian Basin and the Lomonosov Ridge in the zone of junction with the continental margin of Siberia (the Laptev and East Siberian Seas). Interpretative analysis of the materials shows that the base of the sedimentary cover of the southern segment of the Eurasian Basin, where there are no regular linear magnetic anomalies, is predominantly represented by strongly stretched blocks of the continental basement. The formation the axial zone of spreading of the Gakkel Ridge here took place according to a three-stage development scheme: rifting in the Aptian-Alba and its telescoping development in the Late Cretaceous — Paleocene-Eocene, inherited valley formation in late neotectonic time. The development of the Severny Basin located in the zone of junction of the Lomonosov Ridge with the continental margin is similar to the scenario for the formation of pull-apart basins. Its formation was interconnected with the simultaneously opening adjacent extreme southeastern segment of the Amundsen Basin of the Eurasian Basin.

Deposition History and Paleo‐Current Activity on the Southeastern Lomonosov Ridge and its Eurasian Flank Based on Seismic Data

AbstractA regional seismic survey on the southeastern Lomonosov Ridge (LR) and adjacent basins provides constraints on the coupled evolution of ocean circulations, depositional regime, and tectonic processes. First, Mesozoic strata on the LR, its faulted flanks and the initial Amundsen Basin were covered with syn‐rift sediments of Paleocene to early Eocene age. Numerous vertical faults indicate differential compaction of possibly anoxic sediments deposited in the young, still isolated Eurasian Basin. The second stage, as indicated by a prominent high‐amplitude‐reflector sequence covering the ridge, was a time of widespread changes in deposition conditions, likely controlled by the ongoing subsidence of the LR and gradual opening of the Fram Strait. Episodic incursions of water masses from the North Atlantic probably were the consequences and led to the deposition of thin sedimentary layers of different lithology. The third stage is marked by continuous deposition since the early Miocene (20 Ma). At that time, the ridge no longer posed an obstacle between the Amerasia and Eurasia Basins and pelagic sedimentation was established. Drift bodies, sediment waves, and erosional structures indicate the onset of circulation. Faulting on the ridge slope has led to a series of terraces where sediment drifts have accumulated since the early Miocene. It is suggested that ongoing sagging of the ridge and currents may have shaped the steep sediment free flanks of the terraces. Lastly, a sequence of high‐amplitude reflectors marks the transition to the early Pliocene large‐scale Northern Hemisphere glaciations.

Annual cycle of downward particle fluxes on each side of the Gakkel Ridge in the central Arctic Ocean.

Two mooring arrays carrying sediment traps were deployed from September 2011 to August 2012 at ∼83°N on each side of the Gakkel Ridge in the Nansen and Amundsen Basins to measure downward particle flux below the euphotic zone (approx. 250 m) and approximately 150 m above seafloor at approximately 3500 and 4000 m depth, respectively. In a region that still experiences nearly complete ice cover throughout the year, export fluxes of total particulate matter (TPM), particulate organic carbon (POC), particulate nitrogen (PN), biogenic matter, lithogenic matter, biogenic particulate silica (bPSi), calcium carbonate (CaCO3), protists and biomarkers only slightly decreased with depth. Seasonal variations of particulate matter fluxes were similar on both sides of the Gakkel Ridge. Somewhat higher export rates in the Amundsen Basin and differences in the composition of the sinking TPM and bPSi on each side of the Gakkel Ridge probably reflected the influence of the Lena River/Transpolar Drift in the Amundsen Basin and the influence of Atlantic water in the Nansen Basin. Low variations in particle export with depth revealed a limited influence of lateral advection in the deep barren Eurasian Basin.This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning’.

Shale and pyrite Re-Os ages from the Hornby Bay and Amundsen basins provide new chronological markers for Mesoproterozoic stratigraphic successions of northern Canada

Here we present Re-Os geochronological data from two carbonaceous shale units from the Hornby Bay and Amundsen basins that provide important chronological markers for Mesoproterozoic stratigraphic successions within northern Canada. Shale from the basal Escape Rapids Formation yields a robust depositional age of 1067.3 ± 13.5 Ma for the lowermost Shaler Supergroup of Amundsen basin with a remarkably unradiogenic initial 187Os/188Os value (0.15), implying that it derived from weathering of juvenile source material, such as basalt. Sedimentological features, indicative of large basin conditions, suggest deposition within an extensive epeiric sea that was intermittently mixed with waters from an exterior ocean. This age, and the 720 Ma age of the uppermost Shaler Supergroup, constrain deposition of the ∼4 km-thick succession to a period of approximately 350 myr, recording sedimentation through a complete supercontinent cycle - amalgamation through to break-up of Rodinia. Further, it indicates that the Amundsen basin formed at the same time as the Borden, and Fury and Hecla basins on northern Baffin Island, and the Thule basin in northwestern Greenland, and that these basins were perhaps connected as part of a much larger basin situated in the interior of Rodinia. Diagenetic pyrite from sandstone dykelets in carbonaceous shales of the Fort Confidence Formation yielded a Re-Os model age of 1438 ± 8 Ma, providing a minimum age for deposition of the lower Dismal Lakes Group of the Hornby Bay basin. This age strengthens proposed regional correlation of the Hornby Bay basin with the large intracontinental Athabasca and Thelon basins, located on the Rae craton to the southeast, and is evidence for a Laurentia-wide marine flooding episode ca. 1500 Ma. Both new ages place important constraints on newly discovered eukaryotic microfossils from the Shaler Supergroup and Dismal Lakes Group and calibration points for critical chemostratigraphic data.

Borealization of the Arctic Ocean in Response to Anomalous Advection From Sub-Arctic Seas

An important yet still not well documented aspect of recent changes in the Arctic Ocean is associated with the advection of anomalous sub-Arctic Atlantic- and Pacific-origin waters and biota into the polar basins, a process which we refer to as borealization. Using a 37-year archive of observations (1981–2017) we demonstrate dramatically contrasting regional responses to atlantification (that part of borealization related to progression of anomalies from the Atlantic sector of sub-Arctic seas into the Arctic Ocean) and pacification (the counterpart of atlantification associated with influx of anomalous Pacific waters). Particularly, we show strong salinification of the upper Eurasian Basin since 2000, with attendant reductions in stratification, and potentially altered nutrient fluxes and primary production. These changes are closely related to upstream conditions. In contrast, pacification is strongly manifested in the Amerasian Basin by the anomalous influx of Pacific waters, creating conditions favorable for increased heat and freshwater content in the Beaufort Gyre halocline and expansion of Pacific species into the Arctic interior. Here, changes in the upper (overlying) layers are driven by local Arctic atmospheric processes resulting in stronger wind/ice/ocean coupling, increased convergence within the Beaufort Gyre, a thickening of the fresh surface layer, and a deepening of the nutricline and deep chlorophyll maximum. Thus, a divergent (Eurasian Basin) gyre responds altogether differently than does a convergent (Amerasian Basin) gyre to climate forcing. Available geochemical data indicate a general decrease in nutrient concentrations Arctic-wide, except in the northern portions of the Makarov and Amundsen Basins and northern Chukchi Sea and Canada Basin. Thus, changes in the circulation pathways of specific water masses, as well as the utilization of nutrients in upstream regions, may control the availability of nutrients in the Arctic Ocean. Model-based evaluation of the trajectory of the Arctic climate system into the future suggests that Arctic borealization will continue under scenarios of global warming. Results from this synthesis further our understanding of the Arctic Ocean’s complex and sometimes non-intuitive Arctic response to climate forcing by identifying new feedbacks in the atmosphere-ice-ocean system in which borealization plays a key role.

Border of the Continental Margin of the Central Arctic Uplifts in Amundsen Basin in Siberia

The seismic structure of the complex underlying the stratified sedimentary cover in the Amundsen Basin in Siberia indicates that the formation of a large-scale rift-related basin lying on a hyper-extended continental crust preceded the spreading process in the Eurasian Basin.

Late Quaternary sedimentary processes in the central Arctic Ocean inferred from geophysical mapping

Cryospheric events in the Arctic Ocean have been largely studied through the imprints of ice sheets, ice shelves and icebergs in the seafloor morphology and sediment stratigraphy. Subglacial morphologies have been identified in the shallowest regions of the Arctic Ocean, up to 1200 m water depth, revealing the extent and dynamics of Arctic ice sheets during the last glacial periods. However, less attention has been given to sedimentary features imaged in the vicinity of the ice-grounded areas. Detailed interpretation of the sparse available swath bathymetry and sub-bottom profiles from the Lomonosov Ridge and the Amundsen Basin shows the occurrence of mass transport deposits (MTDs) and sediment waves in the central Arctic Ocean. The waxing and waning ice sheets and shelves in the Arctic Ocean have influenced the distribution of MTDs in the vicinity of grounding-ice areas, i.e. along the crest of Lomonosov Ridge. Due to the potential of Arctic sediments to hold gas hydrates, their destabilization should not be ruled out as trigger for sediment instability. Sediment waves formed by the interaction of internal waves that propagate along water mass interfaces with the bathymetric barrier of Lomonosov Ridge. This work describes the distribution and formation mechanisms of MTDs and sediment waves in the central Arctic Ocean in relation to grounding ice and internal waves between water masses, respectively. The distribution of these features provides new insight into past cryospheric and oceanographic conditions of the central Arctic Ocean.

Variability of the thermohaline structure and transport of Atlantic water in the Arctic Ocean based on NABOS (Nansen and Amundsen Basins Observing System) hydrography data

Abstract. Conductivity–temperature–depth (CTD) transects across continental slope of the Eurasian Basin and the St. Anna Trough performed during NABOS (Nansen and Amundsen Basins Observing System) project in 2002–2015 and a transect from the 1996 Polarstern expedition are used to describe the temperature and salinity characteristics and volume flow rates (volume transports) of the current carrying the Atlantic water (AW) in the Arctic Ocean. The variability of the AW on its pathway along the slope of the Eurasian Basin is investigated. A dynamic Fram Strait branch of the Atlantic water (FSBW) is identified in all transects, including two transects in the Makarov Basin (along 159∘ E), while the cold waters on the eastern transects along 126, 142, and 159∘ E, which can be associated with the influence of the Barents Sea branch of the Atlantic water (BSBW), were observed in the depth range below 800 m and had a negligible effect on the spatial structure of isopycnic surfaces. The geostrophic volume transport of AW decreases farther away from the areas of the AW inflow to the Eurasian Basin, decreasing by 1 order of magnitude in the Makarov Basin at 159∘ E, implying that the major part of the AW entering the Arctic Ocean circulates cyclonically within the Nansen and Amundsen basins. There is an absolute maximum of θmax (AW core temperature) in 2006–2008 time series and a maximum in 2013, but only at 103∘ E. Salinity S(θmax) (AW core salinity) time series display a trend of an increase in AW salinity over time, which can be referred to as an AW salinization in the early 2000s. The maxima of θmax and S(θmax) in 2006 and 2013 are accompanied by the volume transport maxima. The time average geostrophic volume transports of AW are 0.5 Sv in the longitude range 31–92∘ E, 0.8 Sv in the St. Anna Trough, and 1.1 Sv in the longitude range 94–107∘ E.

Under‐Ice Phytoplankton Blooms in the Central Arctic Ocean: Insights From the First Biogeochemical IAOOS Platform Drift in 2017

AbstractIn April 2017, the first Ice Atmosphere Arctic Ocean Observing System (IAOOS) platform equipped with biogeochemical sensors was deployed near the North Pole. Over the next 8 months it meandered southward with the ice, collecting measurements in the upper 250 m of Amundsen Basin, the Gakkel Ridge, Nansen Basin, and Fram Strait. Two distinct periods of primary production were encountered. The first bloom developed in the mixed layer in Amundsen Basin in May, reaching maximum chlorophyll a concentrations on the order of ~ 0.5 mg m−3 by the end of June. This bloom occurred earlier in the year than any other recorded bloom in Amundsen Basin, despite very limited under‐ice light due to a thick layer of snow. The second bloom, encountered in Nansen Basin in August, was significantly larger (mean profile maximum chlorophyll a was 1.45 mg m−3). Examinations of the optical community index and colored dissolved organic matter concentrations suggest that the spring bloom consisted of small phytoplankton which may have been mixotrophic, while the summer bloom contained a greater diversity of planktonic size classes. The data set demonstrates the heterogeneity of Arctic under‐ice primary production, and is a valuable resource for validating coupled Earth System models.

Decrease in <sup>230</sup>Th in the Amundsen Basin since 2007: far-field effect of increased scavenging on the shelf?

Abstract. This study provides dissolved and particulate 230Th and 232Th results as well as particulate 234Th data collected during expeditions to the central Arctic Ocean (GEOTRACES, an international project to identify processes and quantify fluxes that control the distributions of trace elements; sections GN04 and GIPY11). Constructing a time series of dissolved 230Th from 1991 to 2015 enables the identification of processes that control the temporal development of 230Th distributions in the Amundsen Basin. After 2007, 230Th concentrations decreased significantly over the entire water column, particularly between 300 and 1500 m. This decrease is accompanied by a circulation change, evidenced by a concomitant increase in salinity. A potentially increased inflow of water of Atlantic origin with low dissolved 230Th concentrations leads to the observed depletion in dissolved 230Th in the central Arctic. Because atmospherically derived tracers (chlorofluorocarbon (CFC), sulfur hexafluoride (SF6)) do not reveal an increase in ventilation rate, it is suggested that these interior waters have undergone enhanced scavenging of Th during transit from Fram Strait and the Barents Sea to the central Amundsen Basin. The 230Th depletion propagates downward in the water column by settling particles and reversible scavenging.

Will low primary production rates in the Amundsen Basin (Arctic Ocean) remain low in a future ice-free setting, and what governs this production?

Abstract The study is based on a very extensive data-set of physical, biological, and optical parameters from below the sea ice in the western Amundsen Basin, central Arctic Ocean, in August–September 2012 during the record low sea ice extent. The water column was strongly stratified at all stations related to salinity differences between a surface layer of reduced salinities ( 34). A nitrate utilization-based budget in the surface layer gave a primary production of 67.5 mg C m−2 d−1, which reduced to 3.9 mg C m−2 d−1 in August 2012. Amundsen Basin primary production rates are lower than rates determined for other Arctic Ocean deep-water basins, and also lower compared to rates on the shelf. Below ice phytoplankton was well adapted to low light conditions in the Amundsen Basin and the photosynthetic potential was high, but limited by the low nutrient fluxes induced by the strong stratification. Amundsen Basin is foreseen to be ice-free in summer in 3–4 decades, and the question whether primary production will increase when ice-free was resolved with a coupled physical-biogeochemical model. Results showed that production will increase 10 to 14 times from the present 3.9 mg C m−2 d−1 to 37.4 and 55.2 mg C m−2 d−1 for an ice-free August and July–August, respectively. The study substantiates that both present and future ice-free low production rates were related to the strong stratification, reduced nutrient fluxes, and deep lying nutrient rich waters. Low production rates and strong stratification are discussed in the view of parameters that increase this stratification as higher freshwater run off or reduce stratification as wind.

Предварительная оценка сейсмической опасности района Арктического хребта Гаккеля и окрестностей

This study describes primary data, methods of estimation and final results of the preliminary seismic hazard assessment in the region of the Gakkel Ridge that is a northernmost seismogenic zone of the Earth. According to geological data, the region is considered potentially oil and gas, but its industrial development has not yet begun. These authors for the Baltic Sea did the similar work earlier. At the first stage of this study, the earthquake catalog unified in magnitude Mw was compiled for the period from 1912 to 2014. Information on seismic events from historical sources and the ISC catalog was used, as well as the results of observations of the Arkhangelsk seismic network in the Arctic for 2014–2018. The representative part of earthquake data was revealed and the seismic regime has been studied. By seismicity origin, the region is divided into the highly active rift zone of the Gakkel Ridge and the continental slopes of the Barents, Kara and eastern Laptev seas with weaker activity, separated and framed by aseismic areas like the Nansen, Amundsen basins and the Lomonosov Ridge. The seismic zoning of the study region was carried out based on structural analysis of geological and geophysical data. The mapping of probabilistic seismic hazard in terms of maximum accelerations of PGA soil movements for a return period of 500 and 100 years (10% probability of exceedance in the next 50 and 10 years) was conducted using the CRISIS program. As expected, the most dangerous was the Gakkel zone about 200 km wide.

On the reproduction of two deep-sea Arctic holothurians, Elpidia heckeri and Kolga hyalina (Holothuroidea:Elpidiidae)

ABSTRACTInternal gonad morphology was examined in two species of elpidiid holothurians, Elpidia heckeri and Kolga hyalina, commonly occurring in the Central Arctic Ocean. The holothurians were sampled in August – September 2012 and in August 2018, at several locations in the Nansen and Amundsen Basins, in areas under the constant ice cover and in areas free of ice. Both species were observed at different reproductive stages and both are presumed to reproduce periodically. In 2012, spawning occurred at the beginning of September in Elpidia heckeri and from late August to early September in Kolga hyalina. There are reasons to believe that gametogenesis in K. hyalina depends on the availability of freshly deposited phytodetritus. The maximum oocyte size in Kolga hyalina (240 μm) was smaller than in Elpidia heckeri (390 μm).

Deep-sea megabenthos communities of the Eurasian Central Arctic are influenced by ice-cover and sea-ice algal falls.

Quantitative camera surveys of benthic megafauna were carried out during the expedition ARK-XXVII/3 to the Eastern Central Arctic Basins with the research icebreaker Polarstern in summer 2012 (2 August-29 September). Nine transects were performed for the first time in deep-sea areas previously fully covered by ice, four of them in the Nansen Basin (3571-4066m) and five in the Amundsen Basin (4041-4384m). At seven of these stations benthic Agassiz trawls were taken near the camera tracks for species identification. Observed Arctic deep-sea megafauna was largely endemic. Several taxa showed a substantially greater depth or geographical range than previously assumed. Variations in the composition and structure of megabenthic communities were analysed and linked to several environmental variables, including state of the sea ice and phytodetritus supply to the seafloor. Three different types of communities were identified based on species dominating the biomass. Among these species were the actiniarian Bathyphellia margaritacea and the holothurians Elpidia heckeri and Kolga hyalina. Variations in megafaunal abundance were first of all related to the proximity to the marginal ice zone. Stations located closer to this zone were characterized by relatively high densities and biomass of B. margaritacea. Food supply was higher at these stations, as suggested by enhanced concentrations of pigments, organic carbon, bacterial cell abundances and nutrients in the sediments. Fully ice-covered stations closer to the North Pole and partially under multi-year ice were characterized by lower concentrations of the same biogeochemical indicators for food supply. These stations nevertheless hosted relatively high density and biomass of the holothurians E. heckeri or K. hyalina, which were observed to feed on large food falls of the sea-ice colonial diatom Melosira arctica. The link between the community structure of megafauna and the extent and condition of the Central Arctic sea-ice cover suggests that future climate changes may substantially affect deep ocean biodiversity.

New Hydrographic Measurements of the Upper Arctic Western Eurasian Basin in 2017 Reveal Fresher Mixed Layer and Shallower Warm Layer Than 2005–2012 Climatology

AbstractIn 2017, Ice Atmosphere Ocean Observing System autonomous drifting platforms provided extensive physical and biogeochemical data in the upper 350 m of the western Eurasian Basin through their 8‐month drift across the Amundsen Basin, the Gakkel Ridge, the Nansen Basin and western Fram Strait. Comparison with WOA13 climatology indicates a fresher surface layer and shallower warm layer in 2017 than in 2005–2012. The Ice Atmosphere Ocean Observing System 2017 data feature two halocline eddies in the Amundsen Basin and two Atlantic Water (AW) mesoscale structures in the Nansen Basin. Analysis of the global (1/12)° Mercator Ocean operational system suggests that the halocline eddies resulted from instabilities in the frontal zone between fresher Makarov waters and saltier Eurasian waters. This frontal region appears to have shifted further southeast in 2017 (near 88°N, 10°E) compared to 2005–2012. The operational system depicts the large AW structure in the Nansen Basin (140 km crossed as far as 83.7°N, 34.5°E) as an AW meander from the Arctic Circumpolar Boundary Current, which turned into an anticyclonic eddy about a month after the platforms drifted away. The AW structure at 82.8°N, 3°W, northwest of the Yermak Plateau, corresponds to an AW recirculating branch detaching from the Yermak Plateau slope back toward Fram Strait.

Sensitivity of centennial mass loss projections of the Amundsen basin to the friction law

Abstract. Reliable projections of ice sheets' future contributions to sea-level rise require models that are able to accurately simulate grounding-line dynamics, starting from initial states consistent with observations. Here, we simulate the centennial evolution of the Amundsen Sea Embayment in response to a prescribed perturbation in order to assess the sensitivity of mass loss projections to the chosen friction law, depending on the initialisation strategy. To this end, three different model states are constructed by inferring both the initial basal shear stress and viscosity fields with various relative weights. Then, starting from each of these model states, prognostic simulations are carried out using a Weertman, a Schoof and a Budd friction law, with different parameter values. Although the sensitivity of projections to the chosen friction law tends to decrease when more weight is put on viscosity during initialisation, it remains significant for the most physically acceptable of the constructed model states. Independently of the considered model state, the Weertman law systematically predicts the lowest mass losses. In addition, because of its particular dependence on effective pressure, the Budd friction law induces significantly different grounding-line retreat patterns than the other laws and predicts significantly higher mass losses.

Projected Freshening of the Arctic Ocean in the 21st Century

AbstractUsing state‐of‐the‐art models from the Coupled Model Intercomparison Project phase 5 (CMIP5), this study found the upper Arctic Ocean likely to freshen considerably in the future. Arctic Ocean average sea surface salinity is projected to decrease by 1.5 ± 1.1 psu, and the liquid freshwater column is projected to increase by 5.4 ± 3.8 m by the end of the 21st century under the Representative Concentration Pathway 8.5 (RCP8.5) scenario. Most freshening would occur in the Arctic Ocean basins, that is, the Canada, Makarov, and Amundsen basins. Anomalies in freshwater flux from sea ice melt, Bering Strait inflow, net precipitation (P‐E), river runoff, and freshwater through the Barents Sea Opening (BSO) would contribute to Arctic Ocean freshening. CMIP5 historical and RCP8.5 experiments showed that the respective projected contributions from BSO freshwater flux, river runoff, P‐E, and Bering Strait inflow are about 6.4, 5.0, 2.7, and 2.2 times the contribution from sea ice melt averaged throughout the 21st century. Contributions from sea ice melt and Bering Strait inflow would increase and then decrease gradually, while those from BSO freshwater flux, river runoff, and P‐E would increase continuously. The CMIP5 models are able to simulate the Arctic Ocean freshwater system more accurately than CMIP3 models. However, the simulated rate of increase of freshwater content (296 ± 232 km3/yr) is weaker than estimated (600 ± 300 km3/yr) based on observations (1992–2012). Moreover, the simulated BSO and Davis Strait freshwater fluxes still exhibit substantial intermodel spread and they differ considerably from observed values.

Depositional Evolution of the Western Amundsen Basin, Arctic Ocean: Paleoceanographic and Tectonic Implications

AbstractA new stratigraphic model and estimated sedimentation rates of the western Amundsen Basin, Arctic Ocean, are presented based on multichannel seismic reflection data, seismic refraction data, magnetic data, and integrated with the sedimentary sequence from the central Arctic Ocean, obtained during the Arctic Coring Expedition. This places new constraints on the postbreakup Cenozoic depositional history of the basin, the adjacent Lomonosov Ridge, and improves the understanding of the tectonic, climatic, and oceanographic conditions in the central Arctic region. Four distinct phases of basin development are proposed. During the Paleocene‐mid‐Oligocene, high sedimentation rates are linked to terrestrial input and increased pelagic deposition in a restricted basin. Deposition of sedimentary wedges and mass transport into marginal depocenters reflect a period of tectonic instability linked to compression associated with the Eurekan Orogeny in the Arctic. During the late Oligocene‐early Miocene, widespread passive infill associated with hemipelagic deposition reflects a phase of limited tectonism, most likely in a freshwater estuarine setting. During the middle Miocene, mounded sedimentary buildups along the Lomonosov Ridge suggest the onset of geostrophic bottom currents that likely formed in response to a deepening and widening of the Fram Strait beginning around 18 Ma. In contrast, the Plio‐Pleistocene stage is characterized by erosional features such as scarps and channels adjacent to levee accumulations, indicative of a change to a higher‐energy environment. These deposits are suggested to be partly associated with dense shelf water‐mass plumes driven by supercooling and brine formation over the northern Greenland continental shelf.