Mendeleev Ridge Research Articles

Neogene Hydrothermal Fe‐ and Mn‐Oxide Mineralization of Paleozoic Continental Rocks, Amerasia Basin, Arctic Ocean

AbstractRocks dredged from water depths of 1,605, 2,500, 3,300, and 3,400 m in the Arctic Ocean included Paleozoic continental rocks pervasively mineralized during the Neogene by hydrothermal Fe and Mn oxides. Samples were recovered in three dredge hauls from the Chukchi Borderland and one from Mendeleev Ridge north of Alaska and eastern Siberia, respectively. Many of the rocks were so pervasively altered that the protolith could not be identified, while others had volcanic, plutonic, and metamorphic protoliths. The mineralized rocks were cemented and partly to wholly replaced by the hydrothermal oxides. The Amerasia Basin, where the Chukchi Borderland and Mendeleev Ridge occur, supports a series of faults and fractures that serve as major zones of crustal weakness. We propose that the stratabound hydrothermal deposits formed through the flux of hydrothermal fluids along Paleozoic and Mesozoic faults related to block faulting along a rifted margin during minor episodes of Neogene tectonism and were later exposed at the seafloor through slumping or other gravity processes. Tectonically driven hydrothermal circulation most likely facilitated the pervasive mineralization along fault surfaces via frictional heating, hydrofracturing brecciation, and low‐ to moderate temperature Fe‐ and Mn‐rich hydrothermal fluids, which mineralized the crushed, altered, and brecciated rocks.

A resilient ice cover over the southernmost Mendeleev Ridge during the late Quaternary

The presence of a late Quaternary ice sheet/ice shelf over the East Siberian Sea has been proposed in several papers. Here, we further document its duration/resilience based on the sedimentary, bulk mineralogical, and geochemical (organic matter content and its stable isotopic composition, U‐Th series) properties of a core raised from the southernmost Mendeleev Ridge. The chronostratigraphy of the studied core was mainly built from the 230Th excess (230Thxs) distribution and decay downcore. At the core‐top, peaking 230Thxs values during the early MIS 3 and mid‐MIS 1 encompassing an MIS 2 hiatus were observed. As documented in several papers, these peaks suggest seasonally open ice conditions over proximal continental shelves. Below, the interval spanning MIS 4 and possibly MIS 5d records major ice‐rafting events illustrated by overall high coarse‐fraction contents. Underlying MIS 5e, down to MIS 11, the sediment depicts relatively low sand (1.7±2.5 dw%), high clay (33.5±4.7 dw%), and very low organic carbon (0.10±0.06 dw%) contents, and low δ13Corg values (−24.3±0.9‰). This section is interpreted as recording fine sediment transport by deep currents and/or meltwater plumes below a resilient ice cover, only interrupted by a few short‐duration events. These events include (i) detrital carbonate pulses assigned to deglacial events along the NW Laurentide Ice Sheet margin (Termination (T) III), and (ii) intervals with some planktonic foraminifer occurrences, likely relating to their advection from open areas of the Arctic Ocean (MIS 5e, 9 and 11). All Terminations, but TII and the early MIS 3, show peaking Mn/Al values linked to the submergence of Arctic shelves under a rising sea level. We conclude that the resilient ice cover, likely an ice shelf, has been present over the southern Mendeleev Ridge during most of the interval after the Mid‐Pleistocene Transition and was favoured by the low summer insolation of the MIS 14 to 10 interval.

Arctic deep-water anoxia and its potential role for ocean carbon sink during glacial periods

Deep water freshening beneath pan-Arctic ice shelves has recently been proposed based on the absence of excess thorium in glacial Arctic sediments. This profound proposal requires scrutiny of Arctic paleohydrology during past glacial periods. Here, we present structural and geochemical results of inorganic authigenic carbonates in deep-sea glacimarine sediments from the Mendeleev Ridge, western Arctic Ocean over the last 76 kyr. Our results suggest that Polar Deep Water in the western Arctic became brackish and anoxic during stadial periods. We argue that sediment-laden hyperpycnal meltwater discharged from paleo-ice sheets filled much of the water column depending upon the density, substantially reducing the salinity and oxygen content of the Polar Deep Water. Our findings suggest that this phenomenon was more extreme in the western Arctic Ocean and may point to the potential role of the western Arctic Ocean as an additional carbon reservoir in the global carbon cycle across glacial-interglacial cycles.

Canada’s maritime frontier: the science legacy of Canada’s extended continental shelf mapping for UNCLOS

Canada ratified the United Nations Convention on the Law of the Sea (UNCLOS) in 2003. With that ratification is an obligation to submit data and information to the U.N. pertaining to the limits of the country’s extended continental shelf (ECS); the portion of the juridical continental shelf that extends beyond 200 nautical miles. A team of Canadian scientists, managers, and legal experts that included representation from three Federal Departments (Natural Resources Canada, Fisheries and Oceans Canada, and Global Affairs Canada) with additional support from other departments, spent 13 years compiling and acquiring data to provide the scientific evidence to support delineation of Canada’s seaward most maritime limit. The submission has the potential to provide Canada with 2.4 million km2of additional submarine landmass in the Atlantic and the Arctic oceans over which Canada exercises sovereign rights for the purpose of exploring and exploiting its natural resources. Specific information such as the tectonic framework of the continental margin, the geomorphology of the margin and in particular the continental slope, the geologic nature of adjoined ridges, rises, and plateaux, and sediment thickness within adjacent basins are examples of fundamental pieces of geoscientific information needed to substantiate Canada’s outermost maritime limits. This paper highlights a number of segments of Canada’s continental margins to showcase this scientific evidence and how it is applied in the UNCLOS context. In doing so, the paper demonstrates the geologic complexity of Canada’s margins as illustrated in scientific publications that have resulted from these new data collections, while at the same time presenting new scientific evidence and interpretations. This collection of data and information provides a wealth of new knowledge in Canada’s offshore regions. The massive data compilation in the Atlantic led to conception of continental margins, in a source-to-sink scenario, as having an equilibrium base level or graded form, comparable to river systems. Departures from this shape relate to the interplay of sedimentary processes and in particular to those processes that do not fit the source-to-sink paradigm. For example, a significant part of the Atlantic margin is shown to be heavily influenced by along-slope geostrophic currents that generated massive contourite drift deposits. These deposits reflect lateral transport of sediment that had a significant impact on the morphology of the margin. The role of mass transport processes in shaping continental margins is also highlighted, and in particular the collapses of entire segments of the margin were observed. The prominent role mass failure processes play in delivering sediment to the adjacent abyssal plain is also critical in the ECS context. These observations challenge the entrenched notion of a continental margin comprising a shelf, slope, and rise and in particular the concept of the “continental rise”. Prior to 2006, regions of the Arctic Ocean seaward of the Canadian landmass had fewer than 5000 km of seismic reflection data. The massive efforts of Arctic coastal States to map their margins for ECS purposes have led to a leap in technological advances to acquire data in ice-covered seas and have led to a wealth of new geoscientific knowledge. Perhaps foremost amongst this knowledge is demonstration that Canada Basin is indeed a fully developed ocean basin, albeit significantly infilled with sediment. Based on this knowledge and identification of related structures, new realistic tectonic scenarios for opening of the Amerasia Basin are proposed that include a significant component of transform or strike-slip motions. With seismic velocity and rock sample information, the continental nature of Alpha and Mendeleev ridges has been substantiated. Even bathymetric data were lacking in the Arctic and new editions of seafloor maps now support grids of 500 m spacing; although some regions remain sparse. Once thought to be relatively stagnant, sedimentary processes such as found in many ocean basins were discovered in the Arctic Ocean. Evidence of geostrophic currents, sediment mass failures, and deep-sea turbidity current channels were found to be ubiquitous, even in the deepest parts of the Arctic’s basins.

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.

Synoptic 129I and CFC–SF6 Transit Time Distribution (TTD) Sections Across the Central Arctic Ocean From the 2015 GEOTRACES Cruises

AbstractMeasurements of the tracers, 129I, CFC‐11, and SF6 on water samples collected in the Arctic Ocean in 2015 have been used to calculate mean ages, Γ and mixing, Δ parameters using transit time distributions (TTDs) to constrain water circulation and mixing time scales. Values of Γ and Δ determined separately using the two tracer pairs, SF6‐CFC‐11, and 129I‐CFC‐11 are in good agreement for gas solubilities estimated for saturation levels of 0.90, but agreement decreases for other gas saturation levels. Both Γ and Δ increase rapidly with increasing depth below the base of the intermediate water layer (ca. 1,000 m), but maintaining a value of Δ/Γ ≅ 1 supporting the use of this proportionality in applications of TTDs to deep ocean transport of substances such as anthropogenic carbon. Isolines of Γ = 20 years deepening to depths below 1,000 m over the flank of the Mendeleyev Ridge near the North Pole outline the bathymetrically steered, return flow of recently ventilated Atlantic Water toward Fram Strait. Basin interior waters are significantly older with the Γ = 25 years mean age isoline shallowing upward to depths above 500 m in the Makarov, Canada, and Eurasian Basins. Values of Δ remain relatively constant in the 6–10 years range in upper intermediate water across all three basins indicating that flow is principally advective and that the mixing specified by Δ likely occurs upstream of the central basins in regions proximal to the outflow from the Santa Anna Trough.

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.

Changes in Freshwater Distribution and Pathways in the Arctic Ocean Since 2007 in the Mercator Ocean Global Operational System

AbstractLow‐salinity waters in the upper Arctic Ocean, referred to as “freshwaters”, are cold and play a major role in isolating the sea ice cover from the heat stored in the salty Atlantic Waters (AW) underneath. We examined changes in Arctic freshwater distribution and circulation since 2007 using the 1/12° global Mercator Ocean operational model. We first evaluated model simulations over the upper water column in the Arctic Ocean, using nearly 20,000 independent in situ temperature‐salinity profiles over the 2007–2020 period. Simulated hydrographic properties and water mass distributions were in good agreement with observations. Comparison with long‐term mooring data in the Bering Strait and Beaufort Gyre highlighted the model's capabilities for reproducing the interannual evolution of Pacific Water properties. Taking advantage of the good performance of the model, we examined the interannual evolution of the freshwater distribution and circulation over 2007–2020. The Beaufort Gyre is the major freshwater reservoir across the full Arctic Ocean. After 2012 the gyre extended northward and increased the freshwater content in the Makarov Basin, near the North Pole. Coincidentally, the freshwater content decreased along the East Siberian slope, along with the AW shoaling, and the Transpolar Drift moved from the Lomonosov Ridge to align with the Mendeleev Ridge. We found that these changes in freshwater distribution were followed in 2015 by a marked change in the export of freshwater from the Arctic Ocean with a reduction in Fram Strait (−30%) and an increase in the western Canadian Archipelago (+16%).

Oceanic vertical mixing of the lower halocline water in the Chukchi Borderland and Mendeleyev Ridge

Oceanic vertical mixing of the lower halocline water in the Chukchi Borderland and Mendeleyev Ridge

Distribution and Transport of Water Masses in the East Siberian Sea and Their Impacts on the Arctic Halocline

AbstractWe investigated the transport of Arctic shelf water and its impacts on the downstream hydrographic structures in the Amerasian Basin by combining data collected in the summer of 2016 from two international cruises in the Pacific sector. The East Siberian Sea (ESS), in which cold shelf water with salinity of 31.0–32.8 is produced, was identified to be a major source of the near‐freezing temperature water for the Makarov Basin. This relatively low‐salinity shelf water was transported into the Chukchi Abyssal Plain and occupied the subsurface layer (30–60 m), which led to the formation of a subsurface oceanic front stretching along the Chukchi Plateau and northern side of the Mendeleev Ridge. The offshore transport of waters from the ESS caused an intrusion of the Atlantic Water onto the shelf and promoted the formation of the diapycnally mixed Lower Halocline Water (D‐LHW) at the bottom of the outer shelf. Our findings indicate that the source of the D‐LHW in the Arctic Pacific sector might extend from the Chukchi Plateau to as far as the Makarov Basin. We hypothesize that advection of waters from the ESS into the Makarov Basin promotes development of the subsurface halocline in the Makarov Basin. The volume transport of surface waters (salinity less than 30) on the ESS shelf was estimated to be about 0.6 Sv (106 m3 s−1) eastward to the Canada Basin in 2016, consistent with the cyclonic Arctic circulation regime in 2016.

Driving Mechanisms of Sedimentary 230Th and 231Pa Variability in the Western Arctic Ocean Through the Last Glacial Cycle

AbstractConcentration profiles of excess 230Th and 231Pa (230Thxs and 231Paxs) in Arctic Ocean sediment cores exhibit strong temporal variabilities, but the mechanisms responsible for the variabilities remain poorly understood. Here, we present new 230Thxs and 231Paxs profiles from the Mendeleev Ridge to examine downcore 230Thxs and 231Paxs distributions and their connection to environmental conditions in the Arctic. Measurable concentrations of 230Thxs(231Paxs) from the new record display an alternation of “230Thxs(231Paxs)‐rich” and “230Thxs(231Paxs)‐poor” sections as observed in the other cores from the Mendeleev Ridge. Sedimentary characteristics reveal that this alternation does not directly correspond to interglacial‐glacial cycles. The 230Thxs and 231Paxs concentrations show an inverse relationship to their corresponding sedimentation rates, suggesting the distributions of 230Thxs and 231Paxs are affected by variable dilution with terrigenous material. However, changes in lateral transport of these isotopes may also account for the observed oscillations. Sedimentary cumulative inventories of 230Thxs show evidence for a net deficit of 230Th in the sediments during the last glacial period, possibly due to a lack of scavenging material in general.

The effective elastic thickness of the lithosphere in the Amerasia Basin, Arctic Ocean

AbstractAs a proxy for the lithospheric strength at the time of loading, the effective elastic thickness of the lithosphere (Te) can aid in understanding the structure and evolution of the lithosphere. Here, we obtain spatial variations in Te of Amerasia Basin and surrounding regions using the fan‐shaped Morlet wavelet method. Our results show that variations in Te generally agree well with tectonic provinces in the region. The eastern part of Makarov Basin has the same lithospheric strength as Alpha and Mendeleev ridges, which may be attributed to the influence of the High Arctic large igneous province. N‐S and E‐W Te variations in Podvodnikov Basin may reflect two phases of its formation. Western and eastern parts of south Canada Basin have prominent low and high Te values, respectively. This study provides the reliable Te results of central Arctic and improves our understanding of the evolution of the Arctic.

Paleozoic Tectonics and Geodynamics of the De Long Islands and Adjacent Structures of the Verkhoyansk-Chukotka Fold Belt

This work presents finalized results of our paleomagnetic study of Paleozoic rocks of Henrietta, Jeannette and Bennett islands and their implications for tectonic reconstructions. We discuss possible relationships of the Cambrian section of the De Long sedimentary basin with the Verkhoyansk margin of Siberia, the terranes of the Kolyma-Omolon group, the Chukotka-Alaska microcontinent, as well as with the structures of the submerged Mendeleev Ridge. We propose a new tectonic model, in which the De Long structures feature as the frontalmost part of a large transform system that existed on the periphery of the Siberian paleocontinent.

Gas-geochemical studies of gas fields and increased metal concentrations in the East Siberian Sea

Paper presents the results of complex gas-geochemical studies of bottom sediments of the East Siberian Sea along the meridional profile from Cape Billings to the Mendeleev Ridge. Abnormal concentrations of methane (up to 2.4% vol.) and hydrogen (up to 600 ppm) are controlled by neotectonic faults and are typical for the areas of gas hydrate formation. The carbon isotope composition indicates the predominance of the thermogenic component. When studying the chemical composition of sediments, the data helped to identify the permeability zones of neotectonic faults that have favorable conditions for the concentration of a number of elements: Mn, Cu, Ag. Such zones are characterized by the gas anomalies in sediments (methane, hydrogen, etc.). The accumulation of anomalous metal contents is facilitated by specific geological conditions that occur in zones of gas anomalies within tectonically active structures, where fine-grained sediments enriched with organic matter are present. The gas-geochemical fields formed in this pattern can be applied as indicators in forecasting of hydrocarbon accumulations, for mapping permeable fault zones, and for the environmental impact assessing of hydrocarbon anomalies. This approach could be especially effective in the basins with low seismic activity such as seas of East Siberian shelf and some of the marginal seas of Pacific Ocean, for instance, South China Sea (Bien Dong).

Reflection of Large Tectonic Structures of the Eastern Part of the Arctic Ocean in the Lithospheric Magnetic Field

The article considers maps of the field of lithospheric magnetic anomalies—images of the deep structures of the Arctic Ocean—constructed from observations by the CHAMP satellite. The lithospheric magnetic anomalies corresponding to the most significant tectonic structures were revealed: Lomonosov Ridge, Mendeleev Ridge, Alpha Rise, and the Amundsen, Podvodnikov and Makarov basins. The distribution of the regional magnetic anomaly field, which is a complex set of positive and negative segments of various shapes and amplitudes, is discussed in light of the modern geological and geophysical views on the structure of the Arctic Ocean lithosphere. The constructed lithospheric magnetic field model indicates that in the region of conjugation with Eurasia the foot and the slope of Lomonosov Ridge are morphologically related to the continental margin, while the area of Mendeleev and Alpha elevations in central Arctic appears to be a single-block structure with a continental-type crust.

Ice sheet controls on fine-grained deposition at the southern Mendeleev Ridge since the penultimate interglacial

Clay minerals deposited at the southern Mendeleev Ridge in the Arctic Ocean have a unique provenance, which can be used to reconstruct changes in the local sedimentary environment. We show that sediments in core ARC7-E23 record high-frequency changes in clay minerals since the penultimate interglacial. The clay minerals, grain size, and ice-rafted debris indicate the extent of the East Siberia Ice Sheet (ESIS). During the glacial periods of Marine Isotope Stage 2 (MIS2) and MIS4, the southern Mendeleev Ridge was likely covered by an ESIS-extended ice shelf, blocking almost all sediment input from the Canadian Arctic and Laptev Sea, but allowing transport of fine-grained sediments from the East Siberian and Chukchi Sea shelves. After ESIS retreat, the Beaufort Gyre and Transpolar Drift became the primary transport mechanism for the distally sourced sediments. Climate conditions in MIS3 enhanced both the oceanic circulation and sediment transport.

Ice events along the East Siberian continental margin during the last two glaciations: Evidence from clay minerals

A large ice sheet or ice shelf may have developed repeatedly along the East Siberian continental margin since the Middle Pleistocene. In combination with ice-rafted debris, major elements, and foraminifera, clay mineral records from four gravity sediment cores are used to trace ice events along the East Siberian continental margin during the last two glaciations. The clay mineral assemblages show a strong link to the extent of the East Siberian ice sheet (ESIS) rather than the ice shelf or icebergs sourced from the Canadian Arctic. During the last two glaciations, illite-rich sediment from East Siberia was transported mainly toward the deep basin, whereas kaolinite-rich sediment from the Canadian Arctic was mostly blocked from the East Siberian continental margin. We speculate that during marine isotope stage 2 (MIS2), the southern Mendeleev Ridge, Chukchi Basin, and Chukchi Rise may have been covered by an ESIS-extended ice shelf; however, the Chukchi Plateau and Northwind Ridge were dominated by a Laurentide-sourced ice shelf. During MIS4, the ESIS may have undergone a marked expansion, with an extended ice shelf occupying all of the Chukchi Plateau and Northwind Ridge, thereby restricting iceberg discharge from other ice sheets.

Changes in sediment source areas to the Amerasia Basin, Arctic Ocean, over the past 5.5 million years based on radiogenic isotopes (Sr, Nd, Pb) of detritus from ferromanganese crusts

Ferromanganese (FeMn) crusts provide a useful paleoenvironmental archive for studying the poorly understood climatic, oceanographic, and geologic evolution of the Arctic Ocean. This study is based on the identification and temporal reconstruction of sources and inferred transport pathways of terrigenous material in FeMn crusts collected from several sites across the Amerasia Basin. Samples from the Alpha Ridge (AR), Mendeleev Ridge (MR), and Chukchi Borderland (CB) have similar chemical compositions and high detrital contents. Sr, Nd, and Pb isotopic compositions of the terrigenous detritus extracted from the FeMn crust layers show spatial and temporal variability due to the variable proportions of inputs from the North American and East Siberian margins and local submarine outcrops. The temporal resolution is restricted by macroscopic crust layers, which represent times of significant changes in the depositional environment and are used to study first-order environmental changes. During the period of ca. 4.4 to 3.8 Myr ago, detritus in the MR area was derived mostly from the Laptev Sea. Then, until about 1.3 Myr ago, FeMn crusts from MR received a higher volcanic component sourced from the East Siberian Sea, with mixing of detritus from the Okhotsk-Chukotka volcanic belt (OCVB) area and possibly weathering of local MR outcrops, which is mostly High Arctic Large Igneous Province (HALIP) rocks. The period from about 1.3 Myr ago to the present reflects enhanced contributions of detritus from the Canadian Shield, approximately equal to the East Siberian contribution for the MR area. CB crusts show three main periods of distinct detrital input. During the period ca. 5.5–5.0 Myr ago, CB detritus was sourced from local submarine outcrops of the Jurassic-Cretaceous HALIP rocks and possibly from northeast Siberia, including OCVB rocks, while during the period from about 5.0 to 3.6 Myr ago, the CB FeMn crusts had a much greater input of detritus from the North American margin. After 3.6 Myr ago, the CB detritus shows a mixed composition predominantly sourced from North America with some material transported from the Bering Sea and smaller contributions from northeast Siberia. The AR crust was influenced mostly by detritus sourced from the Canadian Shield.This study shows that FeMn crusts from the Arctic ocean are a useful tool and archive for climate reconstruction and depositional history of this polar region.

Origin of the coarse-grained (> 1 cm) clasts from the Mendeleev Ridge area (Central Arctic Ocean)

The morphometric and petrographic characteristics of the coarse-grained clasts (> 1 cm) sampled from the sediments of the Amerasian Basin, Central Arctic Ocean, were studied. Most of the clasts are represented by dolomites (46,4%), sandstones (22,8%) and limestones (19,8%); the amount of other rocks fragments (chert, shale, igneous) is about 10%. A variety of lithological types were identified among the studied rock fragments. Limestones and dolomitic limestones often contain fragments of fauna. The majority of clasts is poorly rounded and characterized by a wide variety of shapes. More than half of the studied clasts have a size of 1-2 cm, a quarter - 2-3 cm, and larger clasts only occur in insignificant amounts. Geophysical surveys across the sampling sites showed a lack of bedrock outcrops, so the studied coarse-grained clasts are not of local origin. It is concluded that they were predominantly delivered from the Canadian Arctic Archipelago (likely from the platform area, e.g., Victoria Island), mainly due to iceberg rafting during deglaciation periods. The maximum possible contribution of the clasts from the Siberian sources is less than 23%. Distribution of the coarse-grained clasts argues for the existence of a quite stable ice drift path in the past, which is similar to the modern Beaufort Gyre.

Middle to Late Pleistocene Arctic paleoceanographic changes based on sedimentary records from Mendeleev Ridge and Makarov Basin

Three sediment cores from the Makarov Basin and Mendeleev Ridge off the Siberian margin were analyzed for a number of litho-, bio- and magnetostratigraphic proxies. Correlation to previously studied Arctic records from Mendeleev to Lomonosov ridges enables paleoceanographic reconstructions for the Middle to Late Pleistocene (estimated ∼0.8 Ma) on orbital time scales. Cyclic changes in lithology corroborated by multiple stratigraphic constraints were tuned to global climate variations based on the manganese content that increases during interglacials, while calcium and zircon were used as proxies for the Laurentide and Eurasian ice sheet inputs. The combined evaluation of glacial and interglacial proxy variabilities provides a coherent picture of the western Arctic Ocean development in the Pleistocene. We suggest that major cyclic changes were driven by summer processes, such as sea-ice retreat and enhanced biological production vs. suppressed ice melt in warmer and colder summers, respectively. A potential link to low latitudes via atmospheric moisture and heat transport is indicated by similarities in the western Arctic Ocean and East Asian monsoon records. Long-term cooling is signified by biogenic proxies related to sea-ice expansion over the western Arctic Ocean with step changes at MIS 11 and 7 (∼0.4 and 0.2–0.3 Ma). This transition also features better fossil preservation indicating less corrosive/oxidative bottom waters, probably resulting from a decrease in organic export production and/or weakening of bottom water ventilation under perennial sea ice. The latter was possibly enhanced by atmospheric cooling and increased meltwater fluxes with the growth of circum-Arctic ice sheets.