Ice Rafting Research Articles

Датирование современной осадочной толщи краевых бассейнов Восточного Мурмана (Кольский полуостров) на примере губ Ярнышная и Зеленецкая

The authors have studied sediment genesis in the Kola Peninsula bays, the most popular and developed part of the coastal shelf by the population. The main tool used is the method of radioisotope chronology of sedimentary layers based on changes in the specific activity of the natural isotope 210Pb and technogenic 137Cs. It has been revealed that the marginal basins of Murman are characterized by monotonous sedimentation in recent centuries. The average sedimentation rate is 0.15—0.17 cm/year. The sedimentary sequence is composed of silty-pelite-sized material. It has been established that the role of ice rafting in the formation of bottom sediment in coastal bays has decreased over the past centuries.

A Study on the Ice Resistance Characteristics of Ships in Rafted Ice Based on the Circumferential Crack Method

In previous studies of ship–ice interactions, most studies focused on ship–level ice interactions, overlooking potential rafted ice conditions in extreme ice conditions. The purpose of this study is to develop a numerical model for predicting ship resistance in rafted ice regions. Numerical modeling of rafted ice was carried out using preset grid cells. By comparing the model test results, the accuracy and reliability of the numerical model are verified. On this basis, we undertook the analysis of the impacts of different ice thicknesses, ship speeds, bending strengths, and crushing strengths on the ice resistance of ships under level and rafted ice conditions. The results show that the ice resistance of ships is significantly higher than that of rafted ice under the condition of level ice; however, level ice and rafted ice have different effects on ship ice resistance. Compared with level ice, the ice resistance of ships navigating in rafted ice is more concentrated. The findings of the present research can serve as a technical reference for studies focused on predicting ship resistance in rafted ice regions.

Ice rafts, debris flows and slumps along a glaciated basin margin: the Carboniferous El Imperial Formation in the San Rafael basin, western Argentina

Ice rafts, debris flows and slumps along a glaciated basin margin: the Carboniferous El Imperial Formation in the San Rafael basin, western Argentina

Assessment of climate extremes at the regional scale during the last millennium using an annually resolved stalagmite record

In public awareness, extraordinary floods, heat-waves, or drought episodes rank amongst the most tangible indications of a changing climate. Forecasting the magnitude of such extreme events at regional and local scales is a major challenge. Development of reliable small-scale climate models requires testing against past climate records, but these are often incomplete, or cover only partial aspects in a spectrum of environmental parameters, e.g., due to seasonal bias as it is the case for tree-ring records. Here, we study a stalagmite from the “Kleine Teufelshöhle” (KTH) in the Franconian karst (Germany) to obtain independent information on local climate conditions at multi-annual to multi-decadal time scales. Radiocarbon and 230Th/U dating provide constraints for linear growth models, which were then successfully refined by correlating high spatial resolution O isotope data for the KTH stalagmite with a published regional tree-ring width record at annual resolution for the past 1000 years. Besides the availability of the tree-ring record, the KTH site offers the additional advantage that direct comparison between tree-ring and speleothem climate proxies with historical documents is possible due to the proximity of the speleothem and tree-ring sites to the former “Free imperial city” of Nuremberg. Historical documents provide precisely dated records of extreme events, in particular for destructive floods. Higher O isotope values in the stalagmite characterize the “Litte Ice Age" (LIA), an interval coinciding with frequent and devastating floods, sometimes associated with river blockage related to accumulation of ice rafts. Prior to approximately 1500 Common Era (CE), O isotope values were generally lower and only show minor overlap with historical flooding accounts. Conversely, some known drought events for Central Europe as well as the climatic influence of large-volume volcanic eruptions with hemispheric to global impact may be recorded in the speleothem as negative excursions in speleothem O isotopic values. The correlated tree-ring and speleothem data for the first time provide a complementary paleohydrology record that is uniquely suited to resolve extreme short-term climate events at the regional scale.

Reinterpreting glacial deposits as a MTD ponding sediment gravity flows in the upper Mojón de Hierro Formation (Tepuel–Genoa Basin, Argentina)

The Late Paleozoic Ice Age (LPIA) was the longest and most extensive glacial interval during the Phanerozoic, lasting from the Late Devonian to the late Permian (362–254 Ma). Numerous questions remain on the spatial and temporal extent of the ice centers, the timing of expansion and contraction of the ice sheets, and the distribution of ice across Gondwana through time. The Tepuel–Genoa Basin, in Patagonia (Argentina) was situated within the paleo South Polar Circle and contains a nearly continuous lower Carboniferous (Tournasian) to lower Permian (Artinskian) succession. The Mojón de Hierro Formation has been described as both non-glacial and glacial in origin, but lacks detailed sedimentological descriptions. This research investigates a ∼50 m succession with sedimentological and geochemical analysis within strata in the upper portion of the Mojón de Hierro Formation at Arroyo Garrido (Chubut Province). Five stratigraphic sections were measured and the strata were categorized into five lithofacies: 1) deformed sandstones, 2) thin-bedded diamictites, 3) graded rhythmites, 4) laminated mudstones with dispersed clasts, and 5) laminated mudstones. Deposition occurred in a slope environment from debris flows, turbidity currents, mass-movements, hemipelagic sedimentation, and ice rafting. The Chemical Index of Alteration (CIA) indicates values similar to average marine shales, which suggests that the muds were supplied from temperate terrestrial sediment sources. This indicates that icebergs from a distant source transited the basin and grounded glaciers were not present in the catchment area. Vanadium/chromium ratios (V/Cr) indicate dysoxic (restricted water circulation) values in samples between the mass-transported blocks and oxic values (well circulated water) in samples stratigraphically above the filled/leveled ponded mini basin. These oxygenation conditions suggest that the mass-transport deposit formed a topography on the sea floor, resulting in a mini basin that ponded sediments within the blocks. A local glacial source was not substantiated for the upper Mojón de Hierro Formation at Arroyo Garrido neither through the detailed facies analysis nor through geochemical proxies.

Biological responses to change in Antarctic sea ice habitats

Sea ice is a key habitat in the high latitude Southern Ocean and is predicted to change in its extent, thickness and duration in coming decades. The sea-ice cover is instrumental in mediating ocean–atmosphere exchanges and provides an important substrate for organisms from microbes and algae to predators. Antarctic krill,Euphausia superba,is reliant on sea ice during key phases of its life cycle, particularly during the larval stages, for food and refuge from their predators, while other small grazers, including copepods and amphipods, either live in the brine channel system or find food and shelter at the ice-water interface and in gaps between rafted ice blocks. Fish, such as the Antarctic silverfishPleuragramma antarcticum, use platelet ice (loosely-formed frazil crystals) as an essential hatching and nursery ground. In this paper, we apply the framework of the Marine Ecosystem Assessment for the Southern Ocean (MEASO) to review current knowledge about relationships between sea ice and associated primary production and secondary consumers, their status and the drivers of sea-ice change in this ocean. We then use qualitative network modelling to explore possible responses of lower trophic level sea-ice biota to different perturbations, including warming air and ocean temperatures, increased storminess and reduced annual sea-ice duration. This modelling shows that pelagic algae, copepods, krill and fish are likely to decrease in response to warming temperatures and reduced sea-ice duration, while salp populations will likely increase under conditions of reduced sea-ice duration and increased number of days of >0°C. Differences in responses to these pressures between the five MEASO sectors were also explored. Greater impacts of environmental pressures on ice-related biota occurring presently were found for the West and East Pacific sectors (notably the Ross Sea and western Antarctic Peninsula), with likely flow-on effects to the wider ecosystem. All sectors are expected to be impacted over coming decades. Finally, we highlight priorities for future sea ice biological research to address knowledge gaps in this field.

Middle to Late Quaternary changes in ice rafting and deep current transport on the Alpha Ridge, central Arctic Ocean and their responses to climatic cyclicities

To address the Arctic Ocean response to the Quaternary climate change, three sediment cores from the Alpha Ridge, central Arctic Ocean, were analyzed for proxies of sediment transport and depositional mechanisms. The identified end-member grain-size components - clayey EM1, fine-silty EM2, and silty-sandy EM3 - are interpreted as proxies of sea ice transport, near-bottom current, and iceberg inputs, respectively. The sea-ice and iceberg transport proxies, EM1 and EM3, have opposing patterns peaking in the interglacials/interstadials and glacials/stadials, respectively. The near-bottom current proxy EM2 shows an overall gradual decrease with secondary variations indicating current slow-down during intervals of enhanced iceberg discharge. EM3 peaks during Marine Isotope Stages (MIS) 12, 10, 8, and MIS 7 stadials co-occur with high Ca content indicative of iceberg inputs from the Laurentide ice sheet via the Beaufort Gyre. Other EM3 peaks, notably during MIS 6 and MIS 4/3 likely originated from the Siberian margin based on elevated Zr content, thus indicating expanded Eurasian ice sheet and the Transpolar Drift extension towards the central Arctic Ocean. High coherences in the spectral analysis between the sea ice and iceberg proxies versus the global ice volume and orbital parameters on the 100-kyr and 23-kyr time bands illustrate the strong responses of sea ice and glaciations to eccentricity and precessional cycles. A persistent precessional impact of low latitude processes on high Arctic environment indicates a potential link to the low latitudes via atmospheric moisture and heat transport.

Bond events in the Tarim Basin: The loess record

Regional responses to Bond events during the Holocene are important components of paleoclimate and environmental change research. However, there are few records of Bond events during the Holocene in arid central Asia; such events remain poorly understood in this region because of a lack of continuous and high-resolution deposits. Here, new magnetic and grain-size records from a loess sediment sequence located on the southern margin of the Tarim Basin (TB) are provided, which exhibit strengthened dust activity at 9.6–9.4, 8.2–7.8, and 5.5–5.2 ka BP. These periods of strengthened dust activity during the early and middle Holocene in the TB are consistent with Bond events revealed from ice raft detritus in the North Atlantic, and abrupt climate events recorded by loess deposits, lake deposits, and stalagmite oxygen isotopes in arid central Asia and east Asia. We suggest that a delayed seasonal jet transition, resulting from the intensification and southward-shift of the westerly jet during Bond events, plays a key role in creating conditions in the TB that favor strengthened dust activity.

Millennial‐scale oscillations and an environmental regime shift around the Middle to Late Holocene transition in the North Atlantic region based on a multiproxy record from Isfjorden, West Spitsbergen

Palynological and sedimentological analyses were performed on the sediment core HH16‐1205‐GC retrieved from the central Isfjorden, West Spitsbergen. The sequence, which spans the last 7000 years, revealed an overall cooling trend with an important climate shift between 4.4 and 3.8 cal. ka BP, in addition to millennial‐scale oscillations. Sea‐surface reconstruction from dinocyst assemblages indicates a decrease in summer sea‐surface temperature, from 2.5 to 1.5 °C, and primary productivity, from 750 to 650 gC m−2 a−1 over the last 7000 years. From around 6.8 to 5.8 cal. ka BP, the sedimentological and palynological data suggest a predominant sediment supply from the inner part of the fjord, ice rafting, dense sea ice cover, strongly stratified water masses and high primary productivity. The interval from 4.4 to 3.8 cal. ka BP is marked by a layer of coarser material and a significant decrease in the grain‐size mode. Our geochemical data show large‐amplitude fluctuations after 2.0 cal. ka BP, while an increase in the dinocysts Impagidinium pallidum and Spiniferites elongatus from 2.0 to 1.2 cal. ka BP suggests enhanced Atlantic Water inflow. The dinocyst‐based reconstructions also reveal large‐amplitude millennial fluctuations in sea ice cover, summer sea‐surface temperature and salinity. Wavelet analysis and cross‐wavelet analysis on K/Ti ratio coupled with sea‐ice estimates confirm a strong signal with a periodicity of 1200–1500 years.

Linking North Atlantic and Alpine Last Glacial Maximum climates via a high-resolution pollen-based subarctic forest steppe record

During the last glaciation the Eurasian continent was impacted by the global reorganization of atmospheric circulation caused by North Atlantic events, through changes in moisture storm tracks modulated by orography. Our research explores the linkages between climate evolution in the North Atlantic and in the Alps between 30.6 and 12.5 ka, This period, centered on the Last Glacial Maximum, experienced the most recent phase of maximum global ice volume, corresponding to a millennia-long phase of sea-level lowstand, minima in reconstructed sea surface temperatures and isotopic values from polar ice. We present a high-resolution multiproxy record from Lake Fimon at the southern Alpine foothills, where a larch forest-steppe persisted throughout the coldest spells. We analyse the fine climate structure of the MIS 3–2 transition, of the Alpine Last Glacial Maximum stepped by HS2, and provide an unprecedented fine resolution and accurate 14C dating of the Last Glacial Maximum and the early Alpine Lateglacial. We reconstructed quantitative climate parameters based on sensitive pollen descriptors, and derived co-registered sedimentary proxies for dust flux. Reconstructed series suggest that extreme continental spells in the Alps and cold boreal winters match the timing of Heinrich Stadials 2 and 1, episodes of iceberg discharge and release of rock fragments (Ice Rafted Debris) in the North Atlantic. The initiation of the LGM at 27.3 ka is marked by a 4 °C drop in July temperatures, likely responsible for the onset of glacial build-up in the Southern Alpine piedmont. The first extreme continental spell coincides with the British-Irish-sourced IRD event at 26.2 ka and with glacier retreat in the Southern Alps, persisting in the subsequent HS2. Late Glacial glacier collapse and extreme drought occurred during the first half of the HS1, 17.4–16.2 ka. The comparison of the results obtained at Lake Fimon with other terrestrial Eurasian records and with recalibrated marine chronologies from the North Atlantic suggests in-phase reactions of the British-Irish Ice Sheet and of Alpine glaciers and ecosystems to changes in the Atlantic Meridional Circulation. Asymmetric ecoclimatic effects produced by North Atlantic events at the windward and leeward sides of the Alps confirm a southern provenance of moist air masses throughout the LGM.

A revised core-seismic integration in the Molloy Basin (ODP Site 909): Implications for the history of ice rafting and ocean circulation in the Atlantic-Arctic gateway

Today's cryosphere reflects an extreme climate state that developed through stepwise global Cenozoic cooling. In this context the opening of the Fram Strait, the Atlantic-Arctic Gateway (AAG), enabled deep-water exchange between the northern North Atlantic and the Arctic Ocean and thereby influenced global ocean circulation and climate. Here we present a new age model for Ocean Drilling Program Site 909 located in the Molloy Basin, a key site to investigate the late opening phase of the central Fram Strait and the early history of oceanic circulation in the AAG. Our results are based on a revised magnetostratigraphy calibrated by new palynomorph bioevents, which shifts previously used stratigraphies for Site 909 to significantly younger ages in the time interval from c. 15 Ma to 3 Ma. The revised late Miocene to present chronology combined with an improved core-log-seismic integration leads to a new high-resolution seismic stratigraphy for the central Fram Strait that allows a more comprehensive correlation with seismic markers from the western Barents Sea margin and also the adjacent Yermak Plateau. The new stratigraphy implies that prominent maxima in coarse sand particles and kaolinite, often interpreted as evidence for ice rafting in the Fram Strait occur at c. 10.8 Ma, c. 3 Myr later as previously inferred and thus well after the Middle Miocene Climate Transition (c. 15–13 Ma). In the late Tortonian (<7.5 Ma), sediment transport became current controlled, mainly through a western, recirculating branch of the West Spitsbergen Current. This transport was strongly enhanced between c. 6.4 and 4.6 Ma and likely linked to the subsiding Hovgaard (Hovgård) Ridge and the widening of the AAG. Late Pliocene to Pleistocene seismic reflectors correlate with episodes of elevated ice-rafted detritus input related to major steps in Northern Hemisphere ice sheet growth such as the prominent glacial inception MIS M2 that predates the mid-Piacenzian Warm Period and the intensification of Northern Hemisphere glaciation starting at c. 2.7 Ma. At the beginning of the Mid Pleistocene Transition (c. 1.2–0.8 Ma), sediment accumulation in the Fram Strait significantly decreased. • A new age model for Ocean Drilling Program Site 909 in the Molloy Basin. • Seismic reflectors mark major climate events since the late Miocene. • Onset of current controlled sediment transport in the central Fram Strait at ~7.5 Ma. • Enhanced transport between 6.4 and 4.6 Ma linked to the subsiding Hovgaard Ridge.

Evaluating the paleoenvironmental significance of sediment grain size in Bering Sea sediments during Marine Isotope Stage 11

Grain size is an important textural property of sediments and is widely used in paleoenvironmental studies as a means to infer changes in the sedimentary environment. However, grain size parameters are not always easy to interpret without a full understanding of the factors that influence grain size. Here, we measure grain size in sediment cores from the Bering slope and the Umnak Plateau, and review the effectiveness of different grain size parameters as proxies for sediment transport, current strength, and primary productivity, during a past warm interval (Marine Isotope Stage 11, 424-374 ka). In general, sediments in the Bering Sea are hemipelagic, making them ideal deposits for paleoenvironmental reconstructions, but there is strong evidence in the grain size distribution for contourite deposits between ~408-400 ka at the slope sites, suggesting a change in bottom current transport at this time.We show that the grain size of coarse (&gt;150 micrometers) terrigenous sediment can be used effectively as a proxy for ice rafting, although it is not possible to distinguish between iceberg and sea ice rafting processes, based on grain size alone. We find that the mean grain size of bulk sediments can be used to infer changes in productivity on glacial-interglacial timescales, but the size and preservation of diatom valves also exert a control on mean grain size. Lastly, we show that the mean size of sortable silt (10-63 micrometers) is not a valid proxy for bottom current strength in the Bering Sea, because the input of ice-rafted silt confounds the sortable silt signal.

Abrupt Quaternary Ocean-ice Events in the Arctic: Evidence from the Ostracode Rabilimis

The Arctic Ocean has experienced orbital and millennial-scale climate oscillations over the last 500 kilo-annum (ka) involving massive changes in global sea level and components of the Arctic cryosphere, including sea-ice cover, land-based ice sheets and ice shelves. Although these climate events are only partially understood, micropaleontological studies utilizing ostracodes and benthic foraminifera have demonstrated that major changes in faunas have occurred at different timescales that signify ecosystem regime changes linked to sea-ice cover, surface productivity, bottom temperature and other factors. In addition to faunal changes characterizing glacial-interglacial cycles, Arctic sediments contain several unusual faunal events that cannot be explained by orbital-scale sea level and cryospheric changes. One indicator of such events involves the ostracode Rabilimis mirabilis (Brady 1868), a shallow-water species that inhabits continental shelves in the modern Arctic. We conducted studies of the stratigraphic distribution of R. mirabilis in cores from the Northwind, Mendeleev, Lomonosov, and Alpha Ridges; the Siberian and North American (Beaufort Sea) continental margins; and the Lincoln Sea off North Greenland and in the northern Greenland Sherard Osborn Fjord. Evidence from these records suggests that this species occurs as a fossil in deeper water sediment cores on the upper parts of submarine ridges (mainly 700-900 meters water depth, mwd), in significant numbers (from 1%to 50% of total ostracodes) during Marine Isotope Stages (MIS) 5a (125-109 ka), MIS 4 (71-57 ka), and MIS 3 (57-29 ka). Furthermore, it occurs in cores from various depths on the Siberian margin, the Beaufort and Lincoln Seas during MIS 1 (the Holocene, approx. 11-0 ka). These occurrences involve well-preserved, stratigraphically consistent adult and juvenile populations, which are autochthonous in nature and not caused by downslope transport or ice rafting. Based on their age and associated paleoceanographic conditions in the Arctic, we interpret these R. mirabilis events as signifying basin-ward migration during abrupt changes in growth and decay of massive ice shelves and may be useful as biostratigraphic markers.

Grain Size Distribution of the Surface Layer of Bottom Sediments in the Barents Sea

The paper presents data from a granulometric analysis based on the results of studying of 44 surface (0–5 cm) sediment samples obtained on cruise 67 of the R/V Akademik Mstislav Keldysh (2016) from different parts of the Barents Sea. Pebble and gravel material is found in surface sediments as an admixture scattered throughout the sea. Such a chaotic distribution pattern is apparently associated with ice rafting. Coarse material is most common in the Barents Sea off the coast of the Kola Peninsula, Novaya Zemlya, and Svalbard, where it accumulates due to coastal abrasion, glacial exaration and bottom erosion. In addition, the >1 mm fraction is widespread at depths where fine fractions are stirred and washed away. The most common sediments in coastal shallow waters are sands. Sands (0.1–1 mm) are widespread in the southern and southeastern regions of the sea, in the region of the Pechora test area, the Kanin Shoal, the Kola Peninsula shallows; and in the northwest, off the coast of Svalbard. With increasing depth, coarse sediments are replaced by mixed sediments with a low pelite admixture. Fine-grained sediments are prevalent in the central part of the sea. Sediments with a pelitic fraction (<0.01 mm) of more than 50% occupy about 70% of the Barents Sea. They are widespread in deep-sea depressions and troughs, as well as in the numerous fiords of the Northern Island of Novaya Zemlya and Franz Josef Land.

A multiproxy record of sedimentation, pedogenesis, and environmental history in the north of West Siberia during the late Pleistocene based on the Belaya Gora section

AbstractRecent revision of the Pleistocene glaciation boundaries in northern Eurasia has encouraged the search for nonglacial geological records of the environmental history of northern West Siberia. We studied an alluvial paleosol-sedimentary sequence of the high terrace of the Vakh River (middle Ob basin) to extract the indicators of environmental change since Marine Oxygen Isotope Stage (MIS) 6. Two levels of the buried paleosols are attributed to MIS 5 and MIS 3, as evidenced by U/Th and radiocarbon dates. Palynological and pedogenetic characteristics of the lower pedocomplex recorded the climate fluctuations during MIS 5, from thePicea-Larixtaiga environment during MIS 5e to the establishment of the tundra-steppe environment due to the cooling of MIS 5d or MIS 5b and partial recovery of boreal forests withPiceaandPinusin MIS 5c or MIS 5a. The upper paleosol level shows signs of cryogenic hydromorphic pedogenesis corresponding to the tundra landscape, with permafrost during MIS 3. Boulders incorporated in a laminated alluvial deposit between the paleosols are dropstones brought from the Enisei valley by ice rafting during the cold MIS 4. An abundance of eolian morphostructures on quartz grains from the sediments that overly the upper paleosol suggests a cold, dry, and windy environment during the MIS 2 cryochron.

Life on the ice-edge: Paleoenvironmental significance of the radiolarian species Amphimelissa setosa in the northern hemisphere

The high-latitude Northern Hemisphere is a key region in the global climate balance. Variations in sea-ice extent affect biological productivity, CO2 exchange and carbon drawdown. Marine proxies indicative of proximity of the ice-marginal zone are therefore essential to understand these processes. Amphimelissa setosa is nowadays a dominant radiolarian species in the Arctic basin and very abundant in the high-latitude North Atlantic. This species, now absent from the North Pacific, has been widely used as a qualitative proxy of modern and past environmental conditions in the high-latitude Northern Hemisphere. Using our new and published data on the distribution of A. setosa in plankton, sediment trap, surface sediment and downcore samples, we provide a quantitative ecological context for the occurrence of this species. We find that the optimal depth and season of A. setosa in the modern North Atlantic and the Chukchi Sea are 160 m and the late boreal summer/early fall (August–October), respectively. A regression model combining environmental variables (temperature, salinity, silicate and chlorophyll-a concentrations, apparent oxygen utilization, sea-ice) at that season and depth, are able to explain 43% of the distribution of this species in surface sediments. Based on these new findings, we conclude that the presence of A. setosa in surface sediments is closely related to high primary production in the proximity of the sea-ice and areas of ice rafting. The onset of this species started at ca. 1.5 Ma in the North Pacific, linked to a gradual cooling, increased silica availability and southward advance of the ice-margins since the Early Pleistocene. Amphimelissa setosa’s decline in this region was likely caused by the development of a quasi-permanent halocline, perennial sea-ice and depletion of silica during marine isotope stage 4. In the high-latitude North Atlantic, the relative abundance of A. setosa appears to be related to cooling and supply of dissolved silica from the continent during ice-rafting events. The comprehensive approach taken in this study suggest that A. setosa is a useful proxy to explore past variations in the ice-cover in the high-latitude Northern Hemisphere.

Seasonal changes in sediment erodibility associated with biostabilization in a subarctic intertidal environment, St. Lawrence Estuary, Canada

The sediment dynamics and the long-term evolution of intertidal environments depend on the erosion threshold, which is often increased by microbial biofilms. Intertidal biostabilization has been well studied in temperate and subtropical environments, but little is known for subarctic conditions with strong seasonal changes in vegetation cover and sea ice presence in winter because of the difficulty in studying the sediments under ice cover. The present project investigated the role of biofilms and their seasonal as well as spatial variations for sediment biostabilization in a subarctic intertidal area near Rimouski (Quebec, Canada). Grain size distribution reflected a seaward increasing trend in sand content and mean grain size. The poor sorting of sediments results from the contribution of sediment transport by ice rafting in addition to the hydrodynamic transport processes in this subarctic intertidal area. A Cohesive Strength Meter (CSM) was used to measure the erosion threshold (τcrit) across the entire intertidal area. Chlorophyll a (Chl a) and extra polymeric substances (EPS) were measured in the top 3 mm. The high marsh had a far higher τcrit than the low marsh, but there was much less variation between low marsh, mudflat and sandflat. τcrit was significantly correlated with Chl a, elevation, and organic matter content. EPS was not a good indicator for τcrit, except for the low marsh/high marsh difference. The results showed a seasonal cycle in intertidal bed erodibility. Lower intertidal areas had highest τcrit in autumn during the storm season, followed by lowest τcrit in winter under sea-ice cover, and no major difference between summer and spring. This study highlights the characteristics of cold climate intertidal areas, where low temperatures in autumn and land-fast ice cover during winter severely affect biofilms and intertidal vegetation, and ultimately their capacity to shield sediments from erosion.

Enhanced, Climate‐Driven Sedimentation on Salt Marshes

AbstractWith Arctic amplification (enhanced polar warming) possibly increasing periods of intense winter freezing and global warming producing more powerful extratropical storms, winter sedimentation on northern salt marshes may likely increase in the future. Here, we show that a large ice‐rafting event in northern Massachusetts delivered the equivalent of 15 years of mineral deposition to the marsh surface in a single storm. During an intense extratropical cyclone in January 2018, sediment‐laden ice was rafted onto the Great Marsh, Massachusetts, by an ~1‐m storm surge coinciding with high astronomical tides. The muddy sand content combined with abundant shells indicates that the sediment originated from proximal bays and tidal flats. Sediment layers delivered by individual ice rafts averaged 3.19 cm in thickness, 12 times the combined organic and inorganic yearly vertical accretion rate on high marshes. This previously underappreciated vector for sediment deposition is likely to help marsh resiliency to sea‐level rise.

Sedimentological traces of ice-raft grounding in a Weichselian glacial lake near Dukuli (NE Latvia)

A clearly erosional, asymmetrical structure with a large concentration of unsorted clasts (ranging from gravel to boulder size) in its deepest point is present in Weichselian glaciolacustrine, mainly fine-grained sediments exposed in an outcrop near Dukuli (NE Latvia). No traces of currents that were sufficiently strong to be capable to erode the sediments significantly have been encountered in the sedimentary succession under study, and such currents would certainly not have been capable to transport boulder-sized clasts. Neither are traces present of mass movements that could deposit the boulder-sized clasts. The glaciolacustrine setting of the succession, the deep scouring and the high concentration of large clasts must therefore be ascribed to erosion of the lake bottom by the keel of an ice raft that became grounded and gradually melted; the debris that was carried along by the ice raft was released and concentrated at the deepest point of the depression that had been eroded by it.

Serra Sul diamictite of the Carajás Basin (Brazil): A Paleoproterozoic glaciation on the Amazonian craton

Abstract This paper reports the discovery of glacial deposits of likely Siderian–Rhyacian age (2.58–2.06 Ga) in South America (Carajás Basin, Brazil), thereby expanding the potential reach of Paleoproterozoic glaciations to the Amazonian craton for the first time. Glacially derived diamictites are stacked within a hitherto unrecognized ∼600-m-thick siliciclastic succession, here named the Serra Sul Formation. Well-preserved textures, with evidence of glaciotectonism and ice rafting, indicate deposition in a coastal subglacial to glacial-fed submarine fan system, in which the immediately underlying units (banded iron formation and volcanic rock) were the main source and bedrock. The Serra Sul diamictite may be correlated with any of the known Paleoproterozoic glaciations, or with none of them.