Ice Rafting Research Articles

Spatial and Temporal Sediment Dynamics in a Subarctic Salt Marsh (Gulf of St. Lawrence, Canada)

ABSTRACT Gibeault, C.; Neumeier, U., and Bernatchez, P., 2016. Spatial and temporal sediment dynamics in a subarctic salt marsh (Gulf of St. Lawrence, Canada). Sediment transport in the salt marsh and the lagoon of Penouille is driven by tidal currents, waves, wind, and ice rafting, which interact with topography and vegetation to control the spatial distribution of surface sediments. Multiple linear regressions of 203 samples of marsh surface sediments show that grain size gets finer and organic matter content increases with distance from the lagoon inlet, the sandy peninsula, the closest creek, and the lagoon. The vegetation parameters and surface microtopography have only a minor influence. Locally, grain size can get finer with elevation, while organic matter content increases. However, across the entire marsh, grain size exhibits the opposite trend with elevation, a consequence of aeolian transport and ice rafting. The 94 samples covering the lagoon show a trend of finer grain size with distance from...

Icebergs Fertilize Southern Ocean, Sequester Carbon

Huge, drifting ice rafts (the white spot on the satellite image below ) shed minerals as they melt, painting trails of nutrients, teeming phytoplankton, and chlorophyll across hundreds of kilometers of ocean.

A Miniscule System of a ‘Sailing Rock’

The ‘Sailing Rocks’ of Death Valley, California are known worldwide, and the enigma of how they move over a desert ground without intervention of animal or human forces, still remains one of the great natural mysteries. There exist different theories about how these movements happen, the most recent was that ice-rafts build up below and around the rocks, which are then pushed forward by moving ice floes. Earlier theories regarding this phenomenon describe the influence of wind, and flows of melting water on fine soil material (Hypothesis of ‘Sailing Rocks’). We describe here the same phenomenon, observed not in a large rock weighing several hundreds of kilograms, but in a small pebble of about 2 cm in size. A clear analysis of the conditions under which its movement took place has allowed the formulation of the minimum conditions under which a rock starts moving. Ice rafts may be sufficient but are not necessary. Here we show that pebbles sail driven by the wind over a watery surface that covers an icy but water saturated substrate, similar to an aquaplaning car. The same physical rules apply whatever the scale, to this small system as well as to the large sailing rocks of Death Valley.

Similar millennial climate variability on the Iberian margin during two early Pleistocene glacials and MIS 3

Although millennial-scale climate variability (<10 ka) has been well studied during the last glacial cycles, little is known about this important aspect of climate in the early Pleistocene, prior to the Middle Pleistocene Transition. Here we present an early Pleistocene climate record at centennial resolution for two representative glacials (marine isotope stages (MIS) 37–41 from approximately 1235 to 1320 ka) during the “41 ka world” at Integrated Ocean Drilling Program Site U1385 (the “Shackleton Site”) on the southwest Iberian margin. Millennial-scale climate variability was suppressed during interglacial periods (MIS 37, MIS 39, and MIS 41) and activated during glacial inceptions when benthic δ18O exceeded 3.2‰. Millennial variability during glacials MIS 38 and MIS 40 closely resembled Dansgaard-Oeschger events from the last glacial (MIS 3) in amplitude, shape, and pacing. The phasing of oxygen and carbon isotope variability is consistent with an active oceanic thermal bipolar see-saw between the Northern and Southern Hemispheres during most of the prominent stadials. Surface cooling was associated with systematic decreases in benthic carbon isotopes, indicating concomitant changes in the meridional overturning circulation. A comparison to other North Atlantic records of ice rafting during the early Pleistocene suggests that freshwater forcing, as proposed for the late Pleistocene, was involved in triggering or amplifying perturbations of the North Atlantic circulation that elicited a bipolar see-saw response. Our findings support similarities in the operation of the climate system occurring on millennial time scales before and after the Middle Pleistocene Transition despite the increases in global ice volume and duration of the glacial cycles.

Marine record of Holocene climate, ocean, and cryosphere interactions: Herbert Sound, James Ross Island, Antarctica

The sediment record offshore James Ross Island, northeast Antarctic Peninsula presents an unparalleled opportunity to directly compare marine and terrestrial climate records spanning the Holocene in maritime Antarctica. An 11 m drill core was collected between Herbert Sound and Croft Bay as part of the SHALDRIL NBP-0502 initiative and produced the southernmost sediment record from the eastern side of the AP. Thirty-eight radiocarbon ages are used to construct an age model of centennial-scale resolution. Multi-proxy records, including magnetic susceptibility, pebble content, particle size, total organic carbon, and diatom assemblages, were interrogated in the context of nearby Holocene-age ice core, lake, and drift records from James Ross Island. Differences in the timing and expression of Holocene events reflect marine controls on tidewater glaciers, such as water mass configurations and sea ice. Glacial behavior mimics ice core paleotemperatures during the Holocene, with the exception of distinct ocean warming events. Herbert Sound was fully occupied by grounded ice during the Last Glacial Maximum, and experienced rapid lift-off, followed by a floating ice phase. The canopy of floating ice receded by 10 ± 2.4 cal kyr BP, presumably in response to Early Holocene warming. Herbert Sound and Croft Bay fully deglaciated by 7.2 cal kyr BP, when the Mid Holocene Hypsithermal commenced and the sound became open and productive. An extreme peak in productivity ∼6.1 cal kyr BP indicates an oceanic warming event that is not reflected in atmospheric temperature or lacustrine sediment records. Increase in sea ice cover and ice rafting mark the onset of the Neoglacial ∼2.5 cal kyr BP, when pronounced atmospheric cooling is documented in the James Ross Island ice core. Our comparison facilitates more holistic understanding of atmosphere-ocean-cryosphere interactions that may aid predictions of glacial response to future warming and sea-level scenarios.

Pulsed iceberg delivery driven by Sturtian ice sheet dynamics: An example from Death Valley, California

AbstractThe Kingston Peak Formation is a Cryogenian sedimentary succession that crops out in the Death Valley area, California, USA. It is widely accepted to record pre‐glacial conditions (KP1), followed by two glaciations of pan‐global extent, the older Sturtian (KP2 to KP3) and younger Marinoan glaciation (KP4). In the type area (the Kingston Range), detailed facies analysis of the Sturtian succession reveals that a basal diamictite unit and an upper boulder conglomerate were deposited by proglacial subaqueous sediment gravity flows. An olistostrome unit punctuating the succession is interpreted to result from tectonically induced downslope mobilization during isostatic rebound, triggered by significant ice‐meltback. Focussing on strata onlapping the olistostrome, this article provides insight into the processes of glacial re‐advance following an intra‐Sturtian glacial minimum. The first 50 m of strata above the olistostrome are thinly bedded turbidites that are devoid of lonestones. A trend towards thicker graded beds upsection, in concert with the gradual appearance and then abundance of lonestones, testifies to the influence of ice rafting and to the resumption of a direct ice sheet influence upon sedimentation. Stratigraphic organization into thickening and coarsening upward bedsets over a multi‐metre scale reveals a subaqueous gravity flow‐dominated succession composed of a spectrum of high to low density turbidites, with thick graded boulder‐conglomerates at intervals. The finer‐grained facies assemblage is heterolithic: current ripple cross‐laminated sandstones intercalated with shales that bear delicate granule to pebble‐sized dropstones in abundance. Intervals of dropstone‐bearing and dropstone‐free strata are attributable to dynamic oscillation of the ice margin in the hinterland. Integrating palaeocurrent data with observations from neighbouring outcrop belts allows a detailed palaeogeographic map of the eastern Death Valley area to be compiled for the first time.

Variations of Okhotsk Sea oxygen minimum zone: Comparison of foraminiferal and sedimentological records for latest MIS 12–11c and latest MIS 2–1

Abstract Benthic foraminiferal assemblage compositions, foraminiferal and sedimentological proxies were analyzed in core MD01-2415 from the northern slope of the Okhotsk Sea to compare variations of productivity and oxygen minimum zone (OMZ) intensity during the latest marine isotope stage (MIS) 12–11c and latest MIS 2–1. The benthic assemblages reveal close similarity between the two climatic cycles. The absence of benthic assemblages during the latest MIS 12 and the presence of the low-productivity Angulogerina angulosa assemblage during the latest MIS 2 suggest the disappearance of the OMZ. This regime was related to almost perennial ice cover with periods of active ice rafting during the latest MIS 12, while it was attributed to prolonged seasonal ice cover, low surface productivity and enhanced formation of well-oxygenated water masses in the Okhotsk Sea during the latest MIS 2. In deglacial times, the OMZ gradually intensified, as evidenced by the high-productivity Uvigerina akitaensis assemblage during the early termination (T) TV and early TI and the low oxygen-tolerant Bolivina spissa assemblage during the late TV and late TI. The orbitally forced similar rises of the global sea level during TV and TI caused a large offshore supply of organic matter. Synchronously, nutrients from the melting sea ice and shelf erosion promoted high surface (mainly carbonate) productivity. As a result, a high and sustained flux of particulate, degraded and refractory organic matter enhanced oxygen consumption in bottom waters and sediments, leading to the similar gradual OMZ intensifications. The B. spissa assemblage also points to expansion of oxygen-depleted water mass from the North Pacific into the Okhotsk Sea, fostering the OMZ intensifications. The phytodetritus-related Islandiella norcrossi assemblage indicates weakening of the OMZ during full interglacial times of MIS 11c and MIS 1. After stabilization of the global sea level, nutrients were mainly delivered by regional upwelling and fluvial discharge, favoring increased biogenic opal and carbonate production in the surface water, similar to the present. In this way, moderate to high (although less than deglacial) and pulsed flux of predominantly particulate organic matter caused the weakening of the oxygen consumption and OMZ. Notably, during MIS 11c, the benthic assemblage with the dominance of the taxa with dissolution-resistant tests, such as Miliammina herzensteini, Karreriella baccata and Martinottiella communis, reflects carbonate dissolution events in sediments. These events might have been resulted from an interruption of the local surface carbonate production and inflow of more carbonate-corrosive water masses from the North Pacific driven by a drawdown of the global ocean carbonate saturation.

Sedimentation in the submarine Shirshov Ridge area (Bering Sea) during the last 180–185 ka (Penultimate glaciation-Holocene)

The lithological analysis of sediments from Core SO201-2-85KL (18 m long) taken from the Shirshov Ridge in the western part of the Bering Sea (57°30.30′ N, 170°24.79′ E, water depth 968 m), which recovered the section spanning from the penultimate glaciation till Holocene, revealed their mostly terrigenous composition with several intercalations of diatomaceous ooze. The latter was accumulated mainly during relatively warm epochs (last interglacial and Holocene) with elevated bioproductivity of surface waters. Sedimentation during the penultimate glaciation was strongly influenced by bottom currents. Ice rafting of detrital material was intensified during cold marine isotope stages (MIS 6, MIS 4, MIS 2). Glaciations were accompanied by increased sedimentation rates probably due to the glacioeustatic sea level falls, desiccation of the Bering Sea shelf, and enhanced influx of sedimentary material transported by large rivers immediately to the deepwater basin.

Particle sources and downward fluxes in the eastern Fram strait under the influence of the west Spitsbergen current

The carbon cycle of the Arctic Ocean is tightly regulated by land–atmosphere–cryosphere–ocean interactions. Characterizing these environmental exchanges and feedbacks is critical to facilitate projections of the carbon cycle under changing climate conditions. The environmental drivers of sinking particles including organic carbon (OC) to the deep-sea floor are investigated with four moorings including sediment traps and currentmeters at the Arctic gateway in the eastern Fram Strait, which is the area where warm anomalies are transported northwards to the Arctic. Particles fluxes were collected over one year (July 2010–July 2011) and have been analysed to obtain the content of the lithogenic fraction, calcium carbonate, OC and its stable isotopes, opal, and the grain size. Records of near bottom current speed and temperature along with satellite observations of sea ice extent and chlorophyll-a concentration have been used for evaluation of the environmental conditions.We found increased lithogenic fluxes (up to 9872mgm−2d−1) and coarsening grain size of settling particles in late winter–early spring. At the same time, intensifications of the northward flowing west Spitsbergen current (WSC) were recorded. The WSC was able to resuspend and transport northwards sediments that were deposited at the outlet of Storfjordrenna and on the upper slope west of Spitsbergen. The signal of recurrent winnowing of fine particles was also detected in the top layer of surface sediments. In addition, an increased arrival of sea ice transported ice rafted detritus (>414 detrital carbonate mineral grains larger than 1mm per m2) from the southern Spitsbergen coast along with terrestrial organic matter was observed beyond 1000m of water depth during winter months. Finally, the downward particle fluxes showed typical temporal variability of high latitudes, with high percentages of the biogenic compounds (opal, organic carbon and calcium carbonate) linked to the phytoplankton bloom in spring–summer. However, on an annual basis local planktonic production was a secondary source for the downward OC, since most of the OC was advected laterally by the WSC. Overall, these observations demonstrated the sensitivity of the downward flux of particles to environmental conditions such as hydrodynamics, sea ice rafting, and pelagic primary production. Future alteration of the patterns of natural drivers due to climate change is thus expected to cause major shifts in the downward flux of particles, including carbon, to the deep sea ecosystems.

Evolution of the central Nordic Seas over the last 20 thousand years

The deep and surface water paleoceanographic evolution of the central Nordic Seas over the last 20 thousand years was reconstructed using various micropaleontological, isotopic and lithological proxy data. These show a high spatial and temporal complexity of the oceanic circulation when compared with other records from the region. During early deglaciation a collapse of ice sheets surrounding the Nordic Seas released large amounts of freshwater that affected both the surface and bottom water circulation and significantly contributed to Heinrich stadial 1. During the Younger Dryas, the central Nordic Seas were affected by a last major freshwater plume which probably originated from the Arctic Ocean. When major ice rafting had ceased around 11 ka subsurface temperatures started to rise. However, Atlantic Water advection and subsurface temperatures reached their maximum in the central Nordic Seas later than along the eastern continental margin. That spatio-temporal offset is explained by a gradual re-routing and westward expansion of the Atlantic Water flow during times when the Greenland Sea gyre system became more steadily established. In the Greenland Basin, the Holocene thermal maximum ended c. 5.5 ka, and time-coeveal with an increase in sea-ice export from the Arctic. In the Lofoten Basin the cooling occurred later, after 4 ka, and together with a weakening of the overturning processes. The Neoglacial cooling was reached c. 3 ka, together with low solar irradiance, expanding sea ice and a slight decrease in deep convection. At c. 2 ka subsurface temperatures began to rise again due to an increasing influence of Atlantic Waters.

Biogeochemical drivers of the fate of riverine mercury discharged to the global and Arctic oceans

AbstractRivers discharge 28 ± 13 Mmol yr−1 of mercury (Hg) to ocean margins, an amount comparable to atmospheric deposition to the global oceans. Most of the Hg discharged by rivers is sequestered by burial of benthic sediment in estuaries or the coastal zone, but some is evaded to the atmosphere and some is exported to the open ocean. We investigate the fate of riverine Hg by developing a new global 3‐D simulation for Hg in the Massachusetts Institute of Technology ocean general circulation model. The model includes plankton dynamics and carbon respiration (DARWIN project model) coupled to inorganic Hg chemistry. Results are consistent with observed spatial patterns and magnitudes of surface ocean Hg concentrations. We use observational constraints on seawater Hg concentrations and evasion to infer that most Hg from rivers is sorbed to refractory organic carbon and preferentially buried. Only 6% of Hg discharged by rivers (1.8 Mmol yr−1) is transported to the open ocean on a global basis. This fraction varies from a low of 2.6% in East Asia due to the barrier imposed by the Korean Peninsula and Japanese archipelago, up to 25% in eastern North America facilitated by the Gulf Stream. In the Arctic Ocean, low tributary particle loads and efficient degradation of particulate organic carbon by deltaic microbial communities favor a more labile riverine Hg pool. Evasion of Hg to the Arctic atmosphere is indirectly enhanced by heat transport during spring freshet that accelerates sea ice melt and ice rafting. Discharges of 0.23 Mmol Hg yr−1 from Arctic rivers can explain the observed summer maximum in the Arctic atmosphere, and this magnitude of releases is consistent with recent observations. Our work indicates that rivers are major contributors to Hg loads in the Arctic Ocean.

What was the transport mode of large boulders in the Campine Plateau and the lower Meuse valley during the mid-Pleistocene?

The Campine Plateau in northeastern Belgium, a remnant of an alluvial fan deposited by the Meuse River during the mid-Pleistocene, is characterised by the presence of boulders with maximum dimensions of up to 2m, embedded in a gravel matrix. These boulders originated in the Ardennes region and are generally assumed to have been transported by ice-rafting processes. This paper investigates for the first time quantitatively the possibility of purely hydraulic transport of the boulders, taking into account channel and flow characteristics in the boulder provenance area during the mid-Pleistocene. Empirical transport relations that describe incipient motion thresholds in nonuniform river beds as a function of the relative grain size, or the ratio between the grain size of interest and the median grain size of the channel bed, are applied in order to calculate critical water depths for transport of boulders of various sizes. Results indicate that hydraulic transport of boulders with intermediate diameters <1m could have occurred within limited reaches of the palaeoriver, more specifically in the palaeo-Amblève tributary; whereas the small slope gradient of the palaeo-Meuse most probably inhibited boulder movement by hydraulic forces only. Although calculations of the ice volume required to lift a boulder to the water surface and comparison of the ice floe's dimensions with palaeochannel morphology do suggest that ice rafting is theoretically possible, several alternative, more probable transport mechanisms for the larger boulders of the Campine Plateau are proposed, requiring much smaller critical ice volumes and water depths than ice-rafting processes or purely hydraulic transport. These hypotheses include decreased bed friction and effective boulder density caused by a limited ice layer attached to the river bed and the boulder, hence lowering hydraulic transport thresholds, as well as the formation of ice jams and dams inducing catastrophic flooding and mechanical impacts by ice floes upon failure.

Sediment transport dynamics on an ice-covered lake: the ‘floating’ boulders of Lake Hoare, Antarctica

AbstractBetween 1995 and 2011 a global positioning system survey of 13 boulders and three ablation stakes (long stakes frozen in the ice) on the frozen surface of Lake Hoare was undertaken. Data interpretation illustrates complexities of post-depositional transport dynamics of boulders. Earlier studies on comparable datasets have suggested linear ‘conveyor’ type transport mechanisms for lake surface boulders. Yet explanations for non-linear boulder displacements or ‘walks’ and the mechanisms responsible for movements are inadequate. Two modes of boulder specific movement were observed. First, localized changes in the ice surface promote individual boulder movement (rolling). Second, ice rafting, which indicates the displacement of ‘plates’ of lake ice on which the boulder is located. Ablation stakes used as fixed survey control points support the hypothesis that ice cover moves as discrete plates rather than as a single homogenous mass. Factors that create the conditions to generate either of the two modes of movement may be related to location specific energy budgets. A relationship between average orientations and prevailing wind direction was also observed. The investigation describes the local-scale behaviour of surveyed boulders, and offers methodologies and interpretive frameworks for additional studies of modern and ancient sediment transportation dynamics in Antarctic lacustrine environments.

Multi-day anchor ice cycles and bedload transport in a gravel-bed stream

Summary River dynamics in cold regions and the physical processes involving ice formation and release remain relatively understudied topics. Current research suggests that anchor ice forms in diurnal cycles and has the potential to move sediments when released from the bed. Given the importance of river ice dynamics, studies are needed to describe the physical processes of anchor ice and its impact on sediment transport. The study presented in this paper investigated in situ anchor ice formation on the Stoke River in Quebec, Canada. In the fall of 2012, we observed multi-day anchor ice formation cycles and release, which usually ended with a small runoff event. During a cycle, there was little or no release of anchor ice, thus allowing its gradual growth as the air temperature remained cold. Surface and anchor ice layers also often merged, leading to the formation of thick ice masses. Sediment transport was monitored using bed particles individually tagged with passive integrated transponders (PIT-tags). Movement of the experimental particles, measured after each multi-day cycle event, ranged from 0.5 to 4 m. Possible transport mechanisms include: (1) anchor ice rafting, (2) ice jam breakup creating a jave with high erosive capacity, (3) flow corridors through anchor ice masses with high erosive capacity, (4) mechanical pushing of particles by drifting ice blocks during ice breakup, and (5) entrainment by flow or drifting ice blocks of anchored ice with strong ice-pebble bonds. Given the coincidental occurrence of anchor ice release and runoff events, it is uncertain which specific mechanism caused the particles to move. This study is a step forward in understanding the behavior of anchor ice processes and ice-related sediment transport.

Pleistocene Deep Sea ostracods from the Bering Sea (IODP expedition 323)

The study presents the first Pleistocene (0–1.9Ma) record of Deep Sea ostracods from the Bering Sea, derived primarily from Integrated Ocean Drilling Program Expedition 323, Site U1344 (59°3.0′N, 179°12.2′W, 3171m of water depth). Deep Sea ostracod abundances in the Bering Sea sediments are some of the lowest that have been recorded in bathyal and abyssal marine environments (<1 specimen per sediment gram). In comparison, benthic foraminifera are several orders of magnitude more abundant in the same samples. The humble ostracod assemblage at Site U1344 is predominantly composed of deep water species Krithe sawanensis, Fallacihowella sp. A, Cytheropteron spp., Eucytherura sp., Argilloecia toyamaensis, and Bradleya mesembrina. Less abundant taxa include Munseyella melzeri, Munseyella ristveti, Cluthia sp., Robertsonites hanaii, and Microcythere mediostriata. Some of these taxa (e.g. Fallacihowella sp. A, Bradleya mesembrina, Microcythere mediostriata) are reported for the first time in the North Pacific. The predominance of the genera Krithe, Fallacihowella, Cytheropteron and Argilloecia indicates cold, ventilated bottom waters. The deep Bering Sea ostracod assemblage shares many common and closely related species with continental slope faunas from the Gulf of Alaska, the Okhotsk Sea, the Arctic Ocean, and even the subpolar North Atlantic. A few continental shelf ostracods, such as species of Munseyella and Robertsonites, are present at Sites U1344 and U1343, in the northern slope of the Aleutian Basin. The presence of shallow water ostracods at the Bering Sea slope sites is possibly explained by sea ice rafting. Exceptionally low ostracod abundance in the U1344 record did not permit evaluating links between ostracod faunas and paleoceanographic conditions; however, an increase in ostracod occurrences throughout the middle Pleistocene at Site U1344 appears to correlate with general sea ice expansion in the Bering Sea. High primary surface productivity, high food flux and high sedimentation rates are considered to be the main factors diluting or suppressing Deep Sea ostracods in the Bering Sea, suggesting that ostracods may prefer living in more oligotrophic deep water environments with well oxygenated waters and moderate food supply.

Late quaternary climate changes around the elephant islands, Antarctic Peninsula

Sixty-seven species of diatoms of 28 genera were identified in Core GC03-C2 acquired from the north slope of Elephant Island, Antarctic Peninsula. The number of diatom valves per gram of dry sediment ranged from 0.2∼17.3×107g−1, and these were dominated by Fragilariopsis kerguelensis (65.8%). Diatom assemblage analysis reconstructed the Quaternary paleoclimatic change the Elephant Islands. Four diatom assemblage zones were identified according to the frequency of critical taxa as follows: zone I, from 830 to 710 cm (Antarctic Cold Reversal); zone II, from 700 to 550 cm (Deglaciation zone); zone III, from 540 to 260 cm (warm period; Holocene); and zone IV, from 250 to 0 cm (cool period; Holocene). The high abundance of reworked species includes Actinocyclus ingens, Denticulopsis hustedtii, D. praedimorpha, and D. dimorpha appeared in Zone I by turbidity currents and ice rafting in the area during the glaciations-deglaciation event.

Bolla Bollana boulder beds: A Neoproterozoic trough mouth fan in South Australia?

AbstractThe Bolla Bollana Formation is an exceptionally thick (ca 1500 m), rift‐related sedimentary succession cropping out in the northern Flinders Ranges, South Australia, which was deposited during the Sturtian (mid Cryogenian) glaciation. Lithofacies analysis reveals three distinct facies associations which chart changing depositional styles on an ice‐sourced subaqueous fan system. The diamictite facies association is dominant, and comprises both massive and stratified varieties with a range of clast compositions and textures, arranged into thick beds (1 to 20 m), representing stacked, ice‐proximal glaciogenic debris‐flow deposits. A channel belt facies association, most commonly consisting of normally graded conglomerates and sandstones, displays scour and fill structure of ca 10 m width and 1 to 3 m depth: these strata are interpreted as channelized turbidites. Rare mud‐filled channels in this facies association bear glacially striated lonestones. Finally, a sheet heterolithics facies association contains a range of conglomerates through sandstones to silty shales arranged into clear, normally graded cycles from the lamina to bed scale. These record a variety of non‐channelized turbidites, probably occupying distal and/or inter‐channel locations on the subaqueous fan. Coarsening and thickening‐up cycles, capped by dolomicrites or mudstones, are indicative of lobe build out and abandonment, potentially as a result of ice lobe advance and stagnation. Dropstones, recognized by downwarped and punctured laminae beneath pebbles to boulders in shale, or in delicate climbing ripple cross‐laminated siltstones, are clearly indicative of ice rafting. The co‐occurrence of ice‐rafted debris and striated lonestones strongly supports a glaciogenic sediment source for the diamictites. Comparison to Pleistocene analogues enables an interpretation as a trough mouth fan, most probably deposited leeward of a palaeo‐ice stream. Beyond emphasizing the highly dynamic nature of Sturtian ice sheets, these interpretations testify to the oldest trough mouth fan recorded to date.

Sedimentology, ichnology, ecology and anthropogenic modification of muddy tidal flats in a cold-temperate environment: Chignecto Bay, Canada

Abstract In Chignecto Bay, upper Bay of Fundy, Canada, the muddy tidal flats exhibit distinctive sedimentological and ichnological characteristics indicative of winter conditions and the development of ice. From late spring to autumn (May–October: mean temperature +13.6 °C), the mud flats sustain a high infaunal biomass and sediment deposition is dominated by tidal processes. Neap–spring tidal rhythmites, fluid-mud deposition and high levels of bioturbation are all characteristic of summer deposits. During the winter, temperatures remain below zero (December–March: mean temperature −6.3 °C), and ice forms in the bay and periodically on the mud flats. Ice rafts and blocks are common on the tidal-flat surface, and these blocks deform the muddy sediment, cut deep scours and deposit allochthonous sediment (including gravel) across the flats. Intermittent storms in the autumn and winter also contribute to sediment scouring and erosion. Annual die-offs of infauna are reflected by reduced bioturbation in winter deposits. Each spring, renewed larval recruitment and opportunistic colonization results in increasing levels of bioturbation, and the trace suite is dominated by a low-diversity assemblage of diminutive, vertical burrows. The summer–winter cyclicity in infaunal colonization manifests, ichnologically, as a distinctive bioturbated–non-bioturbated bedset character. Infaunal populations in Chignecto Bay peak in the late spring and in the late summer–early autumn. Both population peaks are exploited by vertebrates, including migrating Atlantic sturgeon (late spring) and migratory shorebirds (late summer–early autumn). In particular, the upper Bay of Fundy, including Chignecto Bay, is a staging ground for approximately 42–74% of the world’s Semipalmated sandpipers ( Calidris pusilla ) on their annual migration from the Arctic to South America. Fortunately, anthropogenic modification of Chignecto Bay, and its associated bays and rivers (e.g. construction and dismantling of a causeway over the Petitcodiac River), has not had a significant effect, to date, on the mud-flat habitat or the infaunal biomass.

Magnetic signatures of Heinrich-like detrital layers in the Quaternary of the North Atlantic

Magnetic parameters are useful for distinguishing North Atlantic Heinrich-like detrital layers from background sediments. Here we compare magnetic properties with XRF scanning data back to 700ka and 1.3Ma at IODP Sites U1302–U1303 and U1308, respectively. Multi-domain magnetite, with grain sizes >20µm, is characteristic of both Ca- and Si-rich detrital layers, as defined by XRF core scanning, confirming the contribution of ice rafting. Reflectance spectra and magnetic parameters distinguish Ca- and Si-rich IRD layers due the presence of high coercivity hematite in Si-rich layers. Heinrich layer 6 (H6) at Site U1302–U1303 is unlike other detrital layers, being marked by a 45-cm thick homogeneous cream-colored clay layer underlain by a thin (5-cm) graded coarse-sand. Comparison of Site U1302/03 and Site U1308 detrital layers implies a dominant Laurentide source for both Ca- and Si-rich detrital layers. At Site U1308, low benthic δ13C values during stadials are in-step with magnetic grain-size coarsening associated with Si-rich detrital layers back to 1.3Ma, indicating a link between deep-sea ventilation and ice rafting. The surface-sediment tan-colored oxic layer (~2m thick at Site U1308) yields magnetic hysteresis ratios that are offset from the single-domain to multi-domain (SD–MD) magnetite mixing-line in hysteresis-ratio diagrams. This offset is attributed to maghemite grain-coatings, that form on magnetite in surface sediment, and undergo dissolution as they pass through the oxic/anoxic boundary.

Reprint of: Reconstruction of inflow of Atlantic Water to Isfjorden, Svalbard during the Holocene: Correlation to climate and seasonality

The distribution patterns of benthic foraminifera faunas, stable isotopes and ice rafted debris (IRD) have been studied in piston core JM98-845-PC from Isfjorden, western Svalbard to reconstruct changes in the flow of Atlantic Water during the Holocene interglacial. The paleoenvironmental conditions in Isfjorden and the inflow of the Atlantic Water followed closely the changes in insolation with strong seasonality in the early Holocene and weaker seasonality in the middle to late Holocene. A summer temperature maximum of the bottom water occurred in the early Holocene 10,500–c. 8200 years BP. Cooling began at 9000 years BP with stepwise coolings at c. 8200, 7400 and 4000 years BP. The cooling correlated with an increase in ice rafting and increased influence of polar water and seasonal sea ice cover. Between 4000 and 2000 years BP bottom water temperatures reached a minimum. After 2000 years BP episodic inflow of warmer Atlantic Water at the bottom of the fjord occurred and conditions became more unstable. The data indicate close interaction of the flow of Atlantic Water with the development of the terrestrial climate and the amount of polar meltwater and sea ice.