Riko Noormets

The International Bathymetric Chart of the Arctic Ocean (IBCAO) Version 3.0

The International Bathymetric Chart of the Arctic Ocean (IBCAO) released its first gridded bathymetric compilation in 1999. The IBCAO bathymetric portrayals have since supported a wide range of Arc ...

Sedimentology of rocky shorelines: 3.: Hydrodynamics of megaclast emplacement and transport on a shore platform, Oahu, Hawaii

Abstract Megaclasts of well cemented coralgal limestone have been emplaced onto a shore platform by large waves on the North Shore of Oahu, HI. Emplacement and movements of the largest, ca. 96 ton megaclast have been dated using aerial photographs. Hydrodynamic forces at the low submerged shoreline cliff are computed using design wave characteristics based on linear wave theory and experimental results, considering the local wave climate and near-shore bottom topography. Force exerted by a tsunami has been estimated based on the 1946 Aleutian Tsunami. The computed forces are evaluated in terms of the initial fracturing along a given failure plane that is required in order for the detachment to occur. The analysis shows that tsunami, as well as large swell waves, are capable of quarrying the megaclast, provided that sufficient initial fracturing is present. Dislodgement and emplacement most likely occurred in a sequence during impact of a single wave on the shoreline cliff. Swell waves, however, are seldom capable of emplacing large blocks onto the platform due to their rapid disintegration after breaking, so that most blocks quarried from the cliff edge fall back onto the submarine terrace. Emplacement of large boulders seems to require extreme sea waves with periods longer than storms or North Pacific swells usually produce. Transport mechanisms on the shore platform vary, depending on megaclast shape. Sliding is a common mechanism of transport for larger and irregular megaclasts, whereas somewhat smaller and platy megaclasts are occasionally found in overturned positions.

An extensive and dynamic ice sheet on the West Greenland shelf during the last glacial cycle

Considerable uncertainty surrounds the extent and timing of the advance and retreat of the Greenland Ice Sheet (GIS) on the continental shelf bordering Baffin Bay during the last glacial cycle. Here we use marine geophysical and geological data to show that fast-flowing ice sheet outlets, including the ancestral Jakobshavn Isbrae, expanded several hundred kilometers to the shelf edge during the last glaciation ca. 20 ka. Retreat of these outlets was asynchronous. Initial retreat from the shelf edge was underway by 14,880 calibrated (cal) yr B.P. in Uummannaq trough. Radiocarbon dates from the adjacent Disko trough and adjoining trough-mouth fan imply later deglaciation of Jakobshavn Isbrae, and, significantly, an extensive readvance and rapid retreat of this outlet during the Younger Dryas stadial (YD). This is notable because it is the first evidence of a major advance of the GIS during the YD on the West Greenland shelf, although the short duration suggests that it may have been out of phase with YD temperatures.

Past ice-sheet flow east of Svalbard inferred from streamlined subglacial landforms

The pattern of late Weichselian (ca. 20 ka) ice flow in the northern Barents Sea is not well known, due mainly to a lack of marine data east of Svalbard. Several years with little summer sea ice have allowed acquisition of swath-bathymetric imagery of well-preserved subglacial landforms characterizing late Weichselian ice-flow directions over ∼150,000 km2 of the northwestern Barents Sea. We show that a major ice dome was located on easternmost Spitsbergen or southern Hinlopen Strait, at least 500 km west of its previously inferred position in the northern Barents Sea. This dome controlled the regional flow pattern; ice flowed eastward around Kong Karls Land into Franz Victoria Trough and north through Hinlopen Strait. An ice dome west of Kong Karls Land is required to explain the observed ice-flow pattern, but does not preclude an additional ice dome to the southeast. Discrepancies with earlier ice-sheet reconstructions reflect the lack of previous seafloor observations, with evidence limited mainly to past ice loading and postglacial rebound. The new pattern of ice-flow directions shows predominantly eastward rather than northward flow, with Franz Victoria Trough a major drainage pathway with a full-glacial balance flux of >40 km3 yr−1.

Evidence for an ice shelf covering the central Arctic Ocean during the penultimate glaciation

The hypothesis of a km-thick ice shelf covering the entire Arctic Ocean during peak glacial conditions was proposed nearly half a century ago. Floating ice shelves preserve few direct traces after their disappearance, making reconstructions difficult. Seafloor imprints of ice shelves should, however, exist where ice grounded along their flow paths. Here we present new evidence of ice-shelf groundings on bathymetric highs in the central Arctic Ocean, resurrecting the concept of an ice shelf extending over the entire central Arctic Ocean during at least one previous ice age. New and previously mapped glacial landforms together reveal flow of a spatially coherent, in some regions >1-km thick, central Arctic Ocean ice shelf dated to marine isotope stage 6 (∼140 ka). Bathymetric highs were likely critical in the ice-shelf development by acting as pinning points where stabilizing ice rises formed, thereby providing sufficient back stress to allow ice shelf thickening.

Geophysical constraints on the dynamics and retreat of the Barents Sea ice sheet as a paleobenchmark for models of marine ice sheet deglaciation

Our understanding of processes relating to the retreat of marine-based ice sheets, such as the West Antarctic Ice Sheet and tidewater-terminating glaciers in Greenland today, is still limited. In particular, the role of ice stream instabilities and oceanographic dynamics in driving their collapse are poorly constrained beyond observational timescales. Over numerous glaciations during the Quaternary, a marine-based ice sheet has waxed and waned over the Barents Sea continental shelf, characterized by a number of ice streams that extended to the shelf edge and subsequently collapsed during periods of climate and ocean warming. Increasing availability of offshore and onshore geophysical data over the last decade has significantly enhanced our knowledge of the pattern and timing of retreat of this Barents Sea ice sheet (BSIS), particularly so from its Late Weichselian maximum extent. We present a review of existing geophysical constraints that detail the dynamic evolution of the BSIS through the last glacial cycle, providing numerical modelers and geophysical workers with a benchmark data set with which to tune ice sheet reconstructions and explore ice sheet sensitivities and drivers of dynamic behavior. Although constraining data are generally spatially sporadic across the Barents and Kara Seas, behaviors such as ice sheet thinning, major ice divide migration, asynchronous and rapid flow switching, and ice stream collapses are all evident. Further investigation into the drivers and mechanisms of such dynamics within this unique paleo-analogue is seen as a key priority for advancing our understanding of marine-based ice sheet deglaciations, both in the deep past and in the short-term future.

Massive remobilization of permafrost carbon during post-glacial warming

Recent hypotheses, based on atmospheric records and models, suggest that permafrost carbon (PF-C) accumulated during the last glaciation may have been an important source for the atmospheric CO2 rise during post-glacial warming. However, direct physical indications for such PF-C release have so far been absent. Here we use the Laptev Sea (Arctic Ocean) as an archive to investigate PF-C destabilization during the last glacial–interglacial period. Our results show evidence for massive supply of PF-C from Siberian soils as a result of severe active layer deepening in response to the warming. Thawing of PF-C must also have brought about an enhanced organic matter respiration and, thus, these findings suggest that PF-C may indeed have been an important source of CO2 across the extensive permafrost domain. The results challenge current paradigms on the post-glacial CO2 rise and, at the same time, serve as a harbinger for possible consequences of the present-day warming of PF-C soils.

Debris entrainment and landform genesis during tidewater glacier surges

The englacial entrainment of basal debris during surges presents an opportunity to investigate processes acting at the glacier bed. The subsequent melt-out of debris-rich englacial structures during the quiescent phase produces geometrical ridge networks on glacier forelands that are diagnostic of surge activity. We investigate the link between debris entrainment and proglacial geomorphology by analyzing basal ice, englacial structures, and ridge networks exposed at the margins of Tunabreen, a tidewater surge-type glacier in Svalbard. The basal ice facies display clear evidence for brittle and ductile tectonic deformation, resulting in overall thickening of the basal ice sequence. The formation of debris-poor dispersed facies ice is the result of strain-induced metamorphism of meteoric ice near the bed. Debris-rich englacial structures display a variety of characteristics and morphologies and are interpreted to represent the incorporation and elevation of subglacial till via the squeezing of till into basal crevasses and hydrofracture exploitation of thrust faults, reoriented crevasse squeezes, and preexisting fractures. These structures are observed to melt-out and form embryonic geometrical ridge networks at the base of a terrestrially grounded ice cliff. Ridge networks are also located at the terrestrial margins of Tunabreen, neighboring Von Postbreen, and in a submarine position within Tempelfjorden. Analysis of network characteristics allows these ridges to be linked to different formational mechanisms of their parent debris-rich englacial structures. This in turn provides an insight into variations in the dominant tectonic stress regimes acting across the glacier during surges.

Marginal Fluctuations of a Svalbard Surge-Type Tidewater Glacier, Blomstrandbreen, Since the Little Ice Age: A Record of Three Surges

ABSTRACT Previous advances and retreats of Blomstrandbreen within the cold period known as the Little Ice Age, between approximately 1400 and 1920, are relatively well documented. The seafloor characteristics associated with these glacier fluctuations, and their importance for the identification of similar surge-type tidewater glaciers, are discussed. We use detailed multibeam-bathymetric data acquired within Nordvågen, the marine area offshore of Blomstrandbreen, to provide a new understanding of the style and pattern of deglaciation around Blomstrandhalvøya since Blomstrandbreens neoglacial maximum. Glacial landforms on the seafloor of Nordvågen comprise overridden moraines, glacial lineations, terminal moraines, and annual recessional moraines. Crevasse-fill ridges, which are often regarded as a characteristic landform of surging tidewater glaciers, are present on only restricted areas of Nordvågen. Significantly, this study shows that large terminal surge moraines and numerous crevasse-fill ridges may not always be well developed in association with glacier surges, with implications for the identification of surges in the geological record. Using historical observations, aerial photographs, and satellite imagery of Blomstrandbreen, we have correlated former ice-marginal positions with mapped submarine landforms. Three surge events occurred during a pattern of overall retreat, with a spacing of about 50 years between active advance phases; this represents a relatively short quiescent phase for Svalbard glaciers. Average retreat rates of 10–50 m yr-1 are typical of the quiescent phase of the surge cycle, whereas surge advances vary from 200 m to over 725 m.

Bedrock geology and topography of the Lake Peipsi depression, eastern Estonia

Abstract Seismic reflection profiling in the western part of Lake Peipsi, eastern Estonia, revealed two major units in the sedimentary bedrock, namely Devonian terrigenous rocks on top of Ordovician carbonate rocks. In the Ordovician sequence several subhorizontal reflectors and subvertical faults were recorded. The Ordovician-Devonian contact carries signs of erosion and has an inclination towards the south to southeast by 2-2.2 m per km in the northern and central parts and 4.5- 4.9 m per km in the southern part of the lake. The thickness of the Devonian terrigenous rocks increases rather abruptly in the southern part of the lake, approximately within the same zone where the inclination of the Ordovician-Devonian contact increases. Gently sloping depressions and elevations were distinguished at the bedrock surface, which dips towards the center of the lake. A buried valley in Devonian sedimentary bedrock was found in the northwestern part of the lake. The magnitude of the glacial erosion in the central part of the depression, as related to the altitude of the bedrock surface in the surrounding onshore areas, is estimated to at least 50-60 m. The glacial erosion has not been compensated by subsequent glacial accumulation and late- to postglacial lake sedimentation. Because of this, the main features of the bedrock topography are still traceable at the till surface as well as in the lake bottom topography.