Monica Winsborrow

Late Pliocene–Pleistocene development of the Barents Sea Ice Sheet

The late Pliocene–Pleistocene paleoenvironment has been reconstructed based on three-dimensional seismic data from the southwestern Barents Sea continental margin. During the late Pliocene–early Pleistocene, continental slope sediments were predominantly deposited from meltwater overflows and underflows. The seismic stratigraphy of the early–middle Pleistocene shows both glaciomarine sediment input from channelized meltwater discharge and the first indications of large debris-flow deposits on the continental slope, originating from input of subglacial deformation till eroded and transported by ice streams. During the middle–late Pleistocene, large debris flows dominated the slope succession. From the above results we infer the following evolution in the Barents Sea: (1) a temperate Barents Sea Ice Sheet with channelized meltwater flow developed during the late Pliocene–early Pleistocene; (2) alternating glacial periods of ice with channelized meltwater flow and the first periods of ice, including ice streams, characterized the early and middle Pleistocene; and (3) more polar ice conditions and a Barents Sea Ice Sheet that mainly included large ice streams, with little or no channelized meltwater flow, occurred in the middle and late Pleistocene.

Signature of ice streaming in Bjørnøyrenna, Polar North Atlantic, through the Pleistocene and implications for ice-stream dynamics

Abstract The geomorphology of palaeo-ice-stream beds and the internal structure of underlying tills can provide important information about the subglacial conditions during periods of fast flow and quiescence. This paper presents observations from three-dimensional seismic data, revealing the geomorphology of buried beds of the Bjørnøyrenna (Bear Island Trough) ice stream, the main drainage outlet of the former Barents Sea ice sheet. Repeated changes in ice dynamics are inferred from the observed successions of geomorphic features. Megablocks, aligned in long chains parallel to inferred ice-stream flowlines, and forming dipping plates that are thrust one on top of another, are taken as evidence for conditions of compressive ice flow. Mega-scale glacial lineations (MSGL) and pull-apart of underlying sediment blocks suggest extensional flow. The observed pattern of megablocks and rafts overprinted by MSGL indicates a change in ice dynamics from a compressional to an extensional flow regime. Till stiffening, due to subglacial freezing, is the favoured mechanism for creating switches in sub-ice-stream conditions. The observed pattern of geomorphic features indicates that periods of slowdown or quiescence were commonly followed by reactivation and fast flow during several glaciations, suggesting that this may be a common behaviour of marine ice streams.

Ice-stream flow switching during deglaciation of the southwestern Barents Sea

Ice streams dominate the discharge of continental ice sheets. Recent observations and reconstructions have revealed that large-scale reorganizations in their flow trajectory (flow switching) can occur over relatively short time scales. However, the underlying causes of such behavior, and the extent to which they are predictable, are poorly known. This paper documents a major episode of ice-stream flow switching during the late Weichselian deglaciation of the southwestern Barents Sea and explores various hypotheses for its causation. Regional bathymetric data show that two ice streams that had similar, adjoining, topographically constrained source areas had very different trajectories and dynamics on the outer shelf. At the late Weichselian maximum, the Hakjerringdjupet ice stream flowed westward along the cross-shelf trough of Hakjerringdjupet, while the Soroya Trough ice stream flowed northward into Ingoydjupet, forming a tributary of the Bjornoyrenna ice stream. Initial retreat of the Hakjerringdjupet ice stream was rapid but with episodic periods of grounding. As it retreated onto the higher, rougher topography of the inner shelf, we infer a reduction in ice velocity and a dramatic decrease in the pace of retreat, as recorded by nested sequences of recessional moraines. Following (and probably in response to) this, we suggest that there was a short-lived surge/readvance of an adjacent lobe onto Fugloybanken. In contrast, the adjacent Soroya Trough ice stream remained active throughout deglaciation, before retreating rapidly, with no stillstands or readvances. We argue that the different retreat histories of the ice streams were determined by variations in bed topography/bathymetry, which modulated the grounding line response to sea-level variation. Such a mechanism is likely to be an important control on the long-term behavior of marine-based ice streams and outlet glaciers in Antarctica and Greenland and suggests that gathering data on their subglacial topography should be a priority.

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.

Asynchronous response of marine-terminating outlet glaciers during deglaciation of the Fennoscandian Ice Sheet

This is the accepted manuscript version. Published version is available at http://dx.doi.org/10.1130/G35299.1

Massive blow-out craters formed by hydrate-controlled methane expulsion from the Arctic seafloor

Massive methane blow-outs may be responsible for clusters of kilometer-wide craters in the Barents Sea. Methane takes the quick way out Accounting for all the sources and sinks of methane is important for determining its concentration in the atmosphere. Andreassen et al. found evidence of large craters embedded within methane-leaking subglacial sediments in the Barents Sea, Norway. They propose that the thinning of the ice sheet at the end of recent glacial cycles decreased the pressure on pockets of hydrates buried in the seafloor, resulting in explosive blow-outs. This created the giant craters and released large quantities of methane into the water above. Science, this issue p. 948 Widespread methane release from thawing Arctic gas hydrates is a major concern, yet the processes, sources, and fluxes involved remain unconstrained. We present geophysical data documenting a cluster of kilometer-wide craters and mounds from the Barents Sea floor associated with large-scale methane expulsion. Combined with ice sheet/gas hydrate modeling, our results indicate that during glaciation, natural gas migrated from underlying hydrocarbon reservoirs and was sequestered extensively as subglacial gas hydrates. Upon ice sheet retreat, methane from this hydrate reservoir concentrated in massive mounds before being abruptly released to form craters. We propose that these processes were likely widespread across past glaciated petroleum provinces and that they also provide an analog for the potential future destabilization of subglacial gas hydrate reservoirs beneath contemporary ice sheets.

Large subglacial meltwater features in the central Barents Sea

During the last glacial period large parts of the Arctic, including the Barents Sea, north of Norway and Russia, were covered by ice sheets. Despite several studies indicating that melting occurred beneath much of the Barents Sea ice sheet, very few meltwater-related landforms have been identified. We document ∼200 seafloor valleys in the central Barents Sea and interpret them to be tunnel valleys formed by meltwater erosion beneath an ice sheet. This is the first account of widespread networks of tunnel valleys in the Barents Sea, and confirms previous predictions that large parts of the ice sheet were warm based. The tunnel valleys are interpreted to be formed through a combination of steady-state drainage and outburst floods close to the ice margin, as a result of increased melting within a period of rapid climate warming during late deglaciation. This is the first study documenting widespread tunnel valley formation at the northern reaches of a Northern Hemisphere paleo–ice sheet, during advanced deglaciation and beneath a much reduced ice sheet. This indicates that suitable conditions for tunnel valley formation may have occurred more widely than previously reported, and emphasizes the need to properly incorporate hydrological processes in current efforts to model ice sheet response to climate warming. This study provides valuable empirical data, to which modeling results can be compared.

Retreat patterns and dynamics of the former Bear Island Trough Ice Stream

Covering one of the widest continental shelves in the world, the epicontinental Barents Sea is characterized by several shallow banks separated by troughs that open towards the Norwegian Sea in the west and the Arctic Ocean in the north (Fig. 1a). The bank areas have typical water depths of 100–200 m and the troughs 300–500 m. The most prominent cross-shelf trough, the Bear Island Trough (Bjornoyrenna), extends over 750 km from Storbanken (‘the large bank’ in Norwegian) in the NE to the shelf break in the SW (Fig. 1a). It is 150–200 km wide and spans water depths of 300–500 m. Ice sheet reconstructions over the last decades have recognized that a major ice sheet covered the whole Barents Sea during the Last Glacial Maximum (LGM; Mangerud et al. 1992; Svendsen et al. 2004). Large trough-mouth fans (TMFs; Vorren et al. 1989) appear as seaward-convex bulges in the bathymetry at the mouth of troughs that extend to the shelf break. The largest of these, the Bear Island Trough-Mouth Fan (Fig. 1a), contains up to 3–4 km of Plio-Pleistocene glacial sediments. The location of a major ice stream in the Bear Island Trough, draining the former Barents Sea and Fennoscandian ice sheets and delivering large amounts of sediments to the Bear Island TMF during the LGM (Fig. 1a), has been inferred from seafloor geomorphology and palaeo-ice-sheet geometry (Denton & Hughes 1981; Solheim et al. 1990; Ottesen et al. 2005; Andreassen et al. 2007), from studies of the fan itself (e.g. Vorren & Laberg 1997) and from borehole data (Saettem 1994). Two late glacial maxima have been inferred in the SW Barents Sea, one before 22 cal ka BP and one after 19 cal ka (Vorren & Laberg 1996). Fig. 1. Bathymetry, geomorphology, Last Glacial …

Shallow carbon storage in ancient buried thermokarst in the South Kara Sea

Geophysical data from the South Kara Sea reveal U-shaped erosional structures buried beneath the 50–250 m deep seafloor of the continental shelf across an area of ~32 000 km2. These structures are interpreted as thermokarst, formed in ancient yedoma terrains during Quaternary interglacial periods. Based on comparison to modern yedoma terrains, we suggest that these thermokarst features could have stored approximately 0.5 to 8 Gt carbon during past climate warmings. In the deeper parts of the South Kara Sea (>220 m water depth) the paleo thermokarst structures lie within the present day gas hydrate stability zone, with low bottom water temperatures −1.8 oC) keeping the gas hydrate system in equilibrium. These thermokarst structures and their carbon reservoirs remain stable beneath a Quaternary sediment blanket, yet are potentially sensitive to future Arctic climate changes.