Description: Ice supersaturation (ISS) is the prerequisite condition for cirrus cloud formation. To examine multi-scale dynamics’ influences on ISS formation, we analyze in-situ aircraft observations (~200 m scale) over North America in coordinates relative to dynamical boundaries in the upper troposphere and lower stratosphere (UT/LS). Two case studies demonstrate that ISS formation is likely influenced by mesoscale uplifting, small-scale waves and turbulence. A collective analysis of 15 flights in April – June 2008 shows that the top layers of ISS and ice crystal distributions are strongly associated with thermal tropopause height. In addition, the occurrence frequencies of ISS and ice crystals on the anti-cyclonic side of the jet stream are ~1.5–2 times of those on the cyclonic side. By defining five cirrus evolution phases based on the spatial relationships between ice-supersaturated and in-cloud regions, we find that their peak occurrence frequencies are located at decreasing altitudes with respect to the thermal tropopause: (phase 1) clear-sky ISS around the tropopause, (phase 2) nucleation phase around 2 km below the tropopause, (phases 3 and 4) early and later growth phases around 6 km below the tropopause, and (phase 5) sedimentation/sublimation around 2–6 km below the tropopause. Consistent with this result, chemical tracer correlation analysis shows that the majority (~80%) of the earlier cirrus phases (clear-sky ISS and nucleation) occur inside the chemical tropopause transition layer, while the later phases happen mostly below that layer. These results shed light on the role of dynamical environment in facilitating cirrus cloud formation and evolution.

Description: Based on swath bathymetry, sediment echosounding, seismic profiling and sediment coring we present results of the RV „Polarstern“ cruise ARK-XIII/3 (2008) and RV "Araon" cruise ARA02B (2012), which investigated an area between the Chukchi Borderland and the East Siberian Sea between 165°W and 170°E. At the southern end of the Mendeleev Ridge, close to the Chukchi and East Siberian shelves, evidence is found for the existence of Pleistocene ice sheets/ice shelves, which have grounded several times in up to 1200 m present water depth. We found mega-scale glacial lineations associated with deposition of glaciogenic wedges and debris-flow deposits indicative of sub-glacial erosion and deposition close to the former grounding lines. Glacially lineated areas are associated with large-scale erosion, accentuated by a conspicuous truncation of pre-glacial strata typically capped with mostly thin layers of diamicton draped by pelagic sediments. Our tentative age model suggests that the youngest and shallowest grounding event of an ice sheet should be within Marine Isotope Stage (MIS) 3. The oldest and deepest event predates MIS 6. According to our results, ice sheets of more than one km in thickness continued onto, and likely centered over, the East Siberian Shelf. They were possibly linked to previously suggested ice sheets on the Chukchi Borderland and the New Siberian Islands. We propose that the ice sheets extended northward as thick ice shelves, which grounded on the Mendeleev Ridge to an area up to 78°N within MIS 5 and/or earlier. These results have important implication for the former distribution of thick ice masses in the Arctic Ocean during the Pleistocene. They are relevant for global sea-level variations, albedo, ocean-atmosphere heat exchange, freshwater export from the Arctic Ocean at glacial terminations and the formation of submarine permafrost. The existence of km-thick Pleistocene ice sheets in the western Arctic Ocean during glacial times predating that of the Last Glacial Maximum (LGM) also implies significantly different atmospheric circulation patterns, in particular availability and distribution of moisture during pre-LGM glaciations.

Description: Recently glacial landforms were presented and interpreted as complex pattern of Pleistocene glaciations in the western Arctic Ocean along the continental margin of the East Siberian and Chukchi seas, (Niessen et al. 2013, Dove et al. 2014). These landforms include moraines, drumlins, glacigenic debris flows, till wedges and mega-scale glacial lineations. Orientations of some of the landforms suggest the presence of former ice sheets on the Chukchi Borderland and the East Siberian shelf. Here we present a tentative age model for some of the younger glacial events by correlation of sediment cores with glacial landforms as seen in subbottom profiles. The database was obtained during RV „Polarstern“ cruise ARK-XIII/3 (2008) and RV "Araon" cruise ARA03B (2012), which investigated an area between the Chukchi Borderland and the East Siberian Sea between 165°W and 170°E. The stratigraphic correlation of sediment cores is based on physical properties (wet-bulk density and magnetic susceptibility), lithology and color. The chronology of the area has been proposed by Stein et al. (2010)for a core from the Chukchi Abyssal Plain (PS72/340-5) and includes brown layers B1 to B9 (marine isotope stages MIS 1 to MIS 7), which are used as marker horizons for lateral core correlation. Our tentative age model suggests that the youngest and shallowest (480 m below present water level; mbpwl) grounding event of an ice sheet on the Chukchi Borderland is younger than B2 (interpreted as Last Glacial Maximum; LGM). There is no clear evidence for a LGM glaciation along the East Siberian margin because intensive post LGM iceberg scouring occurred above 350 m present water level. On the slopes of the East Siberian Sea two northerly directed ice advances occurred, both of which are older and younger than B2 and B3, respectively. The younger advance grounded to about 700 m present water depth along the continental slope and the older to 900 m and 1100 m on the Arlis Plateau and the East Siberian continental margin, respectively. We interpret these advances as Middle Weichselian glaciations on the Beringian shelf (MIS 4 to 3). Two older glaciations can be dated as Early Weichselian (MIS 5b to 5d), of which the younger event is older and younger than B3 and B4, respectively. This glaciation can be traced by glacial wedges, streamlined lineations in up to 1200 mbpsl and subglacial diamicton along the East Siberian margin, the Arlis Plateau, and the Mendeleev Ridge. There are at least three older glaciation visible in acoustic images from the East Siberian continental margin, which probably predate the Weichselian. The available cores did not penetrate these events and the ages remain speculative.

Description: In the western Arctic Ocean glacial landforms are interpreted as a complex pattern of Pleistocene glaciations along the continental margin of the East Siberian Sea and the Chukchi borderland. These landforms include moraines, drumlinized features, glacigenic debris flows, till wedges, mega-scale glacial lineations (MSGL), and iceberg plough marks. Orientations of some of the landforms suggest the presence of former ice sheets on the Chukchi Borderland and the East Siberian shelf. In seismic and sub-bottom profiles as well as sediment cores, there is evidence that glaciations have occurred repeatedly. Typically, several generations of glacial wedges intercalate with well-stratified (interglacial) sediments in ice-distal locations. MSGL of former ice grounding in present water depths of more than 1200 m suggests that some ice sheets developed significant thickness and size. The extent of glacial features and deposits into the Arctic Ocean decreased with time. We interpret this as indication that ice sheets in the western Arctic Ocean were thicker and larger during earlier times of the Pleistocene and became restricted to the Chukchi Borderland during the most recent glaciation (Last Glacial Maximum, LGM). Finally, icebergs intensively ploughed the sediments along the Chukchi and East Siberian margin in a range from 350m to 80m present water depth. In water depth shallower than 80m, sub-bottom profiles in the East Siberian Sea exhibit acoustic facies more typical for submarine permafrost. Discontinuous (permafrost) reflectors mask sub-bottom strata beneath an unfrozen 10m thick top sediment layer. In places, unfrozen sediment-filled depressions (taliks) are visible to about 20m below the seafloor, which may be related to former thermokarst and/or channels. We suggest that only during the LGM permafrost formed in the exposed area of the entire East Siberian Sea, whereas some areas have been largely covered by ice sheets during previous glacial periods.