Radiocarbon ages, dose rates, water isotopic composition and hydrochemistry of samples from Komakuk Beach and Herschel Island

Terrestrial permafrost archives along the Yukon Coastal Plain (northwest Canada) have recorded landscape development and environmental change since the Late Wisconsinan at the interface of unglaciated Beringia (i.e. Komakuk Beach) and the northwestern limit of the Laurentide Ice Sheet (i.e. Herschel Island). The objective of this paper is to compare the late glacial and Holocene landscape development on both sides of the former ice margin based on permafrost sequences and ground ice. Analyses at these sites involved a multi-proxy approach including: sedimentology, cryostratigraphy, palaeoecology of ostracods, stable water isotopes in ground ice, hydrochemistry, and AMS radiocarbon and infrared stimulated luminescence (IRSL) dating. AMS and IRSL age determinations yielded full glacial ages at Komakuk Beach that is the northeastern limit of ice-free Beringia. Herschel Island to the east marks the Late Wisconsinan limit of the northwest Laurentide Ice Sheet and is composed of ice-thrust sediments containing plant detritus as young as 16.2 cal ka BP that might provide a maximum age on ice arrival. Late Wisconsinan ice wedges with sediment-rich fillings on Herschel Island are depleted in heavy oxygen isotopes (mean d18O of -29.1 per mil); this, together with low d-excess values, indicates colder-than-modern winter temperatures and probably reduced snow depths. Grain-size distribution and fossil ostracod assemblages indicate that deglaciation of the Herschel Island ice-thrust moraine was accompanied by alluvial, proluvial, and eolian sedimentation on the adjacent unglaciated Yukon Coastal Plain until ~11 cal ka BP during a period of low glacio-eustatic sea level. The late glacial-Holocene transition was marked by higher-than-modern summer temperatures leading to permafrost degradation that began no later than 11.2 cal ka BP and caused a regional thaw unconformity. Cryostructures and ice wedges were truncated while organic matter was incorporated and soluble ions were leached in the thaw zone. Thermokarst activity led to the formation of ice-wedge casts and deposition of thermokarst lake sediments. These were subsequently covered by rapidly accumulating peat during the early Holocene Thermal Maximum. A rising permafrost table, reduced peat accumulation, and extensive ice-wedge growth resulted from climate cooling starting in the middle Holocene until the late 20th century. The reconstruction of palaeolandscape dynamics on the Yukon Coastal Plain and the eastern Beringian edge contributes to unraveling the linkages between ice sheet, ocean, and permafrost that have existed since the Late Wisconsinan.

(Table 2) Radiocarbon ages on Arctic box and multicores

Paleo-sea-ice history in the Arctic Ocean was reconstructed using the sea-ice dwelling ostracode Acetabulastoma arcticum from late Quaternary sediments from the Mendeleyev, Lomonosov, and Gakkel Ridges, the Morris Jesup Rise and the Yermak Plateau. Results suggest intermittently high levels of perennial sea ice in the central Arctic Ocean during Marine Isotope Stage (MIS) 3 (25-45 ka), minimal sea ice during the last deglacial (16-11 ka) and early Holocene thermal maximum (11-5 ka) and increasing sea ice during the mid-to-late Holocene (5-0 ka). Sediment core records from the Iceland and Rockall Plateaus show that perennial sea ice existed in these regions only during glacial intervals MIS 2, 4, and 6. These results show that sea ice exhibits complex temporal and spatial variability during different climatic regimes and that the development of modern perennial sea ice may be a relatively recent phenomenon.

Comparison between radiocarbon ages obtained on coexisting planktonic foraminifera

As a test of the reliability of paleocean ventilation rates reconstructed from radiocarbon age differences between planktonic and benthic foraminifera, measurements have been made on coexisting species of planktonic foraminifera. While ideally no differences should exist, we do find them. In this paper we discuss the possible causes for these differences and attempt to evaluate their impact on the interpretation of benthic-planktonic age differences.

(Table 2) AMS radiocarbon dates from sediment cores obtained during James Clark Ross cruise JR142, Kvitøya Trough

High-resolution geophysical and sediment core data are used to investigate the pattern and dynamics of former ice flow in Kvitøya Trough, northwestern Barents Sea. A new swath-bathymetric dataset identifies three types of submarine landform in the study area (streamlined landforms, meltwater channels and cavities, iceberg scours). Subglacially produced streamlined landforms provide a record of ice flow through Kvitøya Trough during the last glaciation. Flow directions are inferred from the orientations of streamlined landforms (drumlins, crag-and-tail features). Ice flowed northward for at least 135 km from an ice divide at the southern end of Kvitøya Trough. A large channel-cavity system incised into bedrock in the southern trough indicates that subglacial meltwater was present at the former ice-sheet base. Modest landform elongation ratios and a lack of mega-scale glacial lineations suggest that, although ice in Kvitøya Trough was melting at the bed and flowed faster than the likely thin and cold-based ice on adjacent banks, a major ice stream probably did not occupy the trough. Retreat was relatively rapid after 14-13.5 14C kyr B.P. and probably progressed via ice sheet-bed decoupling in response to rising sea level. There is little evidence for still stands during ice retreat or of ice-proximal deglacial sediments. Relict iceberg scours in present-day water depths of more than 350 m in the northern trough indicate that calving was an important mass loss mechanism during retreat.

Radiocarbon ages and last occurrences of Globorotalia menardii flexuosa and Globoquadrina hexagona in Atlantic sediment cores

Ranges of Globorotalia menardii menardii, Gr. menardii flexuosa and Globoquadrina hexagona at time of climatostratigraphic intervals representing oxygen-isotope stages 5-1 are determined by study of dozens of Late Quaternary sediment cores involving constructing paleotemperature curves from planktic foraminifera and oxygen isotope curves for their shells, as well as radiocarbon dating. Gr. menardii flexuosa and Gq. hexagona disappeared about 27-25 ky ago from the subtropical and tropical Atlantic and about 18 ky ago from the equatorial area.

(Table 2) Radiocarbon age determination from the polar planktonic foraminifera Neogloboquadrina pachyderma s for sediment core HU90-013-029

An additional Heinrich ice-rafting event is identified between Heinrich events 5 and 6 in eight cores from the Labrador Sea and the northwest Atlantic Ocean. It is characterized by sediment rich in detrital carbonate (40% CaCO3) with high concentration of floating dropstones, high coarse-fraction (% > 150 µm) content, and has a sharp contact with the underlying but grades into the overlying hemipelagic sediment. It also shows lighter d18ONpl values, indicating freshening due to iceberg rafting and/or meltwater discharge. This event is correlated with Dansgaard-Oeschger event 14 and interpreted as an additional Heinrich event, H5a. The thickness of H5a in the Labrador Sea reaches up to 220 cm. This additional Heinrich event has also been reported in cores PS2644 and SO82-5 from the northern North Atlantic. With the recognition of H5a the temporal spacing between Heinrich events 1 to 6 becomes more uniform (~7 ka).

Pollen content and radiocarbon ages of profile Delorett, Austria

A pollen profile from the highest known peatbog in the Alps is presented. The peatbog started to grow about 8000 years ago and over the last 5000 years. The influence of man on the vegetation is documented. Before the beginning of the bronze age pasturing started.