Christian Hjort

Antarctic Glacial History Since the Last Glacial Maximum: An Overview of the Record on Land

This overview examines available circum-Antarctic glacial history archives on land, related to developments after the Last Glacial Maximum (LGM). It considers the glacial-stratigraphic and morphologic records and also biostratigraphical information from moss banks, lake sediments and penguin rookeries, with some reference to relevant glacial marine records. It is concluded that Holocene environmental development in Antarctica differed from that in the Northern Hemisphere. The initial deglaciation of the shelf areas surrounding Antarctica took place before 10 000 14 C yrs before present( BP ), and was controlled by rising global sea level. This was followed by the deglaciation of some presently ice-free inner shelf and land areas between 10 000 and 8000 yr BP . Continued deglaciation occurred gradually between 8000 yr BP and 5000 yr BP . Mid-Holocene glacial readvances are recorded from various sites around Antarctica. There are strong indications of a circum-Antarctic climate warmer than today 4700–2000 yr BP . The best dated records from the Antarctic Peninsula and coastal Victoria Land suggest climatic optimums there from 4000–3000 yr BP and 3600–2600 yr BP , respectively. Thereafter Neoglacial readvances are recorded. Relatively limited glacial expansions in Antarctica during the past few hundred years correlate with the Little Ice Age in the Northern Hemisphere.

Holocene glacial history and sea-level changes on James Ross Island, Antarctic Peninsula

A reconstruction of deglaciation and associated sea-level changes on northern James Ross Island, Antarctic Peninsula, based on lithostratigraphical and geomorphological studies, shows that the initial deglaciation of presently ice-free areas occurred slightly before 7400 14 C yr BP. Sea-level in connection with the deglaciation was around 30 m a.s.l. A glacier readvance in Brandy Bay, of at least 7 km, with the initial 3 km over land, reached a position off the present coast at ca. 4600 yr BP. The culmination of the advance was of short duration, and by 4300 yr BP the coastal lowlands again were ice-free. A distinct marine level at 16- 18 m a.s.l. was contemporaneous with or slightly post-dates the Brandy Bay advance, thus indicating the relative sea-level around 4600-4500 yr BP. Our results from James Ross Island confirm that over large areas in this part of Antarctica the last deglaciation occurred late.

STRATIGRAPHIC AND PALEOCLIMATIC STUDIES OF A 5500-YEAR-OLD MOSS BANK ON ELEPHANT ISLAND, ANTARCTICA

Analyses of a core from the deepest known moss peat bank in Antarctica, on Elephant Island, South Shetlands, show that this Chorisodontium aciphyllum-dominated bank began to grow ca. 5500 14C yr BP. Combined with other studies in the region the present study indicates more extensive glaciation before 5000 to 6000 BP than today on some of the South Shetland Islands. The main hypothesis is that these frozen moss banks contain important paleoclimatic information. The stratigraphic parameters analyzed included degree of humification, organic and mineral matter content, bulk density, chronology, volumetric growth and organic accumulation rates, carbon and nitrogen concentrations, C/N ratios, nitrogen accumulation rates, and finally magnetic analyses to detect tephra horizons. A discussion of the interrelationships between these parameters is followed by theoretical calculations of annual net primary productivity combined with multivariate analysis of the data set. Results of the analysis show that three calculated productivity peaks coincide with three periods of milder and more humid summers, at 4150-3900, 3180-3030, and 2030-1840 BP. However, the period with possibly the warmest summers, 3180-3030 BP, is interpreted also to have been characterized by cold winters. The data suggest that the periods with the coldest summers (and possibly also winters) prevailed at the earliest stage of the moss bank development, at ca. 3500 BP, and 2500 BP.

The north Taymyr ice-marginal zone, arctic Siberia—a preliminary overview and dating

he North Taymyr ice-marginal zone (NTZ) is a complex of glacial, glaciofluvial and glaciolacustrine deposits, laid down on the northwestern Taymyr Peninsula in northernmost Siberia, along the front of ice sheets primarily originating on the Kara Sea shelf. It was originally recognised from satellite radar images by Russian scientists; however, before the present study, it had not been investigated in any detail. The ice sheets have mainly inundated Taymyr from the northwest, and the NTZ can be followed for 700-750 km between 75 degrees N and 77 degrees N, mostly 80-100 km inland from the present Kara Sea coast. The ice-marginal zone is best developed in its central parts, ca. 100 km on each side of the Lower Taymyr River, and has there been studied by us in four areas. In two of these, the ice sheet ended on land, whereas in the two others, it mainly terminated into ice-dammed lakes. The base of the NTZ is a series of up to 100-m-high and 2-km-wide ridges, usually consisting of redeposited marine silts. These ridges are still to a large extent ice-cored; however, the present active layer rarely penetrates to the ice surface. Upon these main ridges, smaller ridges of till and glaciofluvial material are superimposed. Related to these are deltas corresponding to two generations of ice-dammed lakes, with shore levels at 120-140 m and ca. 80 m a.s.l. These glacial lakes drained southwards, opposite to the present-day pattern, via the Taymyr River valley into the Taymyr Lake basin and, from there, most probably westwards to the southern Kara Sea shelf. The basal parts of the NTZ have not been dated; however, OSL dates of glaciolacustrine deltas indicate an Early-Middle Weichselian age for at least the superimposed ridges. The youngest parts of the NTZ are derived from a thin ice sheet (less than 300 m thick near the present coast) inundating the lowlands adjacent to the lower reaches of the Taymyr River. The glacial ice from this youngest advance is buried under only ca. 0.5 m of melt-out till and is exposed by hundreds of shallow slides. This final glaciation is predated by glacially redeposited marine shells aged ca. 20,000 BP ( (super 14) C) and postdated by terrestrial plant material from ca. 11,775 and 9500 BP ( (super 14) C)-giving it a last global glacial maximum (LGM; Late Weichselian) age.

A sea correction for East Greenland

Abstract Samples of shells collected as live specimens in East Greenland around the turn of the century were dated at the Lund Radiocarbon Laboratory, in order to study their apparent ages. The mean value for dates corrected for the original isotopic fractionation (C13) was 570 years, and that for uncorrected dates was 155 years. These were rounded off to 550 and 150 years respectively and are suggested (with a negative prefix) as preliminary sea corrections for the central parts of East Greenland.

Glacial and Climate History of the Antarctic Peninsula since the Last Glacial Maximum

Abstract During the Last Glacial Maximum (LGM), ice thickened considerably and expanded toward the outer continental shelf around the Antarctic Peninsula. Deglaciation occurred between >14 ka BP and ca. 6 ka BP, when interglacial climate was established in the region. Deglaciation of some local sites was as recent as 4–3 ka BP. After a climate optimum, peaking ca. 4–3 ka BP, a distinct climate cooling occurred. It is characterized at a number of sites by expanding glaciers and ice shelves. Rapid warming during the past 50 yr may be causing instability of some Antarctic Peninsula ice shelves. Detailed reconstructions of the glacial and climatic history of the Antarctic Peninsula since LGM are hampered by scarcity of available archives, low resolution of many datasets, and problems in dating samples. Consequently, the configuration of LGM ice sheets, pattern of subsequent deglaciation, and environmental changes are poorly constrained both temporally and spatially.

Glaciation, climate history, changing marine levels and the evolution of the Northeast Water polynya

Abstract The morphology of the bank- and trough system in the Northeast Water (NEW) polynya area is to a large extent a result of glacial erosion and deposition by an extended Inland Ice which reached the continental shelf. The last deglaciation of the fjords and forelands adjacent to the polynya took place shortly before 9000 radiocarbon yr B.P. At that time the marine level stood 80 m higher than today, which meant a water depth over much of the Ob Bank and parts of the Belgica Bank twice as deep as presently. This probably gave the East Greenland Current a more unimpeded southward flow, with less eddy effects. In combination with the generally warmer climate of the early Holocene (with less ice coming out of the Polar Basin, a larger influx of Atlantic Water leading to more ice melting in the NEW area, and probably no ice drift hindering fast-ice barrier in the south), this probably meant that no polynya existed at that time. Instead there was a more general open water summer situation. After 5000 yr B.P. climate deteriorated and glaciers expanded. As the isostatic rise of land and near coast bottoms continued, resulting in shallowing banks, both the bathymetric and climatic situation responsible for the existence of todays polynya gradually came into existence. The shallowing Ob Bank began to obstruct ice drift from the north and an ice-drift hindering fast-ice barrier was created in the south. But even after 500 yr B.P. it is likely that shorter spells of more generally open water existed, during which marine based paleoeskimos (Independence II) and neoeskimos (Thule) immigrated along the at other times totally ice-bound coasts of North Greenland.

Annual bird ringing totals and population fluctuations

Ringing of migrant birds has taken place at Ottenby, Oland, southeast Sweden since 1947. The effect of the winter weather in northwestern Europe on the wren Troglodytes troglodytes, of precipitation in the Sahel zone on the common whitethroat Sylvia communis and of the use of alkyl mercury compounds in Swedish agriculture on the yellowhammer Emberiza citrinella are clearly seen in the ringing figures. These examples illustrate the usefulness of ringing statistics for migrant birds in environmental monitoring.

Holocene and pre-Holocene temporary disappearance of the George VI ice shelf, Antarctic Peninsula

We present evidence for the absence of the George VI Ice Shelf during a brief period in the mid-Holocene and during one or more earlier interstadials or interglacials. Barnacle Bathylasma corolliforme shells sampled from ice shelf moraines at Two Step Cliffs on Alexander Island have been dated to c, 5750–6000 14C yr BP(c. 6550–6850 cal yr BP) and imply seasonally open water in the George VI Sound during this period. Other shells are beyond the range of radiocarbon dating and imply open water during one or more previous interglacial or interstadial period, prior to 40 000 14C yr BP. Our results show that the ongoing collapse of some Antarctic Peninsula ice shelves is not unprecedented.

A re-evaluated glacial chronology for northern East Greenland

Abstract The combination of geomorphological, lithostratigraphical and pollen-analytical data from southern Hochstetter Forland and Shannon o. northern East Greenland, together with 14 C datings and a time/temperature evaluation of amino-acid racemization data, leads to a re-evaluation of the glacial chronology of that area. The most extensive Weichselian glaciation probably culminated not long before 15 000 BP, during the Nanok I stadial. This was a fairly limited glacial advance, compared to older recognized stages represented by older sediments and more weathered landforms. During Nanok I sizeable ice-free lowland areas existed and there was a number of nunataks in the western mountains. Initial deglaciation of the southern rim of Hochstetter Forland took place well before 13000 BP, during the Artemisia interstadial characterized by a pioneer type vegetation. At ca. 10 100 BP the glaciers advanced again, reaching almost the same frontal positions as during Nanok I time. This stadial, Nanok II, ended wi...