Ólafur Ingólfsson

The Preboreal oscillation around the Nordic Seas: terrestrial and lacustrine responses

The occurrence of an early Preboreal climatic cooling/oscillation (PBO) in lacustrine and glacial records from northwest Europe, Iceland and Greenland is reviewed and documented. The often subtle response of the proxy records to this oscillation, in combination with its short duration, make it difficult to detect. Owing to its chronostratigraphic position between the 10 000-9900 and 9600-9500 14 C plateaux (c. 11 300-11 150 calendar yr BP) it is also difficult to 14 C date with precision. We find that the vegetation response to the PBO varies between sites and regions. In contrast to the pioneer vegetation in Iceland and southern Sweden, the expanding birch-pine forest in Germany-Denmark was more susceptible to deteriorating growing conditions. The combined lacustrine, tree-ring and glacial records imply that the PBO was characterised by cool and humid conditions throughout northwestern and central Europe. This is documented by vegetation changes, decreased aquatic production, increased soil erosion, increased 2 H and 13 C content in tree-rings, readvances or stillstands of the ice sheet in Norway and Finland, and ingression of brackish water into the Baltic. Icelandic proxy records from lake sediments and glacial moraines imply cooler conditions than during the previous Preboreal period, but not as extreme as during the Younger Dryas. Greenland records suggest that the early Preboreal was characterised by ice readvances, as an effect of cool climate and increased precipitation (in relation to the Younger Dryas). It was not until the end of the PBO that climate was warm enough to melt the land-based ice sheet. This Preboreal oscillation, found on both sides of the Nordic Seas, is interpreted as an effect of increased freshwater forcing on the thermohaline circulation in the Nordic Seas, which is implied by a simultaneous and distinct rise in the atmospheric 14 C/ 12 C ratio. A slow-down of the thermohaline circulation may tempor- arily have pushed the Polar Front further south.

FLUCTUATIONS OF THE SVALBARD–BARENTS SEA ICE SHEET DURING THE LAST 150 000 YEARS

Abstract On Spitsbergen, western Svalbard, three major glacial advances have been identified during the Weichselian. All three reached the continental shelf west of the Svalbard archipelago. Radiocarbon, luminescence and amino acid dating of interbedded interstadial and interglacial sediments indicate that these glacial advances have Early (Isotope Stage 5d), Middle (Stage 4), and Late Weichselian ages (Stage 2). An additional, more local, advance has been dated to Isotope Stage 5b. The Late Weichselian ice sheet expanded across the entire Barents Sea. However, in the south-western Barents Sea, the Late Weichselian till is the only till above Eemian sediments, indicating that the Early- and Middle Weichselian ice advances were restricted to the Svalbard archipelago and the northern Barents Sea. A major problem with the onshore sites is the dating of events beyond the range of the radiocarbon method. To overcome this, the onshore record has been correlated with marine cores from the continental slope and the deep-sea west of Svalbard, where a chronology has been established by oxygen isotope stratigraphy. Ice rafted detritus (IRD) was used as the main monitor of glaciation. The IRD record closely mirrors the glaciation history as interpreted from the onshore sections. During the Late Weichselian, the largest IRD peak occurred during deglaciation, a pattern also postulated for the earlier events. Given this, the results from the marine cores indicate that the ages for the first glacial advances during the Weichselian were a few thousand years older than interpreted from the onshore stratigraphy.

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.

Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change

Polar bears (PBs) are superbly adapted to the extreme Arctic environment and have become emblematic of the threat to biodiversity from global climate change. Their divergence from the lower-latitude brown bear provides a textbook example of rapid evolution of distinct phenotypes. However, limited mitochondrial and nuclear DNA evidence conflicts in the timing of PB origin as well as placement of the species within versus sister to the brown bear lineage. We gathered extensive genomic sequence data from contemporary polar, brown, and American black bear samples, in addition to a 130,000- to 110,000-y old PB, to examine this problem from a genome-wide perspective. Nuclear DNA markers reflect a species tree consistent with expectation, showing polar and brown bears to be sister species. However, for the enigmatic brown bears native to Alaskas Alexander Archipelago, we estimate that not only their mitochondrial genome, but also 5–10% of their nuclear genome, is most closely related to PBs, indicating ancient admixture between the two species. Explicit admixture analyses are consistent with ancient splits among PBs, brown bears and black bears that were later followed by occasional admixture. We also provide paleodemographic estimates that suggest bear evolution has tracked key climate events, and that PB in particular experienced a prolonged and dramatic decline in its effective population size during the last ca. 500,000 years. We demonstrate that brown bears and PBs have had sufficiently independent evolutionary histories over the last 4–5 million years to leave imprints in the PB nuclear genome that likely are associated with ecological adaptation to the Arctic environment.

Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear

The polar bear has become the flagship species in the climate-change discussion. However, little is known about how past climate impacted its evolution and persistence, given an extremely poor fossil record. Although it is undisputed from analyses of mitochondrial (mt) DNA that polar bears constitute a lineage within the genetic diversity of brown bears, timing estimates of their divergence have differed considerably. Using next-generation sequencing technology, we have generated a complete, high-quality mt genome from a stratigraphically validated 130,000- to 110,000-year-old polar bear jawbone. In addition, six mt genomes were generated of extant polar bears from Alaska and brown bears from the Admiralty and Baranof islands of the Alexander Archipelago of southeastern Alaska and Kodiak Island. We show that the phylogenetic position of the ancient polar bear lies almost directly at the branching point between polar bears and brown bears, elucidating a unique morphologically and molecularly documented fossil link between living mammal species. Molecular dating and stable isotope analyses also show that by very early in their evolutionary history, polar bears were already inhabitants of the Artic sea ice and had adapted very rapidly to their current and unique ecology at the top of the Arctic marine food chain. As such, polar bears provide an excellent example of evolutionary opportunism within a widespread mammalian lineage.

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.

Active drumlin field revealed at the margin of Múlajökull, Iceland: A surge-type glacier

ABSTRACT Recent marginal retreat of Mulajokull, a surge-type outlet glacier of Hofsjokull, Iceland, has revealed a drumlin fi eld consisting of more than 50 drumlins. The drumlins are 90–320 m long, 30–105 m wide, 5–10 m in relief, and composed of multiple beds of till deposited by lodgment and bed deformation. The youngest till layer truncates the older units with an erosion surface that parallels the drumlin form. Thus, the drumlins are built up and formed by a combination of subglacial depositional and erosional processes. Field evidence suggests each till bed to be associated with individual recent surges. We consider the fi eld to be active in the sense that the drumlins are shaped by the current glacial regime. The Mulajokull fi eld is the only known active drumlin fi eld, and is therefore a unique analogue to Pleistocene drumlin fi elds.

Glacial and climatic events in iceland reflecting regional north atlantic climatic shifts during the Pleistocene-Holocene transition

Abstract This paper presents a summary of the evidence for glacial and climatic changes during the late Pleistocene-early Holocene transition in Iceland. The deglaciation during the Bolling-Allerod event was interrupted by a short-lived Older Dryas glacial advance. A biostratigraphical record from northern Iceland shows significant climate warming in late Allerod, when mean July temperatures were at least as warm as those of today. An abrupt cooling marked the beginning of the Younger Dryas event. It was characterised by a cold and stable polar climate and an extensive glaciation, before the postglacial warming of climate set in. The Icelandic paleoclimatic record is discussed in the light of climatic oscillations recorded from the GRIP ice-core, from the Greenland Inland Ice, and with reference to major shifts in the oceanic front systems, recorded in the Troll 8903 marine sediment core from the North Sea. The Vedde Ash gives a unique opportunity to address the chronological problems and correlate event stratigraphies of the different proxies. It is concluded that the Icelandic record of glacial and climatic changes during the late Pleistocene—early Holocene transition largely reflects the climatic development in the North Atlantic region.

Late Quaternary stratigraphy of western Yamal Peninsula, Russia: New constraints on the configuration of the Eurasian ice sheet

Ice-sheet reconstructions for the last glacial maximum in northern Eurasia range from nearly complete coverage by a contiguous marine-based ice sheet to large unglaciated areas. Stratigraphic records from Yamal Peninsula, Russia, provide new insight on the eastern limit of the Eurasian ice sheet in the Kara Sea. Radiocarbon and infrared stimulated luminescence ages from coastal cliff sections date the emplacement of the Kara diamicton as older than ca. 40 ka, reflecting regional glaciation. Ice-wedge growth, peat accumulation, and eolian and fluvial deposition characterized the past ~40 k.y. and do not support coverage of Yamal Peninsula by an ice sheet or proximity to a glacier margin. Thus, the late Weichselian Eurasian ice sheet was largely confined to Barents Sea and its global sea-level contribution is reduced to ~7 m of sea-level equivalent.