Previous Interglacials Research Articles

Pleistocene development of the SE Nordic Seas margin

Throughout the Pleistocene the sedimentary environment on the SE Nordic Seas continental slope/outer shelf, off western Norway, has been strongly controlled by variability in the Norwegian Atlantic Current (NwAC), glaciations of the shelf areas and sea level changes. Acoustic and core data from the southern Vøring Plateau show a Pleistocene sequence characterised by hemipelagic sediments interfingered by diamictons on the upper slope. The area of the 7.25 14C ka BP Storegga Slide shows evidence of a long history of Pleistocene mega-slides. During the last interglacial, and most likely also during previous interglacials, the slide region has been the locus of rapid deposition between water depths of 800 and 1200 m, as a result of NwAC winnowing along the upper slope. The North Sea Fan region is strongly influenced by glacigenic debris flows (GDFs) deposited during glacial advances reaching the shelf edge, when the Norwegian Channel was occupied by the Norwegian Channel Ice Stream. It appears that GDF activity was initiated at ca. Marine Isotope Stage 12. Interbedded between the debris flow sequences, mega-slide events such as the Møre and Tampen slides have been identified. During glaciations, when the entire SE Nordic Seas continental shelf was covered by extensive grounded ice sheets, basal till were transported to the shelf edge from where subsequent mass movement occurred. During late glaciation/early deglaciation meltwater plumes were released at the time of disintegration of ice streams in the Norwegian Channel, as is evidenced from the last deglaciation of the margin at ca. 15 14C ka BP. The plume material was transported northwards by currents, before rapidly deposited as a thick package within the Storegga Slide area and on the south Vøring Plateau. Based on identification and dating of iceberg scourings, glacial erosion surfaces and delta deposits on the shelf, subsidence rates between 0.7 and 1.2 m/ky have been calculated for the last ca. 250 ka.

Pleistocene reduction of polar ice caps: Evidence from Cariaco Basin marine sediments

Sea level is projected to rise between 13 and 94 cm over the next 100 yr due to continued climate warming. The sea-level projections assume that polar ice sheets will remain stable or even increase on time scales of centuries, but controversial geologic evidence suggests that current polar ice sheets have been eliminated or greatly reduced during previous Pleistocene interglacials indicating that modern polar ice sheets have become unstable within the natural range of interglacial climates. Sea level may have been more than 20 m higher than today during a presumably very warm interglacial about 400 ka during marine isotope stage 11. Because of the implications for future sea level rise, additional study of the conflicting evidence for warmer conditions and higher sea level during marine isotope stage 11 is needed. Here we present microfossil and isotopic data from marine sediments of the Cariaco Basin supporting the interpretation that global sea level was 10–20 m higher than today during marine isotope stage 11. The increased sea level requires reduction in modern polar ice sheets and is consistent with the interpretation that the West Antarctic ice sheet and the Greenland ice sheet were absent or greatly reduced during marine isotope stage 11. Our results show a warm marine isotope stage 11 interglacial climate with sea level as high as or above modern sea level that lasted for 25 to 30 k.y. Variations in Earth9s orbit around the sun (Milankovitch cycles) are considered to be a primary external force driving glacial-interglacial cycles. Current and marine isotope stage 11 Milankovitch forcing are very similar, suggesting that the present interglacial (Holocene) that began ca. 10 ka will continue for another 15 to 20 k.y. Therefore any anthropogenic climate warming will accelerate the natural process toward reduction in polar ice sheets. The potential for increased rates of sea level rise related to polar ice sheet decay should be considered as a potential natural hazard on centennial time scales.

Montane climate and vegetation dynamics in easternmost Beringia during the Late Quaternary

New and published palynological data are used to investigate the Quaternary vegetation history of high-elevation sites in the Mackenzie Mountains of easternmost Beringia (Northwest Territories, Canada). During some previous interglacials, sites that are presently treeless supported spruce forest, indicating conditions were warmer than present and possibly warmer than at any time in the Holocene. No information is available on vegetation in unglaciated portions of the Mountains during the Last Glacial Maximum, but in the Lateglacial (ca 12,000–10,000 14C yr BP), a shrub- and herb-dominated vegetation, characterized in the pollen record by Artemisia, Betula, Salix, gramnoids and herbs, grew at all elevations, and extended in a continuous corridor southwards to Alberta. Populus balsamifera probably grew up to and above present treeline between about 11,000 and 9000 yr BP. The late Pleistocene vegetation of the Mackenzie Mountains contrasts with the rest of eastern and central Beringia, which supported a Betula-dominated shrub tundra by 12,000 yr BP. Betula shrub tundra expanded at all elevations in the mountains by 10,000 yr BP, followed by the expansion of Picea glauca and Picea mariana forest at ca 8000 yr BP. There is evidence for limited elevational shifts in treeline during the Holocene. Picea population densities in the forest–tundra were probably greater than at present in the early and mid-Holocene. Vegetation changes in the Mackenzie Mountains during this time are consistent with increased summer insolation and temperatures during the early to mid-Holocene. Higher resolution analysis (decadal to century scale) of small lake basins at treeline in this region also provide evidence for a late Holocene increase in forest–tundra density and/or a treeline rise centered on 3000 yr BP. Slight warming at this time has been noted at sites in the southern Rocky Mountains, and provides evidence for century-scale climatic fluctuations superimposed on the longer-term changes resulting from variations in insolation.

Glacial cirque formation in northern Scandinavia

Passglaciären is a small cirque glacier in the Kebnekaise massif, northern Sweden. It is frozen to its bed over more than 70% of its area, and under present climatic conditions has little effect on cirque formation. More favourable conditions for cirque glacier erosion during the Holocene are of short duration. Assuming similar conditions during previous interglacials, it is suggested that forms such as the Passglaciären cirque developed mainly during the initial phases of glacials when they were part of networks of large valley glaciers or of a small warm-based mountain-centred ice sheet. Passglaciären has been examined in order to evaluate its erosive capacity and its association with the subglacial cirque morphology. The methods used are radar surveys and direct ice-temperature measurements. Erosion is restricted to a small section of the glacier bed, at present resulting in only partial deepening of the cirque and erosion of the backwall. In cold, arid regions with extensive permafrost, small cirque glaciers are largely frozen to the bed, and therefore cannot contribute significantly to cirque formation. In such regions glacial erosion by larger temperate glaciers is more likely to be the major cause of cirque excavation.

Glacial cirque formation in northern Scandinavia

Passglaciären is a small cirque glacier in the Kebnekaise massif, northern Sweden. It is frozen to its bed over more than 70% of its area, and under present climatic conditions has little effect on cirque formation. More favourable conditions for cirque glacier erosion during the Holocene are of short duration. Assuming similar conditions during previous interglacials, it is suggested that forms such as the Passglaciären cirque developed mainly during the initial phases of glacials when they were part of networks of large valley glaciers or of a small warm-based mountain-centred ice sheet. Passglaciären has been examined in order to evaluate its erosive capacity and its association with the subglacial cirque morphology. The methods used are radar surveys and direct ice-temperature measurements. Erosion is restricted to a small section of the glacier bed, at present resulting in only partial deepening of the cirque and erosion of the backwall. In cold, arid regions with extensive permafrost, small cirque glaciers are largely frozen to the bed, and therefore cannot contribute significantly to cirque formation. In such regions glacial erosion by larger temperate glaciers is more likely to be the major cause of cirque excavation.

Fire, prehistoric humanity, and the environment

Abstract An association between our hominid ancestors and fire extends back over a million years. Along with speech and the ability to use tools, fire has allowed humanity an unprecedented power to manipulate its environment. Archaeological evidence suggests that modern humans were present on most habitable continents, firestick in hand, by at least 40 000 years ago and probably earlier. Humans were therefore in a position to exert considerable influence over vegetation patterns as well as at least regional climate, at a time when the earth was changing from glacial to interglacial mode and 'natural' ecosystems were destabilised. Emerging evidence suggests that the interior of Australia may be arid today because of prehistoric anthropogenic burning that inhibited the re-establishment of forests over northern Australia at the end of the last glaciation. On a global scale, the present interglacial period is arid in comparison with what is known of previous interglacials in both Australasia and Saharan Afric...

Fire, prehistoric humanity, and the environment

An association between our hominid ancestors and fire extends back over a million years. Along with speech and the ability to use tools, fire has allowed humanity an unprecedented power to manipulate its environment. Archaeological evidence suggests that modern humans were present on most habitable continents, firestick in hand, by at least 40 000 years ago and probably earlier. Humans were therefore in a position to exert considerable influence over vegetation patterns as well as at least regional climate, at a time when the earth was changing from glacial to interglacial mode and 'natural' ecosystems were destabilised. Emerging evidence suggests that the interior of Australia may be arid today because of prehistoric anthropogenic burning that inhibited the re-establishment of forests over northern Australia at the end of the last glaciation. On a global scale, the present interglacial period is arid in comparison with what is known of previous interglacials in both Australasia and Saharan Africa, an observation which has yet to be adequately explained. The possibility that we have been living in a global environment partly of our own making since long before the advent of modern concerns over 'global change' in the twentieth century merits serious consideration.

Late Pleistocene and Holocene aeolian dust deposition in North China and the Northwest Pacific Ocean

Rates of dust deposition in the North Pacific and the Central Loess Plateau of China show a poor correlation during the last 30,000 yr, although in both cases the dust was derived from deserts in northern China. Loess accumulation rates were high at the time of the last glacial maximum, while dust flux to the North Pacific was low at this time. Maximum dust flux to the North Pacific is recorded in the mid-Holocene, but Holocene rates of loess accumulation have been low. These differences are shown to reflect the different nature of the wind conditions responsible for dust transport to the Loess Plateau and the North Pacific. Dust transport responsible for formation of the loess occurred mainly in the lower 1.5 km of the atmosphere and was accomplished by cold, low-level northwesterly winds associated with cold waves emanating from the Mongolian anticyclone. At present such conditions occur quite frequently in winter but during the coldest phases of the Pleistocene they probably also occurred in the spring and summer. Long-range dust transport from northern China to the North Pacific requires that the dust is lifted to high levels (above 3 km) where it can be incorporated into the strong mid-latitude westerlies. Strong vertical motions of the atmosphere occur most frequently in the spring when the Mongolian anticyclone begins to weaken and frontal depressions move across the deserts of northern China. Such frontal depressions were probably more frequent during the climatic optimum of the mid-Holocene and during previous interglacials. Changes in continental aridity have probably played a minor role in controlling the magnitude of the dust flux to the North Pacific.

Quaternary History of Deciduous Forests of Eastern North America and Europe

Quaternary History of Deciduous Forests of Eastern North America and Europe

The natural breakdown of the present interglacial and its possible intervention by human activities

On the premise that climate responds in a stable and well-behaved manner to changes of terrestrial or extraterrestrial conditions of environment, the question of anticipating when the present interglacial will break down is explored from two different viewpoints. The concept, that the timing of the breakdown can be inferred through a comparison of the duration of the present interglacial (to date) with the duration of previous interglacials of similar warmth in the upper Quaternary, is examined from the viewpoint of the chronology of changes of solar radiation received at the top of the atmosphere, generally recognized as a likely causative agent in the development of interglacials. It is found that the radiation changes around the time of the present interglacial are somewhat different in character from those that accompanied the last interglacial, but that the radiation changes around the time of the last interglacial were more nearly similar to those which may have accompanied earlier interglacials. From this viewpoint, comparisons drawn between the present and previous interglacials are judged to be of uncertain value as a predictive tool for the future. The principal effects of man's activities on present-day climate are then reviewed. and the thermal effects of anticipated future increases of atmospheric carbon dioxide and particle loading are compared. It is concluded that the net impact of human activities on the climate of future decades and centuries is quite likely to be one of warming, and therefore favorable to the perpetuation of the present interglacial.