Late Wisconsin climate in northeastern USA and southeastern Canada,reconstructed from fossil beetle assemblages

Mean July and January temperatures are reconstructed from radiocarbon-dated fossil beetle assemblages, yielding a synthesis of palaeoclimatic history of the regions south of the Laurentide Ice Sheet in North America from 35 000 to 8500 yr BP. Mean July temperatures close to the last glacial maximum were 11–12°C colder than present; mean January temperatures were possibly 10–19°C colder. Mutual climatic range analyses of the beetle assemblages show warming of mean summer temperatures as early as 13.7 kyr, although ice-proximal sites were consistently about 5°C cooler than ice-distal sites. Late-glacial mean summer temperatures peaked between 12 and 11 kyr, then remained fairly constant through the early Holocene. Mean winter temperatures did not reach modern values until after 10 kyr.     

Interhemispheric synchrony of Late-glacial climatic instability as recorded in proglacial Lake Mascardi,Argentina

Several high-resolution continental records have been reported recently in sites in South America, but the extent to which climatic variations were synchronous between the northern and southern hemispheres during the Late-glacial–Holocene transition, and the causes of the climatic changes, remain open questions. Previous investigations indicated that, east of the Andes, the middle and high latitudes of South America warmed uniformly and rapidly from 13 000 ¹⁴C yr BP, with no indication of subsequent climate fluctuations, equivalent, for example, to the Younger Dryas cooling. Here we present a multiproxy continuous record, radiocarbon dated by accelerated mass spectroscopy, from proglacial Lake Mascardi in Argentina. The results show that unstable climatic conditions, comparable to those described from records obtained in the Northern Hemisphere, dominated the Late-glacial–Holocene transition in Argentina at this latitude. Furthermore, a significant advance of the Tronador ice-cap, which feeds Lake Mascardi, occurred during the Younger Dryas Chronozone. This instability suggests a step-wise climatic history reflecting a global, rather than regional, forcing mechanism. The Lake Mascardi record, therefore, provides strong support for the hypothesis that ocean–atmosphere interaction, rather than global ocean circulation alone, governed interhemispheric climate teleconnections during the last deglaciation. © 1997 John Wiley & Sons, Ltd.     

The fossil Coleoptera of the Two Creeks forest bed,Wisconsin

A 21-kg sample of plant detritus from the Two Creeks forest bed at the junction of Kewaunee and Manitowoc Counties, Wisconsin, has produced a small, but well-preserved fauna of Coleoptera, Hymenoptera, and Diptera. Among the Coleoptera the majority of Carabidae (ground beetles) indicate moderately moist to fairly dry, open ground conditions with a sparsely vegetated substrate. Another significant beetle group is represented by the Scolytidae (bark beetles) nearly all of which inhabit spruce. The restricted number of Coleoptera species (21) is believed to represent an in situ assemblage and is part of about 50 taxa. Most of the Coleoptera identified to the species level have a transcontinental distribution, with the exception of two carabids which are extreme western forms, and nearly all reside within the boreal forest. The beetles indicate that the mean July temperature at Two Creeks approximately 11,850 yr B.P. was 14° to 16°C.     

Late-glacial and Holocene vegetation and climatic history of the Cass Basin, central South Island, New Zealand

Lithology, pollen, macrofossils, and stable carbon isotopes from an intermontane basin bog site in southern New Zealand provide a detailed late-glacial and early Holocene vegetation and climate record. Glacial retreat occurred before 17,000 cal yr B.P., and tundra-like grassland–shrubland occupied the basin shortly after. Between 16,500 and 14,600 cal yr B.P., a minor regional expansion of forest patches occurred in response to warming, but the basin remained in shrubland. Forest retreated between 14,600 and 13,600 cal yr B.P., at about the time of the Antarctic Cold Reversal. At 13,600 cal yr B.P., a steady progression from shrubland to tall podocarp forest began as the climate ameliorated. Tall, temperate podocarp trees replaced stress-tolerant shrubs and trees between 12,800 and 11,300 cal yr B.P., indicating sustained warming during the Younger Dryas Chronozone (YDC). Stable isotopes suggest increasing atmospheric humidity from 11,800 to 9300 cal yr B.P. Mild (annual temperatures at least 1°C higher than present), and moist conditions prevailed from 11,000 to 10,350 cal yr B.P. Cooler, more variable conditions followed, and podocarp forest was completely replaced by montane Nothofagus forest at around 7500 cal yr B.P. with the onset of the modern climate regime. The Cass Basin late-glacial climate record closely matches the Antarctic ice core records and is in approximate antiphase with the North Atlantic.     

Rapid late-glacial atmospheric CO2 changes reconstructed from the stomatal density record of fossil leaves

The Younger Dryas stadial (11 000-10 000 yr BP) was an abrupt return to a glacial climate during the termination of the last glaciation. We have reconstructed atmospheric CO₂ concentrations from a high-resolution sequence of fossil Salix herbacea leaves through this climatic oscillation from Kråkenes, western Norway, using the relationship between leaf stomatal density and atmospheric CO₂ concentration. High Allerød CO₂ values (median 273 ppmv) decreased rapidly during 130–200 ¹⁴C-years of the late Allerød to ca. 210 ppmv at the start of the Younger Dryas. They then increased steadily through the Younger Dryas, reaching typical interglacial values once more (ca. 275 ppmv) in the Holocene. The rapid late Allerød decrease in CO₂ concentration preceded the Younger Dryas temperature drop, possibly by several decades. This striking pattern of changes has not so far been recorded unambiguously in temporally coarse measurements of atmospheric CO₂ from ice cores. Our observed late-glacial CO₂ changes have implications for global modelling of the ocean-atmosphere-biosphere over the last glacial-interglacial transition.     

Late-glacial sedimentological and morphological changes in a lowland river in response to climatic change: the Maas,southern Netherlands

The late Pleniglacial and Late-glacial Maas valley, south of Nijmegen, contains four terraces. Three river systems are described based on the morphology of channel scars on these terrace surfaces and by sediment characteristics. The River Maas reacted to climatic warming at the start of the Weichselian Late-glacial by changing its river system slowly, from a braided system to a transitional phase between braiding and meandering and finally to a highly sinuous meandering system. The Maas reacted rapidly to the Younger Dryas climate deterioration by again establishing a braiding system. At the onset of the Holocene, the river changed abruptly to a meandering river without a transitional phase. The triggering factor for change in the Maas river pattern is almost certainly the changing climate in the Late Glacial. Gradient lines on the terrace surfaces show that tectonic activity did not modify the morphology of the channels. A division of the terraces is shown, the morphological, sedimentological and petrographical characteristics are presented and the linking of changing fluvial patterns with climatic changes or tectonic movements is discussed. © 1997 John Wiley & Sons, Ltd.     

加拿大阿萨巴斯卡地区Mildred泥炭柱脂类化合物的组成特征

利用气相色谱-质谱联用仪(GC-MS)系统地分析了阿萨巴斯卡地区Mildred泥炭柱37个样品的脂类化合物,研究了它们的组成特征及可能来源。结果表明,样品中检出的正构烷烃、正烷酮、正烷醛、正烷醇、脂肪酸和脂肪酸甲酯均由高碳数化合物构成,并具明显的奇碳或偶碳优势,GC-MS质量色谱图中甾类和萜类极性化合物呈现显著的高峰。根据泥炭分子地球化学分析,Mildred泥炭柱沉积有机质主要来源于原地堆积的高等植物,苔藓类、水生植物对泥炭有机质也有一定程度的贡献,其中松柏、杜鹃花科等木本植物、莎草科草本植物以及泥炭藓类是主要的成炭植物。萜类和甾类极性化合物的分布存在明显差异,萜类化合物主要集中于剖面的上部,其形成与泥炭藓类植物存在联系;甾类化合物来源不具专属性,多与高等植物的输入有关,也可能是受微生物的改造作用而形成。     

Mid-Wisconsin seasonal temperatures reconstructed from fossil beetle assemblages in eastern North America: comparisons with other proxy records from the Northern Hemisphere

Mutual climatic range (MCR) analysis was applied to 15 North American beetle assemblages spanning the interval from > 52 000 to 17 200 yr BP, bracketing a Mid-Wisconsin interstadial interval. The analyses yielded estimates of mean July (TMAX) and mean January (TMIN) temperatures. The oldest assemblage (> 52 ka) yielded TMAX values 7.5–8°C lower than present and TMIN values 15–18°C lower than present. A Mid-Wisconsin interstadial warming dating from 43.5–39 ka was rapid and intense. At the peak of the warming event, about 42 ka, TMAX values were only 1–2°C lower than modern. This level of amelioration apparently lasted only about 2000–3000 yr. By 23.7 ka, TMAX values declined to 11.5–10°C lower than modern, but another, small-scale amelioration is indicated by assemblages dating from 20.5 to 19.7 ka. The interstadial event recorded from the site at Titusville, Pennsylvania closely matches the timing and intensity of the climate change estimated from British beetle faunas in the Upton Warren interstadial. Another warm interval (ca. 31–32.5 ka) has been documented from fossil beetle assemblages in Europe and North America. Copyright © 1999 John Wiley & Sons, Ltd.     

Pollen analysis of fossil packrat middens from the Sonoran Desert

Pollen contained in 22 fossil packrat middens from the Sonoran Desert provides a complementary, but differing, view of the paleoenvironment from that derived by analysis of the associated plant macrofossils. The regional component of the pollen data is in sharp contrast to the locally oriented macrofossils. A total of 84 macrofossil taxa and 47 pollen taxa were identified; only 18 taxa were common to both. The low Index of Similarity, 0.4, indicates that the two sources of fossil information are providing different sets of paleobotanical data. When combined with plant macrofossils and good radiocarbon dating control, the pollen spectra derived from fossil middens are compatable with other paleoenvironmental sequences.     

Primary compositional range and alteration trends of microlite from the yellowknife pegmatite field,Northwest territories,Canada

Summary Microlite occurs as a rare accessory mineral in beryl-columbite, beryl-columbitephosphate, complex spodumene, albite-spodumene and amblygonite type rare-element granitic pegmatites in the Archean Yellowknife pegmatite field of the Canadian Shield. The chemistry of microlite is variable but consistent with the accepted structural formula A_2–mB₂X₆Y_1–n pH₂O, where generally A = Ca,Na; B = Ta,Nb; X = O; Y = O,OH, F; m = 0 - 2; n = 0 - 1 and p = 0 - l. The chemistry of the Yellowknife microlite is dominated by Ca, Na, Ta, and Nb with minor amounts of U, Pb, Fe, Mn, and Ti. The compositions of microlite are interpreted to reflect primary variability and effects of late-stage alteration.Two principal types of microlite can be distinguished by their primary composition and alteration trends. U-poor microlite originated by the metasomatic replacement of pre-existing manganocolumbite, manganotantalite, and ferrotapiolite; with progressive alteration, its composition evolves from early Ca-rich, Fe,Mn-poor members to late Ca,Na-poor, Fe,Mn-enriched members. In contrast, U-bearing microlite formed from U-enriched, moderately fractionated pegmatitec fluids acting upon ferrocolumbite, manganocolumbite, and manganotantalite; with progressive alteration, its composition evolves from U,Ca,Na-enriched members to U,Ca,Na-poor, Fe,Mn-enriched members.

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Primärer zusammensetzungsbereich und umwandlungstrends der mikrolithe aus dem yellowknife pegmatitfeld, Northwest Territories, Kanada

Zusammenfassung Primärer Zusammensetzungsbereich und Umwandlungstrends der Mikrolithe aus dem Yellowknife Pegmatitfeld, Northwest Territories, Kanada Mikrolith kommt in den Beryll-Columbit-, Beryll-Columbit-Phosphat-, komplexen Spodumeri-, Albit-Spodumen- und Amblygonit-Typen der Seltene-Element-Granitpegmatite im archäischen Yellowknife Pegmatitfeld des Kanadischen Schildes als seltenes akzessorisches Mineral vor. Der Chemismus des Mikroliths variiert, ist aber mit der gebräuchlichen Strukturformel A_2–mB₂X_1–mY_1–n·pH₂O verträglich, mit im allgemeinen A = Ca,Na, B = Ta,Nb, X = O, m=0–2, n =O–1 und p=0–1. Der Chemismus des Yellowknife Mikroliths wird durch Ca, Na, Ta und Nb dominiert, U, Pb, Fe, Mn und Ti treten in kleineren Mengen auf. Die Zusammensetzungen des Mikroliths spiegeln die primäre Variabilität sowie die Auswirkungen späterer Umwand lungen wieder.Zwei Haupttypen des Mikroliths können nach ihrer primären Zusammensetzung und den Umwandlungstrends unterschieden werden. U-armer Mikrolith entstand durch metasomatischen Ersatz von früherem Manganocolumbit-Manganotantalit und Ferrotapiolith, mit fortschreitender Umwandlung entwickelt sich seine Zusammensetzung von frühen Ca-reichen, Fe,Mn-armen Gliedern zu späten Ca,Na-armen, Fe,Mn-angereicherten Gliedern. Im Gegensatz dazu bildete sich U-haltiger Mikrolith aus an U angerereicherten, mäßig fraktionierten pegmatitischen Fluiden, die auf Manganocolumbit-Manganotantalit einwirkten, mit fortschreitender Umwandlung entwickelt sich seine Zusammensetzung von U,Ca,Na-angereicherten Gliedern zu U,Ca,Na-armen, Mn,Feangereicherten Gliedern.

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Late-glacial vegetational and climatic history of the Allegheny Plateau and the Till Plains of Ohio and Indiana, U.S.A.

Shane, Linda C. K. 1987 03 01: Late-glacial vegetational and climatic history of the Allegheny Plateau and the Till Plains of Ohio and Indiana, U.S.A. Boreas , Vol. 16, pp. 1–20. Oslo. ISSN 0300–9483.
Pollen evidence from the Allegheny Plateau and the Till Plains south of the Great Lakes shows marked post-glacial vegetation gradients. C. 15,500–11,000 B.P .: On the Plateau, spruce forest was rapidly established, persisted for 2,000 years, then began a gradual change to deciduous-conifer forest. On the Till Plains, open spruce forest tundra closed slowly over 1,000 years, declined rapidly c . 13,500 B.P., and a deciduous open woodland developed. C. 11,000–10,300 B.P .: On the Till Plains, a brief cooling is recorded by increases in the abundance of spruce and fir, contemporaneous with the European Younger Dryas. No clear change is seen on the Plateau. 10,300–4,000 B.P .: Warming and/or drying occurred in both areas, as hemlock and jack/red pine trees immigrated, followed by white pine. Conifers disappeared from the Till Plains by 9,800 B.P., but pine and hemlock trees may have persisted on the Plateau. After 10,000 B.P. mixed deciduous forest was established across the entire region. Between 8,000 and 4,000 B.P., further warming/drying is indicated on the Till Plains with development of open oak forest and lake shallowing, and on the Plateau by a minor increase in herbs, lake shallowing, and reduction in pine.     

The first fossil species of subfamily Piestinae (Coleoptera: Staphylinidae) from the Lower Cretaceous of China

Two new Early Cretaceous fossil rove beetle species, Paleosiagonium brevelytratum n. gen. n. sp. and Paleosiagonium adaequatum n. gen. n. sp. are described from the Lower Cretaceous Yixian Formation of Liaoning Province, China. They are clearly placed in the extant Piestinae according to the following characters: body elongate and flat; anterior coxae small and globose; abdomen long and parallel-sided, with six visible sterna and one pair of paratergites on segments III–VII. This is the first report on the occurrence of the coleopteran subfamily Piestinae in the fossil record. The new discoveries indicate that Piestinae have originated at least by the Early Cretaceous, at about 125–120 Ma.     

Evidence of climatic change from oxygen and carbon isotope variations in sediments of a small arctic lake,Canada

Radiocarbon dating of the organic-rich sediments of Lake Illisarvik in the outer Mackenzie Delta indicates that formation of the lake occurred approximately 9500yr BP, with maximum expansion around 6000 yr BP. Sedimentation rates have remained relatively constant at an average of 0.3mm/yr. ¹³C results on biogenic and inorganic carbonates and organics indicate a change from dominantly terrestrial organics (?27 to ?28%₀) to submerged aquatic vegetation or plankton (?18 to ?23%₀) upon formation of the lake (9500yr BP), and a dramatic return to dominantly terrestrial organics at 5800yr BP (δ¹³C = ?27 to ?30%₀). This latter shift is accompanied by a drastic reduction in the macroflora and fauna populations. ¹⁸O results suggest that a warmer climate than today existed prior to the shift at 5800yr BP.     

Groundwater and climate change: a sensitivity analysis for the Grand Forks aquifer,southern British Columbia,Canada

The Grand Forks aquifer, located in south-central British Columbia, Canada was used as a case study area for modeling the sensitivity of an aquifer to changes in recharge and river stage consistent with projected climate-change scenarios for the region. Results suggest that variations in recharge to the aquifer under the different climate-change scenarios, modeled under steady-state conditions, have a much smaller impact on the groundwater system than changes in river-stage elevation of the Kettle and Granby Rivers, which flow through the valley. All simulations showed relatively small changes in the overall configuration of the water table and general direction of groundwater flow. High-recharge and low-recharge simulations resulted in approximately a +0.05 m increase and a –0.025 m decrease, respectively, in water-table elevations throughout the aquifer. Simulated changes in river-stage elevation, to reflect higher-than-peak-flow levels (by 20 and 50%), resulted in average changes in the water-table elevation of 2.72 and 3.45 m, respectively. Simulated changes in river-stage elevation, to reflect lower-than-baseflow levels (by 20 and 50%), resulted in average changes in the water-table elevation of –0.48 and –2.10 m, respectively. Current observed water-table elevations in the valley are consistent with an average river-stage elevation (between current baseflow and peak-flow stages).

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Resumen El acuífero de los Grand Forks, situado al sur de la Columbia Británica central (Canadá) ha sido utilizado como lugar de estudio para modelar la sensibilidad de un acuífero a los cambios en la recarga y el caudal de los ríos de acuerdo con escenarios previstos de cambio climático en la región. Los resultados sugieren que las variaciones en la recarga al acuífero bajo los diversos escenarios, que han sido modelados en régimen estacionario, tienen un impacto mucho menor en las aguas subterráneas que los cambios en el caudal de los ríos Kettle y Granby, que discurren por el valle. Todas las simulaciones muestran diferencias relativamente pequeñas en la configuración regional de los niveles freáticos y en la dirección general del flujo subterráneo. Las simulaciones de recarga elevada y baja causan un incremento de 0,05 m y un decremento de 0,025 m, respectivamente, en los niveles del acuífero. Los cambios de la elevación del río, simulados para reflejar niveles de flujo mayores que los valores pico (en un 20% y un 50%) resultan en cambios medios de los niveles del acuífero de 2,72 m y 3,45 m, respectivamente. Los cambios simulados en la elevación del río para flujos inferiores al caudal de base (en un 20% y en un 50%) provocan descensos en los niveles de 0,48 y 2,10 m, respectivamente. Los niveles actuales del acuífero en el valle son coherentes con una elevación media del nivel en el río (entre el caudal de base actual y los picos de caudal).

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Résumé L'aquifère de Grand Forks, situé en Colombie britannique (Canada), a été utilisé comme zone d'étude pour modéliser la sensibilité d'un aquifère à des modifications de la recharge et du niveau de la rivière, correspondant à des scénarios envisagés de changement climatique dans cette région. Les résultats font apparaître que les variations de recharge de l'aquifère pour différents scénarios de changement climatique, modélisées pour des conditions de régime permanent, ont un impact sur le système aquifère beaucoup plus faible que les changements du niveau des rivières Kettle et Granby, qui coulent dans la vallée. Toutes les simulations ont montré des différences relativement faibles dans la configuration d'ensemble de la nappe et dans la direction générale des écoulements. Des simulations de conditions de recharge forte et de recharge faible produisent respectivement une remontée de 0,05 m et un abaissement de 0,025 m, approximativement, des cotes de la nappe pour l'ensemble de l'aquifère. Des changements simulés de la cote du niveau de la rivière, pour refléter des niveaux plus hauts que ceux des pics de crues (de 20 et de 50%), produisent respectivement des remontées de la nappe de 2,72 et 3,45 m en moyenne. Des changements simulés de l'altitude du niveau de la rivière, pour refléter des niveaux plus bas que ceux de basses eaux (de 20 et de 50%), produisent respectivement des abaissements de la nappe de 0,48 et 2,10 m en moyenne. Les altitudes courantes observées de la nappe dans la vallée sont cohérentes avec une cote moyenne du niveau de la rivière (entre les niveaux courants de basses eaux et de crues).

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Chronologic and environmental implications of a new genus of fossil deer from late Wisconsin deposits at Toronto,Canada

A new cervine deer (Torontoceros hypogaeus), described from a partial cranium with portions of the main antler beams, has been recovered from deposits of early Lake Ontario age on the exposed bench of Glacial Lake Iroquois at Toronto, Ontario, Canada. The deer was about the size of a caribou, has heavy antlers that lie chiefly in a nearly horizontal plane, and its beams are bowed anteriorly. The tines are not flattened, the brow tines are asymmetrical, and no evidence of surface roughening or palmation of the beam is found. A ¹⁴C date of 11,315 ± 325 yr B.P. obtained on the antler allows the date at which Glacial Lake Iroquois drained to be revised to before 11,400 yr B.P. Spruce (Picea), pine (Pinus), and sedges (Cyperaceae) are major components of the associated pollen spectrum, which implies a typically interstadial or postglacial climate in which mixed forests grew in the Toronto area.     

Palaeolimnological conditions inferred from fossil diatom assemblages and derivative spectral properties of sediments in thermokarst ponds of subarctic Quebec,Canada

Thermokarst ponds are widespread in arctic and subarctic regions, but little is known about their temporal evolution prior to human observations. This paper presents a pioneer biostratigraphic study conducted at a subarctic site with limnologically contrasted ponds located on the eastern shore of Hudson Bay, Canada. Fossil diatom and visible near infrared (VNIR) derivative spectral analyses were performed on short sediment cores, confirming the occurrence of three distinct stratigraphic facies as already inferred from an anterior sedimentological study: a lacustrine upper facies (UF) and a marine lower facies (LF), separated by an organic‐rich/peat transitional zone (TZ). Diatoms were almost absent from LF, but increased significantly in both TZ and UF. Identified diatom taxa were mainly benthic species (e.g. genera Fragilaria, Pinnularia), and their down‐core distribution appeared to be related to dissolved organic carbon (DOC) and possibly pH conditions. Diatom‐inferred DOC showed a decreasing trend towards the surface (potentially associated with an increase in pH), inverse to the general trend in this region, suggesting the action of other mechanisms on DOC, such as exhaustion of external inputs from limited catchments and the role of discontinuous peat layers (former surfaces of permafrost mounds) during the initial stages of pond formation. These bryophilous substrates in aerophilic habitats probably controlled diatom community composition. The combination of diatom and VNIR data revealed similar trends between (i) opal (amorphous silica) and diatom abundances; (ii) eukaryotic/prokaryotic algae ratio and anoxia or hypoxia in bottom waters; and (iii) limonite (iron oxide) and redox conditions in surface sediments. These findings indicate that diatom community changes and pond limnological evolution in the recent past were controlled mainly by autogenic processes (e.g. local vegetation/soil development, peat accumulation and erosion), rather than by allogenic forcing mechanisms (e.g. precipitation and temperature, geochemical leaching of the surrounding glaciomarine sediments).     

Late Quaternary climatic history of the Chilean Channels based on fossil pollen and beetle analyses,with an analysis of the modern vegetation and pollen rain

Fossil beetles and pollen were examined from an intermorainal bog at Puerto Edén, Isla Wellington, Chile (latitude 49°08'S, longitude 74°25'W). Wood from near the base of the section has an age of 12 960 ± 150 yr BP. Occurrence of flightless beetle species in the basal peat sample is evidence that some members of the biota survived the last glacial maximum in refugia. The assumption that the Chilean Channels were entirely ice-covered is incorrect. Plants and insects that invaded the deglaciated terrain were those of an Empetrum heathland in which patches of Nothofagus forest were restricted to sheltered locations. The climate supporting the heathland is inferred to have been windier and probably drier than that of the present day. From 13 000 yr BP to 9500 yr BP Nothofagus forest expanded, possibly in response to less windiness and more available moisture. Neither the fossil beetle nor pollen data support a return to significantly colder conditions between 11 000 and 10 000 yr BP at the time of the Younger Dryas Stade. From 9500 to 5500 yr BP the climate was as wet as that of the present day, based on an increased representation of the pollen of moorland plants and of aquatic beetle species. From 5500 to 3000 yr BP the climate was drier, as indicated by the expansion of Empetrum heath and the reduction in mesic habitats. From 3000 yr BP to the present-day mesic habitats dominated as the climate returned to a wetter mode. The alternatively wetter and drier episodes are attributed to latitudinal shifts in the position of storm tracks in the belt of Southern Westerlies.     

Holocene environments and climate in the Mongolian Altai reconstructed from the Hoton-Nur pollen and diatom records: a step towards better understanding climate dynamics in Central Asia

This study presents the results of the palynological and diatom analyses of the sediment core recovered in Hoton-Nur Lake (48°37′18″N, 88°20′45″E, 2083 m) in 2004. Quantitative reconstruction of the Holocene vegetation and climate dynamics in the semiarid Mongolian Altai suggests that boreal woodland replaced the primarily open landscape of northwestern Mongolia at about 10 kyr BP (1 kyr = 1000 cal yr) in response to a noticeable increase in precipitation from 200–250 mm/yr to 450–550 mm/yr. A decline of the forest vegetation and a return to a predominance of open vegetation types occurred after 5 kyr BP when precipitation sums decreased to 250–300 mm/yr. Prior to 11.5 kyr BP diatom concentrations are relatively low and the lake is dominated by planktonic Cyclotella and small Fragilariaceae, suggesting the existence of a relatively deep and oligotrophic/mesotrophic lake. The great abundance of Staurosirella pinnata from the beginning of the record until 10.7 kyr BP might imply intensified erosion processes in the catchment and this is fully consistent with the presence of scarce and dry vegetation and the generally arid climate during this period. From about 10.7 kyr BP, more planktonic diatom taxa appeared and increased in abundance, indicating that the lake became more productive as diatom concentration increased. This change correlates well with the development of boreal woodland in the catchment. Decrease in precipitation and changes in the vegetation towards steppe are reflected by the rapid increase in Aulacoseira distans from about 5 kyr BP. The Holocene pollen and diatom records do not indicate soil and vegetation cover disturbances by the anthropogenic activities, implying that the main transformations of the regional vegetation occurred as a result of the natural climate change. Our reconstruction is in agreement with the paleomonsoon records from China, demonstrating an abrupt strengthening of the summer monsoon at 12 kyr BP and an associated increase in precipitation and in lake levels between 11 and 8 kyr BP, followed by the stepwise attenuation of the monsoon circulation and climate aridization towards the modern level. The records from the neighboring areas of Kazakhstan and Russia, situated west and north of Hoton-Nur, demonstrate spatially and temporally different Holocene vegetation and climate histories, indicating that the Altai Mountains as a climate boundary are of pivotal importance for the Holocene environmental and, possibly, habitation history of Central Asia.     

加拿大阿萨巴斯卡地区泥炭脂类分子记录的古气候信息

为了进一步了解泥炭中脂类分子化合物的分布与植被演化和气候变化的响应关系,对采自加拿大阿尔伯塔省东北部阿萨巴斯卡地区的JPH4、Mildred和McMurray泥炭柱样品中的脂类分子进行测定。根据正构烷烃、正烷酮、甾类和萜类等分子化合物的地球化学分析,结合年代学资料,重建了研究区近千年来的古气候演化过程。现代暖期以来(1990年以来),区内大范围的锈色泥炭藓发育,指示温暖干旱的气候条件;现代暖期早期(1900年以来),陆源高等植物和狭叶泥炭藓共存于干-湿交替的气候环境下;JPH4泥炭柱记录了小冰期期间寒冷干旱的气候环境,该时期莎草科等陆生维管植物发育;上述结论与前人记录的区域植被和气候变化具有较好的可比性。研究区发育的锈色泥炭藓种类特殊,多形成于干旱环境且具有C₃₁正构烷烃优势分布,ACL、P_aq、C₂₃/(C₂₇+C₃₁)正构烷烃指标能够有效表征植被输入特征和气候变化。正烷酮主要来源于相应正构烷烃的微生物氧化作用,正烷酮指标CPI-ket、ACL-ket和(K₂₃