39 research outputs found
Early-Holocene warming in Beringia and its mediation by sea-level and vegetation changes
No full textArctic land-cover changes induced by recent global climate change (e.g., expansion of woody vegetation into tundra and effects of permafrost degradation) are expected to generate further feedbacks to the climate system. Past changes can be used to assess our understanding of feedback mechanisms through a combination of process modelling and paleo-observations. The sub-continental region of Beringia (Northeast Siberia, Alaska, and northwestern Canada) was largely ice-free at the peak of deglacial warming and experienced both major vegetation change and loss of permafrost when many arctic regions were still ice covered. The evolution of Beringian climate at this time was largely driven by global features, such as the amplified seasonal cycle of Northern Hemisphere insolation and changes in global ice volume and atmospheric composition, but changes in regional land-surface controls, such as the widespread development of thaw lakes, the replacement of tundra by deciduous forest or woodland, and the flooding of the Bering–Chukchi land bridge, were probably also important. We examined the sensitivity of Beringia’s early Holocene climate to these regional-scale controls using a regional climate model (RegCM). Lateral and oceanic boundary conditions were provided by global climate simulations conducted using the GENESIS V2.01 atmospheric general circulation model (AGCM) with a mixed-layer ocean. We carried out two present day simulations of regional climate, one with modern and one with 11 ka geography, plus another simulation for 6 ka. In addition, we performed five ? 11 ka climate simulations, each driven by the same global AGCM boundary conditions: (i) 11 ka “Control”, which represents conditions just prior to the major transitions (exposed land bridge, no thaw lakes or wetlands, widespread tundra vegetation), (ii) sea-level rise, which employed present day continental outlines, (iii) vegetation change, with deciduous needleleaf and deciduous broadleaf boreal vegetation types distributed as suggested by the paleoecological record, (iv) thaw lakes, which used the present day distribution of lakes and wetlands; and (v) post-11 ka “All”, incorporating all boundary conditions changed in experiments (ii)–(iv). We find that regional-scale controls strongly mediate the climate responses to changes in the large-scale controls, amplifying them in some cases, damping them in others, and, overall, generating considerable spatial heterogeneity in the simulated climate changes. The change from tundra to deciduous woodland produces additional widespread warming in spring and early summer over that induced by the 11 ka insolation regime alone, and lakes and wetlands produce modest and localized cooling in summer and warming in winter. The greatest effect is the flooding of the land bridge and shelves, which produces generally cooler conditions in summer but warmer conditions in winter and is most clearly manifest on the flooded shelves and in eastern Beringia. By 6 ka continued amplification of the seasonal cycle of insolation and loss of the Laurentide ice sheet produce temperatures similar to or higher than those at 11 ka, plus a longer growing season
Holocene thermal maximum in the western Arctic (0-180°W)
No full textThe spatio-temporal pattern of peak Holocene warmth (Holocene thermal maximum, HTM) is traced over 140 sites across the Western Hemisphere of the Arctic (0–180°W; north of ~60°N). Paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer-than-present conditions at 120 of these sites. At the 16 terrestrial sites where quantitative estimates have been obtained, local HTM temperatures (primarily summer estimates) were on average 1.6±0.8°C higher than present (approximate average of the 20th century), but the warming was time-transgressive across the western Arctic. As the precession-driven summer insolation anomaly peaked 12–10 ka (thousands of calendar years ago), warming was concentrated in northwest North America, while cool conditions lingered in the northeast. Alaska and northwest Canada experienced the HTM between ca 11 and 9 ka, about 4000 yr prior to the HTM in northeast Canada. The delayed warming in Quebec and Labrador was linked to the residual Laurentide Ice Sheet, which chilled the region through its impact on surface energy balance and ocean circulation. The lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change. The spatial asymmetry of warming during the HTM resembles the pattern of warming observed in the Arctic over the last several decades. Although the two warmings are described at different temporal scales, and the HTM was additionally affected by the residual Laurentide ice, the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing. Unlike the HTM, however, future warming will not be counterbalanced by the cooling effect of a residual North American ice sheet
Paleoenvironmental dynamics of Western Beringia - New studies from the Yedoma key site Duvanny Yar (Lower Kolyma River, Siberia)
No full textDuvanny Yar is a stratigraphic key site for the late Quaternary in Western Beringia. It is characteristic for ice-rich permafrost sequences of the so-called Yedoma Suite in north-east Siberia (e.g. KAPLINA et al. 1978; SHER et al. 1979) and is an important reference site for the late Pleistocene history of Beringia (HOPKINS 1982). The aim of our study was to reconstruct the paleoenvironmental dynamics at the Duvanny Yar site during the late Quaternary using its terrestrial archive. A multidisciplinary approach using geocryological, geochronological, sedimentological, hydrochemical, isotope geochemical, and paleoecological methods was applied to obtain multiproxy records. Sediment samples were analysed for ice contents, grain size parameters, biogeochemistry (total carbon, total organic carbon, total nitrogen, stable carbon isotopes), mineral density, mass specific magnetic susceptibility, and for radiocarbon age. Stable isotopes of water were measured for ground ice (ice wedges, segregated ice, and pore ice), modern surface waters and modern precipitation. Six profiles along the riverbank were sampled in August 2008. They contained Eemian lacustrine deposits, long sequences of Ice Complex deposits of the Late Pleistocene Yedoma, Holocene lacustrine and boggy deposits in thermokarst depressions. All profiles showed very bad sorted sediment of fine to coarse silt. A homogenous and polymodal grain size distribution for the ice rich (~30 to 60 wt %) Yedoma Suite revealed a polygenetic origin and disproves the pure arctic loess hypothesis for these deposits. Measurements of bulk density, ice content and total organic carbon content (TOC) enable for a relative TOC content in Ice Complex deposits at Duvanny Yar. The mean value of organic carbon at Duvanny Yar is 16 +- 11 kg/m³. Geochronological results based on 11 new AMS ages revealed that the Yedoma Suite was continuously formed from the end of the Middle Weichselian (~ 40000 years BP) and at least until the Late Glacial Maximum (~ 20000 years BP). Stable water isotopes measured in ice wedges, segregated ice and ice lenses were used to estimate paleotemperature changes. Isotopic signals revealed cold temperatures for the whole period with relative warm peaks in the Eemian and Holocene times. During Yedoma Suite formation the climate was stable and cold. A better understanding of the paleoenvironmental dynamics at Duvanny Yar may provide a basis for more reliable predictions of future reaction on global warming of organic-bearing ice-rich permafrost in Siberia which is considered as a potential greenhouse gas source permafrost (DUTTA et al. 2006; ZIMOV et al. 2006).ReferencesDutta, K., Schuur, E.A.G., Neff, J.C. and Zimov, S.A. (2006). "Potential carbon release from permafrost soils of Northeastern Siberia", Global Change Biology 12(12): 2336-2351.Hopkins, D.M. (1982). Aspects of the paleogeography of Beringia during the Late Pleistocene. In: Paleoecology of Beringia. Hopkins, D. M., Matthews, J. V., Schweger, C. E. and Young, S. B., Academic Press: 3-28.Kaplina, T.N., Giterman, R.E., Lakhtina, O.V., Abrashov, B.A., Kiselyov, S.V. and Sher, A.V. (1978). "Duvannyy Yar - a key section of Upper Pleistocene deposits of the Kolyma lowland", Bulletin of Quaternary Research Commission 48: 4965. (in Russian)Sher, A.V., Kaplina, T.N., Giterman, R.E., Lozhkin, A.V., Arkhangelov, A.A., Kiselyov, S.V., Kouznetsov, Y.V., Virina, E.I. and Zazhigin, V.S. (1979). Late Cenozoic of the Kolyma Lowland. 14th Pacific Science Congress, Khabarovsk, Academy of Science, USSR.Zimov, S.A., Davydov, S.P., Zimova, G.M., Davydova, A.I., Schuur, E.A.G., Dutta, K. and Chapin III, F.S. (2006). "Permafrost carbon: Stock and decomposability of a globally significant carbon pool", Geophysical Research Letters 33(20): L20502
2508 POSTER Pseudomyxoma Peritonei – the Role of Cytoreductive Surgery in the Combination With Fluorescental Laparoscopy and Intraperitoneal Photodynamic Therapy
No full textArterial blood supply to the liver, celiac and mesenteric pool (literature review) [Артериальное кровоснабжение печени, целиако-мезентериальный бассейн (обзор литературы)]
No full textKnowledge of the variant vascular anatomy of the celiac and mesenteric basin is of paramount importance in operative interventions in the hepatopancreatobiliary zone, stomach and duodenum, liver transplantation. The article presents various classifications of vessels of the celiac and mesenteric basin, created by the authors as a result of numerous studies using various techniques. The main advantages and disadvantages of the main classifications of blood vessels are considered, the features of application in clinical practice are given. © 2021 The Author(s
