Clumped isotope paleothermometry using pedogenic carbonates is a powerful tool for investigating past climate changes. However, location-specific seasonal patterns of precipitation and soil moisture cause systematic biases in the temperatures they record, hampering comparison of data across large areas or differing climate states. To account for biases, more systematic studies of carbonate forming processes are needed. We measured modern soil temperatures within the San Luis Valley of the Rocky Mountains and compared them to paleotemperatures determined using clumped isotopes. For Holocene-age samples, clumped isotope results indicate carbonate accumulated at a range of temperatures with site averages similar to the annual mean. Paleotemperatures for late Pleistocene-age samples (ranging 19-72 ka in age) yielded site averages only 2 degrees C lower, despite evidence that annual temperatures during glacial periods were 5-9 degrees C colder than modern. We use a 1D numerical model of soil physics to support the idea that differences in hydrologic conditions in interglacial versus glacial periods promote differences in the seasonal distribution of soil carbonate accumulation. Model simulations of modern (Holocene) conditions suggest that soil drying under low soil pCO2 favors year-round carbonate accumulation in this region but peaking during post-monsoon soil drying. During a glacial simulation with lowered temperatures and added snowpack, more carbonate accumulation shifted to the summer season. These experiments show that changing hydrologic regimes could change the seasonality of carbonate accumulation, which in this study blunts the use of clumped isotopes to quantify glacial-interglacial temperature changes. This highlights the importance of understanding seasonal biases of climate proxies for accurate paleoenvironmental reconstruction. Reconstructing the amount of temperature change associated with past climate changes for individual regions is important for understanding their climate vulnerability. Carbonate minerals developed naturally in desert soils record past temperatures in the numbers of their rare isotopes, called clumped isotopes. However, the temperature recorded in soil minerals is linked to the time of year they form, which varies greatly from winter to summer, so understanding the timing is key to interpreting past climate. We measured underground temperatures in the southern Rocky Mountains, compared them to mineral temperatures from young soils, and found that they record mean annual soil temperature. In contrast, temperatures recorded by soil minerals during the last ice age were only 2 degrees C colder than young soil temperatures, despite evidence that ice age air temperatures were 5-9 degrees C colder. We performed numerical modeling to predict the seasonal timing of soil carbonate accumulation under interglacial and glacial climate states and found that carbonate likely forms year-round during interglacial states but forms during the summertime under glacial conditions due to delayed melting of snow under colder temperatures. This lowers the difference between glacial and interglacial temperatures, which is important to account for when quantifying past climate change for the region. Clumped isotope temperatures for soil carbonate are biased to different seasons in different regions and time periods depending on climate In the San Luis Valley, USA, monitoring, modeling, and isotope results suggest carbonate accumulation throughout the year in the Holocene In the glacial late Pleistocene, clumped isotopes and soil modeling indicate longer snow cover shifted carbonate accumulation to the summer

A range of polycyclic aromatic hydrocarbons has been identified, and regularities of their vertical distribution in the peatland of hummock-hollow complexes in the southern tundra - forest tundra and northern tundra - southern tundra ecotones of the European Arctic zone have been determined. Benzo[ghi]perylene, naphthalene, pyrene, fluorene, phenanthrene, benzo[b]fluoranthene and benzo[a]pyrene are displayed most in the peatlands under study. Regarding the peatland profile the vertical polyarene distribution is similar - in 150-175 cm permafrost layers (site 1) and 50(70)-210(250) cm layers (site 2), and on the border between the active layer and permafrost 35-50(60) cm (site 1) and 30(42) - 50 cm (site 2) a significant increase of HCO-accumulated PAHs weight fraction is observed. PAHs content maximums in tundra peatland horizons are associated both with 4-, 5- and 6-nuclear structures at both sites under the analysis, and with a larger amount of 2- and 3-nuclear polyarenes in the peatlands on the northern tundra-southern tundra ecotone. Aeration-exposed seasonally thawing peatland layers are subject to continuous formation of primarily light 2- and 3-nuclear PAHs of natural origin resulting from microbiological decomposition of plant residues, which are subsequently involved in equilibrium cycles of chemical and biochemical transformation, with their total capacity remaining almost unchanged and constituting ?200-500 ng/g. Owing to low productivity of plant communities and absence of tree vegetation in the seasonally thawed layer, accumulation of the sum of 4-, 5- and 6-nuclear PAHs weakens significantly. One can detect dependencies between individual PAHs and the botanic composition of peat through higher weight fraction of 4-, 5- and 6-nuclear polyarenes being lignin transformation products generated more as the share of tree vegetation grows. The PAHs composition is a paleoclimatic marker reflecting adequately both changing paleovegetation stages and the degree of peat decomposition.

The VAMPERS (Vrije Universiteit Amsterdam Permafrost Snow Model) has been coupled within iLOVECLIM, an earth system model. This advancement allows the thermal coupling between permafrost and climate to be examined from a millennial timescale using equilibrium experiments during the Last Glacial Maximum (21 ka) and transient experiments for the subsequent deglaciation period (21-11 ka). It appears that the role of permafrost during both stable and transitional (glacial-interglacial) climate periods is seasonal, resulting in cooler summers and warmer winters by approximately +/- 2 degrees C maximum. This conclusion reinforces the importance of including the active layer within climate models. In addition, the coupling of VAMPERS also yields a simulation of transient permafrost conditions, not only for estimating areal changes in extent but also total permafrost gain/loss.

Heterogeneous Holocene climate evolutions in the Northern Hemisphere high latitudes are primarily determined by orbital-scale insolation variations and melting ice sheets. Previous inter-model comparisons have revealed that multi-simulation consistencies vary spatially. We, therefore, compared multiple model results with proxy-based reconstructions in Fennoscandia, Greenland, north Canada, Alaska and Siberia. Our model-data comparisons reveal that data and models generally agree in Fennoscandia, Greenland and Canada, with the early-Holocene warming and subsequent gradual decrease to 0 ka BP (hereinafter referred as ka). In Fennoscandia, simulations and pollen data suggest a 2 degrees C warming by 8 ka, but this is less expressed in chironomid data. In Canada, a strong early-Holocene warming is suggested by both the simulations and pollen results. In Greenland, the magnitude of early-Holocene warming ranges from 6 degrees C in simulations to 8 degrees C in delta O-18-based temperatures. Simulated and reconstructed temperatures are mismatched in Alaska. Pollen data suggest strong early Holocene warming, while the simulations indicate constant Holocene cooling, and chironomid data show a stable trend. Meanwhile, a high frequency of Alaskan peatland initiation before 9 ka can reflect a either high temperature, high soil moisture or large seasonality. In high-latitude Siberia, although simulations and proxy data depict high Holocene temperatures, these signals are noisy owing to a large spread in the simulations and between pollen and chironomid results. On the whole, the Holocene climate evolutions in most regions (Fennoscandia, Greenland and Canada) are well established and understood, but important questions regarding the Holocene temperature trend and mechanisms remain for Alaska and Siberia. (C) 2017 Elsevier Ltd. All rights reserved.

Declining sea-ice extent is currently amplifying climate warming in the Arctic. Instrumental records at high latitudes are too short-term to provide sufficient historical context for these trends, so paleoclimate archives are needed to better understand the functioning of the sea ice-albedo feedback. Here we use the oxygen isotope values of wood cellulose in living and sub-fossil willow shrubs (delta O-18(wc)) (Salix spp.) that have been radiocarbon-dated (C-14) to produce a multi-millennial record of climatic change on Alaska's North Slope during the Pleistocene-Holocene transition (13,500-7500 calibrated 14C years before present; 13.5-7.5 ka). We first analyzed the spatial and temporal patterns of delta O-18(wc) in living willows growing at upland sites and found that over the last 30 years delta O-18(wc) values in individual growth rings correlate with local summer temperature and inter-annual variations in summer sea-ice extent. Deglacial delta O-18(wc) values from 145 samples of subfossil willows clearly record the Allerod warm period (similar to 13.2 ka), the Younger Dryas cold period (12.9-11.7 ka), and the Holocene Thermal Maximum (11.7-9.0 ka). The magnitudes of isotopic changes over these rapid climate oscillations were similar to 4.5 parts per thousand, which is about 60% of the differences in delta O-18(wc) between those willows growing during the last glacial period and today. Modeling of isotope-precipitation relationships based on Rayleigh distillation processes suggests that during the Younger Dryas these large shifts in 6180,c values were caused by interactions between local temperature and changes in evaporative moisture sources, the latter controlled by sea ice extent in the Arctic Ocean and Bering Sea. Based on these results and on the effects that sea-ice have on climate today, we infer that ocean-derived feedbacks amplified temperature changes and enhanced precipitation in coastal regions of Arctic Alaska during warm times in the past. Today, isotope values in willows on the North Slope of Alaska are similar to those growing during the warmest times of the Pleistocene-Holocene transition, which were times of widespread permafrost thaw and striking ecological changes. (C) 2017 Elsevier Ltd. All rights reserved.

Northern Tibetan Plateau uplift and global climate change are regarded as two important factors responsible for a remarkable increase in dust concentration originating from inner Asian deserts during the Pliocene-Pleistocene period. Dust cycles during the mid-Pliocene, last glacial maximum (LGM), and present day are simulated with a global climate model, based on reconstructed dust source scenarios, to evaluate the relative contributions of the two factors to the increment of dust sedimentation fluxes. In the focused downwind regions of the Chinese Loess Plateau/North Pacific, the model generally produces a light eolian dust mass accumulation rate (MAR) of 7.1/0.28 g/cm(2)/kyr during the mid-Pliocene, a heavier MAR of 11.6/0.87 g/cm(2)/kyr at present, and the heaviest MAR of 24.5/1.15 g/cm(2)/kyr during the LGM. Our results are in good agreement with marine and terrestrial observations. These MAR increases can be attributed to both regional tectonic uplift and global climate change. Comparatively, the climatic factors, including the ice sheet and sea surface temperature changes, have modulated the regional surface wind field and controlled the intensity of sedimentation flux over the Loess Plateau. The impact of the Tibetan Plateau uplift, which increased the areas of inland deserts, is more important over the North Pacific. The dust MAR has been widely used in previous studies as an indicator of inland Asian aridity; however, based on the present results, the interpretation needs to be considered with greater caution that the MAR is actually not only controlled by the source areas but the surface wind velocity.

To evaluate the isotopic record of climate change and carbon sequestration in the Late Paleozoic, we have compiled new and published oxygen and carbon isotopic measurements of more than 2000 brachiopod shells from Carboniferous through Middle Permian (359-260 Ma) strata. We focus on the isotopic records from the U.S. Midcontinent and the Russian Platform because these two regions provide well-preserved marine fossils spanning a broad time interval. Both regions show a delta O-18 increase at the Mid-Carboniferous boundary (ca. 318 Ma) that roughly correlates with geologic evidence for an expansion of Gondwanan glaciers. Only the Russian Platform record shows a delta O-18 maximum during the glacial maximum in the Asselian. In contrast to a previous study [Korte, C., Jasper, T., Kozur, H.W., and Veizer, J., 2005. delta O-18 and delta C-13 of Permian brachiopods: a record of seawater evolution and continental glaciation. Palaeogeogr. Palaeoclimatol. Palaeoecol. 224, 333-351.], our data show no oxygen isotope evidence for glacial retreat in the early Permian, but instead show increasing delta O-18 values related to arification. Dissimilarity in the delta O-18 trends for the epicontinental seas of North American and the Russian Platform suggests that at least one region experienced periodic restriction that altered regional salinities and seawater delta O-18 values. These results highlight the need for complementary proxies to identify restricted circulation in epicontinental seas. Carbon isotopic compositions of carbonates exhibit substantial regional variation with low values in western North America, intermediate values in the midcontinent, and high values in the Sverdrup Basin, Russian Platform, and northern Spain. Nevertheless, both U.S. Midcontinent and Russian Platform records show a late Serpukhovian minimum, a sharp increase across the Mid-Carboniferous boundary, and a minimum centered on the Kasimovian. The correlative increase in brachiopod delta C-13 and delta O-18 values at the Mid-Carboniferous boundary is our best isotopic evidence for a link between the carbon burial and glaciation in the Permo-Carboniferous. (c) 2008 Elsevier B.V. All rights reserved.

Active-layer monitoring and the permafrost thermal regime are key indicators of climate change. The results of 3 years (1997-1999) of active-layer monitoring at one high-mountain site (La Foppa, 46degrees28' 42 N; 10degrees11' 18 E, 2670 m a.s.l.) and at one Antarctic site (Boulder Clay, 74degrees44' 45 S; 164degrees01' 17 E, 205 m a.s.l) are presented. The initial analysis of a thermal profile in a borehole (100.3 m deep) within mountain permafrost at Stelvio (3000 m a.s.l., 46degrees30' 59N; 10degrees28' 35 E) is also presented. At the alpine site, the active-layer thickness variations (between 193 and 229 cm) relate to both the snow cover and to the air temperature changes. By contrast, at the Antarctic site, there is a strong direct linkage only between air temperature fluctuations and active-layer variations. At the alpine (La Foppa) site, the relationship between climate and active-layer thickness is complicated by thermal offset that is almost negligible at both the Stelvio and Antarctic sites. The permafrost temperature profile at Stelvio site contains a climate signal suitable for paleoclimate reconstruction. The permafrost at this site has a mean annual ground surface temperature (MAGST) of - 1.9degreesC (during 1998/1999), an active layer of about 2.5 in thick and a total thickness of - 200 m. Analysis of the MAGST history, obtained by applying a simple heat conduction one-dimensional model, revealed the occurrence of a cold period from 1820 to 1940 followed by a warming period until 1978. Since the beginning of the 1980s, temperature dropped (less than 2degreesC) until the middle 1990s, when a new period of warming started. All these climatic changes fit well with the glacial fluctuations in the area and with other paleoclimatic information derived from different proxy data. (C) 2003 Elsevier B.V. All rights reserved.