Climatic Reversal Research Articles

The Encke comet impact/airburst and the Younger Dryas Boundary: Testing the impossible hypothesis (YDIH)

Many have tagged the Younger Dryas Impact Hypothesis (YDIH), a supposition, lacking convincing evidence in support. The core of criticism lies squarely on uniformitarianism, that is, uniform processes moving uniformly with no room for catastrophic events, specifically cosmic catastrophic forces. Beyond philosophically based aversion to the YDIH, specific criticism comes from megafauna and archaeologic corners, related to the near coeval disappearance of specific Late Pleistocene species on the one hand, and relation to disruption and temporary disappearance of the Clovis people et al. on the other. The Younger Dryas geologic paradigm, originally in place with meltwater release into the Atlantic in tow, attention slowly drifted to explanation of an innocuous looking, thin (~1-3 cm), black sediment bed found in lacustrine and fluvial deposits of the American southwest, and other intercontinental places. Thus, with thin dark beds of Gubbio in mind, the quest to explain black mat (BM) beds took on a cosmic aspect, one with growing, supporting evidence on several continents. The impossible hypothesis, now the probable explanation of the Younger Dryas climatic reversal, is at center stage, set at 12.8 ka, with a burgeoning corpus of evidence its cornerstone.

Multi-proxy record of the Austrian Upper Triassic Polzberg Konservat-Lagerstätte in light of the Carnian Pluvial Episode

We present a multi-proxy investigation of a lower Carnian basinal succession from Polzberg in the Northern Calcareous Alps (Lower Austria). A section comprising a unique Konservat-Lagerstätte was studied based on bio- and chemostratigraphy along with geophysical methods, yielding a detailed and robust stratigraphic calibration of the Polzberg succession. The Polzberg section revealed the paleoceanographic history and helped to identify a global climatic reversal, the Carnian Pluvial Episode. The age of the Upper Triassic Reingraben formation in the Northern Calcareous Alps is refined as the Austrotrachyceras austriacum Zone within the lower Carnian (Julian 2). Ammonoids and conodonts provide a detailed biostratigraphic subdivision that serves as a basis for analyses of the faunal distribution and the paleoenvironmental evolution of the Upper Triassic Reifling Basin. The succession includes lithological and facies changes similar to those of coeval units in the Tethys. The Carnian was characterized by a weak (~ 1‰) positive δ13C trend, punctuated by a negative shift during the lower Carnian corresponding to the initiation of the Carnian Pluvial Episode, a period representing the onset of early/late Carnian transitional global greenhouse conditions. Organic maturity parameters and the conodont alteration index (CAI) show that the thermal overprint of the Polzberg section is low. Biomarker proxies suggest that the organic matter of the uppermost Göstling formation is a mixture of marine and terrestrial material deposited in a dysoxic environment. Within the overlaying Reingraben formation, the amount of marine biomass decreased gradually upwards. Oxygen-depleted conditions, probably due to water-column stratification, continued during deposition of the Reingraben formation. Bacterial sulfate reduction played a major role in organic matter degradation.

A diminished North Atlantic nutrient stream during Younger Dryas climate reversal

The high rate of biological productivity in the North Atlantic is stimulated by the advective supply of nutrients into the region via the Gulf Stream (nutrient stream). It has been proposed that the projected future decline in the Atlantic Meridional Overturning Circulation (AMOC) will cause a reduction in nutrient supply and resulting productivity. In this work, we examine how the nutrient stream changed over the Younger Dryas climate reversal that marked the transition out of the last ice age. Gulf Stream nutrient content decreased, and oxygen content increased at the Florida Straits during this time of weakened AMOC. The decreased nutrient stream was accompanied by a reduction in biological productivity at higher latitudes in the North Atlantic, which supports the link postulated in theoretical and modeling studies.

A 10Be chronology of the Esmark Moraine and Lysefjorden region, southwestern Norway: Evidence for coeval glacier resurgence in both polar hemispheres during the Antarctic Cold Reversal

From 10Be surface-exposure dating we make the case that classic late-glacial moraines in southwestern Norway, including the iconic Esmark moraine, and in the Southern Hemisphere middle latitudes, including the iconic Birch Hill moraine of the Southern Alps of New Zealand, were constructed coevally within dating uncertainties. This finding of simultaneous glacier resurgence in the two polar hemispheres carries the implication that the millennial-scale climate reversal of the last deglaciation was global and required a change in the energy budget of the earth's climate system.

New 14C data of megafaunal remains from Lithuania – implications for the palaeoenvironmental interpretation of the Middle Weichselian

Palaeobiological data, supplemented by new 14C dates in conjunction with palaeobotanical and lithological information, have allowed reconstruction of Middle Weichselian (MIS 3) environmental fluctuations in the southern Eastern Baltic region. Palaeoenvironmental reconstructions implying non-glacial conditions during the Middle Weichselian (MIS 3) are supported by the spatial and temporal context of recently discovered remains of Mammuthus primigenius Blumenbach and Rangifer tarandus Linnaeus, 1758. Recording both cold and warm climatic reversals of MIS 3, representatives of the megafauna thrived in an environment characterized by a heterogeneity of vegetation and climate. 14C dating shows that the majority of the megafaunal remains analysed represent the 38–45 cal kyr BP time-interval, which correlates with the Nemunas 2c cold interval (cryomer), and the 31–34 cal kyr BP or Mickñnai 3 thermomer. From pollen data, the palaeovegetation pattern varied from tree-less tundra to birch-predominating forest with an admixture of temporal tree species providing additional information about the diet and habitat preferences of these herbivores in the context of the MIS 3 climatic events.

Terminal Pleistocene human occupation of the upper Copper River basin, southern Alaska: Results of test excavations at Nataeł Na’

After decades of debate, the homeland of the First Americans is now generally understood to be northeast Asia; however, the process of Late Pleistocene peopling remains unresolved. As more archaeological sites south of the continental ice sheets are discovered that predate the opening of the interior “ice-free” corridor, interest in a coastal Pacific dispersal route has grown, and previously overlooked regions proximal to the Pacific coast have become a central focus of exploration efforts. The Copper River basin of southern Alaska is one such region. Here we present the results of 2019 archaeological excavations at Nataeł Na’, a buried and stratified archaeological site situated along the upper Copper River. The site contains a robust occupation dating to the late Younger Dryas climate reversal as well as an earlier occupation dating to the late Allerød interstadial. This discovery demonstrates that Pleistocene hunter-gatherers inhabited the Pacific basin of southern Alaska during the same time Clovis peoples inhabited temperate North America. The occupations at Nataeł Na’ join a growing body of evidence suggesting that the early inhabitants of eastern Beringia were geographically more widely dispersed than previously documented.

Mid to late Holocene climate variability, forest fires and floods entwined with human occupation in the upper Ganga catchment, India

The present study attempts to understand the geomorphic response in the upper Ganga catchment to the mid-late Holocene (neoglacial) climate variability. The study infers five major phases of millennial-scale climate variability with centennial-scale inversions using geochemical and magnetic proxies from relict Lesser Himalayan Lake sediments. Phase-1 (6–4 ka) is marked by enhanced precipitation/runoff (increased allochthonous contribution) under a stronger Indian Summer Monsoon (ISM). The prominent reversal in the trend between ∼5 and 4 ka includes global arid events such as 4.2 ka. Phase-2 (4–2.2 ka) shows a declining precipitation/runoff (decreased allochthonous input) under declining ISM with a prominent dip after ∼3 ka. After phase-2 the climate reversals are distinct and of shorter (centennial) duration. For example, in Phase-3 (2.2–1.4 ka) improved ISM is inferred; Phase-4 (1.4–1.0 ka) is marked by a sharp decline in the ISM, and Phase-5 (<1.0 ka) includes centennial-scale events of Medieval Climate Anomaly (MCA) and the onset of Little Ice Age (LIA). The relative increase (decrease) in the concentration of geochemical and magnetic proxies is indicative of strengthened (weakened) ISM where relatively drier phases are in sync with the North Atlantic climate perturbations. We observed clustering of optically dated flood events around 6.5, 4.5, 2.6, 1.4, 0.8, and 0.4 ka which corresponds to periods of moderate ISM thus, suggesting a coupling between warm-humid monsoon and relatively dry westerlies. The relatively higher concentration of micro-charcoal in the lake sediments indicates widespread forest fires around 5.9–5.3, 4.5–4.3, 3.4–3.0, 2.0–1.5 and ∼1 ka. Given the archaeological evidence of sedentary settlements since ∼3 ka in the upper Ganga catchment, the study speculatively argues anthropogenic forcing for forest fires after 3 ka. Further, the highest probability flood phases succeed the fire events and may be indicative of enhanced vulnerability of the catchment to floods due to vegetation loss (enhanced erosion and surface runoff).

Ice‐sheet deglaciation and Loch Lomond Readvance in the eastern Cairngorms: implications of a Lateglacial sediment record from Glen Builg

We present new lithostratigraphic, pollen-stratigraphic and tephrochronological data obtained from a sediment sequence in a small lake basin in Glen Builg, eastern Cairngorms, with mapping of the surrounding glacial geomorphological features. The collective evidence indicates that basin organic sediment accumulation started during the Windermere Interstadial at ~14.3 ka. The new results reaffirm the conclusions of Clapperton et al. (1975), that any occupation of the eastern Cairngorms by Loch Lomond (‘Younger Dryas’) Stadial glacier ice was spatially restricted. The record also suggests that harsh climatic conditions prevailed throughout the Windermere Interstadial, probably due to the relatively high altitude of the site (~460 m) and its proximity to major Cairngorm peaks. Our new, more comprehensive study provides a robust chronology, and reveals several palaeoenvironmental signals congruent with other recently reported Scottish Highlands records. Key similarities with regional records are: (i) a short climatic reversal corresponding to GI-1d in the Greenland stratotype sequence; (ii) a two-phased Loch Lomond Stadial, with a transition around the Vedde Ash, dating to ~12.1 ka; and (iii) a delayed response to Early Holocene climatic warming of ~250 years, before soils were sufficiently stabilised to permit shrub vegetation establishment.

Sedimentation and organic content in the mires and other sites of sediment accumulation in the Sydney region, eastern Australia, in the period after the Last Glacial Maximum

This research reports on a synthesis of radiocarbon (14C) dates and the organic content from a variety of depositional sites in a relatively small region in humid eastern Australia centred on Sydney. We focused on basal dates, changes in accumulation rates and the organic content of these sediment records in the period post-dating 21 thousand years ago to make inferences about past environmental conditions, or to infer the timing of past environmental change. We found that low rates of sediment and organic accumulation at the Last Glacial Maximum continued well into the late Pleistocene. The average rate of sediment deposition and organic content increased from about 14.5 calibrated kiloanni before present (cal ka BP), perhaps coincident with Meltwater Pulse 1A, but this was then checked by dry conditions during the Antarctic Climatic Reversal chronozone. An abrupt increase in the average rate of sediment accumulation coincided with the Pleistocene-Holocene boundary (at 11.7 cal ka BP), and this continued until about 9.2 cal ka BP, largely coinciding with the Antarctic thermal maximum, and probably reflecting reduced moisture availability in the Sydney region. Indication of depressed temperature and a more positive moisture availability, from about 9 to 7.6 cal ka BP, witnessed the fastest increase in the number of sites than at any other time. An overall drier early Holocene abruptly altered at about 7.5 cal ka BP and a mid-Holocene ‘optimum’ in moisture availability extended to 6.2 cal ka BP. The compilation of organic contents of the mid-late Holocene suggests environmental variability, but El Niño frequency or strength is not implicated as a driving force. Increased organic productivity, especially after 2.8 cal ka BP, corresponded to a peak in summer insolation, and associated seasonality of insolation and this is likely to have resulted in increased, or more consistent, summer rainfall and easterly air flow into the Sydney region. In contrast to previous generalisations about changes in south-eastern Australia during the Holocene, which have often relied on palaeoenvironmental records from further south, the southward migration of the intertropical convergence zone during the Holocene had vastly different synoptic outcomes in the Sydney region, with the early Holocene drier as continental air dominated, and the late Holocene wetter, as the synoptic conditions saw more tropical and easterly airflow. This work demonstrates the danger in making generalisations across regions and highlights the utility of the compilation of data at a homoclimatic, regional scale.

Sedimentary signatures of the abrupt deglacial rise in sea level from the East China Sea inner shelf

Understanding the link between deglacial sea-level jumps and abrupt climate change may provide crucial insights into future ice-climate feedbacks. However, much ambiguity remains surrounding many of the last deglacial meltwater pulses. Here, we present a complete sedimentary succession from the inner shelf of the East China Sea, which has clearly documented multiple abrupt rises in the deglacial sea level. The results suggest that the postglacial sedimentary sequence, resting on a late Pleistocene basement of stiff clay, formed during a three-stage process marked by characteristic lithology and foraminiferal fauna. The abrupt shifts in the sedimentary facies indicate that seawater firstly intruded into the inner shelf just before the onset of the slowdown in sea-level rise caused by the Younger Dryas cooling event and changed the inner shelf into a tidal flat environment, as demonstrated by the sudden appearance of both planktonic foraminifera and typical brackish-water species of benthic foraminifera. Meltwater pulse 1B (MWP-1B) is depicted by a sharp upward transition to a nearshore subtidal environment at 11.62 kyr. This episode of sea-level rise induced the initial flooding of the inner shelf, which is well documented by the sudden reduction in brackish-water foraminifera species and evident increase in inner-shelf benthic species. The prominent sea-level rise at approximately 7.54 kyr was responsible for the shift to stable inner shelf conditions and resulted in the maximum flooding of the inner shelf area, initiating subsequent formation of the alongshore mud wedge. Additionally, obvious peaks in both the benthic foraminifer abundance and the marine algae concentration at approximately 8.13 kyr confirm the previously identified sea-level jump at approximately 8.2 kyr, which coincides with the weakening of the Asian monsoon demonstrated by the peak grain size in our record, further indicating its apparent connection to the 8.2 kyr climatic reversal.

Evidence for centennial‐scale Lateglacial and early Holocene climatic complexity from Quoyloo Meadow, Orkney, Scotland

AbstractThe influence of the North Atlantic on the margins of Europe means the region is particularly sensitive to changes in the ocean–atmospheric system. During the Last Glacial–Interglacial Transition (16–8 cal ka bp) this system was repeatedly disrupted, leading to a series of abrupt and short‐lived shifts in climate. Despite much research, the number and magnitude of these ‘centennial‐scale’ events is not well understood. To address this, we expand upon investigations at Quoyloo Meadow, Orkney, Scotland, one of the best chronologically constrained palaeoclimate records in northern Britain. By coupling stable isotope and chironomid fossil analyses with existing data, this study identifies multiple phases of centennial‐scale disturbance at: c. 14.0, 11.1, 10.8, 10.5, 10.45 and 10.3 cal ka bp, with the events at 14.0 and 10.3 exhibiting a particularly pronounced cold‐climate signature. During the Holocene, the strongest response to climate forcing was at c. 10.3–10.0 cal ka bp, expressed as a two‐stage drop in mean July temperatures, a shift in pollen spectra indicative of ‘less‐stable’ climatic regimes, and a depletion in δ18O values. We interpret this as the first reliably dated incidence of the ‘10.3‐ka event’ in the British Isles and consider the wider impact of this climatic reversal in other Holocene records.

Tracking an exotic raw material: Aboriginal movement through the Blue Mountains, Sydney, NSW during the Terminal Pleistocene

A compliance-based excavation on Parramatta River (western Sydney) found evidence of a brief visitation by Aboriginal people during the terminal Pleistocene (c.14 ka), from which an exotic raw material–medium-grained porphyroblastic andalusite-cordierite hornfels–was recovered. This raw material is rare in the region, only found in the Megalong Valley situated some 75 km west of the site in Parramatta, and separated from the site by the Blue Mountains–a 40 km wide, 1 km high dissected sandstone upland. Historical observations and geological evidence suggest that the Coxs River, which runs through the upland, formed the probable connection between the two locales. The formation of the site at c.14 ka, along with our broader understanding of the region, suggests that the upland was only explored after the Last Glacial Maximum (LGM) and that the Great Dividing Range may have formed a significant barrier to early peopling of Sahul. Further, a delay between the end of the LGM and the exploration of the region implies populations were severely affected by the event and required considerable time to recover. The coincident timing of rapid sea-level rise from Meltwater Phase Pulse 1A and an increased supply of moisture associated with the Antarctic Climate Reversal at the time of many site initiations may have been factors in the upland and south west slopes visitation. Finally, the movement of the unusual hornfels artefact provides a coarse indication of the ranging territory for hunter-gatherers living in a temperate region (c. 8,000 km2) at this time. This is probably a lower estimate but begins to provide a quantitative value with which to begin to ratify the increasing divergence between archaeological and genomic studies in their definitions of mobility, sedentism and regional nomadism.

Changes in upper ocean hydrography and productivity across the Middle Eocene Climatic Optimum: Local insights and global implications from the Northwest Atlantic

Abstract The Middle Eocene Climatic Optimum (MECO; 40.5–40.0 Ma) was a pronounced climatic reversal during a long-term global cooling period. Despite conspicuous signals of warming and ocean acidification in global marine sedimentary records, changes in upper ocean thermal stratification and productivity have been less comprehensively understood across the MECO. Here we examined stratigraphic records of the middle Eocene succession (43.1–38.6 Ma) at Integrated Ocean Drilling Program Site U1408 in the Northwest Atlantic, to provide a detailed local environmental reconstruction and to bring new insights on spatiotemporal evolution of coupled hydrography and productivity across the MECO based on global comparison. The present temperature–oxygen isotope calibration inferred at least ~3 °C warming in the upper ocean during the MECO at the study site. Whereas further inter-proxy calibration is required, the present estimation supports expanded meridional surface temperature gradient in both hemispheres during the late middle Eocene. Vertical gradients in planktic foraminiferal oxygen isotope ratios revealed that relatively mixed upper water column prevailed during and immediately (~150 kyr) before the MECO. The observed ~102 kyr-scale local shifts in hydrography was possibly driven by the ~2.4-myr eccentricity modulation in the development of seasonal thermocline, rather than greenhouse gas forcing. Planktic and benthic foraminiferal accumulation rates at Site U1408 decreased and increased during the MECO and adjacent cooling episodes, respectively. The relationships between carbonate productivity and local hydrography varied through time in the Northwest Atlantic. Instead, we suggest that marine carbonate productivity and ocean chemistry was controlled by multiple eccentricity-scale changes in weathering intensity and nutrient supply, widely across the major ocean basins. Hypothesized prevailing decline in productivity agrees with a possible warming mechanism of the MECO, although it is challenged by several existing micropaleontological records suggesting increased bio-silica production and eutrophication at varying oceanographic settings. Further integration of different types of proxies and quantitative assessments will help understand complex biotic changes during significant global carbon cycle perturbations in a greenhouse world.

Effects of climate variability on mercury deposition during the Older Dryas and Younger Dryas in the Venezuelan Andes

We present a high-resolution record of Late Glacial and Early Holocene mercury (Hg) accumulation within the sediments of Laguna de Los Anteojos, a small headwater alpine lake in Venezuela. Our sediment core spans the Older Dryas (OD) and Younger Dryas (YD) climate reversals, providing new insight into the effects of abrupt climatic transitions on atmospheric Hg deposition. Sediment Hg concentrations and accumulation rates (fluxes) increased sharply after regional deglaciation and then peaked immediately prior to onset of both the OD and YD. Mercury concentrations and fluxes then decreased through cold OD and YD conditions. Both Hg concentrations and fluxes increased rapidly with warming, rising two-fold in about 100 years after the OD and nearly four-fold in about 500 years during the latter-half of the YD. This lake-sediment record of Hg dynamics during the Glacial–Holocene transition suggests atmospheric Hg cycling is affected by abrupt climate change.

Tree growth patterns associated with extreme longevity: Implications for the ecology and conservation of primeval trees in Mediterranean mountains

Abstract This study analyzed with dendrochronology 177 Heldreich’s pines growing on the Pollino Massif in southern Italy for understanding climatic and human impacts on old trees. Most of the large-diameter trees currently living became established in the late Medieval to Renaissance periods under a snowy wet climate and low anthropic influence. Millennium-old (i.e., > 900 years of age) trees in remote sites escaped Medieval human impacts, then a wave of pine stands established in the late 14th and 16th centuries following recurrent human plague epidemics. Stem growth histories showed that both millennium-old and the majority of century-old trees grew along similar trajectories. These old trees have survived long-lasting climatic reversals, clearly a sign of their resilience to extreme events. Cliff habitats played a strategic environmental role for tree conservation during periods of land exploitation; such biodiversity refugia may serve as stepping stones for rewilding mountain landscapes. In recent decades, land abandonment following the collapse of sheep-herding, together with climate warming, have led to a new pulse of tree recruitment. Since 1850, low-frequency variability (50-70-year periods) in tree growth has been in synchrony with the Atlantic Multidecadal Oscillation. Recently observed growth increases counter widespread reports of tree and forest decline in Mediterranean environments, and suggest that extreme longevity does not necessarily reduce stem increment. Discovering, studying, and preserving primeval trees in forest landscapes remains a priority for biodiversity conservation in the Anthropocene. Heldreich’s pine resilience to current global changes bodes well for sustainable development in the Mediterranean mountains they inhabit, and similar studies are needed for threatened habitats and iconic trees of other ecoregions in order to assess their probable survival into the future.

Late-Glacial Paleoecology of the Middle Susitna Valley, Alaska: Environmental Context for Human Dispersal

We present here the results of multi-proxy analyses (sediment geochemistry, diatoms, and pollen) from sediment cores collected at four lakes in the middle Susitna Valley, Alaska. These lakes form a transect from the tundra to the boreal forest. The retrieved cores span from ~12,000 cal yr BP to the present, with age control provided by radiometric dates and tephra deposits, some of which are newly identified. Results indicate that deglaciation occurred before 12,000 cal yr BP and that by that time, the lakes were deep, productive, and surrounded by shrub tundra. The lake with the highest sampling resolution indicates a brief climatic reversal ~11,500 cal yr BP with decreased diatom-inferred lake level and lowered lake productivity, and reduced shrub presence. During the early to middle Holocene, all of the sedimentary records provide evidence of climatic amelioration with tree expansion and productive lakes. A middle to late Holocene climatic deterioration with reduced trees and a shallower, less productive lake is also indicated. In addition, the prominent Watana tephra at ~4,000 cal yr BP likely reduced lake productivity and affected the vegetation. Even though the region was relatively productive soon after deglaciation, people did not occupy the region until ~11,000 cal yr BP, about 1000 years later, and then only sparsely. By the middle and late Holocene, the region was more densely populated and this shift in human occupancy presumably reflects changes in resource abundance, especially caribou. Whether the Watana ashfall influenced caribou abundance and thus people, is still under investigation, but given the tephra’s effect on vegetation and lake productivity, it seems likely.

Climate history of the southwestern United States based on Estancia Basin hydrologic variability from 69 to 10 ka

Newly acquired UTh dates on gypsum sands contained within Pleistocene lake sediments allow us to extend the record of climatic change in the Estancia Basin of central New Mexico back to ∼69 ka from a previous limit of ∼29 ka. Using a hydrologic balance index (HBI) that reflects variations in sediment mineralogy and bedding characteristics, we find strong similarities to speleothem oxygen isotopic records for the southwestern United States as well as to isotopic variations in Greenland ice cores. Intervals of wetter climate in the Estancia Basin generally appear to correlate to colder conditions in the North Atlantic and conversely. The existence of a pronounced dry period from 18.1 to 17 ka matches in timing the Extrapolar Climate Reversal of Asmerom et al. (2017) at Fort Stanton cave in southern New Mexico and confirms a different origin for this stadial-like event. Furthermore, the new UTh dates support the existing Estancia Basin interpretation of a dry Bølling/Allerød followed by a wet Younger Dryas, a result consistent with speleothem and wet meadow evidence from New Mexico and Arizona (Asmerom et al., 2010, 2017; Polyak et al., 2004; Hall et al., 2012; Wagner et al., 2010) but in conflict with an interpretation of speleothem evidence from central Texas (Feng et al., 2014).

Reconstruction of the mid-to Late- Holocene history of vegetation and land-use in Petrešiūnai, north-east Lithuania: Implications from palaeobotanical and archaeological data

This study presents the results of multi-proxy investigations of mid-to late-Holocene vegetation dynamics and land-use history associated with the Petrešiūnai archaeological site, north-east Lithuania. Identified vegetation changes are discussed in the context of both natural and anthropogenic factors determined within the site and its surroundings. The initial stages of colonisation and the subsequent land-use dynamics are discussed by applying the results of biostratigraphical and archaeological-historical data. To explore these issues, a lacustrine sediment core obtained from the stand-scale sedimentary basin was studied using a microbotanical (pollen and microcharcoal) survey and 14C and magnetic susceptibility (MS) measurements. Associations between proxies were statistically tested. In addition, archaeological excavations were conducted within the area of the Petrešiūnai hill fort. The results obtained show the presence of a heavily forested landscape with broad-leaved deciduous trees dominating the taxa existed in the area until approximately 4800-4700 cal BC. Low magnitude fire activity mainly caused by the natural factors was recorded. The expansion of spruce and the gradual decrease in thermophilous taxa suggests a local-scale climatic reversal started prior to the general decline of the HTM (Holocene Thermal Maximum). The decay of broad-leaved taxa followed by the pronounced culmination of Picea-dominated forest (2100-1500 cal BC) confirms the general re-organisation of the forest structure in the area. A small-scale vegetation stress, accompanied by an increase in fire interference suggestive of episodic Neolithic-Bronze Age human activity, was noted in the area at approximately 2800-2200 cal BC, 1800 cal BC and 1500 cal BC. Biostratigraphical and sedimentological data suggest agriculture practices and animal husbandry were introduced and the landscape was increasingly utilised between 1300 and 1000 cal BC, which preceded the archaeological record of the Petrešiūnai hill fort by a few hundred years. However, as indicated by pollen data, pastoral farming predominated the local economy until approximately 400 AD. The establishment of regular cultivation practices, including crop rotation, and the formation of permanent fields with introduction of winter cereals began in the surroundings of Petrešiūnai by the 6th - 7th c. AD. At that time, the secondary open Betula-dominant vegetation successions flourished in the area. A highly diversified landscape with large agriculture fields, open grounds, meadows and pastures culminated in the area between 1000 and 1300 AD. After this period, the intensity of the human-induced vegetation changes apparently decreased, which confirms re-organisation of the settlement pattern and land-use in this area.

14C chronology and stable isotopes on Lymnaea viatrix shells in northwest Patagonia, Argentina. Do they express the Antarctic climatic reversal?

A multi-isotope environmental record comprising an upper section of the late Pleistocene and the lower and middle Holocene, including isotopic data on 30 samples of peat, marl, tufa, Lymnaea viatrix shells, and Hippidion sp. teeth, is described from a 15 m profile at Arroyo Leuto Caballo, Neuquen Province, Argentina. The chronology of the Pleistocene sequence was derived from five modeled 14C ages. δ18O of L. viatrix, assumed as a proxy for δ18O of meteoric water isotopic composition and thus sensitive to air temperature changes, showed a warming period from 14.03 cal ka BP until ca. 13.90 cal ka BP, followed by a rapid decline in temperature, attaining a minimum between 13.79 and 13.56 cal ka BP and a subsequent warming reestablishment. In addition, a “specular pattern” of δ13C of L. viatrix peaking in the same time span would be probably showing aridity. This pattern developed within the globally defined Antarctic Cold Reversal-ACR-time span (from 14.6–2.8 cal ka BP), and prior to the onset of the Huelmo Mascardi Cold Reversal episode, HCMR, from ca 13.30–1.87 cal ka BP. Given the characteristics and the time span covered, it could be related to a continental expression of the ACR event.

Implications for conifer glacial refugia and postglacial climatic variation in western Beringia from lake sediments of the Upper Indigirka basin

A c. 13 800 cal. a BP palynological record from Tschuch'ye Lake (eastern Yakutia), when combined with other palaeobotanical records from western Beringia, supports previous conclusions that Larix and Pinus pumila spread from multiple cryptic refugia during the Lateglacial period and middle Holocene, respectively. The antiquity and spatial distribution of Larix in many of these records suggest that full‐glacial landscapes probably had a small but important tree component. The Tschuch'ye record further indicates the absence of a Younger Dryas climatic reversal and is in accord with the preponderance of other data from western Beringia, which indicate warmer than present climates between c. 12 900 and 9100 cal. a BP. The subsequent spread of Pinus pumila, which has similar temperature requirements as Larix, denotes a shift to wetter but still warm climates during the middle Holocene, with palynological data suggesting optimal conditions in far western areas of western Beringia c. 7000–5000 cal. a BP. The postglacial occurrence of relatively warm/dry and warm/wet intervals is consistent with results of a regional climate‐model simulation that indicates warmer than present temperatures and decreased effective moisture at 11 000 cal. a BP and persistence of warm conditions but with greater moisture and longer growing season at 6000 cal. a BP.