Early Holocene Warming Research Articles

Late-glacial and Holocene shifts in the mountain landscapes of the Cantabrian range (northern Spain) in response to changing climate, fire occurrence and land use

A new well-dated (18 14C dates on short-lived terrestrial plant macrofossils, 210Pb and 137Cs dating) fine-resolution palaeoecological sequence (pollen, coprophilous fungi, conifer stomata, macrofossils, charcoal) from Lago de Isoba (1400 m a.s.l.) spanning the past c. 17,000 years provides novel insights into the long-term dynamics of the high-elevation Cantabrian landscape in response to climate variability and human activities. Vegetation changes were mainly climate-driven during the Late-glacial and the Early Holocene. Thus, steppe-like environments prevailed during the Greenland Stadial 2.1a (GS-2.1a ≈ ‘Oldest Dryas’). Warmer and moister conditions during the Greenland Interstadial 1 (GI-1 ≈ ’Bølling-Allerød’) enabled successive expansions of Juniperus, Betula and Pinus. Steppe-like vegetation re-expanded during the cooler and drier Greenland Stadial 1 (GS-1 ≈ ‘Younger Dryas’). Afterwards, the Early Holocene warming triggered the rapid spread of mixed pine-broad-leaved deciduous mountain forests. Pollen, stomata and macrofossils indicate continuous, long-lasting (c. 10,850–1250 cal yr BP) local occurrence of Pinus sylvestris stands. The records of Cerealia and Plantago lanceolata pollen types (first Holocene occurrences dated at c. 5100 cal yr BP) evidence increased land use during the Late Holocene, which triggered substantial changes in forest ecosystems. Following a phase of intensified arable and pastoral farming, which involved the use of fire to create and maintain forest clearings, pinewoods expanded and reached their maximum development between c. 3600 and 3300 cal yr BP. Pinus probably outcompeted temporarily other woody taxa thanks to its resilience to low-intensity disturbances. Later, pinewoods underwent a stepwise, human-induced decline ending in their final massive collapse after c. 1150 cal yr BP. The highest intensity of agricultural and pastoral farming took place during the Middle Ages, following maximum fire occurrence. We conclude that the present-day open landscape has been shaped by millennia of land use that partly blurred the impact of climate variability.

Holocene paleoclimatic records from Chakrata area, Northwest Himalaya

We present monsoon variability records for the Holocene using multi-proxy approach (environmental magnetism, carbon isotope, and total organic carbon) from a 146 cm thick sedimentary profile in the Kotikanasar meadow (Chakrata), Northwest Himalaya. The chronology of the record was constrained by five AMS 14C ages. The carbon isotope (δ13C) and Total Organic Carbon (TOC) data highly variable which vary between -26.62‰ and -22.46‰ (C3-plants) and 0.1 to ∼4%, respectively, indicating paleo-vegetation history and productivity of the studied area. The environmental magnetics indicates highly fluctuating in the Early Holocene with high concentrations of magnetic minerals during high monsoon conditions and vice-versa. Intense Indian Summer Monsoon (ISM) phases were identified during the Early and Late Holocene i.e., ∼9.2 to 7.4 ka, and ∼4.8 ka to Modern which shows warm and wet climate. While decline in the ISM intensity during ∼7.4 to 4.8 ka which indicates cold and dry climatic condition in the Northwest Himalayan regions. From ∼9.2 to 7.4 ka highly fluctuating climate linked with the Early Holocene warming. Sediment profile exhibits aridity in climate accompanying with the high influence of mid-latitude westerlies during ∼7.4 to 4.8 ka from Northwest Indian regions (Enzel et al., 1999). Hence the long-term fluctuation in the climate governed by the changes in the North Atlantic Ocean circulation as well as variations in the incoming solar radiations.

A method for estimating paleotemperatures from pollen assemblages applied to the quantification of the Early Holocene warming in Central Brazil

This study presents a method for the quantification of paleotemperatures from pollen records based on the predominant leaf margin trait of the genera represented in the pollen assemblages. The method presented here uses the well-known and widely used relationship between leaf margin and mean annual temperature, which is commonly applied to macrofloras to reconstruct paleotemperatures. Here, genera occurring in the arboreal vegetation of Central Brazil were analyzed respective to the presence or absence of species bearing toothed leaves. Using floristic surveys, a correlation between the proportion of genera without toothed species and the mean annual temperature was investigated. A positive linear correlation between these two variables was found for forest sites, but not for the arboreal savannas. The equation derived from the linear regression between the two variables - proportion of genera without toothed species and mean annual temperature (MAT) - was applied to pollen assemblages collected on forest sites. The equation was used to calculate estimated MATs for surface samples in order to test the validity of its application to pollen assemblages. The equation was then applied to fossil records to estimate paleoMATs for two Brazilian sites located respectively at 15.5° and 28° of latitude, during the Pleistocene-Holocene transition (between 13.6 and 10 kyr cal BP). While MAT increases of around 3 °C were found for the higher latitude site, no significant differences between final Pleistocene and early Holocene MATs were found for the near-equatorial site.

Sedimentary DNA reveals the link between microbial community dynamics and climate during the late last glaciation in the offshore region of the Zambezi River, Southwest Indian Ocean

Reconstructing the relationship between microbial communities and past abrupt climate change is of great importance for understanding current biodiversity patterns and predicting changes under future climate scenarios. However, little is currently known about how microbial communities respond to changes in key environmental stages due to a lack of research in this area. Here, we examine the variability in the communities of bacteria, archaea, and fungi from sediments deposited offshore region of the Zambezi River between 21.7 and 9.6 thousand years ago (ka) (covering the last glacial maximum, or LGM, and the early Holocene) using DNA metabarcoding approach via high-throughput sequencing. The results showed that (1) microbial assemblages differed across three key time intervals, with the last deglaciation having the most homogeneous prokaryotic assemblages, while for fungal communities in the LGM, and the early Holocene and LGM differing the most; (2) the warm early Holocene showed the highest diversity, whereas the lowest diversity was found in the LGM; and (3) the selected indicator species better reflected the climatic characteristics of different environmental stages. These results highlight the power of ancient sedimentary DNA to refine our understanding of microbial dynamics in marine sedimentary systems near large rivers, thus providing a basis for better modeling ecological processes in further research.

Relative importance of forcings and feedbacks in the Holocene temperature conundrum

The pre-industrial Holocene provides the backdrop for the emergence of civilisations and the starting point of anthropogenic climate change. Several reconstructions show an early Holocene warming that was followed by cooling for several thousand years before Industrialisation. In contrast climate simulations show warming throughout the Holocene. Whilst reconstructed trends over ocean can be reconciled with warming through either seasonality or uncertainties, a consistent explanation for cooling trends over some land areas is missing. We present a suite of transient Holocene climate model simulations with a coupled general circulation model and show that a widespread mid- to late-Holocene cooling emerges over some regions of the northern hemisphere with the inclusion of anthropogenic land-use. This is mostly because in regions of prescribed late Holocene deforestation, the simulated early to mid-Holocene is characterised by a lower albedo than the late Holocene. Whilst this cooling through time can quantitatively explain some regional aspects of the reconstructions, the model-data agreement remains imperfect, and differences between reconstructions also hinders the evaluation. Moreover, model-dependency in the response of several feedbacks, particularly sea-ice, but potentially also clouds, means that it is difficult to uniquely attribute Holocene temperature evolution to specific factors. Future work should aim to derive a consensus-signal including uncertainties from the available proxy data which could be used to fingerprint simulations covering a range of plausible feedback strengths.

Elevated Grand Canyon groundwater recharge during the warm Early Holocene

Elevated Grand Canyon groundwater recharge during the warm Early Holocene

Responses of Caribbean Mangroves to Quaternary Climatic, Eustatic, and Anthropogenic Drivers of Ecological Change: A Review.

Mangroves are among the world's most threatened ecosystems. Understanding how these ecosystems responded to past natural and anthropogenic drivers of ecological change is essential not only for understanding how extant mangroves have been shaped but also for informing their conservation. This paper reviews the available paleoecological evidence for Pleistocene and Holocene responses of Caribbean mangroves to climatic, eustatic, and anthropogenic drivers. The first records date from the Last Interglacial, when global average temperatures and sea levels were slightly higher than present and mangroves grew in locations and conditions similar to today. During the Last Glaciation, temperatures and sea levels were significantly lower, and Caribbean mangroves grew far from their present locations on presently submerged sites. The current mangrove configuration was progressively attained after Early Holocene warming and sea level rise in the absence of anthropogenic pressure. Human influence began to be important in the Mid-Late Holocene, especially during the Archaic and Ceramic cultural periods, when sea levels were close to their present position and climatic and human drivers were the most influential factors. During the last millennium, the most relevant drivers of ecological change have been the episodic droughts linked to the Little Ice Age and the historical developments of the last centuries.

Lateglacial paleoglacier and paleoclimate reconstructions in the north-western Italian Alps

Lateglacial (19.0–11.7 ka ago) paleo-temperature records in the Western European Alps document several short-term cooling episodes within the general post-Last Glacial Maximum warming trend, in phase with North Hemisphere climatic oscillations. Alpine paleo-precipitation reconstructions are instead rare, and further constraints are needed in order to assess whether Lateglacial cold periods were associated with a modified atmospheric circulation pattern over the European Alps. The Alpine paleoglacial record offers a quantitative framework to investigate Lateglacial paleoclimatic conditions and glacier sensitivity in response to both temperature and precipitation changes. Through the combination of 10Be surface-exposure dating of glacial landforms and deposits constraining ice front and surface, together with numerical glacier simulations (iSOSIA), our study aims to reconstruct in space and time the main Lateglacial ice stages and associated paleo-climatic conditions in three tributary valleys (Valpelline, Valsavarenche and Val di Cogne) located within the Dora Baltea catchment (north-western Italian Alps). Our dating-modelling approach reveals in all the three investigated sectors two distinct paleoglacier stages at ca. 13 and 11 ka, documenting respectively the ice configurations at the transition between the Oldest Dryas cold period and the Bølling-Allerød interstadial, and between the Younger Dryas cold period and the early Holocene warming. Numerical ice-simulation outcomes suggest a similar-to-today precipitation pattern during the two ice stages, with either same absolute or homogeneously decreased precipitation values over the Dora Baltea catchment, although a unique quantitative solution of paleo-precipitation magnitudes could not be constrained from our dataset. Using present-day precipitation pattern and magnitude, our results provide paleo-temperature offsets from present-day between −3.3 and −3.8 °C for the older glacial stage, and −2.7 to −3.2 °C for the younger glacial stage, coinciding with the upper range of paleo-temperature reconstructions from other paleoclimatic proxies. Finally, Alpine glaciers’ sensitivity to climate fluctuations differ significantly between the investigated catchments, with a much higher glacier sensitivity to changes in the Equilibrium Line Altitude in the northern (Valpelline) compared to the southern (Valsavarenche and Val di Cogne) tributaries, reflecting different topographic and/or climatic conditions between the three valleys.

Holocene seasonal temperature evolution and spatial variability over the Northern Hemisphere landmass

The origin of the temperature divergence between Holocene proxy reconstructions and model simulations remains controversial, but it possibly results from potential biases in the seasonality of reconstructions or in the climate sensitivity of models. Here we present an extensive dataset of Holocene seasonal temperatures reconstructed using 1310 pollen records covering the Northern Hemisphere landmass. Our results indicate that both summer and winter temperatures warmed from the early to mid-Holocene (~11–7 ka BP) and then cooled thereafter, but with significant spatial variability. Strong early Holocene warming trend occurred mainly in Europe, eastern North America and northern Asia, which can be generally captured by model simulations and is likely associated with the retreat of continental ice sheets. The subsequent cooling trend is pervasively recorded except for northern Asia and southeastern North America, which may reflect the cross-seasonal impact of the decreasing summer insolation through climatic feedbacks, but the cooling in winter season is not well reproduced by climate models. Our results challenge the proposal that seasonal biases in proxies are the main origin of model–data discrepancies and highlight the critical impact of insolation and associated feedbacks on temperature changes, which warrant closer attention in future climate modelling.

Late- and postglacial vegetation and climate history of the central Kola Peninsula derived from a radiocarbon-dated pollen record of Lake Kamenistoe

A radiocarbon-dated sediment core collected from the small freshwater Lake Kamenistoe, in the central part of the Kola Peninsula, provides a pollen record of vegetation and climate history of this part of Fennoscandia and the European Arctic during the past ca. 13,000 years. In contrast to existing Scandinavian Ice Sheet reconstructions, the record shows that the study site was ice-free at 13 cal. kyr BP, thus allows to improve our knowledge on deglaciation dynamics in North Europe. The biome reconstruction results together with other pollen records from the wider region suggest that forest-tundra surrounded Kamenistoe at the end of the Bølling-Allerød interstadial and that the reconstructed presence of trees is not determined by far-distance pollen transport. The spread of pine in the study region started ca. 9.3 cal. kyr BP, and maximum pollen percentages during 8.2–4.2 cal. kyr BP mark the Holocene thermal optimum. Progressive climate cooling accompanied by increasing moisture levels from 6 cal. kyr BP is indicated by the spread of spruce (boreal evergreen conifer), reflecting the expansion of taiga forests. In contrast to some previous interpretations, we argue that the spread of pine in the Early Holocene and spruce in the Middle Holocene did not follow zonal expansions, but rather originated from scattered small populations withing the study region. Archaeological records from northern Fennoscandia suggest that postglacial human occupation on the Kola Peninsula began no later than 10,000 years ago. This northward expansion of hunter-gatherers was likely related to the continuous Early Holocene warming, which not only resulted in less harsh climatic conditions for human occupation, but may have also pushed reindeer populations to the study region. This game animal, which has been a major resource for humans, prefers July temperatures below 12–13 °C and thus may have migrated to cooler environments during the Early Holocene.

A ∼14 000-year record of environmental change from Lake Simcoe, Canada

While the evolution of the Laurentian Great Lakes has been researched intensively, fewer studies have focused on reconstructing the history of Lake Simcoe, the fourth largest lake wholly within southern Ontario. This study uses proxy data, including ostracod valves, contained in lake sediments to reconstruct a ∼14 000-year history of paleoenvironmental change in the Lake Simcoe basin. First, oxygen-isotope compositions of ostracod valves (δ18Ovalve) are used to estimate the oxygen-isotope composition of ancient lake water, δ18Olake water. In Lake Simcoe, δ18Olake water is insensitive to changes in lake level but sensitive to shifts in temperature, capturing periods such as the early Holocene warming. This δ18Olake water record demonstrates that the delivery of meltwater from Lake Algonquin to Lake Simcoe began to slow at ∼12 050 cal yr BP, supporting previous research based on geophysical data. It also suggests that an isolated pulse of 16O-rich water from Early Lake Mattawa may have occurred from ∼10 699 to 10 591 cal yr BP. Next, stable carbon-isotope compositions of ostracod valves (δ13Cvalve) are used to estimate the stable carbon-isotope composition of dissolved inorganic carbon (δ13CDIC) in Lake Simcoe. These data, combined with ostracod assemblages, grain size, magnetic susceptibility (MS) and sediment mineralogy, support our interpretation of temperature-sensitive variation in δ18Olake water and provide further insights into in-lake conditions. A pronounced increase in accumulation rate, and subtler increases in grain size and detrital carbonate contents, between ∼8300 and ∼8000 cal yr BP may reflect the abrupt transition from cold and dry to wet and warm conditions resulting from the collapse of the Laurentide Ice Sheet (LIS) at ∼8200 cal yr BP. In summary, this study refines our understanding of the evolution of Lake Simcoe and provides a holistic picture of environmental change over the last ∼14 000 years.

Simulating the Holocene deglaciation across a marine-terminating portion of southwestern Greenland in response to marine and atmospheric forcings

Abstract. Numerical simulations of the Greenland Ice Sheet (GrIS) over geologic timescales can greatly improve our knowledge of the critical factors driving GrIS demise during climatically warm periods, which has clear relevance for better predicting GrIS behavior over the upcoming centuries. To assess the fidelity of these modeling efforts, however, observational constraints of past ice sheet change are needed. Across southwestern Greenland, geologic records detail Holocene ice retreat across both terrestrial-based and marine-terminating environments, providing an ideal opportunity to rigorously benchmark model simulations against geologic reconstructions of ice sheet change. Here, we present regional ice sheet modeling results using the Ice-sheet and Sea-level System Model (ISSM) of Holocene ice sheet history across an extensive fjord region in southwestern Greenland covering the landscape around the Kangiata Nunaata Sermia (KNS) glacier and extending outward along the 200 km Nuup Kangerula (Godthåbsfjord). Our simulations, forced by reconstructions of Holocene climate and recently implemented calving laws, assess the sensitivity of ice retreat across the KNS region to atmospheric and oceanic forcing. Our simulations reveal that the geologically reconstructed ice retreat across the terrestrial landscape in the study area was likely driven by fluctuations in surface mass balance in response to Early Holocene warming – and was likely not influenced significantly by the response of adjacent outlet glaciers to calving and ocean-induced melting. The impact of ice calving within fjords, however, plays a significant role by enhancing ice discharge at the terminus, leading to interior thinning up to the ice divide that is consistent with reconstructed magnitudes of Early Holocene ice thinning. Our results, benchmarked against geologic constraints of past ice-margin change, suggest that while calving did not strongly influence Holocene ice-margin migration across terrestrial portions of the KNS forefield, it strongly impacted regional mass loss. While these results imply that the implementation and resolution of ice calving in paleo-ice-flow models is important towards making more robust estimations of past ice mass change, they also illustrate the importance these processes have on contemporary and future long-term ice mass change across similar fjord-dominated regions of the GrIS.

Pollen-Based Holocene Thawing-History of Permafrost in Northern Asia and Its Potential Impacts on Climate Change

As the recent permafrost thawing of northern Asia proceeds due to anthropogenic climate change, precise and detailed palaeoecological records from past warm periods are essential to anticipate the extent of future permafrost variations. Here, based on the modern relationship between permafrost and vegetation (represented by pollen assemblages), we trained a Random Forest model using pollen and permafrost data and verified its reliability to reconstruct the history of permafrost in northern Asia during the Holocene. An early Holocene (12–8 cal ka BP) strong thawing trend, a middle-to-late Holocene (8–2 cal ka BP) relatively slow thawing trend, and a late Holocene freezing trend of permafrost in northern Asia are consistent with climatic proxies such as summer solar radiation and Northern Hemisphere temperature. The extensive distribution of permafrost in northern Asia inhibited the spread of evergreen coniferous trees during the early Holocene warming and might have decelerated the enhancement of the East Asian summer monsoon (EASM) by altering hydrological processes and albedo. Based on these findings, we suggest that studies of the EASM should consider more the state of permafrost and vegetation in northern Asia, which are often overlooked and may have a profound impact on climate change in this region.

Early Holocene palaeoceanographic and glaciological changes in southeast Greenland

Sediment core ER11-16 from Køge Bugt in Southeast Greenland is used to assess early Holocene palaeoceanographic changes and sediment rafting from icebergs calved from the large outlet glaciers in the area. Diatom analysis reconstructs variability in surface water temperature, salinity and sea-ice concentrations, and benthic foraminiferal assemblages is used to reconstruct subsurface ocean conditions. We report Holocene Thermal Maximum in Southeast Greenland during the early Holocene (at least since onset of the record 9100 cal yr BP) until around 4500 cal yr BP, which contrasts with a delay until the mid-Holocene of the Holocene Thermal Maximum in South and Southwest Greenland. The early Holocene warming in Southeast Greenland was caused by a combination of high solar insolation and a weakened subpolar gyre, both of which served to warm the Irminger Current waters subducting onto the shelf. At the same time, the surface temperature was relatively high and sea-ice cover in the polar surface waters of the East Greenland Current was relatively low. High levels of iceberg rafting occurred in Køge Bugt during the early Holocene, synchronously with these warm oceanic temperatures. This is attributed to an increase in iceberg production from the extensive, but retreating, Greenland Ice Sheet. The warm surface conditions were interrupted by a marked and short-lived increase in sea ice around 8200 years ago, providing the first evidence of this global cold episode in Southeast Greenland. After 4500 cal. yr BP, sea-ice cover increased with an expansion of the East Greenland Current, suppressing the inflow of warmer subsurface Irminger Current water to the Southeast Greenland shelf. We relate this oceanic shift to the decreased Northern Hemisphere summer solar insolation. Multi-centennial variability is observed in the grain size spectrum of iceberg rafted debris; a finding we interpret in the context of palaeoceanographic changes.

Adaptions in subsistence strategy to environment changes across the Younger Dryas - Early Holocene boundary at Körtiktepe, Southeastern Turkey

The site of Körtiktepe in southeastern Turkey is one of few sites in the Upper Mesopotamia basin that attests continuous, permanent occupation across the boundary from end of the colder, drier Younger Dryas (YD) into the comparatively wetter and warmer Early Holocene (EH). This allows for the study of the degree of environmental change experienced on a local level over this boundary as well as for the study of the adaptations that the occupants of the site undertook in response to these changes. The mammal assemblage of Körtiktepe remains relatively stable across the YD – EH transition with the main contributors to diet being mouflon ( Ovis orientalis) and red deer ( Cervus elaphus) in approximately the same quantities, although the contribution of aurochs ( Bos primigenius) increases in the EH. The most significant changes can be seen in the shift in avifauna remains, with a sharp increase of waterbirds during the EH. It is proposed that these shifts reflect changes in the local environment with an increase in woodland cover as well as expansion of local waterways, which is generally consistent with previously published archaeobotanical studies. In terms of species exploited, mortality profiles as well as size distribution of mammals, a great deal of continuity is observed. This suggests that over this particular period the local impact of the beginning of the Early Holocene was not overly dramatic, allowing for cultural continuity of previously established subsistence strategies.

Late Glacial and Holocene environmental variability, Lago Trasimeno, Italy

Lago Trasimeno (central Italy), 10 km wide and < 6m deep, fills a basin of tectonic origin, and is one of Europe's few endorheic lakes. We report on a multidisciplinary stratigraphic study based on seismic reflection profiles and two sediment cores, aimed at providing information on the vegetational, lithological, and climate history of this area. Trace elements, palynology, macrofossils, organic carbon, and C isotopes, coupled with AMS C-14 dating, describe environmental changes from Late-Glacial to present. The base of the cores records a Late-Glacial steppic vegetation (Poaceae, Artemisia, Amaranthaceae), with dry and cold conditions and a high charcoal/pollen ratio. An ensuing shift to a warmer and moister climate is shown by the rise of Pinus and Quercus, and shallow water aquatics such as Nitella. Early Holocene warming, indicated by Quercus, Oleaceae, Corylus, and Betula, is followed by a hiatus suggesting one or more severe drought events between 9000 and 3000 yr BP. Late Holocene presence of Alnus and Fagus indicates increased moisture, probably in winter, which would have increased lake level. Heavy metal pollution indicators (Pb, Cu, Zn) in the upper portion of the cores imply industrialization. Due to its location, to intensive and uninterrupted anthropogenic impacts since proto historical times, and to its shallow depths, Lago Trasimeno provides an important observation point for climate and environmental changes in the central Mediterranean region during and before Holocene times.

A radiocarbon chronology of Holocene climate change and sea-level rise at the Delmarva Peninsula, US Mid-Atlantic Coast

Radiocarbon dates from 176 sites along the Delmarva Peninsula record the timing of deposition and sea-level rise, and non-marine wetland deposition. The dates provide confirmation of the boundaries of the Holocene subepochs (e.g. “early-middle-late” of Walker et al.) in the mid-Atlantic of eastern North America. These data record initial sea-level rise in the early Holocene, followed by a high rate of rise at the transition to the middle Holocene at 8.2 ka, and a leveling off and decrease in the late-Holocene. The dates, coupled to local and regional climate (pollen) records and fluvial activity, allow regional subdivision of the Holocene into six depositional and climate phases. Phase A (&gt;10 ka) is the end of periglacial activity and transition of cold/cool climate to a warmer early Holocene. Phase B (10.2–8.2 ka) records rise of sea level in the region, a transition to Pinus-dominated forest, and decreased non-marine deposition on the uplands. Phase C (8.2–5.6 ka) shows rapid rates of sea-level rise, expansion of estuaries, and a decrease in non-marine deposition with cool and dry climate. Phase D (5.6–4.2 ka) is a time of high rates of sea-level rise, expanding estuaries, and dry and cool climate; the Atlantic shoreline transgressed rapidly and there was little to no deposition on the uplands. Phase E (4.2–1.1 ka) is a time of lowering sea-level rise rates, Atlantic shorelines nearing their present position, and marine shoal deposition; widespread non-marine deposition resumed with a wetter and warmer climate. Phase F (1.1 ka-present) incorporates the Medieval Climate Anomaly and European settlement on the Delmarva Peninsula. Chronology of depositional phases and coastal changes related to sea-level rise is useful for archeological studies of human occupation in relation to climate change in eastern North America, and provides an important dataset for future regional and global sea-level reconstructions.

Holocene mountain landscape development and monsoon variation in the southernmost Russian Far East

A c. 9720‐year history of a mountain lake–mire complex on the Shufan Plateau (southern Primorye, Russian Far East) is based on a multi‐proxy study of peat deposits. The chronology is based on eight radiocarbon dates in addition to two dated tephra horizons. The sediment contains eight cryptotephra layers attributable to Baitoushan, a volcano on the Korea/China border. Pollen, diatom and botanical macrofossil analyses indicate climate fluctuations during the Holocene and a compositional response of biodiverse mountain forests to changes in temperature and humidity. Widespread, diverse broadleaf‐dominated forest marked a warm Early Holocene (~9500–9280 cal. a BP). Korean pine (Pinus koraiensis) rose to dominance in the Middle Holocene (after c. 7630 cal. a BP). Evergreen conifers, fir (Abies) in particular, became widespread after 2.9 ka BP and show a positive response to the Little Ice Age cooling. Over the c. 10 000‐year record, moisture appears largely controlled by gradually declining intensity of the summer monsoon, trending from moist conditions in the Early Holocene to cooler and drier conditions in the Late Holocene with shorter‐term hydrological changes superimposed. Peat composition indicates significant changes in mire vegetation, and diatom assemblages indicate four stages of inundation and three dry stages in the basin, probably controlled by precipitation changes. The initial importance of atmospheric precipitation shifted over time to a greater input from groundwater. During a long episode in the Late Holocene the basin may have dried out. Forest fires occurred during much of the Middle and Late Holocene, and their prominence, as marked by macro‐charcoal, is related to changes in humidity. Human activities mark the past few centuries. Medieval fires were probably anthropogenic, and there is evidence of agricultural impact on the landscape during the Jin Empire settlement period 900–700 years ago and of more recent forest exploitation, particularly a reduction in conifer taxa.

Food availability and temperature optima shaped functional composition of chironomid assemblages during the Late Glacial–Holocene transition in Northern Europe

Non-biting midges (Chironomidae) are the most diverse and abundant invertebrate group in boreal lakes and are strongly responsive to climate change, thus they are a valuable palaeoecological proxy for studying aquatic biodiversity response in the face of climate change. Here, we aim to decipher the influence of climate-induced changes on temporal patterns in chironomid assemblages. We apply a novel approach combining traditional taxon-based analysis and species-trait framework to subfossil chironomid assemblages in a sediment core covering the Late Glacial–Holocene transition in Northern Europe. We produce pollen-based July and January temperature reconstructions to characterize past climate fluctuations that show a distinct Early Holocene warming for continuous 1700 years with considerably greater warming in winter (ca. 10 °C) than summer (ca. 4 °C) temperature. Overall, chironomid taxonomic changes were mainly induced by the temperature tolerances and optima of species, as temporal dynamics showed agreement with temperature reconstructions. Our results suggest that vegetation dynamics could also play a role in structuring chironomid assemblages by selecting taxa with the highest ability to exploit available food resources, thus potentially confounding chironomid-based summer climate reconstructions. We demonstrate that the study of the hidden chironomid functional composition could help to identify secondary drivers and thus strengthen the reliability of climate reconstruction based on subfossil chironomids.

Late-Holocene: Cooler or warmer?

Early studies on the evolution of glaciers argue that the warm Early Holocene transitioned into a Late-Holocene Neoglacial, with numerous glacial advances due to decreasing solar forcing in the boreal summer. The assumption of decreasing temperatures based on the glacier studies was supported by several proxy-based reconstructions as well as by simulations. Under the keyword “Holocene temperature conundrum” this fact is questioned. In particular, it is argued that the proxy studies are biased on time series dominated by a seasonal bias in the form of predominant summer temperatures (mainly marine). After a presentation of the facts, the weaknesses of both hypotheses (cooler or warmer) are briefly presented.