Younger Dryas Holocene Transition Research Articles

Deglaciation of the highest mountains in Scandinavia at the Younger Dryas–Holocene transition: evidence from surface exposure‐age dating of ice‐marginal moraines

Surface exposure–age dating was applied to rock surfaces associated with ice‐marginal moraines at elevations of ~1520–1780 m a.s.l. on the slopes of Galdhøpiggen and Glittertinden, the two highest mountains in Scandinavia located in the Jotunheimen mountains of central southern Norway. This is important for understanding the pattern and timing of wastage of the Scandinavian Ice Sheet at the Younger Dryas–Holocene transition. Cosmogenic exposure dating (here 10Be dating) of boulders from the moraine ridges yielded overall mean ages (corrected for glacio‐isostatic uplift, surface erosion and snow shielding) of ~11.6 ka from Galdhøpiggen and ~11.2 ka from Glittertinden. Similar 10Be ages were also obtained from additionally collected proximal and distal erratic boulders and bedrock samples. These enabled age calibration of Schmidt‐hammer R‐values and independent Schmidt‐hammer exposure‐age dating (SHD) of the moraine ridges, which yielded comparable mean SHD ages of ~10.8 and ~10.6 ka from the Galdhøpiggen and Glittertinden sites, respectively. Taking account of the age resolution and other limitations of both dating techniques, the results suggest that the two sets of moraines have approximately the same age but that neither technique can distinguish unambiguously between moraine formation in the late Younger Dryas or Early Holocene. Together with features of moraine‐ridge morphology and estimates of equilibrium‐line altitude depression of ~360–575 m (corrected for land uplift), the results imply moraine formation during short‐lived re‐advances of active glaciers, at least the lower reaches of which were warm‐based. It is concluded that the local glaciers remained active and advanced during deglaciation either very late in the Younger Dryas or very early in the Holocene, possibly in response to the Preboreal Oscillation at ~11.4 ka. The study supports the concept of a thin Younger Dryas ice sheet and places time constraints on the timing of final deglaciation in southern Norway.

Common chironomids drive the biodiversity–temperature relationship during the Younger Dryas-Holocene transition in a southern Baltic coastal lake

The Younger Dryas-Holocene transition represents a period of significant thermal change, comparable in magnitude to modern warming, yet in a colder context and without the effect of anthropogenic disturbance. This is useful as a reference to tackle how biodiversity is affected by temperature in natural conditions. Here, we addressed the thermal change during this period in a southern Baltic coastal lake (Konarzewo Lake, Poland), as inferred by chironomid remains. We evaluated changes in chironomid communities and used Hill numbers to explore how commonness and rarity underlie biodiversity changes attributable to warming. We found evidence of warming at Konarzewo Lake during the Younger Dryas-Holocene transition, with inferred temperatures in the Younger Dryas period supporting the NW–SE gradient in Younger Dryas summer temperatures across Europe. Chironomid communities drastically changed during the thermal transition. However, Hill numbers showed no response to temperature when rare morphotypes were emphasized (order q = 0) or a weak response when they were balanced with common morphotypes (order q = 1). Hill number of order q = 2, emphasizing the most common morphotypes, consistently increased with temperature across different sample sizes or coverages. This illustrates how common morphotypes, rather than the rare ones, may boost biodiversity facing warming.

Younger Dryas and early Holocene climate in south Greenland inferred from oxygen isotopes of chironomids, aquatic Moss, and Moss cellulose

Ice core records have long indicated that the Younger Dryas began and ended with large, abrupt climate shifts over Greenland. Key climatic features remain unknown, including the magnitude of warming during the Younger Dryas-Holocene transition along with the seasonality and spatial variability of Younger Dryas climate changes across Greenland. Here, we use geochemical and paleoecological proxies from lake sediments at Lake N14 in south Greenland to address these outstanding questions. Radiocarbon dating and diatom assemblages confirm early deglaciation and isolation of Lake N14 before ∼13,600 cal yr BP, consistent with previous work. Oxygen isotope ratios (δ18O) of chironomid head capsules, bulk aquatic moss, and aquatic moss-derived cellulose are used to reconstruct oxygen isotopes of past lake water and annual precipitation. Oxygen isotope proxies indicate annual precipitation δ18O values increased by 5.9–7.7‰ at the end of the Younger Dryas. Following the Younger Dryas, moss and cellulose δ18O values show a clear decline in precipitation δ18O values of 2–3‰ from ∼11,540–11,340 cal yr BP that may correspond with the Preboreal Oscillation. Reconstructed precipitation δ18O values then gradually increased from 11,300–10,100 cal yr BP. All three aquatic organic materials register similar shifts in precipitation δ18O values over time, and they closely parallel the δ18O shifts observed in ice cores. This evidence strongly supports the utility of these methods for reconstructing lake water δ18O, and furthermore precipitation δ18O values where lake water reflects precipitation. The relatively large shift in isotopic composition of precipitation at Lake N14 suggests that shifts in temperature, precipitation seasonality, and/or moisture sources at the end of the Younger Dryas were even larger in south Greenland than they were in central Greenland, most likely because of the proximity to major changes in North Atlantic Ocean circulation. The annual air temperature change estimated at Lake N14 at the end of the Younger Dryas is also very large (∼18 ± 7 °C) compared to the summer warming previously inferred from chironomid species assemblages there (∼6 °C). This indicates that the strongest warming at the end of the Younger Dryas occurred in the winter season, consistent with past observations of intensified Younger Dryas seasonality at Lake N14 and elsewhere in Greenland.

Sediment cascades and the entangled relationship between human impact and natural dynamics at the pre‐pottery Neolithic site of Göbekli Tepe, Anatolia

ABSTRACTThis study presents a meta‐analysis of radiocarbon ages for the environs of Göbekli Tepe – one of the oldest monumental structures worldwide – using cumulative probability functions to diachronically assess phases of geomorphodynamic activity as controlled by natural or anthropogenic drivers. We employ sediment cascades as a heuristic framework to study the complex responses of the geomorphological system to various triggers at local to supra‐regional scales. Possible triggers include climatic variability as documented by supra‐regional hydroclimatic proxy data, regional demographic trends, and local to regional socioeconomic developments such as the emergence of sedentism or the introduction and dispersal of livestock herding. Our results show that phases of intensified geomorphodynamic activity occurred between ca. 7.4–7.0 and 5.8–3.3 ka BP. These phases roughly coincide with phases of population growth in southern Turkey and climatic variations in Turkey and the Levant. The phase between ca. 5.8–3.3 ka BP also corresponds to the time when organized agriculture and the seeder plough were introduced. Also, the identified phases are in agreement with the general trend of varying geomorphodynamic activity in the Eastern Mediterranean as driven by human impact and climatic change. However, neither the Younger Dryas–Holocene transition nor the development of herding during the Pre‐Pottery Neolithic left a clear signature. We demonstrate how the different depositional environments in the studied landscape compartments vary with respect to their spatiotemporal coverage and discuss challenges when trying to understand processes that once shaped landscapes of past societies. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

The Late Pleistocene–Early Holocene palaeoenvironmental evolution in the SE Baltic region: a new approach based on chironomid, geochemical and isotopic data from Kamyshovoye Lake, Russia

The Kamyshovoye Lake sedimentary record in the southeastern Baltic Sea region was studied to reconstruct climatic fluctuations and the abiotic responses to them during the Lateglacial and Early Holocene. New results from chironomid, isotopic, palaeomagnetic and geochemical data analyses were correlated with earlier evidence of lithological and palynological changes in the Kamyshovoye Lake record. The section of the record that was studied covered the interval between 15 200 and 6500 cal. a BP. Palaeoclimatic reconstructions showed that during the Younger Dryas there was a two‐step decrease in the mean July temperature. The temperature dropped by 3 °C in the period from ̃12 650 to 12 300 cal. a BP, and then it dropped by another 0.5 °C to a minimum of 11.5 °C at 11 900 cal. a BP. During the Younger Dryas–Holocene transition, a temperature increase of 3 °C can be seen over a period of several centuries, inferred from the chironomid data. Further, the temperature curve showed that significant fluctuations continued until ̃9500 cal. a BP. During the coolings, the average July temperature dropped to values that were typical for the Younger Dryas, while values characteristic for the Allerød were only reached at around 9700 cal. a BP. After 9500 cal. a BP, a more stable, gradual increase in temperature was recorded. The short‐term Early Holocene climatic oscillations are clearly traced in the Kamyshovoye sequence, although the responses of the natural components are sometimes asynchronous. In the case of the Kamyshovoye study, the geochemical data seem to be a sensitive indicator of the climatic and environmental changes despite the absence of an evident response to the Holocene onset at ̃11 700 cal. a BP. Considerable changes in the geochemical pattern are recorded later, at ̃11 500 cal. a BP, coinciding with noticeable changes in the development of vegetation. The results obtained in this study contribute to a deeper understanding of how global climatic trends are manifested on a local scale.

The radiolarian fauna during the Younger Dryas–Holocene transition in Andfjorden, northern Norway

We report on the changing radiolarian faunas from the Younger Dryas (ca.12.9 calibrated thousands of years BP [cal. Ky BP]) and into the Holocene (ca. 10.3 cal. Ky BP) in the core JM99-1200 from Andfjorden, northern Norway. Temperature reconstructions using both the Q-mode factor analysis and modern analogue technique methods show stable, cold temperatures below ca. 410 cm core depth, followed by abrupt warming into a relatively stable Holocene temperature regime. Age-depth modelling with three different methods gives an age of ca. 11.9–12.0 cal. Ky BP at this core depth, clearly older than the Younger Dryas–Holocene transition at ca. 11.65 cal. Ky BP according to ice core chronology. Considering that the age models may be insufficiently informed in an interval without radiometric dates, it is possible that the base of the Holocene is indeed at 410 cm, as indicated by the radiolarian and other core data. Such a change in the chronology would have implications for previously published work on the JM99-1200 core. Alternatively, the abrupt warming in Andfjorden predated the Younger Dryas–Holocene transition by a few hundred years. A distinct cold pulse at ca. 315–335 cm, or 11.4–11.5 cal. Ky BP is interpreted as the Preboreal Oscillation. The Preboreal Oscillation has not previously been detected in temperature curves based on planktic foraminifera in the same core, indicating that Radiolaria may be a more sensitive temperature indicator in this region.

Leaf wax <i>n</i>-alkane distributions record ecological changes during the Younger Dryas at Trzechowskie paleolake (northern Poland) without temporal delay

Abstract. While of higher plant origin, a specific source assignment of sedimentary leaf wax n-alkanes remains difficult. In addition, it is unknown how fast a changing catchment vegetation would be reflected in sedimentary leaf wax archives. In particular, for a quantitative interpretation of n-alkane C and H isotope ratios in terms of paleohydrological and paleoecological changes, a better understanding of transfer times and dominant sedimentary sources of leaf wax n-alkanes is required. In this study we tested to what extent compositional changes in leaf wax n-alkanes can be linked to known vegetation changes by comparison with high-resolution palynological data from the same archive. We analyzed leaf wax n-alkane concentrations and distributions in decadal resolution from a sedimentary record from Trzechowskie paleolake (TRZ, northern Poland), covering the Late Glacial to early Holocene (13 360–9940 yr BP). As an additional source indicator of targeted n-alkanes, compound-specific carbon isotopic data have been generated in lower time resolution. The results indicated rapid responses of n-alkane distribution patterns coinciding with major climatic and paleoecological transitions. We found a shift towards higher average chain length (ACL) values at the Allerød–Younger Dryas (YD) transition between 12 680 and 12 600 yr BP, coevaled with a decreasing contribution of arboreal pollen (mainly Pinus and Betula) and a subsequently higher abundance of pollen derived from herbaceous plants (Poaceae, Cyperaceae, Artemisia), shrubs, and dwarf shrubs (Juniperus and Salix). The termination of the YD was characterized by a successive increase in n-alkane concentrations coinciding with a sharp decrease in ACL values between 11 580 and 11 490 yr BP, reflecting the expansion of woodland vegetation at the YD–Holocene transition. A gradual reversal to longer chain lengths after 11 200 yr BP, together with decreasing n-alkane concentrations, most likely reflects the early Holocene vegetation succession with a decline of Betula. These results show that n-alkane distributions reflect vegetation changes and that a fast (i.e., subdecadal) signal transfer occurred. However, our data also indicate that a standard interpretation of directional changes in biomarker ratios remains difficult. Instead, responses such as changes in ACL need to be discussed in the context of other proxy data. In addition, we find that organic geochemical data integrate different ecological information compared to pollen, since some gymnosperm genera, such as Pinus, produce only a very low amount of n-alkanes and for this reason their contribution may be largely absent from biomarker records. Our results demonstrate that a combination of palynological and n-alkane data can be used to infer the major sedimentary leaf wax sources and constrain leaf wax transport times from the plant source to the sedimentary sink and thus pave the way towards quantitative interpretation of compound-specific hydrogen isotope ratios for paleohydrological reconstructions.

Biotic controls on Holocene fire frequency in a temperate mountain forest, Czech Republic

ABSTRACTMountain spruce–beech–fir mixed forests are an important type of vegetation at higher elevations in temperate Europe. We aimed to determine how fire disturbances have affected long‐term vegetation dynamics and to assess their contribution to soil formation. We detected fire episodes using a soil charcoal record extensively dated based on 14C and combined with pollen and macrocharcoal records from a local peat bog. Altitudinal shifts of the timberline during the Younger Dryas–Holocene transition seem to be responsible for an abrupt occurrence of fire at 11 200 cal a BP. The minimum fire frequency estimation based on dated soil charcoal particles showed variation during the early to mid‐Holocene in response to climatic changes. A marked decrease of fire frequency since 6200 cal a BP is attributed to the transformation of vegetation from Picea abies‐dominated forests into mixed Fagus sylvatica–Abies alba stands. Once Fagus sylvatica established a dense canopy a profound alteration of the disturbance regime occurred, leading to the exclusion of fire, and has indirectly accelerated the process of podzolization. Thus, the synergistic effects of biotic change are capable of amplifying a climatic impulse, illustrating the important influence of bottom‐up controls on fire regimes and soil development.

Abrupt mortality of marine invertebrates at the Younger Dryas-Holocene transition in a shallow inlet of the Goldthwait Sea

The late Pleistocene–early Holocene transition period was characterized by rapid environmental change. Here, we investigate the impact of these changes on the marine invertebrates living in a shallow inlet of the post-glacial Goldthwait Sea. The site is located near Baie-Comeau (QC, Canada), where a number of remarkably well-preserved shell deposits are found along the Rivière aux Anglais Valley on the north shore of the St. Lawrence maritime estuary. Seven phyla of marine invertebrates with a minimum of 25 species or taxa were inventoried in a shell deposit, dominated by a community of Hiatella arctica with Mytilus edulis and barnacles composing the subcommunity. The majority of taxa identified in the shell deposit are boreal and sub-Arctic species; however, temperate species that exist today in the St. Lawrence maritime estuary have not been found. Based on marine invertebrate diversity and δ18O(CaCO3) of Mytilus edulis, the water in the shallow inlet of the Goldthwait Sea must have been cold and saline. The range of AMS 14C ages from 15 Mytilus edulis, constrained to 10,900 and 10,690 cal. yr BP, and exceptional state of preservation of adult and juvenile molluscan specimens suggest the abrupt mortality of entire invertebrate communities due to changing hydrodynamic conditions that included the combined effect of freshwater discharge from the receding Laurentide Ice Sheet and rapid isostatic uplift.

An assessment of biomarker-based multivariate classification methods versus the PIP25 index for paleo Arctic sea ice reconstruction

The development of various combinative methods for Arctic sea ice reconstruction using the sympagic highly branched isoprenoid (HBI) IP25 in conjunction with pelagic biomarkers has often facilitated more detailed descriptions of sea ice conditions than using IP25 alone. Here, we investigated the application of the Phytoplankton-IP25 index (PIP25) and a recently proposed Classification Tree (CT) model for describing temporal shifts in sea ice conditions to assess the consistency of both methods. Based on biomarker data from three downcore records from the Barents Sea spanning millennial timescales, we showcase apparent and potential limitations in both approaches, and provide recommendations for their identification or prevention. Both methods provided generally consistent outcomes and, within the studied cores, captured abrupt shifts in sea ice regimes, such as those evident during the Younger Dryas, as well as more gradual trends in sea ice conditions during the Holocene. The most significant discrepancies occurred during periods of highly unstable climate change, such as those characteristic of the Younger Dryas–Holocene transition. Such intervals of increased discrepancy were identifiable by significant changes of HBI distributions and correlations to values not observed in proximal surface sediments. We suggest that periods of highly fluctuating climate that are not represented in modern settings may hinder the performance and complementary application of PIP25 and CT-based methods, and that data visualisation techniques should be employed to identify such occurrences in downcore records. Additionally, due to the reliance of both methods on biomarker distributions, we emphasise the importance of accurate and consistent biomarker quantification.

Vegetation responses to the warming at the Younger Dryas-Holocene transition in the Hengduan Mountains, southwestern China

The Younger Dryas (YD) is one of the most abrupt climatic events in Earth's recent history. The warming at the end of the YD, in particular, is considered to be comparable to the global warming seen in the 21st century. However, the YD termination has received little attention, particularly in the Hengduan Mountains of Southwestern China, a low latitude temperate biodiversity hotspot. Here we present evidence for a rapid response in the diversity and composition of vegetation to the warming at the YD termination, based on a continuous, well-dated pollen sequence and loss-on-ignition data (12.9–9.2 cal. ka BP) from Haligu wetland in the Hengduan Mountains. Our data indicate that variations in plant diversity were correlated with relative humidity during this period, and suggest a distinct shift from Pinus-Abies-Picea forest to Pinus-dominated forest at the YD-Holocene transition, accompanied by an increase in coverage of generally temperate taxa such as Salix and Betula. This finding provides insights that may be of relevance to biodiversity conservation under future warming scenarios in similar mountain ecosystems worldwide.

A multi-proxy view of exceptionally early postglacial development of riparian woodlands with Ulmus in the Dniester River valley, western Ukraine

This article presents the first multi-proxy study of palaeoecosystem changes during the Late Weichselian and Early Holocene in western Ukraine. The site is a former oxbow lake situated in the Dniester River valley (western Ukraine). We hypothesised that this area may have been one of the nuclei for the Holocene expansion of elm (Ulmus), a primary arboreal taxon in riparian woodlands, in the Central and Eastern European lowlands. Our study was undertaken on a profile encompassing the period ca. 12,900–9880cal. BP. We applied selected palaeobotanical, palaeozoological and geochemical analyses. During the Younger Dryas the area was dominated by well-developed Pinussylvestris-Betula woodlands. Drier habitats were overgrown by Juniperus shrublands and steppe communities. Patches of tundra with Betulanana were also present. Ulmus started to spread on the lower river terraces before ca. 11,840cal. BP i.e., during the late Younger Dryas, which suggests this area as a nucleus for its Holocene expansion. A chironomid-based reconstructed mean July temperature (MJT) of 15–17°C is suggested during the Younger Dryas. The main Ulmus expansion started ca. 11,160cal. BP and its optimum in local woodlands lasted until ca. 10,340cal. BP, when Pinussylvestris rapidly expanded. The reconstructed MJT varied between 18 and 21°C during the Ulmus optimum; however, it rapidly declined to 16°C around ca. 10,800cal. BP. Fire activity increased before ca. 10,870cal. BP, concurrent with the climate warming at the Younger Dryas-Holocene transition, but did not affect the population of Ulmus, a taxon intolerant of fires. Wildfires probably affected patches of steppe vegetation and Pinussylvestris-Betula woodlands occupying higher elevations of the valley and watersheds. However, changes in the fire activity may have stimulated Ulmus retreat at ca. 10,340cal. BP.

A multi-proxy reconstruction of oceanographic conditions around the Younger Dryas–Holocene transition in Placentia Bay, Newfoundland

Abstract A marine sediment core from Placentia Bay off the south coast of Newfoundland was studied to reconstruct past oceanographic conditions based on a multiproxy approach, including diatoms, benthic foraminifera, grain size analysis, IP25, X-Ray fluorescence, calcium carbonate and total organic carbon content. Based on 6 radiocarbon dates, the 513.5 cm long core spans the age interval from ca. 13 to 10 cal kyrs BP and thus captures the transition from the Younger Dryas stadial into the Holocene at sub-decadal resolution. The Younger Dryas was characterized by cold Arctic water taxa of diatoms and benthic foraminifera and a stratified water column with high sea ice cover and low productivity due to strong influence of the Labrador Current. During the second half of the Younger Dryas, after ca. 12.2 cal kyrs BP, the influence of the Labrador Current gradually decreased and sea ice conditions became more variable. The transition towards the Holocene is characterized by a sequence of events starting with turnovers in the biogenic proxies followed by an abrupt retreat of sea ice and a rise in productivity. These events are believed to be related to a northward migration of the Gulf Stream–Labrador Current oceanic front. The Younger Dryas termination is directly followed by a detrital carbonate layer associated with the final phase of the Heinrich 0 event, linked to Laurentide Ice Sheet dynamics. During the early Holocene productivity remained high, with minimal sea ice cover, reduced stratification and stronger influence of warmer, Gulf Stream-derived waters.

Socio-economic organization of Final Paleolithic societies: New perspectives from an aggregation site in Western France

Overlooked in larger European syntheses for some time, northwestern France now plays an important role in a dynamic research program investigating the very end of the Lateglacial in Western Europe. The discovery of the well-preserved open-air site of La Fosse has allowed for significant advances in our understanding of different aspects of the Younger Dryas-Holocene transition in this region. This homogenous lithic assemblage adds further precision to the Lateglacial chrono-cultural sequence and provides essential new information for investigating techno-economic changes that appeared during this period. A techno-functional study of the lithic material combined with a spatial analysis of artifact distribution provides insights concerning the site’s function. Several lines of evidence also shed light on occupation duration, activities carried out on-site, and the likely composition of the groups who occupied the site. The combination of the above lead us to interpret La Fosse as a large residential site. Following this, we propose a new mobility and land-use model for hunter-gatherer groups from the Younger Dryas-Preboreal transition in which La Fosse functioned as an aggregation site. This model confirms several previous hypotheses emphasizing the logistical character of mobility strategies of these societies. Finally, this scenario adds further details and precision concerning both the status and connections between different groups of sites within a complex socio-economic system.

Late Glacial and Early Holocene lake level fluctuations in NE Poland tracked by macro-fossil, pollen and diatom records

Late Glacial and Early Holocene vegetation history and lake level fluctuations in NE Poland were described based on high-resolution plant macrofossils analyses and records of pollen, diatoms, and radiocarbon dates. The development of Lake Kojle-Perty began in the Allerød (GI-1c). Cooling during the Younger Dryas (GS-1) led to the development of communities of heliophyte herbaceous plants. Palaeobotanical and lithostratigraphical records demonstrate that abrupt climate changes at approximately 11,600 and 9450 cal. BP resulted in distinct changes in the vegetation and water level of Lake Kojle-Perty. Climate warming in the Younger Dryas–Holocene transition (11,600 cal. BP) caused the development of the terrestrial vegetation dominated by cold deciduous forest (Betula), and appearance of thermophylious aquatic plants: Typha latifolia, Najas marina, and Ceratophyllum demersum. A gradual decrease in the lake level since 11,200 cal. BP is evident from the appearance of plants from shallow lake environments (Potamogeton natans and Hippuris vulgaris). The low water levels since 10,700 cal. BP resulting from increased temperature and evaporation caused the development of peat on limnic sediments. The dry and warm climatic phase with lowest lake level in the lakes of NE Poland were coeval with the expansion of temperate deciduous forests (Corylus and Ulmus) that commenced at approximately 10,500 cal. BP. A distinctive increase in lake levels at approximately 9450 cal. BP is suggested by the development of rush plants such as Cladium mariscus and aquatic plants such as Najas marina, Chara sp., and Nuphar lutea in the shallow parts of Lake Kojle-Perty. The increased lake levels correspond with the expansion of Alnus glutinosa in North-East (NE) Poland. Our reconstructions of the vegetation development and lake level changes align with results from other studies in the region.

Corylus expansion and persistent openness in the early Holocene vegetation of northern central Europe

The warming at the Younger Dryas–Holocene transition (∼11,600 cal. BP) triggered the rearrangement of vegetation across Europe. To better understand that response, we analysed both an extended fossil pollen data set and robust present-day site parameters from NE Germany in a data model comparison approach. This approach allowed to quantify past plant abundances in sub-regional landscape patterns, as defined by soil types and relief. Betula was the first tree taxon to expand from scattered stands that already existed during the Younger Dryas. The major tree taxa Betula and Pinus fully established after 200–400 years as largely separated stands of Pinus on well drained sandy and Betula on fertile morainic soils. Corylus started to spread after 11,200 cal. BP, yet the final expansion was delayed until ∼10,800 cal. BP. Corylus established on gleyic soils, from which it largely expelled Betula. Throughout the first two millennia of the Holocene, open grasslands existed on fine grained soils, where seasonal dryness prevented the establishment of deciduous trees.

Younger Dryas – early Holocene transition in the south‐eastern Iberian Peninsula: insights from land snail shell middens

ABSTRACTThe land snail Sphincterochila candidissima from archeological records in Villena (SE Spain) was studied isotopically to estimate the Younger Dryas (YD)–early Holocene transition in the western Mediterranean. Live‐collected individuals exhibited body (−21.8 ± 1.6‰) and shell (−5.8 ± 1.4‰) δ13C values typical of a C3 plant diet, probably combined with carbonate ingestion. Calculations of a carbon flux balance‐mixing model suggest that living specimens experienced similar metabolic rates, with comparable ratio of input and output fluxes of bicarbonate from the snail hemolymph. All fossil shells showed comparable δ13C values among each other, but values were ∼2‰ higher than living specimens. This may be explained by higher water stress at the YD–Holocene transition or by the Suess effect. Shell δ18O values averaged +1.3 ± 0.8‰ for living individuals, −0.5 ± 0.8‰ for Holocene (8.4–10.2 cal ka BP) specimens and +0.4 ± 0.6‰ for YD (12.0–12.4 cal ka BP) snails. An oxygen flux balance‐mixing model suggests that YD shells precipitated during relative humidity (RH) values of ∼79–82%, after which RH increased gradually reaching maximum values of ∼87–88% at ∼8.4–8.6 cal ka BP and, from there, RH eventually declined to present values of ∼82%. Comparisons with other snail data suggest that the xerophilous Sphincterochila records different environmental signatures fro other contemporaneous taxa.

Lateglacial vegetation development in Denmark – New evidence based on macrofossils and pollen from Slotseng, a small-scale site in southern Jutland

This paper presents the first unambiguous terrestrial palaeoecological record for the late glacial “Bølling warming” in Denmark. Pollen and macrofossil stratigraphies from pre-Bølling to 10,800 cal yr BP are presented from a small kettle hole in Southwest Denmark, during which the lake basin developed from an immature stage after the deglaciation to complete infilling in the early Holocene. Results show that the recently deglaciated landscape bore a discontinuous vegetation of pioneer plants. After the Bølling warming, an open Dryas octopetala- Betula nana community developed with Helianthemum oelandicum. Subarctic species were dominant and local successions were probably delayed by relatively unstable and infertile soils. There is no indication of a climate cooling during the period corresponding to the Older Dryas, but the occurrence of several drought tolerant and steppe species indicates that the period was relatively dry. In the Allerød period the Dryas- B. nana vegetation was initially replaced by an open Salix and grass dominated vegetation and some 400 years later, the first tree birches were documented presumably occupying moist and sheltered soils while drier land remained open. In the Younger Dryas period trees disappeared and the vegetation became open again and dominated by subarctic species. Following climate warming at the Younger Dryas–Holocene transition a shrub community of Empetrum and Juniperus developed. After approximately 200 years it was replaced by birch forest. Overall, the late-glacial vegetation cover had a more open and patchy character than inferred from previous pollen studies as assessment of the vegetation succession based on macrofossil evidence is essential. The inferred general vegetation development corresponds well with results of other studies in the region. Canonical ordinations (RDA) indicate that vegetation changes at the landscape scale during the Lateglacial period were driven by changes in climate, soils and competition for light.

Lateglacial vegetation and palaeoenvironment in W Norway, with new pollen data from the Sunnmøre region

Kruger, L. C., Paus, A., Svendsen, J. I. & Bjune, A. E. 2011: Lateglacial vegetation and palaeoenvironment in W Norway, with new pollen data from the Sunnmore region. Boreas, 10.1111/j.1502-3885.2011.00213.x. ISSN 0300-9483. Two sediment sequences from Sunnmore, northern W Norway, were pollen-analytically studied to reconstruct the Lateglacial vegetation history and climate. The coastal Dimnamyra was deglaciated around 15.3 ka BP, whereas Lokjingsmyra, further inland, became ice-free around 14 ka BP. The pioneer vegetation dominated by snow-bed communities was gradually replaced by grassland and sparse heath vegetation. A pronounced peak in Poaceae around 12.9 ka BP may reflect warmer and/or drier conditions. The Younger Dryas (YD) cooling phase shows increasing snow-bed vegetation and the local establishment of Artemisia norvegica. A subsequent vegetation closure from grassland to heath signals the Holocene warming. Birch forests were established 500–600 years after the YD–Holocene transition. This development follows the pattern of the Sunnmore region, which is clearly different from the Empetrum dominance in the Lateglacial interstadial further south in W Norway. The Lateglacial oscillations GI-1d (Older Dryas) and GI-1b (Gerzensee) are hardly traceable in the north, in contrast to southern W Norway. The southern vegetation was probably closer to an ecotone and more susceptible to climate changes.

Abrupt sea surface pH change at the end of the Younger Dryas in the central sub-equatorial Pacific inferred from boron isotope abundance in corals (<i>Porites</i>)

Abstract. The "δ11B-pH" technique was applied to modern and ancient corals Porites from the sub-equatorial Pacific areas (Tahiti and Marquesas) spanning a time interval from 0 to 20.720 calendar years to determine the amplitude of pH changes between the Last Glacial Period and the Holocene. Boron isotopes were measured by Multi-Collector – Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) with an external reproducibility of 0.25‰, allowing a precision of about ±0.03 pH-units for pH values between 8 and 8.3. The boron concentration [B] and isotopic composition of modern samples indicate that the temperature strongly controls the partition coefficient KD for different aragonite species. Modern coral δ11B values and the reconstructed sea surface pH values for different Pacific areas match the measured pH expressed on the seawater scale and confirm the calculation parameters that were previously determined by laboratory calibration exercises. Most ancient sea surface pH reconstructions near Marquesas are higher than modern values. These values range between 8.19 and 8.27 for the Holocene and reached 8.30 at the end of the last glacial period (20.7 kyr BP). At the end of the Younger Dryas (11.50±0.1 kyr BP), the central sub-equatorial Pacific experienced a dramatic drop of up to 0.2 pH-units from the average pH of 8.2 before and after this short event. Using the marine carbonate algorithms, we recalculated the aqueous pCO2 to be 440±25 ppmV at around 11.5 kyr BP for corals at Marquesas and ~500 ppmV near Tahiti where it was assumed that pCO2 in the atmosphere was 250 ppmV. Throughout the Holocene, the difference in pCO2 between the ocean and the atmosphere at Marquesas (ΔpCO2) indicates that the surface waters behave as a moderate CO2 sink or source (−53 to 20 ppmV) during El Niño-like conditions. By contrast, during the last glacial/interglacial transition, this area was a marked source of CO2 (21 to 92 ppmV) for the atmosphere, highlighting predominant La Niña-like conditions. Such conditions were particularly pronounced at the end of the Younger Dryas with a large amount of CO2 released with ΔpCO2 of +185±25 ppmV. This last finding provides further evidence of the marked changes in the surface water pH and temperature in the equatorial Pacific at the Younger Dryas-Holocene transition and the strong impact of oceanic dynamic on the atmospheric CO2 content.