Dry Younger Dryas Research Articles

Late Pleistocene-Holocene sea level and climate changes in the Gulf of Saros: Evidence from seismostratigraphic record and sediment core data

A detailed record of sea level changes and climate oscillations in the Gulf of Saros during the late Pleistocene to the Holocene is provided by high-resolution seismic reflection profiles and analysis of a sediment core. The seismic stratigraphy reveals the formation of four main depositional units bounded by prominent reflection surfaces, reflecting high sea level variations. The Last Glacial Maximum period in the gulf is associated with the lowstand sea level, forming the deepest marine terrace at −148 m. The increasing sea level due to post-glacial warming was accompanied by the deposition of transgressive units characterized by coastal onlaps together with local channel-fills. This transgressive phase was modulated by three brief still-stands sea levels at 17 cal ka BP, 14.6 cal ka BP and 13.6 cal ka BP, producing the younger marine terraces in the gulf at −135 m, −112 m and −90 m, respectively. A comparison of their depths with the global sea level curve reveals elevation differences with decreasing subsidence rates from the older to younger ages, implying tectonic subsidence along the gulf floor.The close correlation of multi-proxy data from core SAG-22 in the gulf with the northern Aegean and Anatolia data strongly indicates that the timings of the past climate events generally agree with the global and regional climate patterns. The warm climate around the gulf during the late phase of Bølling/Allerød (13.3–12.6 cal ka BP) is represented by high marine biological productivity and improved ventilation in the deeper water due to subsequent transgression. Permafrost formation in the catchment area during the following cold and dry Younger Dryas period (12.6–11.7 cal ka BP) resulted in reduced soil erosion and sediment input to the gulf. Noticeable warming during the Early Holocene between 11.2 cal ka BP and 9.4 cal ka BP is recorded by the multi-proxy data of the core when the enhanced marine biological productivity occurred above the poorly oxygenated deep water column. The climatic deterioration to a cooler and drier phase during the Early to Middle Holocene transition (9.4–7.8 cal ka BP) resulted in intense physical weathering and erosion, causing the highest sedimentation in the gulf. The later period of the Holocene is associated with the formation of two discrete sapropels at 7.8 cal ka BP and 5.4 cal ka BP that were accompanied by warm and wet climates around the gulf.

Pollen evidence for a wet Younger Dryas in northern NE China

The regional expression of moisture conditions during the Younger Dryas (YD) event and its forcing mechanism in northeastern (NE) China are controversial and more evidence is needed. Here, we present a reconstruction of changes in vegetation and precipitation during the YD event, based on pollen data from two well-dated, parallel sediment cores (TC2 and GQ2) from Tianchi Crater Lake in NE China. The elevated tree pollen abundance during the YD indicates an abrupt shift from steppe to forest, and therefore wetter conditions in Northeast China. Also, a pollen-based quantitative climatic reconstruction suggests that the climate was significantly colder and wetter, with a 3–5 °C decrease in annual temperature (Tann) and a 200 mm increase in annual precipitation (Pann) during the YD. To characterize the spatial distribution of moisture conditions in NE China during the YD, we also compiled 26 records from 14 sites in the region. Based on well-dated proxies, we conclude that the YD throughout northern NE China was characterized by wet conditions—significantly different from the consensus of a dry YD in southern NE China. We propose that the increased precipitation in northern NE China during the YD was a response to the strengthening of the Okhotsk High.

North Iberian temperature and rainfall seasonality over the Younger Dryas and Holocene

Several stalagmite records have yielded important but discontinuous insights into northern Iberian climate since the Last Glacial. Here we present the first continuous Iberian stalagmite-based reconstruction of climate since the Bølling-Allerød interstadial, from a single stalagmite sample (GAR-01 from La Garma Cave, Cantabria). The ∼13.5 ka GAR-01 record provides the opportunity for replication, continuation, and aggregation of previously published records from northern Spain. The GAR-01 record reveals shifts in oxygen isotope ratios that are inexplicable by appealing to a single control (i.e., exclusively temperature, rainfall amount, etc.). Herein we explore the potential role of rainfall and temperature seasonality shifts on the new δ18O record using a simple Monte Carlo approach to estimate the seasonal distribution of rainfall and the annual temperature range at 100-year timeslices across the record. This model is corroborated by intervals of monthly-resolved laser ablation trace element data, providing glimpses into past Iberian seasonality shifts. The most salient features of the modelled results include extremely dry Younger Dryas winters (∼12.9–11.6 ka BP) and several intervals during the mid-Holocene with almost no summer rainfall (e.g., at 4.2 and 9.0 ka BP). By 1.6 ka BP, a near-modern rainfall seasonality was established. According to the modelling results, seasonal rainfall and temperature distribution variability can account for 95% of the record. The model presented here provides a new tool for extracting critical missing seasonality information from stalagmite δ18O records. Intervals where the model does not converge may represent transient climate anomalies with unusual origins that warrant further investigation.

Holocene Vegetation, Climate, and Carbon History on Western Kodiak Island, Alaska

At Phalarope Pond, western Kodiak Island, a multidisciplinary study using pollen and spores, macrofossils, stable isotopes, and carbon accumulation provides the Holocene vegetation and climate history following the deglaciation that began over 16,000 cal years ago (yr BP) [years Before Present, as calibrated from 1950]. Following a cold and dry Younger Dryas, a warm and wet early Holocene was characterized by abundant ferns in a sedge tundra environment with maximum carbon accumulation, similar to high latitude peatlands globally. About 8,700 cal yr BP sedge and ferns declined and climate remained warm as drier conditions prevailed, limiting carbon sequestration. The abrupt shift in D/H (Deuterium/Hydrogen) isotopes of about 60 percent indicates a shift to cooler conditions or a more distal moisture source. Neoglaciation beginning about 3,700 cal yr BP is evident from increases in Artemisia, Empetrum and Betula, signifying cooler conditions, while Alnus declines, paralleling regional trends.

Late Pleistocene to Holocene paleoenvironmental evolution of Lake Hazar, Eastern Anatolia, Turkey

High-resolution seismic reflection profiles and core analyses in Lake Hazar provide a detailed record of the lake level fluctuations and the robust chronology of paleoclimatic events of the Eastern Anatolia during the late Pleistocene to Holocene. The earlier period of Marine Isotope Stage 3 (MIS-3) prior to 48 cal ka BP was accompanied by considerable lake level drop below −95 m, whereas the lake level increase during the later period (ca. 48-29 cal ka BP) gave rise to deposition of a transgressive unit with typical of aggradational architecture in the seismic reflection profiles. High climate variability with the Greenland interstadials and stadials (Dansgard-Oeschger and Heinrich events) in Lake Hazar are sensitively recorded in the core sediments by using multi-proxy analyses. Adjustment of seismic units correlating with the radiocarbon-dated chronostratigraphic units in the studied cores implies that the early Marine Isotope Stage 2 (MIS-2) is marked by an another lowstand lake level existed at ca. −90 m during 29–23 cal ka BP. In comparision to MIS-3 stage, the multi-proxy analyses reveal a general dry evaporative condition during most of the Last Glacial Maximum. In Lake Hazar, the maximum humidity in the late glacial period existed during 14.9–13.5 cal ka BP. The existence of a hiatus in the sedimentary record is documented in the seismic data that coincides with the cold and dry Younger Dryas period, implying a remarkable lake level drop. The multi-proxy records of the Holocene sediments reveal that a maximum precipitation in the early Holocene period prevailed during 10.1–9.3 cal ka BP, leading water level rise in the lake. In Lake Hazar, the middle Holocene until 4.9 cal ka BP is represented by highly climate variations, indicating a series of shorter wet and longer dry climate periods. The late Holocene is accompanied by lake level rises under a general wet climate condition that was interrupted by short dry climate intervals during 3.7–3.3 cal ka BP, 2.8–2.6 cal ka BP and 2.1–1.8 cal ka BP.

Stable carbon isotopes (δ13C) of total organic carbon and long-chain n-alkanes as proxies for climate and environmental change in a sediment core from Lake Petén-Itzá, Guatemala

Sediment core PI-6 from Lake Peten Itza, Guatemala, possesses an ~85-ka record of climate and environmental change from lowland Central America. Variations in sediment lithology suggest large and abrupt changes in precipitation during the last glacial and deglacial periods, and into the early Holocene. We measured stable carbon isotope ratios of total organic carbon and long-chain n-alkanes from the core, the latter representing a largely allochthonous (terrestrial) source of organic matter, to reveal past shifts in the relative proportion of C3–C4 terrestrial biomass. We sought to test whether stable carbon isotope results were consistent with other paleoclimate proxies measured in the PI-6 core, and if extraction and isotope analysis of n-alkanes is warranted. The largest δ13C variations are associated with Heinrich Events. Carbon isotope values in sediments deposited during the last glacial maximum indicate moderate precipitation with little fluctuation. The deglacial was a period of pronounced climate variability, e.g. a relatively warm and moist Bolling–Allerod, but a cool and dry Younger Dryas. Arid periods of the deglacial were inferred from samples with high δ13C values in total organic carbon, which reflect times of greater proportions of C4 plants. These inferences are supported by stable isotope measurements on ostracod shells and relative abundance of grass pollen from the same depths in core PI-6. Similar trends in carbon stable isotopes measured on bulk organic carbon and n-alkanes suggest that carbon isotope measures on bulk organic carbon in sediments from this lake are sufficient to infer past climate-driven shifts in local vegetation.

Environmental changes during the late Pleistocene and the Holocene in the Gonghe Basin, north-eastern Tibetan Plateau

Different types of aeolian sediments are widespread in Gonghe Basin on the north-eastern Tibetan Plateau. Despite that individual sections are often discontinuous, the analysis of a large number of individual sections distributed in the entire basin provides a valuable archive for the reconstruction of environmental changes since the late Pleistocene. This study presents 43 new OSL (optical stimulated luminescence) ages and combines them with 39 previously published ages from the Gonghe Basin for the last 16ka. From around 15 to 14ka onwards an increase in moisture facilitated the formation of a vegetation cover and the fixation of loess sediments on the northern and southern side of the basin. After enhanced mobility of aeolian sediments during the dry Younger Dryas greater than before precipitation fixated also the sands in the lower parts of the basin. The increase in moisture can be attributed to the strengthening of the Asian summer monsoon. A comparison with the timing of aeolian sediment accumulation in the neighbouring basins probably indicates a time-transgressive strengthening of the monsoon on the north-eastern Tibetan Plateau. Aeolian sands were first permanently stored from around 17ka at the Qinghai Lake in the north-east of the area and at 10.5ka at the Donggi Cona in the south-west. The Gonghe Basin is located in an intermediate position. Highest moisture values occurred during the mid-Holocene from around 7.5 until probably 2.5ka in the Gonghe Basin. In most parts of the basin aeolian activity was low during this time. In the central part of the basin aeolian sand movement occurred sporadically. Since 2.5ka aeolian activity in most parts of the basin resumed due to the late Holocene weakening of the Asian summer monsoon. However, the parallel accumulation of aeolian sands and silts indicates an increased human influence.

Late glacial and Holocene history of the Penobscot River in the Penobscot Lowland, Maine

When the Laurentide ice sheet retreated rapidly (~150 m/a) across the Penobscot Lowland between ~16 and ~15 ka, the area was isostatically depressed and became inundated by the sea. Silt and clay were deposited, but no significant moraines or deltas were formed. The Penobscot River was reborn at ~14 ka when ice retreated onto land in the upper reaches of the river’s East Branch. As isostatic rebound exceeded sea level rise from melting ice, the river extended itself southward. Between ~13.4 and 12.8 ka, it established a course across marine clay and underlying glacial till in the Lowland. Its gradient was low as differential rebound had not begun. Discharge, however, was higher and the river transported and deposited outwash gravel. During the cold, dry Younger Dryas, ~11 ka, eolian sand began to accumulate in dunes in the Lowland. Some of this sand, along with fluvial sediment from the headwaters, was redistributed into terraces along gentler stretches of the river and into a paleodelta in Penobscot Bay. Eolian activity continued to ~8 ka and aggradation in terraces until ~6 ka. The climate became wetter and warmer after ~6 ka, the dunes were stabilized by vegetation, the river began to downcut, and braiding became less intense. Pauses in the downcutting are reflected in discontinuous strath terraces. In due course, the river re-encountered the old outwash gravels, marine clay, glacial till, and, in a few places, bedrock. Its profile is now stepped, with gentle, gravel-bedded reaches between bedrock ribs that form rapids.

Alkenone-based temperature and climate reconstruction during the last deglaciation at Lake Dangxiong Co, southwestern Tibetan Plateau

The climate of the Tibetan Plateau is influenced by the monsoons and the Westerlies due to land-sea thermal contrasts. However, few quantitative temperature reconstructions have been made for the region, and this hinders our ability to validate climate models and to understand the coupling process between monsoon, ocean and ice sheets through time. Here we report a quantitative temperature reconstruction for the last deglaciation from a terrace of Lake Dangxiong Co in the southwestern Tibetan Plateau. Based on the unsaturation index of long-chain alkenones, the reconstructed mean annual air temperature during the late Last Glacial Maximum was about 0.2 °C, while the Bølling/Allerød (B/A) period was characterized by a rapid increase in temperature from 0.8 °C to 2.4 °C, (2.9) followed by an abrupt decrease in temperature to −0.9 °C in the early Younger Dryas (YD). The alkenones and n-alkanes record indicates that Dangxiong Co experienced a cold and wet climate in the early part of the last deglaciation after the Last Glacial Maximum, a warm-dry B/A, and a cold and dry YD. Our temperature reconstruction confirms that the thermal and dynamical effects of the Tibetan Plateau are one of the main elements contributing to abrupt changes in the Indian Summer Monsoon.

Late quaternary bottom-current activity in the south Aegean Sea reflecting climate-driven dense-water production

The Eastern Mediterranean Transient (EMT) represents a recent (early 1990s to today) water circulation phenomenon, during which more arid climatic conditions in the Mediterranean lead to the production of very dense, surface water masses in the north Aegean Sea. The produced north Aegean dense water (NADW) sinks to seafloor, flows to the south Aegean Sea and, then, spills into the Eastern Mediterranean Sea through the Cretan Straits, contributing to the rejuvenation of the bottom water. A large dataset of vintage high-resolution seismic-reflection profiles indicates that the floors of the basins in the southwest Aegean Sea are characterised by late Quaternary erosional gullies, abraded surfaces, and channel-related sediment drifts. Such structures provide evidence for the presence of strong bottom currents, which originate from the southward propagation of NADW masses. The basins have depths of 300m to 1000m and are connected to each other by narrow (a few km) straits and sills. The dominance of abraded surfaces and erosional gullies on the northern flank of the basins and in the narrow straits/sills between them provides evidence that the southward propagation of the NADW occurs by the filling and flooding of each basin by dense water, which then spills and cascades into the next basin southward. Seismic stratigraphy suggests that the formation of the drifts started at marine isotope stage (MIS) 11 (420ka BP). The coincidence of at least three erosional surfaces within the sediment drifts with interglacial MIS 11, 5, and 1, respectively, suggests that bottom currents were mostly active during high sea-level stands and sluggish during glacial low sea-level stands. A 90–100m drop of the sea level below the current level would have resulted in the emergence of the eastern Cyclades Plateau and, thus, in the disconnection of the southwest Aegean Sea from the north Aegean Sea. Several (0.05–0.5m thick) beds of gravelly sand with algal and mollusc debris, laminated to mottled sand, and mottled mud are observed in the sediment cores analysed in this paper; these beds can be confidently correlated from core to core, and are interpreted as bottom-current deposits. The separation of these beds by hemipelagic sediments indicates that bottom-current activity in the southwest Aegean is not permanently established but episodic. Six episodes of intense bottom-current activity are recorded in the MIS 2–4 (10–80ka) sediments and they appear to correlate well with the cold and dry Younger Dryas and Heinrich 3, 4, 5, 6, and 7 events. In contrast, at least three episodes of intense bottom-current activity are recorded in the cores during the Holocene. The higher frequency of strong bottom-current episodes in the southwest Aegean Sea during the Holocene is attributed to the higher sea-level stand that makes the circulation of water masses in the Aegean Sea more sensitive to climatic changes.

Evidence for insolation and Pacific forcing of late glacial through Holocene climate in the Central Mojave Desert (Silver Lake, CA)

Silver Lake is the modern terminal playa of the Mojave River in southern California (USA). As a result, it is well located to record both influences from the winter precipitation dominated San Bernardino Mountains – the source of the Mojave River – and from the late summer to early fall North American monsoon at Silver Lake. Here, we present various physical, chemical and biological data from a new radiocarbon-dated, 8.2m sediment core taken from Silver Lake that spans modern through 14.8calkaBP. Texturally, the core varies between sandy clay, clayey sand, and sand-silt-clay, often with abrupt sedimentological transitions. These grain-size changes are used to divide the core into six lake status intervals over the past 14.8calkaBP. Notable intervals include a dry Younger Dryas chronozone, a wet early Holocene terminating 7.8 – 7.4calkaBP, a distinct mid-Holocene arid interval, and a late Holocene return to ephemeral lake conditions. A comparison to potential climatic forcings implicates a combination of changing summer – winter insolation and tropical and N Pacific sea-surface temperature dynamics as the primary drivers of Holocene climate in the central Mojave Desert.

A modern pollen–climate calibration set from central‐western Mongolia and its application to a late glacial–Holocene record

AbstractAimFossil pollen spectra from lake sediments in central and western Mongolia have been used to interpret past climatic variations, but hitherto no suitable modern pollen–climate calibration set has been available to infer past climate changes quantitatively. We established such a modern pollen dataset and used it to develop a transfer function model that we applied to a fossil pollen record in order to investigate: (1) whether there was a significant moisture response to the Younger Dryas event in north‐western Mongolia; and (2) whether the early Holocene was characterized by dry or wet climatic conditions.LocationCentral and western Mongolia.MethodsWe analysed pollen data from surface sediments from 90 lakes. A transfer function for mean annual precipitation (Pann) was developed with weighted averaging partial least squares regression (WA‐PLS) and applied to a fossil pollen record from Lake Bayan Nuur (49.98° N, 93.95° E, 932 m a.s.l.). Statistical approaches were used to investigate the modern pollen–climate relationships and assess model performance and reconstruction output.ResultsRedundancy analysis shows that the modern pollen spectra are characteristic of their respective vegetation types and local climate. Spatial autocorrelation and significance tests of environmental variables show that the WA‐PLS model for Pann is the most valid function for our dataset, and possesses the lowest root mean squared error of prediction.Main conclusionsPrecipitation is the most important predictor of pollen and vegetation distributions in our study area. Our quantitative climate reconstruction indicates a dry Younger Dryas, a relatively dry early Holocene, a wet mid‐Holocene and a dry late Holocene.

Land–sea climatic variability in the eastern North Atlantic subtropical region over the last 14,200 years: Atmospheric and oceanic processes at different timescales

High-temporal resolution analysis of different climatic tracers (pollen, foraminiferal-based winter sea surface temperature (SST), benthic foraminiferal δ18O) from marine core MD95-2042, retrieved off SW Iberia, allows us to directly compare, without any chronological ambiguity, Mediterranean vegetation and eastern North Atlantic winter SST changes for the last 14.2 kyr. We identify on land and in the ocean several climatic phases such as the end of the warm and humid Bølling–Allerød, the cold and dry Younger Dryas, and the warm and humid Holocene with the Mediterranean forest (MF) optimum between 9.6 and 8.1 kyr. This record shows that, at multi-centennial timescale (~800 years), declines in forest cover generally related to dry and cool periods in southern Iberia are synchronous with cold SST in the eastern part of the North Atlantic subtropical gyre. At multi-centennial timescale, changes in thermohaline circulation, via freshwater content fluctuations, appear to be responsible for the coupling between dryness in Iberia and SST cooling in eastern North Atlantic subtropical gyre. In contrast, some Holocene events include centennial-scale oscillations (~100 years) marked by MF declines in southern Iberia concomitant with SST warming in the eastern North Atlantic subtropical gyre. This climatic pattern is similar to that observed at decadal timescale under the influence of the positive mode of the North Atlantic Oscillation (NAO). We suggest, therefore, that synchronous SW Iberian dryness and SST warming at centennial timescale could be explained by atmospheric fluctuations related to NAO changes.

031 Pollen evidence for a cool, dry Younger Dryas and warm, wet Early Holocene in the Southeastern United States

031 Pollen evidence for a cool, dry Younger Dryas and warm, wet Early Holocene in the Southeastern United States

A 25,000-year record of climate-induced changes in lowland vegetation of eastern equatorial Africa revealed by the stable carbon-isotopic composition of fossil plant leaf waxes

The debate of climate versus CO2 in controlling the long-term dynamics of tropical African vegetation has focused on events at the upper tree-line, since the relevant paleodata tend to be from mid-elevation sites (~2000–3000m). Less well known is the relative importance of CO2 in regulating the dynamics of tropical lowland (<1500m) vegetation, particularly that of the dry open woodlands, bush- and grasslands covering much of eastern equatorial Africa. Here we examine the stable carbon isotopic composition of n-alkanes in the sediment record of Lake Challa, a lowland crater lake near Mt. Kilimanjaro, covering the last 25,000years. The distributions of the n-alkanes, with dominance of the long-chain odd-carbon-numbered components, and their isotopic composition reveal a mixed origin. The C23 and C25n-alkanes are depleted in 13C, with δ13C values between −30 and −50‰. In shallow lakes these n-alkanes are thought to derive from non-emergent aquatic plants, but this is unlikely in this steep-sided crater lake as it lacks a significant littoral habitat. The C27+n-alkanes are predominantly derived from leaf wax lipids of terrestrial plants, brought into the lake predominantly by local soil run-off. Their δ13C values, in particular that of the n-C31 alkane, reveal a marked transition in local lowland vegetation from being dominated (~70–100%) by C4 plants during the glacial period until 16.5cal kyr BP, to a more mixed C3/C4 composition (~30–60% C4) during the Holocene. The start of the late-glacial trend towards a greater proportion of C3 plants coincided with the start of increasing monsoon rainfall, ~1500years after the onset of the rise in atmospheric CO2 and ~3500years after the onset of post-glacial warming. The transition was interrupted during the dry Younger Dryas period (13.0–11.7cal kyr BP), when C4 plants again became much more prevalent, almost reaching their glacial-period abundance. Notably, the principal trend in leaf-wax δ13C values infers C4 dominance during both wet and dry phases of the glacial period, and a mixed C3/C4 vegetation during both wet and dry phases of the Holocene. Our results indicate that long-term variation in pCO2 exerted important control on the composition of drought-adapted savanna vegetation in this currently semi-arid lowland region of equatorial East Africa.

Late Glacial to Holocene environments in the present-day coldest region of the Northern Hemisphere inferred from a pollen record of Lake Billyakh, Verkhoyansk Mts, NE Siberia

Abstract. In this study, a radiocarbon-dated pollen record from Lake Billyakh (65°17' N, 126°47' E; 340 m a.s.l.) in the Verkhoyansk Mountains was used to reconstruct vegetation and climate change since about 15 kyr BP. The pollen record and pollen-based biome reconstruction suggest that open cool steppe (STEP) and grass and sedge tundra (TUND) communities with Poaceae, Cyperaceae, Artemisia, Chenopodiaceae, Caryophyllaceae and Selaginella rupestris dominated the area from 15 to 13.5 kyr BP. On the other hand, the constant presence of Larix pollen in quantities comparable to today's values points to the constant presence of boreal deciduous conifer (CLDE) trees in the regional vegetation during the Late Glacial. A major spread of shrub tundra communities, including birch (Betula sect. Nanae), alder (Duschekia fruticosa) and willow (Salix) species, is dated to 13.5–12.7 kyr BP, indicating a noticeable increase in precipitation toward the end of the Last Glaciation, particularly during the Bølling-Allerød Interstadial. Between 12.7 and 11.4 kyr BP pollen percentages of herbaceous taxa rapidly increased, whereas shrub taxa percentages decreased, suggesting strengthening of the steppe communities associated with the relatively cold and dry Younger Dryas Stadial. However, the pollen data in hand indicate that Younger Dryas climate was less severe than the climate during the earlier interval from 15 to 13.5 kyr BP. The onset of the Holocene is marked in the pollen record by the highest values of shrub and lowest values of herbaceous taxa, suggesting a return of warmer and wetter conditions after 11.4 kyr BP. Percentages of tree taxa increase gradually and reach maximum values after 7 kyr BP, reflecting the spread of boreal cold deciduous and taiga forests in the region. An interval between 7 and 2 kyr BP is noticeable for the highest percentages of Scots pine (Pinus subgen. Diploxylon), spruce (Picea) and fir (Abies) pollen, indicating mid-Holocene spread of boreal forest communities in response to climate amelioration and degradation of the permafrost layer.

Holocene altitudinal shifts in vegetation belts and environmental changes in the Sierra Madre Occidental, Northwestern Mexico, based on modern and fossil pollen data

A modern pollen rain study was performed in a 300 km-long altitudinal transect (~ 28° N latitude) from 300 to 2300 m elevation. The higher elevation modern communities: epithermal oak–pines, pine–oak forest, pine forest, and mixed conifer forest were easy to distinguish from their pollen content. In contrast, lower elevation subtropical communities: thornscrub and tropical deciduous forest were difficult to separate, because they share many pollen taxa. Nevertheless we identify high frequencies of Bursera laxiflora as an important component of the tropical deciduous forest. Additionally, fossil pollen was analyzed at three sites located between 1700 and 1950 m altitude at ~ 28° latitude north in the Sierra Madre Occidental of northwestern Mexico. The sites were in pine–oak ( Pinus–Quercus), pine, and mixed-conifer forests respectively. Shifts in the altitudinal distribution of vegetation belts were recorded for the last 12,849 cal yr BP, and climate changes were inferred. The lowest site (pine–oak forest) was surrounded by pine forest between 12,849 and 11,900 cal yr BP, suggesting a cold and relatively dry Younger Dryas period. The early Holocene was also cold but wetter, with mixed conifer forest with Abies (fir) growing at the same site, at 1700 m elevation, 300 m lower than today. After 9200 cal yr BP, a change to warmer/drier conditions caused fir migration to higher elevations and the expansion of Quercus at 1700 m. At 5600 cal yr BP Abies was growing above 1800 m and Picea (spruce) that is absent today, was recorded at 1950 m elevation. Fir and spruce disappeared from the 1950 m site and reached their present distribution (scattered, above 2000 m) after 1000 cal yr BP; we infer an episodic Holocene migration rate to higher elevations for Abies of 23.8 m/1000 cal yr and for Picea of 39.2 m/1000 cal yr. The late Holocene reflects frequent climate oscillations, with variations in the representation of forest trees. A tendency towards an openness of the forest is recorded for the last 2000 yrs, possibly reflecting human activities along with short-term climate change.

10Be dating of alluvial deposits from Southeastern Iran (the Hormoz Strait area)

A key area for the understanding of the southeastern Iran geodynamics, the Hormoz Strait area, was investigated. Tectonically active, it provides useful sequences of Quaternary deposits whose deposition may have ended either when the climate dried or when the tectonic activity uplifted the riverbed. In-situ produced cosmogenic 10Be provides surface abandonment ages at 5.6 ± 0.6, 8.4 ± 1.0, 12.8 ± 1.0, 20.1 ± 1.5, and 44.0 ± 3.4 10Be kyr. Located at the transition between the Mediterranean Winter Rain and the Indian Monsoon domains, and south of the northernmost position of the ITCZ (InterTropical Convergence Zone), the studied area is very sensitive to climate changes. Some of the calculated 10Be abandonment ages are within the entire investigated area coincidental with large-scale climate shifts: the offset of the mid-Holocene humid period (5.6 ± 0.6 10Be kyr) , the onset of the dry Younger Dryas cold and dry episode (12.8 ± 1.0 10Be kyr), and the end of the Last Glacial Maximum (LGM; 21 ± 2 kyr) at 20.1 ± 1.5 10Be kyr. These events correspond to large-scale changes in atmospheric circulation linked to global climate changes, and/or to sea-level fluctuations in the Persian Gulf, which was totally dry during the LGM. Locally observed abandonment ages at 8.4 ± 1.0 and 44.0 ± 3.4 10Be kyr may likely be related to enhanced tectonic activity without regional impact.

Climate of the last 53,000 Years in the eastern Mediterranean, based on soil-sequence Stratigraphy in the coastal plain of Israel

In this paper we present a detailed record of proxy-climatic events in the coastal belt of the eastern Mediterranean during the past 53,000 years. A sequence of alternating palaeosols, aeolianites, and dune sands, which have been dated by luminescence and by 14C, was studied by the magnetic susceptibility, particle-size distribution, clay mineralogy and soil micromorphology. Thirteen proxy-climatic events, demonstrating fluctuations of relatively dry and wet episodes, were recognized. The soil parent materials, as well as the different soil types, were rated in a semi-quantitative “dry” to “wet” scale. The palaeosol sequence is compared to a proxy-climatic record of oxygen and carbon isotopes in speleothems from a karstic cave in central Israel and to a record of lake levels of Lake Lisan and its successor, which is known as the Dead Sea. A genuine red Mediterranean Soil (Rhodoxeralfs), localy designated as “Hamra Soil” developed during the Last Glacial Stage, from 40 to 12.5 thousand calendar years BP. Climatic fluctuations that were recorded in speleothems and in changing lake levels were not preserved in this soil. During the cold and dry Younger Dryas, ca 12.5 to 11.5 calendar ka BP, a thick bed of loess material, deriving from atmospheric dust of the Sahara and Arabian deserts, covered the entire coastal belt. During this phase Lake Lisan was desiccated and turned into the modern, smaller Dead Sea. During the early Holocene, some 10–7.5 calendar ka BP, a second Red Hamra soil developed in warm and wet environments, associated with a relatively high stand of the Dead Sea level. A depletion of δ 18O and a significant enrichment of δ 13C in the speleothems were recorded during this episode. This event was in phase with the widespread distribution of freshwater lakes in the Sahara Desert and the accumulation of the S1 Sapropel in the eastern Mediterranean. Several small-scale dry and somewhat wet fluctuations of the Late Holocene, from 7.5 calendar ka BP to the present, were recorded in the coastal belt. Changes in human history, as reflected in archaeological records, are associated with proxy-climatic fluctuations. Periods of desertification and deterioration are coupled with dry episodes; periods of relative human prosperity are coupled with wetter episodes.

Terrestrial evidence for a spatial structure of tropical–polar interconnections during the Younger Dryas episode

The Younger Dryas chronozone, recognised in northern high-latitude areas as a cold event between 11 000 and 10 000 14C yr BP (12 900–11 600 cal. yr BP), seems to manifest itself globally in different ways. Here, we examine well-dated stratigraphic sequences together with high-resolution proxy data plots from sites across our study area, the arid–semi-arid transition zone in northern China. This climatically sensitive area of China records a cold, dry Younger Dryas climate which was punctuated by a brief period of summer monsoon precipitation. We have since found that similar climatic sequences have been reported from the Sahel and the equatorial region of Africa. Based on evidence from these sites, together with other published data, we postulate that precipitation during the Younger Dryas chronozone was indicative of a low-latitude driving force superimposed on the high-latitude cold background. This rain belt rearrangement was most probably caused by an interaction between cold air advection and summer moisture transport across the tropical Pacific Ocean. Examination of high-resolution proxies suggests short-term climate fluctuations indicative of a global teleconnection involving moist air transportation patterns from the tropics to higher latitudes, varying with ENSO and other tropical factors.