Younger Dryas Episode Research Articles

Response of East Asian summer monsoon climate to North Atlantic meltwater during the Younger Dryas

The Younger Dryas (YD) event, recognized as one of the most typical abrupt climate changes on the millennial time scale, results in striking cooling in most regions of the North Atlantic. The most acceptable hypothesis believes that this event is related to a large volume of meltwater fluxes injected into the North Atlantic. In remote Asia, various paleoclimate reconstructions have revealed that the East Asian summer monsoon (EASM) is significantly depressed during the cold YD episode. However, the effect of North Atlantic meltwater-induced cooling on the whole downstream Eurasian regions and its potential dynamics remains been not fully explored till now. In this study, the responses of Asian climate characteristics during the YD episode, especially the EASM, are evaluated based on modeling data from the Simulation of the Transient Climate of the Last 21,000 years (TraCE 21ka). The results show that the cooling signal during the YD, which is mainly caused by meltwater flux, spreads from the North Atlantic to the whole Eurasia. In agreement with the paleoclimatic proxies, the simulated EASM is obviously weakened. The summer precipitation is also suppressed over East, South, and Central Asia. Dynamically, the North Atlantic cooling produces an eastward propagated wave train across the mid-latitude Eurasia, which facilitates weaker EASM circulation. The weakened land-sea thermal contrast over East Asia also contributes to the monsoon decrease during YD cooling.

Seasonal evolution differences of east Asian summer monsoon precipitation between Bølling-Allerød and younger Dryas periods

The rapid warming trend during the last deglaciation triggered significant global climate instabilities due to a complex non-linear response of the climate system to the gradual increase in insolation over the northern hemisphere. Although climate impacts can be detected globally, major regional imprints such as seasonal evolution and changes of the East Asian Summer Monsoon (EASM) during the last deglaciation are still poorly constrained due to a lack of comprehensive proxy data. In this study, we compare how the extreme climate shifts are linked to changes in EASM precipitation over China between the unusually warm Bolling-Allerod (BA) interstadial and the following strong cooling of the Younger Dryas (YD) stadial. Our analysis is based on the transient atmosphere-ocean simulations of TraCE-21ka, in addition to new results from high-resolution simulations of the CESM1 model for the BA and YD time slices. We find that the earlier onset and stronger intensity of the EASM in the BA interstadial lead to more precipitation in early summer (May–June) but drier conditions during mid-summer (July–August) over Southern China compared to a stadial climate during the YD episode. For Northern China, we find the opposite response. The insolation change in spring and the forced response of the atmospheric system are thought to be responsible for these differences. Relative to the YD episode, the hemispheric temperature gradient during the BA period is enhanced due to the asymmetric warming between the two hemispheres, leading to an intensified northward equatorial cross flow. Combined with a stronger sensible heating of the Tibetan Plateau in spring and the related earlier northward shift of the westerly jet, the early onset of the EASM is triggered. The latent heat release, which is accompanied by the onset of the EASM and the sudden increasing precipitation over Southern China in early summer, contributes to the westward shift of the Western Pacific Subtropical High (WPSH) and eastward movement of the South Asia High (SAH) in mid-summer. Under the above conditions, Southern China experiences a hot and dry climate, while Northern China receives more precipitation. Additionally, the La Nina-like pattern of the equatorial Pacific also partly contributes to the strong EASM in the warm period by influencing the WPSH location and Pacific-North American (PNA) teleconnection pattern.

Time-transgressive Holocene maximum of temperate and Mediterranean forest development across the Iberian Peninsula reflects orbital forcing

We describe a new pollen and Sea Surface Temperature (SST) record with sub-centennial to centennial resolution from a well-dated sediment core (D13882) obtained from the Atlantic margin of the southwest Iberian Peninsula. The core provides new insights into the dynamics of five ecological pollen-based groups (temperate forests, Mediterranean forests, pinewoods, heathlands and steppe) since the Late Pleistocene (last 13,260 cal yr BP). The record shows that the Early Holocene was marked by an increase of temperate and Mediterranean taxa, accompanied by a reduction of steppic plants, reflecting warm and relatively moist conditions when compared with the previous Younger Dryas episode. The timing of this Temperate and Mediterranean Forest (TMF) maximum which reflects optimum conditions of temperature and precipitation is regionally specific, as detected through a comparison of three marine cores across the Iberian Peninsula. Maximum TMF development occurs between 9760 and 7360 cal yr BP in D13882 whereas at a site to the northwest (MD03-2697), the maximum occurs some centuries earlier, and at a southeast site (MD95-2043), the maximum is evident later. The duration of the forest maximum is also progressively longer from northwest to southwest to southeast. A transition from forested ecosystems to heathland over western Iberia characterises the Middle Holocene, starting from around 7360 cal yr BP in D13882, suggesting reduced seasonality and a more Atlantic climate character with more moisture evenly available through the year. This contrasts with the aridity recorded in southeast Iberian sites and reflects a stable precipitation gradient between the wetter west and drier east of the Iberian Peninsula. Against this backdrop, pinewoods had a continuous presence, albeit with a trend of steady decline through the Holocene. The time-transgressive pattern in the TMF maximum is best explained by a less favourable moisture balance in the drier southeast sector, which was particularly important during the Early Holocene. Despite the relatively short duration of the Holocene interglacial (one half precession cycle), our observations support a key role for orbital forcing in the temporal expression of major vegetation groups, specifically the decline in summer insolation and the precession-related passage of perihelion season from summer to winter.

Cosmic impact or natural fires at the Allerød–Younger Dryas boundary: A matter of dating and calibration

Wittke et al. (1) present evidence of a major cosmic impact at the onset of the Younger Dryas (YD) episode, including some markers found in the top of the well-known Usselo marker horizon (UH). This finding is contrary to our extensive radiocarbon dating effort from this horizon (2), which shows that the UH at Aalsterhut postdates the onset of the YD. Furthermore, Wittke et al. (1) misinterpret the origin of the UH: this horizon is a well-defined paleosoil that formed during the Allerod and the early YD in the top part of coversand. This coversand was deposited before the Allerod, during cold and dry conditions, and is part of the European Sandbelt. Wildfires were common and occurred throughout this period, rather than synchronously with the onset of the YD (3).

Neotectonic and climatic impressions in the zone of Trans Himadri Fault (THF), Kumaun Tethys Himalaya, India: A case study from palaeolake deposits

Late Quaternary tectonic activity on a NW-SE trending fault within the Trans Himadri Fault (THF) zone in lower reaches of the Milam glacier (Indian Tethys Himalaya) resulted in development of a lake around 23 ka BP. The remnants of the ancient lake are preserved in form of a 25.6 m thick lacustrine profile, consisting of muds and sands. The palaeolake seems to have breached around 11 ka BP possibly due to revival of a further event of neotectonic activity. The geomorphic consequences of the tectonic movements in the fault zone are manifest in form of palaeo-landslide cones, unpaired terraces, fault facets, soaring waterfalls, deep gorges and slope failures etc. The soft sedimentary structures, e. g., micro-faulting and flame structures in the exposed profile also point to a possible reactivation of the THF in the Late Pleistocene. We present the first palynological results from otherwise a totally unexplored area in the eastern part of the Indian Tethys Himalaya. The preliminary results indicate that the area experienced cold desertic climatic conditions from ca. 22.9 to 15.7 ka BP covering a period of Last Glacial Maximum (LGM) and during which the sediment accumulation rate was also extremely slow. This phase was followed by deglaciation together with amelioration of climate between ca. 15.7 and 14.5 ka BP. A dry period from ca. 14.5 to 13.8 ka BP can be associated with the Older Dryas. The area underwent wetter/moist conditions from ca. 13.8 to 12.8 ka BP, followed by a century scale dry event (ca. 12.8 to 12.7 ka BP) which may be linked to the Younger Dryas episode.

Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago

Airbursts/impacts by a fragmented comet or asteroid have been proposed at the Younger Dryas onset (12.80 ± 0.15 ka) based on identification of an assemblage of impact-related proxies, including microspherules, nanodiamonds, and iridium. Distributed across four continents at the Younger Dryas boundary (YDB), spherule peaks have been independently confirmed in eight studies, but unconfirmed in two others, resulting in continued dispute about their occurrence, distribution, and origin. To further address this dispute and better identify YDB spherules, we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry, morphologies, origins, and processes of formation. We investigated 18 sites across North America, Europe, and the Middle East, performing nearly 700 analyses on spherules using energy dispersive X-ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization. Twelve locations rank among the world's premier end-Pleistocene archaeological sites, where the YDB marks a hiatus in human occupation or major changes in site use. Our results are consistent with melting of sediments to temperatures >2,200 °C by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere; they are inconsistent with volcanic, cosmic, anthropogenic, lightning, or authigenic sources. We also produced spherules from wood in the laboratory at >1,730 °C, indicating that impact-related incineration of biomass may have contributed to spherule production. At 12.8 ka, an estimated 10 million tonnes of spherules were distributed across ∼50 million square kilometers, similar to well-known impact strewnfields and consistent with a major cosmic impact event.

Geochemical signatures of sediments documenting Arctic sea-ice and water mass export through Fram Strait since the Last Glacial Maximum

Elemental (Ca, Zr, Th, etc.) and radiogenic isotope (Pb, Nd, Sr) measurements in leachates and residues from deep-sea sediments of core MC16 (WarmPast Program) in central Fram Strait were used to document the geochemical signatures of outflowing Arctic water masses and ice rafted debris (IRD) since the Last Glacial Maximum. In addition, the elemental distribution among the three main sedimentary fractions (terrigenous, biogenic and authigenic) was quantified. Elements dominated by the terrigenous fraction display a change at ∼13 ka assigned to an early Younger Dryas (YD) event. In the authigenic fraction, migration of the redox front, perhaps spurred by discontinuous delivery of organic matter to the sediment, has led to the mobility of elements such as Mn. Fe contents display lesser variability within the sediment suggesting that Fe experienced only minor redox-related redistribution. Authigenic Pb and Nd, thought to be hosted primarily by Fe-oxyhydroxides, also show little evidence of mobility, suggesting that their isotopic compositions should reliably record the isotopic compositions of past bottom water.We have broadly identified the isotopic signatures of the three major source areas of IRD, the Russian, Canadian and Greenland margins. The elemental and isotopic residue records from core MC16 display distinct trends prior to and after the YD. The pre-YD interval, with ɛNd values between −10.1 and −13.2, and 87Sr/86Sr ratios from 0.715 to 0721, reflects a mixture of IRD from the Russian and Canadian margins. The YD episode stands out with sediments originating mostly from the Canadian end-member, displaying the lowest ɛNd values and highest 87Sr/86Sr ratios. This suggests enhanced sea-ice production and/or drifting along the Beaufort Gyre at that time. The post-YD interval, i.e. the Holocene, is characterized by a less variable mixture of IRD material, with ɛNd values and 87Sr/86Sr ratios centred at −12.2 and 0.718 respectively. This material was derived from several sources including northwestern Canada (Mackenzie River), the western Arctic, the East Siberian/Chukchi Seas, and the more proximal Greenland margin.The isotopic compositions of leachates are mostly linked to boundary exchange processes near major meltwater and freshwater source areas. Due to its longer residence time relative to Pb, Nd preserves the isotopic signature of more distal areas where high-density particulate fluxes may occur. In the studied core, Nd-isotope leachate data illustrate the influence of the western Russian margins prior to the YD event and that of the East Siberian and Chukchi Sea margins following this event. This study illustrates that complementary information on IRD sources and water-mass histories can be obtained from isotopic analyses of inherited (residual) and exchangeable (leachable) fractions in deep Arctic Ocean sediments. Higher resolution cores are now needed to illustrate more rapid variations of paleo sea-ice and water mass circulation.

DYNAMICAL SIMULATION ON TIDAL CURRENT FIELD IN ANCIENT YELLOW SEA DURING YOUNGER DRYAS EPISODE

During Younger Dryas episode(12.9～11.6 cal.ka),the sea level oscillated at about-66 m depth contour in the continental shelf seas in eastern China and the seawater got into the south of the present North Yellow Sea,so Yellow Sea became a long narrow channel gulf.To simulate the tidal current of this special gulf,Environmental Fluid Dynamics Computer Code is used in this paper.The calculation combines curvilinear orthogonal coordinates in the horizontal direction with Sigma mapping coordinates in the vertical direction,and it uses a method which transforms a problem from three-dimension into two-dimension.The results show that the tidal current was still very strong in Yellow Sea during Younger Dryas episode,entering the special gulf along the right side entrance.From both shallow sides to deep centre,rotating flow changes reciprocating flow.Under the effect of reciprocating flow,the floor is eroded and the mud and sand are carried away.Progressive wave turns into rotating wave and two ateliotic amphidromic points appear on the western side of the gulf after tidal wave coming into Yellow Sea.At the same time,tidal residual current flows to southeast and east with relatively great velocity at the west and interior gulf and it becomes weakly near the entrance.The current may carry the bottom sediments and discharge them to the present mud sediment zone in north East China Sea.But at eastern Yellow Sea the tidal residual current flows to north and forms an anticlockwise circulation at centre of the eastern gulf which is corresponding to the eddy mud zone in Southeast Yellow Sea(Heishan mud zone).The anticlockwise circulation captures suspended sediment and may gestate the present mud sediment zone.The extension direction of tidal sand ridges and the size composition of the tidal current sand in eastern South Yellow Sea have a close relationship with the tidal current field of the ancient Yellow Sea,so the less fluctuating sea level changing and the relative stable tidal current field are the favorable factors for the tidal sand ridges during Younger Dryas episode in ancient Yellow Sea.It can be said that the ocean current,tidal current and tidal residual current figure the pattern of the present Yellow Sea during Younger Dryas episode.

Coversand and Pleistocene palaeosols in the Landes region, southwestern France

AbstractNew sections in the coversand of the Landes region, southwestern France, show at least two main depositional phases corresponding to the Upper Pleniglacial and the Lateglacial, which are separated by palaeosols. The lower palaeosol, a gleyic to histic cryosol overlying a net of sand wedges and dated to ca. 23 14C ka BP, testifies to a short occurrence of permafrost. Impeded drainage due to the frozen subsoil is assumed to be the main factor involved in lowered aeolian transport and soil formation. Pollen analysis indicates a shrub tundra‐type environment. The overlying coversand unit is associated with small transverse ridges or sheet‐like deposits, and corresponds to the maximal extension of the sands, Upper Pleniglacial in age. An incipient podzol developed on the dunes under a boreal pine forest, and has been dated to 11.5–12 14C ka BP, i.e. to the Allerød period. This has been buried by the second coversand unit during the Younger Dryas, typified by abundant denivation features and root imprints. Although preliminary, the chronology of sand deposition in the Landes region appears thus to be roughly similar to that found for the other European coversands, showing that all were the result of similar western European climatic changes, i.e. repeated episodes of increasing aridity related to the Upper Pleniglacial and the Younger Dryas episode. Copyright © 2008 John Wiley & Sons, Ltd.

Loess record of the Pleistocene–Holocene transition on the northern and central Great Plains, USA

Various lines of evidence support conflicting interpretations of the timing, abruptness, and nature of climate change in the Great Plains during the Pleistocene–Holocene transition. Loess deposits and paleosols on both the central and northern Great Plains provide a valuable record that can help address these issues. A synthesis of new and previously reported optical and radiocarbon ages indicates that the Brady Soil, which marks the boundary between late Pleistocene Peoria Loess and Holocene Bignell Loess, began forming after a reduction in the rate of Peoria Loess accumulation that most likely occurred between 13.5 and 15 cal ka. Brady Soil formation spanned all or part of the Bølling-Allerød episode (approximately 14.7–12.9 cal ka) and all of the Younger Dryas episode (12.9–11.5 cal ka) and extended at least 1000 years beyond the end of the Younger Dryas. The Brady Soil was buried by Bignell Loess sedimentation beginning around 10.5–9 cal ka, and continuing episodically through the Holocene. Evidence for a brief increase in loess influx during the Younger Dryas is noteworthy but very limited. Most late Quaternary loess accumulation in the central Great Plains was nonglacigenic and was under relatively direct climatic control. Thus, Brady Soil formation records climatic conditions that minimized eolian activity and allowed effective pedogenesis, probably through relatively high effective moisture. Optical dating of loess in North Dakota supports correlation of the Leonard Paleosol on the northern Great Plains with the Brady Soil. Thick loess in North Dakota was primarily derived from the Missouri River floodplain; thus, its stratigraphy may in part reflect glacial influence on the Missouri River. Nonetheless, the persistence of minimal loess accumulation and soil formation until 10 cal ka at our North Dakota study site is best explained by a prolonged interval of high effective moisture correlative with the conditions that favored Brady Soil formation. Burial of both the Brady Soil and the Leonard Paleosol by renewed loess influx probably represents eolian system response that occurred when gradual change toward a drier climate eventually crossed the threshold for eolian activity. Overall, the loess–paleosol sequences of the central and northern Great Plains record a broad peak of high effective moisture across the late Pleistocene to Holocene boundary, rather than well-defined climatic episodes corresponding to the Bølling-Allerød and Younger Dryas episodes in the North Atlantic region.

Inland aeolian deposits of the Iberian Peninsula: Sand dunes and clay dunes of the Duero Basin and the Manchega Plain. Palaeoclimatic considerations

This paper describes the latest research on the geomorphological characteristics, formation environment and chronology of the main inland aeolian deposits from the south-eastern Duero Basin (DB) and the Manchega Plain (MP) of the Iberian Peninsula. Similarities and differences between the aeolian deposits of these two locations are summarised. Wind deflation from the Guadiana and Júcar alluvial systems created the aeolian deposits of the MP. These deposits are mainly composed of quartz sands. However, in the San Juan alluvial plain (MP) there is a large extent of clay dunes formed by exposure to prevalent winds of seasonal playa-lakes with salt and clay sediments. In the DB, wind remobilisation of the small particles from Quaternary terraces and Tertiary arkosic sediments left aeolian deposits of quartz–feldspar sands. Textural parameters of the aeolian deposits show large variations depending on the location and the original deposit. Thus, in the DB the aeolian sands derived from the deflation of fluvial sediments are better sorted and smaller in grain size than those created by the deflation of arkosic sediments. Morphologically, simple and compound parabolic dunes (U–V forms, hemicyclic, lobate and elongate), crescentic and linear dunes, climbing dunes and blowout dunes have been recognized at both sites. Barchan and dome dunes are present only in the DB while “lunette lunette-clay dunes” are found only in the MP. In both locations, the large extent of aeolian sand sheets and the predominance of simple and compound parabolic dunes indicates the active role of sparse vegetation cover in the formation of this aeolian system. In the DB, dunes were formed by southwest and west winds, while in the MP the aeolian morphologies indicate that the prevalent winds were west and northwest. The chronology of the dune deposits is being determined with luminescence (TL-OSL) dating and Mass Spectrometry Analysis ( 14C-AMS). In this way, the aeolian activity and stabilisation stages can be established, the latter well marked in the DB through soil A horizon development. Thus, the main sand dune formation in the DB and the eastern regions of the MP occurred between 13.5 and 7 ka BP, during the cold and arid Younger Dryas episode and the Early Holocene. The clay dunes of the MP accumulated mainly from 29 to 19 ka BP that corresponds with Heinrich events HE-3 and HE-2 and the Last Glacial Maximum. However, clay dunes were also formed between 13.5 and 7 ka BP. In both locations, there have been reactivations of some sand deposits in the recent Holocene, with maximum activity around 5–2 ka BP and 0.5–0.2 ka BP. On the other hand, three marked stages of stabilisation of the DB aeolian system have been established with 14C-AMS, around 10.2, 6.2 and 1.2 ka BP. Finally, the main winds contributing to dune construction were also responsible for the deflation processes with the formation of erosional depressions.

The Younger Dryas and the Sea of Ancient Ice

We propose that prior to the Younger Dryas period, the Arctic Ocean supported extremely thick multi-year fast ice overlain by superimposed ice and firn. We re-introduce the historical term paleocrystic ice to describe this. The ice was independent of continental (glacier) ice and formed a massive floating body trapped within the almost closed Arctic Basin, when sea-level was lower during the last glacial maximum. As sea-level rose and the Barents Sea Shelf became deglaciated, the volume of warm Atlantic water entering the Arctic Ocean increased, as did the corresponding egress, driving the paleocrystic ice towards Fram Strait. New evidence shows that Bering Strait was resubmerged around the same time, providing further dynamical forcing of the ice as the Transpolar Drift became established. Additional freshwater entered the Arctic Basin from Siberia and North America, from proglacial lakes and meltwater derived from the Laurentide Ice Sheet. Collectively, these forces drove large volumes of thick paleocrystic ice and relatively fresh water from the Arctic Ocean into the Greenland Sea, shutting down deepwater formation and creating conditions conducive for extensive sea-ice to form and persist as far south as 60°N. We propose that the forcing responsible for the Younger Dryas cold episode was thus the result of extremely thick sea-ice being driven from the Arctic Ocean, dampening or shutting off the thermohaline circulation, as sea-level rose and Atlantic and Pacific waters entered the Arctic Basin. This hypothesis focuses attention on the potential role of Arctic sea-ice in causing the Younger Dryas episode, but does not preclude other factors that may also have played a role.

High-resolution numerical simulation of Younger Dryas glaciation in Scotland

We use a 500 m resolution three-dimensional thermomechanical ice-sheet model forced by a scaled GRIP temperature pattern to retrodict the extent of glaciers during the Younger Dryas episode in Scotland. Using empirical data from sources spanning half a century we systematically perturb temperature depression, precipitation distribution, and the amount of basal sliding to identify the parameter space that most closely reproduces the glacier margins identified from field investigations. Arithmetic comparison of predicted ice cover with empirical glacier extent enables mismatch to be quantified and an ‘optimum-fit’ timeslice to be identified. This ‘best-fit’ scenario occurs with a maximum mean annual temperature depression from present of 10 ∘ C and steep eastward and northward gradients imposed on a modern precipitation distribution, coupled with restricted basal sliding. Even small deviations around these values produce considerably less well-fitting glacier configurations, suggesting that only a narrow range of optimal parameterisation exists. Mismatch between modelled and empirically reconstructed glacier extents occurs as a consequence of local conditions not accommodated by the model, such as wind-blown snow accumulation and lake (loch) bathymetry, and where geological evidence is equivocal. At the domain scale, however, our simulation suggests that inception of Scottish glaciers occurs rapidly, and leads to a coherent ice cap within 400 years of initial climatic cooling. According to our model, the ice cap begins to decay relatively early in the stadial, probably as a result of increasing aridity leading to net thinning and recession of many of its margins, with final and catastrophic collapse of the ice cap occurring largely within a century.

Influence of the intertropical convergence zone on the East Asian monsoon

The Asian-Australian monsoon is an important component of the Earth's climate system that influences the societal and economic activity of roughly half the world's population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bølling-Allerød warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period ad 700 to 900 (refs 4-6), suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America.

Late Glacial and Holocene vegetational history of the Altai Mountains (southwestern Tuva Republic, Siberia)

Two lakes of glacial origin (Grusha at 2413 m and Akkol at 2204 m) are located on a broad high-mountain plateau on the dry eastern end of the Russian Altai Mountains in the Republic of Tuva, just north of Mongolia. The present vegetation of the area is an open high-mountain tundra–steppe mosaic, with patches of Larix and Pinus sibirica forests on north-facing slopes 30 km northwest of the lakes. Alpine meadows occur near streams and snow patches. Pollen diagrams and radiocarbon dates (16 for Grusha and 12 for Akkol) were used to reconstruct the vegetational history since the last glaciation. The core from Grusha contains about 50 cm of Late Glacial sediment of relatively low organic content, mostly correlated with the Younger Dryas episode because of the high percentages of Artemisia, Gramineae, and Chenopodiaceae, along with a diversity of non-arboreal pollen types indicating tundra–steppe and meadow-steppe. In the earliest Holocene, starting with increased organic matter at about 195 cm (12,000 cal. yr BP), the Artemisia–Gramineae–Chenopodiaceae pollen assemblage continued, along with an increase in shrub Betula but without the diversity of minor non-arboreal types. The interpretation of dry and warm climate at this time is supported by the unconformity at Akkol, implying a dried lake. Then after about 1000 years the increase of Pinus sibirica, P. sylvestris, Picea obovata, and Abies sibirica suggests that forests developed in response to increased humidity, first at the higher-elevation Grusha, and then at Akkol. After about 6000 cal. yr BP the role of forests decreased sharply in the area. The Picea and Abies components almost disappeared, as they did also in more westerly areas of the Altai Mountains. Steppe elements increased along with alpine herbs, especially after 2000 cal. yr BP as a result of cooling or increased aridity, as well as probable human activity. Correlation of the vegetation sequence for the Tuva sites with those to the west in the central Altai Mountains indicates that the Late Glacial open landscapes persisted in the earliest Holocene but then were invaded by coniferous forests as a result of increased moisture and temperature associated with Atlantic storm systems as well as with insolation-enhanced Asian monsoon. Cooling and drying after about 6000 cal. yr BP brought the decline of Abies and Picea and the expansion of tundra–steppe.

The Younger Dryas episode and the radiocarbon chronologies of the Lake Huleh and Ghab Valley pollen diagrams, Israel and Syria

Pollen diagrams from the former Lake Huleh in Israel and the Ghab Valley in Syria are the most important records of vegetation change in the Levant during the Lateglacial and early Holocene. Environmentally deterministic explanations of the development of agriculture in this region therefore rely on the accuracy of the diagrams radiocarbon chronologies. Radiocarbon results at both sites are subject to large reservoir effects, however: the 14C content of modern water from the Huleh basin implies that the Huleh radiocarbon results require corrections of up to 5500 14C years. A revised chronology of the latest Huleh pollen diagram is proposed. This is consistent with the regional vegetation sequence recorded in eastern Mediterranean marine sediments. The regional sequence also provides the most plausible chronology for the Ghab pollen diagrams.

Oceanic and atmospheric variations during the last deglaciation in the Fram Strait (Arctic Ocean): a coupled high-resolution organic-geochemical and sedimentological study

From a high sedimentation rate core, obtained from the Yermak slope (Arctic Ocean), we deduced short-term climatic fluctuations of the last deglaciation lasting from 17,000 to 9000 years BP. By using a coupled organic-geochemical and sedimentological approach, we reconstructed near-bottom current conditions and assessed their significance in relation with comparable global thermohaline circulation patterns and surface water conditions. Highest concentrations and fluctuations of specific marine phytoplankton biomarkers were obtained in the Bølling-Allerød, when the major deglaciation of the Svalbard-Barents-Sea-Ice-Sheet (SBIS) occurred. The marine biomarker record and the sedimentological record are consistent with climate reconstructions obtained from Greenland ice cores and thermohaline circulation variations. However, beginning with the Younger Dryas (YD) episode, bottom currents decreased during this time period, but surface waters remained very productive. The atmospheric conditions of the western Fram Strait (reconstructed from biomarker data) and thermohaline circulation signals (reconstructed from sedimentological data) were decoupled in the YD. Maximum insolation values of solar radiation and “Föhn”-like winds enabled open water conditions and increased productivity values in the northern Fram Strait, at least in summer.

Late Quaternary Climate and Vegetation of the Sudanian Zone of Northeast Nigeria

The Lake Tilla crater lake in northeastern Nigeria (10°23′N, 12°08′E) provides a ca. 17,000 14C yr multiproxy record of the environmental history of a Sudanian savanna in West Africa. Evaluation of pollen, diatoms, and sedimentary geochemistry from cores suggests that dry climatic conditions prevailed throughout the late Pleistocene. Before the onset of the Holocene, the slow rise in lake levels was interrupted by a distinct dry event between ca. 10,900 and 10,500 14C yr B.P., which may coincide with the Younger Dryas episode. The onset of the Holocene is marked by an abrupt increase in lake levels and a subsequent spread of Guinean and Sudanian tree taxa into the open grass savanna that predominated throughout the Late Pleistocene. The dominance of the mountain olive Olea hochstetteri suggests cool climatic conditions prior to ca. 8600 14C yr B.P. The early to mid-Holocene humid period culminated between ca. 8500 and 7000 14C yr B.P. with the establishment of a dense Guinean savanna during high lake levels. Frequent fires were important in promoting the open character of the vegetation. The palynological and palaeolimnological data demonstrate that the humid period terminated after ca. 7000 14C yr B.P. in a gradual decline of the precipitation/evaporation ratio and was not interrupted by abrupt climatic events. The aridification trend intensified after ca. 3800 14C yr B.P. and continued until the present.

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.

The Birth of the Gods and the Origins of Agriculture by Jacques Cauvin, translated by Trevor Watkins (New Studies in Archaeology.) Cambridge: Cambridge University Press, 2000; ISBN 0-521-65135-2 hardback £37.50 & $59.95 Reviewed by Ian Hodder, Gary O. Rollefson, Ofer Bar-Yosef with a response by Trevor Watkins

When, almost a century ago, Raphael Pumpelly put forward the ‘oasis theory’ for the origins of farming in the Near East, his was one of the first in a long series of explanations which looked to environment and ecology as the cause of the shift from hunting and gathering to cultivation and animal husbandry. Pumpelly envisaged climatic desiccation at the end of the last Ice Age as the primary factor, forcing humans, plants and animals into ever closer proximity as the arid zones expanded around them. Subsequent fieldworkers took the closer investigation of environmental changes as a key aim of their research, both in the Near East and elsewhere, and this has remained a fundamental theme in theories for the emergence of agriculture. More recent advances in our understanding of environmental change have placed particular emphasis on the cold Younger Dryas episode, at the end of the last Ice Age. The impact of this sudden reversal of climate warming on the complex Natufian hunter-gatherers of the Levant may, it is argued, have forced or encouraged these communities to explore novel subsistence modes.Not everybody accepts such a chain of reasoning, however, and in The Birth of Gods and the Origins of Agriculture, French archaeologist Jacques Cauvin rejects this emphasis on ecology and environment as the cause of change. Instead, he argues that primacy should be accorded to a restructuring of human mentality from the thirteenth to the tenth millennium BC, expressed in terms of new religious ideas and symbols. Cauvin's book, originally published in French in 1994 under the title Naissance des divinités, naissance de l'agriculture, adopts an ideological approach to explaining the Neolithic which is at odds with many traditional understandings, but which resonates closely with the idea that the Neolithic is much more than an economic transition, and coincided with a transformation in the world view of the prehistoric societies concerned. The present English translation appeared in 2000, and is based on the second French edition (1997) with the addition of a postscript summarizing relevant discoveries made since that date.Owing to illness, Jacques Cauvin has been unable to contribute to this Review Feature as had been hoped, but we are fortunate that his translator, Trevor Watkins, has agreed to draft a response to the comments made by our invited reviewers. These include Ian Hodder, whose own work on the Neolithic transition has been influenced by Cauvin's research, and Ofer Bar-Yosef and Gary Rollefson, both specialists in the prehistory of the Levant. At Dr Watkins' suggestion, the introductory piece which opens the Review Feature is a translated extract from Jacques Cauvin's contribution to a similar review treatment in Les Nouvelles de l'Archéologie (No. 79, 2000, 49–53). As our reviewers make clear, the significance of the book, and the debate which it has initiated, will make it akey text for many years to come.