Palaeoclimate Archives Research Articles

Using ratios in cave guano to assess past environmental changes

Abstract Stable isotope ratios of cave bat guano reflect environmental influences at the soil and plant level within the foraging range of the bats. Carbon, nitrogen and hydrogen isotopes have utility in reconstructing precipitation and temperature as well as in tracing the influence of large-scale atmospheric circulation and climate variability. Guano-derived stable isotope time series have been demonstrated to effectively provide high-resolution records of palaeoenvironmental variation from the late Pleistocene to present. Owing to their influence on these isotopes, factors such as bat ecology, species foraging range and site location must be considered when interpreting such records. Research analysing the isotopic composition of guano has provided a highly resolved history of vegetation and climate dynamics previously less understood. In this chapter we discuss the factors influencing the δ 13 C, δ 15 N and δ 2 H values in guano and provide an up-to-date review of cave guano as palaeo-climate archive.

Holocene tephrostratigraphic framework and monsoon evolution of East Asia: Key tephra beds for synchronising palaeoclimate records

In East Asia our understanding of Holocene climate change, forcing mechanisms and propagation, require the precise chronological control of palaeoclimate records to allow robust integration of data sets. The existing chronologies, predominantly based on 14C method, however, are not sufficient to constrain key questions about abrupt climate shifts that occur within a century in the transitions between states. Widely dispersed tephra layers allow precise dating and synchronisation of sedimentary archives, providing a chronological framework for integrating records, especially where the visible tephra record is complemented by the addition of cryptotephra (non-visible ash). Despite significant tephra studies in this region, however, a comprehensive Holocene tephra framework is not available. To address this issue, we carry out a thorough review on Holocene tephra investigations undertaken in Japan. Using widespread tephra beds we present an integrated tephra framework and suggest the way forward for establishing this as a wider approach for East Asia. The framework is based on twenty-two ash layers that are mainly from Japan, and to a lesser extent China/N Korea, S Korea and Russia. Each tephra is assessed in terms of chronology, geochemistry and distribution. The framework is compared with high resolution palaeoclimate records from East Asia. Using this we demonstrate regional variations in monsoon evolution and more importantly, the potential of tephra isochrons in constraining such variations. Given the scarce identification of tephra layers in those well-resolved palaeoclimate records, we advocate a more systematic employment of the cryptotephra method, which would potentially lead to a significant advance in East Asian tephrochronology and the correlation of palaeoclimate archives in the region.

Speleothem record attests to stable environmental conditions during Neanderthal-modern human turnover in southern Italy.

The causes of Neanderthal-modern human (MH) turnover are ambiguous. While potential biocultural interactions between the two groups are still little known, it is clear that Neanderthals in southern Europe disappeared about 42 thousand years ago (ka) after cohabitation for ~3,000 years with MH. Among a plethora of hypotheses on Neanderthal extinction, rapid climate changes during the Middle to Upper Palaeolithic transition (MUPT) are regarded as a primary factor. Here we show evidence for stable climatic and environmental conditions during the MUPT in a region (Apulia) where Neanderthals and MH coexisted. We base our findings on a rare glacial stalagmite deposited between ~106 and ~27 ka, providing the first continuous western Mediterranean speleothem palaeoclimate archive for this period. The uninterrupted growth of the stalagmite attests to the constant availability of rainfall and vegetated soils, while its δ13C-δ18O palaeoclimate proxies demonstrate that Apulia was not affected by dramatic climate oscillations during the MUPT. Our results imply that, because climate did not play a key role in the disappearance of Neanderthals in this area, Neanderthal-MH turnover must be approached from a perspective that takes into account climatic and environmental conditions favourable for both species.

Testing polymineral post-IR IRSL and quartz SAR-OSL protocols on Middle to Late Pleistocene loess at Batajnica, Serbia.

The loess–palaeosol sequence of Batajnica (Vojvodina region, Serbia) is considered as one of the most complete and thickest terrestrial palaeoclimate archives for the Middle and Late Pleistocene. In order to achieve a numerical chronology for this profile, four sets of ages were obtained on 18 individual samples. Equivalent doses were determined using the SAR protocol on fine (4–11 μm) and coarse (63–90 μm) quartz fractions, as well as on polymineral fine grains by using two elevated temperature infrared stimulation methods, pIRIR 290 and pIRIR 225. We show that the upper age limit of coarse quartz OSL and polymineral pIRIR 290 and pIRIR 225 techniques is restricted to the Last Glacial/Interglacial cycle due to the field saturation of the natural signals. Luminescence ages on coarse quartz, pIRIR 225 and pIRIR 290 polymineral fine grains are in general agreement. Fine quartz ages are systematically lower than the coarse quartz and pIRIR ages, the degree of underestimation increasing with age. Comparison between natural and laboratory dose response curves indicate the age range over which each protocol provides reliable ages. For fine and coarse quartz, the natural and laboratory dose response curves overlap up to ~150 and ~250 Gy, respectively, suggesting that the SAR protocol provides reliable ages up to c. 50 ka on fine quartz and c. 100 ka on coarse quartz. Using the pIRIR 225 and pIRIR 290 protocols, equivalent doses up to ~400 Gy can be determined, beyond which in the case of the former the natural dose response curve slightly overestimates the laboratory dose response curve. Our results suggest that the choice of the mineral and luminescence technique to be used for dating loess sediments should take into consideration the reported limited reliability.

Detecting dynamical anomalies in time series from different palaeoclimate proxy archives using windowed recurrence network analysis

Abstract. Analysing palaeoclimate proxy time series using windowed recurrence network analysis (wRNA) has been shown to provide valuable information on past climate variability. In turn, it has also been found that the robustness of the obtained results differs among proxies from different palaeoclimate archives. To systematically test the suitability of wRNA for studying different types of palaeoclimate proxy time series, we use the framework of forward proxy modelling. For this, we create artificial input time series with different properties and compare the areawise significant anomalies detected using wRNA of the input and the model output time series. Also, taking into account results for general filtering of different time series, we find that the variability of the network transitivity is altered for stochastic input time series while being rather robust for deterministic input. In terms of significant anomalies of the network transitivity, we observe that these anomalies may be missed by proxies from tree and lake archives after the non-linear filtering by the corresponding proxy system models. For proxies from speleothems, we additionally observe falsely identified significant anomalies that are not present in the input time series. Finally, for proxies from ice cores, the wRNA results show the best correspondence to those for the input data. Our results contribute to improve the interpretation of windowed recurrence network analysis results obtained from real-world palaeoclimate time series.

On the recent hiatus of tropical cyclones landfalling in NSW, Australia

It is well known that severe storms result in some of the costliest natural disasters for New South Wales (NSW), Australia. However, it is not widely acknowledged that some of these events are, in fact, a result of landfalling tropical cyclones (TCs). Indeed, the intense focus of TC research within the tropics generally disregards landfalling TC events in the mid-latitude regions of Australia. This is likely due to the perceived infrequency of these events compared to other more susceptible regions. Therefore, in this study, we review this assumption by developing a 150-year record of TC activity, based on a range of digitised and analogue historical datasets and identify 30 individual landfalling TCs that have impacted NSW. Periods of enhanced and reduced TC activity are observed, with a defined hiatus (absence of landfalling TCs) after approximately 1980. The recent decrease in TC activity is subsequently linked to an increase in El Niño activity and warming of north-west Australian sea-surface temperatures during this time. Importantly, it is possible that a return to enhanced TC activity could occur again in the future if the Pacific conditions align. We also propose that pre-instrumental data on TC activity need to be developed to appropriately quantify TC risk for the study region via the development of local palaeoclimate archives. This study provides a significant contribution to understanding the risks of NSW landfalling TCs and expands upon our knowledge of environmental conditions that influence landfalling TCs in NSW.

High resolution OSL dating of the late quaternary loess from Central Shandong Mountains in eastern China and the paleoclimatic implications

The loess deposits widely distributed in the Central Shandong Mountains are significant terrestrial palaeoclimatic archives in the alluvial plain of eastern China. It is also sensitive records of East Asian monsoon changes for situating in a key region connected to the Eurasian plate and the Pacific Ocean. However, research into the climate changes in CSM loess is restricted by the lack of independent age control. High resolution samples of Optically Stimulated Luminescence (OSL) dating results are presented from a new Heishan loess section over ~34 ka. Our results reveal the following: (1) The boundary age defined by the sedimentation rates model between the Pleistocene and the Holocene at Heishan loess section is 10.9±0.6 ka. (2) A marked hiatus in the record is identified between ~30 and ~17 ka, probably resulting from deflation; this has never been raised in previous CSM loess researches and indicates that the study area is the wind erosion area during this time. (3) The relatively rapid sedimentation rate spanning 10.9±0.6 to 8.5±0.4 ka may be related to the post-depositional disturbance induced by more monsoon precipitation in the CSM region.

Stable isotopic composition of rice grain organic matter marking an abrupt shift of hydroclimatic condition during the cultural transformation of Harappan civilization

Several hypotheses have been proposed to solve the conundrum of the cause of transition of Harappan civilization to a de-urbanized form in its Late Phase. In view of this, high-resolution off-site palaeoclimatic records along with archaeological findings provide strong evidence of an abrupt climate change ∼4000 yr BP (before present) that coincides with the civilization's cultural transition. The present study investigates whether this climatic shift recorded in palaeoclimate archives (speleothem, lake and marine sediments) at remote locations can be traced to the human settlements of Harappan civilization. This was accomplished by analyzing the remains of rice cereal-a highly water sensitive crop. The analysis involved measuring carbon and nitrogen isotopic compositions (δ13C, δ15N) of well-preserved rice grains recovered from archaeological sites belonging to the Harappan civilization and other contemporary regional cultures, representing the time windows between 4520 and 3400 yr BP. The parameter of intrinsic water use efficiency (WUEi) was used as an index for water availability in the rice crop's growth environment and was ascertained based on δ13C values measured in the bulk grain organic matter (OM). The observed WUEi values ranged between 49 and 69 μmol mol−1 and captured the variation in water availability. The δ15N values ranged from 5‰ to 6.5‰, thereby allowing us to confirm the role of climate in controlling δ13C variation in the archaeological grains. The present findings will extend the usage of archaeobotanical cereal remains to deduce environmental changes at human settlements and will further our understanding of the process of Harappan cultural adaptation in response to decline in southwest monsoon at the Middle-Late Holocene boundary.

A Window into Africa’s Past Hydroclimates: The SISAL_v1 Database Contribution

Africa spans the hemispheres from temperate region to temperate region and has a long history of hominin evolution. Although the number of Quaternary palaeoclimatic records from the continent is increasing, much of the history of spatial and temporal climatic variability is still debated. Speleothems, as archives of terrestrial hydroclimate variability, can help reveal this history. Here we review the progress made to date, with a focus on the first version of the Speleothem Isotopes Synthesis and AnaLysis (SISAL) database. The geology of Africa has limited development of large karst regions to four areas: along the northern coast bordering the Mediterranean, eastern Africa and the Horn of Africa, southwestern Africa and southern Africa. Exploitation of the speleothem palaeoclimate archives in these regions is uneven, with long histories of research, e.g., in South Africa, but large areas with no investigations such as West Africa. Consequently, the evidence of past climate change reviewed here is irregularly sampled in both time and space. Nevertheless, we show evidence of migration of the monsoon belt, with enhanced rainfall during interglacials observed in northeast Africa, southern Arabia and the northern part of southern Africa. Evidence from eastern Africa indicates significant decadal and centennial scale rainfall variability. In northwestern and southern Africa, precession and eccentricity influence speleothem growth, largely through changing synoptic storm activity.

The water cycle of the mid‐Holocene West African monsoon: The role of vegetation and dust emission changes

During the mid‐Holocene (6 kyr BP), West Africa experienced a much stronger and geographically extensive monsoon than in the present day. Changes in orbital forcing, vegetation and dust emissions from the Sahara have been identified as key factors driving this intensification. Here, we analyse how the timing, origin and convergence of moisture fluxes contributing to the monsoonal precipitation change under a range of scenarios: orbital forcing only; orbital and vegetation forcings (Green Sahara); orbital, vegetation and dust forcings (Green Sahara‐reduced dust). We further compare our results to a range of reconstructions of mid‐Holocene precipitation from palaeoclimate archives. In our simulations, the greening of the Sahara leads to a cyclonic water vapour flux anomaly over North Africa with an anomalous westerly flow bringing large amounts of moisture into the Sahel from the Atlantic Ocean. Changes in atmospheric dust under a vegetated Sahara shift the anomalous moisture advection pattern northwards, increasing both moisture convergence and precipitation recycling over the northern Sahel and Sahara and the associated precipitation during the boreal summer. During this season, under both the Green Sahara and Green Sahara‐reduced dust scenarios, local recycling in the Saharan domain exceeds that of the Sahel. This points to local recycling as an important factor modulating vegetation‐precipitation feedbacks and the impact of Saharan dust emissions. Our results also show that temperature and evapotranspiration over the Sahara in the mid‐Holocene are close to Sahelian pre‐industrial values. This suggests that pollen‐based paleoclimate reconstructions of precipitation during the Green Sahara period are likely not biased by possible large evapotranspiration changes in the region.

A review of climate reconstructions from terrestrial climate archives covering the first millennium AD in northwestern Europe

AbstractLarge changes in landscape, vegetation, and culture in northwestern (NW) Europe during the first millennium AD seem concurrent with climatic shifts. Understanding of this relation requires high-resolution palaeoclimate reconstructions. Therefore, we compiled available climate reconstructions from sites across NW Europe (extent research area: 10°W–20°E, 45°–60°N) through review of literature and the underlying data, to identify supraregional climatic changes in this region. All reconstructions cover the period from AD 1 to 1000 and have a temporal resolution of ≤50 yr. This resulted in 22 climate reconstructions/proxy records based on different palaeoclimate archives: chironomids (1), pollen (6), Sphagnum mosses (1), stalagmites (8), testate amoebae (4), and tree rings (2). Comparing all temperature reconstructions, we conclude that summer temperatures between AD 1 and 250 were relatively high, and the period between AD 250 and 700 was characterised by colder summer conditions. The period from AD 700 to 1000 was again characterised by warmer summers. These temperature shifts occurred in the whole of NW Europe. In contrast, the compilation of precipitation reconstructions does not show a common pattern across NW Europe either as a result of a heterogeneous precipitation pattern or the lack of suitable and consistent precipitation proxies.

Southwestern Gondwana’s Permian climate amelioration recorded in coal-bearing deposits of the Moatize sub-basin (Mozambique)

The postglacial climate history of Gondwana represents the most prominent climate amelioration in the Phanerozoic, ranging from severe icehouse conditions in the Late Carboniferous (Pennsylvanian) to extreme hothouse conditions in the Early Triassic. Here we report new sedimentological and palynological data from a 350m thick coal-bearing succession intersected by borehole 945L_0022, drilled in the eastern Tete Province of Mozambique, which documents southwestern Gondwana’s Permian postglacial climate amelioration. Palynofacies data further support the environmental reconstructions interpreted from the sedimentary succession. Changes in the palynomorph assemblage document a shift from cold, to cool-temperate, to warm-temperate climatic conditions. This climate signature corresponds with observations from other depositional environments elsewhere in southern Africa, and thus enables correlation on a regional to interregional scale. Thick lacustrine deposits are described within the lower coal-bearing succession, reflecting the final glacial retreat, with melt waters supplying fresh water to the incipient lacustrine system. Lakes as characteristic postglacial sedimentary sinks are excellent palaeoclimate archives and the high Total Organic Carbon content of these fine-grained clastic sediments also makes them potential source rock targets for oil and gas.

Patagonian ash on sub‐Antarctic South Georgia: expanding the tephrostratigraphy of southern South America into the Atlantic sector of the Southern Ocean

ABSTRACTFingerprinting non‐visible volcanic ash (cryptotephra) enables precise dating and time synchronization of palaeoclimate archives. Recent analytical advances allow us to strengthen and expand existing tephrostratigraphical frameworks and harness the full potential of this powerful geochronological tool. Here, we present geochemical (electron microprobe) and chronological (14C) evidence to show that ash found in a peat section on the sub‐Antarctic island of South Georgia correlates to the ∼2950 cal a BP Alpehué eruption of the Chilean Sollipulli volcano. This discovery marks the first tephra fingerprint on South Georgia and expands the recently refined tephrostratigraphy of Patagonia ∼3000 km into the Southern Ocean. As deposition of this tephra horizon coincides with a major shift in regional atmospheric conditions, future finds may yield information on the spatiotemporal pattern of Southern Ocean climate dynamics.

Tracing marine cryptotephras in the North Atlantic during the last glacial period: Improving the North Atlantic marine tephrostratigraphic framework

Tephrochronology is increasingly being recognised as a key tool for the correlation of disparate palaeoclimatic archives, underpinning chronological models and facilitating climatically independent comparisons of climate proxies. Tephra frameworks integrating both distal and proximal tephra occurrences are essential to these investigations providing key details on their spatial distributions, geochemical signatures, eruptive sources as well as any available chronological and/or stratigraphic information. Frameworks also help to avoid mis-correlation of horizons and provide important information on volcanic history. Here we present a comprehensive chronostratigraphic framework of 14 tephra horizons from North Atlantic marine sequences spanning 60-25 cal ka BP. Horizons previously discovered as visible or coarse-grained deposits have been combined with 11 newly recognised volcanic deposits, identified through the application of cryptotephra identification and characterisation methods to a wide network of marine sequences. Their isochronous integrity has been assessed using their physical characteristics. All horizons originated from Iceland with the vast majority having a basaltic composition sourced from the Grímsvötn, Kverkfjöll, Hekla/Vatnafjöll and Katla volcanic systems. New occurrences, improved stratigraphic placements and a refinement of the geochemical signature of the NAAZ II are reported and the range of the FMAZ IV has been extended. In addition, several significant geochemical populations that further investigations could show to be isochronous are reported. This tephra framework provides the foundation for the correlation and synchronisation of these marine records to the Greenland ice-cores and European terrestrial records to investigate the phasing, rate, timing and mechanisms controlling rapid climate changes that characterised the last glacial period.

Moving beyond the age–depth model paradigm in deep-sea palaeoclimate archives: dual radiocarbon and stable isotope analysis on single foraminifera

Abstract. Late-glacial palaeoclimate reconstructions from deep-sea sediment archives provide valuable insight into past rapid changes in ocean chemistry. Unfortunately, only a small proportion of the ocean floor with sufficiently high sediment accumulation rate (SAR) is suitable for such reconstructions using the long-standing age–depth model approach. We employ ultra-small radiocarbon (14C) dating on single microscopic foraminifera to demonstrate that the long-standing age–depth model method conceals large age uncertainties caused by post-depositional sediment mixing, meaning that existing studies may underestimate total geochronological error. We find that the age–depth distribution of our 14C-dated single foraminifera is in good agreement with existing bioturbation models only after one takes the possibility of Zoophycos burrowing into account. To overcome the problems associated with the age–depth paradigm, we use the first ever dual 14C and stable isotope (δ18O and δ13C) analysis on single microscopic foraminifera to produce a palaeoclimate time series independent of the age–depth paradigm. This new state of the art essentially decouples single foraminifera from the age–depth paradigm to provide multiple floating, temporal snapshots of ocean chemistry, thus allowing for the successful extraction of temporally accurate palaeoclimate data from low-SAR deep-sea archives. This new method can address large geographical gaps in late-glacial benthic palaeoceanographic reconstructions by opening up vast areas of previously disregarded, low-SAR deep-sea archives to research, which will lead to an improved understanding of the global interaction between oceans and climate.

Climatic signals in stable carbon isotope ratios of Juniper and Oak tree rings from northern Iran

Abstract Stable isotope ratios in tree rings are increasingly used as palaeoclimatic archive and ecophysiological indicator. We used cross-dated tree-ring series from different tree functional types, i.e. the evergreen conifer Juniperus polycarpus and the deciduous broadleaved Quercus macranthera. The samples were collected from Chaharbagh Gorgon forest in northern Iran, where oaks and junipers grow on north-facing and south-facing slopes, respectively. We extracted α-cellulose from the whole wood of annually separated tree rings and evaluated their potential for palaeoclimate reconstructions by examining the relationships between variations of δ13C in tree-ring cellulose and climate parameters. Based on tree-ring δ13C, we calculated intrinsic water use efficiency (iWUE) and evaluated changes in water availability at the study site over the past 50 years. We found significant negative relationships between tree-ring δ13C values in oak and juniper and precipitation in April and spring (only in junipers), while no significant correlations of tree-ring δ13C with temperature occurred. A strong negative relationship between tree-ring δ13C and summer Palmer Drought Severity Index (PDSI) was found at the drier site, indicating that juniper growing on south-facing slopes is a better indicator of drought stress in semiarid environments. A continuous increase in iWUE in both species was observed. δ13C variations in both species reflect the combined influence of climate and local site conditions. The interplay between climatic conditions and species behavior determines the inter-annual δ13C patterns of oak and juniper trees. Due to its more enriched mean δ13C (i.e., less negative average δ13C) and its higher climatic sensitivity, juniper is a more suitable proxy for paleoclimatic reconstructions in northern Iran.

Quaternary fossil coral communities in uplifted strata along the Balochistan coast of Pakistan: understanding modern coral decline in the Arabian Sea

Uplifted reefs due to being important palaeoclimate archives and a rich source of information on past physical and geochemical changes globally have become the centre of marine research. The uplifted fossil Quaternary coral communities of Jiwani and Gwadar are perfect places to study the palaeoclimatic and geological changes that have shaped the Balochistan coast. Studies on the palaeodiversity of corals along the Makran coast of Pakistan are lacking. In the present study, the samples collected using line intercept method from four uplifted sites (Balochistan coast: one at Gwadar and three at Jiwani) were analysed. The relative distribution and diversity of scleractinian fossil corals was determined, and the factors responsible for coral decline along Pakistan coast were compared with modern coral distribution and diversity. A total of 48 fossil coral species were recorded in nine families and 22 genera. High coral diversity was recorded in the uplifted landward sites of Jiwani and Gwadar headland. Terraces close to the shore at Jiwani had lower diversity. The corals seem to be Quaternary: most likely Pleistocene to Holocene. The modern fauna lacks many species recorded in the fossil community, thus suggesting a faunal turnover in diversity and redistribution of coral fauna which may be linked with past geological events and increasing anthropogenic pressure.

Mediterranean Outflow Water variability during the Early Pleistocene

Abstract. Gaining insights into the evolution of Mediterranean Outflow Water (MOW) during the Early Pleistocene has been so far hampered by the lack of available palaeoclimatic archives. Here we present the first benthic foraminifera stable oxygen and carbon isotope records and grain-size data from IODP Expedition 339 Site U1389 presently located within the upper core of the MOW in the Gulf of Cadiz for the time interval between 2.6 and 1.8 Ma. A comparison with an intermediate water mass record from the Mediterranean Sea strongly suggest an active MOW supplying Site U1389 on glacial–interglacial timescales during the Early Pleistocene. We also find indication that the increasing presence of MOW in the Gulf of Cadiz during the investigated time interval aligns with the progressive northward protrusion of Mediterranean sourced intermediate water masses into the North Atlantic, possibly modulating the intensification of the North Atlantic Meridional Overturning Circulation at the same time. Additionally, our results suggest that MOW flow strength was already governed by precession and semi-precession cyclicity during the Early Pleistocene against the background of glacial–interglacial variability.

Last glacial tephra layers in the Talos Dome ice core (peripheral East Antarctic Plateau), with implications for chronostratigraphic correlations and regional volcanic history

Tephra isochrons offer considerable potential for correlating diverse palaeoarchives and highlighting regional climatic differences. They are especially useful when applied to polar ice records encompassing the last glacial, as these clearly portray the pronounced millennial-scale climate variability that characterised this period. Here we present the continuous record of primary fallout tephra layers in the East Antarctic Talos Dome ice core (72°49′S, 159°11′E), developed upon examination of the core sections spanning the glacial period 16.5 to 71 ka. A total of ca. 45 discrete tephra deposits precisely positioned stratigraphically relative to the temperature record for the core and dated using the AICC2012 timescale, were identified. Quantitative grain size, particle morphology, major and trace element composition using Coulter Counter, SEM, EPMA-WDS, and LA-ICP-MS analytical methods were studied as diagnostic features for tephra characterisation. The tephrostratigraphic framework provides a reference for future precise comparison between ice and sediment sequences across the Antarctic continent. Indeed, several potential markers characterised by distinct volcanic glass geochemistry and/or particular stratigraphic location (e.g., a 17.6-ka ash layer deposited during the well-known major acidity event) are now available for the direct linkage of palaeoclimatic archives. The Talos Dome tephra sequence, dominated by mid-distal pyroclastic products from the nearby Northern Victoria Land volcanoes, also represents the most comprehensive and best time-constrained record of regional Antarctic volcanism yet developed. It documents nearly continuous sustained explosive activity during the considered time interval and, combined with previous ice-core tephra results for the last and the current interglacial periods, suggests progressive compositional shift through time.

A 16-ka oxygen-isotope record from Genggahai Lake on the northeastern Qinghai-Tibetan Plateau: Hydroclimatic evolution and changes in atmospheric circulation

Moisture source history and changes in atmospheric circulation are the most important attributes for portraying past climate changes and for estimating possible future trends. However, few climate records reflecting these attributes are available from the marginal zones of the Asian summer monsoon. Here, we present a record of the oxygen isotopic composition of authigenic carbonates (δ18Ocarb) of sequential sediments from Genggahai Lake in the northeastern Qinghai-Tibetan Plateau (QTP). Isotopic analyses were performed on the fine-grained carbonate fraction (<38 μm), mollusc shells, and stem encrustations from submerged plants. The stratigraphic variations of the δ18O record from the different carbonate components exhibit a remarkably similar pattern, probably reflecting the fact that δ18Ocarb variability was controlled primarily by changes in the oxygen isotopic composition of the lake water (δ18OLW). Disequilibrium effects and water temperature are precluded as major factors affecting the δ18Ocarb variations. Genggahai Lake is hydrologically open and characterized by a rapid discharge rate, as indicated by analysis of the hydrological setting of the lake system and by the observed significant positive correlation between δ18OLW and the oxygen isotopic composition of the inflowing water (δ18OI). Under such hydrological conditions, we argue that the isotopic signals of different moisture sources should be reflected in the carbonate isotopic composition. Furthermore, placing the δ18Ocarb record in the context of regional palaeoclimate archives, we found that the isotopic signals, particularly the negative shifts from the average values, cannot be interpreted consistently, despite a process of evaporative enrichment at the lake surface. During the early- to mid-Holocene, low δ18Ocarb values during 10.6–9.4 and 7.4–6.3 ka were associated with higher lake levels, and thus the record may have been significantly affected by a strengthened Asian summer monsoon at those times and hence by a positive moisture balance. Isotopic depletions with similar magnitudes occurred at 15–14.5, 13.8–13.3, 12.5–11.4, 5.3–4.8, 3.7–3.4, 2.8–2.3, 1.7, 1.3, and 0.6 ka, i.e., within the Lateglacial and the late Holocene. These negative shifts in δ18Ocarb corresponded to lower lake-level phases, and most likely were a response to intensification of the mid-latitude westerly circulation that may have transported significantly more 18O-depleted moisture to the region, compared to that from the Asian summer monsoon. Overall, our results suggest that the alternating influence of the Asian summer monsoon and the westerlies played a significant role in determining the pattern of atmospheric circulation on the northeastern QTP.