Lacustrine Record Research Articles

Lacustrine leaf wax hydrogen isotopes indicate strong regional climate feedbacks in Beringia since the last ice age

The Late-Quaternary climate of Beringia remains unresolved despite the region's role in modulating glacial-interglacial climate and as the likely conduit for human dispersal into the Americas. Here, we investigate Beringian temperature change using an ∼32,000-year lacustrine record of leaf wax hydrogen isotope ratios (δ2Hwax) from Arctic Alaska. Based on Monte Carlo iterations accounting for multiple sources of uncertainty, the reconstructed summertime temperatures were ∼3 °C colder (range: −8 to +3 °C) during the Last Glacial Maximum (LGM; 21-25 ka) than the pre-industrial era (PI; 2–0.1 ka). This ice-age summer cooling is substantially smaller than in other parts of the Arctic, reflecting altered atmospheric circulation and increased continentality which weakened glacial cooling in the region. Deglacial warming was punctuated by abrupt events that are largely synchronous with events seen in Greenland ice cores that originate in the North Atlantic but which are also controlled locally, such as by the opening of the Bering Strait between 13.4 and 11 ka. Our reconstruction, together with climate modeling experiments, indicates that Beringia responds more strongly to North Atlantic freshwater forcing under modern-day, open-Bering Strait conditions than under glacial conditions. Furthermore, a 2 °C increase (Monte Carlo range: −1 to +5 °C) over the anthropogenic era reverses a 6 °C decline (Monte Carlo range: −10 to 0 °C) through the Holocene, indicating that recent warming in Arctic Alaska has not surpassed peak Holocene summer warmth.

Recurrent outburst floods and explosive volcanism during the Younger Dryas–Early Holocene deglaciation in south Iceland: evidence from a lacustrine record

ABSTRACTLake sediment records give valuable insight into the dynamic events that characterized the last deglaciation in Iceland. Here, we focus on the well‐dated sediment record from Hestvatn, a low‐elevation lake in south Iceland, that features six graded bedding events deposited by outburst floods from glacial lakes dammed by the decaying Iceland Ice Sheet (IIS) in the time period of the Vedde Ash and the G10ka Series tephra. Using climate proxies preserved in the sediment cores, in conjunction with regional glacial geomorphology, we reconstruct the retreat of the IIS in south Iceland, from a marine‐based glacier during the Younger Dryas to a land‐based glacier during the Preboreal. As the ice sheet margin withdrew to the central highlands, ice‐dammed lakes formed along glacier margins. The ice‐dams were occasionally breached, generating large‐scale jökulhlaups (catastrophic outburst floods) that deposited thick turbidite sequences preserved in the sediment record of Hestvatn. The high concentration of volcanic material incorporated within deglacial sediments indicates that along with IIS retreat, subglacial volcanic activity may have helped initiate some of the jökulhlaups. Onset of more stable Holocene conditions was reached after the final turbidite at ~10 ka bp, when the IIS had withdrawn from most of the highlands of Iceland.

Using multiple chronometers to establish a long, directly-dated lacustrine record: Constraining >600,000 years of environmental change at Chew Bahir, Ethiopia

Despite eastern Africa being a key location in the emergence of Homo sapiens and their subsequent dispersal out of Africa, there is a paucity of long, well-dated climate records in the region to contextualize this history. To address this issue, we dated a ∼293 m long composite sediment core from Chew Bahir, south Ethiopia, using three independent chronometers (radiocarbon, 40Ar/39Ar, and optically stimulated luminescence) combined with geochemical correlation to a known-age tephra. The site is located in a climatically sensitive region, and is close to Omo Kibish, the earliest documented Homo sapiens fossil site in eastern Africa, and to the proposed dispersal routes for H. sapiens out of Africa. The 30 ages generated by the various techniques are internally consistent, stratigraphically coherent, and span the full range of the core depth. A Bayesian age-depth model developed using these ages results in a chronology that forms one of the longest independently dated, high-resolution lacustrine sediment records from eastern Africa. The chronology illustrates that any record of environmental change preserved in the composite sediment core from Chew Bahir would span the entire timescale of modern human evolution and dispersal, encompassing the time period of the transition from Acheulean to Middle Stone Age (MSA), and subsequently to Later Stone Age (LSA) technology, making the core well-placed to address questions regarding environmental change and hominin evolutionary adaptation. The benefits to such studies of direct dating and the use of multiple independent chronometers are discussed.

Environmental change recorded by radionuclides and organic geochemical signatures in a sediment core from Lake Daihai, North China

Grasslands are one of the most important terrestrial ecosystems, acting as carbon sinks and thus mitigating greenhouse gas emissions. However, large uncertainties remain regarding the effects of long-term anthropogenic activities on the carbon cycling in grassland ecosystems, due to a dearth of high-resolution records. Here we present an organic geochemical lacustrine record from Lake Daihai, in the arable-pastoral ecotone of North China, in order to investigate the response of biogeochemical processes to anthropogenic activities. The chronology, based on radionuclides (210Pb and 137Cs), shows that the record spanning the last ~80 years, with a temporal resolution of 0.3–7.6 years per centimeter. The total organic carbon (TOC) and total nitrogen (TN) molar ratio and the stable carbon isotopic composition of the total organic carbon (δ13CTOC) range from 13.9 to 19.1 and from −27.0 to −23.9‰, respectively, suggesting that the sedimentary organic matter is mainly derived from the terrestrial vegetation-soil system. The significant negative correlation between TOC concentrations and their δ13CTOC is consistent with those in soil profiles, suggesting the δ13CTOC enrichment is mainly due to microbial decomposition of the organic carbon in the vegetation-soil system. From the 1980s to the 2000s CE, rapid agricultural expansion and over-grazing in the watershed reduced the organic carbon input from vegetation to soils, and enhanced the soil organic carbon decomposition, hence affecting the restoration of the carbon sink function of the surrounding grasslands. Furthermore, organic carbon burial rates within Lake Daihai sediments increase significantly with anthropogenic activities, from 4.2 to 39.3 g m−2 yr−1 with a sediment focusing correction. The geochemical record as a whole clearly shows that anthropogenic activities have dominated biogeochemical processes of carbon in the arable-pastoral ecotone of North China over the last ~80 years.

Sedimentology of a Late Quaternary lacustrine record from the south‐eastern Carpathian Basin

ABSTRACTThe Upper Pleistocene geoarchives in the south‐eastern Carpathian Basin are represented predominantly by loess–palaeosol records. In 2015, a 10 m sediment core composed of clay‐rich lacustrine sediments was recovered by vibracoring a dry lake basin located between the Vršac Mountains (Serbia) and the Banat Sands in the south‐eastern Carpathian Basin; a location relevant for placing regional archaeological results in a palaeoenvironmental context. Here, we present results from geoelectrical prospection and a lithostratigraphic interpretation of this sequence supported by a detailed granulometric study supplemented by ostracod analysis. An age model based on luminescence dating is discussed against sedimentological proxy data and its implication for palaeoenvironmental change. The cores show a stratigraphy of lighter ochre‐coloured and darker greyish sediment, related to the deposition of clay and silt trapped in an aquatic environment. Geophysical measurements show ~20 m thick lacustrine sediments. The grain‐size distributions including the variability in fine clay are indicative of a lacustrine environment. Fine particles were brought into the depositional environments by aquatic input and settled from suspension; also, direct dust input is constrained by grain‐size results. Riverine input and aeolian dust input interplayed at the locality.

Climatic and environmental change in the western Tibetan Plateau during the Holocene, recorded by lake sediments from Aweng Co

Understanding the strength and extent of the Asian summer monsoon (including the East Asian summer monsoon and the Indian summer monsoon) in the Tibetan Plateau (TP) region is crucial for predicting possible changes in the regional eco-environment and water resources under global warming. Due to the lack of well-dated and high-resolution paleoclimate records, long-term monsoon dynamics are still not well understood in the western TP, which is currently influenced by both the Indian summer monsoon (ISM) and the westerlies. Here we present a multi-proxy lacustrine record covering the past 10,500 years from Aweng Co, an alpine lake at the northern limit of the modern ASM in western Tibet. Our results show that the western TP was mainly controlled by the ISM during the Holocene and the regional ecosystem/environment was sensitive to climate change. The climate was the wettest between 10.5 and 7.3 cal kyr BP, when terrestrial plants in the catchment were productive and the biomass of benthic algae was low possibly due to limited sunlight at the lake bottom due to high lake level. From 7.3 to 5.0 cal kyr BP the climate shifted towards drier conditions, resulting in a decline in terrestrial plant cover. Between 5.0 and 3.1 cal kyr BP, the climate became even drier, resulting in a further decline in vegetation cover in the catchment. Between 4.6 and 3.1 cal kyr BP, 100% endogenic dolomite precipitated from the lake water, possibly induced by high Mg/Ca ratios. After 3.1 cal kyr BP, the climate was the driest and frequent centennial-scale droughts occurred. The lake level was low and would have resulted in more light reaching the lake bottom, favoring the growth of benthic algae. The reconstructed lake level change of Aweng Co agrees well with the paleo-shoreline records in the southern TP, demonstrating that the ISM evolution played a key role in lake hydrological processes in this region. A comparison of paleoclimate records shows the ISM reached 34.5° N in the western TP during the Holocene.

Upper Cretaceous paleoenvironmental changes and petrophysical responses in lacustrine record (Songliao Basin, NE China) and marine sedimentary deposit (Goban Spur Basin, NW Europe)

AbstractThe Cretaceous interval is marked by several important geological changes whose prints are buried in both continental and marine sytems. Although significant paleoenvironmental details of this period have been inferred from biological and geochemical indicators, little is known about the physical proxies. Through scientific borehole data, petrophysical properties of Upper Cretaceous Songliao Basin (SB) in NE China and Goban Spur Basin (GSB) in NW Europe were intercorrelated to investigate the critical geological paleoenvironmental shifts and their petrophysical responses, through statistical, wavelet and spectral approaches. The results demonstrated that petrophysical features, particularly gamma-ray and resistivity reactivities, were responsive to past environmental changes in both terrestrial and marine systems. Shifts in organic-rich shale deposition and brine bearing shale showed a correlation to a probable period of seawater incursion in SB, while the gamma log, resistivity and density reactivities were interrelated to the basin paleo-structuration. At GSB, the gamma-ray and resistivity reactivities are tied-up to the Mid-Atlantic seabed motion, marine-water level shifts and paleoceanographic instabilities. In both paleo-basins, a decrease in the gamma-ray reactivity occurred from Turonian to Maastrichtian and is consistent with a regional or global increase in hydrodynamic energy. The oceanic/lacustrine anoxic events related to low sedimentation rate occurred in both basins and are associated with high gamma-ray and resistivity signals (SB); high gamma-ray and low resistivity signals (GSB). These changes correlated with geochemical evidence, suggesting that gamma-ray and resistivity can represent alternative means for marine and continental paleoenvironmental comparison.

Deep lake water balance by dual water isotopes in Yungui Plateau, southwest China

Stable isotopes of water (δ18O and δ2H) serve as a practical tool in the understanding of regional hydrological cycle, in particular, where the regional water cycle is stressed by both climate change and human activities. Lake water in southwest China is believed to play an important role in the local hydrology and possible modification of local climate. Water isotopes in lake systems are influenced by both the input water isotope and surface evaporation, such that they can be used as a natural tracer to evaluate these effects. Here we present a detailed water isotope balance study in a plateau lake – Fuxian Lake, the deepest and largest water body in Yungui Plateau, southwest China, with analogues of a steady state of water isotope equilibrium. The lake water isotopes are significantly enriched in heavy isotope due to evapoconcentration during the long residence time. We simulated the dual isotopes in the lake water using the Craig and Gordon model constrained by the observed water isotopes in lake water, river water, and atmospheric vapor. Results show consistent results for both δ18O and d-excess with values of −2.6‰ and −9.0‰ at the present relative humidity and E/I ratio. The test simulations also demonstrate how potential change in climate and/or lake water E/I ratio could impact lake water isotopes, and how the dual water isotope approach can unequivocally define the climatic and hydrological controls on lake water isotopes. These findings may be relevant for anticipating and detecting hydrological response to future climate change and water source management, e. g. inter - basin diversion projects. The results from this study highlight the potential of water isotopes in regional water cycle work, paleo climate reconstruction from lacustrine record, as well as human activity on water management.

A 700-year multiproxy reconstruction on the Argentinian Pampas inferred from the sediments of Laguna Blanca Grande

The Pampean region is a crucial area to obtain sensitive paleoclimatic lacustrine archives due to the presence of shallow environments in a territory non impacted by humans until the last centuries. In this study, we provide a paleoecological reconstruction for the last ca. 700 years based on a multiproxy lacustrine record from Laguna Blanca Grande, in Olavarría (Buenos Aires, Argentina). Our inferences, which were based on sedimentary properties, diatom, cladoceran and ostracod assemblages, offered interesting information about hydroclimatic variability and nutrient increase. Changes in relative abundances on diatoms, specifically on Aulacoseira granulata and Aulacoseira granulata var. angustissima and fragilariods, were used to infer shifts in nutrient conditions. The remainder proxies together indicated small lake level changes. Reconstructed hydroclimatic conditions in Laguna Blanca Grande are consistent with previous paleoecological inferences indicating a humid phase around ca. AD 1450 and progressive drier conditions ca. AD 1530–1900. A flood gate construction and an increase of nutrients in the lake revealed a higher human pressure due to population increase and land-use changes during the last century. Further studies on taxonomy and autecology of microcrustaceans are needed to effectively unlock the information contained in biological proxies from Sudamerican records.

A lacustrine record of the early Aptian climate change by oceanic anoxic event (OAE) 1a in the Jiaolai Basin, Eastern China

The early Aptian climate change resulted in changes in both the ocean and terrestrial ecosystems. This global climate change is well documented in marine sediments in the form of the oceanic anoxic event (OAE) 1a, but it has rarely been reported from the inland lakes in the East Asian continent. In this study, carbon and oxygen isotopes, quantitative palaeosalinity, clay mineral composition and contents, organic matter content, and detrital zircon ages from the Shuinan and Zhifengzhuang Formations in the Jiaolai Basin were determined to examine whether this global climate change was recorded in inland lakes. This study showed that in the early depositional stage of the Shuinan Formation, the paleo-lake in the Jiaolai Basin was a saline lake with long detention and strong sealing. The climate of the Jiaolai Basin became hotter and drier in the early Aptian (~120 Ma) and the evaporation increased, which increased the palaeosalinity of the Shuinan Formation. High-salinity water, high temperatures, and strong evaporation led to a positive migration of the δ13C values. The extreme dry and hot climate documented in the black shale of the Shuinan Formation might serve as a response of inland lakes to OAE 1a. This study provided not only a lacustrine record of an Early Aptian global climate change event but also a new perspective to deepen the understanding of the effects of the Cretaceous climate change in terrestrial ecosystems.

A 450-year lacustrine record of recurrent seismic activities around the Fuyun fault, Altay Mountains, Northwest China

Seismic hazards and risks around an individual fault can be assessed from complementary paleoseismic data. A high-resolution sediment core recovered from the Yileimu Lake revealed 11 potential paleoearthquakes around the Fuyun fault in northern Xinjiang. Among them, three events recorded by ductile deformation and turbidites or coseismic subsidence were possibly correlated with Ms ≥ 7.0 earthquakes, and the others characterized by turbidites or coarse deposits were likely related to Ms ≥ 5.8 or Ms ≥ 5.5 earthquakes. Shaking intensities of 8.60 and 5.51 MMI were required to cause extensive landslides and turbidites, respectively; while 6.89, 6.35 and 5.34 MMI were likely to be required to cause coseismic subsidence, ductile deformation and coarse deposits related to landscape shattering in the catchment area, respectively. Absolute radiocarbon dating and historical earthquake-event markers disclosed a recurrence interval of 20–60 years for Ms ≥ 5.5 earthquakes, and a potential interval of 100 years for Ms ≥ 7.0 earthquakes during the last 450 years. A seismic quiescence of 89 years since the 1931/08/11 Ms8.0 Fuyun earthquake might have significantly increased the potential of a strong earthquake related to the fault in the near future.

Late Holocene hydroclimate changes in the eastern Sierra Nevada revealed by a 4600-year paleoproduction record from June Lake, CA

Well-resolved lake sediment records are key to answering questions about past hydroclimate variability. These questions are particularly relevant in California (USA), where a recent drought stoked fears of water scarcity and caused significant agricultural and other economic losses in this populous state. To contextualize recent and past cycles of aridity, we utilized a Late Holocene lacustrine record from the eastern Sierra Nevada. This study presents geochemical and sedimentological data from June Lake, California to link changes in organic matter production to environmental variability over the last ∼4600 years. The earliest part of our record is characterized by relatively high productivity and wetter conditions than the modern lake system. This interval is followed by a series of distinct and prolonged droughts from ∼3600 to 1700 cal yr BP, an interval that includes the regionally pervasive Late Holocene Dry Period that is recorded in June Lake as enhanced carbonate precipitation and lower primary production. The interval from ∼1700 to 130 cal yr BP, encompassing both the Medieval Climate Anomaly and the Little Ice Age, is characterized by less-frequent droughts and generally high production in a wetter climate. In contrast, sediments of the past 130 years record an abrupt shift to drier conditions, indicated by marked declines in nearly all production indicators. This divergence is likely influenced by anthropogenic warming and suggests that the modern lake system is anomalous with respect to the longer record of change in the basin.

Lacustrine record from the eastern Tibetan Plateau associated with Asian summer monsoon changes over the past ~ 6 ka and its links with solar and ENSO activity

A multi-parameter, absolute-dated lacustrine record from the eastern Tibetan Plateau provides a continuous history of the Asian summer monsoon (ASM) over the past ~ 6000 years. The record broadly follows the trend in northern hemisphere summer insolation. However, it diverges during a period of increased moisture and thermal conditions during the past ~ 1300 years, broadly corresponding to the ASM “2-kyr shift”. This shift as referred by Cheng et al. (Nature 534:640–646, 10.1038/nature18591, 2016) may be related to Atlantic Meridional Overturning Circulation variations, and potentially to increased forcing by greenhouse gases coincident with reduced ENSO activity. The record is punctuated by several weak monsoon intervals. Cross-correlation of the sub-millennial-scale monsoon record with solar activity and ENSO records shows that most of the monsoon variability potentially results from changes in solar irradiation, and has a strong teleconnection with ENSO activity.

Lacustrine Record of 1954 Flood Event on Begnas and Rupa Lake, Central Nepal

AbstractIn recent decades, there have been discussions and predictions regarding the impact of climate on floods, due to its socioeconomic and environmental consequences. For accurate prediction of future flood events and their impacts, it is crucial to have an improved understanding on past flood events. Lacustrine sediments have been used as a natural archive to study the past flood events. Here, we study the impact of 1954 flood event on the lacustrine environment of Bengas and Rupa Lake in central Nepal based on X‐ray fluorescence spectrometry (XRF) element analysis, magnetic susceptibility (MS), total organic carbon (TOC) and the biomarker molecular compositions. Results showed that 1954 flood event had significant impacts on the two lakes in terms of detrital input, organic matter deposition and aquatic production. Before the flood event, both two studied lakes had relatively lower catchment erosion rate, lower organic matter deposition and aquatic production. During the flood event, catchment erosion and aquatic production increased in both lakes due to mass transport deposits and the increased nutrition loading attributed to flood event. Following the flood event, Begnas Lake showed the sharp increase in organic matter deposition, whereas in the Rupa Lake organic matter deposition showed minor changes. The difference in organic matter deposition in lakes during flooding event is likely due to detrital material brought and deposited by the flood activity. Overall our results suggest that lacustrine sediments are sensitive to the extreme event and would be an ideal archive for the reconstruction of flood events.

Quantitative precipitation reconstructions from Chagan Nur revealed lag response of East Asian summer monsoon precipitation to summer insolation during the Holocene in arid northern China

East Asian summer monsoon (EASM) variability is primarily responsible for climate change in East Asia, influencing over a billion lives. The pattern and mechanisms of EASM change during the Holocene are intensely debated. In this study, we produce a high-quality age-depth model for a 11.7 m lacustrine record (CG18B core) from central Chagan Nur (a closed-basin lake located on the southern Mongolia plateau at the fringe of the modern EASM region) using 27 radiocarbon dates on organic materials and 10 quartz optically stimulated luminescence (OSL) ages on the lake sediments. Multiple-proxy analyses of pollen, grain size, total organic content, and CaCO3 content are used to reconstruct Holocene monsoonal precipitation change in the region. Reconstructed Chagan Nur level changes during the Holocene are based on 18 quartz OSL ages from well-preserved paleolake shorelines at different elevations on the northwestern basin margin. A combination of the lacustrine pollen records and a paleohydrology lake level change model is used to define quantitative precipitation change at the fringe of the EASM dominated region during the Holocene. Our results show that a lake highstand of 15-12 m higher than present occurred at 11–5 ka, followed by a decrease in lake levels to 8-7 m higher than present at 4–3 ka, and a general lake lowering during the late Holocene. EASM dominated annual precipitation varied between 420 mm and 280 mm during the Holocene period in the basin, indicating a generally semi-arid environment on the southern Mongolian Plateau, and corresponding to prevailing open steppe vegetation in the basin throughout the Holocene. EASM precipitation intensity increased from the early Holocene to the beginning of the mid-Holocene, with a maximum precipitation 30–50% higher than present at 8–6 ka, followed by a decrease in precipitation during the late Holocene. EASM precipitation shown in these records lags northern Hemisphere solar insolation shifts by ∼3 ka, indicating that variation in EASM intensity did not respond directly to northern Hemisphere summer insolation, but rather was likely modulated by high latitude forcing of ice volume and greenhouse gases.

Organic matter geochemical signatures of sediments of Lake Ngoring (Qinghai-Tibetan Plateau): A record of environmental and climatic changes in the source area of the Yellow River for the last 1500 years

The climate history of the source area of the Yellow River (SAYR) is important to understanding the pacing and processes of climate evolution on the northeastern Qinghai-Tibetan Plateau (QTP). To date, results of research in this alpine region are limited. Herein, we provide a first lacustrine record from the largest freshwater lake on the QTP that archives climatic and environmental variations in the SAYR for the last 1500 yr. The bulk organic carbon and nitrogen isotopic compositions (δ13Corg and δ15Ntot) are combined with atomic carbon/nitrogen (C/N) ratios to indicate that the sedimentary organic matter in Lake Ngoring originates from autochthonous algae and that the terrestrial input is minor. Comparison of the record of organic carbon isotope values with published records of ice accumulation rate (IAR) of Dunde and Guliya glaciers from the northern QTP, the total solar irradiance (TSI), δ18O variations in the Greenland ice core and biologically sensitive indicators (C37:4 alkenone, crenarchaeol and bGDGT concentrations) from Lake Qinghai indicate that variations in the δ13Corg values in Lake Ngoring sediments reflect lake level fluctuations that are associated with variations in the amount of precipitation and glacier meltwater on the SAYR. High lake level phases indicated by positive shifts of δ13Corg values correspond to higher values of the North Atlantic Oscillation Index (NAOI) and lower values of the Southern Oscillation Index (SOIpr). This relation suggests that the hydrologic history in the SAYR is linked to changes in the Indian monsoon and the Westerlies and that both air masses delivered more moisture to the SAYR during high TSI stages. In contrast, variations in the δ15Ntot values resemble the multiple paleoclimate proxy records obtained from ice cores, peat sequences, lake sediments, and integrated paleo-records from other QTP sites, suggesting that the δ15Ntot values archived in Lake Ngoring sediments provide a history of past lake-water temperature variations that is consistent with the variation history of the temperature-dependent parameters from regional paleo-climatic studies. Global climatic events including the Dark Ages Cold Period, the Medieval Climate Anomaly, the Little Ice Age and Modern Warming in 20th century could be clearly observed in the δ15Ntot variations. In general, the δ13Corg and δ15Ntot values in Lake Ngoring sediments appear to be useful indicators for paleo-environment and paleo-climate reconstructions in alpine oligotrophic freshwater lakes such as this one.

Lacustrine record of high magnitude flood events and climate variability during mid to late Holocene in the semiarid alluvial plains, western India

Long-term data on flooding events are necessary for understanding the flood frequency and magnitude variations, but the available instrumental records span only a few decades. Recently, lake sediment sequences have emerged as a relatively untapped archive for high-magnitude floods over longer timescales. In the present study, multi-proxy data from a lacustrine sequence at Timbi in the Dhadhar river basin were analysed to determine high magnitude floods in this semi-arid region of western India in response to southwest monsoon variability. The results indicate the occurrence of discreet high magnitude flood events and depositional phases corresponding to different climatic conditions during the mid to late Holocene. Three large flash floods between 4830 and 2730 cal yr B.P. (Phase-I) have been identified as coeval with flood events from fluvial archives and correlated to the widespread period of aridity and weak monsoon recorded in the lakes of north western India. A shift in the depositional environment from fluvial to lacustrine occurred around 2730 cal yr B.P. as inferred from an abrupt change in the sediment character and palynofacies caused by high intensity flash floods and high sediment influx. Phase II (2730–1730 cal yr B.P.) was marked by periodic erosion and deposition in the catchment and sediment influx during pulses of higher precipitation as evident in the dominance of Glomus sp. and Alternaria sp., and relatively low values in concentration-related magnetic parameters and S-Ratio. A high magnitude flood event occurred at 1770 cal yr B.P. towards the end of phase II. During phase III (1730–880 cal yr B.P.) the monsoon was enhanced and climate was warmer as suggested by low χfd%, SIRM/χ, S-Ratio, higher Ba/Sr ratio and presence of aquatic palynomorphs. A high percentage of structured Organic Matter (OM), low fungal remains and increased Ba/Sr ratios between 880 and 360 cal yr B.P. indicate higher lake waters and better monsoon condition during phase IV. The phase V (360–0 cal yr B.P.) corresponds to lake level lowering, soil formation and eutrophication. The Timbi lake sediments demonstrate the potential for lakes in semi-arid alluvial plains of western India to preserve proxy flood records providing additional archives for generating long-term datasets. The flood record generated substantiates palaeoflood data from the fluvial archive suggestive of regional-scale southwest monsoon variability. Low frequency-high magnitude floods occurred during arid periods and low magnitude-high frequency events during relatively strengthened monsoon and thus have implication for linking the palaeoflood events to the southwest monsoon intensity in the semi-arid continental regions.

BIOEROSION ON SPINICAUDATA SHELLS FROM A TRIASSIC FRESHWATER PALEOLAKE, MENDOZA, ARGENTINA

ABSTRACTBioerosion by microorganisms is termed microbioerosion. In aquatic systems, microbioerosion affects various substrates, most commonly relatively soluble ones such as sulfates, phosphates, and carbonates. However, the record of microbioerosion in fossil calcareous organisms in lake environments is limited. In northern Mendoza Province, at the locality Paramillos de Uspallata, Triassic sedimentary rocks outcrop, including the Agua de la Zorra Formation, which was deposited in a deltaic and lacustrine system. This study assesses possible bioerosion by microorganisms on spinicaudata shells from the lacustrine sequence and discusses their preservation potential in lacustrine environments. Spinicaudatans from the Agua de la Zorra Formation were analyzed under a binocular microscope and via scanning electron microscopy. Traces, the product of microbioerosion, preserved as molds, were assigned as Type 1 and 2 and were then compared with those generated by cyanobacteria, bacteria, chlorophyte, and fungi that inhabit fresh and brackish water environments. Microbioerosion occurs post-mortem and enhances dissolution of the spinicaudata shells. This, together with bias in extraction methodologies and sampling, is considered the reason for the rarity of spinicaudatans with traces of bioerosion in the lacustrine record.

Scientific drilling of Lake Chalco, Basin of Mexico (MexiDrill)

Abstract. The primary scientific objective of MexiDrill, the Basin of Mexico Drilling Program, is development of a continuous, high-resolution ∼400 kyr lacustrine record of tropical North American environmental change. The field location, in the densely populated, water-stressed Mexico City region gives this record particular societal relevance. A detailed paleoclimate reconstruction from central Mexico will enhance our understanding of long-term natural climate variability in the North American tropics and its relationship with changes at higher latitudes. The site lies at the northern margin of the Intertropical Convergence Zone (ITCZ), where modern precipitation amounts are influenced by sea surface temperatures in the Pacific and Atlantic basins. During the Last Glacial Maximum (LGM), more winter precipitation at the site is hypothesized to have been a consequence of a southward displacement of the mid-latitude westerlies. It thus represents a key spatial node for understanding large-scale hydrological variability of tropical and subtropical North America and is at an altitude (2240 m a.s.l.), typical of much of western North America. In addition, its sediments contain a rich record of pre-Holocene volcanic history; knowledge of the magnitude and frequency relationships of the area's explosive volcanic eruptions will improve capacity for risk assessment of future activity. Explosive eruption deposits will also be used to provide the backbone of a robust chronology necessary for full exploitation of the paleoclimate record. Here we report initial results from, and outreach activities of, the 2016 coring campaign.

Holocene evolution of the Indian Summer Monsoon inferred from a lacustrine record of Lake Wuxu, south‐east Tibetan Plateau

ABSTRACTA continuous sediment record since 12.3 cal ka bp from Lake Wuxu (south‐eastern Tibetan Plateau) was investigated in terms of the Holocene evolution of the Indian Summer Monsoon. The molar C/N ratio and stable C isotope were used to identify the source of the organic matter as well as climate conditions. The evolution of Lake Wuxu was summarized wihtin two periods. During the first period (early to mid‐Holocene), the lake received increased fluvially transported materials, reflecting variation in the summer monsoon with solar insolation. The lake level declined and water residence time increased because of reduced river discharge during the second period (late Holocene) corresponding to a weakening of the summer monsoon. The organic material revealed a major contribution from lake primary productivity, which showed identical patterns with a high‐resolution isotope record from Dongge Cave, as well as total solar irradiance. Our record from Lake Wuxu indicates that the Holocene evolution of the Indian Summer Monsoon has been driven by the solar forcing at decadal/centennial to millennial time scales. Furthermore, an abrupt decline in the monsoon was detected at around 4.0 cal ka bp, which is probably caused by an increased frequency of EI Nino‐Southern Oscillation events. Copyright © 2019 John Wiley & Sons, Ltd.