Cal Kyr BP Research Articles

Vegetation and climate changes since the middle MIS 3 inferred from a Lake Ailike pollen record, xinjiang, arid central asia

Marine Isotope Stage (MIS) 3 is the latest interstadial to be a significant period for paleoclimate research. The mild interstadial climate in MIS 3 can shed light on studies of the current warming period. However, it is uncertain whether or not the humidity in MIS 3 exceeds that in Holocene level, and forcing mechanisms of paleoclimate evolution remains controversial. In this paper, we use pollen and AMS 14C dating data of a sediment core Ailike-2019 (4.49 m) recovered from Lake Ailike, Xinjiang in arid central Asia, to reconstruct the vegetation and climate changes in Lake Ailike over the last 40,000 years. The results show that Lake Ailike region has been dominated by desert vegetation since the middle MIS 3. The vegetation coverage shows that climate was arid in the middle MIS 3 (40.2-33.2 cal kyr BP), ameliorated during the late MIS 3 (33.2-26.7 cal kyr BP) but still dry, extremely dry during MIS 2 (26.7-12 cal kyr BP), and relatively wet during Holocene (12-0 cal kyr BP). The climate in the Holocene was wetter than in the mid-late MIS 3 in Lake Ailike, which is generally consistent with the paleoclimate records from arid central Asia and Europe. This climate pattern might be attributed to amounts of vapor transported by the westerlies and Siberian high pressure. Low/high Northern Atlantic sea-surface temperatures (SSTs) induced by decreased/increased Northern Hemisphere summer insolation resulted in westerlies transporting less/more amount of vapor to the arid central Asia during MIS 3/Holocene. Meanwhile, intensified/weakened Siberian high pressure resulting from the expanding/melting of ice sheet might induce dry/wet climate conditions in the mid-late MIS 3/the Holocene.

The Last Glacial Maximum and Deglacial History of the Seno Skyring Ice Lobe (52°S), Southern Patagonia

There are still many uncertainties about the climatic forcing that drove the glacier fluctuations of the Patagonian Ice Sheet (PIS, 38–55°S) during the last glacial period. A key source of uncertainty is the asynchrony of ice lobe fluctuations between the northern, central, and southern PIS. To fully understand the regional trends requires careful mapping and extensive geochronological studies. This paper presents geomorphological and geochronological reconstructions of the glacial and deglacial landforms formed during the last glacial period at the Seno Skyring lobe, southernmost Patagonia (52°S, 71°W). We present a detailed geomorphological map, where we identify two moraine systems. The outer and older is named Laguna Blanca (LB) and the inner Río Verde (RV). The LB moraines were built subaerially, whereas parts of the RV were deposited subaqueously under the palaeo lake Laguna Blanca, which developed during deglaciation. We conducted surface exposure 10Be dating methods on boulder samples collected from LB and RV glacial margins. The moraine LB III and LB IV formed at 26.3 ± 2.3 ka (n = 5) and 24.3 ± 0.9 ka (n = 3), respectively. For the inner RV moraine, we obtained an age of 18.7 ± 1.5 ka (n = 6). For the palaeo Laguna Blanca evolution, we performed 10Be exposure ages on shoreline berms and optically stimulated luminesce dating to constrain the lake levels, and 10Be depth profile dating on an outwash deposit formed by a partial lake drainage event, which occurred at 22 ± 3 ka. For the RV moraine deglaciation, we performed radiocarbon dating of basal sediments in a peat bog, which indicates that the glacier retreated from the terminal RV moraine by at least c. 16.4 cal kyr BP. Our moraine geochronology shows an asynchrony in the maximum extents and a different pattern of ice advances between neighbouring lobes in southern Patagonia. We speculate that this may be due, at least in part, to the interaction between topography and the precipitation carried by the southern westerly wind belt. However, we found broad synchrony of glacial readvances contemporaneous with the RV moraine.

Tropical environmental change in North Sumatra at the Last Glacial Maximum: Evidence from the stable isotope composition of cave guano

Insular Southeast Asia is of critical importance to global biodiversity, climate, and socioeconomics. Past environmental histories are a crucial factor influencing contemporary patterns and, in this region, fluctuating sea-level and climate change during the Last Glacial Period (LGP) resulted in drastic changes in both land extent and vegetation cover. However, understanding the past environments of this tropical region is hampered by its sheer size and relative lack of traditional continental proxy records, such as lake sediments. To provide further insight into the response of vegetation to the changing environmental conditions, we use the stable carbon isotope composition (δ13C) of a bat guano sequence from Liang Mbelen Cave in North Sumatra, to determine whether tropical grass (C4 vegetation) or forest (C3 vegetation) was present in the area from c. 30 to 10 cal kyr BP. Our results confirm the presence of C4 vegetation during the LGP; however, we find that the timing of grass expansion was just prior to the Last Glacial Maximum (LGM; 23–19 kyr BP). Comparing Liang Mbelen with the other evidence from across the region, we find a generally consistent pattern of peak tropical grasses often occurring prior to the LGM in Sundaland, with the timing of forest encroachment variable, although beginning at c. 25 cal kyr BP. We find that the timing of the shift from savanna to forest systems in Sundaland is similar to changes in obliquity, likely resulting in a decrease or elimination of the dry season that favoured forest expansion. Understanding past vegetation history will help the management and protection of the fragile and ecologically valuable ecosystems of SE Asia.

History of Anvers-Hugo Trough, western Antarctic Peninsula shelf, since the Last Glacial Maximum. Part II: Palaeo-productivity and palaeoceanographic changes during the Last Glacial Transition

Following the Last Glacial Maximum (LGM; ca. 23-19 calibrated [cal.] kyr before present [BP]), atmospheric and oceanic warming, together with global sea-level rise, drove widespread deglaciation of the Antarctic Ice Sheet, increasing the flux of freshwater to the ocean and leading to substantial changes in marine biological productivity. On the Antarctic continental shelf, periods of elevated biological productivity, often preserved in the sediment record as laminated (and sometimes varved) diatomaceous oozes (LDO), have been reported from several locations and are typically associated with the formation of calving bay re-entrants during ice sheet retreat. Understanding what drives the formation and deposition of LDOs, and the impact of deglacial processes on biogenic productivity more generally, can help inform how Antarctic coastal environments will respond to current and future ice sheet melting. In this study we utilise a suite of sediment cores recovered from Anvers-Hugo Trough (AHT), western Antarctic Peninsula shelf, which documents the transition from subglacial to glacimarine conditions following retreat of an expanded ice stream after the LGM. We present quantitative absolute diatom abundance (ADA) and species assemblage data, to investigate changes in biological productivity during the Last Glacial Transition (19-11 cal kyr BP). In combination with radiocarbon dating, we show that seasonally open marine conditions were established on the mid-shelf by 13.6 cal kyr BP, but LDOs did not start to accumulate until ∼11.5 cal kyr BP. The ∼1.4 kyr delay between the onset of seasonally open marine conditions and LDO deposition indicates that physiographic changes, and specifically the establishment of a calving bay in AHT, is insufficient to explain LDO deposition alone. LDO deposition in AHT coincides with the early Holocene climatic optimum (∼11.5 – 9.0 kyr) and is therefore explained in terms of increased atmospheric/ocean temperatures, high rates of sea and glacial ice melt and the formation of a well-stratified water column in the austral spring. An implication of our study is that extensive bathymetric mapping in conjunction with detailed core analyses is required to reliably infer environmental controls on LDO deposition.

Palaeoclimate dynamics within the Summer Rainfall Zone of South Africa

South Africa has a particularly spatially heterogeneous climate driven by a complex set of ocean-atmospheric factors. This results in three distinct regions of rainfall seasonality: the summer- (SRZ), winter- (WRZ) and year-round (YRZ) rainfall zones. Even within these zones there is considerable spatio-temporal variability in precipitation patterns, especially in the SRZ which covers the largest terrestrial area. Over the past several thousand years the position of the boundaries of the southern extent of the SRZ have shifted due to the influences of major climate systems, as well as changes in the characteristics of the adjacent oceanic currents. Here we use the Modern Analogue Technique on a new synthesis of pollen records from across the SRZ to quantitatively reconstruct four climate variables (Mean Temperature of the Coldest month, Mean Temperature of the Warmest Month, Mean Summer Precipitation and Winter Climatic Moisture Index), over the past 36,000 years. The climate variables and number of analogues used were selected after thorough examination of the modern dataset. The results provide evidence for three key features: first, an early deglaciation following a cold, dry and prolonged Last Glacial Maximum. Second, a clear manifestation of the African Humid Period in the north-eastern part of the zone, supported by reconstructed sea surface temperatures. And third, a short-lived cold period at around 10 cal kyr BP, which may have been obscured by larger statistical errors in past studies. As it is still mostly within the margin of error for our study, this period requires further study. Our results show that the technique can be used successfully on large and diverse compilations of South African records, improving comparability with other regions.

Holocene fire history in southwestern China linked to climate change and human activities

Global warming and human activities have significantly increased the wildfire risk in highly populated subtropical East Asia over recent decades; however, the factors driving fire occurrence on multiple timescales remain unclear. We present and analyze an observational dataset of fire for the Yunnan Plateau in southwestern China for the past 20 years, together with a Holocene fire history documented by four sedimentary black carbon records from four lakes on the Yunnan Plateau: lakes Dian, Xingyun, Qilu, and Yilong. Our aims were to explore the linkage between wildfires and natural and anthropogenic variables driving wildfire occurrence on multiple timescales. The results show that on a short timescale, more wildfires occurred in drier seasons, and on a longer timescale, more wildfires occurred during 6–2.8 cal kyr BP, when humidity decreased. In contrast, there was a moderate wildfire incidence during the relatively humid interval from 11.7 to 6 cal kyr BP. These observations indicate that fire activity is controlled by the humidity, against the background of the abundance of biofuel. The lowest degree of biomass burning occurred after ∼2.8 cal kyr BP and was linked to an anthropogenic reduction of biomass availability, despite an increase in drought. This suggests the importance of active fire management and prescribed clearance measures, especially given projections of continued drought and increased vegetation cover.

Reconstruction of the holocene climate and environmental changes of Niayes peat bog in northern coast of Senegal (NW Africa) based on stable isotopes and charcoals analysis

A core from a peat bog in the “Niayes” region, northwest of Senegal, provides a valuable insight into the evolution of the landscape and fire regime related to the West African Monsoon rainfall and to human impact over the Holocene. The high-resolution multiproxy approach based on stable carbon isotopic composition (δ13C), Rock-Eval® thermal analysis, and charcoal analysis, provides an understanding of vegetation composition (C3/C4) and fire regime in the continental watershed. The analysis of our data indicates a more woody landscape and a higher water table level, related to increased monsoonal rainfall during the African Humid period (AHP) between 9.5 and 5.5 cal kyr BP. High insolation and orbital changes account for a northward migration of the African Rain Belt and a reinforced West Africa monsoon. The amount of forest fires was low and most likely of natural occurrence, as human density was very low. A short dry event interrupts the AHP and gives rise to a weakly grassy landscape at around 8.2 cal kyr BP. This drought of centennial scale (400 years) occurring between 8.4 and 8 cal kyr BP is concomitant with the general cooling observed in the Northern Hemisphere. After a period of transition, the millennial-scale Late Holocene dry period begins at 4.8 cal kyr BP. The low water table level, resulting from a decrease in precipitation, coincides with the appearance of a grassy savanna landscape. Insolation decrease and reduced vegetation were likely driven by a southward migration of the African Rain Belt and a weakening of the monsoon. During the late Holocene the “Niayes” zone evolved into a phase of environmental stress often interrupted by Little Humid Periods (LHP) of centennial scale (500 and 600 years) respectively between 4.4 and 3.9 and between 1.1 and 0.5 cal kyr BP. After 0.5 cal kyr BP the decrease in precipitation and the increase in savanna fires, probaly of anthropogenic origin, favor the landscape opening.

The early Holocene ecology of hilly terrain reconstructed by plant remains from Ping'an Cave in northern China

Against the background of climate change during the late Pleistocene and early Holocene, human society gradually transitioned from a hunting/foraging economy to an agricultural economy. The role of climate change in the development of millet-based agriculture, which originated in northern China, is of major scientific interest. Millet agriculture is thought to have originated in areas of hilly terrain, because most of the early millet agricultural sites in northern China occur in such areas. Hence, understanding the ecological background of hilly terrain during the early Holocene is important for understanding the climatic and ecological mechanisms associated with the origin of millet agriculture. In this study, we reconstructed the ecological background of Ping'an Cave, an early Neolithic site in the hilly area of Beijing, based on analyses of pollen, fungal spores, and plant macrofossils recovered from the cave sediments. At ~11,000 cal kyr BP, deciduous broadleaved forest-grassland developed in the hilly areas of northern China, where the climate was warm and relatively dry. Nut shells of Juglans and Celtis were found, associated with an increase in Quercus pollen, indicating the development of a wetter climate at ~8000 cal kyr BP. Open deciduous broadleaved forest–grassland was the main vegetation type in the hilly areas of northern China during the early Holocene. The geographical characteristics of these areas were favorable for prehistoric humans, providing ecological resources from both forest and grassland.

A southern refugium for temperate tree species in the Mediterranean mountains of El Port massif (NE Iberia): Charcoal analysis at Cova Del Vidre

This study focuses on the analysis of wood charcoal from Cova del Vidre (Roquetes, Tarragona, Spain). This cave, located at an altitude of 1000 m a.s.l., is a key site for understanding the transition from hunting and gathering to farming in a mountainous region of north-eastern Iberia so far unexplored by archaeobotany. The anthracological study is based on four archaeological levels from the Epipalaeolithic to the Early Neolithic dated between 12.9 and 6.8 kyr cal BP, that is, from the Late Glacial period to the Middle Holocene. During the Younger Dryas, the record is dominated by a little varied spectrum of cryophilous pines ( Pinus tp. sylvestris) and thorny shrubs revealing an open landscape where Prunus may prefigure the spread of temperate taxa. In the Early Holocene, a significant change in temperature and precipitation is evidenced, which promoted successional changes in plant communities, with a decline of pine and the sudden rise of temperate/Mediterranean taxa, such as Prunus and Quercus coccifera/ilex. During the Middle Holocene, other temperate taxa appear, such as Quercus deciduous, Buxus sempervirens and Taxus baccata, with yew finally dominating the anthracological record. The appearance and early development of temperate or Mediterranean species as early as the Younger Dryas and the Early Holocene periods, in a mountain context that had long been favourable for Scots pine, allows us to argue the local existence of a glacial refugium. Differences with the nearby sites in the lowlands demonstrate that altitudinal differences in vegetation were already established in the Holocene. Furthermore, the site helps to define the palaeodistribution of Taxus baccata, Abies alba and Fagus sylvatica in southern Europe.

Late quaternary hydrological changes in the southeastern amazon basin from n-alkane molecular and isotopic records in sediments of Saci lake, Pará state (Brazil)

The molecular distributions and isotopic ratios (δ13C and δD) of n-alkanes in sediments deposited during the last 35 cal kyr BP in Saci Lake, southeastern Amazonia, have been measured and interpreted for their paleoclimate information. A terrestrial origin for the odd carbon-numbered long-chain (>C27) n-alkanes has been inferred from molecular distributions. A shift in the n-alkane δ13C values across the Pleistocene-Holocene boundary coincides with a climate-induced floral change previously deduced from palynological analyses. Dry conditions during the late Pleistocene and mid-Holocene were confirmed by several n-alkane indices used to estimate the availability of moisture and by larger δD values. High ACL values indicate relatively dry conditions from 29 to 18.2 cal kyr BP and again from 8.0 and 5.0 cal kyr BP with a decrease in precipitation and an increase of grasses. Changes in the n-alkane C-29 isotopic compositions (δD and δ13C) were observed during the Pleistocene-Holocene transition, coinciding with a floristic change induced by climatic conditions that is also observed in palynological results from the same lake. The n-alkane C-29 δ13C values remain between −33.2‰ and − 35.7‰, which is representative for n-alkanes derived from C3 vegetation. These lines of evidence suggest a continuous gallery forest cover in the Saci Lake catchment for the last ~35 cal kyr BP in contrasted with the observed palynological changes which represent the dynamics of the vegetation on the upland areas. The n-alkane C-29 δD values are less negative in the Pleistocene and more negative during the Holocene in the lake record. These characteristics suggest more humid conditions during the Holocene than in the Pleistocene, probably associated with the South American Monsoon System (SAMS). However, an increase of approximately 10‰ occurs during the late Holocene, which coincides with the forest expansion around 5 cal kyr BP that is recorded in palynological data of Saci Lake and other records of the Amazon Forest. This change suggests that the D/H ratio is also influenced by the increase of the plant biomass, which would provide an enrichment of D in the isotopic signal as a function of regional moisture recycling caused by evapotranspiration.

Influences of summer monsoon variations on the terrigenous influx, bioproductivity and early diagenetic changes in the southwest Bay of Bengal during Late Quaternary

The sediment core (MGS11/02), collected from the lower boundary of the Bay of Bengal (BoB) oxygen minimum zone (890 m) is studied to understand the influences of summer monsoon variations on the terrigenous sediment influx and its relation to bioproductivity and early diagenetic changes in the southwest BoB during the Late Quaternary. The high content of terrigenous controlled metals increased, kaolinite/chlorite increased, and K/Rb decreased after 19 cal kyr BP, indicating a simultaneous increase in the terrigenous influx and intensity of chemical weathering during this period. The variations in summer monsoon and land influx observed in the core sediments are also reflected in the sources of organic matter. High values of δ13C observed at 40 to 19 cal kyr BP suggests higher influence of marine organic matter and/or presence of C4 terrestrial plant. The negative shift of δ13C between 19 and 6 cal kyr BP could be the result of the change in terrestrial plant biomass to C3 photosynthesis and increased marine organic matter. Rock magnetic properties and redox-sensitive metals distribution suggests the prevalence of reduced environment during 52 to 19 cal kyr BP and oxic condition during 19 cal kyr BP to present. The low sedimentation during glacial period allowed sufficient time for pyritization which resulted in the low magnetic signals during 52 to 20 cal kyr BP and led to reducing conditions in the sediment interval. The δ15N values in the core sediments are suggesting the absence of denitrification in the water column during last 52 cal kyr BP. Previous studies suggest that the BoB is a geochemical ‘tipping point’ because the slight changes in water column denitrification by anthropogenic or climatic impact can accelerate BoB nitrogen contribution to the global nitrogen budget. This study indicates the absence of water column denitrification signals during last 52 cal kyr BP even though drastic climatic changes happened in the BoB in the Late Quaternary.

A Chironomid Record of Early-Middle Holocene Environmental Evolution in the Darhad Basin, Northern Mongolia.

Simple SummaryThe Holocene humidity evolution presents spatial heterogeneity in the Mongolian Plateau where multiple circulation systems converged. Inconsistent records of regional environmental evolution affect accurate assessment of lake ecosystems. Chironomid larvae are one of the most abundant benthic invertebrates in lakes. The taxa composition of their functional groups is largely determined by aquatic habitats. Therefore, analyzing the changes of their functional traits can effectively indicate the hydrological dynamics in the past. In this study, we analyzed the early-middle Holocene (9.0–4.5 cal kyr BP) subfossil chironomid assemblages of a sedimentary sequence from the Darhad Basin in northern Mongolia. At 9.0 cal kyr BP, the community structure suddenly changed from littoral taxa to sublittoral/profundal taxa, reflecting an environmental transition from a river or shallow lake condition to a deep lake environment, which lasted until 4.5 cal kyr BP. Those hydrological patterns are consistent with the humidity evolution in the Westerlies dominated region, except that the onset of wetness occurred one thousand years earlier when comparing our results with previous ones, which may be related to the melting of regional glaciers and permafrost caused by increased summer solar insolation.Under the influence of various circulation systems, the Holocene humidity conditions on the Mongolian Plateau are spatially heterogeneous and the underlying mechanism is still ambiguous. The complexity of climate change may affect the accuracy of assessing lake ecosystem evolution. In this study, based on the precise chronology, a chironomid assemblage sequence from the Darhad Basin in northern Mongolia is analyzed to elucidate the hydroclimate variation during the early-middle Holocene. The results show that the chironomid communities changed suddenly from littoral taxa to sublittoral/profundal taxa at about 9 cal kyr BP, reflecting an environmental transition from a river or shallow lake condition to a deep lake environment. Thereafter, most parts of the paleolake remained at a relatively high level until 4.5 cal kyr BP. This hydrological pattern resembles the typical humidity variations in the Westerlies affected regions, except that the onset of wetter conditions occurred one thousand years earlier as reflected in our results. The melting of glaciers and permafrost in the basin resulting from the early increased summer solar insolation could be a feasible explanation for these time advances.

The late Holocene hydroclimate variability in the Northwest Himalaya: Sedimentary clues from the Wular Lake, Kashmir Valley

• Wular Lake levels, redox conditions , and organic matter supply significantly altered over the last ∼4.2 kyr BP. • Hydroclimate varied from early wetter to relatively drier conditions and severe drought before the wet Little Ice Age. • Latitudinal migration of inter-tropical convergence zone drove seasonal precipitation turnover in Kashmir Valley. • A spatially coherent hydroclimate is found in the Northwest Himalaya during the late Holocene . This study focuses on the immensely debated pre-instrumental centennial–millennial scaled forcing of the late Holocene hydroclimate variability in the Northwest Himalaya (NWH). The Wular Lake is ideally situated close to the modern intertropical convergence zone (ITCZ) in the NWH and fed by the Jhelum River draining the vast Kashmir Valley, which receives seasonal precipitations from the Indian Summer Monsoon (ISM) and Western Disturbances (WD). In this study, size segregated lithic fractions (sand, silt, and clay) and elemental concentrations (TOC, TN, Ba, Al, Mn, and Fe) have been measured in the 14 C-AMS dated NWC sediment core collected from this lake. The continuous NWC sedimentary records reveal temporal environmental changes (water level, organic detritus supply, and redox conditions) in this open mega-lake system suggesting prevailed hydroclimate variations in the valley during the late Holocene. The captured hydroclimate variations are a wetter phase (∼4.2–3.4 cal kyr BP) followed by relatively drier conditions and extreme drought before the wet Little Ice Age (LIA). The plausible forcing seems to be the latitudinal ITCZ migration regulated seasonal precipitations from the tropical/subtropical moisture sources. The ISM to WD moisture turnover dominated winter over summer precipitations in the valley during the former warm and wet phase but unequivocally reflected during the cold and wet LIA. Additional hydroclimate control comes from permafrost ice melting episodes of a few centuries duration during high solar activity events (sunspot number ≥50). A comparison with the regional studies further indicates that NWC records capture hydroclimate signatures widespread in the NWH.

Holocene hydroclimate changes revealed by multiple proxies from an alpine lake in the central Tianshan Mountains, Northwest China

Alpine wetland ecosystems are of importance in water conservation and animal husbandry in the arid central Asian. However, there is limited research on their formation and relationship with climate. In this study, we evaluated the long-term interactions of vegetation, hydrology, and climate based on multiple proxies (lithology, grain size, loss-on-ignition (LOI) at 550 °C, and pollen) obtained from a sediment core (BY10A) from Lake Swan, an alpine lake in the central Tianshan Mountains of northwest China (2541 m a.s.l.). Our records show that Holocene sediment facies change considerably within core BY10A, with the succession from bottom to top comprising fluvial, peat, and lake sediments. The fluvial deposits were characterized by coarser sediment, which was probably reworked from extensive sand dunes developed over the basin during the early Holocene (12.0–7.5 cal kyr BP). Regional biological productivity was low at this time as indicated by the low LOI value, and the remains of vegetation was degraded under the dry climate. A warm and wet climate prevailed from 7.5 to 2.0 cal kyr BP, resulting in the establishment of peat. The ability of the peat surface to efficiently trap fine particles was enhanced when regional sand dune bodies (coarser part) became largely stabilized by vegetation growth in response to increases in moisture availability. After 2.0 cal kyr BP, a rise in water level, leading to the formation of Lake Swan, indicated a further shift to a wetter climate. During this interval, there was an increasing influence of lacustrine settling and/or wave action on the grain size distribution and the pollen assemblage. This study indicates a stepwise wetting trend from the early to late Holocene. Findings show that changes in moisture, regulated by the prevailing westerlies, exerted a significant influence on the evolution of vegetation communities and lake level in our study area.

Summer temperature drives the lake ecosystem during the Late Weichselian and Holocene in Eastern Europe: A case study from East European Plain

East European lake-river systems have hydrological regimes typical for continental climate zones. The Postglacial development of the basins and regional palaeoclimatic pattern in the Holocene implied a specific succession of biota communities passing through lakes’ subsequent stages in the water level, trophic state and habitat availability. The Great Serteya Palaeolake Basin is the largest palaeolake within the Serteyka lake-river system, which has been functioning since the Late Weichselian (Vistulian). Presented below is a multi-proxy study on the sequence from the paleolake within the present-day valley. During the Late Weichselian, it formed an astatic, and later a permanent, water body of Postglacial origin. The associated melting, flooding and aeolian processes acted as sources of various Late Weichselian sediments. Summer mean air temperature drove the lake ecosystem development. The Holocene Thermal Optimum in 8.5–7.7 kyr cal BP was followed by subsequent lower temperature. There is weak evidence of coole oscillations at 8.2 kyr cal BP, 7.0–6.8 kyr cal BP and 5.8–5.9 kyr cal BP and the Little Ice Age (480–395 yr cal BP), until modern time (−65 yr cal BP). Five stages of lake ecosystem development were identified based on the biota: 1) an initial stage during the Late Weichselian with glaciolacustrine and later lacustrine accumulation of inorganic deposits, 2) the stage with Cladocera as a leading indicators of mesotrophic water body with a well-developed pelagic zone supplied by early spring floods of the Serteyka River, carrying melting snow and ice floes to the lake basin, 3) the stage dominated by Chironomidae and diatoms typical to high trophic states reflecting widespread open bottom sediment and macrophyte habitats 4) a stage with distinct Neolithic community impact reflected by macrofossils remain after pile-dwelling settlement, fish and aquatic plants consumption 5) a final stage dominated by diatoms typical to shallow, semi-permanent, eutrophic bulrush swamp.

Hydroclimate and vegetation changes in southeastern Amazonia over the past ∼25,000 years

Stable isotope analysis of plant waxes (δ13Cwax and δDwax) along with detailed pollen data provide a new perspective on vegetation and precipitation variability in Serra Sul de Carajás, southeastern (SE) Amazonia, over the past ∼25 cal kyr BP. The δDwax record indicates drier conditions during the Last Glacial Maximum (LGM) and a transition to wetter conditions into the Holocene, while the δ13Cwax data reveal that vegetation did not experienced C3/C4 shifts and remained dominated by C3-plant communities. Under glacial conditions C3-savanna was prominent on the Serra Sul plateau with tropical forest areas limited to the lower slopes. Tropical forests expanded upslope and forest patches developed on the plateau as a response to more humid and warmer conditions during the Holocene. Pollen data indicate a shift towards more open landscape of savanna, woodlands, and open forests during the mid-Holocene. The δDwax record exhibits a distinct moisture variability during the Holocene, not always coherent with the vegetation data (pollen), especially during the mid-Holocene interval. Our study confirms the complexity and the overall lack of coherence among Holocene moisture proxy records throughout the monsoon domain in South America and suggest that Holocene local moisture conditions might not follow the regional monsoonal variability. Our data further stress the need for more multi-proxy reconstructions of hydroclimate patterns in SE Amazonia.

Indian monsoon variability during the past ∼8.5 cal kyr as recorded in the sediments of the northeastern Arabian Sea

We analyzed benthic and planktic foraminifera, total organic carbon and stable isotopes ratios of carbon and oxygen from 100 samples of marine sediment Core ABP, 25/03, off Gujarat in the northeastern Arabian Sea. Factor and cluster analyses of thirty highest-ranked benthic foraminifer species enabled to identify seven biofacies in the study sequence characterizing major changes in deep-sea environments during the past ∼8.5 cal kyr. The benthic foraminiferal data has been combined with population abundances of upwelling indicator planktic foraminifer Globigerina bulloides and mixed layer species from Core ABP, 25/03 and compared with that from ODP Hole 723A and Core ABP, 25/02, western and eastern Arabian Sea, respectively. A major change in benthic foraminifera occurs during ∼8.5 to 6 cal kyr BP, when the summer monsoon was stronger whereas during 6 to 2 cal kyr BP, the data suggest a weak summer monsoon. Planktic foraminifer Globigerina bulloides does not show any significant trend in the study core, but deep thermocline species Neogloboquadrina dutertrei and mixed layer species show significant trend with an inverse correlation with the TOC. This core records high TOC from ∼6.5 to 4 cal kyr BP and during the last 2 cal kyr and low TOC during ∼8.5 to 6.5 cal kyr BP and 4 to 2 cal kyr BP. It is interesting to note intervals of high mixed layer species coincide with high Neogloboquadrina dutertrei percentages and low TOC values at Core ABP, 25/03 during 8.5 to 6.5 cal kyr BP.The faunal data is supported by carbon and oxygen isotope ratios (δ13C and δ18O) of shells of benthic foraminifer Cibicides spp. to strengthen our understanding of changes in the eastern Arabian Sea since ∼8.5 cal kyr BP.

Small mammals of the Holocene sequence of Postes Cave (SW Spain): biogeographic and palaeoenvironmental implications for southwestern Iberia

ABSTRACT Postes Cave in SW Spain (Extremadura) displays a Holocene archaeostratigraphic sequence radiocarbon-dated to between c. 8.5–4 cal kyr BP (Mesolithic-Chalcolithic). The sediments collected at two areas within the cave yielded small mammal remains belonging to eleven species of rodents, ‘insectivores’, lagomorphs and chiropterans. This small mammal fossil assemblage is one of the few known to date in the southwest of the Iberian Peninsula. Preliminary taphonomic analysis and previously published data suggest that the remains were accumulated by the predatory activity of Eurasian eagle owls and small to medium-sized carnivores such as red foxes. The palaeoenvironmental and palaeoclimatic reconstructions performed show stable conditions with slight variations during its time span, suggesting an initial scenario of open forested habitats during the Mesolithic and Neolithic which progressively decreased in favour of more open habitats during the Chalcolithic coinciding with human-induced changes of the landscape and with the aridification trend of this period. The presence of Microtus cabrerae and Crocidura suaveolens in the assemblage is noteworthy since they are now extinct in this region. The plausible causes and the processes of these extirpations are discussed.

Lake eutrophication in northeast China induced by the recession of the East Asian summer monsoon

Lakes are one of the most important freshwater resources on Earth and they provide a wide range of ecosystem services. However, due to rapid economic development and the intensification of human activities, many lakes have become eutrophic, which may threaten their status as water resources. Human activities have played a significant role in lake eutrophication, but whether this role is independent of, or coupled with, natural climate change requires further study. We selected Dali Lake, a large lake affected by human activity within the ancient warfare borders, to clarify the ecological response of a lake to climate change and human activity. We used analyses of sedimentary n-alkanes and AMS 14C dating to reconstruct the paleolimnological evolution of Dali Lake since 15 cal kyr BP, and specifically to assess the timing and causes of eutrophication. The results show that the short-chain n-alkanes (C17–19-alkanes) in Dali Lake are mainly produced by bacteria and algae within the lake, and that the sedimentary absolute abundance of short-chain n-alkanes (A17–19-alkanes) can be used as a proxy for assessing the ecological status of the lake. The ecological status of Dali Lake was the most stable during the early to middle Holocene, when the East Asian summer monsoon was strong, but bacterial and algal outbreaks occurred during three episodes of a weakened summer monsoon—the Older Dryas, Younger Dryas, and the Common Era—when the lake experienced different degrees of eutrophication. During the recession of the East Asian summer monsoon, the weakening of precipitation recharge of the lake led to a reduction in lake area and an increase in nutrient concentrations in the lake water, while aeolian dust input was an additional nutrient source, leading to bacterial and algal outbreaks. During the Common Era, lake eutrophication occurred in the context of both summer monsoon recession and enhanced human activities, but their combined effects did not lead to more intense lake eutrophication than was caused by monsoon recession during the Younger Dryas. We conclude that, although human activities have enhanced the eutrophication of Dali Lake, the reduction in lake size due to monsoon recession and the resulting increase in the salinity and nutrient concentration of the lake water, combined with increased aeolian inputs, were a more important trigger of lake eutrophication.

Summer sea-ice variability on the Antarctic margin during the last glacial period reconstructed from snow petrel (<i>Pagodroma nivea</i>) stomach-oil deposits

Abstract. Antarctic sea ice is a critical component of the climate system affecting a range of physical and biogeochemical feedbacks and supporting unique ecosystems. During the last glacial stage, Antarctic sea ice was more extensive than today, but uncertainties in geological (marine sediments), glaciological (ice core), and climate model reconstructions of past sea-ice extent continue to limit our understanding of its role in the Earth system. Here, we present a novel archive of past sea-ice environments from regurgitated stomach oils of snow petrels (Pagodroma nivea) preserved at nesting sites in Dronning Maud Land, Antarctica. We show that by combining information from fatty acid distributions and their stable carbon isotope ratios with measurements of bulk carbon and nitrogen stable isotopes and trace metal data, it is possible to reconstruct changing snow petrel diet within Marine Isotope Stage 2 (ca. 24.3–30.3 cal kyr BP). We show that, as today, a mixed diet of krill and fish characterizes much of the record. However, between 27.4 and 28.7 cal kyr BP signals of krill almost disappear. By linking dietary signals in the stomach-oil deposits to modern feeding habits and foraging ranges, we infer the use by snow petrels of open-water habitats (“polynyas”) in the sea ice during our interval of study. The periods when consumption of krill was reduced are interpreted to correspond to the opening of polynyas over the continental shelf, which became the preferred foraging habitat. Our results show that extensive, thick, and multiyear sea ice was not always present close to the continent during the last glacial stage and highlight the potential of stomach-oil deposits as a palaeoenvironmental archive of Southern Ocean conditions.