Lakes are sensitive to climate change and their sediments play a pivotal role as environmental recorders. The oxygen and carbon isotope composition (δ
O and δ
C) of carbonates from alkaline lakes is ...featured in numerous studies attempting a quantitative reconstruction of rainfall, temperature and precipitation-evaporation changes. An often-overlooked challenge consists in the mineralogically mixed nature of carbonates themselves. We document a large variability of carbonate components and their respective distinct δ
O and δ
C values from sediments of Lake Van (Turkey) covering the last 150 kyr. The carbonate inventory consists of primary (1) inorganic calcite and aragonite precipitating in the surface-water, (2) biogenic calcite ostracod valves; and post-depositional phases: (3) dolomite forming in the sediment, and previously overlooked, (4) aragonite encrustations formed rapidly around decaying organic matter. We find a systematic relation between the lithology and the dominant deep-water carbonate phase formed recurrently under specific hydrological conditions. The presence of the different carbonates is never mutually exclusive, and the isotopic composition of each phase forms a distinctive cluster characteristic for the depth and timing of their formation. Our findings stretch the envelope of mechanisms forming lacustrine carbonates and highlight the urge to identify and separate carbonate components prior to geochemical analyses.
A high-resolution multi-proxy record from Lake Van, eastern Anatolia, derived from a lacustrine sequence cored at the 357m deep Ahlat Ridge (AR), allows a comprehensive view of paleoclimate and ...environmental history in the continental Near East during the last interglacial (LI). We combined paleovegetation (pollen), stable oxygen isotope (δ18Obulk) and XRF data from the same sedimentary sequence, showing distinct variations during the period from 135 to 110ka ago leading into and out of full interglacial conditions. The last interglacial plateau, as defined by the presence of thermophilous steppe-forest communities, lasted ca. 13.5ka, from ~129.1–115.6ka BP.
The detailed palynological sequence at Lake Van documents a vegetation succession with several climatic phases: (I) the Pistacia zone (ca. 131.2–129.1ka BP) indicates summer dryness and mild winter conditions during the initial warming, (II) the Quercus-Ulmus zone (ca. 129.1–127.2ka BP) occurred during warm and humid climate conditions with enhanced evaporation, (III) the Carpinus zone (ca. 127.2–124.1ka BP) suggest increasingly cooler and wetter conditions, and (IV) the expansion of Pinus at ~124.1ka BP marks the onset of a colder/drier environment that extended into the interval of global ice growth. Pollen data suggest migration of thermophilous trees from refugial areas at the beginning of the last interglacial. Analogous to the current interglacial, the migration documents a time lag between the onset of climatic amelioration and the establishment of an oak steppe-forest, spanning 2.1ka. Hence, the major difference between the last interglacial compared to the current interglacial (Holocene) is the abundance of Pinus as well as the decrease of deciduous broad-leaved trees, indicating higher continentality during the last interglacial. Finally, our results demonstrate intra-interglacial variability in the low mid-latitudes and suggest a close connection with the high-frequency climate variability recorded in Greenland ice cores.
•The new high-resolution pollen data reflects an intra-interglacial instability.•Lake Van proxy data suggest a strong teleconnection with the North Atlantic circulation.•LI is no analogue to the climate condition of the current interglacial.•Correlations have been established between the Lake Van and southern Europe.
Tropical rainfall variability is closely linked to meridional shifts of the Intertropical Convergence Zone (ITCZ) and zonal movements of the Walker circulation. The characteristics and mechanisms of ...tropical rainfall variations on centennial to decadal scales are, however, still unclear. Here, we reconstruct a replicated stalagmite-based 2,700-y-long, continuous record of rainfall for the deeply convective northern central Indo-Pacific (NCIP) region. Our record reveals decreasing rainfall in the NCIP over the past 2,700 y, similar to other records from the northern tropics. Notable centennial- to decadal-scale dry climate episodes occurred in both the NCIP and the southern central Indo-Pacific (SCIP) during the 20th century Current Warm Period (CWP) and the Medieval Warm Period (MWP), resembling enhanced El Niño-like conditions. Further, we developed a 2,000-y-long ITCZ shift index record that supports an overall southward ITCZ shift in the central Indo-Pacific and indicates southward mean ITCZ positions during the earlyMWP and the CWP. As a result, the drying trend since the 20th century in the northern tropics is similar to that observed during the past warm period, suggesting that a possible anthropogenic forcing of rainfall remains indistinguishable from natural variability.
Abstract
Natural disturbances such as volcanic eruptions provide a unique opportunity to study past ecological dynamics. Here we illustrate the response of terrestrial and aquatic ecosystems to ...volcanic eruptions in connection to prevailing climate conditions. We selected five volcaniclastic depositions in the Lake Van (Turkey) sediments from different interglacial/glacial periods (Marine Isotope Stages 3 to 9e). Using high-resolution pollen data, non-pollen palynomorphs, and microscopic charcoal particles we attempted to disentangle climatic and volcanic forcing of natural environmental disturbances. Our results highlights that the thickness of subsequent volcanic deposits and the respective climatic conditions strongly influence the impact on terrestrial and aquatic ecosystems. The most common response to ash deposition is a shift towards herbaceous taxa and abrupt fire activity. The affected herbaceous vegetation recovers to pre-eruption levels in 20 to 40 varve-years. The lake water experiences intensified productivity due to subsequent nutrient input and significant increase in aquatic microfossils. Our findings pave the way for disentangling climatic and volcanic forcing of natural environmental disturbances.
Lake Van sediment cores from the Ahlat Ridge and Northern Basin drill sites of the ICDP project PALEOVAN contain a wealth of information about past environmental processes. The sedimentary sequence ...was dated using climatostratigraphic alignment, varve chronology, tephrostratigraphy, argon–argon single-crystal dating, radiocarbon dating, magnetostratigraphy, and cosmogenic nuclides. Based on the lithostratigraphic framework, the different age constraints are compiled and a robust and precise chronology of the 600,000 year-old Lake Van record is constructed. Proxy records of total organic carbon content and sediment color, together with the calcium/potassium-ratios and arboreal pollen percentages of the 166-m-long event-corrected Ahlat Ridge record, mimic the Greenland isotope stratotype (NGRIP). Therefore, the proxy records are systematically aligned to the onsets of interstadials reflected in the NGRIP and synthesized Greenland ice-core stratigraphy. The chronology is constructed using 49 age control points derived from visual synchronization with the Greenland ice-core stratigraphy using the GICC05 timescale, an absolutely-dated speleothem timescale (e.g., Hulu, Sanbao, Linzhu cave) and the Epica Dome C timescale. In addition, the uppermost part of the sequence is complemented with four ages from Holocene varve chronology and three calibrated radiocarbon ages. Furthermore, nine argon–argon ages and a comparison of the relative paleointensity record of the magnetic field with reference curve PISO-1500 confirm the accuracy of the age model. Also the identification of the Laschamp event via measurements of 10Be in the sediment confirms the presented age model. The chronology of the Ahlat Ridge record is transferred to the 79-m-long event-corrected composite record from the Northern Basin and supplemented by additional radiocarbon dating on organic marco-remains. The basal age of the Northern Basin record is estimated at ∼90 ka. The variations of the time series of total organic carbon content, the Ca/K ratio, and the arboreal pollen percentages illustrate that the presented chronology links ice-marine-terrestrial stratigraphies and that the paleoclimate data are suited for reconstructions and modeling of the Quaternary and Pleistocene climate evolution in the Near East at millennial timescales. Furthermore, the chronology of the last 250 ka can be used to test other dating techniques.
•Long lacustrine drill cores were dated using a suite of age constraints.•A robust chronology of the 600,000 year-old Lake Van record is constructed.•Proxy records are aligned to the NGRIP/GLT syn ice-core stratigraphy.•Ar/Ar ages, 14C, relative paleointensity and 10Be confirm the age model.
A well-dated suite of Lake Van climate-proxy data covering the last 360 ka documents environmental changes over 4 glacial/interglacial cycles in Eastern Anatolia, Turkey. The picture of cold and dry ...glacials and warm and wet interglacials emerging from pollen, organic carbon, authigenic carbonate content, elemental profiling by XRF and lithological analyses is inconsistent with classical interpretation of oxygen isotopic composition of carbonates pointing to a more complex pattern in Lake Van region. Detailed analysis of glacial terminations allows for the constraining of a depositional model explaining different patterns observed in all the proxies. We hypothesize that variations in relative contribution of rainfall, snowmelt and glacier meltwater recharging the basin have a very important role for all sedimentary processes in Lake Van. Lake level of glacial Lake Van, predominantly fed by snowmelt, was low, the water column was oxic, and carbonates precipitating in the epilimnion recorded the light isotopic signature of inflow. During terminations, increasing rainfall and significant supply of mountain glaciers' meltwater contributed to lake level rise. Increased rainfall enhanced density gradients in the water column, and hindered mixing leading to development of bottom-water anoxia. Carbonates precipitating during terminations show large fluctuations in their isotopic composition. Full interglacial conditions in Lake Van are characterized by high or slowly falling lake level. Rainfall and snowmelt feed the lake but due to re-established mixing, the isotopic composition of authigenic carbonates is heavier and closer to that of evaporation-influenced lake water than that of runoff representing snowmelt and atmospheric precipitation.
•Lake Van climate-proxy data document changes in Turkey for the last 360 ka.•Switch from snowmelt-fed to rainfall-fed runoff control the δ18O of carbonates.•Most Lake Van data imply dry glacial/wet interglacial conditions in Eastern Mediterranean.•But oxygen isotopes show a more complex pattern.
Some of the earliest evidence for the presence of modern humans in rainforests has come from the fossil deposits of Lida Ajer in Sumatra. Two human teeth from this cave were estimated to be 73-63 ...thousand years old, which is significantly older than some estimates of modern human migration out of Africa based on genetic data. The deposits were interpreted as being associated with a rainforest environment based largely on the presence of abundant orangutan fossils. As well as the main fossil-bearing chamber, fossil-bearing passages are present below a sinkhole, although the relationship between the different fossil deposits has only been tenuously established. Here, we provide significant new sedimentological, geochronological and palaeoecological data aimed at reconstructing the speleological and environmental history of the cave and the clastic and fossil deposits therein. Our data suggest that the Lida Ajer fossils were deposited during Marine Isotope Stage 4, with fossils from the lower passages older than the main fossil chamber. Our use of stable carbon and oxygen isotope analyses of mammalian tooth enamel demonstrates that early humans probably occupied a closed-canopy forest very similar to those present in the region today, although the fossil orangutans may have occupied a slightly different niche. This article is part of the theme issue 'Tropical forests in the deep human past'.
Fossil long-chain alkenones have been used for several decades to reconstruct past ocean surface water temperatures and gained recent interest as a paleotemperature proxy for continental lake ...settings. However, factors besides temperature can affect alkenone distributions in haptophyte algae, and alkenone compositions can differ between haptophyte species. Alkenone-biosynthesizing haptophyte algae are genetically much more diverse in lakes than in the marine realm, and species-level variations in alkenone compositions could have implications for alkenone paleothermometry. Here, we performed a paired analysis of alkenone distributions and haptophyte species compositions using ancient DNA in up to 270 ka-old sediments of Lake Van in Turkey to reveal a possible species-effect on fossil alkenone distributions and paleotemperature estimates. The same predominant haptophyte in Lake Van today prevailed also since the last ∼100 ka. However, a calibration of alkenone paleotemperature especially in the oldest analyzed intervals is complicated due to a more complex haptophyte species composition predominated by a haptophyte (LVHap_6), which is phylogenetically different from sequences recovered from currently existing lakes including Lake Van and from haptophyte species existing in culture. The predominance of LVHap_6 coincided with the presence of alkenone MeC38:3 and relatively high MeC37:3/4 (2.4) and MeC38:4/5 ratios (3.0). Uk37 index values in the sediment core over the last 270 ka reflect relative changes in past temperature and are additionally linked to haptophyte species composition. A sustained period of high salinity, as indicated by pore-water salinity measurements, could potentially have triggered the succession of haptophytes as sources of alkenones in Lake Van.
•Alkenone analyses in a record from Eastern Anatolia covering the past 270 ka.•New insight into the influence of haptophyte species on alkenone distribution.•Semi-quantitative link between Uk37 index and past temperatures in Lake Van.
Abstract
The Caucasus region is key for understanding early human dispersal and evolution in Eurasia, and characterizing the environmental contrast between Last Glacial Maximum and Holocene is ...crucial for investigating human adaptation strategies to large climatic shifts. However, a paucity of high-resolution paleoclimate records leave this context largely unknown for early human populations in the Caucasus region. Based on our model-proxy comparison of high- and low-resolution records of 24 stalagmites from three caves, we find spatially distinct changes in vegetation and seasonality of precipitation, especially under glacial conditions. Supported by modern oxygen-isotope data and climate modeling, we identify a supraregional cold-season temperature control for oxygen isotopes in Black Sea speleothems, which previously had been interpreted as a local moisture-source signal. Carbon-isotope and trace-element data further suggest disproportionate changes in vegetation cover and soil dynamics at high altitudes, which would have resulted in a reduction but not a disappearance of human refugia during the Last Glacial Maximum, relative to the current interglacial. Our findings imply that abrupt climatic pressures from harsh conditions were overcome by adaptive strategies in the past.
Processes within the Earth shape and influence the surface environment and the emergence and evolution of life. Our Editorial board members outline recent advances and future directions in our ...attempt to understand the history of our planet and its environment.Processes within the Earth shape and influence the surface environment and the emergence and evolution of life. Our Editorial board members outline recent advances and future directions in our attempt to understand the history of our planet and its environment.
PDF
