Paleoclimatic Signals Research Articles

Stable Isotopic Evidence of Paleorecharge in the Northern Gulf Coastal Plain (USA)

Stable isotope abundances (δ18O and δ2H) in regional aquifers can provide important paleoclimate information. However, identifying paleoclimate signals can be complicated by cross-formational mixing and, potentially, by isotopic diffusion between aquifers and confining units. We examine controls on δ18O and δ2H distributions in the Wilcox aquifer of the northern Gulf Coastal Plain (USA). We sampled groundwater for δ18O, δ2H, Cl−, and 36Cl along a ~300 km downgradient transect. We developed a simplified, 1D numerical model of groundwater flow and 18O transport to assess the possible importance of isotopic diffusion between the aquifer and its confining units. Along the inferred flowpath, δ18O and δ2H values were depleted by as much as 1.3 and 8.2‰, respectively, as the Wilcox aquifer transitioned from unconfined to confined. However, they then gradually rose farther downgradient by up to 1.1 and 8.6‰. Chlorine-36 analyses and 14C analyses (from other studies) indicate that groundwater ages range from ~103 yr to ~8 × 105 yr. Modeling results indicate that the effect of diffusion on isotopic abundances is limited, whereas Cl− data indicate that cross-formational flow is limited. Therefore, we posit that confined groundwater in our study reflects a Pleistocene paleorecharge signal.

Hot Tropical Temperatures During the Paleocene‐Eocene Thermal Maximum Revealed by Paired In Situ δ13C and Mg/Ca Measurements on Individual Planktic Foraminifer Shells

AbstractThe Paleocene‐Eocene thermal maximum (PETM, 56 Ma) is an ancient global warming event closely coupled to the release of massive amounts of 13C‐depleted carbon into the ocean‐atmosphere system, making it an informative analogue for future climate change. However, uncertainty still exists regarding tropical sea‐surface temperatures (SSTs) in open ocean settings during the PETM. Here, we present the first paired δ13C:Mg/Ca record derived in situ from relatively well‐preserved subdomains inside individual planktic foraminifer shells taken from a PETM record recovered in the central Pacific Ocean at Ocean Drilling Program Site 865. The δ13C signature of each individual shell was used to confirm calcification during the PETM, thereby reducing the unwanted effects of sediment mixing that secondarily smooth paleoclimate signals constructed with fossil planktic foraminifer shells. This method of “isotopic screening” reveals that shells calcified during the PETM have elevated Mg/Ca ratios reflecting exceptionally warm tropical SSTs (∼33–34°C). The increase in Mg/Ca ratios suggests ∼6°C of warming, which is more congruent with SST estimates derived from organic biomarkers in PETM records at other tropical sites. These extremely warm SSTs exceed the maximum temperature tolerances of modern planktic foraminifers. Important corollaries to the findings of this study are (a) the global signature of PETM warmth was uniformly distributed across different latitudes, (b) our Mg/Ca‐based SST record may not capture peak PETM warming at tropical Site 865 due to the thermally‐induced ecological exclusion of planktic foraminifers, and (c) the record of such transitory ecological exclusion has been obfuscated by post‐depositional sediment mixing at Site 865.

Quantitative Insights on Impurities in Ice Cores at the Micro‐Scale From Calibrated LA‐ICP‐MS Imaging

AbstractUnderstanding the microscopic variability of impurities in glacier ice on a quantitative level has importance for assessing the preservation of paleoclimatic signals and enables the study of macroscopic deformational as well as dielectric ice properties. Two‐dimensional imaging via laser‐ablation‐inductively‐coupled‐plasma‐mass‐spectrometry (LA‐ICP‐MS) can provide key insight into the localization of impurities in the ice. So far, these findings are mostly qualitative and gaining quantitative insights remains challenging. Recent advances in LA‐ICP‐MS high‐resolution imaging now allow ice grains and grain boundaries to be resolved individually. These resolutions require new adequate quantification strategies and, consequently, accurate calibration with matrix‐matched standards. Here, we present three different quantification methods, which provide a high level of homogeneity at the scale of a few tens of microns and are dedicated to imaging applications of ice cores. One of the proposed methods has a second application, offering laboratory experiments to investigate the displacement of impurities by grain growth, with important future potential to study ice‐impurity interactions. Standards were analyzed to enable absolute quantification of impurities in selected ice core samples. Calibrated LA‐ICP‐MS maps indicate similar spatial distributions of impurities in all samples, while impurity levels vary distinctly: Higher concentrations were detected in glacial periods and Greenland, and lower levels in interglacial periods and samples from central Antarctica. These results are consistent with ranges from complementary meltwater analysis. Further comparison with cm‐scale melting techniques calls for a more sophisticated understanding of the ice chemistry across spatial scales, to which calibrated LA‐ICP‐MS maps now contribute quantitatively.

Mediterranean marine sediment core database: Unlocking paleoclimate signals for the last 20,000 years.

Mediterranean marine sediment core database: Unlocking paleoclimate signals for the last 20,000 years.

Holocene climate dynamics and ecological responses in Kaas Plateau, Western Ghats, India: Evidence from lacustrine deposits

The Kaas lake is one of the unique sedimentary basins in the Northern region of the Western Ghats escarpment of India and preserves paleoclimate signals of the Holocene in its cover sequence over the lateritic basement. The signatures preserved in the lacustrine deposits, such as pollen, non-pollen palynomorphs, diatoms, and geochemistry of the sediments, were decoded and used to reconstruct the Holocene environmental changes of the Kaas Plateau in the Western Ghats. The main stages in the lake's evolution indicate the following: (1) low nutrient level and strongly changeable water levels in the early Holocene, (2) persistence of the oligotrophic state of the lake up to the last 2000 yr, and (3) strong cultural eutrophication of the lake in the recent period (c. 1000 years). The localized depression or a seasonal-lake favoured freshwater accumulation since 8.0 kyr BP and was probably dried sometimes after 2.0 kyr BP. A late Holocene peak of planktonic diatom and high organic recovery at the top level in the recent past (c. 1000 years) represents an event of eutrophication, possibly due to human impact. Being a seasonal lake, the plateau surface allowed clay accumulation with an extremely slow rate of 2.5 cm/10 kyr. Diatoms, pollen, and non-pollen palynomorphs together showed periodic fluctuations in lake hydrology, which was probably in response to climate-controlled vegetation on the Kaas Plateau. The diatom and sediment anomalies point to an ecological shift towards the mid–Holocene, supporting the hypothesis of increased Southwest Monsoons during the early Holocene around 8.0 kyr BP and the relative weakening of Northeast Monsoon after 2.5 kyr BP.

Modeling the decomposition signal and correcting bulk organic data from a peat deposit, a case study at low latitudes (Cameroon)

Organic compounds are widely used for paleoclimatic and paleoenvironmental reconstructions. Bulk organic proxies, however, are more complicated to interpret due to the multiple causes of variation in climatic and environmental conditions and the degree of diagenetic alteration. As labile compounds, rich in easily degradable function and generally richer in heteroatoms such as oxygen and nitrogen, decompose, the remaining organic matter becomes progressively richer in refractory carbon and its carbon content increases. Thus, in a peat deposit composed almost entirely of organic matter, total organic carbon (TOC) is expected to increase with time and depth, which could mask the paleoclimatic signal. We propose a simple model for peat sediments to remove the decomposition signal based on a logarithmic function fitted with a partial dataset where decomposition is the main parameter. The subtraction of the obtained logarithmic function to the raw data (i.e., measured data) leads to “residual” data. We discuss the influence of different parameters (water table depth, vegetation, microbial community) on the “residual” data and their possible link to paleoclimatic and paleoenvironmental variations. This method is tested on bulk elemental and isotopic data obtained from a new peat core from the Adamawa Plateau (North-East Cameroon) covering nearly 10 ka cal BP. Comparison with Rock-Eval® parameters highlights similar variations between the Hydrogen Index and residual TOC variations, supporting the interpretation based on residual TOC. Our approach allows to extract paleoenvironmental information from decomposition-prone bulk organic proxies and can be generalized to peat deposits where decomposition plays a major role in controlling bulk data.

Oxygen isotopes of the Japanese stalagmites as global and local paleoclimate proxies

AbstractStalagmite oxygen isotopes (δ18O) have been used to reconstruct terrestrial paleoclimates during the late Pleistocene and Holocene. However, the interpretation of the δ18O is not straightforward when determining the factor controlling δ18O; temperature or water δ18O. In addition, the water δ18O changes with rainfall intensity (amount effect), rainfall seasonality, and some other factors. Here, we first review the hydrochemical processes and behaviors of the oxygen isotopes and the other proxies in a cave system, which are fundamental for interpretating the paleoclimatic signals. We then introduce the oxygen isotope records of Japanese caves. Some of the Japanese stalagmites demonstrated a δ18O profile that represented a similar pattern to the Chinese stalagmite records, but had relatively small δ18O amplitudes, which can be explained mainly by temperature changes rather than the amount effect. This demands a reversal of the relationship between climate and rainwater δ18O across the Japanese Islands. Using δ18O data for rainwater samples from four sites in Japan (in Niigata, Fukuoka, Gifu and Mie Prefectures), we presents the results of model calculations to verify how the rainfall intensity and the seasonality relate with the δ18O of rainwater. A significant correlation coefficient was observed in Niigata, where the rainfall δ18O decreases with an increase in the annual amount of rainfall, and with a decrease in the winter rainfall. Similar trends were observed in Fukuoka, whereas while the results of Gifu and Mie exhibited no significant trends. Temperature change was would be the main factor controlling the stalagmite δ18O at the latter two sites. For a better understanding of the stalagmite δ18O records, the measurement of fluid inclusions and carbonate clumped isotopes can be used to evaluate the effect of temperature on the stalagmite δ18O, as well as to reconstruct the water δ18O. We predict that the 17O excess in stalagmites reconstructs the seasonal shift in the vapor sources.

Irreversible intensification of the East Asian winter monsoon and aridity in Asian interior linked to the uplifted Tibetan Plateau: Evidence from magnetoclimatology of the Xining loess since 1.3 Ma

The impact of the uplifted Tibetan Plateau on the East Asian monsoon (EAM) evolution is not an uncontroversial topic. Disentangling complex paleoclimatic signals derived from solar insolation, global ice volume, and the rise of Tibetan Plateau in the EAM domain is challenging because of the paucity of continuous Quaternary sedimentary records shed from the Tibetan Plateau. The 260-m-thick Xining loess deposits in the northeastern Tibetan Plateau (NETP) with ultra-high average accumulation rate of ∼23 cm/ka during 0.2–1.3 Ma provide an excellent opportunity to delineate the long-term influence of the uplifted Tibetan Plateau on paleoclimatic variations in the western margin of the EAM. High-resolution grain size and mineral magnetic analyses for this unique record reveal a much more significant long-term increasing trend of the East Asian winter monsoon strength and aridification compared with those of typical loess records on the Chinese Loess Plateau, confirming that the uplifted Tibetan Plateau may play an important part in enhancing the EAM circulation and aridity of Asian interior under global cooling. It is further proposed that the negative correlation between physical erosion and chemical weathering in the NETP does not contradict the prevailing uplift-weathering hypothesis, but probably suggests a strong climate-silicate-weathering feedback in this tectonically active region. This result provides robust evidence that the existence of the NETP and adjacent topography exerts a profound impact on the irreversible intensification of Asian interior aridity.

Utility of Dendrochronology Crossdating Methods in the Development of Arctic Coralline Red Algae Clathromorphum compactum Growth Increment Chronology for Sea Ice Cover Reconstruction

Paleoclimate and paleoenvironmental reconstructions from increment-yielding archives strongly depend on precise age models. Like bivalves, corals, trees, and speleothems, the coralline alga Clathromorphum compactum produces annual growth increments and shows considerable promise as an environmental archive for arctic and subarctic regions. Though their growth increment widths correlate with temperature and sea ice cover in high Arctic regions, existing timeseries have not been crossdated. In fact, previous studies have shown a lack of inter-sample correlation in non-crossdated timeseries suggesting possible age model dating errors. Here, we use dendrochronology crossdating techniques and COFECHA software to ensure and validate synchrony between C. compactum timeseries (<141 years) from three specimens collected near Beechey Island, Nunavut, Canada. Results showed that non-crossdated timeseries constructed by four coralline red algae researchers using annual increments of the same C. compactum samples were highly variable and showcase the likelihood of dating errors in non-crossdated timeseries. Crossdating improved inter-series correlations, and correlations to sea ice-related records, suggesting that at least three crossdated timeseries are required to isolate paleoclimate signals. Our findings suggest that future reconstructions with C. compactum should employ crossdating techniques to reduce dating errors and allow for more precise climate reconstructions.Lay AbstractLong-term environmental records provide a critical baseline to examine how humans have impacted Earth’s natural climate. An important piece to consider is sea ice’s role in natural climate variability because its brightness limits warming by reflecting solar irradiation back to space. However, instrumental records of sea ice rarely extend beyond the early satellite era (late 1970s), limiting our understanding of how sea ice affects natural climate variability in the preindustrial era. A lack of historical baseline prompted the development of sea ice proxies, including the long-lived marine alga, Clathromorphum compactum. Similar to tree-rings, C. compactum produces a new mineralized layer each year, and layer thicknesses have been shown to respond to sea ice cover, making them useful to record long-term sea ice variability. However, a recent study showed that records had replicability problems, maybe due to dating mistakes. Our study applies tree-ring dating methods (dendrochronology) to match annual algal growth layers across algal specimens. Results showed that these new methods reduced dating errors, allowing for more precise past sea ice cover reconstructions.

Application of cave monitoring to constrain the value and source of detrital 230Th/232Th in speleothem calcite: Implications for U-series geochronology of speleothems

Cave calcite deposits (“speleothems”) are widely-used archives of terrestrial paleoclimate signals, in large part because they can be accurately and precisely dated using 238U-series disequilibria (U-series dating) measured by mass spectrometry. Difficulties arise when growth layers in stalagmites incorporate detrital material during calcite deposition that contains significant amounts of 230Th (“detrital 230Th”). U-series ages must be corrected for detrital 230Th either by 1) assuming a detrital 230Th content (expressed as the ratio 230Th/232Th), or by 2) developing isochron models for each stalagmite. We examine two alternative correction approaches for detrital 230Th/232Th: analysis of calcite grown on artificial substrates in central Texas caves (i.e., present-day or “zero-age” calcite) and analysis of soil leachates from the recharge zones of the caves. Samples collected over three years yield elevated detrital atomic 230Th/232Th values, ranging from 5.4 to 28.1 parts per million (ppm) ± 0.5 ppm (1σ uncertainty for a typical measurement), compared with the commonly used “bulk earth” value of 4.4 ppm. Agreement between soil leachate and calcite 230Th/232Th values suggests that soil material is a source of detrital Th in the stalagmites in central Texas. Correlations between Fe, Mn, and Th concentrations in soil leachates from above caves suggests that Th sorbs on to Fe and Mn colloidal material. Independent estimates of detrital 230Th/232Th in a young (< 100 years) stalagmite from central Texas reveal higher 230Th/232Th (~4 to 11 ppm) than bulk earth, consistent with measurements from zero-age calcite and soil (~5 to 20 ppm). These results offer a new method for improving U-series chronologies of stalagmites.

Significance of sedimentary provenance reconstruction based on borehole records of the North China Plain for the evolution of the Yellow River

The Yellow River (YR) flows from Northwest China to North China, crossing three topographic steps: the Tibetan Plateau, the Chinese Loess Plateau (CLP), and the North China Plain (NCP). Determining the river's geomorphological evolution is significant for understanding the formation of macro-geomorphic patterns in China. However, the formation age of YR channelization into the Bohai Sea is highly controversial and uncertain, with estimates ranging from the Late Miocene to the Late Pleistocene. This study conducted grain size analysis, whole-rock geochemistry, and detrital zircon U-Pb dating on ~5.2 Ma sediments from borehole G4 drilled in the Bohai Bay. The geochemical results from borehole G4 indicate that chemical weathering was weak during 5.2–1.6 Ma, but showed diverse characteristics since ~1.6 Ma. Moreover, concordant detrital zircons from borehole G4 (~1.6 Ma) yield a major age population of 350–550 Ma, which, however, is scarcely produced by detrital zircons from the nearby Yanshan and Taihang Mountains. Geochemical signals and detrital zircon U-Pb geochronology suggest that the YR reached the NCP after cutting through the Sanmen Gorge and entered the Bohai Bay since ~1.6 Ma. The massive presence of sediments with an age population of 650–1350 Ma in the top of borehole G4 demonstrates that the present topography of the middle and upper reaches of the YR initially formed at ~0.6 Ma. Subsequently, changes in provenance since ~1.6 Ma influenced the records of paleoclimatic signals indicated by elemental geochemistry in the Bohai Bay. Because of detrital inputs from the YR, the (La/Sm)N and (La/Yb)N ratios of borehole G4 abruptly increased after ~1.6 Ma. Other geochemical indicators exhibited no noticeable variation trend, which differs from the decreasing trend of deep-sea δ18O records, the Chemical Index of Alteration (CIA), and the 87Sr/86Sr ratio of the loess from the Chinese Loess Plateau after the YR entered the NCP at ~1.6 Ma. This paper proposes that the YR gradually triggered fluvial erosion up to its upper reaches after entering the NCP, which is primarily in response to climate change.

Modelling and reconstructing tree ring growth index with climate variables through artificial intelligence and statistical methods

Climate variables play an important role in the increase of tree ring width (TRW), which is one of the primary paleoclimate signals. In this study, we apply new methods for understanding tree growth responses to climate variables under anthropogenic climate change. This study uses regression and artificial intelligence (AI) modelling techniques to develop a model describing the relationships between climate variables and the tree ring standardised growth index (TRSGI) in northeast South Korea. We examined data from 1901 to 1998, and then used data from 1999 to 2019 to reconstruct the TRSGI. This study comprised four major steps. In the first step, we evaluated the performance of the Climate Reach Unit (CRU) TS4.03 in comparison to synoptic station in situ data by using three well-known bias correction methods, namely delta, quantile mapping, and empirical quantile mapping. CRU TS4.03 obtained the highest cross-correlation coefficients (r2 > 0.90) within the synoptic station data. In the second step, different temperatures, precipitation, vapour pressure, and two drought indices were analysed using TRSGI. In the third step, the Mann-Kendall test was run to detect climate and TRSGI data trends, and it showed that all variables had increasing trends at a significance level of 0.05 during 1901–1998. Finally, in the fourth and final step, by selecting the most significant factors from the statistical tests, four models were developed: multiple linear regression (MLR), stepwise regression (SR), nonlinear autoregressive with exogenous input (NARX), and NARX de-noised wavelet, the last of which was a model we developed by combining NARX and de-noised wavelet models. The results indicated that MLR with r = 0.44 (p < 0.003) and SR with r = 0.27 (p < 0.001) were not strong enough to reconstruct TRSGI. In contrast, the NARX de-noised model (r = 0.80) performed better than the NARX model (r = 0.78), showing highly similar trends to the observed TRSGI in the case study area. This research highlights the high performance of AI approaches for modelling TRW data in comparison to regression models. AI techniques may improve the accuracy of climate variable estimations and reconstructions that use TRW data. The presented artificial neural network method has the potential to be a highly effective method for TRW studies.

Measurement and research status of stable hydrogen isotopes in tree-ring lignin methoxyl groups

The primary hydrogen (H) source for all organic compounds in the biosphere is from water, and then participates in biogeochemical cycles through photosynthesis and plant physiological metabolism. As a new proxy of paleoclimate and paleoenvironment, stable hydrogen isotope ratios in wood lignin methoxyl groups (δ2HLM) show great advantages in the studies of paleoclimatic change and have been used to reconstruct precipitation stable hydrogen isotope ratios and paleoclimate signals in many regions. Based on the lignin application mechanism and analysis method of δ2HLM, we evaluated the stability and effectiveness of δ2HLM-measurement method from lignin content and lignin monomer composition, and expounded the tree lignin methoxyl groups' stable isotope proxies of current research outcomes. In the middle latitudes, the tree-ring δ2HLM had great potential in recording temperature signals and precipitation stable hydrogen isotope ratios. However, the study of tree-ring δ2HLM was still in its infancy as evidenced by following reasons: 1) The study area was limited to the middle latitude of the northern hemisphere, and the study subjects were limited to conifer species; 2) To compensate for the limitation of hydrogen isotopic records of nitrocellulose, high resolution tree-ring δ2HLM would be studied; 3) The potential of tree-ring δ2HLM utilization in plant physiology and forest ecology remained to be exploited.

Paleoclimate Signals and Groundwater Age Distributions From 39 Public Water Works in the Netherlands; Insights From Noble Gases and Carbon, Hydrogen and Oxygen Isotope Tracers

AbstractKnowing the age distribution of water abstracted from public water supply wells helps to ensure customer trust in drinking water sources and underpin predictions of water quality evolution. We sampled the mixed water of 39 large public supply well fields for major ion chemistry, 3H, 3He, 18O, 2H, 14CDIC, 13CDIC and noble gases and determined the Noble Gas Temperature (NGT). We used a discrete travel time distribution model to quantify the age distributions using 3H, 4He, 14C apparent age and NGT as the 4 distinctive tracers. Helium‐4 and NGT provided information on the older part of the age distributions and showed that the 14C apparent ages are often the result of mixing of waters ranging between 2,000 and 35,000 years old, instead of being discrete ages with a limited variance as previously assumed. The results reveal a large range of age distributions, comprising vulnerable well fields with &gt;60% young water (&lt;100 years) and well fields with &gt;30% paleo groundwater (&gt;25,000 years) and all forms of intermediate distributions. The age distributions match the hydrogeological setting; well fields with age distributions skewed towards older ages appear in the Roer Valley Graben structure, where fluvial and marine aquitards and sealed faults provide protection from recent recharge. Waters from this graben exhibit paleoclimate signals, with a clear relation between NGT (range 3.2–9.3 °C), 4He (up to 3.3 × 10−6 ccSTP/g) and δ18O (range −8.5‰ to −5.9‰VSMOW). Moreover, 3He/4He ratios of these graben waters suggest a certain influx of He from mantle origin.

Ice Core Science Meets Computer Vision: Challenges and Perspectives

Polar ice cores play a central role in studies of the earth’s climate system through natural archives. A pressing issue is the analysis of the oldest, highly thinned ice core sections, where the identification of paleoclimate signals is particularly challenging. For this, state-of-the-art imaging by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) has the potential to be revolutionary due to its combination of micron-scale 2D chemical information with visual features. However, the quantitative study of record preservation in chemical images raises new questions that call for the expertise of the computer vision community. To illustrate this new inter-disciplinary frontier, we describe a selected set of key questions. One critical task is to assess the paleoclimate significance of single line profiles along the main core axis, which we show is a scale-dependent problem for which advanced image analysis methods are critical. Another important issue is the evaluation of post-depositional layer changes, for which the chemical images provide rich information. Accordingly, the time is ripe to begin an intensified exchange between the two scientific communities of computer vision and ice core science. The collaborative building of a new framework for investigating high-resolution chemical images with automated image analysis techniques will also benefit the already wide-spread application of laser-ablation inductively-coupled plasma mass spectrometry chemical imaging in the geosciences.

Thermal regelation of single particles and particle clusters in ice.

We investigate the migration by thermal regelation of single particles and clusters of particles surrounded by ice subjected to a temperature gradient. This phenomenon is relevant to the casting of porous materials, to cryopreservation of biological tissue, and to the degradation of paleoclimatic signals held in ice sheets, for example. Using carefully controlled laboratory experiments, we measure the migration rates of single particles and clusters as they approach the freezing front. We find that clusters migrate at a constant rate, while single particles accelerate towards the freezing front. This fundamental difference is attributed to the fact that, during regelation, melt water passes through the interstices of a cluster, limited by its constant permeability, but for a single particle must flow through a thin layer of pre-melted ice whose thickness diverges as the freezing temperature is approached, reducing the viscous resistance to migration. We extend existing theories of particle and cluster migration to include the influences of different thermal conductivities and of latent heat on the local temperature field in and around the particle or cluster. We find that if the specific latent heat is large or the viscous resistance to flow is sufficiently small then the migration rate is determined solely by heat transport.

Weak Influence of Paleoenvironmental Conditions on the Subsurface Biosphere of Lake Ohrid over the Last 515 ka.

Lacustrine sediments are widely used to investigate the impact of climatic change on biogeochemical cycling. In these sediments, subsurface microbial communities are major actors of this cycling but can also affect the sedimentary record and overprint the original paleoenvironmental signal. We therefore investigated the subsurface microbial communities of the oldest lake in Europe, Lake Ohrid (North Macedonia, Albania), to assess the potential connection between microbial diversity and past environmental change using 16S rRNA gene sequences. Along the upper ca. 200 m of the DEEP site sediment record spanning ca. 515 thousand years (ka), our results show that Atribacteria, Bathyarchaeia and Gammaproteobacteria structured the community independently from each other. Except for the latter, these taxa are common in deep lacustrine and marine sediments due to their metabolic versatility adapted to low energy environments. Gammaproteobacteria were often co-occurring with cyanobacterial sequences or soil-related OTUs suggesting preservation of ancient DNA from the water column or catchment back to at least 340 ka, particularly in dry glacial intervals. We found significant environmental parameters influencing the overall microbial community distribution, but no strong relationship with given phylotypes and paleoclimatic signals or sediment age. Our results support a weak recording of early diagenetic processes and their actors by bulk prokaryotic sedimentary DNA in Lake Ohrid, replaced by specialized low-energy clades of the deep biosphere and a marked imprint of erosional processes on the subsurface DNA pool of Lake Ohrid.

Late Pleistocene lake level, glaciation and climate change in the Mongolian Altai deduced from sedimentological and palynological archives

AbstractGlacial and lacustrine sediments from the Mongolian Altai provide paleoclimatic information for the late Pleistocene in Mongolia, for which only a few sufficiently studied archives exist. Glacial stages referred to global cooling events are reported for the last glacial maximum (27–21 ka) and the late glacial period (18–16 ka). Sedimentary archives from the first part of the last glacial period are infrequent. We present proxy data for this period from two different archives (88–63 and 57–30 ka). Due to the limitation of effective moisture, an increase of precipitation is discussed as one trigger for glacier development in the cold-arid regions of central Asia. Our pollen analysis from periods of high paleolake levels in small catchments indicate that the vegetation was sparse and of dry desert type between 42–29 and 17–11 ka. This apparent contradiction between high lake levels and dry landscape conditions, the latter supported by intensified eolian processes, points to lower temperatures and cooler conditions causing reduced evaporation to be the main trigger for the high lake levels during glacier advances. Rising temperatures that cause melting of glacier and permafrost ice and geomorphological processes play a role in paleolake conditions. Interpreting lake-level changes as regional or global paleoclimate signals requires detailed investigation of geomorphological settings and mountain–basin relationships.

Misinterpreting proxy data for paleoclimate signals: A reply to Srivastava and Jovane, 2020

Srivastava and Jovane (2020) have made several comments on our assessment of proxy data and challenged the outcome of Shukla et al. (2020) based mainly on interpretation of environmental magnetic parameters. We respond to their criticisms and re-evaluate our paper, remove ambiguities and validate our conclusions through additional proxies (grain-size and geochemistry). We welcome their comments and do not entirely rule out their interpretation for magnetic mineralogy. We highlight the importance of proxy validation for high-energy environments like Chorabari lake. However, single proxy data correlation is likely to produce biased results with no relevant meaning. The objective of our study was to understand complexities in the glacial-climate system by reconstructing late-Holocene climate variations using the glacial lake sediment records from the Mandakini River Basin, Central Himalaya, India. We presented the complexities in Shukla et al. (2020), and this was also highlighted by Srivastava and Jovane (2020). In response, we provide additional justification of proxy response and substantiate our results with present-day estimates from the Chorabari glacier valley. We disagree with the thesis put forward by Srivastava and Jovane (2020) in their conclusion as they overemphasize the interpretation of a single proxy. We maintain that the investigation of present-day glacial settings is an important precursor of paleoclimatic data interpretation and that this supports our conclusions. We will try to incorporate the important suggestions of Srivastava and Jovne (2020) relating to the interpretation of magnetic data in future work.

Misinterpreting proxy data for paleoclimate signals: A comment on Shukla et al. 2020

Shukla et al. explored paleoclimatic signals from a ~8 m thick profile of a moraine-dammed lake in the central Himalaya exposed due to lake burst from a flash flood in 2013. The main objective of their research work is to understand the complex glacial-climate system during late-Holocene. They attempted a novel multi proxy approach for paleoclimate reconstruction but their work suffers from misinterpretation of various proxies and erroneous/misleading discussion. We therefore report following major points in this comment article. (1) Misinterpretation of magnetic parameters: Magnetic susceptibility (χlf) has been used to interpret changes in magnetic mineralogy rather than concentration of magnetic minerals. Susceptibility of anhysteretic remanence (χARM) has been used at several places to indicate presence of superparamagnetic (SP) and multi domain (MD) ferrimagnetic particles rather than single domain (SD) ferrimagnetic (magnetite) particles. Interpreting erroneous negative values of percentage of frequency dependent susceptibility (χfd%) for climate change. (2) Poor chronology: Overlaps in ages of glacial-lake sediments. (3)