Lake Sediment Sequences Research Articles

Chitinous aquatic invertebrate assemblages in Quaternary lake sediments as indicators of past deepwater oxygen concentration

Chitinous aquatic invertebrate remains are abundant in late Quaternary lake sediment sequences. However, for standard analysis of invertebrate indicator groups usually only the remains of one particular invertebrate group (e.g. water fleas, midges) are systematically isolated from the sediments and analysed numerically. In a calibration study focusing on surface sediment samples from 36 small Swiss lakes, we show that the abundances of other chitinous aquatic invertebrate remains in samples prepared for chironomid analysis provide important information on in-lake oxygen conditions when assessed relative to and together with the overall number of chironomid remains. Our results show that particularly high abundances of chironomid and chaoborid remains are indicative for lakes with high and low deepwater oxygen concentrations, respectively, but that also the remains of other invertebrate groups such as Ceriodaphnia ephippia and Plumatella statoblasts tend to be more abundant in warm, stably stratified and anoxic lakes. Direct gradient analyses of the isolated invertebrate assemblages indicate that deepwater oxygen concentration during late summer is the strongest predictor for assemblage changes in our dataset, followed by variables representing temperature conditions (summer surface water temperature, altitude), summer stratification stability (temperature difference between surface and deepwater layers), water depth and the relative volume of the late summer hypoxic water layer. We show, based on invertebrate assemblages in a Lateglacial sediment sequence from Rotsee, Switzerland, that our new calibration data can provide information on past changes in deepwater oxygen availability from late Quaternary lake sediment sequences, when fossil invertebrate assemblages are added as passive samples to ordinations of the surface sediment calibration data. This provides a new approach for developing assessments of past changes in deepwater oxygen concentrations from late Quaternary sediment samples prepared for chironomid analysis. Such assessments can provide important supplementary information for e.g. interpreting palaeotemperature reconstructions based on chironomid remains.

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

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

A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial

Abstract. Changes in the latitudinal position and strength of the Southern Hemisphere westerlies (SHW) are thought to be tightly coupled to important climate processes, such as cross-equatorial heat fluxes, Atlantic Meridional Overturning Circulation (AMOC), the bipolar seesaw, Southern Ocean ventilation and atmospheric CO2 levels. However, many uncertainties regarding magnitude, direction, and causes and effects of past SHW shifts still exist due to lack of suitable sites and scarcity of information on SHW dynamics, especially from the last glacial. Here we present a detailed hydroclimate multiproxy record from a 36.4–18.6 kyr old lake sediment sequence on Nightingale Island (NI). It is strategically located at 37∘ S in the central South Atlantic (SA) within the SHW belt and situated just north of the marine Subtropical Front (SF). This has enabled us to assess hydroclimate changes and their link to the regional climate development as well as to large-scale climate events in polar ice cores. The NI record exhibits a continuous impact of the SHW, recording shifts in both position and strength, and between 36 and 31 ka the westerlies show high latitudinal and strength-wise variability possibly linked to the bipolar seesaw. This was followed by 4 kyr of slightly falling temperatures, decreasing humidity and fairly southerly westerlies. After 27 ka temperatures decreased 3–4 ∘C, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly-positioned SHW seem to be related to periods of increased CO2 outgassing from the ocean, while changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, our results show that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial. Finally we can conclude that multiproxy lake records from oceanic islands have the potential to record atmospheric variability coupled to large-scale climate shifts over vast oceanic areas.

Stratigraphy and sedimentology of the Orakei maar lake sediment sequence (Auckland Volcanic Field, New Zealand)

Abstract. Global paleo-climate reconstructions are largely based on observations from the Northern Hemisphere despite increasing recognition of the importance of the Southern Hemisphere mid-latitudes for understanding the drivers of the global climate system. Unfortunately, the required complete and high-resolution terrestrial records from the Southern Hemisphere mid-latitudes are few. However, the maar lakes in the Auckland Volcanic Field (AVF), New Zealand, are crucial in this regard as they form outstanding depositional basins due to their small surface-to-depth ratio, restricted catchment, and absence of ice cover since their formation, hence ensuring continuous sedimentation with anoxic bottom water. Significantly, the estimated age of the AVF of ca. 250 ka may allow development of a continuous sediment record spanning the last two glacial cycles. The Orakei maar lake sediment sequence examined in this study spans the Last Glacial Cycle (ca. 126 to ca. 9.5 ka cal BP) from the phreatomagmatic eruption to the crater rim breach due to post-glacial sea-level rise. Two overlapping cores of >100 m sediment were retrieved and combined to develop a complete composite stratigraphy that is presently undergoing a wide range of multi-proxy analyses.

Plants or bacteria? 130 years of mixed imprints in Lake Baldegg sediments (Switzerland), as revealed by compound-specific isotope analysis (CSIA) and biomarker analysis

Abstract. Soil erosion and associated sediment transfer are among the major causes of aquatic ecosystem and surface water quality impairment. Through land use and agricultural practices, human activities modify the soil erosive risk and the catchment connectivity, becoming a key factor of sediment dynamics. Hence, restoration and management plans of water bodies can only be efficient if the sediment sources and the proportion attributable to different land uses are identified. According to this aim, we applied two approaches, namely compound-specific isotope analysis (CSIA) of long-chain fatty acids (FAs) and triterpenoid biomarker analysis, to a eutrophic lake, Lake Baldegg, and its agriculturally used catchment (Switzerland). Soils reflecting the five main land uses of the catchment (arable lands, temporary and permanent grasslands, mixed forests, orchards) were subjected to CSIA. The compound-specific stable isotope δ13C signatures clearly discriminate between potential grasslands (permanent and temporary) and forest sources. Signatures of agricultural land and orchards fall in between. The soil signal was compared to the isotopic signature of a lake sediment sequence covering ca. 130 years (before 1885 to 2009). The recent lake samples (1940 to 2009, with the exception of 1964 to 1972) fall into the soil isotopic signature polygon and indicate an important contribution of the forests, which might be explained by (1) the location of the forests on steep slopes, resulting in a higher connectivity of the forests to the lake, and/or (2) potential direct inputs of trees and shrubs growing along the rivers feeding the lake and around the lake. However, the lake sediment samples older than 1940 lie outside the source soils' polygon, as a result of FA contribution from a not yet identified source, most likely produced by an in situ aquatic source, either algae, bacteria or other microorganisms or an ex-site historic source from wetland soils and plants (e.g. Sphagnum species). Despite the overprint of the yet unknown source on the historic isotopic signal of the lake sediments, land use and catchment history are clearly reflected in the CSIA results, with isotopic shifts being synchronous with changes in the catchment, land use and eutrophication history. The investigated highly specific biomarkers were not detected in the lake sediment, even though they were present in the soils. However, two trimethyltetrahydrochrysenes (TTHCs), natural diagenetic products of pentacyclic triterpenoids, were found in the lake sediments. Their origin is attributed to the in situ microbial degradation of some of the triterpenoids. While the need to apportion sediment sources is especially crucial in eutrophic systems, our study stresses the importance of exercising caution with CSIA and triterpenoid biomarkers in such environments, where the active metabolism of bacteria might mask the original terrestrial isotopic signals.

CYFRA 21.1 in the diagnosis of differents cancer types

The analysis of both the mineralogical and geochemical composition and the stratigraphy of Holocene sediments of Lake Onego provided a novel interpretation of their genesis. Their mineral composition suggests that Holocene bottom sediments generally inherit the composition of terrigenous material that enters the lake with river runoff. The predominant authigenic minerals include biogenic opal from diatom skeletons, Fe-illite, and Fe-chlorite, which are the main iron mineral forms in bottom sediments of Lake Onego. Unstable suspension components (illite and chlorite) transform into their ferruginous varieties under humid climate conditions and high concentrations of iron and silicon in the lake waters. Both the presence of ferromanganese nodules in the uppermost part of bottom sediments of Lake Onego and their formation result from the location of the lake in the northern humid zone, geological composition of Fennoscandian crystalline rocks on the Lake Onego watershed, and the presence of an oxidative geochemical barrier into the lake sediment sequence. Late Holocene bottom sediments are represented by two types of sequences: 1) Fe-Mn enriched layers are formed near the sediment-water interface under oxygenated near bottom waters, and 2) such layers are not formed due to low concentrations of oxygen in water; but there are also non-laminated, homogeneous greyish-green silts, similar to those present in the lower parts of the first type of cores. Sedimentation rates (~0.10 cm y−1) were determined by 210Pb- and 137Cs geochronologies.

Holocene paleomagnetic secular variation (PSV) near 80° N, Northwest Spitsbergen, Svalbard: Implications for evaluating High Arctic sediment chronologies

We present a paleomagnetic study of two Holocene lake sediment sequences from Northwest Spitsbergen on Svalbard, Norway. Core HAP-212 from Lake Hajeren preserves a rare combination of a well-defined Characteristic Remanent Magnetization (ChRM) and robust age control defined by 21 terrestrial macrofossil-derived radiocarbon dates—providing a unique opportunity to assess the stratigraphic potential of High Arctic Holocene paleomagnetic secular variation (PSV) close to 80° N. Core AMP-112 from Hakluytvatnet, which is also well dated, is a poor magnetic recorder and cannot be used for geomagnetic reconstruction. Comparison of the HAP-212 PSV record with poorly dated regional records show similar directional features, suggesting that PSV can be used for chronological synchronization. Comparison of the HAP-212 PSV with well dated records ∼1800 km to the southwest and southeast show that these records are in-phase at millennial timescales, within dating and magnetic uncertainties, indicating that a consistent geomagnetic signal is recorded at these temporal and spatial scales. These findings support the notion that PSV, when optimally recorded, can be used to import chronologies from lower latitudes to the High Arctic, where low organic content, hard water effects, lack of datable material, and other difficulties often preclude more conventional radiocarbon dating.

Bromine as indicator of source of lacustrine sedimentary organic matter in paleolimnological studies

AbstractBromine (Br) to organic matter (OM) concentration ratio is studied in lake sediment sequences to provide information on environmental changes modifying OM production. The sequences studied were extracted from shallow lakes Morenito, El Trébol, Escondido, and Portezuelo; and deep lakes Futalaufquen, Moreno, and Traful (North Patagonia Andean range). Lake Morenito, a former Lake Moreno bay until its closure in AD 1960, showed a decrease in Br:OM ratios from 1.38 to 0.74 after lake closure, associated with an increase of primary autochthonous productivity attributable to the development of submerged and emerging macrophytes. Sedimentary sequences from Lakes Escondido, Portezuelo, and El Trébol (with large participation of macrophytes in primary productivity), and from Lakes Moreno, Futalaufquen, and Traful (with little development of littoral macrophytes), showed Br:OM ratios consistent with the Lake Morenito pattern. Consistently, the morphometric parameters mean depth and shoreline development correlate with Br:OM ratios. Therefore, Br:OM ratios can be associated with the composition of primary autochthonous productivity, with values of about 0.7 associated to significant macrophyte contributions, and higher values associated with more pelagic contributions. Accordingly, Br:OM variations along a sedimentary sequence can be associated with modifications on the composition of the primary autochthonous productivity of the water body, providing information on environmental changes.

Laacher See tephra discovered in the Bohemian Forest, Germany, east of the eruption

Abstract We present a geochemically confirmed finding of the Laacher See tephra (LST), directly east of the Laacher See volcano (LSV). European lake sediment (Rachelsee, Germany) contains the time period of the Laacher See volcanic explosion (LSE) ca. 12900 yr cal BP in the East Eifel volcanic field. This eruption significantly affected the local environment but its impact in regions far east from its source is poorly known. The lake sediment sequence showed an enhancement in magnetic susceptibility and an increase in concentration in niobium and phosphorus. These enhancements correlated with direct tephra particle observations whose geochemical analysis compared well with the published LST data from proximal sites. Detailed analyses revealed a most likely match with tephra known to have been deposited during the middle phase of the eruption. We suggest that contrary to other published studies, the LST cloud traveled directly east, as far as 470 km, from the volcano,. The pollen record shows minimal change during the tephra deposition, thus indicating that the LSE had no noticeable influence on vegetation in eastern Bavaria and the southern part of the Czech Republic. The discovery of the LST at Rachelsee, opens the potential of further tephrochronological studies in regions to the east of the LSV.

Towards a Holocene tephrochronology for the Faroe Islands, North Atlantic

The Faroe Islands hold a key position in the North Atlantic region for tephra studies due to their relative proximity to Iceland. Several tephras have been described over the last 50 years in peat and lake sediment sequences, including the type sites for the Saksunarvatn and Mjáuvøtn tephras. Here we present a comprehensive overview of Holocene tephras found on the Faroe Island. In total 23 tephra layers are described including visible macrotephras such as the Saksunarvatn and Hekla 4 tephras and several cryptotephras. The importance of tephras originally described from the Faroe Islands is highlighted and previously unpublished results are included. In addition, full datasets for several sites are published here for the first time. The Saksunarvatn Ash, now considered to be the result of several eruptions rather than one major eruption, can be separated into two phases on the Faroe Islands; one early phase with two precursor eruptions with lower MgO concentrations (4.5–5.0 wt%) than the main eruption and a later phase with higher MgO concentrations (5.5–6.0 wt%), including the visible Saksunarvatn Ash. The Tjørnuvík Tephra, previously considered to be a primary deposit, is now interpreted as a reworked tephra with material from at least two middle Holocene eruptions of Hekla. Several of the tephras identified on the Faroe Islands provide useful isochrons for climate events during the Holocene.

Radiocarbon Wiggle Matching on Laminated Sediments Delivers High-Precision Chronologies

ABSTRACTHigh-resolution sediment chronologies with the best possible time control are essential for comparing palaeoecological studies with independent high-precision climatic, archaeological or historic data in order to disentangle causes and effects of past environmental, ecological and societal change. We present two varved lake sediment sequences from Moossee and Burgäschisee (Swiss Plateau) that have chronologies developed with Bayesian models and radiocarbon (14C) dating of terrestrial plant macrofossils extracted from sediment samples with constant age ranges. We illustrate the potential of high-resolution14C dating for the construction of robust, high-precision sediment chronologies. The mean 2σ age uncertainties were reduced to±19 cal yr for Moossee and to±54 cal yr for Burgäschisee over the entire period of 3000 cal yr, while 2σ uncertainties of only±13 cal yr and±18 cal yr respectively, were achieved for shorter time intervals. These precisions are better than or comparable to those of previous varve studies. Our results imply that a sophisticated subsampling strategy and a careful selection of short-lived and well-defined terrestrial plant remains are crucial to avoid outlying14C ages. A direct linkage between palaeoeological studies with dendrochronologically dated, local archaeological sites as well as a precise comparison with high-resolution climate proxy data have become feasible.

Controls of climate, catchment erosion and biological production on long-term community and functional changes of chironomids in High Arctic lakes (Svalbard)

Arctic freshwater basins are diversity hotspots and sentinels of climate change, but their long-term variability and the environmental variables controlling them are not well defined. We examined four available lake sediment sequences from High Arctic Svalbard for their subfossil Chironomidae communities, biodiversity and functional traits and assessed the influence of climatic and limnological variability on the long-term ecological dynamics. Our results indicated that collector-filterers had an important role in the oligotrophic sites, whereas collector-gatherers dominated the nutrient-enriched sites with significant bird guano inputs. In the oligotrophic sites, benthic production, taxon richness and taxonomic and functional diversity were highest during the early Holocene, when temperatures showed a rapid increase. An increase in subfossil abundance and diversity metrics was also found in recent samples of the oligotrophic sites, but not in the bird-impacted sites, where the trends were decreasing. When partitioning out the environmental forcing on chironomid communities, the influence of climate was significant in all the sites, whereas in-lake production (organic matter) was significant in two of the sites and catchment erosion (magnetic susceptibility) had only minor influence. The findings suggest that major changes in Arctic chironomid assemblages were driven by climate warming with increasing diversity in oligotrophic sites, but deteriorating ecological functions in environmentally stressed sites. We found that although taxonomic and functional diversity were always coupled, taxonomical and functional turnovers were coupled only in the oligotrophic sites suggesting that the ecological functions operated by chironomids in these low-productivity sites may not be as resilient to future environmental change.

Geochemical records of paleocontamination in late pleistocene lake sediments in West Flanders (Belgium)

ABSTRACTConcentrations of trace elements in Late Pleistocene lake sediments from two localities in West Flanders (Belgium) were studied using inductively coupled plasma-mass spectrometry. The studied sediments were deposited during the transition of the Allerød to the Younger Dryas. Trace elements enter lake environments from a variety of sources, but mostly from bedrock weathering in catchments and as wind-blown dust particles. These sources create a natural trace element background in lake sediment records. The concentration and distribution of trace elements across the studied lake sediment sequences display features consistent with inputs of materials other than those from soil or bedrock. Input from volcanic eruptions, biomass burning and meteorite impacts should be considered among the main sources of non-anthropogenic contamination of lake environments.

A multi-centennial record of past floods and earthquakes in Valle d'Aosta, Mediterranean Italian Alps

Abstract. Mediterranean Alpine populations are particularly exposed to natural hazards like floods and earthquakes because of both the close Mediterranean humidity source and the seismically active Alpine region. Knowledge of long-term variability in flood and earthquake occurrences is of high value since it can be useful to improve risk assessment and mitigation. In this context, we explore the potential of a lake-sediment sequence from Lago Inferiore de Laures in Valle d'Aosta (Northern Italy) as a long-term record of past floods and earthquakes. The high-resolution sedimentological study revealed 76 event layers over the last ca. 270 years; 8 are interpreted as most probably induced by earthquakes and 68 by flood events. Comparison to historical seismic data suggests that the recorded earthquakes are strong (epicentral Medvedev–Sponheuer–Kárník (MSK) intensity of VI–IX) and/or close to the lake (distance of 25–120 km). Compared to other lake-sediment sequences, Lago Inferiore de Laures sediments appear to be regionally the most sensitive to earthquake shaking, offering a great potential to reconstruct the past regional seismicity further back in time. Comparison to historical and palaeoflood records suggests that the flood signal reconstructed from Lago Inferiore de Laures sediments represents the regional and (multi-)decadal variability of summer–autumn floods well, in connection to Mediterranean mesoscale precipitation events. Overall, our results reveal the high potential of Lago Inferiore de Laures sediments to extend the regional earthquake and flood catalogues far back in time.

Holocene treeline and timberline changes in the South Carpathians (Romania): Climatic and anthropogenic drivers on the southern slopes of the Retezat Mountains

Two high-altitude lake-sediment sequences (Lake Lia, 1910 m a.s.l. and Lake Bucura, 2040 m a.s.l.) from the Retezat Mountains (South Carpathians, Romania) were analysed using multi-proxy methods to study responses of treeline, timberline and alpine/subalpine vegetation to climate change and human impact during the past 16,000 years. Woody species ( Pinus mugo, Pinus cembra, Picea abies and Juniperus communis) reached Lake Lia between 12,000 and 11,800 cal. yr BP, whereas P. mugo colonised the shores of Lake Bucura at 9600 cal. yr BP. Lake Lia was in the timberline ecotone between 8000 and 3200 cal. yr BP, in semi-open P. cembra and Picea abies woodland, probably mixed with P. mugo on the steeper slopes. Lake Bucura was surrounded by the upper part of the krummholz zone during the mid-Holocene. The increase in P. cembra after c. 6000 cal. yr BP around Lake Lia suggests that the composition of the timberline forest changed. The disappearance of P. cembra and Picea abies around Lake Lia at ~3000 cal. yr BP reflects descent of the timberline. A large mean July temperature decline between 3300 and 2800 cal. yr BP may have driven or at least contributed to the descent of the Picea abies–P. cembra forests. An increase in human indicator pollen types in Lake Bucura around 4200 cal. yr BP may reflect human impact in the naturally open alpine zone in the Late Bronze Age. In contrast, human impact likely appeared considerably later, around 2650 cal. yr BP (Early Iron Age) around Lake Lia in the upper subalpine zone. Human impact likely intensified after 2200 cal. yr BP at both sites that resulted in the lowering of the krummholz zone. We conclude that climate change and human impact both played an important role in the lowering of the treeline and timberline in the late-Holocene.

A late Holocene record of solar-forced atmospheric blocking variability over Northern Europe inferred from varved lake sediments of Lake Kuninkaisenlampi

Abstract This study presents a new varved lake sediment sequence from Lake Kuninkaisenlampi, Eastern Finland. The record is constituted by alternations of clastic and biogenic laminae and provides a precise chronology extending back to 3607 ± 94 varve yrs. BP. The seasonality of the boreal climatic zone, with cold winters and mild summers, is reflected in the varve structure as a succession of three laminae from bottom to top, (i) a coarse to fine-grained detrital lamina marked by detrital catchment material transported by spring floods; (ii) a biogenic lamina with diatoms, plant and insect remnants reflecting biological productivity during the season of lake productivity; and (iii) a very fine amorphous organic lamina deposited during the winter stratification. The thickness of the detrital lamina in the lake reflects changes in the rate of spring snow melt in the catchment and is, therefore, considered a proxy for winter conditions. Hence, the record allows reconstructing local climate and environmental conditions on inter-annual to the multi-centennial timescales. We find that minerogenic accumulation reflected in the detrital lamina exhibits a high multi-decadal to centennial-scale spectral coherency with proxies for solar activity, such as Δ 14 C, and Total Solar Irradiance, suggesting a strong link between solar variability and sediment transport to the lake basin. Increased catchment erosion is observed during periods of low solar activity, which we ascribe to the development of more frequent atmospheric winter blocking circulation induced by solar-forced changes in the stratosphere. We suggest that soil frost in the catchment of Lake Kuninkaisenlampi related to more frequent winter blocking led to increased surface run-off and ultimately to increased catchment erosion during spring. We conclude that, during the past ca 3600 years, solar forcing may have modulated multi-decadal to centennial variations in sedimentation regimes in lakes from Eastern Finland and potentially in other North European lakes.

Progressive and regressive soil evolution phases in the Anthropocene

Soils have a substantial role in the environment because they provide several ecosystem services such as food supply or carbon storage. Agricultural practices can modify soil properties and soil evolution processes, hence threatening these services. These modifications are poorly studied, and the resilience/adaptation times of soils to disruptions are unknown. Here, we study the evolution of pedogenetic processes and soil evolution phases (progressive or regressive) in response to human-induced erosion from a 4000-year lake sediment sequence (Lake La Thuile, French Alps). Erosion in this small lake catchment in the montane area is quantified from the terrigenous sediments that were trapped in the lake and compared to the soil formation rate. To access this quantification, soil processes evolution are deciphered from soil and sediment geochemistry comparison. Over the last 4000years, first impacts on soils are recorded at approximately 1600yrcal. BP, with the erosion of surface horizons exceeding 10t·km−2·yr−1. Increasingly deep horizons were eroded with erosion accentuation during the Higher Middle Ages (1400–850yrcal. BP), reaching 1000t·km−2·yr−1, and leading to the remobilization of carbonated and poorly weathered material, hence rejuvenating soil development. Erosion exceeded the soil formation rate and constituted a regression in the development of soils. The tolerable erosion limit is thus defined for erosion from 25to 30t·km−2·yr−1. Beyond this limit, the sustainability of the agroecosystem is limited and ecosystem services decrease. Afterwards, pedogenesis evolved again from progressive (700–300yrcal. BP) to regressive (300yrcal. BP-today) phases. Erosion was less important during the last 700years than during the Middle Ages but with the same weathering stages, indicating that soils were deeply affected during the Middle-Age and have yet not recovered. Our results highlight the importance of the human factor in the pedogenesis over last millennia and suggest that the studied agro-ecosystem entered the Anthropocene 1400years ago.

Mid- to late Holocene climate response from the Triloknath palaeolake, Lahaul Himalaya based on multiproxy data

A 2.13-m-thick palaeolake sequence deposited between the right lateral moraine and the valley wall of Triloknath glacier, has been studied at high resolution to understand the past climatic changes in Lahaul Himalaya. The OSL and AMS14C chronology suggests that the lake building activity was initiated post-cooling phase since ~6484cal.YBP. The combination of sedimentological, geochemical, and environmental magnetic parameters helped to infer five first-order climatic zones. The influx of magnetic minerals along with a notable increase in antiferromagnetic mineralogy (appearance of hematite) and higher chemical index of alteration (CIA) values during ~5925cal.YBP to 5676cal.YBP suggests warm and wet conditions owing to enhanced Indian Summer Monsoon (ISM) precipitation corresponding to the mid-Holocene warming. Further, reduction in the magnetic mineral concentration with few peaks of antiferromagnetic mineralogy and higher CIA values suggests warmer episodes within an overall cold climate, and the finer ferrimagnetic grain size suggests intermittent low energy and ponding conditions during ~3904cal.YBP to 2365cal.YBP. This was followed by fluctuating cold arid conditions in response to reduced ISM precipitation during ~2365cal.YBP and 1758cal.YBP. Further, between 1758cal.YBP to the present, episodic warming, pedogenic enhancement, and increased monsoon precipitation have been recorded. A major warm episode (Medieval warm period) as depicted from enhanced antiferromagnetic mineralogy and higher CIA values has been recorded between ~742cal.yrs. BP and 593cal.YBP. The sudden drop in the magnetic mineral concentration as well as the CIA value, around ~300cal.YBP (1650 CE) is likely because of colder conditions prevailing during the Little Ice Age (LIA). The study illustrates high sensitivity of the lake sediment sequence to witness the climatic episodes experienced in the glaciated regime of the Triloknath glacier.

A refined paleotemperature calibration for New Zealand limnic environments using differentiation of branched glycerol dialkyl glycerol tetraether (brGDGT) sources

ABSTRACTBranched glycerol dialkyl glycerol tetraethers (brGDGTs) were abundant in surface sediments of freshwater lakes and in catchment soils at altitudes from 10 to 2020 m in New Zealand. Significant differences in brGDGT compositions between lake sediments and soils indicate sources from separate microbial habitats. An expanded modern calibration dataset comprising 33 lakes has enabled a revised calibration function for determining past mean annual air temperature (MAT) from brGDGTs in New Zealand lake sediments: MAT (°C) = −31.664 × MBTm + 16.252 (n = 30). The function uses a modified methylation index of branched tetraethers (MBTm) that incorporates brGDGT III in the numerator to overcome the lower correlation found between our larger dataset and the unmodified MBT which had been used for previous calibrations. Calibrations combining the cyclization index of branched tetraethers (CBT) and the MBTm or using only certain brGDGTs are possible but have limitations. The revised function shows slightly higher correlation with MAT (R2 = 0.75) than previous calibrations, which were based on nine sites. The refined calibration function is applied to a ∼16 000‐year lake sediment sequence from northern South Island, New Zealand, and yields temperature reconstructions that are consistent with independently derived climate trends from the same sequence.

Environmental and climate reconstruction of the late-glacial-Holocene transition from a lake sediment sequence in Aubrac, French Massif Central: Chironomid and diatom evidence

The analysis of fossil chironomid and diatom assemblages from a sedimentary record from Les Roustières peat bog (Massif Central, France, 1196m asl) allows the reconstruction of past environmental and climate changes during the late-glacial and early Holocene. Chironomid assemblages showed that the infilling of the palaeolake had commenced during the Oldest Dryas (GS-2b) as suggested by the rapid decrease in chironomid species associated with the cold and deep zone of lakes and by their replacement by littoral and eutrophic taxa. Quantitative July temperature reconstructions, based on the chironomid data, suggest that mean July air temperature (Tjul) ranged between 6°C and 11°C at the termination of the Oldest Dryas period (GS-2b). Climate began to warm at the start of the Bølling period (GI-1e), between 15,000 and 14,800calyr BP, with a rise in Tjul of about 4°C. This climate warming is contemporaneous with lake eutrophication as suggested by diatoms and chironomids. Maximum temperatures of 13–14°C were reached around 13,600calyr BP during the Allerød period (GI-1c–GI-1a). The Younger Dryas period (GS-1) is marked by a return to cold conditions with Tjul of about 10°C during a first phase, then 13°C in its terminal part. A probable increase in the duration of the ice-cover may have favoured arctic and alpine diatom species. The early-Holocene climate improvement is marked by a rise in Tjul of about 3°C.