Showing papers on "Varve published in 2021"

Seasonal climate signals preserved in biochemical varves: insights from novel high-resolution sediment scanning techniques

Abstract: . Varved lake sediments are exceptional archives of paleoclimatic information due to their precise chronological control and annual resolution. However, quantitative paleoclimate reconstructions based on the biogeochemical composition of biochemical varves are extremely rare, mainly because the climate–proxy relationships are complex and obtaining biogeochemical proxy data at very high (annual) resolution is difficult. Recent developments in high-resolution hyperspectral imaging (HSI) of sedimentary pigment biomarkers combined with micro X-ray fluorescence ( µ XRF) elemental mapping make it possible to measure the structure and composition of varves at unprecedented resolution. This provides opportunities to explore seasonal climate signals preserved in biochemical varves and, thus, assess the potential for annual-resolution climate reconstruction from biochemical varves. Here, we present a geochemical dataset including HSI-inferred sedimentary pigments and µ XRF-inferred elements at very high spatial resolution (60 µ m, i.e. > 100 data points per varve year) in varved sediments of Lake Żabinskie, Poland, over the period 1966–2019 CE. We compare these data with local meteorological observations to explore and quantify how changing seasonal meteorological conditions influenced sediment composition and varve formation processes. Based on the dissimilarity of within-varve multivariate geochemical time series, we classified varves into four types. Multivariate analysis of variance shows that these four varve types were formed in years with significantly different seasonal meteorological conditions. Generalized additive models (GAMs) were used to infer seasonal climate conditions based on sedimentary variables. Spring and summer (MAMJJA) temperatures were predicted using Ti and total C ( R adj 2 = 0.55 ; cross-validated root mean square error (CV-RMSE) = 0.7 ∘ C, 14.4 %). Windy days from March to December (mean daily wind speed > 7 m s −1 ) were predicted using mass accumulation rate (MAR) and Si ( R adj 2 = 0.48 ; CV-RMSE = 19.0 %). This study demonstrates that high-resolution scanning techniques are promising tools to improve our understanding of varve formation processes and climate–proxy relationships in biochemical varves. This knowledge is the basis for quantitative high-resolution paleoclimate reconstructions, and here we provide examples of calibration and validation of annual-resolution seasonal weather inference from varve biogeochemical data.

Evaluating sedimentary DNA for tracing changes in cyanobacteria dynamics from sediments spanning the last 350 years of Lake Tiefer See, NE Germany

Abstract: Since the beginning of the Anthropocene, lacustrine biodiversity has been influenced by climate change and human activities. These factors advance the spread of harmful cyanobacteria in lakes around the world, which affects water quality and impairs the aquatic food chain. In this study, we assessed changes in cyanobacterial community dynamics via sedimentary DNA (sedaDNA) from well-dated lake sediments of Lake Tiefer See, which is part of the Klocksin Lake Chain spanning the last 350 years. Our diversity and community analysis revealed that cyanobacterial communities form clusters according to the presence or absence of varves. Based on distance-based redundancy and variation partitioning analyses (dbRDA and VPA) we identified that intensified lake circulation inferred from vegetation openness reconstructions, δ13C data (a proxy for varve preservation) and total nitrogen content were abiotic factors that significantly explained the variation in the reconstructed cyanobacterial community from Lake Tiefer See sediments. Operational taxonomic units (OTUs) assigned to Microcystis sp. and Aphanizomenon sp. were identified as potential eutrophication-driven taxa of growing importance since circa common era (ca. CE) 1920 till present. This result is corroborated by a cyanobacteria lipid biomarker analysis. Furthermore, we suggest that stronger lake circulation as indicated by non-varved sediments favoured the deposition of the non-photosynthetic cyanobacteria sister clade Sericytochromatia, whereas lake bottom anoxia as indicated by subrecent- and recent varves favoured the Melainabacteria in sediments. Our findings highlight the potential of high-resolution amplicon sequencing in investigating the dynamics of past cyanobacterial communities in lake sediments and show that lake circulation, anoxic conditions, and human-induced eutrophication are main factors explaining variations in the cyanobacteria community in Lake Tiefer See during the last 350 years.

Siliceous algae response to the “Great Acceleration” of the mid-20th century in Crawford Lake (Ontario, Canada): A potential candidate for the Anthropocene GSSP:

Abstract: Diatom and chrysophyte assemblages from varved sediments of meromictic Crawford Lake, Ontario record major environmental changes resulting from spatially broadening anthropogenic environmental stre...

A record of late Holocene precipitation on the Central Tibetan Plateau inferred from varved lake sediments

Abstract: Lake sediments on the Tibetan Plateau are important natural archives for studying past climate and environment changes. A precise sediment core chronology is a fundamental prerequisite for reconstructing past climate and environment changes. However, radiocarbon dates on bulk carbon in lake sediment cores are subject to “old carbon” reservoir effects, which can cause problems when trying to establish depth-age relations for sediment cores, especially those from the Tibetan Plateau (TP). Here we present a varve chronology that spans the last ~ 2000 years, from Jiang Co, on the central TP. Clastic-biogenic varves in Jiang Co sediments are comprised of two laminae, a coarse-grain and a fine-grain layer, observed by petrographic microscope and Electron Probe Micro Analyzer. The varve chronology is supported by 210Pb and 137Cs dating and was used to estimate the radiocarbon reservoir ages for the past ~ 2000 years. The thickness percentage of the coarse-grain layer within a single varve was used as a proxy for summer precipitation amount, as coarse grains are transported mainly by runoff. During the past 2000 years, the precipitation record inferred from Jiang Co sediments shows centennial-scale fluctuations that are consistent with other records from the region. The varve chronology from Jiang Co provided a valuable opportunity to estimate the “old carbon” reservoir effect in Jiang Co and reconstruct past precipitation on the Tibetan Plateau.

Hydroclimatic variability of opposing late Pleistocene climates in the Levant revealed by deep Dead Sea sediments

Abstract: . Annual and decadal-scale hydroclimatic variability is a key characteristic embedded into climate insitu. It is therefore crucial to study hydroclimatic variability in order to understand its effects on climate derivatives such as hydrological processes and water availability. However, the study of this variability from modern records is limited due to their relatively short span, whereas model simulations relaying on modern dynamics could miss some of its aspects. Here we study annual to decadal hydroclimatic variability in the Levant using two sedimentary sections covering ~ 700 years deposited at ~ 18 and ~ 27 Ka retrieved from the depocenter of the Dead Sea, which has been continuously recording environmental conditions throughout the late Pleistocene. We focus on two ~ 700 years long series of annually-deposited laminated intervals (i.e., varves) representing two episodes of opposing mean climates, deposited during lake level rise and fall at 27 and 18 Ka, respectively. These two series comprise alternations of authigenic aragonite precipitated during summer and flood-derived detrital laminae deposited during winter. Within this record, aragonite laminae serve as a proxy of annual inflow and epilimnion dilution, whereas detrital laminae comprise sub-laminae that record individual floods. The two series depict distinct characteristics with increased mean and variance of annual inflow and flood frequency during "wetter", with respect to the "dryer", conditions. In addition, decades of intense flood frequency are identified (e.g., clusters), suggesting shifts between centennial-scale climatic regimes, which are particularly pronounced during wetter, lake-rising conditions. The combined application of multiple time series analyses indicates that episodes of falling lake levels are characterized by multiple pronounced quasi-periodic components with periodicities of 2–4, 6–8 and ~ 12 years, whereas the rising lake level episode presents weaker, less-persistent periodical components with similar periodicities. Combining these observations with the modern synoptic-scale hydroclimatology indicates shifts in the dominance of key synoptic systems governing rainfall, annual inflow and flood frequency in the eastern Mediterranean over centennial time-scale.

Reconstructing past hydrology of eastern Canadian boreal catchments using clastic varved sediments and hydro-climatic modelling: 160 years of fluvial inflows

Abstract: . Analysis of short sediment cores collected in Grand Lake, Labrador, revealed that this lake is an excellent candidate for the preservation of a laminated sediment record. The great depth of Grand Lake, the availability of fine sediments along its tributaries and its important seasonal river inflow have favoured the formation of a 160-year-long clastic varved sequence. Each varve represents 1 hydrological year. Varve formation is mainly related to spring discharge conditions with contributions from summer and autumn rainfall events. The statistically significant relation between varve parameters and the Naskaupi River discharge observations provided the opportunity to develop local hydrological reconstructions beyond the instrumental period. The combined detrital layer thickness and the particle size (99th percentile) series extracted from each varve yield the strongest correlations with instrumental data ( r=0.68 and 0.75 respectively) and have been used to reconstruct the respective Naskaupi River mean and maximum annual discharges over the 1856–2016 period. The reconstructed Q -mean series suggest that high Q -mean years occurred during the 1920–1960 period, and a slight decrease in Q -mean took place during the second half of the 20th century. Independent reconstructions based on rainfall–runoff modelling of the watershed from historical reanalysis of global geopotential height fields display a significant correlation with the reconstructed Naskaupi River discharge based on varve physical parameters. The Grand Lake varved sequence contains a regional hydrological signal, as suggested by the statistically significant relation between the combined detrital layer thickness series and the observed Labrador region Q -mean series extracted from five watersheds of different sizes.

Atmospherically produced beryllium-10 in annually laminated late-glacial sediments of the North American Varve Chronology

Abstract: . We attempt to synchronize the North American Varve Chronology (NAVC) with ice core and calendar year timescales by comparing records of atmospherically produced 10 Be fallout in the NAVC and in ice cores. The North American Varve Chronology (NAVC) is a sequence of 5659 varves deposited in a series of proglacial lakes adjacent to the southeast margin of the retreating Laurentide Ice Sheet between approximately 18 200 and 12 500 years before present. Because properties of NAVC varves are related to climate, the NAVC is also a climate proxy record with annual resolution, and our overall goal is to place the NAVC and ice core records on the same timescale to facilitate high-resolution correlation of climate proxy variations in both. Total 10 Be concentrations in NAVC sediments are within the range of those observed in other lacustrine records of 10 Be fallout, but 9 Be and 10 Be concentrations considered together show that the majority of 10 Be is present in glacial sediment when it enters the lake, and only a minority of total 10 Be derives from atmospheric fallout at the time of sediment deposition. Because of this, an initial experiment to determine whether or not 10 Be fallout variations were recorded in NAVC sediments by attempting to observe the characteristic 11-year solar cycle in short varve sections sampled at high resolution was inconclusive: short-period variations at the expected magnitude of this cycle were not distinguishable from measurement scatter. On the other hand, longer varve sequences sampled at decadal resolution display centennial-period variations in reconstructed 10 Be fallout that have similar properties as coeval 10 Be fallout variations recorded in ice core records. These are most prominent in glacial sections of the NAVC that were deposited in proglacial lakes and are suppressed in paraglacial sections of the NAVC that were deposited in lakes lacking direct glacial sediment input. We attribute this difference to the fact that buffering of 10 Be fallout by soil adsorption can filter out short-period variations in an entirely deglaciated watershed, but such buffering cannot occur in the ablation zone of an ice sheet. This implies that proglacial lakes whose watershed is mostly glacial may effectively record 10 Be fallout variations. We attempted to match centennial-period variations in reconstructed 10 Be fallout flux from two segments of the NAVC with ice core fallout records. For both records, it is possible to obtain matches that result in acceptable correlation between NAVC and ice core 10 Be fallout records, but the best-fitting matches for the two segments disagree, and only one of them is consistent with independent calendar year calibrations of the NAVC and therefore potentially valid. This leaves several remaining ambiguities in whether or not 10 Be fallout variations can, in fact, be used for synchronizing NAVC and ice core timescales, but these could most likely be resolved by higher-resolution and replicate 10 Be measurements on targeted sections of the NAVC.

Human actions were responsible for both initiation and termination of varve preservation in Lake Vesijärvi, southern Finland

Abstract: The influence of lake restoration efforts on lake bottom-water conditions and varve preservation is not well known. We studied varved sediments deposited during the last 80 years along a water-depth transect in the Enonsaari Deep, a deep-water area of the southernmost Enonselka Basin, Lake Vesijarvi, southern Finland. For the last few decades, the Enonselka Basin has been subject to ongoing restoration efforts. Varve, elemental, and diatom analyses were undertaken to explore how these actions and other human activities affected varve preservation in the Enonsaari Deep. In contrast to most varved Finnish lakes, whose water columns have a natural tendency to stratify, and possess varve records that span thousands of years, varve formation and preservation in Lake Vesijarvi was triggered by relatively recent anthropogenic stressors. The multi-core varve analysis revealed that sediment in the Enonsaari Deep was initially non-varved, but became fully varved in the late 1930s, a time of increasing anthropogenic influence on the lake. The largest spatial extent of varves occurred in the 1970s, which was followed by a period of less distinguishable varves, which coincided with diversion of sewage from the lake. Varve preservation weakened during subsequent decades and was terminated completely by lake aeration in the 2010s. Despite improvements in water quality, hypolimnetic oxygen depletion and varve preservation persisted beyond the reduction in sewage loading, initial aeration, and biomanipulation. These restoration efforts, however, along with other human actions such as harbor construction and dredging, did influence varve characteristics. Varves were also influenced by diatom responses to anthropogenic forcing, because diatoms form a substantial part of the varve structure. Of all the restoration efforts, a second episode of aeration seems to have had the single most dramatic impact on profundal conditions in the basin, resulting in replacement of a sediment accumulation zone by a transport or erosional zone in the Enonsaari Deep. We conclude that human activities in a lake and its catchment can alter lake hypolimnetic conditions, leading to shifts in lake bottom dynamics and changes in varve preservation.

Waterborne GPR mapping of stratigraphic boundaries and turbidite sediments beneath the bottom of Lake Polevskoye, Karelia, NW Russia

Abstract: We used waterborne Ground Penetrating Radar (GPR) for mapping stratigraphic boundaries, sedimentation environments, and specific features of lacustrine sediments of postglacial Lake Polevskoye, which is located in Eastern Fennoscandia, near Lake Onega. The purpose of our study was to examine the spatial structure of the lacustrine sediments including particular facies represented by the thin interbedding of sand and clay in order to understand the conditions for their formation. The lake bottom morphology and stratigraphic boundaries beneath were mapped with a GPR with 150 MHz antenna unit and then made drill holes to verify GPR interpretations. The GPR measurements, verified by drilling, have vertical accuracy in the range of 0.12–0.36 m. We delineation specific GPR facies in the topmost part of varved clays and classified them as lacustrine turbidites by their structure and grain size. Further studies suggested that turbidites could be deposited by hyperpycnal flows during water discharge from glacial Lake Onega in the period 11.4–10.7 cal ka BP. The architectural implications of the turbidity current in the lake include localized layered sediments 0.5 m thick overlying an erosional surface, which contain unconformable sandy layers and plant debris. A GPR-based 3D model helped us to reconstruct the turbidity current direction, establish its confinement to a lake bottom depression, and consider the associated erosion processes. The near-bottom turbidity current passed through Lake Polevskoye from north to south, along with the bottom depression, and the eastern shore was eroded with a hydraulic jump. The hyperpycnal flows in the study site are associated probably with the last phase deglaciation of the terrain and a drop of the level of Lake Onega because these processes were the most prominent in recent geological history.

Insights into the Connaught sequence of the Timiskaming varve series from Frederick House Lake, northeastern Ontario

Abstract: A sub-bottom acoustic profile survey encountered a mass transport deposit (MTD) bed, 5-7 m thick, interbedded within glaciolacustrine deposits of glacial Lake Ojibway at Frederick House Lake, Ontar...

A new ~ 900-year varved record in Lake Walker, Québec North Shore, eastern Canada: insight on late Holocene climate mode of variability

Abstract: This paper presents a new annually laminated record (varves) from Lake Walker, Quebec North Shore (eastern Canada) spanning the period from ~ 3230 to 2320 ± 20 cal BP. A ~ 3.5-m-long composite sequence was established with the best regular and continuous laminated intervals using computed tomography and high-resolution photographs. The varve chronology was built based on two methods: manual multi-parameter counting using the PeakCounter software, and manual counting on thin-section images obtained by scanning electron microscopy (SEM). The latter correlates more closely with the ages derived from AMS radiocarbon dating, suggesting that thin-section analysis is here a more reliable counting technique. Varves are clastic, composed of a silt layer deposited in spring and summer, and a clay layer deposited in winter. Annually resolved grain size obtained using image analysis technique on SEM images of thin sections and elemental composition from X-ray microfluorescence analyses performed on the floating varve chronology suggests that the record is sensitive to the North Atlantic Oscillation (NAO), as revealed by the strong co-variability with another lower resolution record from Greenland. This suggests that periods of negative winter NAO promoted a thicker snow cover that resulted in higher river discharges and stronger clastic component in the varves. In modern times, cooling of the North Atlantic in the mid 1970s to the late 1980s was also characterized by concurrent negative phase of NAO, which condition translated into increase snow precipitation over the region. Overall, these results highlight that the new Lake Walker varve record presents remarkable prospects of developing a longer and high-resolution paleoclimatological reconstruction of the NAO in a region where similar records are scarce.

Odensjön - A new varved lake sediment record from southern Sweden

Abstract: Annually laminated (varved) sediments provide opportunities to reconstruct climate change and human impact on the environment with high temporal resolution. Depending on the character of the varves, such sediments can provide insight into the delivery of both autochthonous and allochthonous material from the lake and its catch- ment and answer questions regarding both regional and local responses to climate- and land-use changes. This the- sis examines a new sediment record from the small Lake Odensjon in southern Sweden. The aim of the study is to describe the sedimentary sequence, determine the type of laminations and establish if these are annual, and to date the sediment sequence using radiometric methods together with varve counts. The uppermost 910 mm of the sedi- ment sequence was collected with a freeze-corer at 20 m water depth in the deepest part of the lake. Dating with 210Pb and 137Cs in combination with identification of the distinctive and well-documented pollution Pb peak associ- ated with the use of leaded petrol in the mid 1970’s confirms that the sediments partly consist of laminations repre- senting annual varves. Radiocarbon dating of the lower part of the record demonstrates its extension to around 450 years before present. The sediments are characterized by X-ray fluorescence, biogenic silica content, total carbon and nitrogen content and microscopic analyses are showing high contents of biogenic silica and organic matter, abundant diatoms and low minerogenic content, demonstrating that the varves are of biogenic origin and annual. The varve counting was limited by thin and indistinct layers in part of the core. The conditions for varve formation have changed through time and substantial variations in the content of macroscopic plant remains give evidence of pronounced changes in catchment land-use. In the late 19th century the catchment most likely was completely de- forested, while rapid colonization by beech, Fagus sylvatica, occurred during the following century as demonstrat- ed by abundant well-preserved leaves. (Less)