Lake Sediment Deposits Research Articles

Quantifying the dynamic processes of soil erosion and lake sediment deposition in the Holocene in China

The Holocene lake sediment records are critical indicators of past global change studies. This study attempts to reconstruct soil erosion rates (SERs) quantitatively based on lake sediment records from 35 lakes in China. Environmental sequences, e.g., precipitation, vegetation and human activity, were reconstructed and used to identify the dynamic processes of soil erosion based on the General Additive Model (GAM). The findings were as follows. First, drivers of soil erosion in lake basins in different sub-regions of China differed markedly due to the evolution of different monsoon systems and regional environments. Second, influenced by the evolution of different monsoon systems and regional environments, soil erosion of the Holocene showed different evolutionary processes in China. For example, SERs gradually increased in the northwestern arid zone and remained relatively stable in the monsoon zone until 2.0 ka (1 ka = 1000 calibrated a B.P.). Third, human activity played a more important role in soil erosion in all sub-regions during the Holocene. Human activities through vegetation/land-use change have led to rapid enhancement of soil erosion in the north, south, and southwest China since 2.0 ka, especially in the last 1000 years. This study contributes to our understanding of the evolution of soil erosion under different climate systems in China.

Soft Sediment Deformation Structures in the Sediment Deposits of Ganga River and Its Oxbow Lakes in Parts of Indo-Gangetic Plain: Implications for Quake Proxies and Paleoseismicity Tracking

The Late Quaternary thick sedimentary fills of the Ganga basin predominantly consist of unconsolidated fluvial sandy deposits which are often intercalated with mud. These deposits at various places record the evidence of earthquakes, which occurred in the recent geological past. The evidence is contained and manifested in the form of Soft Sediment Deformation Structures (SSDSs). Saturated sediments/muds/soils are liquefied by earthquake tremors which either generate SSDS or produce structural discordance in the pre-existing sedimentary structures. The present study reports the occurrence of SSDS, e.g. load and associated flame structures, clastic dikes and sill structures, slump structures and sedimentary breccias, etc. from the Ganga River and adjacent oxbow lake sediment deposits. An attempt has been made to establish the origin of soft sediment structures of this region in accordance with its neotectonic history and in turn, identification of seismic structural proxies to delineate paleoseismic events in this region with futuristic implications. The preservation of soft sediment deformation structures in large numbers with multiple geomorphology and scale, in the river and adjoining lake sediments, is indicative of frequent earthquakes of high magnitude consequent to tectonic activism in the Himalayan region.

A hydraulic modelling approach to study flood sediment deposition in floodplain lakes

AbstractAbandoned river channels on alluvial floodplains represent areas where sediments, organic matter, and pollutants preferentially accumulate during overbank flooding. Theoretical models describing sedimentation in floodplain lakes recognize the different stages in their evolution, where the threshold for hydrological connectivity increases in older lakes as a plug‐bar develops. Sedimentary archives collected from floodplain lakes are widely used to reconstruct ecological and hydrological dynamics in riverine settings, but how floodplain lake evolution influences flow velocities and sedimentation patterns on an event scale remains poorly understood. Here we combine sediment samples collected in and around a floodplain lake with hydraulic modelling simulations to examine inundation, flow velocity, and sedimentation patterns in a floodplain lake along the Trinity River at Liberty, Texas. We focus our analyses on an extreme flood event associated with the landfall of Hurricane Harvey in August 2017 and develop a series of alternative lake bathymetries to examine the influence of floodplain lake evolution on flow velocity patterns during the flood. We find that sediments deposited in the lake after the Hurricane Harvey flood become thinner and finer with distance from the tie‐channel in accordance with simulated flow velocities that drop with distance from the tie‐channel. Flow velocity simulations from model runs with alternative plug‐bar geometries and lake depths imply that sedimentation patterns will shift as the lake evolves and infills. The integration of sediment sampling and hydraulic model simulations provides a method to understand the processes that govern sedimentation in floodplain lakes during flood events that will improve interpretations of individual events in sedimentary archives from these contexts.

Luminescence chronology of fluvial and aeolian deposits from the Emirate of Sharjah, UAE

AbstractQuaternary environments on the Arabian Peninsula shifted between pronounced arid conditions and phases of increased rainfall, which had a profound impact on Earth surface processes. However, while aeolian sediment dynamics are reasonably well understood, there is a lack of knowledge with regard to variability in the fluvial systems. Presented here are the findings from several locations within wadi drainage systems to the west of the Hajar Mountains (United Arab Emirates). The performance of optically stimulated luminescence dating using a customized standardized growth curve approach is investigated, showing that this approach allows reliable determination of ages by reducing the machine time required. Three main periods of fluvial activity occurred at 160–135, 43–34, and ca. 20 ka. Additional ages fall into the latest Pleistocene and Late Holocene. None of the ages coincides with major wet periods in SE Arabia that have been identified in stalagmites and by the deposition of lake sediments. It is shown that fluvial activity was partly contemporaneous (within the given time resolution) with phases of aeolian deposition and was almost continuous, but likely sporadic, during the Middle to Late Pleistocene. This highlights the need for regionally defined paleoenvironmental records to fully understand the response of dryland systems to long-term climatic change.

Fluvial suspended sediment transfer and lacustrine sedimentation of recent flood turbidites in proglacial Eklutna Lake, western Chugach Mountains, Alaska

AbstractSuspended sediment delivery and deposition in proglacial lakes is generally sensitive to a wide range of hydrometeorologic and geomorphic controls. High discharge conditions are of particular importance in many glaciolacustrine records, with individual floods potentially recorded as distinctive turbidites. We used an extensive network of surface sediment cores and hydroclimatic monitoring data to analyse recent flood turbidites and associated sediment transfer controls over instrumental periods at Eklutna Lake, western Chugach Mountains, Alaska. Close to a decade of fluvial data from primary catchment tributaries show a dominating influence of discharge on sediment delivery, with various interconnections with other related hydroclimatic controls. Multivariate fluvial models highlight and help quantify some complexities in sediment transfer, including intra‐annual variations, meteorological controls, and the influence of subcatchment glacierization. Sediments deposited in Eklutna Lake during the last half century are discontinuously varved and contain multiple distinctive turbidites. Over a 30‐year period of stratigraphic calibration, we correlate the four thickest flood turbidites (1989, 1995, 2006, and 2012) to specific regional storms. The studied turbidites correlate with late‐summer and early‐autumn rainstorms with a magnitude of relatively instantaneous sedimentation 3–15 times greater than annual background accumulation. Our network of sediment core data captured the broad extent and sediment variability among the study turbidites and background sediment yield. Within‐lake spatial modelling of deposition quantifies variable rates of downlake thinning and sediment focusing effects, and highlights especially large differences between the thickest flood turbidites and background sedimentation. This we primarily relate to strongly contrasting dispersion processes controlled by inflow current strength and turbidity. Sediment delivery is of interest for this catchment because of reservoir and water supply operations. Furthermore, although smaller floods may not be consistently represented, the lake likely contains a valuable proxy record of regional flooding proximal to major population centers of south‐central Alaska including Anchorage.

Evolutionary inferences from the sedimentary deposits of Lake LH73, Larsemann Hills, East Antarctica

Lakes from the coastal oasis of Larsemann Hills respond to changes in regional climate. They are mainly influenced by the ice-sheet dynamics and regional eustatic variation based on their proximity to either of them or both. The sedimentary sequences of the ice-free regions of Antarctica offer unique climate records (lake sediment deposits). Here, we present the evolution of a lake reconstructed from its sedimentary sequence. A 60 cm long radiocarbon-dated sediment core spanning the last 11.8 kyr from Lake LH73 located in Broknes Peninsula of Larsemann Hills is studied for grain size fractions, magnetic susceptibility, and diatom abundance. Based on the diatom and grain fraction data, it is inferred that the lake persisted as a proglacial lake during the early-Holocene (11.8 to 7.2 ky BP) dammed by ice-sheet towards the south. The lake level was at least 3 m higher than the present level (4 m) as inferred from the diatom transfer function owing to the presence of ice dam. The retreat of the ice-sheet and the collapse of the ice dam between 7.2 and 7 ky BP resulted in the lake transitioning to an isolated lake with the lake level attaining current level (4 m). As a consequence, the catchment area increased and the lake received meltwater from the snowbanks hence largely modulating the sedimentary process. The absence of glacial clay from 5 ky BP suggests that the lake was cut-off from the glacial input. The appearance of diatom at ~ 8.6 ky BP indicates the establishment of optimal condition for productivity in the lake ecosystem. Stauroforma inermis and Plannothidium quadripunctatum (Psammothidium abundans) dominates between 7 ky BP till present (11.8 to 7 ky BP), responding to lake level variation. Ice-sheet retrieval and ablation led to transition of proglacial-to-isolated lake leading to shallowing of the lake as reflected in diatom assemblage changes.

Refined dating using palaeomagnetic secular variations on a lake sediment core from Guozha Co, northwestern Tibetan Plateau

Lake sediments are widely applied to investigate palaeoclimatic and palaeoenvironmental changes on the Tibetan Plateau (TP). Radiocarbon dating of various materials is one of the most commonly used methods to constrain the timing of lake sediment deposition, but it is problematic for most lakes on the TP due to carbon reservoir effects. Because palaeomagnetic secular variations (PSV) share similar patterns at the regional scale, PSV-stratigraphic correlation can provide an alternative approach to date lake sediments. Here, we present a PSV-depth model for a sediment core taken from Guozha Co, a glacial meltwater-fed lake on the northwestern TP. To establish reliable chronological constraints on the sediment core, we compared our PSV-depth model with the PSV stack of East Asia, PSV records of Tangra Yumco and Lake Baikal. The establishment of this new chronology at Guozha Co is a major step toward dating lake records more widely in the TP to reveal palaeoclimatic changes at the world's ‘Third Pole’.

Comparison of published palaeoclimate records suitable for reconstructing annual to sub-decadal hydroclimatic variability in eastern Australia: implications for water resource management and planning

Abstract. Knowledge of past, current, and future hydroclimatic risk is of great importance. However, like many other countries, Australia's observed hydroclimate records are at best only ∼ 120 years long (i.e. from ∼ 1900 to the present) but are typically less than ∼ 50 years long. Therefore, recent research has focused on developing longer hydroclimate records based on palaeoclimate information from a variety of different sources. Here we review and compare the insights emerging from 11 published palaeoclimate records that are relevant for annual to sub-decadal hydroclimatic variability in eastern Australia over the last ∼ 1000 years. The sources of palaeoclimate information include ice cores, tree rings, cave deposits, and lake sediment deposits. The published palaeoclimate information was then analysed to determine when (and where) there was agreement (or uncertainty) about the timing of wet and dry epochs in the pre-instrumental period (1000–1899). The occurrence, frequency, duration, and spatial extent of pre-instrumental wet and dry epochs was then compared to wet and dry epochs since 1900. The results show that instrumental records (∼ 1900–present) underestimate (or at least misrepresent) the full range of rainfall variability that has occurred, and is possible, in eastern Australia. Even more disturbing is the suggestion, based on insights from the published palaeoclimate data analysed, that 71 % of the pre-instrumental period appears to have no equivalent in the instrumental period. This implies that the majority of the past 1000 years was unlike anything encountered in the period that informs water infrastructure, planning, and policy in Australia. A case study, using a typical water storage reservoir in eastern Australia, demonstrates that current water resource infrastructure and management strategies would not cope under the range of pre-instrumental conditions that this study suggests has occurred. When coupled with projected impacts of climate change and growing demands, these results highlight some major challenges for water resource management and infrastructure. Though our case study location is eastern Australia, these challenges, and the limitations associated with current methods that depend on instrumental records that are too short to realistically characterise interannual to multi-decadal variability, also apply globally.

Persistent millennial-scale climate variability in Southern Europe during Marine Isotope Stage 6

Exploring the mode and tempo of millennial-scale climate variability under evolving boundary conditions can provide insights into tipping points in different parts of the Earth system, and can facilitate a more detailed understanding of climate teleconnections and phase relationships between different Earth system components. Here we use fossil diatom and stable carbon and oxygen isotope analysis of lake sediment deposits (core I-284) from the Ioannina basin, NW Greece, to explore in further detail millennial-scale climate instability in southern Europe during Marine Isotope Stage 6 (MIS 6; ca. 185–130 ka). This interval correlates with the Vlasian Stage in Greece and the Late Saalian Substage in northern Europe, which were both characterised by extensive glaciations. The new dataset resolves at least 18 discrete warmer/wetter intervals, many of which were associated with strong Asian Monsoon events and North Atlantic interstadials. A number of cooler/drier intervals are also identified in the I-284 record, which are typically associated with weaker Asian Monsoon events and North Atlantic stadials, consistent with a variable Atlantic Meridional Overturning Circulation. Unlike the subdued changes in tree populations that are observed at Ioannina during mid-to-late MIS 6, the diatom record contains frequent high-amplitude oscillations in species assemblages, pointing to its sensitivity at a time when the lake system must have been close to environmental thresholds. Millennial-scale variability in diatom species assemblages continues into late MIS 6 at Ioannina, contributing important evidence for an emerging picture of frequent and persistent climate instability even at times of high global ice volume.

Stratigraphic evolution of the northern part of the Cretaceous Neungju Basin, South Korea

Several nonmarine basins were formed in SW Korea under an extensional or transtensional tectonic regime during the Cretaceous. The Neungju Basin is one of these basins, filled by alluvial to lacustrine deposits interbedded with thick, amalgamated tuff beds. Facies analysis in the northern part of the basin shows that the basin fill is composed of four facies associations, representing (1) alluvial fan, (2) alluvial plain, (3) sandflat, and (4) marginal playa lake environments. The basin fill can be subdivided into four stratigraphic units by interbedded thick amalgamated tuff beds. The lowermost unit (unit I) is composed of alluvial plain deposits. The lack of any vertical depositional trend indicates a balance between basin subsidence and sediment supply. The middle unit (units II and III) show regional variations. The extensive lake formed in the central to northern parts of the basin, which are later partly filled by proximal sediments from surrounding areas. In contrast, only alluvial plain to sandflat environments occurred in the southern part. The uppermost unit (unit IV) shows a transition from alluvial fan to sandflat, comprising a fining-upward trend that reflects a gradual decrease in sediment supply and slope-gradient during denudation of uplifted source areas. The variations in depositional patterns were possibly caused by the relationship between volcanism-driven sediment supply and basin subsidence. Basin subsidence may have been sufficient to compensate the volcanism-driven sediment supply prior to the deposition of units II and III, resulting in the conformable deposition of lake sediments over thick amalgamated tuff beds. In contrast, volcanism-driven sediment supply may have reduced the generation of accommodation prior to the deposition of unit IV, resulting in the deposition of conglomerates over thick tuff beds. The traditional depositional model composed of syn-eruption aggradation and inter-eruption degradation may not sufficiently explain the development of sedimentary succession, in which the balance between volcanism-driven sediment supply versus basin subsidence coeval to volcanism is variable.

Nonlinear vegetation cover changes in the North Ethiopian Highlands: Evidence from the Lake Ashenge closed basin

Vegetation cover changes in African drylands are often thought to result from population growth, social factors and aridification. Here we show that long-term vegetation proxy records can help disentangling these main driving factors. Taking the case of North Ethiopia, we performed an integrated investigation of land cover changes over the last four centuries around the endorheic Lake Ashenge, as derived from pollen analysis and repeat photography complemented with information from historical sources. Pollen and sediment analysis of radiocarbon-dated lake deposits shows a phase of environmental destabilization during the 18th century, after a more stable previous period. This is evidenced by decreases of tree pollen (Juniperus, Olea, Celtis, Podocarpus <5%), increases in Poaceae (>40%) and deposition of coarser silt lake sediments (>70%). Quantitative analysis of 30 repeated landscape photographs around the lake indicates a gradual decline of the vegetation cover since a relative maximum during the mid-19th Century. Vegetation cover declined sharply between the 1950s and the 1980s, but has since begun to recover. Overall, the data from around Lake Ashenge reveal a nonlinear pattern of deforestation and forest regrowth with several periods of vegetation cover change over the past four centuries. While there is forcing of regional drought and the regional land tenure system, the cyclic changes do not support a simplified focus on aridification or population growth.

The application of sediment fingerprinting to floodplain and lake sediment cores: assumptions and uncertainties evaluated through case studies in the Nene Basin, UK

Fine sediment has been shown to be a major cause of the degradation of lakes and rivers and, as a result, research has been directed towards the understanding of fine sediment dynamics and the minimisation of sediment inputs. The use of tracers within a sediment fingerprinting framework has become a heavily used technique to investigate the sources of fine sediment pressures. When combined with the use of historically deposited sediment, the technique provides the opportunity to reconstruct past changes to the environment. However, alterations to tracer signatures during sediment transport and storage are a major potential source of uncertainty associated with tracer use. At present, few studies have quantified the uncertainties associated with tracer use. This paper investigated uncertainty by determining the differences between sediment provenance predictions obtained using lithogenic radionuclide, geochemical and mineral magnetic signatures when fingerprinting lake and floodplain sedimentary deposits. It also investigated the potential causes of the observed differences. A reservoir core was fingerprinted with the least uncertainty, with tracer group predictions ∼28 % apart and a consistent down-core trend in changing sediment provenance produced. When fingerprinting an on-line lake core and four floodplain cores, differences between tracer group predictions were as large as 100 %; the down-core trends in changing sediment provenance were also different. The differences between tracer group predictions could be attributed to the organic matter content and particle size of the sediment. There was also evidence of the in-growth of bacterially derived magnetite and chemical dissolution affecting the preservation of tracer signatures. Simple data corrections for sediment organic matter content and particle size did not result in significantly greater agreement between the predictions of the different tracer groups. Likewise, the inclusions of weightings for tracer discriminatory efficiency and within-source variability had minimal effects on the fingerprinting results. This paper highlights the importance of tracer selection and the consideration of recognising tracer non-conservatism when using lake and floodplain sediment deposits to reconstruct anthropogenic changes to the environment and changing sediment dynamics. It was recommended that future research focus on the assessment of uncertainty using the artificial mixing of sediment source samples, the limitation of the fingerprinting to narrow particle size fractions and the development of specific particle size and organic matter correction factors for each tracer.

Terrestrial and submarine evidence for the extent and timing of the Last Glacial Maximum and the onset of deglaciation on the maritime-Antarctic and sub-Antarctic islands

This paper is the maritime and sub–Antarctic contribution to the Scientific Committee for Antarctic Research (SCAR) Past Antarctic Ice Sheet Dynamics (PAIS) community Antarctic Ice Sheet reconstruction. The overarching aim for all sectors of Antarctica was to reconstruct the Last Glacial Maximum (LGM) ice sheet extent and thickness, and map the subsequent deglaciation in a series of 5000 year time slices. However, our review of the literature found surprisingly few high quality chronological constraints on changing glacier extents on these timescales in the maritime and sub–Antarctic sector. Therefore, in this paper we focus on an assessment of the terrestrial and offshore evidence for the LGM ice extent, establishing minimum ages for the onset of deglaciation, and separating evidence of deglaciation from LGM limits from those associated with later Holocene glacier fluctuations. Evidence included geomorphological descriptions of glacial landscapes, radiocarbon dated basal peat and lake sediment deposits, cosmogenic isotope ages of glacial features and molecular biological data. We propose a classification of the glacial history of the maritime and sub–Antarctic islands based on this assembled evidence. These include: (Type I) islands which accumulated little or no LGM ice; (Type II) islands with a limited LGM ice extent but evidence of extensive earlier continental shelf glaciations; (Type III) seamounts and volcanoes unlikely to have accumulated significant LGM ice cover; (Type IV) islands on shallow shelves with both terrestrial and submarine evidence of LGM (and/or earlier) ice expansion; (Type V) Islands north of the Antarctic Polar Front with terrestrial evidence of LGM ice expansion; and (Type VI) islands with no data. Finally, we review the climatological and geomorphological settings that separate the glaciological history of the islands within this classification scheme.

Modeling soil erosion and river sediment yield for an intermountain drainage basin of the Central Apennines, Italy

The overall aim of this research was to investigate the spatial patterns of the soil erosion risk. We focused on accelerated soil erosion processes in an Italian central Apennine intermountain watershed using modeling techniques implemented in a GIS environment. Our thorough literature review revealed a gap in research on soil erosion processes in such forested, intermountain watersheds. To gain a better understanding of the soil erosion processes in such landscapes, we proposed an integrated modeling approach applying a RUSLE model and a Turbidity Unit Index. The model outcomes were validated through measurements of lake sediment deposition. Our findings indicate a potential high soil erosion risk. With 1.33Mt−1yr−1 of annual sediment yield, corresponding to an area-specific sediment yield of 32.35tha−1yr−1, the Turano drainage basin belongs to the Italian basins with the highest sediment discharge. The outcomes of the RUSLE model showed that, despite the diverse forms of forests that cover about 62% of the drainage basin area, sizable plots of the investigated area are prone to soil erosion. The validation of the model outcomes revealed that the TU Index model performed significantly better than the RUSLE model with regard to sediment yield prediction. Accordingly, we found that even though rill and interrill processes reach very alarming values (RUSLE), they are not the dominant sediment source within the Turano watershed. Other geomorphological processes contributing to the watershed sediment yield – for instance, megarill, gully, bank and channel erosion and re-entrainment of landslide sediments – were very active in the study area. If both models are used in a combined approach, the amount of river load (TU Index) as well as the relative spatial distribution of rill and interrill erosion processes (RUSLE) can be described with sufficient precision.

Land use and climate change impacts on lake sedimentation rates in western Canada

Fine sediment derived from catchment erosion can adversely impact aquatic ecosystems. Previous studies of lake sediment deposits in western Canada attributed increased sedimentation to land use; however, high catchment variability and short- and long-term climatic responses complicated the interpretation of those regional records. We compiled a large inventory of lake catchment data transecting the Canadian cordillera that comprised 210Pb-based profiles of deposition, GIS-based land use records, and interpolated climate change data. We used these data and mixed-effects modeling to relate sedimentation trends to land use and climate change since the mid-20th century. Although sedimentation was highly variable, increasing trends in accumulation corresponded with cumulative land use and, to a lesser degree, with climate change. Road density was the most important variable, but the inclusion of timber harvesting density further improved model fits significantly. Land use effects were more difficult to discern for the easternmost region of the Alberta Plateau where sedimentation appeared to be cumulatively associated with both timber and energy resource extraction. Stronger relations were obtained with whole catchment (0.50–273 km2) measures of land use, suggesting that fine sediment is efficiently transferred from hillslopes to the lake basins. While accounting for land use, a climate signal was also detected with improved model fits obtained with the inclusion of catchment warming. Further modeling results suggest that at least one critical control of reconstructed sedimentation remains undetermined. Interdecadal rates of lake sedimentation in western Canada have steadily increased during the late 20th century, following patterns of regional environmental change.

A whole-basin, mass-balance approach to paleolimnology

Lake sediments record the flux of materials (nutrients, pollutants, particulates) through a lake system both qualitatively, as changes in the composition of geochemical and biological tracers, as well as quantitatively, through changes in their rate of burial. Burial rates provide a direct link to contemporary (neo-) limnological studies as well as management efforts aimed at load reductions, but are difficult to reconstruct accurately from single cores owing to the spatial and temporal variability of sediment deposition in most lakes. The accurate determination of whole-lake burial rates from analysis of multiple cores, though requiring more effort per lake, can help resolve such problems and improve our understanding of sediment heterogeneity at multiple scales. Partial solutions to these problems also include focusing corrections based on 210Pb flux, co-evaluation of concentration profiles, trend analysis using multiple lakes, and trend replication based on a small number of cores from the same lake. Recent multi-core studies demonstrate that no single core site faithfully records the whole-lake time-resolved input of materials, but that as few as five well-placed cores can provide a reliable record of whole-lake sediment flux for morphometrically simple basins. Lake-wide sediment fluxes can be coupled with reconstructed outflow losses to calculate historical changes in watershed and atmospheric loading of nutrients, metals, and other constituents. The ability of paleolimnology to accurately assess the sedimentary flux and extend the period of reference into the distant past represents an important contribution to the understanding of biogeochemical processes and their response to human and natural disturbance.

Organic, elemental, and geochemical contributions to lake sediment deposits during severe spruce beetle (Dendroctonus rufipennis) disturbances

Recent outbreaks of native bark beetles are unprecedented during the historical period. The aim of this manuscript is to develop a proxy-based methodology to infer past bark beetle outbreaks using lake sediments to provide long-term context for recent outbreaks. We test three hypotheses to determine how the ecological impacts of severe spruce beetle (Dendroctonus rufipennis Kirby) disturbances are recorded in lake sediment deposits. The resulting mortality and defoliation of Engelmann spruce is hypothesized to: (1) decrease the ratio of spruce to fir pollen; (2) reduce canopy interception of precipitation leading to an increase in soil erosion and/or enhanced mobilization of terrestrial carbon; and (3) leach foliar nitrogen and enhance algal productivity resulting in increased nitrogen values in lake sediments. To test these hypotheses, we analyzed sediment cores from six spruce beetle-affected basins in Utah for spruce/fir pollen ratio (hypothesis 1), loss-on-ignition, magnetic susceptibility of sediments (hypothesis 2), and δ13CBOM, δ15NTN, elemental C and N, and the C/N ratio of bulk organic material (hypotheses 2 and 3). The dataset was statistically tested using general linear mixed models (GLMMs) to determine if the response variables differed significantly between outbreak and non-outbreak period. The spruce/fir pollen ratio responded significantly to outbreaks at all sites suggesting that this metric may be the most suitable for identifying past spruce beetle outbreaks. For our second hypothesis we found little support for an influx of terrestrial C due to strongly individualist responses of the basins. For our third hypotheses we found little support for increased sedimentary nitrogen, likely due to alterations to nutrient cycling from human activities. Therefore the host/non-host pollen ratio provides the most promising metric for detecting past outbreaks.

Eastern Beringia and beyond: Late Wisconsinan and Holocene landscape dynamics along the Yukon Coastal Plain, Canada

Terrestrial permafrost archives along the Yukon Coastal Plain (northwest Canada) have recorded landscape development and environmental change since the Late Wisconsinan at the interface of unglaciated Beringia (i.e. Komakuk Beach) and the northwestern limit of the Laurentide Ice Sheet (i.e. Herschel Island). The objective of this paper is to compare the late glacial and Holocene landscape development on both sides of the former ice margin based on permafrost sequences and ground ice. Analyses at these sites involved a multi-proxy approach including: sedimentology, cryostratigraphy, palaeoecology of ostracods, stable water isotopes in ground ice, hydrochemistry, and AMS radiocarbon and infrared stimulated luminescence (IRSL) dating. AMS and IRSL age determinations yielded full glacial ages at Komakuk Beach that is the northeastern limit of ice-free Beringia. Herschel Island to the east marks the Late Wisconsinan limit of the northwest Laurentide Ice Sheet and is composed of ice-thrust sediments containing plant detritus as young as 16.2calkaBP that might provide a maximum age on ice arrival. Late Wisconsinan ice wedges with sediment-rich fillings on Herschel Island are depleted in heavy oxygen isotopes (mean δ18O of −29.1‰); this, together with low d-excess values, indicates colder-than-modern winter temperatures and probably reduced snow depths. Grain-size distribution and fossil ostracod assemblages indicate that deglaciation of the Herschel Island ice-thrust moraine was accompanied by alluvial, proluvial, and eolian sedimentation on the adjacent unglaciated Yukon Coastal Plain until ~11calkaBP during a period of low glacio-eustatic sea level. The late glacial–Holocene transition was marked by higher-than-modern summer temperatures leading to permafrost degradation that began no later than 11.2calkaBP and caused a regional thaw unconformity. Cryostructures and ice wedges were truncated while organic matter was incorporated and soluble ions were leached in the thaw zone. Thermokarst activity led to the formation of ice-wedge casts and deposition of thermokarst lake sediments. These were subsequently covered by rapidly accumulating peat during the early Holocene Thermal Maximum. A rising permafrost table, reduced peat accumulation, and extensive ice-wedge growth resulted from climate cooling starting in the middle Holocene until the late 20th century. The reconstruction of palaeolandscape dynamics on the Yukon Coastal Plain and the eastern Beringian edge contributes to unraveling the linkages between ice sheet, ocean, and permafrost that have existed since the Late Wisconsinan.

Stable lead (Pb) isotopes and concentrations – A useful independent dating tool for Baltic Sea sediments

The prehistory of the Baltic Sea has for a long time suffered from imprecise dating, due to the large uncertainties associated with bulk radiocarbon dating of Baltic Sea sediments. To constrain the timing of environmental changes in the Baltic Sea it is critical to apply new dating approaches. This study identifies lead pollution isochrones in Baltic Sea sediments, which have previously been recorded in lake sediment and peat deposits in northern Europe and ice cores from Greenland. These isochrones have formed through the deposition of atmospheric lead associated with historic lead production and silver mining in Europe, and more recently with the increased industrial emissions that peaked in the 1970’s. Lead concentration and stable lead isotope analyses (206Pb/207Pb ratios) reveal three distinct lead pollution horizons in the Baltic Sea, i.e. a Roman peak at 1 AD, a Medieval peak at 1200 AD and a peak in the 1970s. The new data will improve the chronological accuracy and precision of paleoenvironmental studies in the Baltic Sea, and for the first time, allow synchronization of Baltic Sea geological records within the basin and across Europe and the North Atlantic region (including Greenland).

Optical ages indicate the southwestern margin of the Green Bay Lobe in Wisconsin, USA, was at its maximum extent until about 18,500 years ago

Samples for optical dating were collected to estimate the time of sediment deposition in small ice-marginal lakes in the Baraboo Hills of Wisconsin. These lakes formed high in the Baraboo Hills when drainage was blocked by the Green Bay Lobe when it was at or very near its maximum extent. Therefore, these optical ages provide control for the timing of the thinning and recession of the Green Bay Lobe from its maximum position. Sediment that accumulated in four small ice-marginal lakes was sampled and dated. Difficulties with field sampling and estimating dose rates made the interpretation of optical ages derived from samples from two of the lake basins problematic. Samples from the other two lake basins—South Bluff and Feltz basins—responded well during laboratory analysis and showed reasonably good agreement between the multiple ages produced at each site. These ages averaged 18.2ka (n=6) and 18.6ka (n=6), respectively. The optical ages from these two lake basins where we could carefully select sediment samples provide firm evidence that the Green Bay Lobe stood at or very near its maximum extent until about 18.5ka.The persistence of ice-marginal lakes in these basins high in the Baraboo Hills indicates that the ice of the Green Bay Lobe had not experienced significant thinning near its margin prior to about 18.5ka. These ages are the first to directly constrain the timing of the maximum extent of the Green Bay Lobe and the onset of deglaciation in the area for which the Wisconsin Glaciation was named.