Minerogenic Sediments Research Articles

Environmental variability in northeastern Siberia during the last ~ 13,300 yr inferred from lake diatoms and sediment–geochemical parameters

In northeastern Siberia, Russia, a 1.2m sediment core was retrieved and radiocarbon dated from a small and shallow lake located at the western side of the lower Lena River (N 69°24′, E 123°50′, 81ma.s.l.). The objective of this paper is to reconstruct the palaeoenvironmental variability and to infer major palaeoclimate trends that have occurred since ~13.3cal. kyr BP. We analysed the diatom assemblages, sedimentology (grain-size, total organic carbon (TOC), total nitrogen (TN)), and the elemental and mineralogical composition using X-ray fluorescence (XRF) and X-ray diffractometry (XRD) of the sediment core. Our results show parallel changes in the diatom species composition and sediment characteristics. Enhanced minerogenic sediment input and the occurrence of pyrite is indicative of a cold period between ~12.7 and 11.6cal. kyr BP. The diatom data enable a qualitative inference about the local ecological conditions to be made, and reveal an oligotrophic lake system with alkaline and cold conditions during the earliest Holocene. Moderately warmer climates are inferred for the period from ~9.1 to 5.7cal. kyr BP. The major shift in the diatom assemblage, from dominance of small benthic fragilarioid taxa to a more complex diatom flora with an influx of several achnanthoid and naviculoid diatom species, occurred after a transitional period of about 1400yr (7.1 to 5.7cal. kyr BP) at ~5.7cal. kyr BP, indicating a circumneutral and warmer hydrological regime during the Holocene thermal maximum (HTM). Diatom valve concentrations declined starting ~2.8cal. kyr BP, but have been rising again since ≤600cal. yr BP. This has occurred in parallel to the increased presence of acidophilous diatom taxa (e.g. Eunotia spp.) and decreased presence of small benthic fragilarioid species in the most recent sediments, which is interpreted as the result of neoglacial cooling and subsequent recent climate warming. Our findings are compared to other lake-inferred climate reconstructions along the Lena River. We conclude that the timing and spatial variability of the HTM in the lower Lena River area reveal a temporal delay from north to south.

Tracking hydrological responses of a thermokarst lake in the Old Crow Flats (Yukon Territory, Canada) to recent climate variability using aerial photographs and paleolimnological methods

AbstractRecent studies using remote sensing analysis of lake‐rich thermokarst landscapes have documented evidence of declining lake surface area in response to recent warming. However, images alone cannot identify whether these declines are due to increasing frequency of lake drainage events associated with accelerated thermokarst activity or to increasing evaporation in response to longer ice‐free season duration. Here, we explore the potential of combining aerial photograph time series with paleolimnological analyses to track changes in hydrological conditions of a thermokarst lake in the Old Crow Flats (OCF), Canada, and to identify their causes. Images show that the water level in lake OCF 48 declined markedly sometime between 1972 and 2001. In a sediment core from OCF 48, complacent stratigraphic profiles of several physical, geochemical, and biological parameters from ∼1874–1967 indicate hydro‐limnological conditions were relatively stable. From ∼1967–1989, declines in organic matter content, organic carbon isotope values, and pigment concentrations are interpreted to reflect an increase in supply of minerogenic sediment, and subsequent decline in aquatic productivity, caused by increased thermo‐erosion of shoreline soils. Lake expansion was likely caused by increased summer rainfall, as recorded by increased cellulose‐inferred lake‐water oxygen isotope compositions. Stratigraphic trends defining the lake expansion phase terminated at ∼1989, which likely marks the year when the lake drained. Above‐average precipitation during the previous year probably raised the lake level and promoted further thermo‐erosion of the shoreline soils that caused the lake to drain. These are meteorological conditions that have led to other recent lake‐drainage events in the OCF. Thus, the decline in lake level, evident in the aerial photograph from 2001, is unlikely to have been caused by evaporation, but rather is a remnant of a drainage event that took place more than a decade earlier. After drainage, the lake began to refill, and most paleolimnological parameters approach levels that are similar to those during the stable phase. These findings indicate that combined use of aerial images and paleolimnological methods offers much promise for identifying the hydrological consequences of recent climatic variations on thermokarst lakes. Copyright © 2011 John Wiley & Sons, Ltd.

The Lateglacial and early Holocene vegetation and environment in the Dovre mountains, central Norway, as signalled in two Lateglacial nunatak lakes

By using heavy coring equipment in two high-altitudinal lakes (1253 and 1316 m a.s.l.) at Dovre, Central Norway, 1–1.5 m of unsorted coarsely minerogenic sediments were retrieved below the Holocene organic sediments. The minerogenic sequence contained well-preserved pollen and chironomid remains, revealing new and detailed palaeoenvironmental knowledge of the mountains in Central Norway during the last 5–6000 years of the Lateglacial (LG) period. However, the LG chronology is based on biostratigraphical correlations and not on 14C-dates, due to low organic content in the minerogenic sediments. The emerging LG nunataks, probably indicating a thin and multi-domed Scandinavian ice-sheet, was rapidly inhabited by immigrating species which could explain the present centric distributions of certain arctic-alpine plants. The LG vegetation development included a pre-interstadial dominated by mineral-soil pioneers, an interstadial dominated by shrubs and dwarf-shrubs, and the Younger Dryas cold period with recurring dominance of pioneers. Pollen and stomata of Pinus and Picea indicate their local LG presence at Dovre. LG climate oscillations are indicated by pollen stratigraphy and for the later part of LG also by chironomids. These oscillations could correspond to Heinrich event 1, GI-1d, GI-1b, and the Younger Dryas cold events. The LG interstadial reached July mean temperatures of more than 7–8 °C, similar to the present. Chironomids colonized the lake already during the onset of the interstadial, albeit at very low richness and abundances. Starting from YD, there are sufficient chironomid head capsules to perform a temperature reconstruction. The Holocene warming of about 2 °C initiated a vegetation closure from snow beds and dwarf-shrub tundra to shrubs and forests. Birch-forests established about 10 ka cal BP, slightly earlier than pine forests. Alnus expanded ca 9.2 ka cal BP and a thinning of the local forests occurred from ca 7 ka cal BP. Two short-lasting climate deteriorations found in the pollen record and the chironomid record may represent the Preboreal Oscillation and the 8.2 event. The Holocene Thermal Maximum is indicated around ca 7.8–7.3 ka cal BP showing a chironomid-inferred July mean of at least 11 °C. This is ca 3 °C warmer than today.

Paleoclimate changes inferred from stable isotopes and magnetic properties of organic-rich lake sediments in Arctic Norway

Stable isotope measures in organic matter are frequently used as indicators of past climate change. Although such analyses can provide valuable information, there is considerable uncertainty associated with studies of organic-rich sediments, especially those from Arctic lakes and bogs. We studied stable isotopes of carbon and nitrogen, and magnetic properties in a sediment core from a small alkaline lake with a high sedimentation rate, Lake Nattmålsvatn, Norway. There is good correspondence among the different sediment variables during the late glacial, and they seemingly reflect major climate variations such as the Allerød Interstade and the Younger Dryas, as well as the transition into the current interglacial. During the early Holocene, however, these relationships are more complex and δ13C and δ15N values do not stabilize until ~7,500 cal year BP. A significant excursion in all variables occurs between 6,850 and 6,500 cal year BP and is interpreted to represent climate deterioration. Holocene δ13C values vary little and indicate that isotopically-depleted dissolved inorganic carbon (DIC) in the lake, possibly influenced by methanotrophy and high pCO2, dominated the lake’s carbon cycle. Holocene δ15N is similarly muted, likely due to the availability of abundant dissolved nitrogen. Bulk organic matter is probably dominated by phytoplankton remains produced beneath the ice cover in late spring and during ice breakup when isotopically-depleted DIC, pCO2 and ammonium availability were maximal. Thus, use of δ13C and δ15N as indicators of Holocene paleoclimate and paleoproductivity variation can be challenging in a lake such as Nattmålsvatn, where ice cover isolates the basin for large parts of the year, allowing dissolved respiratory gases to accumulate in the water column. In contrast, magnetic variables appear to better track climate variations. In particular, runoff-driven influx of minerogenic sediments shows high variability that can be attributed to regional changes in Holocene winter precipitation. The most striking shifts occur between 4,000 and 2,300 cal year BP.

Highly episodic fire and erosion regime over the past 2,000 y in the Siskiyou Mountains, Oregon

Fire is a primary mode of natural disturbance in the forests of the Pacific Northwest. Increased fuel loads following fire suppression and the occurrence of several large and severe fires have led to the perception that in many areas there is a greatly increased risk of high-severity fire compared with presettlement forests. To reconstruct the variability of the fire regime in the Siskiyou Mountains, Oregon, we analyzed a 10-m, 2,000-y sediment core for charcoal, pollen, and sedimentological data. The record reveals a highly episodic pattern of fire in which 77% of the 68 charcoal peaks before Euro-American settlement cluster within nine distinct periods marked by a 15-y mean interval. The 11 largest charcoal peaks are significantly related to decadal-scale drought periods and are followed by pulses of minerogenic sediment suggestive of rapid sediment delivery. After logging in the 1950s, sediment load was increased fourfold compared with that from the most severe presettlement fire. Less severe fires, marked by smaller charcoal peaks and no sediment pulses, are not correlated significantly with drought periods. Pollen indicators of closed forests are consistent with fire-free periods of sufficient length to maintain dense forest and indicate a fire-triggered switch to more open conditions during the Medieval Climatic Anomaly. Our results indicate that over millennia fire was more episodic than revealed by nearby shorter tree-ring records and that recent severe fires have precedents during earlier drought episodes but also that sediment loads resulting from logging and road building have no precedent in earlier fire events.

Scanning reflectance spectroscopy (380–730 nm): a novel method for quantitative high-resolution climate reconstructions from minerogenic lake sediments

High-resolution (annual to sub-decadal) quantitative reconstructions of climate variables are needed from a variety of paleoclimate archives across the world to place current climate change in the context of long-term natural climate variability. Rapid, high-resolution, non-destructive scanning techniques are required to produce such high-resolution records from lake sediments. In this study we explored the potential of scanning reflectance spectroscopy (VIS-RS; 380–730 nm) to produce quantitative summer temperature reconstructions from minerogenic sediments of proglacial, annually laminated Lake Silvaplana, in the eastern Swiss Alps. The scanning resolution was 2 mm, which corresponded to sediment deposition over 1–2 years. We found correlations up to r = 0.84 (p < 0.05) for the calibration period 1864–1950, between six reflectance-dependent variables and summer (JJAS) temperature. These reflectance-dependent variables (e.g. slope of the reflectance 570/630 nm, indicative of illite, biotite and chlorite; minimum reflectance at 690 nm indicative of chlorite) indicate the mineralogical composition of the clastic sediments, which is, in turn, related to climate in the catchment of this particular proglacial lake. We used multiple linear regression (MLR) to establish a calibration model that explains 84% of the variance of summer (JJAS) temperature during the calibration period 1864–1950. We then applied the calibration model downcore to develop a quantitative summer temperature reconstruction extending back to AD 1177. This temperature reconstruction is in good agreement with two independent temperature reconstructions based on documentary data that extend back to AD 1500 and tree ring data that extend back to AD 1177. This study confirms the great potential of in situ scanning reflectance spectroscopy as a novel non-destructive technique to rapidly acquire high-resolution quantitative paleoclimate information from minerogenic lake sediments.

Terrestrial and aquatic ecosystem responses to late Holocene climate change recorded in the sediments of Lochan Uaine, Cairngorms, Scotland

We summarise the results of a range of sediment-based studies at Lochan Uaine, a remote corrie lake in the heart of the Cairngorm massif in Scotland. The site lies above the Holocene forest limit and has been minimally affected by human activities. The results presented are mainly based on magnetic measurements, element analysis, granulometry, organic geochemical analysis and pollen analysis carried out over a period of some 15 years. The magnetic properties and element concentrations record a coherent sequence of changes reflecting mainly stages in catchment erosion. In terms of the chronology developed for the sedimentary record from the site, increases in allochthonous, minerogenic sediment delivery to the lake occurred around 1000 BC, AD 330–480 and AD 1260–1410. The only notable change in the pollen diagram records a period of deforestation at lower altitude predating the last of the periods of increased erosion. The organic geochemistry analyses record a series of higher frequency responses in the aquatic ecosystem, already noted in previous papers, e.g. Battarbee et al. (2001). These include fluctuations in organic carbon content and in the concentrations of biomarkers indicative of changing lake productivity. Both the terrestrial and aquatic ecosystem responses are superimposed on a longer-term trend of declining aquatic productivity, progressive catchment weathering and increasing erosion. The sediments of Lochan Uaine thus appear to have recorded complex system responses on three timescales reflecting (a) the long term decline in northern hemisphere insolation during the Holocene, (b) the millennial scale forcing of the kind found in many other mid-late Holocene records and (c) much shorter term, quasi-cyclic but clearly a-periodic sub-millennial fluctuations.

Holocene alpine glaciation inferred from lacustrine sediments on northeastern Baffin Island, Arctic Canada

AbstractWith accelerated melting of alpine glaciers, understanding the future state of the cryosphere is critical. Because the observational record of glacier response to climate change is short, palaeo‐records of glacier change are needed. Using proglacial lake sediments, which contain continuous and datable records of past glacier activity, we investigate Holocene glacier fluctuations on northeastern Baffin Island. Basal radiocarbon ages from three lakes constrain Laurentide Ice Sheet retreat by ca. 10.5 ka. High sedimentation rates (0.03 cm a−1) and continuous minerogenic sedimentation throughout the Holocene in proglacial lakes, in contrast to organic‐rich sediments and low sedimentation rates (0.005 cm a−1) in neighbouring non‐glacial lakes, suggest that glaciers may have persisted in proglacial lake catchments since regional deglaciation. The presence of varves and relatively high magnetic susceptibility from 10 to 6 ka and since 2 ka in one proglacial lake suggest minimum Holocene glacier extent ca. 6–2 ka. Moraine evidence and proglacial and threshold lake sediments indicate that the maximum Holocene glacier extent occurred during the Little Ice Age. The finding that glaciers likely persisted through the Holocene is surprising, given that regional proxy records reveal summer temperatures several degrees warmer than today, and may be due to shorter ablation seasons and greater accumulation‐season precipitation. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Climatic influence on the inter‐annual variability of late‐Holocene minerogenic sediment supply in a boreal forest catchment

AbstractProcesses controlling sediment yield vary over a range of timescales, although most process‐based observations are extremely short. Lake sediments, however, can be used to extend the observational timescale and are particularly useful when annually laminated (varved) sediment is present. The sediment record at Kassjön (N. Sweden) consists of ∼6400 varves, each 0·5–1 mm thick. Image analysis was used to determine grey‐scale variation and varve thickness from which annual minerogenic accumulation rate (MinAR) (mg cm−2 year−1) was inferred for the period 4486 BC – AD 1900. MinAR varies on annual to centennial scales and mainly reflects channel bank erosion by the inflow streams. The mineral input reflects the intensity of the spring run‐off, which is dependent on the amount of snow accumulated during the winter, and hence MinAR is a long‐term record of variability in past winter climate; other factors will be a variable response to catchment uplift, vegetation succession and pedogenesis. A major shift from low to high MinAR occurred ∼250 BC, and peaks occurred around AD 250, 600, 1000, 1350 and 1650. Wavelet power spectrum analysis (confirmed by Fourier analyses) indicated significantly different periodicities throughout the period 4000 BC – AD 1700, including 275 years for the period 4000 BC – 2900 BC, 567 years for the period 2901 BC – 1201 BC, and 350 and 725 years for the period 1200 BC – AD 1700. The long‐term, centennial scale variability (∼350 years) may reflect solar forcing (cf the 385‐year peak in tree‐ring calibrated 14C activity) but interestingly, there is no obvious link to high frequency forcing, such as the North Atlantic Oscillation. The high resolution component of the record highlights the relevance of varved lake sediment records for understanding erosion dynamics in undisturbed forested catchments and their link to long‐term climate dynamics and future climate change. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Repeated short-term bioproductivity changes in a coastal lake on Store Koldewey, northeast Greenland: an indicator of varying sea-ice coverage?

A lacustrine sediment core from Store Koldewey, northeast Greenland, was biogeochemically, biologically and sedimentologically investigated in order to reconstruct long- and short-term climatic and environmental variability. The chronology of the uppermost 189 cm of the record is based on ten 14C AMS age determinations of aquatic mosses. The record covers almost the entire Holocene and revealed changes on multidecadal to centennial scales. Dating of the oldest mosses shows that lacustrine biogenic productivity already began at around 11 cal. kyr BP. This age pre-dates the onset of biogenic productivity in other lakes on Store Koldewey by about 2 kyr. In spite of the early onset of biogenic production organic matter accumulation remained low and minerogenic sedimentation dominated. At about 9.5 cal. kyr BP moss, sulphur, organic carbon and biogenic silica content started to increase, indicating that the environment stabilized and the biogenic production in the lake adjusted to more preferable conditions. Subsequently, the biogenic productivity experienced repeated changes and varied both on long- and short-term scales. The long-term trend shows a maximum during the early Holocene thus responding to increased temperatures during the Holocene Thermal Maximum. Superimposed on the long-term trend, biogenic productivity also experienced repeated short-term fluctuations that match partly the NGRIP temperatures. The most pronounced decrease of biogenic productivity occurred at around 8.2 cal. kyr BP. Perennial lake ice coverage resulting from low temperatures is supposed to have caused decreased lacustrine biogenic productivity. From the middle Holocene to the present repeated decreases of productivity occurred that could be related to periods with severe sea-ice conditions of the East Greenland Current. Besides the dependence on air temperature it therefore demonstrates the sensitivity of lacustrine biogenic productivity in coastal high arctic areas to short-term cold spells that are mediated by the currents emanating from the Arctic Ocean. However, the data also emphasize the difficulties associated with the interpretation of lacustrine records.

An integrated lake-catchment approach for determining sediment source changes at Aqualate Mere, Central England

This paper raises fundamental questions about the sole use of paleolimnological techniques to identify sediment sources and develop catchment management plans. The concept of an integrated lake: catchment framework was established 30 years ago, yet paleolimnologists occasionally fail to appreciate the dynamics of the contributing catchment. This is especially critical when the predominant source of sediment accumulating in a lake is allochthonous. In this paper we argue that a detailed appraisal of catchment sources and investigation of historical documentary evidence is needed to identify and evaluate the relative significance of sediment sources. We used such an approach at Aqualate Mere, Shropshire, UK. Mineral magnetic and radionculide signatures of potential catchment sources and accumulating lake sediments were compared in an attempt to match the sources to sediments deposited in the Mere. Dated lake sediments indicate there has been an increase in sedimentation rate and the relative amount of minerogenic material delivered to the Mere over the last 200 years. In contrast to a previous study at the same site, there is no evidence to attribute this increase to an overspill from a nearby canal. Other catchment disturbances include landscaping in parkland surrounding the Mere in the early nineteenth century and drainage systems installed to improve catchment agriculture over the last ca. 150 years. Both activities may explain the change in sedimentation rates and types, independent of the hypothesized canal origin. Although our results exclude the canal as a major sediment source, identifying the contribution of other potential catchment sources remains problematic. 137Cs inventories for the lake are similar to those recorded at a local reference site, suggesting little influx of 137Cs-bearing topsoil, yet 137Cs activities remain high in the upper 20–30 cm of the lake sediment profile, indicating a topsoil origin. Combined radionuclide and mineral magnetic signatures proved to be relatively poor discriminators of potential sources, and the high atmospheric pollution load from the West Midland conurbation has probably altered recent lake sediment signatures. Although further research is required to identify the origins of recent (last ca. 200 years) minerogenic sediment inputs to the Mere, we suggest that the combined lake: catchment approach offers a more rigorous method for understanding the impact of catchment disturbance than analysis of the paleolimnological record alone.

A 2000 year varve-based climate record from the central Brooks Range, Alaska

Varved minerogenic sediments from glacial-fed Blue Lake, northern Alaska, are used to investigate late Holocene climate variability. Varve-thickness measurements track summer temperature recorded at Atigun Pass, located 41 km east at a similar elevation (r2 = 0.31, P = 0.08). Results indicate that climate in the Brooks Range from 10 to 730 AD (varve year) was warm with precipitation inferred to be higher than during the twentieth century. The varve-temperature relationship for this period was likely compromised and not used in our temperature reconstruction because the glacier was greatly reduced, or absent, exposing sub-glacial sediments to erosion from enhanced precipitation. Varve-inferred summer temperatures and precipitation decreased after 730 AD, averaging 0.4°C above the last millennial average (LMA = 4.2°C) from 730 to 850 AD, and 0.1°C above the LMA from 850 to 980 AD. Cooling culminated between 980 and 1030 AD with temperatures 0.7°C below the LMA. Varve-inferred summer temperatures increased between 1030 and 1620 AD to the LMA, though the period between 1260 and 1350 AD was 0.2°C below the LMA. Although there is no equivalent to the European Medieval Warm Period in the Blue Lake record, two warm intervals occurred from 1350 to 1450 AD and 1500 to 1620 AD (0.4 and 0.3°C above the LMA, respectively). During the Little Ice Age (LIA; 1620 to 1880 AD), inferred summer temperature averaged 0.2°C below the LMA. After 1880 AD, inferred summer temperature increased to 0.8°C above the LMA, glaciers retreated, but aridity persisted based on a number of regional paleoclimate records. Despite warming and glacial retreat, varve thicknesses have not achieved pre-730 AD levels. This reflects limited sediment availability and transport due to a less extensive retreat compared to the first millennium, and continued relative aridity. Overall, the Blue Lake record is similar to varve records from the eastern Canadian Arctic that document a cool LIA and twentieth century warming. However, the occurrence and timing of events, such as the LIA and Medieval Warm Period, varies considerably among records, suggesting heterogeneous climatic patterns across the North American Arctic.

Devensian Late-glacial environmental change in the Gordano Valley, North Somerset, England: a rare archive for southwest Britain

The Late-glacial and Holocene environmental history of the Gordano Valley, North Somerset, UK has been reconstructed using pollen, sediment particle size and mineralogical analyses and radiocarbon dating. A Devensian sediment ridge across the valley confined the waters of a small lake, within which the initial sedimentation was minerogenic. Radiocarbon dating of overlying organic-rich deposits suggests that this began late in the Dimlington Stadial c. 18,000–15,000 Cal. BP. Petrographic analyses indicate the minerogenic sediments were partly wind-blown in origin. Climatic amelioration during the Windermere Interstadial c. 15,000 Cal. BP encouraged a shift from minerogenic to biogenic sedimentation. A brief return to minerogenic sedimentation between c. 10,400 and c. 9,520 Cal. BP was followed by uninterrupted fen peat accumulation throughout the Holocene. The later minerogenic horizon appears to represent the Loch Lomond Stadial. Few stratigraphic sequences preserving the complete Devensian Late-glacial–Holocene transition exist in southwest Britain, making the sedimentary archive of the Gordano Valley valuable regionally for reconstructing Late-glacial climate change.

Late Holocene debris cone development and vegetation and land-use history in the Pasture Beck valley, Lake District, NW England

SUMMARY Investigations of a sedimentary sequence exposed in the distal margin of a small debris cone in the valley of Pasture Beck, eastern Lake District, have yielded detailed information concerning the time and nature of cone aggradation phases and the local vegetation and land-use history. Radiocarbon dates from two buried peat beds indicate that the cone surface was generally stable from the late 8 th to the early 13 th centuries AD (1165–736 cal. BP). An interlude of colluvial gravel deposition interrupted peat accumulation and was probably the result of a single short-lived event. Gravel-dominated beds represent deposition of reworked rockfall debris and glacial drift by cohesionless debris flows, whereas finer-grain beds are probably the result of sheet-flows derived by washout of fines from debris-flow deposits. The stratigraphic evidence indicates that the minerogenic sediments accumulated in quick succession. Pollen and charcoal data from peat samples reveal intensification of land-use before burial of the peats by minerogenic sediments. It is proposed that landscape instability was associated first with Norse-Irish settlement and woodland clearance, and later, probably, to the expansion of upland pastures and more intensive grazing in the 13 th century AD. When the Pasture Beck data is considered along with information from elsewhere in the British Isles, it is apparent that not all patterns of Late Holocene landscape change in upland areas can be ascribed to a single cause.

Reconstruction of Holocene glacier history from distal sources: glaciofluvial stream-bank mires and a glaciolacustrine sediment core near Sota Sæter, Breheimen, southern Norway

This study attempts to reconstruct the Holocene glacier history of Breheimen, southern Norway from four stream-bank mire sites and a downstream lake sediment core near Sota Sæter, based on lithostratigraphy, sediment properties (loss-on-ignition (LOI), magnetic susceptibility (MS), grain size characteristics) and 69 radiocarbon dates. Late-Preboreal deglaciation is indicated by lake-floor accumulation of organic matter (gyttja) largely replacing minerogenic sediment before 9000 cal. BP. A distinct minerogenic layer is thought to represent glacier expansion during the 8.2 ka Finse Event. Following this event until c . 2200 cal. BP, glaciers were mostly smaller than today, indicated by slow accumulation of mainly gyttja with predominantly thin minerogenic layers. A trough in LOI and corresponding small peaks in different silt fractions indicate possible glacial expansion from c. 3500 to 3000 cal. BP. The shorter c. 7500 year stream-bank mire record suggests brief, distinct episodes of Neoglacial glacier activity at c. 5600 and 3650 cal. BP. After c. 2200 cal. BP, up to three possible glacier expansion events are indicated in the lake core compared with up to eight represented in the mire sequences, suggesting greater sensitivity of the latter to small-scale glacier expansion. Marked `suppressed' Neoglacial expansion until after c. 2200 cal. BP contrasts with the more typically reported start at 6000—5300 cal. BP elsewhere in southern Norway. Methodological implications are discussed including: factors responsible for erroneous radiocarbon dates in mire and lake core stratigraphies; high spatial variability in mire stratigraphy; determining the amplitude of glacier contraction episodes; isolating the glacial signal from background `noise'; and further improvement of dating.

Net Methylmercury Production as a Basis for Improved Risk Assessment of Mercury-contaminated Sediments

Sediments contaminated by various sources of mercury (Hg) were studied at 8 sites in Sweden covering wide ranges of climate, salinity, and sediment types. At all sites, biota (plankton, sediment living organisms, and fish) showed enhanced concentrations of Hg relative to corresponding organisms at nearby reference sites. The key process determining the risk at these sites is the net transformation of inorganic Hg to the highly toxic and bioavailable methylmercury (MeHg). Accordingly, Hg concentrations in Perca fluviatilis were more strongly correlated to MeHg (p < 0.05) than to inorganic Hg concentrations in the sediments. At all sites, except one, concentrations of inorganic Hg (2-55 microg g(-1)) in sediments were significantly, positively correlated to the concentration of MeHg (4-90 ng g(-1)). The MeHg/Hg ratio (which is assumed to reflect the net production of MeHg normalized to the Hg concentration) varied widely among sites. The highest MeHg/Hg ratios were encountered in loose-fiber sediments situated in southern freshwaters, and the lowest ratios were found in brackish-water sediments and firm, minerogenic sediments at the northernmost freshwater site. This pattern may be explained by an increased MeHg production by methylating bacteria with increasing temperature, availability of energy-rich organic matter (which is correlated with primary production), and availability of neutral Hg sulfides in the sediment pore waters. These factors therefore need to be considered when the risk associated with Hg-contaminated sediments is assessed.

Electrical Resistivity of Coastal Back-Barrier Sediments from South Devon, South-West England, United Kingdom

Electrical resistivity surveying is a commonly used geophysical method in civil engineering, but few examples of this application exist in palaeoenvironmental reconstructions. Here, we provide an example of the application of the method to the two-dimensional distribution of lithofacies from coastal back-barrier sites of south-west England. The region is particularly important in coastal studies because of the current paucity of stratigraphic data and its position along the fastest subsiding coastline in Britain. Surveys were carried out at four sites along the south Devon coastline with the ABEM LUND Imaging System. Ranges of electrical resistivity of subsurface sediments were interpreted by ground-truthing from boreholes placed along the survey runs. Electrical resistivity surveys carried out at coastal locations can be affected by a range of phenomena—in particular, saltwater intrusion from rising tides and storm-induced overwash events. Conductive groundwater can cause temporal variations in the electrical profiles measured at the surface. This has not been witnessed to any significant degree at our sites. Despite anthropogenic and environmental noise (e.g., fences, tracks, drainage ditches, streams, and the possibility of salt water intrusion), the system has produced encouraging results and demonstrated that the technique has a future in coastal research. In particular, it readily identified the depth of gravel barriers and depth to basal strata. Given the speed and efficiency of data collection and interpretation available in the field, such systems should prove valuable in future research. Integration of the resistivity profiles with borehole information and radiocarbon dates of selected samples show that back-barrier basins began to fill with fine minerogenic sediments and shells ca. 9000 to 8000 Cal BP, and the majority of sites became terrestrialised by ca. 4500 Cal BP.

Wetlands in the Venetian Po Plain (northeastern Italy) during the Last Glacial Maximum: Interplay between vegetation, hydrology and sedimentary environment

In the low Venetian plain (northeastern Italy) thick sequences of silt and sand layers alternate with common, thin layers of peat and organic silt; the organic layers in the topmost 30 m of the Late Pleistocene alluvial series span between 23,000 and 14,000 yr BP (radiocarbon dating), in an area measuring 100 km by 30 km. They indicate broad areas where wetlands developed. We aim to understand the features and the origin of the wetlands by undertaking sedimentological, pollen, non-pollen palynomorph and plant macrofossil analyses. Thirteen cores were drilled in the central zone of the low Venetian plain near the coast of the Adriatic Sea and 79 samples were analysed. The palaeoenvironmental reconstruction based on previous pollen analysis did not emphasize the areas where peat layers were formed, suggesting a homogenous steppe environment, typical of a cold and dry climate. They were probably waterlogged for most of the year allowing the formation of peat and the development of local plant communities of mainly aquatic species. Macrofossil and pollen analyses suggest that herbaceous plants, such as Cyperaceae and Poaceae (probably Carex fusca and Phragmites australis), and brown mosses (mainly Scorpidium scorpioides) were the most important components of wetland communities. Fossils of obligate aquatic organisms indicate open water environments, these include macrofossils of Nymphaea, Characeae, Bryozoa and Potamogeton, and non-pollen palynomorphs such as algal resting cells, free cells and colonies (Zygnemataceae, Spirogyra, Mougeotia, Closterium idiosporum, Type 225, Type 229, Botryococcus, Pediastrum cf. boryanum, P. cf. simplex, Ceratium hirundinella, Tetraedron cf. minimum and Type 333), oocytes of aquatic invertebrates (Type 353A and 353B) and incompletely known types probably of algal origin (Type 303, Type 74, Type 128A and 128B). A discontinuous occurrence of fungal spores and other microfossils (Type 200, Gaeumannomyces (Type 126), Glomus (Type 207), Type 351, Type 79, and incompletely known types) suggests frequent fluctuations of the water depth with periodic emersions of the bottom of the ponds or fens. The water quality preferred by the identified species, or suggested in literature for the fossil types, is mainly eutrophic to mesotrophic and rich in cations. Peatland formed in wide, low-lying areas between the fluvial ridges which were periodically inundated by the fluctuating groundwater. Peat accumulated in continuous layers only where the fen organic deposition prevailed the alluvial minerogenic sediment. When alluvial deposition buried the organic deposit, the peat level was incorporated into the stratigraphic record.

Vashegyite from Gaura cu Muscă Cave (Locvei Mountains, Romania): a new and rare phosphate occurrence

This study investigated the occurrence of vashegyite from a guano-rich deposit located in the Gaura cu Muscă Cave, Romania. Analytical methods used include optical microscopy, X-ray powder diffraction (XRD), scanning electron-microscopy (SEM), inductively coupled plasma-atomic emission spectrometry (ICP-AES), thermal investigations and Fourier-transform infrared (FT-IR) analyses. Vashegyite occurs as friable, chalky white, irregular nodules of up to 2.5 cm in diameter, within a 15 cm thick sequence of organic and minerogenic sediments. The chemical structural formula is: (Al10.91Fe3+0.06Na0.1Ca0.02Mg0.08)Σ=11.17[(PO4)8.78(SiO4)0.056]Σ=8.83(OH)6.17·43.79H2O. Electron microscope images show vashegyite crystals to be flattened on (001). The orthorhombic lattice constants of vashegyite determined by XRD are a = 10.766(2) Å, b = 15.00(4) Å, c = 22.661(1) Å, and V = 3660.62 Å3 (Z = 4). The major weight loss, reflected in 3 endothermic peaks, was observed between 40° and 200°C, corresponding to the removal of water molecules. Vashegyite FT-IR absorption bands are comparable in position and relative intensity to other Al-phosphates. Water percolating through guano becomes strongly acidic and reacts with the clay-rich sediment laid down by the underground stream to form vashegyite. In the lower part of the investigated profile, crandallite and ardealite were also found.

Autocompaction in Holocene coastal back-barrier sediments from south Devon, southwest England, UK

The present-day elevation of superficial horizons situated above a competent basal stratum is likely to be lower than the original height of deposition. This is because sediments such as minerogenic fines and peat undergo a post-depositional reduction in volume as a result of the weight of overlying sediments, the downward movement being due to the cumulative compression of all the sediment below the level in question. This “autocompaction” can affect the palaeoenvironmental interpretation of lithofacies from which a vertical reference is required, e.g., when quantifying the height of a sea-level index point. Geotechnical theory was used to apply a correction to Holocene coastal back-barrier sediments from North Sands and Blackpool Sands in south Devon (UK) and so to return marker horizons to a level approximating their original height of deposition. In this model the total downward movement is calculated by notionally dividing the underlying soil into a number of thin (∼ 0.1 to 0.2 m) layers and calculating the individual compression in each one. The results were then summed to give the total compression. This approach can underestimate the full extent of autocompaction notably due to uncertainties arising from the behaviour of organic-rich facies and from inadequate knowledge of groundwater history. The results must be considered semiquantitative and are usually minimum estimates. Results of vertical corrections from coastal sedimentary units in south Devon range from < 0.1 m in fine-grained sediments situated above basal facies to > 1 m at contacts between minerogenic sediments and peat, increasing to > 2 m in more organic facies.