Dirk Enters

Changes in erosion patterns during the Holocene in a currently treeless subalpine catchment inferred from lake sediment geochemistry (Lake Anterne, 2063 m a.s.l., NW French Alps): The role of climate and human activities

A high-resolution sedimentological and geochemical study was performed on a 20 m long core from the alpine Lake Anterne (2063 m a.s.l., NW French Alps) spanning the last 10 ka. Sedimentation is mainly of minerogenic origin. The organic matter quantity (TOC%) as well as its quality (hydrogen (HI) and oxygen (OI) indices) both indicate the progressive onset and subsequent stabilization of vegetation cover in the catchment from 9950 to 5550 cal. BP. During this phase, the pedogenic process of carbonate dissolution is marked by a decrease in the calcium content in the sediment record. Between 7850 and 5550 cal. BP, very low manganese concentrations suggest anoxic conditions in the bottom-water of Lake Anterne. These are caused by a relatively high organic matter (terrestrial and lacustrine) content, a low flood frequency and longer summer stratification triggered by warmer conditions. From 5550 cal. BP, a decrease in TOC, stabilization of HI and higher sedimentation rates together reflect increased erosion rates of leptosols and developed soils, probably due to a colder and wetter climate. Then, three periods of important soil destabilization are marked by an increased frequency and thickness of flood deposits during the Bronze Age and by increases in topsoil erosion relative to leptosols (HI increases) during the late Iron Age/Roman period and the Medieval periods. These periods are also characterized by higher sedimentation rates. According to palynological data, human impact (deforestation and/or pasturing activity) probably triggered these periods of increased soil erosion.

Historical soil erosion and land-use change during the last two millennia recorded in lake sediments of Frickenhauser See, northern Bavaria, central Germany

Based on palaeolimnological investigations on a 1275 cm long lacustrine sediment record, sediment yields (SY) and denudation rates (DR) for the last 2000 years were reconstructed from Frickenhauser See, a small lake in central Germany. SY and DR generally correspond with the human land-use history derived from pollen analysis and reveal a drastic interruption of natural processes starting in the eleventh century AD. SY increased over 350-fold from 0.9 t/km2 per yr to 328 t/km2 per yr in response to deforestation and subsequent soil erosion. The average denudation rate within the catchment area is 81 mm for the last 2000 years, most of which occurred between AD 1100 and 1870. Taking into account that only one-third of the catchment area is suitable for agriculture, the cumulative soil loss from agricultural fields is around 240 mm (0.31 mm/yr). Historical events such as a widespread abandonment period (AD 1300—1450) and the Thirty Years War (AD 1618—1648) are reflected in the pollen record but cause only minor fluctuations in reconstructed sediment yields. This study demonstrates that by applying a multiproxy approach lacustrine sediments provide an excellent archive to reconstruct the effects of human land use on the geomorphological process system and to identify human—environment interactions.

Construction and validation of calendar-year time scale for annually laminated sediments – an example from Lake Szurpiły (NE Poland)

The composite sediment profile (12.39 m) from Lake Szurpiły (NE Poland) represents an annually laminated organic-carbonaceous gyttja occasionally interrupted by turbidites and massive sand layers. This study focuses on the 7.58-m long and almost continuously varved top section of the profile, which produced a 8410-year long varve chronology with a cumulative counting error of ± 1.24%. The age-depth model was established by multiple microscopic varve counts and improved by the application of independent radiometric dating methods (210Pb, 137Cs). Ten additional AMS 14C dates are consistent with varve counts. In some sections, missing varves were identified as a result of erosional processes related to turbidite deposition. Varve thickness ranges from less than 0.1 to 13.7 mm (mean: 0.83 mm; std: 0.75 mm) with highest variability during the last 1500 years. The accuracy of the varve chronology depends mostly on the regularity of the varve thickness and the distinctness of varve boundaries, and was not influenced by the varve thickness itself. Even though manual and semiautomated varve counting show similar results of the total amount of varves, with the difference of only 0.56%, the comparison between those two methods in intervals of 200 years indicates potential problems, especially for sections with complex lamination and turbidites. We found that semiautomatic varve counting overestimated the varve boundaries in sections with erosive turbidites. Our results confirm the importance of validation of varve chronologies by independent dating methods and caution in relying on automated methods.

Experiences with XRF-Scanning of Long Sediment Records

Lacustrine sediment sequences of more than 100 m in length obtained in the framework of two deep drilling projects (PASADO and El’gygytgyn) were analysed with ITRAX XRF-core scanners. Core length and the long total scan time of 300–8000 h for these records made it necessary to consider downcore lithological changes and tube ageing. To account for these, two different approaches were realised for the acquisition of element data by XRF-scanning. Additionally, X-ray tube ageing was documented by regular measurements of a standard reference glass after each core run. The data obtained suggest that a normalisation of XRF raw-data is advisable for several reasons. (1) To level out effects of the observed long-term decrease in primary tube power that lead to lower count rates and thus lower element intensities. (2) To level out shifts in element profiles between sections that were measured with different X-ray tube current settings. From the tested normalisation procedures a division of element raw intensities by the intensity of coherent radiation (coh) yielded the best results for elements with mid to high atomic numbers. However, elements with low atomic numbers are less affected by a lowering of count rates related to tube ageing, and thus, the coh correction might over-compensate for this for these elements. Thus, for light elements it is advantageous to correct for the apparent energy loss by calculating ratios of elements with comparable atomic numbers (e.g. Si/Ti).

Resilience, rapid transitions and regime shifts: Fingerprinting the responses of Lake Żabińskie (NE Poland) to climate variability and human disturbance since AD 1000:

Rapid ecosystem transitions and adverse effects on ecosystem services as responses to combined climate and human impacts are of major concern. Yet few long-term (i.e. >60 years) quantitative observational time series exist, particularly for ecosystems that have a long history of human intervention. Here, we combine three major environmental pressures (land use, nutrients and erosion) with quantitative summer and winter climate reconstructions and climate model simulations to explore the system dynamics, resilience and the role of disturbance regimes in varved eutrophic Lake Żabińskie (NE Poland) since AD 1000. The comparison between these independent sources of information allows us to establish the coherence and points of disagreements between such data sets. We find that climate reconstructions capture noticeably natural forced climate variability, while internal variability is the dominant source of variability during most parts of the last millennium at the regional scale, precisely at which climate models seem to underestimate forced variability. Using different multivariate analyses and change point detection techniques, we identify ecosystem changes through time and shifts between rather stable states and highly variable ones. Prior to AD 1600, the lake ecosystem was characterised by high stability and resilience against observed natural climate variability. During this period, the anthropogenic fingerprint was small; the lake ecosystem was buffered against the combined human and natural disturbance. In contrast, lake–ecosystem conditions started to fluctuate across a broad range of states after AD 1600. The period AD 1745–1886 represents the phase with the strongest human disturbance of the catchment–lake ecosystem. During that time, the range of natural climate variability did not increase. Analyses of the frequency of change points in the multi-proxy data set suggest that the last 400 years were highly variable and increased vulnerability of the ecosystem to the anthropogenic disturbances. This led to significant rapid ecosystem transformations.

Sedimentological and geochemical responses of Lake Żabińskie (north-eastern Poland) to erosion changes during the last millennium

Increased erosion triggered by land-use changes is a major process that influences lake sedimentation. We explored the record of erosion intensity in annually laminated sediments of Lake Żabińskie, northeast Poland. A 1000-year-long, annually resolved suite of sedimentological (varve thickness, sediment accumulation rate) and geochemical data (scanning XRF, loss on ignition, biogenic silica) was analyzed with multivariate statistics. PCA indicated erosion was a major process responsible for changes in the chemical composition of the sediments. Analysis of sedimentary facies enabled identification of major phases of erosion that influenced lake sedimentation. These phases are consistent with the history of land use, inferred from pollen analysis. From AD 1000 to 1610, conditions around and in Lake Żabińskie were relatively stable, with low erosion intensity in the catchment and a dominance of carbonate sedimentation. Between AD 1610 and 1740, higher lake productivity and increased delivery of minerogenic material were caused by development of settlements in the region and widespread deforestation. The most prominent changes were observed between AD 1740 and 1880, when further land clearance and increased agricultural activity caused intensified soil erosion and higher lake productivity. Landscape clearance also created better conditions for water column mixing, which led to changes in redox conditions in the hypolimnion. The most recent period (AD 1880–2010) was characterized by partial reforestation and a gradual decrease in the intensity of erosional processes.

Environmental changes during the last millennium based on multi-proxy palaeoecological records in a savanna-forest mosaic from the northernmost Brazilian Amazon region

The environmental changes and the dynamics of the savanna-forest mosaic, over the last 1050 years, have been reconstructed by pollen, charcoal, radiocarbon dating mineralogical and geochemical analyses of sediment cores taken from three different Mauritia flexuosapalm swamps in the northernmost part of the Brazilian Amazon region (northern state of Roraima). Studies on the relationship between the modern pollen rain and the regional vegetation provide additional information for the interpretation of the fossil pollen records. The fossil pollen assemblages and geochemical results indicate relatively wet climatic conditions throughout the recorded period. Despite these moist conditions, fires were frequent and are one of the reasons for the dominance of a grassy savanna instead of forest expansion in the study area. Considering the generally wet climatic conditions, these fires were most likely caused by human activities. Even today, fires hinder forest expansion into savanna areas. Sandy hydromorphic soils may also act as an edaphic control to maintain the current sharp boundary between forest and savanna ecosystems.

Contribution of non-pollen palynomorphs to reconstructions of land-use changes and lake eutrophication: case study from Lake Jaczno, northeastern Poland

Abstract Analysis of non-pollen palynomorphs supplemented by pollen analysis, microcharcoal analysis and geochemical data from laminated sediments from Lake Jaczno were used to establish different phases of land-use in the catchment between c.a. AD 1840 and AD 2013. The results show that during the first eighty years the vicinity of the lake was heavily deforested. During this period erosional inputs caused accumulation of abundant fungal spores, indicators of pastures and natural fertilizers (manure) as well as of corroded pollen grains and charcoal. Gradual regeneration of forest cover took place after World War II, when expansion of pioneer trees occurred (Betula, Salix, Carpinus, Populus). At the same time, a considerable increase in the lake trophy was observed, leading to the changes in phytoplankton and macrophyte communities: a decrease in the proportion of Botryococcus and an increase in the Nymphaea alba population. The non-pollen palynomorphs analyses indicate the substantial human impact that caused changing local environmental conditions, compatible with the results based on pollen analysis and geochemical data.