Abstract. We investigate the potential of hyperspectral imaging spectrometry for the analysis of fresh sediment cores. A sediment-core-scanning system equipped with a camera working in the visual to near-infrared range (400 to 1000 nm) is described and a general methodology for processing and calibrating spectral data from sediments is proposed. We present an application from organic sediments of Lake Jaczno, a freshwater lake with biochemical varves in northern Poland. The sedimentary pigment bacteriopheophytin a (BPhe a) is diagnostic for anoxia in lakes and, therefore, an important ecological indicator. Calibration of the spectral data (BPhe a absorption ∼800 to 900 nm) to absolute BPhe a concentrations, as measured by high-performance-liquid-chromatography, reveals that sedimentary BPhe a concentrations can be estimated from spectral data with a model uncertainty of ∼10%. Based on this calibration model, we use the hyperspectral data from the sediment core to produce high-resolution intensity maps and time series of relative BPhe a concentrations (∼10 to 20 data points per year, pixel resolution 70 × 70 μm 2 ). We conclude that hyperspectral imaging is a very cost-and time-efficient method for the analysis of lake sediments and provides insight into the spatiotemporal structures of biogeochemical species at a degree of detail that is not possible with wet chemical analyses. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)

Reconstruction

Hypoxia in freshwater systems is currently spreading globally and putting water quality, biodiversity and other ecosystem services at risk. Such adverse effects are of particular concern in permanently stratified meromictic lakes. Yet little is known about when and how meromixis and hypoxia became established (or vanished) prior to anthropogenic impacts, or how human activities such as deforestation, erosion and nutrient cycling affected the mixing regimes of lakes. We used calibrated hyperspectral imaging (HSI) data in the visible and near infrared range from a fresh, varved sediment core taken in Lake Jaczno, NE Poland, to map sedimentary pigments at very high resolution (sub-varve scale) over the past 1700 years. HSI-inferred bacteriopheophytin a (bphe a, produced by anoxygenic phototrophic bacteria) serves as a proxy for meromixis, whereas HSI-inferred green pigments (chlorophyll a and diagenetic products) can be used as estimators of aquatic productivity. Meromixis was established and vanished long before significant human disturbance in the catchment was observed in the late eleventh century AD. Under preanthropogenic conditions, however, meromixis was interrupted frequently, and the lake mixing regime flickered between dimixis and meromixis. During two periods with intense deforestation and soil erosion in the catchment, characterised by sedimentary facies rich in clay and charcoal (AD 1070-1255 and AD 1670-1710, the lake was mostly dimictic and better oxygenated than in periods with relative stability and a presumably closed forest around the lake, i.e. without human disturbances. After ca. AD 1960, meromixis became established quasi-permanently as a result of eutrophication. The persistent meromixis of the last *60 years is unusual with respect to the record of the last 1700 years.

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Glacial landscapes of the Land of Great Masurian Lakes and Suwałki Lakelands in northeast Poland are characterized by very high abundance of lakes. These two areas were surveyed for lakes containing laminated sediments. Using bathymetry as a criterion, 60 small, deep lakes, representing preferred conditions for formation and preservation of lacustrine non-glacial varves, were selected for gravity coring. We found laminated sediments in 24 of the lakes, 15 in the Land of Great Masurian Lakes and 9 in the Suwałki Lakeland. Seven of these 24 sediment records were laminated in the topmost part only. Analysis of lake morphometric variables showed that the relation between surface area and maximum water depth can be used to identify lakes with laminated sediments. Most of the newly discovered lakes with laminated deposits have surface areas B0.3 km 2 and maximum depths of 15-35 m. Multivariate statistical analysis (Linear Discriminant Analysis) of the lake dataset identified the morphological features of lake basins and their catchments that largely control preservation of laminated sediments. Microscopic and geochemical analyses revealed a biogenic (carbonaceous) type of lamination typical for lakes in northeast Poland. Such lakes are characterized by a spring-summer lamina that is rich in calcium carbonate and an autumn-winter lamina composed of organic and minerogenic detritus. This pattern may be modified by multiple periods of calcite deposition during a single year or substantial contribution of clastic material. Laminations and high sedimentation rates offer the possibility of high-resolution investigation of past climate and environmental changes through application of myriad biological, isotopic and geochemical proxies.

Pollen, charcoal and geochemical investigations were carried out on annually laminated sediments of Lake _ Zabińskie (54°07 0 54.5 00 N; 21°59 0 01.1 00 E) and the results were combined with historical and climate data to better understand the mechanism behind plant cover transformations. A millennium-long record of environmental history at 6-years time resolution permitted an assessment of vegetation responses to past human impact and climate fluctuations. Our results show that the history of the region with repeated periods of warfare, epidemics, famine and crop failures is well reflected by environmental proxies. Before the Teutonic Order crusade (AD 1230-1283), agricultural activities of the Prussian tribes were conducted at a distance from the studied lake and caused slight disturbances of local forests. A stronger human impact was registered after ca AD 1460. We confirm that co-domination of pine forests with spruce and oakhornbeam forests on drier habitats as well as the presence of birch and alder woods on wet surfaces near the lake lasted until AD 1610. We identified a transition period of 20 years between AD 1590 and 1610, when forest cover was significantly reduced and the area was partly transformed into open land used for farming activities. The comparison of our data with other pollen datasets from the region confirms significant spatio-temporal differences in the initiation of large-scale woodland clearings in the Great Masurian Lake District. A strong increase in local cultivation was noted after AD 1750 and became even stronger in the period AD 1810-1940. The last 60 years experienced a succession from arable fields and open grasslands to more tree-covered habitats overgrown by birch and alder.

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