Sand in Early Holocene lake sediments - a microscopic study from Lake Jaczno, northeastern Poland

Deep moraine lakes prevail in northeastern Poland and their sediments serve as a powerful information about past environment. Apart from conventional organic horizons, the sediments may contain clastic and chemically-induced components such as sand, fines or sulphates, thus providing additional insight into the lake history. Sand consists primarily of quartz grains and their study is a well-established method to infer palaeoenvironmental conditions. Quartz grains from lacustrine sediments, however, are still poorly studied. We examined for the first time quartz grains found among organic sediment of Lake Jaczno, northeastern Poland, which deposited before ca 10 700 cal yr BP. By applying the light microscope and scanning electron microscope techniques, we debate about the quartz grain source and discuss possible palaeoenvironmental scenarios. Grains with fresh and sharp edges and diagnostic glacial microtextures coexist with rounded grains with matt surface. Loads of glacial grains originate from the surrounding glaciogenic sediments, which were transported from adjacent steep slopes and further deposited in the lake. Grain rounding and matting, combined with oriented etch pits, result from intense weathering processes associated with a high carbonate content. Part of the microtextures of chemical origin may be due to inflow/outflow activity in the lake. Gypsum crystals occur in the uppermost part of the investigated sediment and likely reflect drier climate conditions at the beginning of the Holocene, which has also been documented in numerous sites in northeastern Poland.