Varve microfacies and varve preservation record of climate change and human impact for the last 6000 years at Lake Tiefer See (NE Germany)

Abstract

The Holocene sediment record of Lake Tiefer See exhibits striking alternations between well-varved and non-varved intervals. Here, we present a high-resolution multi-proxy record for the past ~6000 years and discuss possible causes for the observed sediment variability. This approach comprises microfacies, geochemical and microfossil analyses and a multiple dating concept including varve counting, tephrochronology and radiocarbon dating. Four periods of predominantly well-varved sediment were identified at 6000–3950, 3100–2850 and 2100–750 cal. a BP and AD 1924–present. Except of sub-recent varve formation, these periods are considered to reflect reduced lake circulation and consequently, stronger anoxic bottom water conditions. In contrast, intercalated intervals of poor varve preservation or even extensively mixed non-varved sediments indicate strengthened lake circulation. Sub-recent varve formation since AD 1924 is, in addition to natural forcing, influenced by enhanced lake productivity due to modern anthropogenic eutrophication. The general increase in periods of intensified lake circulation in Lake Tiefer See since ~4000 cal. a BP presumably is caused by gradual changes in the northern hemisphere orbital forcing, leading to cooler and windier conditions in Central Europe. Superimposed decadal- to centennial-scale variability of the lake circulation regime is likely the result of additional human-induced changes of the catchment vegetation. The coincidence of major non-varved periods at Lake Tiefer See and intervals of bioturbated sediments in the Baltic Sea implies a broader regional significance of our findings.

Synchronizing <sup>10</sup>Be in two varved lake sediment records to IntCal13 <sup>14</sup>C

“…1) (Dräger et al, 2017;Ott et al, 2016;Słowiński et al, 2017 (Dräger et al, 2017). JC sediments for these time windows comprise endogenic calcite varves with couplets of calcite and organic/diatom sub-layers, and an additional layer of resuspended littoral material since about 2800 a BP (Ott et al, 2016;Wulf et al, 2013).…”

“…reflecting changes in sediment accumulation and composition (Dräger et al, 2017;Ott et al, 2016;Wulf et al, 2016), to estimate the possible environmental influence ( 10 Be bias ). Only the TOC and Ca time-series with significant p<0.01) (Fig.…”

“…M2015 based on varve chronologies (Dräger et al, 2017;Ott et al, 2016). Complementary sediment accumulation rate (SAR), geochemical X-ray fluorescence (µ-XRF) and total organic carbon (TOC) time-series were constructed using existing high-resolution datasets from the same sediment cores by calculating 10 Be sample averages (Dräger et al, 2017;Ott et al, 2016;Wulf et al, 2016).…”

“…Complementary sediment accumulation rate (SAR), geochemical X-ray fluorescence (µ-XRF) and total organic carbon (TOC) time-series were constructed using existing high-resolution datasets from the same sediment cores by calculating 10 Be sample averages (Dräger et al, 2017;Ott et al, 2016;Wulf et al, 2016). Measured µ-XRF data (cps) were normalized by dividing by the sum of all elements, to reduce the effects of varying sediment properties (Weltje and Tjallingii, 2008).…”

“…Synchronization of 10 Be composites ( 10 Be comp ) from Lakes Tiefer See (TSK) and Czechowskie (JC) for the Maunder-, Homeric-and 5500 a BP grand solar minima with 14 C production rates from the IntCal13 calibration curve (Muscheler et al, 2014). Best fits between the records were calculated using lag-correlation, based on given chronological uncertainties (Dräger et al, 2017;Ott et al, 2016). Significances of the correlations were calculated using 10000 iterations of a non-parametric random 5 phase test taking into account autocorrelation and trend present in the time-series (Ebisuzaki, 1997).…”