Description: Varved lake sediments provide climatic records with seasonal to annual resolution and low associated age uncertainty. Robust and detailed comparison of well-dated and annually laminated sediment records is crucial for reconstructing abrupt and regionally time-transgressive changes as well as validation of spatial and temporal trajectories of past climatic changes. The VARved sediments DAtabase (VARDA) presented here is the first data compilation for varve chronologies and associated palaeoclimatic proxy records. The current version 1.0 allows detailed comparison of published varve records from 95 lakes. VARDA is freely accessible and was created to assess outputs from climate models with high-resolution terrestrial palaeoclimatic proxies. VARDA additionally provides a technical environment that enables us to explore the database of varved lake sediments using a connected data model and can generate a state-of-the-art graphic representation of a multisite comparison. This allows the reassessment of existing chronologies and tephra events to synchronize and compare even distant varved lake records. Furthermore, the present version of VARDA permits the exploration of varve thickness data. In this paper, we report in detail on the data-mining and compilation strategies for the identification of varved lakes and assimilation of high-resolution chronologies, as well as the technical infrastructure of the database. Additional palaeoclimatic proxy data will be provided in forthcoming updates. The VARDA graph database and user interface can be accessed online at https://varve.gfz-potsdam.de (last access: 15 September 2020), all datasets of version 1.0 are available at https://doi.org/10.5880/GFZ.4.3.2019.003 (Ramisch et al., 2019).

Description: This dataset resulted from a parallel monitoring at two lakes, Lake Tiefer See (near Klocksin, TSK; 53° 35.5’ N, 12° 31.8’ E; 62 masl; N Germany) and Lake Czechowskie (Jezioro Czechowskie, JC; 53° 52.4’ N, 18° 14.3’ E; 108 masl; N Poland), and includes four different type of data for both locations: (i) sediment cores microfacies data, (ii) sediment fluxes and composition, (iii) selected water column data, and (iv) selected meteorological information obtained on site. This dual lake monitoring set-up was established in 2012 with the aim to investigate seasonal sedimentation and varve forming processes in detail. The datasets are provided in individual *.csv files, per type of data and per lake. The thin section data from surface sediment cores comprises the thicknesses of the most recent calcite varves’ sub-layers: spring diatom sub-layer, summer calcite sub-layer, and autumn/winter re-suspension sub-layer. The sediment flux data was obtained from sediment traps located in different water depths (epi- and hypolimnion), and the sediment composition is given by the fluxes of total organic carbon (TOC), calcium carbonate (as calculated from total inorganic carbon; TIC), and diatoms & inorganic matter. The water column data comprises water temperature from stationary loggers, and dissolved oxygen measured in ~ 1 meter depth-resolution. The meteorological data includes daily averages of air temperature and mean wind-speed, and summed daily rainfall. Further details about the sampling and analytical methods, data acquisition, and processing are given in Roeser et al. (2021; http://doi.org/10.1111/bor.12506).

Description: Stable carbon isotopes of sediment organic matter (δ13COM) are widely applied in paleoenvironmental studies. Interpretations of δ13COM, however, remain challenging and factors that influence δ13COM may not apply across all lakes. Common explanations for stratigraphic shifts in δ13COM include changes in lake productivity or changes in inputs of allochthonous OM. We investigated the influence of different oxygen conditions (oxic versus anoxic) on the δ13COM values in the sediments of Lake Tiefer See. We analysed (1) a long sediment core from the deepest part of the lake, (2) two short, sediment–water interface cores from shallower water depths, and (3) OM in the water column, i.e. from sediment traps. Fresh OM throughout the entire water column showed a relatively constant δ13COM value of approximately − 30.5‰. Similar values, about − 31‰, were obtained for well-varved sediments in both the long and short, sediment–water interface cores. In contrast, δ13COM values from non-varved sediments in all cores were significantly less negative (− 29‰). The δ13COM values in the sediment–water interface cores from different water depths differ for sediments of the same age, if oxygen conditions at the time of deposition were different at these sites, as suggested by the state of varve preservation. Sediments deposited from AD 1924 to 1980 at 62 m water depth are varved and exhibit δ13COM values around − 31‰, whereas sediments of the same age in the core from 35 m water depth are not varved and show less negative δ13COM values of about − 29‰. The relation between varve occurrence and δ13COM values suggests that δ13COM is associated with oxygen conditions because varve preservation depends on hypolimnetic anoxia. A mechanism that likely influences δ13COM is selective degradation of OM under oxic conditions, such that organic components with more negative δ13COM are preferably decomposed, leading to less negative δ13COM values in the remaining, undegraded OM pool. Greater decomposition of OM in non-varved sediments is supported by lower TOC concentrations in these deposits (~ 5%) compared to well-varved sediments (~ 15%). Even in lakes that display small variations in productivity and terrestrial OM input through time, large spatial and temporal differences in hypolimnetic oxygen concentrations may be an important factor controlling sediment δ13COM.

Description: The Northeast German Lowland Observatory (TERENO‐NE) was established to investigate the regional impact of climate and land use change. TERENO‐NE focuses on the Northeast German lowlands, for which a high vulnerability has been determined due to increasing temperatures and decreasing amounts of precipitation projected for the coming decades. To facilitate in‐depth evaluations of the effects of climate and land use changes and to separate the effects of natural and anthropogenic drivers in the region, six sites were chosen for comprehensive monitoring. In addition, at selected sites, geoarchives were used to substantially extend the instrumental records back in time. It is this combination of diverse disciplines working across different time scales that makes the observatory TERENO‐NE a unique observation platform. We provide information about the general characteristics of the observatory and its six monitoring sites and present examples of interdisciplinary research activities at some of these sites. We also illustrate how monitoring improves process understanding, how remote sensing techniques are fine‐tuned by the most comprehensive ground‐truthing site DEMMIN, how soil erosion dynamics have evolved, how greenhouse gas monitoring of rewetted peatlands can reveal unexpected mechanisms, and how proxy data provides a long‐term perspective of current ongoing changes.