DOI: 10.5880/gfz.3.3.2021.005
title: Dataset on Biogeochemical cycling of Mg and Li isotopes in the Conventwald (the Black Forest, Germany)

description: This data set contains chemical and Mg isotope analyses of time-series creek water, subsurface flow (0-15cm and 15-150cm), vegetation, regolith, clay-sized fraction and exchangeable fraction of regolith from a catchment of the Black Forest, Germany. This dataset is a following work of “Uhlig, D., & von Blanckenburg, F. (2019)", in which major and trace elements concentrations and 87Sr/86Sr isotope data was reported on the same batch of samples. With the new Mg isotope analyses, we investigated the potential controlling factors on water Mg isotopic composition, and we found exchange reactions in our catchment are a primary control on water chemistry. To further interrogate this finding, a batch of adsorption and desorption experiments using soil samples from our study site were carried out. The adsorption and desorption experiment results are also included here. This combination of field research and lab experiments informs about processes fractionating Mg in the critical zone – with the role of the exchangeable pool highlighted as particularly important – and further verifies the potential of Mg isotopes as a tool in tracing continental weathering process. Samples are assigned with International Geo Sample Numbers (IGSN), a globally unique and persistent Identifier for physical samples.
description: Briefly, shallow regolith was sampled at depth increments of 20 cm in a 3 m deep trench. Deeper regolith beyond 3 m was retrieved using diesel-powered wireline core-drilling to ~20 m. Time series water samples were collected from 01.03.2015 to 25.02.2016. Open rainfall and throughfall were collected biweekly in bulk container coved by a netting mesh. Creek discharge was collected daily at midnight by autosampler. Groundwater was sampled daily by an autosampler. The groundwater table level was monitored by a pressure probe installed 8.5 m below the surface. Subsurface flow from subsurface flow collectors was collected at two depths intervals: 0-15 cm, 15-150 cm. All the water samples were acidified and stored at 4 ˚C before analysis. Living wood, beech leaves and spruce needles were collected from representative mature and young trees. All measurements were performed in the Helmholtz Laboratory for the Geochemistry of the Earth Surface (HELGES) at GFZ Potsdam. Soil, saprolite, the extracted clay-sized fraction, primary minerals, and bedrock were dissolved by acid digestion using a mixture of concentrated HF and HNO3 in PFA vials. Aqua regia was also applied to assist digestion after HF and HNO3 treatment. Elemental concentrations of the filtered supernatant, water samples, and acid digested solution were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, Varian 720-ES). Relative uncertainties are better than 5% for Mg based on repeat analyses of the international reference materials SLRS‐6 (river water, NRC CNRC), BHVO-2 (Basalt, USGS), SRM2709a (soil, USGS) and synthetic in-house standards. Mg isotopes were measured via multicollector inductively coupled plasma mass spectrometry (MC-ICPMS, Thermo Neptune) using DSM3 as bracketing standard to correct for instrumental mass bias.