Early stage weathering systematics of Pb and Nd isotopes derived from a high-Alpine Holocene lake sediment record

Abstract

Radiogenic Pb and Nd isotopes are well established tools in palaeoceanographic science tracing ambient climate and continental runoff to the oceans down to sub-millennial timescales. Particularly in case of Pb isotopes, a clear climate dependency of continental isotopic runoff on glacial-interglacial transitions has been observed. Pb isotopes were reported to be released incongruently during initial chemical weathering. This incongruent release implies that Pb isotopic runoff compositions differ from the bulk catchment Pb isotopic signal. Yet only little is known about the processes leading to the incongruent release and the timescales of weathering on the continents. In this study we targeted the adsorbed trace metal signature in sediments from a Swiss high-Alpine lake that have accumulated since the retreat of the large Alpine ice domes during the last deglaciation to investigate initial Pb and Nd isotope weathering processes in a granitic environment. Additionally, selected adsorbed element concentrations and ratios were analysed to complement the isotopic physicochemical weathering information. The integrity of the presented isotope records is supported by further investigation into the lake environment (e.g. oxic/anoxic conditions) and its potential influence on the isotopic record. The Pb isotope records during the early lake phase witnessed high-amplitude isotopic fluctuations linked to the initial chemical weathering of fine glacial substrate. This finding is also supported by the lithology of the core and rapidly decreasing adsorbed Th and U concentrations. Following this early lake phase, the majority of the Holocene traced congruent release of 207 Pb/ 204 Pb and 206 Pb/ 204 Pb and a significant depletion of 208 Pb/ 204 Pb in the adsorbed phase. These findings corroborate earlier suggestions of more effective weathering of uranogenic minerals yet also call for the presence of more weathering-resistant thorogenic minerals in the lake catchment. The latest 2.2 ka of the record are significantly overprinted by anthropogenic Pb deposition coinciding with the rise and fall of the Roman Empire. Finally, our data suggest that Nd isotopes are equally affected by incongruent weathering during the initial deglacial weathering processes, albeit at smaller magnitude than seen for Pb isotopes.

No detectable Weddell Sea Antarctic Bottom Water export during the Last and Penultimate Glacial Maximum

“…Our Pb isotope measurements were performed on a Thermo Scientific Neptune Plus MC-ICP-MS at GEOMAR, Kiel. Mass bias correction during Pb isotope measurements was done externally using the Tl-doping technique 62,63 spiked by a NIST997 Tl standard solution (Pb:Tl =~4:1). Given that Tl and Pb fractionate slightly differently during ionization 64 , 205 Tl/ 203 Tl were determined on a session-by-session basis so that NBS981 Pb isotope compositions matched published compositions 63,65 (Supplementary Data 1).…”

Rare earth element and neodymium isotope tracing of sedimentary rock weathering

“…Boyle et al, 1977), which can be used to help identify the origin of weathered rocks on continents (e.g. von Blanckenburg et al, 2015;Hindshaw et al, 2018;Süfke et al, 2019;Jang et al, 2020). Iron oxides are also ubiquitous weathering products in soils, which can be transported as particulates in rivers prior to deposition as sediments (Bayon et al, 2004;Kraft et al, 2013).…”

Precessional Cyclicity of Seawater Pb Isotopes in the Late Miocene Mediterranean

“…Gutjahr et al () previously raised concerns that anoxic conditions may result in remobilization of Pb. However, a recent study employing this leaching method on anoxic Holocene sediments in an Alpine lake extracted an unperturbed Pb isotope record (Süfke et al, ), suggesting that reliable authigenic Pb isotopic compositions can be extracted from anoxic sediments. Experiments with sediments from a temperate acidic lake have similarly shown that post‐depositional Pb mobility did not occur (Davis & Galloway, ).…”

“…NBS997 Tl standard was added aiming for a Pb/Tl ~ 4 (Belshaw et al, ). The most accurate 205 Tl/ 203 Tl ratio required for mass bias correction was determined on a session‐by‐session basis so that the sum of all offsets of all six Pb isotope pairs for NIST NBS 981 equaled 0 ppm compared with published compositions of Baker et al () (see Süfke et al, ). This always led to slightly higher 205 Tl/ 203 Tl than certified for NIST NBS997 because of the slightly different ionization behavior of Tl compared with Pb (Thirlwall, ).…”