Lithium isotopes as a tracer for changes in interglacial-glacial weathering processes

Final Report Summary - LITING (Lithium isotopes as a tracer for changes in interglacial-glacial weathering processes)

The chemical weathering of silicate rocks is a key feedback mechanism for the stabilisation of Earth’s climate, by regulating the carbon cycle. The primary aim of this research project was to constrain how chemical weathering processes, and in particular chemical weathering intensity, respond to rapid climate change (millennial time-scales). The areas on Earth most susceptible to rapid climate change are those at high latitudes and therefore these regions are where sensitivity to the feedback between silicate weathering and climate is greatest. The high sensitivity is not only due to the current rapid increase in temperature, but also to the waxing and waning of ice-sheets, which have a profound effect on the surface exposure of rock and additionally grinds the rock to a fine powder making it highly reactive.

In order to investigate the effect of glaciation on mineral weathering, the stream water chemistry and the bacterial community composition were analysed in two catchments (Svalbard) containing nominally identical sedimentary formations but which differed in the extent of glaciation. The stream waters were analysed for major ions, δ34S, δ18OSO4 and δ18OH2O and associated stream sediments were analysed by 16S rRNA gene tagged sequencing. Sulphate comprised 72–86% and 35–45% of the summer anion budget (in meq) in the unglaciated and glaciated catchments respectively. This indicates that sulfuric acid generated from pyrite weathering is a significant weathering agent in both catchments. Based on the relative proportions of cations, sulphate and bicarbonate, the stream water chemistry of the unglaciated catchment was found to be consistent with a sulphide oxidation coupled to silicate dissolution weathering process whereas in the glaciated catchment both carbonates and silicates weathered via both sulfuric and carbonic acids.

Stable isotope measurements of sulphate, together with inferences of metabolic processes catalysed by resident microbial communities, revealed that the pyrite oxidation reaction differed between the two catchments. No δ34S fractionation relative to pyrite was observed in the unglaciated catchment and this was interpreted to reflect pyrite oxidation under oxic conditions. In contrast, δ34S and δ18OSO4 values were positively correlated in the glaciated catchment and were positively offset from pyrite. This was interpreted to reflect pyrite oxidation under anoxic conditions with loss of S intermediates. This research suggests that glaciation may alter stream water chemistry and the mechanism of pyrite oxidation through an interplay of biological, physical and chemical factors.

The stable carbon isotopic composition of dissolved organic matter (δ13C-DOC) reveals information about its source and extent of biological processing. In this project we reported the lowest δ13C-DOC values (−43.8‰) measured to date in surface waters. The streams were located in the High Arctic, a region currently experiencing rapid changes in climate and carbon cycling. Based on the widespread occurrence of methane cycling in permafrost regions and the detection of the pmoA gene, a proxy for aerobic methanotrophs, we concluded that the low δ13C-DOC values are due to organic matter partially derived from methanotrophs consuming biologically produced, 13C-depleted methane. These findings demonstrate the significant impact that biological activity has on the stream water chemistry exported from permafrost and glaciated environments in the Arctic. Given that the catchments studied here are representative of larger areas of the Arctic, occurrences of low δ13C-DOC values may be more widespread than previously recognized, with implications for understanding C cycling in these environments.

Microbial eukaryotes are increasingly being recognised for their role in global biogeochemical cycles, yet very few studies have focussed on their distribution in high-latitude stream sediments, an important habitat which influences stream water nutrient chemistry. In this research project, we determined the abundance and phylogeny of 18S rRNA gene fragments recovered from four sediment samples in Svalbard, two from a glaciated catchment and two from an unglaciated permafrost-dominated catchment. Whilst there was no difference between the two catchments in terms of Rao's phylogenetic diversity (0.18-0.04 1SD), the glaciated catchment samples had slightly higher richness (138-139) than the unglaciated catchment samples (67-106). At the phylum level, Ciliophora (heterotrophs) had the highest relative abundance in the samples from the glaciated catchment (32-63%), but only comprised 0-17% of the unglaciated catchment samples. Bacillariophyta (phototrophs) was the most abundant phylum in one of the samples from the unglaciated catchment (43%) but phototrophic microbial eukaryotes only formed a minor component of the glaciated catchment samples (<2%) suggesting that in these environments the microbial eukaryotes are predominantly heterotrophic (chemotrophic). This is in contrast to previously published data from Robertson Glacier, Canada where the relative abundance of chlorophyta (phototrophs) in three samples was 48-57%. The contrast may be due to differences in glacial hydrology and/or geology, highlighting the variation in microbial eukaryote communities between nominally similar environments.

The detailed results from this project, which combine geology, chemistry and biology, give an unprecedented insight into weathering processes occurring in high-latitude environments. This information can be used to improve predictions concerning how these regions will respond to a warming climate, weathering changes induced by glacial-interglacial cycles and how nutrient sources to the Arctic ocean may change: important for those whose livelihoods depend on the ocean.

Website: www.weathering78n.org
Contact: ruth.hindshaw@gmail.com

Final Report Summary - LITING (Lithium isotopes as a tracer for changes in interglacial-glacial weathering processes)

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