The silicon isotopic composition of fine-grained river sediments and its relation to climate and lithology

Abstract

The d 30 Si stable isotopic composition of silicon in soils and fine-grained sediments can provide insights into weathering processes on continents, with important implications on the Si budget of modern and past oceans. To further constrain the factors controlling the distribution of Si isotopes in sediments, we have analysed a large number (n = 50) of separate size-fractions of sediments and suspended particulate materials collected near the mouth of rivers worldwide. This includes some of the world's largest rivers (e.g. Amazon, Congo, Mackenzie, Mississippi, Murray-Darling, Nile, Yangtze) and rivers from the case study areas of the Congo River Basin and Northern Ireland. Silt-size fractions exhibit a mean Si isotopic composition (d 30 Si = À0.21 ± 0.19‰; 2 s.d.) similar to that previously inferred for the upper continental crust. In contrast, clay-size fractions display a much larger range of d 30 Si values from À0.11‰ to À2.16‰, which yield a global d 30 Si clay of À0.57 ± 0.60‰ (2 s.d.) representative of the mean composition of the average weathered continental crust. Overall, these new data show that the Si isotopic signature transported by river clays is controlled by the degree of chemical weathering, as inferred from strong relationships with Al/Si ratios. At a global scale, the clay-bound Si isotopic composition of the world's largest river systems demonstrates a link with climate, defining a general correlation with mean annual temperature (MAT) in corresponding drainage basins. While the distribution of Si isotopes in river sediments also appears to be influenced by the tectonic setting, lithological effects and sediment recycling from former sedimentary cycles, our results pave the way for their use as paleo-weathering and paleo-climate proxies in the sedimentary record.

Ge and Si isotope signatures in rivers: A quantitative multi-proxy approach

“…Similar incongruency might affect riverine Ge/Si diss , δ 30 Si diss , and δ 74 Ge diss of the Watson River, potentially invalidating the modeling results here, given that the model assumes congruent dissolution of primary minerals. However, it is notable that using a global riverine clay dataset Bayon et al (2018) have suggested limited δ 30 Si fractionation in cold and arid environments, in agreement with our results from West Greenland. Overall, it appears that glacial weathering results in distinct geochemical and isotopic signatures in streams, and further research is needed to understand the mechanisms responsible.…”

“…3a,b) -the opposite from an expected weathering signal but expected if hydrodynamic sorting is affecting the bedload composition. Quartz, although not measured in this location, typically has the lowest Ge/Si ratios of all silicate minerals (Mortlock & Froelich, 1987;Kurtz et al, 2002) and higher δ 30 Si relative to fine sediments (Bayon et al, 2018) and becomes progressively enriched in the river bedload relative to the suspended load during downstream transport (Bouchez et al, 2011), explaining the distinctive trend with elevation in Ge/Si and δ 30 Si compositions.…”

The roles of climate and human land-use in the late Holocene rainforest crisis of Central Africa

“…1C). In terms of geological setting, the northern and eastern parts of the Congo Basin are dominated by Archaean and Proterozoic metamorphic basement rocks, while the southwestern part of the watershed is mostly composed of Mesozoic and Cenozoic sedimentary rocks (Bayon et al, 2018). Neogene volcanic rocks associated with the East African Rift system are also encountered in the eastern part of the basin.…”

Silicon Isotope Geochemistry: Fractionation Linked to Silicon Complexations and Its Geological Applications

“…In addition to marine system, the variation of δ 30 Si could help us evaluate the silicon budgets in terrestrial aqueous systems. After the pioneering works on the δ 30 Si of dissolved silicic acid in oceans, rivers, and estuaries [46], the δ 30 Si distributions in various fluids have been clarified, such as freshwater (i.e., −0.1 to +3.4‰) [18,26,46,49,50,51,112,118,141,148,149,150,151], groundwater (−1.43 to +0.43‰) [51], and seawater (i.e., +0.4 to +3.1‰) [13,19,46,47,140,152]. As discussed above, the preferential uptake of light Si isotopes into soil clays and organisms leads to predominately positive Si isotope signatures in continental surface waters.…”