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Establishing Silicon Isotopes as Weathering Tracers for Paleoenvironmental Studies

Final Report Summary - SI-PALEO (Establishing Silicon Isotopes as Weathering Tracers for Paleoenvironmental Studies)

On continents, weathering of silicate rocks acts as a major sink for atmospheric CO2, thought to have played an important role in regulating the Earth’s climate over geological time. Despite the potential importance of this process, and its significance to the global carbon cycle, our ability to reconstruct past variations in silicate weathering remains limited. Built on recent advances in the understanding of the biogeochemical cycling of silicon (Si), the SI-PALEO project aimed at exploring the use of Si isotopes in sediments as a new proxy for past continental weathering. To this end, one of the main objectives was to determine the factors controlling the distribution of Si isotope ratios in fine-grained sediments, in order to better understand how they may relate to chemical weathering and environmental parameters. Another goal of the SI-PALEO project was also to assess, using proxies for chemical weathering and soil erosion, the degree to which prehistoric human activities may have affected past landscapes. There is clear evidence that human imprint on the environment is significantly changing the weathering rates of continental rocks at present, but the impact of humans on prehistoric landscapes remains controversial. To address this issue in the SI-PALEO project, we attempted to assess, via marine sedimentary records, the contribution of climate change versus human activities to two major environmental crises that took place in Central Africa and in Australia, about 2000 and 40,000 years ago, respectively.
A major part of the project relied on the analysis of a large number of modern sediments collected near the mouth of rivers worldwide, including some of the World largest rivers (e.g. Amazon, Niger, Congo, Mississippi, Yellow River). The mineralogical and geochemical composition of these samples had been already fully characterized for clay mineralogy and major/trace element abundances. Taken together with these data, the results obtained during the course of the project for neodymium and hafnium isotopes were first used to provide revised estimates for the geochemical composition of the upper continental crust (Bayon et al., 2015), but also to demonstrate a relationship between the geochemistry of world river sediments and weathering conditions on continents (Bayon et al., 2016). The new data acquired for Si isotopes for the same set of river sediment samples also indicated a clear link between Si isotopic signatures in fine-grained sediments, the degree of chemical weathering, and climatic parameters (rainfall, temperature) in corresponding river basins (Figure 1). While the distribution of Si isotopes in river sediments also appears to be influenced to some extent by lithology, our results now pave the way for their application in marine sedimentary records as proxies for past silicate weathering (Bayon et al., in prep_a).

See Figure 1.

During the SI-PALEO project, we also conducted investigations for two case studies: the Congo Basin and the Murray-Darling Basin (Australia’s largest river system), focusing on the analysis of marine sediment cores and river-borne particles from each river basin. In these two areas, major vegetation changes occurred in the past, contemporaneous with periods of major human settlements. Over the past few years, there has been intense debate as to whether these changes were the consequence of climate change and/or human colonization. To a large extent, this uncertainty reflected the lack of continuous high-resolution records for paleoclimate from both Central Africa and inland Australia during the late Quaternary period. In the SI-PALEO project, by reconstructing the geochemical composition of the sediment exported from the Congo River and the Murray-Darling Basin over the past thousand years, we were able to provide new constraints on the paleohydrological evolution of Central Africa and Australia, demonstrating that in both regions rainfall distribution had been mainly driven by latitudinal migrations of the intertropical convergence zone (ITCZ). These findings allowed us to propose that the abrupt shifts in sediment export patterns and vegetation identified from our proxy records after about 2,000 years (for the Congo Basin) and 38,000 years ago (for Australia) could not due to climate change only, but instead were probably likely the consequence of human activities, presumably via agriculture (for Congo) and enhanced biomass burning (Bayon et al., submitted; Bayon et al., in prep_b).

See Figure 2.