The Paleocene-Eocene Thermal Maximum (PETM; ~55.9 Ma) was a geologically rapid warming period associated with carbon release, which caused a marked increase in the hydrological cycle. Here, we use lithium (Li) isotopes to assess the global change in weathering regime, a critical carbon drawdown mechanism, across the PETM. We find a negative Li isotope excursion of ~3‰ in both global seawater (marine carbonates) and in local weathering inputs (detrital shales). This is consistent with a very large delivery of clays to the oceans or a shift in the weathering regime toward higher physical erosion rates and sediment fluxes. Our seawater records are best explained by increases in global erosion rates of ~2× to 3× over 100 ka, combined with model-derived weathering increases of 50 to 60% compared to prewarming values. Such increases in weathering and erosion would have supported enhanced carbon burial, as both carbonate and organic carbon, thereby stabilizing climate.
17The Ganges river system is responsible for the transportation of a large flux of 18 dissolved materials derived from Himalayan weathering to the oceans. Silicate 19 weathering-driven cooling resulting from uplift of the Himalayas has been 20 proposed to be a key player in Cenozoic climate variation. This study has 21analysed Li isotope (δ 7 Li) ratios from over 50 Ganges river waters and 22 sediments, in order to trace silicate weathering processes. Sediments have δ 7 Li of 23 ~0‰, identical to bulk continental crust, however suspended sediment depth 24 profiles do not display variations associated with grain size that have been 25
Chemical weathering of silicate rocks is a primary drawdown mechanism of atmospheric carbon dioxide. The processes that affect weathering are therefore central in controlling global climate. A temperature-controlled "weathering thermostat" has long been proposed in stabilising long-term climate, but without definitive evidence from the geologic record. Here we use lithium isotopes (δ 7 Li) to assess the impact of silicate weathering across a significant climate-cooling period, the end-Ordovician Hirnantian glaciation (~445 Ma). We find a positive δ 7 Li excursion, suggestive of a silicate weathering decline. Using a coupled lithium-carbon model, we show that initiation of the glaciation was likely caused by declining CO 2 degassing, which triggered abrupt global cooling, and much lower weathering rates. This lower CO 2 drawdown during the glaciation allowed climatic recovery and deglaciation. Combined, the data and model provide support from the geological record for the operation of the weathering thermostat. LetterThe recovery and stabilisation of Earth's climate system from perturbations is central to the continued survival of life. Chemical weathering of continental silicate rocks driving marine carbonate precipitation is the Earth's primary longterm mechanism for removal of atmospheric CO 2 (Berner, 2003 feedback control on weathering rates (i.e. greater temperatures cause higher weathering rates, removing more CO 2 ) would result in a climate-stabilising mechanism. This "weathering thermostat" has long been postulated and assumed in models (Colbourn et al., 2015). However, direct evidence for weathering rate changes in response to climate perturbations has been harder to pin down in the geological record.The Late Ordovician Hirnantian (~445 Ma) records the second largest mass extinction in Earth history. This was likely caused by rapidly decreasing temperatures, culminating in an ice-sheet over Gondwana (Elrick et al., 2013). As such, similarities exist between the Hirnantian and the Late Cenozoic glaciations (Ghienne et al., 2014). The behaviour of atmospheric CO 2 is of particular interest, because of the potential role of declining CO 2 in initiating the glaciation and of increasing CO 2 in terminating it (Vandenbroucke et al., 2010). Either or both could have involved changes in silicate weathering rates (Berner, 2003). The combination of changes in weathering rates and pCO 2 also resulted in a global positive δ 13 C excursion (HICE) (Lenton et al., 2012;Ghienne et al., 2014). Osmium isotopes have suggested a decline in weathering during the glacial maximum (Finlay et al., 2010). However, Os mainly traces weathering provenance, rather than weathering rates or processes. Lithium isotopes are the only tracer available whose behaviour is solely controlled by silicate weathering processes, and therefore give a unique insight into CO 2 drawdown and climate-stabilisation.Lithium isotopes (δ 7 Li) are not fractionated by biological processes (Pogge von Strandmann et al., 2016), and are not affected by carbon...
Silicate weathering is the primary control of atmospheric CO2 concentrations on multiple timescales. However, tracing this process has proven difficult. Lithium isotopes are a promising tracer of silicate weathering. This study has reacted basalt sand with natural river water for ~9 months in closed experiments, in order to examine the behaviour of Li isotopes during weathering. Aqueous Li concentrations decrease by a factor of ~10 with time, and d 7 Li increases by ~19‰, implying that Li is being taken up into secondary phases that prefer 6 Li. Mass balance using various selective leaches of the exchangeable and secondary mineral fractions suggest that ~12-16% of Li is adsorbed, and the remainder is removed into neoformed secondary minerals. The exchangeable fractionation factors have a D 7 Liexch-soln =-11.6 to-11.9‰, while the secondary minerals impose D 7 Lisecmin-soln =-22.5 to-23.9‰. Overall the experiment can be modelled with a Rayleigh fractionation factor of a = 0.991, similar to that found for natural basaltic rivers. The mobility of Li relative to the carbon-cycle-critical cations of Ca and Mg changes with time, but rapidly evolves within one month to remarkably similar mobilities amongst these three elements. Th evolution shows a linear relationship with d 7 Li (largely due to a co-variation between aqueous [Li] and d 7 Li), suggesting that Li isotopes have the potential to be used as a tracer of Ca and Mg mobility during basaltic weathering, and ultimately CO2 drawdown.
Silicon isotope values (δ 30 SiDSi) of dissolved silicon (DSi) have been analyzed in the Lena River and its tributaries, one of the largest Arctic watersheds in the world. The geographical and temporal variations of δ 30 SiDSi range from +0.39 to +1.86‰ with DSi concentrations from 34 to 121 µM. No obvious patterns of DSi concentrations and δ 30 SiDSi values were observed along over 200 km of the two major tributaries, the Viliui and Aldan Rivers. In summer, the variations of DSi concentrations and δ 30 SiDSi values in the water are either caused by biological uptake by higher plants and phytoplankton or by mixing of water masses carrying different DSi concentrations and δ 30 SiDSi values. DSi in tributaries from the Verkhoyansk Mountain Range seems to be associated with secondary clay formation that increased the δ 30 SiDSi values, while terrestrial biological production is likely more prevalent in controlling δ 30 SiDSi values in Central Siberian Plateau and Lena Amganski Inter-River Area. In winter, when soils were frozen, the δ 30 SiDSi values in the river appeared to be controlled by weathering and clay formation in deep intrapermafrost groundwater. During the spring flood, dissolved silicate materials and phytoliths were flushed from the upper thawed soils into rivers, which reset δ 30 SiDSi values to the values observed prior to the biological bloom in summer. The results indicate that the Si isotope values reflect the changing processes controlling Si outputs to the Lena River and to the Arctic Ocean between seasons.The annual average δ 30 SiDSi value of the Lena Si flux is calculated to be +0.86±0.3‰ using measured δ 30 SiDSi values from each season. Combined with the estimate of +1.6±0.25‰ for the Yenisey River, an updated δ 30 SiDSi value of the major river Si inputs to the Arctic Ocean is estimated to be +1.3±0.3‰. This value is expected to shift towards higher values in the future because of the impacts from a variety of biological and geochemical processes and sources under global warming.
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