Insights into chemical weathering of the upper continental crust from the geochemistry of ancient glacial diamictites

Su, Li; Gaschnig, Richard M.; Rudnick, Roberta L.

doi:10.1016/j.gca.2015.12.012

Cited by 53 publications

(27 citation statements)

References 132 publications

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“…This weathering signature appears to be inherited from the sediment source in most cases, rather than reflecting post-depositional changes or weathering during glacial erosion and transport (Gaschnig et al, 2014;Li et al, 2016).…”

Section: Identifying and Seeing Through The Effects Of Chemical Weathmentioning

confidence: 99%

Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites

Gaschnig

Rudnick

McDonough

et al. 2016

Geochimica et Cosmochimica Acta

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102

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Section: Identifying and Seeing Through The Effects Of Chemical Weathmentioning

confidence: 99%

Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites

Gaschnig

Rudnick

McDonough

et al. 2016

Geochimica et Cosmochimica Acta

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116

102

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“…Overall, therefore, this suggests that the Li isotopic excursion represents a primary seawater signal. While carbonates tend to be the usual seawater archive (e.g., Misra and Froelich, 2012;Pogge von Strandmann et al, 2013), silicates have also been investigated (Dellinger et al, 2017), and sediments older than Ordovician are considered to represent pre-depositional (unaltered by diagenesis) weathering signals (Li et al, 2016). Hence, detrital clays (which dominate at Dob's Linn) should reflect changing local continental weathering conditions (see Supplementary Information and Fig.…”

Section: Lettermentioning

confidence: 99%

Global climate stabilisation by chemical weathering during the Hirnantian glaciation

Strandmann¹,

Desrochers²,

Murphy³

et al. 2017

Geochem. Persp. Let.

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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...

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“…By contrast, nearly all of the diamictites record a weathering signature in their chemical compositions (Gaschnig et al, 2014Li et al, 2016). For example, the chemical index of alteration (CIA = molar Al 2 O 3 /(K 2 O + Na 2 O + CaO * ), where CaO * is CaO associated with the silicate fraction of the bulk sample) for most of the diamictites is high ($55 to $75), above those of igneous rocks (<55) .…”

Section: Sample Representativenessmentioning

confidence: 99%

“…For example, the chemical index of alteration (CIA = molar Al 2 O 3 /(K 2 O + Na 2 O + CaO * ), where CaO * is CaO associated with the silicate fraction of the bulk sample) for most of the diamictites is high ($55 to $75), above those of igneous rocks (<55) . Using Li and Pb isotopes, Li et al (2016) showed that the intensity of the weathering signature in the diamictites increases as one goes back in time and concluded that the weathering signature is largely inherited from the provenance of the diamictites; there has been very limited post-or syn-depositional chemical weathering. This inference is supported by correlations between PGE and insoluble transition elements, described in Section 5.2.1, below (see also Figs.…”

Section: Sample Representativenessmentioning

confidence: 99%

“…The difference is that Re, but not Mo, can also be enriched in sediments under suboxic conditions (Crusius et al, 1996;Morford and Emerson, 1999;Miller et al, 2011), suggesting that Re is scavenged more readily than Mo. This difference is reflected in the diamictites, which generally show a weathering signature inherited from their provenance (Li et al, 2016). A weak correlation between Re and total organic carbon (TOC) is observed in the diamictite composites (Fig.…”

Section: Re-os Isotopesmentioning

confidence: 99%

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Platinum-group element abundances and Re–Os isotopic systematics of the upper continental crust through time: Evidence from glacial diamictites

Chen

Walker

Rudnick

et al. 2016

Geochimica et Cosmochimica Acta

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The fine-grained matrix of glacial diamictites, deposited periodically by continental ice sheets over much of Earth history, provides insights into the average composition and chemical evolution of the upper continental crust (UCC) , and references therein). The concentrations of platinum-group elements (PGEs, including Os, Ir, Ru, Pt and Pd) and the geochemically related Re, as well as 187 Re/ 188 Os and 187 Os/ 188 Os ratios, are reported here for globally-distributed glacial diamictites that were deposited during the Mesoarchean, Paleoproterozoic, Neoproterozoic and Paleozoic eras. The medians and averages of PGE concentrations of these diamictite composites decrease from the Mesoarchean to the Neoproterozoic, mimicking decreases in the concentrations of first-row transition elements (Sc, V, Cr, Co and Ni). By contrast, Re concentrations are highly variable with no discernable trend, owing to its high solubility. Assuming these diamictites are representative of average UCC through time, the new data are fully consistent with the previous inference that the Archean UCC contained a greater proportion of mafic-ultramafic rocks relative to younger UCC. Linear regressions of PGEs versus Cr and Ni concentrations in all the diamictite composites from the four time periods are used to estimate the following concentrations of the PGEs in the present-day UCC: 0.059 ± 0.016 ng/g Os, 0.036 ± 0.008 ng/g Ir, 0.079 ± 0.026 ng/g Ru, 0.80 ± 0.22 ng/g Pt and 0.80 ± 0.26 ng/g Pd (2r of 10,000 bootstrapping regression results). These PGE estimates are slightly higher than the estimates obtained from loess samples. We suggest this probably results from loess preferentially sampling younger UCC rocks that have lower PGE concentrations, or PGEs being fractionated during loess formation. A Re concentration of 0.25 ± 0.12 ng/g (2r) is obtained from a regression of Re versus Mo. From this, time-integrated 187 Re/ 188 Os and 187 Os/ 188 Os ratios for the UCC are calculated, assuming an average UCC residence duration of $2.0 Ga, yielding ratios of 20 ± 12 and 0.80 ± 0.38 (2r), respectively.

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Insights into chemical weathering of the upper continental crust from the geochemistry of ancient glacial diamictites

Cited by 53 publications

References 132 publications

Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites

Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites

Global climate stabilisation by chemical weathering during the Hirnantian glaciation

Platinum-group element abundances and Re–Os isotopic systematics of the upper continental crust through time: Evidence from glacial diamictites

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