Global climate stabilisation by chemical weathering during the Hirnantian glaciation

Abstract

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...

The Li isotope composition of marine biogenic carbonates: Patterns and mechanisms

“…n=61). Other carbonate (JLs-1 and in-house marl standard) and rock (BCR-2 and SGR-1) standards are reported in Pogge von Strandmann et al (2013Strandmann et al ( , 2017. The total procedural blank for Li isotopes is effectively undetectable (<0.005 ng Li).…”

“…Furthermore, the residence time of Li in the ocean is about 1-3 Ma, and the marine Li isotopic composition (δ 7 Li = [( 7 Li/ 6 Li)/( 7 Li/ 6 Li) L-SVEC -1] ×1000; expressed in ‰) and concentration are spatially uniform (Angino and Billings 1966). Published Li isotope records in foraminifera (Hathorne and James, 2006;Misra and Froelich, 2012) and bulk carbonates (Lechler et al, 2015;Pogge von Strandmann et al, 2013;Pogge von Strandmann et al, 2017) are characterized by large (several per mil) δ 7 Li variations. These changes have been attributed to past changes in weathering congruency, intensity, or rates (Bouchez et al, 2013;Froelich and Misra, 2014;Li and West, 2014;Wanner et al, 2014).…”

“…In addition, well-preserved planktic foraminifera are not very abundant in the geological record prior to the Cenozoic (Wilkinson, 1979). Finally, Ullmann et al (2013b) for belemnite and Pogge von Strandmann et al, (2017) for brachiopods have shown that the shell of these organisms may preserve Li isotope composition of the ocean over geological timescales.…”

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

“…First, the process of the chemical weathering of silicate minerals is a primary drawdown mechanism of atmospheric CO 2 and is a key process in the geological C cycle (Walker et al, 1981). Therefore, understanding the global Si cycle can provide insight to the functioning of the weathering thermostat (Pogge von Strandmann et al, 2017). Second, DSi is a required nutrient for many organisms, both aquatic (Bowler et al, in review) and terrestrial (Conley and Carey, 2015).…”

Possible patterns of marine primary productivity during the Great Ordovician Biodiversification Event

“…), land‐ice cover (Pogge von Strandmann et al . ) or in response to the advent of the first land plants (Lenton et al . , ; Porada et al .…”