Lithium isotope composition of modern and fossilized Cenozoic brachiopods

Abstract

Seawater lithium stable isotope ratios, represented by δ7Lisw (‰), hold key information about the evolution of global climate and the carbon cycle over geologic time. Reconstructions based on foraminifera suggest an ~9‰ increase in δ7Lisw over the past 60 m.y., interpreted as reflecting changes in the silicate weathering regime that may have contributed to global cooling over this time. However, culture experiments suggest that vital effects in foraminifers might cause varied δ7Li fractionation that could have affected past records. Thus, other archives are needed to determine whether the observed rise in foraminifer δ7Li values over the Cenozoic indeed reflects secular changes in δ7Lisw. Such archives would also open possibilities for robust determination of δ7Lisw deeper in geologic time. We analyzed δ7Li compositions in modern brachiopods (n = 34) and found a relatively narrow range of +24.2‰ to +28.8‰ (mean = +26.8‰; 1σ = 0.5‰), similar to the range reported for inorganic calcite precipitated from seawater. These results confirm that brachiopods are good candidates for providing a representative record of δ7Lisw. We present data from fossil specimens dating from 65 Ma to ca. 700 ka, finding that fossilized brachiopods record a similar magnitude of change in δ7Lisw as foraminifers. These results are the first to corroborate the ~9‰ increase in Cenozoic δ7Lisw and open possibilities for generating robust records ofδ7Lisw in deep time, prior to foraminifer evolution.

A lithium-isotope perspective on the evolution of carbon and silicon cycles

“…Given that carbonates can be a reliable archive of seawater δ 7 Li 18,20,21 , we generated a new carbonate Li isotope record through Earth's history. We analysed over 600 shallow-water marine carbonate samples from 101 stratigraphic units that range in age from 3.0 Ga to the modern (Fig.…”

“…Further, our record suggests that the development of a more modernstyle carbon cycle tied to these ecological transitions was protracted instead of being marked by step changes. 18,20,21,23,30,31,33,[44][45][46]. Grey -calcite, yellow -aragonite, green -dolomite, bluediagenetically altered carbonates, and red -samples from periods of known carbon isotope…”

Lithium Isotopes

“…This Element focuses on pre-Cenozoic records, so we merely note that while the Cenozoic increases in seawater δ 7 Li have been observed in foraminifera (Hathorne and James, 2006;Misra and Froelich, 2012) and reproduced in brachiopods (Washington et al, 2020), the interpretation of this change remains highly controversial (Caves Rugenstein et al, 2019;Li and West, 2014;Vigier and Godderis, 2015;Wanner et al, 2014). This is largely because the Cenozoic is itself a complex time period with substantial long-term changes of both tectonic and climatic conditions, adding complexity to the interaction of the weathering cycle with climate change.…”

“…There have also been reports of pH effects (Roberts et al, 2018), although again this is disputed (Vigier et al, 2015). The conflicting interpretations suggest that there may be complications associated with using foraminifera as a Li isotope archivealthough Cenozoic δ 7 Li records from foraminifera have been independently replicated in brachiopods (Misra and Froelich, 2012;Washington et al, 2020), and in-depth studies similar to those on foraminifera will need to be conducted on other types of biogenic carbonates. Bulk carbonates appear to be fairly robust archives of seawater δ 7 Li (Dellinger et al, 2020;Pogge von Strandmann et al, 2019b), and within such carbonates the proportion of foraminifera to coccoliths does not affect the overall δ 7 Li value, suggesting that both have similar overall fractionation factors (Pogge von Strandmann et al, 2019b).…”

The Influence of Weathering, Water Sources, and Hydrological Cycles on Lithium Isotopic Compositions in River Water and Groundwater of the Ganges–Brahmaputra–Meghna River System in Bangladesh

“…Indeed, the retention of Li by secondary minerals has been a critical process in the long-term evolution of seawater δ 7 Li that links changes in global silicate weathering intensity (Li and West, 2014;Wanner et al, 2014;Caves Rugenstein et al, 2019). The increase in oceanic δ 7 Li during the Cenozoic (Hathorne and James, 2006;Misra and Froelich, 2012;Washington et al, 2020) could be linked to greater 6 Li retention by clays accompanying active secondary clay formation during the uplift of the Himalayan-Tibetan Plateaus and the Andeas (Hoorn et al, 2010), accompanied by floodplain development (Pogge von Strandmann and Henderson, 2015). Chemical compositions of river waters depend on seasonal differences in relative fractions of chemical weathering inputs derived from the high mountains, floodplains, and southern tributaries (Bickle et al, 2018).…”