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A lithium-isotope perspective on the evolution of carbon and silicon cycles - Nature
Abstract . The evolution of the global carbon and silicon cycles is thought to have contributed to the long-term stability of Earth’s climate 1 , 2 , 3 . Many questions remain, however, regarding the feedback mechanisms at play, and there are limited quantitative constraints on the sources and sH?inks of these elements in Earth’s surface environments 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 . Here we argue that the lithium-isotope record can be used to track the processes controlling the long-term carbon and silicon cycles. By analysing more than 600 shallow-water marine carbonate samples from more than 100 stratigraphic units, we construct a new carbonate-based lithium-isotope record spanning the past 3 billion years. The data suggest an increase in the carbonate lithium-isotope values over time, which we propose was driven by long-term changes in the lithium-isotopic conditions of sea water, rather than by changes in the sedimentary alterations of older samples. Using a mass-balance modelling approach, we propose that the observed trend in lithium-isotope values reflects a transition from Precambrian carbon and silicon cycles to those characteristic of the modern. We speculate that this transition was lH?inked to a gradual shift to a biologically controlled marine silicon cycle and the evolutionary radiation of land plants 13 , 14 . You have full access to this article via your institution. Download PDF Download PDF Main . Earth has maintained a clement climate for the vast majority of the past 3.5 billon years, despite large changes in solar luminosity, atmospheric oxygen concentrations and crustal evolution 1 . Climate stability has allowed Earth’s persistent habitation and the proliferation of complex life over billion-year time scales. Feedbacks within Earth’s coupled carbon (C) and silicon (Si) cycles maintain this stability by regulating atmospheric carbon dioxide levels 2 , as exemplified by the continental silicate weathering feedback that removes atmospheric carbon dioxide during weathering and transfers silicon and dissolved inorganic carbon to the ocean. This climate-dependent mechanism is the most commonly invoked process stabilizing Earth’s long-term climate 2 , 3 . However, the idea that terrestrial silicate weathering played the dominant part in climate regulation through Earth’s history has been challenged in the past decade 4 , 5 . For example, there have been several recent suggestions that sedimentary and hydrothermal processes in the marine realm strongly affected atmospheric carbon dioxide levels, especially earlier in Earth’s history 6 , 7 , 8 , 9 . It has been proposed that extensive authigenic clay formation in marine sediments (reverse weathering) in Si-rich oceans was a key factor leading to a warm climate through most of Earth’s history 8 , 10 . In this view, the evolutionary radiation of siliceous organisms (sponges, radiolarians and, later, diatoms) forced a drop in dissolved marine Si levels and hence a marked decrease in the extent of reverse weathering. There has also been extensive debate about whether, how and when land plants transformed the silicate weathering feedback 11 , 12 . Disagreement on fundamental aspects of the long-term C cycle demands new empirical records that provide constraints on the evolution of the C and Si cycles over geological time. The lithium (Li)-isotope system can be used to track processes that control long-term C and Si cycles. Seawater Li-isotope values are strongly influenced by the global extent and dominant modes of clay formation, and therefore can be used to determine global weathering regimes 15 . Lithium in the crust is predominantly found in silicate minerals, and the largest fractionations of Li isotopes occur during the low-temperature formation of secondary silicate minerals—largely clays. Clay minerals preferentially incorporate the lighter Li isotope ( 6 Li), leaving residual waters enriched in the heavier isotope ( 7 Li) 16 . Clay formation occurs on land during incongruent silicate weathering, and in the oceans during off-axis seafloor alteration and during reverse weathering in sediments. Therefore, clay formation in the marine and terrestrial realms has the potential to drive the dissolved seawater Li-isotope signature towards values heavier than those of the average crust (the crustal value is roughly 0‰; ref. 17 ) 15 , 16 , 18 , 19 . By tracking clay formation and its lH?ink to continental and marine weathering processes, the Li-isotope system provides a powerful tool for investigating the long-term controls of the C and Si cycles. Given that carbonates can be a reliable archive of seawater δ 7 Li 18 , 20 , 21 , we generated a new record of carbonate Li isotopes through Earth’s history. We analysed more than 600 shallow-water marine carbonate samples from 101 stratigraphic units that range in age from 3.0 billion years to the present (Fig. 1 and Supplementary Tables 1 and 2 ). Our sampling focused on micritic carbonates, but also included grainstones, reef cements, microbialites and brachiopods. Samples were selected using a combination of standard and cathodoluminescence petrography (see? Supplementary Information for additional information on sampling protocols and selected units). Fig. 1: Isotope records in carbonates through time. a , A compilation of Li-isotope values measured in different types of carbonates, including our new data (open symbols) and previously published data 18 , 20 , 21 , 23 , 30 , 31 , 33 , 44 , 45 , 46 (closed symbols) ( n total ?=?1,396). Grey, calcite; yellow, aragonite; green, dolomite; blue, diagenetically altered carbonates; red, samples from periods of known C-isotope excursions. Shapes denote different types of carbonate archive: squares, cements; crosses, foraminifera; diamonds, brachiopods; triangles, belemnites; stars, corals. b , New filtered Li-isotope data ( n filtered ?=?525; n new ?=?712). Samples with indications of diagenetic alteration or of high detrital input (that is, with Al/Ca ratios of more than 0.00054?ppm?ppm ?1 ) are omitted. Light grey squares denote new data from well preserved marine cements ( n ?=?74). Light grey diamonds denote brachiopods. The light grey solid curve denotes a LOWESS fit of the mean of the data. The light grey dashed curve denotes a LOWESS fit of the lowest ten per cent of the values. c , Oxygen-isotope values measured in carbonates 47 . The blue curve denotes a robust LOESS fit of the data. d , Sr-isotope ratios measured in carbonates 32 , 48 . The red curve denotes a robust LOESS fit of the lowest ten per cent of the values. C, Cainozoic; Mz, Mesozoic; Gyr, billion years. Full size image A major concern with all carbonate-based proxies is whether the samples record primary signals or if they have been overprinted 22 . The Li-isotope composition of shallow-water marine carbonates in the sedimentary record depends on both the primary mineralogy of the sediment (aragonite versus calcite) and the type of alteration during early burial 23 . Additionally, carbonate samples can undergo late-stage alteration. Given the potential uncertainties associated with these processes, we tested the extent to which we can reconstruct seawater values from our carbonate record in four ways (see? Supplementary Information ). First, we generated a complementary record to our samples from well preserved early marine cements—the carbonate component that can be most robustly screened for diagenetic alteration in both Phanerozoic and Precambrian rocks 24 (see? Supplementary Table 1 and Supplementary Information ). Second, to gauge the effects of mineralogy and diagenesis, besides petrology, we analysed all samples for their major, minor and trace element ratios. Third, following ref. 25 , we focused our sampling on carbonate units that are not from carbon-isotope excursions—given that these excursions are interpreted as being a signal for either short-term carbon cycle perturbations or diagenetic events 26 , 27 (see? Supplementary Information ). Finally, we analysed a subset of our sample suite for calcium isotopes, which have emerged as a powerful tracer of the extent and type of diagenetic alteration in shallow-water marine carbonates 28 , 29 . Our dataset suggests a dramatic change in the values of carbonate Li isotopes through time (Fig. 1 and Supplementary Table 2 ). Cainozoic and Mesozoic values range from 14.6‰ to 29.5‰ with an average of 23.1?±?3.8‰ (1 standard deviation, s.d.) ( n ?=?45). This is similar to the values in foraminifera records from the Cainozoic, which range from 20.1‰ to 33.7‰ with an average of 25.9?±?2.7‰ (1?s.d.) ( n ?=?319) 18 , 30 , 31 . Low carbonate δ 7 Li values persist through most of the Palaeozoic with a mean δ 7 Li of 10.1?±?4.3‰ (1?s.d.) ( n ?=?263) (Fig. 1 ). Precambrian values range from ?3.8‰ to 23.5‰ but with a mean δ 7 Li of 7.7?±?5.7‰ (1?s.d.) ( n ?=?217). Basic descriptive statistics suggests that there are statistically significant shifts in carbonate Li-isotope values through time. For instance, a Welch’s analysis of variance (ANOVA) test shows that δ 7 Li values are significantly different ( F ?=?273.6, P ?Mesozoic (0–252 million years ago (Ma), n ?=?45) than for samples spanning the Palaeozoic (252?Ma to 541?Ma, n ?=?263) and for samples spanning the Precambrian (541?Ma to 3,000?Ma, n ?=?217). Crucially, these low values are also found in well preserved micro-drilled marine cements (Figs. 1 , 2 ). The general trend in the δ 7 Li record in carbonates resembles, to a first-order approximation, the trend in the carbonate strontium (Sr)-isotope record through time (Fig. 1c ). Fig. 2: Thin-section photomicrographs of representative well preserved carbonates from this study. a , b , Neoproterozoic carbonates. a , Multiple generations of dolomite (and mimetically dolomitized calcite) marine cements and micrite from the Tonian Devede Formation, Namibia. b , Well preserved dolomitized calcite cements from the Tonian Beck Spring Dolomite, USA. c , Paleozoic carbonate: calcite marine-cemented sponge from the Devonian Napier Formation, Australia. d , Precambrian carbonate: calcite seafloor fans of the Neoarchean Campbellrand Group, South Africa. The presence of well preserved carbonate textures rules out extensive secondary alteration. Full size image Our observed trend in Li-isotope values could be a signal of varying extents of sediment alteration, or a signal for environmental evolution. However, several lines of evidence are inconsistent with the premise that our carbonate Li-isotope record reflects varying extents of alteration. Notably, our observation that low carbonate δ 7 Li values, relative to modern, persist for the majority of Earth’s history—even after the dataset is screened for detrital contamination and diagenetic tracers—suggests an explanatory mechanism other than varying extents of alteration in our samples. Shallow-water carbonate Li-isotope values are likely to be 0‰ to 10‰ lighter than coeval seawater values, depending on the original mineralogy and the mode of burial diagenesis (see? Supplementary Information and ref. 23 ). Yet, crucially, Sr/Ca ratios and δ 44/40 Ca analysis can be used to track the burial offset from sea water in shallow-water carbonates 23 , 32 (see? Supplementary Information ), and there is no evidence for a systematic change in the mode of early marine diagenesis through time that could explain the observed roughly 15‰ increase in mean carbonate δ 7 Li values (or any a priori reason to expect such a change). Additionally, we used a suite of common geochemical filters to constrain primary mineralogy (Sr/Ca, Mg/Ca) and to track detrital contamination (Al/Ca, Rb/Ca) and diagenetic alteration (Mn/Sr, Pb/Ca) (see? Supplementary Information for further detail). Samples screened using these methods show similar trends to the unscreened Li-isotope data (Fig. 1 ; see? Supplementary Information ). Nonetheless, we acknowledge that some units in our study may have experienced late-stage alteration that is not easily screened with typical elemental tracers. Crucially, however, late-stage diagenetic alteration, where it has been studied thus far, appears to result in a shift towards higher δ 7 Li values 33 , indicating that our carbonate record of lighter values in older samples is unlikely to reflect a late-diagenetic bias. Therefore, building upon previous work on the effects of late-stage alteration on the Li-isotope system 33 , we argue that, if detrital contribution can be ruled out, then the lower boundary of the δ 7 Li values will most accurately represent seawater evolution. With this framework, the Li-isotope record would be interpreted in a similar fashion to the scatter in the long-term Sr-isotope records (see, for example, ref. 34 ; Fig. 1d ). Furthermore, carbonate cements with exceptionally well preserved fabrics—samples that could not have undergone extensive alteration after deposition (see refs. 24 , 35 ; Fig. 1 )—display the same trend as our larger dataset from bulk rock. Using our carbonate record (filtered for detrital contamination, and assuming an offset of 4?±?5‰ from sea water, as in ref. 23 ; see? Supplementary Information ), we were able to reproduce previously estimated Cainozoic ( Supplementary Fig. 14 ) as well as Mesozoic and Palaeozoic (Fig. 1a ) δ 7 Li SW values. Assuming this same offset for Precambrian samples, we estimate that Precambrian δ 7 Li SW values were on average 6–16‰—notably lighter than those of the modern oceans (31‰; ref. 36 ). As with other isotope systems 37 , it will be crucial to verify our reconstructed Li-isotope trends in another sedimentary archive. Nonetheless, we propose that the most straightforward explanation for our carbonate Li-isotope dataset is that seawater Li-isotope values changed substantially across Earth’s history. To evaluate the mechanisms that could be driving long-term changes in seawater Li-isotope values, we used a stochastic mass-balance modelling approach (Fig. 3 ). Specifically, we use an isotope mass-balance model to explore plausible solution space. This provides a means of estimating configurations of the Li-isotope system that might be responsible for the long-term shift we observe in estimated δ 7 Li SW values. In our simulations, we solved the Li-isotope mass balance at time intervals of one million years, allowing a wide range of possible values for high- and low- temperature hydrothermal fluxes, riverine fluxes and their isotopic values ( Supplementary Table 3 ). At each time step, we used a Monte Carlo routine to resample the uniformly distributed key parameters 1,000 times, with acceptable solutions being the ones that matched our estimated, error-bounded, Li-isotope record (Fig. 3 ; see? Supplementary Information for the model derivation). Fig. 3: Two-dimensional density heat?map of Li-isotope mass-balance results. Each panel indicates the density of the parameters that successfully match our empirically determined, LOWESS-smoothed Li-isotope record (with LOWESS conducted on the lower ten per cent of data (dashed line in a ), with upper and lower solid filtering bounds) through Earth’s history. Light red represents higher counts per bin; red represents lower counts per bin; and white regions represent solution space that cannot satisfy a steady-state ( F in ?=? F out ) value for Li isotopes in sea water, as determined by our empirical record. a , Li-isotope value of sea water (SW). b , Riverine (riv) Li-isotope value. c , Isotopic fractionation associated with removal of Li from sea water during basalt alteration (Δ 7 Li lowT ). d , Isotopic fractionation associated with removal of Li from sea water during marine authigenic clay formation (Δ 7 Li maac ). e , Estimates of outgassing from refs. 39 , 49 , 50 . f , Riverine Li flux. g , Proportion of Li removed through basalt alteration ( f lowT ). h , Proportion of Li removed through marine authigenic clay formation ( f maac ). High-temperature hydrothermal flux ( F HT ) is scaled linearly to a mean value of outgassing estimates. The LOWESS curve is regressed through our original data with an applied calcite fractionation from sea water (? 7 Li?=??4‰). The lower filtering bound is ?4‰ from the LOWESS curve, representing fluid buffered solutions; the upper bound is +5‰ from the LOWESS curve, representing the potential for any samples to be aragonite. Full size image With our modelling approach, persistently low δ 7 Li SW values in the Precambrian (Fig. 3 and Supplementary Figs. 18–23 ) seem to require changes in terrestrial and marine Li cycling in Earth’s past relative to its present. For instance, the only Earth system (that is, the prominent combination of Li-cycle parameters) that fits our data from the Precambrian (Fig. 3 ) requires rivers with low Li-isotope values (δ 7 Li Riv less than 10‰) together with muted isotope fractionation (? 7 Li less than 10‰) during Li burial in the marine realm through marine authigenic clay formation (maac) and low-temperature basalt alteration (lowT). Low Precambrian δ 7 Li SW values could be related to elevated high-temperature hydrothermal Li fluxes, which are a source of relatively light Li (roughly 6.3‰; ref. 38 ). However, most geophysical models show that near-modern hydrothermal activity was reached by the Paleoproterozoic 39 , and some estimates suggest constant hydrothermal heat flux over the studied time interval 40 , which would lead to approximately constant, long-term hydrothermal Li fluxes. Therefore, consistent with our modelling results, enhanced high-temperature hydrothermal fluxes are unlikely to be responsible for the low δ 7 Li SW values seen through most of Earth’s history. The proliferation and diversification of land plants over much of the Phanerozoic has been hypothesized to have fostered more extensive formation and retention of clay minerals in the terrestrial realm 13 . Our work—which requires an increase through time in δ 7 Li Riv values (Fig. 3 )—supports this idea. There are multiple ways in which plants may have changed weathering, but fostering soil development and increasing water–rock interaction times is one way to increase the probability of clay formation 13 . There is some mineralogical evidence that also supports the hypothesis that before the rise of land plants there was more limited paedogenic formation of clay minerals 41 . Weathering regimes may have continued to shift until the rise to dominance of angiosperms at roughly 80?Ma 14 . Extensive clay formation in the marine sediment column, as has been proposed for the Precambrian 8 , 10 , is one obvious way of changing the marine Li cycle. Our prediction of a more limited effective isotope fractionation during Li burial (Δ 7 Li maac and Δ 7 Li lowT ) earlier in Earth’s history can be lH?inked to rapid rates of clay formation, which could have led to high rates of Li uptake and the reaction sites being in restricted contact with the reactant pool (sea water) 42 . For most of Earth history, without the presence of Si biomineralizers, sea water was highly oversaturated with respect to Si phases, which could have resulted in rapid and extensive clay formation 8 . This style of reverse weathering and Li removal is likely to have limited the effective fractionation of Li isotopes (see? Supplementary Information ; ref. 42 ). Therefore, the progressive decrease in marine Si concentrations over the Phanerozoic 43 , lH?inked to the transition to a more biologically controlled Si cycle, may have driven a shift in seawater Li-isotope values. The apparent common occurrence of low δ 7 Li SW values in the Precambrian and the early Palaeozoic supports the premise that the carbon cycle operated in a fundamentally different way for most of Earth’s history compared with the present day. Although we cannot use Li-isotope values to constrain a single Earth system, our work suggests that there was a major shift in clay factories throughout Earth’s history—with a likely increase in clay formation on land and a decrease in clay formation in the oceans. Clay formation is a key part of the coupled C–Si cycles, suggesting that the mode of climate regulation on Earth has changed dramatically through time. The shift from a Precambrian Earth state to the modern state can probably be attributed to major biological innovations—the radiation of sponges, radiolarians, diatoms and land plants. Further, our record suggests that the development of a more modern-style carbon cycle tied to these ecological transitions was protracted, instead of being marked by step changes. Data availability . All geochemical data generated here are included in? Supplementary Table 2 and are available on Mendeley Data ( https://doi.org/10.17632/ztpkpbm43x.1 ). 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P.A.E.P.v.S. was funded by a European Research Council (ERC) consolidator grant (682760 CONTROLPASTCO2). A.v.S.H. acknowledges funding from an Australian Research Council (ARC) Discovery Early Career Researcher Award (DECRA; DE190100988). B.K.-A. acknowledges financial support from the Yale Institute for Biospheric Studies. We thank J. Utrup, S. H. Butts and the Yale Peabody Museum of Natural History for providing brachiopods and carbonate samples. Author information . Affiliations . Earth and Planetary Sciences, Yale University, New Haven, CT, USA Boriana Kalderon-Asael,?Joachim A. R. Katchinoff,?Noah J. Planavsky,?Eric J. Bellefroid,?Terry T. Isson?&?Dan Asael The University of Melbourne, School of Earth Sciences, Parkville, Victoria, Australia Ashleigh v. S. Hood?&?Malcolm W. Wallace Department of Geography, Durham University, Durham, UK Mathieu Dellinger Amherst College Geology Department, Amherst, MA, USA David S. Jones Department of Geology, University of Johannesburg, Johannesburg, South Africa Axel Hofmann?&?Frantz Ossa Ossa Department of Geosciences, University of Tuebingen, Tuebingen, Germany Frantz Ossa Ossa Department of Earth Sciences, University of California Santa Barbara, Santa Barbara, CA, USA Francis A. Macdonald China University of Petroleum, College of Geosciences, Beijing, China Chunjiang Wang Te Aka Mātuatua, University of Waikato, Tauranga, New Zealand Terry T. Isson Department of Geoscience, Princeton University, Princeton, NJ, USA Jack G. Murphy?&?John A. Higgins Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA A. Joshua West London Geochemistry and Isotope Centre (LOGIC), Institute of Earth and Planetary Sciences, University College London and Birkbeck, University of London, London, UK Philip A. E. Pogge von Strandmann Institute of Geosciences, Johannes Gutenberg University, Mainz, Germany Philip A. E. Pogge von Strandmann Authors Boriana Kalderon-Asael View author publications You can also search for this author in PubMed ? Google Scholar Joachim A. R. Katchinoff View author publications You can also search for this author in PubMed ? Google Scholar Noah J. Planavsky View author publications You can also search for this author in PubMed ? Google Scholar Ashleigh v. S. Hood View author publications You can also search for this author in PubMed ? Google Scholar Mathieu Dellinger View author publications You can also search for this author in PubMed ? Google Scholar Eric J. Bellefroid View author publications You can also search for this author in PubMed ? Google Scholar David S. Jones View author publications You can also search for this author in PubMed ? Google Scholar Axel Hofmann View author publications You can also search for this author in PubMed ? Google Scholar Frantz Ossa Ossa View author publications You can also search for this author in PubMed ? Google Scholar Francis A. Macdonald View author publications You can also search for this author in PubMed ? Google Scholar Chunjiang Wang View author publications You can also search for this author in PubMed ? Google Scholar Terry T. Isson View author publications You can also search for this author in PubMed ? Google Scholar Jack G. Murphy View author publications You can also search for this author in PubMed ? Google Scholar John A. Higgins View author publications You can also search for this author in PubMed ? Google Scholar A. Joshua West View author publications You can also search for this author in PubMed ? Google Scholar Malcolm W. Wallace View author publications You can also search for this author in PubMed ? Google Scholar Dan Asael View author publications You can also search for this author in PubMed ? Google Scholar Philip A. E. Pogge von Strandmann View author publications You can also search for this author in PubMed ? Google Scholar Contributions . B.K.-A., N.J.P., P.A.E.P.v.S. and J.A.R.K. designed the research. E.J.B., A.v.S.H., D.S.J., F.A.M., M.W.W., J.A.R.K., A.H., F.O.O., C.W., M.D. and N.J.P. collected samples. B.K.-A., P.A.E.P.v.S., J.A.R.K., M.D., J.G.M., D.A., F.A.M., A.J.W. and J.A.H. conducted geochemical analyses. J.A.R.K. wrote the Li-isotope mass-balance model. B.K.-A. wrote the Li-isotope diagenetic model. B.K.-A., N.J.P., P.A.E.P.v.S. and J.A.R.K. analysed the data and wrote the paper. All authors contributed to the preparation of the manuscript. Corresponding authors . Correspondence to Boriana Kalderon-Asael or Noah J. Planavsky or Philip A. E. Pogge von Strandmann . Ethics declarations . Competing interests . The authors declare no competing interests. Additional information . Peer review information Nature thanks Jeremy Caves Rugenstein and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Supplementary information . Supplementary Information . The file contains Supplementary Figures 1 – 29; Supplementary Tables 1 – 3; Supplementary Methods; Supplementary Discussion; Global lithium isotope mass balance; Diagenetic modelling and Supplementary References. Supplementary Table 1 . Description of the samples analysed in this study at a) Yale University; b) Oxford and University College London. Supplementary Table 2 . Geochemical data generated in this study: a) δ7Li (in ‰), Li, Mg, Al, Ca, Ti, Mn, Rb, Sr, Pb concentrations (in ppm) and δ44/40Ca (in ‰) of the samples analysed at Yale University; b) δ7Li (in ‰), Li/Ca, Al/Ca, Mn/Ca, Sr/Ca and Mg/Ca elemental ratios of the samples analysed at Oxford and University College London. Rights and permissions . Reprints and Permissions About this article . Cite this article . Kalderon-Asael, B., Katchinoff, J.A.R., Planavsky, N.J. et al. A lithium-isotope perspective on the evolution of carbon and silicon cycles. Nature 595, 394–398 (2021). https://doi.org/10.1038/s41586-021-03612-1 Download citation Received : 14 March 2019 Accepted : 04 May 2021 Published : 14 July 2021 Issue Date : 15 July 2021 DOI : https://doi.org/10.1038/s41586-021-03612-1 Comments . By submitting a comment you agree to abide by our Terms and Community Guidelines . If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate. .
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