Neogene cooling driven by land surface reactivity rather than increased weathering fluxes

Abstract

The long-term cooling, decline in the partial pressure of carbon dioxide, and the establishment of permanent polar ice sheets during the Neogene period1,2 have frequently been attributed to increased uplift and erosion of mountains and consequent increases in silicate weathering, which removes atmospheric carbon dioxide3,4. However, geological records of erosion rates are potentially subject to averaging biases5,6, and the magnitude of the increase in weathering fluxes—and even its existence—remain debated7,8,9. Moreover, an increase in weathering scaled to the proposed erosional increase would have removed nearly all carbon from the atmosphere10, which has led to suggestions of compensatory carbon fluxes11,12,13 in order to preserve mass balance in the carbon cycle. Alternatively, an increase in land surface reactivity—resulting from greater fresh-mineral surface area or an increase in the supply of reactive minerals—rather than an increase in the weathering flux, has been proposed to reconcile these disparate views8,9. Here we use a parsimonious carbon cycle model that tracks two weathering-sensitive isotopic tracers (stable 7Li/6Li and cosmogenic 10Be/9Be) to show that an increase in land surface reactivity is necessary to simultaneously decrease atmospheric carbon dioxide, increase seawater 7Li/6Li and retain constant seawater 10Be/9Be over the past 16 million years. We find that the global silicate weathering flux remained constant, even as the global silicate weathering intensity—the fraction of the total denudation flux that is derived from silicate weathering—decreased, sustained by an increase in erosion. Long-term cooling during the Neogene thus reflects a change in the partitioning of denudation into weathering and erosion. Variable partitioning of denudation and consequent changes in silicate weathering intensity reconcile marine isotope and erosion records with the need to maintain mass balance in the carbon cycle and without requiring increases in the silicate weathering flux.

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Fig. 1: Weathering and carbon cycle results of the CLiBeSO-W model from 16 Myr ago to 0 Myr ago.
Fig. 2: River and sediment δ7Li as a function of silicate weathering intensity.
Fig. 3: The functional form of the weathering feedback at 16 Myr ago and at 0 Myr ago.

Data availability

Results of the CLiBeSO-W model (including carbon and sulfur cycle fluxes, δ7Li, 10Be/9Be, silicate weathering intensity, and E) using the mean of the parameters in the convergent simulations are included as Supplementary Information. Results of all convergent CLiBeSO-W simulations are archived in the ETH Research Collection (https://doi.org/10.3929/ethz-b-000338022). Additional CLiBeSO-W results and data formats can be obtained from the corresponding author on reasonable request.

Code availability

The CLiBeSO-W model is available in the Supplementary Information. Additional code (including scripts for plotting figures and the MCMC scheme) is available from the corresponding author on reasonable request.

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Acknowledgements

We thank S. Gallen, K. Lau, K. Maher, D. Stolper, S. Willett and M. Winnick for discussions regarding erosion, weathering and lithium isotopes. J.K.C.R. is funded by an ETH Fellowship. D.E.I. is supported by the Heising-Simons Foundation.

Reviewer information

Nature thanks Darryl Granger, Lee Kump and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

All authors developed the scientific concept. J.K.C.R. developed the model, conducted the analysis, and led the writing of the manuscript. All authors provided input on the manuscript.

Correspondence to Jeremy K. Caves Rugenstein.

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Extended data figures and tables

Extended Data Fig. 1 Density plots of results from the CLiBeSO-W model.

a, The increase in erosion (E/E0) that would be required over the Neogene to simultaneously match the seawater records of δ7Li and 10Be/9Be and the atmospheric \({p}_{{{\rm{CO}}}_{2}}\) record. b, The consequent change in the silicate weathering flux (Fsilw) given the change in erosion in a. c, Estimated change in weathering intensity (%). The area under each curve sums to 1.

Extended Data Fig. 2 Redox-sensitive weathering fluxes.

a, Normalized global organic weathering (black) and burial (green) flux. The blue shading is the convergent MCMC density for the organic weathering flux. b, Normalized global sulfide weathering (black) and burial (green) flux. The blue shading is the MCMC density for the sulfide weathering flux. c, Seawater S concentration.

Extended Data Fig. 3 Results from the CLiBeSO-W model when pyrite weathering is treated as invariant over the late Cenozoic.

Compare with Fig. 1. Black lines show model ouput when using the mean of the MCMC optimized parameters, and the shading indicates the density of all convergent MCMC iterations. a, Atmospheric \({p}_{{{\rm{CO}}}_{2}}\)with alkenone (yellow) and δ11B (red) proxy and ice-core (blue) data (see Methods). b, Modelled seawater δ7Li. The grey points are planktonic foraminifera δ7Li (ref. 28). c, Modelled marine 10Be/9Be normalized to the value at 0 Myr ago. The grey and blue points are reconstructed marine 10Be/9Be from ref. 7 normalized to the modern 10Be/9Be value of the respective ocean basin. d, Normalized silicate (red) and carbonate (blue) weathering flux and carbonate burial (dashed blue) flux. The grey, dashed lines are the estimated minimum and maximum changes in Fcarbb from sedimentary volumes30. e, Silicate weathering intensity. f, Globally averaged erosion rate (left axis). The solid red and lower and upper dashed lines show the mean, minimum and maximum mountain erosion rate estimates, respectively16 (right axis). The beige line is an estimate of global erosion rate15 and the beige bar at 0 Myr ago indicates the full range of estimated pre-anthropogenic erosion rates15,18 (left axis). In ac, error bars are the published uncertainty for the data (minimum/maximum in a, 1σ in b, c).

Extended Data Fig. 4 Results of the embedded lithium cycle model.

a, Global river δ7Li. b, Change in seawater δ7Li (d/dt δ7Li). c, Normalized river flux of Li. d, Seawater Li concentration. In all panels, the shading indicates relative density of convergent MCMC iterations, with darker colours indicating more iterations. The solid line is the result using the mean of the optimized parameters.

Extended Data Fig. 5 CLiBeSO-W results assuming that only the volcanic and solid Earth degassing flux varies and erosion remains constant.

Here, Fvolc is permitted to vary to optimize the fit with the data, resulting in a 37% decrease in Fvolc (full range: 35–41% decrease). Compare with Fig. 1. The model output is shown by black lines using mean of the MCMC optimized parameters and the shading indicates density of all convergent MCMC iterations. a, Atmospheric \({p}_{{{\rm{CO}}}_{2}}\)with alkenone (yellow) and δ11B (red) proxy and ice-core (blue) data (see Methods). b, Modelled seawater δ7Li. The grey points are planktonic foraminifera δ7Li (ref. 28). c, Modelled marine 10Be/9Be normalized to the value at 0 Myr ago. The grey and blue points are reconstructed marine 10Be/9Be from ref. 7 normalized to the modern 10Be/9Be value of the respective ocean basin. d, Normalized silicate (red) and carbonate (blue) weathering flux, carbonate burial (dashed blue) flux, and volcanic/solid Earth degassing flux (black with blue shading). The grey, dashed lines are the estimated minimum and maximum changes in Fcarbb from sedimentary volumes30. e, Silicate weathering intensity. f, Globally averaged erosion rate (left axis). The solid red and lower and upper dashed lines show the mean, minimum and maximum mountain erosion rate estimates, respectively16 (right axis). The beige line is an estimate of the global erosion rate15 and the beige bar at 0 Myr ago indicates the full range of estimated pre-anthropogenic erosion rates15,18 (left axis). In ac, error bars are published uncertainty for the data (minimum/maximum in a, 1σ in b, c).

Extended Data Fig. 6 CLiBeSO-W model results if erosion decreases and the initial silicate weathering intensity 16 Myr ago is less than 0.05.

Here, erosion decreases by 60% and the initial silicate weathering intensity is set to 0.024. a, Atmospheric \({p}_{{{\rm{CO}}}_{2}}\)with alkenone (yellow) and δ11B (red) proxy and ice-core (blue) data (see Methods). b, Modelled seawater δ7Li. The grey points are planktonic foraminifera δ7Li (ref. 28). c, Modelled marine 10Be/9Be normalized to the value at 0 Myr ago. The grey and blue points are reconstructed marine 10Be/9Be from ref. 7 normalized to the modern 10Be/9Be value of the respective ocean basin. d, Normalized silicate (red) and carbonate (blue) weathering flux and carbonate burial (dashed blue) flux. The grey, dashed lines are estimated minimum and maximum changes in Fcarbb from sedimentary volumes30. e, Silicate weathering intensity. f, Globally averaged erosion rate (left axis). The solid red and lower and upper dashed lines show the mean, minimum and maximum mountain erosion rate estimates16, respectively (right axis). The beige line is an estimate of global erosion rate15 and the beige bar at 0 Myr ago indicates the full range of estimated pre-anthropogenic erosion rates15,18 (left axis). In ac, error bars are published uncertainty for the data (minimum/maximum in a, 1σ in b, c).

Extended Data Fig. 7 CLiBeSO-W model results when marine 10Be/9Be data are excluded from the MCMC inversion.

a, Atmospheric \({p}_{{{\rm{CO}}}_{2}}\)with alkenone (yellow) and δ11B (red) proxy and ice-core (blue) data (see Methods). b, Modelled seawater δ7Li. The grey points are planktonic foraminifera δ7Li (ref. 28). c, Modelled marine 10Be/9Be normalized to the value at 0 Myr ago. The grey and blue points are reconstructed marine 10Be/9Be from ref. 7 normalized to the modern 10Be/9Be value of the respective ocean basin. d, Normalized silicate (red) and carbonate (blue) weathering flux and carbonate burial (dashed blue) flux. The grey, dashed lines are the estimated minimum and maximum changes in Fcarbb from sedimentary volumes30. e, Silicate weathering intensity. f, Globally averaged erosion rate (left axis). The solid red and lower and upper dashed lines show the mean, minimum and maximum mountain erosion rate estimates16, respectively (right axis). The beige line is an estimate of global erosion rate15 and the beige bar at 0 Myr ago indicates the full range of estimated pre-anthropogenic erosion rates15,18 (left axis). In ac, error bars are published uncertainty for the data (minimum/maximum in a, 1σ in b, c). The model output is shown by black lines, using the mean of the MCMC optimized parameters and the shading indicates the density of all convergent MCMC iterations.

Extended Data Table 1 Parameters used in the CLiBeSO-W model and the range of values in perturbed initial parameters of the MCMC runs

Supplementary information

Supplementary Table 1

Supplementary Data Table 1 and a Guide.

Supplementary Data

The CLiBeSO-W.R file and necessary data files.

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Caves Rugenstein, J.K., Ibarra, D.E. & von Blanckenburg, F. Neogene cooling driven by land surface reactivity rather than increased weathering fluxes. Nature 571, 99–102 (2019). https://doi.org/10.1038/s41586-019-1332-y

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