Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Ice retreat in Wilkes Basin of East Antarctica during a warm interglacial

Abstract

Efforts to improve sea level forecasting on a warming planet have focused on determining the temperature, sea level and extent of polar ice sheets during Earth’s past interglacial warm periods1,2,3. About 400,000 years ago, during the interglacial period known as Marine Isotopic Stage 11 (MIS11), the global temperature was 1 to 2 degrees Celsius greater2 and sea level was 6 to 13 metres higher1,3. Sea level estimates in excess of about 10 metres, however, have been discounted because these require a contribution from the East Antarctic Ice Sheet3, which has been argued to have remained stable for millions of years before and includes MIS114,5. Here we show how the evolution of 234U enrichment within the subglacial waters of East Antarctica recorded the ice sheet’s response to MIS11 warming. Within the Wilkes Basin, subglacial chemical precipitates of opal and calcite record accumulation of 234U (the product of rock–water contact within an isolated subglacial reservoir) up to 20 times higher than that found in marine waters. The timescales of 234U enrichment place the inception of this reservoir at MIS11. Informed by the 234U cycling observed in the Laurentide Ice Sheet, where 234U accumulated during periods of ice stability6 and was flushed to global oceans in response to deglaciation7, we interpret our East Antarctic dataset to represent ice loss within the Wilkes Basin at MIS11. The 234U accumulation within the Wilkes Basin is also observed in the McMurdo Dry Valleys brines8,9,10, indicating11 that the brine originated beneath the adjacent East Antarctic Ice Sheet. The marine origin of brine salts10 and bacteria12 implies that MIS11 ice loss was coupled with marine flooding. Collectively, these data indicate that during one of the warmest Pleistocene interglacials, the ice sheet margin at the Wilkes Basin retreated to near the precipitate location, about 700 kilometres inland from the current position of the ice margin, which—assuming current ice volumes—would have contributed about 3 to 4 metres13 to global sea levels.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Fig. 1: Base map of East Antarctica showing bedrock topography.
Fig. 2: Model 234U ingrowth histories in waters from injection by α recoils from the decay of 238U housed within sediments.
Fig. 3: Measured and modelled 234U ingrowth histories for the Wilkes Basin.

Data availability

All data used are included within the Extended Data Tables 14 and Extended Data Figs. 15 and uploaded to https://doi.org/10.26022/IEDA/111548.

Code availability

Any codes used are available upon request.

References

  1. 1.

    Dutton, A. et al. Sea-level rise due to polar ice-sheet mass loss during past warm periods. Science 349, aaa4019 (2015).

    CAS  PubMed  PubMed Central  Google Scholar 

  2. 2.

    Lisiecki, L. E. & Raymo, M. E. A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records. Paleoceanogr. Paleoclimatol. 20, PA1003 (2005).

  3. 3.

    Raymo, M. E. & Mitrovica, J. X. Collapse of polar ice sheets during the stage 11 interglacial. Nature 483, 453–456 (2012).

    ADS  CAS  PubMed  PubMed Central  Google Scholar 

  4. 4.

    Sugden, D. E., Marchant, D. R. & Denton, G. H. The case for a stable East Antarctic ice sheet: the background. Geogr. Ann. Ser. A 75, 151–154 (1993).

    Google Scholar 

  5. 5.

    Sugden, D. E. et al. Preservation of Miocene glacier ice in East Antarctica. Nature 376, 412–414 (1995).

    ADS  CAS  Google Scholar 

  6. 6.

    Refsnider, K. A. et al. Subglacial carbonates constrain basal conditions and oxygen isotopic composition of the Laurentide Ice Sheet over Arctic Canada. Geology 40, 135–138 (2012).

    ADS  CAS  Google Scholar 

  7. 7.

    Chen, T. et al. Ocean mixing and ice-sheet control of seawater 234U/238U during the last deglaciation. Science 354, 626–629 (2016).

    ADS  CAS  PubMed  PubMed Central  Google Scholar 

  8. 8.

    Henderson, G. M., Hall, B. L., Smith, A. & Robinson, L. F. Control on (234U/238U) in lake water: a study in the Dry Valleys of Antarctica. Chem. Geol. 226, 298–308 (2006).

    ADS  CAS  Google Scholar 

  9. 9.

    Hendy, C. H., Healy, T. R., Rayner, E. M., Shaw, J. & Wilson, A. T. Late Pleistocene glacial chronology of the Taylor Valley, Antarctica, and the global climate. Quat. Res. 11, 172–184 (1979).

    Google Scholar 

  10. 10.

    Lyons, W. B. et al. The geochemistry of englacial brine from Taylor Glacier, Antarctica. J. Geophys. Res. Biogeosci. 124, 633–648 (2019).

    CAS  Google Scholar 

  11. 11.

    Mikucki, J. A. et al. Deep groundwater and potential subsurface habitats beneath an Antarctic dry valley. Nat. Commun. 6, 6831 (2015).

    ADS  CAS  PubMed  PubMed Central  Google Scholar 

  12. 12.

    Mikucki, J. A. & Priscu, J. C. Bacterial diversity associated with Blood Falls, a subglacial outflow from the Taylor Glacier, Antarctica. Appl. Environ. Microbiol. 73, 4029–4039 (2007).

    CAS  PubMed  PubMed Central  Google Scholar 

  13. 13.

    Mengel, M. & Levermann, A. Ice plug prevents irreversible discharge from East Antarctica. Nat. Clim. Chang. 4, 451–455 (2014).

    ADS  Google Scholar 

  14. 14.

    DeConto, R. M. & Pollard, D. Contribution of Antarctica to past and future sea-level rise. Nature 531, 591–597 (2016).

    ADS  CAS  PubMed  PubMed Central  Google Scholar 

  15. 15.

    Gasson, E., DeConto, R. & Pollard, D. Antarctic bedrock topography uncertainty and ice sheet stability. Geophys. Res. Lett. 42, 5372–5377 (2015).

    ADS  Google Scholar 

  16. 16.

    Huybrechts, P. Glaciological modelling of the Late Cenozoic East Antarctic ice sheet: stability or dynamism? Geogr. Ann. Ser. A 75, 221–238 (1993).

    Google Scholar 

  17. 17.

    Wilson, D. J. et al. Ice loss from the East Antarctic Ice Sheet during late Pleistocene interglacials. Nature 561, 383–386 (2018).

    ADS  CAS  PubMed  PubMed Central  Google Scholar 

  18. 18.

    Kigoshi, K. Alpha-recoil thorium-234: dissolution into water and the uranium-234/uranium-238 disequilibrium in nature. Science 173, 47–48 (1971).

    ADS  CAS  PubMed  PubMed Central  Google Scholar 

  19. 19.

    Hallet, B. Deposits formed by subglacial precipitation of CaCO3. Geol. Soc. Am. Bull. 87, 1003–1015 (1976).

    ADS  CAS  Google Scholar 

  20. 20.

    Faure, G., Hoefs, J., Jones, L. M., Curtis, J. B. & Pride, D. E. Extreme 18O depletion in calcite and chert clasts from the Elephant Moraine on the East Antarctic ice sheet. Nature 332, 352–354 (1988).

    ADS  CAS  Google Scholar 

  21. 21.

    Hallet, B. Subglacial silica deposits. Nature 254, 682–683 (1975).

    ADS  CAS  Google Scholar 

  22. 22.

    Blackburn, T. et al. Composition and formation age of amorphous silica coating glacially polished surfaces. Geology 47, 347–350 (2019).

    ADS  CAS  Google Scholar 

  23. 23.

    Aharon, P. Oxygen, carbon and U-series isotopes of aragonites from Vestfold Hills, Antarctica: clues to geochemical processes in subglacial environments. Geochim. Cosmochim. Acta 52, 2321–2331 (1988).

    ADS  CAS  Google Scholar 

  24. 24.

    Frisia, S. et al. The influence of Antarctic subglacial volcanism on the global iron cycle during the Last Glacial Maximum. Nat. Commun. 8, 15425 (2017).

    ADS  PubMed  PubMed Central  Google Scholar 

  25. 25.

    Fitzpatrick, J. J., Muhs, D. R. & Jull, A. J. T. Saline minerals in the Lewis Cliff ice tongue, Buckley Island quadrangle, Antarctica. Contrib. Antarctic Res. I 50, 57–69 (1990).

    Google Scholar 

  26. 26.

    Goodwin, I. D. et al. Modern to Glacial age subglacial meltwater drainage at Law Dome, coastal East Antarctica from topography, sediments and jökulhlaup observations. Geol. Soc. Lond. Spec. Publ. 461, 215–230 (2018).

    ADS  Google Scholar 

  27. 27.

    Chutcharavan, P. M., Dutton, A. & Ellwood, M. J. Seawater 234U/238U recorded by modern and fossil corals. Geochim. Cosmochim. Acta 224, 1–17 (2018).

    ADS  CAS  Google Scholar 

  28. 28.

    Berry Lyons, W., Frape, S. K. & Welch, K. A. History of McMurdo Dry Valley lakes, Antarctica, from stable chlorine isotope data. Geology 27, 527–530 (1999).

    ADS  Google Scholar 

  29. 29.

    Poreda, R. J., Hunt, A. G., Lyons, W. B. & Welch, K. A. The helium isotopic chemistry of Lake Bonney, Taylor Valley, Antarctica: timing of late Holocene climate change in Antarctica. Aquat. Geochem. 10, 353–371 (2004).

    CAS  Google Scholar 

  30. 30.

    Warrier, R. B., Castro, M. C., Hall, C. M., Kenig, F. & Doran, P. T. Reconstructing the evolution of Lake Bonney, Antarctica using dissolved noble gases. Appl. Geochem. 58, 46–61 (2015).

    CAS  Google Scholar 

  31. 31.

    Pogge von Strandmann, P. A. E. et al. Riverine behaviour of uranium and lithium isotopes in an actively glaciated basaltic terrain. Earth Planet. Sci. Lett. 251, 134–147 (2006).

    ADS  CAS  Google Scholar 

  32. 32.

    Arendt, C. A. et al. Influence of glacial meltwater on global seawater δ234U. Geochim. Cosmochim. Acta 225, 102–115 (2018).

    ADS  CAS  Google Scholar 

  33. 33.

    Bard, E., Fairbanks, R. G., Hamelin, B., Zindler, A. & Hoang, C. T. Uranium-234 anomalies in corals older than 150,000 years. Geochim. Cosmochim. Acta 55, 2385–2390 (1991).

    ADS  CAS  Google Scholar 

  34. 34.

    Catania, G. A., Neumann, T. A. & Price, S. F. Characterizing englacial drainage in the ablation zone of the Greenland ice sheet. J. Glaciol. 54, 567–578 (2008).

    ADS  Google Scholar 

  35. 35.

    Bøggild, C. E., Brandt, R. E., Brown, K. J. & Warren, S. G. The ablation zone in northeast Greenland: ice types, albedos and impurities. J. Glaciol. 56, 101–113 (2010).

    ADS  Google Scholar 

  36. 36.

    Yamane, M. et al. Exposure age and ice-sheet model constraints on Pliocene East Antarctic ice sheet dynamics. Nat. Commun. 6, 7016 (2015).

    ADS  CAS  PubMed  PubMed Central  Google Scholar 

  37. 37.

    Reyes, A. V. et al. South Greenland ice-sheet collapse during marine isotope stage 11. Nature 510, 525–528 (2014).

    ADS  CAS  PubMed  PubMed Central  Google Scholar 

  38. 38.

    Gomez, N., Mitrovica, J. X., Tamisiea, M. E. & Clark, P. U. A new projection of sea level change in response to collapse of marine sectors of the Antarctic Ice Sheet. Geophys. J. Int. 180, 623–634 (2010).

    ADS  Google Scholar 

  39. 39.

    Fretwell, P. et al. Bedmap2: improved ice bed, surface and thickness datasets for Antarctica. Cryosphere 7, 375–393 (2013).

    ADS  Google Scholar 

  40. 40.

    Greene, C. A., Gwyther, D. E. & Blankenship, D. D. Antarctic Mapping Tools for Matlab. Comput. Geosci. 104, 151–157 (2017).

    ADS  Google Scholar 

  41. 41.

    Pattyn, F. Antarctic subglacial conditions inferred from a hybrid ice sheet/ice stream model. Earth Planet. Sci. Lett. 295, 451–461 (2010).

    ADS  CAS  Google Scholar 

  42. 42.

    Condon, D. J., McLean, N., Noble, S. R. & Bowring, S. A. Isotopic composition (238U/235U) of some commonly used uranium reference materials. Geochim. Cosmochim. Acta 74, 7127–7143 (2010).

    ADS  CAS  Google Scholar 

  43. 43.

    Hamelin, B., Bard, E., Zindler, A. & Fairbanks, R. G. 234U/238U mass spectrometry of corals: how accurate is the U–Th age of the last interglacial period? Earth Planet. Sci. Lett. 106, 169–180 (1991).

    ADS  CAS  Google Scholar 

  44. 44.

    Cheng, H. et al. Improvements in 230Th dating, 230Th and 234U half-life values, and U–Th isotopic measurements by multi-collector inductively coupled plasma mass spectrometry. Earth Planet. Sci. Lett. 371/372, 82–91 (2013).

    ADS  Google Scholar 

  45. 45.

    Andersen, M. B., Erel, Y. & Bourdon, B. Experimental evidence for 234U–238U fractionation during granite weathering with implications for 234U/238U in natural waters. Geochim. Cosmochim. Acta 73, 4124–4141 (2009).

    ADS  CAS  Google Scholar 

  46. 46.

    Prentice, M. L. & Matthews, R. K. Tertiary ice sheet dynamics: the snow gun hypothesis. J. Geophys. Res. Solid Earth 96, 6811–6827 (1991).

    Google Scholar 

  47. 47.

    Christoffersen, P., Bougamont, M., Carter, S. P., Fricker, H. A. & Tulaczyk, S. Significant groundwater contribution to Antarctic ice streams hydrologic budget. Geophys. Res. Lett. 41, 2003–2010 (2014).

    ADS  Google Scholar 

  48. 48.

    Dowdeswell, J. A. & Siegert, M. J. Ice-sheet numerical modeling and marine geophysical measurements of glacier-derived sedimentation on the Eurasian Arctic continental margins. Geol. Soc. Am. Bull. 111, 1080–1097 (1999).

    ADS  Google Scholar 

  49. 49.

    Tulaczyk, S., Kamb, B. & Engelhardt, H. F. Estimates of effective stress beneath a modern West Antarctic ice stream from till preconsolidation and void ratio. Boreas 30, 101–114 (2001).

    Google Scholar 

  50. 50.

    Howat, I. M., Joughin, I., Tulaczyk, S. & Gogineni, S. Rapid retreat and acceleration of Helheim Glacier, east Greenland. Geophys. Res. Lett. 32, L22502 (2005).

  51. 51.

    Howat, I. M., Tulaczyk, S., Waddington, E. & Björnsson, H. Dynamic controls on glacier basal motion inferred from surface ice motion. J. Geophys. Res. Earth Surf. 113, F03015 (2008).

  52. 52.

    Arendt, C. A., Aciego, S. M., Sims, K. W. W. & Aarons, S. M. Seasonal progression of uranium series isotopes in subglacial meltwater: implications for subglacial storage time. Chem. Geol. 467, 42–52 (2017).

    ADS  CAS  Google Scholar 

  53. 53.

    Bréant, C., Martinerie, P., Orsi, A., Arnaud, L. & Landais, A. Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities. Clim. Past 13, 833–853 (2017).

    Google Scholar 

  54. 54.

    Golledge, N. R. et al. The multi-millennial Antarctic commitment to future sea-level rise. Nature 526, 421–425 (2015).

    ADS  CAS  Google Scholar 

  55. 55.

    The Polar Rock Repository. Archive of information about geological samples available for research from the Ohio State University Byrd Polar and Climate Research Center (BPCRC) Polar Rock Repository https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.ngdc.mgg.geology:archived-data_BPCRC-polar-rock-repository (NOAA National Centers for Environmental Information, 2019).

  56. 56.

    Faure, G. & Mensing, T. M. The Transantarctic Mountains: Rocks, Ice, Meteorites And Water (Springer Science & Business Media, 2010).

  57. 57.

    Faure, G. & Taylor, K. S. The geology and origin of the Elephant Moraine on the East Antarctic Ice Sheet. Antarct. J. US 20, 11 (1985).

    Google Scholar 

  58. 58.

    Cassidy, W., Harvey, R., Schutt, J., Delisle, G. & Yanai, K. The meteorite collection sites of Antarctica. Meteoritics 27, 490–525 (1992).

    ADS  Google Scholar 

  59. 59.

    Taylor, K. S. Lithologies and Distribution of Clasts in the Elephant Moraine, Allan Hills South Victoria Land, Antarctica. MSc thesis, Kent State Univ. (1986).

  60. 60.

    Nishiizumi, K., Elmore, D. & Kubik, P. W. Update on terrestrial ages of Antarctic meteorites. Earth Planet. Sci. Lett. 93, 299–313 (1989).

    ADS  CAS  Google Scholar 

  61. 61.

    Lyons, W. B. et al. Halogen geochemistry of the McMurdo Dry Valleys lakes, Antarctica: clues to the origin of solutes and lake evolution. Geochim. Cosmochim. Acta 69, 305–323 (2005).

    ADS  CAS  Google Scholar 

  62. 62.

    Lyons, W. B. et al. Strontium isotopic signatures of the streams and lakes of Taylor Valley, Southern Victoria Land, Antarctica: chemical weathering in a polar climate. Aquat. Geochem. 8, 75–95 (2002).

    CAS  Google Scholar 

  63. 63.

    Brook, E. J. et al. Chronology of Taylor Glacier advances in Arena Valley, Antarctica, using in situ cosmogenic 3He and 10Be. Quat. Res. 39, 11–23 (1993).

    CAS  Google Scholar 

  64. 64.

    Bockheim, J. G., Prentice, M. L. & McLeod, M. Distribution of glacial deposits, soils, and permafrost in Taylor Valley, Antarctica. Arct. Antarct. Alp. Res. 40, 279–286 (2008).

    Google Scholar 

Download references

Acknowledgements

We thank J. Schutt, G. Faure and D. Schmidt for their sample collection at the Elephant Moraine and A. Grunow and the Byrd Polar Rock Repository for providing samples with a ‘PRR’ prefix. We also thank J. Paces, S. Hemming and T. Rasbury for their input. This research was funded by NSF 1644171 to T.B. and S.T.

Author information

Affiliations

Authors

Contributions

T.B. wrote the manuscript, led this study and developed the U-series methods. G.H.E. performed model simulations, tracer calibration and U-series data reduction. S.T. interpreted data and performed modelling. M.S. prepared samples and performed clean laboratory work. G.P. performed clean laboratory work and tracer calibration. N.McL. did the maximum likelihood model construction. B.H. interpreted data. J.C.Z. performed the oxygen isotopic analyses. B.C. did the SEM imaging. J.T.B. prepared samples and performed clean laboratory work.

Corresponding author

Correspondence to T. Blackburn.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature thanks Carli Arendt 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.

Extended data figures and tables

Extended Data Fig. 1 Images of sample PRR16794.

a, b, Sample shown in visible light (with location of dated horizons corresponding to data reported in Extended Data Table 1) (a) and the scanning electron microscope (SEM)/energy-dispersive X-ray spectroscopy (EDS) compositional map showing variations in manganese (green for high Mn, black for low Mn) (b).

Extended Data Fig. 2 Images of sample PRR50489.

a, b, Sample shown in visible light (with location of dated horizons corresponding to data reported in Extended Data Table 1) (a) and the SEM/EDS compositional map showing Ca and Si (b). The sample exhibits an angular unconformity, indicating that the sample physically moved beneath the ice before accumulation began again. Clct, calcite.

Extended Data Fig. 3 Model constraints on 234U ingrowth history of PRR39222.

The inset photograph shows sample PRR39222 under visible light with the location of δ234U measurements marked. The main plot shows the measured δ234U for PRR39222 at three horizons, revealing an increasing δ234U from top to bottom (Extended Data Table 1). Because of the high thorium (Th) contents, we cannot define a formation age and thus cannot identify a reliable δ234Ui. The purple curves in Fig. 3 represent the possible δ234Ui values for the top (higher δ234U) and bottom (lower δ234U) for any formation time. What we do not know is the absolute time at which this sample formed or the duration it formed over. However, we do know that: (1) the δ234Ui for the top and bottom of the sample must lie on these purple lines; (2) the sample must be younger than 1,500 kyr given that the measured δ234U is not in secular equilibrium. In addition to these known conditions, we can assume that the calcite in PRR39222 probably formed very rapidly as indicated by: (1) morphology, specifically radiating clusters of blade-like sparite; (2) lack of unconformities; (3) shared δ18O and δ13C composition with rapidly forming calcite from PRR50489, which is constrained by geochronology. Data from the literature as well as the geochronologic constraints presented in Extended Data Table 1 provide limits on the rate of sub-ice calcite formation (0.5 mm kyr−1 is shallow and 5 mm kyr−1 is steep). Given a known sample dimension of 4.5 cm, any assumed precipitation rate translates to a time duration for sample formation of 10–90 kyr. Assuming these durations, along with the requirement that the bottom and top of the sample intersects the purple curves in Fig. 3, permits us to define possible δ234Ui ingrowth histories (black arrows in Fig. 3). The rate of modelled 234U accumulation as recorded by PRR39222 is strongly controlled by assumed formation age with only a narrow time range yielding 234U ingrowth histories consistent with the other Wilkes Basin fluid histories. For example, if the sample were to have formed at 1,000 ka, we predict a change in δ234Ui of about 300% from the top to the bottom of this sample. Such rapid ingrowth histories result in δ234U compositions that would result in δ234U compositions that far exceeds anything observed in Antarctica (>6,000‰). If, however, the sample were to have formed at about 400 ka, the projected ingrowth histories would match both model projections and measured data for the Wilkes Basin. Only scenarios that place PRR39222 formation at roughly <500 ka yield projected ingrowth histories consistent with the blue curve. In addition to the above analysis, the occurrence of low δ234U (<500‰) in subglacial fluids is apparently rare, having been identified in this region only in samples older than about 300 ka. Collectively, this suggests that the 234U ingrowth history recorded by PRR39222 is at least consistent with formation at about 400 ka.

Extended Data Fig. 4 Long-term results of measurements of NBS 4321 (5.2919 × 10−5 ± 0.013 × 10−5 (0.25%)) at UCSC using an IsotopX X62, TIMS.

All uncertainties are absolute 2σ.

Extended Data Fig. 5 Steady-state activity ratio of 234U and 238U as a function of the flushing timescale for three different values of the 234U ejection factor.

The dotted line shows the assumed level of δ234U in meltwater. The assumed weathering timescale is 100 million years.

Extended Data Table 1 U-series data from EAIS precipitates
Extended Data Table 2 Oxygen and carbon isotopic data from EAIS precipitates
Extended Data Table 3 U-series standard data collected at UCSC and accepted ages
Extended Data Table 4 Legacy U-series recalculated using refined decay constants 2σ

Supplementary information

Supplementary Information

This file contains Supplementary Methods.

Supplementary Data

This file contains source data for Extended Data Table 1.

Supplementary Data

This file contains source data for Extended Data Table 2.

Supplementary Data

This file contains source data for Extended Data Table 3.

Supplementary Data

This file contains source data for Extended Data Table 4.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Blackburn, T., Edwards, G.H., Tulaczyk, S. et al. Ice retreat in Wilkes Basin of East Antarctica during a warm interglacial. Nature 583, 554–559 (2020). https://doi.org/10.1038/s41586-020-2484-5

Download citation

Further reading

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.

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing