Abstract
The storage of organic carbon in the terrestrial biosphere directly affects atmospheric concentrations of carbon dioxide over a wide range of timescales. Within the terrestrial biosphere, the magnitude of carbon storage can vary in response to environmental perturbations such as changing temperature or hydroclimate1, potentially generating feedback on the atmospheric inventory of carbon dioxide. Although temperature controls the storage of soil organic carbon at mid and high latitudes2,3, hydroclimate may be the dominant driver of soil carbon persistence in the tropics4,5; however, the sensitivity of tropical soil carbon turnover to large-scale hydroclimate variability remains poorly understood. Here we show that changes in Indian Summer Monsoon rainfall have controlled the residence time of soil carbon in the Ganges–Brahmaputra basin over the past 18,000 years. Comparison of radiocarbon ages of bulk organic carbon and terrestrial higher-plant biomarkers with co-located palaeohydrological records6 reveals a negative relationship between monsoon rainfall and soil organic carbon stocks on a millennial timescale. Across the deglaciation period, a depletion of basin-wide soil carbon stocks was triggered by increasing rainfall and associated enhanced soil respiration rates. Our results suggest that future hydroclimate changes in tropical regions are likely to accelerate soil carbon destabilization, further increasing atmospheric carbon dioxide concentrations.
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Data availability
All new data produced for this study are from samples from Bengal Fan cores SO93-117KL, -118KL and -120KL. These are available in Supplementary Tables and online in the EarthChem Library (http://www.earthchem.org/). Specifically, these present a compilation of radiocarbon age-dating results from planktonic foraminifera used in derivation of core age-depth models (Supplementary Table 1 and https://doi.org/10.1594/IEDA/111486); results of radiocarbon analyses of bulk organic carbon and calculated reservoir offset and F14R values of bulk and millennial BOC (Supplementary Table 2 and https://doi.org/10.1594/IEDA/111487); results of radiocarbon analyses of fatty-acid homologues and associated calculated reservoir offset and F14R values of bulk homologues and the subset of those cycled on millennial timescales (Supplementary Table 3 and https://doi.org/10.1594/IEDA/111488); and abundances of fatty-acid homologues (Supplementary Table 4 and https://doi.org/10.1594/IEDA/111489). Source data for Figs. 1, 2 and Extended Data Figs. 2–7 are provided with the paper.
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Acknowledgements
We thank C. Johnson (Woods Hole Oceanographic Institution, WHOI) and D. Montluçon (ETH Zürich) for laboratory support. We thank H. Kudrass for assistance with core sampling. This work was supported by a WHOI Coastal Ocean Institute Postdoctoral Fellowship to C.J.H., and by the National Science Foundation (grant numbers OCE-1333826, OCE-1333387 and OCE-1657771). This is contribution number 3868 of the Virginia Institute of Marine Science.
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C.J.H. and V.V.G. designed the study. C.J.H. conducted laboratory analyses with substantial contributions from V.V.G.; most radiocarbon analyses were conducted by M.U. and N.H. at the Laboratory for Ion Beam Physics (ETH). C.J.H. and V.V.G. drafted the manuscript with contributions from T.I.E.
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Extended data figures and tables
Extended Data Fig. 1 Study area and data collection.
a, Major features and tributaries of the Ganges–Brahmaputra drainage basin. The background topographic image is from GeoMapApp (http://www.geomapapp.org). b, Morphology of the Bengal Fan. Sediment is dominantly delivered via turbidity currents that travel along the single-channel channel-levee system. Red circles show sediment cores used here. c, A Parasound seismic-reflection profile crossing cores SO93-117KL and SO93-120KL from west to east. The upper left inset shows locations of the profile and cores with respect to the pathway of the active channel, imaged by multibeam bathymetry. The upper right inset shows Parasound data around core SO93-118 KL. Figure parts are modified from ref. 6, and details of the sediment fan system are described in ref. 55.
Extended Data Fig. 2 Age models for Bengal Fan channel-levee cores.
a–c, Age models for cores SO93-117KL (a; number of dated samples, n = 3), SO93-118KL (b; n = 10) and SO93-120KL (c; n = 12, derived through interpolation between calibrated 14C ages (Supplementary Table 1) and extrapolation to core tops and bottoms. Box widths represent sample depth intervals within cores; box heights represent calibrated age errors. Figure updated from ref. 6.
Extended Data Fig. 3 Summary of bulk BOC and biomarker (long-chain fatty acid) analyses.
a, Radioisotopic compositions of bulk BOC and individual fatty-acid (FA) homologues (n = 47). Vertical error bars indicate propagated measurement uncertainties. b, Comparison between reservoir-age offset (as given by the difference between organic-matter and deposition age, in 14C years) of bulk BOC and C28 plus C24–32 fatty acids (n = 9). Most values fall below the 1:1 line, reflecting the contribution of excess pre-aged organic matter to the bulk BOC pool. Error bars indicate propagated radiocarbon measurement and instrument-correction uncertainties.
Extended Data Fig. 4 Correlation between palaeoclimate proxies with organic-matter age structure.
a–c, Graphs show comparisons of post-glacial precipitation δDP values derived from ice-volume- and vegetation-fractionation-corrected fatty-acid δD values (more-depleted values are indicative of a stronger ISM) and the pre-ageing of organic matter (given as F14R values, in dimensionless Fm units; higher values indicate less pre-ageing). Comparisons shown are: a, bulk BOC versus δDP of C28 fatty acids (n = 30) for the comprehensive data set presented in Supplementary Table 2; b, bulk BOC versus δDP of C28 fatty acids (n = 9) for the subset of samples for which we also have compound-specific (fatty-acid) 14C data (Supplementary Table 3); and c, weighted-average F14R values of C24–32 fatty acid homologues versus weighted-average δDP values of those same C24–32 fatty acids (n = 9). Vertical error bars indicate propagated radiocarbon measurement and instrument-correction uncertainties; horizontal error bars are propagated multimeasurement standard deviation (δD) errors (see ref. 6). OC, organic carbon.
Extended Data Fig. 5 Temporal records of fatty-acid abundance in Bengal Fan core sediments.
Abundance of C28 (closed circles) and C24–32 (open circles) fatty-acid homologues (n = 30) in sediments within Bengal Fan channel-levee cores since the Late Glacial (data given in Supplementary Table 4). Horizontal error bars represent depositional age uncertainties (from core-age models) and are within data points if not shown.
Extended Data Fig. 6 Correlation between fatty-acid abundance and organic-matter age structure.
C28 fatty-acid abundances (Supplementary Table 4) are plotted against F14R values of bulk BOC (open circles, dashed line; n = 30) and of C28 fatty acids (closed circles, solid line; n = 9). Vertical error bars indicate propagated radiocarbon measurement and instrument-correction uncertainties.
Extended Data Fig. 7 Correlation between organic-matter age structure and proxies for sediment and organic-matter composition.
a, b, Bulk BOC F14R values are plotted against: a, bulk sediment TOC values, and b, sediment Al/Si values, for all samples used herein for which both data sets exist (n = 116 and n = 50, respectively). Al/Si values in b are from refs. 6,50. Vertical error bars indicate propagated radiocarbon measurement and instrument-correction uncertainties.
Supplementary information
Supplementary Table 1
Compilation of new and published radiocarbon age-dating results from planktonic foraminifera collected from Bengal Fan cores SO93- 117KL, 118KL, and 120KL, used in derivation of core age-depth models.
Supplementary Table 2
Results of radiocarbon analyses of bulk organic carbon (OC) and calculated reservoir age offset (R) and relative reservoir enrichment (F14R) values of the associated bulk biospheric organic carbon (BOC) and millennial component of that BOC in samples from Bengal Fan cores SO93- 117KL, 118KL, and 120KL.
Supplementary Table 3
Results of radiocarbon analyses of individual fatty acid homologs isolated from sediments in Bengal Fan cores SO93- 117KL, 118KL, and 120KL, and associated calculated reservoir age offset (R) and relative reservoir enrichment (F14R) values of the both the individual homologs and the subset of those cycled on millennial timescales.
Supplementary Table 4
Abundance of individual fatty acid homologs isolated from sediments in Bengal Fan cores SO93- 117KL, 118KL, and 120KL.
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Hein, C.J., Usman, M., Eglinton, T.I. et al. Millennial-scale hydroclimate control of tropical soil carbon storage. Nature 581, 63–66 (2020). https://doi.org/10.1038/s41586-020-2233-9
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DOI: https://doi.org/10.1038/s41586-020-2233-9
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