Methane hydrate dissociation has long been considered as a mechanism for global carbon cycle perturbations, climate change and even mass extinctions in Earth’s history. However, direct evidence of hydrate destabilization and methane release coinciding with such events is scarce. Here we report the presence of diagnostic lipid biomarkers with depleted carbon isotopes from three sites in the Southern Ocean that are directly linked to methane release and subsequent oxidation across the Oligocene–Miocene boundary (23 million years ago). The biomarker evidence indicates that the hydrate destabilization was initiated during the peak of the Oligocene–Miocene boundary glaciation and sea-level low stand, consistent with our model results suggesting the decrease in hydrostatic pressure eroded the base of global methane hydrate stability zones. Aerobic oxidation of methane in seawater consumes oxygen and acidifies the ocean, acting as a negative feedback that perhaps facilitated the rapid and mysterious termination of glaciation in the early Miocene.
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All lipid biomarker and compound-specific carbon isotope data used in this study are available for download from the NOAA National Centers for Environmental Information website (https://www.ncei.noaa.gov/access/paleo-search/study/35113) and are also archived as a Supplementary Data file with the online version of this Article.
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This study used samples and data provided by the International Ocean Discovery Program (IODP). We thank the IODP Gulf Coast Repository and Kochi Core Center for providing the sediment samples from the Southern Ocean, and H. Pfuhl for providing site 1170 foraminiferal stable isotope data. Financial support for this study was provided by Texas A&M University Triads for Transformation Program to Y.G.Z. and Texas Sea Grant Grants-In-Aid of Graduate Research Program (NA18OAR4170088) to B.K. We thank C. Maupin at the Stable Isotope Geosciences Facility, S. Sweet and A. Knap at the Geochemical and Environmental Research Group for their support and advice on the mass spectrometry analyses. We are also grateful to N. Randle for proofreading the manuscript and E. Grossman, Z. Lu and N. Slowey for helpful discussions.
The authors declare no competing interests.
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Extended Data Fig. 1 Potential age shifts of high Methane Index (MI) samples considering ancient SMTZ depths.
a, Sites 1168; b, Site 1170; and c, Site U1356. Gray vertical bars indicate high MI intervals without age adjustment whereas the red vertical bars indicate age adjustment based on the MI-SMTZ depth regression curves and sedimentation rate (Methods; Extended Data Fig. 2). Benthic δ18O from each site were used for comparison, except for Site U1356 where the global benthic stacked δ18O record19 was used. Note that most age shifts are negligible. Blue dashed square highlights the Mi-1 event interval.
Data were compiled from marine sites where both modern SMTZ depth and GDGTs data were available. Red: sites located in tropical-to-temperate oceans, including Gulf of Mexico16,66, Mediterranean Sea67,68, Aarhus Bay69 and Peru margin;65 and blue: sites located in the Arctic Ocean (>60 °N), including Norwegian Sea70, Canadian Beaufort Sea71 and Chukchi Sea72.
Extended Data Fig. 4 Reconstructed 1° x 1° grid paleobathymetry at 23 Ma with the seafloor area of 600-3000 m water depth.
Paleobathymetry data are from Ref. 34. Mid-ocean ridges and terrestrial lakes were excluded from our considerations.
Extended Data Fig. 5 Varying percentage of methane oxidized aerobically and its impact on seawater chemistry (pH and CO2).
The total amount of methane released during OMB was calculated to be 199 ± 35 Gt. Red and blue areas indicate the propagated uncertainties. Gray area indicates the range between AOM-dominant environments (10%) and high methane seepage settings (80%). Refer to “Methods” for full description of our calculations and “Supplementary Table 1” for global ocean seawater parameters used in the calculations.
Extended Data Fig. 6 Benthic carbon isotope records and methane-related biomarkers at Sites 1168 and 1170.
a, carbon isotope (δ13C) of benthic foraminifera at Site 1170 (light blue; Ref. 26) and stacked δ13C (blue; Ref. 19); b, Methane Index (MI) values of Sites 1168 (black) and 1170 (white); and c, concentration of hop-17(21)-ene (white bar; structure shown), archaeol (black bar; structure shown) and their compound-specific carbon isotopic (δ13C) signatures of Site 1168; compounds that are under detection limit are not shown here (Extended Data Table 2). Note that high MI values first appeared at ~ 23 Ma coinciding with a δ13C maxima, and continued into the δ13C decline phase. Yellow area highlights the Mi-1 event, and dashed vertical line indicates the carbonate dissolution event41.
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Kim, B., Zhang, Y.G. Methane hydrate dissociation across the Oligocene–Miocene boundary. Nat. Geosci. 15, 203–209 (2022). https://doi.org/10.1038/s41561-022-00895-5