Abstract
Oceanic anoxic events (OAEs) are historical intervals of global-scale ocean deoxygenation associated with hyperthermal climate states and biological crises1,2. Massive volcanic carbon dioxide (CO2) emissions frequently associated with these events are thought to be a common driver of ocean deoxygenation through several climate-warming-related mechanisms1,3,4. The Early Cretaceous OAE1a is one of the most intense ocean deoxygenation events, persisting for more than 1 Myr (refs. 5,6). However, existing records of marine chemistry and climate across OAE1a are insufficient to fully resolve the timing and dynamics of the underlying processes, thus obscuring cause-and-effect relationships between climate forcing and ocean oxygenation states. Here we show that rapid ocean deoxygenation during OAE1a is linked to volcanic CO2 emissions and the crossing of an associated climate threshold, after which the sluggish pace of the silicate-weathering feedback and climate recovery delayed reoxygenation for >1 Myr. At the end of OAE1a, recrossing this threshold allowed for ocean reoxygenation. Following OAE1a, however, the Earth system remained sufficiently warm such that orbitally forced climate dynamics led to continued cyclic ocean deoxygenation on approximately 100-kyr timescales for another 1 Myr. Our results thus imply a tight coupling between volcanism, weathering and ocean oxygen content that is characterized by a climate threshold.
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Data availability
All source data for this study are available in the Supplementary Information and are publicly available in the Open Science Framework database (https://doi.org/10.17605/OSF.IO/E3KYM).
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Acknowledgements
R. Francois provided information on palaeoproxy behaviour. D. Penman provided information on the LOSCAR model. We thank J. Mossinger for handling our manuscript, whose suggestions greatly helped improve the work. This research used samples provided by the International Ocean Discovery Program (IODP). Funding was provided through a Research Grants Council General Research Fund #17306920 awarded to S.A.C. and N.R.M., a NSERC discovery grant awarded to S.A.C. and PRIN2017RX9XXXY awarded to E.E.
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Conceptualization: K.W.B., S.A.C., E.E., C.B. Methodology: K.W.B., S.A.C., E.E., C.B., C.T.L.C., G.G., N.R.M. Investigation: K.W.B., with support from all authors. Visualization: K.W.B., G.G. Writing – original draft: K.W.B., S.A.C. Writing – review and editing: all authors.
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Extended data figures and tables
Extended Data Fig. 1 Palaeogeographic reconstruction of the early Aptian.
The map was produced and modified using the ODSN Plate Tectonic Reconstruction Service63. The location of the Cismon site is represented by a blue star. Other blue dot locations represent sites at which further evidence for enhanced weathering and expanded oceanic anoxia during OAE1a have been observed. Supplementary Table 2 provides a compilation and summary of this multiproxy geochemical evidence, as well as sites at which variable redox-sensitive element enrichments have been observed across the studied interval. Site locations are as follows: A, Cismon; B, Gorgo a Cerbara; C, DSDP Site 463; D, Kanfanar; E, Resolution Guyot; F, Coppitella; G, Monte Raggeto; H, Santa Maria 4 core; I, La Bédoule; J, DSDP Site 361; K, Cau; L, Poggio le Guaine; M, Santa Rosa Canyon 1.
Extended Data Fig. 2 Lithological log of the studied interval of the Cismon core.
The lithological changes are detailed in the log on the left. In the CaCO3 curve, the black dots indicate the black shale layers. For the box plots, outliers are drawn as filled circles. An outlier is a value that is larger than 1.5 times the interquartile range (IQR) from the median. The IQR is the difference between the first and third quartiles. The whiskers are only drawn to the smallest/largest non-outlier. Extreme outliers that are three times the IQR are drawn as open circles.
Extended Data Fig. 3 Detailed pre-OAE1a and post-OAE1a ocean redox and climate dynamics.
Blue shading represents low ocean oxygen conditions for each palaeoredox proxy. Orange shading represents climate excursions above the pre-OAE1a baseline. a, Detailed view of the onset of OAE1a (phase I to phase II transition). The vertical orange shading represents the timing of a notable volcanic pulse in the Pacific Ocean14. The dashed lines represent the maximum timescale of ocean deoxygenation (about 36 kyr) from the onset of volcanism, based on agreement for all palaeoredox proxies. The red line in the CaCO3 panel represents the superimposed short-eccentricity band-pass-filtered signal scaled to CaCO3 values. b, Detailed view of the end of OAE1a (phase II to phase III transition). c, Detailed view of post-OAE1a proxy dynamics (phase III).
Extended Data Fig. 4 Establishing thresholds for low oxygen conditions.
Low-resolution data are from refs. 14,22. a, On the basis of the conservative Ce/Ce*Bulk > 0.5 and Ce/Ce*HCl > 0.4 thresholds independently established using Fe speciation (panels b–i), the high-resolution whole-rock Ce/Ce* values accurately record changing seawater redox state (that is, the cerium anomaly of seawater is becoming less negative). Both the Ce/Ce*Bulk and Ce/Ce*HCl cross the diagnostic thresholds during OAE1a and the phase III oxygen-poor intervals. b–i, Green data are from within OAE1a and blue data are outside the interval. Samples that are unambiguously deposited under anoxic conditions (FeHR/FeTot > 0.38, Fe/Al > 0.53, FeTot > 0.5 wt%) can be used to establish thresholds in Ce/Ce*Bulk and Ce/Ce*HCl that confidently signal low-oxygen conditions in the Cismon sediments. Conservatively, these thresholds seem to be Ce/Ce*Bulk > 0.5 and Ce/Ce*HCl > 0.4. Use of such Ce/Ce* thresholds to diagnose low-oxygen conditions are directly corroborated by V and MnEFs.
Extended Data Fig. 5 Geochemical cross-plots.
Phase I data are shown in blue, phase II data are shown in green and phase III data are shown in orange, with data having Mn/Ti <2,333 plotted as squares. The linear regression is for all phase III data with the R2 value shown (shading 95% confidence, >1.5 s.d. of the residual outliers removed). We note that the V/Ti x axis has been truncated to better show the data, as phase II includes eight data points with large V enrichments. During phase III, cross-plots between the different redox proxies form a straight line and are strongly correlated, firmly implying a common driver for the enrichment patterns. Moreover, samples during the low-oxygen intervals during phase III cluster with phase II data, again strongly implying deposition under similar conditions, in this case from poorly oxygenated waters.
Extended Data Fig. 6 Multitaper power spectral density estimate (MTM) with robust red-noise modelling of the CaCO3, Rb/Sr and K/Ti records for the pre-OAE1a, syn-OAE1a and post-OAE1a intervals.
Grey-shaded bars highlight the prominent peaks rising above the 95% confidence level.
Extended Data Fig. 7 Comparison of Cismon and DSDP Site 463 Rb/Sr and Os-isotope data.
The Os-isotope data are from ref. 13.
Extended Data Fig. 8 Geochemical cross-plots of palaeoredox and palaeoclimate proxies.
Green data represent the OAE1a interval (phase II) and blue data represent data from before (phase I) and after (phase III) the event. Purple trend represents a fit through all data (phases I–III). a–d, The linear relationships (>1.5 s.d. of the residual outliers removed) are fit through all data with the R2 values indicated and shading representing the 95% confidence bounds. e–l, The pre-OAE1a and post-OAE1a carbonate data are from ref. 6 and within the OAE1a interval are from ref. 23. The linear relationships are fit through all data with the R2 values indicated and shading representing the 95% confidence bounds.
Extended Data Fig. 9 REE fidelity indicators and geochemical cross-plots of REE ratios with other proxies.
Green data represent the OAE1a interval and blue data represent data from before and after the event. a, La anomaly diagram (Ce/Ce* versus Pr/Pr*). Field I, no anomaly; field IIa: positive La anomaly causing an apparent negative Ce anomaly; field IIb, negative La anomaly causing an apparent positive Ce anomaly; field IIIa, genuine positive Ce anomaly; field IIIb, genuine negative Ce anomaly. All samples plot into the field IIIb, indicating a genuine negative Ce anomaly. b, The BSI values. The green and blue box and whisker plots represent the OAE1a and non-OAE1a BSI distributions, respectively. c, Eu/Eu* versus Ce/Ce*. We fit the data with a linear regression with the grey shading representing the 95% confidence bounds. Purple trend represents a fit through all data. A lack of positive correlation (R2 = 0.12) between Ce/Ce* and Eu/Eu* implies that the anomalies are primary. d–k, The linear relationships are fit through all data with the R2 values indicated and shading representing the 95% confidence bounds. Green data represent the OAE1a interval and blue data represent data from before and after the event. Purple trend represents a fit through all data.
Extended Data Fig. 10 Histograms and box plots of the Cismon core geochemical signatures.
The smoothed histograms represent the geochemical data binned by phase, with the phase III data filtered using two independent geochemical thresholds. The panels on the left, outlined in blue, include phase III data filtered by the Mn/Ti threshold (2,333), whereas the data on the right, outlined in red, include phase III data filtered by the Rb/Sr climate threshold (pre-OAE1a = 0.22). Two populations appear, grouping together phase I and phase III oxic/cool-climate samples, which are distinct from phase II and phase III low-oxygen/warm-climate samples. The box plots to the right show a similar analysis but with the P-value results from a two sample t-test between groups reported (alpha = 95%, one-tailed, unequal variance). Values below 0.05 indicate a statistically distinguishable difference between group means. Phase I and phase III oxic samples are statistically similar, whereas phase II and phase III low-oxygen samples are statistically similar.
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Bauer, K.W., McKenzie, N.R., Cheung, C.T.L. et al. A climate threshold for ocean deoxygenation during the Early Cretaceous. Nature (2024). https://doi.org/10.1038/s41586-024-07876-1
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DOI: https://doi.org/10.1038/s41586-024-07876-1
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