Rapid Expansion of Oceanic Anoxia Immediately Before the End-Permian Mass Extinction
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Rapid expansion of oceanic anoxia immediately before the end-Permian mass extinction Gregory A. Brenneckaa,1, Achim D. Herrmanna,b, Thomas J. Algeoc, and Ariel D. Anbara,d aSchool of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287-1404; bBarrett, the Honors College, Arizona State University, Tempe, AZ 85287-1612; cDepartment of Geology, University of Cincinnati, Cincinnati, OH 45221-0013; and dDepartment of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1404 Edited by Donald E. Canfield, University of Southern Denmark, Odense M., Denmark, and approved September 11, 2011 (received for review April 18, 2011) Periods of oceanic anoxia have had a major influence on the evo- anoxia, the flux of reduced U to anoxic facies (such as black 238 lutionary history of Earth and are often contemporaneous with shales) increases, preferentially removing U from seawater. mass extinction events. Changes in global (as opposed to local) The loss of isotopically heavy U drives seawater to lighter isotopic redox conditions can be potentially evaluated using U system compositions (23). Changes in the U isotope ratios of organic- proxies. The intensity and timing of oceanic redox changes asso- rich sediments have been used to study oceanic redox conditions ciated with the end-Permian extinction horizon (EH) were assessed during the Cretaceous (23). Here, we apply this isotope system to from variations in 238U∕235U(δ238U) and Th/U ratios in a carbonate Late Permian and Early Triassic carbonate rocks. Existing evi- section at Dawen in southern China. The EH is characterized by dence indicates that carbonates record the 238U∕235U ratio of the shifts toward lower δ238U values (from −0.37‰ to −0.65‰), indica- seawater in which they were deposited (18, 19), suggesting that tive of an expansion of oceanic anoxia, and higher Th/U ratios ancient carbonates that retain a primary signal of U isotopes can (from 0.06 to 0.42), indicative of drawdown of U concentrations be used to estimate relative changes in ocean oxygenation. in seawater. Using a mass balance model, we estimate that this isotopic shift represents a sixfold increase in the flux of U to anoxic Results and Discussion facies, implying a corresponding increase in the extent of oceanic In the Dawen section, the average U isotopic composition of samples deposited prior to the EH (δ238U ¼ −0.37‰) is very anoxia. The intensification of oceanic anoxia coincided with, or 238 slightly preceded, the EH and persisted for an interval of at least close to that of modern seawater [δ U ¼ −0.41 Æ 0.03‰ (19)]. 40,000 to 50,000 y following the EH. These findings challenge pre- This observation suggests that the fraction of U removed to re- vious hypotheses of an extended period of whole-ocean anoxia ducing sinks during the late Permian was similar to that of the prior to the end-Permian extinction. modern ocean. The Dawen section exhibits an abrupt and signif- icant change in δ238U at the EH (Fig. 2) to values averaging 238 carbonates ∣ uranium isotopes ∣ paleoredox −0.65‰. The δ U ratios of pre- and post-EH samples are sig- nificantly different (p < 0.0001; two-tailed student’s t-test with a α ¼ 0 01 he end-Permian extinction represents the largest mass extinc- significance level of . ). A few isotopically light samples −118 −97 Ttion in Earth history, with the demise of an estimated 90% of are present below the EH ( and cm), which may provide all marine species (1). While it has been extensively studied, the evidence of brief episodes of transient intensification of oceanic exact nature and cause of the end-Permian extinction remains the anoxia preceding the end-Permian mass extinction. This infer- subject of intense scientific debate. Proposed kill mechanisms ence is supported by evidence from additional geochemical SI Text have included a nearby supernova, bolide impacts, periods of proxies in other studies (7) (see for further discussion). extreme volcanism (e.g., Siberian Traps), extensive glaciation, The shift toward lighter U isotopic compositions after the ex- and widespread oceanic anoxia (2). Evidence for shallow-ocean tinction event is consistent with an increase in the deposition of anoxia in conjunction with the end-Permian mass extinction is isotopically heavy U in anoxic facies. The isotopic composition widespread (3–6), but the intensity and timing of oceanic redox of U in seawater is ultimately controlled by the relative sizes changes remain uncertain (7–10). Recent hypotheses have in- and isotopic signatures of the major sources and sinks of U to voked the release of hydrogen sulfide gas (H2S) from seawater the ocean. A simple box model of the oceanic U budget for the as a kill mechanism (11–13). Such models call upon strong expan- modern and end-Permian oceans is shown in Fig. 3. Invoking sion of oceanic anoxia below the oxygenated surface layer to mass balance, we calculated the approximate increase in anoxic allow buildup of H2S, followed by an upward excursion of the sedimentation in the end-Permian ocean as: chemocline that releases the poisonous gas into the atmosphere 238 238 238 238 235 δ U ¼ðð1 − f Þ × δ U Þþðf × δ U Þ: (13). In this study, we examine the U∕ U and Th/U (thorium/ input anoxic other anoxic anoxic uranium) ratios in a carbonate section spanning the end-Permian [1] extinction horizon (EH) to evaluate the timing and scale of these f Here, anoxic represents the fraction of U deposited in anoxic possibilities. Samples for this study were collected from the 238 238 Dawen section of the Yangtze Block in southern China (Fig. 1), facies and δ U represents the δ U values of the anoxic and which has been correlated with the global stratotype section and “other” (i.e., nonanoxic) sinks. Following Montoya-Pino, et al. point (GSSP) of the Permian-Triassic boundary at Meishan (14). (23), we assume: (1) isotopically constant U input from rivers Due to the geochemical properties of U, the ratio of 238U∕235U [the largest source of U to the ocean (24)] over geologic time with can be used as a tool to investigate the history of ocean oxygena- tion at a global scale, as opposed to the local redox information Author contributions: G.A.B., A.D.H., and A.D.A. designed research; G.A.B. and T.J.A. provided by most commonly used proxies. The long residence performed research; G.A.B., A.D.H., and T.J.A. analyzed data; and G.A.B., A.D.H., T.J.A., time (∼500 ky) of U in the oceans leads to a homogeneous U and A.D.A. wrote the paper. concentration in seawater (15, 16), as well as to a homogenous The authors declare no conflict of interest. U isotopic composition (17–19). The low-temperature redox This article is a PNAS Direct Submission. 6þ 4þ 1 transition of U (from U to U ) is the primary cause of To whom correspondence should be addressed. E-mail: [email protected]. GEOLOGY 238 ∕235 U U fractionation on Earth, with the reduced species pre- This article contains supporting information online at www.pnas.org/lookup/suppl/ 238 ferentially enriched in U (18–22). During times of oceanic doi:10.1073/pnas.1106039108/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1106039108 PNAS ∣ October 25, 2011 ∣ vol. 108 ∣ no. 43 ∣ 17631–17634 Downloaded by guest on September 28, 2021 B A C Fig. 1. Location of South China at ∼252 Ma, the time of the end-Permian extinction [Fig. 1A, modified base map from R. Blakey (http://jan.ucc.nau.edu/~rcb7/ 260moll.jpg)] and present-day location of the Dawen section (Fig. 1B, modified from ref. 14). The location of the Meishan GSSP is shown for reference. Lithostratigraphy of the Dawen section is shown in Fig. 1C; see ref. 14 for its correlation to the Meishan GSSP. a value of −0.3‰; (2) a constant isotope fractionation between as insoluble U4þ under reducing conditions (26–28), thus concen- seawater and anoxic/euxinic environments of þ0.5‰; and (3) a trating U relative to Th in anoxic facies. An increase in anoxic constant (þ0.1‰) isotope fractionation between seawater and sedimentation reduces the concentration of U in seawater as the sum of other sinks, including ferromanganese oxide, hydro- more U is sequestered in organic-rich sediments. Because the thermal, and suboxic sediments. Suboxic sediments, which are U concentration of carbonates is related to the U concentration defined by their low oxygen concentrations in the bottom water of the seawater in which they are deposited (29, 30), an increase [e.g., 0.2–2mLO2 per l H2O (25)], are likely to represent the in anoxic sedimentation results in an increase in the Th/U ratio largest nonanoxic sink (24) and are known to accumulate U with of carbonate sediments. At Dawen, average Th/U ratios increase a small fractionation (þ0.1‰) relative to seawater (19). Based from 0.06 below the EH to 0.42 above the EH (Fig. 2). This on the assumptions above, δ238U ¼ −0.3‰, δ238U ¼ increase reflects a decrease in the U content of seawater, possibly input other ∼7× −0 55‰ −0 65 þ 0 1‰ δ238 ¼ −0 15‰ by a factor of if Th concentrations remained constant. A . (i.e., . ), and Uanoxic . (i.e., −0 65 þ 0 5‰ change in seawater U concentrations of this magnitude is consis- . ). These values yield an estimated sixfold increase tent with the sixfold expansion of oceanic anoxic inferred from in the flux of U to anoxic facies in conjunction with the EH.