https://doi.org/10.1130/G46950.1 Manuscript received 20 August 2019 Revised manuscript received 6 December 2019 Manuscript accepted 9 December 2019 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 31 January 2020 Early Mississippian ocean anoxia triggered organic carbon burial and late Paleozoic cooling: Evidence from uranium isotopes recorded in marine limestone Keyi Cheng1, Maya Elrick1 and Stephen J. Romaniello2,3 1 Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA 2 Department of Earth & Space Science, Arizona State University, Tempe, Arizona 85287, USA 3 Department of Earth & Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA ABSTRACT limestone precipitated from that seawater has the The Early Mississippian (Tournaisian) positive δ13C excursion (mid-Tournaisian carbon potential to record global ocean δ238U. isotope excursion [TICE]) was one of the largest in the Phanerozoic, and the organic carbon We applied the δ238U redox proxy to the one (OC) burial associated with its development is hypothesized to have enhanced late Paleozoic of the largest positive δ13C excursion in the Pha- cooling and glaciation. We tested the hypothesis that expanded ocean anoxia drove widespread nerozoic—the Early Mississippian (Tournaisian) OC burial using uranium isotopes (δ238U) of Lower Mississippian marine limestone as a global δ13C isotope excursion, or mid-Tournaisian car- seawater redox proxy. The δ238U trends record a large Tournaisian negative excursion last- bon isotope excursion (TICE) (Saltzman et al., ing ∼1 m.y. The lack of covariation between δ238U values and facies changes and proxies for 2004). Previous TICE studies interpreted that local depositional and diagenetic influences suggests that theδ 238U trends represent a global OC burial generating the positive excursion was seawater redox signal. The negative δ238U excursion is coincident with the first TICE posi- the result of either OC sequestration in foreland tive excursion, supporting the hypothesis that an expanded ocean anoxic event controlled basin deposits (tectonic-sedimentation driver; OC burial. These results provide the first evidence from a global seawater redox proxy that Saltzman et al., 2000, 2004) or oxygen minimum an ocean anoxic event drove Tournaisian OC burial and controlled Early Mississippian zone expansion (marine anoxia driver; Saltzman, cooling and glaciation. Uranium and carbon modeling results indicate that (1) there was an 2003; Buggisch et al., 2008; Liu et al., 2018; Ma- ∼6× increase in euxinic seafloor area, (2) OC burial was initially driven by expanded euxinia harjan et al., 2018a). To test which process was followed by expanded anoxic/suboxic conditions, and (3) OC burial mass was ∼4–17× larger responsible for TICE OC burial, we used δ238U than that sequestered during other major ocean anoxic events. of Tournaisian limestone in Nevada (USA) to generate a global seawater redox curve. INTRODUCTION Traditional tools used for deciphering the mech- Large, repeated positive carbon isotope anism that best explains enhanced OC burial BACKGROUND (δ13C) excursions are a common feature in Pro- events include lithologic and paleobiologic fea- The Early Mississippian spans the climatic terozoic–Phanerozoic marine carbonate and or- tures, elemental geochemistry, Fe speciation, transition between the Devonian greenhouse and ganic carbon isotope records (e.g., Veizer et al., and sulfur and nitrogen isotopes (e.g., Meyer the late Paleozoic ice age (LPIA) (Montañez 1999). Several of the “Big 5” mass extinctions and Kump, 2008); however, these tools reflect and Poulsen, 2013). The timing of initial LPIA were associated with large positive δ13C excur- local rather than global processes. cooling is debated as occurring by the Middle sions, highlighting the fact that these excur- Uranium isotope variations (δ238U) record- to Late Devonian (e.g., Isaacson et al., 2008), sions record key processes and events in Earth ed in marine carbonates provide a novel way Early Mississippian (Buggisch et al., 2008), or history (e.g., Joachimski and Buggisch, 1993). to distinguish between drivers for marine OC the middle-late Mississippian (e.g., Isbell et al., These positive excursions are attributed to an burial by providing an independent estimate of 2003). North American TICE magnitudes range increase in the burial fraction of organic car- globally integrated ocean redox conditions. The from ∼6‰ to 7‰, and most are characterized bon (OC) produced by oxygenic photosynthe- δ238U value of marine carbonates is a global re- by a double spike (Saltzman, 2002; Maharjan sis, resulting in the withdrawal of atmospheric dox proxy because U isotopes fractionate dur- et al., 2018a). TICE magnitudes from Europe 238 CO2, and pO2 buildup. Increased primary pro- ing reduction with U, which is preferentially and Russia range from 4‰ to 6‰ (Saltzman ductivity, expanded marine anoxia, or increased sequestered into marine sediments deposited et al., 2004). The δ18O trends from Tournaisian sedimentation rates have all been suggested as under anoxic conditions, leaving seawater en- apatite, calcite, and carbonate-associated sul- possible drivers for enhanced OC burial during riched in 235U (Weyer et al., 2008). Because the fate indicate a positive ∼1‰–2‰ shift concur- these events (Kump and Arthur, 1999). However, residence time of U in seawater is significantly rent with the TICE, indicating seawater cooled distinguishing among these potential drivers is longer than ocean mixing times, the U isotope as OC was buried (Mii et al., 1999; Buggisch not straightforward, leading to competing inter- composition of open-ocean seawater is homo- et al., 2008; Maharjan et al., 2018b). Previous pretations involving global or local processes. geneous ( Tissot and Dauphas, 2015); as a result, studies using δ15N (Yao et al., 2015; Maharjan CITATION: Cheng, K., Elrick, M., and Romaniello, S.J., 2020, Early Mississippian ocean anoxia triggered organic carbon burial and late Paleozoic cooling: Evidence from uranium isotopes recorded in marine limestone: Geology, v. 48, p. 363–367, https://doi.org/10.1130/G46950.1 Geological Society of America | GEOLOGY | Volume 48 | Number 4 | www.gsapubs.org 363 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/4/363/4972107/363.pdf by guest on 28 September 2021 W E concentrations of Th (<0.6 ppm) indicate that 238 A B SL measured δ U trends were not influenced by these local processes. To evaluate potential ef- PR fects of local reducing bottom water or pore America waters, we compared δ238U versus selected North redox-sensitive metals and Mn/Sr (Fig. 3). The lack of covariation and low Mn/Sr values (<0.3) among these redox proxies indicate that 050 km the δ238U trends were not controlled by local reducing conditions. All samples had Mg/Ca Figure 1. (A) Early Mississippian paleogeography of North American, from Blakey (2013). Red values <0.05, indicating that they have not circle shows location of study area in present-day southern Nevada, USA (GPS coordinates been dolomitized. 37°23’39.1"N, 115°15’42.1"W). (B) Schematic east-west cross section across the Mississip- Previous studies of Bahamian carbonates in- pian carbonate ramp, showing location of study area (PR—Pahranagat Range, Nevada, USA; dicated that early diagenesis results in an aver- SL—sea level; modified from Maharjan et al., 2018a). age ∼0.27‰ enrichment of bulk limestone sedi- ments (Tissot and Dauphas, 2015; Romaniello 34 et al., 2018a) and δ SCAS (CAS—carbonate- et al., 2017; Zhang et al., 2018). U-isotope com- et al., 2013; Chen et al., 2018). We assume that associated sulfate) (Gill et al., 2007; Maharjan positions are reported as: the Lower Mississippian samples were also af- et al., 2018b) to evaluate Tournaisian seawater fected by similar levels of diagenetic U-isotopic 238 235 redox reported mixed results between adjacent 238 (U/U)sample enrichment, and we argue that the samples were δ U =− 238 235 1 ×1000‰, (1) sections, leaving the question of global redox (U/U)standard enriched uniformly throughout the succession, conditions unanswered. based on the early diagenetic closure of the U We sampled the Lower Mississippian Joana where the standard is CRM-145 (Uranyl Nitrate isotopic system due to the low solubility of U4+ Limestone and Limestone X in the Pahranagat Assay and Isotopic Solution; New Brunswick in anoxic pore waters and limited variation in 18 Range of southeastern Nevada, USA (Fig. 1; GPS Laboratory, 2010). Mn/Sr and δ Ocarb values (Fig. 3; Fig. DR2). coordinates 37°23’39.1"N, 115°15’42.1"W), Given the lithologic, sedimentologic, and geo- where previous conodont biostratigraphy and RESULTS chemical results, we interpret the observed δ238U δ13C studies provide a robust temporal frame- Measured δ238U values ranged from −1.02‰ trends to represent original changes in global work and indicate that the ∼250-m-thick succes- to 0.02‰ (Table DR4 in the Data Repository), seawater redox conditions and recognize a clear sion spans ∼4 m.y. (Saltzman, 2002; Maharjan and stratigraphic trends are shown in Figure 2. negative excursion representing a major Tour- 13 et al., 2018a, 2018b). Detailed paleogeographic, The δ Ccarb (carb—carbonate) values ranged naisian ocean anoxic event (OAE). Using con- stratigraphic, and biostratigraphic background, from −1.1‰ to 7.0‰. Cross-plots of δ238U odont biostratigraphy and numeric age control and our methods, are provided in the GSA Data values against proxies of local redox condi- (Buggisch et al., 2008), the duration of the OAE Repository1. tions (U, V, Mo), detrital influx (Al/U, Th/U, was ∼1 m.y. Uranium is sourced from weathered con- Fe, wt% carbonate), nutrients (Fe, P), and dia- The onset and peak of the Tournaisian 18 tinental crust, and the main sinks are subox- genesis (Mn/Sr, δ Ocarb) show no covariation OAE coincide with the onset and first peak of ic-anoxic-euxinic marine sediments, altered (Fig. 3; Fig. DR2). the TICE (Fig.