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 ocean anoxia triggered organic carbon burial and late cooling: Evidence from uranium isotopes recorded in marine Keyi Cheng1, Maya Elrick1 and Stephen J. Romaniello2,3 1Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA 2Department of Earth & Space Science, Arizona State University, Tempe, Arizona 85287, USA 3Department of Earth & Planetary Sciences, University of , Knoxville, Tennessee 37996, USA

ABSTRACT limestone precipitated­ from that seawater has the The Early Mississippian () positive δ13C excursion (mid-Tournaisian carbon potential to record global ocean δ238U. isotope excursion [TICE]) was one of the largest in the , 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 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 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

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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 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 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. 2). This temporal coincidence ocean crust, and biogenic carbonates (Tissot supports the hypothesis that enhanced OC and Dauphas, 2015). Under anoxic conditions, DISCUSSION burial was driven by expanded ocean anoxia, U is reduced to insoluble U(IV) species and Several lines of evidence argue against lo- which drove the positive δ13C shift. In contrast adsorbed by organic ligands or precipitated as cal depositional conditions controlling δ238U to previous studies using local redox prox- U-rich minerals within sediments and is conse- trends. First, each facies deposited in poorly ies (δ15N—Yao et al., 2015; Maharjan et al., 34 quently removed from seawater (Weyer et al., oxygenated to well-oxygenated settings records 2018a; δ SCAS—Maharjan et al., 2018b), these 2008). During reduction, the larger 238U nucleus a wide range of δ238U values; for example, lime results provide the first evidence from a glob- is preferentially concentrated into the reduced mudstone facies deposited under moderately to ally integrated seawater redox proxy, and they U(IV) species due to the nuclear volume ef- poorly oxygenated conditions record the same provide clear evidence that Early Mississippian fect. During times of expanded ocean anoxia spread of δ238U values as the most oxygenated cooling and glaciation, as evidenced by the co- 18 and anoxic seafloor area, more U is reduced, packstone facies (Fig. 3). Second, the eval positive δ Oapatite shift (Mii et al., 1999; and 238U is sequestered into sediments, leav- prominent negative δ238U shift occurs in a rela- Buggisch et al., 2008), were controlled by an ing seawater enriched in 235U, with carbonate tively uniform succession of crinoid packstone anoxia-driven OC burial event. minerals recording the lower 238U/235U ratios. deposited in oxic waters, whereas the return We used a dynamic U cycle model (Lau Because the residence time of U is significantly positive shift occurs within facies deposited in et al., 2016) to quantitatively estimate changes in longer (∼400 k.y.; Dunk et al., 2002) than ocean a mix of poorly, moderately, and well-oxygen- anoxic seafloor area during the Tournaisian OAE mixing times, U isotopes in limestone should ated waters. Third, the complete negative δ238U (see methods in the Data Repository) with the record global seawater redox conditions (e.g., excursion continues across two depositional se- following simplified oxygenation terms: Euxinic Brennecka et al., 2011; Lau et al., 2016; Elrick quence boundaries, indicating that water depths refers to conditions reducing enough to signifi- and depositional environment changes did not cantly fractionate and reduce U, and anoxic/ 238 influenceδ U trends. suboxic refers to low O2 conditions, but with 1GSA Data Repository item 2020104, geologic We cross-plotted δ238U values against Al/U, low reduction and fractionation of U (i.e., deni- background, methods, stratigraphic trends, evaluation Th/U, and wt% carbonate to evaluate the influ- trification zones; Morford and Emerson, 1999). of diagenetic effects, and uranium and carbon mod- eling explanations, is available online at http://www. ence of local detrital or riverine water input To generate the observed 0.3‰ negative shift 238 geosociety.org/datarepository/2020/, or on request on δ U trends (Fig. 3). The lack of covaria- using only anoxic/suboxic sediment sinks ver- from [email protected]. tion among these proxies and the very low sus euxinic sinks, >90% of the global seafloor

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/4/363/4972107/363.pdf by guest on 28 September 2021 Figure 2. Stratigraphy, age, depositional sequences,

M. Miss. Visean redox interpretations, and LOWESS-smoothed 346.7Ma δ238U and δ13C trends S-11 of the studied section PR (Pahranagat Range, S-10 Nevada, USA). Outliers

00 m (open circles) are based S-9 on anomalous elemental Lower G. typicus Osagean concentrations. Gray band S-8 is interpreted Tournaisian ocean anoxic event (OAE)

Limestone X S-7 defined by∼ 1-m.y.-long negative δ238U excur- ~348Ma

S-6 TICE 13

ournaisian sion. Black circles in δ C T S-5S-5 curve are from this study; No diagnosti c fossils Early Mississippian open diamonds are from 10 02 S-4S-4 Maharjan et al. (2018a). Shaded vertical bar left 238

S. crenulata - of δ U curve shows oxy- S-3S-3 genation interpretations Uppe r S. isostich a Kinderhookian based on sedimento-

S-2 logic field observations

Joana Limestone discussed in the Data S. sandberg i ? S-1 Repository (see footnote 0 Sediment derived- -1.0 -0.8 -0.6 -0.4 -0.2 0.00.2 -2 02468 1). TICE—mid-Tournaisian L. De v. Pilot redox interpretation 238 13 carbon isotope excur- Shale δ U (‰) δ C (‰, VPDB)

Conodont sion; VPDB—Vienna Crinoidal packstone Skeletal wackestone Lime mudstone-wackestone Lime mudstone Peedee belemnite stan- dard. Conodont : Depositional Sedimentary redox curve Sedimentary redox curve Sedimentary redox curve S-1 S.—, G.— ​ sequences poorly oxygenated facies moderately oxygenated facies oxygenated facies Gnathus.

would have experienced anoxic/suboxic condi- modern ocean is characterized by <0.5% euxinic of ∼8.4 × 1020 g, which is similar to estimates by

tions. Such large-scale, low-O2 seafloor condi- seafloor (Tissot and Dauphas, 2015). Saltzman et al. (2004). For context, the amount tions are not supported by global Lower Missis- To estimate the amount of OC buried dur- of OC burial at the peak OAE was ∼40% of sippian sedimentary and paleobiologic records ing the Tournaisian OAE and its effects on the the total C flux into the atmosphere-ocean res- (e.g., Davydov et al., 2004); consequently, we global climate, we used the Kump and Arthur ervoir (Kump and Arthur, 1999), and the total modeled the negative δ238U shift using euxinic (1999) carbon model and the observed δ13C Tournaisian OC burial amount was ∼12× larger sediment sinks, which have significantly higher curve (see modeling methods in the Data Re- than that estimated for the OAE-2 fractionation and U flux rates (Table DR3). As- pository). The model curve generated two OC (Owens et al., 2018) and ∼4× to 17× larger than suming euxinia, the model results suggest that burial events corresponding to the TICE double the Late () positive δ13C there was an ∼6× increase in euxinic seafloor spike (Fig. 4F); integrating the curve over the excursion (or HICE; see the Data Repository for area (5% to 30%; Fig. 4B). For comparison, the entire 4 m.y. TICE indicated a total OC burial HICE modeling). The modeled estimate of the

2000 100% 3.0

95% (%)

2.5 t cri pkst 1500 h 90% g i 2.0 85% /U skel wks t e we 1000 t 80%

Al 1.5 Th/U a Facies n 75% lmst-wkst 1.0 bo r

500 a 70% C

lmst 0.5 65%

0 0.0 60% -1.0 -0.8 -0.6 -0.4 -0.2 0.00.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.00.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.00.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.00.2 238 δ23 8U (‰) δ U (‰) δ238U (‰) δ238U (‰)

0.18 2.5 12 0.5 0.16 10 2.0 0.14 0.4 ) ) m)

8 p 0.12 pm (p p

1.5 Sr 0.3

0.10 / (ppm ( n

rb 6 rb carb a a M

c 0.08 c 1.0 o 0.2 V U 4 M 0.06

0.5 0.04 0.1 2 0.02 0.0 0.0 0 0.00

-1.0 -0.8 -0.6 -0.4 -0.2 0.00.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.00.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.00.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.00.2 δ238U (‰) δ238U (‰) δ238U (‰) δ238U (‰)

Figure 3. Cross-plots of δ238U against four main Tournaisian facies, proxies for detrital sediment (Al/U, Th/U, carb wt%), and redox (U, V, Mo, Mn/Sr). Lack of covariation among proxies indicates that measured δ238U values were not influenced by local processes. Lmst—lime mudstone, lmst-wkst—lime mudstone-wackestone, skel wkst—skeletal wackestone, cri pkst—crinoidal packstone.

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200 3 )

150 m.y.

2 (m ) im e( Figure 4. (A) Smoothed δ238U curve. (B) Modeled 100 Mode lt percent euxinic seafloor Thickness changes. (C) Modeled sea- 1 water U concentrations

50 (ppb). (D) Measured Ucarb concentrations (ppm). (E) LOWESS-smoothed 0 13C curve. VPDB— 0 δ -0.9 -0.8 -0.7 -0.6 -0.5 0% 5% 10% 15% 20%25% 30% 0.10.2 0.30.4 0.50.6 0.7 0.00.5 1.02.5 Vienna Peedee belemnite Smoothed δ238U (‰) % Euxinic seafloor Modeled seawater U conc. Measured U conc. standard. (F) Modeled vari- (ppb) (ppm) ations in organic carbon (OC) burial rate estimated E F G 13 oxic using smoothed δ C seafloor curve. (G) Percent eux- anoxic/ suboxic inic, anoxic/suboxic, and seafloor oxic seafloor changes euxinic utilized in U model to seafloor best fit measuredδ 238U and δ13C curves; see the Data Repository for model parameters (see footnote 1). Note that this model curve is not unique.

01234567 10000 14000 18000 22000 0 20 40 60 80 100 Smoothed δ13C(‰, VPDB) Organic carbon % total seafloor burial rate (1012 mol/k.y.)

Tournaisian OC burial flux illustrates its signifi- OC burial) to reoxygenate oceans, and/or (2) ma- in Nevada record an ∼1-m.y.-long negative δ238U cance in cooling the late Paleozoic climate and rine phosphorus inventories eventually declined excursion with an ∼0.3‰ magnitude. The lack further supports an Early Mississippian LPIA below the levels required to sustain expanded of covariation between δ238U values and prox- onset. Comparisons between the TICE and euxinia. The latter explanation is supported by ies for depositional and diagenetic conditions HICE are particularly pertinent to this study be- measured phosphorus concentrations (Fig. DR2). indicates that the δ238U curve represents a global cause extensive HICE OC burial is implicated in The Tournaisian OAE occurred during ongo- seawater redox signal, and that the Tournaisian driving peak Hirnantian cooling/glaciation (e.g., ing late Paleozoic cooling (starting in the De- negative excursion represents an ocean anoxic Hammarlund et al., 2012), when peak glacier vonian), and its occurrence adds to the list of event (OAE). The temporal coincidence between volumes are estimated to have exceeded those OAEs developed during cool/icehouse climates the δ238U negative excursion and the first peak of during the . (Bartlett et al., 2018; White et al., 2018). This the global positive δ13C excursion (TICE) pro- The second broad δ13C TICE peak indicates cooling resulted in increased latitudinal thermal vides the first evidence from a globally integrated continued OC burial for an additional ∼2 m.y. gradients and intensified thermohaline circula- seawater redox proxy that enhanced Tournaisian after the OAE. This peak is best explained by tion and meridional winds, which in turn en- OC burial was driven by an OAE. U modeling re- OC burial occurring in anoxic/suboxic sedi- hanced upwelling- and wind-derived nutrient sults suggest an ∼6× increase in euxinic seafloor

ments, where U sequestration and fractionation flux, increased productivity, and amplified2 O area during the OAE. C modeling suggests that are significantly lower than in euxinic sediments. consumption, leading to the Tournaisian OAE. the continued positive δ13C trends after the OAE We estimated the changes in euxinic, anoxic/ This interpretation is supported by recent reports are best explained by initial euxinia replaced by

suboxic, and oxic seafloor percentages using the of decreased O2 concentrations during the last more anoxic/suboxic conditions, and that the OC observed δ238U curve and U model. The best-fit, two glacial stages, when high-latitude burial amount significantly exceeded that seques- but nonunique model curve includes a gradual downwelling patterns reorganized, leading to tered during other major Phanerozoic OAEs, increase in euxinic seafloor area until the OAE decreased ventilation (e.g., Lu et al., 2016) and leading to Tournaisian cooling and glaciation. peak, followed by a gradual replacement by strengthened efficiency of the global biologic dominantly anoxic/suboxic seafloor (Fig. 4G; see pump, enhancing OC flux to deep oceans. ACKNOWLEDGMENTS the Data Repository for U modeling method). A Funding was provided by the National Science Foun- shift from euxinic to dominantly anoxic/suboxic CONCLUSIONS dation (grant NSF 17–536). Much gratitude goes to conditions may have occurred when (1) atmo- The δ238U trends across an ∼4 m.y. succession Geoff Gilleaudeau for field help and discussions, Gan- qing Jiang and Dev Maharjan for sharing field and data spheric O2 increased to sufficient levels (due to of Lower Mississippian (Tournaisian) limestone

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