Early Mississippian ocean anoxia triggered organic carbon burial and late Paleozoic cooling: Evidence from uranium isotopes recorded in marine limestone

Large, repeated positive carbon isotope (δ¹³C) excursions are a common feature in Proterozoic–Phanerozoic marine carbonate and organic carbon isotope records (e.g., Veizer et al., 1999). Several of the “Big 5” mass extinctions were associated with large positive δ¹³C excursions, highlighting the fact that these excursions record key processes and events in Earth history (e.g., Joachimski and Buggisch, 1993). These positive excursions are attributed to an increase in the burial fraction of organic carbon (OC) produced by oxygenic photosynthesis, resulting in the withdrawal of atmospheric CO₂, and pO₂ buildup. Increased primary productivity, expanded marine anoxia, or increased sedimentation rates have all been suggested as possible drivers for enhanced OC burial during these events (Kump and Arthur, 1999). However, distinguishing among these potential drivers is not straightforward, leading to competing interpretations involving global or local processes. Traditional tools used for deciphering the mechanism that best explains enhanced OC burial events include lithologic and paleobiologic features, elemental geochemistry, Fe speciation, and sulfur and nitrogen isotopes (e.g., Meyer and Kump, 2008); however, these tools reflect local rather than global processes.

Uranium isotope variations (δ²³⁸U) recorded in marine carbonates provide a novel way to distinguish between drivers for marine OC burial by providing an independent estimate of globally integrated ocean redox conditions. The δ²³⁸U value of marine carbonates is a global redox proxy because U isotopes fractionate during reduction with ²³⁸U, which is preferentially sequestered into marine sediments deposited under anoxic conditions, leaving seawater enriched in ²³⁵U (Weyer et al., 2008). Because the residence time of U in seawater is significantly longer than ocean mixing times, the U isotope composition of open-ocean seawater is homogeneous (Tissot and Dauphas, 2015); as a result, limestone precipitated from that seawater has the potential to record global ocean δ²³⁸U.

We applied the δ²³⁸U redox proxy to the one of the largest positive δ¹³C excursion in the Phanerozoic—the Early Mississippian (Tournaisian) δ¹³C isotope excursion, or mid-Tournaisian carbon isotope excursion (TICE) (Saltzman et al., 2004). Previous TICE studies interpreted that OC burial generating the positive excursion was the result of either OC sequestration in foreland basin deposits (tectonic-sedimentation driver; Saltzman et al., 2000, 2004) or oxygen minimum zone expansion (marine anoxia driver; Saltzman, 2003; Buggisch et al., 2008; Liu et al., 2018; Maharjan et al., 2018a). To test which process was responsible for TICE OC burial, we used δ²³⁸U of Tournaisian limestone in Nevada (USA) to generate a global seawater redox curve.

The Early Mississippian spans the climatic transition between the Devonian greenhouse and the late Paleozoic ice age (LPIA) (Montañez and Poulsen, 2013). The timing of initial LPIA cooling is debated as occurring by the Middle to Late Devonian (e.g., Isaacson et al., 2008), Early Mississippian (Buggisch et al., 2008), or the middle-late Mississippian (e.g., Isbell et al., 2003). North American TICE magnitudes range from ∼6‰ to 7‰, and most are characterized by a double spike (Saltzman, 2002; Maharjan et al., 2018a). TICE magnitudes from Europe and Russia range from 4‰ to 6‰ (Saltzman et al., 2004). The δ¹⁸O trends from Tournaisian apatite, calcite, and carbonate-associated sulfate indicate a positive ∼1‰–2‰ shift concurrent with the TICE, indicating seawater cooled as OC was buried (Mii et al., 1999; Buggisch et al., 2008; Maharjan et al., 2018b). Previous studies using δ¹⁵N (Yao et al., 2015; Maharjan et al., 2018a) and δ³⁴S_CAS (CAS—carbonate-associated sulfate) (Gill et al., 2007; Maharjan et al., 2018b) to evaluate Tournaisian seawater redox reported mixed results between adjacent sections, leaving the question of global redox conditions unanswered.

We sampled the Lower Mississippian Joana Limestone and Limestone X in the Pahranagat Range of southeastern Nevada, USA (Fig. 1; GPS coordinates 37°23’39.1"N, 115°15’42.1"W), where previous conodont biostratigraphy and δ¹³C studies provide a robust temporal framework and indicate that the ∼250-m-thick succession spans ∼4 m.y. (Saltzman, 2002; Maharjan et al., 2018a, 2018b). Detailed paleogeographic, stratigraphic, and biostratigraphic background, and our methods, are provided in the GSA Data Repository¹.

Measured δ²³⁸U values ranged from −1.02‰ to 0.02‰ (Table DR4 in the Data Repository), and stratigraphic trends are shown in Figure 2. The δ¹³C_carb (carb—carbonate) values ranged from −1.1‰ to 7.0‰. Cross-plots of δ²³⁸U values against proxies of local redox conditions (U, V, Mo), detrital influx (Al/U, Th/U, Fe, wt% carbonate), nutrients (Fe, P), and diagenesis (Mn/Sr, δ¹⁸O_carb) show no covariation (Fig. 3; Fig. DR2).

Several lines of evidence argue against local depositional conditions controlling δ²³⁸U trends. First, each facies deposited in poorly oxygenated to well-oxygenated settings records a wide range of δ²³⁸U values; for example, lime mudstone facies deposited under moderately to poorly oxygenated conditions record the same spread of δ²³⁸U values as the most oxygenated crinoid packstone facies (Fig. 3). Second, the prominent negative δ²³⁸U shift occurs in a relatively uniform succession of crinoid packstone deposited in oxic waters, whereas the return positive shift occurs within facies deposited in a mix of poorly, moderately, and well-oxygenated waters. Third, the complete negative δ²³⁸U excursion continues across two depositional sequence boundaries, indicating that water depths and depositional environment changes did not influence δ²³⁸U trends.

We cross-plotted δ²³⁸U values against Al/U, Th/U, and wt% carbonate to evaluate the influence of local detrital or riverine water input on δ²³⁸U trends (Fig. 3). The lack of covariation among these proxies and the very low concentrations of Th (<0.6 ppm) indicate that measured δ²³⁸U trends were not influenced by these local processes. To evaluate potential effects of local reducing bottom water or pore waters, we compared δ²³⁸U versus selected 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 the δ²³⁸U trends were not controlled by local reducing conditions. All samples had Mg/Ca values <0.05, indicating that they have not been dolomitized.

Previous studies of Bahamian carbonates indicated that early diagenesis results in an average ∼0.27‰ enrichment of bulk limestone sediments (Tissot and Dauphas, 2015; Romaniello et al., 2013; Chen et al., 2018). We assume that the Lower Mississippian samples were also affected by similar levels of diagenetic U-isotopic enrichment, and we argue that the samples were enriched uniformly throughout the succession, based on the early diagenetic closure of the U isotopic system due to the low solubility of U⁴⁺ in anoxic pore waters and limited variation in Mn/Sr and δ¹⁸O_carb values (Fig. 3; Fig. DR2). Given the lithologic, sedimentologic, and geochemical results, we interpret the observed δ²³⁸U trends to represent original changes in global seawater redox conditions and recognize a clear negative excursion representing a major Tournaisian ocean anoxic event (OAE). Using conodont biostratigraphy and numeric age control (Buggisch et al., 2008), the duration of the OAE was ∼1 m.y.

The onset and peak of the Tournaisian OAE coincide with the onset and first peak of the TICE (Fig. 2). This temporal coincidence supports the hypothesis that enhanced OC burial was driven by expanded ocean anoxia, which drove the positive δ¹³C shift. In contrast to previous studies using local redox proxies (δ¹⁵N—Yao et al., 2015; Maharjan et al., 2018a; δ³⁴S_CAS—Maharjan et al., 2018b), these results provide the first evidence from a globally integrated seawater redox proxy, and they provide clear evidence that Early Mississippian cooling and glaciation, as evidenced by the coeval positive δ¹⁸O_apatite shift (Mii et al., 1999; Buggisch et al., 2008), were controlled by an anoxia-driven OC burial event.

We used a dynamic U cycle model (Lau et al., 2016) to quantitatively estimate changes in anoxic seafloor area during the Tournaisian OAE (see methods in the Data Repository) with the following simplified oxygenation terms: Euxinic refers to conditions reducing enough to significantly fractionate and reduce U, and anoxic/suboxic refers to low O₂ conditions, but with low reduction and fractionation of U (i.e., denitrification zones; Morford and Emerson, 1999). To generate the observed 0.3‰ negative shift using only anoxic/suboxic sediment sinks versus euxinic sinks, >90% of the global seafloor would have experienced anoxic/suboxic conditions. Such large-scale, low-O₂ seafloor conditions are not supported by global Lower Mississippian sedimentary and paleobiologic records (e.g., Davydov et al., 2004); consequently, we modeled the negative δ²³⁸U shift using euxinic sediment sinks, which have significantly higher fractionation and U flux rates (Table DR3). Assuming euxinia, the model results suggest that there was an ∼6× increase in euxinic seafloor area (5% to 30%; Fig. 4B). For comparison, the modern ocean is characterized by <0.5% euxinic seafloor (Tissot and Dauphas, 2015).

To estimate the amount of OC buried during the Tournaisian OAE and its effects on the global climate, we used the Kump and Arthur (1999) carbon model and the observed δ¹³C curve (see modeling methods in the Data Repository). The model curve generated two OC burial events corresponding to the TICE double spike (Fig. 4F); integrating the curve over the entire 4 m.y. TICE indicated a total OC burial of ∼8.4 × 10²⁰ g, which is similar to estimates by Saltzman et al. (2004). For context, the amount of OC burial at the peak OAE was ∼40% of the total C flux into the atmosphere-ocean reservoir (Kump and Arthur, 1999), and the total Tournaisian OC burial amount was ∼12× larger than that estimated for the Cretaceous OAE-2 (Owens et al., 2018) and ∼4× to 17× larger than the Late Ordovician (Hirnantian) positive δ¹³C excursion (or HICE; see the Data Repository for HICE modeling). The modeled estimate of the Tournaisian OC burial flux illustrates its significance in cooling the late Paleozoic climate and further supports an Early Mississippian LPIA onset. Comparisons between the TICE and HICE are particularly pertinent to this study because extensive HICE OC burial is implicated in driving peak Hirnantian cooling/glaciation (e.g., Hammarlund et al., 2012), when peak glacier volumes are estimated to have exceeded those during the Last Glacial Maximum.

The second broad δ¹³C TICE peak indicates continued OC burial for an additional ∼2 m.y. after the OAE. This peak is best explained by OC burial occurring in anoxic/suboxic sediments, where U sequestration and fractionation are significantly lower than in euxinic sediments. We estimated the changes in euxinic, anoxic/suboxic, and oxic seafloor percentages using the observed δ²³⁸U curve and U model. The best-fit, but nonunique model curve includes a gradual increase in euxinic seafloor area until the OAE peak, followed by a gradual replacement by dominantly anoxic/suboxic seafloor (Fig. 4G; see the Data Repository for U modeling method). A shift from euxinic to dominantly anoxic/suboxic conditions may have occurred when (1) atmospheric O₂ increased to sufficient levels (due to OC burial) to reoxygenate oceans, and/or (2) marine phosphorus inventories eventually declined below the levels required to sustain expanded euxinia. The latter explanation is supported by measured phosphorus concentrations (Fig. DR2).

The Tournaisian OAE occurred during ongoing late Paleozoic cooling (starting in the Devonian), and its occurrence adds to the list of OAEs developed during cool/icehouse climates (Bartlett et al., 2018; White et al., 2018). This cooling resulted in increased latitudinal thermal gradients and intensified thermohaline circulation and meridional winds, which in turn enhanced upwelling- and wind-derived nutrient flux, increased productivity, and amplified O₂ consumption, leading to the Tournaisian OAE. This interpretation is supported by recent reports of decreased O₂ concentrations during the last two Neogene glacial stages, when high-latitude downwelling patterns reorganized, leading to decreased ventilation (e.g., Lu et al., 2016) and strengthened efficiency of the global biologic pump, enhancing OC flux to deep oceans.

The δ²³⁸U trends across an ∼4 m.y. succession of Lower Mississippian (Tournaisian) limestone in Nevada record an ∼1-m.y.-long negative δ²³⁸U excursion with an ∼0.3‰ magnitude. The lack of covariation between δ²³⁸U values and proxies for depositional and diagenetic conditions indicates that the δ²³⁸U curve represents a global seawater redox signal, and that the Tournaisian negative excursion represents an ocean anoxic event (OAE). The temporal coincidence between the δ²³⁸U negative excursion and the first peak of the global positive δ¹³C excursion (TICE) provides the first evidence from a globally integrated seawater redox proxy that enhanced Tournaisian OC burial was driven by an OAE. U modeling results suggest an ∼6× increase in euxinic seafloor area during the OAE. C modeling suggests that the continued positive δ¹³C trends after the OAE are best explained by initial euxinia replaced by more anoxic/suboxic conditions, and that the OC burial amount significantly exceeded that sequestered during other major Phanerozoic OAEs, leading to Tournaisian cooling and glaciation.

Funding was provided by the National Science Foundation (grant NSF 17–536). Much gratitude goes to Geoff Gilleaudeau for field help and discussions, Ganqing Jiang and Dev Maharjan for sharing field and data information, and Pat Kelly, Zoe Wiesel, and Monica Charles for sample preparation. Many thanks to the constructive reviews provided by T. Algeo, F. Tissot, and an anonymous reviewer.