Paired carbonate-organic carbon and nitrogen isotope variations in Lower Mississippian strata of the southern Great Basin, western United States

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

“…This temporal coincidence supports the hypothesis that enhanced OC burial was driven by expanded ocean anoxia, which drove the positive δ 13 C shift. In contrast to previous studies using local redox proxies (δ 15 N- Yao et al, 2015;Maharjan et al, 2018a; δ 34 S 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 δ 18 O apatite shift (Mii et al, 1999;Buggisch et al, 2008), were controlled by an anoxia-driven OC burial event.…”

“…The δ 18 O 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 δ 15 N (Yao et al, 2015;Maharjan et al, 2018a) and δ 34 S CAS (CAS-carbonateassociated 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 applied the δ 238 U redox proxy to the one of the largest positive δ 13 C excursion in the Phanerozoic-the Early Mississippian (Tournaisian) δ 13 C 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, 2000Saltzman et al, , 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 δ 238 U of Tournaisian limestone in Nevada (USA) to generate a global seawater redox curve.…”

“…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).…”

“…1; GPS coordinates 37°23'39.1"N, 115°15'42.1"W), where previous conodont biostratigraphy and δ 13 C studies provide a robust temporal framework and indicate that the ∼250-m-thick succession spans ∼4 m.y. (Saltzman, 2002;Maharjan et al, 2018aMaharjan et al, , 2018b. Detailed paleogeographic, stratigraphic, and biostratigraphic background, and our methods, are provided in the GSA Data Repository 1 .…”