Uranium isotope evidence for two episodes of deoxygenation during Oceanic Anoxic Event 2

Abstract

Oceanic Anoxic Event 2 (OAE 2), occurring ∼94 million years ago, was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. It was typified by a rapid rise in atmospheric CO, global warming, and marine anoxia, leading to the widespread devastation of marine ecosystems. However, the precise timing and extent to which oceanic anoxic conditions expanded during OAE 2 remains unresolved. We present a record of global ocean redox changes during OAE 2 using a combined geochemical and carbon cycle modeling approach. We utilize a continuous, high-resolution record of uranium isotopes in pelagic and platform carbonate sediments to quantify the global extent of seafloor anoxia during OAE 2. This dataset is then compared with a dynamic model of the coupled global carbon, phosphorus, and uranium cycles to test hypotheses for OAE 2 initiation. This unique approach highlights an intra-OAE complexity that has previously been underconstrained, characterized by two expansions of anoxia separated by an episode of globally significant reoxygenation coincident with the "Plenus Cold Event." Each anoxic expansion event was likely driven by rapid atmospheric CO injections from multiphase Large Igneous Province activity.

Global marine redox changes drove the rise and fall of the Ediacara biota

“…The U isotope system ( 238 U/ 235 U, denoted as δ 238 U) measured in carbonate sedimentary rocks is a more direct probe of global ocean redox conditions and can be used to place quantitative constraints on the extent of global redox changes (Brennecka, Herrmann, Algeo, & Anbar, ; Clarkson et al, ; Elrick et al, ; Lau et al, ; Tostevin et al, ; Wei et al, ; Zhang, Algeo, Cui, et al, ; Zhang, Algeo, Romaniello, et al, ; Zhang, Romaniello, et al, ; Zhang, Xiao, et al, ). The power of U isotopes as a global proxy derives from the fact that in the modern ocean U has a long residence time, ~500 kyr (Dunk, Mills, & Jenkins, ), and hence, δ 238 U is uniform in the open ocean (e.g., Tissot & Dauphas, ).…”

“…The power of U isotopes as a global proxy derives from the fact that in the modern ocean U has a long residence time, ~500 kyr (Dunk, Mills, & Jenkins, ), and hence, δ 238 U is uniform in the open ocean (e.g., Tissot & Dauphas, ). Although the concentration and residence time of U in seawater would both be reduced during times of expanded marine anoxia, studies suggest that the U isotope composition of open ocean seawater was likely uniform even during periods of expanded anoxia (Clarkson et al, ; Zhang, Algeo, Romaniello, et al, ; Zhang, Xiao, et al, ). Seawater δ 238 U varies with redox conditions because the reduction of dissolved U(VI) to U(IV), which is immobilized in anoxic sediments, results in a large and detectable change in δ 238 U (0.6‰–0.85‰), favoring the 238 U over 235 U in the reduced species (Andersen et al, ).…”

“…These contrasting views likely arise because these paleoredox proxies are inherently local or indirect tracers of oxygenation. The U isotope system ( 238 U/ 235 U, denoted as δ 238 U) measured in carbonate sedimentary rocks is a more direct probe of global ocean redox conditions and can be used to place quantitative constraints on the extent of global redox changes (Brennecka, Herrmann, Algeo, & Anbar, 2011;Clarkson et al, 2018;Elrick et al, 2017;Lau et al, 2016;Tostevin et al, 2019;Wei et al, 2018;Zhang, Algeo, Cui, et al, 2019; terminal Ediacaran period, when recently published U isotope data indicate a return to more widespread ocean anoxia. Taken together, it appears that global marine redox changes drove the rise and fall of the Ediacara biota.…”

Cyanobacteria Proliferation in the Cenomanian‐Turonian Boundary Interval of the Apennine Carbonate Platform: Immediate Response to the Environmental Perturbations Associated With OAE‐2?

“…Taking into account the interpreted origin of the lower laminite interval, it might be speculated that the upper laminite interval records the impact of a second volcanic pulse on the platform carbonate facies during OAE-2. This may have been associated with cooling-induced change in circulation and the southward spread of Arctic water masses Zheng et al, 2013Zheng et al, , 2016 and had a major effect on fauna and water chemistry (Clarkson et al, 2018;Jenkyns et al, 2017;Sweere et al, 2018). This may have been associated with cooling-induced change in circulation and the southward spread of Arctic water masses Zheng et al, 2013Zheng et al, , 2016 and had a major effect on fauna and water chemistry (Clarkson et al, 2018;Jenkyns et al, 2017;Sweere et al, 2018).…”

“…Most likely the renewed increase of pCO 2 and temperature after the PCE reflected continued volcanogenic degassing from the Caribbean LIP (Jarvis et al, 2011, and references therein;Du Vivier et al, 2014) which has been calculated to last for~200-250 kyr after the onset of OAE-2 (Du Vivier et al, 2014Vivier et al, , 2015. Nutrient spiking and resultant productivity increase may also have been sustained by introducing a range of redox-sensitive trace metals in global seawater during a reoxygenation phase occuring during the PCE (Clarkson et al, 2018;Jenkyns et al, 2017;Sweere et al, 2018). The latter was probably sustained by continued remobilization of P from sediments, favored by anoxic deep waters (Kraal et al, 2010;Monteiro et al, 2012;Tsandev & Slomp, 2009;van Cappellen & Ingall, 1994) which would have reduced the N:P ratio in surface waters giving a further advantage to N-fixing cyanobacteria (Monteiro et al, 2012).…”