title: Uranium isotopes distinguish two geochemically distinct stages during the later Cambrian SPICE event.
abstract: Anoxic marine zones were common in early Paleozoic oceans (542-400 Ma), and present a potential link to atmospheric pO2via feedbacks linking global marine phosphorous recycling, primary production and organic carbon burial. Uranium (U) isotopes in carbonate rocks track the extent of ocean anoxia, whereas carbon (C) and sulfur (S) isotopes track the burial of organic carbon and pyrite sulfur (primary long-term sources of atmospheric oxygen). In combination, these proxies therefore reveal the comparative dynamics of ocean anoxia and oxygen liberation to the atmosphere over million-year time scales. Here we report high-precision uranium isotopic data in marine carbonates deposited during the Late Cambrian 'SPICE' event, at ca. 499 Ma, documenting a well-defined -0.18‰ negativeδ238U excursion that occurs at the onset of the SPICE event's positiveδ13C andδ34S excursions, but peaks (and tails off) before them. Dynamic modelling shows that the different response of the U reservoir cannot be attributed solely to differences in residence times or reservoir sizes - suggesting that two chemically distinct ocean states occurred within the SPICE event. The first ocean stage involved a global expansion of euxinic waters, triggering the spike in U burial, and peaking in conjunction with a well-known trilobite extinction event. During the second stage widespread euxinia waned, causing U removal to tail off, but enhanced organic carbon and pyrite burial continued, coinciding with evidence for severe sulfate depletion in the oceans (Gill et al., 2011). We discuss scenarios for how an interval of elevated pyrite and organic carbon burial could have been sustained without widespread euxinia in the water column (both non-sulfidic anoxia and/or a more oxygenated ocean state are possibilities). Either way, the SPICE event encompasses two different stages of elevated organic carbon and pyrite burial maintained by high nutrient fluxes to the ocean, and potentially sustained by internal marine geochemical feedbacks.
authors: Nordic Center for Earth Evolution (NordCEE) and Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen K, Denmark.
authors: Tais W
authors: TW
authors: Dahl
authors: College of Life and Environmental Sciences, University of Exeter, Exeter, UK ; Nordic Center for Earth Evolution (NordCEE) and University of Southern Denmark, Odense, Denmark.
authors: Richard A
authors: RA
authors: Boyle
authors: Nordic Center for Earth Evolution (NordCEE) and University of Southern Denmark, Odense, Denmark.
authors: Donald E
authors: DE
authors: Canfield
authors: Centre for Star and Planet Formation (StarPlan) and Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen K, Denmark.
authors: James N
authors: JN
authors: Connelly
authors: Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA.
authors: Benjamin C
authors: BC
authors: Gill
authors: College of Life and Environmental Sciences, University of Exeter, Exeter, UK.
authors: Timothy M
authors: TM
authors: Lenton
authors: Centre for Star and Planet Formation (StarPlan) and Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen K, Denmark.
authors: Martin
authors: M
authors: Bizzarro
keywords: SPICE event; anoxic marine zones; carbonates; oceanic anoxic events; paleoredox; uranium isotopes