Animal burrowing and sediment-mixing (bioturbation) began during the run up to theEdiacaran-Cambrian boundary 1,2,3 , initiating a transitional interval 4,5 separating stratified Precambrian 6 from well-mixed Phanerozoic 7 sediment archetypes. This rise in bioturbation occurred in between oxygen increases at ~551 million years ago (Ma) 8,9 and ~400(Ma) 10 million years ago(Ma), whilst the global oxygen reservoir was probably smaller than present 10,11 . Phosphorus is the long-term 12 limiting nutrient for oxygen production via burial of organic carbon 13 , and its retention (relative to carbon) within
Anoxic marine zones were common in early Paleozoic oceans (542-400 Ma), and present a potential link to atmospheric pO 2 via 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‰ negative δ 238 U excursion that occurs at the onset of the SPICE event's positive δ 13 C and δ 34 S 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
Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts 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.
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