354-1 Deconvolving multiple Earth system controls on the Phanerozoic surface environment: a new oceanic Ni isotope record and a multi-tracer approach

Abstract:

The evolution of Earth’s biosphere since the late Precambrian has played out and been partially driven by a changing surface Earth environment.  Studies of the interactions between the changing environment and the biosphere it houses have often focused on spectacular, somewhat singular, evolutionary events, such as catastrophic mass extinctions, the Cambrian radiation of animals, or the appearance of vascular plants on the continents. But this evolutionary history is also characterised by long-term temporal trends and long wavelength variability. Though the details of the ways in which environmental change has influenced long-term biological evolution are still vigorously debated, studies of the interactions between tectonics, climate and the carbon cycle have focused attention on two particular drivers, with the biosphere itself feeding back into both: gradual surface Earth oxygenation punctuated by periodic reversions to oceanic anoxia, and the impact of physical and chemical weathering of rocks on climate,  ocean chemistry, and continental regolith generation. Here, we present a new nickel (Ni) isotope record for the ocean over the entire Phanerozoic, based on analysis of ~500 organic-rich sedimentary rocks. Though oceanic Ni isotopes, a relatively new isotope tracer, are at least partially controlled by ocean redox via the preferential sequestration of light isotopes to sediments underlying an oxic water column, we show that the quantitative evolution of the oceanic mass balance must have been strongly impacted by temporal changes in the riverine flux from the continents, controlled by tectonics and the rate, pattern and intensity of chemical weathering. By coupling these new data to multiple complementary records of the surface Earth, we document changes in redox and the nature and rate of chemical weathering. In particular, we use records and mass balance models of multiple metal isotope systems to deconvolve the importance of multiple Earth system processes in controlling their behaviour. We document slow, sometimes transient, emergence from oceanic anoxia in the Ediacaran-Paleozoic, a fundamental reset of the Earth system at the Permian-Triassic boundary (~252 Ma), a Mesozoic Era characterised by variable environmental conditions, and the move to a modern environment through the Cenozoic Era.

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