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Despite a Sun with Lower Luminosity, Earth's Early Climate Was Marked By

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Despite a Sun with Lower Luminosity, Earth's Early Climate Was Marked By Comparative Climatology III 2018 (LPI Contrib. No. 2065) 2045.pdf REVERSE WEATHERING AND EARTH’S CLIMATE. N. J. Planavsky1, T.T. Isson1, and B. Kalderon-Asael1, 1Department of Geology and Geophysics, Yale University, 06511. Introduction and Findings: Despite a Sun with the rise of silica biomineralizing eukaryotes would lower luminosity, Earth’s early climate was marked have initiated a significant waning of reverse weather- by apparent stability with rare catastrophic glacia- ing rates. This transition would have led to a drop in tions. We will make a case that elevated rates of re- baseline atmospheric carbon dioxide concentrations verse weathering—the consumption of alkalinity and and the establishment of a more volatile (responsive) the generation of acidity accompanying clay authi- climate system, directly linked to a decrease in the genesis—dramatically enhanced the retention of car- buffering strength of oceans. Notably, we find this bon within the ocean-atmosphere system, and led to paradigm to be consistent with geologic evidence for the sustenance of a significantly elevated atmospheric a decline in authigenic marine clays and the weaken- carbon dioxide concentrations. Although dampened ing of Earth’s thermostat as indicated by the onset of by sluggish kinetics today, more prolific rates of re- more frequent greenhouse-icehouse oscillations since verse weathering would have persisted under the per- the latest Precambrian. Had siliceous organisms never vasively silica-rich conditions that dominated Earth’s evolved, Earth’s surface environment would be signif- early oceans. Further, with extensive reverse weather- icantly warmer and oceans far more acidic than they ing, the establishment of a potent negative feedback are today, with potential implications for the evolu- between marine pH and authigenic clay formation tion of Phanerozoic life and their associated metabo- would have greatly enhanced climate stability by mit- lisms. More detailed Phanerozoic atmospheric carbon igating large swings in atmospheric carbon dioxide dioxide concentrations trends could reflect, at least in concentrations. The ecological rise of siliceous organ- part, variations to the ecological success of siliceous isms would have dampened the reverse weathering organisms through reverse weathering. Future models buffer, destabilizing Earth’s climate system. seeking to explore climatic evolution on terrestrial In addition we will present a new carbonate Lithi- bodies ought to consider both forward (weathering) um isotope record spanning the last three billon years and backward (reverse weathering) processes govern- of Earth’s history, that reconstructs the evolution of ing global alkalinity budgets and the buffering seawater lithium isotope values and provides a new strength of oceans. perspective on the evolution of the coupled carbon and silica cycles. The global lithium isotope mass balance is controlled foremost by fractionations that occur during incongruent weathering of silicate min- erals in terrestrial environments, and by the formation of authigenic clay minerals in the marine realm (re- verse weathering). We generated Li isotope data from over 700 marine carbonates samples. Our sampling focused on micritic carbonates but included belem- nites, brachiopods, and marine cements. In total, we sampled from 83 stratigraphic units that range in age from 3.0 Ga to the modern. We found a predominance of low Li isotope values in the Precambrian, which indicates weathering intensities and the extent of re- verse weathering in the sediment pile were signifi- cantly elevated relative to modern. Evidence from the geochemical record suggests that organisms have played a critical role in shaping the composition of Earth’s oceans and atmosphere, which in turn also structures the biosphere. Perhaps most notably, methanogens and oxygenic phototrophs have transformed Earth’s surface environments since the Archean. The most recent chapter of Earth’s histo- ry, the Phanerozoic, was one shaped foremost by eu- karyotic life. If our estimates are correct, then the switch to a biologically controlled silicon cycle with .
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