Ocean pH, atmospheric CO2, and habitability of the early Earth
Abstract
Earth's multi-Gyr habitability requires negative feedbacks on atmospheric and ocean composition. Atmospheric CO2 and the geologic carbon cycle are our particular focus because CO2 has wide-ranging influences. Apart from affecting climate, CO2 changes marine chemistry and seawater pH, leaves a record in sedimentary rocks, and provides a substrate for autotrophic generation of organic matter.
On timescales greater than a million years, the carbonate-silicate cycle acts as a global thermostat. Continental weathering buffers pCO2: silicate cations neutralize outgassed CO2, and C is removed in sedimentary carbonate. Seafloor weathering buffers ocean pH and deep-sea temperatures: silicate cations consume H+ ions, depending on deep-sea temperature. Thus, our models show that seafloor weathering stabilized early Earth's climate even when there was no land1. With land, Archean surface temperatures are 0-40°C. A third weathering substrate, important in the Hadean, was impact ejecta. We find a high probability (~70%) that impact ejecta weathering under the faint young Sun created a cold (<0°C), icy, early Hadean with an alkaline ocean2. Possibly, life originated under such conditions. A central problem is the lack of data for basic environmental variables of the early Earth. For the past 100 Ma, enough data exist to do an inverse model of the C cycle3, but this is not practical for a data-sparse Precambrian. CO2 paleosol estimates are few with wide variance, constraints on ocean pH are absent, and temperature proxies are debated. Consequently, we are developing a new, relatively data-rich proxy for Precambrian seawater pH. Complexing of rare earth elements (REEs) by carbonate ions is pH-dependent, allowing pH to be inferred from REE distributions. We have been using pH-dependent REE sorption recorded in sedimentary rocks to reconstruct seawater pH. Results show that since 3.8 Ga, the ocean has been at least circumneutral to alkaline pH. These new results contradict some notions that early seawater pH was acidic but are consistent with moderation of pCO2 levels (and hence pH) by the geologic C cycle1,2. Refs: 1. J. Krissansen-Totton, G. Arney, D. C. Catling, 2018. PNAS 115, 4105-4110. 2. S. Kadoya, J. Krissansen-Totton, D. C. Catling, 2019. EPSL, submitted. 3. Krissansen-Totton J., Catling D. C., 2017. Nat. Comm. 8, 15423.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP53B..06C
- Keywords:
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- 1041 Stable isotope geochemistry;
- GEOCHEMISTRY;
- 1626 Global climate models;
- GLOBAL CHANGE;
- 5225 Early environment of Earth;
- PLANETARY SCIENCES: ASTROBIOLOGY