Habitability for Complex Life and the Development and Self-Limitations of the Biotic Enhancement of Weathering

Abstract:

We submit that the tightly coupled coevolution of biota and climate is a critical driver of the self-organization of the biosphere over geologic time. The long-term carbon biogeochemical cycle includes a major influence of biology relevant to climatic, namely the biotic enhancement of weathering (BEW). According to a meta-analysis of field and experimental evidence, the likely magnitude of the present BEW is roughly two orders of magnitude, the culmination of its progressive increase over geologic time. Within the context of modeling this long-term cycle, this value can be used to estimate the likely abiotic temperature history of the Earth’s surface, assuming plausible initial temperatures, and histories of volcanic outgassing and continental crust growth. The result of this modeling is that the Earth would have been habitable for thermophilic life (growing above 50 deg C) for the past 4.4 billion years, but not for low-temperature life, including plants and animals. Hence biospheric cooling due to biotic actions allowed the emergence of complex life. Much larger increases in BEW are self-limiting, since the atmospheric CO2 level would plunge below the lower limit potentially for photosynthesis, thereby driving a decline in the biological productivity and global BEW, related to reduced plant and soil activities, with the system being kept at this threshold or going back to higher CO2 levels, with scenarios dependent on volcanic outgassing and solar inputs. We will present astrobiological implications of this modeling. References: Schwartzman D (1999, 2002) Life, Temperature, and the Earth: The Self Organizing Biosphere. Columbia Univ. Press; Schwartzman, D. (2013) Keynote: The Geobiology of Weathering: The 13th Hypothesis. Goldschmidt Conference. (Schwartzman D. and Brantley S. (2013) Mineral. Mag. 77(5): 2170); Volk T (1998) Gaia’s Body: Toward a Physiology of Earth. Copernicus.