Kasting, J. F., Howard, M. T., Wallmann, Klaus and Jaffré, J. (2006) Evolution of a habitable planet Paleoclimates, ocean depth, and the oxygen isotopic composition of seawater. [Talk] In: Goldschmidt Conference 2006. , 27.-29.08.2006, Melbourne, Australia .

Abstract

Earth has remained habitable, and inhabited, over most of its 4.5-Gyr history despite an appreciable (30%) increase in solar luminosity over time and despite catastrophic events such as asteroid impacts and ''Snowball Earth'' episodes that have threatened biological survival on a global scale. Life has survived partly because of the resilience of the biota and partly because of feedback mechanisms that help to stabilize Earth's global climate. In particular, buildup of volcanic CO 2 during times when the climate is cold provides a strong negative feedback that helps keep Earth within the temperature regime favorable to life. During the first half of Earth's history, when atmospheric O 2 concentrations were low, CH 4 was probably an important greenhouse gas as well. The Paleoproterozoic glaciations at 2.4 Ga were likely triggered by the rise of O 2 and a corresponding decrease in atmospheric CH 4 concentrations. A possible mid-Archean glaciation at 2.9 Ga may have been caused by the formation of hydrocarbon smog. Both glaciations correspond with anomalies in the sulfur MIF (mass-independent fractionation) record, which has proven to be a wonderful source of information about the nature of the early atmosphere. The same processes that help stabilize Earth's climate should operate on other Earth-like planets, if they exist; thus, it is plausible that life could exist elsewhere. This hypothesis is now on the verge of being tested. NASA's twin Terrestrial Planet Finder (TPF) missions, which could be launched as early as 2015– 2020, will look for Earth-like planets around nearby stars and, if they are found, provide spectroscopic information on their atmospheres. Between them, these missions should be able to look for absorption bands of O 2 , H 2 O, CO 2 , and O 3. Both O 2 and O 3 are considered to be good indicators of life for planets orbiting within the liquid water habitable zone of their parent star. NASA should be encouraged to give these missions high priority, so that we can answer these fundamental questions about the distribution of life in the universe. Oxygen isotope ratios in carbonates are routinely used to estimate paleotemperatures during the last few tens of millions of years. O isotope ratios from both carbonates and cherts have also been used to estimate paleotemperatures in the more distant past. Archean cherts are typically depleted in 18 O by 10& or more; ancient carbonates show smaller, but still substantial, 18 O depletions until as recently as a few hundred million years ago. Taken at face value, these isotope ratios suggest that Earth's mean surface temperature was 70 ± 15 °C in the early Archean and remained significantly elevated until as recently as 350 Ma. While not impossible from a theoretical standpoint, such high temperatures are hard to reconcile with evidence for glaciation at 0.6, 0.75, 2.4, and possibly 2.8 Ga. An alternative way of explaining the O isotope data is if the oxygen isotopic composition of seawater has varied with time. This possibility has been suggested numerous times over the past 40 years but has never garnered widespread support. The O isotopic composition of seawater is controlled primarily by water–rock interactions within the midocean ridge hydrothermal vents. Hot (>350 °C) interactions increase the d 18 O values of seawater; interactions at lower temperatures decrease seawater d 18 O. We suggest here that gradual changes in Earth's tectonic evolution have driven significant changes in this marine isotope control system. Specifically , higher geothermal heat flow in the distant past should have led to shallower oceanic ridges and lower water–rock interaction temperatures within hydrothermal vents. This, in turn, should have caused a corresponding negative shift in the O isotopic composition of seawater. All of this implies that the early Earth was warm, not hot.

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