COLD CRADLES AND WARM GRAVES- HOW TEMPERATURE CONSTRAINS OXYGEN IMPACTING DIVERSITY

We tend to compartmentalize Earth history such that we do not see the most general patterns. For example, Neoproterozoic snowball Earth events seemingly anticipate the evolution of animals, while warm events in the Phanerozoic associate with episodes of extinction. This pattern implies a cold “cradle” for animal diversity, followed by episodic warm “graves,” and this pattern appears to be a function of temperature on oxygen. Here we assess the multiple impacts of temperature on the “effective oxygen” available for complex active multicellular organisms and place these in a single overarching scheme so that events in earth history can be compared to each other and integrated with projections of life on exoplanets on the one hand and human induced climate change on the other. Factors that limit effective oxygen with increasing temperature include reduced thermohaline circulation, reduced solubility of gases in water, increased microbial oxygen uptake, and increased metabolic demand for oxygen by the organisms themselves. These operate in parallel to reduce available oxygen for large complex organisms. In the simplest conception, water-rich worlds have states along a continuum with Snowballs at one end, Polar ice worlds in the middle, and Green/Hot houses at the other extreme. This continuum feeds back on the factors controlling effective oxygen. For example, both snowball and greenhouse earths are expected to have stratified oceans suggesting that partially glaciated worlds should have advantages in effective oxygenation and the evolution of complex multicellular life. This is consistent with an inter-snowball, or post-snowball “Gaskiers’, initiation and radiation of Metazoa. Biodiversity of aqueous exoplanets should then evolve from exclusively “microbial” to inclusion of complex life with transition out of the snowball condition. Arraying Earth’s past climate events along this continuum, provides a heuristic with which to compare events and data. Amount, rate and relative position of change on the continuum are all potentially informative. With this effort towards uniform assessment, events in Earth history can be more consistently compared with each other. Past events can then be used in a coordinated way to predict of the impacts of mankind’s manipulation of climate on Earth’s oceans and marine diversity.