HOTHOUSE CLIMATE PULSES AS DRIVERS OF MULTIPLE, SYSTEMICALLY-LINKED EXTINCTION-FAVORING FACTORS

Multiple extinction-favoring mechanisms developed sequentially as HEATT (Haline Euxinic Acidic Thermal Transgression) episodes ran their course during brief excursions from Greenhouse climate to Hothouse and back again. Many of these mechanisms intensified as the HEATT episodes approached and reached their zenith. Under suitable Greenhouse preconditions, Large Igneous Province (LIP) eruptions triggered a cascade of climatic and biogeochemical changes that were unfavorable to many life forms. More than a dozen Phanerozoic peaks in extinction intensity, including the “Big Five” mass extinctions coincide with HEATT episodes. Factors such as extraterrestrial impact and glaciation take precedence at some extinction horizons.

The onset of a HEATT episode is marked by increased warmth and expansion of anoxia in deep water. Nutrient availability should diminish as pole-equator thermal contrast weakens planetary windbelt velocities, leading to reductions in wind-driven upwelling and delivery of windblown iron-bearing dust to oceans. Rising CO₂ levels may acidify oceans, depending on the degree and rate of buffering. Hypercapnia and reduced atmospheric oxygen become additional threats as HEATT episodes wax. Primitive green-algal plankton, such as prasinophytes, became conspicuous just prior to the HEATT peak. Light ∂¹⁵N values from N fixation by diazotrophic bacteria resulted as these microbes generated reduced N (e.g. ammonium), perhaps from iron supplied from anoxic waters drawn into the photic zone. Ever-larger cyclonic storms may reach increasingly deeply to draw up those waters. LIP volcanism is an additional iron source.

Extinction mostly coincides with the HEATT crescendo, when euxinic waters reach the photic zone. Euxinic waters are deadly in their own right, but mixing them with surface waters may acidify oceans as sulfide oxidizes. Whether HEATT-related extinctions are driven by a single cause such as euxinia or whether intensification of other factors during this pulse of Hothouse conditions result in a deadly combination of systemically-linked factors remains unresolved, but a concatenation of lethal effects cannot, as yet, be ruled out.