LARGE IGNEOUS PROVINCES, THE HEATT (HALINE EUXINIC ACIDIC THERMAL TRANSGRESSION) MODEL, AND PHANEROZOIC EXTINCTIONS

Large Igneous Provinces (LIPs) have propelled the Earth system from a Greenhouse state into a Hothouse state on at least 15 Phanerozoic occasions. Icehouse climates that are sustained by major continental-collision orogenies are marked by sinking polar brines that drive thermohaline circulation. Maintenance of strong pole-driven circulation creates preconditions that deter LIPs from tipping Earth into a Hothouse. However, when such circulation weakens due to minimal polar ice during Greenhouse climates, the oceans are susceptable to a shift to thermohaline circulation in which the dominant deep brine-generating mechanism shifts to middle latitudes where evaporation in restricted shallow embayments and deeper marginal (Mediterranean-like) seas delivers of warm brine to deep waters so as to generate a thermal-expansion-driven transgression that marks the onset of a Hothouse climate.

Phanerozoic Hothouse climates can develop in less than half a million years, and appear to need an ongoing LIP to trigger and sustain them. Hothouses are marked by euxinic waters that can reach into the photic zone. Reduced pole-equator thermal gradients lead to diminished planetary windbelt speeds, weakening ocean-surface circulation and reducing wind-driven upwelling. Sharply reduced delivery of iron dust to oceans limits bacterial fixation of usable nitrogen. Tropical cyclonic storm activity intensifies, and deep-reaching storms that are no longer confined to the tropics become the chief upwelling mechanism for nutrient delivery to surface waters until the HEATT episode climaxes. At that point, cyclones draw up toxic euxinic waters, leading to extinction in warm, euxinic, acidic, and nutrient-depleted oceans. Extreme warmth, hypercapnia, and other factors intensively stress terrestrial life. Mass extinction marks each of the HEATT episodes we have identified.