Phanerozoic Reef Crises and the HEATT Model

Many of the factors that potentially stressed reef communities probably intensified simultaneously during a number of reef crisis in the Phanerozoic. Warming, acidification, anoxia, nutrient crises, hypercapnia, and flooding are all aspects of the HEATT (haline expansion anoxic thermal transgression) model.

In the model, HEATT events occur when global conditions favor a shift from sinking polar brines generated by freezing to sinking mid-latitude brines caused by evaporation. Sinking warm brines deliver oxygen-poor water to the deep ocean. This transformation can shift the planet from a greenhouse state to a hothouse state. Triggers such as large igneous provinces may be critical to pushing global climate to the point at which the deep ocean warms enough to rapidly expand thermally. Such transgressions will be a few tens of meters at most. The falloff in wind speed that occurs during a heat will dramatically reduce erosion of desert dust, leading to widespread iron limitation in many parts of the world ocean.

HEATT events can be geologically rapid. In addition to intense warmth, carbon dioxide triggers ocean acidification, carbonate dissolution and a calcification crisis. Hypercapnia may accompany these events. The transgression will deliver euxinic and nutrient-poor waters to shallow-water settings. We are presently testing this model against extinction horizons in general as well as to reef extinctions and crises.

HEATT events associated with reef extinctions and crises include: 1) the Kellwasser and Hangenberg horizons in the Devonian, 2) the end-Permian, 3) the end-Triassic, 4) the Toarcian, 5) the Albian, and 6) the Cenomanian-Turonian. Some HEATT components have been previously suggested as reef crisis instigators in the Cretaceous (e.g. Johnson et al., 1996). Many of the HEATT model effects are intensifying in our modern global warming experiment.