INFLUENCE OF HOTHOUSE CLIMATE ON SEDIMENTARY PHOSPHATE ACCUMULATION

Phosphate accumulation in Hothouse climates was recently suggested to be represented by phosphorite gaps during productivity shutdowns associated with maximum Hothouse development. This research tests that hypothesis as well as one that predicts accumulation of coated phosphate grains during bursts of productivity just before and/or just after Hothouse climax. The HEATT (Haline Euxinic Acidic Thermal Transgression) model introduced by Kidder and Worsley (2010) explains the pulses of widespread anoxia and euxinia that occur at 15 or more discrete levels in the past 600 million years by connecting a series of global forcing factors and mechanisms to the consequences observed throughout geologic time. Paleocirculation and biogeochemistry distinguish Hothouse climates from Greenhouse climates, and phosphate distribution should provide another way to recognize these different climate states in the geologic record.

During times of anoxic and euxinic conditions, reduced nitrogen (e.g. ammonium) would act as a limiting nutrient. The predicted severe loss in wind erosive power would result in considerable reductions of windblown eolian dust delivery to Hothouse oceans and promote iron-limited oceans that greatly restrict nitrogen fixation and oxygen production. This scarce nitrogen availability and reducing conditions, as well as the limited biota present during extinction at HEATT horizons, would contribute to an excess abundance of phosphorus in the anoxic marine waters. Phosphate here would not get concentrated as phosphorite because widespread and extensive anoxia characteristic of the onset of HEATT episodes would tend to keep phosphate in solution, and an overall lack of metazoan activity would further limit phosphate accumulation so therefore, the peak of the Hothouse should exhibit very little phosphate deposition.

Phosphate expression during many of the organic horizons from the predicted Mesozoic HEATT episodes record the anticipated sedimentologic relationships unique to this model. Promising results to date support the hypothesis at the following intervals : Cenomanian-Turonian, Albian-Aptian, and Early Toarcian.