TECTONIC FORCING OF MARINE BIODIVERSIFICATION AND BIOGEOCHEMICAL CYCLES

Orogeny and tectonic cyclicity during the Phanerozoic Eon forced marine biodiversification and the biogeochemical cycles of carbon, sulfur, and phosphorus. We examined strontium (⁸⁷Sr/⁸⁶Sr) and sulfur isotope (δ³⁴S) ratios, genera origination rates (GOR), phosphorus accumulation rates (PAR), and percent sedimentary carbon (%C) available for three time series: 147-159, 260, and 520 million years (myr). All indicators exhibited periodicities of 30-to-60 myr. These periodicities are similar to the periodicity of emplacement of Large Igneous Provinces (LIPs), which are thought to result from the production of mafic mantle plumes. Mafic igneous rocks have been reported to be enriched in phosphorus and LIP volume correlates (Spearman’s ρ) and cross-correlates with PAR and %C, indicating a link between tectonism, nutrient input, primary productivity, and marine biodiversification.

A generalized framework of these possible relationships is presented. The injection of CO₂ and sulfurous gases to the atmosphere during orogeny and LIP emplacement results in weathering and the input of “new” nutrients to the oceans by runoff. Enhanced nutrient runoff stimulates marine photosynthesis, drawing down CO₂. Enhanced primary productivity and carbon burial in turn promote sulfate reduction by oxyphobic sulfate-reducing bacteria, which mineralize phosphorus from dead organic matter, resulting in authigenic phosphate precipitation while also recycling some phosphorus back to the water column. Variables may interact synergistically or counteract one another over different time spans as a result of their cyclicity, thereby generating lead-lag relationships. Given the geologically slow input of nutrients from land, nutrient recycling may have been critical to maintaining, if not promoting, biodiversification.

These processes appear to lie on a continuum between those recently reported Oceanic Anoxic Events and the end-Permian extinctions.