Iron Fertilization by Volcanic Ash in the Cenomanian/Turonian Western Interior Seaway

Abstract:

Volcanic ash contains 1-10% FeO by weight and can be a significant contributor of Fe to the surface ocean. It is possible that Fe fertilization by volcanic ash has contributed to marine productivity in the past. The Late Cretaceous Eagle Ford Group (Cenomanian/Turonian) contains abundant volcanic ash beds interbedded with black argillaceous limestones and calcareous mudstones, providing opportunities for observation of the influence of ash on productivity and basin chemistry in the Western Interior Seaway. In particular, we hypothesize that volcanic ash from nearby arc volcanoes stimulated productivity by providing reactive iron to the surface ocean that was otherwise nutrient-limited by poor ventilation in a stratified water column. Enhanced productivity likewise reinforced basinal anoxia. To test our hypothesis, we examined the Swenson #1 core (151 feet) from McMullen County, south Texas, which contains 51 visualized ash beds with varying thickness. High resolution x-ray fluorescence spectroscopy (5 mm), x-ray fluorescence microscopy (100 µm) and scanning electron microscopy were performed to examine burial of Fe, trace elements delivered to the sediment by sinking organic matter (Cu and Ba), and paleoredox proxies (Mo and Cr) below, in, and above ash beds.

Ash beds and beds containing admixed ash contain much more Fe than interbedded black shales, with nearly all Fe present in pyrite intergrown with or partially replacing altered ash grains, suggesting that ash transported reactive Fe to otherwise Fe-poor settings. Concentrations of Cu, Ba, Mo, and Cr were significantly greater in beds with admixed ash than in underlying beds. This suggests that input of ash increased marine productivity (Cu and Ba), which in turn enhanced oxygen demand and promoted euxinia (Mo and Cr). We conclude that Fe-bearing volcanic ash fertilized the southern Cenomanian/Turonian Western Interior Seaway, episodically forcing or enhancing euxinia both before and after OAE2.