Diatomaceous mudstones of the Mio-Pliocene Pisco Formation, Perù: implications on vertebrate preservation and role of volcanic ashes in fertilizing ocean surface

Laminated diatomaceous mudstones are the most characteristic lithology of the Mio- Pliocene Pisco Formation, Ica desert, Peru; discrete tuff layers intercalate to them. The Pisco Fm. deposited during the Neogene in the East Pisco Basin, one of the Andean forearc basins which run eastward of the Peru-Chile trench. Nowadays, the Pisco Fm. is one of the most famous Fossillagerstätte in the world. Its popularity is due to the great abundance of exceptionally well preserved marine vertebrates, especially cetaceans, lying in the diatomaceous mudstone. Many specimens are indeed fully articulated and often concentrated in horizontal accumulation layers. This study is meant to understand how diatom deposition may have positively interfered in the genesis of this Fossillagerstätte, by: 1) analyzing diatom laminae and their different assemblages and obtaining information on the seasonality affecting the water column during deposition of the Pisco Fm.; 2) recognizing and evaluating any evidence of exceptional harmful algal blooms (HABs) coinciding with the fossil vertebrate accumulation levels; HABs were recently evoked to explain four discrete horizons showing fossil marine vertebrates accumulation in the late Miocene of Atacama desert, Chile (Pyenson et al., 2014); 3) investigating the hypothesis that diatom and diatom mats deposition may have favored the preservation of vertebrates by lowering the level of dissolved oxygen on the sea floor; indeed, these conditions would have prevented the development of a whale-fall community that generally leads to the fast decay of the carcasses and the final disarticulation of skeletons; 4) evaluating the sedimentation rates of the diatomaceous mudstones to understand how long it would have taken for a carcass to be completely buried, thus, verifying whether fossil preservation is mainly due to the high sedimentation rates of diatomaceous mudstones as suggested by Brand et al. (2004); Moreover, we are investigating diatom assemblages close to tuff levels to verify whether the deposition of volcanic ashes may have enhanced primary production in the Pisco Fm.; indeed, anomalous algal blooms recently observed on the ocean surface following volcanic explosive events have been related to the release of macronutrients such as Fe, NH+ 4,NO-3, NO-2, PO3-4, Si by volcanic ashes reaching the ocean surface (Duggen et al., 2007).