Here, fossil marine vertebrates (mammals, birds, reptiles, and bony and cartilaginous fishes) occur within a semi-continuous, Eocene to Pliocene stratigraphic succession (e.g., Muizon & DeVries, 1985 Brand et al., 2004 Esperante et al., 2008Esperante et al.,, 2015Lambert et al., 2010bLambert et al.,, 2015aParham & Pyenson, 2010 Collareta et al., 2015Collareta et al.,, 2017aCollareta et al.,, 2020bCollareta et al.,, 2021b Bianucci et al., 2016b, c Gioncada et al., 2016 Di Celma et al., 2017 Landini et al., 2017Landini et al.,, 2019Bosio et al., 2021 Boskovic et al., 2021). One of the most privileged locations for unraveling the evolutionary history of cetaceans is undoubtedly the East Pisco Basin of southern Peru. Such conditions also promote high abundances of marine vertebrates and the exceptional preservation of their skeletons in the sediments. calcite) and the lack of sparry cement, are in agreement with those reported for the Peru margin and fall in the fields of either sulphate-reduction or methanogenesis.Īll the data point to dolomite precipitation associated with low-temperature early diagenesis that typically occurs in upwelling settings, where high surface water productivity is responsible for high rates of organic carbon flux to the sea bottom and for the cyclical oxygen depletion at the bottom. δ¹⁸O and δ¹☼ values from the microcrystalline dolomite of two layers, selected based on the absence of other carbonate phases (e.g. As observed in both thin sections and on broken surfaces, microcrystalline dolomite also fills the inner spaces of the diatom areolae replicating their finest ultrastructure, such as foramina and cribra and replace calcite shells of foraminifera.

Two generations of dolomite can be recognised: an early diagenetic microcrystalline one, and a sparry one, filling the large cavities.

The underlying sediments exhibit a yellow layer with sparse dolomite crystals, a black layer with abundant Mn-oxides, and a reddish layer rich in Fe-oxides, indicating redox-related fronts. Dolomite occurs in continuous 10–50 cm thick well cemented layers, formed by sediment of different nature: biogenic, terrigenous, volcanoclastic, and phosphatic. In this work, cemented layers from this sequence were described and sampled for petrographic, chemical, microscopic and isotopic analyses. At Cerro Los Quesos, laterally continuous dolomite layers characterise the diatomaceous sediments of the P2 sequence of the Miocene Pisco Formation, where a large number of marine vertebrates are exceptionally preserved, many enclosed in dolomite nodules. Along the Peruvian coast, the sedimentary succession of the East Pisco Basin is exposed in the Ica Desert.

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