Abstract
The highly productive waters of the Humboldt Current System (HCS) host a particular temperate ecosystem within the tropics, whose history is still largely unknown. The Pisco Formation, deposited during Mio-Pliocene times in the Peruvian continental margin has yielded an outstanding collection of coastal-marine fossils, providing an opportunity to understand the genesis of the HCS ecosystem. We present a comprehensive review, completed with new results, that integrates geological and paleontological data from the last 10 My, especially focusing on the southern East Pisco Basin (Sacaco area). We discuss the depositional settings of the Pisco Formation and integrate new U/Pb radiometric ages into the chronostratigraphic framework of the Sacaco sub-basin. The last preserved Pisco sediments at Sacaco were deposited ~ 4.5 Ma, while the overlying Caracoles Formation accumulated from ~ 2.7 Ma onwards. We identified a Pliocene angular unconformity encompassing 1.7 My between these formations, associated with a regional phase of uplift. Local and regional paleoenvironmental indicators suggest that shallow settings influenced by the offshore upwelling of ventilated and warm waters prevailed until the early Pliocene. We present an extensive synthesis of the late Miocene–Pleistocene vertebrate fossil record, which allows for an ecological characterization of the coastal-marine communities, an assessment of biodiversity trends, and changes in coastal-marine lineages in relation to modern HCS faunas. Our synthesis shows that: (i) typical endemic coastal Pisco vertebrates persisted up to ~ 4.5 Ma, (ii) first modern HCS toothed cetaceans appear at ~ 7–6 Ma, coinciding with a decline in genus diversity, and (iii) a vertebrate community closer to the current HCS was only reached after 2.7 Ma. The genesis of the Peruvian coastal ecosystem seems to be driven by a combination of stepwise transformations of the coastal geomorphology related to local tectonic pulses and by a global cooling trend leading to the modern oceanic circulation system.
Highlights
The East Pisco Basin (EPB) is a forearc basin located along the Peruvian continental margin that has accumulated marine sediments since the Eocene (Fig. 1; Thornburg and Kulm, 1981; Dunbar et al 1990)
The strontium isotopic analysis from the Sacaco locality, the only locality containing independent chronostratigraphic controls from U/Pb datings, has yielded a mean 87Sr/86Sr value of 0.7090005 with a 2σ error lower than ± 0.00002 (Ehret et al 2012). This measured ratio does not deviate from the Messinian average global seawater 87Sr/86Sr that would correspond to three existing independent radiometric dates (Fig. 5), but it is significantly higher than modern strontium values from coastal rivers entering the Pacific Ocean between 14 and 16 oS, which have catchments dominated by igneous rocks that supply very low 87Sr/86Sr
Since the pioneering work of Muizon and DeVries (1985), the EPB has yielded numerous paleontological discoveries that contribute to the understanding of the evolution of different clades
Summary
The East Pisco Basin (EPB) is a forearc basin located along the Peruvian continental margin that has accumulated marine sediments since the Eocene (Fig. 1; Thornburg and Kulm, 1981; Dunbar et al 1990). This measured ratio does not deviate from the Messinian average global seawater 87Sr/86Sr that would correspond to three existing independent radiometric dates (Fig. 5), but it is significantly higher than modern strontium values from coastal rivers entering the Pacific Ocean between 14 and 16 oS, which have catchments dominated by igneous rocks that supply very low 87Sr/86Sr (ranging from 0.70561 to 0.70809, n = 7; Scaffidi et al 2020) This indicates that, the Sacaco sub-basin represents a marginal and shallow area, during the Messinian it did not receive large river inputs with different Sr isotopic composition, and so major salinity fluctuations due to changes in the freshwater supply were unlikely to occur when the sediments from the Sacaco locality were accumulated (~ 5.8–5.7 Ma). These animals are regarded as highly dependent on coastal settings; any disruption of foreshore habitats by Andean tectonics or reduction of neritic zones after sea-level oscillations (see Pimiento et al 2017) might have occurred later, during the 4.5–2.7 Ma sedimentary hiatus (Fig. 10)
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