The decline of the Nasca culture (Peru) as the result of an increasing environmental stress: overcoming the paradigm formulated at Cahuachi of catastrophic mega-floods due to El Niño-Southern Oscillation

Cahuachi, located on the coastal desert of Southern Peru (Nasca Province), represented the main ceremonial centre of the Nasca culture. An hypothesis of destruction of the site related to catastrophic floods (the youngest around the 10th century AD), due to El Niño-Southern Oscillation (ENSO) was proposed by Grodzicki (1990) three decades ago. This paradigm finally led to believe that Cahuachi was covered by huge catastrophic floods accumulating conglomerates up to the top of the highest buildings. By contrast, Orefici (2016) emphasizes progressive abandonment of large areas of the ceremonial site probably due to several environmental stress such as the occurrence of huaycos (i.e. moderate non-catastrophic debris flows) and earthquakes. In order to investigate the origin of the conglomerates at Cahuachi, a stratigraphic succession outcropping on the bedrock of the "Pirámide Sur" was studied by means of mineralogical, petrographic and sedimentological approaches. In addition, a comprehensive overview of the paleoclimatic data from the literature concerning southern Peru (coast vs. Andean Cordillera), such as ENSO cycles, SOI (Southern Oscillation Index) and oxygen isotopes contributed to unravel the cause/s of the Nasca culture decline and demise. The investigated stratigraphic succession consists of mudstones, siltstones, sandstones and conglomerates, all sharing similar silty-clayey components (powder XRD data). Conglomerates, likely resulting from progradation of alluvial fan systems, have the same lithotypes of pebbles (mainly granitoids/diorites, lavas of andesite/dacite/rhyolite compositions, vitrophyric pyroclasts, arenites/quarzarenites). In addition, the conglomerates occurring at the top of the section unequivocally underlie the ceremonial buildings. Along the stratigraphic succession a clear volcaniclastic silty layer (CH3) was also found, just above the two conglomerates at the bottom (CH1-CH2) of the stratigraphic sequence. This volcaniclastic level could also represent a tephrostratigraphic regional marker. Independent paleoclimatic data indicate, around 700 AD, an increase of temperature, driven by the intensification of the ENSO events. At high altitudes strong ENSO implies an increase of precipitation (mainly snow) as confirmed by the SOI. This increase in temperature correspond to dry condition in the lowlands of Southern Peru, where a period of aridity started after 250 AD, culminating after 650 AD, which is also confirmed by an increase of river incision (Eitel et al., 2005). Among the conglomerates of the investigated stratigraphic succession, mineralogical and petrographic studies of the samples do not point out differences in components (i.e. lithology of the pebbles and composition of the fine fraction of the whole samples). This surveyed stratigraphic section well correlates with the Upper Pliocene-Lower Pleistocene "Changuillo Formation" (Montoya et al., 1994). The conglomerate deposits at the top of the section could be also interpreted as a lateral facies of the base of the "Canete Formation" and unequivocally underlie the ceremonial buildings (Delle Rose, 2016). This finding is at odd with the late Holocene age proposed by Grodzicki (1990) for this succession. We suggest that, rather than the mere effect of ENSO-related catastrophic floods, the decline of Cahuachi was related to the increase of aridity and, possibly, other natural disaster events such as earthquakes. A progressive weakness of the resiliency of the Cahuachi population, coupled with possible changes in social acceptance of the environmental stress as motivated by the religious hierarchy, could have played a major role in the decline and demise of the Nasca Culture. References Delle Rose M. 2016. The Geology of Cahuachi (Chapter 3). In