The Nasca Theocracy had in Cahuachi (Southern Peru) and its various temple areas the most important ceremonial site (Lasaponara et al., 2016) whose continuity of use as religious centre over the millennia was highlighted by the discovery of much earlier architectural structures (1700 BCE) than those in the urban context dated in the Paracas-Nasca period (400 BCE-550 CE). The mainstream explanation for the decline and demise of Cahuachi implies the damage and burial of such a ceremonial centre as a consequence of two catastrophic mega-floods, which occurred, respectively, around 600 CE and 1000 CE (Grodzicki, 1990) probably linked to the El Niño-Southern Oscillation (ENSO) events. However, recent geological results achieved at Cahuachi (Delle Rose et al 2016; Delle Rose et al. 2019) hypothesized that the conglomerate at the top of the succession at Piramide Sur and Gran Piramide was not deposited by catastrophic ENSO floods as hypothesized by Grodzicki (1990). During April 2019 a geological survey was carried out at Cahuachi mainly addressed to sample and date by Optical Stimulated Luminescence (OSL) the alluvial sediments supposed to be those of the first millennium CE, thought to be responsible for the decline of the ceremonial site. The OSL analyses were performed using the SAR protocol (Wintle and Murray, 2006) applied on quartz grains (180-250 μm) extracted from the samples. The range of OSL dating were set from different hypothesized weather humidity condition of the samples (dry up to 70% wet). One sandy sample (NZ1a) adjacent to a wall of the Gran Piramide temple has a minimum OSL age of 23-25 ky BP whereas other sandy levels (NZ11) sampled close to the adobe construction along the same type-section studied and considered of paramount importance by Grodzicki (1990) to prove the catastrophic river floods of the first millenium CE give two possible age (due to bimodal distribution of Equivalent Dose obtained) of 30-35 and 23-27 ky. NZ16 sands underlying a conglomerate with imbricated pebbles at Estaqueria (a site 4 km to the west of the main temples) which assumed a dominant role after the abandonment of Cahuachi, gave an OSL age between 18 and 21 ky. All these alluvial sediments adjacent to the adobe ceremonial temples or walls do not match with the first millennium CE. Also, a natural colluvium (NZ3) upon anthropic waste in the eastern sector of Tempio Sur is around 2 ky BP (OSL). This dating closely matches the calibrated radiocarbon age of 1960± 93 years before 1950 CE (conventional radiocarbon age: 2018 ± 35 years BP) of charcoal samples at the bottom of the colluvium itself. Searching more recent alluvial sediments only produced modern ages ≤150-180 years ago (NZ7, NZ8). The OSL survey does not therefore reconcile with any ENSO-related mega-floods in the first millennium CE emphasized by Grodzicki (1990). This author probably made a geomorphological over-interpretation (through aerial photo) which led him to consider the sandy levels and sandy conglomerates as the alluvial deposit reaching the adobe constructions instead of pre-existing geological layers the Cahuachi temples were built upon. It is worth to note that archaeological evidence in the Nasca Phase IV period emphasizes debris flows and at least two heavy rain/alluvial events (around 450 CE, also documented in northern Peru) and intense periods of earthquake activity which are consistent with the progressive weakening of resiliency of the population and abandonment of large areas of Cahuachi (Orefici, 2016). It is thus time to overcome the paradigm of the ENSO catastrophic mega-floods for the decline of Cahuachi and the end of the Nasca Theocracy.
OSL dating of alluvial sediments at Cahuachi (Southern Peru) not reconciling the paradigm of catastrophic mega-floods for the decline and end of the Nasca Theocracy
Delle Rose M.
;
2024
Abstract
The Nasca Theocracy had in Cahuachi (Southern Peru) and its various temple areas the most important ceremonial site (Lasaponara et al., 2016) whose continuity of use as religious centre over the millennia was highlighted by the discovery of much earlier architectural structures (1700 BCE) than those in the urban context dated in the Paracas-Nasca period (400 BCE-550 CE). The mainstream explanation for the decline and demise of Cahuachi implies the damage and burial of such a ceremonial centre as a consequence of two catastrophic mega-floods, which occurred, respectively, around 600 CE and 1000 CE (Grodzicki, 1990) probably linked to the El Niño-Southern Oscillation (ENSO) events. However, recent geological results achieved at Cahuachi (Delle Rose et al 2016; Delle Rose et al. 2019) hypothesized that the conglomerate at the top of the succession at Piramide Sur and Gran Piramide was not deposited by catastrophic ENSO floods as hypothesized by Grodzicki (1990). During April 2019 a geological survey was carried out at Cahuachi mainly addressed to sample and date by Optical Stimulated Luminescence (OSL) the alluvial sediments supposed to be those of the first millennium CE, thought to be responsible for the decline of the ceremonial site. The OSL analyses were performed using the SAR protocol (Wintle and Murray, 2006) applied on quartz grains (180-250 μm) extracted from the samples. The range of OSL dating were set from different hypothesized weather humidity condition of the samples (dry up to 70% wet). One sandy sample (NZ1a) adjacent to a wall of the Gran Piramide temple has a minimum OSL age of 23-25 ky BP whereas other sandy levels (NZ11) sampled close to the adobe construction along the same type-section studied and considered of paramount importance by Grodzicki (1990) to prove the catastrophic river floods of the first millenium CE give two possible age (due to bimodal distribution of Equivalent Dose obtained) of 30-35 and 23-27 ky. NZ16 sands underlying a conglomerate with imbricated pebbles at Estaqueria (a site 4 km to the west of the main temples) which assumed a dominant role after the abandonment of Cahuachi, gave an OSL age between 18 and 21 ky. All these alluvial sediments adjacent to the adobe ceremonial temples or walls do not match with the first millennium CE. Also, a natural colluvium (NZ3) upon anthropic waste in the eastern sector of Tempio Sur is around 2 ky BP (OSL). This dating closely matches the calibrated radiocarbon age of 1960± 93 years before 1950 CE (conventional radiocarbon age: 2018 ± 35 years BP) of charcoal samples at the bottom of the colluvium itself. Searching more recent alluvial sediments only produced modern ages ≤150-180 years ago (NZ7, NZ8). The OSL survey does not therefore reconcile with any ENSO-related mega-floods in the first millennium CE emphasized by Grodzicki (1990). This author probably made a geomorphological over-interpretation (through aerial photo) which led him to consider the sandy levels and sandy conglomerates as the alluvial deposit reaching the adobe constructions instead of pre-existing geological layers the Cahuachi temples were built upon. It is worth to note that archaeological evidence in the Nasca Phase IV period emphasizes debris flows and at least two heavy rain/alluvial events (around 450 CE, also documented in northern Peru) and intense periods of earthquake activity which are consistent with the progressive weakening of resiliency of the population and abandonment of large areas of Cahuachi (Orefici, 2016). It is thus time to overcome the paradigm of the ENSO catastrophic mega-floods for the decline of Cahuachi and the end of the Nasca Theocracy.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
