A 38000-year record of floods and debris flows in the Ilo region of southern Peru and its relation to El Nino events and great earthquakes

摘要

Previous work throughout the Ilo region of south coastal Peru has documented the existence of flood and debris-flow deposits produced by two El Nino events evidently much more severe than any in recent history. These two events have been dated to ca. AD 1300-1400 and AD 1607-08. The Late Pleistocene to Holocene record of older sedimentary deposits in this region is dominated by flood and debris-flow deposits of similar scale. These older deposits have been described and dated from three coastal, alluvial-fan sites. These deposits, which are as old as 38200 years, are dominated by massive debris-flow deposits, several tens of cm thick, typically composed of cobble-and boulder-sized clasts in a matrix of silty sand, with characteristics indicating generation by heavy rainfall in an arid environment. Twenty-two radiocarbon dates and a single infrared-stimulated luminescence date show that particularly severe El Nino events occurred throughout the Late Pleistocene and two of three divisions of the Holocene with significantly different frequencies. The period of greatest activity was during the Early Holocene when at least six such events took place during a period of ca. 3600 years, beginning near the end of the Younger Dryas ca. 12000 years ago. One of these events produced a debris flow that may have caused abandonment of the Paleo-Indian site at Quebrada Tacahuay, one of the oldest on the Andean coast. No severe events took place during the Middle Holocene between ca. 8400 and 5300 years ago, when a wide variety of other paleoclimate proxy records indicate that the El Nino-Southern Oscillation regime was particularly weak. Since ca. 5300 years ago, four of these severe events have taken place. The Late Pleistocene sequence is constrained by only two dates, which indicate that at least ten severe events took place between ca. 38200 and 12900 years ago. Mechanisms probably responsible for generating these large-scale deposits include: (1) 'Mega-Ninos' that produced anomalously heavy rainfall along most or all of the central Andean coast; (2) El Ninos that occurred shortly after great earthquakes that produced large amounts of sediment; or (3) El Ninos that produced anomalously heavy local rainfall. The existence of these large-scale deposits in the Ilo region implies a level of hazard much higher than indicated by the historical record alone.