Fluvial history of the Rio Ilave valley, Peru, and its relationship to climate and human history

摘要

Fluvial strata and landforms in the Rio Ilave valley (Peru) document a history of Holocene aggradation and downcutting that is correlative with regional climatic events and provides an environmental context for human occupation of the river valley. Periods of aggradation correspond to periods of high(or rising) level in Lake Titicaca and elsewhere on the Altiplano, and increased sediment accumulation in the Rio Ilave valley. Downcutting episodes correspond to periods of low level in Lake Titicaca and low or rapidly decreasing sedimentation rates in the Ilave delta. There are five terrace tracts (Tl through T5) present in this southwestern Lake Titicaca tributary. These tracts occur as both paired and unpaired terraces and have average heights from 1.4 to 24.3 m above the valley floor. The major part of the fluvial sequence was deposited during the time period from prior to the Last Glacial Maximum until about 8300 calendar years Before Present (cal BP) - a period of generally high (but variable) precipitation on the Altiplano and high water level in Lake Titicaca. Initial deposition (aggradation) was followed by successive downcutting to the T4 and T3 terrace surfaces. Initial downcutting began immediately after precipitation, runoff, and sediment load decreased while base level dropped. It was followed by a period of episodic equilibrium and minor downcutting that included a prolonged period of soil formation between ～8350 and 6780 cal BP. The major pulses of downcutting likely occurred between ～6000 and 4500 cal BP and were coincident with periods of decreased precipitation on the Altiplano and decreasing levels of Lake Titicaca. Two final periods of infilling, resulting in deposition of the T2 and T1 terrace sediments at ～4000 to 2500 cal BP and ～2000 to 1600 cal BP (during periods of rising water level in Lake Titicaca, lacustrine sedimentation in the Rio Desaguadero valley, and increased sedimentation offshore the Ilave delta), were separated by brief equilibrium stages and a brief downcutting event. This fluvial history, when coupled with regional paleoclimatic data, relates to the region's preceramic through Tiwanakuperiod archeological records. Archeological evidence indicates that humans occupied the Ilave valley as early as 10000 cal BP. The higher terraces (T3, T4 and T5) were occupied for at least 5000 years, but humans did not utilize the lower terraces (T1 and T2) until after ～4400-3700 cal BP. Our results confirm that these lower terraces would not have been available for either occupation or agriculture until after ～4000 cal BP.