LAGOONAL SEDIMENTS AS A PROXY FOR MID-HOLOCENE ENVIRONMENTAL CHANGE IN THE CHICAMA VALLEY, NORTH COASTAL PERU: LINKING EL NIÑO TO FLOODPLAIN EMERGENCE

Humans first settled the coastal valleys of northern Peru over ten thousand years ago (Dillehay 2000). Early Peruvian civilizations are unique in that they originally relied almost entirely on maritime food sources, only later transitioning to intensive agricultural food production in the mid-Holocene (Moseley 1975). Paleoenvironmental research at Huaca Prieta, a coastal archaeological site originally inhabited more than 8500 ybp (Bird 1948), can help uncover why this transition occurred at this time. Previous work on the north coast of Peru suggests a weakened El Niño for some part of the mid-Holocene; however, the timing and effects of this hiatus are disputed (e.g. Sandweiss 2003, Keefer et al. 2003, Rodbell et al. 1999). Sediment coring reveals finely bedded lagoonal muds, carbonates, and organic sediments very different from the typically thick floodplain silts and clays prevalent in today’s El Niño-dominated riverine environment. We interpret this distinct lack of clastic input to represent an absence or weakening of El Niño conditions, and suggest that recurrent flooding and overbank deposition associated with the onset of El Niño allowed for the development of a broad floodplain suitable for extensive agricultural activity. The fossiliferous sediments found here can fill important gaps in the climate record during the mid-Holocene, a formative period in the development of Huaca Prieta.

Paleoecological analysis of microfossils found in these sediments (ostracods, terrestrial gastropods, and charophytes) suggest a freshwater oligotrophic environment with an episodic marine connection. Stable isotope analyses of the microfossils also indicate a freshwater or estuarine environment, and they show a potential decline in aridity during the mid-Holocene. The sedimentology of the lagoonal deposits records changing hydrologic conditions in the area, while radiocarbon dating of the upper and lower extents of the sediments bracket a potential weakening of El Niño conditions in north coastal Peru. Lagoon development occurred by roughly 7300 ybp, with the floodplain emerging by 4500 ybp. Future catchment-scale studies will lead to a more comprehensive understanding of the timing and effects of El Niño in the area.