Nonmarine ostracode shell chemistry from ancient hohokam irrigation canals in central Arizona: A paleohydrochemical tool for the interpretation of prehistoric human occupation in the North American Southwest

Abstract Environmental archaeology studies on samples from an agricultural context provide information that may be used to answer questions of climatic vs. human impact on Hohokam prehistory. Hohokam irrigation systems were fragile and subject to episodic damage and destruction. Ostracodes are sensitive to hydrochemical changes. Their fossil record in Hohokam canals can provide useful data to understand human/environment relationships in the past. Low‐Mg calcite ostracode carapaces form in thermal and chemical equilibrium with the host water. Magnesium and strontium are commonly incorporated into the shell as trace elements, which can be used to track paleotemperature and paleosalinity variations. Thus, fossil ostracodes reflect the water chemistry in which the valves were precipitated. Valves of Limnocythere staplini, Candona patzcuaro , and Cypridopsis vidua were analyzed using inductively coupled plasma mass spectrometry (ICP‐MS) to determine the Mg/Ca and Sr/Ca ratios. Trace element data were used to reconstruct the canal water chemistry and to propose paleoclimatic and human‐impact hypotheses for environmental change. The ostracode record shows some evidence of a progressive salinization process between A. D. 1025 and 1425 that may be the result of human disturbance; this inference is supported by independent evidence from geographic, palynologic, and dendroclimatologic data. In addition, two major climatic events, large enough not to be masked by human activity, were also recorded by ostracode geochemistry: a flood between A. D. 855 and 910, another flood prior to A. D. 1350, and a drought between A. D. 1365 and 1425. © 1994 John Wiley & Sons, Inc.