17Oexcess is the deviation of d17O from the generally accepted 17O-18O mass dependent reference line. In rainfall, 17Oexcess depends mainly on relative humidity at the moisture source region, with lower relative humidity corresponding to higher 17Oexcess. In some cases, however, rainfall 17Oexcess is influenced by atmospheric processes like partial re-evaporation of the raindrops or moisture recycling. We examine how does 17Oexcess in CaCO3 record 17Oexcess of its parent water and apply it to paleo hydrology in Soreq Cave (Israel) and in Devils Hole (Nevada, USA). In Soreq Cave, 17Oexcess of 50 per meg was obtained in the weighted mean modern rainfall, consistent with the low relative humidity at the moisture source region of the Eastern Mediterranean Sea. 17Oexcess of paleo water were reconstructed from Soreq Cave speleothems, at an age range of 0 - 160 ka. In most of the record values are similar to that in modern cave water, but a few events suggest higher relative humidity, consistent with a more marine storm trajectory. The values at the Last Glacial Maximum suggest low relative humidity and likely indicate the penetration of very cold air. In Devils Hole, 17Oexcess in modern and interglacial reconstructed water is higher than expected by relative humidity, suggesting significant moisture recycling in this continental site. In glacial periods, however, 17Oexcess suggest much less evaporation of water from land surfaces.
