δD values of hydrated volcanic glass:: a potential record of ancient meteoric water and climate in New Zealand

Tephra beds that are well drained and have been buried by thin paleosols become hydrated within 2-3000 yr on reaction with meteoric waters. Hence, the absorbed water within silicic volcanic glass shards provides a potential record of deltaD values of ancient meteoric water. Such isotopic records have previously received little investigation. We demonstrate that 1.5-2 in thick tephra beds in central North Island, New Zealand, display uniform deltaD values vertically through their profiles and laterally up to 250 in in outcrop. Reproducibility is not influenced by grain size or age of the tephra. We obtained an average deltaD value of -48 +/- 3parts per thousand for water within the 1.8 ka Taupo Tephra. This is similar to the composition of present-day surface waters. deltaD values of -73 +/-2 and -60 +/- 2parts per thousand for the 25 ka Kawakawa and 30 ka Mangaone Tephra beds are significantly lower than present waters, indicating that they have been hydrated under different surficial conditions. This is consistent with other proxy paleoclimatic indicators that suggest a cooler, drier, and windier climate at the time of their eruption. Tephra beds are a potential source of paleoclimatic data in terrestrial environments that otherwise may lack proxy records.