Low-O-18 terranes tracking Mesozoic polar climates in the South Pacific

Substantially negative delta(18)O values of altered rocks are an unfailing guide to O-18-depleted meteoric water of low mean annual temperature, and therefore of cold climates at times of hydrothermal alteration. However, because water-rock interaction is often incomplete even where it has occurred, the lack of depleted values is poor evidence for lack of cold climate. A now established average surface rock composition of -6.6 parts per thousand delta(18)O, with a lower limit of -16 parts per thousand, for an igneous and metamorphic terrane of some 6,000 km(2) in West Antarctica indicates Cretaceous meteoric water of less, or much less, than -20 parts per thousand. This new anomaly and similar anomalies in New Zealand date from before rifting along the Antarctic-Pacific Rise and are tracking terranes originating from the Mesozoic south polar archipelago. In this area subaerial, or subglacial, hydrothermal isotope exchange has been particularly effective, and/or meteoric waters were isotopically unusually depleted. The discovery, mapping, and dating of further isotopically depleted zones in the geologic record will improve constraints for Paleozoic and Mesozoic greenhouse climates. Copyright (C) 1997 Elsevier Science Ltd.