Low- 18O terranes tracking Mesozoic polar climates in the South Pacific
Abstract
Substantially negative δ 18O values of altered rocks are an unfailing guide to 18O-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%c δ 18O, with a lower limit of -16%, 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%c. 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.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- February 1997
- DOI:
- 10.1016/S0016-7037(96)00350-X
- Bibcode:
- 1997GeCoA..61..569B