THE OXYGEN ISOTOPIC RECORD OF EARTH HISTORY

Oxygen isotope measurements (¹⁸O/¹⁶O) of carbonate and phosphate fossils and microfossils have enabled reconstruction of paleotemperatures of Phanerozoic oceans and yielded a highly refined Neogene marine stratigraphy. This poster presents an overview of the isotopic record of ocean sediments through 3.8 Ga of Earth history based on more than 50,000 analyses of Precambrian rocks and Phanerozoic fossils and microfossils. The record relies and builds upon compilations of others (see poster for references). The Precambrian δ¹⁸O record is based on cherts and limestones and shows decreasing values with age with no distinct, globally correlative events. Phanerozoic fossils and microfossils (brachiopods, belemnites, foraminifera, conodonts) yield isotopic records with moderate to great utility in stratigraphy and paleoclimate research. Data for the Cambrian and much of the Ordovician are low and highly variable, with questionable fidelity. Silurian and Devonian data average lower than those of the remaining Phanerozoic probably because of the absence of continental glaciers and higher temperatures, though slightly lower seawater δ¹⁸O (≤2‰ lower) cannot be ruled out. Potential stratigraphic markers in the Paleozoic record include the latest Ordovician acme, late Silurian acme, mid-late Devonian decline, and early Carboniferous increase. The Jurassic and Early Cretaceous record shows (1) a Toarcian (Jurassic) decline and a Callovian-Oxfordian acme, (2) an increase in the early Cretaceous to a maximum near the Valanginian-Hauterivian boundary, and (3) a decline to a mid Barremian minimum. Foraminiferal data from DSDP and ODP cores show a large δ¹⁸O decrease from Albian to Turonian, followed by an increase to a late Maastrichtian maximum. Highlights of the Cenozoic record based on benthic foraminifera include the Paleocene-Eocene Thermal Maximum (PETM) and Oligocene Oi1 event. While the benthic Cenozoic record shows systematic δ¹⁸O increase reflecting cooling and glaciation, the planktonic record differs depending on whether analyses were performed on “glassy” or less well-preserved foraminifera. Continued development of O-isotopic techniques, diagenetic indicators, and paleotemperature proxies will undoubtedly lead to discovery of new global climate events and applications of ¹⁸O stratigraphy.