Oxygen isotope studies on Jurassic and Cretaceous belemnites from New Zealand and their biogeographic significance

Abstract

Abstract δ (O18) and S(C13) results and isotopic temperatures are presented for transverse sections of New Zealand belemnites (Belemnopsis, Hibolithes, Dimitobelus) obtained from Jurassic strata correlated with Bajocian-Bathovian, Callovian, Kimmeridgian and Lower Tithonian, and Cretaceous strata correlated with Albian-Maastrichtian. Growth layer analyses also were carried out on 15 Jurassic and Cretaceous belemnites. Many of the belemnite specimens were found to be chemically altered to varying degrees, but growth analyses showed that the high isotopic temperatures which alteration is expected to produce were confined to two areas : surrounding the apical line and adjacent to the periphery of the specimen. Between these two areas there are generally plausible isotopic temperatures, and there is no reason to suspect that the temperature record has not been preserved there in its original form. Three or four major growth stages may be recognised in New Zealand belemnite guards and growth layer analyses suggest that these are annual; if this is so then in this respect the belemnites match the modern squids in having a three- or four-year life cycle. Variations in isotopic temperature of the belmnites are comparable with those experienced seasonally by modern squids. Isotopic temperatures for the Jurassic and Cretaceous of New Zealand can be compared with sea water surface temperatures around the present day New Zealand coasts (36°-46° S. Lat.), and inferred temperatures, based on these comparisons, are as follows: Upper Jurassic — temperatures slightly cooler than those of the present day; Albian and Cenomanian — temperatures slightly warmer; Turonian-PConiacian- temperatures markedly warmer; Santonian-Maastrichtian — temperatures slightly warmer in the Santonian, but more so in the Campanian and Upper Maastrichtian. The New Zealand oxygen isotope results indicate a probable midlatitude position, in the warm-temperate climatic zone, for New Zealand in the Jurassic and Cretaceous. New Zealand oxygen isotope results are compared with those obtained from overseas.