Isotopic evidence for palaeotemperatures and depth stratification of Middle Cretaceous planktonic foraminifera from the Pacific Ocean

Stable isotopic measurements have been made on both planktonic foraminifera and coccolithic matrix of Middle Cretaceous (Late Albian–Cenomanian) age from two Pacific low latitude sites. The degree of alteration of the foraminifera has been assessed through the application of chemical analyses, cathodoluminescence and Scanning Electron Microscopy (SEM). The rotaliporid foraminifera display an interspecies range of δ18O values from –2.29 to –3.01 ‰ at Deep Sea Drilling Project (DSDP) Site 463 and from –2.74 to –3.55 ‰ at DSDP Site 305. Hedbergellid foraminifera exhibit a δ18O interspecies variation of –2.52 to –3.02 ‰ at Site 305. Isotopic analysis of individual Hedbergella delrioensis and Rotalipora appenninica foraminifera from single samples shows H. delrioensis to have a surprisingly large spread of δ18O values (–2.492 to –3.097 ‰ from Site 463, –2.454 to –3.344 ‰ from Site 305), whilst δ13C values remain confined to a narrower range. Such a spread of oxygen values may be related to a number of factors, including subtle diagenetic alteration, a wide range of temperature-related depth habitats or growth related changes of primary skeletal calcite. The hedbergellids have consistently lighter oxygen and heavier carbon isotopic values than do the rotaliporid foraminifera and hence provide isotopically derived palaeotemperatures consistent with a thermally stratified ocean. At both sites the oxygen isotopic data are consistent with a gradual warming through Albian–Cenomanian time. However, the results suggest that Middle Cretaceous equatorial oceans were possibly only as warm as those of the present day (or slightly warmer), but did not reach the high temperatures claimed in older literature. * Author for correspondence: [email protected] been the susceptibility of the carbonate shell material to solution and the identification of subtle diagenetic alteration resulting in a masking of the primary palaeoecological and palaeoceanographic signal (see Price, Sellwood & Pirrie, 1996; Huber & Hodell, 1996). Scanning Electron Microscopy (SEM), cathodoluminescence and trace element analysis of the foraminifera, coupled with the stable isotopic analyses have been employed to help eliminate any trends originating from diagenetic overprinting. 2. Geological setting and sampling Samples of Late Albian to Early Cenomanian age have been obtained from DSDP Site 305 (Shatsky Rise, 32.00.13°N; 157.51.00°E). Additionally, Middle–Late Cenomanian samples were obtained from DSDP Site 463 which is located southwest of the Hawaiian Ridge on the Western Mid-Pacific Mountains (21.21.01°N; 174.40.07 °E) (Fig. 1). During Middle Cretaceous times, Sites 305 and 463 were located close to the palaeoequator at ~5°S and 17°S, respectively. They thus represent two sites of great importance in any reconstruction of ocean temperature for the Middle Cretaceous. The lithology of the sampled section at Site 305 consists of a laminated chalk brecciated by drilling. In parts, the only sediment preserved is chalk forming a crust on chert fragments. The lithology of Site 463 consists of uniform white soft chalk/ooze, partly disturbed by drilling. In the lower parts of the section the sediment is slightly more lithified (see Theide et al. 1981). Black and dark grey chert fragments are recorded throughout (Fig. 2). Mineralogically the sediments, from both sites, are composed largely of calcite, with traces of quartz and clay. At both sites planktonic foraminifera are the dominant fossil component. Rotaliporid foraminifera are most abundant in the 211–425 μm size fraction in all samples. Hedbergellids, together with a few benthic foraminifera were also identified. Boersma (1981) has recorded a diverse planktonic fauna from Site 463 including Rotalipora appenninica, R. gandolfi and R. greenhornensis from the sampled section. From this suite of fossils, rotaliporid and hedbergellid planktonic foraminifera, together with individual H. delrioensis and R. appenninica from the 211–425 μm size fraction and matrix samples, were isotopically analysed from each of the sites. 3. Analytical procedures The generally unconsolidated nature of the sediments allowed relatively easy sample disaggregation using an ultrasonic bath. Two-milligram foraminifera and matrix samples were isotopically analysed using standard methods on a VG Sira series II mass spectrometer at the University of Reading. Further isotopic analyses of single specimens of planktonic foraminifera were carried out on a VG PRISM Series II mass spectrometer, with on-line common acid bath system, at the University of Oxford. The δ18O and δ13C compositions are reported in per mil (‰) notation with respect to the PDB international standard. Reproducibility for both δ18O and δ13C was generally better than ± 0.1 ‰, based upon multiple sample analysis. Elemental concentrations (Mn, Sr, Mg and Fe) were determined by inductively coupled plasma (ICP) spectrometry analysis on 2–10 mg subsamples. Reproducibility, based upon replicate analysis, was estimated generally to be less than ± 17 % of the measured concentration for Sr, Mg and Fe and ± 10 % for Mn.