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.