Through most of the Paleogene the planktonic foraminifera called biserial heterohelicids record the most depleted carbon isotope ratios among planktonic foraminiferal species. They are interpreted to occupy an oxygen minimum habitat. Stronger interplanktonic carbon isotope contrasts and more enriched near surface carbon isotope values correspond to higher abundances of biserial heterohelicids. This group appears to have preferred latitudes and times when the oxygen minimum was well developed. Biserial heterohelicid generic and specific diversity are directly related to the interplanktonic carbon isotope contrast. Minimal specific diversity characterizes the Oligocene when interplanktonic and surface to bottom carbon isotope contrasts were consistently low. Diversity and abundance decreases were coeval with increased oceanic ventilation.
展开▼
