Stable carbon and oxygen isotope ratios of bulk sediment, planktonic and benthic foraminifera of the Cenomanian/Turonian section of DSDP Hole 80-551 in the Northeast Atlantic (Table 1)

The CTBI carbon isotope excursion is prominent (up to 2‰ shift in delta13C) in the bulk (coccolith) signal, but less pronounced (approximately 0.5‰ shift in delta13C) in planktonic and benthic foraminifera.

This difference indicates a very steep 13C gradient in the upper water column and a very efficient biological pump during the CTBI carbon isotope excursion. We suggest significantly increased seasonal primary production in the uppermost water column with an enhanced shallow water chlorophyll maximum as a cause for this steep carbon isotope gradient. Deep-water and surface-water temperature changes during the CTBI are estimated using benthic and planktonic foraminiferal oxygen isotopes.