Large terrestrial and marine carbon and hydrogen isotope excursions in a new Paleocene/Eocene boundary section from Tanzania

Abstract

Recent onshore drilling expeditions in Tanzania have yielded sediments spanning much of the Late Cretaceous and Paleogene which show exceptionally good preservation of both calcareous microfossils and organic matter. The interval of the Paleocene–Eocene Thermal Maximum (PETM) was recovered at Tanzania Drilling Project (TDP) Site 14, in 2004. The PETM interval was identified using planktonic foraminifer and nannofossil biostratigraphy. Carbon isotopic measurements of planktonic foraminifera from the genus Subbotina show a maximum measured negative shift in δ 13C values of approximately 4.5‰, which is significantly larger than has been observed for most deep sea foraminifer records. Higher plant derived n-alkanes yield a ca. 6.5‰ negative shift through the same interval. As carbon isotope fractionation in higher plants varies amongst taxa and is influenced by humidity, it is possible that this remarkably large shift in the n-alkane δ 13C values records environmental and vegetation changes as well as changes in the δ 13C value of the ocean–atmosphere reservoir. However, dramatic increases in humidity appear to be unlikely as higher plant biomarker δD values shift to higher rather than lower values. Taken together, the records suggest that the magnitude of the carbon isotope excursion (CIE) in the ocean–atmosphere reservoir may have been larger than has generally been accepted. This has important implications for the source and quantity of 13C-depleted carbon released at the PETM.