Miocene tropical Indian Ocean temperatures: evidence from three exceptionally preserved foraminiferal assemblages from Tanzania

• Published in: Journal of African Earth Sciences
• Main Authors: Stewart, Duncan R. M., Pearson, Paul Nicholas, Ditchfield, Peter W., Singano, Joyce M.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 2004
• Subjects: QE Geology; Pacific; Indian; Planktonic foraminifera
• Online Access: https://orca.cardiff.ac.uk/id/eprint/15240/; https://doi.org/10.1016/j.jafrearsci.2004.09.001

Early to Middle Miocene tropical sea surface temperatures (SSTs) in the equatorial Pacific and Indian oceans have been previously estimated using oxygen isotope palaeothermometry with the shells of surface-dwelling planktonic foraminifera. These estimations are generally lower than in the present day oceans. However, they could be flawed if the foraminiferal tests used for isotopic analysis have been diagenetically altered during shallow burial on the sea floor. This paper presents new isotopic data from three exceptionally well-preserved assemblages from the clay-rich sediments of coastal Tanzania, which constitute a more stringent test for the theory of cool Miocene tropical temperatures. Multiple planktonic and benthic foraminifera morphospecies are analysed for oxygen and carbon isotopes, representing various ecological habitats and isotopic fractionations. The isotopic data are used to determine Indian Ocean tropical sea surface and outer-shelf bottom water temperatures. Oxygen isotope data from the Early Miocene equivalent biozone M1b (22.35 Ma ± 0.85), imply tropical SSTs of approximately 30 °C, while isotope data from Middle Miocene equivalent biozones M9 (12.2 Ma ± 0.3) and M11 (11.55 Ma ± 0.15), suggest SSTs of 27–29 °C. For the assemblage from biozone M1b, isotopic data are used to calculate a shelf bottom water temperature of 21.3 °C, while for biozones M9 and M11–12, bottom water temperatures of 14–15 °C. These palaeotemperature estimates are similar to the modern sea surface and bottom water temperatures in the western Indian Ocean, yet are ∼10 °C higher than palaeotemperatures previously calculated for similar time intervals. These new data compliment recent studies using exceptionally-preserved Cretaceous and Palaeogene foraminiferal assemblages from clay-rich sediments and provide further evidence that micrometer-scale diagenesis can have a significant influence on isotopic ratios in foraminifera shells.