Differential dissolution of Upper Cretaceous planktonic foraminifera from a temperate region of the South Atlantic Ocean

Abstract

Differential dissolution susceptibility is well known in Quaternary planktonic foraminifera, but few quantitative dissolution studies have been carried out on pre-Quaternary species. A dissolution ranking scheme is proposed for 19 of the most common uppermost Cretaceous planktonic foraminifera from a temperate region of the South Atlantic Ocean. The study is based on quantitative faunal variations in an uppermost Maastrichtian segment (about 190 kyr long) from DSDP Site 527 (Leg 74) from the Walvis Ridge. A calcite dissolution index was formulated from degree of fragmentation of planktonic foraminiferal tests. Through this segment, relative abundance of benthonic foraminifera, which are generally known to be more dissolution-resistant than planktonic foraminifera, covary with degree of fragmentation, supporting that degree of fragmentation is a measure of dissolution. A prominent change from a state of moderate dissolution to a state of stronger dissolution takes place near the top of Magnetic Anomaly 30 (66.74 Ma). Some species decrease, whereas others increase, in relative abundance between these dissolution regimes, and dissolution susceptibility was ranked from quantified differences between dissolution regimes. The most susceptible species are in order, from least resistant to most resistant: Heterohelix globulosa, Globigerinelloides multispina, Rugoglobigerina rugosa, Pseudoguembelina costulata, Globotruncanita stuartiformis, and Heterohelix carinata. The following eight species are unaffected by dissolution in the sense that they show no significant difference between regimes: Heterohelix glabrans, Pseudotextularia browni, P. elegans, Racemiguembelina fructicosa, Planoglobulina brazoensis, Gublerina cuvillieri, Globotruncana arca, and Rosita contusa. The most resistant species were ranked in the following order, from least resistant to most resistant: Globigerinelloides subcarinata, Pseudoguembelina palpebra, P. polypleura, Globotruncanella havanensis, and Abathomphalus mayaroensis.