Dissolution susceptibility of Paleocene–Eocene planktic foraminifera : Implications for palaeoceanographic reconstructions

We investigated shell characteristics and differential dissolution susceptibility of planktic foraminiferal species derived from upper Paleocene and lower Eocene deep-sea sequences, Ocean Drilling Program (ODP) Site 865 (Allison Guyot) and Sites 1209B, 1210B and 1212A (Shatsky Rise) in the North Pacific Ocean. The purposes of this study are: 1) assessing the effects of differential dissolution on upper Paleocene-lower Eocene planktic foraminiferal assemblages, at species level and within different biozones, to quantify dissolution susceptibility of genera and species; 2) investigating the differences in shell characteristics; 3) revealing the relationship between shell parameters and dissolution robustness of taxa, and 4) identifying the key shell parameter(s) influencing the dissolution susceptibility of foraminiferal taxa. Two independent experiments were carried out, one focusing on gradual qualitative deterioration of taxa by dissolution and the other documenting the weight loss of taxa. Shell parameters such as wall thickness, porosity and pore size were determined through Scanning Electron Microscopy (SEM) and image analysis (JMicroVision).We found that the large muricate Acarinina and Morozovella are most resistant, followed by the cancellate Subbotina and the small muricate Igorina, confirming results of previous work. At species level, the thick-walled Acarinina soldadoensis, Acarinina subsphaerica and the large Morozovella subbotinae are the most resistant species. Most of the large Morozovella species such as Morozovella aequa, Morozovella formosa-gracilis, Morozovella velascoensis and Morozovella pasionensis, together with Acarinina nitida show intermediate dissolution resistance, whereas the small muricate Igorina species, the cancellate Subbotina velascoensis and the thin-walled Morozovella acuta and Morozovella occlusa are the most vulnerable species.We propose a formula for calculating the dissolution resistance of taxa based on their wall thickness and size - two key parameters in dissolution resistance of a species. Application of this formula reveals good agreement between the calculated and measured dissolution resistance, indicating its robustness. Furthermore, the agreement between our experimental results, in-situ experimental results on live foraminifera and natural quantitative/qualitative records suggests that our experiments accurately mimic natural dissolution processes. Consequently, these experimental results strongly bear on interpretations of foraminiferal dissolution in natural environments, especially in studies on early Paleogene climatic events that are often associated with dissolution phenomena. More generally, a proper assessment of taphonomic alteration by dissolution should be part of every paleoenvironmental reconstruction based on quantitative foraminiferal records.