Quantification of dissolution and diagenetic overgrowth in early Eocene calcareous nannofossils through circular polarised light microscopy

Abstract. Qualitative grading scales are often used to assess the preservation of sediments and biogenic carbonates such as calcareous nannofossils and foraminifera. However, such qualitative indices often conflate the process of dissolution and secondary diagenetic overgrowth, which have different consequences for the analysis of geochemical signals from the primary biogenic calcification environment. Therefore, direct indicators of carbonate microfossil preservation relevant to the geochemical signal are needed. In this study, we assess the morphology of discoasters and placolith-shaped calcareous nannofossil groups from early Eocene (54–50 Ma) sediments using circular polarised light techniques to quantify their thickness and shape factor (thickness for a given size) and assess if these parameters covary with the depositional environment, including carbonate content, palaeowater, and burial depth. We find that the median shape factor of the Discoaster multiradiatus group varies considerably among different sites, with a thicker D. multiradiatus group and a greater proportion of birefringent specimens with increasing sediment carbonate content. We interpret these trends as indicative of increasing secondary overgrowth on the discoasters in sediments of higher carbonate content. In contrast, placolith shape factors are not correlated to the carbonate content, but rather placolith thickness decreases with greater palaeowater depths across the selected settings. We infer that the thickness of Eocene placoliths is driven more by dissolution than overgrowth. We find that across the globally distributed sediment locations, the nannolith to placolith ratio is not a good indicator of dissolution intensity, and we suggest that the ratio of non-birefringent to birefringent D. multiradiatus groups may serve as a more effective proxy for assessing placolith overgrowth.