Journal: Journal of Micropalaeontology

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Copernicus

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0262-821X

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2024 - 20252
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Publications 1 - 2 of 2
  • Diatom and radiolarian biostratigraphy in the Pliocene sequence of ODP Site 697 (Jane Basin, Atlantic sector of the Southern Ocean)
    Item type: Journal Article
    Kato, Yuji; Hernández-Almeida, Iván; Pérez, Lara F. (2024)
    Journal of Micropalaeontology
    Bio- and magnetostratigraphic events are essential to construct age models of marine sedimentary sequences for which no other dating methods are available. In this study, we re-visit Ocean Drilling Program (ODP) Leg 113, Hole 697B (drilled in the Jane Basin, the Atlantic sector of the Southern Ocean; 61°48.626′S, 40°17.749′W), to refine diatom and radiolarian biostratigraphy for the early and middle Pliocene, a warm interval of Earth's history which is often considered a climatic analogue for the future. In total, 16 bioevents were identified in the diatom analysis and 3 in the radiolarian analysis. From these, 8 diatom events and one radiolarian event were identified for the first time in Hole 697B. We correlate the identified bioevents with existing paleomagnetic datums in Hole 697B to recalculate and update the ages of the bioevents. Although most of the calculated ages fall within the range of previously published ages, this study allowed us to narrow the age ranges of a number of bioevents. The updated biostratigraphy, as well as the assemblage data presented here, contributes to strengthening the Pliocene chronological framework at Hole 697B for future paleoceanographic work. In addition, we found an interval characterized by abundant reworked Miocene microfossils (e.g., Denticulopsis spp.) at ca. 4.5-3.7 Ma that may suggest sediment disturbance caused by regional tectonic and/or paleoceanographic events in the Southern Ocean during this time interval.
  • Quantification of dissolution and diagenetic overgrowth in early Eocene calcareous nannofossils through circular polarised light microscopy
    Item type: Journal Article
    Clark, Alexander Johannes; Gonzalez-Lanchas , Alba; Bachmakova , Kyra; et al. (2025)
    Journal of Micropalaeontology
    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.
Publications 1 - 2 of 2