Diatom-based reconstruction of the Subantarctic Front migrations during the late Miocene and Pliocene

The Antarctic Circumpolar Current (ACC) is an influential component of the global climate system, and therefore it is essential to clarify the history of the oceanic fronts associated with it. However, there have been only a few attempts to reconstruct their history during the Neogene, as most previous studies focused on Quaternary ages. This study uses the late Miocene to Pliocene (ca. 9-3 Ma) fossil records of diatoms from the Atlantic sector of the Southern Ocean (Deep Sea Drilling Project Site 513 and Ocean Drilling Program Site 689) to reconstruct the paleo-location of the Subantarctic Front (SAF), which represents the northern limit of the ACC. The abundance of subtropical diatoms at Site 513 shows significant fluctuations during ca. 9-5 Ma, which strongly suggest that there have been repetitive north-south migrations of the SAF across Site 513 during at least ca. 9-5 Ma. In order to reconstruct the SAF migration history in more detail, changes in abundance of a typical subtropical diatom (i.e., Thalassionema nitzschioides var. parva Heiden emend. Moreno-Ruiz) recorded at Sites 513 and 689 are compared to abundance patterns of the same taxon at several drilling sites (Ocean Drilling Program Sites 697, 699, and 704) published in previous data reports. Results suggest several phases of SAF migration involving the following shifts in SAF position (direction of shift is indicated in brackets): ca. 8.5 Ma (northward), 8.0 Ma (northward), 7.0 Ma (southward), 6.1 Ma (northward), 5.4 Ma (the direction differs region by region), 4.9 Ma (northward), 4.6 Ma (southward), 3.8 Ma (northward). The results also show that SAF has experienced north-south migrations with a latitudinal amplitude of ca. 5 degrees or more. Comparison between changes in fossil diatom assemblages at Site 689 and the inferred SAF shifts suggest that the northward/southward shift of the SAF could be induced by the cooling/warming in the high-latitude regions.