South eastern Indian Ocean (ODP Site 752) paleoceanographic conditions during the Middle to Late Miocene based on calcareous nannofossils assemblages

The Middle to Late Miocene represents an important time interval on a global scale. Due to continental reorganisation in the Indian Ocean (IO) a near-modern monsoonal wind system was established, a series of climatic changes linked to the Middle Miocene Climatic Transition occurred and the subsequent shift of the region dominated by the Westerlies to the north took place during the Late Miocene. However, how these processes and forcing mechanisms interact and affect the surface ocean dynamics in the southern subtropical IO is poorly understood. In this regard, Ocean Drilling Program (ODP) Site 752, located on the west flank of Broken Ridge (30° 53.475ˈS/93° 34.652ˈE), represents a key location to study how the above-mentioned processes may have affected the surface currents in the eastern sector of the subtropical IO during the Middle to Late Miocene.

In order to ascertain changes in the surface ocean conditions and their connection to surface currents dynamics between 7.31 and 16.06 Ma, we evaluated the overall changes in the calcareous nannofossil assemblage at Site 752. For this purpose, we performed a quantitative study on 122 calcareous nannofossil samples (with a temporal resolution of ~60 kyr) and performed statistical analyses to observe variations in the assemblage composition. The UPGMA (Bray-Curtis) clustering ordination analyses revealed a total of 5 clusters (Cluster 1-5), defined at a cut-off distance of ~0.76 and a cophenetic correlation coefficient of 0.75. Additionally, Cluster 5 was divided into two sub-clusters (Cluster 5a-5b), at a cut-off distance of ~0.77.

Cluster 1 indicates high nutrient availability due to the abundance of Reticulofenestra minuta. Cluster 2 also indicates high nutrient supply in the region, as per the high abundance of R. minuta, differing from Cluster 1 by the presence of Calcidiscus leptoporus and Coccolithus pelagicus, both indicative of cold surface waters with a deep mixed layer. Cluster 3 is characterised by the concomitant presence of species typical of warm/cold stratified/mixed regions, such as Discoaster spp., C. pelagicus and R. pseudoumbilicus. Cluster 4 is dominated by Reticulofenestra haqii and Reticulofenestra producta, which thrive in warm and oligo/mesotrophic conditions. Reticulofenestra producta is also abundant in Cluster 5a. However, the high abundances of Reticulofenestra perplexa and R. pseudoumbilicus indicate more moderate nutrient conditions with relatively cold surface waters compared to Clusters 1 to 4. Lastly, Cluster 5b is characterised by an increase in the abundance of R. pseudoumbilicus and the common presence of R. haqii, indicating warmer surface water conditions compared with Cluster 5a.

Temporal progression from Cluster 4 to Cluster 2 over our study interval reflects a distinct shift to higher nutrient palaeoceanographic conditions at the Broken Ridge between 15–8 Ma. This change in surface water nutrient availability can be linked to an invigoration of the Antarctic Circumpolar Current and the Southern Indian Ocean Current at ODP Site 752. These shifts indicate the impact of the Middle to Late Miocene northward migration of the Westerlies (from 40 to 30º S) on mid-latitude paleoceanographic conditions and confirm the proposed shift in global nutrient cycling around 11 Ma ago.