Intense deep convective mixing in the southeast Arabian Sea linked to strengthening of the northeast Indian monsoon during the middle Pliocene (3.4 Ma)

摘要

The climate of the Indian Ocean is dominated by monsoon reversals, influencing hydrography and biogeochemistry of the Indian Ocean as well as land vegetation through changes in precipitation. During summer or southwest monsoon season, intense upwelling zones driven by Ekman spiral appear in the western and eastern parts of the Arabian Sea that enhance surface primary production and thus proliferation of distinct fauna and flora. During the winter season, northeast monsoon winds cause deep convective overturning (mixing) that injects nutrients to the surface ocean and increases surface production. As a result, the primary production in the Arabian Sea has bimodal annual distribution. The present study analyses 5.6 Ma record of surface-dwelling planktic foraminifera, Globigerina bulloides, Globigerinoides ruber and Globigerinoides sacculifer from Deep Sea Drilling Project Site 219, southeast Arabian Sea to understand changes in the surface ocean as driven by the Indian monsoon coinciding with the northern hemisphere glaciation (NHG). An increase in mixed-layer species at similar to 3.4 Ma suggests intense deep convective overturning caused by strong NE monsoon winds related to strengthening of NHG. Globigerina bulloides shows a high positive relation with G. ruber during the past 3.4 Ma and a weak relation in the early Pliocene (5.6-3.4 Ma). The high G. bulloides percentages during the past 3.4 Ma could be linked to the injection of nutrients in the top layer by the advecting sub-surface nutrient-rich water.