East Asian Winter Monsoon and enhanced stratification in the western Pacific promoted giant marine diatom (Ethmodiscus rex) blooms during the last deglaciation

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Late Pleistocene thick laminated mats of Ethmodiscus rex (E. rex) have been found on the seafloor of the western tropical Pacific. Exploring the causes of E. rex mats formation can improve our understanding of the role of diatom mats in the global carbon-silicon cycle. Nutrients, such as silicate, are a major factor contributing to the growth of diatoms. However, the origin of the nutrients necessary for E. rex to bloom remains unclear. Here we analysed the 14C ages and silicon isotope compositions (δ30Si) of E. rex mats from deep-sea sediment cores collected in the western Pacific to reveal the nutrient source of the diatom bloom. The results indicate that the age of the cores ranges from 11 ka to 19 ka. This is the first discovery of a last deglacial thick layer of E. rex mats in the western Pacific Hadal zone. Furthermore, the silicon isotope results show that, similar to previous studies in the western Pacific, all δ30Si of E. rex frustules are negative. This is in contrast to the North Atlantic upwelling region where all δ30Si are positive. According to the binary mixing model calculations, eolian dust were responsible for supplying 80%–90% of the silica during the last deglaciation, while upwelling waters accounted for only about 10%–20% of the silica. During Heinrich Stadial 1 (HS1) period, the strengthened East Asian Winter Monsoon (EAWM) transported a lot of eolian dust to the western Pacific, providing nutrients for diatom blooms. In addition, the enhanced stratification in the western Pacific since the late HS1 period was a significant factor that contributes to the blooms of E. rex when the supply of eolian dust material decreased.