| Summary: | The cause of massive blooms of Ethmodiscus rex laminated diatom mats (LDMs) in the eastern Philippine Sea (EPS) during the Last Glacial Maximum (LGM) remains uncertain. In order to better understand the mechanism of formation of E . rex LDMs from the perspective of dissolved silicon (DSi) utilization, we determined the silicon isotopic composition of single E . rex diatom frustules (d 30 Si E . rex ) from two sediment cores in the Parece Vela Basin of the EPS. In the study cores, d 30 Si E . rex varies from -1.23‰ to -0.83‰ (average -1.04‰), a range that is atypical of marine diatom d 30 Si and that corresponds to the lower limit of reported diatom d 30 Si values of any age. A binary mixing model (upwelled silicon versus eolian silicon) accounting for silicon isotopic fractionation during DSi uptake by diatoms was constructed. The binary mixing model demonstrates that E . rex dominantly utilized DSi from eolian sources (i.e., Asian dust) with only minor contributions from upwelled seawater sources (i.e., advected from Subantarctic Mode Water, Antarctic Intermediate Water, or North Pacific Intermediate Water). E . rex utilized only ~24% of available DSi, indicating that surface waters of the EPS were eutrophic with respect to silicon during the LGM. Our results suggest that giant diatoms did not always use a buoyancy strategy to obtain nutrients from the deep nutrient pool, thus revising previously proposed models for the formation of E . rex LDMs.
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