Program

The contemporary southern Mariana Trench is characterized by oligotrophic surface waters, resulting in low primary productivity and well-oxygenated bottom waters. This study seeks to identify whether the redox conditions of bottom water in the southern Mariana Trench have varied during the last glacial maximum (LGM) by examining the sedimentary records and to shed light upon potential causes for the change. We have measured major, trace, and rare earth elements (REE) in three gravity cores (GC03, GC04, and GC05) and one box core (BC11) retrieved from the southern Challenger Deep with water depth from 5289 to 7118 m. The upper sediment layers of both GC05 and BC11 which were taken in proximity to each other from the deepest site are dominated by valve fragments of the giant diatom-Ethmodiscus res, forming laminated diatom mat (LDM). ¹⁴C-AMS dates of bulk organic matter showed that the LDM was generated between 18.4 and 21.8 kyr B.P., corresponding to the LGM. Good correlations between P/Al and REE/Al for all sediment cores possibly indicated that biogenic and/or authigenic apatite are the main carriers of REE. The weak enrichment of middle REE was associated with the cycling of Fe-Mn oxyhydroxides after deposition and was later captured by apatite. Negative Ce anomalies, along with slight to no enrichment of redox sensitive elements (e.g., U, Mo, Cd, As) for non-LDM samples clearly suggest they have deposited through an oxic bottom water condition. In contrast, the point to a suboxic condition when the LDM formed during the LGM. We postulate that the change of bottom water redox condition during the LGM is likely to result from the increased primary productivity induced by enhanced Asian dust input. In fact, the Ethmodiscus res blooming during the LGM in the eastern Philippine Sea was also associated with the increased primary productivity, and it played a significant role in capturing atmospheric CO₂ (Xiong et al., 2012a; Xiong et al., 2013). Consequently, the capacity of atmospheric CO₂ sequestration by giant diatom bloom during the LGM in the tropical western Pacific may be greater than previously envisaged.