| dc.description.abstract | This study attempts to establish the spatial distribution of benthic foraminifera and to discuss the physical and chemical properties of host marine sediments in the upper accretionary wedge off SW Taiwan. Three box cores ranging from 32 cm to 49 cm thick are retrieved onboard R/V Ocean Researcher I during 1092 cruise in 2014 at water depths ranging from 1,135 to 1,586 m lying in between the Good Weather Ridge and the Yuan-An Ridge. Sediments of uppermost 5 cm for each core are immersed in Rose Bengal for staining living benthic foraminifera, spatial species distribution with respect to water depths and temporal down-core variations over the past ~200 years are discussed. Additionally, visual core description, X-radiographs, grain size analyses and 210Pb dating are also carried out in this study.
Analyses on grain size reveal that the sediment size ranges from clay to silt for all sites, fine to well-sorted with the exception of YT1 site where a small percentage of fine sands (< 20%) is found to distribute evenly in a 32 cm-thick box core. X-radiographs of the YT1 core show rare bioturbation with large white spots, probably caused by aggregates of foraminiferal tests. Trace fossil of Chondrites, is found at YT3 core, indicating anoxia condition at YT3 site.
Age of sediment is obtained by using 210Pb dating method. The 210Pb concentration profile decays exponentially down core, indicating sedimentation from suspension. The measured sedimentation rate ranges from 0.013 to 0.26 g/cm2yr. Site YT1 has the lowest sedimentation rate (around 0.013 g/cm2yr), leading to high abundance of individual benthic foraminiferal species. YT1 site locates at a western slope (~17 degree in gradient) of the Good Weather Ridge, whereas the rest two sites (YT2 and YT3) situated in depositional trough. We suggest that the higher slope gradient at YT1 has resulted in lowered sedimentation rate comparing to that for sites at bathymetric trough.
Living foraminiferal individuals were distinguished from dead assemblages by Rose Bengal staining method during the cruise. Our results show that the dominant living species of all studied cores is Chilostomella oolina, with subsidiary occurrences of Bulimina aculeata, Uvigerina auberiana, and Reophax spp. This foraminiferal occurrence indicates suboxic/dysoxic conditions of shallow sub-seafloor and stable hemipelagic sedimentation. Abundance of living foraminiferal assemblages from three cores is of similar, whereas core YT1 exhibits much higher abundance of dead assemblage, due probably to low sedimentation rate.
Cores of YT2 and YT3 are selected for studying down-core variations of foraminiferal assemblages for every 5 cm interval. Results show a remarkable up-core decrease in the abundance of Bulimina and Uvigerina species, indicating suboxic environment, and increasing abundance of Chilostomella, a characteristic species of dysoxic environment. This indicates that the oxygen concentration of the water masses may have decreased since c. 50 year ago.
This study documents living benthic foraminifera distribution in water depths down to c. 1,600 m off SW Taiwan, providing a basis for future benthic foraminiferal studies in the deep-sea around Taiwan. | en_US |