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doi:10.2204/iodp.proc.331.107.2011

Biostratigraphy

Core catcher samples from Site C0017 yielded two zones with microfossils, a shallow one from ~9 to 28 mbsf and a deeper one from ~112 to 145 mbsf (Table T4). The upper layer consists of pumice and microfossils, which are most abundant in the deeper half of this interval (at ~18.5 and 27.2 mbsf). These two samples each possess a large fraction of <63 µm material. Foraminiferal diversity and abundance are generally consistent with surface samples from a similar pumice-foraminifer layer at Site C0014, suggesting it is a modern assemblage. Examination of wet mounts of the <63 µm material using phase contrast microscopy ± cross-polarized light revealed good preservation of coccoliths. The assemblage was dominated by Emiliania huxleyi, Gephyrocapsa oceanica, and Reticulofenestra asanoi, highlighting a modern origin (Raffi et al., 2006; Winter and Siesser, 1994). Transport of coccoliths into the upper microfossil zone was facilitated, in part, by fecal pellets (Fig. F5). Fecal pellets and abundant centric diatoms are notable in the sample from 10.6 mbsf.

Drilling continued through a pumice-rich layer from which core recovery was poor (Table T4) before entering a second interval characterized by microfossils (~112–138 mbsf). This deeper zone of microfossils is also characterized by a large fraction of <63 µm material. Coccoliths at these depths (E. huxleyi and R. asanoi) are poorly preserved and much less abundant (<10%) than they are in the upper layer (8–28 mbsf). Foraminiferal abundances in this layer are also lower than in the upper, modern zone; however, there is a larger proportion (as high as 11%) of benthic foramininfers in the 125 mbsf region. The large proportion of benthic foraminifers suggests the site was more favorable to benthic species in the past than it is today (i.e., hydrothermal activity may have increased at the site).

As at Site C0014, foraminifers observed in both of these layers (~8–28 and 112–138 mbsf) are dominated by planktonic species (Table T5), suggesting a large input of planktonic foraminifers and/or that Site C0017 is a poor habitat for benthic foraminifers (Saidova, 2007), perhaps because it is still too close to the hydrothermal system. Though some fragmentation has occurred, foraminiferal preservation is generally good, with little or no evidence for dissolution and/or overgrowth; diagnostic characteristics are preserved, and many species can be identified. As at Site C0014, the abundance of Neogloboquadrina pachyderma dextral relative to the sinistrally coiled subspecies demonstrates the influence of warmer waters, presumably from the Kuroshio Current (Bandy, 1960; Ericson, 1959). Some foraminifers from 112 mbsf are pyritized (not shown), consistent with previous reports describing infilling of foraminiferal tests (Kohn et al., 1998). It should be noted that these foraminifers are poorly preserved compared with those at Site C0014 (see Figs. F11 and F12 in Expedition 331 Scientists, 2011d).

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