Planktonic foraminifera in late Quaternary turbidite sequences at ODP Leg 139 holes

Late Quaternary planktonic foraminiferal biostratigraphy and paleoceanography were examined at two sites from the east margin of Middle Valley of the northern Juan de Fuca Ridge. Calcareous microfossils are abundant and well preserved in the upper sequences where thermal and hydrothermal alteration is minimal and where deposition occurred after sites were tectonically raised above the turbidite plain of the axial valley. Approximately 200 samples were examined to determine depositional style and foraminiferal content including a census of planktonic foraminifers. Approximately 60% of the samples contain turbiditic material, so paleontologic interpretations must be made with caution. Ocean Drilling Program (ODP) Holes 856Aand 857 A contain abundant foraminifers above 52 and 85 mbsf, respectively. The fossiliferous sequences are divided into four informal coiling direction zones based on the species Neogloboquadrina pachyderma (Ehrenberg) and numbered from 1 to 4 from top to bottom of the sequences. Coiling direction zones 1 and 3 contain 90% sinistral forms and span most of the last two glacial periods. The bases of coiling direction zones 1 and 3 define two datum levels tentatively dated at 11,000 and 125,000 yr ago, respectively. Cluster analysis identified three assemblages: a subarctic assemblage, a transitional assemblage, and a subarctic dissolution assemblage. The subarctic dissolution assemblage occurs infrequently and sporadically throughout the unaltered glacial intervals in Holes 856A and 857A. In contrast, the dissolution assemblage dominates Hole 856A below 22 mbsf and is related to calcite precipitation driven by hydrothermal processes. The transitional assemblage appears in coiling direction zone 3 and records the close approach of the subpolar boundary to the ODP Leg 139 locale during the penultimate interglacial period. The subpolar boundary remained south of the study site at approximately 48°30'N latitude throughout the late Quaternary