Planktonic Foraminiferal Biostratigraphy and Paleoceanography of Late Quaternary Turbidite Sequences at Holes 856A, 857A, and 857C, Leg 139

Abstract

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 include the lower part of the present and penultimate interglacial periods and perhaps part of the glacial termination. Coiling direction zones 2 and 4 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.

Optical thermal maturity parameters and organic geochemical alteration at low grade diagenesis to anchimetamorphism: A review

“…As the wall of the proloculus is thicker and therefore darker in colour than that of succeeding chambers (Stancliffe, 1996) Apart from thermally induced colour changes, both agglutinated and calcareous foraminifera tests respond to increasing burial diagenesis by progressive mineralogical changes (e.g. Brunner, 1994). However, not only burial temperature and burial history can influence diagenetic overprinting of foraminiferal tests but also various other factors such as water depth and temperature, sediment composition or pore water chemistry (e.g.…”

Temperature dependence of the strontium distribution coefficient in calcite; an experimental study from 408 degrees to 2008 degrees C and application to natural diagenetic calcites

“…Age constraints are limited at Site 857, principally because of the high geothermal gradient and the resulting rapid alteration of biogenic components. On the basis of coiling direction of Neogloboquadrina pachyderma, Brunner (1994) placed the base of marine isotope stage 5 (ϳ 125,000 yr) at 40 mbsf. No biostratigraphic age determinations are possible below 40 mbsf, and the total sediment thickness is unknown.…”

Middle Valley: Dead Dog Area (Site 1036)

“…Hole 1036B contains a 5-cm-thick surface layer dominated by residue left after dissolution of anhydrite (see "Lithostratigraphy" section, this chapter) derived from broken hydrothermal chimneys along with abundant radiolarians and diatoms, few agglutinated benthic foraminifers, and very rare planktonic foraminifers. Hole 1036C contains a few centimeters of iron-stained anhydrite with siliceous microfossils and agglutinated foraminifers in a reddish brown oxidized sediment (also reported by Brunner, 1994, andQuinterno, 1994). Beneath this unit, an assemblage of carbonate benthic and planktonic foraminifers is found in olive-gray distal turbidites interlayered with hemipelagic units.…”

“…On the basis of earlier work Brunner, 1994), the Holocene portion of this sequence is known to have accumulated at rates of ~6 cm/k.y. This is substantially slower than the sedimentation rates of ~20 cm/k.y.…”