METHODS

Detailed planktonic foraminiferal analysis was performed on one selected hole for each site (Holes 1209A, 1210A, 1211B, and 1212B). A continuous U-channel taken from across the stratigraphic interval and yielding the clay-rich layer was cut into 1-cm3 samples. The high-resolution centimeter-scale sampling was extended a few centimeters below and above the clay-rich layer. A total of 185 samples were analyzed for planktonic foraminifers. Samples of ~1 cm3 were soaked in water and washed through a 38-Ám sieve. The number of specimens of each species was counted in each sample in the >38-Ám size fraction. Where residues were small, counts of all specimens were performed; in very small residues, <100 specimens could be counted (see Tables T2, T3, T4, T5). Large residues were split and an aliquot was spread over a gridded picking tray so that ~300 specimens were counted on randomly chosen grid squares. Quantitative analysis was plotted as percentages.

Planktonic Foraminiferal Assemblages

The planktonic foraminiferal assemblages in the studied stratigraphic interval are assigned to the G. pseudomenardii (Pl. P1) total range Zone P4 based on the occurrence of the nominate taxon and of Acarinina subsphaerica. In addition, as the FO of the nannofossil H. kleinpellii (marker for the base of Zone CP5) lies (T.J. Bralower, pers. comm., 2002) between the FO of G. pseudomenardii and the last occurrence (LO) of Morozovella conicotruncata, the assemblages are attributed to Subzones P4a–P4b of Berggren et al. (1995).

The assemblages throughout the studied sections are characterized by low diversity (Fig. F2), and they are composed mainly of three species belonging to the genus Igorina (Igorina tadjikistanensis, Igorina pusilla, and Igorina albeari) and two morphotypes (I. pusilla "high trochospire" and I. albeari "chubby") (see the "Appendix"). Morozovellids are less abundant than igorinids in all samples (average = 20%–30%; Fig. F3). The acarininids are rare (Fig. F3) and mainly represented by Acarinina subsphaerica and very rare Acarinina nitida and Acarinina mckannai. Subbotinids compose <30% of the assemblage in all samples (Fig. F4). The smaller size fraction (<150 Ám) contains rare globanomalinids and chiloguembelinids (Fig. F4).

High-resolution quantitative analysis of the planktonic foraminiferal assemblages indicates that fundamental changes in faunal composition occurred before, during, and after deposition of the clay-rich ooze. Changes in diversity, in state of preservation, and in the relative abundance of species can be observed in all the holes and are discussed below in stratigraphic order (from older to younger). Three intervals were identified: interval 1, interval 2, and interval 3, below, within, and above the clay-rich layer, respectively.

Interval 1—below the Clay-Rich Layer

Species richness ranges from 5 species in Hole 1210A to 19 species in Hole 1212B (Fig. F2). In Hole 1211B (the deeper site), an anomalously positive peak in species richness occurs 4 cm below the clay-rich layer (Samples 198-1211B-14H-4, 40–41 cm, to 14H-4, 42–43 cm; 125.00–125.02 mbsf) (Fig. F2). From 125.00 to 125.02 mbsf, igorinids are absent and the assemblage is composed of common M. conicotruncata, Morozovella angulata, Morozovella acutispira, and few G. pseudomenardii, which indicate correlation to Zone P4. Common representatives of the older Zone P3 (Praemurica praecursoria, Praemurica uncinata, Morozovella praeangulata, and Globanomalina ehrenbergi) are also present and clearly indicate significant reworking. Evidence of reworking was also observed in few samples from Holes 1209A and 1212B, as revealed by the presence of sporadic small-sized taxa of Cretaceous age (Hedbergella, Globigerinelloides [Pl. P2], and Schackoina). Preservation ranges from moderate to poor, and common to abundant keel and wall fragments of morozovellids are present throughout (see Pl. P3). The very low number (<100) of total specimens counted in some samples in Holes 1212B and 1211B (see Tables T4, T5) is related to the very small size of the residues.

I. albeari is the most abundant species in this interval, dominating the total assemblages with an average abundance of 50% (Fig. F5). I. pusilla and I. albeari "chubby" are second in abundance and together constitute 40% of the assemblages (Fig. F6). I. albeari "chubby" is also characterized by marked fluctuations in abundance, especially at the shallow-water Hole 1209A. I. tadjikistanensis occurs in low abundance and does not exceed 20% of the total igorinids (Fig. F5). I. pusilla "high trochospire" is rare (<10% of the assemblages) except in Hole 1210A, where its abundance reaches 20% (Fig. F6). Morozovellids are moderately abundant in Hole 1209A (~20% of the total specimens), whereas they are more common in Hole 1212B (40% of the total specimens). The acarininids, mainly represented by A. subsphaerica, show a marked decrease from the base to the top of this interval in Hole 1209A and are very rare in or often absent from the other holes (Fig. F3). The genera Globanomalina, Subbotina, and Chiloguembelina occur in all samples, but combined they do not exceed 30% of the total assemblages. An exception is the genus Globanomalina, which reaches 40% and 60% of the total specimens in the deeper-water sections, Holes 1212B and 1211B, respectively (Fig. F4).

Interval 2—within the Clay-Rich Layer

The assemblages are characterized by very low diversity; species richness ranges from 4 species to 10 species (Fig. F2). An anomalous increase in the number of species occurs within the clay-rich layer in Hole 1211B (Samples 198-1211B-14H-4, 26–27 cm, to 14H-4, 28–29 cm; 124.86–124.88 mbsf). The assemblage from 124.86 to 124.88 mbsf is characterized by common I. albeari, morozovellids, and acarininids. Although the assemblage belongs to Subzone P4a, its composition is different from the surrounding samples that contain common crystals of phillipsite and it resembles the assemblages above the clay-rich layer. Such evidence of local mixing is probably due to drilling disturbance, as is also suggested by the lighter color of the sediment (see Fig. F2). The samples characterized by common to abundant phillipsite contain poorly preserved and few (<300 total specimens in the total residue) planktonic foraminifers (Pl. P3; Tables T2, T3, T4, T5).

The number of I. tadjikistanensis, I. pusilla, and I. albeari "chubby" specimens increases in interval 2 with respect to interval 1 and dominates the assemblages (average abundance = 90%–95% of the total assemblages). I. pusilla "high trochospire" progressively increases in abundance from the base to the top of this interval in Holes 1209A, 1212B, and 1211B, whereas it decreases uphole in Hole 1210A. I. albeari is absent or very rare (<3% of the assemblages) in all of the studied sections (Fig. F5). Morozovellids are generally absent or very rare (<1% of the assemblages) except in Hole 1212B, where the decrease in the number of morozovellid specimens is less abrupt than in the other holes and ranges from 20% to 1% in the lowermost 5 cm of the clay-rich layer. Acarininids are absent or rare (<5% of the assemblages) in Holes 1209A and 1210A, whereas a progressive increase in the number of acarininids from the base (1%) to the top (average = 15%) of the interval is observed in Holes 1212B and 1211B (Fig. F3). The number of globanomalinids decreases from the base (40% of the assemblages) to the top (<1%) of the interval in Holes 1212B and 1211B; globanomalinids are almost absent in the other holes. Chiloguembelinids are generally absent or very rare (<4% of the assemblages) except for an anomalous peak (13%) in Hole 1209A that is coincident with the sudden decrease in igorinid abundance. Subbotinids are generally rare or absent (Fig. F4).

Interval 3—above the Clay-Rich Layer

The number of species ranges from 8 species in Hole 1210A to 18 species in Hole 1212B (Fig. F2). Preservation ranges from poor to moderate, and few to common fragments of morozovellids still are present (Pl. P3). There are slightly fewer I. tadjikistanensis, I. pusilla, and I. albeari "chubby" in interval 3, although they continue to dominate the assemblages in each hole (average = 70%) (Figs. F5, F6). A marked decrease in the abundance of igorinids is observed in Hole 1210A, including an abrupt decrease of I. tadjikistanensis (to 2%–3% of the assemblages), whereas I. albeari increases markedly (as much as 40% of the assemblages). In the other holes, the abundance of I. albeari progressively increases but never exceeds 30% of the total assemblages. The abundance of morozovellids and acarininids is low (<20%) (Fig. F3). Globanomalinids are absent except in Hole 1210A, where they reach 20% of the assemblages. Chiloguembelinids are a minor component of the assemblages in this interval (Fig. F4).

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