Sediment samples were
taken at about every 50-100 cm of the Pliocene-Pleistocene level of Hole 1082A.
The biogenic opal content (see "Appendix")
was determined by two methods based on wet chemical extraction techniques:
1. "OLin" data: At the National Sun Yat-Sen University (Taiwan),
Lin analyzed 80 samples over a depth interval from 121.86 to 261.96 meters
below sea floor (mbsf), following the basic leaching method of Mortlock and
Froelich (1989). This method involves a single extraction of silica with an
alkaline solution at 85° C for 5 hr and the measurement of the dissolved
silicon concentration in the extract by molybdate-blue spectrophotometry.
Instead of the 2-N Na2CO3
suggested by Mortlock and Froelich (1989), 0.5-N NaOH was employed as the
basic leaching chemical to ensure complete dissolution. The HCl was replaced
by a milder 0.5-M glacial acetic acid solution to help prevent the breakdown
of silicates, which would result in overextraction of silicate from the
sediment (Murray et al., 1995).
2. "OPer" data: At the University of Bremen (Federal Republic of
Germany), Pérez analyzed 142 samples from 100.25 to 250.47 mbsf and 104
samples from 298.6 to 399.5 mbsf using the automated leaching method of
Müller and Schneider (1993), which is a modification of the manual
sequential leaching method of DeMaster (1981). While stirred constantly, the
opal is extracted with 1-M NaOH at 85° C in a stainless steel vessel. The
extraction time varies according to the sample type (e.g., sediment type,
opal concentration, and presence of clay minerals), and the increase in
dissolved silica is continuously monitored as a minor portion of the
leaching solution is cycled to an autoanalyzer and analyzed for dissolved
silicon by molybdate-blue spectrophotometry. The resulting absorbance vs.
time plot is then evaluated following the extrapolation procedure of
DeMaster (1981). The weak point in the manual technique is that the
slope—for example, the increase in dissolved silica extracted with
time—and the extrapolated intercept value are based only on a few
measurements. The automated technique of Müller and Schneider (1993)
reduces the degree of uncertainty in the determination of the intercept.
Sediment samples from Hole
1084A were taken at about every 150 cm throughout the uppermost 476 mbsf, which
represents approximately the last 3 m.y. For this site, the biogenic opal
content was determined by the modified "OLin" method only.
Overall diatom abundance
and species composition were determined based on smear-slide analysis with
phase-contrast illumination at a magnification of 400×.
A diatom abundance index (DAI) was established using the following convention: 6
(very abundant or dominant) = >50%; 5 (abundant) = 35%-50%; 4 (common) =
20%-35%; 3 (few) = 5%-20%; 2 (rare) = 1%-5%; 1 (trace) = <1%; 0 (barren) = no
diatoms in sample (Lange et al., 1999).
After converting depths (mbsf)
to meters composite depth (mcd), age models at Sites 1082 and 1084 were derived
according to shipboard magnetostratigraphy and nannofossil datums (Shipboard
Scientific Party, 1998a, 1998b).