Samples (20 cm3) were taken every 10 cm from Cores 167-1011C-1H through 3H, 167-1012B-1H through 4H, 167-1012A-3H and 4H and every 10-20 cm from Cores 167-1018C-1H through 5H. High terrigenous clay and organic matter content of the sediments at these sites made disaggregation of the sediments difficult. Disaggregation began with freeze drying the sediment samples. Following freeze drying, sediments were weighed and then soaked in ~200 mL of deionized water. Sediments were subsequently mechanically agitated using an orbital shaker (Thermolyne Bigger BiII orbital shaker) for 4-16 hr at 170 rpm. Often sediments required further treatment with the addition of 40 mL of buffered Calgon solution (sodium metaphosphate (NaPO3) ~13Na2O) followed by shaking for an additional 2-6 hr. The sand fraction (>63 µm) was separated by passing the disaggregated sediment through a 63-µm sieve in running deionized water with the use of a soft brush. Both coarse (>63 µm) and fine fractions were collected in filter paper and dried overnight at 50°C. The coarse fraction was weighed, and both coarse and fine fractions were transferred into vials for storage.
The coarse fraction was sieved using a 250-µm sieve in preparation for picking of foraminifer shells. Benthic foraminifer species Cibicidoides wuellerstorfi (Sites 1011, 1012, and 1018), Cibicidoides pachyderma (Site 1018), and Uvigerina spp. (Sites 1012 and 1018) were picked from the >250-µm-size fraction. The number of tests analyzed in the mass spectrometer varied from sample to sample depending on the availability of the preferred species C. wuellerstorfi. For Site 1018, most analyses were conducted using one or two C. wuellerstorfi or three or four Uvigerina spp. tests. However, the number of tests of C. wuellerstorfi analyzed ranged from shell fragments to four specimens. In addition, occasional analyses were done using species C. pachyderma when C. wuellerstorfi was scarce. For Site 1012, isotopic analyses were made on a range of one to eight, and an average of two C. wuellerstorfi tests. In occasional intervals barren of C. wuellerstorfi, three or four specimens of Uvigerina spp. were used to supplement the isotope record. For Site 1011, all analyses were conducted on Cibicidoides wuellerstorfi. A range of one to ten and an average of four or five tests were measured.
Before isotopic analysis,
benthic foraminiferal tests were sonicated in methanol for 5 s to clean them of
residual contaminants that may have adhered to the tests. Samples were then
dried in an oven at 50°C
and transferred into stainless steel boats. Isotopic analyses were conducted on
Micromass Mass Spectrometers (models Optima and Prism) in the light stable
isotope laboratory directed by Christina Ravelo, Jim Zachos, and Paul Koch at
the University of California, Santa Cruz. Both instruments are outfitted with
common acid bath automated carbonate devices. Foraminiferal specimens were
reacted at 90°C in
orthophosphoric acid (H3PO4; specific gravity 1.93 g/cm3
at 20°C) and
cryogenically separated from water and noncondensable gases before introduction
into the mass spectrometer. Analytical precision on both instruments is ±0.03
(1 s) for carbon standards and ±0.08
(1 s) for oxygen standards. Carerra Marble, which has been calibrated to
National Institute of Standards and Technology isotopic reference material
NBS-18 and NBS-19 for conversion to (Vienna) Peedee belemnite (VPDB) scale, is
used as the in-house standard. All values reported here are relative to VPDB (Table
1; Fig. 2).
