The upper few core sections from Sites 991, 992, and 993 were re-inspected in the Ocean Drilling Program (ODP) Core Repository in Bremen, Germany. The location of a lithologic discontinuity between undisturbed soft sediments and firm sediments exhibiting signs of distortion was re-established at 2.06 mbsf at Site 991 (Paull, Matsumoto, Wallace, et al., 1996). Site 991 is located between the headwall of the slide and the crest of the diapir (Paull, Matsumoto, Wallace, et al., 1996).
Six sediment horizons from the upper two core sections of Site 991 were selected for 14C age dating (Table 1). Samples, with volumes of ~5 cm3, were obtained from 1-cm-thick zones centered on the depths reported in Table 1. Five samples were collected from above the contact and one below (Fig. 1). Special care was given to avoid material from the outer walls or the cut face. Also, the sediments immediately above (~3 cm) the contact were avoided, because they might be contaminated by older material reworked during the slide event. Similarly, material within 15 cm of the obviously burrowed hiatus surface was also avoided for fear that younger carbonates might be reworked downward (Fig. 2).
The samples were sent to
the National Ocean Sciences Accelerator Mass Spectrometry (AMS) Facility at the
Woods Hole Oceanographic Institution. The CO2 from the carbonate
fraction was extracted from the samples by acidification. The bulk carbonate in
these samples was dominated by nannofossils. The CO2 carbon
was reacted with an Fe/H2 catalytic reductant and converted into
graphite for 14C/12C measurement. Measurements of the 
13C
were made on a VG Optima mass spectrometer at the AMS facility in Woods Hole and
were used to normalize the fraction of modern carbon to National Bureau
Standards (NBS) Oxalic Acid I. The percent of modern carbon and the reported 14C-ages
follow the convention of Stuiver and Polach (1977) and Stuiver (1980).
Radiocarbon ages are calculated using a t1/2 of 5568 yr.
