During the Leg 178 cruise, shipboard scientists actively searched for evidence of the impact of the Eltanin asteroid. This was primarily through an attempt to note unusual sediment deposits in cores from continental rise deposits, particularly in sediments of late Pliocene through early Pleistocene age. Gersonde et al. (1997) dated the impact at 2.15 Ma, just prior to the Reunion Event (Chron C2r.1n), or just before oxygen isotope Stage 82.
The only particularly unusual sediment deposit recovered at the rise sites was a massive, well-sorted sand bed that occurred at 112 meters below seafloor (mbsf) in Hole 1096B (Shipboard Scientific Party, 1999a). This bed was unusual in that it was unique within a partly turbiditic section of silts and muds. It was speculated that this might be from a massive flow triggered by a tsunami, possibly an impact event. Unfortunately, the 1.6-Ma age of this unit precluded the possibility that it might be related to the Eltanin impact event.
The only rise site at which the Reunion Event was tentatively identified was Site 1101 (Shipboard Scientific Party, 1999b), where it occurred at the base of lithostratigraphic Unit II at 142.7 mbsf. Unfortunately, the base of Unit II was defined by the last of a number of foraminifer-bearing interglacial deposits and this particular layer was well calcified. During coring, 4 m of flow-in occurred below this layer and no sediments were recovered just below the Reunion Event.
During postcruise analysis of the record at Site 1096 (Shipboard Scientific Party, 1999a), the author noted a short magnetic reversal to normal polarity near the base of lithostratigraphic Unit II. At Site 1096, Unit II was similar to that at Site 1101 and its base was defined as the last foraminifer-bearing interglacial bed. The short reversal near the base of Unit II had not been interpreted by the Shipboard Scientific Party, but it appears to be a well-defined reversal that occurs across several samples with an onset at Sample 178-1096C-2H-1, 75-80 cm (169.25-169.30 mbsf), and a termination at Sample 2H-2, 35-85 cm (167.35-167.85 m) (G. Acton, pers. comm., 1998). By comparison, the base of Unit II is at 173 mbsf.
Samples were taken from two levels at Site 1096. Six samples were taken from Hole 1096B near the unique sand deposit. These included the sand from the top of the bed and silts from the overlying sediments. If this bed formed as a result of an impact tsunami, it was assumed that meteoritic ejecta settling through the water column would be deposited on the top of the sediment flow, as has been noted at Cretaceous/Tertiary boundary sites with impact wave-triggered deposits along the Gulf of Mexico (e.g., Smit, 1999). Another 85 samples were initially selected from near the base of Unit II and below the short magnetic normal interval in sediments from Holes 1096B and 1096C. These samples ranged from Sample 178-1096B-19H-1, 50-51 cm (151.2 mbsf), to 24X-6, 16-17 cm (191.32 mbsf). Of these samples, 48 were closely spaced (~10-cm intervals) from 168.5 to 174.38 mbsf, thus extending from the magnetic reversal to below the base of Unit II. All these samples were analyzed for Ir concentration in hopes of finding an anomaly indicative of impact ejecta.