The purpose of this data report is to document the nature and occurrence of this early Pliocene Ethmodiscus ooze. However, some features documented using the BSEI analysis are worthy of brief explanation and discussion. The Ethmodiscus laminae comprise layers, or stacks, of frustule fragments. The burrows are horizontal in attitude and appear to run along the Ethmodiscus laminae (Fig. 4C, 4D; marked "x" on Fig. 1A); the bioturbating organism presumably was feeding on the rich store of organic matter in the Ethmodiscus laminae. The flux of Ethmodiscus frustules may have temporarily overwhelmed the benthos, reducing the extent to which the infauna could mix the sediment. The mechanism for the concentration of Ethmodiscus frustules into laminations will continue to be a matter for further debate; the short stratigraphic occurrence of this one interval at Site 1010 precludes any meaningful interpretation. However, the alternation between Ethmodiscus laminae and mixed-sediment laminae is suggestive of a cyclical mode of deposition (Fig. 3F; marked "cy" on Fig. 2A, 2C).
Ethmodiscus cells behave in similar ways to Rhizosolenia chains and mats in that both are positively buoyant and concentrated in the surface waters by low wind speeds (Villareal, 1988, 1992, 1993; Villareal et al., 1993; Villareal and Carpenter, 1989). Rhizosolenia mats are also concentrated along convergent oceanic frontal zones in the Eastern Equatorial Pacific Ocean (Yoder et al., 1994), and Ethmodiscus oozes are often found in swaths beneath tropical seas (Round, 1980). Ethmodiscus is characteristic of oligotrophic waters, so there would appear to be no ecological support for an increase in nutrients facilitating blooms of Ethmodiscus that are sedimented to form oozes (as discussed in Villareal, 1993). Considering the similarity in behavior between the two genera, it is possible that in the geological past Ethmodiscus frustules may have been concentrated along oceanic convergences in a similar way to Rhizosolenia chains and mats today (Kemp, 1995). The Ethmodiscus cells may have been deposited en masse by a similar physical oceanographic process to that proposed for the Neogene Thalassiothrix longissima Cleve & Grunow dominated diatom mat deposits of the Eastern Equatorial Pacific and North Atlantic (Bodén and Backman, 1996; Kemp et al., 1995). The alternation between the Ethmodiscus and mixed-sediment laminae could reflect the periodic movement of the frontal zone. Thorough investigation of other Ethmodiscus ooze deposits from the world's oceans would be required to test this hypothesis. However, it is interesting to note that the early Pliocene Ethmodiscus ooze interval recovered at Site 1010 occurs at the stratigraphic top of a diatom-bearing interval (Miocene to early Pliocene) and directly below a diatom-barren interval (Shipboard Scientific Party, 1997). This could suggest that the productive waters of the California Current are moving shoreward across Site 1010 during the late Miocene to early Pliocene and that the Ethmodiscus ooze was deposited at the outer edge of these productive waters, supporting a possible frontal mechanism of formation.