Ethmodiscus Castracane is a widely distributed, warm-water diatom genus recorded mainly from oligotrophic waters (Villareal, 1993, and references therein). Ethmodiscus cells are the largest known diatoms, reaching 2-3 mm in diameter (Round et al., 1990). In the central and western Pacific, abundance of Ethmodiscus cells in the euphotic zone is on average 0.01-0.1 cells m-3; however, off the coast of Chile, the maximum reported abundance of Ethmodiscus in equatorial waters is 21.7 cells m-3. In the California Current, Ethmodiscus rex (Rattray) Wiseman & Hendey was recorded in 74% of quantitative vertical net tows, with a range in abundance of 0.001-0.25 cells m-3 (Villareal, 1993, and references therein).
Given the low abundance of Ethmodiscus in oceanic surface waters, it is surprising that Ethmodiscus oozes have been reported from the Pacific, Atlantic, and Indian Oceans (Mikkelsen, 1977). The Ethmodiscus oozes are principally found in the large depressions (Ricard, 1970, cited in Rivera et al., 1989) or in belts of sediment beneath tropical oceans (Round, 1980). Two mechanisms have been suggested for accumulation of Ethmodiscus into oozes. The first is differential dissolution of carbonate vs. silica, coupled with resuspension and deposition. The second is that glacial blooms, triggered by high nutrient content from upwelling, are responsible for the oozes (Mikkelsen, 1977; Stabell, 1986). These mechanisms were proposed on the basis of inferences from the sediment record and sediment dissolution experiments (Villareal, 1993); however, more recent work on the growth rate and chemical composition of Ethmodiscus has shown that cells from this genus are positively buoyant (Villareal, 1993; Villareal and Carpenter, 1994). Ethmodiscus cells behave in a similar way to chains and mats of the diatom genus Rhizosolenia Brightwell (Villareal and Carpenter, 1989). Rhizosolenia chains and mats and Ethmodiscus cells are positively buoyant and have decoupled their light and nutrient utilization, taking up nitrate from a depth just below the nutricline and rising to the surface to photosynthesize (Villareal et al., 1993; Villareal and Carpenter, 1994). Indeed, positively buoyant Ethmodiscus cells have been caught by downward-facing sediment traps at 5400-m water depth in the Pacific Ocean (Villareal, 1993), suggesting the possible presence of a hitherto unrecorded population of Ethmodiscus at depth. Positively buoyant populations of Rhizosolenia and Ethmodiscus have been documented as accumulating at the surface under low wind-speed conditions (Collingwood, 1868, cited in Villareal, 1993; Villareal and Carpenter, 1989); therefore, meteorological conditions may be responsible for accumulations of Ethmodiscus in the sediment rather than in situ growth.
The current uncertainty over the depositional mechanism makes it important to document each newly recovered occurrence of Ethmodiscus ooze, particularly because these oozes could have paleoclimatic implications (Stabell, 1986). The purpose of this data report is to document the nature and occurrence of an early Pliocene Ethmodiscus ooze, recovered at Ocean Drilling Program (ODP) Site 1010 located ~100 km north of Guadalupe Island, seaward of Baja California (29°57.9´N, 118°6.0´W; Lyle, Koizumi, Richter, et al., 1997).