Sedimentation Patterns of Meteoritic Ejecta in Eltanin Impact Deposits at Site Ps58/281

Introduction: Deposits of the late Pliocene (2.5 Ma) Eltanin impact are unique in the known geological record. It is the only known km-sized asteroid impact into a deep-ocean (5 km) basin, and the central portion of the impact region is the most meterorite-rich locality known on Earth. Evidence for this deposit was first discovered as an Ir anomaly in sediments from three cores collected in 1965 [1,2] by the USNS Eltanin. These cores contained mm-sized shock-melted asteroidal materials and several percent unmelted meteorite fragments [3]. Based on mineral chemistry of unmelted meteorite fragments, and siderophole element concentrations in the impact melt, the parent asteroid is considered to be a lowmetal (~4%) mesosiderite [4,5]. Two oceanographic expeditions have now explored the impact region, which was in the vicinity of the Freeden Seamounts (57.3 ̊S, 90.5 ̊W). These expeditions used detailed mapping of the seafloor, echosounding of near-surface sediments and collection of sediments using piston cores to examine the nature and extent of the impact deposits. In 1995, Polarstern expedition ANT-XII/4 recovered three cores containing ejecta deposits at the Freeden Seamounts and provided the first detailed understanding of the impact deposit [6]. In 2001 Polarstern expedition ANT-XVIII/5a returned to the impact region and recovered 17 new cores from a region covering ~80,000 km 2 of the ocean floor [7, 8]. Analyses of sediment ages, textures and compositions (size, microfossil, mineralogy, meteorite content), show that sediments as old as Eocene, and probably Paleocene were ripped up by the disturbance from the impact and redeposited in three distinct units. Features of these deposits are elucidated by xray-radiographs taken from slices of the cores. The lowermost unit is a chaotic assemblage of sediment fragments up to 50 cm in size (termed unit SU IV). Above this is a laminated sand-rich unit that was deposited as a turbulent flow (SU III), and this is overlain by a more fine-grained deposit of silts and clays that settled from a cloud of sediment suspended in the water column (SU II) [6]. Meteoritic ejecta are typically found to be concentrated near the base of the uppermost unit, where coarse ejecta caught up with the settling sediment following the impact. It is important to note that the impact deposit contains two distinct components: 1) seafloor sediments disturbed and redeposited by the impact comprise most of the impact deposit, and 2) asteroidal ejecta composed of melted and unmelted asteroid material that was ejected from the ocean, settled through the water column, and deposited in the upper portion of the impact deposit. This Study: In this poster we will illustrate details of the distribution of meteoritic ejecta in a piston core from site PS58/281 (57.4 ̊S, 91.96 ̊W; 4772 m depth), which was recovered from a basin on the NW edge of the seamounts. This core is exceptional in its high concentrations of meteoritic ejecta and may have been close to the actual impact site. We estimate that the impact deposited 2.8 g/cm 2