A Late Miocene dust shower from the break-up of an asteroid in the main belt
SamplesCarbonate sediments were studied from two deep sea cores. A suite of samples spanning from the middle Eocene to ~3 Ma was obtained from Hole 757B in the central Indian Ocean, 17°01.458'S, 88°10.899'E, on the Ninety East Ridge. This hole was cored on Leg 121 of the Ocean Drilling Program and has been described in detail 1 . Over the studied time interval this site is thought to have subsided from a Middle Eocene depth of ~1500 m to its present depth of 1650 m. Sediments from this site are extremely pure carbonate oozes (usually >95% CaCO 3 ) that have accumulated at a fairly slow average rate of 0.35 cm/kyr. The purpose of this suite was to identify any intervals worthy of more detailed study at sites with faster accumulation rates and hence higher temporal resolution. ~140 samples were analyzed at about 1 meter intervals from ~18 to ~ 135 mbsf, except in two areas of interest, in the Late Eocene and the Late Miocene, where sampling density was higher.Based on elevated extraterrestrial 3 He concentrations in the Late Miocene an additional suite of samples was obtained from amongst the best studied cores spanning this interval, from Hole 926B. This hole was drilled on the Ceara Rise, western equatorial Atlantic (3 o 43.148'N, 42 o 54.507'W, 3600 m water depth) on Leg 154 of the Ocean Drilling Program 2 . The Late Miocene interval is a clayey carbonate ooze (~65% CaCO 3 ) with a mean accumulation rate of ~1.7 cm/kyr. 73 samples were analyzed in the interval between 159 and 224 mbsf. The spacing of these samples is ~ 1.5 meters away from the 3 He peak to as close as a few cm at the peak.
MethodsSamples were analyzed for helium isotopes following removal of CaCO 3 using standard procedures at Caltech 3 . In all cases 3 He and 4 He blank levels were sufficiently low compared to sample signals that blank corrections were not required. Many samples were analyzed in duplicate to detect and eliminate anomalously high 3 He values arising from occasional over-sampling of large IDP grains 4 . When preparing the final 3 He and 3 He/ 4 He data set the following procedure was adopted for incorporating replicated analyses. If the duplicates agreed to within a factor of three, the average was used. If the difference exceeded a factor of three, the higher value was excluded. This led to exclusion of about 6% of the 3 He measurements. For 4 He and non-carbonate fraction data the replicates were simply averaged.
Age Models
Site 757The age model for site 757 comes primarily from shipboard biostratigraphy. Event locations are from the shipboard report 1 with updated datum ages from Backman and Raffi 5 . To facilitate comparison with previous wok at the Massignano GSSP,