J. T. Hagstrum, W. R. Premo, T. D. Bullen and D. H. Abbott
Ejecta of multiple impacts found across the K/T boundary in deep-sea cores LL44-GPC3 and DSDP 91-596 from the northern and southern Pacific Ocean (in AGU 2008 fall meeting )
Eos, Transactions, American Geophysical Union(December 2008), 89(53, Suppl.)
Index Terms/Descriptors: Cenozoic; cores; Cretaceous; Deep Sea Drilling Project; deep-sea environment; DSDP Site 596; East Pacific; ejecta; impacts; IPOD; isotopes; K-T boundary; lead; Leg 91; lower Paleocene; marine environment; marine sediments; Mesozoic; metals; microspherules; microtektites; mineral composition; Pacific Ocean; Paleocene; Paleogene; sediments; South Pacific; Southeast Pacific; spherules; stratigraphic boundary; tektites; Tertiary; Upper Cretaceous
Latitude & LongitudeN23°46'24'' - N23°46'24'' and W93°20'46'' - W93°20'46''N30°20'00'' - N30°20'00'' and W157°49'00'' - W157°49'00''
Cores of brown pelagic clay recovered from sites LL44-GPC3 (30°19.9'N, 157°49.4'W) and DSDP 91- 596 (23°51.2'S, 169°39.3'W) in the abyssal Pacific Ocean include the K/T boundary at 20.56 and 20.10 mbsf, respectively. The boundary has been identified in both cores by peak Ir and magnetic susceptibility anomalies, and by ichthyolith fossils. Abundant shocked quartz, magnesioferrite spinels, and microspherules were also found within the cores' K/T boundary sediments. Although lacking high-resolution stratigraphies, abyssal sediment cores have advantages for recovering impact ejecta, mainly slow accumulation rates, uniform composition, and the general absence of coarse-grained detrital minerals. Corliss and Hollister  initially reported finding small (~20 μm) cristobalite "spheres" scattered in the lower part of core GPC3 between 22 and 24 mbsf. Similarly, between 21.79 and 24.12 mbsf, we have found numerous large (often >100 μm) euhedral crystals of feldspar, iron oxide, apatite, and SiO2 (few), in addition to microkrystite spherules, microtektite spherules (some with quenched textures), and several pieces of amorphous carbon. Dozens of the more common feldspar crystals contain smaller iron oxide and/or apatite crystals that were ballistically "shot" into the feldspar crystals, clearly fracturing them. The microkrystites include mineral grains of Cr-rich spinel, olivine, feldspar, Mg-silicate, Fe-sulfide, Fe-Ti oxide, and SiO2. In core 596 between 19.85 and 20.97 mbsf, including the K/T boundary at 20.10 mbsf, we found abundant microspherules and microkrystites at 8 of 9 levels sampled. In addition, Pb spherules were found 17 cm above and 51 cm below the boundary level. The isotopic compositions of the two Pb spherules (206Pb/204Pb=19.23 and 19.27; 207Pb/204Pb=15.67 and 15.72; and 208Pb/204Pb=38.63 and 38.75) are indistinguishable from common MORB Pb isotopic compositions, plotting at the intersection of values for DM, EMI, EMII, and HIMU mantle sources. The Pb composition for the spherules also places them within the Pb isotopic field for modern pelagic sediments. Between 24.00 and 24.46 mbsf in core 596, ~4 m below the K/T boundary, we also found large pieces (>400 μm) of platy FeO and microkrystite spherules. The sedimentation rate in both cores has been estimated to be between 20 to 30 cm per Myr, and we provisionally interpret the ejecta found across the K/T boundary in core 596 to be from multiple impact events that occurred between ~68 and ~64 Ma, some possibly in the ancient Pacific Ocean. An earlier impact event might also have occurred 16 to 18 Myr before (~4 m below) the K/T event. We infer that the pre-K/T mineral crystals, microkrystites, and microspherules in core GPC3 are vapor-phase condensates, microtektites and possibly fine target particulates entrained in the fireball from a large oceanic impact at ~68 Ma, perhaps nearby to the east on seafloor now subducted beneath western North America. Dispersal of the mineral crystals in core GPC3 between 22 and 24 mbsf might have been caused by seismically-induced slumping and megatsunami associated with the nearby impact.  Corliss and Hollister, Nature, 282, p. 707.