Geochemisty at DSDP Hole 62-465A

Very significant enhancements of the element iridium have been observed in association with the Cretaceous/ Tertiary boundary in marine sediments laid down 65 m.y. ago and subsequently uplifted above the ocean's surface. If our hypothesis for the origin of the iridium and the cause of the Cretaceous/Tertiary life extinctions (the asteroid-impact theory) (Alvarez et al., 1980) is correct, the Ir anomaly should be associated with the Cretaceous/ Tertiary boundary region wherever it is intact. The present work was undertaken to search for the Ir anomaly in a deep-sea-drilling core, in order to check this aspect of the asteroid-impact theory.

He isotopes in Late Cretaceous sediments

Late Cretaceous (100-73 Ma) pelagic limestones were measured for helium concentration and isotopic composition to characterize the interplanetary dust flux using 3He as a tracer. In the Bottaccione section near Gubbio, Italy, three intervals of elevated 3He concentration were detected: K1 in the Campanian stage at ~79 Ma, K2 in the Santonian stage at ~ 85 Ma, and K3 in the Turonian stage at ~91 Ma. All three of these episodes are associated with high 3He/4He and 3He/non-carbonate ratios, consistent with their derivation from an enhanced extraterrestrial 3He flux rather than decreased carbonate sedimentation or dissolution. While K2 is modest in magnitude and duration and thus is of limited significance, K1 and K3 are each identified by a few myr interval with an ~4-fold enhancement in mean 3He flux compared with pre-event levels. Samples from ODP Hole 762C in the Indian Ocean spanning both K2 and K3 (93-83 Ma) confirm the presence of a peak in the Turonian stage, suggesting that K3 is a global event. The K1 and K3 3He events are similar in most respects to the two peaks previously detected in the Cenozoic, suggesting a similar origin. These have been attributed to a major asteroid collision in the Late Miocene and to a shower of either comets or asteroids in the Late Eocene. Based on the age and temporal evolution of K1, we suggest that it most likely records the collision which produced the Baptistina asteroid family independently dated at ~80 Ma. The K3 event is less easily explained. It is characterized by an unusually spiky and erratic temporal progression, suggesting an unusual abundance of very 3He rich particles not previously seen in the sedimentary 3He record. We suggest this episode arises either from a comet shower or from an asteroid shower possibly associated with dust-producing lunar impacts.

Isotopic ratios, concentration of noble gases and mineral and bulk composition of extraterrestrial dust particles in Dome Fuji ice core, East Antarctica

Two silicate-rich dust layers were found in the Dome Fuji ice core in East Antarctica, at Marine Isotope Stages 12 and 13. Morphologies, textures, and chemical compositions of constituent particles reveal that they are high-temperature melting products and are of extraterrestrial origin. Because similar layers were found ~2000 km east of Dome Fuji, at EPICA (European Project for Ice Coring in Antarctica)-Dome C, particles must have rained down over a wide area 434 and 481 ka. The strewn fields occurred over an area of at least 3 × 10**6 km**2. Chemical compositions of constituent phases and oxygen isotopic composition of olivines suggest that the upper dust layer was produced by a high-temperature interaction between silicate-rich melt and water vapor due to an impact explosion or an aerial burst of a chondritic meteoroid on the inland East Antarctic ice sheet. An estimated total mass of the impactor, on the basis of particle flux and distribution area, is at least 3 × 10**9 kg. A possible parent material of the lower dust layer is a fragment of friable primitive asteroid or comet. A hypervelocity impact of asteroidal/cometary material on the upper atmosphere and an explosion might have produced aggregates of sub-µm to µm-sized spherules. Total mass of the parent material of the lower layer must exceed 1 × 10**9 kg. The two extraterrestrial horizons, each a few millimeters in thickness, represent regional or global meteoritic events not identified previously in the Southern Hemisphere.

Helium concentrations of late Miocene sediments

Throughout the history of the Solar System, Earth has been bombarded by interplanetary dust particles (IDPs), which are asteroid and comet fragments of diameter 1-1,000 µm. The IDP flux is believed to be in quasi-steady state: particles created by episodic main belt collisions or cometary fragmentation replace those removed by comminution, dynamical ejection, and planetary or solar impact. Because IDPs are rich in 3He, seafloor sediment 3He concentrations provide a unique means of probing the major events that have affected the IDP flux and its source bodies over geological timescales (Farley et al., 1998, doi:10.1126/science.280.5367.1250; Takayanagi and Ozima, 1987, doi:10.1029/JB092iB12p12531; Farley, 1995, doi:10.1038/376153a0; Kortenkamp and Dermott, 1998, doi:10.1126/science.280.5365.874). Here we report that collisional disruption of the >150-km-diameter asteroid that created the Veritas family 8.3 +/- 0.5 Myr ago (Nesvorny et al., 2003, doi:10.1086/374807) also produced a transient increase in the flux of interplanetary dust-derived 3He. The increase began at 8.2 +/- 0.1 Myr ago, reached a maximum of 4 times pre-event levels, and dissipated over 1.5 Myr. The terrestrial IDP accretion rate was overwhelmingly dominated by Veritas family fragments during the late Miocene. No other event of this magnitude over the past 10**8 yr has been deduced from main belt asteroid orbits. One remarkably similar event is present in the 3He record 35 Myr ago, but its origin by comet shower (Farley et al., 1998, doi:10.1126/science.280.5367.1250) or asteroid collision (Tagle and Claeys, 2004, doi:10.1126/science.1098481) remains uncertain.

Benthic foraminifera across the Cretaceous/Paleogene boundary in the Southern Ocean

The impact of an asteroid at the Cretaceous/Paleogene (K/Pg) boundary triggered dramatic biotic, biogeochemical and sedimentological changes in the oceans that have been intensively studied. Paleo-biogeographical differences in the biotic response to the impact and its environmental consequences, however, have been less well documented. We present a high-resolution analysis of benthic foraminiferal assemblages at Southern Ocean ODP Site 690 (Maud Rise, Weddell Sea, Antarctica). At this high latitude site, late Maastrichtian environmental variability was high, but benthic foraminiferal assemblages were not less diverse than at lower latitudes, in contrast to those of planktic calcifiers. Also in contrast to planktic calcifiers, benthic foraminifera did not suffer significant extinction at the K/Pg boundary, but show transient assemblage changes and decreased diversity. At Site 690, the extinction rate was even lower (~3%) than at other sites. The benthic foraminiferal accumulation rate varied little across the K/Pg boundary, indicating that food supply to the sea floor was affected to a lesser extent than at lower latitude sites. Compared to Maastrichtian assemblages, Danian assemblages have a lower diversity and greater relative abundance of heavily calcified taxa such as Stensioeina beccariiformis and Paralabamina lunata. This change in benthic foraminiferal assemblages could reflect post-extinction proliferation of different photosynthesizers (thus food for the benthos) than those dominant during the Late Cretaceous, therefore changes in the nature rather than in the amount of the organic matter supplied to the seafloor. However, severe extinction of pelagic calcifiers caused carbonate supersaturation in the oceans, thus might have given competitive advantage to species with large, heavily calcified tests. This indirect effect of the K/Pg impact thus may have influenced the deep-sea dwellers, documenting the complexity of the effects of major environmental disturbance.

Geochemistry of sediments from the Cretaceous-Tertiary boundary

Siderophilic element concentrations are high in sediments from the Cretaceous-Tertiary boundary. An extraterrestrial source is indicated. Concentrations are too high to be understood in terms of the impact of a chondritic asteroid. Either the projectile was a metal-sulphide core or the infalling material (probably weak cometary matter) was slowed down during atmospheric passage.

Calcareous dinocysts and grain size distribution along profile Stevns-Klint

The distribution patterns of calcareous dinoflagellate cysts were studied in the classic Cretaceous Tertiary (K-T) boundary section of Stevns Klint, Denmark, focusing mainly on the response of the cyst association to an abrupt environmental catastrophe. A major part of the Fish Clay, which covers the K-T boundary at its base and is exposed in the investigated section, contains fallout produced by an asteroid impact. Calcareous dinoflagenate cysts are the best preserved remains of carbonate-producing phytoplankton in this layer. The potential of this group of microfossils for the analysis of survival strategies and extinction patterns has been underestimated. The cyst species of the investigated section can be grouped into four assemblages that represent victims, survivors, opportunists, and specially adapted forms. The victims (Pithonelloideae) were an extremely successful group throughout the Upper Cretaceous, but were restricted to the narrow outer shelf. This restriction minimized their spatial distribution, which generally should be large to facilitate escape from unfavorable conditions. Spatial restriction optimized the population decrease by mass mortality, disabling a successful recovery. In contrast, the survivors that became the dominating group in the Danian had a wide spatial range from the shelf environment to the oceanic realm. A unique calcareous dinocyst assemblage in the Fish Clay shows that even under the stressed conditions immediately following the impact event, some species flourished due to special adaptation or high ecological tolerance. The ability of these dinoflagellate species to form calcareous resting cysts in combination with their generally wide spatial distribution in a variety of environments appears to be the main reason for a low extinction rate at the K-T boundary as opposed to the high extinction rate of other phytoplankton groups, such as the coccolithophorids.

(Table 1) Elemental abundances in DSDP Core 86-577B-1

An Ir anomaly of 61 ng/cm**2 was found in Deep Sea Drilling Project Hole 577B at the same stratigraphic level as the Cretaceous/Tertiary boundary defined by nannoplankton. This close correspondence supports the asteroid-impact theory for the Cretaceous/Tertiary boundary extinctions.

Chemical composition of ocean Fe-Mn ores

The monograph includes the study of chemical and mineral compositions of terrestrial and marine manganese ores, methods of their analysis, dependence of manganese crust geochemistry on tectonic position of their formation, problems of manganese genesis and sources of manganese in ocean ores in connection with geohistorical aspects of ocean formation and development. A hypothesis is offered that formation of giant manganese mineral basins on continental margins resulted from a large asteroid fall to the ocean.

Geochemistry of DSDP Hole 86-577B samples

THE chemical, mineralogical and isotopic characteristics of deposits at the Cretaceous/Tertiary (K/T) boundary are suggestive of a large impact event, the prime candidate (Sharpton et al., 1992, doi:10.1038/359819a0) being the Chicxulub crater in Yucatan, Mexico. Spinel-bearing spherules, which may be associated with such impacts, have been reported (Smit and Romein, 1985, doi:10.1016/0012-821X(85)90019-6) at several K/T boundary sites worldwide, but their origin is still uncertain. We have examined the spinel-bearing material recovered from K/T boundary deposits at site 577 in the Pacific Ocean (Heath et al., 1985, doi:10.2973/dsdp.proc.86.104.1985) and find two distinct populations of particles: spherules with dendritic spinel textures dispersed throughout the grains and irregularly shaped fragments with spinels essentially confined to the rim. The morphology and composition of the particles are characteristic of melted and partially melted meteoritic ablation debris, but their location is difficult to reconcile with an impact on the Yucatan peninsula, some 10,000 km away. We suggest instead that the spinel-bearing particles at site 577 are derived from the impact of a 2-km asteroid in the Pacific Ocean, and that several accretionary events of this type are required to explain the global distribution of spinel-bearing spherules at the K/T boundary.

Element stratigraphy across the Cretaceous-Tertiary boundary in ODP Hole 119-738C

Neutron activation analyses of iridium and other chemical elements were performed across a 1-m-thick, partly nonbioturbated, clay-rich interval at the Cretaceous/Tertiary boundary in ODP Hole 738C. The results show that the boundary interval holds one of the highest Ir enrichments (320 ng Ir/cm2) of all known Cretaceous/Tertiary boundary layers. Iridium concentrations are highest (18 ppb Ir, whole-rock samples) a few centimeters above the base of the clayrich interval and gradually tail off upsection. Compared with background levels the most Ir-rich interval also shows strongly enhanced concentrations of Cr (215 ppm) and slightly elevated Co concentrations (13 ppm). The Ir-rich interval shows low As (< 15 ppm) and Sb (<0.8 ppm) concentrations, a fact that is congruent with absence of abundant authigenic sulfides in the sediment. Irregularly distributed Fe enrichments and a greenish gray color of the Fe-rich intervals may indicate the presence of glauconitic clay minerals and suboxic, slightly reducing conditions during deposition. Rare earth element (REE) abundance patterns change considerably across the Cretaceous/Tertiary boundary interval, reflecting either a change in Cretaceous/Tertiary boundary seawater REE composition or the occurrence of different REE fractionation processes due to changing depositional environment. Element-vs.-element ratios of Hf, Ta, Th, U, Cs, and Sc are similar between the most Ir-rich layers of the boundary section and other levels with lower Ir concentrations. This may imply that the clay fraction of the Ir-rich layers of the Cretaceous/Tertiary boundary interval is made up predominantly of locally derived material. Calculated calcite-free abundances of Hf, Ta, Th, U, Cs, and Sc, on the other hand, are reconcilable with an extraneous origin of the bulk of the clay in the most Ir-rich layers. The Ir in the Cretaceous/Tertiary boundary clay-rich zone in Hole 738C is most likely derived from an earth-impacting asteroid; however, the origin of the clay-rich zone remains enigmatic.

Geochemical studies of the Cretaceous-Tertiary boundary in ODP Holes 113-689B and 113-690C

In a study of ODP Hole 689B no iridium (Ir) anomaly was found in Sections 1 through 6 of Core 25X or in Core 26X from the top down to section 2, 3-12 cm. The background Ir abundance averaged 11 parts per trillion (ppt) and a clay-enriched region had nearly the same average, 26 ± 12 ppt. If the Cretaceous-Tertiary (K-T) contact is in the region studied, then sedimentation was not continuous, and the K-T boundary was probably either not deposited or it was eroded away. In a study of Cores 15X and 16X of ODP Hole 690C, an iridium peak with a maximum abundance of 1566 ± 222 ppt was found in Section 4 of Core 15X at 39-40 cm with a half-width of 6.6 cm. Background abundances were ~15 ppt and distinctly higher Ir abundances were observed from 119 cm below to 72 cm above the main peak. The Ir distribution below the main peak is attributed to bioturbation by organisms with burrows extending at least 0.4 m. The Ir distribution above the main peak may be due to the same cause but other explanations may be significant. There are variable enrichments of clay in the mainly CaCO3 sediment of Core 15X, and the stratigraphically lowest part of the most abundant clay deposits is found (within 2 cm) in the same position as the main Ir peak. The clay deposit, which is estimated to be about 50% of the sediment, extends upward ~19 cm and then slowly decreases to a background level of 10% over 1 m. The degree of homogeneity of the clay-rich interval suggests it was not due to episodic volcanism but may have been due to a decrease of the CaCO3 deposition rate which was possibly triggered by the impact of a large asteroid or comet on the Earth.