Two Large Meteorite Impacts at the Cretaceous-Paleogene Boundary

Two large meteorite impacts at the Cretaceous-Paleogene boundary

David Jolley1*, Iain Gilmour2, Eugene Gurov3, Simon Kelley2, and Jonathan Watson2 1Department of Geology & Petroleum Geology, Kings’s College, Aberdeen AB9 2UE, UK 2Centre for Earth, Planetary, Space and Astronomy Research, Open University, Milton Keynes MK7 6AA, UK 3Institute of Geosciences, National Academy of Sciences of Ukraine, Olesya Gonchara Street 55b, 01601 Kiev, Ukraine

ABSTRACT boundary, although there has been controversy over the interpretation of The end-Cretaceous mass extinction has been attributed by most regional deposits close to the crater (Keller et al., 2004). to a single asteroid impact at Chicxulub on the Yucatán Peninsula, Two of us (Kelley and Gurov, 2002) obtained an Ar-Ar age of 65.17 Mexico. The discovery of a second smaller crater with a similar age ± 0.64 Ma for the 24-km-diameter Boltysh impact crater on the Ukrainian at Boltysh in the Ukraine has raised the possibility that a shower of shield. Boltysh is in the Northern Hemisphere at a latitude similar to that asteroids or comets impacted Earth close to the Cretaceous-Paleogene of the well-characterized North American K-Pg sections 65.6 m.y. ago (K-Pg) boundary. Here we present palynological and δ13C evidence (Fig. 1). Data on terrestrial impacts indicate that one Boltysh-sized crater from crater-fi ll sediments in the Boltysh impact crater. Our analyses forms on continental crust every million years. However, the experimental demonstrate that a post-impact fl ora, formed on the ejecta layer, was error in the Ar-Ar age is too large to constrain whether the two impacts in turn devastated by the K-Pg event. The sequence of fl oral recovery were synchronous, or if not, the order in which they occurred. from the K-Pg event is directly comparable with that in middle North The impact on the land surface of the Ukrainian shield that formed America. We conclude that the Boltysh crater predated Chicxulub by the Boltysh crater (Fig. 2) was unlikely to have contributed substantially ~2–5 k.y., a time scale that constrains the likely origin of the bodies to the worldwide devastation at the end of the Cretaceous. Models indi- that formed the two known K-Pg craters. cate that the ignition zone extended at least 100 km beyond the crater rim (Toon et al., 1997; Kring, 1997) and that an unconsolidated ejecta blanket INTRODUCTION between 120 and 350 m in thickness was deposited close to the crater rim The celestial mechanism responsible for the globally distributed (McGetchin et al., 1973; Collins et al., 2005). This ejecta thinned to 1 m iridium-rich clay layer and shocked quartz associated with the end of the between 50 and 80 km from the crater rim. The crater subsequently fi lled Cretaceous Period and a global mass extinction has been debated since with the sediments (Fig. 3) we used to test both the physical effects of ter- the discovery of the layer (Alvarez et al., 1980; Smit, 1999). The dis- restrial impacts and the single-impact K-Pg boundary hypothesis. covery of the ~180-km-diameter Chicxulub crater, thought to be the origin of the global layer (Hildebrand et al., 1991), intensifi ed the debate. NEW DRILL CORE Alternate hypotheses including a single impacting body (Smit, 1999), a The Boltysh crater was drilled in the 1960s–1980s, but the cores were comet shower (Hut et al., 1987), and an asteroid shower (Zappala et al., not recorded, and have been lost. A 596 m cored borehole (hole 42/11) 1998; Bottke et al., 2007) have been proposed, although the discovery drilled by us in 2008 to the west of the central peak, in the deepest part of meteorite fragments in a Pacifi c Ocean Cretaceous-Paleogene (K-Pg) of the crater, recovered a complete sequence of sedimentary rocks uncon- layer (Kyte, 1998) makes a single asteroid or asteroid shower a more formably overlying suevite breccias (see the GSA Data Repository1). likely explanation. In addition, the asteroid or comet shower hypothesis must be reconciled with the single global iridium (Ir) layer (Alvarez et al., 1990), and lack of any signal of heightened extraterrestrial dust, indicated by levels of 3He in sediments (Mukhopadhyay et al., 2001). For many years following its discovery the Chicxulub structure in Mex- ico was the only confi rmed crater known to have formed at the K-Pg

Figure 1. Location map showing impact sites of Chicxulub and Figure 2. Map showing location of Boltysh impact crater, impact ef- Boltysh, and Deccan Traps large igneous province at time of Creta- fect zone, and area of ejecta blanket. Gray shaded circle represents ceous-Paleogene (K-Pg) events. ejecta thicker than 1 m; dark ring represents edge of ignition zone.

*E-mail: [email protected]. 1GSA Data Repository item 2010232, methods, and statistical treatment of carbon isotope results (Tables DR1–DR5), is available online at www.geosociety.org/ pubs/ft2010.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, September September 2010; 2010 v. 38; no. 9; p. 835–838; doi: 10.1130/G31034.1; 3 fi gures; 1 table; Data Repository item 2010232. 835 Figure 3. Lithological, palynological, and geochemical data from borehole 42/11, Boltysh impact crater. A—base of barren zone; B—base of fern spike; C—fl oodplain ferns; D—mid-succession angiosperms; E—infl ux of marine dinocysts; F—return to freshwater lake; G—oldest pine dominance. Lithological column: all depths are in meters, grain size indicated by horizontal scale (clay, silt, sand). Red shading indicates the extent of the oxidized zone.

Within the basal 5 m of sedimentary rocks, the oldest are thin green-gray are derived from scrubby angiosperms (Batten, 1981; Jolley et al., 2008). silty sands, which are also present in intra-suevite fi ssures and as rip-up Polypodiaceous fern spores and Calamaspora (Equisetum) are also com- clasts in overlying coarse turbidite sandstones. These sandstones pass mon in what is interpreted as temperate early mid-successional vegetation upsection into crudely bedded fi ne silty sandstones and laminated silt- growing on the proximal ejecta fi eld. The lack of any marine component stones interpreted as proximal ejecta blanket material reworked and depos- in this palynofl ora supports an interpretation that the Boltysh meteorite ited by turbidity currents in the anoxic waters of the crater lake. This unit is impacted land, a deep, eutrophic crater lake forming shortly afterward. truncated by the erosional base of the fi rst of a thick sequence of turbidites These moderately diverse assemblages are overlain by a 0.89-m-thick with coarse sandstones at the base (578.75 m), probably representing the interval of sediments that are lithologically similar, but barren of palyno- establishment of an effective fl uvial drainage system from the ejecta blan- morphs (Fig. 3). These sediments have no indication of postdepositional ket into the crater via marginal deltas. oxidation, indicating that the lack of organic material is a depositional feature. A small number of pollen grains recovered from one sample at PALYNOFLORA the base of this zone are probably reworked from the underlying pollen- The 60° unconformity between suevite and sediment is probably due rich unit as part of the rip-up clast assemblage. Palynomorphs reappear at to an uneven crater fl oor, as all other bed boundaries are nearly horizontal. 581.01 m (0.89 m above base), where Echinatisporis species (Selaginella The oldest palynofl oras of the 42/11 core are recovered from immediately or spikemoss) occur with low frequencies of polypodiaceous fern spores. above the suevite, and in a fi ssure fi ll of the same sediment lower down the This infl ux of fern and spikemoss spores is replaced at 580.60 m (1.3 m section (581.9 m, and fi ssure fi ll at 583.4 m) (Fig. 3). They are dominated above base) by assemblages of pteridacean spores marking colonization by Botryococcus braunii, a Chlorophycean algae indicative of eutrophic of the ejecta blanket by a higher biomass early seral succession plant freshwater lakes (Tappan, 1980). Recovered along with these algae is a community. moderately diverse palynofl ora of pollen and spores including species of From 580.35 m (1.55 m above base) mid-successional vegetation is Normapolles pollen, and fagaceous and Platycarya-type pollen, which marked by the Normapolles Plicapollis pseudoexcelsus and Interpollis

836 GEOLOGY, September 2010 supplingensis in association with palm pollen (Arecipites sp.). Immedi- post-impact fl ora by the K-Pg event is recorded in a 0.89-m-thick barren ately above this (579.6 m), penetration of marine water into the crater sequence, exhibiting a signifi cantly higher mean δ13C of −24.9‰ ± 1.5‰ is marked by common occurrences of the dinocyst Areoligera cf. coro- and very low carbon contents. The infl ux of fern and/or moss spores at nata. This marine incursion is probably a manifestation of the post K-Pg 0.89 m above the base, and their succession by fern communities, high- transgression recorded around the Tethys Ocean (Guasti et al., 2005). lights parallels with the North American record of the Chicxulub impact. This transgression possibly originated from the Dneiper depression area While the fern spike record in Boltysh is closely comparable to other K-Pg (Fig. 2), and transformed the Boltysh ejecta blanket vegetation, resulting in boundary examples, Boltysh did not have deposition of carbonaceous sed- a mosaic of early and early-mid successional communities of pteridacean iments or of common fungal spores (Vajda and McLoughlin, 2004), prob- ferns, Normapolles, and palms. Numbers of angiosperm and haploxylo- ably because the low biomass vegetation following the Boltysh impact noid pine pollen increase upsection (above 578.1 m), recording maturing and the subsequent period of little or no vegetation meant that there was community succession. This interval saw freshwater conditions return to insuffi cient rotting organic matter to support saprophytic organisms. the lake, marked by the infl ux of Botryococcus braunii. Comparison of the fern spike in the Boltysh record with the fi rst phase of recovery after the K-Pg in North America is supported by the CARBON ISOTOPE STRATIGRAPHY coincidence of the negative δ13C excursion in bulk organic matter with Bulk sedimentary carbon contents and isotopic compositions in the the infl ux of fern spores (although it postdates their earliest appearance). lowermost 5 m of sediments in the 42/11 core from immediately above the A δ13C excursion of similar magnitude (−1‰ to −2.8‰) is observed in suevite reveal a variation in wt% C and δ13C values upward through the terrestrial K-Pg sequences coincident with a fern spike in the Western section, the C content steadily increasing and marked changes recorded in Interior of North America (Schimmelmann and Deniro, 1984; Beerling δ13C values (Fig. 3). Carbon contents throughout the lowest 5 m of section et al., 2001). A similar excursion has been measured in a higher plant are low, indicating a low biomass within the crater lake drainage, and a biomarker from the marine Caravaca section, Spain (Arinobu et al., lack of deposition of carbonaceous sediments. 1999). In Boltysh, the negative δ13C excursion and adjacent fern spike Immediately above the suevite C contents are very low (<100 ppm); occur 0.9–1.2 m above the base of the barren zone, which is in turn inter- the mean δ13C value is −30.5‰. The C contents then rise slightly (100– preted as recording sedimentation after K-Pg event biotic devastation. 300 ppm between 581.9 m and 580.85 m; 0.0–1.05 m above base) and The erosion of metamorphic carbon from the proximal ejecta blanket is there is a concomitant positive 5.5‰ shift in δ13C values to a mean of recorded in the heavier δ13C values in this zone. Calculating an absolute −24.8‰ through the barren interval with signifi cant variability in δ13C duration for the total fern spike period in the Boltysh core is diffi cult values, probably as a result of a “nugget effect” caused by individual because sedimentation is cyclic, the duration being within two turbidite particles. From 580.9 m to 580.6 m a negative excursion in δ13C values units. However, it is unlikely to have exceeded 5 k.y., and is thus shorter (Fig. 3) at 580.7 m (1.2 m above base) to −28.9‰ occurs within sedi- than the 100 k.y. suggested for the equivalent interval in New Zealand ments, indicating an infl ux of fern spores. This is followed by a return to (Vajda and Raine, 2003), but it is comparable to the duration of early more positive values at 580.6 m (1.3 m above base) in sediments, indicat- successional vegetation on some volcanic terrains (Wolfe and Upchurch, ing a wider colonization of the ejecta blanket by Pteridaceae (Table 1). 1987; Chadwick et al., 1999). Above this, in sediments indicating mid-successional fl ora, carbon contents progressively increase with occasional spikes and δ13C values show IMPLICATIONS FOR CELESTIAL DYNAMICS AT THE K-PG low variability compared with the underlying sequences, and a mean BOUNDARY value of −27.7‰. A second negative δ13C excursion is apparent at 578.1 m The very short period of time, as little as 2–5 k.y., between two large (3.8 m above base) to −32.9‰. asteroid impacts on Earth close to the K-Pg boundary constrains the likely impactor delivery mechanism since it necessitates a high probability of DISCUSSION delivering several large bodies into the inner solar system within a few Borehole 42/11 records minor weathering of the Boltysh impact thousand years. Average cratering rates indicate that craters with diam- suevite prior to the formation of a crater lake and deposition of the oldest eter, D ≥ 20 km are formed on the land surface at a rate of 4 ± 2 every sediments (associated with a mean δ13C value of −30.5‰), accompanied 5 m.y. (Grieve and Pesonen, 1992), yielding a probability of 0.004 that by an early-mid successional community of ferns and angiosperms (Wing a 20 km crater would form somewhere on Earth in a 5 k.y. period. If the and Hickey, 1984). Parallels with inter-lava fl ow durations in large igne- total probability equals P(A ∩ B) (where P is probablilty, A is impact ous provinces (Jolley et al., 2008) and from modern lava fi elds (Vitousek, 1, and B is impact 2), then the probability of 2 craters forming within a 2004) suggest that such communities can occur in sedimentary interbeds 5 k.y. period is <0.001. Because the two impacts were not synchronous, of 2–5 k.y. duration. This comparison suggests that the interval from the the probability that they were a binary pair is signifi cantly reduced, and impact of the Boltysh meteorite to deposition of the earliest palynofl ora another mechanism is required for closely spaced large terrestrial impacts. observed would have been between 2 and 5 k.y. The destruction of this Various mechanisms have been proposed for impact clusters in the Eocene

TABLE 1. MEAN δ13C VALUES BY STRATIGRAPHIC INTERVAL AT THE BASE OF THE CORE Assemblage Description Mean δ13C value Core depth Mean difference in δ13C to previous zone (‰) (m) (‰)

5 Mid-succession angiosperms –27.7 ± 2.1 580.4–577.5 –1.2 (t55 = 0.97, p = 0.34)

4 Fern spike—ﬂ oodplain ferns –26.5 ± 0.4 580.6–580.4 –0.4 (t9 = 0.37, p > 0.5)

3 Fern spike—fern allies –26.1 ± 1.9 581.0–580.6 –1.2 (t20 = 1.68, p = 0.11)

2 Barren zone –24.9 ± 1.5 581.9–581.0 +5.5 (t16 = 7.3, p < 0.001) 1 Botryococcus braunii, Normapolles –30.45 ± 0.7 583–581.9 pollen, Platycaryapollenites, Polypodiaceous fern spores Note: t—number of samples; p—probability.

GEOLOGY, September 2010 837 (Mukhopadhyay et al., 2001), and the Ordovician (Schmitz et al., 1997), (El Kef, Tunisia): Marine Micropaleontology, v. 55, p. 1–17, doi: 10.1016/j focusing on a large collision in the asteroid belt during the Ordovician and .marmicro.2005.01.001. either a comet shower (Hut et al., 1987; Mukhopadhyay et al., 2001) or Hildebrand, A.R., Penfi eld, G.T., Kring, D.A., Pilkington, M., Camargo, A., Ja- cobsen, S.B., and Boynton, W.V., 1991, Chicxulub crater—A possible Cre- an asteroid shower (Claeys et al., 1992; Fritz et al., 2007) in the Eocene. taceous Tertiary boundary impact crater on the Yucatan Peninsula, Mexico: A comet shower is an unlikely explanation for the K-Pg given the Geology, v. 19, p. 867–871. global Ir anomaly and discovery of an asteroid fragment (Kyte, 1998), but Hut, P., Alvarez, W., Elder, W.P., Hansen, T., Kauffman, E.G., Keller, G., Shoe- the very short period between the Chicxulub and Boltysh impacts is also maker, E.M., and Weissman, P.R., 1987, Comet showers as a cause of mass extinction: Nature, v. 329, p. 118–126, doi: 10.1038/329118a0. diffi cult to explain using current models for asteroid showers. A model of Jolley, D.W., Widdowson, M., and Self, S., 2008, Volcanogenic nutrient fl uxes the likely spread of terrestrial impact ages from asteroids expelled from and plant ecosystems in large igneous provinces: An example from the Co- different resonance bands in the asteroid belt (Zappala et al., 1998) dem- lumbia River Basalt Group: Geological Society of London Journal, v. 165, onstrated that the resonance most likely to produce a short burst of aster- p. 955–966, doi: 10.1144/0016-76492006-199. oidal bodies is the J5:2 band (5 asteroid orbits per 2 orbits of Jupiter). Keller, G., Adatte, T., Stinnesbeck, W., Stuben, D., Berner, Z., Kramar, U., and Harting, M., 2004, More evidence that the Chicxulub impact predates the The J5:2 resonance band is thought to have been responsible for the rapid K/T mass extinction: Meteoritics & Planetary Science, v. 39, p. 1127–1144, delivery of many meteorites and possibly larger bodies during the Ordovi- doi: 10.1111/j.1945-5100.2004.tb01133.x. cian Period (Nesvorny et al., 2002). However, no large asteroid family has Kelley, S.P., and Gurov, E., 2002, Boltysh, another impact at the KT boundary: been identifi ed that might be related to the K-Pg boundary and an alterna- Meteoritics & Planetary Science, v. 37, p. 1031–1043, doi: 10.1111/j.1945 -5100.2002.tb00875.x. tive hypothesis that the K-Pg events were the result of a disruption of the Kring, D.A., 1997, Air blast produced by the Meteor Crater impact event and a re- Baptistina asteroid family close to the J7:2 band (Bottke et al., 2007) is construction of the affected environment: Meteoritics & Planetary Science, unlikely to have resulted in two near simultaneous impacts. v. 32, p. 517–530, doi: 10.1111/j.1945-5100.1997.tb01297.x. In summary, the evidence from sediment fi lling the Boltysh impact Kyte, F.T., 1998, A meteorite from the Cretaceous/Tertiary boundary: Nature, crater indicates that at least two large meteorite impacts, separated by as v. 396, p. 237–239, doi: 10.1038/24322. McGetchin, T.R., Settle, M., and Head, J.W., 1973, Radial thickness variation in little as 2–5 k.y., synchronous with the K-Pg boundary and mass extinc- impact crater ejecta: Implications for lunar basin deposits: Earth and Plan- tion, occured and resulted in one identifi able global layer. While there is etary Science Letters, v. 20, p. 226–236, doi: 10.1016/0012-821X(73)90162-3. strong evidence that they were asteroidal impacts, the celestial mechanism Mukhopadhyay, S., Farley, K.A., and Montanari, A., 2001, A short duration of responsible is as yet unclear. the Cretaceous-Tertiary boundary event: Evidence from extraterrestrial helium-3: Science, v. 291, p. 1952–1955, doi: 10.1126/science.291.5510.1952. Nesvorny, D., Morbidelli, A., Vokrouhlicky, D., Bottke, W.F., and Broz, M., 2002, REFERENCES CITED The Flora family: A case of the dynamically dispersed collisional swarm?: Alvarez, L.W., Alvarez, W., Asaro, F., and Michel, H.V., 1980, Extraterrestrial Icarus, v. 157, p. 155–172, doi: 10.1006/icar.2002.6830. cause for the Cretaceous-tertiary extinction: Science, v. 208, p. 1095–1108, Schimmelmann, A., and Deniro, M.J., 1984, Elemental and stable isotope varia- doi: 10.1126/science.208.4448.1095. tions of organic-matter from a terrestrial sequence containing the Creta- Alvarez, W., Asaro, F., and Montanaria, A., 1990, Iridium profi le for 10 million ceous-Tertiary boundary at York Canyon, New Mexico: Earth and Planetary years across the Cretaceous-Tertiary boundary at Gubbio: Science, v. 250, Science Letters, v. 68, p. 392–398, doi: 10.1016/0012-821X(84)90124-9. p. 1700–1702, doi: 10.1126/science.11538083. Schmitz, B., Peucker Ehrenbrink, E., Lindstrom, M., and Tassinari, M., 1997, Arinobu, T., Ishiwatari, R., Kaiho, K., and Lamolda, M.A., 1999, Spike of Accretion rates of meteorites and cosmic dust in the Early Ordovician: Sci- pyrosynthetic polycyclic aromatic hydrocarbons associated with an ence, v. 278, p. 88–90, doi: 10.1126/science.278.5335.88. δ13 abrupt decrease in C of a terrestrial biomarker at the Cretaceous- Smit, J., 1999, The global stratigraphy of the Cretaceous-Tertiary boundary im- Tertiary boundary at Caravaca, Spain: Geology, v. 27, p. 723–726, doi: pact ejecta: Annual Review of Earth and Planetary Sciences, v. 27, p. 75– 10.1130/0091-7613(1999)027<0723:SOPPAH>2.3.CO;2. 113, doi: 10.1146/annurev.earth.27.1.75. Batten, D.J., 1981, Statigraphic, paleogeographic and evolutionary signifi - Tappan, H., 1980, The paleobiology of plant protists: San Francisco, W.H. Free- cance of Late Cretaceous and early Tertiary normapolles pollen: Review man, 1028 p. of Palaeobotany and Palynology, v. 35, p. 125–137, doi: 10.1016/0034 Toon, O.B., Zahnle, K., Morrison, D., Turco, R.P., and Covey, C., 1997, Environ- -6667(81)90104-4. mental perturbations caused by the impacts of asteroids and comets: Re- Beerling, D.J., Lomax, B.H., Upchurch, G.R., Nichols, D.J., Pillmore, C.L., views of Geophysics, v. 35, p. 41–78, doi: 10.1029/96RG03038. Handley, L.L., and Scrimgeour, C.M., 2001, Evidence for the recovery of Vajda, V., and McLoughlin, S., 2004, Fungal proliferation at the Cretaceous- terrestrial ecosystems ahead of marine primary production following a bi- Tertiary boundary: Science, v. 303, p. 1489–1489, doi: 10.1126/science otic crisis at the Cretaceous-Tertiary boundary: Geological Society of Lon- .1093807. don Journal, v. 158, p. 737–740. Vajda, V., and Raine, J., 2003, Pollen and spores in marine Cretaceous/Tertiary Bottke, W.F., Vokrouhlicky, D., and Nesvorny, D., 2007, An asteroid breakup boundary sediments at mid-Waipara River, North Canterbury, New Zea- 160 Myr ago as the probable source of the K/T impactor: Nature, v. 449, land: New Zealand Journal of Geology and Geophysics, v. 46, p. 255–273. p. 48–53, doi: 10.1038/nature06070. Vitousek, P.M., 2004, Nutrient cycling and limitation: Hawai’i as a model system: Chadwick, O.A., Derry, L.A., Vitousek, P.M., Huebert, B.J., and Hedin, L.O., Princeton, New Jersey, Princeton University Press, 223 p. 1999, Changing sources of nutrients during four million years of ecosystem Wing, S.L., and Hickey, L.J., 1984, The Platycarya perplex and the evolution of development: Nature, v. 397, p. 491–497, doi: 10.1038/17276. the juglandaceae: American Journal of Botany, v. 71, p. 388–411. Claeys, P., Casier, J.G., and Margolis, S.V., 1992, Microtektites and mass ex- Wolfe, J.A., and Upchurch, G.R., 1987, Leaf assemblages across the Cretaceous- tinctions—Evidence for a Late Devonian asteroid impact: Science, v. 257, Tertiary boundary in the Raton Basin, New Mexico and Colorado: National p. 1102–1104, doi: 10.1126/science.257.5073.1102. Academy of Sciences Proceedings, v. 84, p. 5096–5100, doi: 10.1073/ Collins, G.S., Melosh, H.J., and Marcus, R.A., 2005, Earth Impact Effects Pro- pnas.84.15.5096. gram: A Web-based computer program for calculating the regional environ- Zappala, V., Cellino, A., Gladman, B.J., Manley, S., and Migliorini, F., 1998, As- mental consequences of a meteoroid impact on Earth: Meteoritics & Plan- teroid showers on Earth after family breakup events: Icarus, v. 134, p. 176– etary Science, v. 40, p. 817–840, doi: 10.1111/j.1945-5100.2005.tb00157.x. 179, doi: 10.1006/icar.1998.5946. Fritz, J., Tagle, R., and Artemieva, N., 2007, Lunar helium-3 in marine sediments: Implications for a late Eocene asteroid shower: Icarus, v. 189, p. 591–594, doi: 10.1016/j.icarus.2007.03.026. Manuscript received 18 January 2010 Grieve, R.A.F., and Pesonen, L.J., 1992, The terrestrial impact cratering record: Revised manuscript received 26 April 2010 Tectonophysics, v. 216, p. 1–30, doi: 10.1016/0040-1951(92)90152-V. Manuscript accepted 30 April 2010 Guasti, E., Kouwenhovenb, T.J., Brinkhuis, H., and Speijerd, R.P., 2005, Paleo- cene sea-level and productivity changes at the southern Tethyan margin Printed in USA

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