Meteorite flux to Earth in the Early Cretaceous as reconstructed from sediment-dispersed extraterrestrial spinels Birger Schmitz1,2*, Philipp R. Heck2, Walter Alvarez3,4, Noriko T. Kita5, Surya S. Rout2, Anders Cronholm1, Céline Defouilloy5, Ellinor Martin1, Jan Smit4,6, and Fredrik Terfelt1 1Astrogeobiology Laboratory, Department of Physics, Lund University, SE-22100 Lund, Sweden 2Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, Illinois 60605, USA 3Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA 4Osservatorio Geologico di Coldigioco, Contrada Coldigioco 4, 62021 Apiro, Italy 5WiscSIMS, Department of Geoscience, University of Wisconsin–Madison, Madison, Wisconsin 53706, USA 6Department of Sedimentary Geology, Vrije Universiteit, 1081 HV Amsterdam, Netherlands ABSTRACT to 470 ± 6 Ma (Korochantseva et al., 2007), but the most precise relative We show that Earth’s sedimentary strata can provide a record of date is given by an abrupt two-orders-of magnitude increase worldwide the collisional evolution of the asteroid belt. From 1652 kg of pelagic in sand-sized L-chondritic chromite grains in mid-Ordovician sediments Maiolica limestone of Berriasian–Hauterivian age from Italy, we (Schmitz et al., 2003; Heck et al., 2016). recovered 108 extraterrestrial spinel grains (32–250 μm) represent- By dissolving 100-kg-sized samples of condensed sediments from ing relict minerals from coarse micrometeorites. Elemental and three different time periods in various acids, the highly refractory extraterres- oxygen isotope analyses were used to characterize the grains, provid- trial spinel minerals can be concentrated. The recovered grains typically ing a first-order estimate of the major types of asteroids delivering contain high concentrations of solar-wind noble gases, indicating that material at the time. Comparisons were made with meteorite-flux they dominantly represent fragments of coarse micrometeorites (Heck time “windows” in the Ordovician before and after the L-chondrite et al., 2008). Pelagic carbonates are the best material for sampling the parent-body breakup. In the Early Cretaceous, ~80% of the extra- population of extraterrestrial chromite grains, because of their low con- terrestrial spinels originated from ordinary chondrites. The ratios tent of detrital minerals that obscure the extraterrestrial fraction and the between the three groups of ordinary chondrites, H, L, LL, appear ease with which they can be dissolved in acid. As a part of a larger effort similar to the present, ~1:1:0.2, but differ significantly from Ordo- to create “windows” into the meteorite flux to Earth at different times vician ratios. We found no signs of a hypothesized Baptistina LL- during the Phanerozoic (Schmitz, 2013), we focus here on a part of the chondrite breakup event. About 10% of the grains in the Maiolica Lower Cretaceous (145–133 Ma) Maiolica limestone in central Italy. This originate from achondritic meteorite types that are very rare (<1%) pelagic limestone is exceptionally “clean,” i.e., having very low contents on Earth today, but that were even more common in the Ordovician. of terrestrial, detrital mineral grains, making it useful for reconstructions Because most meteorite groups have lower spinel content than the of the micrometeorite flux even in the small spinel size ranges, such as the ordinary chondrites, our data indicate that the latter did not domi- 32–63 μm range primarily used here. From a total of 1652 kg of limestone nate the flux during the Early Cretaceous to the same extent as today. from the earliest Berriasian to early Hauterivian Maiolica Formation, we Based on studies of three windows in deep time, we argue that there recovered 108 extraterrestrial spinel grains (Fig. 1). Using three oxygen may have been a gradual long-term (a few hundred million years) isotope and elemental analyses of the grains, we obtained the very first turnover in the meteorite flux from dominance of achondrites in the insights into what types of meteorites fell on Earth at times other than today early Phanerozoic to ordinary chondrites in the late Phanerozoic, and in the mid-Ordovician. These data can be used to test and develop interrupted by short-term (a few million years) meteorite cascades models on the dynamics of meteorite transport from the asteroid belt to from single asteroid breakup events. Earth and how the asteroid belt has evolved over time. For example, here we add perspectives on Bottke et al.’s (2007) hypothesis, based on astro- INTRODUCTION nomical data, that an ~170-km-diameter asteroid broke up between 190 Much knowledge about the history of life, climate, tectonics, magnetic and 140 Ma, leading to the formation of the Baptistina asteroid family, polarity, and chemistry of seawater has accumulated during the past two one of the youngest major asteroid families in the inner main asteroid belt. centuries from studies of Earth’s sedimentary strata. With the discovery of the asteroid impact at the Cretaceous-Paleogene (K-Pg) boundary and MATERIALS AND METHODS its effects on life (Alvarez et al., 1980), an interest has grown in integrat- Samples were collected from 12 beds in two stratigraphically sepa- ing astronomical and geological perspectives. A new approach that can rated groups along the 240-m-thick Monte Acuto section of the Maiolica relate ancient events in space to coeval events on Earth is the search for limestone in central Italy (43°27.83′N, 12°40.27′E). We collected a total of relict spinel minerals from micrometeorites and meteorites in condensed 513 kg from four beds in the Berriasian part of the section, and 1015 kg sediments (Schmitz, 2013). The method has so far been primarily applied from eight beds in the late Valanginian to early Hauterivian part (Fig. 1). in reconstructions of the Ordovician L chondrite parent-body breakup The size of the individual samples varied between 103 and 433 kg (plus (LCPB), the largest documented collisional event in the asteroid belt in one sample of 27 kg). One additional sample from the Bosso section, 12 the past 3 b.y. (Schmitz et al., 2003). This breakup probably led to the for- km northwest of Monte Acuto, weighed 124 kg. The rocks were dissolved mation of one of the major asteroid families (Nesvorný et al., 2009). The in HCl (6 M) and HF (11 M) at room temperature in the Astrogeobiology event has been dated by 40Ar-39Ar analyses of recently fallen L chondrites Laboratory specially built for separation of extraterrestrial minerals from sediments. After sieving at mesh sizes 32 and 63 μm, opaque Cr-spinel *E-mail: [email protected] grains were identified by handpicking under the binocular microscope and GEOLOGY, September 2017; v. 45; no. 9; p. 807–810 | Data Repository item 2017273 | doi:10.1130/G39297.1 | Published online 10 July 2017 ©GEOLOGY 2017 The Authors.| Volume Gold 45 |Open Number Access: 9 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 807 Number of most Cr-spinels from meteorites are rich in V, whereas V-rich (>0.5 wt%) Meters Age ** Sampled EC grains 250 beds no terrestrial Cr-spinels are generally rare. The OC-V and OC grains were 132 Ma found (32-63 µm) and sample size further subdivided based mainly on their oxygen isotopic composition. Ammonite 0 10 20 The three groups of ordinary chondrites, H, L, and LL, have different zones* 133 Ma 17 A. radiatus 133.9 Ma 415 104 kg average values of Δ O (0.73‰, 1.07‰, and 1.26‰, respectively; Clay- Hauterivian 200 406 110 kg 134 Ma ton et al., 1991) and TiO2 (2.2, 2.7, and 3.4 wt%, respectively; Schmitz, N. (T.) 366+367 128 kg 17 S 2013). Around these averages, the O and TiO values are spread fol- pachydicranus 349 27 kg Δ 2 ginian 135 Ma 335 433 kg lowing a Gaussian distribution, but the distributional tails overlap (Heck S. verrucosum alan 150 285 108 kg et al., 2016). The exact ranges for dividing grains based on TiO2 and/or V 136 Ma 270 106 kg 17 tion U. Δ O can in principle be arbitrarily set, but they must be used consistently c ?B. campylotoxus when comparing different time periods. 137 Ma Lithology: o se t inian T. pertransiens a The Maiolica is entirely u T A C E O U g 100 M 138 Ma composed of white RESULTS Ac e alan pelagic limestone in Among the total of 108 extraterrestrial Cr-spinel grains (32–250 μm t T. otopeta V 139.4 beds 5-50 cm thick, with L. 139 Ma large) recovered, 81 are clearly equilibrated ordinary chondritic (EC), and 27 ?? on abundant nodules and M are vanadium-rich grains (OC-V), probably originating from other types of F. boissieri 50 140 Ma beds of black chert. meteorites (Table 1; Tables DR1 and DR2). In the >63 μm fraction, only two *Faraoni et al. extraterrestrial grains were found; both were EC grains. In the 32–63 μm 141 Ma (1997) 39+42 103 kg fraction, based on element analyses alone, we found 79 grains with clear EC W E R C 40 201 kg 0 142 Ma 36 209 kg elemental composition. Oxygen-isotopic analysis was performed on 46 of L O these grains, confirming that they are all ordinary chondritic grains (Fig. 2). **Rough age Berriasian scale based on 143 Ma The division of the 81 recovered EC grains into their three subgroups, stage bounda- based entirely on their TiO content, is given in Table 2. The reason tion 2 ries at 133.9 144 Ma c for using TiO2 is the substantially larger data set for grains having been and 139.4 Ma 145.0 Ma analyzed for TiO compared to 17O.