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Iridium Anomaly and Extraterrestrial Component in the Clays at the Cretaceous-Paleogene Boundary in Denmark, Spain and New Zealand

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Iridium Anomaly and Extraterrestrial Component in the Clays at the Cretaceous-Paleogene Boundary in Denmark, Spain and New Zealand Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/125 Comunicações Geológicas (2012) 99, 2, 27-34 ISSN: 0873-948X; e-ISSN: 1647-581X Iridium anomaly and extraterrestrial component in the clays at the Cretaceous-Paleogene boundary in Denmark, Spain and New Zealand Anomalia de irídio e componente extraterrestre das argilas do limite Cretácico – Paleogénico na Dinamarca, Espanha e Nova Zelândia P. I. Premović1*, B. S. Ilić2 Recebido em 28/09/2011 / Aceite em 11/12/2011 Artigo original Disponível online em Janeiro de 2012 / Publicado em Dezembro de 2012 Original article © 2012 LNEG – Laboratório Nacional de Geologia e Energia IP Abstract: The Cretaceous-Paleogene boundary clays at Højerup been identified and studied at about 345 sites worldwide. Of these, (Denmark), Caravaca (Spain) and Woodside Creek (New Zealand) show 85 sites, representing all depositional environments, contain an Ir anomalous enrichments of iridium compared with the marine sedimentary anomaly. In general, Ir and other platinum-group elements (PGE: rocks. For the average iridium content of 465 ppb of CI carbonaceous collectively the elements Ru, Rh, Pd, Os, Ir and Pt) are invariably chondrite the estimate of CI chondrite proportions in the decarbonated iridium-rich boundary layers, based on the integrated iridium fluencies, is enriched in the prominent boundary clays. Other trace elements (e. about 25 % at Højerup, 15 % at Caravaca and > 30 % at Woodside Creek. g. heavy metals) are also relatively abundant in these clays. These proportions are most likely too high due to a significant Ir influx Many researchers consider that the KPB impactor formed the from the nearby marine or continental site to these sections. ca. 180 km crater at Chicxulub (Yucatan Peninsula, Mexico, Fig. Keywords: Fish Clay, Caravaca, Woodside Creek, Iridium, 1). It has been suggested that the impactor was a carbonaceous Carbonaceous chondrite. chondrite projectile - probably type CI (Kyte, 1998; Shukolyukov & Lugmair, 1998; Frei & Frei, 2002; Quitté et al., 2003; Trinquier Resumo: As argilas do limite Cretácico – Paleogénico em Højerup et al., 2006), though it is still unclear whether it was a (Dinamarca), Caravaca (Espanha) e Woodside Creek (Nova Zelândia) carbonaceous chondrite or a comet. Indeed, comets are believed to mostram enriquecimentos anómalos de irídio comparados com rochas be primitive bodies with a composition like that of carbonaceous sedimentares marinhas. Para o conteúdo médio de irídio de 465 ppb do chondrites (Gelinas et al., 2004). The anomalous Ir associated, condrito carbonáceo CI, a estimativa das proporções condríticas CI nas camadas descarbonatadas ricas em irídio da zona de limite, baseado nas however, with the prominent boundary clay is consistent with the fluências integradas de irídio, é cerca de 25% em Højerup, 15 % em high Ir content typical of most chondritic meteorites and Caravaca e > 30% em Woodside Creek. Estas proporções demasiado inconsistent with the general proposition of comets. Indeed, a elevadas são provavelmente devidas a influxo significativo de Ir a partir simple calculation shows that in the case of the ice-rich (>70 %) das regiões marinhas ou continentais próximas destas secções. comets the amount of Ir produced by an impact energy for a crater Palavras-chave: Argila, Caravaca, Woodside Creek, irídio, condrito of the Chicxulub size could be less than 0.001 % then that of an carbonáceo. asteroid. In the past many researchers used the iridium concentration and/or integrated amount of Ir to estimate the proportion of 1Laboratory for Geochemistry, Cosmochemistry and Astrochemistry, University of Niš, P.O. Box 224, 18000 Niš, Serbia. chondritic component in the renowned boundary clays. This paper 2Department of Pharmacy, Faculty of Medicine, University of Niš, 18000 Niš, aims to re-examine this method using comprehensive Ir data for Serbia. the boundary sections at Højerup, Caravaca and Woodside Creek *Corresponding author / Autor correspondente: [email protected] which are available and published by Schmitz (1988). These stratigraphic sections are well preserved and distal (paleodistance: >7000 km) to the proposed Chicxulub impact site. An Ir analysis 1. Introduction of the decarbonated KPB samples from Denmark, Spain and New Zealand was carried out by Schmitz using instrumental neutron In the original paper Alvarez et al. (1980) have first reported activation analysis (INAA). Relative error in the precision of the anomalously high Ir concentrations in the Cretaceous-Paleogene analyses ranges from 5 % to 10 %. Total uncertainties (including (KPB) boundary clays at Gubbio (central Italy, Fig. 1), Højerup accuracy errors) were up to 20 %. (the eastern Denmark, Fig. 1) and at Woodside Creek (the For the sake of completeness, the Ir data related to the distal northern part of the South Island of the New Zealand, Fig. 1). KPB sections at Agost (Spain), Flaxbourne River (New Zealand), They proposed an impact of extraterrestrial bolide to explain the Gubbio (Italy), Bidart (France), El Kef and Aïn Settara (Tunisia), elevated Ir content at the KPB. Alvarez et al. also suggested that and in Ocean Drilling Program (ODP) Hole 738C (Kerguelen approximately 60 times the mass of the impactor would have been Plateau, southern Indian Ocean) (Fig. 1) are also briefly discussed. ejected into the atmosphere as impact dust. Almost simultaneously Although the sections studied systematically show an Ir with Alvarez et al., Smit & Hertogen (1980) reported an abundance anomaly, they differ from one another because local anomalous Ir in the boundary clay at Caravaca (southeastern sedimentation conditions under a strong continental influence. Spain, Fig. 1). Since the Alvarez et al. discovery, the KPB has Throughout this paper we make five reasonable postulates: (a) Ir 28 P.I. Premović et al. / Comunicações Geológicas (2012) 99, 2, 27-34 is wholly located in the non-carbonate fraction of the boundary carbonaceous chondrites typically contain 406.0 - 849.4 ppb Ir clays studied, i. e. the carbonate fraction of this section is (Table 1). So, a little addition of the CC component would be essentially an Ir diluents; (b) the Ir content of the carbonaceous necessary to critically increase the concentration of Ir in a chondrites range from 406.0 ppb to 849.4 ppb (Table 1); (c) the marine sedimentary rock. Thus, Ir is a very sensitive means of average content of Ir in CI chondrites is 465 ppb (Table 1); (d) all examining the CC contribution to the marine KPB clays such as Ir found in the boundary clays originated from the carbonaceous the Fish Clay. Some authors suggested that Ir in the Fish Clay chondrite (CC) impactor, especially of type CI; and (e) assumed was sourced by the Ir from seawater (Goldberg et al., 1986). density of the boundary clays is about 2 g cm-3. However, the average Ir concentration in the decarbonated BH (ca. 100 ppb, see below) represents an enrichment factor of about 1017 compared to seawater (ca. 2×10-15 ppb, Table 1). Moreover, according to these authors, the residence time of Ir in the oceans is about 1 million years. Thus, excess of Ir in the BH could not, therefore, have been derived from the sea reservoir. Table 1. Concentrations of Ir [ppb] for carbonaceous chondrites, marine sediments and seawater. Tabela 1. Concentrações de Ir [ppb] para os condritos carbonáceos, sedimentos marinhos e água do mar. Fig.1. Geographic location of studied KPB clays. Fig.1. Localização geográfica das argilas KPB estudadas. 2. Results and Discussion Fish Clay. The lowermost Danian Fish clay Member of the Rødvig Formation near the village of Højerup is a classic marine KPB section. The lithology of the Fish Clay within this boundary section characterizes three distinctive layers (from bottom to top): a 3 cm thick (mainly black-to-dark) marl (hereinafter BH) with 0.5 cm-thick basal red (goethite-rich) sublayer, grey-to-brown marl and a light-grey marl (Fig. 2). The red sublayer is underlain by (the latest) Maastrichtian bryozoan-rich limestone (chalk) whereas the top marl is overlain by (early) Danian Cerithium limestone (Schmitz, 1988; Christensen et al., 1973; Schmitz, 1985; Elliott, 1993; Surlyk et al., 2006). Geochemical studies show that the Ir profile (on a whole rock basis) across the Fish Clay column is characterized by a maximum just above the base of BH with an upward gradual decrease (tailing-off) from its maximum (e. g. Schmitz, 1985). This gradual tailing off indicates that the Fish Clay Fig.2. Expanded lithological log of the Fish Clay at Højerup based on Surlyk et al. represent a relatively continuous and complete section. The BH (2006). is considered to constitute the main part of the boundary section, since it contains more than 95 % of the total Ir in the Fig.2. Log litológico expandido das argilas em Højerup baseado em Surlyk et al. Fish Clay (Premović, 2009). The mineralogy of BH is (2006). comparatively simple, smectite and authigenic (mainly biogenic) calcite being the principal components. Geochemical Kyte et al. (1985) estimated that the extraterrestrial component evidence indicates that the BH was deposited under strong in the basal 1 cm part of BH from the measured Ir concentration anoxic conditions but the red sublayer under strong oxic (47.4 ppb) is about 10.5 %, corrected for about 20 % carbonate sedimentation conditions (Premović, 2009). content. According to Grieve (1997), chondrite-normalized relative Premović et al. (2000) inferred that the BH was deposited abundance of Ir indicates that the Fish Clay (on a whole-rock basis) in a shallow marine <100 m water depth at Stevns Klint within contains an admixture of about 10 % chondrite. Trinquier et al. an interval of about 40 years. Wendler & Willems (2002) (2006) have shown that Cr isotopic signature of BH represents a considered that this layer represents the first decades or mixing of the CC matter with terrestrial material in a ratio up to 6.8 centuries of deposition following the KPB impact event.
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