ГЕОХИМИЯ, 2011, № 1, с. 59-70
DISTAL "IMPACT" LAYERS AND GLOBAL ACIDIFICATION OF OCEAN WATER AT THE CRETACEOUS-PALEOGENE BOUNDARY (KPB)
© 2011 P. I. PremoviC
Laboratory for Geochemistry, Cosmochemistry and Astrochemistry, University of Nis, P.O. Box 224, 18000Nis, Serbia e-mail: pavle.premovic@yahoo.com Поступила в редакцию 18.05.2009 г.
The KPB sections at Hojerup in Denmark, Agost and Caravaca in Spain and El Kef in Tunisia and (elsewhere in the world) consists of a very thin reddish biogenic calcite-poor smectite-rich "impact" layer overlain by a thicker smectite-rich marl. The massive amount of impact-generated atmospheric CO2 at KPB would have accumulated globally in the ocean surface, leading to acidification and CaCO3 undersaturation. These chemical changes would have induced a low biocalcification of calcareous plankton and a high dissolution of their shells. The biocalcification/dissolution crises may have played a significant role for the low abundance of biogenic calcite in the "impact" layer of the marine boundary clays at Hojerup, Agost, Caravaca and El Kef (and elsewhere in the world). Experimental data and observations indicate that the deposition of the "impact" layer probably lasted only a few decades at most.
Key words: Cretaceous-Paleogene, "impact" layer, global acidification, ocean water.
INTRODUCTION
Alvarez et al. [1] explained anomalous Ir in the marine KPB clays at three localities (Gubbio in Italy, Hojerup, and Woodside Creek in New Zealand) by proposing a late Cretaceous asteroid impact. This suggestion was followed by reports of the prominent Ir anomaly in many other marine boundary clays around the world. These clays mark one of the most significant impact events in the Phanerozoic, which was probably largely responsible for one of the great extinctions in Earth history.
Many researchers consider that an extraterrestrial im-pactor (ca. 10 km in diameter) formed the ca. 180 km crater at Chicxulub (Yucatan Peninsula, Mexico, Fig. 1) at the KPB. It has been suggested that the impactor was a carbonaceous chondrite-type body [2—6].
In most marine sites now on land, the boundary clay is easily identified based on one or more of the following: (1) a lithology break from the latest Maastrichtian sediment abundant in calcareous plankton to a thin layer of clay-rich deposits (here termed the boundary clay), extremely poor in calcareous plankton; (2) a 2 to 3-mm usually reddish goethite-rich layer (known as the "impact layer", "fireball layer" and the "ejecta layer" at the base of the boundary clay; or (3) anomalously high Ir values, generally concentrated in this basal "impact" layer [7]. Ir in the boundary clay is considered to be mainly of mete-oritic origin [8]. The onset of the marine boundary clay is
coincident with the sudden mass extinction at the KPB, major negative carbon isotopic (8C13) excursion, and a drop in biogenic calcite. Usually, the first appearance of Daman calcareous plankton is found near the base of the boundary clay (the P0 biostratigraphic zone) [9].
In the earliest and most popular scenario of the KPB impact event, the distinct basal "impact" layer of the marine boundary clays was created by thousands of cubic kilometers of impact ejecta dispersed globally and deposited for less than a year [1, 10]. Accordingly, ejecta fallout originated from an impact plume ofa vaporized impactor and target rocks ejected into the stratosphere. The widespread geographical distribution of the "impact" layers at the marine KPB sites throughout the world appears to be strong evidence for such a scenario.
In the following paper, I discuss some of the available data obtained by experimental studies of the marine boundary clays with an "impact" layer at Hojerup, Agost, Caravaca and El Kef (and elsewhere in the world) in order to support the GAOS (Global Acidification of the Ocean Surface) instigated by the KPB impact. The boundary clay sites are distal (>9000 km) to the proposed Chicxulub impact site (Fig. 1). The potential of ocean surface acidification at KPB has been discussed by many authors [e.g., 11—15].
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EXPERIMENTAL DATA AND OBSERVATIONS
Marine boundary clays
The Fish Clay (of earliest Danian age) near the village ofHojerup is a classic KPB lithological unit in Denmark. Basal "impact" layer is a thin layer (2—4 mm thick), overlain by the black-to-grey marl (Fig. 2). These two layers are considered to constitute the main part of the Fish Clay [16, 17]. Notable, thicker (ca. 45 cm) boundary clay with a basal "impact" layer is also found in the Fish Clay at Kulstirenden, another site of Stevns Klint, 7 kilometers from Hojerup [18].
The "impact" layer is sharply underlain by the latest Maastrichtian bryozoan-rich chalk. This layer, mainly made of smectite (>90%), contains goethite-rich micro-spherules, altered nano-size glasses and nano-size goet-hite grains [19, 20]. Bauluz et al. [19] interpreted the go-ethite grains as altered meteorite fragments.
Black-to-grey marl is smectitic and rich in pyritic mi-crospherules, but they are much less abundant than the goethite spherules in the "impact" layer [16]. The marl is overlain by the lowermost Danian Cerithium limestone. The Fish Clay bears many similarities to the records at the marine boundary clays on land now, except that Ir is enriched at the base of the overlying smectite-rich marl, not in the underlying "impact" layer.
Like at Hojerup, at Agost, Caravaca and El Kef the biogenic calcite-rich sediments of the latest Maastrichtian are sharply capped by a reddish "impact" layer (Fig. 2). The KPB at Agost and Caravaca is marked by about 10—12 cm thick dark smectite-rich layer [21, 22] with the 2—3 mm thick "impact" layer [22, 23]. Cos-mogenic Ir and microspherules are mainly confined to this layer. Goethite microspherules at Agost are more abundant than K-feldspar microspherules; in contrast, at Caravaca K-feldspar microspherules are abundant but goethite ones are rare [23]. At El Kef the boundary clay is 55—65 cm thick with the 2-3 mm "impact" layer [9]; this layer contains most of Ir and Ni-rich spinels [24]. Smectite is the main component of the boundary clay at this site [21].
The sections at Agost, Caravaca and El Kef are among the most continuous and complete marine sections for the KPB transition. In addition, the base of the El Kef section has been officially designated as the boundary global stratotype section and point (GSSP) for the KPB [25, 26].
Other continuous and complete marine sections at the distal sites (Fig. 1) are characterized by the boundary clays with a basal "impact" layer (Fig. 2): in Italy (at Gubbio, Forada Creek), Tunisia (Elles and Ain Settara), Spain (Zumaya, Sopelana, Monte Urko) and France
Fig. 2. Correlations of the KPB sections discussed in the appropriate parts of the paper.
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Fig. 3. Distribution of biogenic calcite (as CaCO3, %) across the Fish Clay [16, 38, 39].
(Bidart) [1, 9, 22, 27-34]. The "impact" layer of these sections provides an excellent record of the distal ejecta facies related to the Chicxulub impact
The KPB has been identified in numerous marine sections in the New Zealand region, including well-studied Woodside Creek [35]. Only one of these appears to have near-complete KPB records at Flaxbourne River (Fig. 1). The boundary clay at this location contains the geochemical record of the impact but it is without an "impact" layer [36].
Stratigraphically, the above marine boundary clays correspond to the continental boundary clays with an "impact" layer of the western interior of North America described by [37]. Anomalous Ir and microspherules also occur in these continental clays.
Biogenic calcite distribution
A microscopic examination across the Fish Clay shows that abiotic calcite precipitation is only a minor contributor to total calcite production [7, 38-40]. This also shows that the transition from the calcareous Maas-trichtian bryozoan-rich ooze to the "impact" layer is extremely abrupt and biogenic calcite is almost completely
absent in this layer. The concentration profile of biogenic calcite (as CaCO3) across the Fish Clay is presented in Fig. 3. The concentrations are high in the latest Maas-trichtian bryozoans-rich ooze but decrease sharply in the "impact" layer, reaching a minimum. Up from this layer, the biogenic calcite concentrations increase gradually to much higher levels, reaching the pre-KPB level in the Cerithium limestone.
A similar pattern is observed in the carbonate (mainly biogenic calcite) distribution across the KPB sequences at Agost, Caravaca and El Kef; and, at the other marine settings in Europe (Zumaya, Monte Urko, Sopelana, Gubbio, Forada Creek, Bidart), Africa (Elles, Ain Settara) and New Zealand (Flaxbourne River) (Fig. 4). The carbonate contents of the boundary clays at these sites also decreased considerably at the KPB.
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