RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES6006, doi:10.2205/2008ES000304, 2008

Spatial Distribution of Metallic Iron at the K/T Boundary

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Figure 1
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Figure 2
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Figure 3
[7]  The distribution of metallic iron was studied in the narrowest time interval that is represented by the transitional clay layer at the K/T boundary, which had been accumulating for several thousand years [Grachev et al., 2005]; (D. M. Pechersky et al., in press, 2008a,b). The content of metallic iron in this layer from the five separated sections varies widely: it is absent or, more precisely, has not been detected by TMA, in 19 samples out of 28 studied and varies from 0.0001 to 0.002% in the remaining nine samples [Grachev et al., 2005; Molostovsky et al., 2006; Pechersky, 2008; Pechersky et al., 2006a, 2006b]; (D. M. Pechersky et al., in press, 2008a,b). A detailed imbedded study of the boundary layer from the Gams section showed that metallic iron is present only in the upper and lower parts of this layer (Figure 1). Spherules of metallic nickel were also found in the upper part of the Gams section [Grachev et al., 2005]. The correlation between main magnetic and paramagnetic components of
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Figure 2
sediments from the boundary layer is presented in Figure 2. (It is worth noting that this and the following figures use the logarithmic scale as this notation better suits the approximately lognormal distributions of concentrations of minerals, elements, etc.). The points in Figure 2 form two groups thus highlighting the lack of correlation between metallic iron on one hand and Fe-hydroxides, magnetite, and titanomagnetite on the other and reflecting their different provenance and accumulation history: a) samples where metallic iron, Fe-hydroxides, magnetite, and titanomagnetite are present, and b) samples without metallic iron but with Fe-hydroxides and/or magnetite and titanomagnetite. Thus, certain regularity can be traced: while boundary layer of clay and Fe-hydroxides was accumulating close to the K/T boundary for several thousand years, metallic iron did not accumulate. In contrast, the maximum of cosmic dust accumulation straddles this boundary in sediments from column 596 DSDP in the Pacific (Figure 3) [Peucker-Ehrenbrink, 1996]. Therefore, our data on metallic iron presence in the boundary layer disagree with cosmic dust accumulation close to the K/T boundary. It is worth to ponder on this controversy. The annual global fall of interplanetary dust of ~40times109 g corresponds to accumulation rate of ~0.08times10-9 g cm-2. The accumulation rate of pelagic sediments is 1-2 mm ky-1. A sample of several millimeters in thickness that is needed for TMA had been forming for several thousand years at least. During this interval, the total of ~0.3times10-6 g will accumulate, while the total value for the anomalous sample from the K/T boundary will be by an order of magnitude more, i.e., ~10-5 g. As exemplified by the Gams data, the concentration of metallic iron generally increases close to the K/T boundary and varies from zero to ~0.002%, ~0.001% on average, over an interval of ~20 ky ( pm
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Figure 4
20 cm from the boundary, Figure 4). This corresponds to ~10-6 g for a TMA sample of 0.1-0.2 g. In the upper part of the boundary layer, the sum of metallic iron and nickel is not less than ~0.005%, which is close to 10-5 g [Grachev et al., 2005]. Available data indicate that fragments of chondrites, particles of silicates as well as glass with inclusions of metals and sulfides clearly predominate in the interplanetary dust, while metallic iron and nickel are much rarer. Thus the controversy disappears if a sample with anomalous content of cosmic dust belongs from the boundary clay layer. Note, however, that two adjacent samples of sediments from the hole 596 DSDP show the elevated concentrations of cosmic dust too. It is most likely that a long time interval is averaged during preparation of the probe for analysis. If so, this may partly account for the stability of cosmic dust flux to the Earth over geological time that was noted by many researchers. In contrast to publications on cosmic dust, TMA data provide a much more detailed pattern. For instance, 28 and 50 measurements of metallic iron content were made on the boundary layer alone and within pm 20 cm from the K/T boundary, respectively (Figure 4), whereas there are just 19 analyses of cosmic dust in pelagic sediments over the huge interval of 80 My (Figure 3) [Peucker-Ehrenbrink, 1996]. Let us consider the pm 20 cm interval around the K/T boundary, which is likely to better match averaged data on the Pacific sediments. The concentration of metallic iron varies by one to two orders of magnitude and more for four sections that are 1000 to 5000 km from each other. There are intervals where metallic iron is absent (not determined), for instance, in Danian sediments of the Teplovka and Klyuchi sections; in contrast, metallic iron content reaches 0.004% in uppermost Maastrichtian deposits of the Teplovka section. Therefore, the distribution of metallic iron over the Earth surface was very non-uniform close to the K/T boundary.


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Citation: Pechersky, D. M. (2008), Metallic iron in sediments at the Mesozoic-Cenozoic (K/T) boundary, Russ. J. Earth Sci., 10, ES6006, doi:10.2205/2008ES000304.

Copyright 2008 by the Russian Journal of Earth Sciences

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