RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6001, doi:10.2205/2005ES000189, 2005

Isotopic Compositions of Helium, Carbon, and Oxygen

[87]  Helium isotopic systematics was studied in samples from transitional layer J, in both whole-rock samples and clayey fractions. As follows from Table 13, the 3He/4He ratio in the whole-rock samples and clayey fractions is also lower than the atmospheric one (1.39 times 10-6 ), whereas this value of abyssal oceanic sediments is two orders of magnitude higher [Krylov et al., 1973]. The high 3He/4He ratios of the latter sediments are caused by the addition of extraterrestrial material at an extremely low sedimentation rate. In our samples from layer J of the Gams succession, the fraction of primary solar 3He is very low, because the material of this layer was provided by the erosion of the adjacent source area, whose rocks were enriched in radiogenic He.

[88]  As we possessed only very little material, we failed to obtain enough heavy fractions to examine the He isotopic composition of the uppermost part of layer J, which contains beads of metallic Ni and diamond grains (see below). This task will be hopefully accomplished in our further research. It is particularly interesting to study the He isotopic composition of diamond, because the nature of this mineral in transitional layers at the Cretaceous-Paleogene boundary remains obscure as of yet.

[89]  The isotopic compositions of carbon ( 13C/12C or d13C) and oxygen ( 18O/16O or d18 O) were examined in material taken at intervals of 2 cm in whole-rock samples from intervals starting 4 cm above to 12 cm above the K/T boundary. Within transitional layer J, sampling sites were spaced 0.4 mm apart (the study of the d13C and d18O is still not completed) (Table 14).

2005ES000189-fig26
Figure 26
[90]  The distribution of d13 C in the vertical section is generally controlled by the distribution of the CaO concentrations, and transitional layer J is characterized, as could be expected, by a pronounced minimum in the content of this component (Figure 18) and in d13 C (Figure 26). The variations in d18O in the vertical section reflect the general variations in d13C and show a clearly pronounced shift toward negative values at the K/T boundary, from 5 to 23 PDB d18 O (Figure 26).

[91]  These anomalies in the variations in the carbon and oxygen isotopic composition in the Gams sequence are quite similar to those documented previously elsewhere [Brinkhuis et al., 1998; Magaritz, 1989; Stuben et al., 2002; and others] and resemble most closely the sequences at El Kef in Tunisia and Agost in SW Spain [Keller et al., 1996; Rodrigues-Tovar et al., 2004]. The variations in the temperature cannot be evaluated until the isotopic compositions of the foraminifers is determined.


RJES

Citation: Grachev, A. F., O. A. Korchagin, H. A. Kollmann, D. M. Pechersky, and V. A. Tsel'movich (2005), A new look at the nature of the transitional layer at the K/T boundary near Gams, Eastern Alps, Austria, and the problem of the mass extinction of the biota, Russ. J. Earth Sci., 7, ES6001, doi:10.2205/2005ES000189.

Copyright 2005 by the Russian Journal of Earth Sciences

Powered by TeXWeb (Win32, v.2.0).