RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6001, doi:10.2205/2005ES000189, 2005
[125] The Gams stratigraphic sequence is characterized by a clearly pronounced transitional layer at this boundary, with this layer characterized by pronounced compositional differences from the over- and underlying rocks. The major- and trace-element composition of the rocks display correlations between the variations in the contents of certain elements in the vertical section.
[126] 1. The lower part (units C-I ) has a low concentration of SiO2, Al2O3, and K2O at a high content of CaO. In transitional layer J, the concentrations of SiO2, Al2O3, and K2O dramatically increase, and the contents of CaO and MnO decrease. The transition to the upper part of the sequence occurs through intermediate layer K, whose concentrations of Al2O3, Fe2O3, CaO, Na2O, and K2O differ from those in both the underlying and the overlying rocks. Although the upper part of the sequence (units L-U, except layers S and T ) shows significant variations in the bulk-rock chemical composition, its content of the terrigenous constituent (quartz and feldspathoids) drastically increases.
[127] The extent of the variations in the lower part (below layer J ) significantly differ from that in the upper part, which includes a number of units with anomalous concentrations of siderophile, chalcophile (Zn, Cu, Ni), and some other elements. These variations in the upper part of the sequence were caused by unstable conditions in the source area of the sediments.
[128] The variations in the concentrations of lithophile (Cr, V, Rb, Cs, Ba, Sr, Nb, and Zr), chalcophile (Cu, Zn, and Ga), and siderophile (Co, NI, and Mo) elements demonstrate that the most significant and mutually correlated variations occur in the vicinity of boundary layer J. A remarkable compositional feature of this layer is its elevated contents of K2O, Al2O3, Fe2O3, and TiO2 at a very low concentration of CaO.
[129] 2. The more detailed study of the composition of layer J has revealed that, having a fairly high concentration of clay minerals, this layer has a ratio of clay to non-clay mineral increasing upward in its vertical section. In the lower part of layer J, the fraction of smectite is equal to 60% and systematically decreases upsection, whereas the fraction of illite simultaneously increases by 20%. Taking into account that this layer was detected to contain titanomagnetite of composition corresponding to that of this mineral from basalts, it is reasonable to conclude that the smectite developed after volcanic material. It should be emphasized that the Ir concentration also increases from 5 ppb to 9 ppb in the direction from unit J1 to J4 and then decreases to 3 ppm in J5-J6. The concentrations of As, Ag, Au, and Br change simultaneously with it.
[130] 3. Our detailed magnetological, magneto-mineralogical, and microprobe examination of the Gams sequence have revealed the following tendencies important for understanding the nature of transitional layer J at the K/T boundary:
[131] The lower part of the layer J (unit J1 ) contains notable amounts of titanomagnetite, whose composition is identical to that of titanomagnetite from basalts (the mineral contains ~20-25% TiO2 ). The rocks above unit J4 contain nickel, iron, and iron-nickel alloy, whose composition is similar to that of awaruite (Fe3Ni) and whose amount reaches a maximum in unit J6. The same unit contains beads of metallic Ni. The comparison of the TMA and MPA data reveals the extremely heterogeneous character of the distribution of Ni, Fe, and their alloy in layer J.
[132] Our microprobe studies indicate that the Ni beads consist of practically pure Ni with an outermost film of NiO. Such pure Ni in the form of spherules with a highly crystalline surface has never been found in magmatic rocks or meteorites, whose Ni always contains significant admixtures of other elements. The characteristic structure of the surface of the Ni beads could be produced during the fall of an asteroid (or meteorite), which resulted in the release of energy sufficient not only for the melting of the projectile (asteroid) itself but also for its evaporation. Metal vapors could be lifted by the explosion to a significant altitude (perhaps, to tens of kilometers) and crystallized there.
[133] During the detailed study of layer J (in its uppermost, very thin sedimentary unit J6, whose thickness is not more than 200 m m), we detected not only Ni spherules but also diamond grains. Their number was close to a few dozen, and they ranged from a submicrons to a few microns in size, whereas the number of the Ni spherules amounted to a few hundreds.
[134] The diamond crystals could be formed in the same vapor nebula, during its subsequent cooling. Synthetic diamond is produced by the method of spontaneous crystallization of a carbon solution in a metallic (Fe, Ni) melt above the diamond-graphite equilibrium line. The diamond is obtained in the form of single crystals, twins, polycrystalline aggregates, skeletal crystals, acicular crystals, and anhedral grains [Bokii et al., 1986]. Practically all of the aforementioned typomorphic features of diamond crystals were also identified in our sample.
[135] This led us to conclude that the processes of diamond crystallization were similar in the nebula of nickel-carbon vapor during its cooling and during spontaneous crystallization from a carbon solution in nickel melt in experiments. However, the nature of the diamond can be completely elucidated only after its detailed isotopic studying, which will require a significant amount of the material.
[136] Finally, the mineralogical analysis of the heavy fraction from unit J2 have revealed a few platelets of copper. The microprobe examination of these particles indicated that they consisted not only of pure Cu but also of particles with up to 30% Au and 1-2% Ni. The morphology of these particles is very unusual, they have uneven ragged edges.
[137] Shocked quartz, coesite, and stishovite (minerals serving as indicators of an impact event), as well as spinel with a high Ni concentration, found in boundary layers at the K/T boundary elsewhere [Leroux et al., 1995; Preisinger et al., 1986, 2002] were not detected in the upper part of layer J in the Gams sequence (which does not mean, however, that these minerals are absent in this layer).
[138] The variations in d18 O and d13 C in the vertical section show a pronounced shift toward negative values at the K/T boundary, from 5 to 23 PDB for d18 O. It is important to emphasize that the minimum d18 O and d13 C values are characteristic of the units of layer J occurring below the unit corresponding to the impact event (samples J -6/3 and J -6/4). The most remarkable fact is that no microfauna was found in these units. These anomalies in the variations of the carbon and oxygen isotopic ratios in the Gams sequence are close to the analogous variations documented elsewhere [Brinkhius et al., 1998; Magaritz, 1989; Stuben et al., 2002; and others] and are pronounced most similarly to the variations in the El Kef sequence in Tunisia and Agost sequence in southeastern Spain [Keller et al., 1995; Rodrigues-Tovar et al., 2004]. The temperature variations still are to be evaluated by the isotopic composition of the organogenic carbon in the skeletons of the foraminifers.
[139] Our data on the He isotopic composition of material in transitional layer J (both on whole-rock and clay-fraction samples) indicate that the 3He/4He ratio has the atmospheric values, which provides additional evidence for the contribution of the terrigenous component during the sedimentation. At the same time, it will be interesting to further examine in more detail the upper part of layer J in the context of the problem of fullerenes [Heymann et al., 1994, 2001], whose 3He/4He ratio in sediments at the Permian/Triassic boundary is two orders of magnitude higher than the atmospheric value [Becker et al., 2001).
[140] 4. The distribution of foraminifers in the Cretaceous-Paleogene transitional beds in the Gams section shows that the extinction of foraminifer genera began well before the accumulation of the layer J.
[141] The genera Gansserina, Kuglerina, and Helvetiella consecutively disappeared in the interval A-I. The greatest number of genera ( Abathomphalus, Globotruncanella, and Contusotruncana) were extinct simultaneously just prior to the accumulation of layer J and else two genera Globigerinelloides and Rugoglobigerina probably emigrated from this region. In its lower part the foraminifer assemblage is impoverished and is represented by typical Cretaceous genera Globotruncana, Globotruncanita, Heterohelix, Racemiguembelina, Pseudotextularia, and Hedbergella. Only two genera, Globotruncanita and Pseudotextularia, were extinct there; the benthic foraminifers remained significantly diverse.
[142] In the middle part of layer J (sample J -8/9b) planktonic and benthic foraminifers are missing. The upper part of layer J (sample J -8/9c) contains an impoverished planktonic assemblage dominated by typical Cretaceous Heterohelicidae members (genera Racemiguembelina and Heterohelix) and only genus Racemiguembelina disappeared to the top of this level. Few Globotruncana still occurred and rare typical Paleogene Globoconusa dawbjiergensis appeared. The benthic foraminiferal assemblage is slightly impoverished though rather diverse.
[143] In the upper part of the section (units L - W ) planktonic foraminifers are scarce; the assemblages yield small Paleogene-like forms. The last typical Cretaceous genera Globotruncana disappeared in unit M; the last Heterohelix, in unit Q.
[144] Our investigation showed that in the Gams section the so-called "mass extinction" of foraminifers in the terminal Cretaceous resulted in the disappearance of three genera. The rest genera were gradually extinct below the transitional bed and above it.
Citation: 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).