RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6001, doi:10.2205/2005ES000189, 2005
[21] In order to examine the Gams sequence, the monolith was divided into 23 units (from A through W ) 2 cm thick each (Figure 3), and transitional layer J at the K/T boundary was further split into six units (Figure 4). The methods applied to examine the material involved: the examination of thin sections and monomineralic fractions, conventional chemical analysis, XRF, neutron activation analysis, X-ray diffraction (XRD), analysis of the material for the isotopic composition of helium, oxygen, and carbon, microprobe analysis of minerals (MPA), and petromagnetic analysis.
[22] In order to obtain heavy-fraction minerals, a sample 10-15 g was crushed in a porcelain mortar and screened through a 0.25-mm sieve. Heavy-fraction minerals were separated from the carbonate-clayey mass using a heavy liquid (bromoform CHBr3, density 2.89 g cm-3 ). The heavy and light fractions were washed with alcohol and purified of magnetic minerals (magnetite) by a magnet. Heavy-fraction minerals were then separated according to their electric conductivity (on a magnetic separator) into non-electromagnetic, weakly electromagnetic, and electromagnetic fractions. Zircon and other minerals were hand-picked under a binocular magnifier, using a needle, and were glued to glass platelets with pits. The light fraction was used to obtain quartz (on a magnetic separator).
[23] Major components (SiO2, TiO2, Al2O3, total FeO, MgO, MnO, CaO, Na2O, K2O, P2O5, and total S) were determined by XRF.
[25] Mass spectrometry was conducted on an Elan 6100 DRC analytical system (Elan 6100 DRC, Software Kit, May 2000, Perkin Elmer SCIEX Instrument) in a standard mode, under the following operation conditions: nebulizer argon flow of 0.91-0.93 l min-1, auxiliary argon flow of 1.17 l min-1, plasma-forming argon flow of 15 l min-1, the plasma generator operating at 1270-1300 W, and a voltage of 1000 V at the detector in the counting mode. The sensitivity of the analytical system was calibrated throughout the whole mass scale against standard solutions that included all elements to be analyzed in the samples. The accuracy of the analyses was monitored and the sensitivity drift was taken into account by alternating the analyses of unknown samples and the monitor, which was standard basalt sample BCR-2 (United States Geological Survey). The detection limits (DL) were from 1-5 ppb for elements with heavy and medium masses (such as U, Th, and REE) to 20-50 ppb for light elements (such as Be and Sc). The analyses were conducted accurate to 3-10% for elemental concentrations > 20-50 DL.
[26] In a small number of samples, REE were determined by ICP-MS on a PLASMA QUARD PQ2+TURBO (VG Instruments) quadrupole mass spectrometer, following the procedure described in [Dubinin, 1993].
[28] The phase analysis of clay minerals was accomplished on oriented samples in air-dry and saturated with ethylene-glycol states. Minerals were identified by series of integer reflections OOl. The raw specters were processed with the Jade-6.0 (MDI, United States) computer program. The quantitative proportions of clay minerals were derived from the basal reflections in the saturated state, following the method described in [Biskaye, 1965]. The sum of clay minerals was normalized to 100%.
[32] The petromagnetic measurements involved the determination of the specific magnetic susceptibility c, hysteresis characteristics (saturation field Hs, saturation magnetization Ms, specific remanent saturation magnetization Mrs, coercivity Hc, and remanent coercivity Hcr ), anisotropy Ak and Ars, and coercivity spectra. The magnetic susceptibility and remanent magnetization were measured on susceptibility bridge KLY-2 and spin-magnetometer JR-4, respectively; and the hysteresis characteristics of the samples were examined on a coercivity spectrometer [Burov et al., 1986; Yasonov et al., 1998], which made it possible to obtain normal magnetization line up to 0.5 T in automated mode. The thermomagnetic measurements were carried out with a Curie balance. The concentrations of magnetite, iron, hemoilmenite, nickel, and "goethite" in the sample were estimated from the results of thermomagnetic analysis M(T) as follows. The contributions of each phase to the magnetization was evaluated using the M(T) curve, and this value was divided into the specific saturation magnetization of this mineral. The following values of the Ms of minerals were assumed: ~90 Am2 kg-1 for magnetite, ~200 for iron, ~0.2 for goethite, and from ~4 to ~40 Am2 kg-1 for hemoilmenite at Tc from 300o C to 200oC [Nagata, 1965]. The value of Ms = 0.02 Am2 kg-1 assumed here for goethite is the minimal one, and this leads us to the lower limit for the concentration of iron hydroxides (which include paramagnetic species). Moreover, the saturation field of fully crystalline goethite is much higher than the magnetic field in which the thermomagnetic analysis was conducted.
[33] The values of c, Ms, and Hc are notably contributed by paramagnetic (clays) and diamagnetic (marly limestone) material. This contribution is eliminated from the values of Ms and Hc using the normal magnetization curves.
In the upper, clayey part of the section (units J - W ) planktonic foraminifers occur only in certain intervals. Most of tests in the assemblages are well preserved; intact tests are encountered together with that partially or completely squeezed and those possessing a smoothed-out exterior sculpture. However, specific assignments were defined for most of specimens permitting the exact age estimate of enclosing sediments.
[37] Benthic foraminifers are diverse and numerous in both lower, calcareous marlstone, part of the section (units A - I ) and upper, mainly clayey portion (units J - W ). In the lower part they were identified in all studied units C, D, E, G, H, I. Species of Gaudryina, Marssonella, Rzehakina, Gyroidinoides, Lenticulina, Arenobulimina, Trochammina, Eggrellina, Stensioeina, and Globorotalites are the most common in the benthic assemblages. The benthic foraminifers are well preserved, commonly better than planktonic forms. In the upper, clayey part of the section benthic foraminifers are more numerous than planktonic taxa, however, they are also found only in certain layers. In layer J benthic foraminifers occur in samples J -8/9a and J -8/9c and are missing in sample J -8/9b. Upward from the base they were encountered in units L, M, N, O, Q, R, T, and U.
[38] Planktonic and benthic foraminifers were not encountered in the middle portion of layer J (sample J -8/9b) and are missing in layers K and S. Their absence in the latter units results from unfavorable for test preservation composition of enclosing sediments: such rocks are usually barren of foraminifers. However, the lack of foraminifers in middle part of layer J (sample J -8/9b) can hardly be explained by this reason.
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[41] The studied foraminifer assemblage clearly indicates the correspondence of enclosing sediments to the Abathomphalus mayaroensis or Pseudoguembelina hariaensis zones of standard zonations based on Globotruncanidae and Heterehelicidae, respectively [Hardenbol et al., 1998].
[42] The disappearance of a great majority of typical Cretaceous planktonic foraminifers and especially of Abathomphalus maryaensis and Globotruncanita stuarti, evidences that the upper boundary of this zone corresponds to the top of the similar zone, and evidently to Bed 9, in the Rotwandgraben section in the Eastern Alps [Peryt et al., 1993], to the top of the like zone in the Bavarian Alps [Herm et al., 1981] and Tunisia [Keller, 1988], and probably exactly coincide or is close to the top of the Abathomphalus maryaensis Zone of the standard zonation [Hardenbol et al., 1998].
[44] Barren interval is represented by dark green to black, 0.2-cm-thick clays in the middle part of layer J (Sample J -8/9b) lacking foraminifers. It is still early to judge how geographically wide this interval with missing foraminifers, conventionally named Barren interval, is distributed. Through distinguishing this part of the section as a separate stratigraphic unit we try to call attention to a possible occurrence of the similar bed in other sections.
[45] The Globoconusa daubjergensis Zone (base boundary defined by first occurrence of index species). Dark green to black, noncarbonate, 0.5-cm-thick clays of the upper part of layer J (sample J -8/9c) are referred to the zone. They are characterized by the first occurrence of single specimens of typical Paleogene species Globoconusa daubjergensis (Bronnimann) and Paleogene-like forms close to (?) Hedbergella-Eoglobigerina trivialis (Subbotina). The sediments also yield single deformed shells of Globotruncanita rosetta (Carsey), a typical Cretaceous member of Globotruncanidae, and the Cretaceous Heterohelicidae members Heterohelix lata (Egger), Heterohelix globulosa (Ehrenberg), and Racemiguembelina fructicosa (Egger). It is remarkable that tests of the latter are rather numerous but have a slightly ornamented surface and look somewhat smoothed, i.e. can be redeposited.
[46] In the well-studied sections the earliest findings of Globoconusa daubjergensis are known from the zone P0 (0.8 m above GSSP) in the Elles II section in Tunisia [Keller et al., 2002] and slightly higher, from the zone P1a in Guatemala [Keller and Stinnesbeck, 2000], Egypt [Keller, 2002], Coxquinhui, Mexico [Stinnesbeck et al., 2002, the Koshak section, Kazakhstan [Padro et al., 1999] and from the zone P1c (Bed 23b) in the Stevns Klint section, Denmark (Figure 6). According to the latest records, this species most likely first occurred still earlier and can be considered as one of the earliest Paleogene planktonic foraminifers [Patterson et al., 2004]. It is quite probable that findings of Globoconusa daubjergensis in the Gams section illustrate that very case.
[47] The Subbotina fringa Zone (base defined by first occurrence of index species). The sediments of the zone are represented by greenish-grey, slightly carbonate and carbonate, 14.0-cm-thick clays and siltstones. Planktonic foraminifers occur only in units L, M and Q. The foraminiferal assemblage of that part of the section includes in unit L (Sample L -6) a typical Paleogene species Subbotina fringa (Subbotina) and the Paleogene-like forms close to (?) Morozovella sp. cf. M. conicotruncata (Subbotina) and (?) Morozovella sp. though unsusceptible to exact identification owing to small amount of tests and their moderately poor preservation. The typical Paleogene species (?) Hedbergella-Eoglobigerina trivialis sp. also occur there, together with peculiar Upper Cretaceous forms Heterohelix lata (Egger), Heterohelix planata (Cushman), Heterohelix globulosa (Ehrenberg), Heterohelix lata (Egger), Heterohelix sp., and few Rugoglobigerina sp. and Globotruncana rosetta (Carsey).
[48] Thus there are grounds to believe that the units L and M, according to foraminiferal assemblage and first occurrence of Subbotina fringa, most likely correspond to units 17 and 18 in the Rotwandgraben section (Eastern Alps) and should be correlated with the recognized there Globoconusa conusa Zone [Peryt et al., 1993]. Units L and M can also completely or partially correspond to the Subbotina fringa Zone distinguished in the Bavarian Alps [Graup and Spettel, 1989; Herm et al., 1981] and in Tunisia [Brinkhuis and Zachariasse, 1988].
[49] The Parasubbotina pseudobulloides Zone (base defined by first occurrence of index species). This zone includes light, greenish-grey siltstones of unit R and likely a part of overlying sediments: fine-grained sandstone layers (unit S ) and greenish-grey siltstones of units T - W. Planktonic foraminifers were encountered in unit R (sample R -6), which is characterized by the first occurrence of typical Paleogene Parasubbotina pseudobulloides (Plummer) and few specimens of "Paleogene taxa" defined as Globigerina-like forms that cannot be exactly identified owing to poor preservation.
[50] We consider that unit R corresponds to the recognized there Parasubbotina pseudobulloides Zone and to Bed 22 of the Rotwandgraben section in the Eastern Alps [Peryt et al., 1993], i.e. it can be believed that the base of unit R corresponds to the base of the like zone in schemes by Bolli [1966] and Herm et al. [1981].
[52] Therefore, two species of the family Heterohelicidae constitute almost a half of planktonic foraminiferal assemblage. Racemiguembelina fructicosa and Pseudotextularia elegans occurred there in competitive relations, i.e. the increased number of specimens of one species was accompanied by a reduction of the other. For instance, units D and H are dominated by Racemiguembelina fructicosa, whereas units E and I, by Pseudotextularia elegans. In the lower part of layer J (sample J -8/9a) the assemblage is also dominated by Heterohelicidae, its five species constitute 53% of the total composition but the dominating form is Heterohelix lata and not Racemiguembelina fructicosa and Pseudotextularia elegans, as in units A - I. In that portion of layer J the Globotruncanidae members make up only about 33% of the assemblage and the share of Hedbergellidae species, namely, of Hedbergella-forms, significantly increase compared to the assemblages of units A - I.
[53] As indicated above, the upper part of layer J is marked by though lesser species diversity than its lower portion (sample J -8/9a) and units A - I, but still comparatively high (7 species); however, the most abundant species are Heterohelix lata and Heterohelix globulosa together with Racemiguembelina fructicosa. The Cretaceous Globotruncana and Hedbergella members and Paleogene Globoconusa daubjergensis occur in single specimens.
[54] In the upper part of the discussed section planktonic foraminifers are scarce. However, the assemblage from units L - M also consists of 8-9 species, whereas units O and R are characterized by single specimens of one or two species. This interval is also dominated by Heterohelicidae species Heterohelix globulosa, Heterohelix lata, Heterohelix planata, and Heterohelix sp.
[55] The studied assemblages of planktonic foraminifers contain a number of species, for which inhabiting conditions can be defined according to currently available notion [Abramovich et al., 2003]. For instance, Helvetiella helvetia, Contusotruncana walfischensis, Rugoglobigerina rugosa, Kuglerina rotundata, Pseudoguembelina excolata, Pseudotextularia elegans, and Globotruncana rosetta most likely inhabited the surface+subsurface waters; Globotruncana gagnebini, Gansserina gansseri, Globotruncana arca, Globotruncanita subspinosa, Globotruncanita stuarti, and Racemiguembelina fructicosa occurred nearby the thermocline; Globotruncanita stuartiformis, Globotruncanella havanensis, and Abathomphalus mayaroensis preferred the sub-thermocline waters and are considered to be deep-water species; finally Heterohelix globulosa inhabited both subsurface and thermocline waters [Abramovich et al., 2003]. The distribution of these species in the section is illustrated in Figure 8.
[56] From the distribution, abundance, and disappearance of the listed ecological (bathymetric) groups of planktonic foraminifers we can infer how favorable, and likely stratified, were different layers of the water column in the Gams paleobasin during the transitional Cretaceous-Paleogene time.
[57] Throughout the Cretaceous-Paleogene transition (interval of units ( A )-( J -8/9a) and ( J -8/9c)) favorable conditions constantly occurred in subsurface and thermocline waters. During the accumulation of units C, G, H, and I deep, subthermocline waters were also suitable. The thermocline fauna during the formation of units A - I and J -8/9c excelled the inhabitants of subsurface waters in both number of species and abundance.
[58] The number of planktonic foraminifer specimens from thermocline waters is commonly twice as large as that from subsurface layers. Only during the accumulation of unit D the thermocline inhabitants became extremely numerous and constituted 74% against 3% of subsurface dwellers. This could result from a temporary deterioration in subsurface water environments.
[59] However, it is remarkable that in the period of unit J -8/9a deposition the relationship of the assemblage dominants became inversed. Surface water inhabitants excelled the thermocline dwellers in both number of species (3 against 2) and abundance (42% against 7%). It is likely that in this period the inhabiting conditions could improve, or remain favorable in subsurface layer and deteriorate in the thermocline.
[60] During the accumulation of unit J -8/9c the conditions in subsurface and thermocline waters again became similar to those existed during the deposition of units A - I. The thermocline fauna is more abundant there than the subsurface association.
[61] Thus we can suppose that during the initial accumulation of layer J the thermocline and sub- thermocline environmental conditions were sharply deteriorated.
[62] During the deposition of unit J -8/9b the conditions most likely became still worse and to the thermocline interval the subsurface, sub-thermocline, and bottom waters became barren. The whole water sequence was unsuitable for the existence of planktonic and benthic foraminifers. At the stage of the layer J -8/9c accumulation the environmental conditions in subsurface and thermocline waters regained their original properties similar to that in units A - I. Only sub-thermocline waters remained unfavorable. The low abundance and diversity of planktonic foraminifers indicate that environmental conditions in the thermocline and surface waters though somewhat improved but were not completely suitable for foraminifers and did not manage to become inhabited after the crisis.
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.
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