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

Methods of Material Preparation and Studying

[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.

Analysis for trace elements and REE.
[24]  The sample selected for analysis was decomposed in acids in a MULTIWAVE microwave furnace. 50- to 100-mg batches of the material were decomposed in a Teflon capsule in a mixture of hydrofluoric and nitric acids at a temperature of 240o C and a pressure of no more than 72 bar for 50 min. Fluorides of metals were decomposed by dissolving dry residues in 5-7 ml of 6.2 N HCl at 90o C for 1 h. After this the sample was spiked with an internal standard (In) and mixed with 3 ml of 3 N HNO 3. The chlorides were completely transformed into nitrates for 15-20 min at a temperature of about 200o C. The final concentration of HNO3 was 0.5 N, and the dilution coefficient of the sample was close to 1000. This method ensured the complete decomposition of most minerals contained in the sedimentary rocks, including zircon and monazite (some spinels, such as chromite, were decomposed not fully). All of the acids utilized in this procedure were twice distilled in a quartz (or Teflon for HF) apparatus from starting analytical-grade reactants.

[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].

Analysis of clays.
[27]  The mineralogical analysis of the pelitic fraction < 4  m m was conducted on an D/MAX-2200 (Rigaku, Japan) X-ray diffractometer with Cu monochromatic radiation. The operating conditions were: 40 mA, 40 kV, 0.5o scattering slit, 0.5o and 3 mm receiving slits, 0.02o step, and 2 s collection time at a spot.

[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%.

Microprobe analysis.
[29]  In order to analyze the composition and microstructure of ferromagnetic minerals in the rocks, their samples were examined on a Camebax microprobe. Samples were mounted on a pellet 26 mm in diameter and fixed with Wood's alloy. After this, they were carefully polished and finished using diamond pasts, and were spray sputtered with carbon. The analyses were conducted at an accelerating voltage of 20 kV and a beam current of 10 nA. The effective beam diameter was 2-3  m m and was systematically monitored using small grains. Ore minerals were analyzed for TiO2, FeO, MgO, MnO, Cr2O3, and Al2O3.

Micropaleontoligical analysis.
[30]  Foraminifers from the rocks were examined following the conventional techniques of their separation and preparation. Material for the analysis was taken from units C, D, E, G, H, I, J, K, L, M, N, Q, R, S, T, and U. The 50- to 70-g samples were crushed under a screw-down press into small fragments, which were then put into distilled water with dissolved caustic soda (depending on the rock hardness) and boiled for 30 min to 10 h. After this, the material was washed on various sieves. To identify buoyant hollow shells of planktonic foraminifers before washing on sieves under a water flow, the aqueous solution remaining after boiling was racked, and the solid material was dried. This material was washed on sieves with mesh of 1 mm, 250  m m, and 160  m m, and all the fine grained residue that remained at the sieves was elutriated in a pan. The residues at each of the sieves and pan were dried and weighted. Foraminifers were gathered into Franke cells and examined under an MBS-1 binocular magnified at magnifications of 28 times, 56 times, and 98 times. Structural details of the shell surfaces and ultrastructure of their walls were conducted on a CamScan MV2300 scanning electron microscope, which was also used to obtain images of the shells. The material was preliminarily sputtered with gold at a HV cathode voltage of 5.0 kV. Admixtures on the walls of the shells were analyzed on an INCA Oxford Instruments microprobe.

Petromagnetic study.
[31]  In order to conduct standard pertomagnetic measurements, each of the samples was cut into a series of cubes with edges ranging from 1 cm to 2 cm. Thermomagnetic analysis was carried out in fragments less than 1 cm across.

[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.

Carbon and oxygen isotopic analysis.
[34]  Samples were cut, grounded, and cleaned; and their sections were examined under an optical microscope. The rocks were powdered with a Dremel MiniMite microdrill tool. The amount of the powder prepared for a single analysis was 300  m g or more, depending on expected content of carbonates. Each sample was analyzed at several spots. The spots of duplicate analyses were spaced ~1 cm apart. Carbon isotopes in calcite from the samples were analyzed a Finnigan MAT 253 mass spectrometer with a Gasbench II at the Department of Earth and Space Sciences, University of California, Los Angeles. The carbon isotopic composition is defined as a deviation, in ‰, of the 13C/12C and 18O/16O ratios of a given sample from those of a standard, expressed in the conventional 13C and 18O notations relative to V-PDB and SMOW. The secondary standards were NBS-19 and IAEA-CO-1. The accuracy of the analyses was better than pm 0.1 for carbon and pm 0.2 for oxygen.

Helium isotopic composition.
[35]  Both the isotopic composition and concentration of He were measured on a MI-1201 no. 22-78 mass spectrometer at the Laboratory of Isotopic Analysis of the Geological Institute, Kola Research Center, Russian Academy of Science, in Apatity the sensitivity of 5 times 10-5 A Torr-1 for 3He. The concentrations were deduced from the height of the peaks accurate to 5% ( pm1s ), the errors of the isotopic ratios were pm 20% at 3He/4He = ntimes10-8 and pm 2% at 3He/4He = ntimes10-6. The blank experiments were conducted after the reloading of the holder, under the same conditions as those for the analysis of the samples. The analyses were conducted on a MI-1201 no. 22-78 mass spectrometer using the method of melting, as described in [Kamensky et al., 1990].

Biostratigraphy and Foraminiferal Assemblages

[36]  The studied interval of the section (units A - W ) yields both planktonic and benthic foraminifers. Planktonic foraminifers are common in the lower calcareous marlstone part of the section (units A - I ). The units contain numerous and diverse planktonic foraminifers though their tests are poorly preserved owing to properties of enclosing sediments and to imperfect techniques used for their extraction from these rocks. A poor preservation of many tests is manifested by the fact that certain elements of test morphology, for instance apertures, are covered with rock particles, which are unsusceptible to removal after washing. This fact makes a lot of planktonic foraminiferal shells badly illustrated on photos and hinders their study under electron microscope.

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.

2005ES000189-fig06
Figure 5
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Figure 6
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Figure 7
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Figure 8
[39]  In view of the decisive significance of planktonic foraminifers for studies of stratigraphic position of the Cretaceous--Paleogene boundary and reconstruction of concurrent environmental changes, in this paper the priority is given just to that group (Figures 5, 6, 7, 8, 9, 10, 11, and 12). The benthic foraminifer distribution will be analyzed later. Below is the description of zonal subdivision of the transitional units and substantiation of age of enclosing sediments using planktonic foraminifers (Figure 5).

Uppermost Upper Maastrichtian

2005ES000189-fig10
Figure 9
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Figure 10
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Figure 11
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Figure 12
[40]  The Abathomphalus mayaroensis Zone (upper part) (upper boundary defined by the last occurrence of index species). The sediments are represented by light grey clayey limestones and marls of units A-I. The foraminiferal assemblage includes typical Cretaceous single- and two-keeled taxa of Globotruncanidae, scarce nonkeeled Hedbergellidae, and numerous Heterohelicidae. Among the encountered characteristic species there are Contusotruncana walfischensis (Gandolfi), Globotrincanita stuartiformis (Dalbiez), Globotruncana arca (Cushman), Globotruncana rosetta (Carsey), Globotruncanita andori (Klaus), Globotruncana gagnebini Tilev, Gansserina gansseri (Bolii), Contusotruncana contusa (Cushman), Globotruncanita stuarti (de Lapparent), Globotruncana arca (Cushman), and Rugoglobigerina rugosa (Bronnimann). Additionally the studied assemblages contain rather peculiar species of the family Heterohelicidae: Pseudotextularia elegans (Rzehak) and Racemiguembelina fructicosa (Egger).

[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].

Lowermost Paleogene

[43]  The Guembelitria cretacea Zone-Interval (base defined by last occurrence of Abathomphalus maryaensis; upper boundary, by first occurrence of Globoconusa daubjergensis). Dark green, almost black, noncarbonate, 0.5-cm-thick clays of the lower and middle parts of layer J are referred to the zone. Planktonic foraminifers occur only in the lower part (sample J -8/9a). The middle portion of the bed (sample J -8/9b) is barren of forams. Sediments in the lower part of layer J (sample J -8/9a) yield few specimens of Globotruncanita stuarti (de Lapparent), Globotruncanita sp., and several deformed tests of Globotruncana rosetta (Carsey) from the family Globotruncanidae. The interval also contains numerous Pseudoguembelina excolata (Cushman), Pseudotextularia elegans (Rzehak), Racemiguembelina fructicosa (Egger), Heterohelix lata (Egger), Heterohelix globulosa (Ehrenberg), and Heterohelix aff. semicostata (Cushman) of the family Heterohelicidae. Poorly preserved Hedbergella monmouthensis (Olsson) and Hedbergella holmdelensis (Olsson) were identified as well.

[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].

Paleoecologic Features of Planktonic Foraminiferal Assemblages

[51]  The diversity of planktonic foraminiferal species is comparably high in the lower part of the section (Units A - I, the Abathomphalus maryaensis Zone) ranging from 6-7 to 14-15 species in the assemblage (Figure 7). A similar diversity (11-12 species) is recorded in the lower portion of layer J (Sample J -8/9a). In the middle part of layer J planktonic foraminifers are missing; though in its upper part the species diversity is somewhat lower than in units A - I and lower part of layer J, it still reaches 6-7 species. The total number of planktonic foraminifer specimens in units A-I is high; the maximum amount was recorded in units D and I, the minimum one is in unit G. In the lower part of layer J (sample J -8/9a) the abundance of planktonic foraminifers is as high as in the underlying units. In its upper portion their amount almost twice decreases compared to that of the Maastrichtian (units A - I ) and Early Paleocene (lower part of layer J ) assemblages and slightly exceeds the abundance of the above-lying associations. It is remarkable that in the Maastrichtian assemblage (units A - I ) and lower part of layer J (sample J -8/9a) the members of two families, Globotruncanidae and Heterohelicidae, dominate. Other families - Rugoglobigerinidae and Hedbergellidae - are represented by few species with a low number of specimens. It is notable that Globotruncanidae are characterized by numerous species with comparatively small amount of specimens in the assemblage, whereas Heterohelicidae is represented by two, rarely five species, among which Racemiguembelina fructicosa and Pseudotextularia elegans are extremely abundant.

[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.


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

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