Badenian discoasters from the section in Lenart (Northeast Slovenia, Central Paratethys)

In Slovenske gorice, south of Lenart, a 20 m profile of Middle Miocene strata has been exposed. During previous research numerous discoasters have been found among other coccoliths. In Slovenia Miocene discoasters have only been found in Badenian sediments in Slovenske gorice and they are particularly useful for paleoecological reconstructions. Additional samples were taken from three selected sections in the middle part of the profile, targeting strata with the greatest abundance of discoasters. In two of the three examined sections 9 species of discoasters were identified, the most abundant being D. exilis and D. variabilis. Even though warm water species were found in samples from all three sections, discoasters only occured in two short intervals. This pattern is not a result of temperature changes and is in our opinion connected with the changes in nutrient levels of seawater.


Introduction
The research of Badenian nanoplankton in the territory of Slovenske gorice begun a few years ago, when a large profile of clastic Miocene strata has been exposed on the so- GEOLOGIJA 48/2, 211-223, Ljubljana 2005 uth side of Lenart during construction works ( Figure 1). The profile was sampled and on the basis of nanoplankton assemblage it was established that the sediments are of Badenian age. In the lower part of the sequence poorly preserved shells of eutecosomate pte-opods belonging to the species Vaginella austriaca were found in great numbers, further confirming Badenian age (Pav{i~, 2002). Upper parts of the sequence contain nodules of lithothamniens and sandstone inclusions, which become more frequent in the upward direction. In the uppermost part of the sequence the sandy marlstone contains fossilized plant particles and parts of fish scales and bones. In some parts poorly preserved remains of linear and spiral eutecosomate pteropods were identified (Pav{i~, 2002).
Discoasters have been found in the middle part of the sequence. Those were the first discoasters of Miocene age to be found in Slovenia. We were interested why discoasters are only present in this area and furthermore why they are present in some samples and absent in others, this pattern lacking any association with lithologic changes within the sequence.

Methods and materials
Additional samples were taken from selected sections of the previously examined profile. We targeted the parts of the sequence in which the presence of discoasters was found to be the most significant. Continuous sampling of long sequences within the Badenian strata was impeded by the presence of dense vegetation. For that reason we were compelled to take samples from three separate sections in close proximity. Those sections were sampled in detail at 2, 5 and 10 cm intervals. In this manner we sampled approximately 7 m of marlstone beds in the cen-tral part of the previously examined profile ( Figure 2).
The sampled sequences consist mainly of grey marlstone, weathering to brown on the surface. Samples were collected from unweathered marlstone. The interval we examined is a compound of three short sections. In the first section -A -40 samples were collected at 10 cm intervals , in the second section -B -46 samples were collected at 5 cm intervals  and in the third section -C -50 samples were taken at 2 cm intervals (LJ 1-50). Sequence C is the part of the profile which contained the greatest quantity of discoasters in previous examination.
Marlstone dust was scraped directly onto a glass slide, distilled water was added. Glass slides were then dried on a hotplate and fixed with Canada balsam. Slides were examined under Zeiss MC 80 DX LM with an oil immersion objective of 100 x with a total magnification of 1000 x. We examined one sample (LR-34), which contained diverse and well preserved fossil coccoliths, with a JE-OL SEM.
Nanoplankton species were identified. Discoasters (Discoaster spp.) and coccoliths belonging to the species Coccolithus pelagicus and Helicosphaera carteri were counted. Complete coccoliths and fragments, sufficiently preserved for identification, were counted until minimum of 500. Double counting was avoided by means of zigzag motion along the 22 x 22 mm cover slip. The abundance of sphenoliths (Sphenolithus spp.) was semi quantitatively analysed and rated into 4 different categories according to their abundance.
Calcareous nanoplankton in the samples is relatively well preserved and complete by our appreciation. Considering the level of preservation we believe, that selective dissolution of nanofossils did not take place.

Results
153 samples from 3 proximal profiles were thoroughly examined. Apart from identification of all present species, abundance of four taxa was studied in detail: genus Discoaster, Coccolithus pelagicus, Helicosphaera carteri and genus Sphenolithus. We were interested in their relative abundance and interdependence. iscoasters were absent from the majority of samples, but two culminations in their abundance were noticed. The first was located near the top of the section A (samples LR 32-40) and the other in the upper half of the section B (samples 21-35). Discoasters from section A are in a good state of preservation, while those from B are in a considerably poorer state of preservation, relative to the discoasters from A and to the other species present in the samples from the section B.
In total 31 species and 2 genera were identified (Table 1). 12 species are present in all three sections. The dominating species are Coccolithus pelagicus and Helicosphaera carteri, the first one being more frequent in nearly all the samples examined. Two small (>5μm) Reticulofenestra species were also found to be common in the samples from all three sections. Diversity of assemblage is highest in the samples from section A, where all 31 identified species are present. Genus Discoaster is represented by 9 species, the most common being D. variabilis, D. exilis, D. adamantheus and D. formosus. In the samples from section B, where discoasters were found, their presence is less pronounced, and they are in a poorer state of preservation compared to the rest of the assemblage.
The genus Sphenolithus is represented by two species: S. moriformis and S. heteromorphus. A large majority of all sphenoliths we found belong to the first of the two. S. heteromorphus sphenoliths are rare in sections A and B and absent in section C.
Sphenoliths are common in sections B and C (1-10 sphenoliths / field of view), and abundant in the lower part of section B (>10 sphenoliths / field of view). The samples taken from section A contain fewer sphenoliths. They are present in all examined samples, but are rare (1 sphenolith / 1-10 fields of view) or very rare (<1 sphenolith / 10 fields of view).
The composition of nanoplankton assemblage differs considerably between section A and sections B and C. Samples from section A (LR 1-40) exhibit greater diversity of species and they contain fewer sphenoliths. In the samples taken from the upper portion of the section A a considerable share of discoasters was observed among other coccoliths.

Discussion
The presence of Cyclicargolithus floridanus and the absence of Helicosphaera ampliaperta enables us to place section C into biozone NN 5 or NN 6. The absence of Sphenolithus heteromorphus would imply the age correspondent to the upper part of NN 6. As this species is rare in other sections as well its absence could be attributed to extreme rarity. Moreover in the Mediterranean, intervals of temporary absence of this species are known (F o r n a c i a r i et al., 1996). For that reason we can not give a more precise stratigraphic position of the section C based only on the absence of S. heteromorphus.
Samples from section B contain coccoliths belonging to species S. heteromorphus, D. exilis and D. variabilis as well as Cy. floridanus. All listed species are characteristic of biozones NN 5 and the lower part of NN 6.
The most accurate stratigraphic position can be given for section A. Apart from all the species mentioned above, the samples LR contain D. formosus and D. musicus, stratigraphic markers of biozone NN 5 (P e r c h -N i e l s e n , 1985; B o w n , 1999) and D. moorei, characteristic of the same biozone (B u k r y , 1971). The presence of Helicosphaera species enables us to narrow the interval further: H. intermedia, H. waltrans and H. walbersdorfensis only coexist in a short interval in the upper part of the biozone NN 5 (B o w n , 1999).
The first occurrence of the species Coccolithus pelagicus is known from the Lower Paleogene in equatorial latitude. Today it can be found in the polar and sub polar environments of the Northern hemisphere. It is most common in the North Atlantic  (2004) report, that only the large form (or a sibling species) can be found in subtropical environments while the more common, small form (or species), only lives in cold water. The small form of Coccolithus pelagicus can therefore be used as an indicator of cold water.
In the Middle Miocene the paleoecological preferences of Coccolithus pelagicus were significantly different. The range of the species was far wider than it is today. The small coccolith variety can only withstand temperatures up to 14 °C, whereas it has been found in Middle Miocene sediments from equatorial latitude (B u k r y , 1981). Coccolithus pelagicus from the Badenian can therefore not be used as an indicator of cold water.
The presence of discoasters in the sediment is a characteristic of warm low-nutrient sea environment (C h a p m a n & C h e ps t o w -L u s t y , 1997). Badenian was a relatively warm period, so we would expect onsistent presence of discoasters in all the sections we have examined. Nevertheless discoasters were only found in short intervals and were absent from the majority of samples. According to the state of preservation we believe that the dicoasters found in the samples from section A are autochthonous. We can not be sure of that in the case of discoasters found in section B, as they are in a poorer state of preservation than the accompanying assemblage. The assemblage in section B is much more similar to that in section C than assemblage in section A (apart from containing discoasters of course). This again implies that discoasters in the profile B are not autochthonous.
During Species of the genus Helicosphaera are most common in hemipelagic environments, their presence usually marks the areas of upwelling (P e r c h -N i e l s e n , 1985). Contrary to this discoasters prefer pelagic lownutrient environments (C h a p m a n , C h e ps t o w -L u s t y , 1997). Those ecological preferences are in accordance with the pattern of fluctuations in abundance of the mentioned genera observed in section A. In the upper portion of this section the abundance of discoasters increases. This occurrence coincides with a significant drop in the abundance of helicoliths belonging to the species H. carteri. This incident is a clear indication of a transition from high-nutrient to low-nutrient environment. No similar event can be observed in section B.
Lithostromation is a genus lacking stratigraphic value, nevertheless its paleoecologic preference for hemipelagic environments is known. It is usually not found in sediments deposited far from the shore. The same is presumed for the entire family Pontosphaeraceae (P e r c h -N i e l s e n , 1985). Representatives of the genus Pontosphaera are consistently present throughout the studied material, while Lithostromation perdurum was found in a few samples from sections A and C.
Discoaster and Thoracosphaera are characteristic for pelagic environments. The first of the two is present only in short intervals, the presence of the other is more consistent, but it is very rare.
Nanofossils from all three profiles indicate deposition in a warm epicontinental sea. Sections B and C contain common to abundant sphenoliths. The samples from section A contain fewer sphenolits, but exhibit high assemblage diversity, characteristic of tropical and subtropical environments. Some contain discoasters, indicative of warm water as well as sphenoliths (P e r c h -N i e ls e n , 1985; B o w n , 1999). The absence of discoasters can therefore not be attributed to low temperatures. The changes of nanoplankton assemblage composition can neither be explained by seasonal dynamics, as we frequently find species specific for different seasons according to B e a u f o r t (2001) in a single sample.
Apart from surface water temperature the presence of nutrients in seawater is the most significant factor governing the composition of nanoplankton assemblages. Discoasters are typical of low-nutrient waters. C h a pm a n and C h e p s t o w -L u s t y (1997) describe a correspondence between an increase in diatom abundance and decrease in discoaster abundance. Diatoms are characteristic of high-nutrient waters. Influx of land-derived detritus has a major effect on the nutrient levels in epicontinental seas and it depends largely on precipitation. B ö h m e The reduction in the amount of precipitation could lead to the establishment of low-nutrient environment in the surface waters of Paratethys. This would create conditions that meet the paleoecological requirements of discoasters. On the contrary the increase in the amount of precipitation would cause more nutrients to be washed into the sea and produce a high-nutrient environment, favouring the thriving of genera Sphenolithus and Helicosphaera. The sudden appearance and disappearance of discoasters could therefore be attributed to the variations in the amount of precipitation leading to changes in nutrient levels of seawater.
An influx of nutrients can cause a competitive exclusion or a local extinction event of Discoaster species. Recolonisation in favourable conditions would cause the reappearance of discoasters. This would only be only possible if connections with the surrounding seas existed. In the analysed time interval a connection between the Paratethys and the Mediterranean is known to exist, but is gradually fading. On the basis of a study concerning diatoms H o r v a t (2004) concludes, that a connection between the Paratethys and the Mediterranean persisted until the end of Badenian. The connection between the Eastern Paratethys and the Indian Ocean in the Badenian is uncertain (B i c c h i et al., 2003).

Conclusions
The studied fossil material was deposited in a warm hemipelagic sea environment. Discoasters, characteristic of low-nutrient pelagic environments have been found in two short intervals, but are, by our appreciation, only autochthonous in one. The pattern of changes in their abundance is not a consequence of temperature changes, as warm water species were found in all sections. On the grounds of nanoplankton assemblage we are of the opinion that the observed changes are due to fluctuations in nutrient levels of seawater. A clear indication of such events taking place is a coinciding drop in Helicosphaera carteri abundance and an increase in the abundance of discoasters in section A. Variation in the amount of precipitation, known to coincide with the studied interval, provides the most plansible reason for fluctuations in nutrient levels.