Introduction
Foraminifera are among key microfossils for palaeoenvironmental studies and stratigraphy. However, our knowledge on their pre-Quaternary record from Antarctica is far from satisfactory. Oligocene-Pleistocene foraminifera are relatively well known from the Ross Sea region (e.g. Leckie & Webb Reference Leckie and Webb1985, Strong & Webb Reference Strong and Webb2001, Webb & Strong Reference Webb and Strong2006, as well as several more publications by these authors) as an aftermath of large international drilling projects. Moreover, Pliocene deposits from the McMurdo Dry Valleys were also studied for foraminifera (Webb Reference Webb1974). East Antarctic foraminifera have been less researched. Pliocene assemblages were described from Larsemann and Vestfold hills (Quilty et al. Reference Quilty, Gillieson, Burgess, Gardiner, Spate and Pigeon1990, Quilty Reference Quilty2010), whereas reworked Palaeogene foraminifera were described from the shelf of East Antarctica (Quilty Reference Quilty2001). Foraminiferal studies from pre-Quaternary sediments of West Antarctica are also fragmentary. Oligocene planktonic (Gaździcki Reference Gaździcki1989) and Miocene benthic foraminiferal assemblages (Birkenmajer & ̡uczkowska Reference Birkenmajer and ̡uczkowska1987) were reported from King George Island, South Shetland Islands. Pliocene assemblages were found on Cockburn Island, Antarctic Peninsula (Gaździcki & Webb Reference Gaździcki and Webb1996). Until recently, we have had no information on Eocene foraminiferal communities from West Antarctica. Here, we present foraminiferal data from early Eocene strata of Isla Marambio, Antarctic Peninsula, taking the first step to fill this gap.
The Eocene La Meseta Formation (LMF), exposed in the north-eastern part of Isla Marambio (Fig. 1), is a sequence of shallow-marine, deltaic and/or estuarine, poorly consolidated sandstones and siltstones, accumulated within an incised valley (Sadler Reference Sadler1988, Porębski Reference Pore˛bski1995, Marenssi et al. Reference Marenssi, Santillana and Rinaldi1998, Marenssi Reference Marenssi2006). The LMF is approximately 700 m thick, and it is well known for its exceptional fossil record, which provides a unique insight into Eocene life in what has become a harsh polar environment. Throughout the formation, abundant and diverse microfossils, invertebrates, vertebrates, and plants occur in a number of fossiliferous horizons (Feldmann & Woodburne Reference Feldmann and Woodburne1988, Stilwell & Zinsmeister Reference Stilwell and Zinsmeister1992, Gaździcki Reference Gaździcki1996, Francis et al. Reference Francis, Pirrie and Crame2006, López Cabrera & Olivero Reference López Cabrera and Olivero2011). Among microfossils, foraminifera are very rare in the LMF. So far, only a few specimens of Cibicides encrusting brachiopod shells were reported (Bitner Reference Bitner1996) from Telm2 (equivalent to Acantilados Allomember sensu Marenssi et al. Reference Marenssi, Santillana and Rinaldi1998). In this paper, we describe rich foraminiferal communities from the very base of the LMF (Fig. 2a), Telm1 or Valle de las Focas Allomember sensu Marenssi et al. (Reference Marenssi, Santillana and Rinaldi1998), which is interpreted as early Eocene in age, 52–54 Ma (Marenssi Reference Marenssi2006, López Cabrera & Olivero Reference López Cabrera and Olivero2011).
Fig. 1 Geological map of Isla Marambio showing collection sites from the La Mesesta Formation used in this investigation.
Fig. 2 a. Composite stratigraphic column of the La Meseta Formation on Isla Marambio (north section) adapted from Sadler (Reference Sadler1988), showing the main lithofacies and location of samples with foraminifera. Photographs of b. site ZPAL 2, and c. site ZPAL 5, taken in 1994.
Methods
Fieldwork was carried out by A. Gaździcki during the Argentine-Polish field campaigns on Isla Marambio during the summers of 1987–88, 1991–92, and 1993–94. Rock samples were collected throughout Eocene strata from the north-east side of the island (Fig. 1). After a pilot study, carried out throughout the LMF, detailed study focused on samples from near the base of the section, where some foraminifera were encountered (Fig. 2). Rock samples of approximately 1.5 kg were mechanically crushed and treated with a standard Glauber salt procedure. Disintegrated samples were washed through a series of sieves and dried. Fractions greater than 63 μm were searched for microfossils. All foraminifera were picked and mounted on micropalaeontological slides. Selected specimens of each taxon were investigated under SEM. Generic classification was based on Loeblich & Tappan (Reference Loeblich and Tappan1987). The investigated collection is housed at the Institute of Paleobiology of the Polish Academy of Sciences (Warszawa) under the catalogue number ZPAL F.55.
Results
Foraminifera were found in three samples. Diverse foraminiferal assemblages are present at two locations: ZPAL 2 (64°15′22.530″S, 56°44′22.103″W, 2.2 m a.s.l.), and ZPAL 5 (Fig. 1), however, the later is by far richer (Table I). Only few, poorly preserved specimens were found at ZPAL 11. In total, 1365 foraminiferal specimens were extracted, only three of which represented problematic planktonic forms. Specimens from ZPAL 2 and 11 are shown in Fig. 3 and specimens from ZPAL 5 are shown in Figs 4 & 5. Benthic foraminifera represent at least 26 species and 21 genera. Their assemblages are distinctively different at each location. The assemblage from ZPAL 2 is dominated by Nonionellina and Nonionella, Globocassidulina, Eilohedra, and Cibicides, while that from ZPAL 5 by Globocassidulina, Cribroelphidium, Guttulina, and Lobatula. Only two benthic and all three problematic planktonic foraminifer specimens were found at the most north-eastern location ZPAL 11. They are poorly preserved, making their precise identification rather problematic.
Table I Foraminiferal counts from three samples collected from the La Meseta Formation.
Fig. 3 SEM images of foraminifera from site ZPAL 2: 1. ?Trochammina sp., 2. ?Nonionellina sp., 3.Nonionella bradyi (Chapman, 1916), 4.Eilohedra vitrea (Parker, 1953), 5.Bulimina sp., 6.Bulimina karpatica Szczechura et Pożaryska, 1974, 7.Cibicides williamsoni Garrett, 1941, 8.Lobatula sp., 9.Gyroidina zelandica Finlay, Reference Finlay1939, 10.Globocassidulina subglobosa (Brady, Reference Brady1884), and ZPAL 11: 11.Cribroelphidium aff. saginatum (Finlay, 1959), 12.Cribroelphidium sp., 13. ?globorotalid, 14. ?Planorotalites sp.
Fig. 4 SEM images of foraminifera from site ZPAL 5: 1.Quinqueloculina sp., 2.Neoflabellina sp., 3.–5.Guttulina irregularis (d'Orbigny, 1846), 6.Bulimina sp., 7. ?Saracenaria sp., 8.Lenticulina inornata (d'Orbigny, 1846), 9.Globorosalina sp., 10.Stomatorbina sp., 11.Discorbinella sp., 12. & 13.Lobatula lobatula (Walker et Jacob, 1798), 14.& 15.Lobatula sp., 16.Cibicides aff. ungerianus (d'Orbigny, 1846).
Fig. 5 SEM images of foraminifera from site ZPAL 5: 1. & 2.Cribroelphidium aff. saginatum (Finlay, Reference Finlay1939), 3.–7.Cribroelphidium aff. lauritaense (Todd et Kniker, Reference Todd and Kniker1952): 4c. single aperture, 5c. apertures in the last suture, 6.–7. equatorial and axial sections, 8. ?Nonionellina sp., 9.Nonionella bradyi (Chapman, 1916), 10.Astrononion echolsi Kennett, 1967, 11.Anomalinoides spissiformis (Cushman et Stainforth, 1945), 12.Eilohedra vitrea (Parker, 1953), 13.Buccella sp., 14.Globocassidulina subglobosa (Brady, Reference Brady1884).
Taxonomical notes
Some problematic taxa from the LMF are briefly discussed below, according to their alphabetical order.
Cibicides aff. ungerianus (d'Orbigny, 1846). Our specimens (Fig. 4.16) seem to represent C. ungerianus as shown by Schröder-Adams (Reference Schröder-Adams1991) in pl. 3, figs 5 & 6. However, she provided no side view. Our specimens differ from the holotype pictured by d'Orbigny by rounded, only slightly compressed margin (Fig. 4.16b).
Cribroelphidium aff. lauritaense (Todd et Kniker, 1952). Numerous specimens are present in ZPAL 5 (Fig. 5.3–5.7). In their inflated outline, acutely rounded periphery, number of chambers per whorl, and distributions of sutural bridges, they closely resemble Elphidium lauritaense described by Todd & Kniker (1952, pl. 3, fig. 38a & b), from which our specimens differ by more compressed periphery.
Cribroelphidium aff. saginatum (Finlay, Reference Finlay1939). Our specimens (Fig. 5.1 & 5.2, and perhaps Fig. 3.11) are inflated, with broadly rounded periphery, and have 4–6 broadly inflated sutural bridges per suture in adult chambers. They show more compressed profile and more irregular ornamentation as compared to the type specimen of Finlay (Reference Finlay1939).
Cribroelphidium sp. A single specimen (Fig. 3.12) from ZPAL 11 differs from other specimens of this genus by larger apertures and much broader profile. Unfortunately, its poor preservation prevents precise classification.
Globorosalina sp. A single specimen from ZPAL 5 (Fig. 4.9) resembling Globorotalia reissi Loeblich & Tappan, Reference Loeblich and Tappan1957 reported from New Zealand (Jenkins Reference Jenkins1971) that ranged practically throughout the entire Lower Eocene (Hornibrook et al. Reference Hornibrook, Brazier and Strong1989). It shows distinctively less depressed sutures than Globorotalia reissi of Loeblich & Tappan (Reference Loeblich and Tappan1957), which was assigned to Praepararotalia perclara Loeblich & Tappan, Reference Loeblich and Tappan1957 and interpreted as of benthic habitat (Liu et al. Reference Liu, Olsson and Huber1998). It appears that our specimen belongs to genus Globorosalina established for late Eocene forms from Western Australia (Quilty Reference Quilty1981).
Lobatula sp. In our material, strongly ornamented forms of Lobatula (Fig. 4.14 & 4.15, and possibly Fig. 3.8) dominate. They show ornamentation as in specimen of Brady (Reference Brady1884, pl. 93, fig. 5) classified by Jones (Reference Jones1994) as Cibicides lobatulus. Typical individuals of Lobatula lobatula (Walker & Jacob, 1798) with no ornamentation (Jones Reference Jones1994, pl. 93, fig. 1) are also present in the LMF (Fig. 4.12 & 4.13), but they are significantly less abundant. For counts of both forms refer to Table I.
Neoflabellina sp. (Fig. 4.2). It is represented by three specimens, all incomplete. Almost spherical proloculus and planispiral coil of early chambers are seen in uncoated specimens.
?Nonionellina sp. In ZPAL 2 and ZPAL 5, modest populations of Nonionidae are present. They appear to include two forms, more slender representing Nonionella bradyi (Chapman, 1916) (Figs 3.3 & 5.9) as well as slightly larger and less elongated in side view classified here as ?Nonionellina sp. (Figs 3.2 & 5.8). The latter do show, however, a considerable morphologic variability. The specimen from ZPAL 2 (Fig. 3.2) shows planispiral last coil typical for Nonionellina as well as granules in umbilical area. On the other hand, the specimen from ZPAL 5 (Fig. 5.8) exhibits slightly trochospiral final coil and no granules. Therefore, it is probable that this artificial taxon may represent more than a single species. Unfortunately, our specimens are often compacted and having umbilical area filled with sediment. Therefore, it is impossible to observe their full morphologic variability.
Quinqueloculina sp. This taxon is represented by a single, incomplete specimen (Fig. 4.1).
?Saracenaria sp. It is represented by a single incomplete specimen (Fig. 4.7). It differs from typical Saracenaria by lacking triangular section of latter, rectilinear chambers.
?Trochammina sp. It is represented by a single specimen that was significantly flattened by compaction (Fig. 3.1). It is the only agglutinated foraminifer found in our samples. Its poor preservation prevents establishing its precise taxonomic position. It cannot be ruled out that it is redeposited from underlying strata of the López de Bertodano Formation where diverse agglutinated benthic foraminifera were encountered (Huber Reference Huber1988).
?Planktonic foraminifera. Three specimens come from ZPAL 11. Two incomplete specimens, including one lacking the last chamber (Fig. 3.14), show biconvex outline with weak keel. According to these characteristics, they may represent genus Planorotalites. However, its strongly encrusted outer wall and overall outline bears resemblance with Morozovella lensiformis Subbotina 1953, as pictured in Pearson et al. (Reference Pearson, Olsson, Huber, Hemleben and Berggren2006, pl. 11.9). Another specimen from the same sample shows general globorotalid architecture (Fig. 3.13). Due to its test dissolution and recrystallization, it is difficult to classify.
Discussion
Preservation of foraminiferal assemblages
The two most fossiliferous samples, ZPAL 2 and ZPAL 5, came from coastal cliffs along López de Bertodano Bay west from Cross Valley (Fig. 1c), where the lowermost part of the LMF (Telm1) is exposed. At that location, it cuts into underlying Cretaceous-Palaeocene deposits of the López de Bertodano Formation rich in benthic and planktonic foraminifera (Huber Reference Huber1988). Nevertheless, foraminifera from ZPAL 2 and ZPAL 5 sites of the LMF are distinctively different in taxonomic composition and appear to represent in situ assemblages. They include decent numbers of taxa and specimens that are all similarly well preserved, taking into account rather invasive method of extracting specimens from rock samples. Moreover, the two assemblages are distinctively different, showing few dominating species and a number of accessory taxa (Table I), as is observed among modern living populations, e.g. Majewski (Reference Majewski2005).
The few specimens from sample ZPAL 11 from near Cape Wiman (Fig. 1c) are poorly preserved. They show signs of dissolution and recrystallization, which together with their very low abundance may suggest they are not in situ. The specimen of C. aff. saginatum (Fig. 3.11) is definitely the best preserved foraminifer from this sample and may correspond with the assemblage from ZPAL 5. In contrast, problematic planktonic foraminifera found at ZPAL 11, were not encountered at the two other sites and are all poorly preserved. They differ distinctively from planktonic foraminifera described from the underlying López de Bertodano Formation (Huber Reference Huber1988).
There is also an important question why foraminifera, so abundant in the two samples ZPAL 2 and 5 from the very base of the LMF (Telm1), are practically absent throughout the rest of the Eocene deposits on Isla Marambio. Higher in the LMF (Telm2), only a few specimens of Cibicides were reported (Bitner Reference Bitner1996), however, they were found encrusting brachiopod shells. Other abundant calcareous microfossils (ostracods) are also limited to Telm1. Only a single ostracod specimen was reported from Telm7 (Szczechura Reference Szczechura2001). On the other hand, rich calcareous macrofossils are present throughout the LMF (Feldmann & Woodburne Reference Feldmann and Woodburne1988, Stilwell & Zinsmeister Reference Stilwell and Zinsmeister1992, Gaździcki Reference Gaździcki1996) testifying to a marine setting. It appears, that there was no environmental reason for the absence of foraminifera, thus, they were most probably destroyed during diagenetic processes after deposition. However, there are no clear lithological variations throughout the LMF (see Fig. 2) that could simply explain that variability in preservation.
Palaeonvironmental interpretation
The two foraminiferal assemblages from ZPAL 2 and 5 include almost exclusively hyaline taxa that are characteristic for inner shelf, marine lagoons, and estuarine temperate environments (Murray Reference Murray2006). Numerous Cribroelphidium and Lobatula found in the ZPAL 5 assemblage may suggest shallower and more turbid water conditions than for ZPAL 2. However, C. saginatum is known from throughout New Zealand, where it was interpreted to live in normal marine salinity at upper and mid bathyal depths between c. 200 and 2000 m (Hayward et al. Reference Hayward, Hollis and Grenfell1997), which was rather unusual for this genus. In New Zealand assemblages, it constituted only up to 10% of the total benthic foraminifera, whereas in ZPAL 5 it amounted to 30%, which may suggest more restricted conditions. Moreover, the lack of planktonic foraminifera in ZPAL 2 and ZPAL 5 supports restricted, shallow marine, nearshore conditions at the western limits of the LMF on Isla Marambio. This interpretation is in good agreement with the stratigraphic position of the lowest unit of the LMF, Telm1. It is only 2 m thick and consists of gray to red-brown limonitic, glauconitic, sandy siltstones and pebble conglomerates. These developed as a result of accumulation on an erosional surface, cut into Cretaceous-Palaeocene deposits of the López de Bertodano Formation. This surface was flooded during an early Eocene transgressive cycle (Porębski Reference Pore˛bski1995, Marenssi et al. Reference Marenssi, Santillana and Rinaldi1998). This suggests a restricted, shallow marine habitat.
It appears that at the north-eastern end of the Eocene exposures, at ZPAL 11 (Fig. 1c), there might have been more influence of open marine conditions. The three potentially planktonic foraminifera constitute half of all specimens collected at that site, however, scarcity and poor preservation of these finds may suggest that they represent reworked material, thus having no palaeoenvironmental significance.
Biogeography
Foraminiferal finds of early and mid Palaeogene age from Antarctica are very sparse, therefore it is difficult to relate our assemblages to contemporaneous fauna from nearby geographic locations. Some reworked Palaeogene foraminifera are known from the East Antarctic shelf (Quilty Reference Quilty2001), unfortunately, they lack original population structure and seem to represent mixed associations. Microfauna described from ODP sites at high (Schröder-Adams Reference Schröder-Adams1991) and mid southern latitudes (e.g. Boersma Reference Boersma1985, Nomura Reference Nomura1991), represent deep water biota, rich in Uvigerinidae, and share only limited number of cosmopolitan species with the LMF, i.e. L. lobatula, C. ungerianus, and Eilohedra vitrea (Parker, 1953).
On the other hand, during the Eocene, due to the geographic proximity of Antarctica, Australia, New Zealand, and South America, a considerable overlap in species composition occurred between these regions (Quilty Reference Quilty2001). For comparison with foraminifera from the LMF, it is especially important that in all these regions, relatively shallow water habitats existed. The faunal links between Southern Hemisphere landmasses are well exhibited by the most characteristic component of foraminiferal assemblage from the LMF, Cribroelphidium. Cribroelphidium lauritaense and C. saginatum are known from the Eocene of Patagonia (Todd & Kniker 1952, Malumián Reference Malumián1990) and New Zealand (Finlay Reference Finlay1939, Hayward et al. Reference Hayward, Hollis and Grenfell1997). Guttulina irregularis (d'Orbigny, 1846), also very common in the LMF, was encountered in Patagonian Eocene deposits by Todd & Kniker (1952) as well. Cribroelphidium lauritaense and G. irregularis seem to indicate rather shallow water habitat, as they were not noted from Eocene deep water archives (Boersma Reference Boersma1985, Nomura Reference Nomura1991, Schröder-Adams Reference Schröder-Adams1991).
It is also important to note, that another abundant species (Table I) Globocassidulina subglobosa (Brady, Reference Brady1884) along with some less numerous (E. vitrea, L. lobatula, N. bradyi, Astrononion echolsi Kennett, 1967) long-ranging species were reported from Oligocene-Miocene strata from the Ross Sea (e.g. Leckie & Webb Reference Leckie and Webb1985, Strong & Webb Reference Strong and Webb2001). These taxa remained an important component of Antarctic foraminiferal assemblages until recently (Ward Reference Ward1984, Majewski Reference Majewski2005).
Conclusions
Benthic foraminiferal assemblages from the LMF of Isla Marambio are the first Eocene foraminiferal communities reported from West Antarctica. Their recognition is an important step in reconstructing evolution of Antarctic foraminifera. Two distinctive communities were found at two locations. They represent in situ assemblages inhabiting restricted, shallow water, nearshore environments. The more abundant assemblage from ZPAL 5, dominated by G. subglobosa, C. aff. lauritaense, G. irregularis, Lobatula sp., and C. aff. saginatum may suggest shallower and more turbid water conditions than the assemblage from ZPAL 2, dominated by Nonionellina sp., N. bradyi, G. subglobosa, and E. vitrea. The most characteristic foraminifera of the earlier assemblage, the two species of Cribroelphidium are known only from Eocene formations of New Zealand and Patagonia. A number of other species from the LMF are long-ranging taxa reported from stratigraphically younger Antarctic rocks that are typical also for modern Antarctic foraminiferal communities.
Acknowledgements
We would like to thank Bruce W. Hayward and Brian T. Huber for their reviews, which helped to improve the manuscript. Fieldwork on Isla Marambio was supported by the Instituto Antártico Argentino and the Fuerza Aerea Argentina. The latest stages of this study were supported by a grant of the Polish Ministry of Science and Higher Education No. 2011/01/B/ST10/06956.
