Cecilia R.AMENáBAR,Andrea CARAMéS,Susana ADAMONIS,,Ana DOLDAN,Gabriel MACEIRAS &Andrea CONCHEYRO,,3

1 Instituto de Estudios Andinos “Don Pablo Groeber”.Consejo Nacional de Investigaciones Científicas y Técnicas.Facultad de Ciencias Exactas y Naturales,Universidad de Buenos Aires.Pabellón II.Ciudad Universitaria.CP 1428.Ciudad Autónoma de Buenos Aires,Argentina;

2 Departamento de Ciencias Geológicas,Facultad de Ciencias Exactas y Naturales,Universidad de Buenos Aires,Argentina;

3 Instituto Antártico Argentino,25 de Mayo 1151,3 piso.CP 1650.San Martín,Buenos Aires,Argentina

Abstract A compiled selected literature on some groups of microfossils of the Mesozoic and Cenozoic of the James Ross Basin,eastern Antarctic Peninsula,is presented here,in order to show how the microbiota has been modified over time,triggered by environmental changes.The analyzed microfossils consist of palynomorphs (mostly pollen grains,spores,and dinoflagellate cysts),foraminifers and bryozoans.Dinoflagellate cysts and pollen-spores have been recorded in Jurassic to Pleistocene sedimentary outcrops.Dinoflagellate cysts proved to be good indicators for productivity and/or nutrient availability,surface water temperature and chemistry,the position of ancient shorelines and paleoceanographic trends.Pollen and spores allowed reconstruction of floral community and thus characterization of the climate that prevailed on the continent.Foraminifera,recovered from the Lower Cretaceous to the Pleistocene sedimentary rocks,provided information about the bathymetry,showing different marine settings (e.g.,coastal,inner neritic,outer neritic,upper bathyal) in different localities.The bryozoan record is restricted to the Cenozoic.Their colonial growth-forms reflect several environmental conditions such as shallow waters with a low rate of sedimentation,hard substrate and moderate or strong current action for the analyzed localities.The study of the Antarctic ecosystems based on the fossil microbiota and their response to the climate and the continental configuration changes,allowed understanding of the composition and dynamics of the polar environments,which have an important role in the Earth climate.

Keywords pollen-spores,dinoflagellate cysts,foraminifers,bryozoans,palaeoenvironment,Mesozoic-Cenozoic,James Ross Basin,Antarctic Peninsula

1 Introduction*

Organic and inorganic-walled microfossils are valuable for biostratigraphy,for the study of ancient palaeoenvironments and also for paleoceanographic reconstruction.They can be recovered in large numbers of specimens and diversity from several sedimentary rocks,both marine and continental.

Microbiota recovered from Antarctic sediments constitute an excellent tool for the knowledge of high latitude marine and continental ecosystems that developed from the Mesozoic to Neogene times,along with macrofossils,sedimentological and isotopic data.

In particular,the organic-walled microfossils known as palynomorphs (mostly pollen grains,spores,and dinoflagellate cysts),and the microfossils of inorganic composition,such as foraminifers and bryozoans,found in isolated exposures located in ice-free areas of James Ross Basin,Antarctic Peninsula,helps us to recognize the sedimentary successions,and understand their characteristics and genesis.In this sense,contributions on palynology and micropaleontology supported by good descriptions,illustrations,robust taxonomy,and precise provenance are essential to expand the knowledge of the Southern Hemisphere earth history.

In the last 140 Ma,the Antarctic Peninsula region underwent environmental changes during the final breakup of Gondwana,until its final isolation and location in the South Polar Region.This was accompanied by changes recorded not only in the marine microbiota but also in continental communities.In this respect,dinoflagellate cysts are especially useful in describing the dynamics of Mesozoic and Cenozoic oceans because they are sensitive to water temperature,salinity,depth,productivity and/or nutrient availability at the water surface,so they are also valuable as paleoenvironmental indicators.The same applies to planktonic foraminifera while benthic foraminifera depend on light,the substrate (hard or soft),nutrients and oxygen availability,temperature and salinity.Bryozoans are tightly controlled by substrate,and also by hydrodynamics and sedimentation rate.In contrast,pollen and spores allow reconstruction of coeval vegetation and prevailing climatic conditions in continental areas where parental plants originate.Since each group of microfossils indicates particular environmental parameters,each of them provides different information,and the combination of them all,allow good reconstructions of the past environments.

Therefore,the aim of this contribution is to compile published literature about some microfossil groups(Tables 1-3) to understand how microbiota changed over time,triggered by environmental changes that took place during Mesozoic and Cenozoic times in the James Ross Basin.The selected literature emphasizes those taxa that indicate particular palaeoenvironmental parameters,e.g.,bathymetry,substrate,nutrient levels,oxygenation,figu temperature and salinity of waters,or even positions of ancient shorelines and paleoclimate,among other factors.

2 Geological setting

The James Ross Basin (Figure1a) constitutes one of the most important back-arc basins located in the northeastern sector of the Antarctic Peninsula,between the 63°30' S and 65° S,and 57° W and 58° W (del Valle et al.,1992;Hathway,2000).It includes more than 6000 m of marine sediments deposited in two complete transgressiveregressive marine cycles from Jurassic to Eocene (Marenssi et al.,2012).These sediments are overlain by diamictites and alkaline volcanic rocks from James Ross Island Volcanic Group,JRIVG (Nelson,1975).

Mesozoic sediments include the Jurassic Ameghino Formation (Fm.),also known as Nordenskjold Fm.in English literature (?Oxfordian-Berriasian).This formation is the oldest sedimentary unit within the James Ross Island area and crops out at a number of isolated localities on the eastern coast of the Antarctic Peninsula (Whitham and Doyle 1989).On James Ross Island,the Ameghino Fm.(Figure1b) occurs as small clasts and as large,isolated,allochthonous glide-blocks enclosed within the Cretaceous Gustav Group (Snape,1992).

The Cretaceous marine sediments are included in the Gustav and Marambio groups (Figures 1a,1b).The Gustav Group (Aptian to Coniacian) which is up to 2100 m thick comprises the Lagrelius Point,Kotick Point,Whisky Bay and Hidden Lake Fms.It is composed of conglomerates,sandstones,claystones and shales interpreted as deep marine environments (Ineson et al.,1986).The Marambio Group (Santonian-Danian) is represented by Santa Marta,Rabot,Snow Hill Island,Haslum Crags Sandstones,López de Bertodano and Sobral Fms.This group is 3000 m thick and consists of fine-grained highly fossiliferous sandstones and claystones,subordinated conglomerates,and bioclastic shell beds,deposited in a middle shelf to offshore and inner shelf environments (Rinaldi et al.,1978;Olivero et al.,1986;Crame et al.,1991;Pirrie et al.,1997a;Olivero,2012;Roberts et al.,2014).

The Cenozoic Seymour Island Group (Late Paleocene-?Oligocene) comprises the Cross Valley-Wiman Fm.(Middle to earliest Late Paleocene,Montes et al.,2013),La Meseta Fm.(Late Paleocene to early Middle Eocene,Rinaldi et al.,1978;Marenssi et al.,1998) and the Submeseta Fm.(Middle Eocene to Early Oligocene,Montes et al.,2013).The sediments of this group represent the final filling stage of the James Ross Basin and they were deposited in incised valleys on an emergent marine platform(Marenssi et al.,1998).

For the La Meseta Fm.,Sadler (1988) recognized 7 major lithofacies which he named Telm 1-Telm 7,whereas Marenssi et al.(1998) proposed 6 erosionallybased units (allomembers) namely:Valle de las Focas(equivalent to Telm 1),Acantilados I and II (equivalent to Telm 2 and Telm 3 in part),Campamento (equivalent to Telm 3 in part and Telm 4),andCucullaeaI and II(equivalent to Telm 5 and Telm 6 in part).The uppermost Eocene/?Oligocene sediments previously considered as the upper unit of the La Meseta Fm.by Sadler (1988) and others,were named Submeseta allomember by Marenssi et al.(1998),and more recently it was defined as a new formation,the Submeseta Formation (Montes et al.,2013).It was divided into three allomembers denominated Submeseta I or Laminado (Telm 6 in part and Telm 7 in part),Submeseta II orTurritella(Telm 7 in part) and Submeseta III or Superior (upper Telm 7).Even though in many works the ages mentioned above are the ones considered for the La Meseta Fm.,Douglas et al.(2014),based on dinoflagellate cyst biostratigraphy,pointed out that the unit was deposited during the Middle to Late Eocene(between 45 Ma and 34 Ma),in contrast with previous Early to Late Eocene age assignments.Therefore,we assumed a Middle to Late Eocene age for the La Meseta Fm.(sensuMontes et al.,2013) and an uppermost Eocene/?Oligocene for the Submeseta Fm.(sensuMontes et al.,2013).

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Figure1 a,Location of the James Ross Basin area (after Pirrie et al.(1992);Olivero (2012);Montes et al.(2013)).b,Summary of the lithostratigraphy of the James Ross Basin.References:Gustav Group is after Pirrie et al.(1992);Marambio Group is after Crame et al.(1991),Olivero (2012) and Roberts et al.(2014);Seymour Island Group is after Montes et al.(2013),Douglas et al.(2014).Neogene deposits:Mendel Fm.after Nyvlt et al.(2011);Hobbs Glacier Fm.after Pirrie et al.(1997b);Belén,Gage and Terrapin Fms.after Lirio et al.(2003);Cockburn Island Fm.after Jonkers (1998);Wedell Fm.after Ga?dzicki et al.(2004).Abbreviations:Upper CL Mb-Upper Cape Lamb Mb.;Haslum Crags Sst.-Sandstones Alfa,Beta and Gamma Mbs.-Members;NW JR Is.-northwest of the James Ross Island;SE JR Is.-southeast of the James Ross Island.

The Neogene and Quaternary of the James Ross Basin consist of discontinuous outcrops that are represented by Miocene and volcanic and interbedded sedimentary rocks in the James Ross Island Volcanic Group,JRIVG (e.g.,Nelson,1975;Pirrie et al.,1997b).The sedimentary deposits are dominated by diamict and conglomerate with minor sandstones (Smellie et al.,2006).They are included within the Hobbs Glacier Fm.(Pirrie et al.,1997b) and the Cockburn Island Fm.,previously known as “Pecten-conglomerate”(Jonkers,1998).Other fossiliferous diamictites of the James Ross Basin are the Belén and Gage Fms.(Lirio et al.,2003)and the Mendel Fm.(Nyvlt et al.,2011).The Quaternary deposits are included within Terrapin Fm.(Lirio et al.,2003)and Weddell Fm.(Ga?dzicki et al.,2004).

Ice free outcrops show excellent exposures of the Cretaceous Gustav and Marambio Groups and they are concentrated in the James Ross,Vega,Humps,Cockburn,Seymour,and Snow Hill islands.In these islands,Cenozoic diamictites also crop out (Figure1a).The Seymour Island Group is recognized only in Seymour and Cockburn islands.

The present work focuses on the information provided by the microbiota from the James Ross Basin outcrops.Although cores can also provide further constraints on the Antarctic Peninsula climate changes based on the microbiota,they are beyond the scope of this work.Only microfossils from outcrops will be part of this contribution.Besides,the microbiota considered here consist of palynomorphs,foraminifera and bryozoans.Even though in Antarctic sediments other microfossils are present,such as diatoms,calcareous nannofossils,radiolarians and silicoflagellates;they are excluded from this analysis,since they are out of the scope of the authors’ expertise of the present contribution.The current lithostratigraphical framework of the James Ross Basin is summarized in Figure1b.

3 Fossil microbiotas from the James Ross Basin

3.1 Palynomorphs

Palynomorphs are very abundant in Jurassic to Cenozoic Antarctic sedimentary rocks;both marine and continental(Table1).

The Jurassic Ameghino Fm.(Nordenskj?ld Fm.,in English literature),located in the Antarctic Peninsula,is devoid of recognizable palynomorphs due to local thermal alteration.Nevertheless,the Jurassic palynoflora is known from two sources within James Ross Basin.One of these comes from an allochthonous block on James Ross Island belonging to the Ameghino Fm.that yielded Jurassic index dinoflagellate cyst genera,with typical acavate and proximate forms (a type of cyst based on gross morphology,see Traverse,2007) indicative of a mid-Tithonian age.Other Jurassic-diagnostic species have been recorded from Lower and Upper Cretaceous sediments due to reworking associated with erosion and transport.The terrestrial-derived palynoflora is only known by the record of a gymnosperm(Family Araucariaceae) pollen grain,of warm and wet climate,which was reworked into the Lower Cretaceous Lagrelius Point and Kotick Point Fms.

The Aptian was a time of widespread marine sedimentation within the Antarctic Peninsula-Scotia Arc-Southern Patagonia region,allowing the comparison of the dinoflagellate cyst assemblages and correlation between these key localities.During the Early Cretaceous (Lagrelius Point,Kotick Point and Whisky Bay Fms.),the dinoflagellate cyst assemblages of James Ross Basin had a wide distribution in southern,high latitude,oceanic regions and they are comparable with the Australian zonal scheme proposed by Helby et al.(1987).Furthermore,Dettmann and Thompson (1987) emphasized the resemblance between Antarctic marine assemblages and southern high-latitude regions,specifying that the James Ross assemblages were referred to the three consecutive dinocyst floras (named as superzones by Helby et al.,1987)—Muderongia,HeterosphaeridiumandIsabelidinium—defined by Helby et al.(1987) for the Australian assemblages.The “Muderongiadinocyst flora” is rich in gonyaulacoid (= Order Gonyaulacales) taxa and mainly contain the generaMuderongiaandSpiniferites.Dettmann and Thompson(1987) recognized this flora in the Kotick Fm.(Aptian-Albian) and it also seems to be represented in Lagrelius Point Fm.(Early Aptian) based on the taxonomic composition of the assemblage.The “Heterosphaeridiumdinocyst flora” first occurs during the Middle Albian.It is characterized by gonyaulacoid cysts such asCyclonephelium,HeterosphaeridiumandCribroperidinium,and the increase of peridinoid (= Order Peridiniales) cysts,especiallyDiconodinium.It was recognized in the Middle Albian-Cenomanian units of James Ross Basin (Kotick Point and Whisky Bay Fms.) by Dettmann and Thompson (1987).

The continental community was relatively uniform across southern Gondwana during the Early Cretaceous times.It consisted of gymnosperms (Family Araucariaceae and Podocarpaceae) and Pteridophyte (ferns),both dominating the forests,with subordinated Lycophyte (club mosses) and Bryophyte (mosses) (Cantrill and Poole,2012).The vegetation composition is known by the megaflora and the pollen-spore record (Riding et al.,1998;Riding and Crame,2002,among others).Although the Early Cretaceous Antarctic plant community has the same pollen-spore species as the rest of Gondwana,one of the features that differentiate it,is the low frequency ofClassopollisspp.For example,the Early Albian assemblages of Falkland Plateau yielded many of the pollen-spore species that have also been documented in James Ross Island.However,in the Falkland Plateau,Classopollis(pollen grain of the conifer Cheirolepidaceae)occurs abundantly and other gymnosperm pollen grains (Podocarpaceae,Araucariaceae) are infrequent.This suggests that the climax vegetation of the Falkland Plateau region was predominantly of dry-zone with mangrove cheirolepidacean communities developing,in contrast to the rainforest vegetation of the Antarctic Peninsula.These floral differences may indicate a sharp climatic gradient between the two regions that prevailed during the Early Albian (Dettman and Thompson,1987).The Early Cretaceous floras of the Antarctic Peninsula gradually became more regional relative to those of the earlier Jurassic to earliest Cretaceous.This was due to the development of new oceanic and atmospheric circulation patterns that generated a thermal gradient in hemispheres,prevailing humid conditions and more precipitation in tropical regions,and expanding cold temperate climates at high latitudes (Cantrill and Poole,2012).

In the Early Albian of the James Ross Basin (Kotick Fm.),angiosperms (or flowering plants) first appear as pollen records ofClavatipollenites,a primitive taxon of a shrubby angiospermous habit (Dettmann and Thomson,1987).Leaf records of angiosperms occur in the Late Albian(Cantrill and Poole,2012).By the Coniacian (Hidden Lake Fm.),angiosperms become dominant in the Antarctic floras,developing mixed forests together with conifers.

Returning to the marine realm,towards the Late Cretaceous (Santa Marta,Rabot,Snow Hill Island,López de Bertodano Fms.) other dinoflagellate cyst genera appeared and characterized the marine assemblages.During the Early Santonian-Early Campanian (Santa Marta Fm.) peridinoid cavate forms (Isabelidinium,Chatangiella,Eucladinium,Nelsoniella,Satyrodinium) dominated the dinoflagellate cyst assemblages.This Antarctic assemblage corresponds to the Australian “Isabelidiniumdinocyst flora” proposed by Helby et al.(1987).It was established by the Santonian-Campanian and ranged up to the Maastrichtian,with a circumpolar distribution in southern high latitudes (Dettmann and Thompson,1987).It is characterized by abundant and diverse peridinoid dinoflagellate cysts,with intercalary archeopyles (the archeopyle,a taxonomical important feature,is the place at which excystment occurs and the term intercalary refers to the position of the plate that is lost when the cyst splits).In some Campanian-Maastrichtian localities(e.g.,Sumner,1992;Caramés et al.,2016) dinoflagellate cysts and pollen-spores have been used to recognize the position of the coastline and nutrient supply from the continent to the marine environment.The P/G ratio(peridinioidversusgonyaulacoid cysts;Versteegh,1994) and the S/D ratio (sporomorphs or pollen-sporesversusdinoflagellate cysts;Versteegh,1994) have been applied in order to infer such palaeoenvironmental conditions.

Going back to the terrestrial community,a clear turnover in pollen taxa is recorded between the Santonian and Campanian.The most dramatic change is the increase in abundance and diversity of theNothofagusgroup during the Campanian,not only in pollen record (Nothofagidites) but also in megafossils such as leaves,wood and reproductive organs (Cantrill and Poole,2012).The ancestral pollen grainsNothofagidites endurusandN.senectuswere recorded from the Late Campanian-Maastrichtian sediments from the Santa Marta and Snow Hill Island Fms.(e.g.,Dolding,1992;Keating,1992).By the Campanian-Maastrichtian in Marambio (Seymour) and Vega islands,the genusNothofagiditeswas well-represented by species belonging to ancestral extinct taxa,and also by others that have affinities with fossil and living species of thebrassi-type pollen,grouped mainly in the SubgenusBrassospora.

By the Maastrichtian,the floras continued to increase in abundance and diversity,with theNothofagusgroup diversifying further and other angiospermous taxa appearing.Thus,the Early Cretaceous coniferous (Araucariaceae and Podocarpaceae) rainforests were modified during the mid-Campanian by the appearance ofNothofagus,which rapidly diversified and became an important component of the vegetation.This changed the aspect of the Late Cretaceous southern temperate forest flora,with podocarps and angiosperms being the main canopy elements of the perhumid,tall open forests,in the Campanian-Maastrichtian.

Towards the end of the Cretaceous and in the Early Cenozoic a gradual cooling occurred (Cantrill and Poole,2012).This climatic deterioration is evidenced in the dinoflagellate cyst assemblage prior to the K/Pg boundary(López de Bertodano Fm.) that consisted of dominance of the peridinioid genusManumiellaspp.(and the closely-related taxonIsabelidinium) in the latest Maastrichtian (Thorn et al.,2009).Due to their abundance and diversity within the Late Maastrichtian,the species ofManumiellaare useful biostratigraphical markers.The genus is typical of relatively near-shore,inner-shelf marine environments.Peaks in abundance (acme) ofManumiellarecorded within the López de Bertodano Fm.on Marambio (Seymour) Island,may indicate short-term regressions and/or ocean cooling prior to the K/Pg boundary.Similar spikes in abundance ofManumiellaassociated with that boundary have been detected elsewhere (in both Northern and Southern hemispheres),and have been related to mild cooling and regression.TheManumiellaspike (mostlyManumiella seelandica) in the López de Bertodano Fm.would be indicative of a short-term sea-level change prior to the K/Pg,which favored the habitat preferences of that dinoflagellate cyst and reflected a change in trophic levels (probably nutrient-rich surface waters),water temperature or chemistry(Thorn et al.,2009).

Moreover,in the uppermost López de Bertodano Fm.(on Seymour Island),250 m below the K/Pg boundary,the dinoflagellate cystManumiella seymourensisoccurs together with a great abundance (99% of marine assemblage)of a small spiny palynomorph,which was originally assigned to the acritarch genusMicrhystridium.Later it was identified as the chorate dinoflagellate cystImpletosphaeridium clavusby Bowman et al.(2013a,2013b).In addition to theI.clavusacme recognized 250 m below the K/Pg boundary,two extra abundance peaks have been detected within the 1000 m prior to the boundary (Bowman et al.,2013a).The three conspicuous acmes ofI.clavushave been linked with the presence of Antarctic winter sea-ice during cold episodes in the Maastrichtian,suggesting that temperature could have influenced the blooms of the parental dinoflagellate,and hence theI.clavusacmes.This is based on the fact that no other marine palynomorph exhibited an abundance pattern similar to that ofI.clavusin the uppermost López de Bertodano Fm.Nevertheless,Amenábar et al.(2014)considered that other factors,such as nutrient availability and physico-chemical properties of marine waters,should be taken into account in order to understand the cyst peaks.For example,I.clavusis well represented (70% of the marine assemblage) in the Late Campanian-Early Maastrichtian of the Snow Hill Island Fm.,in Ekel?f Point (James Ross Island)and in Sanctuary Cliffs (Snow Hill Island) (Amenábar et al.,2014).TheI.clavusacme in the Upper Campanian-Lower Maastrichtian of the above-mentioned sections would be related to a cold pulse during the Late Cretaceous,but does not necessarily imply the development of sea-ice cover,as was proposed for the uppermost López de Bertodano Fm.Therefore,the Late CretaceousImpletosphaeridiumacme recorded in high latitudes,could be a combination of cold and nutrient-rich surface waters that would have favoured the increase in number of that species (and also other peridinoid cysts) and finally its dominance in the palynological assemblages towards to the end of the Cretaceous (Amenábar et al.,2014).

The latest Cretaceous-Early Paleogene vegetation of the James Ross Basin remained relatively stable as regards its overall composition.The floral community consisted of mixed podocarp-southern beech temperate rainforest developed under abundant moisture during the Maastrichtian and earliest Danian (Bowman et al.,2014).Pollen-spore records show that Antarctic Peninsula vegetation suffered little at this time of global biotic disturbance as occurred around the K/Pg boundary elsewhere in the Southern Hemisphere (Bowman et al.,2014).

The Paleogene (～65-35 Ma) was a period of substantial climate change,involving Earth’s transition from a greenhouse to an icehouse state;a transition which was not gradual but was characterized by warming intervals,e.g.,the Middle Eocene Climatic Optimum (Zachos et al.,2008).The climatic and tectonic changes that occurred during this period strongly influenced the distribution of dinoflagellate cyst assemblages in the Southern Ocean.During the Paleocene and the Early Eocene (～65-50 Ma) Southern Ocean dinoflagellate cyst assemblages were dominated by cosmopolitan Early Paleogene taxa,while Antarcticendemic assemblage (composed of species that are restricted to latitudes south of 45°S) were rare (Bijl et al.,2013a,2013b).A distinct switch from cosmopolitan- dominated to endemic-dominated assemblages occurred around the Early-Middle Eocene boundary (～50 Ma).In the Middle Eocene,Antarctic-endemic taxa began to dominate the dinoflagellate cyst assemblages south of 45°S and prevailed during the Middle-Late Eocene.It is recognized in Middle Eocene units,not only in the La Meseta Fm.,but also in the Austral Basin,southern Argentina (González Estebenet et al.,2016).In the latest Eocene,the low-diversity of dinoflagellate cyst assemblages evidence a drop in water temperature and a deterioration of the climate conditions compared to those observed in most of the Eocene.Particularly notable is the increase of reworked dinoflagellate cysts of Cretaceous age in the uppermost Eocene Submeseta Fm.(Warny et al.,2018).Towards the Eocene/Oligocene transition (～34 Ma) profound global climatic and oceanographic changes occurred.These changes were accompanied by a rapid transition from greenhouse to icehouse conditions that were reflected in the Antarctic microbiota,both in marine and continental ecosystems.Close to this transition,the previously ephemeral ice sheets in the Antarctic interior coalesced,becoming relatively stable and permanent.This was influenced by the final opening of the Drake Passage and the initiation of a deep circumpolar current (Cantrill and Poole,2012).The Antarctic-endemic dinoflagellate cyst assemblage became extinct in the Eocene/Oligocene transition and was replaced by Oligocene cosmopolitan taxa,typically heterotrophic species (Family Protoperidinaceae)with modern analogues living today in sea-ice environments(Houben et al.,2013).This transition is documented by a deposit on Marambio (Seymour) Island,which is the earliest direct evidence for marine-based Cenozoic glaciation on the Antarctic Peninsula (Ivany et al.,2006).The deposit overlies the Submeseta Fm.(or Telm 7 of La Meseta Fm.) and is beneath the Weddell Sea Fm.(Pliocene-Pleistocene).It consists of a diamict with lower and upper pebbly mudstones in between.Mudstones above the diamict yield scarce Oligocene dinoflagellate cyst together with low abundances of the Antarctic-endemic species and other taxa characteristic of Eocene/Oligocene transition that disappear in the Early and earliest Oligocene,respectively.This deposit is similar in age to the one documented by Warny et al.(2018)which is located elsewhere on Marambio (Seymour) Island.

Since the Oligocene,ice sheets and glacial dynamics shaped the Antarctic Peninsula resulting in the reworking of sediments.Glacial processes increasing in areal extent,with further cooling towards the end of the Miocene,strongly reduced the possibilities of finding any indigenous (orin situ,i.e.,penecontemporaneous with deposition) palynomorphs and so hinder the reconstruction of post-Oligocene marine and continental environments.Geological evidence for Neogene glaciation in the James Ross Basin comes from sporadic terrestrial glacigenic sediments and glaciovolcanic sequences that are exposed on James Ross Island and adjacent areas represented by the Hobbs Glacier and Cockburn Island Fms.together with isolated diamictite deposits (Sandwich and Leal Bluff) on Vega Island.All these diamictites yielded abundant reworked Late Cretaceous dinoflagellate cysts,and some of them also contain scarce Eocene to Oligocene cysts.Some of them (at Cascade Cliffs and Hobbs Glacier localities) also contain rare indigenous dinoflagellate cysts indicative of the Miocene(Bitectatodinium tepikiense).Other marine palynomorphs,such as the acritarch genusLeiosphaeridiaand the dinoflagellate cystImpletosphaeridium,bothrecorded in most of the diamictites,are considered indigenous elements.Leiosphaeridiaindicates the presence of sea-ice and near-modern climate conditions during the Late Neogene(Salzmann et al.,2011).The sporadic occurrence of the sub-Arctic to temperate dinoflagellate cystBitectatodinium tepikiense(in Hobbs Glacier),however,suggests sea surface temperatures might have been substantially warmer during some interglacials.

Considering the Cenozoic floral communities,during the mid-Paleocene mixed forests composed ofNothofagus,Podocarpaceae and Proteaceae,comparable to the modern Valdivian rainforests of South America,were developed in the James Ross Basin,at least until the latest Paleocene(Warny et al.,2018).In the Late Eocene,the climate began to cool in relation to decliningpCO2levels andNothofagusbecame more dominant in these high southern latitude forests (Cantrill and Poole,2012).The interpretation of a cooling climate across the Eocene/Oligocene transition is corroborated by the pollen-spore record,which shows a dramatic decrease in diversity,occurrence and abundance of continental palynomorphs.The impoverishment of the palynological assemblage can be seen in the diamictites of Marambio (Seymour) Island,in Hobbs Glacier Fm.(James Ross Island),Cockburn Fm.(Cockburn Island),and diamictites in Vega Island,where sediments are barren of indigenouspollen-spores or they are scarce.From the Oligocene to the Pliocene,extremely adverse conditions for plant growth prevailed in the James Ross Basin,together with a cyclical glacial-interglacial environment.Thus,by the Early Oligocene,the Antarctic flora had become more impoverished,withNothofagusand Podocarpaceae conifers probably dominating the canopy,and with pteridophytes,lycophytes and some bryophytes contributing to the understory.By the Miocene-Pliocene the vegetation was probably more similar to the tundra shrub vegetation of southern South America,supporting local stands ofNothofagusand a few other angiosperms and conifer taxa that have been developed in coastal regions (Cantrill and Poole,2012).Nevertheless,Salzmann et al.(2011),in the unsuccessful efforts to find indigenous pollen-spores in the Early Pliocene diamictites,concluded that,although the lack of indigenous terrestrial-derived palynomorphs cannot be taken as a definite evidence of non-existence of vegetation,the presence of a Pliocene vegetation cover on James Ross Island is improbable,even during warm interglacials.Thus,their assumption supports previous reconstructions of a permanent ice sheet on the Antarctic Peninsula throughout the Late Neogene.

3.2 Foraminifers

Reports of foraminifera from the James Ross Basin are limited to the Cretaceous,Paleogene,Neogene and Quaternary periods (Table2).

The calcareous and agglutinated benthic foraminifers recovered from Early Cretaceous-Late Cretaceous sediments belonging to Kotick Point,Whisky Bay and Hidden Lake formations are scarce (Mcfadyen,1966;Webb,1972;Hradecká et al.,2011).However,it should be noted that few samples of these units were studied.In particular,the low diversity assemblage recovered from a single sample of the Hidden Lake Fm.,which appears to be richer and more diverse than the other two units,yielded mainly calcareous and cosmopolitan taxa.According to the planktonic together with the benthic species of the generaLenticulina,GavelinellaandValvulineria,the foraminiferal assemblage was interpreted as indicative of outer shelf to an upper bathyal environment with a maximum water-depth of 200-300 m (Hradecká et al.,2011).

The Santonian-Early Campanian Lachman Crags Member of the Santa Marta Fm.,which probably corresponds to the Alpha Member of Olivero et al.(1986),was more intensively sampled.It yielded agglutinated and calcareous cosmopolitan taxa,but an important bias in preservation conditions was suggested on the basis that calcareous foraminifera seem restricted to few stratigraphic levels of tuffaceous sandstones cemented by calcium carbonate,while agglutinated foraminifera together with radiolarians occur in sandstone levels in addition to the tuffaceous sandstone (Florisbal et al.,2013).On the basis of the consistent occurrences ofGyroidinoides globosus,deep-neritic to upper bathyal paleo-depths were suggested(Hradecká et al.,2011;Florisbal et al.,2013).

The Late Campanian and possibly Early Maastrichtian sediments of the Snow Hill Island Fm.,exposed at Ekel?f Point (James Ross Island),yielded agglutinated and calcareous foraminifera.The Ekel?f Costa section,which exposes the lower levels of the unit in the area,yielded low diversity and scarce benthic assemblages mainly composed of cosmopolitan taxa with long stratigraphic ranges and well-known Upper Cretaceous global records (Caramés et al.,2016).A Late Campanian and possibly Maastrichtian age was based on dinoflagellate cysts,and therefore,the section would partially cover the upper part of the NG sequence of Olivero (2012) (Caramés et al.,2016).Outer shelf-upper bathyal environments,in agreement with the sedimentary data and with a record of gonyalaucoid cysts of the genusImpagidinium(indicative of outer neritic and oceanic settings),were deduced from the dominance ofBathysiphon(epifaunal habitat in tranquil bathyal and abyssal settings),even for the last two fertile samples that yielded high proportion ofSpiroplectammina(an active deposit feeder that lived at or just below the sediment surface into shelf to marginal marine environments).Although the abundance ofSpiroplectammina,as well as the S/D and P/G ratios used in palynology,would favour coastal to inner neritic environments interpretation,Caramés et al.(2016) have suggested a deeper environment with a narrow continental shelf which would have allowed the quick transport downslope of the terrestrial palynomorphs,the peridinioid dinoflagellate cysts and the elongate keeled tests ofSpiroplectammina,and their final accumulation in deeper marine environments.

The Chorrillo Leonardo and Ekel?f Ovest sections characterize the middle and upper part of the Cretaceous sequence exposed in the Ekel?f Point area.Although both sections were first assigned to the Santa Marta Fm.,according to the stratigraphy proposed by Olivero (2012),they are part of the Snow Hill Island Fm.Their foraminiferal content was mainly analyzed by Gennari (1997),Concheyro et al.(1997),and Morlotti and Concheyro (1999) to obtain paleoenvironmental information.The species list published by Concheyro et al.(1997) shows assemblages mainly dominated by agglutinated taxa with a mixture of cosmopolitan and indeterminate species.According to the morphological groups recognized in the agglutinated assemblages,Concheyro et al.(1997) inferred outer shelf to slope environments.Based on parallel trends of the generic diversity,the specific diversity and the faunal density along the two sections,Morlotti and Concheyro (1999) interpreted stable bottom environments,good availability of nutrients,bottom water oxygenation and low turbidity.In addition,according to the distribution of morphogroups,with dominant epifaunal (mostly omnivores and opportunists)taxa at the lowest levels and infaunal scavengers increasing upwards and becoming dominant in the upper part of the Ekel?f Ovest section,a generally very low surface water productivity that progressively improved was inferred.

There are a few references to foraminifers recovered from the upper part of the Snow Hill Island Fm.Olivero(1975) described nine indeterminate agglutinated species proceeding from near Nordenskj?ld hut (Refugio Suecia),northwest of Snow Hill Island.Ronchi et al.(2002) mention an almost monotypic and abundant assemblage composed of a very large sized,agglutinated endemic,new taxa,recovered from an outcrop at Cape Lamb on Vega Island.Undoubtedly,both assemblages should be studied in more detail.Huber (1988) recordsCyclamminasp.C.complanataChapman from sediments of the informalRotularia1 unit(Macellari,1988).These sediments were originally included in the López de Bertodano Fm.but currently,based on the stratigraphy redefined by Olivero et al.(2007),they are considered the highest stratigraphic levels of the Snow Hill Island Fm.Regarding the other five species found by Huber(1988) from Snow Hill Island,it is not clear if these species belong to the Snow Hill Island Fm.or the López de Bertodano Fm.,due to the unknown location of the samples.

The Upper Cretaceous and Paleocene marine sedimentary sequence of the James Ross Basin is included in the López de Bertodano Fm.The Cretaceous was intensively sampled and studied by Huber (1988) and yielded one of the most abundant foraminiferal faunas recognized in the basin,with assemblages dominated by agglutinated taxa,followed by calcareous benthic and planktonic species.In contrast,the revised Paleocene outcrops were limited to few samples and only a part of them provided foraminifers,mainly calcareous benthic taxa with only a few Danian diagnostic species.As a result of the foraminiferal studies carried out in the López de Bertodano Fm.,Huber (1988) found that the biostratigraphic resolution based on foraminifers for the Upper Cretaceous in high-latitude regions had particular problems.Among them,the author highlights the absence of the thermophilic keeled planktonic taxa used in the correlation of low latitude regions;the earlier occurrence of some cosmopolitan planktonic species in the James Ross Basin than in lower latitude regions;and in the James Ross Basin several benthic taxa,such asCyclamminaspp.,Frondicularia rakauroana(Finlay),Buliminellaspp.,Ceratolamarckinacf.C.tuberculata(Brotzen),Alabamina westraliensis(Parr) andPlanispirillina subornata(Brotzen),predate any other known record in lower latitudes.According to Huber (1988),the recovered foraminiferal fauna is a mixture of cosmopolitan and provincial taxa.The benthicCyclamminacf.complanataChapman,Gaudryina healyiFinlay,Dorothia elongataFinlay,Karreriella aegraFinlay,Frondicularia rakauroana(Finlay),Bulliminella cretaFinlay andAlabamina creta(Finlay) were highlighted as austral taxa by Huber (1992).The austral character of planktonic fauna is evidenced by the dominance of rugoglobigerines,hedbergellids and heterohelicids informal groups,which are also present in Tethyan and transitional provinces,along with the endemic austral taxaHedbergella sliteriHuber andArchaeoglobigerina mateolaHuber (Sliter,1976;Huber,1992).In regard to the paleoecologic interpretation,Huber considered thatCyclamminacf.C.complanataChapman,Alveolophragmium macellariHuber andHyperammina elongateBrady,which are also the most common foraminifera of the Cretaceous sequence of the López de Bertodano Fm.,were solution-resistant forms,so he warned about the presence of residual assemblages that should not be applied with confidence for paleoenvironmental interpretations.Nevertheless,taking into account high-diversity assemblages that more accurately represent the biocoenosis,Huber (1988) interpreted inner neritic settings for the lower 250 m of the sequence on Seymour Island (the lowest 100 m currently considered as belonging to the Snow Hill Island Fm.and the remaining 150 m to the López de Bertodano Fm.),and outer neritic environments for the upper 950 m of the López de Bertodano Fm.

The presence of foraminifera from the Paleocene Sobral Fm.was exclusively investigated within its informal Unit 1.According to Huber (1988),this unit has the first records ofLenticulina muensteri(Roemer) andBuliminella proceraHuber,as well as some species that have also been recorded in the underlying Cretaceous and Danian stratigraphical levels of the López de Bertodano Fm.The first one is known as an Upper Cretaceous-Danian cosmopolitan species and the second one,which composes as much as 82 % of the total assemblage,has an austral distribution and abundant occurrences from the Maastrichtian to the Eocene in the Austral Basin,south of Argentina (Malumián and Caramés,1994).Furthermore,as the latest stratigraphic interval of the López de Bertodano Fm.,the Sobral Fm.yielded the cosmopolitan Danian index taxonGloboconusa daubjergensis(Br?nnimann) (=Globastica daubjergensisin Huber,1988).An outer neritic interpretation was based on foraminifers and sedimentary facies.It is important to point out that,unlike the Maastrichtian faunas of the López de Bertodano Fm.,the Danian assemblage of the Sobral Fm.,as well as the one recovered from the López de Bertodano Fm.above the dissolution facies,are dominated by calcareous benthic foraminifera (see Table5 in Huber,1988).

Still within the Paleogene sequence,whose foraminiferal fauna until now was only known on Marambio(Seymour) Island,is the La Meseta Fm.which was described by Ga?dzicki and Majewski (2012) as a “unique insight into Eocene life in what has become a harsh polar environment”.In the lowermost section (Telm 1) of the La Meseta Fm.,near the base of the sequence that cuts into the underlying López de Bertodano Fm.,two mainly calcareous benthic assemblages were described by Ga?dzicki and Majewski(2012).The assemblages were interpreted as typical of shallow marine nearshore conditions on the basis of the almost exclusive presence of hyaline taxa characteristic of inner shelf,marine lagoons and estuarine temperate environments.In particular,one of the assemblages that is composed of numerousCribroelphidiumandLobatulasuggests shallower and more turbid waters (Ga?dzicki and Majewski,2012).Both assemblages yielded the typical post-Paleocene generaGlobocassidulinaandCribroelphidium,and the post-Paleocene speciesGlobocassidulina subglobossa(Brady),Anomalinoides spissiformis(Cushman and Stainforth),Cibicidesaff.ungerianus(d’Orbigny) andGyroidina zelandicaFinlay.Ga?dzicki and Majewski (2012)also highlighted the similarities in composition among the La Meseta Fm.and other Eocene foraminiferal assemblages from New Zealand and South America,all of relatively shallow water,withCribroelphidiumaff.lauritaense(Todd and Kniker) andC.aff.saginatum(Finlay),similar to the known species of the Eocene of Patagonia and New Zealand,andGuttulina irregularis(d’Orbigny) that was found in the Agua Fresca Fm.in Chilean Patagonia.

The succeeding units with foraminifera accumulated during interglacial or glacial periods.The shallow marine sediments of the Cockburn Island Fm.typify sediments accumulated during interglacial warm periods (Ga?dzicki and Webb,1996;Jonkers et al.,2002).The other units are considered as glacigenic/glacimarine.Both Cockburn Island Fm.and the other units yielded benthic foraminifers that are autochthonous (those that quickly colonized the sediments),parautochthonous (locally transported in the same environment) or reworked from older units.Once the allochthonous specimens are detected,the post-Early Oligocene foraminiferal assemblages that result are strongly dominated by calcareous taxa and,although each one has diverse population structures,their taxonomic compositions are very similar,including almost all the taxa that are still common in Antarctica.These Neogene-Quaternary sediments containing foraminifers have been deposited on coastal and shelf marine settings.Thus,those shallow-water paleoenvironments would explain the assemblages’similarities.Ammoelphidiellahas been highlighted as a conspicuous taxon because the genus ranges only until the Pliocene.According to Jonkers et al.(2002),two species have been recorded,Ammoelphidiellasp.nov.restricted to the Miocene from Hobbs Glacier and Belén Fms.,andAmmoelphidiella antarcticaConato and Segre from Gage Fm.,Ekel?f Point diamictite and Cockburn Island Fm.Nevertheless,these species should be studied in more detail.

3.3 Bryozoans

Bryozoan assemblages of the James Ross Basin are mainly concentrated in the Cenozoic sediments of the La Meseta Fm.on Marambio (Seymour) Island (Table3).They also appear in other lithostratigraphic units that consist of diamictite deposits cropping out on James Ross Island.Bryozoans of the orders Cyclostomata and Cheilostomata have been reported from the La Meseta Fm.near Cape Wiman in Telm 1-2 (Gazdzicki and Hara,1994).The Cyclostomata are represented by the family Ceriopodidae,and the Cheilostomata by the anascan family Membraniporidae,the latter yielding the largest number of specimens.

Other localities on Marambio (Seymour) Island have been studied in order to find fossil bryozoans.Hara (2001)described new assemblages collected at eight localities(ZPAL 1,5,11,12,6,8,14 and 3) from the La Meseta Fm.Among the seven lithofacies defined by Sadler (1988),only Telm 1,2,6 and 7 have bryozoans.Telm 1 at ZPAL 1 bears the most diverse and abundant assemblage (43 species and 1048 specimens) while Telm 2,6 and 7 show a remarkable impoverishment of taxa (4 species and 9 specimens).The assemblage of Telm 2,indicates stressful marine-estuarine conditions,while the one obtained from Telms 6 and 7 has been related to a cooling event that occurred during the Late Eocene.In contrast to the colonies from Telm 1 that are well-preserved,those from Telm 6 and 7 show poor-preservation.The reported bryozoan fauna includes representatives of Cyclostomata and Cheilostomata,the former being more abundant.Hara (2001) described two new genera and 19 new species:Dennisia eocenicagen.et sp.n.,Reticrescis plicatusgen.et sp.n.,Aimulosia lamellosasp.n.,Aspidostoma multiformissp.n.,Aspidostoma pyriformissp.n.,Aspidostoma taylorisp.n.,Celleporaria australissp.n.,Celleporaria gondwanaesp.n.,Celleporaria mesetaensissp.n.,Celleporaria ovatasp.n.,Ceriopora hemisphaericasp.n.,Disporella marambioensissp.n.,Neofungella capitulasp.n.,Osthimosia globosasp.n.,Reptomulticava clavaeformissp.n.,Reptomulticava seymourensissp.n.,Retecrisina antarcticasp.n.,Rhynchozoon quadratussp.n.andSmittoidea gazdzickiisp.n.Some genera represent the first record for Antarctica:Ceriopora,Retecrisina,Borgella,Crassohornera,Reptomulticava,MetroperiellaandCelleporaria.Others,such asBorgella,Neofungella,Melicerita,Smittina,Smittoidea,Celleporaria,Aimulosia,Metroperiella,Osthimosia,ReteporellaandRhynchozoonrepresent the earliest fossil record of these genera for Antarctica.Hara (2002) erected a new species of the family Macroporidae,for massive,multilamellar colonies recovered from Telm 1 at ZPAL 1.The new species,Macropora antarctica,represents the oldest record of the genus.Hara(2015) described zoecial moulds from Telm 1 at ZPAL 2,belonging toBeaniacf.mirabilis,Beaniaaff.inermis,Beaniasp.,?Vasignyellasp.and ?Malakosariasp.,together with some specimens of the Family Savignyellidae.The genusBeania,the Family Savignyellidae,Catenicellidae and ?Malakosariapresent in La Meseta Fm.constitute the earliest fossil record of these taxa in Antarctica.

The bryofauna from Telm 4 (NRM1 Site) and Telm 5(Rocket Site,Marsupial Site and Ungulate Site) described by Hara et al.(2018) and Taylor et al.(2008) consist of Cyclostomata represented by the family Cerioporidae and the Cheilostomata families such as Microporidae,Lunulitidae,Otionellidae and Brydonellidae.The last ones are represented by new species such asMicropora nordeskjoeldi,Lunulitesmarambionis,Otionellina antarctica,Otionellina eocenicaand a new genus and species of an encrusting bryozoan with a different umbonuloid frontal shield,Uharella seymourensis.This species along with a biostrome ofSmittinasp.,have been recorded from Telm 6 (Localities ZPAL and IAA 1/93),while Telm 7 (Localities ZPAL 3 and DPV 13/84) yields few specimens of ?Idmidroneasp.,Celleporaria mesetaensis,?Goodoniasp.andReticresis plicatus(Hara,2001;Hara et al.,2018).

Regarding the association of colonial morphotypes,Hara (2002,2015) and Gazdzicki and Hara (1994) made inferences about the paleoenvironment of the La Meseta Fm.based on the close relation between the colonial growth forms and various environmental factors (Nelson et al.,1988;Hageman et al.,1998).Many colonial growth forms have been documented,the dominants being the cerioporid and celleporiform types.Less abundant zoarial forms are membraniporiform,adeoniform,reteporiform,retiform,petraliform,catenicelliform and very few cellariform and crissiid colonial types.The occurrence of all these growth forms suggests shallow waters with a low rate of sedimentation,hard substrate and moderate or strong bottom currents for the La Meseta Formation.

The ?Late Miocene Hobbs Glacier Fm.,at Rabot Point(James Ross Island) yields bryozoan colonies belonging to Cyclostomata and Cheilostomata represented byDisporella,Microporella,a species with affinities withDengordonia,an indet.species of a celleporoid and an indet.Cheilostomata species (Pirrie et al.,1997b).In the same unit,but at Hamilton Point (James Ross Island),Concheyro et al.(2007)reported bryozoan encrusting and erect colonies ofMicroporella,Melicerira,Hornera,Polirhabdotos,Membranicellariaand one species showing affinities withCellarinella stellaepolaris,being the last three of the oldest records for the James Ross Basin.

Marenssi et al.(2010) mentioned the presence of bryozoan colonies encrusting a metamorphic rock clast from a diamictite that crops out on Marambio (Seymour) Island.This unit,assigned a Late Miocene age by fossil content and isotopic studies,belongs to the Hobbs Glacier Fm.

The Late Pliocene Cockburn Island Fm.at Cockburn Island,yields encrusting bryozoan faunas represented by at least 14 families of Cheilostomata and Cyclostomata(Hennig,1911;Hara and Crame,2010).The Weddell Sea Fm.on Marambio (Seymour) Island is also Late Pliocene in age,and an encrusting unilamellar,fan-shaped colony ofEscharellahas been described by Gazdzicki et al.(2004),being the first record of this genus for Antarctica.

On Vega Island,the Cape Lamb diamictite deposits,dated as Pleistocene,yield a bryozoan assemblage characterized by encrusting colonies that include 4 species of Cheilostomata of the families Calloporidae,Microporidae,Hippothoidae,Microporellidae and 1 Cyclostomata of the family Crisiidae.The presence ofMicroporella stenoporta,Hippothoa flagellum,Ellisina antarcticaandMicropora notialisconstitute the first record of these bryozoan taxa in Cenozoic diamictites of the Antarctic Peninsula (Adamonis et al.,2015).This fossil fauna resembles the one reported from the Pliocene Cockburn Island Fm.(Hara and Crame,2010) as well as the recent assemblage registered from Admiralty Bay,King George Island,South Shetlands (Pabis et al.,2014).

4 Final considerations

The comprehensive review presented herein reveals that a considerable number of palynological and micropaleontological studies have been carried out on the James Ross Basin over more than 100 years.

Most researches undertaken in the basin has been palynological studies of sedimentary rocks of Jurassic to Pleistocene age.Studies regarding foraminifera follow those of palynology and comprise records from Lower Cretaceous to Pleistocene sediments.Lastly,bryozoan research is limited to Cenozoic deposits.

The large number of localities that have been studied in order to recover palynomorphs may be due in part to the fact that pollen-spores and dinoflagellate cysts are frequently more abundant in sediments than most other microfossils.Palynomorphs are generally well preserved in all type of sediments with fine silt or clay matrix that is usually appropriate for good preservation of organic-walled microfossils,but highly oxidizing conditions in some continental deposits can cause their destruction.On the other hand,biogenic minerals such as calcium carbonate and silica of the skeletal parts of foraminifers and bryozoans are susceptible to dissolution,leading to a poor representation of their assemblages and consequently underrepresenting the original faunas.

Among the disciplines here summarized,palynology is currently the only one that provided information about the Upper Jurassic of the basin.Therefore,the search for foraminifera should be intensified.Palynological data were essential for reconstructing the Early Cretaceous paleogeography and also allowed the comparison with other southern high latitude regions.The limited information on foraminifera for this period indicates dissolution problems,but the scarcity of studies could also be responsible for the lack of their records.If the search for foraminifera was intensified,the findings could provide relevant paleoenvironmental information,such as in the case of the Lower Albian of the Austral Basin (Patagonia,southern Argentina),where the high planktonic content of foraminifera and calcareous nannoplankton,along with some benthic foraminifera,have revealed high productivity waters and deficient oxygen conditions of the seafloor (Malumián,1990).

In general,dinoflagellate cysts proved to be good indicators for productivity and/or nutrient availability,surface water temperature or chemistry,the position of ancient shorelines and paleoceanographic trends.Pollen and spores allowed reconstruction of the floral community and thus characterization of the climate that prevailed in the continent.Foraminifera provided information about the bathymetry,showing different marine settings (e.g.,coastal,inner neritic,outer shelf,upper bathyal) in different localities.The bryozoan colonial growth forms reflected several environmental factors such as shallow waters with a low rate of sedimentation,hard substrate and a moderate or strong current action on the Antarctic sediments.This proves that interdisciplinary studies have clear advantages.For example,for the Santonian-Campanian of the Lachman Crags Member of the Santa Marta Formation,Florisbal et al.(2013) interpreted a sub-bathyal neritic environment based on the calcareous foraminifera fauna in a section showing a strong bias in the microfossil preservation at some of the levels.The presence of the ostracodMajungaellaconfirmed the shelf environment with warm waters and normal salinity.Whereas,the record of agglutinated foraminifera along with the first radiolarians described for the James Ross Island region at other levels,suggests a Late Cretaceous age.By the same token,an integrated analysis of dinoflagellate cyst and foraminifera conducted in Late Cretaceous strata of Ekel?f Point has produced good results.The dinoflagellate cysts allowed proposing a Late Campanian and possibly Maastrichtian age,while the foraminifera supported outer shelf-upper bathyal environments with a narrow continental shelf.

The microbiota changes triggered by environmental variations that occurred during Mesozoic and Cenozoic times in the James Ross Basin can be synthesized as follows.

From the Late Jurassic up to Late Cretaceous (Early Maastrichtian),the dinoflagellate cyst assemblages had a wide distribution in southern,high latitude,oceanic regions and resemble Australian assemblages.At the end of the Cretaceous (latest Maastrichtian) and in the Early Cenozoic a gradual climatic decline occurred which is evidenced by dinoflagellate cyst assemblages dominated by peridinioid speciesof the generaManumiellaspp.

The Paleocene and Early Eocene dinoflagellate cyst assemblages were dominated by cosmopolitan taxa with a small number of Antarctic-endemic species.But in the Middle Eocene,this trend is reversed,with Antarcticendemic taxa dominating the assemblages of several circum-Antarctic sites.Their distribution reflects an ocean-circulation pattern characterized by wide clockwise gyres surrounding Antarctica that were disrupted by the opening and deepening of the Tasmanian and Drake passages towards the Eocene/Oligocene transition.In general,during the Paleogene,the assemblages suffered a gradual species turnover,showing a decrease in the number of species since the Oligocene up to the present.

Latest Maastrichtian foraminifera fauna was dominated by abundant agglutinated taxa together with calcareous benthic and planktonic species.During the Cretaceous/Paleogene transition,the high calcareous dissolution prevented evaluation of the changes in the foraminiferal fauna.Upwards,in the Danian,the foraminiferal assemblage only provided scarce calcareous benthic taxa with few age diagnostic species.Eocene foraminifera assemblages,different from all the preceding ones,have a similar composition to those of South America and New Zealand.

During the Eocene,the bryozoan fauna shows a decrease in richness and abundance linked with the advance of cooler conditions that prevailed towards the Late Eocene/Oligocene.

Neogene sediments yielded very scarce indigenous palynomorphs and foraminifera,whereas the fossil bryozoans,although scarce,are autochthonous.

During the Early Cretaceous,the floral community of the James Ross Basin was represented mainly by conifers.The first occurrence ofNothofagusin the mid-Campanian,changed the appearance of the southern forest flora,composed equally of gymnosperms and angiosperms.The latest Cretaceous-Early Paleogene vegetation remained relatively stable in overall composition,and by the Oligocene the flora became impoverished but withNothofagusand Podocarpaceae remaining as dominant taxa.The Miocene and Pliocene flora was similar to the tundra shrub vegetation that develops today in southern South America.

Due to the sporadic nature of the outcrops,the study of several disciplines is essential to provide a complete biostratigraphic framework,to reconstruct past environments and to outline paleogeographic schemes.Thus,it would be advisable to conduct an interdisciplinary fieldwork program in order to obtain different microfossils from the same sections and outcrops.This approach will allow a more detailed reconstruction of the marine and continental microbiota of the Mesozoic and Cenozoic of the James Ross Basin.

AcknowledgmentsThe authors are most grateful to the Instituto Antártico Argentino and the Argentinian Air Force for the scientific and logistic support during the Argentine Summer Antarctic Expeditions that enabled sampling for palynological and micropaleontological studies that were incorporated into this contribution.We thank the two reviewers,Juan Perez Panera and Peter Sadler,for their valuable suggestions that allowed us to improve the manuscript.We also thank C.Acosta Hospitaleche (Museo de La Plata,Argentina) and J.N.Gelfo (Museo de La Plata,Argentina) for inviting us to contribute to the Special Issue on “Geology and palaeontology of the James Ross Basin,Antarctic Peninsula”.This paper is contribution number R-287 of the Instituto de Estudios Andinos ‘Don Pablo Groeber’(IDEAN-CONICET).