Absolute and probable age limits of the Tussac House forest bed

Unlike Argentina (see Barreda Reference Barreda1996a,Reference Barredab, Barreda & Palamarczuk Reference Barreda and Palamarczuk2000a,Reference Barreda and Palamarczukb,Reference Barreda and Palamarczukc, Reference Barredab, Barreda & Palamarczuk Reference Barreda and Palamarczuk2000a,Reference Barreda and Palamarczukb,Barreda et al. Reference Barreda, Palazzesi and Marenessi2009, Amenabar et al. Reference Amenabar, Guerstein, Alperin, Daners, Casadio and Raising2023), no formal pollen and spore-based palynostratigraphy has been developed to date and correlate Palaeogene and Neogene sediments on the Falkland Islands or independently dated marine sediments on the submerged Falkland Plateau or Malvinas Basin (Amenabar et al. Reference Amenabar, Guerstein, Alperin, Daners, Casadio and Raising2023 and references therein). Accordingly, the age limits of the West Point Island forest bed and the Tussac House forest bed (Macphail & Cantrill Reference Macphail and Cantrill2006, Macphail Reference Macphail2021) have been inferred using the time distribution of fossil pollen species (morphospecies) whose NLRs occur in mega- and meso-thermal rainforest in Patagonia, commonly defined as encompassing the Argentinean provinces of Neuquén, Río Negro, Chubut, Santa Cruz and Tierra del Fuego (Quattrocchio et al. Reference Quattrocchio, Volkheimer, Borromei and Martinez2011, Palazzesi et al. Reference Palazzesi, Vizcaíno, Barreda, Cuitiño, del Río and Goin2021 and references therein).

The present study focuses on the time distribution of fossil pollen of three rainforest thermophiles preserved in marine sedimentary formations in Patagonia, viz. Canthiumidites cf. bellus Clavatipollenites and Cupanieidites, whose NLRs, respectively, are Randia (now found in dry forest communities in northern South America), Hedyosmum (Chloranthaceae; now found in lowland to montane rainforest, including disturbed areas at low elevations, from 18°N to 25°S) and Cupania (Sapindaceae; now found north of 30°S in southern South America; see Marchant et al. Reference Marchant, Almeida, Behling, Berrio, Bush and Cleef2002). Isotopic (⁸⁶Sr/⁸⁷Sr, Ar/Ar, U-Pb) foraminifer and marine dinoflagellate dinocysts of these formations provide independent age control for the time distributions of the morphospecies in Patagonia (Guler et al. Reference Guler, Estebenet, Navarro, Fuentes, Cuitino and Palazzesi2021 and references therein, Parras & Cuitino Reference Parras and Cuitino2021).

Supporting evidence is provided by 1) objective (Jaccard coefficient) comparisons of Oligocene-Miocene palynofloras in Patagonia and Tierra del Fuego, 2) the presence or absence of pollen from wind-pollinated plants dominating Patagonian steppe vegetation (e.g. Nanez et al. Reference Nanez, Quattrochio and Ruiz2009, Cornu et al. Reference Cornou, Martinez, Quattrocchio and Asensio2012, Reference Cornou, Quattrocchio and Martinez2014) and 3) stratigraphic distributions of taxa that are now prominent in Patagonian steppe (Amaranthaceae, Asteraceae, Poaceae) and Randia type (Canthiumidites cf. bellus), which may be evidence for sourcing of pollen from northern Argentina in the Oligocene-Miocene.

Complicating the palynostratigraphic record of the establishments, extirpations and extinctions of these and other mesotherm taxa are (Barreda & Palazzesi Reference Barreda and Palazzesi2007, Barreda et al. Reference Barreda, Guler, Palazzesi and Rabassa2008, Quattrocchio et al. Reference Quattrocchio, Volkheimer, Borromei and Martinez2011, Vizcaíno et al. Reference Vizcaíno, Bargo and Fernícola2013, Estebenet et al. Reference Estebenet, Guerstein and Raising2014, Sachse et al. Reference Sachse, Strozyk, Anka, Rodriguez and di Primo2015, Warny et al. Reference Warny, Kymes, Askin, Krajewski and Tatui2019, Parras et al. Reference Parras, Guerstein, Perez Panera, Griffin, Nanez, Cusminsky and Quiroga2020, Palazzesi et al. Reference Palazzesi, Vizcaíno, Barreda, Cuitiño, del Río and Goin2021, Amenabar et al. Reference Amenabar, Guerstein, Alperin, Daners, Casadio and Raising2023): 1) geographical, geomorphic and climatic factors such as the progressive tectonic uplift of the Andes during the Palaeogene-Neogene,) opening of Drake Passage and development of the Antarctic Circumpolar Current in the Late Eocene and earliest Oligocene c. 34–23 Ma, 3) two marine inundation events (c. 22–15 Ma) during the latest Oligocene and Early to Middle Miocene represented by marginal and shallow- to deep-water marine deposits, 4) extinction in Patagonia by the mid-Eocene of previously established mega- and meso-therm taxa followed by the associated emergence of Nothofagus-Podocarpaceae communities in the Middle Eocene to Early Oligocene and by the expansion of arid-adapted Patagonian steppe communities in the Late Oligocene and Miocene and 5) marked differences between the palaeofloras occupying coastal and hinterland regions.

Much of the published age-range data from Argentina come from outcrops deposited during the Late Oligocene to Early Miocene Patagoniense transgression. Representative sections occur from the Andean Foothills in the north-west to Tierra del Fuego in the extreme south of Argentina, although these represent significantly different periods within the Oligocene to Middle Miocene time (Guerstein et al. Reference Guerstein, Guler and Casadio2004, Blisniuk et al. Reference Blisniuk, Stern, Chamberlain, Idleman and Zeitler2005, Parras et al. 2012, Cornou et al. Reference Cornou, Quattrocchio and Martinez2014, Estebenet et al. Reference Estebenet, Guerstein and Raising2014, Kohn et al. Reference Kohn, Strömberg, Madden, Dunn, Evans, Palacios and Carlini2015, Guler et al. Reference Guler, Estebenet, Navarro, Fuentes, Cuitino and Palazzesi2021, Parras & Cuitino Reference Parras and Cuitino2021). For example, the palynostratigraphically well-studied Santa Cruz Formation (Parras et al. Reference Parras, Guerstein, Perez Panera, Griffin, Nanez, Cusminsky and Quiroga2020) began accumulating at c. 17.5 Ma in the south-west, c. 19.8 Ma in the north-west and c. 19. 3 Ma in the south-east in Santa Cruz Province and at c. 16 Ma in the Golfo San George Basin in the adjacent Chubut Province but is absent further to the north and west in the Valdes and Neuguén basins. Underlying Eocene to Early Miocene formations are mostly confined to specific basins in Patagonia (e.g. the Monte Leon, Río Leona, Río Turbio and San Julien formations (Austral-Magallanes Basin), the Chenque and Centinela formations (Golfo San Jorge Basin) and Slogget Formation (Tierra de Fuego); cf. Guerstein et al. Reference Guerstein, Guler and Casadio2004, Parras et al. Reference Parras, Guerstein, Perez Panera, Griffin, Nanez, Cusminsky and Quiroga2020, Parras & Cuitino Reference Parras and Cuitino2021, Amenabar et al. Reference Amenabar, Guerstein, Alperin, Daners, Casadio and Raising2023). Palynostratigraphic data from marine cores on the Argentine continental shelf are limited to two wells, as are data from confirmed Middle and Late Miocene and Pliocene continental outcrops in Patagonia.

For this reason, and due to the possibility that other relevant records are unpublished, the age limits of samples from the Tussac House forest bed are assigned to broad periods of geological time only. The inferred age limits assume that pollen of mesotherm taxa have been transported long distances by wind, birds or insects prior to their extinction or extirpation in continental Patagonia (see Macphail & Cantrill Reference Macphail and Cantrill2006, Macphail Reference Macphail2021). This approach includes caveats that are difficult to test given that the morphospecies are mostly preserved in lignitic sediments where much of the associated palynoflora represents plants growing in or around the site. The reasons for this are as follows:

1. 1) The Falklands flora show a strong affinity with Patagonian South America and Tierra de Fuego and more limited links with the sub-Atlantic Islands, New Zealand and, especially in the past, Australia (see McDowell Reference McDowell2005). However, any colonization of the Falkland archipelago by mega- and meso-thermal plants in the early Palaeogene is most likely to have occurred during periods of global warmth, particularly the Early Eocene hyperthermals. Nevertheless, we recognize that rainforest thermophiles with broad ecological tolerances might have survived after they became extinct in Patagonia (due to increasing aridity) or West Antarctica (due to cold conditions; cf. Zachos et al. Reference Zachos, Dickens and Zeebe2008, Hyland & Seldon Reference Hyland and Sheldon2018, Fernandez et al. Reference Fernadez, Santamarina, Palazzesi, Telleria and Barreda2021, Srivastava et al. Reference Srivastava, Bhatia, Verma, Singh, Utescher and Mehrotra2023). Nevertheless, given the biologically profound impact of the major global cooling event at the Eocene-Oligocene boundary (Mi-1) elsewhere at mid- to high latitudes in the Southern Hemisphere (cf. Macphail et al. Reference Macphail, Alley, Truswell and Hill1994, Reference Macphail, Hill, Carpenter and McKellar2014), we consider it improbable that any thermophiles survived this event given the exposed habitats on the topographically subdued Falkland archipelago. Whether any recolonization of the Falklands by thermophiles growing in northern Argentina occurred during the Middle Miocene Climatic Optimum (MMCO) at c. 15–17 Ma is unknown.

2. 2) Transoceanic dispersal of seeds by wind or water from South America into the Atlantic Ocean is well documented (cf. Renner Reference Renner2004, Turney et al. Reference Turney, Jones, Fogwill, Hatton, Williams and Hogg2016, Thomas et al. Reference Thomas, Jones, Fogwill, Hatton, Williams and Hogg2018, Zwier et al. Reference Zwier, van der Bilt, de Stigter and Bjune2022), but successful establishment on remote oceanic islands such as the Falklands is markedly reduced if the ‘target’ environment(s) differ significantly from those occupied by the parent plants (see McDowell Reference McDowell2005, Ali Reference Ali2017). Most of these constraints do not apply to pollen or spores of temperate and subtropical plants in South America that are dispersed into the south-west Atlantic or Pacific oceans, although the marine depositional conditions tend to favour long-term preservation more than many terrestrial environments (Kappen & Straka Reference Kappen and Straka1988, Montade et al. Reference Montade, Nebout, Kissel and Mulsow2011).

3. 3) The rarity of Canthiumidites cf. bellus, Clavatipollenites and Cupanieidites and the absence of other South American thermophile pollen types in the Tussac House forest bed is likely to reflect deposition in an environment where much of each microflora is derived from locally growing plants (see Barreda et al. Reference Barreda, Palazzesi and Marenessi2009).

4. 4) There is an unknown likelihood that some earlier or later occurrences of Canthiumidites cf. bellus, Clavatipollenites and Cupanieidites in Patagonia are unpublished.

Records of Cupania type (Cupanieidites), Hedyosmum type (Clavatipollenites) and Randia type (Canthiumidites cf. bellus) obtained from review surveys of the published palynostratigraphic literature for Patagonia and adjacent regions in southern South America (see Barreda et al. Reference Barreda, Guler, Palazzesi and Rabassa2008) are given in Table IV. The fossil record of Randia type (Canthiumidites cf. bellus) in Patagonia is notable as this thermophile is an important element in dry forests in northern South America (Marchant et al. Reference Marchant, Almeida, Behling, Berrio, Bush and Cleef2002).

Table IV. Chronostratigraphic distribution of trace records of fossil Cupania-type, Hedyosmum-type and Randia-type pollen in Patagonia, Argentina. (+) indicates species is present; (-) indicates species was not recorded, (cf.) indicates comparable pollen.

^a After Parras & Cuintino (Reference Parras and Cuitino2021) and Amenábar et al. (Reference Amenabar, Guerstein, Alperin, Daners, Casadio and Raising2023).

NA = not applicable.

Maximum age limits of the Tussac House forest bed

On the data available, the parent source(s) of Clavatipollenites but apparently not Cupanieidites or Canthiumidites cf. bellus were present in Patagonia in or by the Middle Eocene. Assuming the former became extinct during the Mi-1 global cooling event, the maximum age limit of the Tussac House forest bed is inferred to be Late Oligocene based on records of Clavatipollenites, Cupanieidites and Canthiumidites cf. bellus in the Austral-Magallanes, Chubut and Golfo San Jorge basins in (or by) the Late Oligocene sediments (Río Leona, Monte Léon and San Julian formations; Barreda Reference Barreda1997, Barreda et al. Reference Barreda, Palazzesi and Marenessi2009, Heredia et al. Reference Heredia, Paez, Guerstein and Parras2012). Whether any of the parent sources had reached as far south as Tierra del Fuego by this time is uncertain.

Minimum age limit of the Tussac House forest bed

The minimum age limit of the Tussac House forest bed is no older than Middle Oligocene based on multiple records of Cupanieidites and Clavatipollenites in the Austral-Magallanes Basin (Monte Leon and San Julian formations) and, less certainly, Golfo San Jorge Basin (Perfiles Mazarredo Formation), but it might be as young as Middle Miocene (Cupanieidites, Canthiumidites cf. bellus) based on ⁸⁶Sr/8⁷SR dates for the Chenque Formation in the Golfo San Jorge (cf. Barreda Reference Barreda1996a, Parras et al. Reference Parras, Guerstein, Perez Panera, Griffin, Nanez, Cusminsky and Quiroga2020). Whether or not the latter age limit is supported by the trace records of pollen representing the wind-pollinated/dispersed families Amaranthaceae (Chenopodipollis chenopodiaceoides) and Poaceae (Graminidites media) in the Tussac House microfloras is uncertain as the parent plants are recorded in Nothofagaceae-Podocarpaceae communities prior to the Late Miocene expansion of steppe vegetation in Patagonia. The absence of Asteraceae (Tubulifloridites spp.) pollen is less easily explained unless this is due to its lower pollen production and more limited dispersal by wind and animals.

Depositional environment

Relative level data indicate that the top depth of the Tussac House lignitic deposit is ~1.5 m above mean high water mark and that the base of the deposit extends below present-day mean sea level and therefore may have accumulated during a period of low relative sea level. The local depositional environment is more likely to have been a damp depression given the virtual absence of obligate or semi-obligate algal cysts or pollen of herbs such as Sparganiaceae (Sparganiaceaepollis) and Menyanthaceae (Striasyncolpites laxus), which occur in ‘marsh’ deposits in Patagonia during the Oligocene-Miocene. One possibility, suggested by the strong east to west alignment of the basement rocks and ongoing tectonic uplift of the archipelago (Stanca et al. Reference Stanca, McCarthy, Paton, Hodgson and Mortimer2022), is a depression on the back slope on a tilted fault block.

Palaeoflora and vegetation

As the microfloras represent essentially the same Podocarpaceae-Nothofagus rainforest or rainforest shrub community (Table II), the data more compelling suggests that the lignites were deposited over a short period of geological time under perhumid cool or, less likely, cold conditions. The closest modern analogues are Magellanic evergreen Nothofagus rainforest (see table 10.1 in Veblen et al. Reference Veblen, Donoso, Kitzberger, Rebertus, Veblen, Hill and Read1996). Shared characteristics include the rarity of Araucaria and the prominence of Nothofagus spp., whose NLRs may be ancestral to extant species in the subgenus Fuscospora (e.g. Nothofagus antarctica, Nothofagus betuloides and Nothofagus pumilo) given the high-latitude position of the Falkland Islands throughout the Late Palaeogene and Neogene. The parent plants of Nothofagidites americanus and Nothofagus teheulchesii may be extinct, as subgenus Lophozonia spp. in South America (Nothofagus alpina, Nothofagus obliqua) are restricted to latitudes above ~40°S in southern Chile and Argentina.

This palaeo-community, however, has no clear modern analogue, as the NLRs of most Podocarpaceae morphospecies are endemic to Tasmania (Lagarostrobos franklinii), New Zealand and/or Southeast Asia (e.g. Dacrycarpus (Dacrycarpites australiensis), Dacrydium (Dacrydiumites florinii) and Phyllocladus (Microalatidites palaeogenicus)). Two podocarp clades are extinct (Podosporites parvus, Trichotomosulcites subgranulosus). The NLRs of most rare ferns and angiosperm shrubs represent humid-demanding species with broad geographical distributions in South America. Examples are the probable tree fern Cyathidites australis (Cyatheaceae), the club-moss Foveotriletes palaequetrus (Huperzia fuegianum) and Embothrium (Diporites granulosus), all of which occur as far south as Tierra del Fuego, and tree ferns in Andean ‘cloud forests’ at lower latitudes (Broughton & McAdam Reference Broughton and McAdam2005). The same is likely to be true of the herb flora whether the unassigned Tricolpites/Tricolporites counts represent herbaceous or woody plants. The absence of Lophosoria (Cyatheacidites annulatus) is surprising given that this ground fern became established elsewhere on sub-Antarctic islands (e.g. the Kerguelen Islands; Cookson Reference Cookson1947) during the Miocene.

Although the scanning electron microscopy analysis of the wood assemblage was able to identify two distinct xylotypes, poor sample preservation precluded a species-level identification and as such is unable to inform on the probable age limits of or genera presented in the Tussac House forest bed. An interesting characteristic observed in the West Point Island deposit was the preponderance of Podocarpaceae pollen (60%) compared to the wood, which is largely cf. Austrocedrus. Although the wood assemblages analysed from the Tussac House deposit were significantly lower in number, a similar trend is apparent. This is probably a reflection of the pollination strategy in Podocarpaceae (wind pollinated) as well as of taphonomic effects. The pollen grains are saccate and tend to float on surface water, so different concentrations of grains from the same vegetation are found depending on whether the grains were deposited from water or onto some sort of peat surface (Traverse Reference Traverse and Traverse2007). Even within standing water the deposition can fluctuate due to differential transport.

Comparison of the Tussac House and West Point Island forest beds

Comparisons between the West Point Island forest bed (Macphail & Cantrill Reference Macphail and Cantrill2006) and Tussac House forest bed microfloras suggest that the latter are slightly older than the former. With few exceptions, all common and most rare morphospecies recorded in the Tussac House forest bed also occur in the West Point Island forest bed. The only significant differences are that the relative abundance of Phyllocladidites mawsonii is lower in the West Point Island microfloras (15–25%) compared to the Tussac House microfloras (34–48%) and that Chenopodipollis chenopodiaceoides, Graminidites and Myrtaceidites eucalyptoides were not recorded in the Tussac House microfloras. Cupanieidites reticularis was not recorded in the West Point Island samples. Although trace occurrences of Myrtaceidites eucalyptoides in the West Point Island microfloras may represent contamination from drilling equipment, the significance of the other differences provides insights into probable age limits given the wide dispersal of Amaranthaceae and Poaceae pollen by wind around the Southern Hemisphere (Macphail Reference Macphail1979) vs the probable dispersal of Cupanieae pollen by insects.

On these data, the probable age limits of the West Point Island forest bed are more likely to be Late Oligocene to Early (Middle?) Miocene than the Middle(?) Miocene to Early Pliocene age limits (Table IV) based in part on Sparganiaceaepollenites and Thymelaepollis sp. (Macphail & Cantrill Reference Macphail and Cantrill2006). If correct, the revised age limits for the West Point Island forest bed indicate that Lagarostrobos populations on the Falkland Islands had undergone a significant contraction during the Early to Middle Miocene (i.e. during the warming period leading up to the MMCO at c. 15 Ma). This seems ecologically improbable given the broad distribution of extant Lagarostrobos franklinii from the lowlands to the subalpine/alpine zone in perhumid western Tasmania. Possible explanations are that the sources of Phyllocladidites mawsonii pollen on the Falkland Islands were now extinct, cold-demanding ecotypes or, less likely, that warming leading up to the MMCO was accompanied by a significant reduction in precipitation across the archipelago. In either case, the most probable period when Lagarostrobos and any other potentially tree-sized podocarps became extirpated from the Falkland Islands is the Late Miocene, a conclusion that is ecologically consistent with the earlier development of tundra on the South Shetland Islands during the Mi-1 glaciation (Warny et al. Reference Warny, Kymes, Askin, Krajewski and Bart2016).

Climate modelling and interpretation

Anomaly-based reconstructions of annual mean temperature and annual mean precipitation over the Falkland Islands were developed from palaeoclimate modelling scenarios of the Miocene and Oligocene generated by Farnsworth et al. (Reference Farnsworth, Lunt, Robinson, Valdes, Roberts and Clift2019) using the HadCM3BL coupled climate model to provide further context for the palaeoflora evidence. Although the modern annual mean temperature of the Falklands is ~6°C, the reconstructed values are significantly warmer, ranging from ~12°C to 13°C in the Early to Middle Miocene (13.8–23.0 Ma), up to 14–15°C in the Late Oligocene (23.0–27.8 Ma) and then cooler again (11–12°C) in the Early Oligocene (27.8–33.9 Ma; Supplemental Fig. 3). Precipitation varies significantly, from ~500 mm/year in the modern down to 300–400 mm/year for the Middle Miocene, becoming significantly wetter (~700–900 mm/year) in the Early Miocene (20.4–23.0 Ma) and Late Oligocene (23.0–27.8 Ma) and then trending lower again in the Early Oligocene (27.8–33.9 Ma; Supplemental Fig. 4). These patterns are consistent with a hypothesized decrease in precipitation from the Early to Middle Miocene, a possible cause of the changes in Lagorostrobos pollen discussed above.

We also use the temperature and precipitation climatologies to generate Köppen climate classifications for each of the Miocene and Oligocene time slices (Supplemental Fig. 5). Based on WorldClim version 2 data (Fick & Hijmans Reference Fick and Hijmans2017), the modern Falklands climate is predominantly subpolar oceanic (Cfc), with areas of polar tundra (ET) at higher elevations. Going back in time, the Falklands are classified as predominantly cold steppe (BSk) for both the Langhian (13.8–16.0 Ma) and Burdigalian (16.0–20.4 Ma) due to relatively low precipitation and slightly warmer average temperatures. For the Aquitanian (20.4–23.0 Ma) and Chattian (23.0–27.8 Ma) the Falklands become predominantly temperate oceanic (Cfb; i.e. a similar zone to modern-day western Europe), in accordance with higher precipitation during those periods. Further back to the Rupelian (27.8–33.9 Ma) the Falklands are again dominated by cold steppe (BSk) climate.

Palaeoclimate time series for the closest grid point to Tussac House show summer (December–February), winter (June–August) and annual mean reconstructed temperatures and precipitation rates over the mid- to late Cenozoic (Fig. 6). These time series indicate the substantial variations in precipitation for the palaeo-reconstructions (with periods both wetter and drier than modern conditions), and the temperature reconstructions show that Tussac House was consistently 6–7°C warmer than modern in terms of the annual mean, and with greater seasonal variability, ranging from 6–8°C in winter to 16–18°C in summer. These climatic conditions would have been amenable for the development of the rainforest/scrub communities identified in the pollen spectra within the Tussac House lignite deposit.

Figure 6. Reconstructed a. temperature and b. precipitation time series at Tussac House based on WorldClim v2 and anomaly-based temperature/precipitation reconstructions for the five palaeo-simulations of the Langhian, Burdigalian, Aquitanian, Chattian and Rupelian. Ann = annual; DJF = December, January, February; JJA = June, July, August.