Impact of mid Eocene greenhouse warming on America’s southernmost floras

A major climate shift took place about 40 Myr ago—the Middle Eocene Climatic Optimum or MECO—triggered by a significant rise of atmospheric CO2 concentrations. The biotic response to this MECO is well documented in the marine realm, but poorly explored in adjacent landmasses. Here, we quantify the response of the floras from America’s southernmost latitudes based on the analysis of terrestrially derived spores and pollen grains from the mid-late Eocene (~46–34 Myr) of southern Patagonia. Robust nonparametric estimators indicate that floras in southern Patagonia were in average ~40% more diverse during the MECO than pre-MECO and post-MECO intervals. The high atmospheric CO2 and increasing temperatures may have favored the combination of neotropical migrants with Gondwanan species, explaining in part the high diversity that we observed during the MECO. Our reconstructed biota reflects a greenhouse world and offers a climatic and ecological deep time scenario of an ice-free sub-Antarctic realm.

are here used to constrain the extension of the MECO using our recovered dinocyst specimens.Overall, the Río Turbio Formation preserves poorly diverse dinocyst assemblages, mostly composed by endemic Antarctic (e.g.Enneadocysta dictyostila, Vozzhennikovia apertura, Deflandrea antarctica) and cosmopolitan (e.g.Turbiosphaera filosa, Operculodinium spp.and Spiniferites spp.) taxa, with shifts in abundance across the composite section (Supplementary Fig. 2-4).The dinocyst species preserved in our selected 33 samples are assembled within three major intervals: Interval A (samples 1 to 7), Interval B (samples 8 to 25) and Interval C (samples 26 to 32) according to our constrained cluster analysis (Supplementary Fig. 3).
Interval A) includes scarce Enneadocysta dictyostila (up to 10 %), and peaks of both Vozzhennikovia apertura and Deflandrea antarctica.This interval A partially coincides with Sequences I to III defined by Rodríguez Raising (24) for the Río Turbio Fm, recently dated to the mid Eocene (47.1 and 46.3 Myr) based on zircon U-Pb ages near the Cancha Carrera area (25).Our dinocyst assemblage of Interval A correlates with the Río Turbio Dinocyst Zone Interval B) includes peaks in relative abundances of Enneadocysta dictyostila (up to 95%) associated with Hystrichosphaeridium truswelliae and Arachnodinium antarcticum.
Our interval B coincides with Sequences IV to VI of Rodríguez Raising (24), that contains within Sequence VI a thick glauconitic level associated with a Maximum Flooding Surface (MFS).Recently Fosdick et al. (25) dated this glauconitic level in the Cancha Carrera area to the mid Eocene (41.3 Myr).This level was correlated with a Maximum Flooding Surface (MFS) in the Highway 40 sections by Rodríguez Raising (24).We identified this MFS within Interval B (between samples 14-15) based on a closed stratigraphical and palynological (dinocysts composition and abundances) comparison between our section and those of Rodríguez Raising (24) and González Estebenet et al (20,22).The upward samples of this interval continue typically characterized by peak abundances of E. dictyostila, also identified elsewhere in the Austral Basin (i.e.upper part of the equivalent Man Aike Formation (22,27), Antarctic Peninsula (28) and South Tasman Rise ODP Site 1170 (23)

Supplementary Note 3: Spore-pollen assemblages
Our collected samples yielded moderate to large numbers of spores, pollen and dinocysts, in a matrix of well-preserved to carbonized plant debris (including cuticles), finely disseminated charcoal particles and amorphous algal and fungal remains.The material recovered is relatively well-preserved, making identification at morphospecies level possible for most of the spore-pollen and dinocyst elements.The presence of tetrads (e.g.Ericipites sp. 1, Bysmapollis verrucatus) and clusters of pollen grains (e.g.Nothofagidites) suggest lowenergy conditions at the time of the accumulation of the RTF.

Supplementary Note 4: Selected fossil forms with tropical and subtropical affinity
Previous records, botanical affinity and geographical distribution of living representatives of specimens with tropical and subtropical affinity are given.
Geographical distribution of living representatives: Primarily neotropical.Only a few species in Africa-Madagascar and one in south India and islands in the Indian Ocean (49).Botanical affinity: This morphotype is comparable to those of genus Lygodium (Lygodiaceae).It is trilete laesura, medium-large size (50 μm or more), with the sporoderm composed of perispore and exospore.Surface sculpture of exospore ranges from smooth to scabrate, verrucate, rugulate, or reticulate (64).The spores found in the RTF have perispore thin, scabrate and exospore rugulo-reticulate.

Distribution in Río Turbio
Geographical distribution of living representatives: Lygodium species are mainly found in tropical regions; there are only a few temperate outliers in North America and Japan (65)(66).
(Cyatheacea) by having trilete laesura and three large pits (diameter about 5-7 μm) symmetrically arranged at the center of the three sides (82).

Geographical distribution of living representatives: The geographic range of living
Cnemidaria covers the area of Central America, the Greater Antilles, and the northern part of South America (82).
Distribution in Río Turbio Fm: Samples 9 and 20 (all belonging to the MECO interval).
Botanical affinity: The fossil species presents general similarities with the pollen types recognized in Cathedra (Olacaceae) by having amb rounded-triangular, tri-diporate (i.e. the apertures comprise three pairs of pores, and the two members of each pair face each other on opposite hemispheres, without any suggestion of an ectocolpus), pore elliptic close to the corner of the grain in polar view, exine scabrate (88).
Geographical distribution of living representatives: Species of the genus Cathedra are exclusively neotropical.Among the five recognised species, only one is known from wet forests, the other ones are recorded from dryer ecosystems (restinga) (88).Fossil species found in early to middle Miocene of New Zealand (94).

Distribution in Río
Botanical affinity: The analyzed fossil pollen presents general similarities with the pollen of Cupania (Sapindaceae), both types are isopolar, whit3-syncolporate aperture, triangular outline in polar view, peroblate.

Distribution in Río Turbio
Botanical affinity: The pollen fossil analyzed resembles species of living Malpighiaceae, especially those of the Banisteroid group (137).Fossil forms are similar to some species of the Tetrapterid clade within the Banisteroid, currently present in Northern and central Argentina.They share the circular to subcircular outline, the thin colpoids, and a similar number of pores (138).
Geographical distribution of living representatives: The center of diversity of Malpighiaceae is tropical South America (139).Gallardoa fischeri Hicken (clade Cordobioide also included within the Banisteroide) represents the southernmost penetration of the family in the world, reaching 41º S. That species differs from the specimens analyzed by showing verrucate sculpture (138).