Early Paleogene climate at mid latitude in South America: mineralogical and paleobotanical proxies from continental sequences in Golfo San Jorge basin (Patagonia, Argentina)

The Paleocene-Eocene boundary was a period of transient and intense global warming that had a deep effect on middle and high latitude plant groups. Nevertheless, only scarce early Paleogene paleoclimatic records are known from the South American continental sequences deposited at these latitudes. In this contribution clay mineralogy and paleobotanical analyses (fossil woods and phytoliths) were used as paleoclimate proxies from the lower and middle parts of the Río Chico Group (Golfo San Jorge basin, Patagonia, Argentina). These new data may enable to understand the changing climatic conditions during part of the Paleocene-Eocene transition. In this setting, three clay mineral assemblages were identified: S1 assemblage (smectite) dominates the Peñas Coloradas Formation; S2 assemblage (smectite>kaolinite) occurs in the stratigraphic transition to the Las Flores Formation; and S3 assemblage (kaolinite>smectite) dominates the Las Flores Formation. These trend of change in the detrital clay mineral composition is interpreted as resulting mainly from the changing paleoclimatic conditions that shifted from seasonal warm temperate to tropical affecting the same source area lithology. Moreover, the paleobotanical data suggest that the Early Paleogene vegetation in the Golfo San Jorge basin underwent significant composition and diversity changes, ranging from mixed temperate - subtropical forest to mixed subtropical - tropical, humid forest. The integrated analysis of the clay mineral composition and the palaeobotanical assemblages suggests that, in central Argentinean Patagonia, the Paleocene-Eocene climate changed from temperate warm, humid and highly seasonal precipitation conditions to subtropical-tropical, more continuous year-round rainfall conditions

Paleoecology and paleoenvironments of Podocarp trees in the Ameghino Petrified forest (Golfo San Jorge Basin, Patagonia, Argentina): constraints for Early Paleogene paleoclimate

During the Early Paleocene (Danian), Central Patagonia had a warm-temperate climate and was dominated by evergreen coniferous forests. Abundant permineralized conifer woods along with some dicot and palm leaf compressions were found in the Ameghino Petrified Forest, and provide evidence of this type of flora. All the permineralized wood and large trunks recovered were assigned to the species Podocarpoxylon mazzonii. An estimated tree height of 17-29m was calculated on the basis of diameter measurements. Based on 14 ring sequences, with a total of 169 rings, the mean ring width and Mean Sensitivity (MS) were 1.23 and 0.19mm respectively. The growth rings are moderately wide, extremely uniform and complacent, indicating that the environment was favourable and constant, and lacked significant stress factors limiting tree growth. Following the quantitative analysis for conifers outlined by Falcon-Lang, the growth ring anatomy of the Podocarpoxylon mazzonii suggests that these trees had an evergreen habit. The combination of the fossil flora, growth ring, and sedimentological analyses suggest that this mostly evergreen coniferous forest developed under warm-temperate conditions and without limiting factors

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The Salamanca Formation of the San Jorge Basin (Patagonia, Argentina) preserves critical records of Southern Hemisphere Paleocene biotas, but its age remains poorly resolved, with estimates ranging from Late Cretaceous to middle Paleocene. We report a multi-disciplinary geochronologic study of the Salamanca Formation and overlying Río Chico Group in the western part of the basin. New constraints include (1) an 40Ar/39Ar age determination of 67.31 ± 0.55 Ma from a basalt flow underlying the Salamanca Formation, (2) micropaleontological results indicating an early Danian age for the base of the Salamanca Formation, (3) laser ablation HR-MC-ICP-MS (high resolution-multi collector-inductively coupled plasma-mass spectrometry) U-Pb ages and a high-resolution TIMS (thermal ionization mass spectrometry) age of 61.984 ± 0.041(0.074)[0.100] Ma for zircons from volcanic ash beds in the Peñas Coloradas Formation (Río Chico Group), and (4) paleomagnetic results indicating that the Salamanca Formation in this area is entirely of normal polarity, with reversals occurring in the Río Chico Group. Placing these new age constraints in the context of a sequence stratigraphic model for the basin, we correlate the Salamanca Formation in the study area to Chrons C29n and C28n, with the Banco Negro Inferior (BNI), a mature widespread fossiliferous paleosol unit at the top of the Salamanca Formation, corresponding to the top of Chron C28n. The diverse paleobotanical assemblages from this area are here assigned to C28n (64.67–63.49 Ma), ∼2–3 million years older than previously thought, adding to growing evidence for rapid Southern Hemisphere floral recovery after the Cretaceous-Paleogene extinction. Important Peligran and “Carodnia” zone vertebrate fossil assemblages from coastal BNI and Peñas Coloradas exposures are likely older than previously thought and correlate to the early Torrejonian and early Tiffanian North American Land Mammal Ages, respectively

Flammable biomes dominated by eucalypts originated at the Cretaceous-Palaeogene boundary

Fire is a major modifier of communities, but the evolutionary origins of its prevalent role in shaping current biomes are uncertain. Australia is among the most fire-prone continents, with most of the landmass occupied by the fire-dependent sclerophyll and savanna biomes. In contrast to biomes with similar climates in other continents, Australia has a tree flora dominated by a single genus, Eucalyptus, and related Myrtaceae. A unique mechanism in Myrtaceae for enduring and recovering from fire damage likely resulted in this dominance. Here, we find a conserved phylogenetic relationship between post-fire resprouting (epicormic) anatomy and biome evolution, dating from 60 to 62 Ma, in the earliest Palaeogene. Thus, fire-dependent communities likely existed 50 million years earlier than previously thought. We predict that epicormic resprouting could make eucalypt forests and woodlands an excellent long-term carbon bank for reducing atmospheric CO2 compared with biomes with similar fire regimes in other continents

Early Paleogene climate at mid latitude in South America: mineralogical and paleobotanical proxies from continental sequences in Golfo San Jorge basin (Patagonia, Argentina)

The Paleocene-Eocene boundary was a period of transient and intense global warming that had a deep effect on middle and high latitude plant groups. Nevertheless, only scarce early Paleogene paleoclimatic records are known from the South American continental sequences deposited at these latitudes. In this contribution clay mineralogy and paleobotanical analyses (fossil woods and phytoliths) were used as paleoclimate proxies from the lower and middle parts of the Río Chico Group (Golfo San Jorge basin, Patagonia, Argentina). These new data may enable to understand the changing climatic conditions during part of the Paleocene-Eocene transition. In this setting, three clay mineral assemblages were identified: S1 assemblage (smectite) dominates the Peñas Coloradas Formation; S2 assemblage (smectite>kaolinite) occurs in the stratigraphic transition to the Las Flores Formation; and S3 assemblage (kaolinite>smectite) dominates the Las Flores Formation. These trend of change in the detrital clay mineral composition is interpreted as resulting mainly from the changing paleoclimatic conditions that shifted from seasonal warm temperate to tropical affecting the same source area lithology. Moreover, the paleobotanical data suggest that the Early Paleogene vegetation in the Golfo San Jorge basin underwent significant composition and diversity changes, ranging from mixed temperate - subtropical forest to mixed subtropical - tropical, humid forest. The integrated analysis of the clay mineral composition and the palaeobotanical assemblages suggests that, in central Argentinean Patagonia, the Paleocene-Eocene climate changed from temperate warm, humid and highly seasonal precipitation conditions to subtropical-tropical, more continuous year-round rainfall conditions

Paleoecology and paleoenvironments of Podocarp trees in the Ameghino Petrified forest (Golfo San Jorge Basin, Patagonia, Argentina): constraints for Early Paleogene paleoclimate

During the Early Paleocene (Danian), Central Patagonia had a warm-temperate climate and was dominated by evergreen coniferous forests. Abundant permineralized conifer woods along with some dicot and palm leaf compressions were found in the Ameghino Petrified Forest, and provide evidence of this type of flora. All the permineralized wood and large trunks recovered were assigned to the species Podocarpoxylon mazzonii. An estimated tree height of 17-29m was calculated on the basis of diameter measurements. Based on 14 ring sequences, with a total of 169 rings, the mean ring width and Mean Sensitivity (MS) were 1.23 and 0.19mm respectively. The growth rings are moderately wide, extremely uniform and complacent, indicating that the environment was favourable and constant, and lacked significant stress factors limiting tree growth. Following the quantitative analysis for conifers outlined by Falcon-Lang, the growth ring anatomy of the Podocarpoxylon mazzonii suggests that these trees had an evergreen habit. The combination of the fossil flora, growth ring, and sedimentological analyses suggest that this mostly evergreen coniferous forest developed under warm-temperate conditions and without limiting factors

Did increasing seasonality and fire frequency cause the c4 grassland transition in South America (SA)? Investigations from two paleosol sites in NW Argentina using δ13c isotopes, molecular biomarkers, phytoliths, and X-ray fluorescence (XRF)

Recent advances in molecular biomarkers along with traditional bulk δ13C isotopic analysis, phytolith assemblages, and XRF analysis allow for the detailed reconstruction of continental paleoecological and paleoclimatic conditions. We use these methods to reconstruct conditions during the expansion of C4 grasses, considered one of the most important biological events during the Cenozoic. We hypothesize that in SA, the spread of C4 grasses was driven by the intensification of the SA Summer Monsoon and that this increase in C4 vegetation caused an intensification of fire regimes, due to both seasonality fire and fireC4 expansion feedbacks, that has long been suspected but poorly documented and never studied in this context in SA. This study presents resultsfrom welldated paleosols from two sites in N.W. Argentina: La Viña (LV) and Palo Pintado (PP) in the Salta Province. The LV site includes the Jesus Maria Formations, Guanaco and Piquete (~15 4Ma), while the PP site includes the Palo Pintado Formation (~7.5 6 Ma). Previous studies have interpreted these sites as foreland basin sediments deposited in a meandering stream tolacustrine environment. Shifts in the δ13C obtained from bulk organic matter and phytolith assemblages in these paleosols are used to identify vegetation transitions from C3dominatedgrasslands and forests, to C4dominated grasslands. Leaf wax biomarkers include higher order nalkanes (C20 to C36), and are used to independently determine vegetation composition and changes to aridity and seasonality. XRF analysis is carried out to determine the chemical index of alteration in paleosols, which is a comparable proxy for precipitation seasonality. Polycyclic aromatic hydrocarbons (PAH) are used to determine fire regimes, in order to verify whether C4 expansion promoted fires. δ13C analysis of LV reveals a steady change from a C3dominatedecosystem at 15 Ma to a C3C4 intermediary ecosystem by 7.5 Ma. The PP site does not show any directional change in vegetation type during the investigated period between 7.2 Ma to 6 Ma, though δ13C values indicate a mixed C3C4 ecosystem throughout this period. However, results from the PP site show that increasing C4 cover, determined from bulk organic matter δ13C, is positively correlated with an increased fire frequency, determined from increasing PAH concentrations. Ultimately, these data contribute to constraining the causes of C4 expansion in NW Argentina, including the most important climatic controls and feedbacks from fire regimes, in continental SA.Fil: Ghosh, Adit. California State University; Estados UnidosFil: Cotton, Jennifer M.. California State University; Estados UnidosFil: Hyland, Ethan G.. North Carolina State University; Estados UnidosFil: Hauswirth, Scott C.. California State University; Estados UnidosFil: Raigemborn, María Sol. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Tineo, David. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaThe Geological Society of America Connects 2021PasadenaEstados UnidosGeological Society of Americ

The rise of C4 grasses in Argentina: Linking grassland transition to the South American summer Monsoon using sediments, paleosols, phytoliths and isotopes

The expansion of C4 grasses is one of the most dramatic ecological changes in the past 65 million years. Beginning in the late Miocene (~10-5 Ma), these tropical and subtropical grasses began to spread and now cover roughly 25% of the Earth´s surface. C4 grasses include economically and ecologically important crops, but the environmental conditions that drove this global expansion are poorly understood. In this study, we aim to determine the drivers of C4 grass expansion in central South America (Argentina). We hypothesize that this expansion was driven by the strengthening of the South American Summer Monsoon (SASM), which occurred as a result of regional climatic change related to local tectonic and global climatic processes. Using sedimentology/stratigraphy, paleopedology and geochemistry, phytolith and carbon isotope reconstructions, and biomarkers, we present paired reconstructions of environmental and climatic conditions and the abundance of C4 grasses across 7 sites in northern and central Argentina to assess the timing of this ecological transition in South America during the late Miocene-Pliocene. We find that in the absence of temperature and atmospheric CO2 changes, regional hydrologic change is likely the driver of C4 grass abundances in the Miocene. In northwest and central Argentina, our data suggests that increased precipitation seasonality brought about by the intensification of the South American Monsoon drove the expansion of C4 grasses, at least in the more humid regions. These results are supported by Community Earth System Model simulations that suggest an increase in summer precipitation along the eastern foothills of the Andes in Argentina between 8 and 3 Ma. Our data does not show a correlation between fire frequency and C4 grass abundance in Argentina, unlike other regions where this may drive the expansion. We also note that we only find moderate to high abundances of C4 grasses in areas where proxy-based precipitation estimates exceed ~500 mm/yr, and do not observe C4 grasses at sites with modern elevations higher than ~3,000 feet or behind rain shadows, showing that tectonically driven conditions are important controls on the distribution and spread of C4 grasses.Fil: Hyland, E. G.. North Carolina State University; Estados UnidosFil: Cotton, Jennifer M.. California State University; Estados UnidosFil: Ghosh, Adit. University of Southern California; Estados UnidosFil: Hauswirth, S. C.. California State University; Estados UnidosFil: Littleton, S.. California State University; Estados UnidosFil: Azmi, Iffat. North Carolina State University; Estados UnidosFil: Insel, Nadja. Northeastern Illinois University (northeastern Illinois);Fil: Raigemborn, María Sol. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Tineo, David. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaXVIII Reunión Argentina de Sedimentología y IX Congreso Latinoamericano de SedimentologíaLa PlataArgentinaAsociación Sedimentológica Argentin