Oligocene Climate Transition Research Articles

Fossil woods from the Pato Raro Heights, Patagonia National Park, Aysén, Chile: A new paleobotanical assemblage at the Oligocene climate transition

Eleven well-preserved fossil woods were collected from sediments of the San José Formation in the Pato Raro Heights, Chilean Patagonia. Based on their wood anatomy, the specimens were placed in six fossil species: Two conifers of the Podocarpaceae (Phyllocladoxylon antarcticum and a new species of Podocarpoxylon with abundant resin content), one species of the Nothofagaceae (Nothofagoxylon ruei) and one of the Cunoniaceae (Weinmannioxylon trichospermoides), both previously described from Patagonia, a new species of Myrtineoxylon, with the typical anatomy of the family Myrtaceae, and a new taxon with unknown affinity, which was placed in a new fossil genus and species. The assemblage studied provides insights into the Oligocene forests of Patagonia and helps to fill a gap in the knowledge of the Oligocene paleofloras of the region.

Taphonomy and depositional history of the Southfork Quarry (Cypress Hills Formation, late Eocene) in southwestern Saskatchewan, Canada

The Eocene to Miocene Cypress Hills Formation (CHF) spans 28 million years and forms the conglomeratic caprock of the Cypress Hills and Swift Current plateaus in southwestern Saskatchewan and southeastern Alberta. These stacked fluvial, floodplain, and lacustrine deposits preserve the only high latitude, non-polar mammalian fossil assemblage (Uintan to Hemingfordian land mammal stages) in Canada. The Quarry is the oldest CHF (Chadronian 1, late Eocene) site documented in the Cypress Hills region north of the town of Eastend. The Quarry was originally discovered in 1962, after bones were found to be eroding out of the base of a road cut north of the ghost town of South Fork on the southeastern flanks of the Cypress Hills. Numerous field campaigns have resulted in the collection of fossils from a multitaxonomic bonebed. This paper presents a detailed sedimentologic, paleontologic, and taphonomic study to establish a depositional environment framework of the Southfork Quarry. This site was deposited at the onset of the Eocene–Oligocene climate transition, a critical time of climate change during the Paleogene. Six facies and two facies associations are characterized for the Quarry, shifting from a braided-fluvial system to a debris flow-filled incised channel. Patterns of skeletal accumulations and bone surface modification indicate that the assemblage accumulated over a significant interval of time in different depositional environments. This study provides critical insight into environmental shifts driven by climate change and relates these findings to a broader understanding of the Eocene–Oligocene transition in North America.

Isotopic constraints on ocean redox at the end of the Eocene

A multi-million-year decrease in global temperatures during the Eocene was accompanied by large reorganisations to ocean circulation, ocean chemistry and biological productivity. These changes culminated in the rapid growth of grounded ice on Antarctica during the Eocene–Oligocene climate transition (EOT), ∼34 million years ago. However, while it is likely that environmental perturbations of this magnitude altered the oceanic oxygen inventory, the sign and magnitude of the response is poorly constrained. We show that euxinic, hydrographically restricted conditions developed in the Austrian Molasse Basin during the EOT. The isotopic compositions of molybdenum and uranium captured by sediments accumulating in the Molasse Basin at this time reveal that the global extent of sulfidic conditions during the EOT was not appreciably different to that of the Early Eocene greenhouse world. Our results suggest that the early Cenozoic oceans were buffered against extreme long-term changes in oxygenation.

Paleolake salinity evolution in the Qaidam Basin (NE Tibetan Plateau) between ~42 and 29 Ma: Links to global cooling and Paratethys sea incursions

Climate change in the Asian interior during the early Cenozoic remains poorly constrained due to difficulties in distinguishing the impacts of global cooling, the early uplift of the Tibetan Plateau and the retreat of the Paratethys. A quantitative estimation of paleolake salinity enables a better understanding of the regional hydrological cycle and contemporaneous climate change. Here, we present a quantitative record of paleolake salinity in the Qaidam Basin between ~42 and 29 Ma using detailed mineralogical investigations and clay boron content data. This paleolake salinity record generally covers a tectonically less active period in the Asian interior characterized by continuous global cooling, the abrupt Eocene–Oligocene climate transition and the Paratethys transgression/regression cycles, thus offering an opportunity to explore the roles of global cooling and the Paratethys retreat in regulating the regional hydrological cycle. The results of two boron-derived paleosalimeters, equivalent boron and Couch's salinity, collectively indicate a two-stage paleolake salinity evolution, from an oligohaline–mesohaline environment in the middle–late Eocene (~42–~34 Ma) to a mesosaline environment in the early Oligocene (~34–~29 Ma). This transition is also supported by qualitative chloride-based and ostracod-based paleosalinity estimates in the Qaidam Basin. Our quantitative paleolake reconstruction between ~42 and 29 Ma in the Qaidam Basin yields a generally good match with the hydroclimate change in the neighboring Xining Basin, thus indicating a comparable regional drying trend. The synchronous changes in quantitative paleolake salinity and silicate weathering processes derived from the illite weathering index and chlorite content in the studied section suggest climate control of silicate weathering. Global cooling is speculated to have been the first-order driving factor in regulating long-term climatic evolution and weathering responses in the Asian interior between ~42 and 29 Ma. Superimposed on this trend, the Paratethys transgression/regression cycles served as an important factor regulating wet/dry fluctuations in the Asian interior between ~42 and ~34 Ma.

Widespread Antarctic glaciation during the Late Eocene

Marine sedimentary rocks drilled on the southeastern margin of the South Orkney microcontinent in Antarctica (Ocean Drilling Program Leg 113 Site 696) were deposited between ∼36.5 Ma to 33.6 Ma, across the Eocene–Oligocene climate transition. The recovered rocks contain abundant grains exhibiting mechanical features diagnostic of iceberg-rafted debris. Sand provenance based on a multi-proxy approach that included petrographic analysis of over 275,000 grains, detrital zircon geochronology and apatite thermochronometry rule out local sources (Antarctic Peninsula or the South Orkney Islands) for the material. Instead the ice-transported grains show a clear provenance from the southern Weddell Sea region, extending from the Ellsworth–Whitmore Mountains of West Antarctica to the coastal region of Dronning Maud Land in East Antarctica. This study provides the first evidence for a continuity of widespread glacier calving along the coastline of the southern Weddell Sea embayment at least 2.5 million yrs before the prominent oxygen isotope event at 34–33.5 Ma that is considered to mark the onset of widespread glaciation of the Antarctic continent.

Terrestrial responses of low-latitude Asia to the Eocene–Oligocene climate transition revealed by integrated chronostratigraphy

Abstract. The Paleogene sedimentary records from southern China hold important clues to the impacts of the Cenozoic climate changes on low latitudes. However, although there are extensive Paleogene terrestrial archives and some contain abundant fossils in this region, few are accurately dated or have a temporal resolution adequate to decipher climate changes. Here, we present a detailed stratigraphic and paleomagnetic study of a fossiliferous late Paleogene succession in the Maoming Basin, Guangdong Province. The succession consists of oil shale of the Youganwo Formation (Fm) in the lower part and the overlying sandstone-dominated Huangniuling Fm in the upper part. Fossil records indicate that the age of the succession possibly spans the late Eocene to the Oligocene. Both the Youganwo Fm and the overlying Huangniuling Fm exhibit striking sedimentary rhythms, and spectral analysis of the depth series of magnetic susceptibility of the Youganwo Fm reveals dominant sedimentary cycles at orbital frequency bands. The transition from the Youganwo oil shale to the overlying Huangniuling sandstones is conformable and represents a major depositional environmental change from a lacustrine to a deltaic environment. Integrating the magnetostratigraphic, lithologic, and fossil data allows establishing a substantially refined chronostratigraphic framework that places the major depositional environmental change at 33.88 Ma, coinciding with the Eocene–Oligocene climate transition (EOT) at ∼ 33.7 to ∼ 33.9 Ma. We suggest that the transition from a lacustrine to deltaic environment in the Maoming Basin represents terrestrial responses to the EOT and indicates prevailing drying conditions in low-latitude regions during the global cooling at EOT.

Phylogenetic patterns in the geographic distributions of birds support the tropical conservatism hypothesis

AbstractAimRecent research suggests that the latitudinal diversity gradient (LDG) in birds is unlikely to result from faster diversification in the tropics. This puts the focus on other mechanisms, such as dispersal, as the primary drivers for the LDG. We aim to distinguish two prominent models, tropical conservatism (TCH) and out of the tropics (OTM), which make distinct predictions about dispersal across latitudes and the phylogenetic clustering of assemblages in temperate regions.LocationGlobal.MethodsWe used geographic and phylogenetic data for more than 9000 bird species to reconstruct the ancestral latitudinal zone for each node in each of 100 bird phylogenetic estimates. We used methods that consider discrete latitudinal zones, as well as those that consider latitudinal position as a continuous variable. We then estimated the frequency of ancestor–descendant dispersal within and across latitudinal zones. We also quantified phylogenetic clustering in latitudinal zones separately for bird assemblages within the Old World and the New World.ResultsLatitudinal distributions are relatively conserved: 60–96% of nodes had the same inferred latitude as their immediate ancestral node. We find that dispersal events out of the tropics were less frequent (4–5%) than dispersal events into the tropics (15–21%), the opposite of what would be expected under the OTM. Nodes with inferred temperate distributions are generally younger than the Eocene–Oligocene Climate Transition, as expected under the TCH. Phylogenetic clustering shows no regular patterns of association with latitude, and is likely to be largely driven by radiations within a few large forested biomes.Main conclusionsOur results provide support for the expectations of the TCH, but are less consistent with those of the OTM. Both the deeper origins of tropical clades and the comparatively recent but infrequent dispersal events into temperate regions appear to play a role in generating the strong disparity in tropical and temperate species richness in birds.

Clay mineral changes across the Eocene–Oligocene transition in the sedimentary sequence at Xining occurred prior to global cooling

During the Cenozoic, high-quality long-term sedimentary sequences formed in Xining basin at the northeastern margin of the Tibetan Plateau, which are characterized by alternating gypsiferous layers and reddish mudstone beds. The Tashan section was previously dated in detail by magnetostratigraphy, which yielded an age span of 16 to 35.7Ma. In this study, bulk and clay mineral assemblages and magnetic parameters of the 33.1–35.7Ma portion were analyzed using X-ray diffraction and magnetic methods to determine the behavior of the Eocene–Oligocene climate transition. Quartz, calcite, feldspar and clay fractions in sediments that are mainly derived from the surrounding catchments, gypsum and dolomite are authigenic minerals. Illite, chlorite, irregular mixed-layer illite-smectite and smectite are the four main clay-sized fractions (<2μm) in the gypsum/gypsiferous layers and the reddish mudstone beds, which suggest that no significant tectonic activity occurred in the catchment area of the Xining basin during the period of 35.7–33.1Ma. Dominant hematite, illite and gypsum in the sediments indicate that the climate was dominantly hot and dry in the Xining basin. This was controlled by a “planetary” subtropical aridity zonal pattern; the variations in gypsum, dolomite and smectite content are mainly controlled by the climate changes. The characteristics of the clay mineral assemblages suggest that warm and humid fluctuations with hot and dry conditions prevailed during ~35.7–34.1Ma in inner Asia. This changed to cold and dry conditions at ~34.1Ma and remained so from ~34.1 to 33.1Ma. Comparisons with open ocean marine records and Northern Hemisphere continental records indicate that the climate cooling in continental records occurred prior to the main drop of δ18O in foraminifera preserved in marine sediments at Oi-1 (33.545Ma). This demonstrates that the magnitude of Antarctic ice growth during the EOT was not the major contributor to inner Asian aridification, possibly due to the northward moving of subtropical highs that existed in northwest China and was induced by temperate zones moving during the late Eocene to early Oligocene.

Stable isotope paleoecology of White River ungulates during the Eocene–Oligocene climate transition in northwestern Nebraska

We use the carbon and oxygen stable isotope content of mammalian tooth enamel to evaluate ancient biomes, habitat partitioning, and water use among 12 ungulate taxa from the White River Group in Nebraska. The White River Group spans an interval of climate change from the upper Eocene to the lower Oligocene when global climate cooled in a heterogeneous fashion. We test whether changes in biome structure or mammalian ecology occurred during this interval. We find that mean δ13C values for both Eocene and Oligocene faunas are consistent with dry, open biomes (e.g., woodland-savanna or woody scrubland). A significant increase in mean faunal δ13C values (+0.4‰) occurs in the lower Oligocene, primarily from the loss of individuals feeding in wetter–denser parts of the ecosystem. In the upper Eocene, Megacerops sp. (brontothere) and Perchoerus probus (peccary) have low mean δ13C values consistent with feeding in wetter, denser habitats (presumably riparian), whereas significantly higher mean values in the cursorial horse, Mesohippus spp., and the clawed oreodont, Agriochoerus antiquus, are consistent with feeding in more open, drier habitats. The extinctions of Megacerops sp. and Trigonias sp., a rhinocerotid with relatively low δ13C values, along with a decrease in the abundance of P. probus and other taxa, suggests reduction in the extent of wet habitats in the Oligocene. The rare tapir Colodon sp. is the only taxon sampled with a mean δ13C value low enough to indicate consistent feeding in a wet habitat in the Oligocene. However, range-through taxa occupy the same relative positions in isotopic space in both the Eocene and Oligocene faunas suggesting stability in drier habitats. A significant increase also occurs in mean faunal δ18O values (+1.0‰) in the Oligocene, primarily from the loss or reduction in abundance of water dependent species. Variance in δ18O values increases significantly in the Oligocene, which is consistent with greater water stress in plants from drier conditions. Oxygen values increase in A. antiquus (+0.9‰), the most aridity-sensitive species in both faunas, as expected for drier conditions, but values in another aridity-sensitive taxon (Mesohippus spp.) are virtually unchanged. Together, δ13C and δ18O enamel values generally suggest a shift to drier conditions in the early Oligocene although δ13C values yielded more consistent results. The lack of a large negative shift in δ18O values, expected with a decrease in mean annual temperature, suggests that any temperature changes in the early Oligocene were relatively small in the continental interior of North America.

The Eocene–Oligocene climate transition in the Tarim Basin, Northwest China: Evidence from clay mineralogy

Clay mineralogy analyses were carried out on Cenozoic sedimentary rocks collected from the southwestern margin of the Tarim Basin, northwest China, using X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. These investigations were aimed at determining the paleoclimate of the area in early Cenozoic times, especially during the late Eocene to early Oligocene. The proportion of clay mineral species and clay mineral indices, as well as the proportion of non-clay minerals, gypsum, and morphology of the clays showed that the paleoclimate of the Tarim Basin underwent a distinct change from warm and humid to cold and dry during the late Eocene to early Oligocene. The climate evolution derived from the clay mineralogical study in the Tarim Basin is consistent with the effects of Eocene–Oligocene climate cooling. Combined with the retreat of the Neo-Tethys Sea, cooling of global ocean temperatures reduced the moisture supply to continental interiors, leading to cooling and aridification in the Asian interior.

Atmospheric carbon dioxide through the Eocene–Oligocene climate transition

Geological and geochemical evidence indicates that the Antarctic ice sheet formed during the Eocene-Oligocene transition, 33.5-34.0 million years ago. Modelling studies suggest that such ice-sheet formation might have been triggered when atmospheric carbon dioxide levels (pCO2atm) fell below a critical threshold of approximately 750 p.p.m.v., but the timing and magnitude of pCO2atm relative to the evolution of the ice sheet has remained unclear. Here we use the boron isotope pH proxy on exceptionally well-preserved carbonate microfossils from a recently discovered geological section in Tanzania to estimate pCO2atm before, during and after the climate transition. Our data suggest that are reduction in pCO2atm occurred before the main phase of ice growth,followed by a sharp recovery to pre-transition values and then a more gradual decline. During maximum ice-sheet growth, pCO2atm was between approximately 450 and approximately 1,500 p.p.m.v., with a central estimate of approximately 760 p.p.m.v. The ice cap survived the period of pCO2atm recovery,although possibly with some reduction in its volume, implying (as models predict) a nonlinear response to climate forcing during melting. Overall, our results confirm the central role of declining pCO2atm in the development of the Antarctic ice sheet (in broad agreement with carbon cycle modelling) and help to constrain mechanisms and feedbacks associated with the Earth's biggest climate switch of the past 65 Myr.

Paleoclimate implications of high latitude precession-scale mineralogic fluctuations during early Oligocene Antarctic glaciation: the Great Australian Bight record

Sediments from ODP Site 1128 in the Great Australian Bight record isotopic and mineralogic variations corresponding to orbital parameters and regional climate change during the early Oligocene climate transition and Oi1 glacial event. Bulk carbonate stable isotope analyses reveal prominent positive oxygen and carbon isotope shifts related to the inferred major increase in glaciation at approximately 33.6 to 33.48 Ma. The oxygen isotope excursion corresponds to a prolonged period of low eccentricity, suggesting ice-sheet growth during low seasonality conditions. The clay mineralogy is dominated by smectite throughout. The exclusive occurrence of highly crystalline smectite from 33.6 to 33.5 Ma suggests the occurrence of explosive volcanism that correlates with the positive oxygen isotope shift. The dominance of mixed-layer smectite from 33.5 to 33.4 Ma and an increase in illite following 33.4 Ma indicates a transition from cool, wet conditions to cool, dry conditions over Australia during the Oi1 glaciation. Clay mineralogy and carbonate percentages reveal precession-scale oscillations during the Oi1 event. Kaolinite varies inversely with smectite and percent carbonate. Variations in precipitation and runoff, and wind velocities during southern hemisphere summer perihelion and high eccentricity intervals may account for the precession-scale oscillations.