Eocene-Oligocene Climate Transition Research Articles

A tipping point in stable isotope composition of Antarctic meteoric waters during Cenozoic glaciation.

Triple oxygen isotopes of Cenozoic intrusive rocks emplaced along the Ross Sea coastline in Antarctica, reveal that meteoric-hydrothermal waters imprinted their stable isotope composition on mineral phases, leaving a clear record of oxygen and hydrogen isotope variations during the establishment of the polar cap. Calculated O- and H-isotope compositions of meteoric waters vary from -9 ± 2‰ and -92 ± 5‰ at 40 ± 0.6 Ma, to -30 and -234 ± 5‰ at 34 ± 1.9 Ma, and intersect the modern Global Meteoric Water Line. These isotopic variations likely depict the combined variations in temperature, humidity, and moisture source regions, resulting from rearrangement of oceanic currents and atmospheric cooling during the onset of continental ice cap. Here, we report a paleo-climatic proxy based on triple oxygen geochemistry of crystalline rocks that reveals changes in the hydrological cycle. We discuss the magnitude of temperature changes at high latitudes during the Eocene-Oligocene climatic transition.

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.

Formation and preservation of Eocene lacustrine microbialites in the western Qaidam Basin (northeastern Qinghai-Tibetan Plateau, China): Petrological, mineralogical, and geochemical constraints

Studying the lacustrine microbialites in the Qaidam Basin may provide clues to the occurrence and mineralization processes of microbial communities in extremely harsh terrestrial environments. In the Eocene lacustrine systems of the Qaidam Basin, thick thrombolites (~0.4 to 1 m thick) generally formed in near-shore settings with rare to plentiful terrigenous admixtures and are characterized by four different clotted structures (i.e., maze-like, cortoid-like, tightly bound, and aggregated). In contrast, the small-scale stromatolitic, thrombolitic, and composite clotted-laminated microbialites (cm level) exhibit domical, columnar, and stratiform shapes, likely indicating that they developed in very shallow settings (e.g., a lagoon). The early lithification of the outer crusts and the internally laminated and clotted textures facilitated the preservation of the microbialites. The laminated and clotted textures of the Eocene microbialites analyzed in this study were composed of crypto- to micro-crystalline (mimetic) dolomite, which likely formed during the early mineralization process. This was likely caused by the strong evaporative lacustrine conditions on the Qinghai-Tibetan Plateau during the Eocene-Oligocene climate transition. The microbial mineralization process in dolomite-supersaturated pore fluids may also have contributed to early dolomitization. In contrast, the medium to coarsely crystalline components of the microbialites are composed of calcite minerals, implying that these components experienced cementation/recrystallization during diagenesis. In addition, large quantities of authigenic sulfate (celestite-barite) and sulfide (framboidal pyrite) minerals are exclusively distributed within the mimetically-dolomitized, laminated and clotted microbial structures. It is inferred that the saline lake conditions and the mineral dissolution (both carbonate and terrigenous particles) contributed to the SrSO4- and BaSO4-supersaturated fluid conditions and enabled the crystallization of celestite-barite. The development of Eocene lacustrine microbialites on the northeastern Qinghai-Tibetan Plateau provides a case for comparison with widely-distributed records of lake microbial carbonates, and the microstructural characteristics and analytical tools used in this study may offer a new perspective to explore the complex mineralization processes of microbialites.

The Eocene-Oligocene climate transition in the Alpine foreland basin: Paleoenvironmental change recorded in submarine fans

The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifying the transition from Paleocene greenhouse to Oligocene icehouse conditions. Preservation of the sedimentary signal of such an environmental change is most likely in net-depositional environments, such as submarine fans, which are the terminal parts of sedimentary systems. Here, using sedimentary and stable isotope data from the Alpine foreland basin, we assess whether this major climatic transition influenced the stratigraphic evolution of submarine fans. Results indicate that fine-grained deposition in deep-water environments corresponds to positive δ13C excursions and eustatic highstands, while coarse-grained deposition corresponds to negative δ13C excursions and eustatic lowstands during the earliest Oligocene. While alternative explanations cannot be ruled out on the basis of this dataset alone, our results suggest that eustatic fluctuations across the EOT and into the early Oligocene influenced sediment supply to deep-water environments.

Climate transition at the Eocene–Oligocene influenced by bathymetric changes to the Atlantic–Arctic oceanic gateways

The Eocene–Oligocene Transition (∼33.9 Ma) marks the largest step transformation within the Cenozoic cooling trend and is characterized by a sudden growth of the Antarctic ice sheets, cooling of the interior ocean, and the establishment of strong meridional temperature gradients. Here we examine the climatic impact of oceanic gateway changes at the Eocene–Oligocene Transition by implementing detailed paleogeographic reconstructions with realistic paleobathymetric models for the Atlantic–Arctic basins in a state-of-the-art earth system model (the Norwegian Earth System Model [NorESM-F]). We demonstrate that the warm Eocene climate is highly sensitive to depth variations of the Greenland–Scotland Ridge and the proto–Fram Strait as they control the freshwater leakage from the Arctic to the North Atlantic. Our results, and proxy evidence, suggest that changes in these gateways controlled the ocean circulation and played a critical role in the growth of land-based ice sheets, alongside CO2-driven global cooling. Specifically, we suggest that a shallow connection between the Arctic and North Atlantic restricted the southward flow of fresh surface waters during the Late Eocene allowing for a North Atlantic overturning circulation. Consequently, the Southern Hemisphere cooled by several degrees paving the way for the glaciation of Antarctica. Shortly after, the connection to the Arctic deepened due to weakening dynamic support from the Iceland Mantle Plume. This weakened the North Atlantic overturning and cooled the Northern Hemisphere, thereby promoting glaciations there. Our study points to a controlling role of the Northeast Atlantic gateways and decreasing atmospheric CO2 in the onset of glaciations in both hemispheres.

Evolution of Dispersal, Habit, and Pollination in Africa Pushed Apocynaceae Diversification After the Eocene-Oligocene Climate Transition

Apocynaceae (the dogbane and milkweed family) is one of the ten largest flowering plant families, with approximately 5,350 species and diverse morphology and ecology, ranging from large trees and lianas that are emblematic of tropical rainforests, to herbs in temperate grasslands, to succulents in dry, open landscapes, and to vines in a wide variety of habitats. Despite a specialized and conservative basic floral architecture, Apocynaceae are hyperdiverse in flower size, corolla shape, and especially derived floral morphological features. These are mainly associated with the development of corolline and/or staminal coronas and a spectrum of integration of floral structures culminating with the formation of a gynostegium and pollinaria—specialized pollen dispersal units. To date, no detailed analysis has been conducted to estimate the origin and diversification of this lineage in space and time. Here, we use the most comprehensive time-calibrated phylogeny of Apocynaceae, which includes approximately 20% of the species covering all major lineages, and information on species number and distributions obtained from the most up-to-date monograph of the family to investigate the biogeographical history of the lineage and its diversification dynamics. South America, Africa, and Southeast Asia (potentially including Oceania), were recovered as the most likely ancestral area of extant Apocynaceae diversity; this tropical climatic belt in the equatorial region retained the oldest extant lineages and these three tropical regions likely represent museums of the family. Africa was confirmed as the cradle of pollinia-bearing lineages and the main source of Apocynaceae intercontinental dispersals. We detected 12 shifts toward accelerated species diversification, of which 11 were in the APSA clade (apocynoids, Periplocoideae, Secamonoideae, and Asclepiadoideae), eight of these in the pollinia-bearing lineages and six within Asclepiadoideae. Wind-dispersed comose seeds, climbing growth form, and pollinia appeared sequentially within the APSA clade and probably work synergistically in the occupation of drier and cooler habitats. Overall, we hypothesize that temporal patterns in diversification of Apocynaceae was mainly shaped by a sequence of morphological innovations that conferred higher capacity to disperse and establish in seasonal, unstable, and open habitats, which have expanded since the Eocene-Oligocene climate transition.

Helium diffusion systematics inferred from continuous ramped heating analysis of Transantarctic Mountains apatites showing age overdispersion

Application of apatite (U-Th)/He thermochronology has been hindered by incomplete understanding of diffusion systematics that leads to the single-grain age dispersion often displayed by samples, particularly those from older, slowly cooled settings. We applied the continuous ramped heating (CRH) method to an apatite suite from Cathedral Rocks in the Transantarctic Mountains (TAM) that have high age dispersion in order to explain processes that complicate 4He diffusion in apatite. Examining 132 apatite grains from a total of six samples, we confirmed earlier apatite (U-Th)/He results showing that measured AHe ages have at least three-fold intra-sample dispersion with no obvious relationships between ages and effective uranium concentration (eU) or grain size. CRH results on these apatites yielded two groups. Those with younger ages, characterized by unimodal incremental 4He gas-release curves, displayed simple volume diffusion behavior. In contrast, grains with older ages generally show complex gas release in the form of sharp spikes and/or extended gas-release at high temperatures (i.e., ≥800 °C). Simply-behaved apatites still show considerable age dispersion that exceeds what grain size, radiation damage, and analytical uncertainty can explain, but this dispersion appears to be related to variations in 4He diffusion kinetics. The screened AHe ages from simply-behaved younger apatite grains together with kinetic information from these grains suggest that the sampled region experienced slow cooling prior to rapid cooling (rock exhumation) beginning ca. 35 Ma. This interpretation is consistent with other studies indicative of an increase in exhumation rates at this time, possibly related to the initiation of glaciation at the Eocene-Oligocene climate transition. An attempt to correct older apatite ages by simply removing extraneous gas-release components yielded some ages that are too young for the samples’ geologic setting, suggesting that the factors that lead to complex laboratory release behavior can impact both the expected radiogenic component as well as those that are apparently extraneous. From our observations, we infer that many apatite grains contain imperfections of varying kinds that contribute significantly to kinetic variability beyond that associated with radiation damage and conclude that: (1) CRH analysis can serve as a routine screening tool for AHe dating and offers opportunities to reveal first-order kinetic variations; (2) model-dependent age correction may be possible but would require some means of estimating the broad proportions of 4He components incorporated into grains before and after closure to diffusion, and (3) interpretation of highly dispersed AHe ages requires assessment of individual-grain diffusion kinetics beyond that predicted by radiation-damage models.

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.

Changes in calcareous nannoplankton assemblages around the Eocene-Oligocene climate transition in the Hungarian Palaeogene Basin (Central Paratethys)

ABSTRACT The Eocene-Oligocene climate transition (EOT) is the last major greenhouse-icehouse climate state shift in Earth history, ending the warm, ice-free early Palaeogene world and ushering in the Antarctic glaciation. This study is focused on the Hungarian Palaeogene Basin within the Central Paratethys, aiming to characterise the effect of the global cooling event in the calcareous nannoplankton assemblages and to reconstruct the palaeoenvironmental evolution of the region. Calcareous nannoplankton biostratigraphy is focused on documenting the NP21 Zone. Hierarchical cluster analysis allowed us to distinguish five successive assemblages. Thereby defined phases are compared with recently published trends in δ18O values and foraminiferal changes. Taxa with a preference for oligotrophic and warm surface waters dominate the lowest assemblage. The next assemblage contains taxa that indicate oligotrophic conditions but temperate surface water at the onset of the EOT. Nannoplankton abundance drops to a minimum in the third phase, when taxa adapted to cool surface waters gradually became dominant. A gradual rebound of nannoplankton abundance is observed in the fourth phase, possibly reflecting regional climate change related to the uplifting Alpine chain. After the end of the EOT, the youngest assemblage includes mostly eurytopic taxa which could tolerate an increased rate of freshwater and terrestrial influx.

Chlorite chemical composition change in response to the Eocene-Oligocene climate transition on the northeastern Tibetan Plateau

The Eocene-Oligocene climate transition (EOCT) at ~34 Ma is a pronounced environmental reorganization that occurred in the Cenozoic era and has been extensively studied via marine archives. However, to date, there is still a less well-dated continental record across the EOCT, which can mostly be attributed to a lack of suitable proxies used in the varied continental settings in terms of sedimentary facies, provenance and climate, thus resulting in a poor understanding of the continental response to this transition. Located on the northeastern Tibetan Plateau, the Qaidam Basin is filled with continuous and well-exposed Cenozoic fluvio-lacustrine sediments. Cenozoic lacustrine sediment has been proven to be an ideal archive to study the continental climatic response to the EOCT. Here, we present the detailed mineralogical and elemental composition of chlorite, a widespread clay mineral in continental sediments formed by rock weathering, collected from a lacustrine sequence in the northern Qaidam Basin to constrain the onset of the EOCT and the related climatic response. The results show that chlorite abundance, crystallinity indices, intensity ratios of chlorite diagnostic peaks and chemical composition collectively exhibit remarkable changes at ~34.3 Ma. To be more specific, the well-crystallized Fe-Mg-rich chlorites were characterized by a high Fe content before the EOCT and turned into high-Mg-content Mg-chlorites with poor crystallinity. The chlorite chemical composition change is assumed to have been related to regional drying after the EOCT, thus providing new insights into the continental paleoenvironmental change based on this novel proxy.

Eocene–early Oligocene climate and vegetation change in southern China: Evidence from the Maoming Basin

Although the Eocene-Oligocene climate transition marks a critical point in the development of the ‘icehouse’ global climate of the present little is known about this important change in the terrestrial realm at low latitudes. Our palynological study of the Shangcun Formation shows it to be early Oligocene in age: palyno-assemblages in the lower part of the formation indicate a cool interval dominated by conifer pollen in the earliest Oligocene followed by a warmer regime in the second half of the early Oligocene. To quantify middle Eocene to late early Oligocene climate conditions at low (~20°N) palaeolatitudes in southern Asia several thousand leaf fossil specimens from the Maoming Basin, southern China, were subjected to a multivariate (CLAMP) analysis of leaf form. For terrestrial palaeoclimate comparisons to be valid the palaeoaltitude at which the proxy data are obtained must be known. We find that leaves preserved in the Youganwo (middle Eocene), Huangniuling (late Eocene) and Shangcun (early Oligocene) formations were likely to have been deposited well above sea level at different palaeoelevations. In the Youganwo Formation fine-grained sediments were deposited at an altitude of ~1.5km, after which the basin dropped to ~0.5km by the time the upper Huangniuling sediments were deposited. The basin floor then rose again by 0.5km reaching an altitude of approximately 1km in which the Shangcun Formation fine-grained sediments were accumulated. Within the context of these elevation changes the prevailing climates experienced by the Youganwo, Lower Huangniuling, Upper Huangniuling and Shangcun fossil floras were humid subtropical with hot summers and warm winters, but witnessed a progressive increase in rainfall seasonality. By the early Oligocene rainfall seasonality was similar to that of the modern monsoonal climate of Guangdong Province, southern China. All floras show leaf physiognomic spectra most similar to those growing under the influence of the modern Indonesia-Australia Monsoon, but with no evidence of any adaptation to today's South or East Asia Monsoon regimes. The Upper Huangniuling Flora, rich in dipterocarp plant megafossils, grew in the warmest conditions with the highest cold month mean temperature and at the lowest altitude.

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.

The Eocene-Oligocene climate transition in the Central Paratethys

We studied two boreholes (Cserépváralja-1 and Kiscell-1) with continuous sedimentary records across the Eocene-Oligocene climate transition from the Central Paratethyan area. Assemblages of benthic foraminifera display a shift in dominance by epifaunal taxa in the late Eocene to shallow and deep infaunal taxa in the early Oligocene. Using the benthic foraminiferal oxygen index (BFOI), a decreasing trend of bottom-water oxygen levels is established across the Eocene-Oligocene transition (EOT), leading to the development of dysoxic conditions later in the early Oligocene.Trends in δ18O and δ13C values measured on tests of selected benthic and planktic foraminifera roughly parallel those of the global record of stepped EOT δ18O increase and deviate only later in the early Oligocene, related to the isolation of the Paratethys. The overall similarity of the isotope curves and the presence of a planktic-benthic ecological offset suggest that the original isotope trends are preserved, despite the systematically more negative δ18O values. Of different scenarios, a quasi-uniform diagenetic overprint by fluids with low δ18O values, during burial or uplift, appears best supported. We conclude that the globally established isotopic expression of Antarctic ice sheet growth across the EOT may be recognizable in the Paratethys. Deviations from the global trends after the EOT were caused by regional paleoceanographic changes induced by the progressing Alpine orogeny and sea-level change, which led to a restricted connection with the open ocean, freshwater influx from increased precipitation, and gradual development of bottom-water oxygen depletion.

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.

Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, Part III: The heteroconchs

The heteroconch bivalve fauna of the deep-water (>200 m) Keasey Formation in northwestern Oregon records the Eocene–Oligocene climatic transition and replacement of tropical widely-distributed taxa by the cryophilic taxa that dominate modern high-latitude faunas of the North Pacific. Low-diversity assemblages occur in tuffaceous mudstone and siltstone facies of a deep nearshore basin at the onset of subduction on the Cascadia Margin. Six species of anomalodesmatan heteroconchs have been treated separately, and the remaining Keasey heteroconchs treated here include one basal archiheterodont and 13 imparidentian euheterodonts. The families represented are Carditidae, Thyasiridae, Lucinidae, Lasaeidae, Cardiidae, Tellinidae, Basterotiidae, Mactridae and Veneridae. New taxa include the genus Anechinocardium and seven new species: Cyclocardia moniligena, Conchocele bathyaulax, Conchocele taylori, Kellia saxiriva, Kellia vokesi, Moerella quasimacoma and Saxicavella burnsi. Three species lacking adequate material for formal description are treated in open nomenclature. New features useful for taxonomic discrimination include heretofore unrecognized differences in micro-spines and lamellae on the posterior slope in cardiid bivalve genera and presence of a faint subumbonal ridge in three genera of basterotiid bivalves. Four of the new species are in families in which thiotrophic and methanotrophic chemosymbioses have evolved. Specimens are typically articulated. Shell material is well preserved in the massive units but often chalky or highly altered at cold-seep sites. Fossils are never abundant except for local concentrations in carbonate mounds and associated carbonate lenses, blebs and nodules at cold-seep localities. Multiple lines of evidence of both diffuse and robust flow of fluids rich in reduced compounds are reviewed for three sites where the fauna includes an inner core of chemosymbiotic taxa and peripheral zone of taxa that are opportunistic and tolerant of hypoxia.

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.

Quasi-static Eocene–Oligocene climate in Patagonia promotes slow faunal evolution and mid-Cenozoic global cooling

New local/regional climatic data were compared with floral and faunal records from central Patagonia to investigate how faunas evolve in the context of local and global climates. Oxygen isotope compositions of mammal fossils between c. 43 and 21 Ma suggest a nearly constant mean annual temperature of 16 ± 3 °C, consistent with leaf physiognomic and sea surface studies that imply temperatures of 16–18 °C. Carbon isotopes in tooth enamel track atmospheric δ13C, but with a positive deviation at 27.2 Ma, and a strong negative deviation at 21 Ma. Combined with paleosol characteristics and reconstructed Leaf Area Indices (rLAIs), these trends suggest aridification from 45 Ma (c. 1200 mm/yr) to 43 Ma (c. 450 mm/yr), quasi-constant MAP until at least 31 Ma, and an increase to ~ 800 mm/yr by 21 Ma. Comparable MAP through most of the sequence is consistent with relatively constant floral compositions, rLAI, and leaf physiognomy. Abundance of palms reflects relatively dry-adapted lineages and greater drought tolerance under higher pCO2. Pedogenic carbonate isotopes imply low pCO2 = 430 ± 300 ppmv at the initiation of the Eocene–Oligocene climatic transition. Arid conditions in Patagonia during the late Eocene through Oligocene provided dust to the Southern Ocean, enhancing productivity of silicifiers, drawdown of atmospheric CO2, and protracted global cooling. As the Antarctic Circumpolar Current formed and Earth cooled, wind speeds increased across Patagonia, providing more dust in a positive climate feedback. High tooth crowns (hypsodonty) and ever-growing teeth (hypselodonty) in notoungulates evolved slowly and progressively over 20 Ma after initiation of relatively dry environments through natural selection in response to dust ingestion. A Ratchet evolutionary model may explain protracted evolution of hypsodonty, in which small variations in climate or dust delivery in an otherwise static environment drive small morphological shifts that accumulate slowly over geologic time.

Refining our estimate of atmospheric CO2 across the Eocene–Oligocene climatic transition

The Eocene–Oligocene transition (EOT) followed by Oligocene isotope event 1 (Oi-1) is a dramatic global switch in climate characterized by deep-sea cooling and the first formation of permanent Antarctic ice. Models and proxy evidence suggest that declining partial pressure of atmospheric carbon dioxide (CO2atm) below a threshold may explain the onset of global cooling and associated ice formation at Oi-1. However, significant uncertainty remains in the estimated values and salient features of reconstructed CO2atm across this interval. In this study, we present novel carbon isotope records from size separated diatom associated organic matter (δ13Cdiatom) preserved in silica frustules. Physical preservation of this material allows concurrent investigation of isotopic and cell size information, providing two input parameters for biogeochemical models and the reconstruction of CO2atm. We estimate CO2atm in two ways; first we use size and reaction–diffusion kinetics of a cell to calculate a CO2atm threshold. Second we use the calibrated relationship between εp(diatom) and carbon dioxide from culture and field studies to create a record of CO2atm prior to and across the transition. Our study, from site 1090 in the Atlantic sector of the Southern Ocean, shows CO2atm values fluctuating between 900 and 1700±100 p.p.m.v. across the EOT followed by a drop to values in the order of 700 to 800±100 p.p.m.v. just prior to the onset of Oi-1. Our values and magnitude of CO2atm change differ from previous estimates, but confirm the overall trends inferred from boron isotopes and alkenones, including a marked rebound following Oi-1. Due to the intricate nature of the climate system and complexities in constraining paleo-proxies, this work emphasizes the importance of a multi-proxy approach to estimating of CO2atm in order to elucidate its role in the emplacement of Antarctic ice-sheets at the EOT.

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.