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Understanding the climatic effect of vegetation in different regions is important for understanding the climate impact of vegetation change in both the past and future. Here we quantify the climate response to vegetation removal on each continent, except Antarctica, using CESM1.2.2 under pre‐industrial climate condition, and the associated mechanisms are analyzed. Results show that removing the global vegetation lowers the global mean surface temperature (GMST) by 3.65°C. Removing vegetation over Eurasia and North America lowers GMST by 1.83°C and 0.88°C, respectively. They also reduce the global precipitation, but at a much slower rate than that would be caused in the CO2 perturbation experiments. Removing vegetation on all other continents has negligible influence on global climate, but has significant local warming effect due to weakening of evapotranspiration. The removal of low‐latitude vegetation tends to reduce the local precipitation, but increase the precipitation over nearby oceans, especially to the west. The feedbacks of thermodynamic sea ice and oceans amplify the initial direct cooling due to vegetation removal by a factor of >5. The response of ocean circulation has a negligible impact on GMST, but has a significant influence on the pattern of temperature changes by redistributing heat. Without the ocean‐circulation feedback, the Northern Hemisphere would be 1.30°C colder while the Southern Hemisphere 1.17°C warmer when the global vegetation is removed. Plain Language Summary: Vegetation significantly affects climate but many of its effects are not well understood. For example, climatic effects of vegetation on different continents are still unclear. Here, we quantify systematically how the removal of vegetation on each continent would impact on the global climate and demonstrate the corresponding mechanisms using an Earth system model, CESM. Results show that the sum of temperature changes caused by vegetation removal on each continent is smaller than the cooling of 3.65°C when removing global vegetation. Vegetation removal over Eurasia and North America causes a global cooling of 1.83°C and 0.88°C, respectively, which also decreases global precipitation. The influences of vegetation removal on all other continents are negligible on global climate but significant on local climate. Besides, it is found that the precipitation increases over the nearby ocean while local land precipitation decreases after the removal of low‐latitude vegetation. What's more, the initial cooling due to vegetation removal is greatly amplified by the thermodynamic feedback of sea ice and ocean. The change of ocean circulation is such that it warms Northern Hemisphere by 1.30°C and cools Southern Hemisphere by 1.17°C, but has little effect on the global mean surface temperature. Key Points: Removing vegetation over Eurasia or North America induces a global cooling while that over other continents induces a local warming onlyRemoving low‐latitude vegetation reduces local precipitation but increases precipitation over oceans to the westSea‐ice feedback amplifies global cooling, and ocean circulation transports more heat from the Southern Hemisphere to Northern Hemisphere [ABSTRACT FROM AUTHOR]

The aim of this report is to 1) to formally describe Austral and Subtropical Gyre Radiolaria recovered from ODP Hole 123-765C), 2) to compare them with published records of Southern Hemisphere "non-Tethyan" assemblages and 3) to discuss radiolarian paleobiogeography of the Southern Hemisphere and its implications for global climate change during the Jurassic-Cretaceous transition. The Tithonian to Aptian/early Albian radiolarian record recovered from Hole 765C, Cores -62R to -36R in the Argo Abyssal Plain (AAP) is unique in its density of well-preserved samples and in its faunal contents. Radiolaria recovered from claystones yielded the low diversity, ecologically tolerant "Crypto-Archaeo" Assemblage, (chiefly cryptocephalic and cryptothoracic nassellarians and Archeodictyomitra spp.) interpreted herein as originated in the Subtropical Gyre (STG). In contrast, assemblages extracted from radiolarite layers, interpreted as pelagic turbidites derived from the deeper Australian margin, are dominated by Austral taxa. Neotethyan taxa are very rare to absent before the late Hauterivian/Barremian, when they gradually gain in diversity and abundance. Described Austral and STG taxa include 10 families, of which Fusitanellidae n. fam. and Windaliinae n. subfam. are new. Of 18 genera 7 are new (Nodosphaera, Praewindalia, Pachycingula, Archaeotanella, Morchella, Fusitanella, Argofusus) and of 55 species 30 new ones are formally described and 14 new ones are left in open nomenclature. The southern hemisphere Late Jurassic to Early Cretaceous radiolarian biogeography is defined from low- to high latitude: 1. the Neotethyan (NT) and 2. the Central Panthalassan (CP) realms, 3. Eastern Boundary Current (EBC) realm, 4. the Subtropical Gyre (STG) and the Austral (A) circum south-polar realm. Radiolarian biogeography and plate tectonic models support a scenario of palaeoceanographic and global climatic change during the Jurassic-Cretaceous transition related to progressive Pangea break-up with the following consequences: 1. an increased heat transfer to the Southern hemisphere which caused cooling of Neotethyan regions during the Late Tithonian dry event. 2. Anorthward shift of the southern summer Intertropical Convenience Zone reduced the Neotethyan monsoon area and allowed the establishment of a southern Neotethyan subtropical gyre documented by the "Crypo-Archaeo" Assemblage. 3. The south-polar West Wind Drift may have forced a circum Antarctic-Australian cold current through the epicontinental rift between India and Antarctica-Australia since the Berriasian (140 my), transporting Austral Radiolaria into the AAP where they accumulated in radiolarite layers. [ABSTRACT FROM AUTHOR]

The Eocene-Oligocene transition (ca 40–33 Ma) marks a transformation from an ice-free to an ice-house climate mode that is well recorded by oxygen stable isotopes and sea surface temperature proxies. Opening of the Southern Ocean gateways and decline in atmospheric carbon dioxide have been hypothesised as possible triggers of the major climate shift during the Cenozoic. However, identifying the driving mechanisms remains controversial and depends on a better understanding of how the different environmental changes correlate. In this study, we investigate the spatio-temporal variation in export productivity using biogenic Ba (bio-Ba) from different Ocean Drilling Program (ODP) Sites in the Southern Ocean, focusing on possible mechanisms that controlled them as well as the correlation of export productivity changes to changes in the global carbon cycle. We document two significant SO region high export productivity late-Eocene events (ca. 37 and 33.5 Ma) correlated to pronounced changes in global atmospheric p CO2. We propose that paleoceanographic changes that followed Southern Ocean gateway openings, along with more variable increases in circulation driven by episodic expansion and decline of the Antarctic ice sheet, drove enhanced SO export production in the late Eocene through basal Oligocene. These factors may have driven the episodic reduction of atmospheric carbon dioxide and contributed to Antarctic glaciation during the Eocene-Oligocene transition. [ABSTRACT FROM AUTHOR]

Palaeogene sediments of the Hampshire Basin were a sensitive recorder of fluctuations in climate and eustatic sea level as Earth's climate transitioned from the global early Eocene 'hothouse' to the early Oligocene 'coolhouse', accompanied by the first permanent continent‐scale glaciation of Antarctica at the Eocene/Oligocene boundary. A study of the Palaeogene sediments of the Hampshire Basin is not only interesting from a palaeoclimate perspective, but the marine middle Eocene formations are renowned for containing some of the most abundant and diverse Palaeogene fossil assemblages in the world. In this article, I take you on a tour of the geological evolution of the Hampshire Basin from the end of the Paleocene through to the Pleistocene. I highlight some of the best exposures where sediments can be studied at outcrop and representative fossil assemblages can be collected, along with the economic and archaeological significance of these Palaeogene sediments. [ABSTRACT FROM AUTHOR]

A large‐scale nuclear war could inject massive amounts of soot into the stratosphere, triggering rapid global climate change. In climate model simulations of nuclear war, global cooling contributes to an expansion of sea ice in the Northern Hemisphere. However, in the Southern Hemisphere (SH), an initial expansion of sea ice shifts suddenly to a 30% loss of sea ice volume over the course of a single melting season in the largest nuclear war simulation. In smaller nuclear war simulations an expansion in sea ice is instead observed which lasts for approximately 15 years. In contrast, in the largest nuclear war simulation, Antarctic sea ice remains below the long term control mean for 15 years, indicating a threshold that must be crossed to cause the response. Declining sea ice in the SH following a global cooling event has been previously attributed to shifts in the zonal winds around Antarctica, which can reduce the strength of the Weddell Gyre. In climate model simulations of nuclear war, the primary mechanisms responsible for Antarctic sea ice loss are: (a) enhanced atmospheric poleward heat transport through teleconnections with a strong nuclear war‐driven El Niño, (b) increased upwelling of warm subsurface waters in the Weddell Sea due to changes in wind stress curl, and (c) decreased equatorward Ekman transport due to weakened Southern Ocean westerlies. The prospect of sudden Antarctic sea ice loss after an episode of global cooling may have implications for solar geoengineering and further motivates this study of the underlying mechanisms of change. Plain Language Summary: Firestorms from a global nuclear war would generate large amounts of smoke that could enter the Earth's atmosphere and block sunlight. Climate model simulations that inject smoke into the upper atmosphere confirm that this smoke causes rapid global cooling leading to increased sea ice in the Northern Hemisphere. However, sea ice in the Southern Hemisphere actually shrinks in the 2–6 years after a very large nuclear war, mainly caused by a change in the winds around Antarctica. Smoke heats the upper atmosphere and the westerly winds around Antarctica shift closer to the coast. The wind shift causes Ekman transport of the top layer of the ocean away from the coast, which brings up relatively warm water from below, melting sea ice during summer and inhibiting sea ice growth during the winter. A similar response has been found in simulations of supervolcano eruptions. As greenhouse gasses continue to warm up the Earth's oceans, a global cooling event that triggers a similar wind shift may lead to greater reductions in Antarctic sea ice. Key Points: A sudden reduction in Antarctic sea ice occurs despite global cooling after nuclear war in climate model simulationsThe sea ice changes in response to dynamic changes in atmospheric and oceanic circulationEven in a nuclear winter, Antarctic sea ice is vulnerable to wind shifts that cause upwelling of water onto the continental shelf [ABSTRACT FROM AUTHOR]

Global ice volume (sea level) and deep‐sea temperature are key measures of Earth's climatic state. We synthesize evidence for multi‐centennial to millennial ice‐volume and deep‐sea temperature variations over the past 40 million years, which encompass the early glaciation of Antarctica at ∼34 million years ago (Ma), the end of the Middle Miocene Climate Optimum, and the descent into bipolar glaciation from ∼3.4 Ma. We compare different sea‐level and deep‐water temperature reconstructions to build a resource for validating long‐term numerical model‐based approaches. We present: (a) a new template synthesis of ice‐volume and deep‐sea temperature variations for the past 5.3 million years; (b) an extended template for the interval between 5.3 and 40 Ma; and (c) a discussion of uncertainties and limitations. We highlight key issues associated with glacial state changes in the geological record from 40 Ma to present that require attention in further research. These include offsets between calibration‐sensitive versus thermodynamically guided deep‐sea paleothermometry proxy measurements; a conundrum related to the magnitudes of sea‐level and deep‐sea temperature change at the Eocene‐Oligocene transition at 34 Ma; a discrepancy in deep‐sea temperature levels during the Middle Miocene; and a hitherto unquantified non‐linear reduction of glacial deep‐sea temperatures through the past 3.4 million years toward a near‐freezing deep‐sea temperature asymptote, while sea level stepped down in a more uniform manner. Uncertainties in proxy‐based reconstructions hinder further distinction of "reality" among reconstructions. It seems more promising to further narrow this using three‐dimensional ice‐sheet models with realistic ice‐climate‐ocean‐topography‐lithosphere coupling, as computational capacities improve. Plain Language Summary: Global ice volume (hence, sea level) and deep‐sea temperature are important measures of Earth's climatic state. To better understand Earth's climate cycles in response to its orbitally driven insolation cycles, we evaluate and synthesize evidence for ice‐volume (sea‐level) and deep‐sea temperature variations at multi‐centennial to millennial resolution throughout the last 40 million years. These last 40 million years encompass the major build‐up of Antarctic glaciation from about 34 million years ago, and development of extensive Northern Hemisphere ice sheets from about 3.4 million years ago. We present a new template synthesis of ice‐volume (sea‐level) and deep‐sea temperature for the past 5.3 million years, with extension through the interval between 5.3 and 40 Ma with wider uncertainties. We also highlight a number of remaining questions about major climate transitions, including the early glaciation history of Antarctica, the end of the so‐called Middle Miocene Climate Optimum from about ∼14.5 Ma, and the descent over the past several million years into conditions with extensive ice‐age maxima in both hemispheres. Key Points: New synthesis of sea level and deep‐sea temperature over 40 million years, at millennial resolution, across seven different methodsDiscrepancies between Eocene‐Oligocene and Middle Miocene sea‐level reconstructions are highlighted for focussed future researchPlio‐Pleistocene glacial deep‐sea temperature asymptoted to a freezing limit by ∼1.25–0.9 Ma; glacial sea‐level minima decreased ∼linearly [ABSTRACT FROM AUTHOR]

The Eocene‐Oligocene Transition (EOT) at ∼34 Ma marked a climatic shift from greenhouse to icehouse conditions, toward long‐lasting lower global temperatures and a continental ice sheet in the Antarctic. We report on sedimentological and inorganic geochemical results across the EOT at Ocean Drilling Program (ODP) Site 696 in the Weddell Sea, within the Antarctic limb of the Atlantic circulation. The geochemical composition of detrital, authigenic and biogenic marine sediment components, and sortable silt proxies demonstrate the impact of ice growth on high latitude water masses. Sortable silt grain size and Zr/Rb ratios attest to a period of vigorous circulation at ∼36.2–35.8 Ma, coincident with a known warm interval in the Southern Ocean. Across the EOT, detrital provenance suggests that regional ice growth in the western Weddell Sea was stepwise, first expanding in the Antarctic Peninsula, followed by parts of West Antarctica. In conjunction with regional ice growth, high uranium enrichment factors (U EF) in sediments spanning the EOT interval indicate anoxic conditions in the sediment with evidence of carbonate dissolution. Following glacial expansion and sea‐ice formation at ∼33.6 Ma, a return to oxic conditions and carbonate preservation is observed with excess barium and phosphorous indicative of an increase in productivity, and potentially carbon export. Our results highlight the important connections between ice growth and the changing properties of high‐latitude water masses at the EOT with impacts on the global ocean circulation. Plain Language Summary: Here we show how the establishment of a marine ice sheet in Antarctica during a climate transition ca. 34 Ma impacted the oceanography of the Weddell Sea using a paleoclimate archive extracted from a sediment core. The Weddell Sea is an embayment to the south of the Atlantic Ocean where the global ocean circulation interacts with the largest ice shelf of floating land ice in the world. Grain size data and geochemical results on 37 to 32 Myr old sediment provide evidence of an episode of intense ocean circulation preceding ice growth, with slowing currents, high‐latitude ocean stratification and acidification in the Weddell Sea as the ice sheet began to grow. These results show a strong coupling between ice sheet and ocean at high latitude with potential wider implications for the understanding of the climate system. Key Points: Sortable silt and Zr/Rb show vigorous circulation in a warm period ∼36 Ma declining during the Late Eocene onset of Antarctic glaciationMud provenance shows stepwise regional ice expansion with cooling in response to early Oligocene glaciation of West Antarctic terranesTrace element enrichments highlight the impact of Antarctic glaciation on primary productivity, redox and carbonate saturation state [ABSTRACT FROM AUTHOR]

The Antarctic continent reached its current polar location ~83 Ma and became shrouded by ice sheets ~34 Ma, coincident with dramatic global cooling at the Eocene-Oligocene boundary. However, it is not known whether the first Antarctic glaciers formed immediately prior to this or were present significantly earlier. Here we show that mountain glaciers were likely present in the Transantarctic Mountains during the Late Palaeocene (~60–56 Ma) and middle Eocene (~48–40 Ma). Temperate (warm-based) glaciers were prevalent during the Late Eocene (~40–34 Ma) and, in reduced numbers, during the Oligocene (~34–23 Ma), before larger, likely cold-based, ice masses (including ice sheets) dominated. Some temperate mountain glaciers were present during the Miocene Climatic Optimum (~15 Ma), before a widespread switch to cold-based glaciation. Our findings highlight the longevity of glaciation in Antarctica and suggest that glaciers were present even during the Early-Cenozoic greenhouse world. This study finds that glaciers have existed in the Transantarctic Mountains for the past 60 million years, and that warm-based mountain glaciers were present in Antarctica long before ice sheets came to dominate the continent. [ABSTRACT FROM AUTHOR]

Considered one of the most significant climate reorganizations of the Cenozoic period, the Eocene–Oligocene Transition (EOT; ca. 34.44–33.65) is characterized by global cooling and the first major glacial advance on Antarctica. In the southern high latitudes, the EOT cooling is primarily recorded in the marine realm, and its extent and effect on the terrestrial climate and vegetation are poorly documented. Here, we present new, well-dated, continuous, high-resolution palynological (sporomorph) data and quantitative sporomorph-based climate estimates recovered from the East Tasman Plateau (ODP Site 1172) to reconstruct climate and vegetation dynamics from the late Eocene (37.97 Ma) to the early Oligocene (33.06 Ma). Our results indicate three major climate transitions and four vegetation communities occupying Tasmania under different precipitation and temperature regimes: (i) a warm-temperate Nothofagus–Podocarpaceae-dominated rainforest with paratropical elements from 37.97 to 37.52 Ma; (ii) a cool-temperate Nothofagus-dominated rainforest with secondary Podocarpaceae rapidly expanding and taking over regions previously occupied by the warmer taxa between 37.306 and 35.60 Ma; (iii) fluctuation between warm-temperate–paratropical taxa and cool temperate forest from 35.50 to 34.49 Ma, followed by a cool phase across the EOT (34.30–33.82 Ma); and (iv) a post-EOT (earliest Oligocene) recovery characterized by a warm-temperate forest association from 33.55 to 33.06 Ma. Coincident with changes in the stratification of water masses and sequestration of carbon from surface water in the Southern Ocean, our sporomorph-based temperature estimates between 37.52 and 35.60 Ma (phase ii) showed 2–3 ∘ C terrestrial cooling. The unusual fluctuation between warm and cold temperate forest between 35.50 to 34.59 Ma is suggested to be linked to the initial deepening of the Tasmanian Gateway, allowing eastern Tasmania to come under the influence of warm water associated with the proto-Leeuwin Current (PLC). Further to the above, our terrestrial data show the mean annual temperature declining by about 2 ∘ C across the EOT before recovering in the earliest Oligocene. This phenomenon is synchronous with regional and global cooling during the EOT and linked to declining p CO 2. However, the earliest Oligocene climate rebound along eastern Tasmania is linked to a transient recovery of atmospheric p CO 2 and sustained deepening of the Tasmanian Gateway, promoting PLC throughflow. The three main climate transitional events across the studied interval (late Eocene–earliest Oligocene) in the Tasmanian Gateway region suggest that changes in ocean circulation due to accelerated deepening of the Tasmanian Gateway may not have been solely responsible for the changes in terrestrial climate and vegetation dynamics; a series of regional and global events, including a change in the stratification of water masses, sequestration of carbon from surface waters, and changes in p CO 2 , may have also played vital roles. [ABSTRACT FROM AUTHOR]

The role and climatic impact of the opening of the Drake Passage and how it affected both marine and terrestrial environments across the Eocene–Oligocene transition (EOT ∼34 Ma) period remains poorly understood. Here we present new terrestrial palynomorph data compared with recently compiled lipid biomarker (n -alkane) data from Ocean Drilling Program (ODP) Leg 113, Site 696, drilled on the margin of the South Orkney Microcontinent (SOM) in the Weddell Sea, to investigate changes in terrestrial environments and palaeoclimate across the late Eocene and early Oligocene (∼37.6 –32.2 Ma). Early late Eocene floras and sporomorph-based climate estimates reveal Nothofagus-dominated forests growing under wet temperate conditions, with mean annual temperature (MAT) and precipitation (MAP) around 12 ∘ C and 1802 mm respectively. A phase of latest Eocene terrestrial cooling at 35.5 Ma reveals a decrease in MAT by around 1.4 ∘ C possibly linked to the opening of the Powell Basin. This is followed by an increase in reworked Mesozoic sporomorphs together with sedimentological evidence indicating ice expansion to coastal and shelf areas approximately 34.1 Myr ago. However, major changes to the terrestrial vegetation at Site 696 did not take place until the early Oligocene, where there is a distinct expansion of gymnosperms and cryptogams accompanied by a rapid increase in taxon diversity and a shift in terrestrial biomarkers reflecting a change from temperate forests to cool temperate forests following 33.5 Ma. This surprising expansion of gymnosperms and cryptogams is suggested to be linked to environmental disturbance caused by repeat glacial expansion and retreat, which facilitated the proliferation of conifers and ferns. The timing of glacial onset at Site 696 is linked to the global cooling at the EOT, yet the latest Eocene regional cooling cannot directly be linked to the observed vegetation changes. Therefore, our vegetation record provides further evidence that the opening of the Drake Passage and Antarctic glaciation were not contemporaneous, although stepwise cooling in response to the opening of ocean gateways surrounding the Antarctic continent may have occurred prior to the EOT. [ABSTRACT FROM AUTHOR]

Pronounced global cooling around the Eocene–Oligocene transition (EOT) was a pivotal event in Earth's climate history, controversially associated with the opening of the Drake Passage. Using a physical laboratory model we revisit the fluid dynamics of this marked reorganization of ocean circulation. Here we show, seemingly contradicting paleoclimate records, that in our experiments opening the pathway yields higher values of mean water surface temperature than the "closed" configuration. This mismatch points to the importance of the role ice albedo feedback plays in the investigated EOT-like transition, a component that is not captured in the laboratory model. Our conclusion is supported by numerical simulations performed in a global climate model (GCM) of intermediate complexity, where both "closed" and "open" configurations were explored, with and without active sea ice dynamics. The GCM results indicate that sea surface temperatures would change in the opposite direction following an opening event in the two sea ice dynamics settings, and the results are therefore consistent both with the laboratory experiment (slight warming after opening) and the paleoclimatic data (pronounced cooling after opening). It follows that in the hypothetical case of an initially ice-free Antarctica the continent could have become even warmer after the opening, a scenario not indicated by paleotemperature reconstructions. [ABSTRACT FROM AUTHOR]

The Weissert Event ~133 million years ago marked a profound global cooling that punctuated the Early Cretaceous greenhouse. We present modelling, high-resolution bulk organic carbon isotopes and chronostratigraphically calibrated sea surface temperature (SSTs) based on an organic paleothermometer (the TEX86 proxy), which capture the Weissert Event in the semi-enclosed Weddell Sea basin, offshore Antarctica (paleolatitude ~54 °S; paleowater depth ~500 meters). We document a ~3–4 °C drop in SST coinciding with the Weissert cold end, and converge the Weddell Sea data, climate simulations and available worldwide multi-proxy based temperature data towards one unifying solution providing a best-fit between all lines of evidence. The outcome confirms a 3.0 °C (±1.7 °C) global mean surface cooling across the Weissert Event, which translates into a ~40% drop in atmospheric pCO2 over a period of ~700 thousand years. Consistent with geologic evidence, this pCO2 drop favoured the potential build-up of local polar ice. Modelling and sea surface temperature proxy data from the Weddell Sea document a 3–4 °C drop coinciding with the Early Cretaceous Weissert Event. Temperature data worldwide confirm a 3.0 °C global mean surface cooling, equivalent to a ~40% drop in atmospheric pCO2, favouring local polar ice. [ABSTRACT FROM AUTHOR]

Global cooling and glacial–interglacial cycles since Antarctica’s isolation have been responsible for the diversification of the region’s marine fauna. By contrast, these same Earth system processes are thought to have played little role terrestrially, other than driving widespread extinctions. Here, we show that on islands along the Antarctic Polar Front, paleoclimatic processes have been key to diversification of one of the world’s most geographically isolated and unique groups of herbivorous beetles—Ectemnorhinini weevils. Combining phylogenomic, phylogenetic, and phylogeographic approaches, we demonstrate that these weevils colonized the sub- Antarctic islands from Africa at least 50 Ma ago and repeatedly dispersed among them. As the climate cooled from the mid-Miocene, diversification of the beetles accelerated, resulting in two speciesrich clades. One of these clades specialized to feed on cryptogams, typical of the polar habitats that came to prevail under Miocene conditions yet remarkable as a food source for any beetle. This clade’s most unusual representative is a marine weevil currently undergoing further speciation. The other clade retained the more common weevil habit of feeding on angiosperms, which likely survived glaciation in isolated refugia. Diversification of Ectemnorhinini weevils occurred in synchrony with many other Antarctic radiations, including penguins and notothenioid fishes, and coincided with major environmental changes. Our results thus indicate that geoclimatically driven diversification has progressed similarly for Antarctic marine and terrestrial organisms since the Miocene, potentially constituting a general biodiversity paradigm that should be sought broadly for the region’s taxa. [ABSTRACT FROM AUTHOR]

Smithian–lower Anisian strata in Peary Land, North Greenland, were deposited at ∼45° N on the northern margin of Pangaea in offshore to upper shoreface settings. The well-constrained succession (palynology and ammonite biostratigraphy) documents a remarkable shift from lycophyte spore-dominated assemblages in the upper Smithian to gymnosperm pollen-dominated ones in the lower Spathian in concert with a marked shift of +6 ‰ in δ13Corg. Correlation with other Smithian–Spathian boundary sections that record terrestrial floral changes indicates that the recovery of gymnosperms began earlier in the mid-latitudes of the Southern Hemisphere than in the Northern Hemisphere. The lycophyte-dominated Late Smithian Thermal Maximum is here interpreted as reflecting dry and hot climatic conditions with only brief seasonal precipitation unable to sustain large areas of gymnosperm trees, but able to revive dehydrated lycophytes. This suggests that the Late Smithian Thermal Maximum was a time of widespread aridity, which is also supported by red bed deposition in many areas globally, even as far south as Antarctica. The shift to gymnosperm-dominated vegetation during the cooling across the Smithian–Spathian boundary reflects a change to seasonally more humid climatic conditions favouring gymnosperm recovery, and could have been initiated by increased albedo over land due to the widespread aridity during the Late Smithian Thermal Maximum. The recovery of gymnosperm vegetation would have helped to draw down CO2 from the atmosphere and exacerbate global cooling. [ABSTRACT FROM AUTHOR]

The Eocene-Oligocene transition (EOT, ∼33.9 Ma), a period of dramatic global cooling marking the onset of the Antarctic ice sheet. However, paleoclimatic reconstructions indicate a notable spatial heterogeneity in both the marine and terrestrial realms. While limited temporal resolution terrestrial records have hindered the precise understanding of short-term climate events and orbital-scale changes during this transition. Here, we present a chemical weathering data sequence from a fluvial-lacustrine deposition from Lühe Basin, which is located at southeastern margin of the Tibetan Plateau. Based on the radioisotope, paleomagnetic and 405 kyr orbital tuning of the elemental datasets, the deposition age of Lühe Basin is dated to ∼35.5–25.5 Ma. Two weakening stages of the weathering intensity are detected around Eocene-Oligocene boundary: 34.1 to 33.9 Ma and 33.8 to 33.6 Ma, which are temporally equivalent to the two global main climatic steps of EOT (EOT-1 and Oi-1). We find the chemical weathering intensity sequence paced the 1.2 Myr obliquity amplitude modulation and 405 kyr long-eccentricity cycles. Notably, the minima of 1.2 Myr obliquity amplitude and 405 kyr eccentricity are synchronous with the Oil-1 event. These patterns highlight the role of orbital forcing in shaping the cooling trend across the EOT and may trigger Antarctic glaciation. • Chemical weathering intensity and climate in southeastern Tibetan Plateau response to orbital forcing across EOT. • Two stages of chemical weathering intensity decreasing indicate climate cooling occurredduring EOT-1 and Oil-1. • 1.2 Myr obliquity amplitude may played critical role in global cooling and Antarctic glaciation across EEOT. [ABSTRACT FROM AUTHOR]

A large fossil leaf assemblage (>3700 specimens) is reported from the Oligocene Río Leona Formation in the Sierra Baguales of Chilean Patagonia. The association comprises 29 species but is dominated by Nothofagus genera, which constitutes 65% of specimens. The collection can be classified as a Mixed Palaeoflora of the Austral-Antarctic association. Stratigraphic analysis of the assemblage indicates a decrease in species diversity and richness over time, which was accompanied by species turnover. Quantitative studies of foliar morphology (CLAMP, Leaf Margin Analysis) indicates cool-to-cold and dry climatic conditions, with a Mean Annual Temperature of 9.2 °C, a relatively high seasonality in temperature and precipitation, and a Mean Annual Precipitation of 931 mm. Nothofagus only became dominant in southeastern Patagonia during the Rupelian, coinciding with a marked global cooling period linked to the initiation of glaciation in Antarctica about 34 Ma. The decrease in precipitation following this event is attributed to the development of a rain shadow to the east of the rising Southern Patagonian Andes, which must have been of the order of 1000 m or more for topographic climate forcing to take effect. This contrasts with the rain shadow development east of the Andes at lower latitudes, which was mainly manifested after the middle Miocene. • Patagonian fossil plant diversity and climate has been integrated with tectonic and climatic events during Oligocene • Nothofagus genus presence in Patagonia during Oligocene, is the first evidence that this lived under microthermal conditions • Are recorded lowest precipitation, temperature and diversity during Cenozoic in Patagonia • The decline diversity and turnover in Patagonia is resulting from the development of a rain shadow during Cenozoic • Temperature and precipitation drop was conditioned by Drake Passage opening, Patagonian Andes uplift and Oligocene cooling [ABSTRACT FROM AUTHOR]

The Eocene was a critical period of global plate reorganization and it also saw the Earth's climate transition from the warmhouse state to the coolhouse state. Reconstructing the Eocene sedimentary history in the climate-sensitive Southern Ocean is important for understanding paleoenvironmental changes in response to the accelerated Australia/Antarctica separation and global cooling throughout the middle and late Eocene. Here, we present the first detailed multiproxy record of a continuous sequence from International Ocean Discovery Program (IODP) Site U1514 in the Mentelle Basin off southwestern Australia. Our aim is to reconstruct the sediment provenances and paleoenvironmental evolution in response to the abovementioned climatic and tectonic changes in the mid-high southern latitudes during the Eocene. Provenance analyses based on Sr-Nd isotopes, trace elements, and clay mineral assemblages suggest that Eocene sediments at Site U1514 predominantly originated from the southwestern Australian continent and the Naturaliste Plateau. Sediment provenance variations during the middle Eocene indicate that the onset of fast separation between Australia and Antarctica at 43 Ma caused an increased supply of volcanic materials from the Naturaliste Plateau between 43 and 38 Ma. Terrigenous inputs to the Mentelle Basin during the middle Eocene were primarily controlled by paleoclimate changes rather than tectonic processes because coeval clay mineralogical changes (higher kaolinite/smectite ratio and MAR kaolinite) indicate a period of stronger physical erosion and chemical weathering on the western Australian continent that resulted in increased terrigenous materials delivered to the Mentelle Basin. Our results reveal a 5 Myr-long (43–38 Ma) warming reversal in the southern mid-high latitudes, providing an exception to the generally short-lived (10–100 kyr-long) hyperthermals that interrupted the long-term global cooling throughout the middle to late Eocene. As for the late Eocene (38–37 Ma), tectonic processes related to the sudden acceleration in seafloor spreading in the Tasman Sea led to the exposure of shallower areas, resulting in rapid detritus accumulation at the study site. During the late Eocene (37–34 Ma), major sediment provenance shifted from distal source areas (e.g., the Yilgarn Craton) to relatively proximal sources (e.g., the Leeuwin Block and Perth Basin). We interpret that the regional uplift in southwestern Australia and coeval climate cooling resulted in the diversion and inactivation of large drainage systems, thus blocking the transportation of sediment from distant regions. • First detailed record of the Eocene source-to-sink process in the Mentelle Basin. • The onset of fast Australia/Antarctica separation caused increased volcanic input. • Regional warming led to enhanced chemical weathering in Australia during 43–38 Ma. • Regional uplift and global cooling induced sediment source change during 37–34 Ma. [ABSTRACT FROM AUTHOR]

Reconstructing past variability of the Antarctic ice sheets is essential to understand their stability and to anticipate their contribution to sea level change as a result of future climate change. Recent studies have reported a significant decrease in thickness of the East Antarctic Ice Sheet (EAIS) during the last several million years. However, the geographical extent of this decrease and subsequent isostatic rebound remain uncertain. In this study, we reconstruct the magnitude and timing of ice sheet retreat at the Sør Rondane Mountains in Dronning Maud Land, East Antarctica, based on detailed geomorphological survey, cosmogenic exposure dating, and glacial isostatic adjustment modeling. Three distinct deglaciation phases are identified for this sector during the Quaternary, based on rock weathering and 10Be surface exposure data. We estimate that the ice sheet thinned by at least 500 m during the Pleistocene. This thinning is attributed to the reorganization of Southern Ocean circulation associated with global cooling into the Pleistocene, which reduced the transport of moisture from the Southern Ocean to the interior of EAIS. The data also show that since the Last Glacial Maximum the ice surface has lowered less than ca 50 m and that this lowering probably started after ca 14 ka. This suggests that the EAIS in Dronning Maud Land is unlikely to have been a major contributor to postglacial sea-level rise and Meltwater pulse 1A. [ABSTRACT FROM AUTHOR]

Abstract: The early Eocene epoch was characterized by extreme global warmth, which in terrestrial settings was characterized by an expansion of near-tropical vegetation belts into the high latitudes. During the middle to late Eocene, global cooling caused the retreat of tropical vegetation to lower latitudes. In high-latitude settings, near-tropical vegetation was replaced by temperate floras. This floral change has recently been traced as far south as Antarctica, where along the Wilkes Land margin paratropical forests thrived during the early Eocene and temperate Nothofagus forests developed during the middle Eocene. Here we provide both qualitative and quantitative palynological data for this floral turnover based on a sporomorph record recovered at Integrated Ocean Drilling Program (IODP) Site U1356 off the Wilkes Land margin. Following the nearest living relative concept and based on a comparison with modern vegetation types, we examine the structure and diversity patterns of the Eocene vegetation along the Wilkes Land margin. Our results indicate that the early Eocene forests along the Wilkes Land margin were characterized by a diverse canopy composed of plants that today occur in tropical settings; their richness pattern was similar to that of present-day forests from New Caledonia. The middle Eocene forests were characterized by a canopy dominated by Nothofagus and exhibited richness patterns similar to modern Nothofagus forests from New Zealand. [Copyright &y& Elsevier]

Global cooling and the development of continental-scale Antarctic glaciation occurred in the late middle Eocene to early Oligocene (~38 to 28 million years ago), accompanied by deep-ocean reorganization attributed to gradual Antarctic Circumpolar Current (ACC) development. Our benthic foraminiferal stable isotope comparisons show that a large δ13C offset developed between mid-depth (~600 meters) and deep (>1000 meters) western North Atlantic waters in the early Oligocene, indicating the development of intermediate-depth δ13C and O2 minima closely linked in the modern ocean to northward incursion of Antarctic Intermediate Water. At the same time, the ocean's coldest waters became restricted to south of the ACC, probably forming a bottom-ocean layer, as in the modern ocean. We show that the modern four-layer ocean structure (surface, intermediate, deep, and bottom waters) developed during the early Oligocene as a consequence of the ACC. [ABSTRACT FROM AUTHOR]

About 34 million years ago, Earth's climate shifted from a relatively ice-free world to one with glacial conditions on Antarctica characterized by substantial ice sheets. How Earth's temperature changed during this climate transition remains poorly understood, and evidence for Northern Hemisphere polar ice is controversial. Here, we report proxy records of sea surface temperatures from multiple ocean localities and show that the high-latitude temperature decrease was substantial and heterogeneous. High-latitude (45 degrees to 70 degrees in both hemispheres) temperatures before the climate transition were ∼20°C and cooled an average of ∼5°C. Our results, combined with ocean and ice-sheet model simulations and benthic oxygen isotope records, indicate that Northern Hemisphere glaciation was not required to accommodate the magnitude of continental ice growth during this time. [ABSTRACT FROM AUTHOR]

We report a natural wind cycle, the Antarctic Centennial Wind Oscillation (ACWO), whose properties explain milestones of climate and human civilization, including contemporary global warming. We explored the wind/temperature relationship in Antarctica over the past 226 millennia using dust flux in ice cores from the European Project for Ice Coring in Antarctica (EPICA) Dome C (EDC) drill site as a wind proxy and stable isotopes of hydrogen and oxygen in ice cores from EDC and ten additional Antarctic drill sites as temperature proxies. The ACWO wind cycle is coupled 1:1 with the temperature cycle of the Antarctic Centennial Oscillation (ACO), the paleoclimate precursor of the contemporary Antarctic Oscillation (AAO), at all eleven drill sites over all time periods evaluated. Such tight coupling suggests that ACWO wind cycles force ACO/AAO temperature cycles. The ACWO is modulated in phase with the millennial-scale Antarctic Isotope Maximum (AIM) temperature cycle. Each AIM cycle encompasses several ACWOs that increase in frequency and amplitude to a Wind Terminus, the last and largest ACWO of every AIM cycle. This historic wind pattern, and the heat and gas exchange it forces with the Southern Ocean (SO), explains climate milestones including the Medieval Warm Period and the Little Ice Age. Contemporary global warming is explained by venting of heat and carbon dioxide from the SO forced by the maximal winds of the current positive phase of the ACO/AAO cycle. The largest 20 human civilizations of the past four millennia collapsed during or near the Little Ice Age or its earlier recurrent homologs. The Eddy Cycle of sunspot activity oscillates in phase with the AIM temperature cycle and therefore may force the internal climate cycles documented here. Climate forecasts based on the historic ACWO wind pattern project imminent global cooling and in ~4 centuries a recurrent homolog of the Little Ice Age. Our study provides a theoretically-unified explanation of contemporary global warming and other climate milestones based on natural climate cycles driven by the Sun, confirms a dominant role for climate in shaping human history, invites reconsideration of climate policy, and offers a method to project future climate. [ABSTRACT FROM AUTHOR]

Following the early Eocene climatic optimum some 50 million years ago, the marine paleoclimate record indicates that a long interval of global cooling took place, punctuated by a series of short-term warming reversals and culminating with the glaciation of Antarctica at the Eocene-Oligocene transition. We investigate a saline lake record from the Qianjiang and Jinghezhen formations, Jianghan Basin, China for the corresponding continental climate response. A 20.25 Myr long astronomical time scale is constructed based on 405-kyr orbital eccentricity cycle tuning of gamma ray (GR) series measured from the Qianjiang and Jinghezhen formations. The halite-rich interval between 41-40.4 Ma correlates well with the Middle Eocene Climate Optimum recorded in the deep-sea δ 18 O record, confirming predominantly halite deposition during the warming. Silt-mudstone-rich intervals were deposited during ∼2.4 Myr orbital eccentricity minima. The evidence indicates a history of alternating fresh water (humid/cool) and saline water (dry/hot) lake cycles paced by ∼100 kyr orbital eccentricity cycles. Analysis of GR series from the deepest part of the lake indicates strong astronomical forcing of halite deposition throughout the Middle-Late Eocene, and a shift to siliciclastic deposition and cooler climates at ∼33.9 Ma. Early Oligocene wet/cool climate in the Jianghan Basin signals East Asian summer monsoon intensification, and a reduction from northwest winter monsoon in response to Tibetan Plateau uplift. Northern Hemisphere summer insolation minima in the Middle-Late Eocene with temperatures that are 4.6 °C higher than present may provide an analogy for near-future climate change in Jianghan Basin. • Halite-mudstone rhythms paced by ∼100 kyr short eccentricity cycles. • Wet/cold climate in Jianghan Basin in Early Oligocene with East Asian summer monsoon intensification. • Halite deposition warm/dry winters when Perihelion in Northern Hemisphere winter. • ∼600 kyr duration for the MECO event record in Jianghan Basin. • Silt-mudstone-rich intervals occurred during ∼2.4 Myr orbital eccentricity minima. [ABSTRACT FROM AUTHOR]

Formed in the Southern Ocean, Antarctic Intermediate Water (AAIW) plays a fundamental role in the modern climate system as an important sink for atmospheric CO 2 , and as a mid-depth conduit supplying nutrient-rich waters to subtropical and tropical upwelling regions. During the transition from warm Pliocene 'greenhouse' conditions to Pleistocene 'icehouse' conditions around 3 Ma, model simulations suggest dramatic circulation changes in the Southern Ocean AAIW formation region due to cooling and expanded sea ice cover around Antarctica. However, the history and properties of AAIW over this time period are still poorly understood. Here, we trace AAIW source water provenance, temperature, and salinity changes over the last 4 Myr using Nd isotopes of Fe-Mn-oxyhydroxide encrusted foraminifera, benthic foraminiferal Mg/Ca, and stable isotopes from southwest Atlantic DSDP Site 516. Our results show that modern AAIW properties emerged gradually over the last 3 Myr, as evidenced by gradual cooling (6 °C) and freshening of intermediate waters. Over this same interval, εNd-values decrease and diverge away from those of Pacific-sourced waters and converge on values of present-day Circumpolar Deep Water (CDW). These observations are in accordance with model simulations indicating increased deep vertical mixing at the Polar Front associated with sea ice extension. These modified AAIW source waters penetrated increasingly northward into the Atlantic at intermediate depths, enhancing nutrient supply and contributing to surface cooling along subtropical coastal upwelling sites. These circulation changes may have facilitated ocean CO 2 storage, providing a positive feedback amplifying global cooling and intensification of Northern Hemisphere Glaciation. • Multi-proxy records from Site 516 (AAIW depths) during the Plio/Pleistocene. • During 3-0 Ma foraminiferal Nd isotopes decrease and merge towards a Circumpolar Deep Water source. • During the Quaternary benthic foraminiferal Mg/Ca derived temperatures/salinities cool and freshen by 8 °C. [ABSTRACT FROM AUTHOR]

For quite some time, there has been a consensus that a widespread glaciation of theAntarctic continent took place at the Eocene-Oligocene transition. However, the exactconditions and mechanism behind this event are still poorly constrained. What’smore, large variability during the Oligocene/Miocene and indications of Eocene icechallenge the idea of a single abrupt transition within a system characterised by a stronghysteresis.Using newly reconstructed middle-to-late Eocene geography reconstructions and adequatemodel resolution, our simulations can reconcile the seemingly paradoxical existence of iceand warmth on the Antarctic continent. Prior to glaciation, Antarctica featured a monsoonalclimate with warm and wet summers altered by cool and dry winters. Inland regions sawextreme seasonal variation (∼40∘C), while coastal zones were milder and therefore suitableto sustain temperate to sub-tropical vegetation. Meanwhile, near-coastal elevated regionsallowed the formation of localised ice caps mainly thanks to high amounts of precipitation(1-4 m/year). Interestingly, the central Gamburtsev Range is a hostile environment for icegrowth in these simulations despite being suggested as an important focus point for Antarcticglaciation.The results presented here are generally in good agreement with proxies for themiddle-to-late Eocene, explaining indications of regional ice as well as abundant vegetationnear the coasts. This more regional view of the Antarctic greenhouse climate poses newchallenges on the presumed large-scale glaciation at the EOT. While allowing the formationof smaller ice caps, intense summertime melt across the continental interior poses animportant inhibitor on further ice growth. The peculiar climate of an ice-free Antarcticaproves quite resilient to external forcing, including a global cooling (well below the proposed2.5–3× pre-industrial CO2 threshold), geography changes or a shift in the oceaniccirculation (regarding the meridional overturning regime). We therefore suggest that theconventional idea of a mostly CO2-driven critical transition at the EOT needs to berevisited. A rather noise-induced transition with reduced hysteresis can explain aprolonged period of bi-stability following the EOT as well as failed prior glaciation. [ABSTRACT FROM AUTHOR]

The Antarctic ice sheet history dates back to the late Eocene-early Oligocene, a time when carbon dioxide concentrations were declining from values around 1000 ppm towards values below 600 ppm. The continental position of Antarctica was different than today and the Drake Passage and the Tasman Seaway started to open. A large δ18O excursion in deep-sea foraminifera, known as the Oi-1 event (33.9 Myr), is interpreted as a global cooling and major ice sheet formation event. At this time, it is thought that small ephemeral ice sheets grew into a continental scale ice sheet on the Antarctic continent. Up to date there is a debate on how the Antarctic ice sheet developed and what were the decisive actors. The ocean gateway opening theory with the thermal isolation and cooling of the Antarctic continent has been a valid explanation for Antarctic ice sheet formation for a long time. However, the timing is vague, the process is very slow and the simulated cooling is limited. Therefore, ocean gateway opening is thought to have played a secondary role. Carbon dioxide forcing might have been the main driver, but both concentrations and pathways over the transition period show a large uncertainty in the literature. Here we present the Antarctic ice sheet evolution over a million year timescale crossing the Eocene-Oligocene boundary using the Antarctic ice sheet model VUB-AISMPALEO coupled to the emulated climate from HadSM3. The high uncertainty in the CO2 concentrations is investigated using different carbon dioxide pathways from the late Eocene towards the early Oligocene. The role of slightly different initial conditions and the speed of decline of carbon dioxide concentrations are investigated using the emulator. [ABSTRACT FROM AUTHOR]

The article cites various research papers in its discussion of climate cooling and glaciation in Earth's Northern Hemisphere, the latter of which is estimated to have proceeded about 2.75 million years ago. Understanding of the reasons for global cooling is said to be incomplete. The history of cooling in polar regions is discussed. Estimates of prehistoric atmospheric carbon dioxide concentrations made by paleoclimate modelers are noted. The article notes the enigma that low concentrations of atmospheric carbon dioxide thought to have occurred 22 million years ago are not known to have resulted in glaciation.