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The Oligocene (33.9–23.03 Ma) had warm climates with flattened meridional temperature gradients, while Antarctica retained a significant cryosphere. These may pose imperfect analogues to distant future climate states with unipolar icehouse conditions. Although local and regional climate and environmental reconstructions of Oligocene conditions are available, the community lacks synthesis of regional reconstructions. To provide a comprehensive overview of marine and terrestrial climate and environmental conditions in the Oligocene, and a reconstruction of trends through time, we review marine and terrestrial proxy records and compare these to numerical climate model simulations of the Oligocene. Results, based on the present relatively sparse data, suggest temperatures around the Equator that are similar to modern temperatures. Sea surface temperatures (SSTs) show patterns similar to land temperatures, with warm conditions at mid- and high latitudes (∼60 –90°), especially in the Southern Hemisphere (SH). Vegetation-based precipitation reconstructions of the Oligocene suggest regionally drier conditions compared to modern times around the Equator. When compared to proxy data, climate model simulations overestimate Oligocene precipitation in most areas, particularly the tropics. Temperatures around the mid- to high latitudes are generally underestimated in models compared to proxy data and tend to overestimate the warming in the tropics. In line with previous proxy-to-model comparisons, we find that models underestimate polar amplification and overestimate the Equator-to-pole temperature gradient suggested from the available proxy data. This further stresses the urgency of solving this widely recorded problem for past warm climates, such as the Oligocene. [ABSTRACT FROM AUTHOR]

Climate variability is typically amplified towards polar regions. The underlying causes, notably albedo and humidity changes, are challenging to accurately quantify with observations or models, thus hampering projections of future polar amplification. Polar amplification reconstructions from the ice-free early Eocene (∼56 –48 Ma) can exclude ice albedo effects, but the required tropical temperature records for resolving timescales shorter than multi-million years are lacking. Here, we reconstruct early Eocene tropical sea surface temperature variability by presenting an up to ∼4 kyr resolution biomarker-based temperature record from Ocean Drilling Program (ODP) Site 959, located in the tropical Atlantic Ocean. This record shows warming across multiple orbitally paced carbon cycle perturbations, coeval with high-latitude-derived deep-ocean bottom waters, showing that these events represent transient global warming events (hyperthermals). This implies that orbital forcing caused global temperature variability through carbon cycle feedbacks. Importantly, deep-ocean temperature variability was amplified by a factor of 1.7–2.3 compared to the tropical surface ocean, corroborating available long-term estimates. This implies that fast atmospheric feedback processes controlled meridional temperature gradients on multi-million year, as well as orbital, timescales during the early Eocene. Our combined records have several other implications. First, our amplification factor is somewhat larger than the same metric in fully coupled simulations of the early Eocene (1.1–1.3), suggesting that models slightly underestimate the non-ice-related – notably hydrological – feedbacks that cause polar amplification of climate change. Second, even outside the hyperthermals, we find synchronous eccentricity-forced temperature variability in the tropics and deep ocean that represent global mean sea surface temperature variability of up to 0.7 °C, which requires significant variability in atmospheric p CO 2. We hypothesize that the responsible carbon cycle feedbacks that are independent of ice, snow, and frost-related processes might play an important role in Phanerozoic orbital-scale climate variability throughout geological time, including Pleistocene glacial–interglacial climate variability. [ABSTRACT FROM AUTHOR]

During the Pliocene, atmospheric CO2 concentrations (p CO2) were similar to today's and global average temperature was ~3 °C higher. However, the relationships and phasing between variability in climate and p CO2 on orbital time scales are not well understood. Specifically, questions remain about the nature of a lag of p CO2 relative to benthic foraminiferal δ18O in the late-Pliocene Marine Isotope Stage M2 (3300 kiloannum ago, ka), which was longer than during the Pleistocene. Here, we present a multi-proxy paleoceanographic reconstruction of the late-Pliocene subantarctic zone, which is today one of the major ocean sinks of atmospheric CO2. New dinoflagellate cyst assemblage data is combined with previously published sea surface temperature reconstructions, to reveal past surface conditions, including latitudinal migrations of the subtropical front (STF) over the late-Pliocene at ODP Site 1168, offshore west Tasmania. We observe strong oceanographic variability at the STF over glacial-interglacial timescales, especially across the M2 (3320–3260 ka). By providing tight and independent age constraints from benthic foraminiferal δ18O, we find that, much more than benthic δ18O or local SST, latitudinal migrations of the STF are tightly coupled to p CO2 variations across the M2. Specifically, a northerly position of the STF during M2 deglaciation coincides with generally low p CO2. We postulate that the efficiency of the Southern Ocean carbon outgassing varied strongly with migrations of the STF, and that is in part accounted for the variability in p CO2 across M2. [ABSTRACT FROM AUTHOR]

The Oligocene (33.9–23.03 Ma) was characterised by generally warm climates, with flattened meridional temperature gradients while Antarctica retained a significant cryosphere. This makes the Oligocene an imperfect analogue to long-term future climate states with unipolar icehouse conditions. Although local and regional climate and environmental reconstructions of Oligocene conditions are available, the community lacks synthesis of regional reconstructions. In order to provide a comprehensive overview of marine and terrestrial climate and environmental conditions in the Oligocene, as well as a reconstruction of trends through time, we here review marine and terrestrial proxy records and numerical climate model simulations of the Oligocene. Results display weaker temperature gradients during the Oligocene compared to modern times, with generally warm poles and colder-than-modern temperatures around the equator. Sea surface temperatures (SSTs) show similar trends to the land temperatures, with warm temperatures around the mid and high latitudes (~60–90°) of especially the Southern Hemisphere. Vegetation-based precipitation reconstructions of the Oligocene suggest drier conditions compared to modern times, in particular around the equator. When compared to proxy-based data, climate modelling approaches overestimate Oligocene precipitation in most areas, in particular the tropics. Temperature around the mid to high latitudes is generally underestimated in models compared to proxy data and tend to overestimate the warming in the tropics. In line with previous conclusions models underestimate polar amplification and the equator-to-pole heat distribution that prevailed during the Oligocene. Despite prevalent glaciation on Antarctica, the Oligocene "icehouse" experienced warm global average temperatures while still maintaining a unipolar icehouse state. [ABSTRACT FROM AUTHOR]

Climate variability is typically amplified towards polar regions. The underlying causes, notably albedo and humidity changes, are challenging to accurately quantify with observations or models, hampering projections of future polar amplification. Polar amplification reconstructions from the ice-free early Eocene (~56-48 million years ago) can exclude ice albedo effects, but the required tropical temperature records for resolving timescales shorter than multi-million years are lacking. Here, we reconstruct early Eocene tropical sea surface temperature variability by presenting an up to ~4 kyr-resolution biomarker-based temperature record from Ocean Drilling Program Site 959, located in the tropical Atlantic Ocean. This record shows warming across multiple orbitally paced carbon cycle perturbations, coeval with high-latitude-derived deep-ocean bottom waters, showing that these events represent transient global warming events (hyperthermals). This implies that orbital forcing caused global temperature variability through carbon cycle feedbacks. Importantly, deep-ocean temperature variability was amplified by a factor 1.7-2.3 compared to the tropical surface ocean, corroborating available long-term estimates. This implies that fast atmospheric feedback processes controlled meridional temperature gradients on multi-million year, as well as orbital timescales during the early Eocene. Our combined records have several other implications. First, our amplification factor is somewhat larger than the same metric in fully-coupled simulations of the early Eocene (1.1-1.3), suggesting that models slightly underestimate the non-ice related -- notably hydrological -- feedbacks that cause polar amplification of climate change. Second, even outside the hyperthermals, we find synchronous eccentricity-forced temperature variability in the tropics and deep ocean that represent global mean sea surface temperature variability of up to 0.7 °C, and requires significant variability in atmospheric pCO2. We hypothesize that the responsible carbon cycle feedbacks that are independent of ice, snow and frost-related processes might play an important role in Phanerozoic orbital-scale climate variability throughout geological time, including Pleistocene glacial-interglacial climate variability. [ABSTRACT FROM AUTHOR]

It is with great pleasure that we introduce palsys.org (https://palsys.org/genus/ , last access: 8 December 2023), a fully open-access taxonomic, stratigraphic and image database of organic-walled dinoflagellate cysts. Palsys.org started as the in-house database of the Laboratory of Palaeobotany and Palynology (LPP) Foundation over 30 years ago. It is now owned by Utrecht University and has been expanded and transformed into a public online platform for use in research and education. Palsys.org includes the taxonomic descriptions of genera and species of organic walled dinoflagellate cysts, from the (often translated) literature, and emendations and synonymy, mainly following Williams et al. (2017) and the stratigraphic calibrations from DINOSTRAT (Bijl, 2022), and has around 25 000 images of species. Here, in this launch paper, we explain the history of the database, present its current functionalities and explain our set-up of the data quality control. We call upon the community to help us keep palsys.org up to date and complete by, for example, by sending additional information, imagery and feedback in general through the platform. Palsys.org brings dinoflagellate micropaleontology in line with the open-science principles of modern academia. [ABSTRACT FROM AUTHOR]

At present, a strong latitudinal sea surface temperature (SST) gradient of ~16 °C exists across the Southern Ocean, maintained by the Antarctic Circumpolar Current (ACC) and a set of complex frontal systems. Together with the Antarctic ice masses, this system has formed one of the most important global climate regulators. The timing of the onset of the ACC-system, its development towards modern-day strength, and the consequences for e.g., the latitudinal SST gradient around the southern Atlantic Ocean, are still uncertain. Here we present new TEX86-biomarker records, calibrated to SST, from two sites located east of Drake Passage (southern South Atlantic) to assist in better understanding two critical time intervals of prominent climate transitions during the Cenozoic: The Late Eocene–Early Oligocene (ODP Site 696) and Middle–Late Miocene (IODP Site U1536) transitions. Our results overall show rather temperate conditions (20–11 °C) during the Late Eocene to Early Oligocene interval, with a weaker latitudinal SST gradient (~8 °C) across the Atlantic sector of the Southern Ocean compared to present day (~16 °C). We ascribe the regional similarity in SSTs across the Late Eocene–Early Oligocene South Atlantic to a persistent, strong Subpolar Gyre circulation, connecting all sites, which can only exist in absence of a strong throughflow across the Drake Passage. Surprisingly, the southern South Atlantic records show comparable SSTs (~12–14 °C) during both the Earliest Oligocene Oxygen Isotope Step (EOIS, ~33.65 Ma) and the Miocene Climate Optimum (MCO, ~16.5 Ma). Apparently, maximum Oligocene Antarctic ice volume could coexist with warm ice-proximal surface ocean conditions, while at similar ocean temperatures, the Middle Miocene Antarctic ice sheet was strongly reduced. Southern South Atlantic SSTs cooled to ~5 °C at the onset of the Middle Miocene Climate Transition (MMCT, 14 Ma), making it the coldest oceanic region recorded around Antarctica and the likely main location for deep water formation. The already cold southern South Atlantic conditions at MMCT meant it experienced little cooling during the latter part of the Miocene, which contrasts the profound cooling due to northward expansion of the Southern Ocean frontal systems in the lower latitudes and other sectors of the Southern Ocean. [ABSTRACT FROM AUTHOR]

The Neogene (23.04-2.58 Ma) is characterized by progressive buildup of Antarctic and Northern Hemisphere ice volume and climate cooling. Heat/moisture delivery to Antarctica is to a large extent regulated by the strength of meridional temperature gradients. However, the evolution of the Southern Ocean frontal systems remains scarcely studied in the Neogene. Here we present the first long-term continuous sea (sub)surface temperature (SST) record of the subtropical front area in the Southern Ocean at Ocean Drilling Program (ODP) Site 1168 off western Tasmania. This site is at present located near the subtropical front (STF), as it was during the Neogene, despite a 10 degree northward tectonic drift of Tasmania during the Neogene. We analyzed glycerol dialkyl glycerol tetraethers (GDGTs, on 433 samples) and alkenones (on 163 samples) and reconstructed the paleotemperature evolution using TEX86 and Uk'37 as two independent quantitative proxies. Both proxies indicate that Site 1168 experienced a temperate ~25°C during early Miocene (23-17 Ma), reaching ~29 °C during the mid-Miocene Climatic Optimum. The stepwise ~10°C cooling (20-10°C) in the mid-to-late Miocene (12.5-5.0 Ma) is larger than observed in records from lower and higher latitudes. From the Pliocene to modern (5.3-0 Ma), STF SST first plateaus at ~15°C (3 Ma), then decreases to ~6°C (1.3 Ma), and eventually increases to the modern levels around ~16 °C (0 Ma), with a higher variability of 5 degrees compared to the Miocene. Our results imply that the latitudinal temperature gradient between the Pacific equator and STF during late Miocene cooling increased from 4°C to 14°C. Meanwhile, the SST gradient between the STF and the Antarctic margin decreased due to amplified STF cooling compared to the Antarctic Margin. This implies a narrowing SST gradient in the Neogene, with contraction of warm SSTs and northward expansion of subpolar conditions. [ABSTRACT FROM AUTHOR]

Considered as one of the most significant climate reorganisations of the Cenozoic period, the Eocene-Oligocene Transition (EOT; ca. 34.44-33.65) is characterised by global cooling and the first major glacial advance on Antarctica. While in the southern high-latitudes, the EOT cooling is primarily recorded in the marine realm, the extent and effect on terrestrial climate and vegetation is poorly documented. Here, we present a 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 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 37.97-37.52 Ma; (ii) cool-temperate Nothofagus dominated rainforest with secondary Podocarpaceae rapidly expanding and taking over regions previously occupied by the warmer taxa between 37.306-35.60 Ma; (iii) fluctuation between warm temperate - paratropical taxa and cool temperate forest from 35.50-34.49 Ma, followed by a cool phase across the EOT (34.30-33.82 Ma); (iv) post-EOT (earliest Oligocene) recovery characterised by a warm-temperate forest association from 33.55-33.06 Ma. Coincident with changes in stratification of water masses and sequestration of carbon from surface water in the Southern Ocean, our sporomorph-based temperature estimates between 37.52 Ma and 35.60 Ma (phase ii) showed 2-3 ºC 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 mean annual temperature declining by about 2 ºC 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 푝CO2. However, the earliest Oligocene climate rebound along eastern Tasmania is linked to transient recovery of atmospheric 푝CO2 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, but a series of regional and global events, including a change in stratification of water masses, sequestration of carbon from surface waters, and changes in 푝CO2 may have played vital roles. [ABSTRACT FROM AUTHOR]

The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO 2 ) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO 2 beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO 2 record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO 2 thresholds in biological and cryosphere evolution.