Eocene Greenhouse Climate Research Articles

Eocene (50–55 Ma) greenhouse climate recorded in nonmarine rocks of San Diego, CA, USA

Nonmarine rocks in sea cliffs of southern California store a detailed record of weathering under tropical conditions millions of years ago, where today the climate is much drier and cooler. This work examines early Eocene (~ 50–55 million-year-old) deeply weathered paleosols (ancient, buried soils) exposed in marine terraces of northern San Diego County, California, and uses their geochemistry and mineralogy to reconstruct climate and weathering intensity during early Eocene greenhouse climates. These Eocene warm spikes have been modeled as prequels for ongoing anthropogenic global warming driven by a spike in atmospheric CO2. Paleocene-Eocene thermal maximum (PETM, ~ 55 Ma) kaolinitic paleosols developed in volcaniclastic conglomerates are evidence of intense weathering (CIA > 98) under warm and wet conditions (mean annual temperature [MAT] of ~ 17 °C ± 4.4 °C and mean annual precipitation [MAP] of ~ 1500 ± 299 mm). Geologically younger Early Eocene climatic optimum (EECO, 50 Ma) high shrink-swell (Vertisol) paleosols developed in coarse sandstones are also intensely weathered (CIA > 80) with MAT estimates of ~ 20 °C ± 4.4 °C but have lower estimated MAP (~ 1100 ± 299 mm), suggesting a less humid climate for the EECO greenhouse spike than for the earlier PETM greenhouse spike.

Lithogenesis of a phosphatized tephra marker horizon in the Eocene Messel maar lake

The bituminous black pelite of the Messel UNESCO world heritage site is an exceptional palaeobiological archive from the Middle Eocene greenhouse climate. The pronounced homogeneity of the annually laminated pelite through a time interval of 640 kyr complicates the relative stratigraphic classification of fossil remains and is, thus, largely dependent on particular marker horizons within the uniform sedimentary column. We analyzed the most prominent marker horizon M using petrographic microscopy, X-ray powder diffraction, and electron probe microanalyses to identify and characterize its structure and phosphate-dominated mineralogy. Based on our analytical data, we suggest that this phosphatic marker horizon resulted from the exceptional combination of external tephra enclosed in bacteria and algae-rich layers, producing a coupled phosphorus and cation diffusion during diagenesis. Mantienneite (KMg2Al2Ti(PO4)4(OH)3 • 15H2O) is documented for the first time in the Messel fossil deposit. The diagenetic succession of messelite, montgomeryite, and mantienneite precipitation reflects the internal heterogeneities in primary mineral composition of an ash layer. Kerogen maturation and hydrocarbon migration produced acidic, reducing pore fluids with high P concentration, which enhanced the mobility of Al and Ti. The mantienneite-forming reaction marks the change from a regime of reducing to oxidizing conditions. With the deposition and diagenesis of the marker horizon M, a singular event is preserved in a sedimentary sequence otherwise regarded as largely uniform over a time span of about 640 kyr.

Simulation of Arctic sea ice within the DeepMIP Eocene ensemble: Thresholds, seasonality and factors controlling sea ice development

The early Eocene greenhouse climate maintained by high atmospheric CO2 concentrations serves as a testbed for future climate changes dominated by increasing CO2 forcing. In particular, the early Eocene Arctic region is important in the context of future CO2 driven climate warming in the northern polar region and associated shrinking Arctic sea ice. Here, we present early Eocene Arctic sea ice simulations carried out by six coupled climate models within the framework of the Deep-Time Model Intercomparison Project (DeepMIP). We find differences in sea ice responses to CO2 changes across the ensemble and compare the results with available proxy-based sea ice reconstructions from the Arctic Ocean. Most of the models simulate seasonal sea ice presence at high CO2 levels (≥ 840 ppmv = 3× pre-industrial (PI) level of 280 ppmv). However, the threshold when sea ice permanently disappears from the ocean varies considerably between the models (from <840 ppmv to >1680 ppmv). Based on a one-dimensional energy balance model analysis we find that the greenhouse effect likely caused by increased atmospheric water vapor concentration plays an important role in the inter-model spread in Arctic winter surface temperature changes in response to a CO2 rise from 1× to 3× the PI level. Furthermore, differences in simulated surface salinity in the Arctic Ocean play an important role in the control of local sea ice formation. These differences result from different implementations of river run-off between the models, but also from differences in the exchange of waters between a brackish Arctic and a more saline North Atlantic Ocean that are controlled by the width of the gateway between both basins. As there is no geological evidence for Arctic sea ice in the early Eocene, its presence in most of the simulations with 3× PI CO2 level indicates either a higher CO2 level and/or an overly weak polar sensitivity in these models.

Some like it cool: Benthic foraminiferal response to Paleogene warming events

Long-term climatic trends of the Paleogene were interrupted by global perturbations of the carbon cycle, commonly associated with warming of surface and bottom waters and ecosystem disturbance. Most of these perturbations occurred in the Paleocene - Eocene greenhouse climate, but others were superimposed on the transition of greenhouse-to- icehouse conditions during the middle to late Eocene. Here we analyze deep-sea benthic foraminifera to focus on the impact of Paleogene warming events on ocean bottom water ecosystems. We compare changes in assemblage diversity (Fisher-α index) across nine warming events at 16 locations (ocean drilling sites and land sections; 25 case studies) by applying the Epps-Singleton and Fligner-Killeen non-parametric statistical tests. Generally, diversity decreased whereas the coefficient of variation increased during warming events, but not all changes were statistically significant. The declines in diversity may have been due to adaptation to stressed conditions, whereas the higher coefficients of variation may have been caused by unstable environmental conditions. The most severe events, the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma) and Eocene Thermal Maximum 2 (ETM2, ~53.7 Ma), had a higher impact on bottom water ecosystems, as shown in major drops in diversity. Regression analysis of data for ODP Sites 1262 and 1263 (SE Atlantic) shows that only up to 62% of the variability of benthic foraminiferal diversity at these sites depends on δ 13 C and δ 18 O values, indicating that changes in diversity were not exclusively determined by the magnitude of the events as defined by the magnitude of changes in carbon isotope excursion and temperature. No major changes in diversity were identified during middle Eocene warming events, and post-event diversity became significantly more stable, suggesting that the overall cooler middle Eocene conditions supported higher faunal diversity and stability of benthic foraminiferal assemblages. • Declined diversity at warming events correlated to magnitude and local factors. • Background climate conditions influenced the impact of warming on marine biota. • Cooler middle Eocene conditions promoted higher diversity of benthic foraminifera.

The middle to late Eocene greenhouse climate modelled using the CESM 1.0.5

Abstract. The early and late Eocene have both been the subject of many modelling studies, but few have focused on the middle Eocene. The latter still holds many challenges for climate modellers but is also key to understanding the events leading towards the conditions needed for Antarctic glaciation at the Eocene–Oligocene transition. Here, we present the results of CMIP5-like coupled climate simulations using the Community Earth System Model (CESM) version 1. Using a new detailed 38 Ma geography reconstruction and higher model resolution compared to most previous modelling studies and sufficiently long equilibration times, these simulations will help to further understand the middle to late Eocene climate. At realistic levels of atmospheric greenhouse gases, the model is able to show overall good agreement with proxy records and capture the important aspects of a warm greenhouse climate during the Eocene. With a quadrupling of pre-industrial concentrations of both CO2 and CH4 (i.e. 1120 ppm and ∼2700 ppb, respectively, or 4 × PIC; pre-industrial carbon), sea surface temperatures correspond well to the available late middle Eocene (42–38 Ma; ∼ Bartonian) proxies. Being generally cooler, the simulated climate under 2 × PIC forcing is a good analogue for that of the late Eocene (38–34 Ma; ∼ Priabonian). Terrestrial temperature proxies, although their geographical coverage is sparse, also indicate that the results presented here are in agreement with the available information. Our simulated middle to late Eocene climate has a reduced Equator-to-pole temperature gradient and a more symmetric meridional heat distribution compared to the pre-industrial reference. The collective effects of geography, vegetation, and ice account for a global average 5–7 ∘C difference between pre-industrial and 38 Ma Eocene boundary conditions, with important contributions from cloud and water vapour feedbacks. This helps to explain Eocene warmth in general, without the need for greenhouse gas levels much higher than indicated by proxy estimates (i.e. ∼500–1200 ppm CO2) or low-latitude regions becoming unreasonably warm. High-latitude warmth supports the idea of mostly ice-free polar regions, even at 2 × PIC, with Antarctica experiencing particularly warm summers. An overall wet climate is seen in the simulated Eocene climate, which has a strongly monsoonal character. Equilibrium climate sensitivity is reduced (0.62 ∘C W−1 m2; 3.21 ∘C warming between 38 Ma 2 × PIC and 4 × PIC) compared to that of the present-day climate (0.80 ∘C W−1 m2; 3.17 ∘C per CO2 doubling). While the actual warming is similar, we see mainly a higher radiative forcing from the second PIC doubling. A more detailed analysis of energy fluxes shows that the regional radiative balance is mainly responsible for sustaining a low meridional temperature gradient in the Eocene climate, as well as the polar amplification seen towards even warmer conditions. These model results may be useful to reconsider the drivers of Eocene warmth and the Eocene–Oligocene transition (EOT) but can also be a base for more detailed comparisons to future proxy estimates.

Divergent mammalian body size in a stable Eocene greenhouse climate

A negative correlation between body size and the latitudinal temperature gradient is well established for extant terrestrial endotherms but less so in the fossil record. Here we analyze the middle Eocene site of Geiseltal (Germany), whose record is considered to span ca. 5 Myrs of gradual global cooling, and generate one of the most extensive mammalian Paleogene body size datasets outside North America. The δ18O and δ13C isotopic analysis of bioapatite reveals signatures indicative of a humid, subtropical forest with no apparent climatic change across Geiseltal. Yet, body mass of hippomorphs and tapiromorphs diverges rapidly from a respective median body size of 39 kg and 124 kg at the base of the succession to 26 kg and 223 kg at the top. We attribute the divergent body mass evolution to a disparity in lifestyle, in which both taxa maximize their body size-related selective advantages. Our results therefore support the view that intrinsic biotic processes are an important driver of body mass outside of abrupt climate events. Moreover, the taxonomy previously used to infer the duration of the Geiseltal biota is not reproducible, which precludes chronological correlation with Eocene marine temperature curves.

Climate impact on fluvial-lake system evolution, Eocene Green River Formation, Uinta Basin, Utah, USA

AbstractIn light of a modern understanding of early Eocene greenhouse climate fluctuations and new highly seasonal fluvial system faces models, the role of climate in the evolution of one classically-cited continental, terminal lake system is re-examined. Detailed stratigraphic description and elemental abundance data from fifteen cores and seven outcrop regions of the Green River Formation were used to construct a ∼150 km cross section across the Uinta Basin, Utah, USA. Lake Uinta in the Uinta Basin is divided into five lake phases: (1) post-Paleocene Eocene Thermal Maximum, (2) peak Eocene hyperthermal, (3) waning hyperthermal, Early Eocene Climatic Optimum (EECO), (4) post-hyperthermal, and (5) post-EECO regimes, based primarily on climatically driven changes in fluvial style in combination with sedimentary indicators of lacustrine carbonate deposition, organic matter preservation, salinity, and lake depth. Basinwide siliciclastic dominated intervals were deposited by highly seasonal fluvial systems and record negative organic carbon isotope excursions associated with early Eocene abrupt, transient global warming (hyperthermal) events. Carbonate dominated or organic rich intervals record stable, less seasonal climate periods between hyperthermals, with lower siliciclastic sediment supply allowing the development of carbonate and organic matter preservation. The stratigraphic progression from alternating organic rich and lean zones to the overlying organic rich Mahogany and R8 zones represents the global transition out of the pulsed early Eocene hyperthermal climate regime to a time of sediment starvation and lake stratification, sequestering sedimentary organic carbon. This study provides a novel approach to terrestrial paleoclimate reconstruction that relies largely on unique sedimentary indicators and secondarily on isotopic proxy records within the context of a large basin-wide sedimentologic and stratigraphic data set, thus setting the stage for future detailed geochemical terrestrial paleoclimate proxy development.

Major intensification of Atlantic overturning circulation at the onset of Paleogene greenhouse warmth

During the Late Cretaceous and early Cenozoic the Earth experienced prolonged climatic cooling most likely caused by decreasing volcanic activity and atmospheric CO2 levels. However, the causes and mechanisms of subsequent major global warming culminating in the late Paleocene to Eocene greenhouse climate remain enigmatic. We present deep and intermediate water Nd-isotope records from the North and South Atlantic to decipher the control of the opening Atlantic Ocean on ocean circulation and its linkages to the evolution of global climate. The marked convergence of Nd-isotope signatures 59 million years ago indicates a major intensification of deep-water exchange between the North and South Atlantic, which coincided with the turning point of deep-water temperatures towards early Paleogene warming. We propose that this intensification of Atlantic overturning circulation in concert with increased atmospheric CO2 from continental rifting marked a climatic tipping point contributing to a more efficient distribution of heat over the planet.

Eocene greenhouse climate revealed by coupled clumped isotope-Mg/Ca thermometry.

Past greenhouse periods with elevated atmospheric CO2 were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO2, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (Δ47)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. Δ47 is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Casw throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30-36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models.

Paleoenvironment and paleoecology of a Late Paleocene high-latitude terrestrial succession, Arkose Ridge Formation at Box Canyon, southern Talkeetna Mountains, Alaska

Paleogene sedimentary rocks of the Arkose Ridge Formation (Talkeetna Mountains, Alaska) preserve a record of a fluvial–lacustrine depositional environment and its forested ecosystem in an active basin among the convergent margin tectonic processes that shaped southern Alaska. An ~800m measured succession at Box Canyon indicates braid-plain deposition with predominantly gravelly deposits low in the exposure to sandy and muddy facies associations below an overlying lava flow sequence. U–Pb geochronology on zircons from a tuff and a sandstone within the measured section, as well as an Ar/Ar date from the overlying lava constrain the age of the sedimentary succession to between ~59Ma and 48Ma. Fossil plant remains occur throughout the Arkose Ridge Formation as poorly-preserved coalified woody debris and fragmentary leaf impressions. At Box Canyon, however, a thin lacustrine depositional lens of rhythmically laminated mudrocks yielded fish fossils and a well-preserved floral assemblage including foliage and reproductive organs representing conifers, sphenopsids, monocots, and dicots. Leaf physiognomic methods to estimate paleoclimate were applied to the dicot leaf collection and indicate warm temperate paleotemperatures (~11–15±~4°C MAT) and elevated paleoprecipitation (~120cm/yr MAP) estimates as compared to modern conditions; results that are parallel with previously published estimates from the partly coeval Chickaloon Formation deposited in more distal depositional environments in the same basin. The low abundance of leaf herbivory in the Box Canyon dicot assemblage (~9% of leaves damaged) is also similar to the results from assemblages in the meander-plain depositional systems of the Chickaloon. This new suite of data informs models of the tectonostratigraphic evolution of southern Alaska and the developing understanding of terrestrial paleoecology and paleoclimate at high latitudes during the Late Paleocene–Early Eocene greenhouse climate phase.

Short-term fluctuations in vegetation and phytoplankton during the Middle Eocene greenhouse climate: a 640-kyr record from the Messel oil shale (Germany)

The Palaeogene was the most recent greenhouse period on Earth. Especially for the Late Palaeocene and Early Eocene, several superimposed short-term hyperthermal events have been described, including extremes such as the Palaeocene–Eocene Thermal Maximum. Major faunal and floral turnovers in the marine and terrestrial realms were recorded in association with these events. High-resolution palynological analysis of the early Middle Eocene maar lake sediments at Messel, near Darmstadt, Germany, provides an insight into the dynamics of a climax vegetation during the Middle Eocene greenhouse climate in a time span without significant climatic excursions. Numerical techniques like detrended correspondence analysis and wavelet analysis have been applied to recognize cyclic fluctuations and long-term trends in the vegetation through a time interval of approximately 640 kyr. Based on the numerical zoning of the pollen diagram, three phases in the development of the vegetation may be distinguished. Throughout these phases, the climax vegetation did not change substantially in qualitative composition, but a trend towards noticeably less humid conditions probably in combination with a drop of the water level in the lake may be recognized. A shift in algal population from the freshwater dinoflagellate cyst Messelodinium thielepfeifferae to a dominance of Botryococcus in the uppermost part of the core is interpreted as a response to changes in acidity and nutrient availability within the lake. Time series analyses of pollen assemblages show that variations in the Milankovitch range of eccentricity, obliquity and precession can be distinguished. In addition, fluctuations in the sub-Milankovitch range are indicated. This demonstrates that floral changes during steady depositional conditions in the Middle Eocene of Messel were controlled by orbital forcing.

A 600 k.y. record of El Niño–Southern Oscillation (ENSO): Evidence for persisting teleconnections during the Middle Eocene greenhouse climate of Central Europe

The El Nino–Southern Oscillation (ENSO) is a globally important factor in today’s climate dynamics. Annually laminated oil shales from the maar lake of Messel (Germany) provide high-resolution sedimentological and paleoenvironmental data of a time interval of ~600 k.y. during the Eocene greenhouse phase. Individual laminae consist of a light spring and summer algal layer (Tetraedron minimum layer) and a dark winter layer composed of terrigenous background sediment. Four sections were selected from the core of the Messel 2001 well in order to count varves and to measure total varve thickness and the thickness of light and dark laminae. Spectral analyses were done in order to detect possible cyclic fl uctuations in varve thickness. Fluctuations are signifi cant in the quasi-biennial (2.1–2.5 yr) and low-frequency band (2.8–3.5 yr, 4.9–5.6 yr), thus showing that algal growth as well as the background sedimentation were controlled by ENSO effects at least over a time interval of 600 k.y. This confirms the existence of a previously postulated robust Eocene ENSO. Significant peaks within a quasi-decadal (10–11 yr), interdecadal (17–26 yr), and multidecadal band (~52 yr, ~82 yr) show either the enduring influence of more or less cyclic instabilities or the influence of solar cycles.

Reconstructing the Late Miocene climate and oceanic heat flux using the AGCM ECHAM4 coupled to a mixed-layer ocean model with adjusted flux correction

In terms of climate, the Late Miocene represents a transitional phase between the Eocene greenhouse climate and the Quaternary icehouse situation; it is characterised by an extensive Antarctic ice sheet and incipient glaciation in the Northern Hemisphere. In order to better understand the Late Miocene climate, we performed a simulation study for the Tortonian using the complex atmospheric general circulation model ECHAM4 coupled to a mixed-layer ocean model. The boundary conditions such as the orography and the continental ice distribution were adapted to the Tortonian conditions; considering the coarse geographic resolution of the model, the land–sea distribution was assumed to be similar to the present situation and the atmospheric CO 2 concentration was set to 353 ppm. Because the mixed-layer ocean model uses a flux correction tuned to the modern oceanic heat transport a method was developed to estimate the Tortonian palaeo-flux correction based on the zonally averaged SST gradients. As compared to the present-day world, the Tortonian model run shows a significant reduction in the oceanic heat transport, a considerable warming and reduction of sea ice in high latitudes, a weakening of the Asian monsoon, a cooling in the mid-latitudes–caused by the reduced oceanic heat transport–and an increase in precipitation in Southern Europe. Except for the cooling of the mid-latitudes these results are largely consistent with proxy-data and other modelling studies although the proxy data indicate much warmer conditions in high latitudes than calculated in the model. The reasons for the discrepancies between model-based and proxy data reconstructions are discussed.

Heat transport, deep waters, and thermal gradients: Coupled simulation of an Eocene greenhouse climate

For the first time, a coupled general circulation model with interactive and dynamical atmospheric, oceanic, and sea‐ice components, is used to simulate an Eocene (∼50 Ma) “greenhouse” climate. We introduce efficient ocean spin‐up methods for coupled paleoclimate modeling. Sea surface temperatures (SSTs) and salinities evolve unconstrained, producing the first proxy data‐independent estimates for these Eocene climate parameters. Tropical and extratropical model‐predicted SSTs are warmer than modern values, by 3 and 5°C, respectively. Salinity‐driven deep water formation occurs in the North Atlantic and Tethys. The zonal average overturning circulation is weaker than modern. Eocene ocean heat transport is 0.6 PW less than modern in the Northern Hemisphere and 0.4 PW greater in the Southern Hemisphere. The model‐predicted near‐modern vertical and meridional Eocene temperature gradients imply that the dominant theory for maintaining low gradients—increased ocean heat transport—is incorrect or incomplete and other mechanisms should be explored.