Deep-Sea Temperature and Circulation Changes at the Paleocene-Eocene Thermal Maximum

Oxygen rise in the tropical upper ocean during the Paleocene-Eocene Thermal Maximum

The global ocean's oxygen inventory is declining in response to global warming, but the future of the low-oxygen tropics is uncertain. We report new evidence for tropical oxygenation during the Paleocene-Eocene Thermal Maximum (PETM), a warming event that serves as a geologic analog to anthropogenic warming. Foraminifera-bound nitrogen isotopes indicate that the tropical North Pacific oxygen-deficient zone contracted during the PETM. A concomitant increase in foraminifera size implies that oxygen availability rose in the shallow subsurface throughout the tropical North Pacific. These changes are consistent with ocean model simulations of warming, in which a decline in biological productivity allows tropical subsurface oxygen to rise even as global ocean oxygen declines. The tropical oxygen increase may have helped avoid a mass extinction during the PETM.

Diversification dynamics of a common deep-sea octocoral family linked to the Paleocene-Eocene thermal maximum

The deep-sea has experienced dramatic changes in physical and chemical variables in the geological past. However, little is known about how deep-sea species richness responded to such changes over time and space. Here, we studied the diversification dynamics of one of the most diverse octocorallian families inhabiting deep sea benthonic environments worldwide and sustaining highly diverse ecosystems, Primnoidae. A newly dated species-level phylogeny was constructed to infer their ancestral geographic locations and dispersal rates initially. Then, we tested whether their global and regional (the Southern Ocean) diversification dynamics were mediated by dispersal rate and abiotic factors as changes in ocean geochemistry. Finally, we tested whether primnoids showed changes in speciation and extinction at discrete time points. Our results suggested primnoids likely originated in the southwestern Pacific Ocean during the Lower Cretaceous ∼112 Ma, with further dispersal after the physical separation of continental landmasses along the late Mesozoic and Cenozoic. Only the speciation rate of the Southern Ocean primnoids showed a significant correlation to ocean chemistry. Moreover, the Paleocene-Eocene thermal maximum marked a significant increase in the diversification of primnoids at global and regional scales. Our results provide new perspectives on the macroevolutionary and biogeographic patterns of an ecologically important benthic organism typically found in deep-sea environments.

Enhanced phosphorus recycling during past oceanic anoxia amplified by low rates of apatite authigenesis

Enhanced recycling of phosphorus as ocean deoxygenation expanded under past greenhouse climates contributed to widespread organic carbon burial and drawdown of atmospheric CO2. Redox-dependent phosphorus recycling was more efficient in such ancient anoxic marine environments, compared to modern anoxic settings, for reasons that remain unclear. Here, we show that low rates of apatite authigenesis in organic-rich sediments can explain the amplified phosphorus recycling in ancient settings as reflected in highly elevated ratios of organic carbon to total phosphorus. We argue that the low rates may be partly the result of the reduced saturation state of sediment porewaters with respect to apatite linked to ocean warming and acidification and/or a decreased availability of calcium carbonate, which acts as a template for apatite formation. Future changes in temperature and ocean biogeochemistry, induced by elevated atmospheric CO2, may similarly increase phosphorus availability and accelerate ocean deoxygenation and organic carbon burial.

Cenozoic evolution of deep ocean temperature from clumped isotope thermometry

Characterizing past climate states is crucial for understanding the future consequences of ongoing greenhouse gas emissions. Here, we revisit the benchmark time series for deep ocean temperature across the past 65 million years using clumped isotope thermometry. Our temperature estimates from the deep Atlantic Ocean are overall much warmer compared with oxygen isotope-based reconstructions, highlighting the likely influence of changes in deep ocean pH and/or seawater oxygen isotope composition on classical oxygen isotope records of the Cenozoic. In addition, our data reveal previously unrecognized large swings in deep ocean temperature during early Eocene acute greenhouse warmth. Our results call for a reassessment of the Cenozoic history of ocean temperatures to achieve a more accurate understanding of the nature of climatic responses to tectonic events and variable greenhouse forcing.

Lithium isotope evidence for enhanced weathering and erosion during the Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM; ~55.9 Ma) was a geologically rapid warming period associated with carbon release, which caused a marked increase in the hydrological cycle. Here, we use lithium (Li) isotopes to assess the global change in weathering regime, a critical carbon drawdown mechanism, across the PETM. We find a negative Li isotope excursion of ~3‰ in both global seawater (marine carbonates) and in local weathering inputs (detrital shales). This is consistent with a very large delivery of clays to the oceans or a shift in the weathering regime toward higher physical erosion rates and sediment fluxes. Our seawater records are best explained by increases in global erosion rates of ~2× to 3× over 100 ka, combined with model-derived weathering increases of 50 to 60% compared to prewarming values. Such increases in weathering and erosion would have supported enhanced carbon burial, as both carbonate and organic carbon, thereby stabilizing climate.

Greenhouse- and orbital-forced climate extremes during the early Eocene

The Palaeocene-Eocene Thermal Maximum (PETM) was a significant global warming event in Earth's deep past (56 Mya). The warming across the PETM boundary was driven by a rapid rise in greenhouse gases. The event also coincided with a time of maximum insolation in Northern Hemisphere summer. There is increased evidence that the mean warming was accompanied by enhanced seasonality and/or extremes in precipitation (and flooding) and drought. A high horizontal resolution (50 km) global climate model is used to explore changes in the seasonal cycle of surface temperature, precipitation, evaporation minus precipitation and river run-off for regions where proxy data are available. Comparison for the regions indicates the model accurately simulates the observed changes in these climatic characteristics with North American interior warming and drying, and warming and increased river run-off at other regions. The addition of maximum insolation in Northern Hemisphere summer leads to a drier North America, but wetter conditions at most other locations. Long-range transport of atmospheric moisture plays a critical role in explaining regional changes in the water cycle. Such high-frequency variations in precipitation might also help explain discrepancies or misinterpretation of some climate proxies from the same locations, especially where sampling is coarse, i.e. at or greater than the frequency of precession.This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.

Evidence for ephemeral middle Eocene to early Oligocene Greenland glacial ice and pan-Arctic sea ice

Earth’s modern climate is defined by the presence of ice at both poles, but that ice is now disappearing. Therefore understanding the origin and causes of polar ice stability is more critical than ever. Here we provide novel geochemical data that constrain past dynamics of glacial ice on Greenland and Arctic sea ice. Based on accurate source determinations of individual ice-rafted Fe-oxide grains, we find evidence for episodic glaciation of distinct source regions on Greenland as far-ranging as ~68°N and ~80°N synchronous with ice-rafting from circum-Arctic sources, beginning in the middle Eocene. Glacial intervals broadly coincide with reduced CO₂, with a potential threshold for glacial ice stability near ~500 p.p.m.v. The middle Eocene represents the Cenozoic onset of a dynamic cryosphere, with ice in both hemispheres during transient glacials and substantial regional climate heterogeneity. A more stable cryosphere developed at the Eocene-Oligocene transition, and is now threatened by anthropogenic emissions.

With rapidly disappearing ice, understanding the past behavior of the cryosphere is critical. Here, the authors indicate the initiation and disappearance of glaciation on Greenland and Arctic sea ice coincided in the past, synchronous with Antarctic ice and global ice volume, and a CO₂ threshold of ~500 p.p.m.v.

Rapid and recent diversification patterns in Anseriformes birds: Inferred from molecular phylogeny and diversification analyses

The Anseriformes is a well-known and widely distributed bird order, with more than 150 species in the world. This paper aims to revise the classification, determine the phylogenetic relationships and diversification patterns in Anseriformes by exploring the Cyt b, ND2, COI genes and the complete mitochondrial genomes (mito-genomes). Molecular phylogeny and genetic distance analyses suggest that the Dendrocygna species should be considered as an independent family, Dendrocygnidae, rather than a member of Anatidae. Molecular timescale analyses suggests that the ancestral diversification occurred during the Early Eocene Climatic Optimum (58 ~ 50 Ma). Furthermore, diversification analyses showed that, after a long period of constant diversification, the median initial speciation rate was accelerated three times, and finally increased to approximately 0.3 sp/My. In the present study, both molecular phylogeny and diversification analyses results support that Anseriformes birds underwent rapid and recent diversification in their evolutionary history, especially in modern ducks, which show extreme diversification during the Plio-Pleistocene (~ 5.3 Ma). Therefore, our study support that the Plio-Pleistocene climate fluctuations are likely to have played a significant role in promoting the recent diversification for Anseriformes.

Temperature and atmospheric CO2 concentration estimates through the PETM using triple oxygen isotope analysis of mammalian bioapatite

The Paleocene-Eocene Thermal Maximum (PETM) is a remarkable climatic and environmental event that occurred 56 Ma ago and has importance for understanding possible future climate change. The Paleocene-Eocene transition is marked by a rapid temperature rise contemporaneous with a large negative carbon isotope excursion (CIE). Both the temperature and the isotopic excursion are well-documented by terrestrial and marine proxies. The CIE was the result of a massive release of carbon into the atmosphere. However, the carbon source and quantities of CO2 and CH4 greenhouse gases that contributed to global warming are poorly constrained and highly debated. Here we combine an established oxygen isotope paleothermometer with a newly developed triple oxygen isotope paleo-CO2 barometer. We attempt to quantify the source of greenhouse gases released during the Paleocene-Eocene transition by analyzing bioapatite of terrestrial mammals. Our results are consistent with previous estimates of PETM temperature change and suggest that not only CO2 but also massive release of seabed methane was the driver for CIE and PETM.

Sensitivity of the Palaeocene-Eocene Thermal Maximum climate to cloud properties

The Palaeocene-Eocene Thermal Maximum (PETM) was a significant global warming event in the Earth's history (approx. 55 Ma). The cause for this warming event has been linked to increases in greenhouse gases, specifically carbon dioxide and methane. This rapid warming took place in the presence of the existing Early Eocene warm climate. Given that projected business-as-usual levels of atmospheric carbon dioxide reach concentrations of 800-1100 ppmv by 2100, it is of interest to study past climates where atmospheric carbon dioxide was higher than present. This is especially the case given the difficulty of climate models in simulating past warm climates. This study explores the sensitivity of the simulated pre-PETM and PETM periods to change in cloud condensation nuclei (CCN) and microphysical properties of liquid water clouds. Assuming lower levels of CCN for both of these periods leads to significant warming, especially at high latitudes. The study indicates that past differences in cloud properties may be an important factor in accurately simulating past warm climates. Importantly, additional shortwave warming from such a mechanism would imply lower required atmospheric CO2 concentrations for simulated surface temperatures to be in reasonable agreement with proxy data for the Eocene.

Surviving rapid climate change in the deep sea during the Paleogene hyperthermals

Predicting the impact of ongoing anthropogenic CO2 emissions on calcifying marine organisms is complex, owing to the synergy between direct changes (acidification) and indirect changes through climate change (e.g., warming, changes in ocean circulation, and deoxygenation). Laboratory experiments, particularly on longer-lived organisms, tend to be too short to reveal the potential of organisms to acclimatize, adapt, or evolve and usually do not incorporate multiple stressors. We studied two examples of rapid carbon release in the geological record, Eocene Thermal Maximum 2 (∼53.2 Ma) and the Paleocene Eocene Thermal Maximum (PETM, ∼55.5 Ma), the best analogs over the last 65 Ma for future ocean acidification related to high atmospheric CO2 levels. We use benthic foraminifers, which suffered severe extinction during the PETM, as a model group. Using synchrotron radiation X-ray tomographic microscopy, we reconstruct the calcification response of survivor species and find, contrary to expectations, that calcification significantly increased during the PETM. In contrast, there was no significant response to the smaller Eocene Thermal Maximum 2, which was associated with a minor change in diversity only. These observations suggest that there is a response threshold for extinction and calcification response, while highlighting the utility of the geological record in helping constrain the sensitivity of biotic response to environmental change.

Greenhouse gases in the Earth system: a palaeoclimate perspective

While the trends in greenhouse gas concentrations in recent decades are clear, their significance is only revealed when viewed in the context of a longer time period. Fortunately, the air bubbles in polar ice cores provide an unusually direct method of determining the concentrations of stable gases over a period of (so far) 800,000 years. Measurements on different cores with varying characteristics, as well as an overlap of ice-core and atmospheric measurements covering the same time period, show that the ice-core record provides a faithful record of changing atmospheric composition. The mixing ratio of CO(2) is now 30 per cent higher than any value observed in the ice-core record, while methane is more than double any observed value; the rate of change also appears extraordinary compared with natural changes. Before the period when anthropogenic changes have dominated, there are very interesting natural changes in concentration, particularly across glacial/interglacial cycles, and these can be used to understand feedbacks in the Earth system. The phasing of changes in temperature and CO(2) across glacial/interglacial transitions is consistent with the idea that CO(2) acts as an important amplifier of climate changes in the natural system. Even larger changes are inferred to have occurred in periods earlier than the ice cores cover, and these events might be used to constrain assessments of the way the Earth could respond to higher than present concentrations of CO(2), and to a large release of carbon: however, more certainty about CO(2) concentrations beyond the time period covered by ice cores is needed before such constraints can be fully realized.

A geophysiologist's thoughts on geoengineering

The Earth is now recognized as a self-regulating system that includes a reactive biosphere; the system maintains a long-term steady-state climate and surface chemical composition favourable for life. We are perturbing the steady state by changing the land surface from mainly forests to farm land and by adding greenhouse gases and aerosol pollutants to the air. We appear to have exceeded the natural capacity to counter our perturbation and consequently the system is changing to a new and as yet unknown but probably adverse state. I suggest here that we regard the Earth as a physiological system and consider amelioration techniques, geoengineering, as comparable to nineteenth century medicine.

Vital effects and beyond: a modelling perspective on developing palaeoceanographical proxy relationships in foraminifera

This paper mainly reviews our recent work on the biology and geochemistry of foraminifera with respect to their use as palaeoceanographic proxies. Our approach to proxy validation and development is described, primarily from a modeler's point of view. The approach is based on complementary steps in understanding the inorganic chemistry, inorganic isotope fractionation, and biological controls that determine palaeo-tracer signals in organisms used in climate reconstructions. Integration of laboratory experiments, field and culture studies, theoretical considerations and numerical modelling holds the key to the method's success. We describe effects of life-processes in foraminifera on stable carbon, oxygen, and boron isotopes as well as Mg incorporation into foraminiferal calcite shells. Stable boron isotopes will be used to illustrate our approach. We show that a mechanism-based understanding is often required before primary climate signals can be extracted from the geologic record because the signals can be heavily overprinted by secondary, non-climate related phenomena. Moreover, for some of the proxies, fundamental knowledge on the thermodynamic, inorganic basis is still lacking. One example is stable boron isotopes, a palaeo-pH proxy, for which the boron isotope fractionation between the dissolved boron compounds in seawater was not precisely known until recently. Attempts to overcome such hurdles are described and implications of our work for palaeoceanographic reconstructions are discussed.

Environmental precursors to rapid light carbon injection at the Palaeocene/Eocene boundary

The start of the Palaeocene/Eocene thermal maximum--a period of exceptional global warming about 55 million years ago--is marked by a prominent negative carbon isotope excursion that reflects a massive input of 13C-depleted ('light') carbon to the ocean-atmosphere system. It is often assumed that this carbon injection initiated the rapid increase in global surface temperatures and environmental change that characterize the climate perturbation, but the exact sequence of events remains uncertain. Here we present chemical and biotic records of environmental change across the Palaeocene/Eocene boundary from two sediment sections in New Jersey that have high sediment accumulation rates. We show that the onsets of environmental change (as recorded by the abundant occurrence ('acme') of the dinoflagellate cyst Apectodinium) and of surface-ocean warming (as evidenced by the palaeothermometer TEX86) preceded the light carbon injection by several thousand years. The onset of the Apectodinium acme also precedes the carbon isotope excursion in sections from the southwest Pacific Ocean and the North Sea, indicating that the early onset of environmental change was not confined to the New Jersey shelf. The lag of approximately 3,000 years between the onset of warming in New Jersey shelf waters and the carbon isotope excursion is consistent with the hypothesis that bottom water warming caused the injection of 13C-depleted carbon by triggering the dissociation of submarine methane hydrates, but the cause of the early warming remains uncertain.

THE ROLE OF BIOTIC AND ABIOTIC FACTORS IN EVOLUTION OF ANT DISPERSAL IN THE MILKWORT FAMILY (POLYGALACEAE)

A phylogenetic approach was taken to investigate the evolutionary history of seed appendages in the plant family Polygalaceae (Fabales) and determine which factors might be associated with evolution of elaiosomes through comparisons to abiotic (climate) and biotic (ant species number and abundance) timelines. Molecular datasets from three plastid regions representing 160 species were used to reconstruct a phylogenetic tree of the order Fabales, focusing on Polygalaceae. Bayesian dating methods were used to estimate the age of the appearance of ant-dispersed elaiosomes in Polygalaceae, shown by likelihood optimizations to have a single origin in the family. Topology-based tests indicated a diversification rate shift associated with appearance of caruncular elaiosomes. We show that evolution of the caruncular elaiosome type currently associated with ant dispersal occurred 54.0-50.5 million year ago. This is long after an estimated increase in ant lineages in the Late Cretaceous based on molecular studies, but broadly concomitant with increasing global temperatures culminating in the Late Paleocene-Early Eocene thermal maxima. These results suggest that although most major ant clades were present when elaiosomes appeared, the environmental significance of elaiosomes may have been an important factor in success of elaiosome-bearing lineages. Ecological abundance of ants is perhaps more important than lineage numbers in determining significance of ant dispersal. Thus, our observation that elaiosomes predate increased ecological abundance of ants inferred from amber deposits could be indicative of an initial abiotic environmental function.

The Cenozoic palaeoenvironment of the Arctic Ocean

The history of the Arctic Ocean during the Cenozoic era (0-65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm 'greenhouse' world, during the late Palaeocene and early Eocene epochs, to a colder 'icehouse' world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent approximately 14 Myr, we find sedimentation rates of 1-2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (approximately 3.2 Myr ago) and East Antarctic ice (approximately 14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (approximately 45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at approximately 49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (approximately 55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change.

Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum

The Palaeocene/Eocene thermal maximum, approximately 55 million years ago, was a brief period of widespread, extreme climatic warming, that was associated with massive atmospheric greenhouse gas input. Although aspects of the resulting environmental changes are well documented at low latitudes, no data were available to quantify simultaneous changes in the Arctic region. Here we identify the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence obtained during the Arctic Coring Expedition. We show that sea surface temperatures near the North Pole increased from 18 degrees C to over 23 degrees C during this event. Such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. At the same time, sea level rose while anoxic and euxinic conditions developed in the ocean's bottom waters and photic zone, respectively. Increasing temperature and sea level match expectations based on palaeoclimate model simulations, but the absolute polar temperatures that we derive before, during and after the event are more than 10 degrees C warmer than those model-predicted. This suggests that higher-than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms--perhaps polar stratospheric clouds or hurricane-induced ocean mixing--to amplify early Palaeogene polar temperatures.