The causes, consequences and relevance of hyperthermals

09:15-09:35 Introduction and aims

09:35-09:50 Early Cenozoic hyperthermals and the hydrological cycle: Theory versus observations

Professor James (Jim) C. Zachos, University of California at Santa Cruz (UCSC), USA

Abstract

The early Eocene hyperthermals, a series of transient global warming events (2 to 5°C, provide a unique opportunity to assess the sensitivity of the hydrologic cycle to the scale of greenhouse forcing expected over the next several centuries. A growing body of evidence from the most prominent of the hyperthermals, the Paleocene Thermal Maximum (PETM; ~56 Ma), points toward a major mode shift in the intensity and patterns of precipitation. Regionally, the shift in hydrology differs notably, with some regions becoming drier, others wetter.  In many regions both sedimentologic and paleontologic evidence indicate that precipitation became much more seasonal or episodic in character.  In continental fluvial and coastal sections, changes in siliciclastic depositional facies reflect on increased frequency of high-energy events (e.g., extreme flooding), possibly from monsoon-like seasonal rains, and/or from unusually intense and/or sustained extra-tropical storms.  In the open ocean, geochemical data, though still relatively sparse, suggests that the sub-tropical ocean became saltier as a consequence of locally reduced precipitation and/or increased evaporation suggestive of increased meridional vapor transport from low to high latitudes. Indeed evidence, from high latitude oceans suggests reduced salinity.  New data emerging for subsequent smaller hyperthermals show similar patterns. Such observations are consistent with and thus support general theory on the sensitivity of large-scale vapor transport and regional precipitation intensity to extreme greenhouse warming.

09:50-10:00 Discussion

10:00-10:15 Ocean anoxia during the PETM

Dr Alex Dickson, University of Oxford, UK

Abstract

Reconstructions of redox changes in the ocean during the PETM have shown that deoxygenation in its most extreme forms - anoxia and euxinia -  were largely restricted to the Arctic Ocean, Peri-Tethys Ocean, and some shallow marine embayments. This geographic distribution of anoxic conditions points towards the importance of paleogeography, basin restriction, and nutrient fluxes as key controls on the occurrence of extreme deoxygenation. These controls are extremely similar to those highlighted as critical drivers of anoxia during the Mesozoic Oceanic Anoxic Events, suggesting that the PETM should be considered in a similar vein to these older hyperthermals. Characterizing the magnitude of marine anoxia during the PETM has been attempted using redox-sensitive isotope systems such as molybdenum and uranium. While useful in the broadest sense, these isotopic systems currently lack the temporal resolution to discern rates of redox change across the event. Furthermore, by homogenizing the entire ocean into a single metric, they miss important nuances of local and regional scale redox changes that might reflect the activity of climatic feedback processes, such as weathering, ocean circulation change, or temperature change. It will be valuable for future studies to understand the magnitude, rate, and relative temporal phasing of redox changes as they are expressed in spatially diverse locations, in order to provide constraints on the heterogeneity of climatically important feedback processes operating during the PETM, and other hyperthermals.

10:15-10:25 Discussion

10:55-11:10 Temperature extremes and biotic exclusion in the PETM

Professor Paul Pearson, Cardiff University, UK

Abstract

How hot did it get during the PETM, and what were the consequences for life? This presentation suggests that bias in the proxies and gaps in the records could have led to an underestimate of peak PETM warming. The evidence which suggests that extreme environmental conditions caused a mass die-off of ocean plankton in parts of the tropics, will be updated and some of the implications of this will be considered. 

11:10-11:20 Discussion

11:20-11:35 Volcanic drivers of the early Cenozoic hyperthermals

Professor Michael Storey, Natural History Museum of Denmark

Abstract

The climate history of the early Cenozoic is distinguished by multiple short-lived warming events (hyperthermals) that followed large-scale addition of C-based greenhouse gases into the ocean-atmosphere system. Hyperthermals are recorded in carbonate sedimentary sequences by negative C-isotopic excursions, indicating a biogenic source for the gas emissions. The triggering mechanism for early Cenozoic hyperthermals, however, is poorly understood. Any explanation for their origin should be able to account for their timing, duration, recurring nature and the amount of carbon released. Hypotheses include (i) orbital modulation of methane hydrate disassociation and (ii) production of C-based gases during magma-sediment interactions in the formation of the North Atlantic Igneous Province (NAIP). 

Formation of the NAIP commenced around 62 Ma and continued throughout the Cenozoic. A shallow sheet of anomalously hot (Icelandic) asthenosphere has been inferred to have underlain much of the Greenland lithosphere during the Paleocene and early Eocene. Tectonic-magmatic (rift to drift) events on both the West and East Greenland margins are recorded by Paleocene and Early Eocene flood basalts, regional dike swarms, central intrusions and sill complexes in Paleozoic-Mesozoic rift basins that have been exposed by Tertiary uplift. Field observations in these basins testify as to how the magmatism invaded and heated organic-rich sediments, including former oil fields. Here we combine new and published geochronological data for tectonic-magmatic events recorded along the Greenland continental rifted margin to test the hypothesis that the origin of the main Cenozoic hyperthermals, including the PETM, is rooted in plate tectonic, metamorphic and volcanic processes in the North Atlantic region.

11:35-11:45 Discussion

11:45-12:00 Origin of early Cenozoic hyperthermals and their impact on ocean circulation

Dr Philip Sexton, The Open University, UK

Abstract

In the decades following the discovery of the PETM, numerous other hyperthermals have been discovered, marked by coeval excursions in the carbon and oxygen isotope compositions of benthic foraminifera and bulk sediment. These other hyperthermals were comparable in character to the PETM, but less extreme in magnitude and duration. The similarities of these other hyperthermals with the PETM were taken as being suggestive of a common mechanism(s) giving rise to them all. Yet it is not clear that they are all linked in this way. We still do not know what processes triggered hyperthermals, the source(s) of carbon released, and their wider Earth system impacts. This presentation will show evidence that the non-PETM hyperthermals were triggered by orbital pacing of the regular processes that readily redistribute carbon between reservoirs at Earth’s surface. In accord with this view of a minimal role for buried reservoirs of carbon, other data suggest that an interval of active carbon release from Earth’s interior (via large-scale volcanism) gave rise to relative quiescence in the carbon cycle and a consequent abeyance of hyperthermals. Existing and new data together suggest that a range of hyperthermals, spanning the full spectrum of size, were all marked by a transient switch in deep ocean overturning, from a dominant source in the Southern Ocean to one in the North Atlantic.

12:00-12:10 Discussion

13:10-13:25 Causes and consequences of oceanic anoxic events – A focus on ocean nutrient cycling

Dr Fanny Monteiro, University of Bristol, UK

Abstract

Oceanic anoxic events (OAEs) reflect the most dramatic changes in ocean state of the last 250 Ma. Using an (organic geochemical) data - model comparison I provide here detailed insights into the impact of temperature and ocean nutrient inventory, and associated biogeochemical responses to two of the largest OAEs of the Mesozoic, Aptian OAE 1a (~120 Ma) and Cenomanian-Turonian OAE 2 (~93.5 Ma). 

The model-data reconstructions show that the spread of anoxia that both events experienced, mainly resulted from an enhancement in ocean nutrient level (4 and 2 times for OAE 1a and OAE 2 respectively). Enhanced nutrient levels thus increased ocean production in the surface and oxygen consumption in the deep ocean, causing ~50% and at least 40% of the ocean volume to become dysoxic/anoxic during OAE 1a and OAE 2 respectively. The spread of anoxia and euxinia differ between OAEs on their regional distribution as a consequence of paleogeography. 

The model shows that during OAEs the marine nitrogen (N)-cycle operated fundamentally differently to today. While the N:P ratio is closed to Redfield ratio in the modern ocean throughout the water column, OAEs N:P ratio collapses in the deep ocean despite high rates of nitrogen fixation in the surface.

13:25-13:35 Discussion

13:35-13:50 Early Jurassic hyperthermals in the context of a long, continuous, integrated stratigraphy (the JET project)

Professor Stephen Hesselbo, Camborne School of Mines, University of Exeter, UK

Abstract

During the Early Jurassic, the planet was subject to distinctive tectonic, magmatic, and orbital forcing, and fundamental aspects of the modern biosphere were becoming established in the aftermath of the end-Permian and end-Triassic mass extinctions. The breakup of Pangaea was accompanied by biogeochemical disturbances including the largest magnitude perturbation of the carbon-cycle in the last 200 Myr, coeval with the now well-characterised hyperthermal, the Toarcian Oceanic Anoxic Event (T-OAE).  Knowledge of the Early Jurassic is, however, based on scattered and discontinuous datasets, meaning that stratigraphic correlation errors confound attempts to infer temporal trends and causal relationships, leaving us without a quantitative process-based understanding of overall Early Jurassic Earth system dynamics. The Llanbedr (Mochras Farm) borehole in west Wales, originally drilled 50 years ago, provides the basis for placing the T-OAE, and other possible Early Jurassic hyperthermals, in a long-term stratigraphic and timescale context. Here the drillcore represents 27 Myr of Early Jurassic time with sedimentation rate of approximately 5 cm/kyr. Through the Integrated Early Jurassic Timescale and Earth System project (JET), a multi-faceted, international programme of research on the functioning of the Earth system, new data from the old Mochras core will be combined with data from a new core to provide an understanding of global change and quantify the roles of tectonic, palaeoceanographic, and astronomical forcing on hyperthermal (and hypothermal) events at this key juncture in Earth history. This project is funded by the International Continental Scientific Drilling Programme (ICDP) and the UK Natural Environment Research Council (NERC).

13:50-14:00 Discussion

14:00-15:15 pCO2 and temperature during Aptian OAE 1a

Dr David Naafs , University of Bristol, UK

Abstract

The Oceanic Anoxic Events (OAEs) of the Cretaceous represent one of the largest climatic perturbations of the Phanerozoic and share characteristics with the Cenozoic hyperthermals. However, the response of Earth’s climate and ecosystems to OAEs is often much less well constrained. This talk will focus on Aptian Oceanic Anoxic Event (OAE) 1a, which took place about 120 million years ago. Using a range of organic geochemical proxies, combined with computer modeling, this presentation will quantify key-climatic parameters such as pCO2 and temperature across OAE 1a. It will be demonstrated that sustained volcanic outgassing was the primary source of carbon dioxide during OAE 1a and was the ultimate driver of the observed global warming with reconstructed temperatures during OAE 1a being higher than found anywhere during the Cenozoic.

14:15-14:25 Discussion

14:25-14:55 Tea

14:55-15:10 Temperature change and OAEs

Dr Stuart Robinson, University of Oxford, UK

Abstract

Mesozoic oceanic anoxic events (OAEs) have been mechanistically compared with the hyperthermals of the early Cenozoic, with some suggesting that they represent similar, but larger magnitude, perturbations of the Earth system. Conceptual models for explaining hyperthermals, OAEs, and other similar phenomena in Earth history, make specific predictions about the role and pattern of temperature change during such events, which can be tested through comparison with the geological record. Oceanic anoxic event 2 (OAE2) occurred approximately 94 million years ago at the Cenomanian–Turonian boundary and is often considered as the type example of an OAE, as it fulfills many of the predictions of the conceptual models. However, temperature change during OAE2 is largely constrained from Northern Hemisphere sites and, in many cases, is based on qualitative reconstructions. In order to understand the drivers of climate change during OAE2, quantitative estimates of temperature change from many different localities are required. In this presentation, the record of qualitative and quantitative temperature change during OAE2 will be reviewed, including unpublished data from the southern hemisphere. Consideration of these temperature records in the context of short-term carbon cycling, GCM modeling, and longer-term records of Cretaceous and early Cenozoic climate variability, will hopefully lead to better constraints on OAE2 and, more broadly, the understanding of carbon-cycle-driven climate change in the geological record.

15:10-15:20 Discussion

15:20-15:35 Hydrological cycle and cretaceous OAEs

Professor Richard Pancost, University of Bristol, UK

Abstract

Ancient warming events allow us to evaluate the impacts of global warming on the Earth system, including both hydrological and associated biogeochemical feedbacks. There are a diverse range of biological and geochemical signatures that can be interpreted as direct or indirect indicators of hydrological change. Further complicating interpretation is the fact that changes in precipitation and its biogeochemical consequences are often conflated in interpretation of sedimentary signatures, as well as strong evidence for changes in the episodic and/or intra-annual distribution of precipitation which has not widely been considered when comparing proxy data to GCM output. This requires interpretations that integrate proxies holistically with one another and with model simulations. When done so, proxy records and climate models indicate that the response to past global warming was profound, with evidence for global reorganisation of the hydrological cycle and profound local increases and decreases in rainfall; combined with elevated temperatures and terrestrial vegetation change, this appears to often result in warming-enhanced soil organic matter oxidation, chemical weathering and nutrient cycling. All of these responses, however, are spatially and temporally complex. Key challenges, therefore, will be to increasingly: 1) interrogate extreme events in climate simulations; 2) use earth system models to disentangle the complex and multiple controls on proxies; 3) adopt multi-proxy approaches to constrain complex phenomena; and 4) increase the spatial coverage of such records, especially in arid regions, which are currently under-represented.

15:35-15:45 Discussion

15:45-16:45 Summary Discussion

09:30-09:45 Marine anoxia and the end-Permian hyperthermal

Dr Ying Cui, Dartmouth College, USA

Abstract

Exceptionally voluminous and prolonged Siberian Traps volcanism has been widely accepted as the trigger for the end-Permian hyperthermal. The kill mechanism (the mechanism that causes death physiologically in biota) for the extinction event is, however, uncertain due to the many facets of environmental consequences from the volcanism. Recent redox-sensitive elements and their isotopes have shown that the timing and extent of anoxia are consistent with the view that anoxia is the kill mechanism. This hypothesis is tested using an Earth system model of intermediate complexity (cGenie) and explore whether the Siberian Traps volcanism and its interaction with organic-rich sediments can lead to the observed pattern of anoxia. The model was initialized with Late Permian paleogeography and 10× pre-industrial pCO2 level. It was then forced with a global carbon isotope curve for 100 kyr by adding 13C-depleted carbon (-25‰) into the oceans to reproduce the 5-6 °C warming in tropical oceans. The timing and extent of anoxia from the model were comparable to the redox proxies, reinforcing the view that marine anoxia is likely the kill mechanism for the end-Permian mass extinction.

09:45-09:55 Discussion

09:55-10:10 Masses and rates of carbon release during the end-Permian and other hyperthermal events: a role for volcanism?

Professor Andy Saunders, University of Leicester, UK

Abstract

There is a strong association between flood basalt volcanism and perturbations of the Earth’s carbon cycle (as indicated by oceanic anoxic events, mass extinctions, and hyperthermals). Release of carbon (as CO2, CO, or CH4 and related compounds), directly from the volcanism and associated intrusive magmas, and from heating of carbon-bearing country rocks, doubtless acted as important drivers, but constraining the fluxes and the total amounts released is problematic.  Known uncertainties include (1) the content of the primary carbon in the magmas and country rocks; (2) the duration and intensity of magmatism; and (3) the fraction of carbon that is released to the surface (from either the ascending magma or the heated country rocks). Taking a range of realistic values for the end-Permian event suggests that carbon outputs were, on average, between 0.1 and 10 Gt/a.  The lower values are at least 3 times more than the average rate of increase in the atmospheric carbon during the onset of a Pleistocene interglacial period.  The higher figures are similar to anthropogenic C emissions, albeit over a much longer time span (>20 ky).

10:10-10:20 Discussion

10:20-10:45 Coffee

10:45-11:00 Ocean Acidification and anoxia at the Permo-Triassic Mass Extinction: a prelude to innovation?

Professor Rachel Wood, University of Edinburgh, UK

Abstract

Ocean acidification triggered by Siberian Trap volcanism was a possible kill mechanism for the Permian Triassic Boundary (PTB) mass extinction, together with widespread anoxia. Boron isotope data Arabian Margin (Neo-Tethyan Ocean) combined with a quantitative modelling suggest that during the latest Permian, increased ocean alkalinity primed the Earth system with a low level of atmospheric CO2 and a high ocean buffering capacity. The first phase of extinction was coincident with a slow injection of carbon into the atmosphere and ocean pH remained stable. A subsequent earliest Triassic rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota. Fe–S–C systematics for the Late Permian to Early Triassic show that anoxic non-sulfidic (ferruginous), rather than euxinic, conditions were prevalent, and reveal a dynamic history of repeated expansion of ferruginous conditions from anoxia focussed on the distal slope, as well as short-lived episodes of oxia that supported diverse biota.

Environmental fluctuations in redox may reinforce rather than hinder evolutionary transitions, such that variability in near surface oceanic oxygenation can promote morphologic evolution and novelty, followed by innovation, and diversification. We develop a general model for redox controls on the distribution and structure of shallow marine benthos in dominantly anoxic worlds. Assembly of phylogenetic data from the earliest Triassic shows that prolonged and widespread anoxic intervals indeed promoted morphological novelty in soft-bodied benthos, which then provides the ancestral stock for subsequently skeletonised lineages to appear as innovations once oxic conditions became widespread and stable, so in turn promoting major evolutionary diversification. As a result, we propose that so-called ‘Recovery’ intervals after mass extinctions might be better considered as ‘Innovation’ intervals.

11:00-11:10 Discussion

11:10-11:25 The Late Permian mass extinction and ocean anoxia

Professor Richard Twitchett, Natural History Museum, UK

Abstract

The largest extinction event to have impacted animals and plants is intimately associated with evidence of global warming. In common with many such crises throughout Earth history, there is direct evidence from the rock and fossil records for elevated atmospheric CO2, rising temperatures, increased weathering and run-off, sealevel rise, expanded oceanic anoxia as well as other warming-related environmental changes. For more than 25 years, ocean anoxia has been invoked a major cause of the Late Permian mass extinction event, and it is also inferred to have been a key control on the patterns and duration of biosphere recovery. The threat of expanding anoxic ‘dead zones’ is of critical concern at the present day. While there is little doubt that dissolved oxygen concentrations exerted a key control on benthic ecosystems during the past, as they do today, the impact on pelagic ecosystems is less straightforward. Contradictory data, such as the presence of a benthic macrofauna under apparently euxinic conditions, raise questions concerning the intensity and persistence of anoxia and also the validity of certain environmental proxies. The nature of the rock record and sampling biases are frequently ignored when interpreting past climate and environmental data, leading to misleading interpretations. New high-resolution data from Late Permian shallow marine shelf seas, where fluctuating conditions can be examined at the resolution of 1kyr, and potentially less, provide a more nuanced view of the timing and origins of marine anoxia, with wider implications for understanding the rock record of other similar events in Earth history.

11:25-11:35 Discussion

11:35-11:50 Hyperthermals through the Mesozoic

Dr Jessica Whiteside, University of Southampton, UK

11:50-12:00 Discussion

13:00-13:15 Challenges of modeling early Cenozoic hyperthermal and other warm climates in state of the art coupled general circulation models

Professor Matthew Huber, Department of Earth, Atmospheric & Planetary Sciences, Purdue University, USA

Abstract

It is through careful comparison of paleoclimate proxy data and climate models that understanding of past warm climates is improved. This presentation will provide an overview of prior and current model-data agreement and disagreement highlighting key areas of progress and remaining sticking points. Distinct progress has recently been made―while until recently no good solutions to past warm climate problems were evident, now we may be left with too many solutions. The role of proxies in further refining our ability extract further insights from early Cenozoic hyperthermals and other warm climates will be emphasized.

13:15-13:25 Discussion

13:25-13:40 DeepMIP: model-data synthesis of the PETM, Late Paleocene and EECO

Professor Dan Lunt, University of Bristol, UK

Abstract

This presentation provides an overview of the DeepMIP project. Predictions of future climate, essential for safeguarding society and ecosystems, are underpinned by numerical models of the Earth system. These models are routinely tested against, and in many cases tuned towards, observations of the modern Earth system. However, the model predictions of the climate of the end of this century lie largely outside of this evaluation period, due to the projected future CO2 forcing being significantly greater than that seen in the observational record. Indeed, recent work reconstructing past CO2 has shown that the closest analogues to the 22nd century, in terms of CO2 concentration, are tens of millions of years ago, in ‘Deep-Time’.

DeepMIP is dedicated to conceiving, designing, carrying out, analysing, and disseminating, an international effort to improve our understanding of Deep Time climates.
Objectives of the DeepMIP group are:

To foster closer links between the palaeoclimate modelling and data communities.
To design experiments for the MIP, through discussion with both model and data communities.
To carry out such simulations with a wide range of state-of-the-art models.
To create and collate and synthesise datasets where appropriate to enable meaningful model-data comparisons.
To analyse the results with the aims of evaluating the models, understanding the reasons behind the model-model and model-data differences, and, where possible, providing suggestions for model improvements.
To carry out the above in such a way as to facilitate contribution to the IPCC.
DeepMIP is part of the Paleoclimate Modelling Intercomparison Project (PMIP), which itself is affiliated to CMIP6.

13:40-13:50 Discussion

13:50-14:05 Disentangling forcing and feedbacks in warm climates of the Early Eocene

Professor Rodrigo Caballero, University of Stockholm, Sweden

Abstract

Recent work in modelling the warm climates of the Early Eocene shows that it is possible to obtain a reasonable global match between model surface temperature and proxy reconstructions, but only by using extremely high atmospheric CO2 concentrations or more modest CO2 levels complemented by a reduction in global cloud albedo. Understanding the mix of radiative forcing that gave rise to Eocene warmth has important implications for constraining Earth’s climate sensitivity. I will discuss various problems that hamper progress in this direction. One is that climate sensitivity, at least in some climate models, is not fixed but rather depends on the background climate state, increasing rapidly with temperature. Another is the lack of direct proxy constraints on radiative forcing agents other than CO2. I will explore the potential for distinguishing among different radiative forcing scenarios via their impact on regional climate changes, illustrated by a particular case study. Specifically, we compare climate model simulations of two end-member scenarios: one in which the climate is warmed entirely by CO2, and another in which it is warmed entirely by reduced cloud albedo (which we refer to as the “low CO2-thin clouds” or LCTC scenario) . The two simulations have almost identical global-mean surface temperature and equator-to-pole temper-ature difference, but the LCTC scenario has ∼11% greater global-mean precipitation. The LCTC simulation also has cooler midlatitude continents and warmer oceans than the high-CO2 scenario, and a tropical climate which is significantly more El Niño-like. These differences are potentially detectable in the terrestrial proxy record.

14:05-14:15 Discussion

14:15-14:45 Tea

14:45-15:00

Dr Tatiana Ilyina, Max Planck Institute for Meteorology, Germany

15:00-15:10 Discussion

15:10-15:25 Connecting hyperthermals to the Anthropocene

Dr Gavin Schmidt, NASA GISS, USA

Abstract

The eventual geological/geochemical footprint of the Anthropocene will be abrupt, global in nature and multi-variate. It will consist of perturbation in stable isotopes of carbon, nitrogen and oxygen, sedimentological changes, faunal extinctions and expansions, elemental and mineralogical changes. In many ways it will resemble the fingerprints of hyperthermals that have been detected in the geological record, specifically at the PETM, similar Eocene events and ocean anoxic events in the Cretaceous and Jurassic. This presentation shows the similarities and differences and end with a provocative challenge.

15:25-15:35 Discussion

15:35-15:50 Future climate, the Paris Agreement and impacts on society

Dr Dann Mitchell, University of Bristol, UK

Abstract

Recently, under the Paris Agreement on Climate Change, there has been a call for research into impacts associated with a 1.5C or 2C globally-averaged surface temperature anomaly. But how do we understand future climate? Are our current methods suitable to address questions related to the Paris Agreement? This presentation will review different methods for projecting future climate change, showing the sensitivity of global and regional change to the different methods chosen. It will show how our best estimates reveal that even small changes in globally averaged temperature can lead to amplified extremes and localised impacts on society, such as human health, crop failures, hydrology and more. Climate model experiments designed specifically for the Paris Agreement to assess the human impacts associated with extreme climate, will be used. For example, analyses show that in high-population regions, e.g. Central Africa, India and Europe, an additional 10-20 days of heat events can occur on average every year. Modeling the most extreme historical heat-mortality event on record as if it occurred under future climate scenarios shows that for key European cities, stabilising climate at 1.5C would decrease temperature-related mortality by 15-25% per summer compared with stabilisation at 2C, assuming no adaptation and constant vulnerability. Given the detectable impacts of a warmer planet on society, the presentation will argue as to what level of global warming is dangerous for specific sectors.

15:50-16:00 Discussion

16:00-17:00 Summary close