Hyperthermals: rapid and extreme global warming in our geological past

In the context of the archetypal hyperthermal (the PETM), the end-Permian event presents an extreme comparison. Unlike the relatively benign effects of the PETM on the biota (apologies to the benthic foraminifera), the end-Permian event was the largest mass extinction of animals in Earth history. Yet the two events left similar records in the carbon isotope composition of limestones, reflecting potentially quite similar carbon cycle perturbations, both likely triggered by volcanism. Why was one a mass killer and the other not? 

Despite the similar isotope record, there are differences: the end-Permian event occurred at the culmination of Paleozoic-long environmental trends reflected in the sedimentological, isotopic, and geochemical records, reflecting maxima or minima in sea level, continentality, weathering rates, hydrothermal activity, and tectonic uplift. Moreover, the seafloor at the time may have been largely free of the blanket of acid-absorbing calcium carbonate that has existed to varying extents and thicknesses since the Jurassic. Thus, the pre-existing conditions for these two disturbances differ substantially. 

While the carbon isotope records are strikingly similar in magnitude and rate of change, they may have resulted from quite different rates of carbon injection: negative carbon isotope anomalies are the consequence essentially of the product of the rate of carbon addition and its isotopic composition: the same perturbation can result from slower addition of a source depleted in the heavy isotope (methane or volatilized organic matter addition triggered by a small volcanic addition) or a faster addition of carbon from a heavier source (e.g., the volcanism itself). Thus the more acute response in the end-Permian may indicate that much of the carbon addition was magma-sourced. Needed is a second proxy, e.g., the B isotope proxy of pH as being applied to the PETM, to constrain the source and rate of C addition. However, more voluminous release of CO2 and consequent warming could have overwhelmed the ocean uptake, seafloor carbonate dissolution, and silicate weathering feedbacks that more effectively damped the PETM perturbation.

The styles of volcanism potentially differed substantially, with greater subaerial eruption and injection of toxic materials into the atmosphere during the end-Permian Siberian Traps event.

Clearly the duration of C cycle disruption, the scale of biotic impact and delay of recover, and the persistence and extent of anoxia distinguish the end-Permian from the PETM. Whether the difference was in the rate and duration of C addition or in the pre-existing conditions remains to be determined.

The Paleocene-Eocene Thermal Maximum (PETM) is the first of a series of punctuated global warming events, termed hyperthermals, that mark the hot climate conditions of the early Eocene (~56 to ~48 million years ago). The occurrence of these hyperthermals are generally explained by the rapid increase in greenhouse gas forcing (carbon dioxide and methane) as portrayed by distinct negative carbon isotope excursions in both marine and terrestrial realms. So far, approximately 20 hyperthermals were identified based on the positive covariance between oxygen and carbon isotope excursions in high-resolution deep sea benthic foraminiferal records derived in particular from the sedimentary successions of the Walvis Ridge in the southern Atlantic ocean that were drilled during Ocean Drilling Program leg 208 in 2003. Despite that the origin of the carbon released during these events is still highly debated, their regular occurrence points towards a perturbation in the global carbon cycle and associated global temperatures on dominantly eccentricity-paced (i.e. 405 and 95-125 kyr) time scales.

Mass extinctions and transient climate events commonly coincide in time with the formation of Large igneous provinces (LIPs). Classic examples include the end-Permian event which coincides with the Siberian Traps, the end-Triassic with the Central Atlantic Magmatic Event (CAMP), the Toarcian with the Karoo LIP, and the Palaeocene-Eocene Thermal Maximum (PETM) with the North Atlantic Igneous Province. The emplacement of igneous sills into sedimentary basins, and the associated contact metamorphism of the host sedimentary rocks, has emerged as a major player in the understanding of the link between LIPs and past climatic change. This presentation stresses that for these processes to have an environment impact, the gases need to be transferred to the surface and atmosphere on a very short timescale, which is borne out by dating of the sill complexes in question. We have identified a range of different pipe structures that acted as gas transport channels during the end-Permian, the Toarcian, and the PETM, and present a classification and detailed overview of the key parameters governing their formation. This talk shows that the potential for degassing of greenhouse gases, aerosols, and ozone destructive gases in sedimentary basins affected by volcanism is substantial, and can explain the triggering of both transient climatic events and mass extinctions.

The P-E boundary hyperthermal event is considered a geologic analog to our current warming.  However, the rate of carbon release associated with the onset of the P-E event has not been adequately determined by an independent empirical means from an expanded section. Modeling efforts have attempted to address the duration of the onset, but the resultant estimates are only as reliable as the available input records, the majority of which come from slow sedimentation localities in the open ocean.  Here we place an independent empirical constraint on sedimentation rates from two P-E shelf localities on the Atlantic Coastal plain (Wilson Lake-B and IODP Leg 174AX at Millville) by determining the abundance of cosmic spherules in the Vincentown Fm. and overlying Marlboro Clay, the base of which is coincident with the onset of the carbon isotope excursion (CIE) that defines the P-E boundary. Because the flux of cosmic dust to Earth’s surface is known and is relatively constant on short timescales, the accumulation of extraterrestrial material in sediments can provide surprisingly precise estimates of bulk sedimentation rates. Exploiting this known flux, we use the cosmic spherule abundance in the upper Vincentown at Wilson Lake to establish the baseline sedimentation rate as ~1.6 cm/kyr. Against this backdrop, we find a sharp ~70-fold increase in sediment accumulation rate in the lower Marlboro, concomitant with the base of the clay. We note that our rates for the Marlboro are only ~4-5-fold higher than previously published estimates based on foraminifera accumulation.  At this sedimentation rate, the duration of the 3‰ 13Cbulk onset recorded at Wilson Lake spans less than 0.35 kyr. The same exercise applied to micrometeorite counts from Millville yields a comparable result, making the CIE onset recorded there less than ~0.3 kyr. The remarkable agreement between the two sites is a testament to the robustness of the method.  Preliminary counts from open ocean sites show a decrease in sedimentation rates through the onset of the event, opposite of what is observed on the shelves but in agreement with previous estimates based on extra-terrestrial 3He.  These sedimentation rate determinations based on cosmic spherule accumulation are objective and reproducible, and the only noteworthy source of uncertainty is in the flux of extraterrestrial material to the planetary surface.  These expanded shelf sections afford a level of resolution unfamiliar in the deep ocean and demonstrate that the onset of the carbon isotope excursion was at least centennial-scale at these sites. Future modeling efforts should account for these observations.  

It is the rate of current and projected future warming that makes anthropogenic climate change particularly challenging for natural ecosystems. In attempts to use episodes of climatic change in the geologic past as analogs for the future, therefore, timescale is key. The Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma) has been suggested as the best, most recent example of rapid carbon release and warming. The PETM is characterized by a geologically abrupt negative carbon isotope excursion, global warming, and ocean acidification, all of which point to massive carbon release to the atmosphere and/or oceans. Yet the duration of the onset, here defined as the time interval between pre-PETM and minimum carbon isotope values, remains highly debated. Estimates have ranged from decadal to tens of thousands of years. The difficulty of utilizing traditional methods for age determination to constrain rapid events, combined with poor preservation of the PETM onset in many geologic sections, is responsible for the large discrepancy between estimates. Here the results from the Earth System Model cGENIE are used to look for fingerprints of carbon release rate within the atmosphere, oceans, and sediments that may be preserved in the geologic record. These experiments focus on identifying spatial patterns in the propagation timescale for carbon isotopic and temperature anomalies through the ocean. Applied to existing PETM datasets, these estimates support an onset for the PETM of between two to five thousand years.

The Palaeocene-Eocene Thermal Maximum (PETM, ~56 Ma), with its multiple lines of attendant evidence for massive greenhouse gas release and global-scale warming, is regarded as a highly plausible future analogue. However, because the onset of the PETM likely took place at a rate at least one, if not two, orders of magnitude slower than current century-scale anthropogenic warming, it is uncertain what we can learn with regard key societal concerns such as ecological responses to climate change, except perhaps to place a lower limit on potential future disruption. Instead, one might ask what the PETM might reveal regarding the sensitivity of surficial, reduced carbon stores (e.g. vegetation and soil carbon, permafrost, marine hydrates) to warming, and hence the strength of positive feedbacks between atmospheric CO2 and climate change. Indeed, almost all explanations to date for the PETM have relied either solely, or dominantly, on one or more of these carbon sources in conjunction with an initial triggering event. If carbon-climate feedbacks alone can drive 5 degrees C of global warming in the past, what does this mean for the future?

The good news is that the PETM is likely to have been an event arising predominantly as the product of massive volcanism, rather than dominated by feedbacks between climate and reservoirs of reduced organic carbon. The bad news is that cold background climates, such as today, should be characterized by rather larger initial carbon reservoirs than warm climate states, meaning that even a weak feedback could release a substantial quantity of carbon. This presentation will attempt to reconcile both bad and good news and distinguish from any fake news, and discuss what we can and cannot (yet) learn from events such as the PETM about the future warming response to fossil fuel carbon release and the role and strength of feedbacks.

The early Eocene hyperthermals such as the Paleocene-Eocene Thermal Maximum (PETM) and the Eocene Thermal Maximum 2 (ELMO) represent the best analogues for massive carbon release into Earth's surface reservoirs throughout the Cenozoic. The hyperthermals are hence relevant case studies for current and future anthropogenic carbon emissions and climate change. Two aspects of the hyperthermals are of particular interest to our future, i.e., the onset duration of the events and the nature and timing of mechanisms and feedbacks that led to the recovery from the massive carbon cycle-climate perturbations. Are the characteristic time scales and drivers of the hyperthermals applicable to the current man-made perturbation? This presentation will put the hyperthermals into context of past and future background climate and perturbations. The discussion will include the geologic evidence for constraining the duration of the PETM onset and compare the PETM onset and carbon release rate to the time scale and rate of anthropogenic emissions. Similarities and differences between the hyperthermals and the long-term legacy of our fossil-fuel emissions will be pointed out. Finally, the duration of the main- and recovery phase of PETM and ELMO in the context of orbital forcing will be discussed.

Extreme warm periods in the deep past are often formulated in terms of shifts in mean global or regional surface temperatures. Although surface temperature is an important metric for climate change, it is not the only variable that exhibits shifts between changing climate states. Changes in atmospheric state, chemistry, regional water cycles and high temporal frequency phenomena like weather are also key indicators of climate change. Many of these indicators directly affect the sustainability of life on Earth.  Using the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM), the presentation will explore two particular hyperthermal periods of Earth’s deep past: the end-Permian (~252 Ma) and the Paleocene-Thermal Maximum (~55 Ma). For the end-Permian, it will be shown how simulations of extreme states in atmospheric chemistry significantly impact Earth’s surface climate. For the PETM, high spatial resolution simulations that exhibit extreme weather conditions that potentially persisted during the warm, moist climate of this time will be presented. Finally, simulations from projected future shifts in climate extremes due to increase greenhouse warming and relate these simulations to past hyperthermal events will be discussed.