The causes, consequences and relevance of hyperthermals

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.

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.

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. 

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.

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.

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.

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).

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.

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.

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.