Daniel C. Kelly

Eocene hyperthermal event offers insight into greenhouse warming

What happens to the Earths climate, environment, and biota when thousands of gigatons of greenhouse gases are rapidly added to the atmosphere? Modern anthropogenic forcing of atmospheric chemistry promises to provide an experiment in such change that has not been matched since the early Paleogene, more than 50 million years ago (Ma),when catastrophic release of carbon to the atmosphere drove abrupt, transient, hyperthermal events. Research on the Paleocene-Eocene Thermal Maximum(PETM)—the best documented of these events, which occurred about 55 Ma—has advanced significantly since its discovery 15 years ago. During the PETM, carbon addition to the oceans and atmosphere was of a magnitude similar to that which is anticipated through the 21st century. This event initiated global warming, biotic extinction and migration, and fundamental changes in the carbon and hydrological cycles that transformed the early Paleogene world.

The Palaeocene–Eocene carbon isotope excursion: constraints from individual shell planktonic foraminifer records

The Palaeocene–Eocene thermal maximum (PETM) is characterized by a global negative carbon isotope excursion (CIE) and widespread dissolution of seafloor carbonate sediments. The latter feature supports the hypothesis that the PETM and CIE were caused by the rapid release of a large mass (greater than 2000 Gt C) of 12C-enriched carbon. The source of this carbon, however, remains a mystery. Possible sources include volcanically driven thermal combustion of organic-rich sediment, dissociation of seafloor methane hydrates and desiccation and oxidation of soil/sediment organics. A key constraint on the source(s) is the rate at which the carbon was released. Fast rates would be consistent with a catastrophic event, e.g. massive methane hydrate dissociation, whereas slower rates might implicate other processes. The PETM carbon flux is currently constrained by high-resolution marine and terrestrial records of the CIE. In pelagic bulk carbonate records, the onset of the CIE is often expressed as a single- or multiple-step excursion extending over 104 years. Individual planktonic shell records, in contrast, always show a single-step CIE, with either pre-excursion or excursion isotope values, but no transition values. Benthic foraminifera records, which are less complete owing to extinction and diminutive assemblages, show a delayed excursion. Here, we compile and evaluate the individual planktonic shell isotope data from several localities. We find that the most expanded records consistently show a bimodal isotope distribution pattern regardless of location, water depth or depositional facies. This suggests one of several possibilities: (i) the isotopic composition of the surface ocean/atmosphere declined in a geologic instant (<500 yr), (ii) that during the onset of the CIE, most shells of mixed-layer planktonic foraminifera were dissolved, or (iii) the abundances or shell production of these species temporarily declined, possibly due to initial pH changes.

Large-scale glaciation and deglaciation of Antarctica during the Late Eocene

Approximately 34 m.y. ago, Earths climate transitioned from a relatively warm, ice-free world to one characterized by cooler climates and a large, permanent Antarctic Ice Sheet. Understanding this major climate transition is important, but determining its causes has been complicated by uncertainties in the basic patterns of global temperature and ice volume change. Here we use an unusually well exposed coastal incised river-valley complex in the Western Desert of Egypt to show that eustatic sea level fell and then rose by ∼40 m 2 m.y. prior to establishment of a permanent Antarctic Ice Sheet. This fall in sea level is associated with a positive oxygen isotope excursion that has been interpreted to reflect global cooling, but instead records buildup of an Antarctic Ice Sheet with a volume ∼70% of the present-day East Antarctic Ice Sheet. Both the sea-level fall and subsequent rise were coincident with a transient oscillation in atmospheric CO2 concentration down to ∼750 ppm, which climate models indicate may be a threshold for Southern Hemisphere glaciation. Because many of the carbon emission scenarios for the coming century predict that atmospheric CO2 will rise above this same 750 ppm threshold, our results suggest that global climate could transition to a state like the Late Eocene, when a large permanent Antarctic Ice Sheet was not sustainable.

Oceanographic controls on the diversity and extinction of planktonic foraminifera

Understanding the links between long-term biological evolution, the ocean–atmosphere system and plate tectonics is a central goal of Earth science. Although environmental perturbations of many different kinds are known to have affected long-term biological evolution, particularly during major mass extinction events, the relative importance of physical environmental factors versus biological interactions in governing rates of extinction and origination through geological time remains unknown. Here we use macrostratigraphic data from the Atlantic Ocean basin to show that changes in global species diversity and rates of extinction among planktonic foraminifera have been linked to tectonically and climatically forced changes in ocean circulation and chemistry from the Jurassic period to the present. Transient environmental perturbations, such as those that occurred after the asteroid impact at the end of the Cretaceous period approximately 66 million years ago, and the Eocene/Oligocene greenhouse–icehouse transition approximately 34 million years ago, are superimposed on this general long-term relationship. Rates of species origination, by contrast, are not correlated with corresponding macrostratigraphic quantities, indicating that physiochemical changes in the ocean–atmosphere system affect evolution principally by driving the synchronous extinction of lineages that originated owing to more protracted and complex interactions between biological and environmental factors.

Deciphering the paleoceanographic significance of Early Oligocene Braarudosphaera chalks in the South Atlantic

Abstract The recurrence of Braarudosphaera chalks in the lower Oligocene sequences of the subtropical South Atlantic has been a long-standing conundrum, with many hypotheses having been advanced to explain the genesis of these exotic nannofossil assemblages. Here, we evaluate different paleoceanographic models within the context of stable isotope (δ18O, δ13C) data measured from bulk-sediment samples and well-preserved foraminifera. Two closely-spaced Braarudosphaera layers from a lower Oligocene (foram Subzone P21a, 29.4–28.5 Ma) section drilled in the southeastern Atlantic (DSDP Site 363) are investigated. Maximum durations for the blooms that deposited the lower and upper Braarudosphaera layers are estimated to be 1.1 and 2.2 k.y., respectively. Bulk-sediment samples enriched in braarudosphaerid carbonate exhibit pronounced δ18O increases on the order of 0.6–1.0‰ which we attribute to isotopic disequilibria driven by braarudosphaerid vital effects. The two Braarudosphaera layers straddle a single peak in benthic foraminiferal δ18O values, suggesting that these blooms may recur on glacial/interglacial timescales. This same pair of braarudosphaerid layers also occurs as a couplet bundled with prolonged (∼6.7 k.y.) thermocline cooling, evidence that these stratigraphically distinct deposits may represent a ‘split signal’ for a single paleoceanographic/paleoclimatic event. Subsumed within this episode of subsurface cooling are two short-lived, negative excursions (∼0.5‰) in the δ13C record of a thermocline-dwelling planktonic foraminifer that coincide with the braarudosphaerid layers. Thus, benthic-to-thermocline δ18O and δ13C gradients were reduced during the braarudosphaerid blooms, a hallmark signature for strengthened upwelling. Both braarudosphaerid layers are marked by transient divergences in the stable isotopic signals of two shallow-dwelling species of planktonic foraminifera. These transient δ18O offsets may reflect subtle differences in the depth ecologies of these two mixed-layer species. If so, then braarudosphaerid depositional events may represent ‘subsurface blooms’ that took place within the lower parts of the euphotic zone. Alternatively, these transient δ18O offsets may reflect periods of pronounced seasonality, with braarudosphaerid blooms occurring during spring upwelling. The recurrence of Braarudosphaera blooms on both sides of the South Atlantic is believed to reflect rhythmic changes in the vigor and configuration of gyre circulation. We speculate that the termination of Braarudosphaera blooms in the South Atlantic near the end of the Early Oligocene may be related to paleoceanographic change caused by the crossing of a critical threshold in the tectonic opening of the Drake Passage and the development of the Antarctic Circum-Polar Current.