The Paleocene-Eocene Thermal Maximum (PETM) has been associated with the release of several thousands of petagrams of carbon (Pg C) as methane and/or carbon dioxide into the ocean-atmosphere system within~10 kyr, on the basis of the co-occurrence of a carbon isotope excursion (CIE), widespread dissolution of deep sea carbonates, and global warming. In theory, this rapid carbon release should have severely acidified the surface ocean, though no geochemical evidence has yet been presented. Using boron-based proxies for surface ocean carbonate chemistry, we present the first observational evidence for a drop in the pH of surface and thermocline seawater during the PETM.

The cause of the end-Cretaceous mass extinction is vigorously debated, owing to the occurrence of a very large bolide impact and flood basalt volcanism near the boundary. Disentangling their relative importance is complicated by uncertainty regarding kill mechanisms and the relative timing of volcanogenic outgassing, impact, and extinction. We used carbon cycle modeling and paleotemperature records to constrain the timing of volcanogenic outgassing. We found support for major outgassing beginning and ending distinctly before the impact, with only the impact coinciding with mass extinction and biologically amplified carbon cycle change. Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from postextinction volcanism.

The Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma) represents one of the largest and most abrupt greenhouse warming events in Earth history. Marine and terrestrial records document a global >2.5‰ negative carbon isotope excursion (CIE) [1][2][3] coincident with global mean surface ocean warming of >4°C 4 and geochemical and sedimentological evidence for ocean acidification 5,6 . Collectively, these lines of evidence suggest a rapid (10 3 -10 4 years) and massive (~3,000-10,000 PgC) release of 13 C-depleted carbon into the ocean-atmosphere system 7-9 . The PETM thus offers the opportunity to examine the response and recovery of the global carbon cycle and seawater carbonate chemistry to an ancient CO2 release similar in magnitude to ongoing anthropogenic fossil fuel combustion 10 . Nonetheless, some models 9,10,20 predict that a testable facet of the recovery process from massive carbon cycle perturbation involves an over-deepening of the CCD, and the location of these sites above the pre-PETM CCD means that they cannot directly test for this predicted CCD over-deepening. Direct observational evidence sites deep enough to test for a post-PETM CCD overshoot has thus far remained elusive.Here we present lithology, CaCO3 content, and carbon isotope (δ 13 C) records from recently recovered sediment cores in the North Atlantic (IODP Sites U1403, PETM 6 paleodepth ~4374m and U1409, paleodepth ~2913m 30 ) that provide important constraints on the evolution of the CCD through the PETM, including the first evidence for CCD over-deepening during the PETM recovery. To explore the broader implications of these records for PETM carbon emissions scenarios, we present new carbon release experiments using two carbon cycle models -LOSCAR 9,31 and cGENIE 32,33 and discuss uncertainties in the representation of geological carbon cycling in current models.At Site U1409, the PETM CIE occurs in an interval of variously silicified sediments (siliceous claystones, siliceous limestones and cherts) at 178.9-179.2 mcd ( Figure 1A), contrasting with the nannofossil chalk that characterizes much of the Paleogene at this site 30 . Although likely somewhat condensed, the δ 13 C carb record bears the typical 27 PETM CIE pattern of an abrupt decrease (here of ~2‰) followed by a plateau of low values and then gradual recovery (Fig. 1). Bulk δ 13 Ccarb over the PETM CIE interval sampled a heterogeneous mixture of lithology (clay, carbonate-rich burrows within that clay, and siliceous sediments), with all three lithologies revealing significantly lower δ 13 C within the CIE than pre-event values. The integrity of the bulk δ 13 Ccarb record is also supported by the close structural similarity between it and the equivalent bulk δ 13 Ccarb records from the Southern Ocean and Walvis Ridge (Fig. 1, 2A). The Site U1409 (Figure 2A), so we confidently assign the onset of carbonate sedimentation at Site U1403 to early in the PETM recovery phase.We construct age models by correlating the δ 13 Ccarb records to a compilation of bulk and fine-fraction δ 13 Cc...

No abstract

Data from International Ocean Discovery Program (IODP) Expedition 371 reveal vertical movements of 1–3 km in northern Zealandia during early Cenozoic subduction initiation in the western Pacific Ocean. Lord Howe Rise rose from deep (∼1 km) water to sea level and subsided back, with peak uplift at 50 Ma in the north and between 41 and 32 Ma in the south. The New Caledonia Trough subsided 2–3 km between 55 and 45 Ma. We suggest these elevation changes resulted from crust delamination and mantle flow that led to slab formation. We propose a “subduction resurrection” model in which (1) a subduction rupture event activated lithospheric-scale faults across a broad region during less than ∼5 m.y., and (2) tectonic forces evolved over a further 4–8 m.y. as subducted slabs grew in size and drove plate-motion change. Such a subduction rupture event may have involved nucleation and lateral propagation of slip-weakening rupture along an interconnected set of preexisting weaknesses adjacent to density anomalies.

One contribution of 17 to a theme issue 'Biodiversity and ecosystem functioning in dynamic landscapes'. Pelagic ecosystem function is integral to global biogeochemical cycling, and plays a major role in modulating atmospheric CO 2 concentrations ( pCO 2 ). Uncertainty as to the effects of human activities on marine ecosystem function hinders projection of future atmospheric pCO 2 . To this end, events in the geological past can provide informative case studies in the response of ecosystem function to environmental and ecological changes. Around the Cretaceous-Palaeogene (K-Pg) boundary, two such events occurred: Deccan large igneous province (LIP) eruptions and massive bolide impact at the Yucatan Peninsula. Both perturbed the environment, but only the impact coincided with marine mass extinction. As such, we use these events to directly contrast the response of marine biogeochemical cycling to environmental perturbation with and without changes in global species richness. We measure this biogeochemical response using records of deep-sea carbonate preservation. We find that Late Cretaceous Deccan volcanism prompted transient deep-sea carbonate dissolution of a larger magnitude and timescale than predicted by geochemical models. Even so, the effect of volcanism on carbonate preservation was slight compared with bolide impact. Empirical records and geochemical models support a pronounced increase in carbonate saturation state for more than 500 000 years following the mass extinction of pelagic carbonate producers at the K-Pg boundary. These examples highlight the importance of pelagic ecosystems in moderating climate and ocean chemistry.

The Middle Eocene Climatic Optimum (MECO) was a gradual warming event and carbon cycle perturbation that occurred between 40.5 and 40.1 Ma. A number of characteristics, including greater-than-expected deep-sea carbonate dissolution, a lack of globally coherent negative δ 13 C excursion in marine carbonates, a duration longer than the characteristic timescale of carbon cycle recovery, and the absence of a clear trigger mechanism, challenge our current understanding of the Earth system and its regulatory feedbacks. This makes the MECO one of the most enigmatic events in the Cenozoic, dubbed a middle Eocene "carbon cycle conundrum." Here we use boron isotopes in planktic foraminifera to better constrain pCO 2 changes over the event. Over the MECO itself, we find that pCO 2 rose by only 0.55-0.75 doublings, thus requiring a much more modest carbon injection than previously indicated by the alkenone δ 13 C-pCO 2 proxy. In addition, this rise in pCO 2 was focused around the peak of the 400 kyr warming trend. Before this, considerable global carbonate δ 18 O change was asynchronous with any coherent ocean pH (and hence pCO 2 ) excursion. This finding suggests that middle Eocene climate (and perhaps a nascent cryosphere) was highly sensitive to small changes in radiative forcing.Plain Language Summary Geoscientists often look to periods of global warming in the geological past to understand how the Earth responds to input of atmospheric CO 2 . However, during the Middle Eocene Climatic Optimum (or MECO) 40 million years ago, the Earth did not respond in the way one would expect, given what we know from these earlier warming events. The MECO poses a number of puzzles for geoscientists relating to what caused it and why the Earth system responded in the way it did. Before we can hope to answer these questions, however, we need to know what atmospheric CO 2 levels were in the middle Eocene and how much they changed over the MECO event. Here we use boron isotope ratios in fossil plankton shells to tell us how ocean pH (which predominantly reflects CO 2 levels) changed over the MECO. We show that relatively little change in CO 2 at this time were associated with large-scale changes in climate. This suggests that during the Eocene, when CO 2 levels were similar to those likely to be reached by the end of this century, the Earth's climate (and possibly ice sheets) was very sensitive to minor disturbances.

During the Paleocene-Eocene Thermal Maximum (PETM, ca. 56 Ma), thousands of gigatons of carbon were released into the ocean and atmosphere over several thousand years, offering the opportunity to study the response of ocean biogeochemistry to a carbon cycle perturbation of a similar magnitude to projected anthropogenic CO 2 release. PETM scenarios typically invoke accelerated chemical weathering of terrestrial silicate rocks as a significant negative feedback driving the recovery and termination of the event. However, the implications of this mechanism for the geochemical cycling of silica during the PETM have received little attention. I use "back-of-the-envelope" calculations and a simple two-box geochemical model of the marine silica cycle to demonstrate that the sequestration of thousands of gigatons of carbon by enhanced silicate weathering during the PETM would have dramatically increased the riverine supply of dissolved silica (H 4 SiO 4) to the oceans. This would have elevated seawater [H 4 SiO 4 ], encouraging both increased opal (SiO 2) production by siliceous organisms and enhanced preservation of SiO 2 in the water column and sediments. Both of these factors would have promoted a prompt (due to the relatively short oceanic residence time of silica) increase in sedimentary opal burial, thus balancing the marine silica budget. Several recently recovered deep-sea sedimentary records from the central North Atlantic demonstrate elevated SiO 2 content across the Paleocene-Eocene boundary, which I argue is the result of enhanced production and/or preservation of SiO 2 in response to elevated [H 4 SiO 4 ] in the North Atlantic, representing the ultimate fate of excess Si weathered from the continents during the PETM.