Search

A hallmark of the rapid and massive release of carbon during the Palaeocene–Eocene Thermal Maximum is the global negative carbon isotope excursion. The delayed recovery of the carbon isotope excursion, however, indicates that CO2 inputs continued well after the initial rapid onset, although there is no consensus about the source of this secondary carbon. Here we suggest this secondary input might have derived partly from the oxidation of remobilized sedimentary fossil carbon. We measured the biomarker indicators of thermal maturation in shelf records from the US Mid-Atlantic coast, constructed biomarker mixing models to constrain the amount of fossil carbon in US Mid-Atlantic and Tanzania coastal records, estimated the fossil carbon accumulation rate in coastal sediments and determined the range of global CO2 release from fossil carbon reservoirs. This work provides evidence for an order of magnitude increase in fossil carbon delivery to the oceans that began ~10–20 kyr after the event onset and demonstrates that the oxidation of remobilized fossil carbon released between 102 and 104 PgC as CO2 during the body of the Palaeocene–Eocene Thermal Maximum. The estimated mass is sufficient to have sustained the elevated atmospheric CO2 levels required by the prolonged global carbon isotope excursion. Even after considering uncertainties in the sedimentation rates, these results indicate that the enhanced erosion, mobilization and oxidation of ancient sedimentary carbon contributed to the delayed recovery of the climate system for many thousands of years. Delivery of fossil carbon to the oceans strongly increased about 15 kyr after the onset of the Palaeocene–Eocene Thermal Maximum as a result of oxidation of sedimentary carbon, suggests an analysis of geochemical measurements with a biomarker mixing model.

The “greenhouse” climates of the Paleocene and Eocene have formed the focus for many proxy and modelling studies in recent decades, as they are the closest geological analogues for our future warme...

Eocene Thermal Maximum 2 (ETM-2; 54.1 Ma) was the second largest Eocene hyperthermal. Like the Paleocene-Eocene Thermal Maximum (PETM), ETM-2 was characterized by massive carbon emissions and several degrees of global warming and thus can serve as a case study for assessing the impacts of rapid CO2 emissions on ocean carbonate chemistry, biota, and climate. Marine carbonate records of ETM-2 are better preserved than those of the PETM due to more subdued carbonate dissolution. As yet, however, the magnitude of this carbon cycle perturbation has not been well constrained. Here, we present the first records of surface ocean acidification for ETM-2, based on stable boron isotope records in mixed-layer planktic foraminifera from two midlatitude ODP sites (1210 in the North Pacific and 1265 in the SE Atlantic), which indicate conservative minimum global sea surface acidification of −0.20 +0.12/−0.13 pH units. Using these estimates of pH and temperature as constraints on carbon cycle model simulations, we conclude that the total mass of C, released over a period of 15 to 25 kyr during ETM-2, likely ranged from 2,600 to 3,800 Gt C, which is greater than previously estimated on the basis of other observations (i.e., stable carbon isotopes and carbonate compensation depth) alone.

Microcrystalline calcite (micrite) dominates the sedimentary record of the aftermath of the Cretaceous–Paleogene (K–Pg) impact at 31 sites globally, with records ranging from the deep ocean to the Chicxulub impact crater, over intervals ranging from a few centimeters to more than seventeen meters. This micrite-rich layer provides important information about the chemistry and biology of the oceans after the impact. Detailed high-resolution scanning electron microscopy demonstrates that the layer contains abundant calcite crystals in the micron size range with a variety of forms. Crystals are often constructed of delicate, oriented agglomerates of sub-micrometer mesocrystals indicative of rapid precipitation. We compare the form of crystals with natural and experimental calcite to shed light on their origin. Close to the crater, a significant part of the micrite may derive from the initial backreaction of CaO vaporized during impact. In more distal sites, simple interlocking rhombohedral crystals resemble calcite precipitated from solution. Globally, we found unique calcite crystals associated with fossilized extracellular materials that strikingly resemble calcite precipitated by various types of bacteria in natural and laboratory settings. The micrite-rich layer contains abundant bacterial and eukaryotic algal biomarkers and most likely represents global microbial blooms initiated within millennia of the K–Pg mass extinction. Cyanobacteria and non-haptophyte microalgae likely proliferated as dominant primary producers in cold immediate post-impact environments. As surface-water saturation state rose over the following millennia due to the loss of eukaryotic carbonate producers and continuing river input of alkalinity, “whitings” induced by cyanobacteria replaced calcareous nannoplankton as major carbonate producers. We postulate that the blooms grew in supersaturated surface waters as evidenced by crystals that resemble calcite precipitates from solution. The microbial biomass may have served as a food source enabling survival of a portion of the marine biota, ultimately including life on the deep seafloor. Although the dominance of cyanobacterial and algal photosynthesis would have weakened the biological pump, it still would have removed sufficient nutrients from surface waters thus conditioning the ocean for the recovery of biota at higher trophic levels.

The ratio of boron to calcium (B/Ca) in marine biogenic carbonates has been proposed as a proxy for properties of seawater carbonate chemistry. Applying this proxy to planktic foraminifera residing in the surface seawater largely in equilibrium with the atmosphere may provide a valuable handle on past atmospheric CO 2 concentrations. However, precise controls on B/Ca in planktic foraminifera remain enigmatic because it has been shown to depend on multiple physicochemical seawater properties. To help establish a firm inorganic basis for interpreting the B/Ca records, we examined the effect of a suite of chemical parameters ([Ca 2+ ], pH, [DIC], salinity and [PO 4 3− ]) on B/Ca in inorganic calcite precipitated in artificial seawater. These parameters were primarily varied individually while keeping all others constant, but we also tested the influence of pH and [DIC] at a constant calcite precipitation rate ( R ) by concurrent [Ca 2+ ] adjustments. In the simple [Ca 2+ ], pH and [DIC] experiments, both R and B/Ca increased with these parameters. In the pH–[Ca 2+ ] and [DIC]–[Ca 2+ ] experiments at constant R , on the other hand, B/Ca was invariant at different pH and decreased with [DIC], respectively. These patterns agree with the behavior of solution [B Total /DIC] ratio such that, at a fixed [B Total ], it is independent of pH but decreases with [DIC]. Based on these results, R and [B Total /DIC] ratio appear to be the primary controls on B/Ca in inorganic calcite, suggesting that both B(OH) 4 − and B(OH) 3 are possibly involved in B incorporation. Moreover, B/Ca modestly increased with salinity and [PO 4 3− ]. Inorganic calcite precipitated at higher R and in the presence of oxyanions such as SO 4 2− and PO 4 3− in growth solutions often undergoes surface roughening due to formation of crystallographic defects, vacancies and, occasionally, amorphous/hydrous CaCO 3 . These non-lattice sites may provide additional space for B, particularly B(OH) 3 . Consequently, besides the macroscopic influence of R and bulk solution chemistry, molecular-scale processes associated with calcite nucleation can be an important consideration for B incorporation, especially in complex ionic solutions. Lastly, the covariance of B/Ca with [DIC] and salinity observed here qualitatively agrees with those in planktic foraminifers. It follows that their impact on foraminiferal B/Ca is partly inorganically driven, which may explain why the effect is evident across different species.

© 2016 Macmillan Publishers Limited. All rights reserved. Carbon release rates from anthropogenic sources reached a record high of ∼10 Pg C yr-1in 2014. Geologic analogues from past transient climate changes could provide invaluable constraints on the response of the climate system to such perturbations, but only if the associated carbon release rates can be reliably reconstructed. The Palaeocene-Eocene Thermal Maximum (PETM) is known at present to have the highest carbon release rates of the past 66 million years, but robust estimates of the initial rate and onset duration are hindered by uncertainties in age models. Here we introduce a new method to extract rates of change from a sedimentary record based on the relative timing of climate and carbon cycle changes, without the need for an age model. We apply this method to stable carbon and oxygen isotope records from the New Jersey shelf using time-series analysis and carbon cycle-climate modelling. We calculate that the initial carbon release during the onset of the PETM occurred over at least 4,000 years. This constrains the maximum sustained PETM carbon release rate to less than 1.1 Pg C yr-1. We conclude that, given currently available records, the present anthropogenic carbon release rate is unprecedented during the past 66 million years. We suggest that such a 'no-analogue' state represents a fundamental challenge in constraining future climate projections. Also, future ecosystem disruptions are likely to exceed the relatively limited extinctions observed at the PETM.

Geologically abrupt carbon perturbations such as the Palaeocene–Eocene Thermal Maximum (PETM, approx. 56 Ma) are the closest geological points of comparison to current anthropogenic carbon emissions. Associated with the rapid carbon release during this event are profound environmental changes in the oceans including warming, deoxygenation and acidification. To evaluate the global extent of surface ocean acidification during the PETM, we present a compilation of new and published surface ocean carbonate chemistry and pH reconstructions from various palaeoceanographic settings. We use boron to calcium ratios (B/Ca) and boron isotopes (δ 11 B) in surface- and thermocline-dwelling planktonic foraminifera to reconstruct ocean carbonate chemistry and pH. Our records exhibit a B/Ca reduction of 30–40% and a δ 11 B decline of 1.0–1.2‰ coeval with the carbon isotope excursion. The tight coupling between boron proxies and carbon isotope records is consistent with the interpretation that oceanic absorption of the carbon released at the onset of the PETM resulted in widespread surface ocean acidification. The remarkable similarity among records from different ocean regions suggests that the degree of ocean carbonate change was globally near uniform. We attribute the global extent of surface ocean acidification to elevated atmospheric carbon dioxide levels during the main phase of the PETM. This article is part of a discussion meeting issue ‘Hyperthermals: rapid and extreme global warming in our geological past’.

The boron to calcium ratio (B/Ca) in biogenic CaCO3 is being increasingly utilized as a proxy for past ocean carbonate chemistry. However, B/Ca of cultured and core-top foraminifers show dependence on multiple physicochemical seawater properties and only a few of those have been inorganically tested for their impacts. Accordingly, our understanding of the controls on foraminiferal B/Ca and thus how to interpret B/Ca in fossil shells is incomplete. To gain a clearer understanding of the B incorporation mechanism, we performed inorganic calcite precipitation experiments using a pH-stat system. As previously reported, we confirm that B/Ca in calcite increases with both fluid pH and total B concentration (denoted as [BT], where [BT] = [B(OH)3] + [B(OH)4−]). We provide the first evidence that B/Ca also increases with the concentration of total dissolved inorganic carbon (DIC) and calcium ion. With the exception of the [BT] experiments, these chemical manipulations were accompanied by an increase in calcite saturation, and accordingly precipitation rate (denoted as R). But when pH and [Ca2+] were jointly varied at a fixed saturation level to maintain relatively constant R at different pH and [Ca2+] combinations, B/Ca was insensitive to both pH and [Ca2+] changes. These experimental results unequivocally suggest kinetic effects related to R on B/Ca. Furthermore, with a suite of chemical manipulations we found that the B/Ca variability is explicable by just R and the [BT]/[DIC] ratio in the parent fluids. This observation was particularly robust for relatively rapidly precipitated samples, whereas for relatively slowly precipitated samples, it was somewhat ambiguous whether the [BT]/[DIC] or [B(OH)4−]/[HCO3−] ratio provides a better fit to the experimental data. Nonetheless, our experimental results can be considered as indirect evidence for incorporation of both B(OH)4− and B(OH)3 into calcite. We propose a simple mathematical expression to describe the mode of B incorporation into synthetic calcite that depends only on the fluid [BT]/[DIC] ratio and the precipitation rate R. This novel finding has important implications for future calibrations and applications of the B/Ca proxy as well as the δ11B paleo-pH proxy.

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. Planktic foraminifers from a drill site in the North Pacific (Ocean Drilling Program Site 1209) show a ~0.8‰ decrease in boron isotopic composition (δ11B) at the onset of the event, along with a 30–40% reduction in shell B/Ca. Similar trends in δ11B are present in two lower-resolution records from the South Atlantic and Equatorial Pacific. These observations are consistent with significant, global acidification of the surface ocean lasting at least 70 kyr and requiring sustained carbon release. The anomalies in the B records are consistent with an initial surface pH drop of ~0.3 units, at the upper range of model-based estimates of acidification.

A perturbation of the carbon cycle and biosphere, linked to globally increased temperatures about 55.9 million years ago, characterized the Palaeocene–Eocene Thermal Maximum. Its effect on global oceanic productivity is controversial. Here we present records of marine barite accumulation rates that show distinct peaks during this time interval, suggesting a general increase in export productivity. We propose that changes in marine ecosystems, resulting from high atmospheric partial pressure of CO2 and ocean acidification, led to enhanced carbon export from the photic zone to depth, thereby increasing the efficiency of the biological pump. Higher seawater temperatures at that time increased bacterial activity and organic matter regeneration. Through this process much of the sinking particulate organic matter was probably converted to dissolved inorganic and organic carbon. We estimate that an annual carbon export flux out of the euphotic zone and into the deep ocean waters could have amounted to about 15 Gt during the Palaeocene–Eocene Thermal Maximum. About 0.4% of this carbon is expected to have entered the refractory dissolved organic pool, where it could be sequestered from the atmosphere for tens of thousands of years. Our estimates are consistent with the amount of carbon redistribution expected for the recovery from the Palaeocene–Eocene Thermal Maximum. Globally increased temperatures and a perturbation of the carbon cycle and biosphere characterized the Palaeocene–Eocene Thermal Maximum about 55.9 million years ago, but its effect on ocean productivity is controversial. Records of marine barite accumulation rates suggest that carbon sequestration during the event could have been enhanced by an efficient biological pump.

A coupled set of models is used to explore the possibility of long-term internal cycles in the CO 2 –climate–weathering–Antarctic Ice Sheet system. Cycles of this type were found in an earlier study with 0-D box models, and proposed to explain the quasi-periodic oscillations in benthic deep-sea-core records during the Eocene–Oligocene Transition ~ 34 Ma. Here the system is extended using a 3-D Global Climate Model, a 3-D Antarctic ice-sheet model, and two previously published spatially distributed parameterizations of CO 2 consumption by silicate weathering. In 6-million year long simulations across an idealized Eocene–Oligocene Transition, no internal cycles are found, and the coupled system just relaxes from the initial state to the final state, with at most one overdamped half-cycle. The absence of cycles is presumably due to features in this 3-D model system that are absent in the 0-D models: powerful Height Mass-Balance Feedback producing strong ice-sheet expansion after initial growth, and hysteresis in ice-sheet response to climate that damps retreat due to moderate warming. With one of the weathering parameterizations, the models indicate a region of negative slope in the relation between CO 2 level and global weathering consumption, occurring in the range ~ 0.2 to 1.5x PAL (preindustrial atmospheric level). This contrasts with the monotonically increasing relation usually assumed. If confirmed, it would have serious consequences for the well-known CO 2 -weathering thermostat mechanism, at least for CO 2 levels below ~ 1.5x PAL.

The Palaeocene–Eocene Thermal Maximum was associated with a massive release of carbon. Marine sediments suggest a temporary deepening of the calcite compensation depth, indicating extensive silicate weatheringin the aftermath of the event. The Palaeocene–Eocene Thermal Maximum was associated with a massive release of carbon. Marine sediments suggest a temporary deepening of the calcite compensation depth, indicating extensive silicate weatheringin the aftermath of the event. During the Palaeocene–Eocene Thermal Maximum (PETM) about 56 million years ago, thousands of petagrams of carbon were released into the atmosphere and ocean in just a few thousand years, followed by gradual sequestration over approximately 200,000 years. If silicate weathering is one of the key negative feedbacks that removed this carbon, a period of seawater calcium carbonate saturation greater than pre-event levels would be expected during the event's recovery phase. In marine sediments, this should be recorded as a temporary deepening of the depth below which no calcite is preserved — the calcite compensation depth (CCD). Previous and new sedimentary records from sites that were above the pre-PETM CCD show enhanced carbonate accumulation following the PETM. A new record from an abyssal site in the North Atlantic that lay below the pre-PETM CCD shows a period of carbonate preservation beginning about 70,000 years after the onset of the PETM, providing the first direct evidence for an over-deepening of the CCD. This record confirms an overshoot in ocean carbonate saturation during the PETM recovery. Simulations with two earth system models support scenarios for the PETM that involve a large initial carbon release followed by prolonged low-level emissions, consistent with the timing of CCD deepening in our record. Our findings indicate that sequestration of these carbon emissions was most likely the result of both globally enhanced calcite burial above the CCD and, at least in the North Atlantic, an over-deepening of the CCD.

We present modern B/Ca core-top calibrations for the epifaunal benthic foraminifer Nuttallides umbonifera and the infaunal Oridorsalis umbonatu s to test whether B/Ca values in these species can be used for the reconstruction of paleo-Δ[CO 3 2− ]. O. umbonatus originated in the Late Cretaceous and remains extant , whereas N. umbonifera originated in the Eocene and is the closest extant relative to Nuttallides truempyi , which ranges from the Late Cretaceous through the Eocene. We measured B/Ca in both species in 35 Holocene sediment samples from the Atlantic, Pacific and Southern Oceans. B/Ca values in epifaunal N. umbonifera (~ 85–175 μmol/mol) are consistently lower than values reported for epifaunal Cibicidoides (Cibicides) wuellerstorfi (130–250 μmol/mol), though the sensitivity of Δ[CO 3 2− ] on B/Ca in N. umbonifera (1.23 ± 0.15) is similar to that in C. wuellerstorfi (1.14 ± 0.048). In addition, we show that B/Ca values of paired N. umbonifera and its extinct ancestor, N. truempyi, from Eocene cores are indistinguishable within error. In contrast, both the B/Ca (35–85 μmol/mol) and sensitivity to Δ[CO 3 2− ] (0.29 ± 0.20) of core-top O. umbonatus are considerably lower (as in other infaunal species), and this offset extends into the Paleocene. Thus the B/Ca of N. umbonifera and its ancestor can be used to reconstruct bottom water Δ[CO 3 2− ], whereas O. umbonatus B/Ca appears to be buffered by porewater [CO 3 2− ] and suited for constraining long-term drift in seawater B/Ca.

In conjunction with increasing benthic foraminiferal δ 18 O values at the Eocene–Oligocene transition (EOT; ca. 34 Ma), coarse-grained ice-rafted debris (IRD; >425 μm) appears abruptly alongside fossil fish teeth with continentally derived neodymium (Nd) isotope ratios (e Nd ) in Kerguelen Plateau (Southern Ocean) sediments. Increased Antarctic weathering flux, as inferred from two steps to less radiogenic e Nd values, coincides with two steps in benthic foraminiferal δ 18 O values. These results indicate that two distinct surges of weathering were generated by East Antarctic ice growth during the EOT. Weathering by ice sheets during a precursor glaciation at 33.9 Ma did not produce significant IRD accumulation during the first e Nd shift. Glacial weathering was sustained during a terrace interval between the two steps, probably by small high-elevation ice sheets. A large increase in weathering signals the rapid coalescence of small ice sheets into an ice sheet of continental proportions ca. 33.7 Ma. Rapid ice sheet expansion resulted in a suppression of weathering due to less exposed area and colder conditions. Parallel changes in Antarctic weathering flux and deep-sea carbonate accumulation suggest that ice-sheet expansion during the EOT had a direct impact on the global carbon cycle; possible mechanisms include associated changes in silicate weathering on the East Antarctic craton and enhanced fertilization of Southern Ocean waters, both of which warrant further investigation.

The Palaeocene–Eocene Thermal Maximum warm event about 56 million years ago was caused by the release of large amounts of carbon to the ocean and atmosphere. Estimates of the rate of recovery from the event suggest that about 2,000 Pg of the carbon released was sequestered as organic carbon. The Palaeocene–Eocene Thermal Maximum (PETM), an approximately 170,000-year-long period of global warming about 56 million years ago, has been attributed to the release of thousands of petagrams of reduced carbon into the ocean, atmosphere and biosphere1,2. However, the fate of this excess carbon at the end of the event is poorly constrained: drawdown of atmospheric carbon dioxide has been attributed to an increase in the weathering of silicates or to increased rates of organic carbon burial1,3,4,5. Here we develop constraints on the rate of carbon drawdown based on rates of carbon isotope change in well-dated marine and terrestrial sediments spanning the event. We find that the rate of recovery is an order of magnitude more rapid than that expected for carbon drawdown by silicate weathering alone. Unless existing estimates of carbon stocks and cycling during this time are widely inaccurate, our results imply that more than 2,000 Pg of carbon were sequestered as organic carbon over 30,000–40,000 years at the end of the PETM. We suggest that the accelerated sequestration of organic carbon could reflect the regrowth of carbon stocks in the biosphere or shallow lithosphere that were released at the onset of the event.

The upper Paleocene and lower Eocene are marked by several prominent (> 1‰) carbon isotope (δ13C) excursions (CIE) that coincide with transient global warmings, or thermal maxima, including the Paleocene–Eocene Thermal Maximum (PETM). The CIE, which are recorded mainly in marine sedimentary sequences, have also been identified in continental sequences, occurred episodically, and yet appear to be paced or triggered by orbital forcing. To constrain the timing and scale of the CIE relative to long-term baseline variability, we have constructed a 4.52 million year (myr) long, high-resolution (~ 3 kyr) bulk sediment carbon isotope record spanning the lower Eocene to upper Paleocene (C25r–C24n) from a pelagic sediment section recovered at ODP Site 1262 in the southeast Atlantic. This section, which was orbitally-tuned utilizing high-resolution core log physical property and geochemical records, is the most stratigraphically complete upper Paleocene to lower Eocene sequence recovered to date. Time-series analysis of the carbon isotope record along with a high-resolution Fe intensity record obtained by XRF core scanner reveal cyclicity with variance concentrated primarily in the precession (21 kyr) and eccentricity bands (100 and 400-kyr) throughout the upper Paleocene–lower Eocene. In general, minima in δ13C correspond with peaks in Fe (i.e., carbonate dissolution), both of which appear to be in phase with maxima in eccentricity. This covariance is consistent with excess oceanic uptake of isotopically depleted carbon resulting in lower carbonate saturation during periods of high eccentricity. This relationship includes all late Paleocene and early Eocene CIE confirming pacing by orbital forcing. The lone exception is the PETM, which appears to be out of phase with the 400-kyr cycle, though possibly in phase with the 100-kyr cycle, reinforcing the notion that a mechanism other than orbital forcing and/or an additional source of carbon is required to account for the occurrence and unusual scale of this event.

Rapid carbon input into the ocean–atmosphere system caused a dramatic shoaling of the lysocline during the Paleocene–Eocene thermal maximum (PETM), a transient (∼ 170 kyr) global warming event that occurred roughly 55 Ma. Carbon cycle models invoking an accelerated carbonate–silicate feedback mechanism to neutralize ocean acidification predict that the lysocline would subsequently deepen to depths below its original position as the marine carbonate system recovered from such a perturbation. To test this hypothesis, records of carbonate sedimentation and preservation for PETM sections in the Weddell Sea (ODP Site 690) and along the Walvis Ridge depth transect (ODP Sites 1262, 1263, and 1266) were assembled within the context of a unified chronostratigraphy. The meridional gradient of undersaturation delimited by these records shows that dissolution was more severe in the subtropical South Atlantic than in the Weddell Sea during the PETM, a spatiotemporal pattern inconsistent with the view that Atlantic overturning circulation underwent a transient reversal. Deepening of the lysocline following its initial ascent is signaled by increases in %CaCO3 and coarse-fraction content at all sites. Carbonate preservation during the recovery period is appreciably better than that seen prior to carbon input with carbonate sedimentation becoming remarkably uniform over a broad spectrum of geographic and bathymetric settings. These congruent patterns of carbonate sedimentation confirm that the lysocline was suppressed below the depth it occupied prior to carbon input, and are consistent with the view that an accelerated carbonate–silicate geochemical cycle played an important role in arresting PETM conditions.

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 12 C-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 10 4 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 (

Transient greenhouse warming events in the Paleocene and Eocene were associated with the addition of isotopically-light carbon to the exogenic atmosphere–ocean carbon pool, leading to substantial environmental and biotic change. The magnitude of an accompanying carbon isotope excursion (CIE) can be used to constrain both the sources and amounts of carbon released during an event, as well as to correlate marine and terrestrial records with high precision. The Paleocene Eocene Thermal Maximum (PETM) is well documented, but CIE records for the subsequent warming events are still rare especially from the terrestrial realm. Here, we provide new CIE records for two of the smaller hyperthermal events, I1 and I2, in paleosol carbonate, as well as two additional records of ETM2 and H2 in the Bighorn Basin. Stratigraphic comparison of this expanded, high-resolution terrestrial carbon isotope record to the deep-sea benthic foraminifera records from ODP Sites 1262 and 1263, Walvis Ridge, in the southern Atlantic Ocean corroborates that the Bighorn Basin fluvial sediments record global atmospheric change. The stratigraphic thicknesses of the eccentricity-driven hyperthermals in these archives are in line with precession-forcing of the 7 m thick fluvial overbank-avulsion sedimentary cycles. Using the CALMAG bulk oxide mean annual precipitation proxy, we reconstruct similar or slightly wetter than background soil moisture contents during the four younger hyperthermals, in contrast to drying observed during the PETM. Soil carbonate CIEs vary in magnitude proportionally with the marine CIEs for the four smaller early Eocene hyperthermals. This relationship breaks down for the PETM, with the soil carbonate CIE ~ 2–4‰ less than expected if all five linearly relate to marine CIEs. If the PETM CO2 forcing was similar but scaled to the younger hyperthermals, photosynthetic isotope fractionation or soil environmental factors are needed to explain this anomaly. We use sensitivity testing of experimentally determined photosynthetic isotope discrimination relationships to show that factors other than the recently demonstrated pCO2 sensitivity of C3 plants carbon isotope fractionation are required to explain this anomaly.

An abrupt climate warming of 5 to 10 degrees C during the Palaeocene/Eocene boundary thermal maximum (PETM) 55 Myr ago is linked to the catastrophic release of approximately 1,050-2,100 Gt of carbon from sea-floor methane hydrate reservoirs. Although atmospheric methane, and the carbon dioxide derived from its oxidation, probably contributed to PETM warming, neither the magnitude nor the timing of the climate change is consistent with direct greenhouse forcing by the carbon derived from methane hydrate. Here we demonstrate significant differences between marine and terrestrial carbon isotope records spanning the PETM. We use models of key carbon cycle processes to identify the cause of these differences. Our results provide evidence for a previously unrecognized discrete shift in the state of the climate system during the PETM, characterized by large increases in mid-latitude tropospheric humidity and enhanced cycling of carbon through terrestrial ecosystems. A more humid atmosphere helps to explain PETM temperatures, but the ultimate mechanisms underlying the shift remain unknown.

The earliest Oligocene (∼33.5 Ma) is marked by a major step in the long-term transition from an ice-free to glaciated world. The transition, characterized by both cooling and ice-sheet growth, triggered a transient but extreme glacial period designated Oi-1. High-resolution isotope records suggest that Oi-1 lasted for roughly 400,000 yr (the duration of magnetochron 13N) before partially abating, and that it was accompanied by an ocean-wide carbon isotope anomaly of ∼0.75‰. One hypothesis relates the carbon isotope anomaly to enhanced export production brought about by climate-induced intensification of wind stress and upwelling, particularly in the Southern Ocean. To understand how this climatic event affected export production in the Southern Ocean, biogenic silica (opal) and carbonate accumulation rates were computed for the sub-polar Indian Ocean using deep-sea cores from ODP Site 744, Kerguelen Plateau. Our findings suggest that net productivity in this region increased by several fold in response to the Oi-1 glaciation. In addition, calcareous primary producers dominant in the Late Eocene were partially replaced by opaline organisms suggesting a trend toward seasonally greater surface divergence and upwelling in this sector of the Southern Ocean. We attribute these changes to intensification of atmospheric/oceanic circulation brought about by high-latitude cooling and the appearance of a full-scale continental ice-sheet on East Antarctica. Higher terrigenous sediment accumulation rates support the idea that wind-induced changes in regional productivity were augmented by an increased supply of glacial dust and debris that provided limiting micro-nutrients (e.g., iron-rich dust particles). We speculate that the rapid changes in biogenic sediment accumulation in the Southern Ocean and other upwelling-dominated regions contributed to the ocean-wide positive carbon isotope anomaly by temporarily increasing the burial rate of organic carbon relative to carbonate carbon. The changes in burial rates, in turn, may have produced a positive feedback on climate by briefly drawing down atmospheric pCO2.

Over the next few centuries, with unabated emissions of anthropogenic carbon dioxide (CO 2 ), a total of 5000 Pg C may enter the atmosphere, causing CO 2 concentrations to rise to approximately 2000 ppmv, global temperature to warm by more than 8 ° C and surface ocean pH to decline by approximately 0.7 units. A carbon release of this magnitude is unprecedented during the past 56 million years—and the outcome accordingly difficult to predict. In this regard, the geological record may provide foresight to how the Earth system will respond in the future. Here, we discuss the long-term legacy of massive carbon release into the Earth's surface reservoirs, comparing the Anthropocene with a past analogue, the Palaeocene–Eocene Thermal Maximum (PETM, approx. 56 Ma). We examine the natural processes and time scales of CO 2 neutralization that determine the atmospheric lifetime of CO 2 in response to carbon release. We compare the duration of carbon release during the Anthropocene versus PETM and the ensuing effects on ocean acidification and marine calcifying organisms. We also discuss the conundrum that the observed duration of the PETM appears to be much longer than predicted by models that use first-order assumptions. Finally, we comment on past and future mass extinctions and recovery times of biotic diversity.

[1] We have compiled the first stratigraphically continuous high-resolution benthic foraminiferal stable isotope record for the Paleocene from a single site utilizing cores recovered at Pacific ODP Site 1209. The long-term trend in the benthic isotope record suggests a close coupling of volcanic CO2 input and deep-sea warming. Over the short-term the record is characterized by slow excursions with a pronounced periodic beat related to the short (100 kyr) and long (405 kyr) eccentricity cycle. The phase relationship between the benthic isotope record and eccentricity is similar to patterns documented for the Oligocene and Miocene confirming the role of orbital forcing as the pace maker for paleoclimatic variability on Milankovitch time scales. In addition, the record documents an unusual transient warming of 2°C coeval with a 0.6‰ carbon isotope excursion and a decrease in carbonate content at 61.75 Ma. This event, which bears some resemblance to Eocene hyperthermals, marks the onset of a long-term decline in δ13C. The timing indicates it might be related to the initiation of volcanism along Greenland margin.

An understanding of sediment redox conditions across the Paleocene?Eocene thermal maximum (PETM) (?55 Ma) is essential for evaluating changes in processes that control deep?sea oxygenation, as well as identifying the mechanisms responsible for driving the benthic foraminifera extinction. Sites cored on the flanks of Walvis Ridge (Ocean Drilling Program Leg 208, Sites 1262, 1266, and 1263) allow us to examine changes in bottom and pore water redox conditions across a ?2 km depth transect of deep?sea sediments of PETM age recovered from the South Atlantic. Here we present measurements of the concentrations of redox?sensitive trace metals manganese (Mn) and uranium (U) in bulk sediment as proxies for redox chemistry at the sediment?water interface and below. All three Walvis Ridge sites exhibit bulk Mn enrichment factors (EF) ranging between 4 and 12 prior to the warming, values at crustal averages (Mn EF = 1) during the warming interval, and a return to pre?event values during the recovery period. U enrichment factors across the PETM remains at crustal averages (U EF = 1) at Site 1262 (deep) and Site 1266 (intermediate depth). U enrichment factors at Site 1263 (shallow) peaked at 5 immediately prior to the PETM and dropped to values near crustal averages during and after the event. All sites were lower in dissolved oxygen content during the PETM. Before and after the PETM, the deep and intermediate sites were oxygenated, while the shallow site was suboxic. Our geochemical results indicate that oxygen concentrations did indeed drop during the PETM but not sufficiently to cause massive extinction of benthic foraminifera.

In the deep-sea, the Paleocene–Eocene Thermal Maximum (PETM) is often marked by clay-rich condensed intervals caused by dissolution of carbonate sediments, capped by a carbonate-rich interval. Constraining the duration of both the dissolution and subsequent cap-carbonate intervals is essential to computing marine carbon fluxes and thus testing hypotheses for the origin of this event. To this end, we provide new high-resolution helium isotope records spanning the Paleocene–Eocene boundary at ODP Site 1266 in the South Atlantic. The extraterrestrial ^3He, ^3He_(ET), concentrations replicate trends observed at ODP Site 690 by Farley and Eltgroth (2003). By assuming a constant flux of ^3He_(ET) we constrain relative changes in accumulation rates of sediment across the PETM and construct a new age model for the event. In this new chronology the zero carbonate layer represents 35 kyr, some of which reflects clay produced by dissolution of Paleocene (pre-PETM) sediments. Above this layer, carbonate concentrations increase for ~165 kyr and remain higher than in the latest Paleocene until 234 ^(+48)/_(−34) kyr above the base of the clay. The new chronology indicates that minimum δ^(13)C values persisted for a maximum of 134 ^(+27)/_(−19) kyr and the inflection point previously chosen to designate the end of the CIE recovery occurs at 217 ^(+44)/_(−31) kyr. This allocation of time differs from that of the cycle-based age model of Rohl et al. (2007) in that it assigns more time to the clay layer followed by a more gradual recovery of carbonate-rich sedimentation. The new model also suggests a longer sustained δ^(13)C excursion followed by a more rapid recovery to pre-PETM δ^(13)C values. These differences have important implications for constraining the source(s) of carbon and mechanisms for its subsequent sequestration, favoring models that include a sustained release of carbon after an initial pulse.

CHANGES in the isotope content of the large marine carbon reservoir can force shifts in that of the smaller carbon pools in the atmosphere and on land. The carbon isotope compositions of marine carbonate sediments from the late Palaeocene vary considerably, exhibiting a sudden decrease close to the Palaeocene/Eocene boundary which coincides with deep-sea benthic extinctions1 and with changes in ocean circulation. Here we report that these fluctuations in the marine carbon isotope record are closely tracked by the terrestrial records provided by palaeosol carbonates and mammalian tooth enamel. In using palaeosol carbonates to reconstruct the CO2 content of the ancient atmosphere2, isotope shifts of this sort will have to be taken into account. The sharp decrease in 13C/12C ratios in the late Palaeocene provides a datum for precise correlation of marine and continental records, and suggests that abrupt climate warming at this time may have played an important role in the evolution of land mammals.

[1] The Paleocene Eocene Thermal Maximum (PETM) coincided with a negative carbon isotope excursion (CIE) in carbonate and organic matter, in terrestrial and marine records. Consequently the PETM global warming of 5–6°C has been attributed to the rapid emission of a large amount of isotopically light carbon into the ocean-atmosphere system, and the magnitude of the CIE has been used to estimate the amount of carbon greenhouse gas emitted. However, there are large discrepancies between the magnitude of the CIE in marine and in terrestrial material, in different types of marine carbonate records (bulk carbonate, planktic and benthic foraminiferal carbonate), and in marine carbonate records from different locations. The marine carbonate record of the CIE may be incomplete in most deep-sea (bathyal-abyssal) locations because of ocean acidification and widespread dissolution of seafloor carbonates. We demonstrate that the CIE in benthic foraminiferal stable isotope records is relatively abrupt and not as gradual as portrayed in bulk carbonate records along the Walvis Ridge depth transect (SE Atlantic). The benthic CIE is about −1.5‰ at 3600 m paleodepth, in contrast to about −3.5‰ at 1500 m paleodepth, consistent with depth-dependent truncation by carbonate dissolution. Strong covariance between the benthic record and the stable carbon isotope values of terrestrial n-alkanes isolated from the same core (i.e., Site 1263) supports a relatively rapid excursion of at least −3.5‰ but no more than −5.0‰.

The impacts of the Paleocene-Eocene thermal maximum (PETM) (?55 Ma), one of the most rapid and extreme warming events in Earth history, are well characterized in open marine and terrestrial environments but are less so on continental margins, a major carbon sink. Here, we present stable isotope, carbonate content, organic matter content, and C:N ratio records through the PETM from new outcrop sections in California and from cores previously drilled on the New Jersey margin. Foraminifer ? 18O data suggest that midlatitude shelves warmed by a similar magnitude as the open ocean (5°C–8°C), while the carbon isotope excursion (CIE), recorded both in carbonate and organic matter ? 13C records, is slightly larger (3.3–4.5‰) than documented in open ocean records. Sediment accumulation rates increase dramatically during the CIE in marked contrast to the open ocean sites. In parallel, mass accumulation rates of both organic and inorganic carbon also increased by an order of magnitude. The estimated total mass of accumulated carbon in excess of pre-CIE rates suggests that continental margins, at least along North America, became carbon sinks during the CIE, mainly because of weathering feedbacks and rising sea level. This result is significant because it implies that the negative feedback role of carbon burial on continental margins was greater than previously recognized.

[1] The Paleocene-Eocene Thermal Maximum, which occurred � 55.5 Ma, was caused by a massive release of carbon, as indicated by an � 3% negative carbon isotope excursion recorded in the marine, atmospheric, and terrestrial reservoirs. One suggested source for the carbon, a cometary impactor, is based on the sudden appearance and high concentration of single-domain (SD) magnetite in Paleocene-Eocene (P-E) boundary cores from the North Atlantic continental margin. We evaluate the potential sources of SD magnetite at the P-E boundary by presenting new magnetic hysteresis, low-temperature magnetic remanence, and transmission electron microscopy data from the North Atlantic coastal ocean. Our results show a similar increase in SD material but demonstrate that the magnetic material has a biogenic origin. These findings indicate that the high concentrations of SD magnetite immediately above the P-E boundary are the result of unusual accumulations and/or preservation of magnetotactic bacteria. Such bacteria typically occupy the oxic-anoxic transition zone near the sediment-water interface or in the water column. The high abundances of SD magnetite in sediments from across the shelf may be an artifact of nonsteady state redox conditions and exceptional preservation of SD magnetite. It may also indicate that the oxic-anoxic redox boundary shifted into the water column. The latter explanation implies transient eutrophy of the coastal ocean in this region, most likely due to seasonally enhanced runoff, and increased stratification and nutrient loading.

[1] The Paleocene-Eocene thermal maximum (PETM) is one of the best known examples of a transient climate perturbation, associated with a brief, but intense, interval of global warming and a massive perturbation of the global carbon cycle from injection of isotopically light carbon into the ocean-atmosphere system. One key to quantifying the mass of carbon released, identifying the source(s), and understanding the ultimate fate of this carbon is to develop high-resolution age models. Two independent strategies have been employed, cycle stratigraphy and analysis of extraterrestrial helium (HeET), both of which were first tested on Ocean Drilling Program (ODP) Site 690. These two methods are in agreement for the onset of the PETM and initial recovery, or the clay layer (“main body”), but seem to differ in the final recovery phase of the event above the clay layer, where the carbonate contents rise and carbon isotope values return toward background values. Here we present a state-of-the-art age model for the PETM derived from a new orbital chronology developed with cycle stratigraphic records from sites drilled during ODP Leg 208 (Walvis Ridge, Southeastern Atlantic) integrated with published records from Site 690 (Weddell Sea, Southern Ocean, ODP Leg 113). During Leg 208, five Paleocene-Eocene (P-E) boundary sections (Sites 1262 to 1267) were recovered in multiple holes over a depth transect of more than 2200 m at the Walvis Ridge, yielding the first stratigraphically complete P-E deep-sea sequence with moderate to relatively high sedimentation rates (1 to 3 cm/ka, where “a” is years). A detailed chronology was developed with nondestructive X-ray fluorescence (XRF) core scanning records on the scale of precession cycles, with a total duration of the PETM now estimated to be ∼170 ka. The revised cycle stratigraphic record confirms original estimates for the duration of the onset and initial recovery but suggests a new duration for the final recovery that is intermediate to the previous estimates by cycle stratigraphy and HeET.

[1] The Paleocene-Eocene thermal maximum (PETM, ∼55 Ma ago) was marked by widespread CaCO3 dissolution in deep-sea sediments, a process that has been attributed to massive release of carbon into the ocean-atmosphere system. The pattern of carbonate dissolution is key to reconstructing changes in deep sea carbonate chemistry and, ultimately, the rate, magnitude, and location of carbon input. Here we show that during the PETM, the deep-sea undersaturation was not homogeneous among the different ocean basins. Application of a sediment model to a suite of data records from different sites and ocean basins shows that a globally uniform decrease in deep-sea carbonate ion concentration ([CO32−]) is inconsistent with the data. Rather, we demonstrate that deep-sea [CO32−] increased from the Atlantic through the Southern Ocean into the Pacific. Our results show that the PETM deep-sea [CO32−] basin gradient during dissolution was reversed relative to the modern.

[1] The carbonate saturation profile of the oceans shoaled markedly during a transient global warming event known as the Paleocene-Eocene thermal maximum (PETM) (circa 55 Ma). The rapid release of large quantities of carbon into the ocean-atmosphere system is believed to have triggered this intense episode of dissolution along with a negative carbon isotope excursion (CIE). The brevity (120–220 kyr) of the PETM reflects the rapid enhancement of negative feedback mechanisms within Earth's exogenic carbon cycle that served the dual function of buffering ocean pH and reducing atmospheric greenhouse gas levels. Detailed study of the PETM stratigraphy from Ocean Drilling Program Site 690 (Weddell Sea) reveals that the CIE recovery period, which postdates the CIE onset by ∼80 kyr, is represented by an expanded (∼2.5 m thick) interval containing a unique planktic foraminiferal assemblage strongly diluted by coccolithophore carbonate. Collectively, the micropaleontological and sedimentological changes preserved within the CIE recovery interval reflect a transient state when ocean-atmosphere chemistry fostered prolific coccolithophore blooms that suppressed the local lysocline to relatively deeper depths. A prominent peak in the abundance of the clay mineral kaolinite is associated with the CIE recovery interval, indicating that continental weathering/runoff intensified at this time as well (Robert and Kennett, 1994). Such parallel stratigraphic changes are generally consonant with the hypothesis that enhanced continental weathering/runoff and carbonate precipitation helped sequester carbon during the PETM recovery period (e.g., Dickens et al., 1997; Zachos et al., 2005).

The relation between the partial pressure of atmospheric carbon dioxide ( p CO 2 ) and Paleogene climate is poorly resolved. We used stable carbon isotopic values of di-unsaturated alkenones extracted from deep sea cores to reconstruct p CO 2 fromthe middle Eocene to the late Oligocene (∼45 to 25 million years ago). Our results demonstrate that p CO 2 ranged between 1000 to 1500 parts per million by volume in the middle to late Eocene, then decreased in several steps during the Oligocene, and reached modern levels by the latest Oligocene. The fall in p CO 2 likely allowed for a critical expansion of ice sheets on Antarctica and promoted conditions that forced the onset of terrestrial C 4 photosynthesis.

Several recent papers have applied correlation analysis to climate-related time series in the hope of finding evidence for causal relationships. For a critical discussion of correlations between solar variability, cosmic rays, and cloud cover, see Laut [2003]. A prominent new example is a paper by Shaviv and Veizer [2003], which claims that fluctuations in cosmic ray flux reaching the Earth can explain 66% of the temperature variance over the past 520 m.y.,and that the sensitivity of climate to a doubling of CO2 is less than previously estimated.

[1] Changes in carbon and sulfur cycling over geologic time may have caused considerable modification of atmospheric and oceanic composition and climate. Here we calculate pyrite sulfur (Spy) and organic carbon (Corg) burial rates from recently improved Cenozoic stable isotope records, and from these rates we infer global changes in Corg burial environments. Given predominantly normal shelf-delta organic carbon burial, the global Spy burial flux should be coupled to Corg burial. However, we find that the major early Cenozoic peak in Corg burial coincides with a minimum in Spy burial. Although the calculated magnitude of variations in global pyrite burial flux is sensitive to our assumptions about the concentration of sulfate in paleoseawater, a non-steady-state isotope mass balance model indicates very low Spy burial rates during the Paleocene and a dramatic increase starting near the Paleocene-Eocene boundary, dropping off to a fairly constant Cenozoic rate beginning in the middle Eocene. High Corg/Spy burial ratios (C/S mole ratio ≈15–30) coinciding with the Paleocene carbon isotope maximum most likely reflect enhanced accumulation of terrestrial organic carbon in Paleocene terrestrial swamps. We suggest that rapid burning of accumulated Paleocene terrestrial organic carbon could have significantly contributed to the short-lived negative carbon isotope excursion at the Paleocene-Eocene boundary in addition to or possibly even as an alternative to release of gas hydrates from the continental slopes. An early Eocene minimum in calculated Corg/Spy burial ratios (C/S mole ratio ≈2–4) suggests that the predominant locus of organic carbon burial shifted to euxinic environments in a warm early Eocene ocean.

Avoiding environmental damage from ocean acidification requires reductions in carbon dioxide emissions regardless of climate change.

Sudden global warming 55 million years ago provides evidence for high climate sensitivity to atmospheric CO, but the source of the carbon remains enigmatic.

Distinct magnetic properties of marine sediments that record the Palaeocene–Eocene thermal maximum (PETM) have been suggested to be due to a bacterial magnetofossil signal that is linked to enhanced weathering conditions during the PETM. We document the dominance of bacterial magnetite in deep-sea sediments from southern Kerguelen Plateau (Ocean Drilling Program Hole 738C, southern Ocean) not only during the PETM, but also before and after the thermal event. This occurrence of magnetofossils throughout the PETM indicates that the occurrence of bacterial magnetosomes is not due to a preservation effect. Instead, we suggest that it is due to sustained mild iron-reducing conditions that dissolved the most labile aeolian-derived iron, which favoured continued magnetotactic bacterial activity without being strong enough to dissolve the less reactive magnetite and haematite. Enhanced aeolian haematite abundances at the beginning of the PETM indicate drier conditions on the neighbouring Antarctic continent at those times. Our results provide evidence that iron fertilisation by aeolian dust was the main limiting factor that conditioned proliferation of magnetotactic bacteria in the deep sea at the southern Kerguelen Plateau, with the exception of two short periods of rapidly changing palaeoenvironmental conditions at the onset and termination of the PETM. Increased iron supply from aeolian dust, that enhanced oceanic primary productivity and subsequent delivery of organic carbon to the seafloor, along with mild iron-reducing diagenetic conditions, seem to have been necessary to provide the iron needed for magnetite biomineralization by magnetotactic bacteria to drive their marked increase in abundance in the studied PETM record from southern Kerguelen Plateau. Our analyses of a deep-sea PETM record from Hole 1051B at Blake Nose (Atlantic Ocean) failed to identify magnetofossils despite evidence for the occurrence of magnetite and haematite of probable aeolian origin. Contrasting magnetic properties at these PETM sections indicate that further work is needed to understand the palaeoenvironmental and diagenetic factors whose interactions lead to production and preservation of magnetofossils in deep-sea sediments, Unidad de Zaragoza, Instituto Geológico y Minero de España, España, Research School of Earth Sciences, The Australian National University, Canberra, Australia, School of Ocean and Earth Science, National Oceanography Centre, University of Southampton, Reino Unido, Palaeomagnetic Laboratory ‘Fort Hoofddijk’, Institute of Earth Sciences, Universiteit Utrecht, Paises Bajos, Department of Geological Sciences, San Diego State University, Estados Unidos, Scripps Institution of Oceanography, University of California, Estados Unidos, Department of Earth and Planetary Sciences, University of California, Estados Unidos

RECONSTRUCTIONSof early Eocene climate depict a world in which the polar environments support mammals and reptiles, deciduous forests, warm oceans and rare frost conditions 1–5. At the same time, tropical sea surface temperatures are interpreted to have been the same as or slightly cooler than present values6. The question of how to warm polar regions of Earth without noticeably warming the tropics remains unresolved; increased amounts of greenhouse gases would be expected to warm all latitudes equally7. Oceanic heat transport has been postulated as a mechanism for heating high latitudes8–10, but it is difficult to explain the dynamics that would achieve this7,11. Here we consider estimates of Eocene wetland areas and suggest that the flux of methane, an important greenhouse gas, may have been substantially greater during the Eocene than at present. Elevated methane concentrations would have enhanced early Eocene global warming, and also might specifically have prevented severe winter cooling of polar regions because of the potential of atmospheric methane to promote the formation of optically thick, polar stratospheric ice clouds12–14.

The causes of mass extinctions and the importance of major bio-events in the history of life are subjects of considerable scientific interest. A large amount of geological, geochemical, and paleontological information now exists for the Cretaceous/Tertiary (K/T) boundary (66 Myr BP). These data are used here to constrain a newly developed time-dependent biogeochemical cycle model that is designed to study transient behavior of the earth system. Model results suggest significant fluctuations in ocean alkalinity, atmospheric CO2, and global temperatures at the K/T boundary, brought about by the extinction of calcareous plankton and reduction in pelagic CaCO3 and organic carbon sedimentation rates. Oxygenisotope analyses and other paleoclimatic data provide some evidence that such climatic fluctuations may have occurred, but stabilizing feedback processes may have acted to reduce the ocean-alkalinity and carbon dioxide fluctuations.

Dramatic warming and upheaval of the carbon system at the end of the Paleocene Epoch have been linked to massive dissociation of sedimentary methane hydrate. However, testing the Paleocene-Eocene thermal maximum hydrate dissociation hypothesis has been hindered by the inability of available proxy records to resolve the initial sequence of events. The cause of the Paleocene-Eocene thermal maximum carbon isotope excursion remains speculative, primarily due to uncertainties in the timing and duration of the Paleocene-Eocene thermal maximum. We present new high-resolution stable isotope records based on analyses of single planktonic and benthic foraminiferal shells from Ocean Drilling Program Site 690 (Weddell Sea, Southern Ocean), demonstrating that the initial carbon isotope excursion was geologically instantaneous and was preceded by a brief period of gradual surface-water warming. Both of these findings support the thermal dissociation of methane hydrate as the cause of the Paleocene-Eocene thermal maximum carbon isotope excursion. Furthermore, the data reveal that the methane-derived carbon was mixed from the surface ocean downward, suggesting that a significant fraction of the initial dissociated hydrate methane reached the atmosphere prior to oxidation.

Geochemical and paleontological evidence indicate that marine primary productivity decreased rapidly at the Cretaceous-Tertiary boundary resulting in the selective elimination of those organisms directly dependent upon the flux of organic matter as a food source (filter and suspension feeders). Detritus and deposit feeders, however, suffered relatively fewer extinctions, apparently utilizing the reservoir of organic matter stored within the sediments. Lower rates of oceanic productivity might have continued for at least 1.5 m.y. following the initial decrease despite the rapid evolution of fauna and flora during the early Paleocene. Although these results can be viewed as being compatible with the bolide impact hypothesis, the extended period of low productivity afterwards suggests some longer term effects on the biosphere than predicted by such a model.

We report the results of new stable oxygen and carbon isotope analyses on carbonate fine fraction, whole rock, and benthic foraminifers, CaCO3 and coarse fraction percentage determinations, and trace element (Sr) analyses on carbonate constituents across the Cretaceous/Tertiary (K/T) boundary in Deep Sea Drilling Project (DSDP) sites 47.2, 356, 384, and 577 and compare them with published results from K/T boundary sections in other DSDP sites. We used the trace element data and scanning electron microscope examination to evaluate possible diagenetic alteration and relative preservation of the samples analyzed in this study. The ∂18O data when interpreted as isotopic paleotemperatures indicate relative stable surface water and deepwater temperatures in the late Maestrichtian followed by somewhat fluctuating temperatures in the early Paleocene. However, there is no indication of either a significant warming or cooling at or following the K/T boundary. Several sites do exhibit somewhat heavier ∂18O values by about 0.5‰ across the boundary, which might suggest a 2-3°C cooling at most. However, we interpret these somewhat heavier ∂18O values as reflecting slightly better preservation of ∂18O of carbonate constitutents in relatively clay-rich intervals (e.g., lower diagentic potential) characteristic of the K/T boundary. The ∂13C values of carbonate fine fraction and planktonic foraminifers indicate a major negative excursion in surface water total dissolved carbon across the boundary. The surface water and deepwater ∂13C values from benthic foraminifers combined with calculated decreases in carbonate accumulation rates at all sites in the earliest Paleocene are consistent with a major decrease in productivity across the K/T boundary. The decrease in CaCO3 accumulation rates is due not to increased rates of dissolution but to decreased production in surface waters. The low-productivity episode lasted at least 1 m.y. beyond the K/T boundary crisis, suggesting that hypothesized catastrophic events alone, such as impact of an apollo object, are not the only explanation for the observed long-term decrease in productivity. There is also an indication in the ∂13C curves from several sites that the surface water ∂13C decrease began in the latest Maestrichtian, suggesting that the decrease in productivity may have been independent of a hypothesized impact event.

Exposed near Braggs, Alabama, is one of the few well-studied, nearly continuous shallow-marine Cretaceous/Tertiary boundary sections; it allows a glimpse of the biotic and environmental changes that occurred in the latest Cretaceous to earliest Paleocene. Paleomagnetic, strontium isotopic, and biostratigraphic data closely constrain the age of a series of lithologic, geochemical, and biotic variations and suggest that no more than 100–200 ka could be missing at the boundary. A major reduction in macrofaunal diversity associated with lithofacies changes occurs prior to but within 300 ka of the nannofossil-defined boundary. Approximately 40% of the apparent faunal reduction is attributed to the “Lazarus effect.” Faunal and floral assemblages, trends in carbon isotopic composition of benthic invertebrates, and lithologic characteristics indicate that a latest Maestrichtian regression culminated near the boundary (Chron C29R; Micula murus zone), significantly later than recent estimates. Water depths at this site remained shallow during the subsequent early Paleocene (zone NP1) transgression and did not reach depths equivalent to those of the late Maestrichtian until zone NP2. Relatively minor climatic changes across the boundary are suggested by a ≤4 °C cooling trend seen in the oxygen-isotope paleotemperatures. A high-resolution 87 Sr/ 86 Sr record from well-preserved macrofossil calcite shows a pattern of smooth variation and elevated values near the boundary; however, the early Paleocene “spike” of other workers was not found.