Rise In Atmospheric pCO2 Research Articles

THE MIDDLE EOCENE CLIMATIC OPTIMUM (MECO) IMPACT ON THE BENTHIC AND PLANKTIC FORAMINIFERAL RESILIENCE FROM A SHALLOW-WATER SEDIMENTARY RECORD

We present here new quantitative analyses of planktic and benthic foraminifera to assess the impact of the Middle Eocene Climatic Optimum (MECO, ~40 Ma) on these biotic groups studied along a shallow-water succession rich in larger benthic foraminifera (Sealza, Liguria, NW Italy). The MECO is one of the major Eocene global warming events, characterized by ~4–6°C warming, shifts in the global carbon cycle, and rise in atmospheric pCO2. The Sealza succession is interpreted as the product of a drowning ramp influenced by tectonic activity and provides an exceptional chance to compare biotic variations in shallow-water assemblages with deep-water communities across the MECO. In the section, the MECO interval is tentatively constrained by stable isotope oxygen data and calcareous plankton biostratigraphy. The marked decline in abundance of the epifaunal benthic Cibicidoides across the lower-middle part of the MECO suggests a decrease in oxygenation at the seafloor. Further evidence of oxygen depletion is the increase in organic matter content (TOC) of the sediment and the presence of infaunal genera Uvigerina and Bolivina. The planktic foraminiferal assemblages record the MECO warming in the upper water column as the mixed-layer warm index genera Acarinina and Morozovelloides markedly increase in abundance. In the post-MECO interval, here poorly exposed, cooler conditions are indicated by the dominance of the cold-water index genus Subbotina. Remarkably, Acarinina decline in abundance in the upper MECO interval and never recover. The MECO perturbance permanently impacted the benthic and planktic communities at Sealza that exceeded the tipping point to move to a new regime, thus proving the fauna to be not resilient, but also not recording any extinctions.

Astronomically paced climate evolution during the Late Paleozoic icehouse-to-greenhouse transition

Late Paleozoic deglaciation is Earth's first icehouse-to-greenhouse transition with extensive vegetation, but the response of tropical climate to this transition has not yet been fully addressed. Here, cyclostratigraphic analysis was conducted on the magnetic susceptibility (MS) record of a deep marine carbonate succession in South China to construct the astrochronology of late Cisuralian (Early Permian), and decipher the evolutionary response to the climate transition in the MS and δ13Ccarb time series. The constructed astronomical time scale indicates an age of 272.83 ± 0.2 Ma for the Cisuralian–Guadalupian boundary in South China. Synchronizing global records demonstrates a complicated linkage between low and high latitudes, involving secular changes in atmospheric pCO2, ice volume, tropical climate and carbon cycling. In the final stage of the Early Permian icehouse (~290–288.2 Ma), the delivery of moisture from low to high latitudes was reduced in comparison with that in the icehouse apex. A more humid climate facilitated the development of tropical coal forests and more storage of 12C on land. In a full greenhouse condition (~281–272 Ma), the tropical responses to a rise in atmospheric pCO2 involved continental drying and ocean stagnation. On a shorter time scale, the nodes of 1.36 Myr obliquity cycles triggered ice-sheet expansion and enhanced tropical precipitation during an icehouse condition. With the gradual transition to an arid greenhouse, the insolation-climate relationship began to change at ~285.1 Ma, and the obliquity nodes became associated instead with terrestrial aridity and marine anoxia. These results bring into a focus pattern of shifting dynamics involving Earth's astronomical parameters and climate change for icehouse and greenhouse worlds in the Late Paleozoic Era.

An overview on isotopic divergences – causes for instability of tree-ring isotopes and climate correlations

Abstract. Climatic reconstructions based on tree-ring isotopic series convey substantial information about past conditions prevailing in forested regions of the globe. However, in some cases, the relations between isotopic and climatic records appear unstable over time, generating the “isotopic divergences”. Former reviews have thoroughly discussed the divergence concept for tree-ring physical properties but not for isotopes. Here we want to take stock of the isotopic divergence problem, express concerns and stimulate collaborative work for improving paleoclimatic reconstructions. There are five main causes for divergent parts in isotopic and climatic series: (1) artefacts due to sampling and data treatment, relevant for dealing with long series using sub-fossil stems; (2) stand dynamics, including juvenile effects mostly occurring in the early part of tree-ring series; (3) rise in atmospheric pCO2, which can directly influence the foliar behaviour; (4) change in climate, which may modify the isotope–climate causal links; and finally (5) atmospheric pollution, which may alter leaf and root functions. Future paleoclimate research would benefit from interdisciplinary efforts designed to develop further process-based models integrating multi-proxy inputs so as to help identify causes of isotopic divergences and circumvent some of them in inverse applications.

Ventilation of the Deep Ocean Carbon Reservoir During the Last Deglaciation: Results From the Southeast Pacific

AbstractCoeval changes in atmospheric CO2 and 14C contents during the last deglaciation are often attributed to ocean circulation changes that released carbon stored in the deep ocean during the Last Glacial Maximum (LGM). Work is being done to generate records that allow for the identification of the exact mechanisms leading to the accumulation and release of carbon from the oceanic reservoir, but these mechanisms are still the subject of debate. Here we present foraminifera 14C data from five cores in a transect across the Chilean continental margin between ~540 and ~3,100 m depth spanning the last 20,000 years. Our data reveal that during the LGM, waters at ~2,000 m were 50% to 80% more depleted in Δ14C than waters at ~1,500 m when compared to modern values, consistent with the hypothesis of a glacial deep ocean carbon reservoir that was isolated from the atmosphere. During the deglaciation, our intermediate water records reveal homogenization in the Δ14C values between ~800 and ~1,500 m from ~16.5–14.5 ka cal BP to ~14–12 ka cal BP, which we interpret as deeper penetration of Antarctic Intermediate Water. While many questions still remain, this process could aid the ventilation of the deep ocean at the beginning of the deglaciation, contributing to the observed ~40 ppm rise in atmospheric pCO2.

Equatorial Pacific seawater pCO2 variability since the last glacial period

The ocean may have played a central role in the atmospheric pCO2 rise during the last deglaciation. However, evidence on where carbon was exchanged between the ocean and the atmosphere in this period is still lacking, hampering our understanding of global carbon cycle on glacial–interglacial timescales. Here we report a new surface seawater pCO2 reconstruction for the western equatorial Pacific Ocean based on boron isotope analysis—a seawater pCO2 proxy—using two species of near-surface dwelling foraminifera from the same marine sediment core. The results indicate that the region remained a modest CO2 sink throughout the last deglaciation.

CO<sub>2</sub> drawdown due to particle ballasting by glacial aeolian dust: an estimate based on the ocean carbon cycle model MPIOM/HAMOCC version 1.6.2p3

Abstract. Despite intense efforts, the mechanisms that drive glacial–interglacial changes in atmospheric pCO2 are not fully understood. Here, we aim at quantifying the potential contribution of aeolian dust deposition changes to the atmospheric pCO2 drawdown during the Last Glacial Maximum (LGM). To this end, we use the Max Planck Institute Ocean Model (MPIOM) and the embedded Hamburg Ocean Carbon Cycle model (HAMOCC), including a new parameterization of particle ballasting that accounts for the acceleration of sinking organic soft tissue in the ocean by higher-density biogenic calcite and opal particles, as well as mineral dust. Sensitivity experiments with reconstructed LGM dust deposition rates indicate that the acceleration of detritus by mineral dust played a small role in atmospheric pCO2 variations during glacial–interglacial cycles – on the order of 5 ppmv, compared to the reconstructed ∼80 ppmv rise in atmospheric pCO2 during the last deglaciation. The additional effect of the LGM dust deposition, namely the enhanced fertilization by the iron that is associated with the glacial dust, likely played a more important role; although the full iron fertilization effect can not be estimated in the particular model version used here due to underestimated present-day non-diazotroph iron limitation, fertilization of diazotrophs in the tropical Pacific already leads to an atmospheric pCO2 drawdown of around 10 ppmv.

Warm Terrestrial Subtropics During the Paleocene and Eocene: Carbonate Clumped Isotope (Δ47) Evidence From the Tornillo Basin, Texas (USA)

AbstractRecords of subtropical climate on land from the early Paleogene offer insights into how the Earth system responds to greenhouse climate conditions. Fluvial and floodplain deposits of the Tornillo Basin (Big Bend National Park, Texas, USA) preserve a record of environmental and climatic change of the Paleocene and the early Eocene. We report carbon, oxygen, and clumped isotopic compositions (δ13C, δ18O, and Δ47) of paleosol carbonate nodules from this basin. Mineralogical, geochemical, and thermal modeling evidence suggests that the measured isotopic values preserve primary environmental signals with a summer bias with the exception of data from two nodules reset by local igneous intrusions. The unaltered nodules record Δ47 temperatures of 25 ± 4 and 32 ± 2 °C for the Paleocene and early Eocene nodules, respectively, showing an increase in average summer temperatures of 7 ± 3 °C. Calculations of δ18O of soil water are −2.8 ± 0.7‰ and −0.8 ± 0.4‰ (standard mean ocean water) for the early‐mid‐Paleocene and late Paleocene‐early Eocene, showing an increase of 2.0 ± 0.9‰. The increase in temperature and δ18O values likely relates to a rise in atmospheric pCO2, although we cannot rule out that changes in paleosol texture and regional precipitation patterns also influence the record. Comparison with Δ47 estimates of summer temperature from the Green River and Bighorn Basins (WY) highlights that terrestrial surface temperatures are heterogeneous, and latitudinal temperature gradients on land remain undetermined. Previously published paleoclimate models predict summer temperatures that are 2 to 6 °C higher than our estimate; discrepancies between climate models and proxy data persist at lower latitudes.

Refined modeling and 14C plateau tuning reveal consistent patterns of glacial and deglacial 14C reservoir ages of surface waters in low‐latitude Atlantic

AbstractModeling studies predict that changes in radiocarbon (14C) reservoir ages of surface waters during the last deglacial episode will reflect changes in both atmospheric 14C concentration and ocean circulation including the Atlantic Meridional Overturning Circulation. Tests of these models require the availability of accurate 14C reservoir ages in well‐dated late Quaternary time series. We here test two models using plateau‐tuned 14C time series in multiple well‐placed sediment core age‐depth sequences throughout the lower latitudes of the Atlantic Ocean. 14C age plateau tuning in glacial and deglacial sequences provides accurate calendar year ages that differ by as much as 500–2500 years from those based on assumed global reservoir ages around 400 years. This study demonstrates increases in local Atlantic surface reservoir ages of up to 1000 years during the Last Glacial Maximum, ages that reflect stronger trades off Benguela and summer winds off southern Brazil. By contrast, surface water reservoir ages remained close to zero in the Cariaco Basin in the southern Caribbean due to lagoon‐style isolation and persistently strong atmospheric CO2 exchange. Later, during the early deglacial (16 ka) reservoir ages decreased to a minimum of 170–420 14C years throughout the South Atlantic, likely in response to the rapid rise in atmospheric pCO2 and Antarctic temperatures occurring then. Changes in magnitude and geographic distribution of 14C reservoir ages of peak glacial and deglacial surface waters deviate from the results of Franke et al. (2008) but are generally consistent with those of the more advanced ocean circulation model of Butzin et al. (2012).

Effects of eustatic sea-level change, ocean dynamics, and nutrient utilization on atmospheric <i>p</i>CO<sub>2</sub> and seawater composition over the last 130 000 years: a model study

Abstract. We have developed and employed an Earth system model to explore the forcings of atmospheric pCO2 change and the chemical and isotopic evolution of seawater over the last glacial cycle. Concentrations of dissolved phosphorus (DP), reactive nitrogen, molecular oxygen, dissolved inorganic carbon (DIC), total alkalinity (TA), 13C-DIC, and 14C-DIC were calculated for 24 ocean boxes. The bi-directional water fluxes between these model boxes were derived from a 3-D circulation field of the modern ocean (Opa 8.2, NEMO) and tuned such that tracer distributions calculated by the box model were consistent with observational data from the modern ocean. To model the last 130 kyr, we employed records of past changes in sea-level, ocean circulation, and dust deposition. According to the model, about half of the glacial pCO2 drawdown may be attributed to marine regressions. The glacial sea-level low-stands implied steepened ocean margins, a reduced burial of particulate organic carbon, phosphorus, and neritic carbonate at the margin seafloor, a decline in benthic denitrification, and enhanced weathering of emerged shelf sediments. In turn, low-stands led to a distinct rise in the standing stocks of DIC, TA, and nutrients in the global ocean, promoted the glacial sequestration of atmospheric CO2 in the ocean, and added 13C- and 14C-depleted DIC to the ocean as recorded in benthic foraminifera signals. The other half of the glacial drop in pCO2 was linked to inferred shoaling of Atlantic meridional overturning circulation and more efficient utilization of nutrients in the Southern Ocean. The diminished ventilation of deep water in the glacial Atlantic and Southern Ocean led to significant 14C depletions with respect to the atmosphere. According to our model, the deglacial rapid and stepwise rise in atmospheric pCO2 was induced by upwelling both in the Southern Ocean and subarctic North Pacific and promoted by a drop in nutrient utilization in the Southern Ocean. The deglacial sea-level rise led to a gradual decline in nutrient, DIC, and TA stocks, a slow change due to the large size and extended residence times of dissolved chemical species in the ocean. Thus, the rapid deglacial rise in pCO2 can be explained by fast changes in ocean dynamics and nutrient utilization whereas the gradual pCO2 rise over the Holocene may be linked to the slow drop in nutrient and TA stocks that continued to promote an ongoing CO2 transfer from the ocean into the atmosphere.

Evolution of South Atlantic density and chemical stratification across the last deglaciation

Explanations of the glacial-interglacial variations in atmospheric pCO2 invoke a significant role for the deep ocean in the storage of CO2. Deep-ocean density stratification has been proposed as a mechanism to promote the storage of CO2 in the deep ocean during glacial times. A wealth of proxy data supports the presence of a "chemical divide" between intermediate and deep water in the glacial Atlantic Ocean, which indirectly points to an increase in deep-ocean density stratification. However, direct observational evidence of changes in the primary controls of ocean density stratification, i.e., temperature and salinity, remain scarce. Here, we use Mg/Ca-derived seawater temperature and salinity estimates determined from temperature-corrected δ(18)O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation (i.e., 22-2 ka before present). We find that a major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide, as indicated by the chemical tracers of benthic foraminifer δ(13)C and foraminifer/coral (14)C. Our results indicate that chemical destratification likely resulted in the first rise in atmospheric pCO2, whereas the density destratification of the deep South Atlantic lags the second rise in atmospheric pCO2 during the late deglacial period. Our findings emphasize that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed.

Radiocarbon evidence for a possible abyssal front near 3.1 km in the glacial equatorial Pacific Ocean

We investigate the radiocarbon ventilation age in deep equatorial Pacific sediment cores using the difference in conventional 14C age between coexisting benthic and planktonic foraminifera, and integrate those results with similar data from around the North Pacific Ocean in a reconstruction for the last glaciation (15 to 25 conventional 14C ka). Most new data from both the Equatorial Pacific and the Emperor Seamounts in the northwestern Pacific come from maxima in abundance of benthic taxa because this strategy reduces the effect of bioturbation. Although there remains considerable scatter in the ventilation age estimates, on average, ventilation ages in the Equatorial Pacific were significantly greater below 3.2 km (∼3080±1125 yrs, n=15) than in the depth interval 1.9 to 3.0 km (∼1610±250 yrs, n=12). When compared to the average modern seawater ΔC14 profile for the North Pacific, the Equatorial Pacific glacial data suggest an abyssal front located somewhere between 3.0 and 3.2 km modern water depth. Above that depth, the data may indicate slightly better ventilation than today, and below that depth, glacial Equatorial Pacific data appear to be as old as last glacial maximum (LGM) deep water ages reported for the deep southern Atlantic. This suggests that a glacial reservoir of aged waters extended throughout the circumpolar Southern Ocean and into the Equatorial Pacific. Renewed ventilation of such a large volume of aged (and, by corollary, carbon-rich) water would help to account for the rise in atmospheric pCO2 and the fall in ΔC14 as the glaciation drew to a close.

Responses of ocean circulation and carbon cycle to changes in the position of the Southern Hemisphere westerlies at Last Glacial Maximum.

We explore the impact of a latitudinal shift in the westerly wind belt over the Southern Ocean on the Atlantic meridional overturning circulation (AMOC) and on the carbon cycle for Last Glacial Maximum background conditions using a state-of-the-art ocean general circulation model. We find that a southward (northward) shift in the westerly winds leads to an intensification (weakening) of no more than 10% of the AMOC. This response of the ocean physics to shifting winds agrees with other studies starting from preindustrial background climate, but the responsible processes are different. In our setup changes in AMOC seemed to be more pulled by upwelling in the south than pushed by downwelling in the north, opposite to what previous studies with different background climate are suggesting. The net effects of the changes in ocean circulation lead to a rise in atmospheric pCO2 of less than 10 μatm for both northward and southward shift in the winds. For northward shifted winds the zone of upwelling of carbon- and nutrient-rich waters in the Southern Ocean is expanded, leading to more CO2outgassing to the atmosphere but also to an enhanced biological pump in the subpolar region. For southward shifted winds the upwelling region contracts around Antarctica, leading to less nutrient export northward and thus a weakening of the biological pump. These model results do not support the idea that shifts in the westerly wind belt play a dominant role in coupling atmospheric CO2 rise and Antarctic temperature during deglaciation suggested by the ice core data.

Atmospheric Δ14C reduction in simulations of Atlantic overturning circulation shutdown

A rapid reduction in the Atlantic meridional overturning circulation (AMOC) can significantly disrupt the global heat transport and likely triggered abrupt climate change during the last glacial cycle. A slowdown in AMOC has long been assumed to inhibit the exchange of carbon between the atmosphere and the deep ocean and thus cause radiocarbon (14C), which is produced in the atmosphere, to accumulate in the atmosphere. Indeed previous model studies have demonstrated that a reduction in AMOC leads to higher atmospheric 14C abundance (Δ14C). However, this seems inconsistent with the observed rise in atmospheric pCO2 during Heinrich 1 and the Younger Dryas stadial events and the emerging view that this CO2 rise resulted from the deep ocean venting “old” carbon. Using an Earth system model, we offer an alternative scenario that AMOC slowdown and an accompanying dynamical response in the south (i.e., the bipolar seesaw) can in fact lead to a decline in atmospheric Δ14C. This decline is realized in the model when the bipolar seesaw and thus the flux of old carbon from the Southern Ocean are sufficiently large so as to overcome the accumulation of 14C in the atmosphere as AMOC is reduced. The bipolar seesaw we describe invokes an oceanic teleconnection, whereby a freshwater perturbation in the North Atlantic drives a southern Δ14C response, but this does not necessarily preclude an atmospheric teleconnection.

High nitrate to phosphorus regime attenuates negative effects of rising <i>p</i>CO<sub>2</sub> on total population carbon accumulation

Abstract. The ongoing rise in atmospheric pCO2 and consequent increase in ocean acidification have direct effects on marine calcifying phytoplankton, which potentially alters carbon export. To date it remains unclear, firstly, how nutrient regime, in particular by coccolithophores preferred phosphate limitation, interacts with pCO2 on particulate carbon accumulation; secondly, how direct physiological responses on the cellular level translate into total population response. In this study, cultures of Emiliania huxleyi were full-factorially exposed to two different N:P regimes and three different pCO2 levels. Cellular biovolume and PIC and POC content significantly declined in response to pCO2 in both nutrient regimes. Cellular PON content significantly increased in the Redfield treatment and decreased in the high N:P regime. Cell abundance significantly declined in the Redfield and remained constant in the high N:P regime. We hypothesise that in the high N:P regime severe phosphorous limitation could be compensated either by reduced inorganic phosphorous demand and/or by enzymatic uptake of organic phosphorous. In the Redfield regime we suggest that enzymatic phosphorous uptake to supplement enhanced phosphorous demand with pCO2 was not possible and thus cell abundance declined. These hypothesised different physiological responses of E. huxleyi among the nutrient regimes significantly altered population carrying capacities along the pCO2 gradient. This ultimately led to the attenuated total population response in POC and PIC content and biovolume to increased pCO2 in the high N:P regime. Our results point to the fact that the physiological (i.e. cellular) PIC and POC response to ocean acidification cannot be linearly extrapolated to total population response and thus carbon export. It is therefore necessary to consider both effects of nutrient limitation on cell physiology and their consequences for population size when predicting the influence of coccolithophores on atmospheric pCO2 feedback and their function in carbon export mechanisms.

The Roles of CO2 and Orbital Forcing in Driving Southern Hemispheric Temperature Variations during the Last 21 000 Yr*

Abstract Transient climate model simulations covering the last 21 000 yr reveal that orbitally driven insolation changes in the Southern Hemisphere, combined with a rise in atmospheric pCO2, were sufficient to jump-start the deglacial warming around Antarctica without direct Northern Hemispheric triggers. Analyses of sensitivity experiments forced with only one external forcing component (greenhouse gases, ice-sheet forcing, or orbital forcing) demonstrate that austral spring insolation changes triggered an early retreat of Southern Ocean sea ice starting around 19–18 ka BP. The associated sea ice–albedo feedback and the subsequent increase of atmospheric CO2 concentrations helped to further accelerate the deglacial warming in the Southern Hemisphere. Implications for the interpretation of Southern Hemispheric paleoproxy records are discussed.

Late Paleozoic tropical climate response to Gondwanan deglaciation

Research Article| September 01, 2007 Late Paleozoic tropical climate response to Gondwanan deglaciation Christopher J. Poulsen; Christopher J. Poulsen 1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Search for other works by this author on: GSW Google Scholar David Pollard; David Pollard 2Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar Isabel P. Montañez; Isabel P. Montañez 3Department of Geology, One Shields Avenue, University of California–Davis, Davis, California 95616, USA Search for other works by this author on: GSW Google Scholar David Rowley David Rowley 4Department of Geophysical Sciences, 5734 South Ellis Avenue, University of Chicago, Chicago, Illinois 60637, USA Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (9): 771–774. https://doi.org/10.1130/G23841A.1 Article history received: 27 Feb 2007 rev-recd: 02 Apr 2007 accepted: 05 Apr 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Christopher J. Poulsen, David Pollard, Isabel P. Montañez, David Rowley; Late Paleozoic tropical climate response to Gondwanan deglaciation. Geology 2007;; 35 (9): 771–774. doi: https://doi.org/10.1130/G23841A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Coupled climate-biome model simulations of the late Paleozoic were developed to determine the response of Pangean tropical climate to Gondwanan deglaciation. The model simulations predict substantial changes over equatorial Pangea including continental drying, a reversal of equatorial winds, warming, heavier δ18O values of meteoric precipitation, and the expansion of deserts and the contraction of forests. The magnitude of these tropical responses is sensitive to the extent of Gondwana continental ice and the deglacial rise in atmospheric pCO2, boundary conditions that are not well known for the late Paleozoic. Nonetheless the model predictions are consistent with climatic and environmental trends determined from terrestrial proxy data, implying that the deglaciation of Gondwana was a transformational climate event in tropical Pangea. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Quantitative interpretation of atmospheric carbon records over the last glacial termination

The glacial/interglacial rise in atmospheric pCO2 is one of the best known changes in paleoclimate research, yet the cause for it is still unknown. Forcing the coupled ocean‐atmosphere‐biosphere box model of the global carbon cycle BICYCLE with proxy data over the last glacial termination, we are able to quantitatively reproduce transient variations in pCO2 and its isotopic signatures (δ13C, Δ14C) observed in natural climate archives. The sensitivity of the Box model of the Isotopic Carbon cYCLE (BICYCLE) to high or low latitudinal changes is comparable to other multibox models or more complex ocean carbon cycle models, respectively. The processes considered here ranked by their contribution to the glacial/interglacial rise in pCO2 in decreasing order are: the rise in Southern Ocean vertical mixing rates (>30 ppmv), decreases in alkalinity and carbon inventories (>30 ppmv), the reduction of the biological pump (∼20 ppmv), the rise in ocean temperatures (15–20 ppmv), the resumption of ocean circulation (15–20 ppmv), and coral reef growth (<5 ppmv). The regrowth of the terrestrial biosphere, sea level rise and the increase in gas exchange through reduced sea ice cover operate in the opposite direction, decreasing pCO2 during Termination I by ∼30 ppmv. According to our model the sequence of events during Termination I might have been the following: a reduction of aeolian iron fertilization in the Southern Ocean together with a breakdown in Southern Ocean stratification, the latter caused by rapid sea ice retreat, trigger the onset of the pCO2 increase. After these events the reduced North Atlantic Deep Water (NADW) formation during the Heinrich 1 event and the subsequent resumption of ocean circulation at the beginning of the Bølling‐Allerød warm interval are the main processes determining the atmospheric carbon records in the subsequent time period of Termination I. We further deduce that a complete shutdown of the NADW formation during the Younger Dryas was very unlikely. Changes in ocean temperature and the terrestrial carbon storage are the dominant processes explaining atmospheric δ13C after the Bølling‐Allerød warm interval.

A transient rise in tropical sea surface temperature during the Paleocene-Eocene thermal maximum.

The Paleocene-Eocene Thermal Maximum (PETM) has been attributed to a rapid rise in greenhouse gas levels. If so, warming should have occurred at all latitudes, although amplified toward the poles. Existing records reveal an increase in high-latitude sea surface temperatures (SSTs) (8 degrees to 10 degrees C) and in bottom water temperatures (4 degrees to 5 degrees C). To date, however, the character of the tropical SST response during this event remains unconstrained. Here we address this deficiency by using paired oxygen isotope and minor element (magnesium/calcium) ratios of planktonic foraminifera from a tropical Pacific core to estimate changes in SST. Using mixed-layer foraminifera, we found that the combined proxies imply a 4 degrees to 5 degrees C rise in Pacific SST during the PETM. These results would necessitate a rise in atmospheric pCO2 to levels three to four times as high as those estimated for the late Paleocene.

Biogeochemical constraints on the Triassic‐Jurassic boundary carbon cycle event

The end‐Triassic mass extinctions represent one of the five most severe biotic crises in Earth history, yet remain one of the most enigmatic. Ongoing debate concerns the environmental effects of the Central Atlantic Magmatic Province (CAMP) eruptions and their linkage with the mass extinction event across the Triassic‐Jurassic boundary. There is conflicting paleo‐evidence for changes in atmosphericpCO2during the extrusion of the CAMP basalts. Studies on sediments from European and Pacific localities have, however, identified a substantial negative isotopic anomaly (up to −3.5‰) across the TR‐J boundary, providing an important indicator of changes in the operation of the ancient global carbon cycle. We sought to explain the paleo‐evidence by utilizing a carbon cycle model for the “hothouse” world of the end‐Triassic that emphasizes the chemical weathering of silicate and carbonate rocks and the ocean carbonate chemistry. We find that volcanic CO2outgassing fails to fully account for either a sufficient rise in atmosphericpCO2(indicated by the stomata of fossil leaves) or the sedimentary isotopic fingerprint. Instead, the scenario that best fits all of the geologic evidence is a positive feedback loop in which warming, due to a buildup of volcanically derived CO2, triggers destabilization of seafloor methane hydrates and the catastrophic release of CH4[Pálfy et al., 2001]. We calculate that this carbon cycle perturbation was huge, involving the release of ∼8000–9000 Gt C as CO2during the CAMP basaltic eruptions and ∼5000 Gt C as CH4. In the model the initial isotopic excursion is assumed to take place over ∼70 kyr, while complete reequilibration of the ocean‐atmosphere system with respect to CO2is accomplished over 700–1000 kyr. Our results thus provide a preliminary theoretical explanation for the bioevents, estimatedpCO2changes, and isotopic excursions observed in marine and continental sediments at this time.