CO2 Proxy Research Articles

The Syabru‐Bensi hydrothermal system in central Nepal: 1. Characterization of carbon dioxide and radon fluxes

AbstractThe Syabru‐Bensi hydrothermal system (SBHS), located at the Main Central Thrust zone in central Nepal, is characterized by hot (30–62°C) water springs and cold (<35°C) carbon dioxide (CO2) degassing areas. From 2007 to 2011, five gas zones (GZ1–GZ5) were studied, with more than 1600 CO2 and 850 radon flux measurements, with complementary self‐potential data, thermal infrared imaging, and effective radium concentration of soils. Measurement uncertainties were evaluated in the field. CO2 and radon fluxes vary over 5 to 6 orders of magnitude, reaching exceptional maximum values of 236 ± 50 kg m−2 d−1 and 38.5 ± 8.0 Bq m−2 s−1, with estimated integrated discharges over all gas zones of 5.9 ± 1.6 t d−1 and 140 ± 30 MBq d−1, respectively. Soil‐gas radon concentration is 40 × 103 Bq m−3 in GZ1–GZ2 and 70 × 103 Bq m−3 in GZ3–GZ4. Strong relationships between CO2 and radon fluxes in all gas zones (correlation coefficient R = 0.86 ± 0.02) indicate related gas transport mechanisms and demonstrate that radon can be considered as a relevant proxy for CO2. CO2 carbon isotopic ratios (δ13C from −1.7 ± 0.1 to −0.5 ± 0.1‰), with the absence of mantle signature (helium isotopic ratios R/RA < 0.05), suggest metamorphic decarbonation at depth. Thus, the SBHS emerges as a unique geosystem with significant deep origin CO2 discharge located in a seismically active region, where we can test methodological issues and our understanding of transport properties and fluid circulations in the subsurface.

Refining the pedogenic carbonate atmospheric CO2 proxy and application to Miocene CO2

Quantifying uncertainty is essential for any paleoenvironmental proxy. Recent work on propagating error through the equation used to determine atmospheric CO2 concentrations from paleosol carbonates yields conflicting results. Small magnitude uncertainty from Gaussian error propagation contrasts with larger magnitude uncertainty from Monte Carlo error propagation. The discrepancy is reconciled here by revising partial differential equations for the Gaussian approach. Uncertainties calculated using the two approaches are compared and applied to Miocene calcic paleosols. Monte Carlo-propagated errors are asymmetrical but otherwise agree with Gaussian errors. Three methods for assigning soil-respired CO2 concentrations (S(z)) to paleosols are also compared. A revised calibration of depth to the Bk horizon as a direct proxy for S(z) is presented and although it results in lower atmospheric CO2 concentrations than the original calibration, it results in significantly higher calculated atmospheric CO2 than the use of mean annual precipitation as a proxy for S(z) or assigning S(z) values based on soil order. Averaging atmospheric CO2 concentrations calculated using soil-order based S(z) values from >10 penecontemporaenous and independent paleosols will likely result in useful constraints on Earth system sensitivity. The improvement of proxies for S(z) is needed to increase the accuracy and precision and to resolve potential volatility in CO2 time series.

Stomatal density and index data of Platanus neptuni leaf fossils and their evaluation as a CO2 proxy for the Oligocene

Platanus neptuni (Ettingshausen) Bůžek, Holý and Kvaček is a deciduous and preferentially azonal taxon of temperate to warm-temperate vegetation in Europe from the Late Eocene to the Late Miocene. The high fossilization potential of its leaves and easily identifiable stomata and epidermal cells make P. neptuni an excellent source of stomatal data that can be utilized as a CO2 proxy. Moreover, it was found in former studies that CO2 data based on stomatal frequency data of P. neptuni overlapped to a high degree with CO2 results which are provided by other, contemporaneous taxa. In this study, the stomatal CO2 signal of P. neptuni is expanded to include the early Oligocene and is analyzed in more detail with three aims: 1) to evaluate the CO2 signal of P. neptuni stomatal data, 2) to check SI and SD data of P. neptuni for consistency, and 3) to contribute additional terrestrial CO2 data to the Oligocene record. During the Oligocene, full scale Antarctic glaciation occurred, punctuated by various distinct glaciation events. There is evidence that Oligocene glaciation was coupled to atmospheric CO2 level. Presently, the main proxy sources for Oligocene CO2 levels are alkenones and boron-isotope data, both obtained from marine sediments.Since P. neptuni is an extinct taxon, CO2 was reconstructed by using an ecophysiological modeling approach to plant gas exchange which utilizes various other data in addition to stomatal density. Material was considered from sites which are dated to the following time intervals: early Oligocene — 33.9 to 32Ma, late Oligocene — 27 to 26.2Ma and 25.3 to 23Ma, and latest Oligocene — around 24Ma. Comparison of raw SI and SD data of P. neptuni revealed partially conflicting results, with the SD data indicating a decrease in CO2 from the early to the late Oligocene whereas SI data indicate an increase. In contrast, CO2 results calculated with the gas exchange model indicate relatively stable CO2 for the considered time intervals, with levels of about 400ppm. The reconstructed CO2 data points are similar to other proxy data and are consistent with the general climate development during the Oligocene.

Effect of fluid topology on residual nonwetting phase trapping: Implications for geologic CO2 sequestration

This work examines the influence of initial (i.e. post drainage) nonwetting (NW) fluid topology on total residual (i.e. after imbibition) NW phase saturation. Brine and air (used as a proxy for supercritical CO2) flow experiments were performed on Bentheimer sandstone; results were quantified via imaging with X-ray computed microtomography (X-ray CMT), which allows for three dimensional, non-destructive, pore-scale analysis of the amount, distribution, and connectivity of NW phase fluid within the sandstone cores. In order to investigate the phenomenon of fluid connectivity and how it changes throughout flow processes, the Bentheimer sandstone results are compared to previously collected X-ray CMT data from similar experiments performed in a sintered glass bead column, a loose packed glass bead column, and a column packed with crushed tuff. This allows us to interpret the results in a broader sense from the work, and draw conclusions of a more general nature because they are not based on a single pore geometry. Connectivity is quantified via the normalized Euler number of the NW fluid phase; the Euler number of a particular sample is normalized by the maximum connectivity of the media, i.e. the Euler number of the system at 100% NW phase saturation. General connectivity-saturation relationships were identified for the various media. In terms of trapping, it was found that residual NW phase trapping is dependent on initial (i.e. post-drainage) NW phase connectivity as well as imbibition capillary number for the Bentheimer sandstone. Conversely, the sintered glass bead column exhibited no significant relationship between trapping and NW topology.These findings imply that for a CO2 sequestration scenario, capillary trapping is controlled by both the imbibition capillary number and the initial NW phase connectivity: as capillary number increases, and the normalized initial Euler number approaches a value of 1.0, capillary trapping is suppressed. This finding is significant to CO2 sequestration, because both the drainage (CO2 injection) and imbibition (subsequent water injection or infiltration) processes can be engineered in order to maximize residual trapping within the porous medium. Based on the findings presented here, we suggest that both the Euler number-saturation and the capillary number-saturation relationships for a given medium should be considered when designing a CO2 sequestration scenario.

Exploring the usability of isotopically anomalous oxygen in bones and teeth as paleo-CO2-barometer

Fluctuations in atmospheric pCO2 may have played the key role in global climate throughout Earth’s history. For the quantification of past variations in atmospheric pCO2, several geological proxy approaches and geochemical models have been developed. Here, we evaluate a new CO2 proxy approach that is based on the triple oxygen isotope composition (16O, 17O, 18O) of skeletal apatite of terrestrial mammals. Our approach utilizes the relation between an anomaly in 17O of tropospheric air O2 and atmospheric pCO2. The anomaly is transferred from inhaled air O2 to skeletal apatite of mammals. Hence, triple oxygen isotope data of mammalian bioapatite provide information regarding pCO2 during the animal’s lifetime. The approach was calibrated with a detailed mass balance model that was verified by analyses on a set of recent mammals. We evaluate the potential of this new independent terrestrial paleo-CO2 proxy in a case study including Eocene to Miocene samples. The present investigation provides promising results that are in good agreement with existing proxy- and model data. The uncertainty intrinsic to the proxy is mainly due to uncertainties in physiological parameters.

A simple empirical model estimating atmospheric CO<sub>2</sub> background concentrations

Abstract. A simple empirical CO2 model (SECM) is presented to estimate column-average dry-air mole fractions of atmospheric CO2 (XCO2) as well as mixing ratio profiles. SECM is based on a simple equation depending on 17 empirical parameters, latitude, and date. The empirical parameters have been determined by least squares fitting to NOAA's (National Oceanic and Atmospheric Administration) assimilation system CarbonTracker version 2010 (CT2010). Comparisons with TCCON (total carbon column observing network) FTS (Fourier transform spectrometer) measurements show that SECM XCO2 agrees quite well with reality. The synthetic XCO2 values have a standard error of 1.39 ppm and systematic station-to-station biases of 0.46 ppm. Typical column averaging kernels of the TCCON FTS, a SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY), and two GOSAT (Greenhouse gases Observing SATellite) XCO2 retrieval algorithms have been used to assess the smoothing error introduced by using SECM profiles instead of CT2010 profiles as a priori. The additional smoothing error amounts to 0.17 ppm for a typical SCIAMACHY averaging kernel and is most times much smaller for the other instruments (e.g. 0.05 ppm for a typical TCCON FTS averaging kernel). Therefore, SECM is well suited to provide a priori information for state-of-the-art ground-based (FTS) and satellite-based (GOSAT, SCIAMACHY) XCO2 retrievals. Other potential applications are: (i) near real-time processing systems (that cannot make use of models like CT2010 operated in delayed mode), (ii) "CO2 proxy" methods for XCH4 retrievals (as correction for the XCO2 background), and (iii) observing system simulation experiments especially for future satellite missions.

Stomatal pore length change in leaves of Eotrigonobalanus furcinervis (Fagaceae) from the Late Eocene to the Latest Oligocene and its impact on gas exchange and CO2 reconstruction

It has repeatedly been reported that stomatal pore length changes with changing atmospheric CO2 concentration. Since stomatal pore length affects stomatal conductance, the question arises of how pore length change interacts with the application of stomatal density as a CO2 proxy. In this study, stomatal density, pore length and maximum stomatal conductance are evaluated for Eotrigonobalanus furcinervis (Fagaceae) from the late Eocene, late Oligocene and latest Oligocene. Then a model that simulates plant gas exchange by treating transpiration and photosynthesis as coupled processes and allows also for CO2 reconstruction was applied to the data of E. furcinervis. The model is based on an optimization approach which calculates plant gas exchange as providing a maximum amount of photosynthesis rate under given climate conditions. Stomatal density increases from the late Eocene onwards in E. furcinervis and stomatal pore length decreases. Maximum stomatal conductance which results from both stomatal pore length and density is highest for the late Eocene and lowest for the latest Oligocene. Since both stomatal conductance and stomatal density are usually negatively correlated with CO2, the stomatal data of E. furcinervis appear to be in conflict with a CO2 decrease over that period, as inferred from other evidence. The model results indicate that the stomatal data of E. furcinervis are in fact linked to a decrease of CO2 from the late Eocene to the Oligocene, in accordance with other proxy data although the late Eocene CO2values of this study are lower than usually found for this time period (range between 259 and 640ppm for the late Eocene, 213–581 ppm for the late Oligocene and 258–611 ppm for the latest Oligocene). The seemingly inconsistency between changes of stomatal density, stomatal pore length and stomatal conductance as found in this study is due to the effect on gas exchange exerted by other climate parameters than CO2. The results of this study strongly suggest that any interpretation of stomatal parameters as indicators of CO2 should consider the influence of climate on stomatal conductance and gas exchange.

Bennettitalean leaf cuticle fragments (here Anomozamites and Pterophyllum) can be used interchangeably in stomatal frequency‐based palaeo‐CO2 reconstructions

Abstract: Bennettites are an abundant and frequently well‐preserved component of many Mesozoic fossil floras, often playing an important ecological role in flood plain vegetation communities. During a recent study focusing on stomatal indices of Triassic–Jurassic fossil plants, it became evident that the leaf fragments of two bennettite genera Anomozamites Schimper (1870) emend. Harris (1969) and Pterophyllum Brongniart (1825) display a significant overlap of leaf shape as well as cuticular characters. Owing to the preference of recognition of single taxa (ideally species) for the stomatal method, we use a database of 70 leaf fragments of Anomozamites and Pterophyllum compressions from five isotaphonomic Late Triassic sedimentary beds of Astartekløft in East Greenland to test whether leaf and cuticle fragments of the two genera can be separated using a range of quantitative and qualitative morphological and statistical analyses. None of the observed characters – including stomatal frequencies – could be applied to separate the fragments of the two genera into well‐defined groups. Our results therefore indicate that fragmented material and dispersed cuticles cannot be utilized to distinguish between Anomozamites or Pterophyllum at the genus level, but that instead these cuticle fragments may be used interchangeably as stomatal proxies. Classification of fossil leaves into either of these genera is thus only possible given adequate preservation of macro‐morphology and is not possible based solely on cuticle morphology. We suggest that this large inter‐ and intra‐generic morphological variation in both leaf and cuticle traits within Anomozamites and Pterophyllum may be related to the bennettites’ role as understory plants, experiencing a range of micro‐environmental conditions, perhaps depending mainly on sun exposure. Based on the results obtained in this study, we conclude that Anomozamites and Pterophyllum cuticle fragments can be employed interchangeably in palaeo [CO2] reconstructions based on the stomatal method, thus potentially annexing a plethora of bennettitalean fossil plant material as CO2 proxies, including dispersed cuticles.

The stomatal CO2 proxy does not saturate at high atmospheric CO2 concentrations: evidence from stomatal index responses of Araucariaceae conifers

The inverse relationship between the number of stomata on a leaf surface and the atmospheric carbon dioxide concentration ([CO(2)]) in which the leaf developed allows plants to optimise water-use efficiency (WUE), but it also permits the use of fossil plants as proxies of palaeoatmospheric [CO(2)]. The ancient conifer family Araucariaceae is often represented in fossil floras and may act as a suitable proxy of palaeo-[CO(2)], yet little is known regarding the stomatal index (SI) responses of extant Araucariaceae to [CO(2)]. Four Araucaria species (Araucaria columnaris, A. heterophylla, A. angustifolia and A. bidwillii) and Agathis australis displayed no significant relationship in SI to [CO(2)] below current ambient levels (~380 ppm). However, representatives of the three extant genera within the Araucariaceae (A. bidwillii, A. australis and Wollemia nobilis) all exhibited significant reductions in SI when grown in atmospheres of elevated [CO(2)] (1,500 ppm). Stomatal conductance was reduced and WUE increased when grown under elevated [CO(2)]. Stomatal pore length did not increase alongside reduced stomatal density (SD) and SI in the three araucariacean conifers when grown at elevated [CO(2)]. These pronounced SD and SI reductions occur at higher [CO(2)] levels than in other species with more recent evolutionary origins, and may reflect an evolutionary legacy of the Araucariaceae in the high [CO(2)] world of the Mesozoic Era. Araucariacean conifers may therefore be suitable stomatal proxies of palaeo-[CO(2)] during periods of "greenhouse" climates and high [CO(2)] in the Earth's history.

Quantitative uncertainty analyses of ancient atmospheric CO2 estimates from fossil leaves

The relationship between atmospheric CO2 and ancient climate is of fundamental importance for gauging the climate sensitivity of the Earth system to a changing CO2 regime. One of the most widely adopted paleobiological CO2 proxies for reconstructing Earth's atmospheric CO2 history exploits the inverse relationship between leaf stomatal index, the fraction of leaf epidermal cells that are stomatal structures, and atmospheric CO2. However, fossil leaf-based CO2 reconstructions make a priori assumptions about the form of the empirical relationship between SI and CO2 required for transfer functions and have failed to correctly propagate error terms. These effects can translate into erroneous interpretations that undermine the value of the proxy. Here we report the development and application of a rigorous generalized statistical framework overcoming these limitations that generates probability density functions for each atmospheric CO2 estimate. The utility of our statistical tools is demonstrated by showing how they revise earlier atmospheric CO2 estimates from fossil cuticles of Ginkgo and Metasequoia trees during the early Eocene and middle Miocene warm periods upwards by +150 to 250 ppm to 450 to 700 ppm. The revised CO2 reconstructions therefore help to resolve the paradox of warm Paleogene and Neogene greenhouse climates co-existing with near present-day levels of CO2 and support the emerging view from independent paleoclimate studies for a high climate sensitivity of the Earth system. The statistical tools presented are sufficiently versatile to permit their use in other investigations of paleoCO2 estimates from fossil leaves.

Closer but not there yet: models in child injury research

Medicine is full of examples of things we measure as proxies for things we can’t measure directly because of unacceptable risk or cost, limitations in technology, or ethical constraints. End-tidal CO2 is a reasonably good proxy for alveolar CO2, unless you have, say, an air embolus. In that case, a second proxy, arterial PCO2, may give you close enough information to effectively guide clinical care. Other proxy measurements are further removed from the actual target of interest. Anesthesiologists wishing to avoid recall in their patients may use a combination of measurement of the percent of end-tidal anesthetic agent concentration, patient vital signs, physiological changes such as sweating, and computer processed electroencephalography to approximate the risk of recall during a given procedure, but none of these proxy measures directly measure likelihood of recall. An indirect, or proxy, measurement has value in proportion to its ability to predict the presence or likelihood of the target event, and most such measurements have inherent limitations. Mathematical models for prediction also appear frequently in medicine. Pharmacokinetic modeling predicts optimal drug dosing, generally with excellent but not perfect results. Mathematical modeling for hydrocephalus has become more sophisticated and takes into account an increasing number of factors but, as yet, has not resulted in perfect treatments for this condition. In their paper in this issue, Coats and Margulies continue a long line of efforts to use anthropomorphic models to answer some of the fundamental questions about loading conditions and prediction of injury in human in fants. This paper has a narrow focus: to use a more refined model of a 6-week-old human infant, with skull and neck properties that more closely replicate those physical features in actual infants, to measure the impact forces and angular decelerations in 3 dimensions from occiput-first drops from low heights onto various surfaces. These “worst-case scenario” data will be incorporated into existing mathematical models that are then validated against witnessed low-height falls analyzed in actual children. In this way the mathematical model can be refined continuously to improve its ability to predict the injuries resulting from documented clinical cases. In this instance, the specific injury type targeted is the prediction of skull fracture. Models, whether in anesthesia, pharmacology, or surgery, are not static. They are constantly refined, tested against other proxy measurements of what is difficult or impossible to measure directly and, hopefully, improved so that they get increasingly closer to a reliable prediction of complex biological phenomena. Although the experience of many clinicians with anthropomorphic models is limited to their training in cardiopulmonary resuscitation or some lay familiarity with crash test dummies, this modeling scheme is used widely in many biomedical applications. In this specific instance, the authors fully acknowledge, and are actively engaged in, the steps necessary to get from anthropomorphic models used for measurement of loading conditions, to an understanding of specific tissue and location injury thresholds, to mathematical modeling, to model validation against a large series of welldocumented witnessed events in human children. Only when this circuit is complete does this approach have a chance of correctly “working backwards” from a history of an event to a prediction of the probability that the witnessed injuries would be likely to arise from a reported mechanism. For those who have been waiting for a model of an infant that correctly predicts all injuries, the wait isn’t over yet. For those who believe that modeling as an approach is fundamentally flawed, a look at how much proxy measurement and modeling in practice that we use and accept daily may be helpful to put models in perspective. In child injury, we’re not there yet. But for those who are interested, this paper gives a glimpse into the steps that get us closer.

Timing and prediction of climate change and hydrological impacts: periodicity in natural variations

Hydrological impacts from climate change are of principal interest to water resource policy-makers and practicing engineers. Predictive climatic models have been extensively investigated to quantify the impacts. Palaeoclmatic investigations, on the other hand, show unequivocal and strong periodicity of climate variations in proxy evidence. Yet how to use the periodicity in future hydroclimatic timing and forecasting has received less attention. This paper examines the periodicity in Pleistocene–Holocene glacial–interglacial events and in modern precipitation records, and discusses a way in which the periodicity is used for hydroclimatic predictions. The analysis, based on published CO2, ΔT (δ2H) and δ18O proxy data of polar ice cores and deep oceanic benthic fossils, shows a periodicity in a ~100, ~40 or 25 kyear duration consistent with Milankovitch orbital regulations during the glacial–interglacial periods. On a fine time scale, millennium and multi-decadal periodicity is observed in high-resolution proxy variations of Greenland ice cores and in instrumental precipitation records of the contiguous USA. A basic periodicity of decadal and multi-decadal changes in ~20 and ~10–15 year duration is apparent in wavelet frequency analysis of both ice core proxy and precipitation data. While the kyear-scale periodicity is found of global prevalence, the millennium and decadal variations vary in space and are region-specific. Based on these findings, a generalized time-downscaling hierarchy of periodicity is proposed as a potential approach for timing and forecasting future hydroclimatic conditions at a resolution relevant to the water resources engineering and management.

Siberian glaciation as a constraint on Permian–Carboniferous CO2 levels

Reconstructions of Phanerozoic CO2 levels have generally relied on geochemical modeling or proxy data. Because the uncertainty inherent in such reconstructions is large enough to be climatically significant, inverse climate modeling may help to constrain paleo-CO2 estimates. In particular, we test the plausibility of this technique by focusing on the climate from 360 to 260 Ma, a time in which the Siberian landmass was in middle to high latitudes, yet had little or no permanent land ice. Our climate model simulations predict a lower limit for CO2—the value beneath which Siberia acquires “excess” ice. Simulations provide little new information for the period in which Siberia was at a relatively low paleoaltitude (360–340 Ma), but model results imply that paleo-CO2 levels had to be greater than 2–4× modern values to be consistent with an apparently ice-free Siberia in the late Permian. These results for the later period in general agree with soil CO2 proxies and the timing of Gondwanan deglaciation, thus providing support for a significant CO2 increase before the end-Permian boundary event. Our technique may be applicable to other time intervals of unipolar glaciation.

Evolution of a Submarine Magmatic-Hydrothermal System: Brothers Volcano, Southern Kermadec Arc, New Zealand

Brothers volcano, which is part of the active Kermadec arc, northeast of New Zealand, forms an elongate edifice 13 km long by 8 km across that strikes northwest-southeast. The volcano has a caldera with a basal diameter of ~3 km and a floor at 1,850 m below sea level, surrounded by 290- to 530-m-high walls. A volcanic cone of dacite rises 350 m from the caldera floor and partially coalesces with the southern caldera wall. Three hydrothermal sites have been located: on the northwest caldera wall, on the southeast caldera wall, and on the dacite cone. Multiple hydrothermal plumes rise ~750 m through the water column upward from the caldera floor, originating from the northwest caldera walls and atop the cone, itself host to three separate vent fields (summit, upper flank, northeast flank). In 1999, the cone site had plumes with relatively high concentrations of gas with a ΔpH of −0.27 relative to seawater (proxy for CO2 + S gases), dissolved H2S up to 4,250 nM, high concentrations of particulate Cu (up to 3.4 nM), total dissolvable Fe (up to 4,720 nM), total dissolvable Mn (up to 260 nM) and Fe/Mn values of 4.4 to 18.2. By 2002, plumes from the summit vent field had much lower particulate Cu (0.3 nM), total dissolvable Fe (175 nM), and Fe/Mn values of 0.8 but similar ΔpH (−0.22) and higher H2S (7,000 nM). The 1999 plume results are consistent with a magmatic fluid component with the concentration of Fe suggesting direct exsolution of a liquid brine, whereas the much lower concentrations of metals but higher overall gas contents in the 2002 plumes likely reflect subsea-floor phase separation. Plumes above the northwest caldera site are chemically distinct, and their compositions have not changed over the same 3-year interval. They have less CO2 (ΔpH of −0.09), no detectable H2S, total dissolved Fe of 955 nM, total dissolved Mn of 150 nM, and Fe/Mn of 6.4. An overall increase in 3He/4He values in the plumes from R/RA = 6.1 in 1999 to 7.2 in 2002 is further consistent with a magmatic pulse perturbing the system. The northwest caldera site is host to at least two large areas (~600 m by at least 50 m) of chimneys and sub-cropping massive sulfide. One deposit is partially buried by sediment near the caldera rim at ~1,450 m, whereas the other crops out along narrow, fault-bounded ledges between ~1,600 and 1,650 m. Camera tows imaged active 1- to 2-m-high black smoker chimneys in the deeper zone together with numerous 1- to 5-m-high inactive spires, abundant sulfide talus, partially buried massive sulfides, and hydrothermally altered volcanic rocks. 210Pb/226Ra dating of one chimney gives an age of 27 ± 6 years; 226Ra/Ba dating of other mineralization indicates ages up to 1,200 years. Formation temperatures derived from Δ34Ssulfate-sulfide mineral pairs are 245° to 295° for the northwest caldera site, 225° to 260°C for the southeast caldera and ~260° to 305°C for the cone. Fluid inclusion gas data suggest subsea-floor phase separation occurred at the northwest caldera site. Alteration minerals identified include silicates, silica polymorphs, sulfates, sulfides, Fe and Mn oxide and/or oxyhydroxides, and native sulfur, which are consistent with precipitation at a range of temperatures from fluids of different compositions. An advanced argillic assemblage of illite + amorphous silica + natroalunite + pyrite + native S at the cone site, the occurrence of chalcocite + covellite + bornite + iss + chalcopyrite + pyrite in sulfide samples from the southeast caldera site, and veins of enargite in a rhyodacitic sample from the northwest caldera site are indicative of high-sulfidation conditions similar to those of subaerial magmatic-hydrothermal systems. The northwest caldera vent site is a long-lived hydrothermal system that is today dominated by evolved sea-water but has had episodic injections of magmatic fluid. The southeast caldera site represents the main upflow of a relatively well established magmatic-hydrothermal system on the sea floor where sulfide-rich chimneys are extant. The cone site is a nascent magmatic-hydrothermal system where crack zones localize upwelling acidic waters. Each of these different vent sites represents diverse parts of an evolving hydrothermal system, any one of which may be typical of submarine volcanic arcs.

Fossil bryophytes as recorders of ancient CO2 levels: Experimental evidence and a Cretaceous case study

Biological and geochemical CO2 proxies provide critical constraints on understanding the role of atmospheric CO2 in driving climate change during Earth history. As no single existing CO2 proxy is without its limitations, there is a clear need for new approaches to reconstructing past CO2 concentrations. Here we develop a new pre‐Quaternary CO2 proxy based on the stable carbon isotope composition (δ13C) of astomatous land plants. In a series of CO2‐controlled laboratory experiments, we show that the carbon isotope discrimination (Δ13C) of a range of bryophyte (liverwort and moss) species increases with atmospheric CO2 across the range 375 to 6000 ppm. Separate experiments establish that variations in growth temperature, water content and substrate type have minor impacts on the Δ13C of liverworts but not mosses, indicating the greater potential of liverworts to faithfully record past variations in CO2. A mechanistic model for calculating past CO2 concentrations from bryophyte Δ13C (White et al., 1994) is extended and calibrated using our experimental results. The potential for fossil liverworts to record past CO2 changes is investigated by analyzing the δ13C of specimens collected from Alexander Island, Antarctica dating to the “greenhouse” world of the mid‐Cretaceous. Our analysis and isotopic model yield mid‐Cretaceous CO2 concentrations of 1000–1400 ppm, in general agreement with independent proxy data and long‐term carbon cycle models. The exceptionally long evolutionary history of bryophytes offers the possibility of reconstructing CO2 concentrations back to the mid‐Ordovician, pre‐dating all currently used quantitative CO2 proxies.

Fossil leaves: Effective bioindicators of ancient CO2 levels?

[1] Past variations in the concentration of the greenhouse gas CO2 are thought to have played a major role in controlling Earth’s climate on preQuaternary and Quaternary timescales. To identify the contribution of CO2 to past climatic change requires accurate quantification of its content in the ancient atmosphere, and a number of proxies have been developed for this purpose (for a review see Royer et al. [2001a]). For the Late Quaternary, there is the unique opportunity to measure directly the composition of fossil air samples trapped in bubbles preserved in the polar ice sheets. Results from Antarctic ice cores reveal that the glacialinterglacial changes characterizing Quaternary climate were accompanied by variations in the atmospheric concentration of CO2 [Petit et al., 1999]. Although detection of phase relations between isotope-derived temperature estimates and trace gas concentration values is hampered by the difference in age of ice and air from the same ice sample, it is believed that CO2 lags glacial-interglacial temperature change and has acted as an amplifier of orbitally forced changes in temperature [Petit et al., 1999; Shackleton, 2000; Mudelsee, 2001].

Hydrothermal venting along Earth's fastest spreading center: East Pacific Rise, 27.5°–32.3°

During March/April 1998 we conducted detailed mapping and sampling of hydrothermal plumes along six segments of Earth's fasting spreading mid‐ocean ridge, 27.5°–32.3°S on the East Pacific Rise. We compared the distribution and chemistry of hydrothermal plumes to geological indicators of long‐term (spreading rate) and moderate‐term (ridge inflation) variations in magmatic budget. In this large‐offset, propagating rift setting, these geological indices span virtually the entire range found along fast spreading ridges worldwide. Hydrothermal plumes overlaid ∼60% of the length of superfast (>130 km/Myr) spreading axis surveyed and defined at least 14 separate vent fields. We observed no plumes over the slower spreading propagating segments. Finer‐scale variations in the magmatic budget also correlated with hydrothermal activity, as the location of the five most intense plumes corresponded to subsegment peaks in ridge inflation. Along the entire ridge crest, the more inflated a ridge location the more likely it was to be overlain by a hydrothermal plume. Plume chemistry mostly reflected discharge from mature vent fields apparently unperturbed by magmatic activity within the last few years. Plume samples with high volatile/metal ratios, generally indicating recent seafloor volcanism, were scarce. Along‐axis trends in both volatile (3He; CH4; ΔpH, a proxy for CO2; and particulate S) and nonvolatile (Fe, Mn) species showed a first‐order agreement with the trend of ridge inflation. Nevertheless, a broad correspondence between the concentration of volatile species in plumes and geological proxies of magma supply identifies a pervasive magmatic imprint on this superfast spreading group of ridge segments.

Re-examining the reliability of tree-ring isotope ratio as a historical CO2 proxy

To examine the reliability of using tree ring δ13C and the model of isotopic fractionation in reconstructing atmospheric CO2 levels, we studied the variations of some important parameters of several subtropical species under natural field conditions. It was found that, different from other researchers’ results, leaf conductance to CO2 transfer,g, did not change in proportion to the change in rate of CO2 assimilation,A, with the result that intercellular concentration of CO2, Ci, could not keep constant. Thus, we conclude that the use of tree-ring isotope ratios in the reconstruction of atmospheric CO2 variation based on the presupposition that Ci keeps constant during assimilation is not reliable under current circumstances.

Influence of atmospheric CO2 on the decline of C4 plants during the last deglaciation

CHANGES in atmospheric carbon dioxide concentrations in the past may have caused changes in vegetation type1,2, and it has been suggested2 that the isotopic signature of such vegetation shifts, preserved in palaeosols3, might be used as a proxy for past CO2 variations. But the connection between palaeosol isotopic signatures and atmospheric CO2 concentrations has been difficult to establish, partly because of the unreliability of CO2 proxies and partly because of the difficulty in ruling out other potential causes of vegetation changes, such as climate4. Here we present palaeosol carbon isotope ratios that reveal a shift from C4-dominated grasses to C3-dominated shrubs about 7–9 kyr ago on an alluvial fan system in the Chihuahuan desert, New Mexico. This coincides with a rapid increase in atmospheric CO2 concentration recorded in Antarctic ice cores5–8 and increased aridity recorded by geo-morphic reconstructions9–11 and packrat remains12. Palaeosol oxygen isotope ratios, which depend on temperature and moisture, were relatively constant during the vegetation shift, suggesting that the CO2 change, rather than climate, was the dominant cause. We conclude that the carbon isotope ratios of ancient soils can indeed be used as a proxy for past CO2 changes.