Composition Of Atmospheric CO2 Research Articles

Continuous Measurements of Temporal and Vertical Variations in Atmospheric CO2 and Its δ13C in and above a Subtropical Plantation

Atmospheric CO2 dynamics in forest ecosystems are dependent on interactions between photosynthesis, respiration, and turbulent mixing processes; however, the carbon isotopic composition of atmospheric CO2 (δ13C) is not well established due to limited measurement reports. In this study, a seven-inlet profile system with a Picarro analyzer was developed to conduct continuous in situ measurements of CO2 and its δ13C in and above a subtropical plantation from 2015 to 2017. Results showed that ecosystem CO2 concentration was the lowest in the afternoon and reached its peak at dawn, which mirrored variations in its δ13C in and above the canopy. Inverse seasonal variations were apparent between CO2 and its δ13C in and above the canopy, and δ13C was positive during the peak growing season and negative at other times. Diel and seasonal variations in ecosystem CO2 and its δ13C were mainly affected by the vapor pressure deficit, followed by photosynthetic active radiation, temperature, and the enhanced vegetation index in and above the canopy; however, environmental and physiological factors had reverse or no effects near the forest floor. Nocturnal gradients of vertical variations in atmospheric CO2 and its δ13C were greater than diurnal variations due to weak turbulent mixing under more stable atmospheric conditions overnight. These results implicate that photosynthesis and respiration dominated CO2 dynamics above the canopy, while CO2 recycling by photosynthesis and turbulent mixing changed CO2 dynamics in the canopy.

Human Activity Recorded in Carbon Isotopic Composition of Atmospheric CO2 in Gliwice Urban Area and Surroundings (Southern Poland) in the Years 2011–2013

ABSTRACTIn this paper, a record of the 14C and 13C isotope content of atmospheric CO2 for Gliwice is presented for samples collected on a weekly basis in the years 2011–2013. In addition, measurements were performed on the early and late wood from the annual rings of pine trees from five sites located 3–6 km from the atmospheric CO2 sampling point. The concentration of 14C in CO2 samples from the air was much lower relative to the concentration of this isotope in “clean air,” indicating a pronounce Suess effect, with a mean Δ14C lower by ca. 60‰ than Jungfraujoch data when the 15% of the highest differences are excluded, which leads to the FFCO2 estimate of 5.8%. In winter, the main source of fossil CO2 was fuel combustion, as confirmed by significant correlations with air pollutants. In the vegetation seasons, the Δ14C was highly variable due to biogenic influence and more variable winds. The isotopic results were also affected by an additional significant CO2 source for the Gliwice air, which was a closed mine shaft. The Δ14C and δ13C in tree rings did not record a strong Suess effect in the years 2008–2013 in woodland areas around Gliwice city.

Potential of global croplands and bioenergy crops for climate change mitigation through deployment for enhanced weathering

Conventional row crop agriculture for both food and fuel is a source of carbon dioxide (CO2) and nitrous oxide (N2O) to the atmosphere, and intensifying production on agricultural land increases the potential for soil C loss and soil acidification due to fertilizer use. Enhanced weathering (EW) in agricultural soils-applying crushed silicate rock as a soil amendment-is a method for combating global climate change while increasing nutrient availability to plants. EW uses land that is already producing food and fuel to sequester carbon (C), and reduces N2O loss through pH buffering. As biofuel use increases, EW in bioenergy crops offers the opportunity to sequester CO2 while reducing fossil fuel combustion. Uncertainties remain in the long-term effects and global implications of large-scale efforts to directly manipulate Earth's atmospheric CO2 composition, but EW in agricultural lands is an opportunity to employ these soils to sequester atmospheric C while benefitting crop production and the global climate.

Collapse of the tropical and subtropical North Atlantic CO2 sink in boreal spring of 2010

Following the 2009 Pacific El Niño, a warm event developed in the tropical and subtropical North Atlantic during boreal spring of 2010 promoted a significant increase in the CO2 fugacity of surface waters. This, together with the relaxation of the prevailing wind fields, resulted in the reversal of the atmospheric CO2 absorption capacity of the tropical and subtropical North Atlantic. In the region 0–30°N, 62–10°W, this climatic event led to the reversal of the climatological CO2 sink of −29.3 Tg C to a source of CO2 to the atmosphere of 1.6 Tg C from February to May. The highest impact of this event is verified in the region of the North Equatorial Current, where the climatological CO2 uptake of −22.4 Tg for that period ceased during 2010 (1.2 Tg C). This estimate is higher than current assessments of the multidecadal variability of the sea-air CO2 exchange for the entire North Atlantic (20 Tg year−1), and highlights the potential impact of the increasing occurrence of extreme climate events over the oceanic CO2 sink and atmospheric CO2 composition.

Evolution of the stable carbon isotope composition of atmospheric CO2 over the last glacial cycle

AbstractWe present new δ13C measurements of atmospheric CO2 covering the last glacial/interglacial cycle, complementing previous records covering Terminations I and II. Most prominent in the new record is a significant depletion in δ13C(atm) of 0.5‰ occurring during marine isotope stage (MIS) 4, followed by an enrichment of the same magnitude at the beginning of MIS 3. Such a significant excursion in the record is otherwise only observed at glacial terminations, suggesting that similar processes were at play, such as changing sea surface temperatures, changes in marine biological export in the Southern Ocean (SO) due to variations in aeolian iron fluxes, changes in the Atlantic meridional overturning circulation, upwelling of deep water in the SO, and long‐term trends in terrestrial carbon storage. Based on previous modeling studies, we propose constraints on some of these processes during specific time intervals. The decrease in δ13C(atm) at the end of MIS 4 starting approximately 64 kyr B.P. was accompanied by increasing [CO2]. This period is also marked by a decrease in aeolian iron flux to the SO, followed by an increase in SO upwelling during Heinrich event 6, indicating that it is likely that a large amount of δ13C‐depleted carbon was transferred to the deep oceans previously, i.e., at the onset of MIS 4. Apart from the upwelling event at the end of MIS 4 (and potentially smaller events during Heinrich events in MIS 3), upwelling of deep water in the SO remained reduced until the last glacial termination, whereupon a second pulse of isotopically light carbon was released into the atmosphere.

The response of the18O/16O composition of atmospheric CO2to changes in environmental conditions

[1] This study investigates the response of the global mean and spatial variations of the δ18O value of atmospheric CO2 (δCa) to changes in soil CO2 hydration rates, relative humidity, the δ18O value of precipitation and water vapor, visible radiation, temperature, and ecosystem flux partitioning. A three-dimensional global transport model was coupled to a mechanistic land surface model and was used to calculate isotopic fluxes of CO2 and H2O and the resulting δCa. The model reproduced the observed global mean and north-south gradient in δCa. The simulated seasonal amplitude and phases of CO2 and δCa agreed well at some but not all locations. Sensitivity tests with relative humidity increased by 3.2% from its original value decreased δCa by 0.21‰. Similarly, a global 3.3‰ decrease in the isotopic composition of both precipitation and water vapor (δWP and δWAV, respectively) caused a 2.6‰ decrease in δCa. A 1 K increase in atmospheric temperatures also affected δCa, but there was a very small δCa response to realistic changes in light levels. Experiments where leaf and soil CO2 fluxes were repartitioned revealed a nontrivial change to δCa. The predicted north-south δCa gradient increased in response to an increase in soil CO2 hydration rates. However, the δCa gradient also had a large response to global changes in δWP and δWAV. This result is particularly important since most models fail to deplete δWP enough at middle and high latitudes, where the influence of δWP and δWAV on the δCa gradient is strongest.

Evidence for atmospheric carbon injection during the end-Permian extinction

The end-Permian mass extinction is marked by pronounced terrestrial ecosystem turnover and a severe loss of marine invertebrate biodiversity. This extinction event is accompanied by a prominent negative carbon-isotope excursion indicating massive changes in the global carbon cycle across the Permian-Triassic boundary. In this study, we present organic carbon-isotope data from land plant cuticles, fossil wood fragments, and bulk organic matter recovered from the Amb section in the Salt Range, Pakistan. We apply δ 13 C data from cuticles as a proxy record for the carbon-isotope composition of atmospheric CO 2 across the Permian-Triassic boundary. The data show an ∼5.5‰ negative excursion in terrestrial organic matter, reflecting the change in carbon-isotope composition of atmospheric CO 2 . Our data demonstrate that these atmospheric changes coincide with biotic (mass extinction) and abiotic (carbonate carbon-isotope perturbation) changes in the marine realm, hence affecting the entire ocean-atmosphere system.

Photosynthetic carbon isotope discrimination and its relationship to the carbon isotope signals of stem, soil and ecosystem respiration

• Photosynthetic carbon (C) isotope discrimination (Δ(Α)) labels photosynthates (δ(A) ) and atmospheric CO(2) (δ(a)) with variable C isotope compositions during fluctuating environmental conditions. In this context, the C isotope composition of respired CO(2) within ecosystems is often hypothesized to vary temporally with Δ(Α). • We investigated the relationship between Δ(Α) and the C isotope signals from stem (δ(W)), soil (δ(S)) and ecosystem (δ(E)) respired CO(2) to environmental fluctuations, using novel tuneable diode laser absorption spectrometer instrumentation in a mature maritime pine forest. • Broad seasonal changes in Δ(Α) were reflected in δ(W,) δ(S) and δ(E). However, respired CO(2) signals had smaller short-term variations than Δ(A) and were offset and delayed by 2-10 d, indicating fractionation and isotopic mixing in a large C pool. Variations in δ(S) did not follow Δ(A) at all times, especially during rainy periods and when there is a strong demand for C allocation above ground. • It is likely that future isotope-enabled vegetation models will need to develop transfer functions that can account for these phenomena in order to interpret and predict the isotopic impact of biosphere gas exchange on the C isotope composition of atmospheric CO(2).

Wetting and drying cycles drive variations in the stable carbon isotope ratio of respired carbon dioxide in semi-arid grassland

In semi-arid regions, where plants using both C(3) and C(4) photosynthetic pathways are common, the stable C isotope ratio (delta(13)C) of ecosystem respiration (delta(13)C(R)) is strongly variable seasonally and inter-annually. Improved understanding of physiological and environmental controls over these variations will improve C cycle models that rely on the isotopic composition of atmospheric CO(2). We hypothesized that timing of precipitation events and antecedent moisture interact with activity of C(3) and C(4) grasses to determine net ecosystem CO(2) exchange (NEE) and delta(13)C(R). Field measurements included CO(2) and delta(13)C fluxes from the whole ecosystem and from patches of different plant communities, biomass and delta(13)C of plants and soils over the 2000 and 2001 growing seasons. NEE shifted from C source to sink in response to rainfall events, but this shift occurred after a time lag of up to 2 weeks if a dry period preceded the rainfall. The seasonal average of delta(13)C(R) was higher in 2000 (-16 per thousand) than 2001 (20 per thousand), probably due to drier conditions during the 2000 growing season (79.7 mm of precipitation from April up to and including July) than in 2001 (189 mm). During moist conditions, delta(13)C averaged -22 per thousand from C(3) patches, -16 per thousand from C(4) patches, and -19 per thousand from mixed C(3) and C(4) patches. However, during dry conditions the apparent spatial differences were not obvious, suggesting reduced autotrophic activity in C(4) grasses with shallow rooting depth, soon after the onset of dry conditions. Air and soil temperatures were negatively correlated with delta(13)C(R); vapor pressure deficit was a poor predictor of delta(13)C(R), in contrast to more mesic ecosystems. Responses of respiration components to precipitation pulses were explained by differences in soil moisture thresholds between C(3) and C(4) species. Stable isotopic composition of respiration in semi-arid ecosystems is more temporally and spatially variable than in mesic ecosystems owing to dynamic aspects of pulse precipitation episodes and biological drivers.

C4 photosynthetic isotope exchange in NAD-ME- and NADP-ME-type grasses

Monitoring photosynthetic isotope exchange is an important tool for predicting the influence of plant communities on the global carbon cycle in response to climate change. C(4) grasses play an important role in the global carbon cycle, but their contribution to the isotopic composition of atmospheric CO(2) is not well understood. Instantaneous measurements of (13)CO(2) (Delta(13)C) and C(18)OO (Delta(18)O) isotope exchange in five NAD-ME and seven NADP-ME C(4) grasses have been conducted to investigate the difference in photosynthetic CO(2) isotopic fractionation in these subgroups. As previously reported, the isotope composition of the leaf material (delta(13)C) was depleted in (13)C in the NAD-ME compared with the NADP-ME grasses. However, Delta(13)C was not different between subtypes at high light, and, although Delta(13)C increased at low light, it did so similarly in both subtypes. This suggests that differences in leaf delta(13)C between the C(4) subtypes are not caused by photosynthetic isotope fractionation and leaf delta(13)C is not a good indicator of bundle sheath leakiness. Additionally, low carbonic anhydrase (CA) in C(4) grasses may influences Delta(13)C and should be considered when estimating the contribution of C(4) grasses to the global isotopic signature of atmospheric CO(2). It was found that measured Delta(18)O values were lower than those predicted from leaf CA activities and Delta(18)O was similar in all species measured. The Delta(18)O in these C(4) grasses is similar to low Delta(18)O previously measured in C(4) dicots which contain 2.5 times the leaf CA activity, suggesting that leaf CA activity is not a predictor of Delta(18)O in C(4) plants.

Anthropogenic emissions of CO 2 and CH 4 in an urban environment

Regular observations of atmospheric mixing-ratios of carbon dioxide and methane in the urban atmosphere, combined with analyses of their carbon-isotope composition (δ 13C, δ 14C), provide a powerful tool for assessing both the source strength and source partitioning of those gases, as well as their changes with respect to time. Intense surface fluxes of CO 2 and CH 4, associated with anthropogenic activities result in elevated levels of these gases in the local atmosphere as well as in modifications of their carbon-isotope compositions. Regular measurements of concentration and carbon-isotope composition of atmospheric CO 2, carried out in Krakow over the past two decades, were extended to the period 1995–2000 and also to atmospheric mixing-ratios of CH 4 and its carbon-isotope composition. Radiocarbon concentrations (δ 14C) in atmospheric CO 2 recorded at Krakow are systematically lower than the regional background levels. This effect stems from the addition of 14C-free CO 2 into the local atmosphere, originating from the burning of fossil fuels. The fossil-fuel component in the local budget of atmospheric carbon calculated using a three-component mixing model decreased from ca. 27.5 ppm in 1989 to ca. 10 ppm in 1994. The seasonal fluctuations of this component (winter–summer) are of similar magnitude. A gradually decreasing difference between the 14CO 2 content in the local atmosphere and the regional background observed after 1991 is attributed to the reduced consumption of 14C-free fuels, mostly coal, in southern Poland and the Krakow municipal area. The linear regression of δ 13C values of methane plotted versus reciprocal concentration, performed for the data available for Krakow sampling site, yields the average δ 13C signature of the local source of methane as being equal to −54.2‰. This value agrees very well with the measured isotope signature of natural gas being used in Krakow (−54.4±0.6‰) and points to leakages in the distribution network of this gas as the main anthropogenic source of CH 4 in the local atmosphere.

The contribution of C3 and C4 plants to the carbon cycle of a tallgrass prairie: an isotopic approach.

The photosynthetic pathway composition (C(3):C(4) mixture) of an ecosystem is an important controller of carbon exchanges and surface energy flux partitioning, and therefore represents a fundamental ecophysiological distinction. To assess photosynthetic mixtures at a tallgrass prairie pasture in Oklahoma, we collected nighttime above-canopy air samples along concentration and isotopic gradients throughout the 1999 and 2000 growing seasons. We analyzed these samples for their CO(2) concentration and carbon isotopic composition and calculated C(3):C(4) proportions with a two-source mixing model. In 1999, the C(4) percentage increased from 38% in spring (late April) to 86% in early fall (mid-September). The C(4) percentages inferred from ecosystem respiration measurements in 2000 indicate a smaller shift, from 67% in spring (early May) to 77% in mid-summer (late July). We also sampled daytime CO(2 )concentration and carbon isotope gradients above the canopy to determine ecosystem discrimination against (13)CO(2) during net uptake. These discrimination values were always lower than corresponding nighttime ecosystem respiration isotopic signatures would suggest. After accounting for the isotopic disequilibria between respiration and photosynthesis resulting from seasonal variations in the C(3):C(4) mixture, we estimated canopy photosynthetic discrimination. The C(4) percentage calculated from this approach agrees with the percentage determined from nighttime respiration for sampling periods in both growing seasons. Isotopic imbalances between photosynthesis and respiration are likely to be common in mixed C(3):C(4 )ecosystems and must be considered when using daytime isotopic measurements to constrain ecosystem physiology. Given the global extent of such ecosystems, isotopic imbalances likely contribute to global variations in the carbon isotopic composition of atmospheric CO(2).

Using <sup>14</sup>C, <sup>13</sup>C, <sup>18</sup>O and <sup>17</sup>O isotopic variations to provide insights into the high northern latitude surface CO inventory

Abstract. Measurements of the complete isotopic composition of atmospheric CO (3CO, 14CO, C17O, C18O) have been carried out at the high northern latitude stations Spitsbergen, Norway, and Alert, Canada. The annual changes of the isotope signatures reflect the seasonally varying contributions from the individual CO sources and the OH sink. Short-term variability is small at the remote sampling locations. Nevertheless, the interannual variability is considerable, in particular for the summer minimum. The most prominent event was a strong increase in CO in 1998 that persisted for several months. Using the isotope signatures it is possible to clearly identify extraordinarily strong biomass burning during that season as the cause for this large-scale CO anomaly. In 1997, on the other hand, biomass burning emissions were very low, leading to an unusually low summer minimum and corresponding isotope signatures. The results underscore that monitoring of CO and its isotopic composition at remote high latitude stations is a valuable tool to better understand long-term variations of CO that are representative for the whole high northern latitude region.

The isotopic composition of soil and soil-respired CO 2

The isotopic composition of soil CO 2 has been investigated for the past three decades by earth scientists in a variety of disciplines. Carbon dioxide is produced in soil through biological processes (organic matter decomposition and root respiration) and is transported to the overlying atmosphere via molecular diffusion. Research has focused on both the isotopic composition of respired CO 2 (CO 2 that diffuses across the soil–atmosphere interface) and soil CO 2 (CO 2 that exists at any given soil depth). It is recognized that most soils are at quasi-steady state, and that the C isotopic composition of respired CO 2 is the same as that of the biological sources in the soil. Research on the isotopic composition of soil CO 2 has successfully used steady-state production/diffusion models to explain the common observation that the isotopic composition of soil CO 2 generally shows a gradual change from atmospheric isotopic values at the soil surface to values closer to biological sources with increasing depth. The complexity of the production/diffusion models needed to describe soil CO 2 is simplest with stable C isotopes, and increases in complexity and uncertainty with radiocarbon and O isotopes. For radiocarbon and O isotopes this is due to the fact that there can be a variety of isotopic sources within a given soil. Furthermore, for O isotopes, equilibration processes can alter the composition of the CO 2 as it diffuses through the soil. While much remains to be learned about the natural variations in the isotopic composition of soil and respired CO 2, interest in this topic will likely increase due to the magnitude of soil respiration on a global scale, and its role in the processes controlling the isotopic composition of atmospheric CO 2.

Monitoring the isotopic composition of atmospheric CO2: Measurements from the NOAA Global Air Sampling Network

The stable isotopic composition of atmospheric CO2 is being monitored via measurements made at the University of Colorado‐Institute of Arctic and Alpine Research, using air samples collected weekly by the Global Air Sampling Network of the NOAA Climate Monitoring and Diagnostics Laboratory. These measurements, in concert with the monitoring of atmospheric CO2 mixing ratios, offer the potential to characterize quantitatively the mechanisms operating in the global carbon cycle, by recording the isotopic signatures imparted to CO2 as it moves among the atmosphere, biosphere, and oceans. This data set increases the number of measurements of atmospheric CO2 isotopes by nearly an order of magnitude over those previously available. We describe the analytical techniques used to obtain and calibrate these data and report measurements from 25 land‐based sites, and two ships in the Pacific Ocean, from samples collected during 1990–1993. The typical precision of our mass spectrometric technique is 0.03‰ for δ13C and 0.05‰ for δ18O. Collecting the flask samples without drying leads to loss of δ18O information at many sites. The seasonal cycle in δ13C at sites in the northern hemisphere is highly correlated with that of the CO2 mixing ratio, with amplitudes approaching 1‰ at high latitudes. The seasonal cycle in δ18O is of similar amplitude, though variable from year to year and lags the other species by 2–4 months. Interhemispheric differences of the 1992 and 1993 means of the isotopic tracers are in strong contrast: the north pole‐south pole difference for δ13C is −0.20‰, which though highly quantitatively significant is dwarfed by the −2‰ difference for δ18O. In contrast to the record of atmospheric δ13C during the 1980s we observe no significant temporal trend in annual mean δ13C during 1990–1993.

Speleothems as proxy for the carbon isotope composition of atmospheric CO2

We have measured the stable isotope ratios of carbon in a suite of recent cave deposits (<200 years) from the San Saba County, Texas, USA. The methodology for dating these deposits using excess 210Pb was recently established [Baskaran and Iliffe, 1993]. The carbon isotope ratios of these samples, spanning the time period ∼1800–1990 AD, reflect the carbon isotope ratio of atmospheric CO2 for the same period. The pathways by which the δ13C of atmospheric CO2 is imprinted on these speleothems can be explained using a model developed by Cerling (1984). The results suggest that the carbon isotope ratios of speleothems can be used to develop long‐term, high‐resolution chronologies of the δ13C of atmospheric CO2 and, by implication, the concentration of the atmospheric CO2.

Concentration and carbon isotopic composition of atmospheric CO<sub>2</sub> in southern Poland*

The isotopic composition of atmospheric CO 2 and its concentration in the Krakow region has been measured since 1983. Air samples have been collected in bi-weekly intervals close to the center of Krakow. The six years records (1983–88) of these data reveal multi-annual linear trends and winter-summer oscillations. The calculated parameters of the individual regression lines (intercept and slope) for the measured Δ 14 C, δ 13 C and CO 2 concentration, C, are: 173‰ (January 1983) and − 10.7‰ yr -1 , − 9.58‰ (January 1983) and – 0.038‰ yr -1 , 371 ppmv (January 1983) and 1.0 ppmv yr -1 , respectively. The 14C activity in tree leaves collected in a mountain region, about 60 km from Krakow was also assessed. The parameters of the linear decrease of 14 C activity in tree leaves are: 239‰ (January 1983) and 21‰ yr -1 . DOI: 10.1034/j.1600-0889.1991.t01-4-00003.x

The isotopic composition of atmospheric carbon monoxide

The concentration and the carbon and oxygen isotopic composition of atmospheric CO of the northern hemisphere show regular seasonal variations. The isotopic pattern is different from that for CO from automobile engine combustion, whether compared to combustion in the same region as the air sample or to an estimated world average. These results indicate that there are several natural sources, one of which is much greater than anthropogenic emissions. The most likely nature of these sources are discussed and the production rates estimated.

The effects of atmospheric NO 2 on the measured isotopic composition of atmospheric CO 2

The consequences of the effects of the slight concentration of N/sub 2/O in the atmosphere in the isotopic studies of CO/sub 2/ are discussed. N/sub 2/O has the same molecular masses as CO/sub 2/ (44, 45, 46), and being isoteric with CO/sub 2/, has almost exactly the same physical properties, including vapor pressure. Thus N/sub 2/O is condensed in liquid N/sub 2/ during removal of CO/ sub 2/ from air, follows the CO/sub 2/ through purification by fractional distillation, and enters the mass spectrometer as a contaminant with identical mass numbers but quite a different isotope ratio. Indicated corrections to be added to the measured isotopic delta values of atmospheric CO/sub 2/ were determined to be +0.6% for C/sup 13/ and +0.8% for O/sup 18/. (P.C.H.)