Land biospheric carbon exchange associated with respiration and photosynthesis exerts a major control on the oxygen isotope composition (δ
18
O) of atmospheric CO
2
especially with respect to the ...seasonal cycle. In particular, an important feature that requires our attention is the phase of the seasonal cycle of δ18O which lags CO
2
by one month in the Arctic. We have developed a global parameterization of the land biotic exchange of
18
0 in CO
2
, which has been prescribed in an atmospheric 3-D transport model in order to simulate the global atmospheric distribution of δ18O. Furthermore, we have separated in the model the specific contribution of different regions of the globe to the seasonal and latitudinal variation of δ
18
O. The model simulated values are compared in detail with atmospheric observations made at 22 different remote stations. The respective role of respiration vs. photosynthesis in determining the phase and amplitude of the δ
18
O seasonal cycle is also analysed. Based on a good agreement between our model simulation and the atmospheric observations, we observe that the large seasonal cycle of δ
18
O at high latitudes is mainly due to the respiratory fluxes of all extra-tropical ecosystems while for CO
2
the relative contributions of photosynthesis and respiration to the overall seasonal cycle are similar. Geographically, the CO
2
exchanges with the northern Siberian ecosystem dominate the δ
18
O seasonality at all remote stations of the northern hemisphere, reflecting the strongly continental climate of that region.
Spatial and temporal variations of atmospheric CO$_2$ concentrations contain information about surface sources and sinks, which can be quantitatively interpreted through tracer transport inversion. ...Previous CO$_2$ inversion calculations obtained differing results due to different data, methods and transport models used. To isolate the sources of uncertainty, we have conducted a set of annual mean inversion experiments in which 17 different transport models or model variants were used to calculate regional carbon sources and sinks from the same data with a standardized method. Simulated transport is a significant source of uncertainty in these calculations, particularly in the response to prescribed "background" fluxes due to fossil fuel combustion, a balanced terrestrial biosphere, and air-sea gas exchange. Individual model-estimated fluxes are often a direct reflection of their response to these background fluxes. Models that generate strong surface maxima near background exchange locations tend to require larger uptake near those locations. Models with weak surface maxima tend to have less uptake in those same regions but may infer small sources downwind. In some cases, individual model flux estimates cannot be analyzed through simple relationships to background flux responses but are likely due to local transport differences or particular responses at individual CO$_2$ observing locations. The response to the background biosphere exchange generates the greatest variation in the estimated fluxes, particularly over land in the Northern Hemisphere. More observational data in the tropical regions may help in both lowering the uncertain tropical land flux uncertainties and constraining the northern land estimates because of compensation between these two broad regions in the inversion. More optimistically, examination of the model-mean retrieved fluxes indicates a general insensitivity to the prior fluxes and the prior flux uncertainties. Less uptake in the Southern Ocean than implied by oceanographic observations, and an evenly distributed northern land sink, remain in spite of changes in this aspect of the inversion setup.
Techniques for inverse modeling using atmospheric tracer transport models are able to provide important information concerning regional carbon sources and sinks based on analyses of observed ...atmospheric carbon dioxide concentrations. Estimates of surface-atmosphere CO sub(2) fluxes are compared from an intercomparison of diverse atmospheric CO sub(2) inversion models. The systems considered in the project include 16 transport models and model variants. Findings from the project indicated that the uptake of CO sub(2) in the southern extratropical ocean is less than the value determined using ocean measurement data. This result was not sensitive to transport models of methodological techniques. Evidence of a northern land C sink distributed across the continents of the Northern Hemisphere is considered.
In this study, using a three‐dimensional (3‐D) tracer modeling approach, we simulate the δ18O of atmospheric CO2. In the atmospheric transport model TM2 we prescribe the surface fluxes of 18O due to ...vegetation and soils, ocean exchange, fossil emissions, and biomass burning. The model simulations are first discussed for each reservoir separately, then all the reservoirs are combined to allow a comparison with the atmospheric δ18O measurements made by the National Oceanic and Atmospheric Administration‐University of Colorado, Scripps Institution of Oceanography‐Centrum Voor Isotopen Onderzoek (United States‐Netherlands) and Commonwealth Scientific and Industrial Research Organisation (Australia) air sampling programs. Insights into the latitudinal differences and into the seasonal cycle of δ18O in CO2 are gained by looking at the contribution of each source. The isotopic exchange with soils induces a large isotopic depletion over the northern hemisphere continents, which overcomes the concurrent effect of isotopic enrichment due to leaf exchange. Compared to the land biota, the ocean fluxes and the anthropogenic CO2 source have a relatively minor influence. The shape of the latitudinal profile in δ18O appears determined primarily by the respiration of the land biota, which balances photosynthetic uptake over the course of a year. Additional information on the phasing of the terrestrial carbon exchange comes from the seasonal cycle of δ18O at high northern latitudes.
Concentrations of dissolved organic carbon (DOC) and particulate organic carbon (POC) were measured in an alpine and a subalpine lake over a 4-year period. Spring snowmelt entered the alpine lake as ...shallow groundwater, while 90 per cent of the subalpine drainage area was upstream and incoming snowmelt was channelized. There was a strong statistical correlation between DOC and dissolved aluminium during spring snowmelt in the alpine (but not the subalpine) lake, and stable carbon isotope ratios from fulvic acids suggested that DOC originated mainly from soils during this period. DOC contributed more than 75 per cent of the organic matter content of the subalpine lake where POC concentrations were correlated with chlorophyll-a only after snowmelt. In the alpine lake POC was a major constituent of organic matter, and was related to phytoplankton abundance throughout the year. The source of organic carbon was predominantly autochthonous in the alpine lake and allochthonous in the subalpine lake, and stable carbon isotope ratios showed that phytoplankton was an important source of DOC for both lakes in winter while terrestrial sources increased during snowmelt and summer. Possible mechanisms for the removal of organic material from the lakes are discussed. There are 55 references.
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