Tsung Hung Peng

Fate of fossil fuel carbon dioxide and the global carbon budget

The fate of fossil fuel carbon dioxide released into the atmosphere depends on the exchange rates of carbon between the atmosphere and three major carbon reservoirs, namely, the oceans, shallow-water sediments, and the terrestrial biosphere. Various assumptions and models used to estimate the global carbon budget for the last 20 years are reviewed and evaluated. Several versions of recent atmosphere-ocean models appear to give reliable and mutually consistent estimates for carbon dioxide uptake by the oceans. On the other hand, there is no compelling evidence which establishes that the terrestrial biomass has decreased at a rate comparable to that of fossil fuel combustion over the last two decades, as has been recently claimed.

Effect of elevated CO2 on the community metabolism of an experimental coral reef

[1]xa0The effect of elevated pCO2 on the metabolism of a coral reef community dominated by macroalgae has been investigated utilizing the large 2650 m3 coral reef mesocosm at the Biosphere-2 facility near Tucson, Arizona. The carbonate chemistry of the water was manipulated to simulate present-day and a doubled CO2 future condition. Each experiment consisted of a 1–2 month preconditioning period followed by a 7–9 day observational period. The pCO2 was 404 ± 63 μatm during the present-day pCO2 experiment and 658 ± 59 μatm during the elevated pCO2 experiment. Nutrient levels were low and typical of natural reefs waters (NO3− 0.5–0.9 μM, NH4+ 0.4 μM, PO43− 0.07–0.09 μM). The temperature and salinity of the water were held constant at 26.5 ± 0.2°C and 34.4 ± 0.2 ppt. Photosynthetically available irradiance was 10 ± 2 during the present-day experiment and 7.4 ± 0.5 mol photons m−2 d−1 during the elevated pCO2 experiment. The primary producer biomass in the mesocosm was dominated by four species of macroalgae; Haptilon cubense, Amphiroa fragillisima, Gelidiopsis intricata and Chondria dasyphylla. Algal biomass was 10.4 mol C m−2 during the present-day and 8.7 mol C m−2 and during the elevated pCO2 experiments. As previously observed, the increase in pCO2 resulted in a decrease in calcification from 0.041 ± 0.007 to 0.006 ± 0.003 mol CaCO3 m−2 d−1. Net community production (NCP) and dark respiration did not change in response to elevated pCO2. Light respiration measured by a new radiocarbon isotope dilution method exceeded dark respiration by a factor of 1.2 ± 0.3 to 2.1 ± 0.4 on a daily basis and by 2.2 ± 0.6 to 3.9 ± 0.8 on an hourly basis. The 1.8-fold increase with increasing pCO2 indicates that the enhanced respiration in the light was not due to photorespiration. Gross production (GPP) computed as the sum of NCP plus daily respiration (light + dark) increased significantly (0.24 ± 0.03 vs. 0.32 ± 0.04 mol C m−2 d−1). However, the conventional calculation of GPP based on the assumption that respiration in the light proceeds at the same rate as the dark underestimated the true rate of GPP by 41–100% and completely missed the increased rate of carbon cycling due to elevated pCO2. We conclude that under natural, undisturbed, nutrient-limited conditions elevated CO2 depresses calcification, stimulates the rate of turnover of organic carbon, particularly in the light, but has no effect on net organic production. The hypothesis that an increase pCO2 would produce an increase in net production that would counterbalance the effect of decreasing saturation state on calcification is not supported by these data.

Rates of benthic mixing in deep-sea sediment as determined by radioactive tracers

A series of closely spaced radiocarbon measurements on a carbonate-rich box core from the western equatorial Pacific show a mixed layer at least 7 cm thick, with 14C ages between 4000 and 5000 years, and an orderly progression of ages below this layer, indicating an average sedimentation rate of about 2 cm/103 yr. The profile can be simulated using a numerical extension of the mixing model of Guinasso and Schink (1975) and a numerical exponential mixing model. The best-fit iteration indicates an apparent mixing coefficient of K = 120 cm2/103 yr which also fits well the excess 210Pb distribution. The best-fit also indicates that a small amount of sediment was lost on the top, and that there was a reduction in sedimentation rate within the early Holocene.

Carbon cycle; 1985 glacial to interglacial changes in the operation of the global carbon cycle.

The hottest topic for those interested in the earths carbon cycles is the change in atmospheric CO/sub 2/ content between glacial and interglacial time. What caused it. What is its role in glacial cycles. The authors evaluate here the hypotheses that have been put forward to explain the CO/sub 2/ change with evidence from deep sea sediments. They conclude that all the hypotheses have serious drawbacks and that much effort will have to be expended in gathering more data from ice-cores and ocean sediments before they will be pointed toward the correct scenario. Also, thoughtful modeling aimed at depicting the ties between pCO/sub 2/, O/sub 2/, /sup 13/C//sup 12/C, /sup 14/C//sup 12/, and nutrient constituents in the sea for various modes of circulation will have to do done before the evidence from ocean cores can be properly interpreted.

The vertical distribution of radon in the Bomex area

Measurements of the vertical distribution of222Rn in the Bomex area yield a number of important pieces of oceanographic information. They yield a gas exchange piston velocity of 700 m/yr (equivalent to a stagnant boundary layer thickness of 60 μ). They allow a limit of greater than 160 cm2/sec to be placed on the coefficient of vertical eddy diffusion in the 0 to 20 m depth zone and of less than 4 cm2/sec in the 30 to 40 m depth zone. They also provide a vertical profile of the226Ra concentration in sea water.

A strategy for the use of bomb-produced radiocarbon as a tracer for the transport of fossil fuel CO2 into the deep-sea source regions

Abstract Because the equilibration time for CO 2 gas between the surface ocean and atmosphere is an order of magnitude shorter than the equilibration time for the isotopes of carbon, bomb-produced radiocarbon cannot be simply used as a tracer for fossil fuel CO 2 . This difference is especially important for the areas of deep water formation. In this paper we derive the relationship between the isotopic equilibration time and chemical equilibration times for carbon between sea and air. We also show that if the distribution of a second transient tracer is measured, it is possible to circumvent the ambiguity associated with this exchange time difference. Tritium, while adequate in purely diffusive models, is not ideal as the second transient in models involving advection (i.e., downwelling). For such models 85 Kr or freon is a much better choice.

A direct comparison of 14C and 230Th ages at Searles Lake, California☆

Thorium-230 dating on saline of the Lower Salt unit in pluvial Searles Lake, California, shows that this unit was formed between 24,500 and 32,000 years ago. The initial apparent 14C age of the lake is estimated to be about 900 years. After correcting for nonradiogenic 230Th and for the initial 14C age, excellent agreement between 230Th and 14C ages is obtained. The reliability of 230Th dating on salt deposit opens a new way for continuation of absolute chronology below the Lower Salt in Searles Lake.

Factors controlling the rate of CaCO3 precipitation on Great Bahama Bank

Measurements by Langdon et al. [2000] in the man-made mesocosm coral reef at Biosphere 2s ocean reveal a strong dependence of calcification rate on the degree of supersaturation of CaCO 3 in seawater. A similar trend was previously encountered on the Bahama Banks, where Halimeda and other calcifiers are likely responsible for aragonite precipitation [Broecker and Takahashi, 1966]. In this paper we compare these two sets of results and conclude that the dependence on saturation state is significant but less strong in the Bahamas. However, it must be kept in mind that to some extent, the reduction in CaCO 3 precipitation on the Bahama Banks may be due to impact of higher salinity on the growth of the calcifying algae. However, if, as many sedimentologists are convinced, the precipitation of CaCO 3 on the Bahama Banks is inorganic [Macintyre and Reid, 1992; Milliman et al., 1993], then the comparison of the Bahamas and Biosphere 2 results for dependence of calcification rate on saturation state is telling us something quite different.

The distribution of bomb-produced tritium and radiocarbon at GEOSECS station 347 in the eastern North Pacific

Following Roether et al. [1] an upwelling model has been tested to explain the distribution of bomb-produced tritium at the GEOSECS I test station off Baja, California. We have extended their treatment to include the time histories for tritium and for bomb radiocarbon now available for this station. If the CO2 gas exchange rate at this station is similar to the ocean average value of 20 moles/m2 yr, then the upwelling rate must be quite small (<3 m/yr). However, a satisfactory match to the time histories of14C and of3H is achieved only if an upwelling rate of 40 m/yr is used. In this case, however, a CO2 exchange rate of 70 moles/m2 yr would be required to match the observed surface water14C/C ratios and a tritium input 4 times the expected value would be needed. The inconsistency in the bomb14C time history obtained using the accepted CO2 exchange rate is likely the result of horizontal effects which void the validity of one dimensional modeling in this region of the ocean.