Eric T. Sundquist

The global carbon dioxide budget

The increase in atmospheric CO[sub 2] levels during the last deglaciation was comparable in magnitude to the recent historical increase. However, global CO[sub 2] budgets for these changes reflect fundamental differences in rates and in sources and sinks. The modern oceans are a rapid net CO[sub 2] sink, whereas the oceans were a gradual source during the deglaciation. Unidentified terrestrial CO[sub 2] sinks are important uncertainties in both the deglacial and recent CO[sub 2] budgets. The deglacial CO[sub 2] budget represents a complexity of long-term dynamic behavior that is not adequately addressed by current models used to forecast future atmospheric CO[sub 2] levels.

Dynamics of soil carbon during deglaciation of the Laurentide ice sheet

Deglaciation of the Laurentide Ice Sheet in North America was accompanied by sequestration of organic carbon in newly exposed soils. The greatest rate of land exposure occurred around 12,000 to 8,000 years ago, and the greatest increase in the rate of carbon sequestration by soils occurred from 8,000 to 4,000 years ago. Sequestration of carbon in deglaciated peat lands continues today, and a steady state has not been reached. The natural rate of carbon sequestration in soils, however, is small relative to the rate of anthropogenic carbon dioxide production.

Steady- and non-steady-state carbonate-silicate controls on atmospheric CO2

Two contrasting hypotheses have recently been proposed for the past long-term relation between atmospheric CO2 and the carbonate-silicate geochemical cycle. One approach (Berner, 1990) suggests that CO2 levels have varied in a manner that has maintained chemical weathering and carbonate sedimentation at a steady state with respect to tectonically controlled decarbonation reactions. A second approach (Raymo et al., 1988), applied specificlly to the late Cenozoic, suggests a decrease in CO2 caused by an uplift-induced increase in chemical weathering, without regard to the rate of decarbonation. According to the steady-state (first) hypothesis, increased weathering and carbonate sedimentation are generally associated with increasing atmospheric CO2, whereas the uplift (second) hypothesis implies decreasing CO2 under the same conditions. An ocean-atmosphere-sediment model has been used to assess the response of atmospheric CO2 and carbonate sedimentation to global perturbations in chemical weathering and decarbonation reactions. Although this assessment is theoretical and cannot yet be related to the geologic record, the model simulations compare steady-state and non-steady-state carbonate-silicate cycle response. The e-fold response time of the ‘CO2-weathering’ feedback mechanism is between 300 and 400 ka. The response of carbonate sedimentation is much more rapid. These response times provide a measure of the strength of steady-state assumptions, and imply that certain systematic relations are sustained throughout steady-state and non-steady-state scenarios for the carbonate-silicate cycle. The simulations suggest that feedbacks can maintain the system near a steady state, but that non-steady-state effects may contribute to long-term trends. The steady-state and uplift hypotheses are not necessarily incompatible over time scales of a few million years.

Carbon Dioxide in the Ocean Surface: The Homogeneous Buffer Factor

The amount of carbon dioxide that can be dissolved in surface seawater depends at least partially on the homogeneous buffer factor, which is a mathematical function of the chemical equilibrium conditions among the various dissolved inorganic species. Because these equilibria are well known, the homogeneous buffer factor is well known. Natural spatial variations depend very systematically on sea surface temperatures, and do not contribute significantly to uncertainties in the present or future carbon dioxide budget.

10.9 – The Geologic History of the Carbon Cycle

Geologists, like other scientists, tend to view the global carbon cycle through the lens of their particular training and experience. This chapter describes the behavior of the carbon cycle prior to human influence. It describes events and processes that extend back through geologic time and include the exchange of carbon between the Earths surface and the long-term reservoirs in the lithosphere. This chapter begins with an overview of the carbon exchanges and processes that control the variations observed in the geologic record of the carbon cycle. Then examples of past carboncycle change are described, beginning with the most recent variations seen in cores drilled from glaciers and the sea floor, and concluding with the distant transformations inferred from the rock record of the Precambrian.

Total individual ion activity coefficients of calcium and carbonate in seawater at 25°C and 35%. salinity, and implications to the agreement between apparent and thermodynamic constants of calcite and aragonite

Abstract We have calculated the total individual ion activity coefficients of carbonate and calcium, γ TCO 3 2− and γ TCa 2+ , in seawater. Using the ratios of stoichiometric and thermodynamic constants of carbonic acid dissociation and total mean activity coefficient data measured in seawater, we have obtained values which differ significantly from those widely accepted in the literature. In seawater at 25°C and 35%. salinity the (molal) values of γ TCO 2 3− and γ TCa 2+ are 0.038 ± 0.002 and 0.173 ± 0.010 , respectively. These values of γ TCO 3 2− and γ TCa 2+ are independent of liquid junction errors and internally consistent with the value γ TCl − = 0.651 . By defining γ TCa 2+ and γ TCO 3 2− on a common scale ( γ TCl − ), the product γ TCa 2+ γ TCO 3 2− is independent of the assigned value of γ Cl − and may be determined directly from thermodynamic measurements in seawater. Using the value γ TCa 2+ γ TCO 3 2− = 0.0067 and new thermodynamic equilibrium constants for calcite and aragonite, we show that the apparent constants of calcite and aragonite are consistent with the thermodynamic equilibrium constants at 25°C and 35%. salinity. The demonstrated consistency between thermodynamic and apparent constants of calcite and aragonite does not support a hypothesis of stable Mg-calcite coatings on calcite or aragonite surfaces in seawater, and suggests that the calcite critical carbonate ion curve of Broecker and Takahashi (1978, Deep-Sea Research25, 65–95) defines the calcite equilibrium boundary in the oceans, within the uncertainty of the data.

Oil Shales and Carbon Dioxide

During retorting of oil shales in the western United States, carbonate minerals are calcined, releasing significant amounts of carbon dioxide. Residual organic matter in the shales may also be burned, adding more carbon dioxide to the atmosphere. The amount of carbon dioxide produced depends on the retort process and the grade and mineralogy of the shale. Preliminary calculations suggest that retorting of oil shales from the Green River Formation and burning of the product oil could release one and one-half to five times more carbon dioxide than burning of conventional oil to obtain the same amount of usable energy. The largest carbon dioxide releases are associated with retorting processes that operate at temperatures greater than about 600�C.

Investigators share improved understanding of the North American Carbon Cycle

The U.S. North American Carbon Program (NACP) sponsored an “all-scientist” meeting to review progress in understanding the dynamics of the carbon cycle of North America and adjacent oceans, and to chart a course for improved integration across scientific disciplines, scales, and Earth system boundaries. The meeting participants also addressed the need for better decision support tools for managing the carbon cycle of North America, so that strong science can inform policy as interest in taking action increases across the nation. Herein we report on themes to integrate the diversity of NACP science and fill significant gaps for understanding and managing the North American carbon cycle: integration among disciplines involving land, atmosphere, and ocean research; strengthening data management infrastructure to support modeling and analysis; identification of study regions that are critical for reducing uncertainties in the North American carbon balance; and integrating biophysical science with the human dimensions of carbon management and decision support.