Ocean Alkalinity Research Articles

Spectral Modeling of Sulfonephthalein Indicators: Application to pH Measurements Using Multiple Indicators

The visible spectra of five sulfonephthalein indicators (thymol blue, bromophenol blue, bromocresol green, bromocresol purple, and phenol red) were deconvoluted into four Gaussian components. The spectra of the basic form of the indicator were described by three components and the acidic form by one component. The combination of the spectral model, pH, and pK data allowed quantative prediction of indicator spectra as a function of pH. A general equation was derived for the calculation of pH from two absorbance measurements of a solution which contains one or more indicators. The application of multiple indicators significantly expands the pH range over which pH measurements can be made. With the use of the modeled spectra for several indicators, optimal indicator combinations and measurement wavelengths for specific pH ranges were determined. The combination of phenol red and bromocresol green allowed determination of seawater pH over the range 3.0 to 8.2, which is suitable for oceanic pH and alkalinity determinations.

The role of vertical chemical fractionation in controlling late Quaternary atmospheric carbon dioxide

The 30% decrease in atmospheric carbon dioxide during glacial maxima must be driven by some change in the chemical circulation of the ocean. Here, a new model for late Quaternary CO2 variability is presented which resolves some problems occurring in previous models (including the timing of carbon dioxide response and changes in the oxygen content of the deep ocean). The primary driving factor in this model is a rearrangement of chemical distributions in the ocean whereby labile nutrients and metabolic CO2 are concentrated in deep waters rather than in intermediate waters as observed in the modern ocean. This new “bottom‐heavy” chemical structure does not affect atmospheric carbon dioxide directly. Instead, CO2‐induced acidity lowers the deep ocean carbonate ion concentration and temporarily increases carbonate dissolution rates. Oceanic alkalinity then rises until the deep ocean carbonate ion is restored to its steady state value. The resulting increase in oceanic alkalinity draws CO2 out of the atmosphere into the ocean. Alkalinity and atmospheric CO2 lag several thousand years behind the change in oceanic chemical structure; this delayed response is determined by the limited rate of continental weathering and deep ocean carbonate dissolution relative to the oceanic alkalinity inventory. It is proposed that characteristic deglacial and preglacial states of the ocean are leading factors driving glacial‐interglacial climate changes. This concept reinterprets the carbon isotope contrast between surface water and deep waters [Δδ13C(P‐B)] as due to a shift of light metabolic carbon from intermediate waters into deep waters. The process is illustrated here by a simple equilibrium five box ocean model. The observed intermediate‐depth nutrient depletion is not sufficient in itself to determine which of several possible mechanisms are operating, and some mechanisms are not as effective in changing atmospheric CO2 as others. The largest response is seen from deepening the regeneration cycle of organic carbon; the least response is seen from converting North Atlantic Deep Water into intermediate water.

Vertical oceanic nutrient fractionation and glacial/interglacial CO2cycles

Ice-core studies have established that atmospheric carbon dioxide is about 80 parts per million by volume (p.p.m.v.) lower during cold glacial climates than it is during warm interglacial times1–8. A number of models have been offered for this phenomenon9–12. These models are consistent with observation of an enhanced carbon isotope contrast between surface and deep waters, but they also predict dissolved oxygen depletions which are inconsistent with the widespread occurrence of glacial-age fossils from aerobic benthic organisms. Recent observations on changes in the vertical oceanic chemical structure provide a new mechanism to control glacial/interglacial CO2 change. Palaeochemical evidence shows that nutrients and metabolic CO2 are shifted from intermediate waters into deeper waters during glacial times13–15. At the outset of this redistribution, increased deep dissolved CO2 concentrations raise carbonate dissolution above that required for a steady-state input/output balance. Oceanic alkalinity then increases until steady-state dissolution rates are restored. Here I propose that atmospheric CO2 decreases in direct response to increased oceanic alkalinity. Although the carbonate response factor in atmospheric CO2 has been noted before, it has not been linked with vertical nutrient rearrangements. The withdrawal of nutrients from inter-mediate waters of the ocean also prevents oxygen depletion in these waters.

Processes affecting the oceanic distributions of dissolved calcium and alkalinity

Recent studies of the CO2 system have suggested that chemical processes in addition to the dissolution and precipitation of calcium carbonate affect the oceanic calcium and alkalinity distributions. Calcium and alkalinity data from the North Pacific have been examined both by using the simple physical‐chemical model of previous workers and by a study involving the broader oceanographic context of these data. The simple model is shown to be an inadequate basis for these studies. Although a proton flux associated with organic decomposition may affect the alkalinity, previously reported deviations of calcium‐alkalinity correlations from expected trends appear to be related to boundary processes that have been neglected rather than to this proton flux. The distribution of calcium in the surface waters of the Pacific Ocean is examined.