W.G. Mook

CARBON ISOTOPE FRACTIONATION BETWEEN DISSOLVED BICARBONATE AND GASEOUS CARBON-DIOXIDE

Abstract The fractionation of the stable carbon isotopes between gaseous CO 2 and dissolved bicarbonate has been measured from 5 to 25°C by a method of high precision. Additional measurements of lower precision were made up to 125°C. The results are well represented by the relation: e b (g) = −(9.483 ± 0.22)10 3 /T+(23.89 ± 0.75)‰ . Using earlier values for the equilibrium between gaseous and dissolved CO 2 , the isotope fractionation between dissolved HCO 3 − and dissolved CO 2 is: e b (a) = −(9.866 ± 0.23)10 3 /T+(24.12 ± 0.78)‰ . Tables are given for the carbon isotope fractionation in the system CO 2 (g)-CO 2 (aq)-HCO 3 − at normal temperatures.

Isotopic fractionation between gaseous and dissolved carbon dioxide

AbstractThe isotopic fractionation between gaseous carbon dioxide and an equilibrated aqueous solution of the gas has been measured at temperatures between 0° and 60 °C for the carbon isotopes and close to 0 °C for the oxygen isotopes.n13C16O2 is slightly less, and12C16O18O slightly more soluble than12C16O2, the actual values for the fractionation,ɛ, being −(1.18−0.0041 ·t)‰ (temperature,t, in °C) for the carbon isotopes and about 0.8‰ at 0°C for the oxygen isotopes.A theoretical expression is derived for the vapour-solute equilibrium system by treating it in a similar manner as the pure vapour-liquid system. The calculated isotopic fractionation for carbon dioxide, using data on the pure vapour-liquid system, is in satisfactory agreement with the experimental results.

The measuring procedure and corrections for the high-precision mass-spectrometric analysis of isotopic abundance ratios, especially referring to carbon, oxygen and nitrogen

In order to determine isotopic abundance ratios such as 13C/12C, 18O/16O or 15N/14N, a mass spectrometer reads the ratio of the voltages which are generated by the isotopic ion beams, trapped by a double collector system. n nThe effect of the gas inlet pressure on the voltage ratio in the case of nitrogen and carbon dioxide is determined experimentally, using a Varian M86. A knowledge of this pressure dependence provides a method of determining the voltage ratio, which is independent of the inlet pressure. n nThis paper partly reviews the work of Craig and gives a detailed treatment of the measuring procedure, of the corrections to be applied to the voltage ratios, viz. for pressure effects, of the way in which the voltage ratios are transformed into isotopic abundances, and of disturbing factors such as spectrometer background and sample contamination. n nThe corrections will be discussed successively in the order in which they are to be applied. In every case the exact equation is deduced, while often an approximation is given for use with natural samples. n nThe major contributions to the technique of mass-spectrometric analysis of isotopic abundance ratios are due to Craig (1957) and Dansgaard (1961). The original choice of the latter to use absolute differences between a sample and standard, rather than relative, has been abandoned. Craig made in his correction formula approximations applicable to samples deviating not more than a few percent from the standard and based on an accuracy of 0.1‰, while nowadays the precision is often better than that. Consequently, we thought it necessary to review the whole procedure and make revisions, where necessary. n nAlthough the stable isotopes of carbon, oxygen and nitrogen in CO2 and N2 respectively are specifically dealt with, the general equations also apply to isotopic analyses on other gases, such as argon (36Ar/40Ar) and SO2 (34S/32S).

Paleotemperatures and chlorinities from stable carbon and oxygen isotopes in shell carbonate

A procedure is proposed by which meaningful water temperatures can be derived from the carbon and oxygen isotopic composition of shell carbonate, even if the habitat of the shell animals is not purely marine as in the case of estuaries or brackish coastal waters. n nSince these temperatures reflect the average temperature during the growth period of the organisms in question, it should also be possible to deduce reliable paleotemperatures from fossil samples of these species and others under certain conditions. n nTwo main aspects are considered: (1) the relation between the isotopic composition of shell carbonate and that of the surrounding water; and (2) the significance of the temperatures deduced from the 13C and 18O content of shells. n nIn a Dutch estuary the respective variations in 13C and 18O contents of the shell carbonate on the one hand and the dissolved bicarbonate and water on the other are found to correspond. If the δ13ue5f8δ18 relation is extrapolated to the isotopic composition of carbonate precipitated in isotopic equilibrium with pure sea water, water temperature can be deduced. The temperature derived from such an extrapolation, is a reasonable average for the growth period of the molluscs. The δ13ue5f8δ18 relations of shells from other nearby estuaries reflect, when similarly extrapolated, the same temperature. n nFrom this it is concluded that some molluscs deposit their carbonate in isotopic equilibrium with their environment, and that the difference in carbon and oxygen isotopic composition between carbonate on the one hand and dissolved bicarbonate and water on the other is determined solely by temperature. Reasonable temperatures are also derived from some coastal samples of molluscs. n nThe procedure developed for determining the growth temperature of series of living molluscs can also be applied to fossil shells. Besides, the extrapolation procedure provides a method for estimating the brackishness of the water in which the molluscs lived.

CHEMISTRY OF DISSOLVED INORGANIC CARBON IN ESTUARINE AND COASTAL BRACKISH WATERS

Abstract The rapid increase of the first and second apparent dissociation constants of carbonic acid with salinity results in a remarkable pH distribution in an estuary. On going down-stream, instead of a gradual continuous increase in pH from fresh water values of 7·0 to 7·5 to a marine value of 8·2, the pH shows a certain minimal value at low salinity. With high pH fresh water a sharp decrease at low salinity is observed. Depending on the alkalinity ratio of the fresh river water and the sea water, the carbonate ion concentration in an estuary can remain surprisingly low up to relatively high salinities. This phenomenon might affect the shell growth of certain molluscs. In order to obtain the proper dissociation constants for brackish waters of low chlorinities, a practical adjustment of well known data for pure water and sea water is made: p K 1 ′ ( for 0 ⩽ Cl ⩽ 9‰) = 3404·71 T + 0·032786T − 14·8435 − 0·08921Cl 1 2 p K 2 ′ ( for 0 ⩽ Cl ⩽ 0·005‰) = 2902·39 T + 0·02379T − 6·4980 − 0·7531Cl 1 2

Shell characteristics, isotopic composition and trace-element contents of some euryhaline molluscs as indicators of salinity

The shell characteristics, δ13C−δ18 and trace-element contents of five euryhaline molluscs from the Dutch coastal waters are considered in relation to salinity. Shape and weight of the shells may be influenced by salinity but are too variable to show a significant correlation. Maximum size and maximum age decrease with salinity. Between δ13C−δ18O and salinity there is a strong correlation allowing salinity to be determined within 0.5‰ Cl′. Also the average number of ribs on Cardium edule shell is strongly related to salinity, which can be determined from the average rib number within ±1.5‰Cl′. The results on the relation between trace-element contents and salinity are contradictory. The various relationships with salinity are not clear and need further elucidation.

ENRICHMENT OF RADIOCARBON FOR DATING SAMPLES UP TO 75,000 YEARS

Abstract The range of the radiocarbon dating method can be extended by about 20,000 years by means of thermal diffusion isotopic enrichment. A description of the Clusius-Dickel type enrichment columns and their operation is given. Prior to enrichment the carbonaceous sample material is combusted to CO2 and then reduced to CO. The enriched sample is re-oxidized to CO2 . The combustion-, reduction-and oxidation-apparatus and their operation are described. The performance of the thermal diffusion columns has been tested and an empirical, volume independent relation between the isotope separation factors of 14C 16O and all molecules with mass 〈30〉 is given q14 - 1 = (1.13 ± 0.05) (q30 - 1) + (0.035 ± 0.005) (q30 - 1)2 . The influence of contaminations on the reability of 14C dates obtained with this method is discussed. Laboratory contaminations can be kept below the detection limit as is shown by the preparation of two anthracite samples. Still there may be some influence on dates obtained for very old samples. As far as laboratory contaminations are concerned, dates up to 75,000 years can be obtained.