Ice volume effect

It is expected that the temperature of deep-water masses is more or less constant, as long as ice caps exist at the poles. Thus, the oxygen isotope composition of benthic organisms should preferentially reflect the change in the isotopic composition of the water (ice-volume effect), while the 5 0-values of planktonic foraminifera are affected by both temperature and isotopic water composition. 

Data from planktonic foraminiferans can provide information on sea-surface temperature (Epstein et al. 1953). This information can be more difficult to interpret because it is superimposed on the ice-volume effect and because sea-surface temperature patterns and the factors that affect them can be quite complex. A major factor is the balance between evaporation and precipitation, which affects salinity. All other conditions being constant, a fall of 1 °C results in an increase in 5180 of c.0.2%o. If independent estimation of surface-water temperature is possible (e.g. U ), 5180 values for planktonic foraminiferans can... 

A 17 Ma trend toward cooler conditions followed, as expressed by a 3%c rise in 5 0, which can be attributed to a 7°C decline in deep-sea temperatures. All subsequent changes reflect a combined effect of ice-volume and temperature. 

Although sea level is closely related to continental ice volume, it is also a function of other variables, most notably isostatic effects (see Clark et al. (1978) and review of Peltier (1998) and references therein). Sea-level change can be divided into a eustatic component, which depends... 

Figure 12 Alkenone data acquired with planktonic foraminiferal can be used to remove the temperature effect from oxygen isotopic signals in order to assess global (ice volume) and regional (evaporation/precipitation balance) contributions to isotopic change. Data come from an equatorial Atlantic core studied by Schneider et al. (1996) the isotopic deconvolution was made by the present author. Note that the structure and amplitude of the temperature-corrected planktonic record are consistent with current thinking on the global ice volume signal for the late...

Much of what is currently understood about the Cenozoic history, of deep-sea temperature, carbon chemistry, and global ice volume, has been gleaned from the stable isotope ratios of benthic foraminifera. Benthic foraminifera extract carbonate and other ions from seawater to construct their tests. In many species, this is achieved near carbon and oxygen isotopic equilibrium. Kinetic fractionation effects tend to be small and constant (Grossman, 1984, 1987). As a result, shell fi C and strongly covary with the isotopic... 

The molecular theory of structural change from low-temperature liquid water to that of ice Ih structure is yet incomplete. Eisenberg and Kauzmann (1969) suggested that the observed temperature dependence of the molar volume of water can be explained qualitatively based on two competing effects (1) continuation of the strengthening of the four-coordination of molecules keeps the ice form ih an open structure (2) the expected decrease in all the anhar-monic vibrations will lead to a decrease in the ice volume. Probably, it is the dominance of the first effect that keeps ice Ih in a low density state. 

The incorporation of oxygen isotopes into biogenic carbonate is complex and as Wefer et al. (1999) point out, the 8 0 value of a carbonate is a function of temperature, ice volume, selective growth in space and time, vital effects and post-depositional dissolution. The carbonate chemistry and pH of a benthic foraminifera s microenvironment or microhabitat also plays an important role in test isotopic composition (e.g. Spero et al. 1997 Bemis et al. 1998 Bijma et al. 1999 Wolf-Gladrow et al. 1999), with a 0.2 increase in pH resulting in a 8 0 shift of approximately — 0.22%o (Zeehe 1999). 

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