10 Influence of Geochemical Processes on Stable Isotope Distribution in Marine Sediments
342
of water masses containing different 18 O/ 16 O
ratios such as melt water, river runoff, etc., and the
global isotope content of the oceans (Craig and
Gordon 1965; Broecker 1974). Since the salinity of
seawater is similarly affected by these processes,
Craig and Gordon (1965) and later Fairbanks et al.
(1992) defined a set of regression relationships
between salinity and δ 18 O w with different slopes
for several modern water masses, varying between
0.1 for humid tropical and 1.0 for arid polar surface
water masses with a global mean of 0.49 (Fig.
10.1). Higher slopes represent areas where evaporation exceeds precipitation, and vice versa. However, the extent of oxygen isotope enrichments
due to evaporation is limited due to the recycling
of atmospheric moisture by the different pathways
of precipitation. The slope of the global trend
extrapolates to a δ 18 O-value of –17‰ at zero
salinity, reflecting the influx of high-latitude precipitation and glacial meltwater. For the Antarctic
continental ice, even δ 18 O-values as low as –54‰
have been determined (Weiss et al. 1979; Jacobs et
al. 1985). However, the slope of the δ 18 O w -salinity
relationship may have changed through geological time (see discussion below).
For water masses deeper than 2000 m, Zahn
and Mix (1991) obtained a slope as high as 1.53.
This gradient is explained with the formation of
sea-ice in the source areas especially for southern
component water masses. Since the freezing of polar surface waters raises the salinity, but does not
fractionate oxygen isotopes, southern source
deep water masses, like the Antarctic Bottom
Water, exhibit relatively low δ 18 O w values, and so
do other water masses, which are derived from the
admixture of south polar water masses (Mackensen 2001).
Modern Range of Values and Historical Variability
The modern δ 18 O w values of seawater are close
to 0‰ (V-SMOW) and vary only within narrow
limits. From the GEOSECS δ 18 O sections for the
today’s world oceans, compiled by Birchfield
Fig. 10.1 Relationship between salinity and δ 18 O w of major water masses (after Craig and Gordon 1965; deep water
line according to Zahn and Mix 1991).
342
of water masses containing different 18 O/ 16 O
ratios such as melt water, river runoff, etc., and the
global isotope content of the oceans (Craig and
Gordon 1965; Broecker 1974). Since the salinity of
seawater is similarly affected by these processes,
Craig and Gordon (1965) and later Fairbanks et al.
(1992) defined a set of regression relationships
between salinity and δ 18 O w with different slopes
for several modern water masses, varying between
0.1 for humid tropical and 1.0 for arid polar surface
water masses with a global mean of 0.49 (Fig.
10.1). Higher slopes represent areas where evaporation exceeds precipitation, and vice versa. However, the extent of oxygen isotope enrichments
due to evaporation is limited due to the recycling
of atmospheric moisture by the different pathways
of precipitation. The slope of the global trend
extrapolates to a δ 18 O-value of –17‰ at zero
salinity, reflecting the influx of high-latitude precipitation and glacial meltwater. For the Antarctic
continental ice, even δ 18 O-values as low as –54‰
have been determined (Weiss et al. 1979; Jacobs et
al. 1985). However, the slope of the δ 18 O w -salinity
relationship may have changed through geological time (see discussion below).
For water masses deeper than 2000 m, Zahn
and Mix (1991) obtained a slope as high as 1.53.
This gradient is explained with the formation of
sea-ice in the source areas especially for southern
component water masses. Since the freezing of polar surface waters raises the salinity, but does not
fractionate oxygen isotopes, southern source
deep water masses, like the Antarctic Bottom
Water, exhibit relatively low δ 18 O w values, and so
do other water masses, which are derived from the
admixture of south polar water masses (Mackensen 2001).
Modern Range of Values and Historical Variability
The modern δ 18 O w values of seawater are close
to 0‰ (V-SMOW) and vary only within narrow
limits. From the GEOSECS δ 18 O sections for the
today’s world oceans, compiled by Birchfield
Fig. 10.1 Relationship between salinity and δ 18 O w of major water masses (after Craig and Gordon 1965; deep water
line according to Zahn and Mix 1991).