90
Peter Stille and Graham Shields
information back to the Jurassic Period. For information about even more ancient
oceans we must turn to well preserved sediments on land. Relating the
composition of now lithified rocks to the composition of a paleo-ocean brings
many potential problems that we will discuss later using case studies. Please note
that this chapter deals primarily with the radiogenic isotopes. There are several
good teaching texts already available on the subject of stable isotopes in the
marine environment, e.g. Hoefs (1980) and Arthur and Anderson (1983).
5.1 S r I s o t o p i c C o m p o s i t i o n o f S e a w a t e r : T e c t o n i c P r o x y a n d
S t r a t i g r a p h i c T o o l
Strontium is present in seawater as a trace element with a concentration of 7-8
mo~l... Due to its very long residence time (about >3 Ma) relative to the ocean
mixing time (about I500 years: Broeker and Peng, 1982), the isotopic
composition of Sr does not change detectably from place to place in the open
ocean: 87Sr/86Sr = 0.709165 +/- 0.000020, (this value has been normalized to the
commonly reported value for the standard NBS (National Bureau of Standards)
987 of 0.710250). Only in restricted basins, partly closed to seawater, are we
likely to find any significant deviation from the seawater ratio. For example,
seawater and therefore also shells and carbonate cement from sabkha
environments or restricted seaways such as the Gulf of Bothnia in the Baltic Sea
may not possess a seawater isotopic signature (Aberg and Wickman, 1987).
significantly anomalous isotopic ratios may also be observed in areas of high
hydrothermal activity, for example, along the mid-ocean ridge (AlbarSde et al.,
1981). Investigations of hydrotherrnal vents of the "East Pacific Rise" near to the
"Galapagos Spreading Center" make it clear that the seawater Sr isotopic
signature there has been influenced by mantle Sr. Fig. 5.1 helps to illustrate these
exchange mechanisms.
The three samples of hydrothermal waters (open circles) show variable
87Sr/86Sr and MffSr ratios which are positively correlated with each other. Pure,
non-hydrothermally contaminated seawater lies on the mixing line with the
characteristic ratio of 0.70916 and with high Mg/Sr ratios of about 600. Mg in the
mixing system is likely to come from seawater only. A non-seawater
contaminated hydrothermal system has a Mg/Sr of about zero and a 87Sr/86Sr
value similar to the basalts, i.e. about 0.703.
It is interesting to note that there is a direct correlation between the temperature
of the investigated waters and their Sr isotope composition. This correlation
allows us to reconstruct an initial temperature of about 340 +/- 30"C for the
hydrothermal fluid phase before any exchange with seawater.
Because of its high absolute mass, strontium does not experience any isotopic
fractionation or 'vital effect' during shell formation as does carbon. Sr is also not
Peter Stille and Graham Shields
information back to the Jurassic Period. For information about even more ancient
oceans we must turn to well preserved sediments on land. Relating the
composition of now lithified rocks to the composition of a paleo-ocean brings
many potential problems that we will discuss later using case studies. Please note
that this chapter deals primarily with the radiogenic isotopes. There are several
good teaching texts already available on the subject of stable isotopes in the
marine environment, e.g. Hoefs (1980) and Arthur and Anderson (1983).
5.1 S r I s o t o p i c C o m p o s i t i o n o f S e a w a t e r : T e c t o n i c P r o x y a n d
S t r a t i g r a p h i c T o o l
Strontium is present in seawater as a trace element with a concentration of 7-8
mo~l... Due to its very long residence time (about >3 Ma) relative to the ocean
mixing time (about I500 years: Broeker and Peng, 1982), the isotopic
composition of Sr does not change detectably from place to place in the open
ocean: 87Sr/86Sr = 0.709165 +/- 0.000020, (this value has been normalized to the
commonly reported value for the standard NBS (National Bureau of Standards)
987 of 0.710250). Only in restricted basins, partly closed to seawater, are we
likely to find any significant deviation from the seawater ratio. For example,
seawater and therefore also shells and carbonate cement from sabkha
environments or restricted seaways such as the Gulf of Bothnia in the Baltic Sea
may not possess a seawater isotopic signature (Aberg and Wickman, 1987).
significantly anomalous isotopic ratios may also be observed in areas of high
hydrothermal activity, for example, along the mid-ocean ridge (AlbarSde et al.,
1981). Investigations of hydrotherrnal vents of the "East Pacific Rise" near to the
"Galapagos Spreading Center" make it clear that the seawater Sr isotopic
signature there has been influenced by mantle Sr. Fig. 5.1 helps to illustrate these
exchange mechanisms.
The three samples of hydrothermal waters (open circles) show variable
87Sr/86Sr and MffSr ratios which are positively correlated with each other. Pure,
non-hydrothermally contaminated seawater lies on the mixing line with the
characteristic ratio of 0.70916 and with high Mg/Sr ratios of about 600. Mg in the
mixing system is likely to come from seawater only. A non-seawater
contaminated hydrothermal system has a Mg/Sr of about zero and a 87Sr/86Sr
value similar to the basalts, i.e. about 0.703.
It is interesting to note that there is a direct correlation between the temperature
of the investigated waters and their Sr isotope composition. This correlation
allows us to reconstruct an initial temperature of about 340 +/- 30"C for the
hydrothermal fluid phase before any exchange with seawater.
Because of its high absolute mass, strontium does not experience any isotopic
fractionation or 'vital effect' during shell formation as does carbon. Sr is also not