I06 Peter Stille and Graham Shields
Thereafter, seawater 87Sr/86Sr appears to have dropped gradually to around
0.7081 (Fig. 5.13). At the same time. C isotope ratios rose from around -4 to +5
%c PDB. It is here that the potential of this stratigraphic technique can be seen.
The authors recognized that such isotopic trends had not been recorded elsewhere
for the lowermost Cambrian. The other major database available was from Siberia
(Fig, 5.13) and showed only an abrupt change in Sr isotope ratio from 0.7085 to
0.7080 and no C isotope values as high as +5%c. Although inconclusive on their
own, the data would permit the conclusion that a significant unconformity in
Siberia and elsewhere had masked the true course of biologic evolution in the
Early Cambrian. Indeed sections of this age which do contain sharp increases in
faunal diversity also show signs of hiatus within them and are commonly
condensed. The authors argue that the relatively gradual nature of the 'Cambrian
explosion' in shelly fossil diversity in Mongolia may be closer to the truth and
that there was no massive faunal turnover at this point as suggested elsewhere. In
this case, the fortunate tectonic situation of a rapidly subsiding basin in Mongolia
avoided the major unconformity seen in most other carbonate shelf successions.
This helped smooth out the isotopic trends already recorded elsewhere.
Sr isotope stratigraphy, combined with C isotope-, bio- and event stratigraphy
can provide us with high resolution correlation of events as far back as the
Neoproterozoic. Not only that but by establishing trends in geochemical
parameters through time, we can help answer some of the fundamental questions
regarding past global environmental change and evolution.
5.1.4 Mass Balance Calculations
Faure et al. (1965) were the first to try to quantify the fluctuations in the Sr
isotopic composition of seawater that took place over geologic time. In their
model the authors assume the existence of three sources of St, which are all the
consequences of chemical weathering, either on the surface of the continents o r o n
the seafloor which could bring Sr into the oceans. The resulting 87Sr/86Sr ratio of
seawater can therefore be considered as a product of the mixing of these three
components. The authors suggested the following mass balance calculation for
seawater isotopic composition:
('7Sr/S' Sr)sw= S(87Srl s6 Sr)s + V( ~7 Sr/~ Sr)v + M( '7 Sr/8~ Sr)m
S,V, and M represent that proportion of Sr which derives from crustal (s), volcanic
(v) and marine carbonate (m) sources respectively (s+v+m = 1).
(STSr/86Sr) sw
(87Sr/86Sr) s
(8"7Sr/S6Sr) v
(87Sr/86Sr) m
: seawater isotopic ratio
: Sr isotopic ratio from volcanic rocks (0.704)
: Sr isotopic ratio of crustal rocks (0.720)
: Sr isotopic ratio of marine carbonate and sulfate
Thereafter, seawater 87Sr/86Sr appears to have dropped gradually to around
0.7081 (Fig. 5.13). At the same time. C isotope ratios rose from around -4 to +5
%c PDB. It is here that the potential of this stratigraphic technique can be seen.
The authors recognized that such isotopic trends had not been recorded elsewhere
for the lowermost Cambrian. The other major database available was from Siberia
(Fig, 5.13) and showed only an abrupt change in Sr isotope ratio from 0.7085 to
0.7080 and no C isotope values as high as +5%c. Although inconclusive on their
own, the data would permit the conclusion that a significant unconformity in
Siberia and elsewhere had masked the true course of biologic evolution in the
Early Cambrian. Indeed sections of this age which do contain sharp increases in
faunal diversity also show signs of hiatus within them and are commonly
condensed. The authors argue that the relatively gradual nature of the 'Cambrian
explosion' in shelly fossil diversity in Mongolia may be closer to the truth and
that there was no massive faunal turnover at this point as suggested elsewhere. In
this case, the fortunate tectonic situation of a rapidly subsiding basin in Mongolia
avoided the major unconformity seen in most other carbonate shelf successions.
This helped smooth out the isotopic trends already recorded elsewhere.
Sr isotope stratigraphy, combined with C isotope-, bio- and event stratigraphy
can provide us with high resolution correlation of events as far back as the
Neoproterozoic. Not only that but by establishing trends in geochemical
parameters through time, we can help answer some of the fundamental questions
regarding past global environmental change and evolution.
5.1.4 Mass Balance Calculations
Faure et al. (1965) were the first to try to quantify the fluctuations in the Sr
isotopic composition of seawater that took place over geologic time. In their
model the authors assume the existence of three sources of St, which are all the
consequences of chemical weathering, either on the surface of the continents o r o n
the seafloor which could bring Sr into the oceans. The resulting 87Sr/86Sr ratio of
seawater can therefore be considered as a product of the mixing of these three
components. The authors suggested the following mass balance calculation for
seawater isotopic composition:
('7Sr/S' Sr)sw= S(87Srl s6 Sr)s + V( ~7 Sr/~ Sr)v + M( '7 Sr/8~ Sr)m
S,V, and M represent that proportion of Sr which derives from crustal (s), volcanic
(v) and marine carbonate (m) sources respectively (s+v+m = 1).
(STSr/86Sr) sw
(87Sr/86Sr) s
(8"7Sr/S6Sr) v
(87Sr/86Sr) m
: seawater isotopic ratio
: Sr isotopic ratio from volcanic rocks (0.704)
: Sr isotopic ratio of crustal rocks (0.720)
: Sr isotopic ratio of marine carbonate and sulfate