108 Peter Stille and Graham Shields
A more probable situation is represented by point B. In this case M = 0.6, V -
0.2.5 and S = 0.15. The very high Sr contents of marine carbonates (up to 6000
ppm), their high solubility and their worldwide distribution on the continents
would tend to suggest that a significant proportion of the strontium in the ocean
comes from this source. Apart from weathering, strontium is also released from
carbonates in another way. Elderfield and Greaves (1981) observed tl-.,at
submarine carbonate recrystallization can exert a strong influence on the seawater
isotopic composition. Sr is released during the diagenetic recrystallization of
aragonite to calcite. Our estimation that up to 60% of the strontium that is found
dissolved in the oceans originates from marine carbonate does not appear
improbable. This would mean that 25% of the strontium derives from volcanic,
basaltic sources and only 15% from crustal, sialic rocks at the present day.
In the Permian and Jurassic periods and in the late Precambrian the Sr seawater
curve reached minimum values (Figs. 5.2; 5.8). If we assume that the proportion
of Sr deriving from marine carbonate was 60%, we can calculate that at these
times, the proportion of strontium from the mantle was 40% and that from crustal,
sialic sources approached zero (point D). This mantle influence could have been
provided by enhanced rates of ocean spreading and intensive exchange between
seawater, newly produced oceanic basalts and hydrothermal fluid phases. It is
possible that this enhanced activity had some connection to the breaking up of the
supercontinents Pangaea and Rodinia (Proto-Pangaea) respectively, which are
likely to have occurred at this time (Piper. 1982). We must remember that such an
hydrothermal event can only be recognizable using 87Sr/86Sr if there was
relatively insignificant input of radiogenic Sr from the continent, i.e. relatively
low weathering rates. Low weathering rates were made possible by the existence
of a peneplained supercontinent as has been envisaged for both late Permian times
(Pangaea) and Late Precambrian times (Rodinia). The beginning of the breaking
up of a supercontinent may lead initially to low 87Sr/S6Sr ratios because of the
enhanced rifting associated with break-up but this wilt soon be reversed by the
start of mountain building due to subsequent doming and collision. The erosion
and weathering of granitic rocks, which have been uplifted by orogeny, will serve
to increase seawater 875r/86Sr. The two steep rises in 87Sr/86Sr in the geologic
record (Cenozoic till today and latest Precambrian-Cambrian) are both considered
to represent orogeny and erosion and exposure of the ancient granitic roots of a
mountain belt (high 87Sr/86Sr ratios). The Himalayan-Tibet uplift, is considered to
have controlled the evolution of seawater 87Sr/86Sr over the last 30 Ma.
5.1.5 The Influence of Groundwater on the Sr Isotopic Composition of
Seawater
In order to understand long-term seawater isotopic variations better, Chauduri and
Clauer (1986) compared these with the calculated and derived sea level
fluctuations for the Phanerozoic of Vail et al. (1977).
A more probable situation is represented by point B. In this case M = 0.6, V -
0.2.5 and S = 0.15. The very high Sr contents of marine carbonates (up to 6000
ppm), their high solubility and their worldwide distribution on the continents
would tend to suggest that a significant proportion of the strontium in the ocean
comes from this source. Apart from weathering, strontium is also released from
carbonates in another way. Elderfield and Greaves (1981) observed tl-.,at
submarine carbonate recrystallization can exert a strong influence on the seawater
isotopic composition. Sr is released during the diagenetic recrystallization of
aragonite to calcite. Our estimation that up to 60% of the strontium that is found
dissolved in the oceans originates from marine carbonate does not appear
improbable. This would mean that 25% of the strontium derives from volcanic,
basaltic sources and only 15% from crustal, sialic rocks at the present day.
In the Permian and Jurassic periods and in the late Precambrian the Sr seawater
curve reached minimum values (Figs. 5.2; 5.8). If we assume that the proportion
of Sr deriving from marine carbonate was 60%, we can calculate that at these
times, the proportion of strontium from the mantle was 40% and that from crustal,
sialic sources approached zero (point D). This mantle influence could have been
provided by enhanced rates of ocean spreading and intensive exchange between
seawater, newly produced oceanic basalts and hydrothermal fluid phases. It is
possible that this enhanced activity had some connection to the breaking up of the
supercontinents Pangaea and Rodinia (Proto-Pangaea) respectively, which are
likely to have occurred at this time (Piper. 1982). We must remember that such an
hydrothermal event can only be recognizable using 87Sr/86Sr if there was
relatively insignificant input of radiogenic Sr from the continent, i.e. relatively
low weathering rates. Low weathering rates were made possible by the existence
of a peneplained supercontinent as has been envisaged for both late Permian times
(Pangaea) and Late Precambrian times (Rodinia). The beginning of the breaking
up of a supercontinent may lead initially to low 87Sr/S6Sr ratios because of the
enhanced rifting associated with break-up but this wilt soon be reversed by the
start of mountain building due to subsequent doming and collision. The erosion
and weathering of granitic rocks, which have been uplifted by orogeny, will serve
to increase seawater 875r/86Sr. The two steep rises in 87Sr/86Sr in the geologic
record (Cenozoic till today and latest Precambrian-Cambrian) are both considered
to represent orogeny and erosion and exposure of the ancient granitic roots of a
mountain belt (high 87Sr/86Sr ratios). The Himalayan-Tibet uplift, is considered to
have controlled the evolution of seawater 87Sr/86Sr over the last 30 Ma.
5.1.5 The Influence of Groundwater on the Sr Isotopic Composition of
Seawater
In order to understand long-term seawater isotopic variations better, Chauduri and
Clauer (1986) compared these with the calculated and derived sea level
fluctuations for the Phanerozoic of Vail et al. (1977).