5 Isotopic Composition of Seawater I11
River waters, according to this newer equation, deliver strontium which is derived
from continental sialic rocks as we]] as marine carbonates and young volcanic
rocks exposed on the surface of the Earth. Faure, in his equation, applied three
different fluxes for these components. The groundwater flux calculations show
that the global contribution of Sr into the oceans from groundwater would match
that from rivers.
What kind of influence could groundwater have had during the Phanerozoic?
An important factor that controls the amount of groundwater flow into the oceans
is the variation in length of coastline and fluctuations in sea level. Fragmentation
and the splitting up of continents or regression result in an increase in the length
of coastline, whereas continent-continent collision and continent-island arc
collision or transgression lead to a reduction in the amount of coastline.
Transgressions or regressions would be likely to have the same effect on the Sr
contribution from both river water and groundwater. Continental fragmentation or
collision would however be likely to affect the Sr contribution from river water far
less than that from groundwater.
Such mass balance considerations allow us to explain some of the more
noticeable Sr variations in the Phanerozoic. For example, the rise in seawater
87Sr/86Sr ratio after the Cretaceous parallels the further breaking apart of the
supercontinent Pangaea as well as eustatic regression. These mechanisms have the
consequence that the flux of groundwater to the oceans increases in particular and
with it, the flux of radiogenic, continental Sr. The Sr isotopic composition may
have increased additionally due to the relative decrease in mid-ocean ridge
hydrothermal activity which may have ted to the regression.
The rise in 87Sr/86Sr ratio during the Cretaceous occurred dunng a period of
sustained transgression and likely enhanced sub-aquatic hydrothermal activity,
which would tend to lower the Sr isotopic ratio of seawater. Therefore, this rise is
likely to relate wholly to a compensating increase in continental input of
strontium. In Cretaceous times, 35% of today's land mass was covered by water
and the supercontinent Pangaea was still more or less intact. The amount of
coastline must have been a great deal shorter than that of today. As a consequence,
much less continental strontium could have made it into the oceans than during
the Cenozoic, although still much more than at the Jurassic-Cretaceous transition.
In Jurassic-Cretaceous times, fragmentation of the supercontinent was limited to
the barest minimum and so the contribution of groundwater strontium must have
also been less. The 87Sr/86Sr ratios reached a minimum during this time.
Apart from a few fluctuations in seawater 87Sr/86Sr ratio that can be correlated
with important orogenic phases, the Paleozoic is marked by a general fall from
high to low ratios from the Cambrian period until the Permian (545 - 250 Ma).
The formation of the supercontinent Pangaea led to a decrease in the groundwater
flow into the oceans which may have caused a general decrease in the contribution
of continental strontium.
There are few data for the Precambrian. What can be drawn from what little we
know is that the breaking up of the >800 Ma old supercontinent (Rodinia) was
River waters, according to this newer equation, deliver strontium which is derived
from continental sialic rocks as we]] as marine carbonates and young volcanic
rocks exposed on the surface of the Earth. Faure, in his equation, applied three
different fluxes for these components. The groundwater flux calculations show
that the global contribution of Sr into the oceans from groundwater would match
that from rivers.
What kind of influence could groundwater have had during the Phanerozoic?
An important factor that controls the amount of groundwater flow into the oceans
is the variation in length of coastline and fluctuations in sea level. Fragmentation
and the splitting up of continents or regression result in an increase in the length
of coastline, whereas continent-continent collision and continent-island arc
collision or transgression lead to a reduction in the amount of coastline.
Transgressions or regressions would be likely to have the same effect on the Sr
contribution from both river water and groundwater. Continental fragmentation or
collision would however be likely to affect the Sr contribution from river water far
less than that from groundwater.
Such mass balance considerations allow us to explain some of the more
noticeable Sr variations in the Phanerozoic. For example, the rise in seawater
87Sr/86Sr ratio after the Cretaceous parallels the further breaking apart of the
supercontinent Pangaea as well as eustatic regression. These mechanisms have the
consequence that the flux of groundwater to the oceans increases in particular and
with it, the flux of radiogenic, continental Sr. The Sr isotopic composition may
have increased additionally due to the relative decrease in mid-ocean ridge
hydrothermal activity which may have ted to the regression.
The rise in 87Sr/86Sr ratio during the Cretaceous occurred dunng a period of
sustained transgression and likely enhanced sub-aquatic hydrothermal activity,
which would tend to lower the Sr isotopic ratio of seawater. Therefore, this rise is
likely to relate wholly to a compensating increase in continental input of
strontium. In Cretaceous times, 35% of today's land mass was covered by water
and the supercontinent Pangaea was still more or less intact. The amount of
coastline must have been a great deal shorter than that of today. As a consequence,
much less continental strontium could have made it into the oceans than during
the Cenozoic, although still much more than at the Jurassic-Cretaceous transition.
In Jurassic-Cretaceous times, fragmentation of the supercontinent was limited to
the barest minimum and so the contribution of groundwater strontium must have
also been less. The 87Sr/86Sr ratios reached a minimum during this time.
Apart from a few fluctuations in seawater 87Sr/86Sr ratio that can be correlated
with important orogenic phases, the Paleozoic is marked by a general fall from
high to low ratios from the Cambrian period until the Permian (545 - 250 Ma).
The formation of the supercontinent Pangaea led to a decrease in the groundwater
flow into the oceans which may have caused a general decrease in the contribution
of continental strontium.
There are few data for the Precambrian. What can be drawn from what little we
know is that the breaking up of the >800 Ma old supercontinent (Rodinia) was