Carbon Species in Water
75
ious authors to be 0.5 to 1.5%0, in IgO. Going back from Recent to
Paleozoic time it seems rather certain that ocean water, within small
variations, had a constant isotopic composition. Greater variations of
several per mil and more may have been possible in Precambrian time.
(PERRY, 1967, suggested that the low 180-contents of Precambrian massive cherts reflect lower 180-contents in the Precambrian oceans.) Assuming a constant rate of weathering and sedimentation throughout the
earth's history, SAVIN and EpSTEIN (1970b) concluded from material
balance computations that sedimentary processes have probably caused
a depletion of about 3%0 in 18 0 in the world's oceans. (As will be discussed under "Sedimentary Rocks", sediments, especially their authigenic components, are appreciably heavier in 18 0 than weathered igneous rocks.) SA YIN and EpSTEIN (1970b) argued that most of the depletion of 18 0 took place during Precambrian time, and only about 0.6%0
since the end of the Precambrian.
Since connate waters have been defined as ancient ocean water
trapped in the sediments, they should have an isotopic composition
similar to ocean water. However, the processes involved in the development of saline formation waters (brines) are complicated by the extensive
chemical changes that have taken place in the brines after sediment
deposition. CLAYTON et al. (1966) concluded from the relationships between £5D and £5180-variations and chemical compositions of the brines
that the deuterium content has not been greatly altered by exchange or
fractionation processes but that extensive oxygen exchange has taken
place between water and reservoir rocks.
CLAYTON et al. (1966) demonstrated that oil field saline formation
waters are meteoric and not connate waters. However, the problem of
the origin of oil field brines still seems problematical. Many natural
brines, originally interpreted as connate waters, may be meteoric waters
that have undergone isotopic exchange and enrichments in salts.
2. The Isotopic Composition of Dissolved Compounds in Natural
Water
a) Carbon Species in Water
In addition to organic carbon, four other carbon species exist in
natural water: dissolved CO2 , H2C03 , HCO) , and CO) - , all of which
tend to equilibrate with each other. As has already been mentioned, the
concentration and the isotopic composition of the single compounds
vary with temperature and pH.
HCO) is the dominant species in ocean water. Assuming that
pH = 8.2, and T= 25° C, the isotopic composition of ocean water in
equilibrium with atmospheric CO2 should be around zero relative to
75
ious authors to be 0.5 to 1.5%0, in IgO. Going back from Recent to
Paleozoic time it seems rather certain that ocean water, within small
variations, had a constant isotopic composition. Greater variations of
several per mil and more may have been possible in Precambrian time.
(PERRY, 1967, suggested that the low 180-contents of Precambrian massive cherts reflect lower 180-contents in the Precambrian oceans.) Assuming a constant rate of weathering and sedimentation throughout the
earth's history, SAVIN and EpSTEIN (1970b) concluded from material
balance computations that sedimentary processes have probably caused
a depletion of about 3%0 in 18 0 in the world's oceans. (As will be discussed under "Sedimentary Rocks", sediments, especially their authigenic components, are appreciably heavier in 18 0 than weathered igneous rocks.) SA YIN and EpSTEIN (1970b) argued that most of the depletion of 18 0 took place during Precambrian time, and only about 0.6%0
since the end of the Precambrian.
Since connate waters have been defined as ancient ocean water
trapped in the sediments, they should have an isotopic composition
similar to ocean water. However, the processes involved in the development of saline formation waters (brines) are complicated by the extensive
chemical changes that have taken place in the brines after sediment
deposition. CLAYTON et al. (1966) concluded from the relationships between £5D and £5180-variations and chemical compositions of the brines
that the deuterium content has not been greatly altered by exchange or
fractionation processes but that extensive oxygen exchange has taken
place between water and reservoir rocks.
CLAYTON et al. (1966) demonstrated that oil field saline formation
waters are meteoric and not connate waters. However, the problem of
the origin of oil field brines still seems problematical. Many natural
brines, originally interpreted as connate waters, may be meteoric waters
that have undergone isotopic exchange and enrichments in salts.
2. The Isotopic Composition of Dissolved Compounds in Natural
Water
a) Carbon Species in Water
In addition to organic carbon, four other carbon species exist in
natural water: dissolved CO2 , H2C03 , HCO) , and CO) - , all of which
tend to equilibrate with each other. As has already been mentioned, the
concentration and the isotopic composition of the single compounds
vary with temperature and pH.
HCO) is the dominant species in ocean water. Assuming that
pH = 8.2, and T= 25° C, the isotopic composition of ocean water in
equilibrium with atmospheric CO2 should be around zero relative to