Marine Carbonates
95
3) the isotopic preservation of primary oxygen in the carbonates.
1) As has already been mentioned, it may be concluded from several
arguments that ocean water has had a constant isotopic composition
within small limits during geological history. Glacial periods during the
history of the earth are, however, excluded.
We may, therefore, reasonably assume that ancient ocean water had a
180j160 ratio similar to that of today. However, a crucial point is the
question of "paleosalinities". We must know if the organism to be analyzed has lived in ocean water of 35%0 salinity. Ocean water of higher
salinities has a higher 180-content, because during evaporation 16 0 is
preferentially concentrated in the vapor phase; ocean water of lower
salinities has a lower 180-content, because it is diluted by fresh waters.
EpSTEIN and MAYEDA (1953) estimate that variations in salinities of one
unit in 35%0 salinity would be accompanied by 1
0
C error in temperature
determinations in a nonglacial period of the history of the earth.
2) Some organisms, e.g., mollusks (LLOYD, 1964) and brachiopods
(LOWENSTAM, 1961), apparently deposit calcite or aragonite in isotope
equilibrium with ocean water. In contrast, other organisms, e.g., echinoderms, asteroidea, ophiuroidea, and crinoidea (WEBER and RAUP,
1966a, b; WEBER, 1968) do not precipitate their carbonates in equilibrium with their environment. These observations are accounted for by
an isotope-exchange reaction between respiratory CO2 and dissolved
bicarbonate at or near the site of skeletal deposition. The fact that metabolic fractionation occurs in many organisms is most important in respect to the problem of paleotemperature determinations with the help of
ammonites and belemnites and other species (TOURTELOT and RYE,
1969; SPAETH et aI., 1971).
A knowledge of the ecologic behavior of shell-secreting organisms is
essential. If still-existing species are used for thermometry, the assumption must be made that their depth habitats have not changed with time.
EICHLER and RISTEDT (1966) reported variations in the 180rO ratio of
the early shell and septa of two Nautilus specimens, which are in part
ascribed to migrations from warmer to cooler water after a certain stage
of development. How complicated these processes are can be demonstrated in the measured difference between right- and left-coiled foraminifera, observed by LONGINELLI and TON GIORGI (1964). One explanation might be the preferential growth of one form or the other within a
certain temperature range.
Another important point is the question whether the calcium-carbonate-secreting organisms grow shells only during a portion of the
local temperature range or throughout the entire range. EpSTEIN and
LOWENSTAM (1953) have shown that growth of skeletons of most species
does not take place during the entire year. The majority of the pelecy-
95
3) the isotopic preservation of primary oxygen in the carbonates.
1) As has already been mentioned, it may be concluded from several
arguments that ocean water has had a constant isotopic composition
within small limits during geological history. Glacial periods during the
history of the earth are, however, excluded.
We may, therefore, reasonably assume that ancient ocean water had a
180j160 ratio similar to that of today. However, a crucial point is the
question of "paleosalinities". We must know if the organism to be analyzed has lived in ocean water of 35%0 salinity. Ocean water of higher
salinities has a higher 180-content, because during evaporation 16 0 is
preferentially concentrated in the vapor phase; ocean water of lower
salinities has a lower 180-content, because it is diluted by fresh waters.
EpSTEIN and MAYEDA (1953) estimate that variations in salinities of one
unit in 35%0 salinity would be accompanied by 1
0
C error in temperature
determinations in a nonglacial period of the history of the earth.
2) Some organisms, e.g., mollusks (LLOYD, 1964) and brachiopods
(LOWENSTAM, 1961), apparently deposit calcite or aragonite in isotope
equilibrium with ocean water. In contrast, other organisms, e.g., echinoderms, asteroidea, ophiuroidea, and crinoidea (WEBER and RAUP,
1966a, b; WEBER, 1968) do not precipitate their carbonates in equilibrium with their environment. These observations are accounted for by
an isotope-exchange reaction between respiratory CO2 and dissolved
bicarbonate at or near the site of skeletal deposition. The fact that metabolic fractionation occurs in many organisms is most important in respect to the problem of paleotemperature determinations with the help of
ammonites and belemnites and other species (TOURTELOT and RYE,
1969; SPAETH et aI., 1971).
A knowledge of the ecologic behavior of shell-secreting organisms is
essential. If still-existing species are used for thermometry, the assumption must be made that their depth habitats have not changed with time.
EICHLER and RISTEDT (1966) reported variations in the 180rO ratio of
the early shell and septa of two Nautilus specimens, which are in part
ascribed to migrations from warmer to cooler water after a certain stage
of development. How complicated these processes are can be demonstrated in the measured difference between right- and left-coiled foraminifera, observed by LONGINELLI and TON GIORGI (1964). One explanation might be the preferential growth of one form or the other within a
certain temperature range.
Another important point is the question whether the calcium-carbonate-secreting organisms grow shells only during a portion of the
local temperature range or throughout the entire range. EpSTEIN and
LOWENSTAM (1953) have shown that growth of skeletons of most species
does not take place during the entire year. The majority of the pelecy-