Oxygen Isotope

and

Paleoclimatic Information

B. Oxygen Isotope studies of calcareous

marine fauna

A. Paleoclimatic information from biological

material in ocean cores

C. Oxygen Isotope stratigraphy

D. 18O / Ice volume / Sea-level changes

A. Paleoclimatic information from biological material in ocean cores

1. Paleoclimatic inferences from biogenic material in ocean

sediments derive from assemblages of dead organisms

(thanatocoenoses). However, thanatocoenoses may not

be representative of the biocoenoses in the overlying

water column—selective dissolution of thin-walled

specimens at depth, differential removal of easily

transported species by scouring bottom currents, and

occasional contamination by exotic species transported

over long distances by large-scale ocean currents.

ACD

CCD

Lysocline

2. Biogenic oozes are made up primarily of the calcareous

or siliceous skeletons (tests) of marine organisms,

which may have been planktic (passive floating

organisms living near the surface [0-200m]) or benthic

(bottom dwelling).

(a) Calcareous materials— foraminifera, coccolith,

dinoflagellate

(b) Siliceous materials— radiolarian, silicoflagellate,

diatom

3. Paleoclimatic influences from the remains of calcareous

and siliceous organisms have resulted from basically

three types of analysis:

(a) the oxygen isotopic composition of calcium carbonate in

foram test;

(b) the relative abundance of warm and cold water species

or quantitative interpretations of species assemblages

and their spatial variations through time

(c) morphological variations in a particular species resulting

from environmental factors

N. pachyderma

G. bulloides

diatom

coccolithdino-flagellate

1. Urey(1948): if an animal deposits calcium carbonate in

equilibrium with the water in which it lies, and the shell

sinks to the bottom of the sea…it is only necessary to

determine the ratio of the isotopes of oxygen in the shell

today in order to known the temperature at which the

animal lived.

2. Isotopic composition of a sample

B. Oxygen Isotope studies of calcareous marine fauna

(18O/16O)sample (18O/16O)standard

(18O/16O)standard

18O= x103 (permil)

3. Isotopic composition of the ocean

T=16.94.2(cw)0.13(c w)2

4. Changes in the 18O of the ocean

(a) Emiliani(1955, 1966): Amplitude of isotopic variation

related to glacial and interglacial periods is 1.8‰.

70%changes in temperature (5~6C);

30%changes in the isotopic composition of ocean

water.

(c) Dansgaard and Tauber(1969): They estimated the

isotopic composition of glacial age ocean water as

+1.2‰ , thus accounting for ~70% of the observed

isotopic change in foraminiferal carbonate at that

time.

(b) Shackleton(1967): Analyzed benthic foraminifera to

show an increase of bottom water in 18O during

glacial times being similar to that of surface water

(between 1.4~1.6 ‰).

The isotopic changes recorded in benthic fora

miniferal tests are primarily a record of changing

terrestrial ice volumes, or a “paleo-glaciation”

record.

5. problems:

(a) the isotopic composition of the water is unknown

(b) vital effect:

In some cases, the carbonate would not be formed in

isotopic equilibrium with the water. (ex: Globigerinoides

ruber give isotopic values 0.5‰ lighter than expected.)

(c) variations in the depth habitat:

Water density is of prime importance to individual

species, as the same species may be found in different

areas living at different depths, but in water of the

same temperature and salinity.

(d) gametogenesis:

Foram tests from sea floor are significantly enriched

with 18O compared to their living counterparts. This

is apparently due to calcification of the tests at depth

(>300m) considerably below the upper mixed layer.

It may count for ~20% of foram test weight in sample

from the sea floor, because calcium carbonate has

been extracted from water which is much cooler

than that nearer the surface, the overall d18O values

indicate a mean temperature significantly lower than

the near-surface temperature.

(e) salinity effect:

Any change in salinity due to large-scale dilution effects

(because of ice sheet melting) or to local changes in

the precipitation-evaporation (P-E) relationship will also

be recorded in foraminifera.

(f) dissolution:

The effect of dissolution in the thanatocoenoses is a

pervasive factor with implications not only for isotopic

studies, but for all paleoclimatic studies based on floral

and faunal assemblages. Most importantly, dissolution

does not effect all species uniformly, selective removal

of the more fragile.