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.  18 O / 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 coccolith dino- 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 ( 18 O/ 16 O) sample  ( 18 O/ 16 O) standard ( 18 O/ 16 O) standard  18 O= x10 3 (permil)

3. Isotopic composition of the ocean T=16.9  4.2(  c  w )  0.13(  c   w ) 2 4. Changes in the  18 O of the ocean (a) Emiliani(1955, 1966): Amplitude of isotopic variation related to glacial and interglacial periods is 1.8‰. 70%  changes in temperature (5~6  C); 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  18 O during glacial times being similar to that of surface water (between 1.4~1.6 ‰).

The isotopic changes recorded in benthic foraminiferal 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 18 O 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 d 18 O 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.