220.127.116.11. Palaeoclimatic reconstruction from biogenic material
Biogenic sea floor sediments are called oozes, and are usually either calcareous or siliceous in nature. Calcareous oozes consist mainly of the carbonate tests (skeletons) of millions of marine organisms, whilst siliceous oozes are made up of their silicate counterparts. For palaeoclimatic purposes, the most important materials are the tests of foraminifera (calcareous zooplankton), coccoliths (calcareous algae), radiolarians and silicoflagellates (siliceous zooplankton), and diatoms (siliceous algae).
Palaeoclimate reconstruction from the study of calcareous and siliceous tests has resulted from basically three types of analysis:
a) the oxygen isotope composition of calcium carbonate;
b) the relative abundance of warm- and cold-water species;
c) the morphological variations in particular species resulting from environmental factors.
Most work has concentrated on the study of the foraminifera, in particular oxygen isotopic analyses (e.g. Emiliani, 1977, 1978; Shackleton, 1977; Duplessy & Shackleton, 1985; Shackleton, 1988).
If calcium carbonate (of a marine organism) is crystallised slowly in water, 18O is slightly concentrated in the precipitate relative to that remaining in the water. This fractionation process is temperature dependent, with the concentrating effect diminishing as temperature increases. When the organism dies, the test sinks to the sea bed and is laid down, with millions of other tests, as sea floor sediment (calcareous ooze), thus preserving a temperature signal (in the form of an oxygen isotopic ratio) from a time when the organism lived. If a record of oxygen isotope ratios is built up from cores of ocean sediment, and the cores can be accurately dated, this will provide a method of palaeoclimate reconstruction (Urey, 1948).
As for isotope ratios from ice cores, the oxygen isotopic composition of a sample is generally expressed as a departure, d18O, from the 18O/16O ratio of an arbitrary standard, 18O/16OSMOW (see equation 12, section 18.104.22.168). The fractionation effect is much smaller than that which occurs during evaporation/condensation of water, and typically, d18O values are no more than a few parts per mille (‰) above or below the SMOW isotopic ratio.
Empirical studies relating the isotopic composition of calcium carbonate deposited by marine organisms to the temperature at the time of deposition have demonstrated the following relationship:
T = 16.9 - 4.2 (dc - dw) + 0.13 (dc - dw)2
where T is the water temperature (°C), dc is departure from SMOW of the carbonate sample and dw is the departure from SMOW of the water in which the sample precipitated (Shackleton, 1974). For modern analyses, dw can be measured directly in ocean water samples; in fossil samples, however, the isotopic composition of sea water is unknown and cannot be assumed to have been the same as it is today. In particular, during glacial times, sea water was isotopically heavier (i.e. enriched in 18O) compared to today; large quantities of isotopically lighter water were land-locked as huge ice sheet formations. Thus, the expected increase in dc due to colder sea surface temperatures during glacial times, is complicated by the increase in dw at these times.
By analysing isotopic records of deep water organisms, it is possible to resolve how much of the increase in dc for surface organisms was due to decreases in surface temperature and how much due to continental ice sheet formation. It is expected that bottom water temperatures (» -1°C to 2°C) have changed very little since glacial times (the last glacial maximum being 18,000 Ka) and increases in dc for deep water organisms would reflect only changes in the isotopic composition of the glacial ocean. On this basis, Duplessy (1978) has concluded that 70% of the changes in the isotopic composition of surface dwelling organisms was due to changes in the isotopic composition of the oceans, and only 30% due to temperature variations.
Unfortunately, changes in the isotopic composition of the ocean reservoirs are not the only complications affecting a simple temperature interpretation of dc variations. The assumption that marine organisms precipitate calcium carbonate from sea water in equilibrium is sometimes invalidated. Certain vital effects of marine organisms, such as the incorporation of metabolically produced carbon dioxide, may cause a departure from the thermodynamic equilibrium of carbonate precipitation (Urey, 1947). However, by careful selection of species either with no vital effects or where the vital effects may be quantified, this problem can be avoided.
In addition to stable isotope analyses, the reconstruction of palaeoclimates can also be achieved by studying the relative abundances of species, or species assemblages (e.g. Williams & Johnson, 1975), and their morphological variations (e.g. Kennett, 1976). In the case of the latter, test coiling directions (either right-coiling (dextral) or left-coiling (sinistral)) often reveal useful proxy information about palaeo-temperatures of the oceans. Other variations include differences in test size, shape and surface structure.