Molluscan isotope sclerochronology in marine palaeoclimatology: Taxa, technique and timespan issues

Study of the accretionary biomineralised hardparts of organisms (sclerochronology) can make a useful contribution
to palaeoclimatology. Ontogenetic sequences of isotopic data (δ18O and Δ47 values) from the shells of
marine molluscs are a source of information on seasonal sea-surface temperatures that can be used for detailed
and rigorous evaluation of the outputs of numerical climate models. In situations where there is significant
seasonality, and where shell preservation is adequate, accurate information about winter and summer surface
temperature can be obtained from shallow-water benthic forms (bivalves and gastropods), in particular the early
ontogeny of fast-growing species. Accurate information about winter surface temperature can also be obtained
from individuals that lived at mid-shelf depths (20–40 m), but summer seafloor values from these need upward
adjustment to derive a plausible surface temperature if thermal stratification of the water column occurs in this
season. Ontogenetic δ18O profiles from planktonic pteropod gastropods are a potential source of insight into
seasonal surface temperatures in the ocean basins; these organisms merit investigation for provision of information
to complement shelf data.
Temperature profiles constructed from shell δ18O require an estimate of the δ18O value of ambient seawater,
which can be derived by back-calculation from the Δ47-temperature supplied by the same shell material.
Alternatively, through appropriate sampling and data processing, seasonal temperatures can be obtained directly
from Δ47 profiles. Climate parameters are defined in terms of the mean state over a period of 30 consecutive
years, a statistic (e.g., for seasonal temperatures) which can be derived from the long isotopic temperature records
obtainable from bivalve species that live for many tens or hundreds of years. Efforts should be made to
acquire such records, especially averaged data from crossdated shells, to specify climate parameters for precise
times in the past. Information for precise times would be of particular value for icehouse intervals like the late
Cenozoic, characterised by high frequency (high amplitude) climate fluctuation. Short records from noncrossdated
shells can nevertheless provide useful insights into climate, particularly if a large dataset is obtained,
supplying a reliable picture of the mean state and range of variation in climate parameters over the
interval represented by the shells.