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Otoliths and temperature history

Learn how we reconstruct otolith temperature histories with different approaches, using isotopes and relative temperature histories.

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Reconstruction approaches

Reconstructing temperature histories of individual fish is of considerable value to those studying:

In principle, the chemical composition of the daily growth increments formed in the otolith records the temperature history of the fish on a daily basis.

Based largely on previous studies of coral, 2 approaches have been employed for otolith-based temperature reconstruction:

Early applications of Sr:Ca ratios were promising, and appeared to indicate a reasonable relationship with temperature. However, subsequent research has indicated that there is no generalized relationship between Sr:Ca and temperature, with the possible exception of cold-water larvae. Therefore, attention has shifted to analyses of oxygen isotope ratios.

Calculating temperatures with isotopes

The basis for temperature reconstructions is that oxygen incorporated into carbonates (such as the otolith) includes both major isotopes (18O and 16O).

Normally, we’d expect them to be deposited in equilibrium with their concentration in the water. However, there is a physical fractionation due to temperature at the time of deposition, such that the proportion becomes increasingly depleted as temperature increases. As a result, if the 18O:16O in the ambient water is known, and if the 18O:16O in the otolith can be measured, we can calculate the temperature. With the recent development of otolith micromilling devices, small regions of the otolith (around 50 micrometres in width) can be sampled and assayed. This provides temporal resolution on the order of months.

Relative temperature history

The major constraint to the application of oxygen isotope assays is the requirement for knowledge of the ambient 18O:16O in the water during the period of interest.

Where this is known, otolith assays should provide an accurate and unambiguous temperature history of the fish. If the isotope ratio of the water is unknown, salinity can often be used as a proxy for the ratio. However, the temperature history may then become a relative one, rather than absolute.

Sometimes, reconstruction of the temperature history is likely to be flawed. This includes instances where fish migrate through various water masses characterized by large, unmeasured differences in oxygen isotope composition. Nevertheless, the microsampled isotope ratios will still record the migration through the various water masses on an age-structured basis.

Temperature history projects

Recent projects have included:

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