STABLE ISOTOPES, SCLEROCHRONOLOGY AND GROWTH HIATUSES: SOURCES OF PALEOENVIRONMENTAL, PALEOECOLOGICAL AND EVOLUTIONARY DATA

Seasonal cessations of growth are common features in bivalve mollusk shells. They occur when environmental conditions exceed growth tolerances and are generally recognized by incomplete isotopic and/or sclerochronologic records. By restricting the range of conditions recorded in the shell these growth halts set the limits for environmental reconstructions. However, gaps can also be informative, providing environmental, ecological and evolutionary data.

For example, environmental reconstructions based on a single taxon that stops growing for some interval may underestimate the full range of actual environmental variability. However, if another species can be found that does not shut down during this interval, they can be used in conjunction to reconstruct the complete range of paleoenvironmental variability.

Integrated sclerochronologic and isotopic profiles also can be useful sources of ecological data. The period of shell deposition between successive growth halts represents an interval of favorable environmental conditions. Because daily increment width profiles can be used to estimate the exact duration of this interval, this technique can detect regional or temporal changes in the growth interval.

Because growth halts occur when a physiological tolerance has been crossed, they can be a useful tool in evolutionary studies. That is, physiological thresholds can be compared within and across evolutionary lineages. This information can be used to trace changes in environmental tolerances within a group or to document how environmental conditions affect different species.

These examples illustrate that growth hiatuses, rather than detracting from shell archives, are a valuable component of biogenic records of ancient environments.