OXYGEN-ISOTOPE AND CARBON-ISOTOPE ANALYSES OF THE ARTICULATE BRACHIOPOD LAQUEUS-CALIFORNIANUS - A RECORDER OF ENVIRONMENTAL-CHANGES IN THE SUBEUPHOTIC ZONE

This study is the first systematic attempt to use oxygen and carbon is otopes in modern brachiopods to investigate seasonality and growth rat es. A comparison of oxygen-isotope analyses of shells of living specim ens of Laqueus californianus dredged from 80 and 130 m off Santa Catal ina Island, California, to available hydrographic data indicates that this articulate brachiopod secretes its shell in or close to oxygen is otopic equilibrium with ambient seawater. Periodic oxygen-isotope depl etions appear to result from EI Nino events. Unexpectedly low delta(13 )C values associated with the 1983 El Nino may be explained by increas ed bacterial activity or organic loading into the Southern California Eight associated with these warm-water pulses. Growth rates determined from annual cycles in delta(18)O records are variable, but generally average between 2 and 3 mm yr(-1) for mature individuals. Because of t he longevity of L. californianus, O-18 profiles provide high-resolutio n seasonal temperature records spanning one to two decades. Our data s uggest that oxygen isotopes in brachiopod shells can be utilized as mo nitors of environmental change in the subeuphotic zone.