THE IMPACT OF SEASONALITY ON THE BENTHOS AS REFLECTED IN THE ASSEMBLAGES OF DEEP-SEA FORAMINIFERA

A surface sediment study of benthic foraminiferal assemblages in the e astern Pacific (Loubere, 1994) yielded a regression equation for estim ating surface ocean productivity from assemblage composition with an r (2) = 0.98. This equation was tested with samples from locations outsi de the calibration data set area and yielded estimates with an error o f about 11% of the estimate. The estimation equation has an r(2) = 0.8 6 for the non-calibration data set samples. This equation was then app lied to surface sediment samples from the Indian Ocean, which represen t benthic conditions identical to those of the Pacific calibration dat a set, except that surface ocean productivity in the Indian Ocean is h ighly seasonal. There is relatively little seasonal variation in produ ctivity over the areas sampled for the Pacific calibration data. Altho ugh it is difficult to quantify average annual surface ocean productiv ity in the Indian Ocean, it appears that the benthic foraminiferal tra nsfer function yields estimates of only qualitative value. At highest productivities it is likely that the equation underestimates productiv ity. Using Discriminant Function Analysis to compare sample groups fro m the Pacific and Indian Oceans representing identical environmental c onditions, except for seasonality, shows that the Indian Ocean samples are increasingly anomalous, or ''no-analog'', from the Pacific Ocean perspective as productivity increases. At higher productivities, India n Ocean samples are deficient in Uvigerina species, Chilostomella sp., Pullenia sp. and E. tumidulus while having an excess of E. exigua, C. hooperi, Gyroidina grp. species, B. mexicana and Nonion species, from the Pacific viewpoint. The differences in benthic foraminiferal assem blages between the two oceans can be ascribed to changes in benthic co mmunity structure that occur as organic carbon flux to the seabed beco mes increasingly episodic or seasonal. Overall, Indian Ocean foraminif eral assemblages follow compositional trends similar to those seen in the Pacific Ocean as surface productivity changes. Hence, a Pacific ba sed benthic foraminiferal transfer function can still reasonably estim ate productivity gradients from Indian Ocean assemblages. However, the re are sufficient assemblage differences between the two oceans to mak e seasonality a variable that should be incorporated in a calibration data set, and to make estimation of paleo-seasonality a possibility. ( C) 1998 Elsevier Science Ltd. All rights reserved.