The impact of seasonality on the benthos as reflected in the assemblages of deep-sea foraminifera

Abstract

A surface sediment study of benthic foraminiferal assemblages in the eastern Pacific (Loubere, 1994) yielded a regression equation for estimating surface ocean productivity from assemblage composition with an r 2=0.98. This equation was tested with samples from locations outside the calibration data set area and yielded estimates with an error of about 11% of the estimate. The estimation equation has an r 2=0.86 for the non-calibration data set samples. This equation was then applied to surface sediment samples from the Indian Ocean, which represent benthic conditions identical to those of the Pacific calibration data set, except that surface ocean productivity in the Indian Ocean is highly seasonal. There is relatively little seasonal variation in productivity over the areas sampled for the Pacific calibration data. Although it is difficult to quantify average annual surface ocean productivity in the Indian Ocean, it appears that the benthic foraminiferal transfer function yields estimates of only qualitative value. At highest productivities it is likely that the equation underestimates productivity. Using Discriminant Function Analysis to compare sample groups from the Pacific and Indian Oceans representing identical environmental conditions, 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, Indian 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 assemblages between the two oceans can be ascribed to changes in benthic community structure that occur as organic carbon flux to the seabed becomes increasingly episodic or seasonal. Overall, Indian Ocean foraminiferal assemblages follow compositional trends similar to those seen in the Pacific Ocean as surface productivity changes. Hence, a Pacific based benthic foraminiferal transfer function can still reasonably estimate productivity gradients from Indian Ocean assemblages. However, there are 4 sufficient assemblage differences between the two oceans to make seasonality a variable that should be incorporated in a calibration data set, and to make estimation of paleo-seasonality a possibility.