Abstract: Planktonic-Benthonic Ratio in Foraminifera as Paleobathymetric Tool--Quantitative Evaluation

Ramil G. Wright

For 20 years the ratio between planktonic and benthonic foraminiferal tests has been recognized as a useful qualitative tool in paleobathymetric analysis. Positive correlation between water depth and the proportion of planktonic tests can be made in many parts of the world. However, distribution of planktonic tests in the waters over the shelf and slope depends not only on depth per se but also is controlled in part by other factors such as distance from shore, upwelling water masses, and variations in salinity and turbidity. Nevertheless, "depth" or depth-related factors can explain two-thirds of the variation in the ratio. It is possible to derive the relation between the ratio and depth and to use this ratio to estimate quantitatively the depth of deposition of the sed ment containing the foraminiferal tests. An evaluation of 688 samples of recent sediments from various areas showed that the relation follows a logarithmic logistic curve but could not be described completely by simple linear-regression techniques because of peculiar boundary conditions. However, a simplified weighted linear-regression model allows the prediction-estimation of depth for samples in which planktonic tests comprise between 5 and 93% of the foraminiferal fauna: D = e^{0.0418 P + 3.4823} where D = water depth in meters and P = percentage of planktonic tests in the sample. Although variation within the data is relatively high, the method tests successfully against several recent and fossil sequences whose depth is known or estimated by independent means.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC