Abstract: Coiling Ratios in Planktonic Foraminifera: Adaptive Strategy and Paleoenvironmental Interpretation

Jere H. Lipps

Coiling ratios in various species of planktonic Foraminifera have been used as environmental, paleoenvironmental, and biostratigraphic indicators. Yet there is no satisfactory explanation of why coiling ratios vary, and so conclusions based on them are suspect. Twelve hypotheses can be listed. Most of these are dismissed for lack of evidence; for example, temperature and salinity, although possibly significant, have no direct effect on coiling ratios. Instead the Foraminifera must respond to such factors in some way that has adaptive advantage for the species. In any animal, reproductive and feeding strategies are especially important for species survival.

Evidence from some benthic Foraminifera indicates that sexual and asexual individuals coil in different directions. Coiling ratios also may reflect sexual-asexual ratios in planktonic Foraminifera, and the advantages conferred on populations dominated by one form or the other may provide clues to paleoenvironmental interpretation.

I suggest that coiling-ratio variations are the result of reproductive strategies that confer selective advantage on the species in different trophic regimes. In seasonally fertile, upwelling regions of the open ocean, a strategy that increases the intrinsic rate of population growth (r) would be favored when food becomes abundant. High r is accomplished most easily by asexual reproduction. In areas or at times of decreased fertility, a population can grow more leisurely and slowly, and engage in more elaborate sexual reproductive strategies. Coiling-ratio variations in space and time, therefore, may indicate changes in upwelling intensity, not changing temperature, salinity, genetics, water depth, and so on. If this is valid, then paleoenvironmental and biostratigraphic interpretatio s using coiling ratios must consider oceanic fertility, especially in the Tertiary and recent populations that lived in areas subject to upwelling, such as California. Variations may occur on different sides of points, embayments, or oceanic fronts at the same time, or at various places along a coast as the climate changes, annually or over geologically significant time periods. Thus, correlation using coiling ratios may be erroneous even in local basins. Large asexual populations may be an indication of sites of significant organic material in the geologic past.

AAPG Search and Discovery Article #90976©1976 AAPG-SEPM-SEG Pacific Section Meeting, San Francisco, California