DID HIGH-CALCIUM SEA WATER IN THE LATE CRETACEOUS CAUSE ANOMALOUSLY HIGH RATIOS OF PLANKTONIC TO BENTHIC FORAMINIFERA?

Absolute water depth has been estimated for ancient deposits based on the ratio of planktonic to benthic foraminiferal tests (the P/B ratio) in today's seas. In the present study, the formula proposed by Van der Zwaan et al. (1990, Marine Geology 95:1-16) was used to estimate absolute water depth for the Upper Cretaceous Mooreville, Demopolis, and Prairie Bluff Chalks in west-central Alabama. Planktonic individuals made up 70-80% of the total of ~300 specimens in each sample. The resulting absolute paleodepth estimates were as follows: 557m (mean of 3) for the Mooreville Chalk, 601 m for the Demopolis Chalk, and 436m for the Prairie Bluff Chalk. Other investigators have reported similar and even higher P/B ratios for Cretaceous deposits in the Gulf Coastal Plain and Western Interior Seaway, leading to depth estimates in excess of 500 m. In our study, these extreme depths are contraindicated by lithologic evidence (e.g., >19% sand in the Prairie Bluff Chalk), megafossil and ichnofossil assemblages, and even the microfossil assemblages themselves. Foraminifera were low-diversity faunas, dominated by Heterohelix, Globigerinelloides, and Hedbergella. These are interpreted to be shallow-shelf opportunists. Deeper-water, single and double-keeled taxa such as Globotruncana and Globotruncanita were relatively rare in our samples.

Cretaceous seas were unlike those of today in several respects including the extreme sea-level highstand. Another significant difference was sea-water chemistry. Recent research (Stanley, Ries, and Hardie, 2005, Geology, 33:593-596) suggests that the high concentration of Ca2+ and high proportion of Ca2+ vs Mg2+ in Cretaceous seas promoted coccolithophore productivity and induced chalk formation. We note that both coccolithophores and planktonic foraminifera have maximum species diversity in the Late Cretaceous. Although biomineralization in foraminifera is more complex than in algae, increased availability of calcium would have aided in test construction. Moreover, as heterotrophs, foraminifera would have benefited from the increased primary productivity and overall enrichment of pelagic communities. We suggest that the abundance of Cretaceous plankton relative to that of today's seas seriously limits the use of foraminiferal P/B ratios in estimating absolute water depths.