Neritic and Bathyal Planktonic Foraminiferal Paleobathymetric Indicators

Roberta K. Smith, Marcia M. Gowing

A comparative study was made of planktonic foraminifers standing stocks from a set of 35-µm mesh net tows and from flux collected by particle interceptor traps from various depths between 0 and 2,000 m at Vertex station 5A in the north-central Pacific. Different groups of taxa were found living at different depths. A Globigerinoides group, including Globigerinoides, Orbulina, Globigerinella, and characteristic Globigerina, inhabited the euphotic zone (approximately 0-125 m, or roughly upper to mid-neritic depths). Globorotalia s.s. and several species of Globorotalia s.l. inhabited the lower euphotic and upper aphotic zones (approximately > 50-200 m, or roughly mid-neritic depths). A deeper aphotic depth range, approximately 250-2,000 m (maximum depth sampled) was inhabited by the hitherto little-known, small Berggrenia(?) clarkei Rogl and Bolli and B. (?) cf. B. (?) clarkei.

Berggrenia (?) clarkei s.l. has previously largely been lost or disregarded because of its small size (130 maximum diameters ranged from 65 to 130 µm, commonly 85-105 µm) and being mistaken for juveniles of other taxa. Its time range is unknown, but may extend down into the Neogene. The value of the form as a paleobathymetric and paleotemperature and possibly paleochemical indicator is just emerging. It appears to live at lower neritic and bathyal depths. Although it is possible that Berggrenia (?) walls dissolve relatively readily below 1,000-1,500 m (data are inconclusive), Berggrenia has been seen in sediments. This is significant because to the extent that these three water-depth-related groups fossilize, a record of their depths and habitats is preserved by their presen e and by ¹⁸O/¹⁶O and other elemental and isotopic measurements in the CaCO₃ walls (with some time-related and diagenetic changes).

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.