Planktonic foraminifers from the Southwestern Atlantic (30 to 60°S): species-specific patterns in the upper 50 m.

BOLTOVSKOY ESTEBAN; BOLTOVSKOY DEMETRIO; CORREA NANCY; BRANDINI, FREDERICO

MARINE MICROPALEONTOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 1996 vol. 28 p. 53 - 72

0377-8398

Planktonic foraminifers were studied in 96 samples collected in the southwestern Atlantic (30 to 60°S, along 53°W) in November 1993, mainly from depths between 0 and 50 m. Very high proportions of juveniles (unidentified) were present throughout the area, especially north of 37°S, where they accounted for up to 70-80% of all shells recorded. For most species no clear vertical specific stratification was detected in the 0-50 m layer. Zoogeographic grouping of the 18 species identified allowed defining 5 distinct zones along the transect: Subtropical (north of 31°S, 80% warm water individuals); Warm-Transitional (34-37°S, 35% warm water); Transitional (37-49°S, 99% cold water); Subantarctic (49-55°S, 100% cold water); and Antarctic (south of 56°S, 100% cold water). Boundaries between foraminiferal assemblage zones are in good agreement with hydrological fronts described for the area. Comparison of the present data with planktonic collections from the North Atlantic show large differences in the proportions of various taxa. In the 14-24°C range, G. bulloides is much more abundant in the northern hemisphere than in the southern one, whereas G. quinqueloba, G. inflata and G. rubescens show the opposite trend. On the other hand, temperature-related percentage contributions within the 14-24°C range indicate that the preferred thermic régimes of the 9 species considered are remarkably similar in the North and South Atlantic collections compared. The southernmost planktonic distributional ranges of selected warm water taxa are roughly coincident with those established previously on the basis of surface sediments, disagreements being chiefly attributable to selective dissolution on the bottom. In contrast, on the bottom percentages of cold water foraminifers (G. bulloides, G. pachyderma) are significantly enhanced with respect to their planktonic populations, and their sedimentary northward limits extend well beyond their maximum ranges in the upper-layer plankton. It is suggested that submergence of these cold water species and northward displacement at subsurface depths is chiefly responsible for the mismatching patterns observed. Dissimilar species proportions in both hemispheres and plankton-sediments uncouplings can conceivably engender erroneous conclusions when distributional data are used for paleoecologic reconstructions with the aid of numerical analyses such as the Imbrie-Kipp transfer-function technique.