Quantitative analysis was performed on the Quaternary planktonic foraminiferal fauna from Site 516, near the crest of the Rio Grande Rise, and Site 518, оп the lower western flank of the Rise. From Hole 516, 46 samples were taken, and from Hole 518, 80 samples were taken. Тhе mean interval between samples is 20 to 25 cm. About 50 species of Quaternary and Pliocene planktonic foraminifers were ... [Show full abstract] identified. Quaternary sediments, dated bу the initial evolutionary appearance of Globorotalia truncatulinoides and other criteria, have thickness of 9.8 m in Hole 516 and 16 m in Hole 518.
The Globorotalia truncatulinoides Zone is subdivided into four subzones or biostratigraphic horizons (from lower to upper): (1) Globorotalia crassaformis viola, (2) Globorotalia crassaformis hessi, (3) Globigerina calida calida, and
(4) Globigerinoides ruЬer (pink). Thickness of these horizons in Hole 516 establishes the age of the boundaries between them as 1.47, 0.81, and 0.28 Ма, respectively.
All the Quatemary planktonic foraminiferal complexes sampled are subtropical. Тhе region of the Rio Grande Rise, therefore, has beеn within the southem subtropical gyre continuously for the last 2 Ма.
The average annu.al surface water temperatures were reconstructed for the Quaternary at both sites. А micropaleontologic method for the paleotemperature analysis of the thanatocoenoses registers an average Quatemary temperature of 21.2°С at Site 516 and 21.7°С at Site 518. The temperature fluctuations increase up to 3.5°С during the accumulation of the two last horizons (since 0.81 Ма). Temperature peaks are tentatively compared with oxygen isotopic stages and with continental glaciations. Levels at which planktonic foraminiferal species disappear correspond to coldwater intervals.
In the Quaternary of Site 518, some layers show signs of dissolution. Corrosive to СаСО3, the northward flow of Antarctic Bottom Water through the Vema Channel increases during the cold periods. Site 518 has two layers of redeposited foraminiferal sand with Pliocene foraminifers. The average rate of the Quaternary sedimentation in Hole
516 is 0.52 cm per thousand years, and in Hole 518 it is 0.84 cm per thousand years.