THE RECORD OF GARNETS AND DICTYOCORYNE (PROFUNDA?) AT SITES TN057-6-PC4 AND ODP 177-1090 (AGULHAS RIDGE, SOUTH ATLANTIC)

Cores from two sites on the Agulhas Ridge (AR), South Atlantic (TN057-6-PC4 and ODP 177-1090) have been spliced together; our analysis of ice-rafted debris (IRD) from these cores covers MIS 5c through MIS 14 so far. Counts were carried out on the same samples that were used to establish the stable-isotope stratigraphy. Garnets that are visually indistinguishable from those found in East Antarctica occur throughout both cores. Metamorphic garnets must be terrigenous, and could not be transported to these sites, above the influence of bottom-currents and far from any land, other than by icebergs. Garnets in IRD deposits close to Antarctica are cited as evidence for an East Antarctic origin (Diekmann and Kuhn, 1999). In our AR cores, garnets are found during both glacials and as isolated grains during strong stadials within interglacials. Thus ice-rafting continued throughout the interval studied, indicating the stability of at least the East Antarctic Ice Sheet.

A second notable feature of the studied interval is the presence of the radiolarian Dictyocoryne (profunda?) during the warmest intervals only. So far, MIS 9-14 at ODP 177-1090 and MIS 5c, 10-12 at TN057-6-PC4 have been counted. D. profunda has been considered to be a warm-water indicator (Casey, 1971), although specimens have been found in cores from more southerly latitudes (Brathauer, 1996). However, in that study all sizes from >40 microns up were counted, while we studied only the size interval from 250 microns to 2 mm. Matsuoka and Anderson (1992) and Sugiyama and Anderson (1997) investigated the effects of silica, salinity and temperature on growth and survival of the closely-related D. truncatum, and established that temperature is the critical variable affecting growth. It is possible that the pattern of occurrence of D. (profunda?) that we observe reflects a similar effect of temperature on size. If the pattern persists in our cores, this idea should be further investigated.