Sediments of the Leg 177 sites are dominated by calcareous and siliceous biogenic components comprising foraminifers, nannofossils, diatoms and subordinate radiolarians, silicoflagellates, and sponge spicules (Fig. 14). Almost pure calcareous sediments were recovered at Sites 1088 and 1092 situated well above the regional CCD in intermediate water depths on the Agulhas Ridge (2083 m) and the Meteor Rise (1976 m). South of the Subantarctic Front, diatom-rich sediments predominate at Sites 1091 (4361 m), 1093 (3624 m), and 1094 (2808 m), within the circumantarctic opal belt. At all sites, the terrigenous sediment fraction mainly consists of siliciclastic silt and clay. At the southern Sites 1091-1094, sand- to gravel-sized ice-rafted detritus (IRD) represents a minor but ubiquitous constituent of the sediments.

Pelagic calcareous sediments at Site 1088 (Agulhas Ridge) consist of Quaternary foraminifer nannofossil ooze that grades into nannofossil ooze in Pliocene to middle Miocene sediments. Opal and siliciclastics represent minor components. Downhole lithological change is associated with an increase in clay-sized particles in the terrigenous fraction.

Site 1089 (4624 m) is located on a drift deposit at the northern flank of the Agulhas Ridge. Quaternary to Pliocene calcareous sediments contain the highest concentrations (up to 50%) of terrigenous silt and clay encountered at Leg 177 sites, permitting the study of current strengths of paleo-bottom water. Lithological alternations between mud- and carbonate-rich sediments probably reflect sedimentary cycles attributed to glacial-interglacial cycles that triggered oscillations in carbonate production and/or terrigenous sediment supply.

At Site 1090 (3699 m) on the southern flank of the Agulhas Ridge, we recovered a 400-m-thick sediment succession that yields a long-term record of lithological and paleoenvironmental change from the Quaternary to the middle Eocene, interrupted by several hiatuses. A hiatus at 70 mcd, marked by a redeposited tephra layer and color change to redder sediments below, separates a Pleistocene to lower Pliocene calcareous ooze from lower Miocene sediments that are more opal- and mud-rich. The older sediments contain high opal concentrations (as much as 50%) in the late Eocene. The lower part of the section is composed of zeolite-bearing calcareous ooze of middle Eocene age.

In addition to Site 1088, Site 1092 on the Meteor Rise provides a low-sedimentation-rate record of pelagic calcareous deposits, spanning the Pleistocene to Miocene. The Quaternary part of the section reveals distinct variations in opal and siliciclastics, probably associated with glacial-interglacial cycles.

Sites 1091 and 1093 yielded diatom oozes deposited at high sedimentation rates from the Holocene to Pliocene. They represent typical pelagic deposits of the southern abyssal portion of the southeast Atlantic (Fig. 9). Distinct carbonate-rich intervals probably indicate peak interglacial periods, which were more frequent in the late Pleistocene to Pliocene record. At Site 1093, the Pliocene part of the section accumulated at lower sedimentation rates and is marked by an increase in terrigenous silts and clays. Several millimeter-thick marker beds, consisting of sand-sized foraminifer ooze, are intercalated in the section of Site 1091 and probably represent turbidites.

Rapidly deposited diatom ooze of Pleistocene age was also obtained at Site 1094, which was drilled in 2808 m of water in a small sedimentary basin north of Bouvet Island. In contrast to Sites 1091 and 1093, only a few carbonate-bearing intervals were found. Downhole lithological variations are marked by pronounced changes in the abundance of siliciclastics, as illustrated by fluctuations in magnetic susceptibility that show peak values in glacial intervals.

Four porcellanite horizons were penetrated at Site 1094 and form discrete layers as documented in Parasound seismograms. Porcellanite mainly is present as brownish amorphous fragments derived from the crushing of concrete porcellanite layers during coring, and also as individual loaf-shaped concretions, as much as 6 cm in diameter, exhibiting internal bedding structures that indicate an early diagenetic growth within the host sediment. Fragments of porcellanite were also found as cavings in cores of Site 1093. Shipboard X-ray diffraction (XRD) measurements indicate an opal-CT composition for the porcellanites. Joint investigations among Leg 177 scientists on the geochemical and mineralogical properties of porcellanite, pore-water, and host sediment composition in the context of regional heat flow and spatial distribution patterns of porcellanite layers will provide information on the conditions under which these young porcellanites formed.

Sediments from Sites 1091, 1093, and 1094 contain scattered IRD throughout the entire sections and should provide a high-temporal resolution record of past ice-rafting activity in response to terrestrial ice-sheet dynamics. IRD mainly consists of volcaniclastic particles along with minor quartz and crystalline rock fragments.

A significant proportion of the sediment at the southern Sites 1091, 1093, and 1094, which have the highest accumulation rates, consists of mats of the needle-like diatom Thalassiothrix. These diatom mats, which proved difficult to recover with the APC or XCB coring systems, occur as intervals of laminated sediment as much as 20 m thick (Fig. 15), as intermittently laminated sediment, or as bioturbated mat fragments or burrow-fills of mat material. Stratigraphically, this mat sediment is common in the transitions to and from interglacial, carbonate-rich sediment resulting in expanded sections in these intervals (e.g., the 5-m-thick MIS 12/11 boundary at Site 1093). At the two southernmost sites (1093 and 1094), diatom mats were recovered in the upper and mid-Pleistocene sediment. At Site 1091, located in the PFZ, the youngest diatom mats were noted at the lower/mid-Pleistocene boundary (Fig. 11). At both Sites 1091 and 1093, the most significant diatom mat sediment was deposited in the late early and mid-Pleistocene. Diatom mats also occur in the mid-Pliocene. The Leg 177 Thalassiothrix diatom mat deposits are remarkably similar to the vast Neogene laminated diatom mat deposits of the eastern equatorial Pacific Ocean (Kemp and Baldauf, 1993). Such deposits are thought to form beneath intense frontal zones (Kemp et al., 1995) and, in the Leg 177 sites, the Thalassiothrix mat intervals may track the paleoposition of the Southern Ocean frontal systems. These laminated sequences also represent a paleosediment trap that preserves individual flux events and provides the potential to generate pelagic records of climate/ocean change at key time intervals at a resolution that rivals that of ice cores.

To 177 Summary - Biostratigraphy

To 177 Table of Contents

ODP Publications

ODP Home