During examination of residues of samples prepared for foraminifers, observations of other fossil groups and various lithologic characteristics of sediments were made. These observations are summarized in Table AT1.
The site was drilled on the continental rise; therefore, glauconite and well-rounded terrigenous grains found above Core 188-1165B-26X (which originated in shallow marine settings) were probably reworked from older sections on the continental shelf.
Glauconite is a common component of Paleogene sediments on the Mac. Robertson Shelf (Quilty et al., 1999) and is unlikely to be forming in the modern shelf environment around Antarctica. It is present in sediments at Site 1165 in several forms: individual rounded grains or clusters of grains; elongate, curved "concertina" form; and in one instance, pseudomorphic after a foraminifer. It appears almost continuously above Core 188-1165B-26X but only rarely below that depth.
Sample 188-1165B-2H-CC contains a notable quantity of volcanic glass shards in the 63- to 125-µm fraction. Possible sources include a South Sandwich Islands eruption (followed by transport to this site via currents south of the Antarctic Divergence), a local source from an unknown eruption (Gaussberg is only 800 km east of this site and has been active in the last 50 k.y.), or some other source such as the McDonald Islands west of Heard Island.
Smaller amounts of volcanic glass are present in several other samples and seem to represent three different types. No information is available on possible genetic relationships between the different types of glass. One is clear and consists of thin walled glass that surrounds bubbles. A second, widespread but rare form, is dark brown with fewer bubbles. Occasionally it appears to be solid. A third type is in the form of white grains, usually somewhat elongate, with highly parallel structure, probably resulting from flow of a viscous lava and elongation of bubbles.
Crystalline pyrite in small irregular clusters, or pseudomorphic after organic remains, is present in the interval from Samples 188-1165B-32X-CC through 45X-CC. It appears to be diagenetic and not detrital. In some samples (e.g., 188-1165B-37X-CC), it is in the form of simple rods (length:diameter = 5:1) but in others it is in the form of semicylinders, suggesting growth in the lower half of a horizontal cylindrical fossil on (or slightly under) the seafloor. This has been observed in modern samples in the vicinity of Prydz Bay (Harris et al., 1997a).
Larger cylindrical bodies of pyrite (up to 2.5 mm × 1.0 mm) are present in the form of framboidal aggregates in Sample 188-1165B-45X-CC. These are pseudomorphic after fossils, perhaps echinoid spines, suggesting that calcite-secreting organisms lived on the seafloor but are not represented by fossils.
Several samples yielded small amounts of unidentifiable fine bone fragments; however, each of those in Samples 188-1165B-38X-CC, 55X-CC, and especially 33R-CC also yielded several teeth that may be attributable to notothenioid fish, the only fish family now occupying the waters around Antarctica.
Chert is present in several samples in and below Sample 188-1165B-40X-CC. It is in the form of chert chips (probably from small nodules formed in situ) in samples such as 188-1165B-67X-CC and commonly in samples below that depth. In most shallower occurrences, it is found as partly chertified sediment with high organic silica content. This material seems to be in the early stages of formation. It appears to be restricted to a discrete depth range because it is absent from Hole 1165C cores, except for Sample 188-1165C-10R-CC, where it is in the form of partly chertified sediment rather than solid chert.
Using the data tabulated in Table AT1, it is possible to identify a series of levels at which the paleoenvironment changed. These are indicated on the table. Horizons and events are discussed in an uphole (i.e., time) sequence.
Below this depth, residues contain no evidence of a terrigenous component nor of siliceous microfossils. The uniformity of the sequence is broken only by the occasional presence of chert, in the form of chips (in the residue) of dark gray chert, probably fragments of small diagenetic nodules, essentially in situ.
The interval between Samples 188-1165B-56X-CC and 53X-CC marks a transition from small chert-dominated residues to more terrigenous grains. Some samples contain residues of terrigenous and biogenic origin, including sponge spicules and minor amounts of bone.
This interval of six cores is barren of pyrite but contains minute residues, commonly dominated by terrigenous content, and may include minor incipient chert. The distinction between it and the interval below is the greater volume of residue.
Between this sample and 188-1165B-45X-CC, samples contain pyrite, orosphaerid radiolarians, and in one case, significant bone. The presence of pyrite indicates reducing conditions near the sediment surface (a seafloor feature), and the presence of the large (up to 2-3 mm) planktonic radiolarians suggests changes either in the planktonic realm or in preservation conditions on the seafloor.
Above this depth residues are generally larger, but the notable feature is the presence in most samples of glauconite—usually only in trace quantities, but in two samples (188-1165B-11H-CC and 12H-CC) in noteworthy amounts. The increase in glauconite at this level in the hole suggests a shift in sediment source to a Paleogene section. This increase may reflect a change in either source or transporting mechanism.
This level approximates the horizon above which abundant carbonate is present.
In situ diagenetic chert is absent from samples above Sample 188-1165X-40X-CC, suggesting that this is an important horizon from a diagenetic perspective. Another significant level is at the base of Core 188-1165B-2H, where volcanic glass is notable. It coincides roughly with the base of abundant carbonate and, with the carbonate above, provides scope for dating from the surface to some level in Core 188-1165B-3H using radiometric and biochemical means.