Site 1093 was drilled to a TD of 597.7 mbsf, corresponding to 623.8 meters composite depth (mcd). The recovered sediments are diatom-rich pelagic deposits of Quaternary (0-1.8 Ma) to early Pliocene (~3.6 Ma) age, and late Miocene (~6-7 Ma) mud rocks and diatomites in the two deepest cores of Hole 1093D, below a hiatus at 563.9 mbsf (590 mcd). The Pliocene/Pleistocene boundary is probably also marked by a hiatus (434.40 mbsf, 460.51 mcd) representing a gap of ~200 k.y. The thick Quaternary section was deposited at extremely high average sedimentation rates of ~250 m/m.y. A distinctive feature of the diatom-rich Quaternary sediments are intervals of Thalassiothrix diatom mats up to several meters thick, which also appear in sediment cores of Site 1091 located further north in the northern part of the circum-Antarctic opal belt. The probable Pliocene/Pleistocene hiatus is associated with a downhole increase in relative mud content deposited under lower sedimentation rates of 80 m/m.y.
Two lithostratigraphic units are present. The upper unit was further subdivided into two subunits (Fig. F3), on the basis of downhole compositional variations and the degree of sediment consolidation. Lithostratigraphic units and subunits are in accordance with chronostratigraphic subdivisions. Lithostratigraphic Subunit IA (Holocene-Pleistocene) consists of diatom ooze with varying proportions of mud, foraminifers, and nannofossils. Subunit IB (Pliocene) is represented by consolidated muddy diatom ooze, including several carbonate-rich intervals and few unconsolidated diatom ooze layers. Diatomites and mud rocks constitute lithostratigraphic Unit II (Miocene).
Diatoms are the most prominent sedimentary component of this site with varying abundances of foraminifers, nannofossils, and siliciclastic mud. Radiolarians and silicoflagellates are present in trace amounts. A minor but ubiquitous component is dispersed sand- to gravel-sized siliciclastic material, mainly of volcanogenic origin, that probably represents IRD.
Sediment composition was estimated from smear-slide analysis (see "Site 1093 Smear Slides"). In addition, weight proportions of carbonate, opal, and siliciclastics were inferred from coulometric bulk carbonate determination (see "Geochemistry") and by quantitative X-ray diffraction (XRD) analysis of opal concentrations in the carbonate-free fraction (Table T2, also in ASCII format in the TABLES directory). Downhole compositional variations deduced from visual smear-slide analysis do not correlate well with compositional data obtained from coulometry and XRD measurements (Fig. F4). Total carbonate concentrations in smear slides are usually 10%-20% higher than weight percentages of carbonate determined by coulometry, but relative downhole carbonate variations are in good agreement. Concentrations of siliciclastics in smear slides are generally underestimated because the fine-grained mud particles are often masked by larger biogenic particles. Hence, fluctuations of opal and siliciclastics are not well represented by smear-slide data. For the description of the lithostratigraphic units we refer to compositional data inferred from coulometry and XRD (given in average values ± standard deviations).
Recovery in Holes 1093A-1093C was good and penetrated the upper 200 to 250 m of the Quaternary sediments (Fig. F3). Recovery in the deeper interval of Hole 1093D between 250 and 420 mbsf was low to nil, probably because of problems associated with coring extensive diatom-mat sequences. An almost complete recovery marks the upper Pliocene interval between 430 and 500 mbsf. Recovery was low again in the lowermost part of Hole 1093D, yielding only remnants of lower Pliocene and uppermost Miocene mud rocks and diatomites. Holes 1093E and 1093F were drilled to fill recovery gaps in Holes 1093A, 1093B, and 1093C.
Intervals: 177-1093A-1H through 34X (0-309.4 mbsf; 0-330.24 mcd); 177-1093B-1H through 24H (0-221.8 mbsf; 5.28-252.11 mcd); 177-1093C-1H through 18H (0-169.5 mbsf; 7.06-196.49 mcd); 177-1093D-1H through 34X (136.0-434.4 mbsf; 152.49-460.51 mcd); 177-1093E-1H through 2H (4.0-42.5 mbsf; 4.59-48.98 mcd); 177-1093F-1H (34.0-43.5 mbsf; 36.92-46.42 mcd). Note: 0-136.0 mbsf in Hole 1093D, 0-4.0 mbsf in Hole 1093E, and 0-34.0 mbsf in Hole 1093F were drilled ("washed") without coring.
Age: Holocene to Pleistocene
Subunit IA consists of diatom-rich sediments and is made up of alternations of four sediment types: (1) dark olive mud-bearing diatom ooze, (2) olive foraminifer-bearing diatom ooze, (3) olive gray or tan to pale tan diatom ooze (diatom mats), and (4) pale gray foraminifer diatom ooze. Subunit IA comprises the upper 460.51 mcd and extends down to the Pliocene/Pleistocene hiatus. Total carbonate concentrations vary between 0 and 53 wt%, opal averages 66 ± 13 wt%, and siliciclastics average 18 ± 12 wt%. Porcellanite fragments were found in the top sections of Cores 177-1093D-28X, 46X, and 50X. However, they are probably not in place and indicate the presence of porcellanite horizons higher in the section.
Downhole distribution patterns of sediment types were used for interhole correlations (Fig. F5). Dark olive mud-bearing diatom ooze contains 1 ± 1 wt% carbonate, 67 ± 12 wt% opal, and 32 ± 13 wt% siliciclastics. These relatively mud-rich sediments also contain high relative concentrations of IRD that mainly consists of volcanic particles including basalt and dolerite, brownish green glass shards, pumice, lappilli tuff, and welded ash fragments. Volcanic clasts consisting of feldspar and pyroxene phenocrysts, together with garnet grains embedded in a hyaline ground mass, were also identified. Other subordinate IRD components are quartz grains, gneiss fragments, and metaquartzites. In Section 177-1093C-11H-5, a 30-cm-thick residual layer of gritty IRD was encountered (Fig. F6).
The mud-bearing diatom ooze is overlain either by foraminifer-bearing diatom ooze with 9 ± 3 wt% carbonate, 67 ± 3 wt% opal, and 24 ± 13 wt% siliciclastics, or by diatom mats. The average composition of the diatom mats is 10 ± 7 wt% carbonate, 73 ± 10 wt% opal, and 14 ± 10 wt% siliciclastics. The diatom mats include the highest opal contents of the four different sedimentary types, are characterized by a rough spongy surface texture that was observed after scraping the split cores (Fig. F7), and show sparsely to well-developed color laminations and mottles (Fig. F8). They often contain a near-monospecific diatom assemblage of Thalassiothrix sp. Individual intervals of diatom mats range between 1 and 10 m in thickness and are widely distributed in the upper 120 m of lithostratigraphic Subunit IA. Another dense cluster of Thalassiothrix sp. diatom mats appears between 200 and 300 mcd and probably below 300 mcd, throughout the remaining lower part of Subunit IA. These mats likely caused technical problems in core recovery.
In many core intervals the diatom mats grade into pale gray foraminifer diatom ooze that also contains minor quantities of nannofossils. The average composition is 29 ± 12 wt% carbonate, 57 ± 10 wt% opal, and 14 ± 10 wt% siliciclastics. The carbonate-rich units rarely exceed thicknesses >1 m and become more common below 100 mcd. Diatom mats and foraminifer-bearing diatom ooze are usually thinner above the carbonate-rich sediments than in the intervals below the carbonate-rich sediments and grade upward into olive mud-bearing diatom ooze.
Intervals: 177-1093D-34X through 47X (434.4-563.5 mbsf; 461.10-589.60 mcd)
Age: late to latest early Pliocene
This subunit consists of mud diatom ooze with highly variable carbonate concentrations between 0 and 60 wt%, average opal concentrations of 43 ± 14 wt%, and high average siliciclastic mud contents of 49 ± 10 wt%. Compared to lithostratigraphic Subunit IA, the sediments of lithostratigraphic Subunit IB, apart from pure diatom ooze layers, show a higher degree of consolidation. Condensed and compacted diatom mats are not abundant but are still present as intercalations (Fig. F9). Because sediment recovery was <20% in the lowermost 60 m of the subunit, we are unable to determine possible lithologic changes in Cores 177-1093D-40X through 47X.
Intervals: 177-1093D-49X through 50X (578.4-597.7 mbsf; 604.50-623.80 mcd)
Age: late Miocene
Recovery was poor in lithostratigraphic Unit II below the Pliocene/Miocene hiatus, which represents a time gap of ~3-4 m.y. Only lithified remnants of uppermost Miocene laminated mudrocks and diatomites were recovered in the uppermost sections of Cores 177-1093D-49X and 50X, respectively.
Site 1093 is located in the central circum-Antarctic opal belt and was dominated by high diatom export production with terrigenous background sedimentation through bottom-water current transport, ice rafting, and eolian sediment supply. Assuming that the observed alternations of distinct sedimentary types represent glacial-interglacial cycles, interglacial maxima were characterized by high biogenic carbonate accumulation that was preceded and followed by extensive deposition of diatom mats during interglacial-glacial and glacial-interglacial transitions. Deposition of diatom mats accounts for the high sedimentation rates at this site.
The composition of IRD suggests provenance from the volcanic island provinces of the Scotia Arc and the Bransfield Strait rather than a major supply from Antarctic crystalline rocks. Rare volcanic clasts, including garnet xenoliths that have an earth mantle origin, indicate ice-rafted delivery from regions with hotspot volcanism such as Bouvet Island. However, we have no clear indications of provenance and modes of ice rafting (sea ice vs. icebergs) so far.
Sedimentation rates were lower during the Pliocene. Increased concentration of siliciclastic mud may be the result of a higher supply of terrigenous matter or decreased dilution by biogenic components. Calculation of biogenic and siliciclastic accumulation rates may elucidate the controlling factor after establishing a precise chronostratigraphy.