Generally, the low variability of the physical properties in the holes can be attributed to biogenic mixing. This effect is particularly noticeable at Site 1099, where the low-variability material between 40 and 87 mbsf is almost completely bioturbated. The porosity in both holes increases in variability toward their bases; this may be a result of gas fracturing on depressurization and warming on deck. However, it may equally be a result of the increase in pebbles >0.5 cm in both holes at these depths, as reported by the shipboard party (Barker, Camerlenghi, Acton, et al., 1999). Hole 1098C has the additional possibility that the porosity variation reflects the increased slumping observed in the beds toward the base of the hole.
Lower absolute levels of porosity below 25 mbsf at both sites are associated with the poorer diatom preservation noted by the shipboard party (Barker, Camerlenghi, Acton, et al., 1999), whereas the high-porosity, diatom-rich, laminated sediments have better preserved diatom chains and spines than the bioturbated units. It is therefore likely that the low density values reflect a semi-interlocked skeleton of biosiliceous material. Little laboratory research has been done in this area, but it would appear possible on the basis of the rapid change in attributes here that consolidation does not take place in such high diatom concentration sediments until the overburden stress reaches the yield strength of the diatom chain links and spines that support the skeleton.
The porosity and consolidation properties of diatom-containing materials were found to be complex and only partly related to overburden stress by Bryant and Rack (1990) and Pittenger et al. (1989). However, neither study dealt with the depth range in question here, examined diatom damage, or looked at such high concentrations of biogenic material; therefore, the question will have to remain open until laboratory studies can be completed. Both the quoted studies note that at greater depths the internal space of the diatoms gives sediments a higher than expected porosity, and this will also be acting here. In addition, the homogeneity of the diatom species and limited size range in the laminated layers will allow a more porous structure to develop in the same manner as a well-sorted mineral skeleton.
Such factors may explain why the MGS and porosity appear to be inversely related in both holes. The high water content resulting from these processes reduces the bulk density and susceptibility (Bryant and Rack, 1990). Bioturbated layers have poorer diatom preservation, and the effect on porosity may be an additional reason for the finding by Leventer et al. (1996) that bioturbated intervals tended to have a high MGS in the upper 6 mbsf of Basin I. It is likely, however, that the MGS and porosity signals are linked in a manner other than simple volume considerations. For example, a high biogenic content will produce a higher porosity; contrary to this, Leventer et al. (1996) observed that it may also produce a geochemical environment suitable for the removal of susceptible minerals. It is likely, therefore, that the relative importance of volume effects will change with time (and hence, depth). Given the nonlinear relation with depth and without a more detailed study of the mineralogy, it is not appropriate to estimate the effect of porosity on MGS through normalization of the latter's record.
Holes 1098C and 1099A both reveal a trough in the MGS signal and a corresponding high in the porosity data between ~8 and 25 mbsf. Leventer et al. (1996) place the highest point of this trough at 2600 corrected radiocarbon years before present (6 mbsf in their core). Shipboard calculations place the low point at ~11,000 yr before present. Higher MGS and lower porosity levels below this lie directly above diamict units (Hole 1098C [43-45 mbsf] and Site 1099 [32-35 mbsf]). Although the return to high values downcore is obliterated in Hole 1099A by a turbidite, the pattern can be estimated from the MGS levels in the turbidite material (25-32 mbsf) and the material directly below the diamict sequence.
It might be suggested that such a coincidence across holes represents further dating-independent evidence that the diamict at the base of Hole 1098C and that at ~32 mbsf in Hole 1099A (the midbasin reflector) (Barker, Camerlenghi, Acton, et al., 1999) are the same unit. However, there are some inconsistencies between the low-MGS periods in the two holes. The porosity levels inversely match the MGS variation at Site 1099; in Hole 1098C, a porosity rise is less clear and the Hole 1098C MGS signal may also be affected by lower terrigenous inputs at these depths, as seen by the shipboard party (Barker, Camerlenghi, Acton, et al., 1999).
Leventer et al. (1996) have suggested that this trough represents a raised biogenic sedimentation rate during this period, based largely on the presence of raised levels of total organic carbon (TOC) in the portion of the trough they recovered. Shipboard analyses suggest the TOC levels for both holes (Barker, Camerlenghi, Acton, et al., 1999) show peak values matching the complete troughs in both holes. A higher biogenic sedimentation rate convincingly explains both the higher biogenic component of the sediment and the porosity, which would reflect a more complete biogenic skeleton and lack of sufficient time to undergo normal consolidation. Such an explanation must await further dating and sedimentary analysis for confirmation but may link the two diamicts more convincingly and could push back the start of the climatic optimum, which Leventer et al. (1996) claim is responsible for the sedimentation rate change.
The laminated ooze (35-40 mbsf) immediately below the diamict in Hole 1099A carries a signal continuous with that of the diamict and distinct from surrounding sediment. Shipboard smear slides suggest sand and silt levels increase up through these materials with high variabilities (Barker, Camerlenghi, Acton, et al., 1999), which are well reflected in the physical properties. Such a continuum between the two lithologies may well point to the diamict being deposited through a water column and the lithology being controlled solely by ice proximity. It could be suggested that the two lithologies are partly mixed and the diamict terrestrial. However, as they are associated by petrophysical classification, the argument for glaciomarine/lacustrine deposition can be enhanced by examining the porosity of the materials, which is relatively large for a subglacial diamict even compared with high porosity subglacial material (cf. 40%) (Ronnert and Mickelson, 1992). Thus, the evidence is consistent with Basin III being a subglacial lake during the diamict deposition; however, the porosity of the diamict at the base of Hole 1098C is much more consistent with terrestrial subglacial material, possibly suggesting the ice was grounded in the shallower Basin I.
The turbidites in the sequences appear to be strongly associated with an increase of MGS and a decrease in porosity with depth. This is to be expected, with coarse-grained terrigenous material being concentrated at the sequence base, giving it a higher susceptibility, and the high-energy deposition going some way to destroying any diatomaceous sediment skeleton. However, in petrophysical Unit C of Hole 1098C and Unit F of Hole 1099C, there are turbidites for which the ramping of these attributes does not coincide with the lithologic limits of the turbidite. The turbidites appear to have affected the material below them without affecting their lithologic appearance. An example is in Hole 1098C, where this process includes the preservation of laminated sediments, suggesting that the material below the turbidites is overprinted with the turbiditic signature. This may occur by settling of small amounts of the high-MGS material into the shear liquefied and unconsolidated seabed sediments that are overrun or by compression and/or flow parallel shear. Given the order of magnitude difference in magnetic susceptibility, any small quantity of terrigenous sediment introduced to the diatomaceous material may swamp the original MGS record with little consequence for the particle size range of the material. This effect is most noticeable in Hole 1098C, where the material between 27 and 28 mbsf is overprinted by the turbidite. The material between 28 mbsf and the top of the next turbidite at 29.5 mbsf could reasonably be assumed identical to that in petrophysical Unit B. The mismatch between the turbidite material at 71 mbsf in Holes 1099A and 1099B and the associated events in the MGS and porosity records between 70 and 74 mbsf may be due to a similar process; however, the more homogeneous nature of the material below the lithologic trace of the turbidite in this case makes it impossible to stipulate that this lower material is not a reworked flow.