Site 1132 is the shallowest site (Hole B, 218.5 m) in the western transect and supports mounded features (Holbourn et al., in press). The two older dates (26.45 and 36.77 ka) are from benthic foraminifers and are contemporaneous with the bryozoan mound growth, indicating a sediment accumulation rate of 76 cm/k.y. (Tables T1, T2). At 2.3 mbsf, a 21.50-ka date from total bulk sediment was obtained indicating a sediment accumulation rate of 11 cm/k.y. from this point in time until the present. These calculations indicate a reduction of sediment accumulation rate at this site by seven times since the byrozoan mounds were terminated at MIS 2 (James et al., 2000). The sediments in the upper 20 m of this site are alternating white, light gray, and pale yellow bryozoan floatstones, grainstones, and packstones—all heavily bioturbated. The allochems are mostly broken debris of delicate and robust branching bryozoans and benthic foraminifers. The matrix consists of nannofossils, foraminifers, and sponge and tunicate spicules. All sediment descriptions are from Feary, Hine, Malone, et al. (2000).
A total of 12 AMS dates were obtained from Site 1130 (Hole 1130A), located in 486.7 m of water downslope of the buried bryozoan mounds at Site 1132 (Table T1). AMS dates from the <63-µm fraction, the sand fraction, and total bulk sediment were obtained. The <63-µm fraction dates were always younger than the dates obtained from the bulk sand fraction. At 5.73 mbsf, dates were obtained from the <63-µm fraction (19.0 ka), bulk sand fraction (27.6 ka), and total bulk sediment (24.5 ka). This was the only location where dates were obtained from these three different grain size fractions. The total bulk sediment date fell between the dates obtained from the <63-µm fraction and the bulk sand fraction. The dominant sediment is a moderately to heavily bioturbated pale olive and light gray bioclastic packstone. The coarse fraction is dominantly unidentifiable skeletal debris, benthic formainfers, and sponge spicules. The matrix consists of nannofossils, tunicate and sponge spicules, small quartz grains, and minor clay.
Various sediment accumulation rates were calculated based upon the grain size of the material dated (Table T2). These rates varied over a wide range of values for similar, but not exactly the same, periods of time, indicating that different sediment sources, different transport mechanisms, and bioturbation were involved and active. However, these dates generally show that sediment accumulation rates decreased toward the top of the succession.
Site 1134 (Hole 1134A), in the deepest water on this transect (701.0 m), provided a 38.1-ka date at the uppermost sample (1.95 mbsf). Extending down to 10.95 mbsf, a date of 44.2 ka was obtained making the age difference of this section 6.1 ka. Note that there is stratigraphic inversion within the dates at this site. All dates were from <63-µm bulk sediment. The sediment accumulation rate from the youngest date obtained at this site to present is 5 cm/k.y., making it the lowest of all calculated in this study. The dominant sediments are alternating light gray, light olive gray, and white nannofossil oozes with planktonic foraminfer and bioclasts. These sediments are texturally wackestone and are moderately to heavily bioturbated.
Site 1131 (Hole 1131A in 333.6 m water depth; Hole 1131B in 331.4 m water depth) is the intermediate site of a three-site depth transect termed the eastern transect. No data were obtained from Site 1129, the shallowest site along the eastern transect, because of the proximity of the bryozoan reef mounds in the shallow subsurface. James et al. (2000) point out that the bryozoan reef mounds were active no later than 26.61 k.y. at 11.2 mbsf and were covered by a 7- to 10-m-thick drape that lacks the prolific bryozoans found in the mounds beneath. This drape is a massive, homogenous, light gray, unlithified bioclastic packstone. The bryozoan reef mounds below are texturally floatstone (James et al., 2000). AMS dates from this drape were obtained from both the <63-µm bulk sediment and from the bulk sand fraction.
Hole 1127B was the deepest hole along the eastern transect lying in 479.3 m of water. There were no bryozoan reef mounds encountered here. The uppermost 6 m of the section consisted of a nannofossil ooze overlying a foraminiferal ooze. Below 6 mbsf, a thick section of alternating bioclastic wackestone- and packstone-dominated packages were predominant. Radiocarbon AMS dates reveal that the <63-µm bulk sediment fraction provided dates that were younger or the same as age as dates from the bulk sand fraction (Table T1). Calculated accumulation rates from the <63-µm bulk sediment fraction and the bulk sand fraction are similar except that the oldest date from the bulk sand fraction is stratigraphically inverted and contrasts sharply with the 275 cm/k.y. rate calculated from the two older dates obtained from the <63-µm bulk sediment fraction (Table T2).
Site 1132 grain size data indicate a coarsening upward trend from ~9 mbsf as a result of increased gravel- and sand-sized fractions (Fig. F2; Table T3). At 7.25 mbsf there is a pronounced increase in coarser material. This location within the hole corresponds approximately to the top of the bryozoan reef mounds. There is another coarser unit at 2.7 mbsf. Carbonate content is quite variable over the top 9-m-long section displayed with values ranging from 86.7 to 98.4 wt%. Carbonate mineralogy is mostly invariant with HMC being dominant extending down to the bottom of the section examined here at 19.8 mbsf.
Grain size data at Site 1130 reveals two pronounced sand-dominated units at 5.73 and 9.73 mbsf (Fig. F3; Table T4). Both of these occur approximately within the time frame bracketed by the 19.00- to 26.90-ka AMS dates from the <63-µm bulk sediment fraction (Table T1). These dates lie within MIS 2 and the last glacial maximum. The sediments are dominantly HMC, but there is an overall increase in LMC as compared to Site 1132.
Grain size at Site 1134 is much finer grained although both gravel- and sand-sized sediments are found (Fig. F4; Table T5). The 14.45 mbsf section examined has a coarser-grained upper unit and a finer grained lower unit, with the transition occurring at ~6 mbsf. Carbonate content is generally between 90 and 95 wt% but drops to below 84 wt% at the bottom of the section examined at this site.
The upper 10 mbsf at Site 1129 is sand dominated with some gravel (Fig. F5; Table T6). Sand increases in content within the uppermost 2.5 m of this section. Where there is a noticeable drop in carbonate content, there is a corresponding increase in total organic content. At some point below this studied section lie the bryozoan reef mounds.
At Site 1131 there is also an increase in sand from ~10.36 mbsf, the base of this studied section, to the top (Fig. F6; Table T7). This encompasses almost all of the sedimentary cover or drape that lies on top of the buried bryozoan reef mounds, which are encountered at ~11.2 mbsf. Note the same relationship between carbonate content and total organic carbon as seen at Site 1129. Overall, the sediments here at Site 1131 are finer grained than those at the shallower Site 1129.
At Site 1127, the deepest site along the eastern transect, this sedimentary section is finer grained than both those sections examined at Site 1129 and Site 1131 (Fig. F7; Table T8). Grain size trends are less variant. Again, there is an inverse relationship between carbonate content and total organic carbon.