Visual examination of the sediment core sections from Core 178-1098A-6H reveals that the thickness of the orange-brown and blue-gray laminae decreases upcore from interval 178-1098A-6H-3, 43 cm, through 6H-1, 0 cm. Laminations range in thickness from 1 mm to 3 cm in Section 178-1098A-6H-1, and from 2 to 3 cm in interval 178-1098A-6H-3, 0-29 cm. Between interval 178-1098A-6H-3, 110 cm, and 6H-3, 43 cm, the blue-gray diamict takes on an orangey hue with four identifiable orange-brown laminae (intervals 178-1098A-6H-3, 52-53.5, 67-68, 96-97, and 109-110 cm). Below interval 178-1098A-6H-3, 110 cm, there are no orange-brown laminae and the orangey hue disappears from the sediment. Detailed comparison and correlation of Hole 1098A with Holes 1098B and 1098C (Table T1) reveals that Hole 1098A has an almost complete sample of the postglacial laminated unit, which permits the radiocarbon chronology that was developed for Hole 1098C (Domack et al., 2001) to be used on Hole 1098A.
BSEI analysis concentrated on samples from Section 178-1098A-6H-2, so the large-scale reduction in average lamina thickness from interval 178-1098A-6H-3, 43 cm, through 6H-1, 0 cm, is not detected, but analysis did reveal that the sediment is laminated on two scales. There is a primary alternation between thick laminae of diatom ooze and diatom-bearing terrigenous sediments (Fig. F2) and also a smaller-scale inclusion of thin biogenic sublaminae within the diatom-bearing terrigenous lamination.
Contacts between the two lamina compositional types vary from sharp (Fig. F3A) to gradational (Fig. F3B) to bioturbated (Fig. F3C). Bioturbation is on a subcentimeter scale and is not seen during visual core inspection. Sharp, gradational, and bioturbated transitions of lamination composition over submillimeter distances is clearly seen using both BSEI and optical light microscopy.
All of the diatom-ooze laminations (average thickness = 1.02 cm; standard deviation [SD] = 0.84 cm; range = 0.1-3.6 cm; N = 124) examined in Core 178-1098A-6H are dominated by Chaetoceros Ehrenberg resting spores (CRS) (Fig. F4). BSEI and light microscopy show that the ooze is variable over distances of <100 µm, with areas of densely packed CRS, more porous patches, and silt/clay-rich lenses (Fig. F4B). SEI of horizontal fracture surfaces shows that the CRS-ooze laminae can vary between almost exclusively CRS (Fig. F4C) to CRS with high percentages (up to 50%) of detached setae from the Chaetoceros vegetative stages (Fig. F4D). The diatom-ooze laminae also can show a gradation between ooze dominated by CRS and detached setae at the base to pure CRS at the top. SEI of vertical fracture surfaces revealed a very porous fabric characterized by open pore spaces and microtunnels (Fig. F4E). Other diatom taxa are very subordinate to the CRS and are present as isolated valves or as clusters (Fig. F5A).
Thinner diatom-ooze sublaminae (submillimeter) within the thick diatom-bearing terrigenous laminae are dominated by either almost monogeneric assemblage of centric diatoms (Fig. F5C) or, more commonly, Chaetoceros setae (Fig. F5E). Centric diatom sublaminae are usually found close to the top of the diatom-bearing terrigenous laminae, including ones produced by Corethron criophilum (Fig. F5D). Sublaminae composed of Chaetoceros setae are very difficult to detect using BSEI or light microscopy (being only a few microns in thickness); however, fractured surfaces through diatom-bearing terrigenous laminae reveal that setae-rich sublaminae are common (Fig. F5E).
Light microscope analysis of the diatom-bearing terrigenous laminae (average thickness = 1.08 cm; SD = 1.55 cm; range = 0.1-10.4 cm; N = 122) shows that a significant amount of bioturbation affects the fabric (Fig. F3C). SEI analysis of the laminae reveals that the diatom assemblage is much more diverse than within the almost monogeneric ooze. Diatoms are sometimes present as monogeneric clusters within the terrigenous laminae (Fig. F5B). CRS are still common, but the proportion of other centric taxa increases. Chaetoceros setae occur throughout the laminae, as well as in discrete sublaminae (Fig. F5E). In general, the frustules within the diatom-bearing terrigenous laminae are less well preserved than frustules in the ooze.
All methods of analysis used revealed that microbioturbation occurs intermittently through the examined Sections 178-1098A-6H-1 to 6H-3, redistributing material across lamina boundaries (Fig. F3) as well as increasing porosity within the diatom ooze (Fig. F4). The redistribution of sediment occurs over distances of up to 1 cm and, as stated earlier, is not obvious from visual core inspection.