2. The uppermost 40-50 m consist of very well laminated diatomaceous muds deposited over the past ~7000 years. These sediments are characterized by lamina "triplets" made up of: gray silty mud, deposited during late fall to early spring; olive diatomaceous ooze, deposited in late spring to early summer; and dark olive gray diatomaceous mud, deposited in late summer and early fall. These "triplets" (varves) range in thickness from about 3 to 15 mm, averaging ~6 mm. Well-preserved plant debris and charcoal are plentiful throughout this interval, and macroscopic fish remains are common.
3. Below the very well-laminated interval, sediments become progressively less distinctly laminated and contain abundant whole bivalves and shell fragments, reflecting better oxygenated bottom water conditions between approximately 12000 and 7000 calendar (cal) years BP. At this time relative sea level is believed to have been at least 15 m below that of today; the sill would thus have stood at 55 m water depth or less compared to 70 m today.
4. The Holocene laminated diatomaceous muds are interrupted by occasional massive intervals, a few centimeters to several decimeters in thickness. These are essentially structureless diatomaceous muds, commonly with a basal zone of discontinuous or fragmented laminae and a capping of diatomaceous ooze.
5. A 1.0 to 1.5 cm-thick, light gray ash layer, interpreted to be the Mazama Ash (7645 cal years BP), based on AMS radiocarbon dates, was cored in most holes, providing an invaluable stratigraphic datum.
6. The earliest Holocene is marked by a nearly structureless 40-cm thick gray mud unit with an extremely sharp basal contact and gradational upper contact. It was cored in all holes at both sites and has been dated at ~11000 cal years BP. It apparently represents an abrupt discharge event.
7. The olive gray diatomaceous muds are gradationally underlain by more massive gray silty mud containing graded sand units, dropstones, and contorted silt and sand laminae. This late Pleistocene glaciomarine unit is older than ~12000 cal years BP; the oldest AMS date obtained from this unit is ~14500 cal years BP, ~25 m above the base of the drilled section at the more northerly Site 1034.
8. The upper parts of the sediment section at both sites were highly disturbed due to gas expansion resulting in generally poor quality acoustic velocity data. The bulk density in the upper well laminated
interval is generally low (1-1.3 Mg/m3), with short intervals of higher bulk density characterized by high
magnetic susceptibility; shear strengths are low in this unit (~10 kPa).
The underlying late Pleistocene unit of gray mud has significantly higher bulk densities (1.9-2.1 Mg/m3) and shear strengths (~40 kPa) than the Holocene well laminated unit. Despite the higher shear strengths in this interval, there is no indication of any ice loading or erosion.
9. The organic geochemistry of the sediments segregate into two clear diagenetic zones at 50 mbsf
(Site 1033) and 80 mbsf (Site 1034). The combination of changes in organic supply and oxygenation in the overlying water column are likely responsible for this situation.
The uppermost zone has anaerobic, organic-rich sediments (1-2.5 wt% Corg) and enhanced remineralization. This is indicated by the intense enrichment of nutrient concentrations and extensive bacterial sulfate reduction to sulfide and methanogenesis. The toxic sulfide accumulations preclude infauna in the upper zone, and as a result, the sediments are laminated and not bioturbated.
In the deeper zone, the organic matter is largely refractory and an order of magnitude lower than in the shallower zone (~0.2 wt% Corg). The limited diagenesis is indicated by dissolved sulfate close to full marine concentrations (26 mM), low nutrient levels, and an absence of methane. The clearly oxidized conditions support benthic communities that can bioturbate the sediments and destroy any record of lamination.
10. Preliminary diatom analyses from the well-laminated Holocene show similar species,
abundances, and compositions to previous sediment studies and phytoplankton investigations. Dominant
species include: Paralia sulcata, Thalassionema nitzschoides, Skeletonema costatum, Thalassiosira
gravida, Chaetoceros radicans (spores) and Thalassiosira eccentrica.
Bivalves in the lowermost part of the Holocene are dominated by Macoma calcarea and Axinopsida serricata.
Identified wood samples include Western Red Cedar (Thuja plicata) and Pacific Yew (Taxus brevifolia).
To 169S Table of Contents