Leg 169S--Saanich Inlet

Co-Chief: Brian Bornhold	Cruise Dates: 19-21 August, 1996
Staff Scientist: John Firth	Operations Superintendent: Eugene Pollard

An improved understanding of the climate-ocean system, and in particular the global carbon cycle, require ultra-high resolution studies of rapidly deposited sediments in a variety of geographic settings. Such sites record climatic and oceanographic conditions on an annual or seasonal basis permit calibration and refinement of fully ocean-coupled General Circulation Models, as well as lead to a better appreciation of the links among oceanographic processes, climatic parameters, terrestrial vegetation, and marine biota in coastal areas of the world.

Sediments of Saanich Inlet, British Columbia, contain a virtually continuous record of Holocene climatic and oceanographic change, with seasonal resolution, together with a possible record of paleoseismicity associated with the Cascadia convergent margin. Situated in a fjord near Victoria, British Columbia, the Leg 169S sites record both terrestrial floral change since deglaciation (9,000 to 11,000 years B.P.) as well as marine biological productivity variations in a temperate latitude coastal setting. The Saanich Inlet sites will provide an important complement to the high-resolution record obtained at Site 893 (Leg 146) in the Santa Barbara Basin.

Objectives

Obtain an ultra-high resolution record of Holocene climate, oceanography, marine productivity, and ecology and terrestrial vegetation;
Attempt to establish the frequency of earthquakes (particularly very large events; greater than magnitude 8) in this region of the Cascadia convergent margin;
Advance the understanding of diagenesis in organic-rich sedimentary basins and especially of the role of microbial processes. The predominantly finely laminated (varved) sediments which are believed to have accumulated in the inlet since deglaciation will be cored at two sites in the deeper axial region of the fjord; these two sites (at 200 and 225-m water depth) have significantly different organic contents and accumulation rates. Saanich Inlet will provide an important companion to the high-resolution Site 893 drilled in Santa Barbara Basin.

Preliminary results

During the leg, only the A-hole from each site was run through the MST, split and photographed on board. The B and C hole were MST'd during Leg 169 and returned to ODP/TAMU for further work. Sampling and core description occurred at ODP/TAMU from November 11-15, with the Hole Summaries and Preliminary Report due before the 169S post-cruise meeting in February.

The upper 15 m at both sites consist of dark gray to black, very soft, gassy, highly disturbed sediments. Below this, core disturbance was minimal, and preservation of laminated sequences is superb. The laminated sequence extends to a depth of about 55 mbsf at Site 1033 and about 80 mbsf at Site 1034. Varve thickness at Site 1033 averages about 6 mm, and about 8-10 mm at Site 1034. Varves as thick as 1.5 cm occur at Site 1034. The lamina thicknesses will enable very detailed paleoenvironmental investigations to be undertaken. Long intervals of uninterrupted laminations characterize much of this unit. In the lower part of the laminated unit there is evidence of possible river flood events in the occurrence of thin gray silty-clay bands within normal laminae couplets. The laminated sequences are periodically interrupted by intervals with indistinct laminae, grading to massive structureless muds with overall similar composition to the laminae. Thin units of burrowed sediments apparently reflect periods of basin ventilation. Wood and plant fragments, shells, and fish remains are scattered throughout the laminated interval. Color variations within the laminated sequence display a decade to century scale of variability. The Mazama Ash (about 6800 years B.P.; about 1-1.5 cm thick) was detected in all holes; it was encountered at about 50 mbsf at Site 1033 and about 60 mbsf at Site 1034. (See Mazama Ash core photograph.)

Above the base of the laminated sequence (30-40 cm), there is a 30-40 cm thick interval of gray silty clays at both sites, which possibly reflects a sudden cooling, and conditions of landscape instability in the latest Pleistocene/earliest Holocene, followed by a return to anoxic conditions and laminated sediments.

A 4-cm thick carbonate concretion was recovered at Site 1034 at approximately 80 mbsf. This concretion appears to occur at a depth characterized by significantly higher methane contents. This interval of higher methane can be correlated between the two sites.

The laminated sequence is underlain by gray, largely homogeneous, stiff glacio-marine muds. There is evidence of debris flows (tens of cm thick) and silt to medium sand horizons (up to 10 cm thick) in the lower part of this unit at Site 1034. Occasional granule-sized ice-rafted debris was noted at Site 1034.

Gas analyses revealed no higher order hydrocarbons. Extremely low methane contents were detected in the gray glacio-marine unit, possibly reflecting aerobic conditions. Hydrogen sulfide gas was persistent only in the upper 4 m at each site.

Insitu temperature measurements revealed a linear thermal gradient. The maximum temperature recorded was 11°C at about 80 mbsf at Site 1034.

High resolution magnetic susceptibility and color reflectance data will be extremely valuable for correlation purposes and for construction of composite stratigraphic sections.

See photograph of Leg 169S-1034B-8H5 which may record the beginning of the younger Dryas cooling event.

Operations

Port call activities included a dockside display area, tours for the local media and VIPs, and a luncheon for invited guests on Friday, 16 Aug. On Saturday and Sunday, public tours of the JOIDES Resolution were conducted. Approximately 1200 people toured the ship.