Sediment texture of the continental shelf and upper slope off the west coast of South Island, New Zealand

Abstract

Surface sediments on the continental shelf and upper slope off the west coast of the South Island, New Zealand, from north of Karamea Bight (40°45'S) to the Whataroa River mouth (43°07'S) consist almost entirely of sands and muds. Inshore (< 50 m depth) sediment is predominantly fine sand to coarse silt. Silt-clay content increases towards mid-shelf depths, and areas of mud occur at depths of about 90-140 m. Sediments tend to become coarser again on the outer shelf, and sandy mud or muddy sand covers the outer shelf and upper slope. The west coast shelf experiences a high input of fluvial sediment, probably c. 12-26 × 10 6 tonnes y -1 for the study area. Sedimentation rate on the shelf is estimated to average at least 1-2 mm y -1 . Sedimentation is probably appreciably lower than this in the north of the area and higher in the south, and this is reflected in shelf grain-size parameters and sediment carbonate content. There are north-to-south decreases in mean grain size and in the ranges of sorting and skewness: shelf sediments tend to become finer and more uniform in texture towards the south. Carbonate content ranges from 0-70%, generally increasing with depth, but with a marked north-south contrast. In the north, carbonate values increase to c. 15% by 200 m depth and to 60-70% by 650-700 m, whereas off the southernmost transect, carbonate values are < 5% by 500 m and reach only c. 40% by 1100 m. Shelf

Macrobenthic polychaete assemblages of the continental shelf and upper slope off the west coast of the South Island, New Zealand

“…This is because of a southward increase in silt content; the clay content of Transect 4 stations is generally lower than at stations on Transects 1-3 (Table 1). The rate of modern sedimentation increases towards the south of the study area as there is a north-south increase in input of riverine sediment and a corresponding decrease in shelf width (Probert & Swanson 1985). The increase of sediment carbonate content with water depth also indicates greater dilution of carbonate by detrital sediment to the south (Probert & Swanson 1985).…”

“…1). The shelf is c. 85 km wide off Karamea Bight, but narrows to 30-35 km at the southern end of the study area; the shelf edge lies at c. 187-225 m (Carter 1980;Probert & Swanson 1985). Apart from the constriction caused by the Hokitika Canyon, the shelf is relatively uninterrupted.…”

“…Sediment samples were analysed by standard sieving and pipette procedures, and carbonate content was determined by back titration (see Probert & Swanson 1985). A subcore was also taken for determination of sediment adenosine triphosphate (ATP) (see Probert 1986).…”

“…Sediments tend to become coarser again on the outer shelf (> 100-150 m), and sandy mud or muddy sand covers the outer shelf and upper slope, except south-west of Hokitika Canyon where there is a less pronounced coarsening of sediments at outer shelf depths. Towards the south of the study area, rates of sedimentation are generally higher and shelf sediments tend to become finer and more uniform in texture (Probert & Swanson 1985). The study area is characterised by highly variable changes in freshwater inflow and episodic upwelling (Stanton & Moore 1992).…”

“…included in a more detailed sedimentological study of the area (Probert & Swanson 1985). The mean depth of penetration of the box cores was 25 cm, and sample volume declined as mean grain diameter decreased (Probert 1984).…”