Sediment accumulation patterns and fine-scale strata formation on the Waiapu River shelf, New Zealand

Abstract

Multiple sediment transport and reworking processes influence fine, cm-scale strata formation and long-term accumulation on the Waiapu River shelf, New Zealand. Gravity cores collected during two cruises, in August 2003 and May 2004, were analyzed using 7Be and 210Pb geochronologies, bulk carbon, δ 13C, X-radiographs, and grain-size to investigate sediment mixing and accumulation patterns. The presence of 7Be on the inner- and mid-shelf regions (< ~ 80m) indicated recent (within the last 4–5 months) deposition of fluvial muds, whereas the distribution of excess 210Pb accumulation rates revealed that the middle to outer shelf (50–130 m) acted as fine sediment repositories on longer time scales. Excess 210Pb accumulation rates were high, with an area weighted average of 1.1 ± 0.1 cm/yr and ranging between 0.2 and 3.5 cm/yr, yet were localized such that only an estimated ~ 23% (ranging between 17 and 38%) of the fluvial load was retained on the shelf between 40 and 200 m depths over the last 80 to 100 yr. Sediments not retained on the shelf were either transported to deeper waters or along the shelf beyond the sampling area. Several cores collected from the high sediment accumulation zone on the middle to outer shelf exhibited non-steady state excess 210Pb profiles, suggesting that multiple transport processes influenced fine-scale strata formation. Layers of low excess 210Pb activity and predominantly terrestrial δ 13C and C/N values were likely formed during floods, when sediments were rapidly deposited and buried on the shelf. These event layers were sufficiently thick (up to ~ 20 cm), such that all or a portion of the initial flood layer immediately transited through the surface mixing layer, ensuring preservation in the sediment record. Sediments inter-bedded with these event layers reflected a relatively marine source indicating either that they were not deposited rapidly or were significantly bioturbated. A gradient of physical and biological mixing signatures, extending radially from the Waiapu River mouth, suggested that high background accumulation rates and flood deposits negatively impacted the preservation of biological structures and enhanced preservation of event-produced beds.