A 2400 yr record of natural events and anthropogenic impacts in intercorrelated terrestrial and marine sediment cores: Waipaoa sedimentary system, New Zealand

摘要

The Waipaoa sedimentary system spans 100 km from terrestrial upland to continental rise. Alluvial buffering has little effect on sediment flux at the outlet of this mesoscale dispersal system, and hinterland-to-margin transport is accomplished rapidly. Because of this synergy, the floodplain and shelf depocenters are sensitive to changes in sediment production in the hinterland, and natural and anthropogenically forced changes in sediment source dynamics that occur at several temporal and spatial scales leave distinctive signals in the stratigraphic record. Manifested as variations in sediment properties, these signals appear in intercorrelated sediment cores from a headwater riparian storage area and the major terrestrial and marine repositories for sediment discharged during the past 2.4 k.y. The signals represent the landscape response to vegetation and land-use change, short-term fluctuations in climate that affect surface properties and processes, and extreme storms and subduction-thrust earthquakes. Extreme storms are the minimum geomorphologically effective event preserved in the sediment records. Lower-magnitude storms that are integral components of the prevailing hydrometeorological regime create high-frequency fluctuations in sediment properties and collectively contribute to event sequences of >100 yr duration. Events and event sequences comprise a hierarchy of temporally sensitive phenomena, the impacts of which are conditioned by frequency and magnitude. By contrast, vegetation disturbance is a spatially sensitive phenomenon that directly impacts sediment source areas and lowers the threshold of landscape sensitivity to erosion. For this reason, the Taupo eruption of 1.718 ka and the piecemeal vegetation changes that occurred after the arrival of Polynesian settlers also generated strong depositional signals. After European colonization, deforestation of the hinterland altered landscape sensitivity and precipitated the transition to an erosional regime that impacted sediment production and dispersal across the entire magnitude-frequency spectrum of events, regulating sediment delivery to and transport in stream channels. No other perturbation had such a profound impact on the Late Holocene depositional record.