Signatures of natural catastrophic events and anthropogenic impact in an estuarine environment, New Zealand

The sedimentary record of known natural catastrophic events and anthropogenic activity in an estuarine environment is assessed using sedimentological, chemical and geochronological techniques. Shallow cores collected from intertidal and salt marsh sediments in Ahuriri Estuary, Hawke's Bay, reveal a variety of signatures of natural and human disturbance. Evidence for the 1931 Hawke's Bay earthquake, which resulted in an uplift of one to two metres in the Napier area, is given by a change from silt- to sand-dominated sediment in the lower estuary, which is consistent with a shift toward higher energy depositional conditions following uplift. However, based on physical properties of sediments, the 1931 uplift event does not appear to have caused major changes in depositional conditions in the upper estuary. Similarly, no changes were recorded at one site in the lower estuary, which seems to represent an area of low energy depositional environment. Although the 1960 Chilean tsunami resulted in structural damage in Napier, it did not produce any recognisable sedimentological and geochemical signature in the sedimentary record, suggesting that the study sites were possibly beyond the Limit of sedimentation of the tsunami. Post-European settlement impact is mainly restricted to the lower estuary, where increased concentrations of Zn, Cr, Pb and Cu are attributed to industrial discharges. Evidence of agricultural runoff is shown by an increase in Cu concentrations within a fine-grained depositional environment that is distal from industrial sources in the town of Napier. Chemical data (Cl and S) suggest a change in the depositional environment in the upper estuary due to increased freshwater influx and/or decrease in seawater influence. Dating by Pb-210 suggests that this occurred around the middle part of the 19th century, and might be attributed to river flooding in the region at that time. (C) 2000 Elsevier Science B.V. All rights reserved.