Earthquake and human impact on the sedimentology and geochemistry of Ahuriri Estuary, Hawke's Bay

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Chague-Goff, C.; Nichol, S.L.; Ditchburn, R.G.; Trompetter, W.J.; Sutherland, V.T. 1998 Earthquake and human impact on the sedimentology and geochemistry of Ahuriri Estuary, Hawke's Bay. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 98/23 43 p.

Abstract: Three cores were collected from the intertidal and salt marsh sediments in Ahuriri Estuary, Hawke's Bay, and analysed by sedimentological, chemical and geochronological techniques. Signatures of various events, of both natural and anthropogenic origin, were identified. 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 in grainsize distribution in a core from the southern shore of the Lower Estuary. The change to a high energy environment, similar to the present one in the Lower Estuary, has resulted in deposition of sand over fine silt. The data suggest increased sediment accumulation rates following the uplift event, which might be attributed to increased erosion in the upper catchment. There is no evidence of the earthquake at the two other sites sampled, which is probably due to their more sheltered location in the estuary. Post-European settlement impact is mainly restricted to the Lower Estuary, where increased concentrations of Zn, Cr, Pb and Cu may be due to industrial discharges. Evidence of agricultural runoff is given by an increase in Cu concentrations near the Poraiti Hills. The 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 210Pb infers that this occurred prior to 1931, but the origin and timing of the event are still to be determined. Sediment accumulation rates have averaged 2.5 mm/yr for the last 45 years in the Lower Estuary and 3.8 mm/yr for the last 70 years or so in the Upper Estuary. The variation probably reflects the difference in depositional environment, from a high energy environment dominated by tidal and wave action to a low evergy environment with additional organic and fine sediment input from stream runoff. The various signatures identified are based on known events but may be used for identifying events in other less well documented sedimentary sequences. (auth)