Techniques for estimation of tidal elevation and con” nement (∼salinity) histories of sheltered harbours and estuaries using benthic foraminifera: examples from New Zealand

Abstract

Techniques for estimating tidal elevation and con” nement (proxy for salinity) using modern benthic foraminiferal census data from New Zealand harbours and lower-salinity estuaries are described and assessed for their utility for reconstructing the depositional settings of late-Holocene sequences. We describe a simple modern analogue technique for estimating tidal or subtidal elevation of Holocene fossil faunas, utilizing the modern data set most applicable to the Holocene setting (sheltered tidal harbours and inlets, or lower-salinity estuaries). Canonical correspondence analysis was used to extract a synthetic con” nement gradient from for aminiferal census data in transects down” ve New Zealand estuaries. This gradient was used to develop a New Zealand Con” nement Index that can be computed for New Zealand modern and Holocene estuarine and harbour samples based on their foraminiferal composition. The value of the method for estimating Holocene elevational and con” nement (palaeosalinity) histories was assessed for a tidal inlet and the middle reaches of an estuary. Two earthquake-related vertical displacements are recognized in a 1.1 m core from Ahuriri Inlet, Hawkes Bay: (1) a 1–2 m subsidence (c. 500 BP) from low tidal to subtidal; (2) a 1.5–2.5 m uplift (1931 Napier Earthquake) from subtidal to high tidal, followed by progressively increasing con” nement in a much smaller inlet. A substan tial con” nement increase (probable salinity decrease) is recognized in a 0.8 m core from Rangitopuni Estuary, Auckland, accompanying the widespread disappearance of cockle beds. Palynology shows that this event is associated with forest clearance in the watershed following earliest signi” cant human settlement (c. 600–800 BP). Taphonomic dissolution of calcareous tests was a signi” cant factor in the lesser accuracy of elevational estimates in our estuarine study core, although dissolution appears to correspond with increased freshwater runoff. Taphonomic disaggregation and loss of agglutinated foraminifera did not appear to be signi” cant in these short cores.