Geomorphological, palaeoenvironmental and managerial implications for the conservation of the fossil forest at Titahi Bay

Abstract

The remains of a Quaternary fossil forest consisting of broken off tree stumps retaining the normal characteristics of wood is preserved on the beachface and inner surf zone along the length of the shore at Titahi Bay Bench, on the west coast of the North Island, New Zealand. The tree stumps remain in-situ, within a mud of sheltered marine origin. The results of a luminescence date from this material indicate an age of deposition of 96.0 ± 10.7 ka. BP. Buried and preserved forests provide valuable palaeoenvironmental evidence for interpreting past climate and sea level shifts and several examples exists along New Zealand's coast. Due to the geographic location and popularity of the area as a recreational resource, the example at Titahi Bay provides interest to both quaternary scientists and coastal planners. Several issues regarding the use and management of the beach remain unresolved, and until now the significance and vulnerability to natural processes of the fossil forest has not been considered as part of the overall conservation of Titahi Bay Beach. This study provides a foundation from which future conservation plans can develop by focussing on quantifying the active processes which are operating in Titahi Bay and how these processes affect the exposure of the fossil trees. The rate of deterioration of the tree stumps due to natural and human induced effects is proportional to the amount of tree stump exposure. Waves are the greatest contributing factor in determining the level of exposure at Titahi Bay, with wave generated currents being responsible for driving sediment transport within the bay. Development of a wave climate using historical wind data and wave hindcasting techniques indicate a mean breaker height of 0.9 m and closure depth of 2.4 m, showing for the great majority of the time, sediment transport is restricted to an area shoreward of the embayment entrance, thereby decreasing the opportunity for irrecoverable erosion to occur. Support for this theory is derived from extensive surveys of the beach topography in which sediment volume changes could be monitored on the beachface and inner surf zones over a nine month period. Episodes of beach rotation operating over periods of three to four weeks may be responsible for the alternating exposure of the forest from one end to the other in the short term. Breaker heights on the order of 1.1 m eroded sand from the southern end and deposited it at the northern end. The opposite transport pattern was recorded during the recovery phase. Vertical bed elevation changes were far more significant in the middle of the beach than at the ends. Over longer time scales (three to four years) fluctuations in the Southern Oscillation Index may be responsible for the total volume of sand present on the beach face. An accumulation rate of 61 m3 per day was recorded during the study which was conducted during a strong La Niña phase which typically produces less wave generating energy for Titahi Bay. The strengthening of an El Niño phase will likely reverse this accumulation trend. Further study using a fully morphodynamic approach preferably during an El Niño phase is required to test the robustness of these observations. Management issues at Titahi Bay are complex, particularly if it is decided to attempt to reduce the rate of deterioration of the fossil wood as part of an overall conservation plan. Implementation of such measures will inevitably cause conflict amongst the many different beach users, and it is essential to gain an understanding of the publics attitudes towards the beach as a recreational resource and area of scientific interest before such plans can proceed.