A tephra-dated record of palaeoenvironmental change since ~ 5,500 years ago from Lake Rotorua, North Island, New Zealand

Abstract

A palaeolimnological study was carried out on a high-resolution, 7.62 m-long core (RU188-07) from northern Lake Rotorua, North Island. The core consists predominantly of olive diatomaceous ooze, laminated in places, and contains five tephras including Tarawera (1886 A.D.), Kaharoa (c. 1314 A.D.), Taupo (c. 233 A.D.) and Whakatane (c. 5500 cal. years B.P.). The core terminated in Whakatane Tephra giving the sediment a maximum age of 5530 60 cal. years B.P. An age model for the sediment was developed using tephrochronology. Radiocarbon dates obtained on the sediment returned ages too old because of contamination by old CO2 or CH4, or both. Investigations carried out on the core included spectrophotometric, sedimentological and geochemical analyses, and diatom identifications, which provided a number of proxies from which inferences were made about lake history, catchment development, and palaeoclimate since c. 5500 cal. years B.P. The laminations, evident only in the upper, post-Kaharoa Tephra part of the record, comprise alternations of thin, dark, detrital deposits and pale, relatively fine-grained diatom assemblages. Sediment geochemistry indicates that the Rotorua catchment has undergone several changes since c. 5500 cal. years B.P., alternating between periods of variable and stable environmental conditions. Following the Whakatane and Waimihia eruptions and up to approximately 3000 cal. years B.P., the catchment surrounding Lake Rotorua was rather unstable. Fluctuations in many of the proxies during this period are likely to be associated with a variable climate with periods of storminess, coinciding with the establishment of ENSO conditions in New Zealand. A notable feature of the record is two phases of stability, the first following the Taupo eruption (from c. 1700 cal. years B.P. to c. 630 cal. years B.P.) and the second from c. 580 cal. years B.P. to c. 300 cal. years B.P. The latest, most significant event in the catchment history of Lake Rotorua was the settlement by Polynesians. M.S. McGlone implied from pollen profiles (from Holden's Bay) that initial settlement took place around the time of the Kaharoa eruption (c. 630 cal years B.P.; c. 1314 A.D.), but the sediment chemistry and erosion profiles obtained here, from the northern part of Lake Rotorua, indicate that although there may have been some early clearing in the northern catchment for tracks or buildings, large-scale clearing in the area probably did not occur until considerably later, c. 300 cal. years B.P. Also contained within the sediments are three layers of reworked tephric material that probably originate from the transfer of coarse grained tephra from shallow to deeper water during large storms at c. 1300 cal. years B.P, c. 520 cal. years B.P, and c. 220 cal. years B.P. Each event coincides with storm events inferred from records from Lake Tutira in eastern North Island. Because of Lake Rotorua's inland position, these inferred storm events probably represent only the largest cyclonic events (e.g. ex-tropical cyclones).