Environmental Change of the Waipaoa Watershed Shown by Marine Sedimentary Signals; North Island, New Zealand

Abstract

The small mountainous watershed of the Waipaoa River has undergone environmental changes throughout the Holocene, including volcanic eruptions, deforestation, land use change, and increased erosion induced by anthropogenic activities. Analysis of the sedimentological and biogeochemical signals in three long piston cores recovered from the continental shelf of New Zealand provides a record of these environmental changes. The shelf cores are generally fine, clayey muds, but contain a prominent interval of coarser, silty sediment that begins about the level of the Waimihia tephra (3.4 ka BP) and contains both the Taupo (1.7 ka BP) and Kaharoa (0.7 ka BP) tephras. Concurrent with the coarsening of grain size, the clay-sized fraction in this zone shows a marked increase in organic carbon content and mineral surface area, as well as more positive δ13C values. Higher smectite content associated with the tephric sediment is likely a key player in these patterns.

Above the Kaharoa tephra, fining of the shelf sediments signals anthropogenic impacts on the Waipaoa watershed. 14C data from charcoal fragments also demonstrates strong anthropogenic influence over the past several hundred years. The charcoal fraction is significantly older than other organic fractions throughout most of the core, indicating a contribution of ancient sedimentary charcoal from sources in the watershed. In the anthropogenically influenced sediments, however, very young charcoal produced from combustion of contemporaneous biomass overprints the natural charcoal signal.