Utilisation of the sedimentological and hydrochemical dynamics of the Stump Bay Wetland along Lake Taupo, New Zealand, for the recognition of paleo-shoreline indicators

Abstract

Terrestrial sediments are generally simple to differentiate from lacustrine sediments, but the interface between the two is difficult to recognise when clean beach sand is absent. In addition, tracing the dispersal of pollutant trace metals at this interface can be a major problem because toxic and natural metal cycles often take the same path. To solve these problems, we studied the natural geochemical processes in a wetland along the shore of Lake Taupo, New Zealand to characterise the surficial sediments that could be used as indicators of shoreline processes and to determine the variability of natural metals in the system. Lake Taupo is New Zealand's largest lake. The Stump Bay Wetland is part of the South Taupo Wetland, which is situated along the southern edge of Lake Taupo and has an area of 15.4 km2. Although much of the surrounding shoreline has been modified by human activity, the Stump Bay Wetland has retained its natural configuration. To understand the variability at the terrestrial–lacustrine interface, we characterised the surficial sediments using shallow cores to learn the stratigraphy and geochemical make up of the sediment using X-ray Fluorescence (XRF) for major elements and selected metals (Fe, Mn, Zn, Cu, Ni, Ba). In addition, we installed shallow piezometers to relate the stratigraphy and geochemistry to groundwater flow and chemical composition (pH, alkalinity, dissolved Fe). Silt deposits derived from floods alternate with peat accumulations in shallow cores taken in the centre of the wetland. The reduction of ferric iron to ferrous iron controls the South Taupo wetland groundwater pH and total alkalinity. Sediment cores from the wetland have high iron concentrations (up to 15 wt.%). The abundance of iron in the cores does not correlate with either loss on ignition (LOI) or silica abundance, indicating that the iron is derived from a combination of decay of organic matter and leaching from volcanic rock material. Concentrations of sodium, potassium and calcium and magnesium tend to increase with depth, and reflect the increasing amount of inorganic matter in the core. Sulphur concentrations in the cores are relatively low, and there is a weak correlation with LOI. Although conditions for iron sulphide precipitation are possible in the wetland sediments, it does not occur within the upper 1 m of the sediment. The low topographic and groundwater gradient of the wetland area has led to a close interaction between sediment build-up and the hydrochemistry of the water flowing into the lake. In particular, zones of iron-rich diagenetic alteration of sediments at the surface water–groundwater interface could be used to fix the position of the shoreline in paleo-lake studies, as long as the chemical interactions along the groundwater flow path are understood.