Field trip guide to Oligocene Limestones and Caves in the Waitomo District

The field guide runs from Hamilton to Waitomo to Te Anga and return in limestone-dominated country developed in transgressive sedimentary deposits of the Oligocene Te Kuiti Group – a world class example of a temperate shelf carbonate depositional system. Attention focuses on the nature, distribution and paleoenvironmental controls of the main limestone facies and some of the mixed terrigenous-carbonate facies in the Group. Along the way features of the Waitomo karst landscape are noted and the trip concludes by going underground in the Ruakuri Cave to discuss cave origins and the evidence for paleoenvironmental changes locked up in speleothems

University of Waikato radiocarbon dates I

This date list reports on samples submitted by University of Waikato researchers and assayed in the Waikato laboratory mainly between 1979 and 1985. Most dates reported here relate to the deposition of distal airfall tephras in lakes and peats in central and northern North Island, New Zealand. Most of the tephras have been correlated with named eruptive units elsewhere using diagnostic mineralogic and chemical criteria, together with stratigraphic and age relationships.The dates listed in Section 2 were obtained on carbonaceous matter associated with the Hinuera Formation, an extensive low-angle fan of volcanogenic alluvium that was deposited in several phases in the Waikato and Hauraki basins before and during the last stale (isotope stage 2) of the last glaciation. In Section 3, the samples comprise materials associated with peat bog growth or local sedimentation that postdates the deposition of the Hinuera Formation, ie, < ca 15,000 BP. Samples in both Sections 2 and 3 are grouped into series according to geographic location, and, where appropriate, arranged stratigraphically with uppermost samples shown first

Historical and contemporary perspectives on the sediments of Lake Rotorua

Lake Rotorua is probably the oldest continuously inundated lake in New Zealand, occupying a caldera formed by or closely associated with the eruption of the Mamaku ignimbrite and the collapse of the Rotorua caldera (Healy, 1975; Lowe and Green, 1991). The lake has undergone drastic changes in size and depth as a result of tectonics, volcanic activity and erosion. Since the Rotoehu eruption, (~60 kyr), the lake level has fluctuated between 120 m above present (280 m asl) and 10 m below present level. The modern lake covers an area of 79 km2 and has a mean depth of 10 m. Despite its long history of sedimentation, Lake Rotorua has an irregular bathymetry with features including faulted blocks, slumps, hydrothermal explosion craters, springs and large methane discharge pock marks

Lake-floor sediment texture and composition of a hydrothermally-active, volcanic lake, Lake Rotomahana

Young volcanic lakes undergo a transition from rapid, post-eruptive accumulation of volcaniclastic sediment to slower pelagic settling under stable lake conditions, and may also be influenced by sublacustrine hydrothermal systems. Lake Rotomahana is a young (129 year-old), hydrothermally-active, volcanic lake formed after the 1886 Tarawera eruption, and provides a unique insight into the early evolution of volcanic lake systems. Lake-bottom sediment cores, 20–46 cm in length, were taken along a transect across the lake and characterised with respect to stratigraphy, facies characteristics (i.e., grain size, componentry) and pore water silica concentrations. The sediments generally comprise two widespread facies: (i) a lower facies of light grey to grey, very fine lacustrine silt derived from the unconsolidated pyroclastic deposits that mantled the catchment area immediately after the eruption, which were rapidly reworked and redeposited into the lake basin; and (ii) an upper facies of dark, fine-sandy diatomaceous silt, that settled from the pelagic zone of the physically stable lake. Adjacent to sublacustrine hydrothermal vents, the upper dark facies is absent, and the upper part of the light grey to grey silt is replaced by a third localised facies comprised of hydrothermally altered pale yellow to yellowish brown, laminated silt with surface iron-rich encrustations. Microspheres, which are thought to be composed of amorphous silica, although some may be halloysite, have precipitated from pore water onto sediment grains, and are associated with a decrease in pore water silicon concentration. Lake Rotomahana is an example of a recently-stabilised volcanic lake, with respect to sedimentation, that shows signs of early sediment silicification in the presence of hydrothermal activity

Dynamics of silicon in lakes of the Taupo Volcanic Zone, New Zealand, and implications for diatom growth

Intact sediment cores were taken from the deepest basins of 9 lakes in the Taupo Volcanic Zone, New Zealand, to investigate the factors controlling silicon (Si) concentrations in sediment pore waters and the flux of Si to the overlying lake water. The lakes ranged in trophic state from oligotrophic to highly eutrophic. A Si vertical flux model simulated Si gradients in the pore water with high precision (r2 > 0.95, p < 0.01) in lakes with no volcanic tephra layers or significant geothermal inflows. The ubiquitous presence of diatom frustules in the sediment was likely responsible for release of Si to the pore waters and subsequent diffusion to overlying lake waters. Fluxes of Si were related to the trophic status of the lake and were greatest in eutrophic lakes where diatom populations reduced epilimnetic Si concentrations to <1 mg L-1. Temporal variations in the concentrations of Si, nitrogen (N), and phosphorus (P) suggest that over most of the year diatom growth in the oligotrophic lakes is limited by N, or co-limited by N and P. In some eutrophic lakes, Si may limit diatom growth during homothermal conditions, and P and/or N may limit growth when the lake is stratified. The reduction of surface Si concentrations to <0.1 mg L-1 following homothermy in some eutrophic lakes is likely to restrict the growth of diatoms, potentially resulting in increased dominance of nonsiliceous flagellated species and cyanobacteria

Dynamics of silicon in lakes of the Taupo Volcanic Zone, New Zealand, and implications for diatom growth

Intact sediment cores were taken from the deepest basins of 9 lakes in the Taupo Volcanic Zone, New Zealand, to investigate the factors controlling silicon (Si) concentrations in sediment pore waters and the flux of Si to the overlying lake water. The lakes ranged in trophic state from oligotrophic to highly eutrophic. A Si vertical flux model simulated Si gradients in the pore water with high precision (r2 0.95, p &lt;0.01) in lakes with no volcanic tephra layers or significant geothermal inflows. The ubiquitous presence of diatom frustules in the sediment was likely responsible for release of Si to the pore waters and subsequent diffusion to overlying lake waters. Fluxes of Si were related to the trophic status of the lake and were greatest in eutrophic lakes where diatom populations reduced epilimnetic Si concentrations to &lt;1 mg L&minus;1. Temporal variations in the concentrations of Si, nitrogen (N), and phosphorus (P) suggest that over most of the year diatom growth in the oligotrophic lakes is limited by N, or co-limited by N and P. In some eutrophic lakes, Si may limit diatom growth during homothermal conditions, and P and/or N may limit growth when the lake is stratified. The reduction of surface Si concentrations to &lt;0.1 mg L&minus;1 following homothermy in some eutrophic lakes is likely to restrict the growth of diatoms, potentially resulting in increased dominance of nonsiliceous flagellated species and cyanobacteria

Carbon dioxide emissions and sediment organic carbon burials across a gradient of trophic state in eleven New Zealand lakes

Lakes are known to be important to the global carbon balance as they are both CO₂ sources to the atmosphere and also accumulate large amounts of carbon in their sediment. CO₂ flux dynamics across the air–water interface in 11 lakes of varying trophic state in the Rotorua region, New Zealand, derived from measured alkalinity, pH and wind speed at given temperature, showed that lakes may shift from being atmospheric CO₂ sources to sinks due to seasonal changes in phytoplankton productivity and lake mixing dynamics. Decreases in trophic state (i.e. improved water quality) in some of the lakes over the eight-year monitoring period were associated with increased surface water CO₂ concentrations and, as a consequence, increased CO₂ flux to the atmosphere. Organic carbon content analysis of the bottom sediments revealed that lakes with high phytoplankton productivity, indicated by high chlorophyll a biomass, generally had high rates of carbon deposition to the sediments, but not all deposited carbon was permanently buried. Remineralization of the organic carbon accrued in productive lakes may potentially generate CO₂, as well as CH₄, which promotes these lakes to act as greenhouse gas emitters