Short-term climate variability in the mid-Miocene: A Micropalaeontological and sedimentological investigation of a high-resolution maar lake sediment near Hindon, East Otago, New Zealand
Bates, N. W. (2018). Short-term climate variability in the mid-Miocene: A Micropalaeontological and sedimentological investigation of a high-resolution maar lake sediment near Hindon, East Otago, New Zealand (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/12083
Permanent Research Commons link: https://hdl.handle.net/10289/12083
Maars are deep, steeply-sloped, small-diameter craters formed by phreato-magmatic eruptions. Phreatomagmatic eruptions result when rising magma intersects a body of water such as an aquifer. Maars often form deep closed-basin lake systems that become repositories for detrital material from the local environment. Tens of metres of detritus representing thousands of years of accumulation may be anoxically-preserved in maar lake sediments for millions of years. The Hindon Maar Complex (HMC) is located in East Otago, on the South Island of New Zealand. The HMC hosts the highest-resolution mid-Miocene climate archive in the Southern Hemisphere. The date of its formation (14.6±0.1 Ma) coincides with the mid-Miocene climate transition (MMCT), a unique period when global temperatures were decreasing following an anomalous short-lived peak at the mid-Miocene climate optimum. Sensitivity to atmospheric pCO₂ levels and the confluence of the three main Milankovitch orbital cycle minima caused the Antarctic ice sheet (AIS) to advance and retreat. Concurrently, the Antarctic circumpolar currents (ACC) and atmospheric circumpolar westerlies also underwent latitudinal shifts while interacting with warm subtropical currents in the vicinity of vestigial Zealandia. With little rain-shadow effect over the low-lying land mass and exclusively ocean between the latitudes of Zealandia and the polar ice cap, the Hindon Maar Complex was ideally placed in the mid-latitudes to experience and record the subtle climate changes that resulted. An 18.5 m core drilled from Hindon Maar #1 contained ~9.5 m of highly organic lake sediments with varying concentrations of biosiliceous microfossils. Physical properties of the core (magnetic susceptibility, gamma attenuation density, and spectrophotometry) measured using a Geotek multi-sensor core logger identified two lithofacies distinct from each other at ~5.7 mcd. High resolution mm-scale surveys of core split surfaces revealed core attributes that corroborated the Geotek findings. The lower facies is black laminated sapropelic material throughout while the upper facies is dark brown diatomite that exhibits light laminae and bedding structures related to turbidite mass flow events. An age-depth model was developed by measuring lamina-couplet thickness in 10 lake sediment samples throughout the core. The lake sediments in the HMC M1-2 core represent ~10,400 years of deposition. The lamina-couplet thickness also identifies a boundary between depositional environments in the vicinity of 6.5 mcd, where deposition below represents a high annual flux of organic material to the lake, while deposition above represents biosiliceous deposition and much reduced flux of organic material. 51 approximately-equidistant lake sediment samples were subjected to FTIRS and BioSi wet chemistry compositional analyses and siliceous microfossil smear slide surveys. All analyses identify a change in deposition environment at ~6.5 mcd, from sediments with high organic input with some terrigenous material and few biosiliceous microfossils to sediments with reduced organic and terrigenous input and copious biosiliceous material. Changes in the biosiliceous microfossil assemblages upcore were used to infer surface water pH changes. Changes in diatom abundance were indicative of access to nutrients sequestered in the profundal zone of the meromictic lake. Accessibility to nutrients was equated to climates that would promote or inhibit the transfer of nutrients from monimolimnion to mixolimnion across the chemocline. Therefore, changes in the lake sediment deposit were equated to climate-driven changes in the meromictic lake dynamics. This Hindon Maar #1 lake sediment deposit represents two main climate-driven depositional environments. Warm wet climate conditions that promoted deciduous forest growth and inhibited nutrient accessibility in the lake transitioned over ~1300 years (~6.5 to 5.7 mcd) to windy, cool and dry climate conditions that shifted the range of the forests away from the HMC and allowed nutrients sequestered in the lake to become accessible to diatom algae, increasing the productivity of the lake. The climates represented in the HMC M1 lake sediments are then projected back to the latitudinal shifts of the ACC and circumpolar westerlies, driven by the expansion and retreat of the AIS during the MMCT.
The University of Waikato
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