Palaeolimnology of Adelaide Tarn, a ~14,000-year-old low-alpine glacial lake, northwestern South Island, New Zealand
Foster, C. R. (2013). Palaeolimnology of Adelaide Tarn, a ~14,000-year-old low-alpine glacial lake, northwestern South Island, New Zealand (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/7896
Permanent Research Commons link: https://hdl.handle.net/10289/7896
A palaeolimnology study has been carried out on a ~14,000 calendar [cal]-year-old low-alpine glacial lake, Adelaide Tarn. The lake is located in a cirque basin at ~1260 m elevation in the Tasman Mountains, just below present-day treeline, within Kahurangi National Park, northern South Island, New Zealand. A 5.6-m long sediment core was retrieved from the lake by staff of GNS Science. The chronology of the core was constructed from 15 ¹⁴C dates obtained via AMS on 14 samples of in-situ plant macrofossils and one sample of bulk organic sediment. The core was divided into lithozones 1, 2, and 3 from the base through to the top of the core. Lithozone 1 (5.6–4.8 m) comprises inorganic (carbon content 0.3 to 4%) grey silts with gravel (Munsell colour 5Y 6/1) and these sediments, primarily composed of angular particles identifed by scanning electron microscopy (SEM), date to a little earlier than ~13,932 cal yr BP. Lithozone 2 consists of organic (carbon content up to 15%) brownish black (10YR 2/3) silt and clay and extends from ~13,932 to ~7709 cal yr BP. In lithozone 2, diatoms appear, as identified by SEM. Lithozone 3 is made up of dark brown (10YR 3/4) organic silt (carbon content 5 to 10%) and extends from ~7709 cal yr BP through to the top of the core which has an age estimated to be a little older than ~700 cal yr BP on the basis of a ¹⁴C date (~926 cal yr BP) at 13 cm depth and a pollen record that shows neither a Polynesian deforestation signal (i.e. the sediments likely pre-date ~700 cal yr BP) nor European adventives (i.e. the sediments pre-date ~1840 AD). Lithozone 3 is diatom-rich with subordinate angular particles. Intermittent yellow/brown (10YR 6/8) laminae occur throughout lithozones 2 and 3; one lamination occurs near the top of lithozone 1. These yellow/brown laminae comprise mainly angular clastic particles (evident in SEM micrographs) and show a slight increase in sand compared with non-laminated sediments, and the laminae are inferred to reflect terrigenous input to the lake as a consequence of storms or during intense rainfall events. These possible storm events may correspond with ENSO events as described/identified in Lake Tutira in Hawke’s Bay. Multiple components of the sediment archive were analysed to reconstruct the history of the lake and its catchment. Properties included were X-radiography, grey-scale, magnetic susceptibility, grain size, plant macrofossil assemblages, organic carbon content, and isotopes δ¹⁵N and δ¹³C. The results show four phases of climate variability (Adelaide Tarn climate events, AT-CEs) from the onset of lake formation ~14,000 cal yr BP. AT-CE1: early sediments ~14,000 cal yr BP show enhanced terrigenous input (fine-grained grey silts, with gravel layers, of lithozone 1), consistent with fluvio-glacial in-wash during retreat from the Adelaide Tarn basin of a local cirque glacier. AT-CE2: from ~13,932 to ~10,000 cal yr BP, erosion of the catchment is much diminished, indicated by decreases in modal grain size and magnetic susceptibility, and lake productivity concomitantly commences as shown by the δ¹³C and δ¹⁵N isotope records. Poaceae species dominate the macrofossil plant assemblages with no tree-species found. AT-CE2 (~13,932 to ~10,000 cal yr BP) is inferred to be one of warming but the catchment was not yet stable and little or no soil formation occurred that would allow habitation by tree species. AT-CE3: the third palaeoenvironmental phase, from ~10,000 to ~2400 cal yr BP, is marked by an expansion of Nothofagus (Nothofagus menziesii and Nothofagus fusca) forest with minor constituents of Libocedrus biwillii and Phyllocladus alpinus. This phase is inferred to be one of sustained warm conditions and a stable catchment. Adelaide Tarn is situated 250 m higher than the present-day altitudinal limit of N. fusca and the presence of macrofossils of this species in the sediment record suggests that between ~6400 and ~2400 cal yr BP the mean annual temperature was possibly ~1.2˚C warmer than present based on an observed environmental lapse rate of 0.47˚C/100 m. There was slight increase in denitrification from ~8000 through to ~3000 cal yr BP, suggesting that primary productivity increased in the lake. AT-CE4: from ~2400 cal yr BP, the fourth phase, the plant assemblage shifted to Poaceae-dominated; cold-sensitive taxa were forced to descend, marked by disappearance of forest species from the plant macrofossil assemblage. The climate is inferred to have deteriorated and the tree-line descended to near the present-day position. The Adelaide Tarn record is one of only a few spanning the last ~14,000 years in central New Zealand, and is especially useful in adding to the plant macrofossil records, which are rare. The Adelaide Tarn record was compared with the newly-published New Zealand climate event stratigraphy (NZ-CES), with the proposed tripartite subdivision of the Holocene, and with a number of records across New Zealand that are based on various proxies including speleothems and pollen assemblages. None of the NZ-CES events were clearly evident in the Adelaide Tarn record (apart from NZce-1), and the proposed boundaries of the subdivided Holocene (at ~8.2 ka and 4.2 ka) were not evident. A palynological record spanning the last ~12,000 years from Cropp Valley, western South Island, showed close consistency with the Adelaide Tarn record.
University of Waikato
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