The Holocene Glacial History of Dart Glacier, Southern Alps, New Zealand

<p>Mountain glaciers are sensitive climate indicators, as climate variability drives mass changes that are expressed in glacier length fluctuations. These length changes are preserved in the geological record, thus offering the potential to generate new palaeoclimate proxy data that can be used to extend instrumental climate records. This study presents geomorphological mapping and cosmogenic ¹⁰Be surface exposure dating of the Holocene moraines at Dart Glacier, New Zealand. These findings show that an early Holocene advance (~6 km longer than present-day) took place ~7817 ± 336 years ago. Moraine ages also show that a more restricted glacier readvance (~4 km longer than present-day) occurred ~321 ± 44 years ago. Through better constraining the timing and magnitude of Holocene glacier length changes, we extend the ~100-year history of observational records in the upper Dart valley. Net retreat of Dart Glacier during the Holocene is consistent with other moraine chronologies from New Zealand, which supports existing hypotheses that suggest summer insolation was the dominant driver of multi-millennial climate change at southern mid-latitudes during the current interglacial. Individual moraine forming events at Dart Glacier also coincide with moraine ages from several other catchments in the Southern Alps and likely reflect shorter-term (decadal-centennial-scale) climatic changes. The new geological record constraints of length changes at Dart Glacier offer the opportunity to test such hypotheses more formally using physics-based modelling. Connecting Holocene moraine records to historical glacier observations using ¹⁰Be surface exposure dating requires consistently low background levels of this rare isotope. Systematic blank experiments show that concentrated analytical grade hydrofluoric acid and reused beakers are likely the largest contributors of ¹⁰Be to the average process blank in the VUW Cosmogenic Laboratory. Based on these findings I recommend small methodological improvements that could be implemented to lower process blank ratios for routine application of ¹⁰Be surface exposure dating to near-historic glacial landforms.</p>

The Holocene Glacial History of Dart Glacier, Southern Alps, New Zealand

<p>Mountain glaciers are sensitive climate indicators, as climate variability drives mass changes that are expressed in glacier length fluctuations. These length changes are preserved in the geological record, thus offering the potential to generate new palaeoclimate proxy data that can be used to extend instrumental climate records. This study presents geomorphological mapping and cosmogenic ¹⁰Be surface exposure dating of the Holocene moraines at Dart Glacier, New Zealand. These findings show that an early Holocene advance (~6 km longer than present-day) took place ~7817 ± 336 years ago. Moraine ages also show that a more restricted glacier readvance (~4 km longer than present-day) occurred ~321 ± 44 years ago. Through better constraining the timing and magnitude of Holocene glacier length changes, we extend the ~100-year history of observational records in the upper Dart valley. Net retreat of Dart Glacier during the Holocene is consistent with other moraine chronologies from New Zealand, which supports existing hypotheses that suggest summer insolation was the dominant driver of multi-millennial climate change at southern mid-latitudes during the current interglacial. Individual moraine forming events at Dart Glacier also coincide with moraine ages from several other catchments in the Southern Alps and likely reflect shorter-term (decadal-centennial-scale) climatic changes. The new geological record constraints of length changes at Dart Glacier offer the opportunity to test such hypotheses more formally using physics-based modelling. Connecting Holocene moraine records to historical glacier observations using ¹⁰Be surface exposure dating requires consistently low background levels of this rare isotope. Systematic blank experiments show that concentrated analytical grade hydrofluoric acid and reused beakers are likely the largest contributors of ¹⁰Be to the average process blank in the VUW Cosmogenic Laboratory. Based on these findings I recommend small methodological improvements that could be implemented to lower process blank ratios for routine application of ¹⁰Be surface exposure dating to near-historic glacial landforms.</p>

Pteridophytes as primary colonisers after catastophic events through geological time and in recent history

Pteridophytes reproduce by producing vast numbers of spores that may be dispersed over considerable distances, helping the plants colonise new areas. Being resistant to desiccation, fern spores can often survive for many years as spore banks in soil. After disturbance, such spores can germinate and subsequently colonise the area. These factors help pteridophytes to become primary colonisers on barren land, such as volcanic islands or land that has been devastated by some cataclysmic event. A further method of rapid colonisation is provided through the preservation and possible scattering of fragments of rhizomes in particular of horsetails. Similar rapid colonising by pteridophytes has been documented in the geological record following several major extinction events. These distinct, but short-lived, fern populations are recognisable by fern spikes in the microfossils. This paper brings together information on the reasons for pteridophyte success in colonising barren land, and examples taken from both the historic and geological records.

Mafic Archean continental crust prohibited exhumation of orogenic UHP eclogite

The absence of ultrahigh pressure (UHP) orogenic eclogite in the geological record older than c. 0.6 Ga is problematic for evidence of subduction having begun on Earth during the Archean (4.0–2.5 Ga). Many eclogites in Phanerozoic and Proterozoic terranes occur as mafic boudins encased within low-density felsic crust, which provides positive buoyancy during subduction; however, recent geochemical proxy analysis shows that Archean continental crust was more mafic than previously thought. Here, we show via petrological modelling that secular change in the composition of upper continental crust (UCC) would make Archean continental terranes negatively buoyant in the mantle before reaching UHP conditions. Subducted or delaminated Archean continental crust passes a point of no return during metamorphism in the mantle prior to the stabilization of coesite, while Proterozoic and Phanerozoic terranes remain positively buoyant at these depths. UHP orogenic eclogite may thus readily have formed on the Archean Earth, but could not have been exhumed, weakening arguments for a Neoproterozoic onset of subduction and plate tectonics. Further, isostatic balance calculations for more mafic Archean continents indicate that the early Earth was covered by a global ocean over 1 kilometre deep.