Carbon and nitrogen isotopes in lichen as a geothermal exploration tool

• Main Author: Asher, Cameron Michael
• Language: en
• Published: University of Canterbury. Geological Sciences 2014
• Subjects: Ngatamariki; Lichen; Isotopes; Geochemistry; Environmental; Geothermal; Exploration; Geology; TVZ; carbon; nitrogen
• Online Access: http://hdl.handle.net/10092/9651

Lichen have been used as indicators of atmospheric pollutants since Grindon (1859) observed lichen populations declining in a polluted Southern Lancashire in the mid-1800s. Since then lichen have been used in a number of atmospheric studies. A study by Tozer et al. (2005) attempted to use nitrogen isotopes of lichen and free-living algae as indicators of geothermal ‘pollution’ near Rotorua and the Te Kopia Geothermal Area, but was unable to show a correlation with distance to geothermal features. This thesis aims to build from Tozer et al. (2005) and use both carbon and nitrogen isotopes in lichen as an exploration tool in geothermal areas. Three transects were completed: one across the South Island from Christchurch to Greymouth (non-geothermally influenced area), and two along (north-south) and across (east-west) the Taupo Volcanic Zone (TVZ) in the North Island (geothermally influenced area). In addition to these three transects, sampling at higher spatial resolution was conducted in the immediate vicinity of the Orakonui Stream geothermal springs at the Ngatamariki Geothermal Area. The three transects showed large variation, largely due to the type of land use from which the sample was collected. The highest nitrogen contents (1.62 ± 0.39%) and less negative nitrogen isotopic compositions (-9.44 ± 0.39‰) were found over farmland, while both exotic and native forests had low nitrogen (1.08 ± 0.35% and 1.03 ± 0.44‰, respectively) and highly negative isotopic compositions (-12.94 ± 0.26‰ and -12.09 ± 0.45‰, respectively). The statistical difference between land use classes is hypothetically explained by variations in nitrogen sources, with intensive farmland volatilizing NH3 with δ15N values of -6 to -10‰ (Tozer et al., 2005), while forest areas are expected to produce biogenic nitrogen from decomposition with more negative δ15N. At Ngatamariki, δ13C and δ15N isoscapes were produced, with both showing a large isotopic anomaly (>-23.5 and >-8‰, respectively) to the north and north-west of the study area, correlating with areas of farmland, although in some places the δ15N values exceed 0‰, which is unexplained. A study by Hanson (2014) identified diffuse soil flux using δ13C in the vicinity of the Orakonui South Main Crater to have a geothermal signature, the same location in which a small relatively less-negative δ13C anomaly (>-23.5‰) is seen in lichen isotopes. While this could be attributed to a geothermal influence, it could also be due to the effect of substrate the lichen lives on and a reduction in carbon sourced from biogenic respiration. Ultimately, there is the potential for isotopes in lichen to be used as a geothermal exploration tool, although this method needs to be investigated in a higher flux geothermal area, such as Rotokawa, 7km to the south of Ngatamariki.