After water vapour, CO₂ is the most abundant gas associated with magmatic hydrothermal systems. The detection of anomalous soil temperature gradients, and/or a significant flux of magmatic volatiles, is commonly the only surface signature of an underlying high temperature reservoir. For both heat (as water vapour) and gas to ascend to the surface, structural permeability must exist, as the unmodified bulk permeability of reservoir rock is too low to generate the focussed fluid flow typical of magmatic hydrothermal systems. This thesis reports the investigation into the surface heat and mass flow of the Ohaaki hydrothermal field using detailed surface measurements of CO₂ flux and heat flow. Detailed surface measurements form the basis of geostatistical models that quantify and depict the spatial variability of surface heat and mass flow, across the surface of both major thermal areas, as high resolution pixel plots. These maps, in conjunction with earlier heat and mass flow studies, enable: (i) estimates of the pre-production and current CO₂ emissions and heat flow for the Ohaaki Field; (ii) interpretation of the shallow permeability structures governing fluid flow, and; (iii) the spatial relationships between pressure-induced ground subsidence and permeability. Heat flow and CO₂ flux surveys indicate that at Ohaaki the soil zone is the dominant (≥ 70% and up to 99%) pathway of heat and mass release to the atmosphere from the underlying hydrothermal reservoir. Modelling indicates that although the total surface heat and mass flow at Ohaaki is small, it is highly focused (i.e., high volume per unit area) relative to other fields within the Taupo Volcanic Zone (TVZ). Normalised CO₂ emissions are comparable to other volcanic and hydrothermal fields both regionally and globally. Despite 20 years of production, there is little difference between pre-production and current CO₂ emission rates. However, the similarity of CO₂ emission rates masks a 40% increase in CO₂ emissions from new areas of intense steaming ground that have developed in response to production of the field for electrical energy production. This increase in thermal ground emissions is offset by emission losses associated with the drying up of all steam heated pools and alkali-Cl outflows from the Ohaaki West (OHW) thermal area, in response to production-induced pressure decline. The location of surface thermal areas is governed by the occurrence of buried or partially emergent lava domes, whereas the magnitude of CO₂ flux, mass flow, and heat flow occurring within each thermal area is determined by the proximity of each dome (thermal areas) to major upflow zones. Buried or partially emergent silicic lava domes act as cross-stratal pathways for fluid flow, connecting the underlying reservoir to the surface, and bypassing several hundred metres of the poorly permeable Huka Falls Formation (HFF) caprock. For each dome complex the permeable structures governing fluid flow are varied. At Ohaaki West, thermal activity is controlled by a deep-rooted concentric fracture zone, developed during eruption of the Ohaaki Rhyolite dome. Within the steam-heated Ohaaki East (OHE) thermal area, flow is controlled by a high permeability fault damage zone (Broadlands Fault) developed within the apex of the Broadlands Dacite dome. Structures controlling alkali-Cl fluid flow at OHW also iii appear to control the occurrence and shape of major subsidence bowls (e.g., the Main Ohaaki Subsidence Bowl), the propagation of pressure decline to surface, and the development and localization of pore fluid drainage. Across the remainder of the Ohaaki field low amplitude ground subsidence is controlled by the extent of aquifer and aquitard units that underlie the HFF, and proximity to the margins of the hot water reservoir. The correlation between the extent of low amplitude ground subsidence and the margins of the field reflects the coupled relationship between the hot water reservoir and reservoir pressure. Only where thick vapour-phase zones buffer the vertical propagation of deep-seated pressure decline to the surface (i.e., OHE thermal area), is ground subsidence not correlated with subvertical structural permeability developed within the HFF. This thesis makes contributions to regional and global research on geothermal and hydrothermal systems by: (i) quantifying the origin, mass, and upward transport of magmatic carbon from geothermal reservoirs; (ii) assessing the changes to the natural surface heat and mass flow of the Ohaaki Field following 20 years of production; (iii) establishing the utility of surface CO₂ flux and heat flow surveys to identify major upflow zones, estimate minimum mass flow, and determine the enthalpy of reservoirs; (iv) providing insight into the hydrothermal, structural and lithological controls over hydrothermal fluid flow; (v) demonstrating the influence of extinct silicic lava domes as important structural elements in the localisation of hydrothermal fluid flow; (vi) identifying the hydrostructural controls governing the spatial variability in the magnitude of pressure-induced ground subsidence, from which predictive models of subsidence risk may be defined, and; (vii) developing new technologies and characterising methods used for detailed assessment of surface heat and mass flow.

Abstract Air temperatures in high-latitude regions are anticipated to rise by several degrees by the end of the century and result in substantial northward shifts of species. These changes will likely affect the source and sink dynamics of greenhouse gases and possibly lead to a net carbon release from high-latitude soils to the atmosphere. However, regional differences in carbon cycling depend highly on the vegetation community composition, which may be controlled by the abundance of herbivores. I investigated whether mammalian herbivores, mainly reindeer and rodents, alter ecosystem carbon storage through their impacts on vegetation and on dominant plant functional traits. I combined observations of recent changes in ecosystem carbon with experimental field manipulations of both herbivory and climate change and measured carbon storage in vegetation and soil, the uptake and release of carbon dioxide, microbial activity and compared these to plant community composition. Results of my PhD thesis show that under ambient conditions, the impacts of herbivory on both above- and belowground carbon storage ranged from positive to negative. Herbivory altered dominant plant functional traits and these were fairly good predictors of the changes in soil carbon. When combined with experimental warming, herbivory continued to exert control on the dominant plant functional traits but the strong effects of warming on ecosystem carbon storage mostly concealed the impact of herbivory. Interestingly, herbivory–nutrient interactions that were not linked to dominant functional traits determined the consequences of warming on soil carbon. Taken together, I show clear and site-specific impacts of herbivores on vegetation and ecosystem carbon storage and the processes that govern them. Therefore, I suggest that an improved understanding of the role of herbivory in the global carbon cycle could improve estimations of global carbon–climate feedbacks. / Tiivistelmä Vuosisadan loppuun mennessä arktisten alueiden lämpötilan odotetaan nousevan usealla asteella ja johtavan lajien siirtymiseen yhä pohjoisemmaksi. Nämä muutokset todennäköisesti muuttavat pohjoisten ekosysteemien kykyä vapauttaa ja sitoa ilmakehän hiiltä ja saattavat johtaa siihen, että yhä enemmän hiiltä vapautuu tundramailta ilmakehään. Kuitenkin paikallisesti hiilenkierto on riippuvainen kasviyhteisöstä ja erityisesti kasvien funktionaalisista ominaisuuksista. Väitöskirjassani tutkin, voivatko herbivorit, pääasiassa porot sekä jyrsijät, muokata hiilenkiertoa muuttamalla kasvillisuutta. Tutkimuksissani seurasin kuinka alueen laidunnushistoria on muokannut hiilivarastoja ja hiilenkiertoa tällä hetkellä ja pyrin arvioimaan herbivorien vaikutusta lämpenevässä ilmastossa kokeiden avulla, joissa manipuloidaan sekä herbivoriaa että lämpötilaa tai ravinteiden saatavuutta. Tulokseni perustuvat arvioihin hiilen varastoista, hiilidioksidin vapautumisesta ja sitoutumisesta sekä mikrobien aktiivisuudesta, joita vertaan kasviyhteisöön. Tulokseni osoittavat, että herbivoria voi joko lisätä tai vähentää ekosysteemin hiilivarastoja sekä maan päällä että maan alla. Muutokset hiilivarastoissa selittyivät varsin hyvin herbivorien tuottamilla kasvillisuusmuutoksilla ja valtalajien funktionaalisilla ominaisuuksilla. Herbivoria muokkasi kasviyhteisöä myös kokeellisen lämmityksen yhteydessä, mutta lämmityksen välittömät vaikutukset hiilivarastoihin peittivät suureksi osaksi alleen herbivorian vaikutukset. Kuitenkin herbivorian ja lannoituksen kasvillisuusmuutoksista riippumattomat yhdysvaikutukset määrittivät lämpenemisen seuraukset maan hiileen. Kaiken kaikkiaan, tutkimukseni osoittaa, että herbivorit voivat paikkakohtaisesti muokata kasvillisuutta, ekosysteemin hiilivarastoja sekä hiilenkierron prosesseja. Näiden tulosten myötä ehdotan, että parempi ymmärrys herbivorian vaikutuksista maailmanlaajuisesti voisi parantaa nykyisiä ennusteita siitä, kuinka ilmaston lämpeneminen muuttaa hiilenkiertoa.