Soil CO2 flux baseline in an urban monogenetic ... - Springer Link

Bull Volcanol (2013) 75:757 DOI 10.1007/s00445-013-0757-7

COLLECTION: MONOGENETIC VOLCANISM

Soil CO2 flux baseline in an urban monogenetic volcanic field: the Auckland Volcanic Field, New Zealand Agnès Mazot & Elaine R. Smid & Luitgard Schwendenmann & Hugo Delgado-Granados & Jan Lindsay

Received: 20 December 2012 / Accepted: 1 September 2013 / Published online: 8 October 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract The Auckland Volcanic Field (AVF) is a dormant monogenetic basaltic field located in Auckland, New Zealand. Though soil gas CO2 fluxes are routinely used to monitor volcanic regions, there have been no published studies of soil CO2 flux or soil gas CO2 concentrations in the AVF to date or many other monogenetic fields worldwide. We measured soil gas CO2 fluxes and soil gas CO2 concentrations in 2010 and 2012 in varying settings, seasons, and times of day to establish a baseline soil CO2 flux and to determine the major sources of and controlling influences on Auckland's soil CO2 flux. Soil CO2 flux measurements varied from 0 to 203 g m−2 day−1, with an average of 27.1 g m−2 day−1. Higher fluxes were attributed to varying land use properties (e.g., landfill). Using a graphical statistical approach, two populations of CO2 fluxes were identified. Isotope analyses of δ13CO2 confirmed that the source of CO2 in the AVF is biogenic with no volcanic component. These data may be used to assist with eruption forecasting in the event of precursory activity in the AVF, and highlight the importance of knowing land use history when assessing soil gas CO2 fluxes in urban environments. Editorial responsibility: I.E.M. Smith, Guest Editor This paper constitutes part of a topical collection: Smith IEM, Nemeth K, and Ross P-S (eds) Monogenetic volcanism and its relevance to the evolution of volcanic fields. A. Mazot (*) GNS Science, Private Bag 2000, Taupo, New Zealand e-mail: [email protected] E. R. Smid : L. Schwendenmann : J. Lindsay School of Environment, University of Auckland, Private Bag 92019, Auckland, New Zealand H. Delgado-Granados Departamento de Vulcanología, Instituto de Geofísica, Universidad Nacional Autónoma de MéxicoCiudad Universitaria, 04510 México, D.F., México

Keywords Auckland Volcanic Field . Soil gas CO2 flux . Soil gas δ13CO2 . Soil gas CO2 concentration . Soil temperature . Volcanic hazards . Volcano monitoring

Introduction Soil CO2 flux is a useful indicator of the state of volcanic activity and has been used in volcano monitoring programs for decades (e.g., Carbonelle and Zettwoog 1982; Allard et al. 1991; Giammanco et al. 1998; Farrar et al. 1995; Gerlach et al. 1998; Hernandez et al. 2001; Notsu et al. 2006; Bloomberg et al. 2012). More recently, soil CO2 flux has been suggested as a key input parameter in eruption forecasting models (Lindsay et al. 2010). With its high abundance and low solubility in magma, CO2 is the first gas to exsolve from a rising melt, and does so in easily detectable quantities and with an identifiable δ13CO2 isotopic signature (Baubron et al. 1991; Cerling et al. 1991; Mori et al. 2001; Bruno et al. 2001; Granieri et al. 2003; Badalamenti et al. 2004). These properties make CO2 one of the most studied of the volcanic gases emitted through the soil (e.g., Granieri et al. 2003; Chiodini et al. 2008; Inguaggiato et al. 2012). Though soil CO2 flux monitoring has been used successfully in polygenetic volcanic settings, to date few monitoring programs or baseline estimates have been established for the many monogenetic volcanic fields worldwide. These fields cover broad areas where future eruption sites are relatively unconstrained. Soil CO2 flux could therefore be a potentially useful tool to identify the likely area of surface breakout, particularly in regions with active fault systems (DelgadoGranados and Villalpando-Cortes 2008; Delgado-Granados 2009). In addition to volcanic monitoring, measurements of soil gas CO2 flux have been used to characterise subsurface structures, such as active faults and fissures, within geothermal and volcanic areas in numerous studies (e.g., Finlayson

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1992; Barberi and Carapezza 1994; Giammanco et al. 1998; Baubron et al. 2002; Fu et al. 2005). These structures are thought to be natural conduits for magma and gases to move through the crust from deep-seated sources (Baubron et al. 2002). Anomalously high soil CO2 fluxes in a monogenetic field could therefore reflect the location of substructures which may be pathways for magma during future eruptions. The method of monitoring soil CO2 flux in active monogenetic volcanic fields to identify underlying structures was developed and used by Delgado-Granados and VillalpandoCortes (2008), Delgado-Granados et al. (2011), and DelgadoGranados (this volume) in the Mexican Chichinautzin and Xalapa monogenetic fields. For example, Delgado-Granados and Villalpando-Cortes (2008) measured soil gas CO2 fluxes that ranged from 0 to 8.6 g m−2 day−1 in the Chichinautzin Volcanic Field in 2005. They found two zones with anomalous fluxes, one of which measured 16.1 g m−2 day−1, in an area that experienced a shallow tectonic earthquake 5 months later. This method faces a new challenge when quantifying a baseline in a dormant volcanic field in a busy urban environment such as Auckland. Carbon dioxide may originate from biogenic, anthropogenic, or volcanic sources (Heiligmann et al. 1997). Environmental conditions, including soil moisture, air and soil temperature, barometric pressure, wind speed, geologic setting, and vegetation, all play large roles in controlling the soil CO2 flux (Reimer 1980; Hinkle 1994; Chiodini et al. 2008). Quantifying a baseline that is adequately representative of a particular volcanic field requires numerous and repeated measurements, as soil CO2 fluxes can be variable even within a small area (Gunn and Trudgill 1982; Maljanen et al. 2002). Here, we attempt to establish a baseline of soil CO2 fluxes for the Auckland Volcanic Field (AVF) for potential use in future eruption forecasting. We examine several influencing variables and address the concomitant issues faced when surveying soil CO2 flux in a large area with limited natural ground, anthropogenic CO2 sources, mixed biological ecosystems, and a heavily human-altered landscape.

Background The AVF is a dormant monogenetic basaltic field comprised of approximately 50 volcanoes spanning a ∼360 km2 area in Auckland, New Zealand's most populous city (Fig. 1; Kermode 1992; Allen and Smith 1994). The Auckland region lies in a temperate zone, receiving an average of 1,240 mm of rainfall each year and has an average temperature of 15.1 °C (NIWA Science, 2000). In 2007, the majority of the land in the Auckland region was used for rural commercial endeavors (∼55 %), such as pastureland for livestock or pine forests for logging (Thompson and Hicks 2009). Roughly, 27 % of

Bull Volcanol (2013) 75:757

Auckland was covered by conservation or park areas with both native and exotic species (17 % scrub, 7 % forest, and 3 % coastal or wetland plants). Urban cover such as buildings and pavement comprised approximately ∼14 % of the Auckland region; it is underneath this urbanised area that much of the AVF lay. The remainder of the land (∼4 %) was naturally unvegetated, largely lakes or beaches. Underlying sediments in Auckland are predominantly composed of sandstones and mudstones, unconsolidated volcaniclastic deposits, and lava flows (e.g., Kermode 1992). Given the young age of the AVF (∼200 ky) as compared with analog monogenetic fields such as the South AVF (1.5– 0.5 Ma), future eruptions are expected, with an estimated warning period as short as a few days (Németh et al. 2012; Sherburn et al. 2007). The non-uniform characteristics of the AVF render the timing and location of future eruptions difficult to predict (Bebbington and Cronin 2011). Furthermore, volcanic eruptions in the AVF usually begin with phreatomogmatic explosions and base surges, increasing the risk posed to the city and its 1.5 million inhabitants (Magill et al. 2005; Houghton et al. 1999). It is estimated that an eruption in Auckland today could directly affect the population and infrastructure within a 5-km+ radius surrounding the vent (Sandri et al. 2012). Considering the likelihood of magmatic soil gas CO2 emissions prior to a future eruption, it is critical to monitor soil gas emissions in the AVF to establish a baseline CO2 flux to recognise any anomalies during future unrest episodes (Lindsay et al. 2010). No direct measurements or surveys of soil CO2 flux in the Auckland area have been published to date (Lindsay et al. 2010). Given the short warning period and likelihood of explosive activity expected, early detection of an impending eruption could improve lead time, which is crucial to saving lives (Smith et al. 2008).

Methods Over a 2-year period (2010–2012), we carried out a pilot study, followed by a more intensive study focused on the most populated areas of the AVF, such as Auckland's Central Business District, as an eruption in these areas would pose the greatest risk to the population and infrastructure (Fig. 1). During the 2010 pilot study, 443 soil CO2 flux measurements were collected over a 4-day period, from 16 to 19 November 2010, in seven sites spanning the AVF. Sites during this campaign were chosen to include areas with confirmed or suspected faults or volcanic lineaments. In addition, 72 soil gas CO2 concentration measurements were taken from three sites: Albert Park, Victoria Park, and Ihumatao (Fig. 1). During the 2012 intensive study in the most populated areas, 651 measurements were taken over 3 days, 8–10 February 2012. Measurement locations were limited to public green spaces, such as parks. During the 2012 study, diurnal variation


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Rangitoto

N

AVF

Meola Reef Reserve

Auck

Victoria Park

land

CBD

Albert Park

200 km

Domain Western Park

Seddon Fields Western Springs Fowlds Park

Basque Reserve

Outhwaite Park

Mt Eden

Thomas Bloodworth/ Newmarket Park

Hendon Park One Tree Hill

Big King Reserve

Lloyd Elsmore Park

Keith Hay Park Waikowhai Park

Whitford

AVF Volcano

Ihumatao

2010 Study Site Puhunui

2012 Study Site 2010 and 2012 Study Site

5 km

Current AVF Boundary

Fig. 1 Soil CO2 flux measurement sites in the AVF; inset, location of the field within the North Island of New Zealand. Volcanoes are represented by triangles; left-shaded circles indicate 2010 study locations; right shaded circles represent 2012 study sites; black circles indicate locations

where measurements were taken in both the 2010 and 2012 studies. Auckland's Central Business District is shaded in grey. The hashed ellipse represents the approximate current boundary of the volcanic field

in soil CO2 flux was investigated by measuring fluxes approximately every 1 to 1.5 h at nine sites in Outhwaite Park over 7 h, from 9:30 a.m. to 4 p.m., over 1 day. In 2012, samples for δ13C isotope analysis were taken from three sites with the highest measured fluxes: in Victoria Park, Albert Park, and Meola Reef Reserve (Fig. 1).

Amesbury, MA, USA) as described in detail in Chiodini et al (1998). Soil CO2 flux (F CO2 , in grams per square meter per day) was calculated as follows:

Soil CO2 flux

where dc/dt is the change in concentration with time (in parts per million per second); k is a constant (155.87 m−3) to convert parts per million per second to grams per square meter per day; P is the measured pressure (in kilopascals); T is the measured temperature (in Kelvin); V is the volume of the chamber; and A is the area of the base of the chamber. The

Soil CO2 fluxes during the two sampling campaigns were measured by the accumulation chamber technique using portable non-dispersive infrared systems (WS-LI820-CO2: West Systems S.r.l., Pontedera (PI), Italy; EGM-4: PP Systems,

F CO2 ¼ k ðV =AÞðT 0 =T ÞðP=P0 Þðdc=dt Þ;

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T 0 and P 0 are 298 K and 101.3 kPa, respectively. It is noteworthy to mention that the use of these units (in grams per square meter per day) might be misleading at some point because the results reported in this study represent the nearinstantaneous average of measurements taken over approximately 5 min, not the average over an entire day. However, these units are the most widely used in the volcanological literature, allowing the comparison of the order of magnitude of these fluxes with others reported in the literature (e.g., Chiodini et al. 1998, Cardellini et al. 2003). An estimate of the flux variations over an entire day for each of the 1,000+ measurement sites is beyond the scope of this study. One WS-LI820-CO2 unit was used during the 2010 study. During the 2012 campaign, three WS-LI820-CO2 and one EGM-4 were used concurrently. The four instruments were compared by measuring the soil CO2 flux at a single location. Soil CO2 flux in the comparison site ranged from 26.6 to 39.9 g m−2 day−1. The three WS-LI820-CO2 instruments measured CO2 flux within 4 % of one another, whereas the CO2 flux measured by the EGM-4 instrument was 30 % lower than the flux measured by the other instruments. Subsequent values from the latter instrument were corrected by adding 30 %. Site descriptions were recorded (e.g., grass; Table 1). Soil gas CO2 concentrations were measured at selected sites during the 2010 study by pumping the gas from the soil (25–40 cm deep) using a duralumin-customised probe with several perforations above a pointed base. Tubing connected the probe to the infrared analyser. Air and soil temperatures (the latter at 10 cm depth) were recorded at each location with a thermocouple TX10 (Yokogawa Electric Corporation, Tokyo, Japan). Soil moisture (0–12 cm depth) was measured at a subset of sites (Newmarket Park, Thomas Bloodworth Park, and Outhwaite Park; Fig. 1) with a Hydrosense II (Campbell Scientific Inc., Logan, UT, USA). We used Spearman-Rank correlation

analysis to evaluate the relationship between soil CO2 flux and abiotic soil characteristics (soil temperature and soil moisture). Samples for δ13C isotope analysis were collected on 20 February 2012 from Albert Park, Victoria Park, and Meola Reef Reserve, sites where soil CO2 flux was comparatively high (>70 g m−2 day−1). At each location, we collected samples of soil CO2 and ambient air. Air samples were collected using a syringe and then introduced into 0.5 L Tedlar bags. Soil CO2 samples were withdrawn from the accumulation chamber after 2 to 5 min. The samples were analysed for CO2 and CH4 concentrations and δ13CO2 using an isotopic CO2 analyser (G2131-i Isotopic Carbon Analyser, Picarro Inc., Santa Clara, CA, USA). Calculation of total CO2 output The distribution of degassing areas over Albert Park, Victoria Park, the Domain, and estimates of the total CO2 discharge in the AVF, with associated errors, were derived by sequential Gaussian simulation (sGs) (Deutsch and Journel 1998). Details of the application of sGs simulations to CO2 flux data have been described in detail by Cardellini et al. (2003). There were not enough flux measurements collected at each area to perform these analyses; Golden Software's Surfer programme was used to map the remaining sites with 20 or more measurements. Probability distribution analysis Analysis of the CO2 flux data using a graphical statistical approach (GSA) (Chiodini et al. 1998, 2001; Cardellini et al. 2003), permits differentiation of various CO2 degassing mechanisms. The GSA consists of the partitioning of soil CO2 flux data into different log-normal populations. The proportion, the mean, and the standard deviation of each population are

Table 1 Results of AVF soil CO2 flux survey across each site with 20 or more CO2 flux measurements Site

Area (m2)

Number of Average CO2 flux Mean CO2 output Mean T (10 cm (°C)) Dominant vegetation type measurements (g m−2 day−1) (t day−1)

Albert Parka Victoria Parka Domaina Hendon Parkb

41,000 78,600 411,500 141,315

209 96 104 41

26.8±0.6 34±1.1 20.6±0.4 30.2

1.1±0.02 2.68±0.09 8.5±0.18 4.3

20±1.1 22.6±0.24 NAc 24

Grass Grass Forest and grass Grass

Meola Reef Reserveb Western Parkb Thomas Bloodworth Parkb Auckland Volcanic Field

254,502 60,502 134,200 83,830,000

83 37 37 1,094

39.4 1.6 3.2 27.1

10 26.8 23.8 61.75 ±2.15

24.3 22.6 23.8 21.5

Grass Mature trees and leaf litter Grass and bare patches Grass

a

Mean CO2 output estimated by 100 simulations from sGs in the Auckland Volcanic Field

b

Mapped using surfer NA = Not Analyzed

c


estimated following the procedure introduced by Sinclair (1974).This method was used to study soil CO2 flux data from 2010 and 2012. Two populations were determined and their percentages validated by combining both populations in the proportion of 80 % A and 20 % B at various levels of log F CO2 . We used Sichel's t estimator (David 1977) to estimate the arithmetic mean of CO2 flux and the central 95 % for each population, following Chiodini et al. (1998).

Results

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Possible factors influencing the soil CO2 flux Vegetation type Though many measurement locations were spatially heterogeneous, biologically, within a given site (i.e., with mixed grass of varying heights and species, forests of varying maturity and composition, and bare patches with no vegetation), grass was the dominant vegetation type within the study area. The CO2 flux measured on grass only ranged from 22 to 43 g m−2 day−1. Differences in grass height did not influence soil CO2 flux significantly.

Average soil CO2 flux and total CO2 output A total of 1,094 soil CO2 flux measurements were made in Auckland during the two campaigns. No significant differences in flux rates were found within diurnal monitoring experiments (see ‘Temporal changes’) and an extrapolation from the measurements taken over approximately five minutes to daily values is considered valid. Soil CO2 fluxes ranged from 0 to 202.9 m−2 day−1, with an average soil flux of 27.1 g m−2 day−1 across all sites (Fig. 2). No faults were detected. The mean of the 100 total simulated CO2 outputs as determined by sGs, 61.75±2.15 t day-1, represents the estimation of the total CO2 output from the studied area. A summary of the measurements and average characteristics at the most detailed surveyed sites can be found in Table 1. Probability distribution of the soil CO2 flux In the combined dataset for the two sampling periods, 2010 and 2012, we identified two populations of CO2 flux data: population A (Fig. 3b) corresponding to 80 % of the data, with an estimated mean flux value of 31 g m−2 day−1 (30– 33 g m−2 day−1) and population B (Fig. 3b) corresponding to 20 % of the data with a mean F CO2 of 20 g m−2 day−1 (16–26 g m−2 day−1). Carbon isotopic composition of CO2 Six locations at Albert Park, Victoria Park, and Meola Reef Reserve (Fig. 1) exhibited the highest soil CO2 fluxes and were chosen for isotopic evaluation to determine source characteristics. The δ13CO2 values of ambient air (394–422 ppmv) and of CO2 collected inside the accumulation chamber (581– 1,451 ppmv) ranged from −11.3 to −12.2‰ and −15.1 to −24.2‰, respectively. Plotting the δ13CO2 values versus 1/CO2 concentration (Keeling 1958; Cerling et al. 1991) resulted in an intercept of −25.9‰ (Fig. 4) that corresponds to the source of CO2.

Soil temperature and moisture Soil temperature (at ∼10 cm depth) ranged from 14.8 to 34.4 °C (in 2010) and from 17.2 to 30.2 °C (in 2012). No statistically significant correlation was found between soil CO2 flux and soil temperature in 2010. By contrast, soil CO2 flux and soil temperature were positively correlated (r =0.447, p =0.000, n =495) for the 2012 campaign. The diurnal variation studied in detail at Outhwaite Park (see ‘Temporal changes’) during 2012 ranged from 19.3 °C (at 9 am) to 20.5 °C at 4 pm. The combined datasets from 2010 and 2012 have a weak positive correlation (r =0.267, p =0.000, n =741). The soil CO2 flux at Outhwaite Park was also positively correlated with soil moisture (r =0.315, p =0.020, n =54). Soil moisture measured in Outhwaite Park during the diurnal study (see ‘Temporal changes’) ranged from 18 to 22 %, with the highest values measured at 4:00 p.m. and the lowest at 2:30 p.m. local time.

Soil CO2 concentrations Soil CO2 concentrations measured between 25 and 40 cm depth in Albert Park, Victoria Park, and Ihumatao ranged from 399 to 10,140 ppmv in 2010 (Fig. 1). The soil CO2 concentration increased with depth (r =0.552, p =0.000, n =72). A significant positive correlation was found between soil CO2 concentrations measured at 30 cm depth and soil CO2 flux (r =0.548, p =0.007, n =23).

Temporal changes Repeated CO2 flux measurements at nine spatially heterogeneous sites over one day at Outhwaite Park showed no statistically significant changes (Fig. 5). Soil temperature and soil moisture also did not show significant changes across the day.

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Fig. 2 Example of CO2 flux maps for the northern 2012 study sites surveyed in AVF: a Victoria Park, b Albert Park, c Domain. Black points represent individual CO2 flux measurement locations

Discussion The soil CO2 concentrations measured in this study are within the range of values measured in the southern Hauraki area in New Zealand (Francis 2013), from sites in Northland (Newton et al. 1996), and the Chichinautzin Volcanic Field in Mexico (Delgado-Granados and Villalpando-Cortes 2008). They are also within the range commonly observed across different forest and grassland ecosystems (from 0.2 to 21 g m−2 day−1; Raich and Tufekcioglu 2000), which suggests the absence of a volcanic source. Fluxes between 1 and 52 g m−2 day−1 (29–590 mg C m−2 h−1) have been measured from urban lawns and green spaces in the USA (Kaye et al. 2005). As both A and B flux populations, found by the GSA method, were low (31 and 20 g m−2 day−1), this suggests that they both represent background CO2 emissions, mainly controlled by biological CO2 production in the soil. Varley and Armienta (2001) also found no volcanic component in the soil CO2 flux during a survey of Popocatépetl volcano, Mexico, which was actively erupting

and emitting up to 60,000 tons a day of CO2 at the summit plume. Soil gas CO2 concentrations in the flux at the volcano were up to 14 vol.%, however isotopic studies (δ13CO2 values between −24 to −34‰) indicated that the source of these relatively high concentrations was of biological origin. Significance of the two CO2 flux populations Some areas of high CO2 flux are likely due to high rate of decomposition relatively common in urban areas (Lorenz and Lal 2009). For example, during our study, a measurement of soil CO2 flux over a pile of leaf litter at Hendon Park (Fig. 1) was 71.3 g m−2 day−1(18 μmol CO2 m−2 s−1). Studies elsewhere that have revealed high CO2 fluxes suggest a high production of CO2 in the soil but also a high diffusivity/permeability dominated by advective gas transport. It is postulated that the difference between populations A and B is controlled by the permeability of the substrate, such as the low permeability of phreatomagmatic deposits, which prevent CO2 from escaping to the surface. Phreatomagmatic eruptions


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Fig. 5 Diurnal variation in soil CO2 flux on 9 February 2012 at the Outhwaite Park. Values (mean±standard deviation) were calculated from six measurements spread out over nine locations over 7 h in 1 day

Fig. 3 Histogram (a) and probability plot (b) of CO2 flux data (black circles). Populations A (open triangles) and B (open squares) are shown as straight lines. The inflection point is indicated by an arrow and corresponds to 80 % of population A and 20 % of population B

in the AVF produced wide tuff rings with relatively thin deposits with gently dipping beds (Allen et al. 1996). These deposits are frequently fine grained and indurated, inhibiting soil development and creating an impermeable layer in much

of the study area. In several parks (Victoria Park, Western Park, Thomas Bloodworth Park, Newmarket Park, Basque Road Reserve, Mount Eden, and Fowlds Park; Fig. 1), despite high soil CO2 concentrations, there were no sites with high CO2 flux (Fig. 2). These areas are covered by thin phreatomagmatic deposits that create an impermeable layer, possibly preventing the CO2 from escaping to the surface. The high soil CO2 concentrations measured show that the soil is impermeable in areas with this underlying geology. Effect of land use on CO2 flux Meola Reef Reserve (Fig. 1) showed the highest CO2 fluxes measured (203 g m−2 day−1) during our study. It is located on a lava flow from the Te Kopuke (Mt. St. John) volcano (Hayward et al. 2011). The area was the site of an Auckland city landfill for many years (B. Hayward, personal communication), which explains the high CO2 flux measured at this site. CO2 emissions measured at California landfill sites were on average 135± 117 g m−2 day−1 (Bogner et al. 2011), on the same order of magnitude as at Meola Reef Reserve. The high CH4 concentrations measured at various locations in Meola Reef Reserve (up to 52 ppb) is also an indicator of landfill emissions (Le Mer and Roger 2001). This demonstrates how land use characteristics may affect CO2 fluxes in urban environments. Sources of CO2

Fig. 4 Keeling plot of the isotopic composition of soil-respired CO2 in Auckland. The δ13C intercept for CO2 accumulated in the chamber (i.e., respired from the soil) is −25.9‰, suggesting biogenic sources

Soil CO2 was depleted in 13C (δ13CO2, −15 to −25‰) compared with ambient CO2 (δ13CO2, −11 to −12‰). The δ13CO2 from CO2 emitted from the soil was estimated to be −25.9‰, typical of a main biogenic source, namely heterotrophic and autotrophic respiration (Smith et al. 2003). By contrast, δ13CO2 values for magmatic gases are typically enriched. For example, δ13CO2 values of around −2‰ have been

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measured in gas sampled from fumaroles at White Island volcano, New Zealand, and δ13CO2 values around −4‰ have been measured in gas sampled at Mount Etna, Italy (Marty and Giggenbach 1990; Giggenbach and Matsuo 1991; Allard et al. 1997).

Conclusions This study is the first to assess and establish the background level of CO2 emitted from the soil in Auckland. From the 1,094 CO2 flux measurements made in green spaces in Auckland, we estimated a mean CO2 flux of 27.1 g m−2 day−1 and a total CO2 output of 61.75±2.15 t day−1. Isotopic composition of soil CO2 revealed no magmatic component. Soil gas CO2 concentration and flux baselines for the AVF as measured in this study are typical of those produced by biogenic sources and for New Zealand. The values reported here would constitute a reference for future studies, representing a background level to assess future unrest in the AVF, particularly if magma tries to open its way to the surface in the AVF. The variations measured in CO2 flux in this study confirm the important role that setting plays when assessing soil gas CO2 fluxes in urban landscapes. We suggest that future studies in similar urban areas take prior and current land use into consideration when analysing their results for signs of volcanism. Acknowledgments The authors are grateful to Jo Hanley (Royal Society of New Zealand Primary Science Teaching Fellow), Tracy Howe, Madison Frank, Mary Anne Thompson, Jia Liu, Andrew Wheeler, and Karine Tan for field assistance. Thoughtful reviews by Deborah Bergfeld and an anonymous reviewer were most helpful in improving the manuscript. This work was carried out under the umbrella of the Determining Volcanic Risk in Auckland (DEVORA) project, which is financially supported by the Earthquake Commission and the Auckland Council.

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