TÍTULO I DE LOS PRINCIPIOS GENERALES CAPÍTULO ÚNICO DEL ÁMBITO, PRINCIPIOS Y FINES
ENCUESTA A REPRESENTANTES LEGALES
4.4.3.1 Bedrock carbon isotope values
Rock samples were taken from inside and outside the cave entrance and from a collapsed roof section within the cave. These rock samples were dolomitic with calcite intergrown (section 3.1). The carbon isotope values from the rock samples exhibit a range of 4.69‰ between -2.44‰ and +2.25‰ (table 4.5).
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Table 4.5: Carbon isotope values from rock samples.
Rock sample type Rock sample
δ13C (‰)
Rock from cave entrance RCE1 1.96 RCE2 0.73 Rock from outside cave ROC1 2.25 ROC2 1.61 Rock from cave roof RCR1 -2.44
RCR2 1.57
4.4.3.2 Cave atmosphere CO2 concentration
Carbon dioxide was monitored hourly within Cueva de las Perlas between Feb-15 and Dec-17 (figure 4.12A). The gaps within the dataset relate to battery malfunctions within the logger.
Average annual cave CO2 concentrations were 421ppm, 425ppm and 456ppm in 2015
(number of days in average (n) = 119), 2016 (n= 190) and 2017 (n= 207) respectively. In 2015 and 2016 CO2 values had a range of 505ppm and 590ppm. However, in 2017
the CO2 range was almost five times greater than previous years, at 2490ppm (table
4.6).
CO2 concentrations are shown to fluctuate daily (figure 4.12B) with an average
variability of 182ppm (n=507). The daily CO2 variability exhibits a range between 30-
2430ppm.
Table 4.6: Annual and seasonal CO2 concentrations.
Year Annual average (ppm) Annual range (ppm) Summer average (ppm) Winter average (ppm) 2015 421 505 422 418 2016 425 590 424 426 2017 456 2490 486 434
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Figure 4.12: Hourly and daily cave CO2 concentration (A) and daily variability (B).
Figure 4.12 demonstrates the variability within the CO2 record which is apparent
throughout the year. Seasonality is minimal as CO2 concentrations fluctuate on a
diurnal basis. Additionally, summer and winter CO2 averages (except for summer
2017) both lie close to annual CO2 averages (table 4.6).
4.4.3.3 Cave air carbon isotopes
Cave air was sampled between Feb-15 and Jan-18 and results are presented in figure 4.13. Annual averages for 2015 (n= 15) and 2016 (n= 22) were both -12.81‰ and -
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12.59‰ in 2017 (n= 8). Carbon isotope values demonstrated intra-annual variability of 5.17‰, 4.35‰ and 3.06‰ in 2015, 2016 and 2017 respectively. Variability in carbon isotope values occurs throughout the year with no distinctive seasonal variation.
Figure 4.13: Carbon isotope composition of cave air.
4.4.3.4 Cave carbon dynamics interpretation
Carbon dioxide
Cave carbon dioxide concentrations are variable on an hourly scale throughout the year and do not show distinct seasonal variations. CO2 concentrations are compared
to internal and external hourly temperature in figure 4.14. Daily variability in CO2
corresponds to external temperatures crossing the internal cave temperature threshold which occurs throughout the year. When external hourly temperature falls below internal hourly temperature a density-driven ventilation causes a rapid influx of CO2-poor air into the cave. As external temperatures exhibit a large diurnal range
and appear to cross internal temperatures nearly every day, this switch in ventilation causes significant daily variability in CO2 values.
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Typically, this dynamic ventilation leads to low CO2 concentrations in the cave
throughout the year. However, in Jun-17 there is a distinctive increase in CO2
concentration within the cave. Figure 4.15 demonstrates that when external temperatures remain higher than internal temperatures for an extended period, CO2
concentrations rise within the cave as a result of reduced ventilation. This is the primary reason for the high CO2 values in Jun-17. However, a secondary influence of
pressure will also be assessed in section 4.4.3.5.
Figure 4.14: Hourly cave CO2 concentrations and external and internal hourly air temperature.
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Figure 4.15: Cave hourly CO2 and internal and external temperatures Jun-17. Shading represents a prolonged period of external temperatures exceeding internal cave temperatures.
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Cyclicity in cave CO2 concentration
Spectral analysis was undertaken on months where over 14 days of CO2 data was
collected to determine whether oscillations in CO2 concentration are diurnal. The
majority of months demonstrate a peak suggesting seven cycles per week which would suggest daily oscillations in CO2. Interestingly, no cyclicity is evident in Jun-17
which supports section 4.4.3.4 (figure 4.15) as no cyclicity would be expected as external temperatures are consistently higher than internal cave temperatures and subsequently CO2 rises within the cave.
A case study from Nov-16 is used herein to assess the validity of the spectral analysis. Figure 4.17A demonstrated a pronounced peak at seven cycles per week. However, when the individual data are presented alongside cave and external temperatures it is clear that the cyclicity is not persistent throughout the record (figure 4.17B). CO2 is
clearly variable on a diurnal basis and most likely a response to external temperatures crossing the internal cave temperature threshold. The degree to which this pattern is cyclical requires further analysis.
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Figure 4.16: Spectral plots for monthly CO2 data.
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Figure 4.17: Spectral analysis case study from Nov-16. A: Spectral plot from Nov-16. B: CO2 concentration, internal and external hourly temperature for Nov-16.
Carbon isotopes
The carbon isotope composition of cave air lies upon a mixing line between the soil air and external air carbon isotope compositions (figure 4.18). Cave air carbon isotope composition lies closer to the external air isotope composition shown through lower CO2 concentrations and relative enrichment in 13C compared to soil air.
The close relationship between internal cave air and external air supports the model proposed above that the cave is being continuously ventilated with atmospheric CO2-
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poor air throughout the year. The trend was constructed using average EA, CA and SA data and predicts a soil δ13C end member of -23.5‰.
Figure 4.18:Carbon isotope values for cave air (CA), external air (EA) and soil air (SA) plotted against individual CO2 measurements.
4.4.3.5 Pressure-induced ventilation
CO2 values have been shown to oscillate on a daily basis due to external hourly
temperature fluctuating around cave temperature (section 4.4.3.4). Variations in pressure have also been proposed as a mechanism driving CO2 concentrations within
the cave void on an hourly basis (Denis et al., 2005; James et al., 2015, Smith et al.,
2015). The potential for this hourly influence of pressure on cave carbon dynamics is explored in figure 4.19 through assessing the relationship between CO2
concentrations, external/internal temperature and pressure variability. Jun-17 shows an increase in cave CO2 during a period of consistently higher external temperatures
relative to cave temperature as well as a reduction in pressure. This is highlighted on figure 4.19 by the grey shading. Periods of high CO2 concentrations coincide with
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external temperatures being consistently higher than cave temperatures which would reduce density-driven advection of air into the cave. However, periods of high CO2
concentrations also correspond to decreases in cave pressure. A reduction in external pressure causes a pressure gradient between the cave and external air. To ensure pressure equalisation between the internal and external environments, air will flow out of Cueva de las Perlas. Consequently, the CO2 of the cave will increase due to a
larger proportion of CO2-rich air being drawn down from the karst and therefore
increasing CO2 concentration within the cave void (Denis et al., 2005; James et al.,
2015; Smith et al., 2015).
Therefore, a model of ventilation is proposed for cave carbon dynamics where the external versus internal temperature dynamics drive ventilation on an hourly-scale and pressure acts as a secondary forcing mechanism. This model will be further discussed in section 4.5.
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Figure 4.19: Cave CO2 concentration, cave pressure and external and internal temperature for Jun-17. Shading represents heightened influence of pressure on cave air CO2 concentrations.
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