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| Titel |
Modelling stalagmite growth rate and oxygen isotope composition |
| VerfasserIn |
Alexander Baker, James Baldini, Dave Mattey |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056048
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| Zusammenfassung |
| Two numerical forward models based on monitoring data from New St Michael’s Cave,
Gibraltar, available between 28/04/2004 and 24/02/2008 (Mattey et al., 2010) were created to
investigate the growth rate and oxygen isotope (δ18O) composition of stalagmite calcite, with
an aim of replicating the record from stalagmite Gib04a (Mattey et al., 2008). Model outputs
for stalagmite growth rate variability are based on high-frequency fluctuations in cave
atmosphere pCO2, temperature, and dripwater [Ca2+], and pseudoproxy stalagmite δ18O data
outputs are based on step-wise isotopic fractionation associated with seawater evaporation,
moisture migration and rainout.
High-frequency fluctuations in cave atmosphere pCO2 and, to a lesser extent,
temperature, affect stalagmite growth rates, implying that cave atmosphere dynamics are an
important control on stalagmite growth. As a result of seasonality present in dripwater [Ca2+]
and cave air pCO2, modelled Gib04a growth rates vary seasonally. In addition,
higher-frequency fluctuations in both dripwater [Ca2+] and pCO2 are apparent in
the monitoring data and in modelled growth rates. The modelling assumed that
the measured drip water [Ca2+], drip discharge, and cave air pCO2 patterns are
representative of the entire growth period of Gib04a (~53 years). However, modelled total
growth (~3 mm) considerably underestimates actual growth (~45 mm). If drip
water [Ca2+] is raised uniformly by 75 ppm, the required growth is achieved. On
intra-annual timescales, modelling also suggests that stalagmites frequently experience
temporary cessations of growth, even if dripwater is supplied continuously to the
stalagmite.
The ‘typical’ oceanic source regions for Gibraltar precipitation between 1951 and
2004 were inferred from ensemble back trajectory analyses. Seawater δ18O, sea
surface temperature, and relative humidity data for these regions allow synthetic
precipitation δ18O and pseudoproxy stalagmite δ18O time series datasets to be
generated by the numerical forward model constructed. Modelled monthly meteoric
precipitation δ18O data are found to correlate with estimated monthly amounts of
hydrologically-effective precipitation at Gibraltar (r2 = 0.66); this correlation is comparable
to that determined from Global Network for Isotopes in Precipitation (GNIP) data (r2
= 0.63). Additionally, modelled precipitation δ18O data replicate the amplitude
of GNIP δ18O variability. Some pseudoproxy stalagmite δ18O outputs based on
distinct source regions replicate the amplitude of variability in actual Gib04a δ18O
moderately well, although others exhibit lower amplitude. This suggests that the regions
on which they are based do not contribute to precipitation in Gibraltar frequently.
Additionally, adjustments of only the fraction of rainout within realistic boundaries are
sufficient to produce satisfactory fits between pseudoproxy and actual Gib04a δ18O
data.
This research illustrates that modelling can help evaluate the parameters that
ultimately affect stalagmite growth and stable isotope records, and which are otherwise
difficult to constrain. Future work includes incorporating continentally-derived
precipitation into the model in order to better quantify the changing contributions of
different precipitation source regions through time evident in stalagmite palaeoclimate
records.
References
Mattey, D. P., Fairchild, I. J., Atkinson, T. C., Latin, J.-P., Ainsworth, M., and Durell, R.,
2010. Seasonal microclimate control of calcite fabrics, stable isotopes and trace
elements in modern speleothem from St Michaels Cave, Gibraltar. In Pedley, H.
M. and Rogerson, M. (Ed.) Tufas and Speleothems: Unravelling the Microbial
and Physical Controls. Geological Society of London Special Publication 336. pp.
323-344.
Mattey, D. P., Lowry, D., Duffet, J., Fisher, R., Hodge, E., and Frisia, S., 2008. A 53 year
seasonally resolved oxygen and carbon isotope record from a modern Gibraltar speleothem:
Reconstructed drip water and relationship to local precipitation. Earth and Planetary Science
Letters 269, 80-95. |
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