Seasonal microclimate control of calcite fabrics, stable isotopes and trace elements in modern speleothem from St Michaels Cave, Gibraltar - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 317

Seasonal microclimate control of calcite fabrics, stable isotopes
and trace elements in modern speleothem from
St Michaels Cave, Gibraltar

DAVID P. MATTEY 1,* , IAN J. FAIRCHILD 2 , TIM C. ATKINSON 3 , JEAN-PAUL LATIN 4 ,
MARK AINSWORTH 4 & RICHARD DURELL 4
1 Department of Earth Sciences, Royal Holloway University of London, TW20 0EX, UK
2 School of Geography, Earth &Environmental Sciences, University of Birmingham B15 2TT, UK
3 Department of Earth Sciences, University College London, WC1E 6BT, UK
4 Cliffs and Caves Section, Gibraltar Ornithological and Natural History Society, Gibraltar
*Corresponding author (e-mail: [e-mail])

Abstract: Detailed monitoring of three drip sites in New St Michael's Cave, Gibraltar, reveals a
strongly coherent seasonal pattern of dripwater chemistry despite each site having significantly
different flow paths and discharge patterns. Calcite saturation is closely linked to regular seasonal
variations in cave air pCO 2 caused by seasonally reversing ventilation driven by temperature differ-
ence between the cave interior and the air outside. A coupled model of CO 2 degassing and calcite
precipitation links seasonal d 13 C variations in coexisting dripwater, cave air CO 2 and speleothem
calcite to large variations in pCO 2 that are driven by cave ventilation. The relationships between
stable isotope ratios, Sr/Ca and speleothem fabrics across annually formed calcite laminae are
consistent with a degassing - calcite precipitation process in which rapid degassing controls the
d 13 C of both drip water DIC and calcite whereas a much slower rate of calcite precipitation
causes seasonal cycles of Sr in a more complex manner. By demonstrating the causes of laminated
speleothem fabrics plus trace element and isotope cycles in modern speleothem from a closely
monitored cave, this study provides clear links between the local microclimate and the proxy
record provided by speleothem geochemistry. In Gibraltar, low cave air pCO 2 in summer is
unusual compared to what has been revealed by cave monitoring carried out elsewhere and
shows that caution is needed when linking paired speleothem fabrics to specific seasons without
knowledge of local processes operating in the cave.

Speleothems provide continuous and precisely
dated records of past environmental change which
have advanced understanding of climate variability
on timescales from glacial - interglacial cycles
(Wang et al. 2008) down to seasonal patterns of
precipitation (Borsato et al. 2007). Speleothem
deposition in stable cave environments can record
changes in surface climate as variations in proper-
ties such as extension rate, trace element abun-
dances and stable isotopes (McDermott 2004;
Fairchild et al. 2006a) but the causal relationships
between these proxies and climate are not always
fully understood. For some proxies they appear
to be straightforward, for example the dependence
of extension rate on the amount of rainfall
(Baker et al. 2008), but for others such as stable
isotopes interpretations have often been based on
assumptions and guesswork regarding the aspects
of climate that are most closely reflected.
Ideally, any proxy-climate transfer function
used should be based on a full understanding of
the

climate - karst-cave system and its influences on
the recording process.
Careful, multi-annual monitoring of the cave
microclimate, dripwater chemistry and calcite
growth mechanisms reveals some of the local
effects that may modify the climate recording
process. Some of the important issues include the
relationships between precipitation, recharge, drip
rates and solute chemistry (Bottrell & Atkinson
1991; Genty & Deflandre 1998; Baker & Brunsdon
2003; Tooth & Fairchild 2003; Cruz et al. 2005;
Baldini et al. 2006; Genty 2008), the role of seasonal
ventilation and degassing (Ek & Gewelt 1985;
Bar-Matthews et al. 1996; Sp¨tl et al. 2005;
Banner et al. 2007; Baldini et al. 2008) and the
impact of kinetic factors such as fast degassing or
crystal chemical effects on CaCO 3 growth (Hendy
1971; Mickler et al. 2004, 2006). Knowledge of
these local processes and their stability through
time are a critical step in the derivation of reliable
climate - proxy transfer functions that can be used
for quantitative climate reconstruction. physico-chemical workings of the local