Studying cave environments and their formation provides a unique insight into our prehistoric climate and is an important step towards a quantitative interpretation of palaeo climate.
In our field campaign to the Jenolan Caves in New South Wales, we monitored air gases and drip water through the year, using isotope techniques for our measurements. A detailed picture of seasonal cave formation (speleothem) emerged, illustrating growth patterns of stalactites and stalagmites, and thus helped to further the interpretation of palaeo-climate recordsWhy we study caves
Evidence for global warming and Earth’s climatic history comes from long continuous Greenland and Antarctic ice-core records [1,2]. These palaeo-climate records from the poles show the Earth experienced four major cycles of glaciation with short intervening periods of relative warmth over the preceding 500,000 years.
At present, Earth is in a warm inter-glacial period. This global pattern of glacial cycles has contrasting and divergent impact in different regions across the globe because of the influence of changing ocean currents, atmospheric weather circulation patterns, and latitude.
If we want to better understand regional impacts of global warming on our weather long high-resolution palaeoclimate records from mid and low latitudes are required. Cave formations (speleothems) are long, widespread, continental palaeoclimate archives.
At issue is the quantitative translation of speleothem isotopic and trace element abundances into reliable records of past temperature and rainfall. Studies of modern speleothem growth linked to weather and climate records are used to improve speleothem translation.
Whilst there are many good qualitative speleothem palaeo-climate records, a universal quantitative transfer function from external weather / climate to speleothem record is elusive. Studies of cave environments and speleothem growth are an important step towards quantitative speleothem palaeo-climate interpretation.
The growth looks at the net accumulation of CaCO3 (speleothem growth), but in order to interpret this we need to understand the Gas-Aqueous-Solid equilibrium conditions in the cave environment (dissolved ions in water, temperature T, pressure P, carbon dioxide concentration in air pCO2).
The largest change to equilibrium conditions in a ventilated cave environment causing speleothem growth is fluctuating carbon-dioxide concentration as a response to the cave-air exchange, driven by external temperature. Figure 1 shows the cycle and summarises isotope tracing in speleothems.
The scheme also indicates that the cave ventilation system, i.e. the location of the speleothem, will influence carbon-dioxide concentration - quantative measurements (pCO2) are shown in Figure 2 - and therefore influence speleothem growth pattern and palaeo-climate records.
Field campaign in Jenolan Caves
Continuous CO2 monitoring records from different caves at Jenolan (NSW, Australia) show different ventilation patterns ranging from slow drainage at week-month long time-scales (cave opening “Temple of Baal” one opening without through-air flow) to large daily (diurnal) fluctuations (“Katie’s Bower” chamber between a top and bottom opening) dependent upon the configuration of cave openings, see Figure 1 [3]. Seasonal differences are also apparent at Katie’s Bower with summer peak CO2 reaching 5,000 ppm compared to a winter range from 400 ppm to 1,000 ppm.
An intense 3-week field campaign in May (winter) continuously measured (5 min) trace gases (CO2, CH4, N2O) H2O and isotopic contributions of CO2δ13CCO2 using a Fourier Transform Infrared (FTIR) spectrometer. Simultaneous drip-water pH, air flow, temperature, pressure, and relative humidity were logged by sensors in the cave pressure, and relative humidity.
Drip water was sampled with CO2 maxima and minima (Figure 3 shows dissolved inorganic carbonate (DIC), δ13CDIC, dissolved organic carbonate (DOC),δ13CDOC, alkalinity, anions, and cations). Further spot samples were taken for drip-water stable isotopes, 14CDIC, and 3H. Our results on soil and drip water and their isotopic constraints are shown in Figure 3 – they imply that speleothem δ13C is largely influenced by prior calcite precipitation, and hence factors that change speleothem growth will have a consequent impact on speleothem δ13C in palaeo-climate records.
At Katie’s Bower with a strong ventilation pattern, speleothem growth rate varies through the diurnal cycle and between seasons (Figure 4). Low pCO2 in the morning, cave air causes rapid speleothem growth with CO2 exsolved to the cave atmosphere lowering drip-water pH. pCO2 increases to an evening maxima and slows speleothem growth before early morning temperature-induced ventilation decreases pCO2.
We find, see Figure 4, that (i) δ13CCO2 has an antithetic relationship with CO2, (ii) dripwater remains constant throughout the winter experiment. This suggests that CO2 is not redissolving into drip-water to dissolve speleothems nor complicate interpretation of speleothem δ13C palaeo-climate records. Summer speleothem growth may have a different δ13C incorporation pattern from a higher diurnal peak CO2 (up to 5,000 ppm).
Current climate records start from the assumption of a regular annual growth rate for the speleothem and a formation age error plus or minus decades at best. However, our study shows that the growth patterns of speleothems have a diurnal
and seasonal component as large as the climatic range.
This means climate records need much more fine-tuning since the growth rate not only varies over years, but also within a year. Highresolution speleothem records can also take advantage of the seasonal isotopic variations to provide annual markers, thereby allowing highresolution chronology and growth rates, similar to ice-core and tree-ring analysis.
We plan to demonstrate the linkage between weather records and speleothem isotopes with highresolution (monthly) analysis of a speleothem grown at Jenolan between 1938 and 1998 on a wire strung across a chamber.
Authors
Chris Waring1, Stuart Hankin1, David Griffith2, Stephen Wilson2 and Samantha Hurry2
1ANSTO and 2University of Wollongong, Australia
References
- Augustin et al., Nature, 429 (2004) 623-628.
- Anderson et al., Nature, 431 (2004) 147-151.
Published: 12/11/2009