Palaeoenvironmental record from a speleothem in Victoria Fossil Cave, Naracoorte, SA — speleothems tell an exciting tale

Dr. Albert Goede, School of Geography & Environmental Studies, University of Tasmania, GPO Box 252-78, Hobart, Tasmania 7001, AUSTRALIA, e-mail: Albert.Goede@utas.edu.au

"In my day speleothems were called something else and were collected from caves by souvenir hunters, not by scientists"

INTRODUCTION

Like all of us, cave managers face a rapidly changing world. One of these changes has been a growing number of requests from earth scientists to collect speleothems for scientific study. Most cave managers do not have a scientific background, yet they are called upon to make a judgement on whether a particular research project, that may involve the collection of speleothems, should or should not be supported. In this paper the author will attempt to explain in relatively simple terms what is involved in the study of speleothems and why there has been a rapidly growing interest in this field of research during the last decade.

THE ICE AGE STORY

We are living in an ice age, known to earth scientists as the Quaternary Ice Age, which has been with us for the last 2.5 million years. During most of the earth's history the planet has been free of ice but from time to time conditions favour the accumulation of one or more ice sheets and such periods are known as ice ages. Each ice age may last up  to 50 million years and one hypothesis to explain their origin is that they occur when continental drift causes one or more of the continents to be concentrated in or move close to a polar position as is the case today. The Quaternary Ice Age is characterised by long cold periods called glacials that are separated by short and much milder periods called interglacials. During the last 12,000 years we have been living in an interglacial - the Holocene epoch. It is this time that has seen the rapid growth of agricultural and industrial technology and the consequent explosive growth of human populations. We have now reached a stage where our activities are beginning to influence significantly the pattern of climate change, especially due to the ever-growing emissions of greenhouse gases. Until recently climatic changes have been entirely due to natural causes. The pattern of glacials and interglacials is strongly influenced by variations in the path of the earth around the sun that are known as the Milankovich Cycles. Shorter term variations in the climate are due in part to variations in the energy output from the sun, sunspot activity and periodic changes in ocean circulation such as the El Nino phenomenon.

In recent years numerous attempts have been made to use global computer models to predict climate change. Such models are known as General Circulation Models (GCMs). Initially the models were tested against the instrumental records of climate but such records are short and very unevenly distributed over the surface of the earth. The search has been on for much longer records of climate change from the geological record that are referred to as proxy records and the numerical data they provide as proxy data. The most important sources of such information have been cores taken from the deep oceans and from the ice sheets of Greenland and Antarctica. They have provided vital information on global climate changes during the Quaternary but as the models grow more sophisticated, it becomes more and more important to obtain proxy data from individual regions and information from speleothems is beginning to play an important role in this.

SPELEOTHEMS AS INFORMATION SOURCES

A number of aspects of speleothems have been investigated to test their suitability to yield proxy data on climatic and other environmental changes. Some of the most significant are:

1. Mineralogy
2. Frequency of speleothem formation
3. Stable isotope ratios
4. Luminescence
5. Trace elements
6. Organic content
7. Colour

1. Mineralogy
   In humid areas the minerals deposited in caves are predominantly carbonate minerals and the most common varieties are calcite and aragonite. In semi-arid regions cave minerals tend to form due to evaporation. Sulphates and halides tend to be the dominant deposits. Common minerals formed are gypsum (CaSO4.2H2O) and rock salt (NaCl). Some of the Nullarbor caves are a good example. More than 400,000 years ago speleothem deposition was predominantly carbonate but since then there has been a progressive drying trend with gypsum and rocksalt becoming the dominant minerals. In some of the caves they are still being deposited today.

   Researchers in western Europe have recently discovered that annual laminations occur in some European stalagmites. They can be seen under the microscope as alternating layers of different types of calcite: dark compact laminae alternate with white porous laminae. They are probably due to seasonally changing hydrological and chemical environments. In areas where the bedrock is rich in magnesium (magnesian limestone or dolomite) seasonal laminations in speleothems may consist of alternating layers of calcite and aragonite. Examples have been found in Botswana, Africa, in a tropical wet-dry climate.

2. Frequency of speleothem deposition.
   The frequency of speleothem deposition appears to vary with climate. To study this successfully requires accurate and precise age determination of a large number of speleothems. It has been done very successfully in north-western Europe where a large number of dated speleothems is available. It was shown that in that region temperature is the dominant control factor with most abundant deposition during interglacials and little or no deposition under glacial maximum conditions. In Australia only two small sets of samples have been analysed so far, one from Tasmania and another from the Naracoorte region. The Naracoorte data in particular appear to indicate that in the Australian environment variations in available moisture are at least as important as are variations in temperature.

3. Stable isotope ratios
   This is the most widely used source of proxy data obtained from speleothems. Carbonate minerals contain the elements oxygen and carbon as well as calcium and magnesium. Oxygen has three stable isotopes (16O, 17O and 18O) and carbon has two (12C and 13C). The ratios 18O/16O and 13C/12C are frequently measured at intervals along the growth axes of speleothems since they have been shown to vary with changing environmental conditions. The exact interpretation of these ratios remains a matter of debate and can vary from one region to another. Information is preserved only under certain conditions - deposition under conditions of isotopic equilibrium - that require the absence of evaporative conditions including strong draughts.

   Under tropical cave conditions oxygen isotope ratios give an indication of variations in the amount of moisture. In temperate caves they usually indicate variations in temperature or variations in the geographical location of moisture source areas or a combination of the two. When the direct temperature effect is dominant, oxygen isotope ratios have a negative relationship with temperature as is the case at Naracoorte. Where changes in moisture source areas are dominant, the ratios have a positive relationship with temperature as happens to be the situation in Tasmanian caves. The quantitative estimation of temperature changes from oxygen isotope ratio variations is still a very contentious issue.

   Interpretations of carbon isotope ratios are also controversial. Most workers in the field believe that the dominant factor is the amount of vegetation activity on the surface above the cave. This in turn may reflect changes in climatic conditions such as available moisture and temperature. In tropical and sub-tropical areas, variations have also been interpreted in terms of changes in vegetation type between predominantly tropical grasses (C4 plants) on the one hand and tree, shrub and herb vegetation (C3 plants) on the other. This complicates interpretation of carbon isotope ratios in areas such as Naracoorte where C4 plants may at times have been dominant in the past, but not in places such as Tasmania and eastern Victoria where this is unlikely to have been the case.

4. Luminescence
   It refers to the emission of visible light by some minerals when they are exposed to as source of ultra-violet radiation. A common type of luminescence in speleothems appears to be due to the presence of certain types of organic substances that may be present in only small amounts. Variations in the intensity of luminescence appear to be due to changes in the rate of breakdown of organic matter in the soil above the cave, controlled in turn by climatic and vegetation conditions. Some speleothems have been shown to contain annual variations in luminescence - a very useful characteristic. In a speleothem that was actively growing when collected, it is possible to count back the number of years from the present as is done in tree-ring studies. This can provide a very accurate and precise timescale and allows proxy data to be compared directly with climatic data from nearby climatic stations on the surface. It also enables the study of year to year variation in the amount of speleothem growth and can relate it to changing climatic conditions on the surface.

5. Trace elements
   Many elements occur as trace impurities in the mineral calcite. When measured throughout the stratigraphy of a speleothem they frequently show periodic variations that are clearly in some way related to changes in environmental conditions at the surface above the site. The elements most widely studied so far have been magnesium, strontium, barium and uranium but methods of chemical analysis are now so refined that a large number of trace elements can be measured simultaneously with a high degree of precision. Their interpretation is even more controversial than that of stable isotope ratios, partly because their study in speleothems is a more recent phenomenon. Although magnesium concentrations in calcite are known to be influenced by the temperature of deposition, this does not appear to be the dominant factor in causing variation. The dominant factor may be the time the seepage water spends in contact with the bedrock before it reaches the cave and that in turn would be influenced by climatic conditions at the surface.

   Strontium content is also quite variable although it is usually present in much smaller concentrations. In the case of a Tasmanian speleothem the author and others have shown that variations are due predominantly to the addition of strontium from dust precipitated from the atmosphere. In caves found in coastal areas one would expect the magnesium and strontium content to be influenced by the addition of seasalt washed out by rain so that concentrations may vary with proximity to the sea. That could change over time with fluctuations in sealevel which in their turn are in most areas controlled predominantly by climate changes.

   Other trace elements such as chlorine and bromine can also be derived from sea-salt but in inland areas their concentration may vary due to changes in the frequency and intensity of bushfires since combustion of vegetation is known to release significant quantities of these elements to the atmosphere. This is a promising field of future research in Australia where fires have long played an important role in modifying vegetation patterns. Uranium content of speleothems is also known to vary significantly but in many cases is so small that it is difficult to measure precisely. The variations appear to be related, at least in part, to changing vegetation activity on the surface above the sites.

   An exciting recent scientific advance has been published by Mark Roberts and others in Great Britain. They used very high resolution sampling of a stalagmite from a Scottish cave to demonstrate that magnesium, strontium and barium all show annual variations in their concentrations. The knowledge that such variations exist will make it easier to study the factors that cause variation since we now know that some of them at least are related to seasonal changes.

6. Organic content
   The study of organic impurities is still at an early stage because the concentrations involved are usually quite small and their analysis requires sophisticated chemical analytical techniques. Some interesting work has already been done in Australia. Julia James and others at the University of Sydney have studied the amino-acid composition of stromatolitic stalagmites from cave entrances in NSW. Pyramo Marionelli and others have recently measured the soil organic acid concentrations in a tall stalagmite collected from Victoria Fossil Cave at Naracoorte and have also determined the carbon isotope composition of the organic samples. This speleothem has grown intermittently over the last 500,000 years and humic acid content has decreased over time, possibly reflecting a gradual drying of the climate.

7. Colour
   Colour changes in speleothems remain poorly studied but it is known that many colours, especially brown, yellow and black, are due to the presence of organic matter. Colour is often, but not always, a reflection of organic content. Many caves in the Mundrabilla region of the Nullarbor show a phase of black calcite deposition that is clearly related to distinctive environmental conditions but at present we are uncertain about the physical nature of that environment.

AGE DETERMINATION

The value of proxy data on environmental change obtained from speleothems depends very much on how well they can be dated. Reliable, accurate and precise dating enables speleothem records to be compared with recorded climatic data, with each other and with other sources of proxy data such as tree-rings, lake sequences, marine cores and ice cores.

Although many methods of dating have been tried with varying degrees of success, there are only two that meet all the requirements. The first is known as uranium-thorium dating and relies on the decay of uranium-234 to thorium-230. Originally, this method was based on the counting of alpha particles that are produced by the radioactive decay process. This is an inherently inefficient analytical process and required large samples for dating. During the last decade it has been replaced increasingly by mass spectrometry (MS) techniques that allow the concentrations of the radioactive isotopes to be measured directly. This technique has made it possible to date much smaller samples and has dramatically improved the accuracy and precision of age determinations.

The second method can be applied only to speleothems that were actively forming when collected and that also show annual banding. The bands are counted backwards from the present but it may be necessary to demonstrate that the bands are indeed annual and this can sometimes be done by very precise uranium-thorium MS dating.

RESEARCH IN THE NARACOORTE AREA

The caves at Naracoorte have become a centre of speleothem research in recent years with at least three inter-related projects.

Linda Ayliffe and others have examined the frequency of speleothem deposition over the last 500,000 years and have found that it is concentrated during a limited number of time periods when there was abundant moisture. In the process they were also able to place age limits on the time of deposition of some bone deposits containing the remains of extinct megafauna.

Pyramo Marianelli and others have investigated variations in organic content and carbon isotope composition of a tall stalagmite collected from Victoria Fossil Cave.

Joel Desmarchelier and others have analysed the stable isotope and trace element composition (magnesium, strontium) of a 545mm tall stalagmite from Spring Chamber in Victoria Fossil Cave that grew between 185,000 and 157,000 years ago. The stable isotope record shows a considerable amount of climate variation during the period and a paper on this has been accepted for publication. Magnesium and strontium concentrations also show much variation but the results will not be published until they are better understood.

REFERENCES

As this is an informal paper, no scientific references are given. Further information on any aspect of the research discussed here can be obtained by contacting the author by e-mail at the following address: Albert.Goede@utas.edu.au

The following is a very informative general reference that touches on many aspects discussed in this paper:

Hill, C. and Forti, P. 1997. Cave Minerals of the World. Second Edition, National Speleological Society, Huntsville, USA, 463 pp.