Introduction: be around precipitated carbonates, as we will

Introduction:

Caves are natural cavities existing beneath the Earth’s surface in lithified rock (Ford, and Cullingford 1976; Pate, and Ronal, 2017) varying in diameter, depth, material and even spatially (Gillieson, 1996). Caves can be classified as either, ‘exogenic,’ which are shallower caves located in hillsides, or ‘endogenic,’ caves (Lowe, and Walker, 1997) which are generally deeper. Caves have long been recognised for their stability over geological time even if the landscape changes, making them valuable to students of the quaternary (Collcutt, 1979).

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Cave sediments are well preserved due to the natural protection from subaerial weathering and erosion. They can be classified depending on location, so ‘autochthonous’ meaning derived from within the cave or ‘allochthonous’ meaning derived from outside the cave. Cave sediments can be broken into three categories:

1.      Clastic Detritus – which includes normal rock, rubble, and cave earth.

2.      Organic Detritus & Fossilised Remains – such as skeletal matter, animals and plant remains.

3.      Precipitated Carbonates – which includes, speleothems (which are secondary cave deposits formed from reprecipitated carbonates CaCO3), dripstones, stalactites, stalagmites or flow/drip stones (William, 2007).

The relative proportion of each category of sediment is dependent on a range of factors including the size of fissures, groundwater regimes, topographical or geological context. Much of the focus will be around precipitated carbonates, as we will be looking at karstic landscapes and speleothems provide a range of proxies that can offer rich archives and high-quality research utilised by new state of the art sampling methods and dating (Fleitmann et al., 2008).   

How are cave sediments and fossilised remains used to reconstruct climate?

There is a relationship between the growth of carbonate precipitates and climatic conditions. Speleothems grow slowly between 1mm to 0.001mm a year, for thousands of years (Fairchild, 2013), however extreme climatic conditions can either increase or decrease the growth rate. Rain water reacts with the CO 2 in the soil to create an acidic solution which travels through the porous parts of the limestone until it drips from cracks forming a speleothem. We can study the level of CO2 to determine the climatic conditions for speleothem growth. In warm, wet conditions, there is more CO2 lost from the water percolating through the cave system which leads to more calcium carbonate being deposited therefore faster speleothem growth. Whereas in colder conditions speleothem growth halts due to less percolation and biogenetic activity which results in less CO2. During glacial conditions when ice melts, it causes cave galleries to flood which prevents carbonate precipitation as seen in Norway which instead led to the deposition of clastic material (Lowe and Walker, 2015).

The oxygen inside the precipitated water becomes locked within the speleothem structure. We can use this proxy to reveal past temperatures, atmosphere, and rainfall. The two isotopes studied are 16O and 18O which is more isotopically heavier. We can conduct fluid inclusions into the speleothem, and determine the concentration of these isotopes which shows regional precipitation. We can also compare caves located in different areas to find the global trend (Riebeek,2005). The thickness of each speleothem layer can be reveal the level of precipitation but can also be dated very precisely by uranium series dating. You can also date the records by uranium series or correlate it with marine cycles, in fact Lowe and Walker (1997) call speleothems and oceans isotopes the best ‘geothermometers’. However, there are several processes which complicate the oxygen isotope signal, for instance, isotope fractionation.

This is when oxygen isotopes fractionate leaving speleothems with a higher ?18O values than the water from when the speleothem was formed. If isotope fractionation has occurred, scientists must evaluate the cave environment beforehand as well as extent at which it has occurred (Smith, 2016). By studying the fossilised remains of snails such as Helix figulina from the Franchthi Cave in Greece we can determine the paleoclimatic and paleoenvironmental conditions by studying the isotopic composition of shells at the time of human occupation of those caves. We can gather the level of carbon which indicates the type of vegetation, c3 or c4 was consumed by the organism. The shell also provides information on the ‘evaporative model,’ where oxygen isotope ?18O shows humidity and rainfall (Colonese, 2013).

How are cave sediments and fossilised remains used to reconstruct environmental change?

The cave sediments and the speleothems can also be indicators as to what is happening on our dynamic planet. Much of the proxies used to assess climatic change can also be used for environmental change, due to the positive correlation between the two. For example, isotope analysis can show whether there was some biogenic activity occurring enabling us to estimate the vegetation cover. We can also study multi-element geochemistry to investigate how the environment was changing. From speleothems, we can find the fingerprints of past volcanic eruptions by looking for sulphur, which is emitted from eruptions to oxidise into a sulphate which lands on the soils and is propagated into caves (Frisia,et al. 2008). This is found in stalagmites from Sofular Cave in the North of Turkey between 1600 – 1650 BC, which show significant peak in sulphur, bromine and molybdenum, due to the Minoan eruption. Its also important to note that there can be delay of several years to decades from the registration of the eruption to the signature in the stalagmites (Badertscher, et al. 2014). We can also see evidence for tectonic activity by locating fissures which show karstic dissolution features such as a layer of calcite, that are due to normal processes, such as seen in Devils hole cave in Nevada (Lowe and Walker, 2015).

Submarines caves in karstic coastal areas can reveal about what the sea level was like, ideally for speleothem development the sea level must be below the caves. Dating speleothems in submerged caves around reef islands indicate how speleothems is dependent on global sea levels. By dating the start and cessation of speleothems growth shows us the amplitude and rate of global sea level variations during succession glacial and interglacial cycles to be reconstructed. This data is valuable for tests of models of global ice volume used on oxygen isotope ratios from marine microfossils. As changes in sea level it also causes change in ice volume changed deduced from the marine isotope records (Lowe and Walker 1997). Speleothems formation can also give an indicate of the level of denudation. Denudation is an erosive process where rocks are broken away from the earth surface. We can infer the rate of denudation and bedrock incisions by looking by dating the speleothems in different cave levels as done in China.

How are cave sediments and fossilised remains used to reconstruct human activities? 

Over this quaternary period, we have seen phenomenal changes to planet on a colossal scale, especially in the 200,000 years since the existence of our species. Humans have been modifying the planet through being hunters and gathers, to the use of fire, quarrying and through cultivation (Matthews, and Matthews 2017) and organic detritus and fossilised remains are key to understanding human activity. Caves are valuable as superimposed in their sediment lies the truth behind what these successive cultures (Lowe and Walker, 2015). From the sediments and fossilised remains we can see evidence for the Mesolithic-Neolithic revolution. In caves we can study pollen remains to date when the transition has occurred, for example in Grotta dell’Uzzo a cave site in Sicily shows in the Mesolithic layers which was around 8500-6000BC, scientists found grass, wild strawberry, olive and grape remains. Whereas in the early Neolithic layers dated to 6000BC they found einkorn, emmer, bread and wheat. As pollen is often blown in can interpret what the conditions were like outside of the caves and the crop diversity and evidence of mixed farming (Cowan, 2006).

Plant macroremains can be studied to determine the environmental and climatic conditions which allowed growth of these plants. By studying the spatial distribution of plant remains in caves, we can track the migration of agricultural practises, which revealed it had occurred in a ‘wave and advance’ model across the Mediterranean Basin, as the transition spread westwards (Zeder, 2008). However, as noted by Triglavca (N. D.), in the Mediterranean sites, that there can be a temporal gap between the two occupational eras, from radiocarbon dating evidence suggests that this was due to a population decline which was replaced with indigenous foragers. Caves also have organic matter interstratified with clastic materials such as skeletal remains of those who occupied the cave and their prey. The analysis of these sediments enables us to get information on their diet which shows what the paleoenvironment was like (Lowe and Walker, 2015), however a significant barrier to this method is flooding can reduce the sweep away remains (Gillieson, 1996).  

Decayed parts of animals have also been instrumental in tracing human activity as you can study their carcasses for method of death, excreta and charcoal deposits from past hearths. The Neolithic also saw the settle community life take form and the domestication of animals, in which we can see the evidence in cave rock art as early as 8000 years ago found in Shuwaymis and Jubbah in North-western Saudi Arabia (Guagnina, 2017). Caves also show how humans have used them to stable animals in upland areas which where rich in pasture, this has been proven by the cave sediment being rich in phosphates which are derived from coprolite which can be dated (Anderson, 2013).

Conclusion

From caves across the globe, although they are short in number, they provide a rich tapestry for what the past climatic, and environmental conditions. Caves also allow us to trace the activities of our ancestors by looking at the range of proxies available. We explored at how many of the indicators that we use for one factor can reveal information for the other factors showing interconnection as the climate has an obvious influence on the environment and these two factors will play upon humans affecting their activities and distribution. Also mentioned is the fact that there are obviously inconsistencies within the data as the nature of our subject it is difficult to get records that are full and uninterrupted. Although caves provide some of the longest, detailed high resolution paleoenvironmental records, we must also critically analyse the validity the signals, especially for isotope analysis. Therefore, in order to get a representative depiction, we often have to correlate the conclusions made from caves and correlate it with the finding with marine and ice core records. From our research speleothems have provided the most information regarding the paleoenvironment, speleothem growth is important as its specifically triggered by certain environmental conditions and that fossilised remains are the most useful when it came to reconstruction human activity.