Trace elements in speleothems as recorders of environmental change

Quaternary Science Reviews 28 (2009) 449–468

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Trace elements in speleothems as recorders of environmental change Ian J. Fairchild a, *, Pauline C. Treble b, c a

School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia c Institute for Environmental Research, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 September 2008 Received in revised form 14 November 2008 Accepted 14 November 2008

Speleothems are now established as important palaeoenvironmental archives and contain a number of suitable proxies, although trace elements have been much less widely used than oxygen and carbon isotopes. The complexity of the cave environment helps to explain this since the fluids from which speleothems form vary greatly in composition in space (even within a cave chamber), seasonally, and over longer periods. Understanding the forcing factors for this variability is the key to decoding the significance of the trace element records. A variety of techniques are available for trace element work and it is important to understand the strengths and limitations of each and also to seek an understanding, e.g. by micro-imaging techniques, of whether the elements are associated with inclusions in the CaCO3, or are isolated within the crystal lattice. For some elements there is a more-or-less predictable relationship between element ratios to Ca in the water and in the calcite. Individual trace elements may be derived from atmospheric deposition, superficial deposits or bedrock and can be recycled in soil processes before being transferred to the cave. Some components show an instantaneous response to water infiltration, whereas others are only leached by slow-flowing seepage waters. Changing in the proportion of water from fracture-fed and seepage-flow aquifer compartments is an important factor in influencing trace element supply. High flows lead to higher fluxes of soil-derived colloidally transported elements. Conversely, under relatively dry conditions, degassing of CO2 results in ‘‘prior calcite precipitation’’ upflow of the site of speleothem deposition and leads to higher ratios of Sr/Ca and Mg/Ca. Some trace element variations in speleothems over time are induced during crystal growth whereby faster growth leads to a greater departure from equilibrium element partitioning. Despite the demonstrated temperature-dependence of Mg partitioning into calcite, attempts at deriving palaeotemperature records from speleothems have been so far confounded by variations in solution Mg/ Ca and/or crystallographic effects. A number of case studies have effectively used trace elements such as speleothem Mg as records of palaeo-aridity, using supporting arguments from modern monitoring or covariations with other parameters such as stable isotopes. Sr and U isotopes can also be indicators of palaeohydrology, although Sr isotope variations can also reflect varying aeolian input. Considerable progress has been made in decoding the meaning of annual trace element variations using criteria for understanding dripwater hydrology and pH. This should lead in the future to more specific interpretations of how seasonality evolves through time. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Calcareous speleothems (stalagmites, stalactites and flowstones) are widespread in karstic environments and grow from dripwaters that degas excess carbon dioxide upon entering caves. Speleothem geochemical records are playing an increasingly important role in shaping our understanding of past climate. In

* Corresponding author. Tel.: þ44 1214144181; fax: þ44 1214145528. E-mail addresses: [email protected] (I.J. Fairchild), pauline.treble@anu. edu.au (P.C. Treble). 0277-3791/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2008.11.007

general terms, they represent possibly the best opportunity to obtain highly detailed (seasonal to decadal resolution), continuous terrestrial records with precise and accurate chronologies, outside of the high polar latitudes where ice cores can be drilled. A firm basis of their appeal is also the wealth of information available in the multiple environmental and climate-sensitive proxies preserved in speleothems. The most commonly measured variable, oxygen isotopes (d18O), has long been utilized to reconstruct cave temperature or properties of meteoric precipitation (amount, moisture source, synoptic meteorology; Hendy and Wilson, 1968; McDermott, 2004). Records of speleothem d18O have been used to challenge the ice core-based timing of glacial/interglacial

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transitions (Winograd et al., 1992; Spo¨tl et al., 2002) and the shortlived Heinrich events (Wang et al., 2001) of the late Pleistocene. Other information recovered from speleothems includes 1) soil/ vegetation dynamics from carbon isotopes (d13C, Dorale et al., 1992; Genty et al., 2003), 2) hydrological information from trace elements (Treble et al., 2003; Borsato et al., 2007), 3) annual lamina (Tan et al., 2006; Baker et al., 2008), 4) growth rate and dating information from 14C measurements over the bomb pulse period (Genty et al., 1998), 5) fluorescent organic components (Baker et al., 1993; McGarry and Baker, 2000) and 6) atmospheric fluxes via strontium isotope and compositions (Goede et al., 1998). The potential of biomarkers to trace vegetation and other organic-based matter (Blyth et al., 2008), noble gases preserved in fluid inclusions for palaeothermometry (Kluge et al., 2008), as well as sulphur concentrations and sulphur and oxygen isotopes from sulphate to determine the sources and biogeochemical cycling of S-species (Frisia et al., 2005; Wynn et al., 2008) were also recently recognized. These multiple proxies preserved in speleothems provide an impressive toolbox for reconstructing a comprehensive history of rainfall, temperature, soil conditions, cave pCO2 and other environmental conditions. Trace elements represent a large proportion of these measurable proxies; however, they are not as commonly used and receive less attention than stable isotopes. To understand why, we can compare the situation with other Quaternary records. Trace element concentrations in ice cores are a direct measure of fluxes from bulk precipitation and hence have been widely used to interpret air mass origins and component mass fluxes (Wolff et al., 2007). In contrast, dendrochemical records of atmospheric pollutants are more problematic since element mobility within the tree sap makes them more difficult to interpret (Pearson et al., 2005 give some exceptions). Element mobility is not usually a problem for speleothems, except where aragonite converts to calcite after deposition; however, the ion ratios in formation waters do vary. In contrast, the fixed ratios in seawater have permitted a number of important trace element applications to be developed in calcareous marine organisms. These include carbonate ion determination (Yu and Elderfield, 2007) and various forms of palaeothermometry in corals and forams in particular (Henderson, 2002). Nutrient elements do vary in concentration in the sea and analyses of calcareous organisms have allowed proxy records of water masses to be developed, but the vital effects controlling fluid composition at the site of precipitation in body fluids are crucial. Thus Cd concentration in planktic forams largely reflects temperature (Rickaby and Elderfield, 1999) whilst in benthic forams it is controlled by pressure or carbonate ion saturation (Marchitto et al., 2005); meanwhile P in cold-water corals apparently directly reflects aqueous P concentration (Montagna et al., 2006). Although speleothems lack the complications of vital effects, they cannot be divided into distinct species with specified behaviour. Instead they display a wide range of growth rates (from a few microns to a mm or so per year) that can influence trace element composition (Morse and Bender, 1990) and also can show variable growth geometries. Unlike the regionally coherent patterns of sedimentation of deep-sea calcareous oozes, caves are appropriately perceived as complex environments since there is a wide range in climatic, geologic and geomorphic environments in which they occur. Hence trace element compositions vary significantly between sites, and even within the same cave chamber, and can often change significantly within a year. The way to minimize the resulting uncertainty is to study and raise awareness of the key processes that control the concentrations of trace elements in speleothems at particular sites and how these may be better characterized and constrained. A parallel set of issues arises for lake carbonates (Kober et al., 2007). In the case of speleothems, five types of influences on geochemistry were distinguished by

Fairchild et al. (2006a), all of which apply to trace elements. They are atmospheric input, vegetation/soil, karstic aquifer, primary speleothem crystal growth, and secondary alteration. This article provides an overview of trace element theory, analytical techniques, and cave processes, and uses case studies to review the progress of the science and to highlight studies that have successfully used trace elements as palaeoenvironmental proxies. In Section 2, we describe analytical techniques, carbonate trace ion theory, crystal host structure and illustrate both the sources of elements and the environmental processes that manipulate and impart (or obscure) an environmental signal to the trace element composition of speleothems. Section 2.1 explains the different forms in which trace elements are present in CaCO3 and how they can be distinguished. Section 2.2 identifies the different sources from which the elements can be derived opening the possibility for using speleothems to monitor the change in each source over time. Section 2.3 explains dissolution processes and how relatively wet or dry periods can lead to enrichments in particular trace elements. Section 2.4 focuses on the key process of prior calcite precipitation causing solutions to evolve in composition during drier seasons or climatic periods. In Section 2.5, the principles by which elements are partitioned into CaCO3 are discussed leading to an understanding of the extent to which the CaCO3 composition mirrors the trace element composition in the water. Section 3 focuses on a critical evaluation of case studies which have identified an overriding environmental control to particular trace elements, allowing palaeoenvironmental inferences to be made. The environmental controls discussed are temperature (Section 3.1), aridity (Section 3.2), vegetation change (Section 3.3) and atmospheric processes of dry and wet deposition (Section 3.4). Section 3.5 summarizes cases where non-linear effects of CaCO3 crystallography are dominate trace element behaviour: although difficult to interpret environmentally, these effects are seasonally variable, and hence allow annual cycles to be delimited and growth rate to be determined as explained in Section 3.6. 2. Controls on trace element composition 2.1. Analytical methods and the presentation of elements in speleothem carbonates Recent work on speleothems has extended our general knowledge of the capability of CaCO3 minerals to incorporate trace species (Kuczumow et al., 2001, 2003, 2005; Fairchild et al., 2001; Ortega et al., 2005; Borsato et al., 2007). The richness of trace element patterns arises both from the range of species that are detectable, but also their spatial variation. For example, Fairchild et al. (2001) asserted that it was normal for speleothems to display some form of chemical variation on the annual scale, regardless of whether or not annual laminae could be seen. In this section we compare the capabilities of the different analytical techniques and illustrate how the location of the elements within the speleothem constrains how to interpret the analyses. 2.1.1. Analytical techniques for elements Modern instrumentation allows a choice of many techniques (acronyms and techniques are summarized in Table 1). There are trade-offs between the volume of material analyzed (and hence the spatial scale of analysis), the precision and accuracy of the results, and the speed and cost of the analytical process. All techniques require standardization. This is straightforward for the electron microprobe (Potts, 2003), but detection limits are high. Both SIMS and laser-ablation ICP-MS require standards to be matrix-matched to samples to ensure consistent removal of material and ionisation of species. In practice, locating or creating CaCO3 standards that are homogeneous on the scale of in situ analyses is challenging (Craig

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451

Table 1 Instrumental techniques for analysis of inorganic trace constituents in speleothems (modified from Fairchild et al., 2006a). Instrument

Typical detection limit

Elements detected

Typical spatial resolution/sample size

Electron microprobe (analysis of X-rays stimulated by electron beam) Secondary ionisation mass spectrometer (SIMS), i.e. ion microprobe (primary negative oxygen ion beam causes sputtering of secondary ions)

100 ppm

Mg and sometimes other divalent ions

1 mm spot (3 mm excited diameter)

H, F, Na, Mg, Si, P, Ca, Fe, Mn, Pb, Sr, Y, Ba, U and potentially others (e.g. REE)

1.8–10 mm spot (2–3 mm depth), with automated line scan

Micro X-ray fluorescence spectrometer using synchrotron radiation Laser-ablation inductively-coupled plasma mass spectrometer (ICP-MS)

0.01–10 ppm (primary Csþ beam optimal for electronegative elements) 0.1–1 ppm

Most elements (light elements less easily detected) 0.1–10 ppm In principle, most elements, but if spot size is large, some elements may be compromised by analysis of non-carbonate inclusions ppt–ppb in solution, (i.e. Most elements, limited mainly by specific interferences ppb–ppm in CaCO3) ppb–100 ppb in solution Na, Mg, Ca, Sr, Ba (ppm–100 ppm in carbonates)

ICP-MS with sample dissolved in dilute acid dissolution from drilled sample powders AA (atomic absorption spectrometer) or ICP-AES (inductively-coupled plasma atomic emission spectrometer)

et al., 2000; Johnson et al., 2006). For ICP-MS, silicate standards are routinely used to provide approximate concentrations (Treble et al., 2003) which may be sufficient in most cases when the interest lies only in the relative variations within a sample transect, but ideally development of matrix-matched standards should remain a high priority. More effort has been made to address this using calcite and apatite standards in SIMS applications (Hinton, 1995; Roberts et al., 1998; Fairchild et al., 2001; Borsato et al., 2007). Fast high-resolution analyses have recently been obtained by micro X-ray fluorescence stimulated by synchrotron radiation (Frisia et al., 2005; Kuczumow et al., 2005; Borsato et al., 2007); this technique also allows elemental speciation studies to be carried out. However, synchrotron facility access is not routine, and there are limitations at a given beam-line in terms of the range of elements that can be analyzed; the results are precise but quantification is complex. Both SIMS and laser-ablation ICP-MS provide sub-annual resolution, except for slow-growing stalagmites, but the rapid acquisition of data using laser-ablation ICP-MS suggests that it may become a routine technique of choice in the future (Treble et al., 2003; Desmarchelier et al., 2006). Solution-based analysis, most conveniently by ICP-AES and/or ICP-MS for most elements, offers the greatest analytical accuracy in principle, but in practice requires much attention from trained analysts to achieve reliable results. The most robust solutional methods have been developed by workers on foraminiferal calcite where high levels of accuracy and precision are vital (de Villiers et al., 2002; Greaves et al., 2005; Yu et al., 2005). Analyses can be carried out routinely at dilutions of around 1 in 4000. Solutionbased analyses could be paired with stable isotope analyses micromilled as prismatic trenches 0.4–0.5 mm wide in the growth direction (Fairchild et al., 2006a), although this would be highly time-intensive for long traverses. Alternatively these analyses could be used to validate standardization procedures for micro-analyses (e.g. Borsato et al., 2007), but in that case the sampling of powder should cover several years of growth to obtain a representative chemistry. 2.1.2. Element incorporation in calcium carbonate and imaging techniques Most studies of CaCO3 focus on elements which form divalent cations in solution and which substitute for Ca in the carbonate crystal lattice, particularly Mg, Sr and Ba (together with Mn and Fe in reducing waters). For such species, a simple equation defining a distribution or partition coefficient (Morse and Bender, 1990) can be used to relate solution and mineral compositions:



Tr=CaCaCO3



¼ KTr ðTr=CaÞsolution

(1)

2 mm 20–1000 mm diameter ablated spot (potentially down to 1 mm); depth of pit ca 1–5 mm (Excimer laser) or ca 60–80 mm (Nd–YAG laser). 100–5000 mg powder 100–5000 mg powder

where Tr is the trace ion and KTr is the distribution coefficient, which may vary to a greater or lesser extent with temperature, precipitation rate, crystal morphology, or other aspects of solution composition. Busenberg and Plummer (1985) suggested that a similar relationship applies to sulphate substitution for carbonate ion. KTr is essentially empirical, although values can be roughly predicted using thermodynamic and kinetic theory (Rimstidt et al., 1998; Curti, 1999; Watson, 2004). KTr values differ for aragonite and calcite and hence it is important to carefully distinguish these phases in the studied sample (Fig. 1a; McMillan et al., 2005; Ortega et al., 2005). Other elements may be incorporated in CaCO3 by different mechanisms. For example, the element may be transported by water and incorporated in the speleothem associated with either fine detrital particles, or as sub-micron-sized colloids (Fig. 2). Higher concentrations of such elements would be expected in water and in associated speleothems during periods of higher infiltration. If chemical species are suspected to be concentrated in particles, a form of selective or specific analysis is needed to identify this association (Fairchild et al., 1988). Backscattered electron microscopy or X-ray mapping would effectively reveal minor phases down to sub-micron resolution, whereas transmission electron microscopy (TEM) may be needed for smaller colloids (Mavrocordatos et al., 2000; Lead and Wilkinson, 2006). To quantify these patterns either the sample needs to be gently leached and particles and colloids removed by filters and/or resins, or microanalysis is needed. Ideally a map on a microscopic scale is generated to reveal the pattern of elements. Fig. 1b illustrates enrichments of a variety of trace elements coinciding with visible laminae containing fluorescent humic substances in a stalagmite from Ernesto Cave in NE Italy (Borsato et al., 2007), but whether the trace elements are still bonded to the organic molecules is not known. In the same samples, Mason et al. (2007) demonstrated using Nuclear Magnetic Resonance spectroscopy that P is present both as individual phosphate ions, but also as crystalline monetite (CaHPO4), which presumably grew at the same time as its enclosing calcite. It is difficult to analyze colloids in waters as they aggregate in solution within days and tend to adsorb onto surfaces. The complexities of transport of U and Th by colloids are thought to contribute to uncertainties in U-series ages of speleothems in some cases (Whitehead et al., 1999). It is important to be aware that in acid leachates of speleothems, colloidal material will be aspirated and volatilized during ICP-MS analysis, and hence will be determined by this technique (e.g. Zhou et al., 2008). Some elements (e.g. Na and Cl) are probably preferentially associated with aqueous inclusions in the speleothems, but typically, such inclusions are too small and insufficiently saline to be analyzed individually. Different leaching protocols have revealed

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Ti Mn Al Si Fe

COLLOIDS

PARTICLES

held in 0.45 µm filter passes 0.45 µm filter

SOLUTES

Mg Ca

Y Pb

SiO2 (quartz), Tio2, Fe2O3, Fe(OH)3, MnO2, Na, K, Ca aluminosilicates (feldspar), sheet silicates with various cations (clays) Oxides, humic and other organic substances, some silicates, composite entities Inorganic hydrated cations, simple-and oxy-anions, small organic molecules K Na

Br P

Fig. 2. Modes of transport of trace elements: particles, colloids, and solutes in karstic waters. Note that dripwaters and hence speleothems are depleted in particles compared with general karstic waters. Colloids are defined as entities with at least one dimension in the size range of 1 nm to 1 mm (Lead and Wilkinson, 2006). The tendency of various elements to be transported in a given mode is shown schematically on the right: for example, Si is transported in abundance as insoluble particles, but is also present in colloids and in solution. For a more comprehensive discussion of colloid types and characterization methods see Lead and Wilkinson (2006).

Fig. 1. a) Distinction of aragonite and calcite using electron backscatter diffraction (EBSD). The image shows columnar aragonite crystals (dark grey) within calcite of varying optical orientation (light grey) of a stalagmite (McMillan et al., 2005) from Clamouse Cave. Black areas are unresolved mineralogy; pixel size is 4.5 mm (black scale bar is 500 mm). For a false-colour version of this image see http://www.geos.ed.ac.uk/ facilities/sem/polymorph.html. b) Micro-XRF map using synchrotron radiation (equivalent to that in Borsato et al., 2007); note concentration scale is normalized between minimum values (white) and maximum (black) for each 5  2 mm pixel. Pb, Zn and Y have maximum values and Sr minimum values corresponding to changes in transmittance which correspond to UV-fluorescent, visible laminae (‘‘Tran’’ has an inverse scale so that black ¼ minimum transmittance). A Pb-rich particle is also shown beneath the lowest high-Pb lamina.

major differences in behaviour between elements in corals where organic- and fluid-associated elements are important (Mitsuguchi et al., 2001). A simpler protocol exists for crushing and leaching inorganic CaCO3 samples (McGillen and Fairchild, 2005). SIMS analysis has shown that in some speleothems, hydrogen contents

determined vary annually (Fairchild et al., 2001). It is not known if the hydrogen is in the form of nano-inclusions of water (McDermott et al., 2005) or as hydrogen-bearing ions. Elements such as Na also tend to be incorporated at defects in the crystal (Busenberg and Plummer, 1985) and phosphate adsorbs strongly at such sites (Meyer, 1984). Understanding the sites where elements will be located is a frontier area of research using specialized techniques, but a clue to the richness of trace elements in CaCO3 lies in the ability of many ions either to substitute for calcium or carbonate, or to occur at defect sites where the exact ion size is less important (e.g. rare earth elements, Elzinga et al., 2002). Also single or triply  charged ions (e.g. Y3þ, Naþ, PO3 4 , Br ) substitute as couples to preserve charge balance. The abundance of elements at defect sites is much greater if the crystal growth surfaces are rough, which is a feature of faster growth from more strongly supersaturated solutions (Frisia et al., 2000). A good example of the importance of understanding how the trace element is hosted is provided by Si. Klein and Walter (1995) and Hu et al. (2005) found that silica adsorption and co-precipitation into calcareous substances could be described by a distribution coefficient, as in Eq. (1). In a study in Heshang Cave in central China, Hu et al. (2005) were careful to distinguish the analysis of dissolved silica, by molybdate blue reaction, from total silica by ICP-AES (which would include any fine suspensates). They showed that coprecipitated silica in the speleothems was initially adsorbed from solution and that it was higher in the past during drier glacial periods when arguably high aeolian supply of reactive silica would have been present. The climatic significance of this result is the opposite of that which would have applied if the Si were predominantly in the form of fine silicate particles, since their abundance should vary positively with the amount of dripping water. 2.2. Sources of elements Fig. 3 illustrates a variety of sources for chemical species including aeolian particles, dry and wet atmospheric deposition, bedrock, superficial sediment deposits, and inorganic soil constituents, and elements recycled via soil biota. Whereas the atmosphere is the primary source for speleothems’ oxygen isotope signal (and indirectly via biological carbon fixation for their light carbon isotopic composition), it is only a subordinate source for trace

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species. However, Sr isotope signals have been shown to relate to the supply of aeolian dust in speleothems from Tasmania and Israel (Goede et al., 1998; Frumkin and Stein, 2004; Section 3.2.2). Sea-salt aerosol in wet atmospheric deposition can be an important source for certain trace species in karst waters, sometimes including Mg or Sr where bedrock supply is limited (Baker et al., 2000; Fairchild et al., 2000). Temporal changes in sulphate in dripwaters (e.g. Fig. 4) can be linked to changes in atmospheric pollutants (Spo¨tl et al., 2005) and this can be captured in speleothems under favourable circumstances (Frisia et al., 2005). Wynn et al. (2008) have succeeded in isolating sulphate from speleothems in Ireland and the

4

2000

2001

2002

Cond.

2

Drip rate

1 0

-8 -10 δ13C

-12

(‰ , VPDB)

-6

-2.0 -2.2

60 Ca

50

-2.4 -2.6

40

-2.8 30

-3.0

20

-3.2

pCO2

10

-3.4

0

12 11 10 9 8 7 6 5 4 3 2 1

-3.6

-9

-10

δ18O SO4

-11

(‰ , VSMOW)

-4

70

18O

-2 Mg

b

d

0

DIC

11 10 9 8 7 6 5 4 3 2 1 0

13C

Mg (mg/l)

c

300 280 260 240 220 200 180 160 140 120 100

logpCO2

3

2003

SO4 (mg/l)

5

Conductivity (umS/cm)

Drips per minute

a

UK for sulphur and oxygen isotopic analysis which provides a clear discrimination of sulphate sources. Despite the above exceptions, the primary source of calcium and most trace elements in speleothems is the bedrock and overlying regolith, including bedrock fragments in the soil, and dissolution is focused in the zone where pCO2 is at a maximum. Depositional and tectonic processes give each cave site, and indeed each drip at a given time, a unique trace element signal of bedrock dissolution. For certain elements, it may be small quantities of a minor mineral phase that are the major supply. For example, the primary source of P is likely to be the mineral apatite, although it is likely to be extensively recycled biogeochemically (Huang et al., 2001). Mg and Sr are normally derived primarily from carbonate bedrock, but at the Tartair site in NW Scotland the high Sr concentrations in calcite speleothems (Roberts et al., 1998; Fuller, 2007) arise from localized veins associated with a thin igneous sill in the overlying Sr-poor dolomite. Where clay minerals are abundant, it is normal for the Sr to be supplied both from carbonates and silicates, and for the Sr isotope signal to give information about the relative amounts dissolved from each (e.g. Banner et al., 1996; Verheyden et al., 2000; Li et al., 2005). Where the dolomite and calcite phases in bedrock are reasonably homogeneous and are the main supply for Mg and Sr, their compositions can be used to help understand the ways in which chemical signals are generated and modified in the karstic system (Fairchild et al., 2000). One factor is the differential rates of dissolution of dolomite and calcite. Although both dissolve briskly in strongly undersaturated solutions, dolomite dissolution slows dramatically whilst still far from equilibrium. In mixed dolomitic– calcitic bedrock, solutions reach equilibrium with calcite with an Mg/Ca ratio significantly lower than that of the mean bedrock composition, a relationship which is accentuated at low pCO2 when solute loads are less (Fig. 5). If the solution remains in contact with dolomite long enough to reach saturation, the solution, now with slightly enhanced Mg/Ca, becomes slightly supersaturated for calcite, a relationship found in some regional groundwaters, although there is doubt whether water residence times are long enough above active caves for this state to be achieved. Fig. 6 illustrates the construction of a base mixing line between dolomite and calcite phases in terms of Mg/Ca and Sr/Ca ratios at

Ca (mg/l)

Fig. 3. Schematic flow chart to indicate sources of elements and processes involved in their transport and deposition in cave systems. Arrows denote element fluxes as particulates, colloids or solutes in aqueous solutions.

453

-12

Fig. 4. Temporal trends in drip PH1, Obir Cave, Austria. Clear annual patterns are shown by the carbonate system patterns related to strong seasonal ventilation. Other parameters such as sulphate and d18Owater show weak secular trends (simplified from Spo¨tl et al., 2005).

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log pCO2 = -3.5 Leachate composition at calcite saturation

+

log pCO2 = -2.6 +

+

+

+

+

Bedrock composition 10 1

10

30 100

50

1

a

log pCO2 = -1.6

1000 Sr/Ca

454

70 90 % dolomite 1000

1000 Mg/Ca Fig. 5. Diagram to illustrate that dripwaters tend to have lower Mg/Ca ratios than mixed dolomite-limestone bedrock under conditions where simple dissolution (at various pCO2 values) to calcite saturation occurs (after Fairchild et al., 2000).

0.1

1

10

100

1000

1000 Mg/Ca

2.3. Dissolution processes and the karst system The transmission of trace element species into karstified bedrock is dependent on two factors: their chemical mobilization in the overlying soil and underlying highly fractured epikarst and the hydrological processes influencing infiltration of surface and soil waters into the epikarst. Studies using soil lysimeters (e.g. Tooth and Fairchild, 2003) can be instructive in characterizing the fluids near the soil base that are likely to be recharging the underlying karst. However, there are a number of limitations, including the presence of significant unconsolidated deposits under the soil zone and the inability of lysimeters to sample adequately colloidal and particulate species. A key principle is that carbonate dissolution to saturation happens in response to high pCO2, so the maximum solute loads are reached where pCO2 is highest. This can be either within the soil or in the epikarst depending on the distribution of carbonate and of decomposing organic matter (Atkinson, 1977; Drake, 1983; Fairchild et al., 2000, 2007). A range of behaviours in relation to chemical mobilization are present. Most rapid is the virtually instantaneous entrainment of pre-existing solutes, mobilized particles and colloids, together with dissolution of salts and leaching of weakly-sorbed species from organic and inorganic substrates. Intermediate in behaviour is calcite dissolution, although this is sufficiently rapid that equilibrium would be expected within a few hours of infiltration (undersaturated dripwaters therefore represent very fast travel times). Slowest are ions released by chemical weathering of less soluble phases such as dolomite, silicates, phosphates and oxides, and sulphides. Some of these processes are bacterially mediated, including the release of inorganic acids from sulphide oxidation (Sharp et al., 1999). The impact of such slow dissolution processes is most important for the matrix of the soil and karst system where seepage flow dominates; such processes have been studied closely

b 1000 Sr/Ca (corrected for marine aerosol)

two cave sites (Ernesto, NE Italy and Clamouse, southern France). Using ratios is useful because it allows the bedrocks to be compared directly with expected solution compositions arising by congruent (total) dissolution of different bedrock mixtures. In both cases, waters lie above the mixing line, pointing to the existence of additional processes. One possible process at Ernesto (Fairchild et al., 2000) is the selective release of Mg and Sr from freshly broken calcite surfaces (McGillen and Fairchild, 2005). Alternatively redissolution of Mg and Sr-bearing salts that were precipitated in drying soils in the summer may occur. On the millennial timescale, changes in the composition of the regolith can be important. For example, Fairchild et al. (2007) interpreted changes in Mg/Ca in a Holocene record from Ernesto in relation to cut-off of supply of slope-derived limestone fragments and their subsequent preferential dissolution.

First chamber drip (F1)

Main chamber pools

First chamber pools

Bedrock analyses

Main chamber drips

Mixing trend of Bedrock compositions

1

0.1

0.01 10

100

1000

10000

1000 Mg/Ca (corrected for marine aerosol) Couloir blanc Salle de gour sec Drip C2 Drip C12 Other Grand Carrefour

Drip D1 Bedrock analyses Mixing trend of bedrock compositions Experiment C4 leachates

Fig. 6. Comparison of bedrock compositions and karstic waters. a. Data from Grotta di Ernesto and b. data from Grotte de Clamouse. The enrichment in element ratios compared with the bedrock mixing line shows that processes other than simple dissolution have occurred (after Fairchild et al., 2000).

in glacial karstic settings (Fairchild et al., 1994, 1999). Where elements are derived from two or more sources with different dissolution behaviours, the effects of a changed rainfall and temperature regime on the relative efficiency of leaching from each can be inferred. Verheyden et al. (2000) used such arguments in the context of a study on a Holocene stalagmite from Belgium, but it is easier to establish the relationships where the chemical trends can be matched to independent evidence of major climatic shifts. For example, Li et al. (2005) infer from a 70–260 ka multiproxy record from Buddha Cave in the loess plateau area of northern China that warm, wet interglacial conditions give rise to a higher proportion of leaching from the 87Sr-rich silicate component of loess compared with carbonate in loess and carbonate bedrock. Hydrological changes are also thought to control variations in 234U/238U activity ratios (Kaufman et al., 1998; Zhou et al., 2005). Dripwaters may develop characteristic enrichments in trace species related to the interaction between dissolution processes

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and hydrological behaviour under both low and high flow conditions. Absolute enrichment of trace species in dripwaters at low flow can be attributed to longer reaction times in low-permeability seepage-flow soil and aquifer compartments (e.g. Baker et al., 2000). However, Fairchild et al. (2006a) argued that the key issue was the hydrological routing. Hence when changes in a parameter such as Mg are found over time and related to varying contributions of dolomite dissolution, it is unlikely that flow times through a specific aquifer compartment are varying, but that different mixtures of water from different types of karstic porosity with different characteristic flow times are represented (Tooth and Fairchild, 2003; Fairchild et al., 2006b; Fig. 7a). When flow first increases after a period of low-flow, higher concentrations may persist due to either a flush of rapidly entrained material from the soil, or piston-flow through the aquifer, or more likely, an enhanced entrainment of ions from matrix by interaction with fracture-flow waters. Element ratios can be predicted from the hydrology using linear systems mixing models qualitatively, but not quantitatively, (Fig. 7b and Fairchild et al., 2006b). High flows can also be associated with trace element enrichments, but primarily of species thought to be transported with colloids, rather than the alkaline earth elements discussed above. The evidence for this is primarily from speleothems (Huang et al.,

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2001; Borsato et al., 2007; Zhou et al., 2008) rather than primary observations of waters because very little is known about colloidal transport of elements in karstic systems. Most attention has been paid to relatively coarse colloids mobilized by storm events in karst conduits and springs (Atteia and Kozel, 1997; McCarthy and Shevenell, 1998). However, these are dominated by silicate and carbonate phases that are scarce in dripwaters and less efficient at elemental binding than colloids such as humic substances, polysaccharides, and inorganic oxides of Al, Fe and Mn, materials requiring specialized techniques to characterize (Lead and Wilkinson, 2006). A rare example of a karstic study where this has been done is that of Mavrocordatos et al. (2000) who demonstrate an evolution of colloid characteristics by the growth of Fe–Ca colloids (100–500 nm in diameter) on humic substance cores in a karstic system fed by peat. Furthermore they show that these colloids are strongly retained in karstic fractures, illustrating the importance of understanding substrate interaction processes, which are likely to differ from those in particulate siliceous porous media. In cave dripwaters, most attention has been paid to the transfer of UV-fluorescent organic matter from the soil zone (Baker et al., 1997; Tan et al., 2006), but element transport is the topic of current research. For example, Zhou et al. (2008) have provided evidence of the co-abundance of colloidal Al and Mn in cave dripwater and

a

b

c

d

Fig. 7. Geochemical relationships at Brown’s Folly Mine. a. Hydrological model illustrating the role of processes such as fluid mixing and prior calcite precipitation (PCP), b. comparison of model predictions with data from a dripsite; deviations are due to non-linear mixing effects, c, d. diagrams to determine the nature of processes causing chemical changes at low flow. Low Ca is found at low flows; in some cases this relationship can simply be explained by PCP (e.g. Mg/Ca data at site F3 and Sr/Ca data at sites B and F5). In the other cases, PCP must be augmented by enhanced trace element composition of the aquifer segment being drawn on at low flow (after Fairchild et al., 2006b).

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link colloidal transport to zones of enrichment of Mn, P and rare earth elements in parts of stalagmites forming in wetter climatic periods. Phosphorus is an example of a key element that was recognized (Fairchild et al., 2001; Huang et al., 2001; Treble et al., 2003) to show important annual variations in speleothems. Phosphorus may be preferentially mobilized from soils in the autumn because of the coincidence of vegetation die-back and high infiltration in some climatic zones. For example, Heathwaite (1997) stressed that limited P uptake in soils during the summer period of soil water deficit enhances the release of dissolved P in the first autumnal storms. The fate of this P is complicated because analysis reveals that even the ‘‘dissolved’’ form that should predominate in dripwaters exists in several different inorganic and organic forms (Chapman et al., 1997). There has been relatively little attention paid to redox-sensitive elements, but Richter et al. (2004) have shown the presence in speleothems of specific laminae exhibiting Mn-stimulated cathodoluminescence that they link to periods of strong infiltration and more reducing conditions favouring Mn-transport (see also Zhou et al., 2008). Perrette et al. (2000) studied the link between the Mn2þ concentration and fluorescence and electron paramagnetic resonance spectrometry properties of organic matter, again suggesting that a link of higher Mn2þ with enhanced water fluxes could be made. Under phreatic conditions, anoxic conditions may be more readily established and both Mn2þ and Fe2þ may be present. For example, Immenhauser et al. (2007) demonstrate how in phreatic calcites forming near the interface of meteoric and saline hydrothermal waters in Oman, there are cyclic variations in chemistry reflecting fluctuations in the intensity of the monsoonal climatic regime. 2.4. Prior calcite precipitation In the previous section, several processes occurring in the karstic environment were discussed, including organic matter decomposition, dissolution, desorption and adsorption of colloids, and mixing processes. An additional and fundamental property of

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speleothem-forming karstic environments is that the infiltrating waters will pass from a dissolution regime to a precipitation regime (Fairchild et al., 2006a) primarily because they encounter a gas phase with lower pCO2 than that which they have previously equilibrated. The resultant degassing leads to supersaturation of the water for CaCO3 and a tendency for calcite precipitation. The effects will be accentuated if evaporation occurs within the cave. It is common to find cases where the dripwaters feeding a speleothem have already undergone some change as a result of precipitation upflow. Hence, this is referred to as prior calcite precipitation (PCP). The effect of calcite precipitation is to remove cations from the water in the proportion in which they are incorporated into calcite (Holland et al., 1964). Since values for KTr in Eq. (1) are typically much less than one, the result is a much larger reduction in Ca than that of the trace element and hence an increase in the ratio of trace element to Ca in solution. This relationship can be modelled given a knowledge of KTr, although the results are not sensitive to small variations in KTr where it is <0.1. Fig. 7c and d illustrates examples from Brown’s Folly Mine (Fairchild et al., 2006b) where drips show the effects of PCP plus or minus enrichments in Mg or Sr related to differing chemistries of aquifer compartments. Where PCP is important, there can be a strong enrichment in Mg and Sr compared with host bedrock compositions, as is the case at the Clamouse site (Fig. 6b). Since PCP is enhanced where descending karst waters are able to degas, it has been argued to be a process that should be enhanced in dry climatic periods (Fairchild et al., 2000) where aerated zones in the aquifer will become more important. There are now a number of examples of sites where dripwater monitoring has shown that such a relationship exists (Tooth and Fairchild, 2003; McDonald et al., 2004, Fig. 8; Fairchild et al., 2006a; Karmann et al., 2007), although by no means all drip sites exhibit the behaviour at a given site (Tooth and Fairchild, 2003). Multi-year variability in drip chemistry at some sites can confound attempts to relate PCP to climate parameters (Baldini et al., 2006). Strong systematic Mg–Sr covariations in speleothems can be quantitatively inferred to originate via PCP (McMillan et al., 2005;

01/04

Month/year Fig. 8. a. Janece McDonald at a dripwater monitoring site in Wombeyan Caves, New South Wales, August 2006. b. Relationship between dripwater Mg/Ca and Sr/Ca composition, hydrological balance and drip rate at site K1 in Wombeyan Caves, illustrating annual and non-annual climate effects with PCP related to dry conditions at this dripsite (after McDonald et al., 2004).

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Johnson et al., 2006). The relationship between degassing and prior calcite precipitation also results in systematic rises in d13C values. Positive covariations in chemical parameters arising from PCP were illustrated by Verheyden et al. (2000), McDermott et al. (2005) and Treble et al. (2005b) and also modelled by Johnson et al. (2006) and Fairchild and McMillan (2007). It is important to look for such relationships in speleothems when looking for explanations of secular change in d13C signatures (McDermott et al., 2005). Johnson et al. (2006) demonstrated strongly sub-annual correlations of Mg, Sr and d13C signatures in a speleothem that they related to PCP, and were able to use the accompanying d18O signal in the speleothem to show that the PCP effect was least shortly after the onset of the monsoon season in May. Both these authors and McMillan et al. (2005) emphasized that if such a mechanism has been established on the sub-annual scale, and related to seasonal dryness, longerterm records with similar covariations could be interpreted in an analogous manner. Similar correlated relationships apply to surface precipitates in the form of fluvial calcitic tufas (Andrews, 2006). Nevertheless, less dependably annual patterns will arise where there is inter-annual rainfall variability, for example where modu˜ o Southern Oscillation events (McDonald et al., lated by the El Nin 2004). PCP is likely to be required to enhance Mg/Ca ratios and allow aragonite to form (Fig. 9, Frisia et al., 2002). McMillan et al. (2005) found arguably annual variations in U, Sr and Ba in aragonite from Clamouse Cave. This might be related to varying PCP upflow, but if prior aragonite precipitation were to occur, Sr levels should not have been affected since KSr is close to one in aragonite.

2.5. Incorporation into carbonate minerals Despite a huge classical literature on element partitioning into calcite and aragonite and many important recent advances in understanding specific growth processes, many phenomena remain poorly understood. This applies even to divalent ions substituting for Ca in the lattice, whilst the experimental literature

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that can provide guidance in predicting the incorporation of other species is sparse. We now understand that the distribution coefficient approach is inadequate when we consider the actual sites where ions are incorporated. Hence different types of crystal face have a different trace element chemistry, giving rise to sector zoning (Reeder and Grams, 1987). This is particularly important for aragonite, an orthorhombic mineral, which typically develops a combination of forms and this is likely to explain the complex lateral changes found by Finch et al. (2003). Aside from hydrothermal settings, speleothem calcite is simpler, typically growing as the unit rhombohedron, although large crystals are typically composites of myriad micron-scale crystallites (Frisia et al., 2000; Fairchild et al., 2001). An extensive body of experimental literature (e.g. Reeder, 1996; Reeder et al., 2001) shows that there are different growth sites with different preferences for trace elements around spiral growth hillocks on single crystal faces. However, the hillocks typically seem too small on natural calcites to be determined even by microanalysis and so their effects are averaged out; hence the distribution coefficient approach continues to be useful, but it is probable that KTr will vary with growth conditions. The ‘‘best behaved’’ ion is thought to be Mg, which under typical karstic conditions, has a KMg value that depends largely on temperature (Fig. 10; Huang and Fairchild, 2001), although there are irregularities of behaviour (Treble et al., 2005a; Borsato et al., 2007) and some experimental work hinting at complex growth mechanisms (Wasylenski et al., 2005). In a study on a 20th century stalagmite, MND-S1 from Moondyne Cave, Treble et al. (2003) established that there was no relationship between Mg and growth rate, consistent with experimental studies (Mucci and Morse, 1983; Huang and Fairchild, 2001), but elemental mapping revealed heterogeneities in speleothem Mg concentration perpendicular to the main growth axis (Fig. 11a; Treble et al., 2005a). In addition to annual Mg cycles traceable along the speleothem growth axis, the concentration of the Mg ion appeared to show greater heterogeneity perpendicular to the growth axis than did other elements such as Ba (Fig. 11b). Treble et al. (2005a) related this to a nonequilibrium uptake of Mg ions related to adsorption effects which vary with Mg/Ca (Mucci and Morse, 1983). In this instance, it was shown that the adsorption effects were less important than climate-driven factors in influencing Mg (Section 3.2.1), but that is not always the case (Section 3.1). Sr is known to be incorporated in larger quantities at high growth rates (Fig. 10; Lorens, 1981; Gabitov and Watson, 2006) and Pingitore and Eastman (1986) proposed that this might relate to its occupancy of defect sites as well as substituting for Ca. Nevertheless Huang and Fairchild (2001) found a discrepancy between Ernesto Cave field values and experimental KSr values, despite similar growth rates (Fig. 10). Sr incorporation was found to be proportional to growth rate in some parts of an Ernesto stalagmite by Borsato et al. (2007), but not in others, and there is no obvious relationship to the calcite fabric types described by Frisia et al. (2000). Gabitov and Watson’s (2006) experiments suggest that growth rate effects become significant above a growth rate equivalent to 0.5 mm/year, a rate that might be exceeded for part of the year, even if total annual growth is much less than 0.5 mm. Hence it may be more practical not to expect partition coefficients to be accurate to within a factor of 2 or 3 for a whole stalagmite lamina, although much smaller relative changes than this within samples could be interpretable in terms of changing fluid composition or, where appropriate, kinetic effects. Where the crystal surface is irregular kinetic effects are more likely and fluid inclusions (Kendall and Broughton, 1978) are also more likely to occur in this case. Borsato et al. (2007) found a rank ordering of trace elements according to how specifically they were associated with fluorescent laminae: Y > (Cu, Zn, Pb) > Br, P. An implication is that Y may be

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0.06 Fig. 11. Speleothem Mg (a) and Ba (b) concentrations mapped across a 0.5  1.5 mm portion of Moondyne Cave stalagmite MND-S1. Magnesium concentrations broadly follow the annual physical layering in MND-S1 (indicated by black lines) with highest Mg coinciding with the driest months of the year, however, heterogeneity parallel to the growth layering indicates non-equilibrium uptake of Mg ions argued to be due to the relative importance of Mg adsorbing to non-lattice sites when Mg/Ca ratios are below 7.5 (Mucci and Morse, 1983) (adapted from Treble et al., 2005a).

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Fig. 10. Partition coefficients determined experimentally (many under high-salinity conditions) and in caves: partition coefficients for Sr versus rate of precipitation and partition coefficients for Mg versus temperature (simplified from Huang and Fairchild, 2001).

largely transported by organic colloids whereas Br and P are transported in more than one form. Free phosphate binds very strongly to calcite surfaces and if present at high concentrations is a strong growth inhibitor. However, at the level of tens of ppb (Huang et al., 2001) that may be more typical of karst settings, a role of PO3 4 may be to bind to defect sites and facilitate the incorporation of other altervalent ions there. The partition coefficient concept appears to be meaningless in these cases particularly since we do not know whether colloidally transported species are incorporated as free ions in calcite, or are incorporated in calcite still attached to their colloidal host. However, it seems likely that such species are incorporated in proportion to some combination of their concentration in the solution and the rate at which they are supplied. Both are likely to increase during strong infiltration events, particularly in autumn, in appropriate climatic zones. The ability of calcite to preserve growth zones over millions of years gives confidence in the likely preservation of primary compositions in speleothem calcite given its low ambient

temperature. Reeder et al. (2001), however, drew attention to the evidence from synchrotron X-ray absorption fine-structure (XAFS) data for extreme disruption to the lattice caused by incorporation of U(VI) as uranyl (UO2þ 2 ) ion and raise the question of its long-term stability. This has obvious implications for U-series dating. Fabrics which have abundant primary inclusions or a high proportion of other impurities would be particularly suspect in this respect. In contrast, U is more readily incorporated in aragonite, with little change in coordination required between uranyl ion and carbonate ions with which it readily complexes in solution (Reeder et al., 2000). Many speleothems show evidence of secondary replacement of aragonite by calcite (Frisia et al., 2002; Ortega et al., 2005), although aragonite can persist for millions of years in some samples and we cannot currently predict how or when this change will occur. 3. Case study review We now draw together key studies that have advanced our understanding of how trace elements in speleothems record environmental information. We divide the discussion using the types of behaviour distinguished by Fairchild et al. (2006a) that in principle could control time series on both sub-annual and longterm timescales: 1. a temperature-controlled pattern where speleothem chemistry is directly controlled by variations in cave temperature (Section 3.1); 2. a fluid-dominated pattern refers to changes in fluid trace element composition (or trace element to calcium ratio) that are mirrored by changes in the stalagmite (Sections 3.2–3.4 and parts of 3.6);

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3. a crystal-dominated pattern refers to changes in crystal composition dictated by crystal growth factors which in turn could be triggered by shifts in fluid characteristics such as pH (Sections 3.5 and parts of 3.6).

3.1. Cave temperature case studies The seminal studies of speleothem trace elements were seeking a palaeotemperature proxy (e.g. Gascoyne, 1983; Goede and Vogel, 1991), based on the observation in early experimental studies of a temperature-dependence of incorporation of Mg in calcite (Katz, 1973). By comparing the Mg/Ca ratio of the tips of actively growing stalagmites from both temperate and tropical cave sites (spanning a variation of 16  C in cave temperature) with the feeding dripwater, Gascoyne (1983) demonstrated a positive relationship between DMg and cave temperature that was close to experimental values. Hence, Gascoyne argued that speleothem Mg could be an effective temperature recorder although he recognized the prerequisite that the dripwater Mg/Ca ratio remains constant over time. Gascoyne (1983) also suggested that any Mg/Ca variations could be corrected for by considering the variation of Mg/Sr rather than simply Mg, assuming that Sr was derived in a similar way to Mg, but that its uptake into calcite was not temperature-dependent. Following Gascoyne’s lead and similar logic employed by Chivas et al. (1986) on marine ostracodes, Goede and Vogel (1991) tested the use of Mg as a palaeothermometer in a stalagmite from Lynds Cave in Tasmania, Australia. They found an overall positive trend in speleothem Mg/Sr in the interval 15–12 ka, consistent with the general warming trend known during this period. However, in a later study involving another Tasmanian stalagmite from Frankcombe Cave, Goede (1994) compared both Mg concentrations and Mg/Sr with speleothem d18O in the interval 98–55 ka and no significant relationships were observed. Goede (1994) concluded that speleothem Mg was unlikely to be a reliable palaeotemperature proxy. Roberts et al. (1998), in the first study to present annual trace element cycles in speleothems, used SIMS analyses of a Holocene stalagmite (growth period 7–1 ka) from Tartair Cave in NW Scotland and attempted to evaluate the temperature hypothesis. While SIMS measurements revealed annual cycles in speleothem Mg concentrations that may have plausibly resulted from variations in temperature (since the stalagmite grew in a passageway exposed to seasonal temperature variations), they recognized that the longerterm Mg variations were too large to be controlled by temperature alone. Instead, Roberts et al. (1998) proposed that these long-term variations reflected changes in the residence time of dripwaters in the dolomite rock above the cave and hence potentially recorded effective rainfall (see Section 2.2). Huang and Fairchild (2001) have, to date, performed the only experimental investigation of the temperature-dependence of KMg under laboratory conditions designed to be analogous to speleothem precipitation. They considered inorganic calcite precipitated from solutions of low ionic strength with Mg concentrations similar to cave dripwaters and calcite precipitated under constant temperature and humidity at typical speleothem growth rates. Huang and Fairchild’s results showed a KMg value of approximately 0.0006/ C in the range of 7–15  C (Fig. 10b), comparable to or slightly larger than previous experimental studies using saline solutions. This small temperature dependency of speleothem KMg would typically be masked by the much larger variations in the concentration of Mg/Ca in dripwater (Sections 2.2 and 2.3). However, a successful example of the approach is from an annually laminated tufa deposit in Australia where trace elements display seasonally varying ratios to Ca and the stream displays a 10  C annual change (Ihlenfeld et al., 2003). Palaeotemperatures

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reconstructed from Huang and Fairchild (2001) relationship show a strong match to measured stream temperatures and residuals were related to plausible changes in aqueous Mg/Ca. No comparable cases of records of annual temperature variations have yet been recorded in caves. A favourable case ought to be the Tartair Cave (Roberts et al., 1998), where monitoring of the modern cave chamber (Fuller, 2007) shows that the dripwater compositions vary little through the year and that temperature varies by 4.5  C. However, detailed scrutiny of a high-resolution Mg time series from a modern Tartair sample (Fairchild et al., 2001 and unpublished) indicates that the seasonal temperature effect is subordinate to noise from other factors. The interpretation of Mg in speleothem studies has thus largely shifted to interpreting its variation in terms of hydrological changes (outlined below). 3.2. Case studies of trace elements interpreted as proxies for aridity 3.2.1. Speleothem Mg A positive outcome of the earlier palaeotemperature investigations was that Mg was identified as being a potential indicator of palaeohydrology, more specifically, that Mg may be useful as a proxy for effective rainfall (meteoric precipitation minus evapotranspiration) at some sites. This is an important finding as trace elements sensitive to rainfall amount are highly useful, particularly when trying to identify trends in d18O related to rainfall amount. While the dripwater Mg/Ca ratio can relate to PCP and is optimized by low-flow conditions it can also be indicative of other processes. For example, this ratio can also be a sign of dilution under conditions of high dripwater flow as well as indicating changes in the source of Mg, with large rainfall events potentially altering the dominant flow path between seepage (pore matrix) and faster (shaft) flow. Also the extent of PCP could be controlled by seasonal variations in cave pCO2, rather than dryness, where caves are wetter or where pCO2 variations are large. Despite some of the potential complexities of using speleothem Mg as a palaeohydrological proxy, many studies have demonstrated a convincing relationship between dripwater and/or speleothem Mg and recorded rainfall (Tooth and Fairchild, 2003; Treble et al., 2003; McDonald et al., 2004; Karmann et al., 2007). Studies such as Treble et al. (2003) and McDonald et al. (2004) demonstrate the potential for using speleothem Mg to reconstruct the past ˜ o and longer, multioccurrence and severity of events such as El Nin decadal dry periods that may have occurred prior to the instrumental climate record. Such baseline climatic information is proving to be particularly important for understanding long-term natural rainfall variability and its implications for water resource planning in nearby cities. In the 2.5-year study of speleothem dripwaters at Wombeyan Caves, New South Wales, Australia, ˜o McDonald et al. (2004) demonstrated that the 2002–2003 El Nin event, which was responsible for widespread drought across eastern Australia, resulted in dripwater Mg/Ca values doubling relative to the previous 14 months (Fig. 8b). Treble et al. (2003) demonstrated that a rise in Mg concentrations observed in a 20th century speleothem from Moondyne Cave in Western Australia reflected an abrupt reduction in rainfall that occurred around 1970 and which has continued to the present time (Fig. 12a). The rise in Mg concentrations after the 1970 rainfall decrease was attributed to an increase in dripwater Mg/Ca ratios owing to increased aquifer residence time in the drier post-1970s period. The possibility of increased PCP in this drier interval is supported by the similarity between higher Mg concentration and higher speleothem Sr and d13C (Treble et al., 2003, 2005b), both of which will also rise if PCP takes place, as outlined in Section 2.4. Treble et al. (2005b) also observed in sub-annual measurements that lower Mg concentrations coincide with lower d18O, implying PCP was at a minimum in the wetter winter months. However, this

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Fig. 12. (a) Winter speleothem Mg, P, U, Sr and Ba concentration anomalies in stalagmite MND-S1 from Moondyne Cave, Australia, with instrumental rainfall and speleothem growth rate. (Anomalies are calculated by subtracting the mean and dividing by the standard deviation of the 1966–1992 period.). (b) Average annual waveform of rainfall and Mg before and after the 1970 rainfall decrease. Speleothem Mg demonstrates an antiphase relationship with rainfall (adapted from Treble et al., 2003).

could not be confirmed by cave monitoring at Moondyne, due to lack of water. Subsequent cave monitoring at another cave in this region (Golgotha Cave, 20 km from Moondyne) identified that dripwater Mg/Ca ratios in Golgotha Cave were highest in winter (Treble et al., 2008) due to a strong fall in cave air pCO2 in this season. Such a study illustrates the importance of cave monitoring to understand prevailing processes. Likewise, the comprehensive monitoring at St. Michael’s Cave, Gibraltar has shown a distinctive pattern of air circulation resulting in strong PCP effects in summer result from a combination both of dryness and low pCO2 (Mattey et al., 2008). McMillan et al. (2005) used arguments both from cave monitoring and speleothem composition to argue for the varying impact on PCP in a late Holocene interval within two Holocene stalagmites from different chambers in Clamouse Cave, southern France. The argument was based both on the strong shifts in water chemistry from bedrock composition in Mg/Ca–Sr/Ca space (Fig. 6b), and the annual rises in Mg and Sr in the speleothem concordant with known effects of summer dryness at this site (Fairchild et al., 2000). A long-term rise in Mg and Sr, 1200–1100 years BP (McMillan et al., 2005) culminated in a switch from calcite to aragonite deposition and was attributed to a major drought phase. McMillan et al. (2005) also quantified an aridity index based on the partitioning of Sr between calcite and its precipitating fluid representing the relative increase in calcite precipitated from fluid before dripwater reaches the speleothem.

We consider now the approaches taken to interpret changes in rainfall from Mg measured in Holocene and Pleistocene periods where no instrumental data exist for comparison and cave monitoring studies may not be possible or the results not applicable (Hellstrom and McCulloch, 2000; Verheyden et al., 2000; Johnson et al., 2006; Cruz et al., 2007). The effectiveness of speleothem Mg as an indicator of rainfall in these studies is typically argued on the basis of its covariation with d13C and/or d18O. Again, a positive relationship between Mg and d13C is used to infer that both proxies may be driven by the effect of increased PCP (owing to longer residence time). This reasoning was adopted by Hellstrom and McCulloch (2000) who interpreted relatively high Mg and d13C values prior to circa 15,000 years BP in a 0–30 ka flowstone record (Fig. 13) from Nettlebed Cave in the Southern Alps, New Zealand as evidence of a drier last glacial maximum climate. Similarly, Verheyden et al. (2000) inferred changes in water excess throughout the Holocene in a Belgian stalagmite by tracking covariation in speleothem Mg and d13C. The relationship between speleothem Mg and d18O is also used to argue that Mg is an indicator of effective water excess where speleothem d18O is assumed to largely reflect changes in rainfall. Cruz et al. (2007) point to an impressive co-variance between these two variables in a speleothem from Botovera´ Cave, Brazil, that grew over the last 116 ka to argue that this speleothem contains a record of past rainfall (Fig. 14). Speleothem d18O is interpreted to record variations in the South American summer monsoon driven by

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Fig. 13. Speleothem MD3 d18O, d13C, Mg and d234U(T), Nettlebed Cave, New Zealand. Magnesium and d234U(T) decrease 1500 years after a sharp change in d13C (as well as Ba and Sr; not shown) at 15 ka while d18O decreases gradually through these transitions. The offset in the timing of these proxies is argued to indicate that temperature rose initially, warming the soil and promoting greater vegetation and soil biological activity (affecting d13C) while rainfall (affecting d18O) rose more gradually before rising sharply at 13.5 ka (adapted from Hellstrom and McCulloch, 2000).

changes in summer insolation. Long-term trends in speleothem Mg (and Sr) bear a positive relationship with d18O and all proxies rise during periods of higher summer insolation (Cruz et al., 2007). The broad similarities between the d18O and trace element proxies lend support that all are recording changes in rainfall characteristics. This is a good example of how combining trace element and stable isotope measurements permit a more detailed interpretation of these records. Speleothem Mg (and Sr) variations were considered to be a direct proxy of rainfall amount with lower values recorded during periods when the South American summer monsoon intensified over southern Brazil during full glacial conditions. Discrepancies in the trace element and d18O record are argued to infer that d18O in subtropical Brazilian speleothems is not directly recording rainfall amount. As a result of the trace element study, the driver of speleothem d18O is argued to be the relative proportion of summer monsoonal and winter extratropical rainfall which have distinct isotopic ratios owing to different moisture source areas (Cruz et al., 2007). Similar hydrological mechanisms affecting Mg are being used to explain climate change on vastly different timescales: very short˜ o), intermediate (multi-decadal to multiterm (seasonal to El Nin centennial) to much longer-term (multi-millennial) climate events. However, it is also clear that, given the potential multitude of factors affecting Mg concentration including dilution of dripwater, marine input, incongruent dissolution, re-routing of fluid causing mixing with other fluids or rock with a different Mg concentration, caution should be exercised in interpreting Mg purely as a straightforward indicator of effective rainfall. This is particularly so when no long-term dripwater studies can be undertaken at the cave site or when the results of such studies may not be relevant owing to strongly different climatic regimes between the present day and past times. For example, during rapid climate events such as Heinrich or Dansgaard–Oeschger events, the relationships between Mg and rainfall may differ from those of the current interglacial period.

Fig. 14. Botovera´ Cave (Brazil) Mg/Ca, Sr/Ca, d18O and summer insolation. These are the first trace element records to demonstrate the effects of insolation. They also constrain the meaning of the d18O record (see text for discussion) (reproduced from Cruz et al., 2007).

On a practical note, it is worth considering when collecting speleothems that the reliability of using the Mg signal to track effective rainfall may be improved by choosing samples from sites with specific hydrologic characteristics (Baldini et al., 2006). For example, dripwaters are more likely to encode a simpler relationship to water excess if seepage flow dominates. Under seepage flow, water moves through the system according to recharge permitting the effects of PCP to dominate with minimal overshadowing by mixing of different age waters. Seepage flow is more common in the highly porous calcarenite host rocks, such as southwest Western Australia (Treble et al., 2003) and the partly dedolomitized rocks at Clamouse (Fairchild et al., 2000). By contrast, dripwater through crystalline, fractured limestones are likely to be dominated by hydrological routing. Host rock depth may also play a role. For example, a dripwater study at the Wombeyan Caves, south-eastern Australia sampled at shallow sites (<20 m) whose response time to surface water excess is on the order of weeks (McDonald et al., 2004). The relatively shallow nature of these limestones may assist

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˜ o signal into speleothem calcite which in the recording of the El Nin can cause large, but typically short-lived rainfall decreases in southeast Australia (typically 1–2 years duration). This relatively short-lived signal could, in the case of slow-growing speleothems or in speleothems fed by deep flow paths enhancing the attenuation of the signal, be undetectable. In contrast, deeper sites (45 m) at the Wombeyan Caves did not show clear short-term hydrogeochemical responses related to the surface hydrology (McDonald et al., 2007). It is notable that the speleothem used in Cruz et al.’s (2007) successful study of multi-millennial changes in the south American summer monsoon comes from a deep (>100 m) cave system which is likely to be less sensitive both to shorter-term climate events and to variations in hydrological re-routing in the cave system. In studies of young speleothems, it may be possible to determine how sensitive the Mg signal is to surface climate events by a carefully constructed dripwater study such as that of McDonald et al. (2007). For older speleothems, adopting a multiproxy approach will help build a more accurate interpretation of past hydrological variation. A largely unexplored area of speleothem Mg studies is the investigation of Mg isotopes. Magnesium isotopes have the potential to identify different sources of Mg, enabling the distinction of atmospherically sourced versus bedrock sourced Mg. Galy et al. (2002) presented measurements of cave dripwaters whose d26Mg value (26Mg/24Mg ratio relative to international standard SRM980) clustered w2& higher than in speleothems and their host limestone, indicating a fractionation between source water and mineral (Galy et al., 2002). This ratio also varied up to 3& within one speleothem. Buhl et al. (2007) examined Mg isotopic data in one speleothem and speculated on the role of PCP as well as silicate weathering, in producing variation in their data. Although these two studies are preliminary in nature, they indicate that Mg isotopes could be a tool for understanding whether source effects, hydrological routing or residence time are dominating at a particular signal. Given the increasing number of studies using Mg concentrations to reconstruct past rainfall and the potential for misinterpreting this variable, a more detailed investigation of Mg isotopes in modern speleothems and their dripwaters appears to be an area worthy of future research. 3.2.2. Speleothem Sr and U In theory, other trace elements whose partition coefficient is less than unity (e.g. Sr, Ba and U) may also be effective palaeohydrological indicators owing to the impact of PCP elevating the dripwater metal/Ca ratios and hence the concentration of the metal in the speleothem (Fairchild et al., 2000; Tooth and Fairchild, 2003; McDonald et al., 2004; Baldini et al., 2006; Karmann et al., 2007; Van Beynen et al., 2008). In speleothems, Sr and Ba concentrations often exhibit a strong, positive correlation over seasonal (Roberts et al., 1998) to millennial timescales (e.g. Hellstrom and McCulloch, 2000) indicating that a common suite of processes can control the concentrations of these ions. However, Sr and Ba have also been shown to display divergent trends over timescales greater than annual (e.g. Treble et al., 2003) as discussed in Section 3.5. The Sr isotopic ratio (87Sr/86Sr) of speleothems and their dripwater are reported as typically falling between that of seawater and the cave host rock or soil (Goede, 1994; Ayalon et al., 1999; BarMatthews et al., 1999; Musgrove and Banner, 2004). Soil 87Sr/86Sr ratio may differ from that of the limestone owing to the incorporation of exogenic sources of Sr, such as aeolian-derived silicates, into the predominantly carbonate soil. The 87Sr/86Sr ratio is interpreted to reflect the relative contribution of weathered material from the limestone versus aeolian sediment end-members, indicating a proxy again for effective rainfall (e.g. Banner et al., 1996; Verheyden et al., 2000; Musgrove and Banner, 2004) and in some cases, aeolian activity (Goede et al., 1998; Bar-Matthews et al., 1999;

Frumkin and Stein, 2004; Li et al., 2005). The latter four studies, conducted on speleothems grown during glacial periods, contained 87 Sr/86Sr ratios higher than that of the host rock, indicating chemical weathering of a detrital phase attributed to higher inputs of dust to the karst soil during glacial periods. On a similar theme, U isotopes have also been used to infer palaeohydrology. For example, two studies of long speleothem records extending from the last glacial period into the Holocene demonstrate the potential use of the 234U/238U ratio as a proxy of water excess. These include the study of Hellstrom and McCulloch (2000) for the Nettlebed Cave record from the Southern Alps in New Zealand and Kaufman et al. (1998) for the Soreq Cave record from Israel. Both records demonstrate a similar trend between d18O (used as a proxy for rainfall) and 234U/238U, and interpret a drop in the 234U/238U ratio as indicating a return to higher discharge as rainfall increased after the last glacial maximum. The study of Hellstrom and McCulloch (2000) is of particular interest as the decrease in the 234U/238U ratio (d234U(T)), accompanying a reduction in speleothem Mg concentrations, occurs 1500 years after a sharp change in other proxies (d13C, Ba and Sr) at 15 ka (Fig. 13). These abrupt shifts in Mg and other proxies bracket a more gradual change in speleothem d18O which reflects both temperature and rainfall O isotopic composition. Hellstrom and McCulloch (2000) argue that the 1500-year offset in the behaviour of these proxies could indicate that temperature rose initially, warming the soil and promoting greater vegetation and soil biological activity (affecting d13C, Ba, and Sr) while rainfall rose more gradually before rising sharply at 13.5 ka (affecting d18O gradually in this lead up and then finally 234U/238U at 13.5 ka; Fig. 13). This study is one of the few that attempts to decouple temperature and rainfall in the speleothem record and in doing so indicates the potential effectiveness of using trace element measurements to disentangle the rainfall and temperature signals. 3.3. Case studies of vegetation change soil-derived elements Normally, carbon isotopes are the primary means by which vegetation characteristics are assessed, but in Section 2.3 we summarized evidence that biogeochemical factors lead to the transport of elements associated with soil-derived colloids into the cave. Following the study of Huang et al. (2001) in which annual P peaks in speleothems from Ernesto Cave (NE Italy) were related to an autumnal flush from the soil, Baldini et al. (2002) highlighted the utility of speleothem P as a recorder of palaeoclimate by demonstrating that a marked decrease in speleothem P occurred in a stalagmite from Crag Cave, Ireland, around the time of the 8.2 ka cold reversal and related this to changes in soil productivity. However, the isotope anomaly which tied the low-P interval precisely to the 8.2 ka event has been found to be an artefact (Fairchild et al., 2006a) and hence the interpretation of the P record is less certain. Nevertheless there is other evidence to link lower speleothem P to cooler and drier conditions reducing both vegetation and soil biological activity. Speleothem P concentrations measured in the southwest Australian speleothem from Moondyne Cave by Treble et al. (2003) demonstrated a close relationship with the instrumental rainfall record. Speleothem P concentrations decreased after the post-1970 rainfall decrease and there were further similarities with the decadal-scale precipitation record prior to this (Fig. 12a). Treble et al. (2003) noted that speleothem U exhibited a similar trend to P (Fig. 12a) and suggested that transport of the uranyl ion was aided by the enhanced solubility and hence transport of the U species as uranyl–phosphate complexes. More recently a suite of elements (F, Y, Pb, Cu, Zn, P and Br) have been found to be associated with the annual autumnal period of maximum soil infiltration at Ernesto (Borsato et al., 2007). This site is currently forested, but a known period of deforestation occurred

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due to military activities in World War 1. This time period is marked by a pronounced increase in the abundance of these elements (Fig. 15) which was attributed by Borsato et al. (2007) to enhanced flushing of the soil into the karst aquifer in the absence of the tree canopy. 3.4. Case studies of speleothems as archives of atmospheric input Goede et al. (1998) used Sr isotopes to identify an exogenic source of Sr in the previously mentioned 84–57 ka stalagmite from Frankcombe Cave, Tasmania. Speleothem Sr concentrations presented a bimodal distribution in this speleothem suggesting switching between two Sr sources (Goede, 1994; Goede et al., 1998). Strontium isotopic measurements confirmed that during times when Sr concentrations were high (>100 ppm) that there was an additional non-limestone source of Sr (Fig. 16a). Goede suggested that the most likely source was marine carbonate material blown

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from the exposed continental shelf between Tasmania and the Australian mainland (Fig. 16b). Higher Sr concentrations and 87 Sr/86Sr ratios during interstadial intervals were interpreted by Goede as evidence of a switching of the dominant wind direction, with northwest winds passing over broad areas of exposed continental shelf to the north of Tasmania and entraining carbonates. In contrast lower Sr concentrations and 87Sr/86Sr ratios during stadials were argued to be due to a much reduced aeolian detrital supply owing to the prevalence of westerly winds passing across a much narrower strip of exposed shelf. Thus, this study effectively combined Sr concentrations and isotopes to interpret a change in prevailing atmospheric circulation patterns during the last glacial period. Similarly, Bar-Matthews et al. (1999) in a study of a 60 ka Israeli speleothem record from Soreq Cave interpreted higher 87 Sr/86Sr ratios as reflecting a greater input of sea spray droplets and dust particles through the last glacial period. In particular, an abrupt drop in 87Sr/86Sr in the early Holocene is attributed to

Fig. 15. Records of trace element concentration in stalagmite ER78 from Ernesto Cave compared with meteorological data for the period 1875–1925. The enhanced trace element concentrations do not coincide with a distinct meteorological anomaly but do correspond with a known period of deforestation at the site. Enhanced transport of trace elements from the soil to dripwater, particularly during autumn infiltration is inferred (reproduced from Borsato et al., 2007).

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Fig. 16. a. Strontium concentrations (1/Sr) versus Sr isotopic ratios measured in a 84–57 ka stalagmite from Frankcombe Cave, Tasmania. b. Regional map with arrows indicating the change in prevailing wind direction between interstadials and stadials inferred from the bimodal distribution of Sr concentrations and Sr isotopic ratios in this speleothem suggesting switching between two Sr sources. See text (reproduced from Goede et al., 1998).

a reduction in sea spray and dust coinciding with the commencement of more intense weathering of the limestone when the eastern Mediterranean climate became warmer and wetter. Speleothem S measurements (both concentration and isotopic) offer the prospect of resolving the history of atmospheric S concentration (e.g. Frisia et al., 2005) and budgets at a wide range of latitudinal environments, complementing the existing S records from ice cores (e.g. Legrand, 1997). Sources of aerosol sulphates include terrestrial and marine biota, volcanic (eruptive and noneruptive) as well as anthropogenic pollution (Legrand, 1997). Potentially, these sources may be distinguishable in speleothems by their S isotopic ratios (Wynn et al., 2008), although such investigations are still in their infancy. Frisia et al. (2005) presented a record of speleothem S concentrations measured by synchroton

radiation over the 1840–1995AD growth period of a speleothem from Ernesto Cave, northeast Italy. An overall trend of rising S from about 1880AD onwards was shown to be consistent with trends in Alpine and Greenland ice cores and is argued to represent the rise in anthropogenically emitted sulphates during the industrial period. Reconstructed speleothem S from prior to the industrial period has also been shown to record significant changes in atmospheric sulphate emissions from non-anthropogenic activities such as volcanic eruptions. For example, multiple short-lived (approximately seasonal) spikes in S concentration recorded in a Holocene stalagmite from Savi Cave in northeast Italy are interpreted as evidence for repeated volcanic events circa 5–5.6 ka that coincide with volcanic activity recorded in Greenland ice cores (Frisia et al., 2005).

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3.5. Case studies of crystal-dominated patterns The climatic interpretation of fluid-dominated pattern speleothem trace elements discussed above should in principle be relatively straightforward and reliable if the uptake of these elements into the speleothem is proportional to the concentration of these elements in the dripwater. This would imply that KTr is constant, but as discussed in Section 2.5, this is often not the case, particularly for crystals with rough surfaces, typically forming relatively quickly from more strongly supersaturated solutions. It may also be possible to detect the effects of speleothem growth rate on trace element uptake in speleothem records themselves, provided that growth rate can be reliably reconstructed. Few studies have considered whether speleothem growth rate could be influencing trace element concentrations, presumably because for the majority of speleothems, growth rate has only been constrained as an average figure between U-series chronologies. Thus the records of speleothem growth rate in such studies are typically too coarse to be closely compared with the more detailed trace element records. Moreover, environmental controls could arguably be affecting growth rate and dripwater ion content simultaneously when significant climatic variation is occurring over longer timescales. A way of examining the possible effects of speleothem growth rate on trace element concentrations is to study speleothems with annual trace element cycles which offer an effective tool for constraining annual growth rate by adopting the cycle wavelength as a measure of linear extension rate (Treble et al., 2003) The Moondyne Cave stalagmite MND-S1 was particularly suitable for investigating detailed changes in speleothem inter-annual growth rate as it was relatively fast growing and growth rate varied significantly over the growth period (1911–1992; approximately 200–600 mm/ a). Speleothem Ba and Sr were highly correlated in each LA-ICP-MS measurement transect (r ¼ þ0.9, p < 0.01) which encodes both intra- and inter-annual variation. After removing the seasonal cycle, the covariation was weaker and the underlying, long-term trends in Ba and Sr dripwater concentrations were revealed (Fig. 12a). The relationship between speleothem growth rate and Ba concentrations appears to have persisted over the inter-annual timescale, e.g. relatively low Ba between 1911 and 1925, rising until 1980 and then falling post-1980 in tune with growth rate, which notably, shows little relationship with the rainfall trend (Fig. 12a). Strontium bore a weaker relationship with speleothem growth rate than did Ba, which conflicts with the experimental data of Tesoriero and Pankow (1996). It appears that Sr was following neither growth rate nor climate in a straightforward manner, but was moderated by both growth rate as well as shifts in the baseline dripwater Sr concentration, forced by a drying climate (Treble et al., 2003). While the effects of speleothem growth rate on Sr and Ba may have been more readily detectable in MND-S1 owing to its unusually high growth rate, it indicates the potential for misinterpreting these signals where speleothem precipitation kinetics may significantly interfere with the preservation of environmental signals. The relative importance of crystallographic factors in controlling the annual-scale variability of trace elements requires further study. One observation in many samples (e.g. Fairchild et al., 2001) is an inverse relationship of Sr and P. A potentially important explanation for this was offered by Borsato et al. (2007) who posited that Sr was out-competed for defect sites in calcite during times of the year when a wide variety of colloidally transported ions such as phosphate were present in dripwaters. Hence the annual variability Sr has a hydrological significance in this case, but this may not apply elsewhere where Sr and P variations are closer to sinusoidal rather than peaked (e.g. Treble et al., 2003). Finally, in order to identify times of the year when growth is fast and when trace element abundances might be more strongly

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kinetically controlled, it would be useful to identify a trace element that reflects the pH and hence saturation of the water (as controlled by the cave air pCO2). Arguably S (as sulphate) fulfils this role since Busenberg and Plummer (1985) propose that a control on its abundance is the SO4/CO3 in water and strong annual variations were seen in Ernesto stalagmites (Frisia et al., 2005) despite constant concentrations of sulphate in cave dripwater through the in water year. At higher pH (faster speleothem growth), CO2 3 increases (at the expense of HCO 3 ) and hence SO4/CO3 in water decreases leading to lower sulphate in the speleothem. 3.6. Case studies of annual trace element cycles as chronological markers The technique of using annual cycles as a chronological tool is in principle, straightforward. Chronologies are adjusted according to how many peaks (or troughs) are counted along a measurement transect parallel to the growth axis and an algorithm for carrying out this procedure objectively is now available (Smith et al., 2009). This technique assumes that there is a seasonality of the cave environment which is imparted to the growing calcium carbonate crystals in a laterally continuous and consistent manner such that the number of cycles in any one transect along the speleothem is representative of the number of years a speleothem has been growing. Despite the potential for using annual trace element cycles to improve the chronology of speleothem records, only a few studies that have measured longitudinal transects of annual trace element cycles between independently determined ages have used this information in a chronological sense (e.g. Baldini et al., 2002; Treble et al., 2003). Comparing cycle count with independent ages is still predominantly used to confirm that the trace element cycles are annual (e.g. Roberts et al., 1998; Finch et al., 2003; Smith et al., 2009) and/or to tie it to isotopic variations (Johnson et al., 2006; Mattey et al., 2008). Few studies have taken full-advantage of the annual cycle information by using it to adjust the chronology for growth rate variations over time. A potential reason for this could be the time-consuming process of measuring multiple transects to reduce potential errors resulting from counting cycles from just one transect. The example from Treble et al. (2005a) in Fig. 11 demonstrates the need to measure more than one transect of trace element cycles and/or examine whether growth is continuous across the speleothem surface. Desmarchelier et al. (2006) used LA-ICP-MS to measure transects of trace element concentrations along the outer sidewall of a soda-straw stalactite from Frankcombe Cave, Tasmania. This was the first study to produce long records from these relatively fast growing, but delicate speleothems. The record of Ba, Sr, Mg, and U variations extending from 1996AD back through 230 (presumed annual) cycles. Clearly this technique of using soda-straw stalactites to build records from very young speleothems shows great promise. 4. Summary and future directions The most reliable palaeoclimatic usage for trace elements so far has been demonstrated in caves where seasonal and long-term changes in water balance result in large and systematic change in Mg/Ca and Sr/Ca in dripwater, signals that are transferred to the speleothems. Often, but not always, these are correlated with d13C. Such signals can help in interpreting the meaning of d18O variations. Where Mg and Sr are not consistent with each other, cave water monitoring data (at a number of drip sites) and a knowledge of bedrock compositions can be highly informative in determining which are the complicating processes (e.g. variable aquifer composition, crystal chemical effects). This can establish which element would be more reliable in palaeohydrological work in a given case. There is plenty of scope for developing long records,

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aided by the spread of laboratories equipped with laser-ablation ICP-MS technology, and supplemented if necessary by high-resolution analysis of short intervals to establish annual-scale patterns (e.g. synchrotron radiation to identify micron-scale events). Hydrological variations can also give rise to relative changes in elements derived from carbonate and non-carbonate sources. The ground-rules need to be established in each case, preferably by analysis of parent materials to derive end-members. Data from isotopes (e.g. 87Sr/86Sr) can be helpful as well as elemental information. It would be useful to develop more quantitative models of leaching behaviour in relation to factors such as pCO2 and residence time for carbonates versus silicates, and to model the comparative dissolution with phases, such as barite, that are not dependent on pCO2. Changes in atmospheric supply of trace elements over time have been recognized to be related in different cases to variable supply of aeolian sediment (e.g. Sr, Si records), atmospheric pollution, or volcanic inputs (e.g. S). For the future, there is also scope for demonstrating anthropogenic metal pollution records over time as has been recognized in peat records (e.g. Tositti et al., 2006). Changes in abundance of colloidally transported, soil-derived elements within a year can reflect infiltration patterns and over longer periods of time can identify periods of deforestation. Work is in progress on demonstrating the transport, speciation and coprecipitation processes of such elements in order to put this area on a firmer basis. High-resolution records of multiple element variations reveal some patterns that are controlled by crystallographic factors. In one sense they are very helpful in accentuating the annual cycle of trace element responses which allows annual growth rate to be determined, which is a palaeoclimatic indicator in itself. On the other hand, a fuller understanding is needed of the crystallographic responses in order to avoid misinterpreting the records. The recent identification of the colloid-transported elements as indicative of high water flux and of low sulphate as indications of high pH (better cave ventilation) and fast growth rate gives new tools for examining annual-scale records and providing information about seasonality at a given time in a record which will be enhanced by seasonally resolved stable isotope data. Acknowledgements Our joint working has been facilitated by a British Council exchange grant in 2007. IJF is indebted to Birmingham colleagues Andy Baker and Jamie Lead, PT to Paul Rustomji and both of us to our wider circle of speleofriends, collaborators and students for their energy, insight and support. Denis Lacelle and Janece McDonald provided useful referee reports. IJF acknowledges financial underpinning from the Natural Environment Research Council (grants NER/T/S/2002/00448 and NE/C511805/1 and ion microprobe and ICP facility grants) and a Leverhulme Trust lamina network; PT acknowledges funding from Land and Water Australia (Y978). References Andrews, J.E., 2006. Palaeoclimatic records from stable isotopes in riverine tufas: synthesis and review. Earth-Science Reviews 75, 85–104. Atkinson, T.C., 1977. Carbon dioxide in the atmosphere of the unsaturated zone: an important control of groundwater hardness in limestones. Journal of Hydrology 35, 111–123. Atteia, O., Kozel, R., 1997. Particle size distributions in waters from a karstic aquifer: from particles to colloids. Journal of Hydrology 201, 102–119. Ayalon, A., Bar-Matthews, M., Kaufman, A., 1999. Petrography, strontium, barium and uranium concentrations, and strontium and uranium isotope ratios in speleothems as palaeoclimatic proxies; Soreq Cave, Israel. The Holocene 9, 715–722. Baker, A., Smart, P., Edwards, R., Richards, D., 1993. Annual growth banding in a cave stalagmite. Nature 364, 518–520.

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