Hydrogeochemistry and fractionation pathways of Mg isotopes in a continental weathering system: Lessons from field experiments

Chemical Geology 300-301 (2012) 109–122

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Hydrogeochemistry and fractionation pathways of Mg isotopes in a continental weathering system: Lessons from field experiments S. Riechelmann a,⁎, D. Buhl a, A. Schröder-Ritzrau b, C. Spötl c, D.F.C. Riechelmann d, D.K. Richter a, T. Kluge e, T. Marx f, A. Immenhauser a a

Ruhr-University Bochum, Institute for Geology, Mineralogy and Geophysics, Universitätsstraße 150, D-44801 Bochum, Germany Heidelberg Academy of Sciences, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany Leopold-Franzens-University Innsbruck, Institute for Geology and Palaeontology, Innrain 52, A-6020 Innsbruck, Austria d Johannes Gutenberg-University Mainz, Institute of Geography, Johann-Joachim-Becher-Weg 21, D-55099 Mainz, Germany e Yale University, Department of Geology and Geophysics, 210 Whitney Avenue, New Haven, CT, 06511, USA f Ruprecht-Karls-University Heidelberg, Institute of environmental physics, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany b c

a r t i c l e

i n f o

Article history: Received 25 July 2011 Received in revised form 13 January 2012 Accepted 18 January 2012 Available online 31 January 2012 Editor: U. Brand Keywords: Magnesium isotopes Speleothems Hydrogeochemistry Calcite Weathering

a b s t r a c t The potential of magnesium isotope records from cave carbonate archives (speleothems) has been documented but remains underexplored. This is due to the limited knowledge regarding the complex suite of physico-chemical and biological disequilibrium fractionation processes affecting meteoric fluids in the soil zone, the carbonate hostrock and calcite precipitation in the cave. This study presents δ 26 Mg data from a monitored cave in Germany (Bunker Cave) including rain water (δ 26 Mg: − 0.70 ± 0.14‰), soil water (δ 26 Mg: − 0.51 ± 0.10‰) and drip waters (δ 26 Mg: − 1.65 ± 0.08‰) sampled between November 2009 and May 2011. Field precipitation experiments, i.e., calcite precipitated on watch glasses (δ 26 Mg: − 3.56 ± 0.26‰; May 2006 to June 2010), were found to be of limited use. This is because of experimental, crystallographic and sampling artefacts. Conversely, variations in soil and drip water δ26 Mg over time are predominantly related to seasonal variations in water availability and air temperature affecting the subtle weathering ratio between Mg-bearing clay minerals in the soil, here mainly chlorite and montmorillonite, and the low-Mg calcite hostrock. Bunker Cave δ26Mgdrip water values display a significant dependency on the air temperature outside the cave. This is because air temperature influences CO2 levels in the soil and hence rock-water interaction. For fast drip sites, the direct correlation of δ26Mgsoil water and δ26Mgdrip water documents a relative short residence time of the fluid in the carbonate aquifer and thus limited isotope equilibration and mixing of different reservoirs. This result is encouraging and adds new evidence to the poorly understood hydro-geochemistry of carbonate aquifers. Slow (seepage flow) drip sites display an annual δ26Mgdrip water pattern that is geochemically unrelated to that of the soil water. Further research, including laboratory experiments, must focus on the complex fractionation between drip water and speleothem calcite Mg isotope record. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Secondary carbonate minerals (speleothems) in carbonate caves precipitate from seeping, dripping or flowing water under a broad range of hydro-geochemical and climatic regimes (Dreybrodt, 1999; Tooth and Fairchild, 2003). Over the past decades, speleothems have been increasingly used as palaeoclimate archives combining geochemical evidence with precise U–Th age dating (Winograd et al., 1992; Wang et al., 2001, 2008; Bar-Matthews et al., 2003; Scholz and Hoffmann, 2008). A main challenge in speleothem-based climate research, however, is that individual drip sites, both within a cave chamber and between different chambers of the same cave may

⁎ Corresponding author. Tel.: + 49 234 3223256. E-mail address: [email protected] (S. Riechelmann). 0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2012.01.025

show considerably different discharge patterns (Baker and Brunsdon, 2003; Banner et al., 2007; Riechelmann et al., 2011). Furthermore, flow paths of rain water percolating from the surface through the soil and the hostrock, the spatial distribution and the volume of different fluid reservoirs (soil, hostrock porosity, fractures/fissures) above the cave, the residence time within and mixing patterns of water masses between different reservoirs affect drip sites in a non-linear manner (Tooth and Fairchild, 2003). As a consequence, the water chemistry and drip characteristics will affect coeval speleothem archives within a given cave (Tooth and Fairchild, 2003). More recently, increasingly sophisticated cave monitoring programmes (Spötl et al., 2005; Banner et al., 2007; Sunqvist et al., 2007; Mattey et al., 2008, 2010; Miorandi et al., 2010; Riechelmann et al., 2011) produced time-series data of cave air temperature, pCO2 and humidity, as well as rain, soil and drip water chemical properties. Other studies investigated recent carbonate precipitates in

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caves in combination with cave monitoring (Mickler et al., 2006; Genty, 2008; Verheyden et al., 2008). Cave monitoring data collected over several years combined with geochemical data from carbonate precipitates at monitored drip sites reveals the full complexity of cave environments and their speleothems (Fairchild et al., 2010; Mattey et al., 2010; Boch et al., 2011). At present, one of the least known aspects in the chain of processes commencing with solution-precipitation patterns in the regolith and ending with the precipitation of carbonate archive in the cave involves the physico-chemical and microbiological processes operating in the karst aquifer. Changes of climate conditions have an influence on carbonate versus silicate weathering ratios. Carbonate weathering is favoured during cooler and wetter conditions whereas silicate weathering is moderately enhanced under warmer and drier conditions (Immenhauser et al., 2010). Based on data shown in Immenhauser et al. (2010), the present study documents the potential of Magnesium isotope (δ26 Mg) analysis of rain-, soil- and drip waters as a previously underexplored proxy for hydro-geochemical processes in soils overlying caves and in their respective carbonate aquifers. The goal of this study is to obtain a more quantitative understanding of the fractionation processes affecting Mg isotopes in karst aquifer waters. This paper addresses questions such as (1) which processes influence soil, aquifer and drip water δ26 Mg? (2) What is the relative significance of each of these processes? (3) To what extent are these processes recorded in the drip water and subsequently in the precipitating calcite? And finally, (4) to which degree do speleothem δ 26 Mg data reflect climatic parameters as opposed to disequilibrium factors? The results shown here are of significance for those concerned with karst systems and their hydro-geochemistry in general but specifically shed light on the complex interaction of different water reservoirs in cave aquifers. 2. Cave setting Bunker Cave is located near the town of Iserlohn-Letmathe in the Rhenish Slate Mountains in the NW part of the Sauerland, Germany (Fig. 1) and is part of the Bunker-Emst-Cave system explored over more than a century (Bunker Cave: 1926; Emst Cave: 1860–1863; Hammerschmidt et al., 1995). The southern entrance of the Bunker– Emst-Cave system (51°22′03″ N/7°39′53″ E) is located 184 m above sea level. The cave formed in ca. 700 m-thick Middle to Upper Devonian massive limestone (von Kamp, 1972; Fig.1) composed of low-Mg calcite. Bunker Cave has a length of about 3500 m and a rock overburden of 15 to 30 m (Grebe, 1993). The thickness of the soil is ca. 70 cm (Riechelmann et al., 2011) and contains quartz (40 wt%), feldspar (19 wt%) and clay minerals (41 wt%) including illite (5 wt%), chlorite (6 wt%), kaolinite (4 wt%), montmorillonite (26 wt%) and very small

amounts of carbonate (XRD analysis). Magnesium is contained in the crystal structure of chlorite and montmorillonite (here smectite; Okrusch and Matthes, 2005) and the same accounts for illite. 3. Monitoring, materials and analytical methods 3.1. Monitoring program in Bunker Cave and materials analyzed The present study makes use of monitoring results from Bunker Cave (Figs. 1 and 2) obtained between August 2006 and October 2010. A detailed description of the monitoring programme is given in Riechelmann et al. (2011) and Riechelmann (2010). Hence, only a limited amount of essential background information is provided here. Two main types of materials were analyzed for their Mg-isotope composition in the context of this study: water and carbonate samples. Water samples were collected from rain, soil and drip water and carbonate samples precipitated from the drip water were collected on watch glasses placed under monitored drip sites. 3.1.1. Rain and soil water Rain water samples were collected at the nearby Dechen Cave (Fig. 1) using a rain gauge (according to DIN 58666C) on the roof of the German Cave Museum Iserlohn. The amount of rain was measured daily and the rain water samples were collected over one month each. The measured amount of rain water as well as data from the nearby weather station Hemer (www.meteomedia.de) were used to calculate the evapotranspiration after Haude (1955) using the following equation:    13 13 Epot ¼ xP  1– F =100

ð1Þ

Epot is the potential evapotranspiration [mm/day], x is the monthly coefficient (depends on vegetation) [mm/day * hPa], P 13 represents the saturation pressure at 1 p.m. [hPa] and F 13 is the relative humidity at 1 p.m. [%]. The saturation pressure was calculated using the equation: 13

P

    13 13 ¼ 6:107101 7:5T = 237 þ T

ð2Þ

whereby T 13 is the temperature at 1 p.m. [°C]. In order to calculate the maximum amount of rain which infiltrates into the soil and afterwards into the cave the following calculation is used: Inf pot ¼ N−Epot

Fig. 1. Geological map with location of the Bunker Cave in Middle/Upper Devonian limestone. Note small inset map at lower right.

ð3Þ

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3.1.2. Drip site characteristics In the context of this study, four drip sites within Bunker Cave were analyzed (Fig. 2). Each of these are characteristically different, a feature that has become obvious in the context of the monitoring programme. The drip rate of the different drip sites was measured manually every month using a stopwatch. The drip site referred to as TS 1 / U I is a fast seasonal drip site (mean: 4.7 ± 5.0 ml/min; Table 1), where drip water was sampled for a few minutes up to several hours during the cave visits. The waters at drip sites TS 2, TS 3 and TS 8 (Fig. 2) were collected over the period of one month due to the slow drip rates. Two of these sites fall in the category of moderately fast drip sites (TS 2 / U II and TS 3 / U VII; Table 1). Drip site TS 2 and U II have a seepage flow characteristic and TS 3 / U VII is a seasonal drip site (Table 1). The fourth monitored drip site (TS 8 / U IV) shows very slow drip rates (mean: 0.002 ± 0.001 ml/min) and has changed from seasonal drip to seepage flow behaviour during the monitoring period (Table 1). 3.1.3. Watch glass precipitation experiments Watch glasses (diameter of 6 cm) were positioned under different drip sites (Fig. 2) with the convex side up in order to mimic the surface morphology of a stalagmite. These glasses were removed every three months and analysed for the amount of calcite precipitated and its δ 13 C and δ 18O signature (Riechelmann, 2010). For the slowly dripping sites TS 2, TS 3 and TS 8, watch glass precipitation experiments performed in parallel with water sampling collection were not feasible. Therefore, drip waters were collected at these sites whilst watch glasses were placed beneath directly adjacent drip sites (TS 2 – U II, TS 3 – U VII, TS 8 – U IV). After drying at room temperature, watch glasses were weighted with a Mettler AT261 Delta Range® (1σ ± 0.015 mg). The weight gain reflects the amount of calcite precipitated during three months. In order to calculate the precipitation rate of the calcite the following equation was used: X½mg=month ¼ ððweight½mg=YÞ365Þ=12

ð4Þ

where Y is the duration (number of days under drip site) of the experiment. The experimental sites were disturbed by cavers and small rodents on rare occasions.

Fig. 2. Map of the Bunker Cave with locations of monitored drip sites (TS) and positions of watch glasses (U).

Infpot is the potentially infiltrating water, which means the potential water excess or deficit [mm/day] and N is the precipitation [mm/day]. Soil water samples were taken at two sites above Chamber 1 of the Bunker Cave using suction probes. Suction probe BW 1 collected soil water samples at a depth of 70 cm and BW 2 at a depth of 40 cm beneath the surface. Magnesium isotopes of soil water BW 1 are reported in Immenhauser et al. (2010), while in this study soil water BW 2 was analysed for its Mg-isotopic composition.

3.1.4. Carbon and oxygen isotope data from drip water and calcite precipitation experiments The direct comparison of drip water and recent calcite precipitates is shown for drip sites TS 1 and 8 and the respective watch glasses U I and U IV (Fig. 2). With reference to drip site TS 1, drip rates were high enough to collect water samples during brief time intervals whilst calcite was precipitated over the remaining monitoring period. Furthermore, the drip rate for drip site TS 8 and for the drip site at watch glass U IV (Fig. 2) are identical and therefore the direct comparison of the calcite C/O-isotopic composition between drip water and precipitated calcite is possible (Riechelmann, 2010). Mean values of δ 13Cdrip water and δ 18Odrip water as well as those of the recent calcite precipitates are shown in Table 1. Carbon isotope ratios of drip waters depend on drip rate, which in turn is linked to karst flow paths. The δ 18Ocalcite and δ 13Ccalcite ratios as obtained

Table 1 Overview of measured factors of the drip sites and watch glasses in context of the monitoring. Number of samples used for statistics varies between 14 and 44.

Mean drip rate [ml/min] Classification of the drip sites After Baker et al. (1997) Mean δ18O [‰ VSMOW] water Mean δ13 C [‰ VPDB] water Mean δ18O [‰ VPDB] calcite Mean δ13 C [‰ VPDB] calcite

TS 1/U I

TS 2/U II

TS 3/U VII

TS 8/U IV

4.7 ± 5.0 Seasonal drip

0.11 ± 0.02/0.05 ± 0.01 Seepage flow

0.03 ± 0.02 Seasonal drip

− 7.8 ± 0.3 − 11.6 ± 0.7 − 6.3 ± 0.3 − 10.6 ± 0.6

− 7.9 ± 0.2 − 7.6 ± 0.5 − 5.9 ± 0.1 − 7.4 ± 0.3

− 7.9 ± 0.3 − 8.9 ± 0.9 − 6.0 ± 0.3 − 8.3 ± 0.6

0.002 ± 0.001 Change from seasonal drip to seepage Flow during monitoring period − 7.8 ± 0.3 − 9.5 ± 1.0 − 5.6 ± 0.2 − 6.1 ± 0.6

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Table 2 Mg-isotope values of all samples; asterisk (*) indicates data from Immenhauser et al. (2010). Δ25Mg′ data were calculated via the equations by Young and Galy (2004) to show the covariance for the internal precision. The values should be close to zero and hence they do not show any abnormality. Sample number

Sample material

δ25 Mg [‰ DSM3]

± 2σ

δ26 Mg [‰ DSM3]

±2σ

Δ25 Mg′ [‰ DSM3]

RW 12/10 RW D 05/11 RW G 05/11 RW M 05/11 RW H 05/11 BW 2 11/09 BW 2 1/10 BW 2 2/10 BW 2 3/10 BW 2 4/10 BW 2 5/10 BW 2 6/10 TS 1 9/09 TS 1 10/09 TS 1 11/09 TS 1 2/10 TS 1 3/10 TS 1 4/10 TS 1 5/10 TS 1 6/10 TS 1 7/10 TS 1 8/10 TS 1 9/10 TS 1 10/10 TS 2 11/09 TS 2 12/09 TS 2 1/10 TS 2 2/10 TS 2 3/10 TS 2 4/10 TS 2 5/10 TS 2 6/10 TS 3 11/09 TS 3 12/09 TS 3 1/10 TS 3 2/10 TS 3 3/10 TS 3 4/10 TS 3 5/10 TS 3 6/10 TS 8 1/10 TS 8 2/10 TS 8 3/10 TS 8 4/10 TS 8 5/10 TS 8 6/10 TS 8 7/10 TS 8 8/10 UI–1 U I - 3* UI–4 UI–5 UI–6 UI–7 UI–8 UI–9 U I – 10 U I – 11 U I – 12 U I – 13 U I – 14 U II – 1 U II – 2 U II – 3 U II – 4 U II – 5 U IV – 1 U IV – 2 U IV – 3 U IV – 4 U IV – 5 U IV – 6 U IV – 7 U IV – 8

Snow Dec 10 Rain water May 11 Rain water May 11 Rain water May 11 Rain water May 11 Soil water Nov 09 Soil water Jan 10 Soil water Feb 10 Soil water Mrc 10 Soil water Apr 10 Soil water May 10 Soil water Jun 10 Drip water Sep 09 Drip water Oct 09 Drip water Nov 09 Drip water Feb 10 Drip water Mrc 10 Drip water Apr 10 Drip water May 10 Drip water Jun 10 Drip water Jul 10 Drip water Aug 10 Drip water Sep 10 Drip water Oct 10 Drip water Nov 09 Drip water Dec 09 Drip water Jan 10 Drip water Feb 10 Drip water Mrc 10 Drip water Apr 10 Drip water May 10 Drip water Jun 10 Drip water Nov 09 Drip water Dec 09 Drip water Jan 10 Drip water Feb 10 Drip water Mrc 10 Drip water Apr 10 Drip water May 10 Drip water Jun 10 Drip water Jan 10 Drip water Feb 10 Drip water Mrc 10 Drip water Apr 10 Drip water May 10 Drip water Jun 10 Drip water Jul 10 Drip water Aug 10 Calcite autumn 06–winter 06/07 Calcite summer 07 Calcite autumn 07 Calcite winter 07/08 Calcite spring 08 Calcite summer 08 Calcite autumn 08 Calcite winter 08/09 Calcite spring 09 Calcite summer 09 Calcite autumn 09 Calcite winter 09/10 Calcite spring 10 Calcite autumn 06–autumn 07 Calcite winter 07/08–winter 08/09 Calcite spring 09–summer 09 Calcite autumn 09–winter 09/10 Calcite spring 10 Calcite autumn 06–spring 07 Calcite summer 07 Calcite autumn 07 Calcite winter 07/08 Calcite spring 08 Calcite summer 08 Calcite autumn 08 Calcite winter 08/09

− 0.71 − 0.34 − 0.37 − 0.33 − 0.49 − 0.23 − 0.36 − 0.23 − 0.28 − 0.28 − 0.24 − 0.35 − 0.83 − 0.80 − 0.76 − 0.75 − 0.80 − 0.82 − 0.77 − 0.86 − 0.82 − 0.81 − 0.80 − 0.85 − 0.91 − 0.91 − 0.90 − 0.90 − 0.89 − 0.85 − 0.87 − 0.88 − 0.91 − 0.89 − 0.89 − 0.84 − 0.89 − 0.88 − 0.87 − 0.86 − 0.87 − 0.89 − 0.87 − 0.90 − 0.92 − 0.87 − 0.81 − 0.89 − 2.16 − 1.49 − 1.80 − 1.76 − 1.53 − 1.86 − 1.89 − 1.66 − 1.50 − 1.07 − 1.38 − 1.89 − 1.73 − 1.95 − 2.07 − 1.96 − 2.01 − 1.89 − 1.98 − 1.90 − 2.05 − 1.73 − 1.64 − 1.92 − 1.93 − 1.82

0.03 0.01 0.02 0.01 0.03 0.03 0.02 0.03 0.02 0.03 0.02 0.02 0.04 0.03 0.04 0.02 0.02 0.03 0.05 0.02 0.03 0.03 0.01 0.04 0.03 0.01 0.02 0.01 0.02 0.02 0.02 0.03 0.04 0.01 0.01 0.01 0.03 0.03 0.02 0.03 0.03 0.05 0.02 0.02 0.01 0.01 0.03 0.06 0.03 0.03 0.02 0.02 0.02 0.02 0.04 0.04 0.02 0.02 0.02 0.03 0.02 0.02 0.05 0.03 0.03 0.02 0.04 0.04 0.04 0.02 0.03 0.02 0.03 0.01

− 1.35 − 0.63 − 0.68 − 0.59 − 0.90 − 0.42 − 0.66 − 0.41 − 0.51 − 0.53 − 0.44 − 0.64 − 1.57 − 1.50 − 1.45 − 1.42 − 1.52 − 1.56 − 1.48 − 1.65 − 1.58 − 1.54 − 1.52 − 1.62 − 1.73 − 1.72 − 1.70 − 1.71 − 1.71 − 1.63 − 1.67 − 1.68 − 1.72 − 1.69 − 1.72 − 1.59 − 1.70 − 1.69 − 1.67 − 1.64 − 1.64 − 1.70 − 1.68 − 1.74 − 1.76 − 1.66 − 1.56 − 1.71 − 4.13 − 2.85 − 3.47 − 3.35 − 2.95 − 3.57 − 3.64 − 3.27 − 2.87 − 2.01 − 2.62 − 3.61 − 3.28 − 3.74 − 4.02 − 3.78 − 3.88 − 3.65 − 3.80 − 3.63 − 3.93 − 3.35 − 3.14 − 3.63 − 3.66 − 3.46

0.04 0.02 0.03 0.02 0.04 0.05 0.04 0.05 0.01 0.03 0.03 0.05 0.06 0.09 0.05 0.04 0.04 0.06 0.10 0.05 0.03 0.06 0.02 0.05 0.06 0.02 0.04 0.02 0.03 0.05 0.04 0.06 0.04 0.02 0.02 0.02 0.04 0.01 0.04 0.05 0.05 0.07 0.04 0.03 0.01 0.05 0.04 0.08 0.04 0.05 0.04 0.04 0.03 0.02 0.06 0.06 0.03 0.02 0.04 0.04 0.02 0.02 0.04 0.04 0.07 0.08 0.09 0.03 0.06 0.04 0.05 0.04 0.03 0.00

− 0.01 − 0.01 − 0.02 − 0.02 − 0.02 − 0.02 − 0.02 − 0.01 − 0.01 0.00 − 0.01 − 0.02 − 0.01 − 0.02 − 0.01 − 0.01 − 0.01 0.00 0.00 0.00 0.00 0.00 − 0.01 0.00 0.00 − 0.01 − 0.01 − 0.01 0.00 0.00 0.00 0.00 − 0.01 − 0.01 0.01 − 0.01 0.00 0.00 − 0.01 − 0.01 − 0.01 − 0.01 0.00 0.00 − 0.01 − 0.01 0.00 0.00 0.00 0.00 0.01 − 0.02 0.01 0.00 0.01 0.04 0.00 − 0.02 − 0.01 − 0.01 − 0.02 0.00 0.03 0.01 0.01 0.01 0.01 0.00 0.00 0.02 0.00 − 0.02 − 0.02 − 0.02

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Table 2 (continued) Sample number U U U U U U U U U U U U U U U U U U U U

IV – 9 IV - 10 IV - 11 IV - 12 IV - 13 IV - 14 VII - 1 VII - 2 VII - 3 VII - 4 VII - 5 VII - 6 VII - 7 VII - 8 VII - 9 VII - 10 VII - 11 VII - 12 VII - 13 VII - 14

Sample material

δ25 Mg [‰ DSM3]

± 2σ

δ26 Mg [‰ DSM3]

±2σ

Δ25 Mg′ [‰ DSM3]

Calcite spring 09 Calcite summer 09 Calcite autumn 09 Calcite winter 09/10 Calcite spring 10 Calcite summer 10 Calcite spring 06 - spring 07 Calcite summer 07 Calcite autumn 07 Calcite winter 07/08 Calcite spring 08 Calcite summer 08 Calcite autumn 08 Calcite winter 08/09 Calcite spring 09 Calcite summer 09 Calcite autumn 09 Calcite winter 09/10 Calcite spring 10 Calcite summer 10

− 1.97 − 1.98 − 2.00 − 2.00 − 1.91 − 1.67 − 2.14 − 1.53 − 1.84 − 2.00 − 2.06 − 2.02 − 1.93 − 2.08 − 1.97 − 1.99 − 1.96 − 1.53 − 1.55 − 2.01

0.02 0.02 0.03 0.03 0.04 0.01 0.01 0.02 0.01 0.06 0.02 0.03 0.03 0.02 0.03 0.03 0.04 0.03 0.03 0.03

− 3.76 − 3.76 − 3.80 − 3.83 − 3.65 − 3.19 − 4.12 − 2.94 − 3.54 − 3.84 − 3.94 − 3.86 − 3.69 − 3.98 − 3.78 − 3.80 − 3.73 − 2.89 − 2.87 − 3.86

0.04 0.05 0.04 0.05 0.07 0.04 0.03 0.04 0.04 0.08 0.05 0.08 0.04 0.04 0.06 0.06 0.04 0.08 0.06 0.04

− 0.01 − 0.02 − 0.02 − 0.01 − 0.01 − 0.01 0.01 0.00 0.01 0.01 0.00 − 0.01 0.00 0.00 0.00 − 0.01 − 0.02 − 0.02 − 0.06 0.00

from watch glass precipitates are equally influenced by the drip rate. These data document that for Bunker Cave equilibrium conditions are the exception rather than the rule and disequilibrium precipitation controls calcite isotope signatures (Riechelmann, 2010). Based on the relations shown above, drip site TS 8 / U IV is characterized by the most pronounced disequilibrium conditions – as revealed by high δ 13Ccalcite values – and the slowest drip rate (Table 1). Conversely, the fast drip site TS 1/U I is least controlled by disequilibrium and may even reach equilibrium conditions for δ13Ccalcite (Table 1). A direct comparison of δ 18Orain water with δ 18Odrip water of TS 1 suggests a lag time of about six months in the seasonal cycle of both O isotopic signals. This implies that summer drip water represents winter rainfall and vice versa, a feature also seen in the soil water data. However, the seasonal variability of the δ18Odrip water is strongly attenuated, which implicates a mixing reservoir in the karst aquifer above the cave. Since none of the drip sites show direct response to precipitation events, the residence time of the water in the karst aquifer above the cave is expectedly long (Riechelmann et al., 2011).

Concerning drip water cations, all four drips sites show nearidentical patterns. The dominant cation is Ca 2+ followed by Na + and then Mg 2+. Correlations of the Mg/Ca ratio with the Sr/Ca ratio and anti-correlations of the Mg/Ca ratio with Ca 2+ as well as anticorrelations of the Mg/Ca ratio with drip rate for drip sites TS 2, TS 3 and TS 8 show evidence for prior calcite precipitation (PCP), i.e. precipitation of calcite in the karst system prior to reaching the stalagmite tip or watch glass (Riechelmann et al., 2011). The pattern of the anions allows dividing the drip sites in two groups. Group one (TS 1) has elevated NO3− and lower SO42− values compared to group two (TS 2, TS 3 and TS 8) with lower NO3− and higher SO42− values. This is probably due to oxidation of pyrite in the karst, leading to high SO42− concentrations and reduction of NO3−. Changes in SO42− and NO3− concentrations between chamber 1 (TS 1; Fig. 2) and chamber 2 (TS 2, TS 3 and TS 8; Fig.2) represent a proxy for water residence time. Specifically, the water residence time is longer for the drip sites in chamber 2 relative to drip sites in chamber 1 (Riechelmann et al., 2011).

3.1.5. Soil and drip water chemistry The soil and drip waters were analysed for their cation and anion concentration at Heidelberg University. A VISTA MXP ICP-OES (Varian) were used to determine the cations (internal 1σ-standard deviation is b1% for Ca 2+, Mg2+ and Sr2+). The standard NIST 1643e with a longterm 1σ-reproducibility of 1 mg/l for Ca 2+, 0.4 mg/l for Mg2+ and 6 μg/l for Sr2+ was used. The anion concentrations were measured with a DX 1120 ion chromatograph (DIONEX) with SPS-NUTR WW1 as standard (long-term 1σ-reproducibility: 0.08 mg/l for NO3− and 0.6 mg/l for SO42−; Riechelmann et al., 2011).

3.2. Tritium age dating of water samples

Table 3 Mean Mg-isotope values, minimum and maximum for water and calcite samples. Number of samples used for statistics varies between 4 and 14. Sample material

Mean δ26 Mg [‰ DSM3]

± 2σ

Min δ26 Mg [‰ DSM3]

Max δ26 Mg [‰ DSM3]

Rain water (RW) Rain water with snow (RW) Soil water (BW 2) Drip water TS 1 Drip water TS 2 Drip water TS 3 Drip water TS 8 Calcite U I Calcite U II Calcite U VII Calcite U IV

− 0.70 − 0.83

0.14 0.31

− 0.90 ± 0.04 − 1.35 ± 0.04

− 0.59 ± 0.02 − 0.59 ± 0.02

− 0.51 − 1.53 − 1.69 − 1.68 − 1.68 − 3.20 − 3.81 − 3.63 − 3.61

0.10 0.07 0.03 0.04 0.06 0.54 0.15 0.42 0.24

− 0.66 ± 0.04 − 1.65 ± 0.05 − 1.73 ± 0.06 − 1.72 ± 0.04 − 1.76 ± 0.01 − 4.13 ± 0.04 − 4.02 ± 0.04 − 4.12 ± 0.03 − 3.93 ± 0.06

− 0.41 ± 0.05 − 1.42 ± 0.04 − 1.63 ± 0.05 − 1.59 ± 0.02 − 1.64 ± 0.05 − 2.01 ± 0.02 − 3.65 ± 0.08 − 2.87 ± 0.06 − 3.14 ± 0.05

The water residence time in the epikarst was determined using the tritium dating method that is based on the concentration difference between infiltrating water and drip water caused by radioactive decay (Kluge et al., 2010). Prerequisite for the application of this method is a constant oxygen isotope drip water ratio over multi-year monitoring periods indicating that water residence times exceeds one year. This pre-condition is essentially given for the Bunker Cave drip sites and water for tritium dating was sampled at drip sites TS 1, 4, 5 and TS 7 from 2007 to 2009 and additionally at drip site TS 2 during March 2011. Tritium was analyzed by decay counting (Weiss et al., 1976; Grothe, 1992) in Heidelberg. Results for TS 1, 4, 5 and 7 are 2.6 ± 0.8 a, 2.6 ± 0.5 a, 2.3 ± 0.5 a and 2.3 ± 0.6 a (Kluge et al., 2010). The water residence time at TS 2 depends on the exact tritium value of the infiltrating water. Assuming a value of 10 TU (equivalent to 2006 and 2007 concentrations) for the time period of 2008 to 2010 a theoretical water residence time of 4.8 ± 0.9 a was obtained. These values, however, include unknown admixtures of old aquifer water and young rain water and represent a mixing age. 3.3. Magnesium isotope analysis The Magnesium isotope composition of watch glass calcites as well as rain, soil and drip water samples was analyzed in the laboratories

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Fig. 3. Overview of Mg-isotope ratios of rain water, snow, soil carbonate, soil water, Devonian hostrock, drip water from sites TS 1, TS 2, TS 3 and TS 8, field experimental calcite precipitates U I, U II, U IV and U VII (watch glass precipitates) and Holocene stalagmite BU 4. Mean value is indicated by thin vertical line, width of horizontal bars indicate data range. Asterisk (*) indicates data from Immenhauser et al. (2010).

of the Ruhr-University Bochum, Germany. As the amount of material required for δ26 Mg analysis depends on the Mg content of the calcite analyzed, the mass of recovered watch glass calcite was determined first.

A minimum of 5 mg was required. Samples were dissolved in 6 M HCl, evaporated to dryness and re-dissolved applying 250 μl of 1:1 mixture of HNO3 and H2O2 (65% : 31%). This solution was evaporated and the sample was re-dissolved in 1.25 M HCl. The Mg fraction was extracted using the ion-exchange resin BioRad AG50 W-X12 (200 to 400 mesh). The Mg fraction was evaporated to dryness and a 500 ppb Mg-solution (in 3.5% HNO3) was prepared, which was analyzed on a Thermo Fisher Scientific Neptune MC-ICP-MS. Water samples (3–18 ml) were directly evaporated to dryness and then re-dissolved in the 1:1 mixture of HNO3 and H2O2 and then treated according to the same protocol as described above. All Mg delta values are given with respect to the isotopic composition of DSM3. The external precision (without chemical sample separation) was determined by repeatedly measuring the mono-elemental solution Cambridge1 (n = 92, δ 25 Mg = − 1.34 ± 0.01‰ and δ 26 Mg = − 2.58 ± 0.03‰; August 2010 to July 2011). The limited amount of sample material available allowed for the determination of the external precision for five repeat samples (n = 2, δ 25 Mg = − 1.62 ± 0.06‰ and δ26 Mg = −3.14 ± 0.19‰; δ25 Mg= −2.03 ± 0.09‰ and δ26 Mg = −3.91 ± 0.14‰; δ25 Mg = −0.81 ± 0.06‰ and δ26 Mg = − 1.54± 0.08‰; δ25 Mg = −0.73± 0.06‰ and δ26 Mg= −1.39 ±0.12‰; δ25 Mg = −0.91 ± 0.07‰ and δ 26 Mg= −1.74 ± 0.11‰). For analytical details please refer to Immenhauser et al. (2010). 4. Description and interpretation of δ 26Mg analytical results 4.1. Rain water 4.1.1. Description Snow has the lowest δ 26 Mg signature of − 1.35 ± 0.04‰, while rain water has values ranging between −0.59 ± 0.02‰ and − 0.90 ± 0.04‰ (mean: − 0.70 ± 0.14‰; Tables 2 and 3; Fig. 3). The mean value of rain water and snow is −0.83± 0.31‰ (Table 3), i.e., a value that is more depleted than that of δ 26Mgsoil water but 26 Mgenriched relative to δ 26Mgsoil (Table 3 and Fig. 3). Relative to δ26Mgdrip water the isotope ratio of rain water is higher (Fig. 3).

Fig. 4. Magnesium-isotope ratios of soil and drip water collected during the monitored interval in 2009 and 2010. A.) Soil water BW 2 (green) and drip water TS 1 (shown in blue with indication of error bar). Error of multiple soil water samples is given by vertical extension of green bars. Green line indicates measured value over monthly sampling period. B.) Drip waters TS 2, TS 3 and TS 8. Note colour code. Measured value is indicated by horizontal line, vertical extension indicates range of error.

4.1.2. Interpretation The δ26Mgrain water from the Bunker Cave region is slightly enriched compared to for example published δ26Mgrain water data from Santa Cruz (California; δ26Mgrain water: −0.79±0.05‰). Rain water data from Santa Cruz are nearly identical to those of seawater δ26 Mg (−0.82±0.06‰, Tipper et al., 2010; −0.80 ±0.04‰ to −0.86

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Fig. 5. Upper graph indicating monitored precipitation, potential water excess/deficit and temperature above Bunker Cave between January 2009 and June 2010. Lower graph displays drip water δ26Mg from drip site TS 1 (black diamonds with indication of error bars) placed against soil water δ26Mg (shaded gray). Note correlation between drip and soil water values. Lag time between rain fall events and the arrival of drip water in the cave is at least 6 months.

Fig. 6. Magnesium ion concentrations of the two soil waters BW 1 and BW 2 and of drip water TS 1 as well as the drip rate of drip site TS 1 during the monitoring period from 2006 to 2010. Note high Mg2+ concentrations of the soil waters after dry phases (no soil water) and the correlation between drip rate of TS 1 with Mg2+ concentration of TS 1.

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±0.12‰, Young and Galy, 2004; -0.83 ±0.06‰, Teng et al., 2010). The main difference between these two settings is the distance of the water sampling site from the coast. While Santa Cruz is directly located at the Pacific coast (Tipper et al., 2010), Bunker Cave lies around 250 km from the North Sea. Considering the limited data set available, a first-order result is that rain water is increasingly enriched in 26 Mg with increasing distance to the marine aerosol source (Fig. 9). Locally different dust patterns and aerosols may have an additional effect on the δ26Mgrain water composition (Fig. 9). Sources of dust and aerosols might include local industry, such as cement works or land and air traffic and heating-related burning of hydrocarbons. Furthermore, depending on the prevailing wind system, the Saharan desert regularly exports airborne dust to western Germany (Stuut et al., 2009). 4.2. Soil water 4.2.1. Description Soil water (sample BW 2) is characterized by δ 26 Mg values between −0.66 (±0.04)‰ and − 0.41 (± 0.05)‰ with a mean value of − 0.51 ± 0.1‰ (Tables 2, 3 and Fig. 3). Soil water values are isotopically higher than those of carbonate in the soil (− 1.03 ± 0.03‰ to − 0.88 ± 0.02‰; Immenhauser et al., 2010). Moreover, soil water samples BW 2 are enriched in 26 Mg relative to values reported in Immenhauser et al. (2010; -1.36 ± 0.02‰ to − 1.06 ± 0.02‰). This probably exemplifies the significance of isotopic soil water depth gradients. Magnesium isotope values reported here are from water samples collected at a depth of 40 cm. In contrast, soil water data reported in Immenhauser et al. (2010) represented epikarst water samples collected at a depth of 70 cm at the soil/hostrock interface. When comparing soil water values with those collected at drip sites, soil water is enriched in 26 Mg relative to drip waters (Fig. 3). As depicted in Fig. 4A, soil water δ 26 Mg-values display a cyclical pattern over the period of eight sampling months (November 2009 to June 2010). Specifically, high values characterize the months November, February and May, intermediate values are found during March and April and low values typify water samples in January and June. 4.2.2. Interpretation Rain water falling on the land surface tends to run off downhill whilst infiltrating slowly into the soil (Hölting, 1996). Due to the gently inclined slope topography (10-14°) above the Bunker Cave, rain water entering the aquifer above the cave has fallen further uphill, runs downhill and slowly infiltrates to soil depths of about 40 cm. In the soil, rain water interacts with organic matter (Tipper et al., 2010), Mg-bearing clay minerals and carbonate (Immenhauser et al., 2010). Given the slow infiltration rate, changes in δ26Mgsoil water probably reflect subtle differences in carbonate versus silicate weathering rates that take place over time periods of several months. Based on oxygen-isotope evidence, δ18Osoil water sampled during winter months represents δ18Orain water falling during the summer months and vice versa (Riechelmann, 2010). With respect to the δ 26Mgsoil water values analyzed, this equally implies at least six (or perhaps 18) months lag time between rainfall event and sampling respectively (cf. Figs. 4 and 5). Assuming a lag time of about six months, the elevated δ 26Mgsoil water value of e.g., the soil water sample BW 2 collected in November 2009 (Fig. 5) thus corresponds to relatively dry and warm climatic conditions during May or June 2009 (slightly increasing silicate weathering). This is supported by the initially high Mg2+ concentrations at different soil water levels (BW 1 and BW 2; Fig. 6) immediately following first rainfall events after dry, low infiltration phases (Fig. 6). During dry time intervals, more Mg was dissolved from clay minerals in the soil zone due to the longer residence time of the water and enhanced soilwater interaction. Increased rain water infiltration rates during

subsequent more humid periods transport dissolved Mg downward into the aquifer. Increased rainfall amounts also result in shorter residence times, reduced soil-water interaction and lower amounts of Mg are leached from the soil zone. As a consequence, decreasing Mg 2+ concentration patterns are found in soil waters (Fig. 6). The November 2009 Mg 2+ concentration in soil water, following a dry summer / early fall period, reaches peak values. Elevated Mg 2+ concentrations and 26 Mg-enriched November 2009 soil water values are in good agreement with the concept of enhanced silicate/clay weathering under more arid conditions. Conversely, lower Mg 2+ concentrations and more depleted δ 26Mgsoil water ratios (e.g., BW 2 from June 2010; Figs. 5 and 6) reflect comparably wetter and cooler conditions (Fig. 5) and point to reduced soil-water interaction and reduced Mg-leaching of chlorite, smectite and illite. It must be emphasized that changes in the silicate versus carbonate weathering ratio are subtle and complex due to the interaction of climatic, biological, chemical and mineralogical parameters (Brenot et al., 2008). In this sense, the interpretation brought forward here is a simplified conceptual model only, but one that is in good agreement with monitoring data shown here and δ 26 Mg

Fig. 7. Conceptual overview of Magnesium isotope fractionation patterns from rain to drip water in the Bunker Cave. Calculated drip water age and mixing ratio of young soil water and old aquifer water is given. Lag time between rainfall event and the arrival of drip water in the cave is either 6 or 18 months, as winter water reaches the cave during summer months and vice versa. A lag time of more than 18 months, i.e., 30 months or 36 months is unlikely. Asterisk (*) indicates data from Immenhauser et al. (2010).

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isotope time series reported in Buhl et al. (2007) and Immenhauser et al. (2010). 4.3. Drip water 4.3.1. Description Drip water δ 26Mg values from four monitored sites (Fig. 2) are shown in Tables 2 and 3 and in Figs. 3 and 4. Drip water from drip site TS 1 is enriched in 26Mg compared to water from the other three drip sites, which all have very comparable Mg-isotopic compositions. Drip site TS 1 reveals the largest variability in δ 26Mg over the monitoring period (Figs. 3 and 4A). It is significant, however, that all drip sites show variations in δ 26Mgdrip water over time (Fig. 4). Drip water isotope ratios from drip site TS 1 follow the annual pattern of δ 26Mgsoil water (BW 2; correlation factor r = 0.98, p = 0.0004, n = 6; Fig. 4A). This observation is considered significant and the offset between drip water and soil water is remarkably stable (1 to 1.04‰; mean: 1.02 ± 0.02‰). The other three drip sites (TS 2, TS 3 and TS 8) conspicuously lack a correlation with annual soil water isotope trends as shown in Fig. 4.

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shorter residence time for drip water at drip site TS 1 and longer residence times for drip water at drip sites TS 2, TS 3 and TS 8 is equally supported by the water chemistry (SO42− and NO3− concentrations). This leads to the conclusion that drip sites in chamber 1 (TS 1; Fig. 2) are fed by water with an overall shorter residence time (s. 3.1.5; Riechelmann et al., 2011) relative to the other cave chamber. Tritium-based ages further support this finding and propose a higher residence time for TS 2 compared to TS 1. Based on these relations, a simple mass balance exercise was undertaken in order to assess the proportion of young soil and old aquifer water for drip site TS 1. The underlying concept is based on the discharge model of Riechelmann (2010) and tested using Mg isotope values reported here. The following equation applies:   26 Fold water  δ Mgaquifer water  caquifer water þ Fyoung water   26  δ Mgsoil water  csoil water   26 ¼ 1  δ Mgdrip water TS 1  cdrip water TS 1

ð5Þ

with 4.3.2. Interpretation Drip water δ 26Mg-values reflect an admixture of the soil water and hostrock isotope values. Soil water, which percolates through the hostrock, is undersaturated and dissolves the limestone. In the hostrock, young downward percolating water mixes with older aquifer water. From this a series of basic rules can be defined and tested with the data set available: (1) Depending on the mixing ratio of young soil water with older aquifer water, drip water predominantly composed of young soil water is expected to be enriched in 26Mg due to the leaching of clay minerals with high δ 26Mg. (2) Conversely, drip water mainly composed of older aquifer water is expected to be depleted in 26Mg due to equilibration with low δ 26Mg limestone values. (3) Moreover, drip water dominated by young soil water follows annual trends in δ 26Mgsoil water, whereas (4) drip water dominated by older, equilibrated aquifer water is expected to deviate from the annual δ 26Mgsoil water pattern. These relationships are illustrated in Figs. 7 and 9 and represent the framework of our conceptual model. As illustrated in Figs. 3 and 4, δ 26Mgdrip water from site TS 1 is characterized by the least negative isotope values of all four drip sites, i.e., the ones that are closest to δ 26Mgsoil water values. Furthermore, annual δ 26Mgdrip water values of site TS 1 are well correlated with fluctuating annual δ 26Mgsoil water changes (Figs. 4 and 5). This implies that in the case of drip site TS 1, soil water percolates over the period of at least one month from a depth of 40 cm in the soil to the cave ceiling. Similar to soil waters drip water TS 1 shows high Mg2+ concentrations in the initial stage of more humid periods that follow dry intervals characterized by low drip rates. In the transition stage from low to high drip rates, higher concentrations of dissolved Mg from enhanced soil zone clay leaching reach the cave (Fig. 6). The correlation factor between the Mg 2+ concentration of drip water TS 1 and the drip rate of TS 1 is r = 0.80 (p= 9,8 * 10− 9, n = 34). Applying the rules formulated above, drip water at site TS 1 is characterized by (i) a short residence time, i.e., it represents younger water, and (ii) is dominated by soil water. This adds support to the hitherto conceptual discharge models of Tooth and Fairchild (2003) for the different drip sites in Bunker Cave (Riechelmann, 2010). According to this model, drip site TS 1 is mainly fed by fissure flow (younger water) and to a lesser degree by seepage flow (older water). Conversely, drip sites TS 2 and U II are characterized by seepage flow only. Drip site TS 3 is intermediate in nature and combines attributes of a seepage flow with these of a fissure flow (dominant). This also accounts for drip site TS 8, but in this case, fissure flow dominates until summer 2009. In 2010 the influence of seepage flow increased and the amount of fissure flow decreased. The concept of a

Fold water ¼ 1–Fyoung water ⇔Fyoung water ¼



ð6Þ

  cdrip water TS 1   26 – δ Mgaquifer water  caquifer water    26 = − δ Mgaquifer water  caquifer water   26 þ δ Mgsoil water  csoil water 26

δ Mgdrip water TS

1

ð7Þ

where Fold water is the fraction of aquifer water, Fyoung water is the fraction of soil water (both in %/100) and c is the Mg-concentration of the different waters. For the Mg-concentration of the aquifer water, beyond the reach of any sampling tool, the Mg-concentration of the most saturated drip water (4.5 ppm) is used. Since the aquifer water is a mixture of the δ 26 Mg of soil water and hostrock, the δ 26 Mg value of the hostrock will be used for aquifer water that has isotopically equilibrated with the hostrock over time (mean δ 26Mghostrock: − 3.72 ± 0.07‰; Immenhauser et al., 2010). The best fit of the data obtained was found at a mixing ratio of 74 ± 1% young soil water and 26 ± 1% old aquifer water (n = 6; Fig. 7).

Fig. 8. Magnesium-isotope ratios from field experimental watch glass experiments placed under monitored drip sites. Data represent monitoring period 2006–2010. Horizontal bar width represents duration of precipitation experiment (e.g., 3 months). Colour code relates data to specific drip sites. A series of experimental artefacts limits the interpretation of these data.

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Fig. 9. Cartoon summarizing δ26Mg fractionation pathways in and above the Bunker Cave. PCP refers to prior calcite precipitation. Asterisk (*) indicates data from Immenhauser et al. (2010).

Drip sites TS 2, TS 3 and TS 8 (Fig. 4) lack a correlation with soil water in terms of their δ 26 Mg ratios. It is assumed, that drip water at drip sites TS 2, TS 3 and TS 8 represents more than 26% old aquifer water and less than 74% young soil water (Fig. 7). Applying the tritium dating method, Kluge et al. (2010) derived a mean age of 2.6 ± 0.8 a for drip water at site TS 1 (2 samples taken in April 2007 and March 2008). Obviously, this age data represents an admixture of aquifer water being older than 2.6 years and soil water being younger than 2.6 years. The arguments brought forward here allow for a more confined age model. Accepting the correlation between δ 26Mgsoil water and δ 26Mgdrip water at site TS 1 and applying the six months offset as indicated by δ 18O values, the following equation results:    Taquifer ½a ¼ Tmixed water ½a– Fyoung water Tyoung water ½a =Fold water

ð8Þ

whereby T is the age of the water, Fyoung water is the fraction of younger soil water and Fold water is the fraction of older aquifer water. Oxygen-isotope data (Riechelmann et al., 2011) indicate a seasonal lag time between a given rainfall event and the arrival of this rain water as drip water in the Bunker Cave. Specifically, winter rainfall reaches the cave as drip water during summer months and vice versa. Oxygen-isotope evidence, however, does not specify if rain falling during the winter months of 2008 or rain falling during the winter months of the year 2009 reaches the cave during the summer months of the year 2010. As such an offset of 6 or 18 months must be considered. Assuming an age of six months for the soil water, a calculated age of approximately 9 years for the aquifer water results. Similarly, assuming an age of 18 months for soil water, a mean age of 6 years for the older aquifer water is computed (Fig. 7). The tritium dating

(4.8 ± 0.9 a) of the drip site TS 2 in March 2011 supports the abovementioned extended residence time for drip sites in chamber 2 characterized by a smaller fraction of young soil water and a larger fraction of old aquifer water. This is nearly twice as old as the age of the drip water at site TS 1 (chamber 1; Fig. 2). In conclusion, four different methods; (i) discharge models (Tooth and Fairchild, 2003; Riechelmann, 2010), (ii) drip water chemistry (Riechelmann et al., 2011), (iii) tritium age dating (Kluge et al., 2010) and (iv) Mg-isotope ratios converge to the same outcome and support that the above considerations are of value. As the balance between silicate and carbonate weathering in the soil and hostrock domain depends among other factors on air temperature and rainfall amount above the cave, correlation factors between δ 26Mgdrip water and outside air temperature, rainfall amount as well as soil water infiltration rate are potentially useful. Air temperature is significant because rates of biological and physico-chemical processes in the soil zone decrease with decreasing temperatures (i.e., during winter months), influencing the CO2 production in the soil zone and therefore dissolution rates of the carbonate hostrock. Correlation factors also aid in assessing the debated six versus 18 month offset between rainfall event and drip water entering the cave. As a test, correlation factors were calculated assuming zero lag time between rainfall and arrival of drip water in the cave. The outcome is a strong negatively correlation (r= −0.87, p = 0.0004, n = 12) and not the expected positive correlation between δ 26Mgdrip water at site TS 1 and outside air temperature. Calculated p-values for other correlations (e.g., rainfall amount, infiltration rate, drip water temperature) are insignificant. As a consequence, the offset between rainfall event and drip water arrival time is poorly constrained and might range from six months to several years. The above considerations exemplify the complexity of factors affecting different drip sites in cave settings (Fig. 9).

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4.4. Watch glass calcite precipitates 4.4.1. Description The δ 26 Mg ratios of recent calcite precipitates on watch glasses are intermediate between those of drip water and those of speleothems. Magnesium isotope values of precipitates are often enriched relative to δ 26Mghostrock (−3.84 ± 0.05 to − 3.67 ± 0.05‰, mean: −3.72 ± 0.07‰; Immenhauser et al., 2010), but in part also similar or even depleted (Tables 2 and 3, Fig. 3). In comparison to a stalagmite (BU 4, -3.91 ± 0.05‰ to −4.34± 0.07‰, mean: −4.20 ± 0.10‰; n = 23) that grew under drip site TS 8, recent calcite precipitates on watch glasses have with only few exceptions a higher Mg-isotope ratio compared to those of stalagmite BU 4 (Fig. 3). Calcite from watch glass U I ranges from −4.13 (±0.04)‰ to −2.01 (±0.02)‰ with a mean of −3.20 (±0.54)‰ (Tables 2 and 3, Figs. 3 and 8). Similar to δ 26Mgdrip water from drip site TS 1, δ26Mgprecipitate from a series of watch glasses subsequently placed under drip site U I display significant variations over the monitored time period, whilst watch glass U II recorded only minor variations (Figs. 3 and 8). Calcite U VII shows - in comparison to calcite U IV - more variability in δ26 Mg over time (Figs. 3 and 8). Calcite U II reveals δ26 Mg ratios between −4.02 (±0.04) to −3.65 (±0.08)‰ with a mean value of −3.81 (±0.15)‰ (Tables 2 and 3). Overall, calcite from drip site U II shows the most depleted mean value and calcite U I the most enriched one. Calcite precipitates from watch glass U IV and calcite U VII are intermediate in δ 26Mgprecipitate (U IV: − 3.93 ± 0.06 to − 3.14 ± 0.05‰; mean: − 3.61 ± 0.24‰; U VII: − 4.12 ± 0.03 to − 2.87 ± 0.06‰, mean: − 3.63 ± 0.42‰; Tables 2 and 3). Fig. 7 summarizes the changing δ 26Mgprecipitate over time. Note, there is no clear correlation between δ 26Mgprecipitate values from different drip sites representing the same time interval. 4.4.2. Interpretation In contrast to laboratory experimental work reported in Immenhauser et al. (2010), δ 26Mgprecipitate values show no significant correlation with precipitation rate. Reasons may include considerably higher saturation indices (0.9–1.3) used to precipitate calcite within hours to a few days (50 to 630 μmol/h) in the laboratory. In contrast, drip water in the Bunker Cave, characterized by low saturation indices of 0.3 to 0.7 (Riechelmann et al., 2011) slowly precipitates calcite over periods of several months (0.045 to 0.342 μmol/h). The δ 26Mgprecipitate values of the watch glasses show greater variations (difference between minimum and maximum ranges from 0.79 to 2.12‰ for U I, U IV and UVII) than stalagmite BU 4 (difference between minimum and maximum is 0.43‰) with the exception of watch glass U II (difference between minimum and maximum is 0.37‰). Hence, watch glass precipitates are probably influenced by factors not present during the precipitation of calcite on top of a stalagmite. The watch glass U II is least affected by these artefacts. This may be due to longer precipitation times (≥3 months, ≤12 months; Fig. 8). 5. Discussion 5.1. Magnesium isotope values of rain water Considering the limited data set, the Mg isotope composition of snow is significant lower than that of rain. The difference between δ 26Mgsnow and δ 26Mgrain water exceeds 0.5‰. A temperature dependency of 0.02‰/AMU/per degree Celsius for Mg isotope fractionation during carbonate precipitation was reported in Galy et al. (2002). From this it seems possible that air temperature differences between summer (18.0 °C) and winter (2.6 °C) months drive at least in part, the observed differences in δ 26 Mg signatures with decreasing temperatures (Fig. 9), similar to the correlation found for the oxygen isotope system in this region (Riechelmann et al., 2011). The differences

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between depleted δ 26Mgsnow relative to that of enriched δ 26Mgrain 26 water and its possible impact on annual changes on δ Mgsoil water merits consideration. Summer drip water, representing winter melt water (snow), is indeed isotopically depleted (Table 2, Fig. 4A). Additionally, Mg-isotope composition is apparently enriched in 26 Mg with increasing distance from the marine aerosol source (Fig. 9). The farther the distance to the ocean, the lesser is the influence of marine aerosols on the δ 26Mgrain water. This pattern is perhaps of significance for the comparison of regionally different study areas and requires attention in future work. Additional factors that require attention include dust and aerosols from the local industry and hydrocarbon burning. This is particularly the case after dry periods. During low rainfall periods dust and aerosols accumulate in the atmosphere (Fig. 9) but quantitative data are lacking. Further work should include the compilation of a δ26Mgrain water timeseries data set spanning dry and wet seasons plotted against Saharan dust events in western Germany.

5.2. Magnesium isotope fractionation in the soil zone Previous studies aiming at unraveling the complex succession of processes active in the karst zone applied several indirect methods. Specifically, drip water chemistry, drip discharge and its relationship to rainfall, age dating of drip water (based on tritium data; Kaufman et al., 2003; Kluge et al., 2010) and 14C models for drip sites and karst (Baker and Brunsdon, 2003; Tooth and Fairchild, 2003; Fairchild et al., 2006; McDonald and Drysdale, 2007) were investigated. Furthermore, processes controlling limestone dissolution patterns and rates (Genty and Massault, 1999) were explored using laboratory and field experiments. The influence of mineral dissolution patterns and rates on δ 26Mg are not well constrained but a recent field and laboratory study from Santa Cruz, California reported that the soil water δ 26Mg ratio is likely an admixture of the Mg leached from mainly 26Mg-enriched smectite, and rain water (depleted in 26Mg; Tipper et al., 2010). For the Bunker Cave data set shown here (Fig. 3; Table 2) data from silicate minerals were not included due to the analytical protocol tailored for the analysis of carbonates. As a consequence, published clay mineral (smectite) δ26Mg ratios from Tipper et al. (2010) are used here, due to the fact that the main Mg bearing mineral in the case of Bunker Cave soil is montmorillonite (smectite). For the Bunker Cave rain water is found to be isotopically higher (−0.70±0.14‰) than soil zone carbonate (−0.93±0.09‰; Immenhauser et al., 2010) and lower than clay minerals (0.11±0.01‰; Tipper et al., 2010) and soil water (−0.51±0.1‰; Fig. 3). The soil carbonate itself is 26Mg-depleted, whilst soil water displays the highest values. Hence, the admixture of 26Mg-depleted rain water with leached 26Mg-enriched montmorillonite (smectite) results in the δ26Mgsoil water composition observed. Further on, biological fractionation of Mg by plants and microbial organisms in the soil zone must not be neglected. Previous work on the Mg-isotope signature of different terrestrial plants included English ivy (Black et al., 2007), wheat (Black et al., 2008), rye flour, sea lettuce, hairgrass, lichen (Bolou-Bi et al., 2009), rye grass, clover (Bolou-Bi et al., 2010) and spinach (Young and Galy, 2004). According to Tipper et al. (2010), the fractionation of Mg-isotopes by grass vegetation in Santa Cruz is ca. 0.21‰. Given that only a small amount of Mg is cycled by grass, the δ 26 Mg composition of soil water is largely unaffected. In contrast, the land surface above Bunker Cave is mainly vegetated by beech and ash trees. Magnesium is incorporated in the chlorophyll of the tree leaves (Porra et al., 1993; Black et al., 2007). The selective removal of isotopically light Mg from soil water via tree roots and the decomposition of leaves shed during the fall period may also influence the δ 26Mgsoil water (Fig. 9) when monitored over a year. Hence, it is very likely that for Bunker Cave the vegetation has a much greater influence on the δ 26 Mg-values of the soil water

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compared to the Santa Cruz study. Nevertheless, quantitative data on Mg recycling from plant debris are, at present, lacking. 5.3. Field precipitation experiments — potentials and pitfalls Whilst the field precipitation approach on watch glasses is well established for carbon and oxygen isotopes (Mickler et al., 2006; Genty, 2008; Riechelmann, 2010), the applicability of this method for isotopes of the element Mg is yet untested. Four main potential problems were identified in the context of field experiments performed in the context of this study. I. A potential problem is related to the fact that a thin water film covers the watch glasses calcite crystals in the cave and water occludes pore space within and between the crystals. When watch glasses are removed from the cave, this water film evaporates and dissolved, inorganic residual Mg not incorporated in the precipitated calcite will remain on the watch glass and affect the bulk isotope ratio. This is because dissolved Mg, in contrast to oxygen will not evaporate as gas phase. Despite the fact that the amount of DIC in the water film is small, drip water is enriched in 26 Mg and so is the calcite precipitating from the evaporating water. Given the very low amount of Mg (ca. 300 ppm) and the small amounts of calcite on an average watch glass, even a thin water film (containing 3–5 ppm of dissolved Mg) may affect the Mg isotope values to some degree. In essence, a calcite-to-water ratio of 1:2 is required to significantly (ca. 0.5‰ shift to heavier values) affect δ 26Mgprecipitate values. This ratio was not found in the context of this study and hence dissolved Mg in the water film has arguably a negligible affect on δ 26Mgprecipitate. II. Cave loam (mean δ 26Mg of carbonate in cave loam: -0.59 ± 0.03‰; Immenhauser et al., 2010), settles as dust on the watch glasses due to perturbation of the dust and loam on the cave floor by geologists and cavers. Small amounts of sinter carbonate precipitate on cave loam particles and occlude pore space in cave loam. Cave loam may be water-rich as it contains expandable clay minerals and these minerals are considerably 26 Mg-enriched relative to carbonate. Direct evidence is found in several cases. Watch glass sample U I, for example, has been accidentally pushed from its site several times by cavers and has fallen on the loamy cave floor. The δ 26Mg value of watch glass sample U I summer 2010 (−1.20 ± 0.02‰) is higher than that of the corresponding drip water (−1.53± 0.07‰). Conversely, the significance of clays transported by drip water from the soil zone are insignificant given that the host limestone is composed of 95.97% CaO (MgO: 1.12%, Al2O3 + Fe2O3: 1.07%, SiO2: 1.84%; von Kamp, 1972). III. Calcite on watch glasses has to overcome nucleation energy (heterogeneous nucleation; Nancollas and Reddy, 1971) and – as recognized under the SEM – is characterized by growth disorders. In contrast, most stalagmites grow continuously and under rather stable conditions as calcite is precipitated on a carbonate substratum. Stalagmites from Bunker Cave show homogenous columnar and dendritic calcite fabric (Riechelmann, 2010). In contrast, crystal faces on watch glasses include lateral crystal faces growing sideward on the loosely populated watch glass surface. In speleothems, lateral crystal faces are confined in a dense fabric. Carbon and oxygen isotope fractionation between fluid and solid is known to respond to differential growth of crystal faces (Dickson, 1991) but similar research dealing with Mg isotopes is lacking. IV. The possibility of in partly hydrous precursor phases on watch glasses such as amorphous calcium carbonates (ACC; Loste et al., 2003) or vaterite (Spanos and Koutsoukos, 1998) transforming into thermodynamically stable calcite within hours

cannot be excluded. Previous work has shown that glass substrates may actually induce nucleation of vaterite. In this case, quantum mechanic theory may predict a fractionation effect on δ 26Mg. In essence, heavier isotopes are enriched in the species in which they are stronger bounded (Mavromatis et al., 2012). For cave environments, this implies that the δ 26Mg values of drip waters, containing Mg in the form of aquocomplexes (Rustad et al., 2010) may be enriched whilst those of the instable precursor calcites are depleted. In essence, this basic consideration might explain - at least partially - the considerable fractionation between drip water (− 1.65 ± 0.08‰) and speleothem calcite (−4.20 ± 0.10‰). In nature, however, quantum mechanical effects are frequently overwhelmed by disequilibrium conditions. In summary, conventional field precipitation experiments are of limited relevance for Mg isotope research. Nevertheless, given the considerable interest of the cave archive community in field precipitation experiments and the increasing number of laboratory precipitation experiments, the lessons learnt here are considered highly meaningful. 5.4. Relevance of monitoring data for the interpretation of δ26Mg time series data from speleothems Published time series records of Mg isotopes from speleothems are rare. Data include the work of Galy et al. (2002) from the Soreq Cave in Israel and caves in France and data shown in Buhl et al. (2007) from a speleothem from the Grotte d'Aoufous in Morocco. Whilst Galy et al. (2002) reported rather invariant time-series data from several dolomite caves, the Moroccan case example displays a cyclical co-variance of speleothem δ 26Mg with δ 13C and δ 18O as well as Sr and Mg elemental abundances (Buhl et al., 2007). As discussed above, three or maybe four main parameters control Mg isotope fractionation in drip water and modern calcite precipitates: (i) the differential input of isotopically heavy Mg from clay minerals and 26Mg-depleted limestones, (ii) disequilibrium factors, and (iii) the prior calcite precipitation and/or dissolution in the hostrock zone and (iv) possibly also biological activity in the soil zone (Fig. 9). Bunker Cave monitoring data (Riechelmann, 2010) compared with carbon and oxygen isotope time series data from the Holocene speleothem BU 4 reveals that stalagmites of this cave lack an annual resolution of climate change. The patterns recognized so far are at least decadal or higher in duration. This raises the question of the applicability of mainly annual changes in monitoring data to decadal and longer time-series data sets. Nevertheless, the interpretation of long-term δ 26Mg cycles as shown for example in Buhl et al. (2007) is not feasible without an improved understanding of seasonal factors driving speleothem δ 26Mg. Bridging the gap between seasonal and decadal patterns is a challenging task for future work. 6. Conclusions (1) Considering the limited data set, the Mg isotope ratio of rain water is – with decreasing influence of marine aerosols – increasingly 26Mg-enriched. Magnesium-isotope ratios from snow melt water are at least 0.5‰ lower than those of rain water from the same area. This effect may in part be explained by temperature-related fractionation patterns. These patterns are significant but require the compilation of time-series rain and snow data. (2) The Magnesium-isotope signature of soil water is controlled by δ 26Mgsoil and specifically by differential leaching rates of Mg-bearing chlorite, montmorrilonite and illite. Furthermore, biological Mg fractionation by plants and soil micro-

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organisms results in the release of 26Mg-depleted values during organic matter decomposition of e.g., leaves shed during the fall period and may also have an additional effect on δ 26Mgsoil water. (3) The interpretation of calcite δ 26Mg data from watch glass field precipitation experiments is hampered by several factors. Specifically, δ 26Mgprecipitate is isotopically higher than δ 26Mg ratios of naturally precipitated speleothems in Bunker Cave. Problems include cave loam and water-rich clay minerals, crystallographic forcing of δ 26Mg fractionation during calcite precipitation on a silica substratum (watch glass) and the potential of metastable precursor phases. (4) The considerable fractionation between fluid (drip water) and solid (calcite precipitate) is perhaps best explained by quantum mechanical laws with specific reference to the dehydratisation of Mg aquo-complexes, overprinted by disequilibrium effects. Given the non-conclusive results from field precipitation experiments, particular processes forcing disequilibrium such as drip rate or calcite precipitation rate await further investigation. (5) Perhaps the most significant outcome of this study is the recognition of very clear correlation patterns between seasonal soil water δ26Mg trends and δ 26Mg ratios of drip water from specific drip sites in the Bunker Cave. This relation sheds light on residence time and mixing patterns of young soil and old aquifer water reservoirs and represents a novel tool for the interpretation of poorly understood processes in the karst aquifer. Further on, by comparing time-series δ 26Mg patterns from different drip sites with drip properties and soil water values, the characteristics (seasonal versus seepage flow) of specific drip sites are better constrained. From this an improved understanding of equilibrium versus disequilibrium processes affecting speleothem geochemistry results. Acknowledgements We thank the Speleogroup Letmathe for supporting monitoring in the Bunker Cave. The student assistants are thanked for their help in monitoring. The Bochum lab team is thanked for technical assistance in the non-traditional isotope laboratory and T. Reinecke is thanked for XRD analysis of the soil. Silvia Frisia is thanked for valuable input regarding disequilibrium in cave precipitation environments. This is a contribution to the collaborative research initiative DAPHNE II, founded by the DFG. We greatly acknowledge the work of associate editor Dr. U. Brand and the very constructive comments by the reviewers Dr. E. Tipper and Dr. M. Norman. References Baker, A., Brunsdon, C., 2003. Non-linearities in drip water hydrology: an example from Stump Cross Caverns, Yorkshire. Journal of Hydrology 277, 151–163. Baker, A., Barnes, W.L., Smart, P.L., 1997. Variations in the discharge and organic matter content of stalagmite drip waters in Lower Cave, Bristol. Hydrological Processes 11, 1541–1555. Banner, J.L., Guilfoyle, A., James, E.W., Stern, L.A., Musgrove, M., 2007. Seasonal variations in modern speleothem calcite growth in central Texas, U.S.A. Journal of Sedimentary Research 77, 615–622. Bar-Matthews, M., Ayalon, A., Gilmour, M., Hawkesworth, C.J., 2003. Sea-land isotopic relationships from planktonic foraminifera and speleothems in the Eastern Mediterranean region and their implications for paleorainfall during interglacial intervals. Geochimica et Cosmochimica Acta 67, 3181–3199. Black, J.R., Yin, Q.-Z., Rustad, J.R., Casey, W.H., 2007. Magnesium isotopic equilibrium in chlorophylls. Journal of the American Chemical Society 129, 8690–8691. Black, J.R., Epstein, E., Rains, W.D., Yin, Q.-Z., Casey, W.H., 2008. Magnesium-isotope fractionation during plant growth. Environmental Science and Technology 42, 7831–7836. Boch, R., Spötl, C., Frisia, S., 2011. Origin and paleoenvironmental significance of lamination in stalagmites from Katerloch Cave, Austria. Sedimentology 58, 508–531. Bolou-Bi, E.B., Poszwa, A., Leyval, C., Vigier, N., 2010. Experimental determination of magnesium isotope fractionation during higher plant growth. Geochimica et Cosmochimica Acta 74, 2523–2537.

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