A radiocarbon dated bat guano deposit from N.W. Romania: Implications for the timing of the Little Ice Age and Medieval Climate Anomaly

Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227

Contents lists available at ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p a l a e o

A radiocarbon dated bat guano deposit from N.W. Romania: Implications for the timing of the Little Ice Age and Medieval Climate Anomaly Vanessa E. Johnston a,⁎, Frank McDermott a, Tudor Tămaş b,c a b c

UCD School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland Faculty of Biology and Geology, Department of Geology, “Babeş-Bolyai” University, Cluj-Napoca, Romania “Emil Racoviţă” Institute of Speleology, Cluj-Napoca, Romania

a r t i c l e

i n f o

Article history: Received 15 October 2009 Received in revised form 20 February 2010 Accepted 24 February 2010 Available online 2 March 2010 Keywords: Bat guano Cosmogenic isotope Radiocarbon Little Ice Age Cl-36 Bomb pulse

a b s t r a c t There is considerable interest in the potential of bat guano as an alternative record of palaeoclimate in regions that are devoid of more commonly utilised archives. In this study, designed originally to evaluate the potential of cave hosted bat guano to preserve temporal variations in the flux of cosmogenic 36Cl, it was found that the guano depositional history is strongly linked to climatic conditions. Radiocarbon measurements on a 2.7 metre long core of bat guano from Măgurici Cave, N.W. Romania indicate a maximum depositional age of 1195 AD for the base of the core. Deposition of the lowermost portion of the accumulation occurred during the Medieval Climate Anomaly. The cave roost was subsequently devoid of bats during a regional cold phase linked to the Little Ice Age, with bats returning when local temperatures increased. The rate of guano accumulation then appears to increase in tandem with anthropogenic warming. This indicates that bat occupation at this roost site in Măgurici Cave is strongly linked to regional climate variability, with habitation during warm periods, possibly associated with the abundance of insects upon which the bats feed. Comparison of large peaks in anthropogenic 14C and 36Cl production associated with nuclear weapons testing indicates downward migration of 36Cl, probably reflecting post-depositional migration within the guano deposit. Elevated 36Cl/Cl at the top of the core in comparison with modern atmospheric values may indicate recycling of bomb 36Cl in vegetation. Therefore, we show that while bat guano contains abundant atmospherically-derived chloride it has severe limitations as a potential archive of atmospherically-derived 36 Cl (a solar proxy), because of post-depositional mobility. However, separation of organically bound chloride, or the use of an alternative cosmogenic isotope 10Be, in bat guano, may offer an unexploited solar proxy that contains contemporaneous environmental signals, such as stable isotopes (e.g. δ13C) and pollen, in association with radiocarbon dating. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The under-exploited archive of bat guano has recently been shown to provide high resolution palaeoclimate data (e.g. Leroy and Simms, 2006; Bird et al., 2007; Wurster et al., 2008). In an attempt to understand solar forcing of the climate, we test, here for the first time, the suitability of bat guano as an archive of atmospheric 36Cl. In principle, this solar irradiance proxy (Bard et al., 2000; Beer et al., 2002; Muscheler et al., 2007) could be used in conjunction with stable isotope data from the guano to circumvent correlation uncertainties associated with deriving solar and climate proxy records from different archives. 36Cl is strongly hydrophilic and is therefore removed rapidly from the atmosphere by wet precipitation. This behaviour contrasts with 14C which becomes incorporated in biogeochemical

⁎ Corresponding author. Tel.: +353 1 7162138; fax: +353 1 2837733. E-mail address: [email protected] (V.E. Johnston). 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.02.031

cycles. 36Cl is thought to enter bat guano through nutrient and water intake by bats, similar to the mechanism proposed previously for packrat middens (Plummer et al., 1997). Insectivorous bat guano is predominantly composed from insect remains, bat hair and mucus (Maher, 2006). Its 36Cl/Cl ratio is therefore expected to reflect the local contemporaneous atmospheric composition. Here, a stratigraphic sequence of bat guano, extracted from Măgurici Cave, N.W. Romania, is measured for 14C and 36Cl/Cl isotopes to assess the possibility of utilising this under-exploited archive as a palaeoclimate and solar irradiance proxy. In Adam Cave, S. Carpathian Mountains, Romania, a 250 cm thick bat guano deposit had been dated previously using radiocarbon to 7600 ± 80 14C yrs BP (Carbonnel et al., 1999); however the 271 cm thick deposit in Măgurici Cave was undated prior to this study. This study presents the first known measurements of 36 Cl/Cl on bat guano, and hence the chemical extraction processes and sampling protocols are also evaluated. The chronology of guano deposits may be used to infer the presence of bats in a cave which in turn can have implications for habitat and environmental changes

218

V.E. Johnston et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227

in a region (e.g. Leroy and Simms, 2006; Maher, 2006; Wurster et al., 2007). 2. Methods 2.1. Sampling Bat guano cores were removed from Măgurici Cave (47.36 °N, 23.55 °E, 319 m a.s.l.) on the Someşan Plateau, Transylvania, N.W. Romania (Fig. 1) in October 2008. The cave is situated close to the river Someş and is surrounded by agricultural land, meadows, and forests that provide excellent foraging habitats for bats. The modern climate conditions within Transylvania are temperate-continental, with mean annual precipitation of c.650 to 700 mm, and a mean annual temperature of 8.5 °C, but with strong seasonal variability (mean temperatures for January and July are −2 and 18 °C, respectively) (Feurdean et al., 2007). Măgurici Cave, 543 m long and 30 m deep, formed in Eocene limestones and is an important natural underground site for bat activity in the region. The cave has recently been subject to several studies of its climate (Borda and Racovita, 2000-2001), bat populations and micro-organisms (Borda et al., 2004), and the peculiar mineralogy relating to its guano deposits (Onac and Veres, 2003). It has no active streams and due to the limestone characteristics (metre thick limestone beds separated by thin clay-marl intercalations), very few dripwater sites and thus few speleothems are formed. The average temperature in the cave is ∼ 8 °C; however due to a rise in the floor of the Bat Passage (Fig. 1), the deeper parts receive no cold air during winters and temperatures are around 11 °C, with very little variation (Borda and Racoviţă, 2000–2001). Within the cave there are a number of significant guano accumulations situated under different bat colonies. Nursing and hibernating colonies are permanently inhabited by Myotis myotis (Greater mouse-eared bat) and Myotis blythii (Lesser mouse-eared bat) situated around 50 m from the entrance, whereas mating colonies are occupied in the autumn by Miniopterus schreibersii (common bent-wing bat), and are located

further into the cave (250 m from the entrance) (Borda et al., 2004). Samples of guano were taken from the largest accumulation (271 cm high conical guano deposit, Fig. 2), formed by a mating colony towards the end of the cave within the ‘Circular room’ (Fig. 1); a room with a domed roof, around 3 m below a small internal cliff. The guano was situated within a part of the cave with no dripwater sites; temperature and relative humidity average 11.5 °C and 97.8%, respectively (Borda and Racoviţă, 2000–2001). A small number of insects were present on the surface of the guano deposit; however there was no evidence for large scale bioturbation at this site.

2.2. Description of the cores The guano deposit was undisturbed previously. Samples were removed using a Russian corer (see Maher, 2006), retrieving semicylindrical cores (5 cm diameter, 1 m long). The core reached a depth of 3 m, and the core sections were labelled MGB1 (upper core; 0–100 cm), MGB2 (middle section; 100–200 cm), MGB3 (lower core; 200–300 cm). The lowest part of the core penetrated the silty clay cave fill at a depth of 271 cm. Fig. 3 shows a stratigraphic log and a description of the Măgurici Cave bat guano core. The guano was composed of uncompacted, moist organic matter, containing insect remains, hairs, and mucus (Fig. 4). The majority of the core was dark coloured, moist, and exhibited subtle stratification. In some parts, millimetre scale limestone and clay inclusions formed thin layers. A marked change in the appearance of the core occurs towards its base. Between 250 and 260 cm depth, the core is dark red-brown in colour and comprises compacted chucks which were broken up during coring, leading to a low recovery from this part of the core. Above this (239 to 250 cm depth), the core changes in appearance to a mahogany colour and contains numerous secondary mineral inclusions and some subtle layering. Although the guano here was less compact, the recovery was substantially higher. Above 239 cm, the guano returned to moist organic matter, dark brown in colour with relatively low compaction and high recovery (Figs. 3 and 4).

Fig. 1. Survey and cross section of Măgurici Cave, N.W. Romania. The core site is shown, along with other guano accumulations. The inset shows the location of the cave within Romania.

V.E. Johnston et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227

219

Fig. 2. Photo of the Măgurici Cave guano deposit. The guano deposit is shown in the process of being cored using a Russian corer. The cliff within the Circular room is shown behind the guano deposit.

2.3. Laboratory sub-sampling In the laboratory, the cores were sub-sampled into 5 cm sections, numbered by depth from the top of the core downwards. To minimise contamination from the packaging or smearing of guano during coring, the outer edges of guano from each section were removed, and only the inner part of the core was transferred to re-sealable bags for transport and storage. The guano sampling and preparation was carried out using stainless steel tools. The highly acidic nature of the guano caused tarnishing of the tools, but this is not thought to have caused contamination of the samples. The samples were refrigerated until further sub-sampling for chloride and radiocarbon was undertaken. Guano samples were imaged with a Hitachi SwiftED-TM tabletop microscope at University College Dublin, Ireland (Fig. 4). Representative aliquots (2–4 g) from 17 sub-samples were placed into pre-weighed ceramic crucibles for loss-on-ignition analyses of their organic matter content. The samples were dried in an oven at 105 °C overnight, and weighed to determine moisture content. The samples were then placed in a muffle furnace at 550 °C for 2 h to incinerate the organics. The remaining ash material was re-weighed to calculate the organic matter content. 2.4. Radiocarbon measurement and calibration Representative aliquots were taken from 13 homogenised samples, transferred into glass vials, and sent to Poznań Radiocarbon Laboratories, Poland (Goslar et al., 2004), for pre-treatment and 14C activity measurements by accelerator mass spectrometry (AMS). Prior to measurement, a three stage process of treatment with acids and base solutions was carried out to remove organic acids and secondary carbonates. The samples were then combusted to produce carbon dioxide and this was converted into graphite cathodes for AMS. Radiocarbon ages were calibrated using Calib 5.0.1 (Stuiver and Reimer, 1993) with the calibration data set IntCal04 (Reimer et al., 2004a). Six samples yielded calibrated radiocarbon dates ranging from 1051 to 1919 AD (see Table 1). A further seven samples exhibited post-bomb radiocarbon activities (Table 1), four of which were calibrated using CALIBomb (Reimer et al., 2004b), with the

calibration dataset of Levin and Kromer (2004), updated by Levin et al. (2008). 2.5. Anion extraction and

36

Cl/Cl measurements

Representative aliquots (c. 5 g) were taken from 17 sub-samples, and processed for chloride analysis. These samples were placed into pre-weighed centrifuge tubes and then re-weighed to determine the wet weight of the guano before adding 15 ml of Milli-Q de-ionised (DI) water to extract the leachable chloride. After leaching for 3 days, with intermittent shaking, the contents of each centrifuge tube was poured into a vacuum filtration system containing a pre-weighed Whatman glass fibre filter paper. A 200 μl aliquot of the filtrate was removed for analysis of leachable anions by Dionex® ion chromatography (IC). Then, the guano samples were rinsed with DI water, in the filtration set-up, and all the washings were collected. The guano and filter paper were then dried in an oven at a low temperature (60 °C), before being transferred to the original centrifuge tube and reweighed to determine the dry weight of the processed guano. The filtrate and washings were processed for 36Cl/Cl measurements by AMS using a pre-cleaned anion exchange resin (Dowex AG 1-X8) column to concentrate the chloride onto the resin, which was then eluted with nitric acid. Silver chloride was precipitated by the addition of silver nitrate solution. To remove the isobar 36S, the precipitate was re-dissolved by addition of ammonium hydroxide solution and barium nitrate solution was used to form insoluble barium sulphate. This was centrifuged and the supernatant transferred into a clean vessel. Acidifying this solution re-precipitated the silver chloride, and this purification step was repeated before rinsing the precipitate in DI water and drying. The unspiked silver chloride targets were subsequently sent to the PRIME Laboratories, Purdue University (Sharma et al., 2000), for 36Cl/Cl measurements by AMS. 3. Results Calibration of the four post-bomb radiocarbon samples with CALIBomb (Reimer et al., 2004b) typically yielded two possible dates from either side of the bomb pulse. Fig. 5 shows the possible

220

V.E. Johnston et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227

Fig. 3. Stratigraphic log of the Măgurici Cave guano deposit. The guano core retrieved was 271 cm in length, at which point it penetrated the altered cave fill. Some stratification is observed within the core. A distinctive change in the guano at 250 cm depth represents a hiatus, confirmed by radiocarbon dating. In some parts of the core, brushite inclusions are found due to reactions between limestone fragments and phosphoric acid. The clay cave fill has been altered to contain taranakite through interactions with phosphoric acid derived from the guano.

V.E. Johnston et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227

221

Fig. 4. A comparison of the bat guano texture from the surface, just above, and below the hiatus, and alteration of the cave fill. A to D show photo-micrographs and E to H show SEM images. A and E are from sample MGB1 0–5 cm which is the top of the guano deposit, showing obvious insect remains. B and F are from sample MGB3 244–250 cm which lie just above the hiatus at 250 cm and contain numerous brushite inclusions. C and G represent material from just below the hiatus, from the sample MGB3 250–255 cm, which shows a slick texture from diagenesis. D and H show the cave fill, originally silty clay, in which taranakite forms white earth masses at the silty clay/guano contact and in desiccation cracks.

dates and our preferred age model. The relative stratigraphic positions allowed selection of the appropriate dates at 1998 and 1955. At a depth of 30–35 cm, the two date options were equally valid. We have selected the date of 1973 in preference to 1962 because this leads to a more linear age–depth calibration. However, the small difference between these two dates would not affect the results of this study or change greatly the age–depth calibration if the incorrect date were selected. Table 1 shows that three additional samples exhibited postbomb radiocarbon activities despite their deeper locations within the core. These samples were taken from the upper part of each 1 m core section, and likely represent contamination from surface material falling into the borehole and becoming incorporated into the core sections. Therefore, these three samples have been omitted from the age–depth reconstruction. Fig. 6 shows an age–depth plot constructed using the dated samples with the probability density functions of the individual

calibrated samples (excluding CALIBomb calibrated samples) below. The calibration of samples using the bomb pulse has constrained the dates at the top of the guano core extremely well. A simple deposition model indicates a constant rate of deposition of 1 cm year−1 to a depth of 250 cm (within the radiocarbon uncertainties, dashed age– depth model, Fig. 6). However, this does not pass exactly through the tightly bomb calibrated ages and it passes through an area of very low probability for the radiocarbon calibration on the sample MGB2 55– 60 cm. With a small decrease in the deposition rate to 0.6 cm year−1 at 1900 AD, the age–depth curve passes through the lower three calibrated samples where the probability is high (solid age–depth model, Fig. 6). In addition, keeping the same age–depth relationship as shown in Fig. 5, the bomb calibrated samples are better constrained, indicating deposition rates of 0.7 to 2.5 cm year-1 in the upper metre of core. Below 250 cm depth (modelled age of 1647 AD) there is a rapid change in the radiocarbon ages to a date of 1285 AD

Table 1 Radiocarbon activities and calibrated ages on Măgurici Cave bat guano samples. Sample name

Depth (cm)

14 C activity (pMC)

14 C age (14C yrs BP)

Age (Cal. yrs AD) From

To

Model

MGB1 MGB1 MGB1 MGB1 MGB1 MGB2 MGB2 MGB2 MGB3 MGB3 MGB3 MGB3 MGB3

2.5 ± 2.5 32.5 ± 2.5 47.5 ± 2.5 62.5 ± 2.5 97.5 ± 2.5 102.5 ± 2.5 157.5 ± 2.5 197.5 ± 2.5 202.5 ± 2.5 222.5 ± 2.5 237.0 ± 2.0 252.5 ± 2.5 268.0 ± 3.0

110.53 ± 0.37 144.10 ± 0.39 115.49 ± 0.39 100.10 ± 0.35 99.44 ± 0.37 102.46 ± 0.32 97.60 ± 0.36 97.91 ± 0.37 109.17 ± 0.35 102.36 ± 0.33 97.72 ± 0.36 91.71 ± 0.40 89.90 ± 0.34

Moderna Moderna Moderna Moderna 45 ± 30 Modernb 195 ± 30 170 ± 30 Modernb Modernb 185 ± 30 695 ± 35 855 ± 30

1996 1973 1954 1950 1694 – 1648 1659 – – 1650 1259 1051

2000 1974 1956 1951 1919 – 1955 1954 – – 1955 1390 1259

1998 1973 1955 1951 1900 1892 1801 1734 1726 1693 1669 1285 1195

a b

0–5 cm 30–35 cm 45–50 cm 60–65 cm 95–100 cm 0–5 cm 55–60 cm 95–100 cm 0–5 cm 20–25 cm 35–39 cm 50–55 cm 65–71 cm

Comprises radiocarbon bomb pulse, age calibrated with CALIBomb (Reimer et al., 2004b). Due to contamination from the coring process.

Age (Cal. yrs AD)

222

V.E. Johnston et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227

Appendix). Leachable sulphate concentrations exhibit no relationship with either moisture or organic content of the guano. The 36Cl/Cl ratios, 36Cl concentrations, and 14C activities of the guano samples are shown in Fig. 8. The 36Cl/Cl ratio and 36Cl concentration exhibit a large peak between approximately 60 and 130 cm depth. In contrast, the radiocarbon profile shows a sharp peak in the first 50 cm of the core, which decreases in the lower sections, with the exception of three radiocarbon activities that are elevated due to contamination from the overlying younger material during the coring operation. The large peaks in the 36Cl/Cl ratio and 36Cl concentration do not coincide with the 14C activity peak within the core, which has implications for the mobility of 36Cl in guano, discussed in Section 4.3 below. 4. Discussion 4.1. Age–depth reconstruction Fig. 5. Calibrated dates against depth of the four bat guano samples affected by the nuclear weapons testing in the 1950s (see Table 1). The calibration was carried out using CALIBomb (Reimer et al., 2004b), applying a 5 yr smoothing to the radiocarbon atmospheric curve (see text). Where samples yield two possible ages, the preferred solution is shown in association with the age–depth curve, and unlikely dates circled.

(Table 1). This is likely associated with an absence of bats at the site and a hiatus in the guano record. Below this hiatus, the apparent deposition rate is only 0.2 cm year-1, but this may be a result of compaction of the guano material. The calibrated radiocarbon age at the base of the core is likely to be 1195 AD (Table 1). The results of moisture and organic matter content, leachable anion concentrations and 36Cl/Cl measurements on 17 guano samples are shown in Table 2. The organic matter content of the Măgurici Cave guano (63-89%) is comparable to that of guano from Tumbling Creek Cave, Missouri, USA (63-94%: Maher, 2006). Leachable chloride concentrations (based on dry weight) are very high (c. 30005000 ppm). This is similar to chloride concentrations measured in packrat middens by Plummer et al. (1997), and presumably associated with biological concentration of anions within urine. Interestingly, in a study measuring evaporative water loss in bats, Miniopterus schreibersii exhibited a level of water economy similar to that of arid-adapted rodents (Baudinette et al., 2000), and therefore the concentration of anions into bat urine may be similar to that for desert rodents, such as packrats. Such high chloride concentrations are favourable for 36Cl chemistry, providing large amounts of sample chloride, allowing simple chloride concentration measurements and no requirement to add an isotopic spike. Importantly, as discussed in Section 4.4 below, the measured 36Cl/Cl ratios (2000–6000 × 10−15) at the top of the core are much greater than those of present-day rainwater from N. Romania (205 ± 19 × 10−15) (Johnston and McDermott, 2008). Fig. 7 shows the organic matter and moisture content, with the leachable anion concentrations of the bat guano samples as a function of depth. Chloride and moisture content appear to follow a similar trend and are positively correlated (R2 = 0.62). Therefore, the leachable chloride analysed here is likely derived from the moisture in the guano deposit, rather than directly from the organic matter itself. The moisture in the guano is derived primarily from bat excretions, and therefore likely reflects the atmospheric chlorine isotope composition. In the lower half of the core, leachable nitrate concentrations also follow the moisture content, but deviate from this in the upper part. Leachable phosphate concentrations appear to only weakly follow moisture content. Low concentrations of leachable phosphate, particularly at the base and in the lower part of the guano deposit, may be associated with the formation of phosphate minerals. X-ray diffraction (XRD) analyses show that the secondary minerals forming inclusions and layers are brushite and taranakite (see the

The radiocarbon measurements highlight an important feature of the guano core record, namely contamination of the top of each core section with material from the walls of the borehole during the coring operation. The dashed vertical lines in Fig. 8 indicate the division between the 1 m long sections of core. The upper approximately 20 cm of each of the lower two cores (MGB2 and MGB3) exhibit high radiocarbon activities (N100 pMC), taken to reflect contamination with material derived from the upper borehole and the guano surface. These contaminated samples are circled in Fig. 8 and excluded from the trend-line and the age–depth model (Fig. 6). The contaminated material appears to have been derived from the upper 60 cm of the guano heap that is characterised by high 14C activities acquired from the bomb pulse (Fig. 8), elevating the 14C activity in the contaminated samples. Simultaneously, the 36Cl signal from the upper guano is transferred into samples at the top of each core section. Since the upper 60 cm of the guano deposit contains a lower 36Cl/Cl ratio than exhibited at depth within the core, the 36Cl/Cl ratio of the sample from 112.5 cm depth appears to have been reduced somewhat by this contamination (note that this sample is from a slightly lower depth than the contaminated radiocarbon sample). Therefore, the 36Cl peak falls somewhere within the base of MGB1 or the top of MGB2 core sections (100–125 cm). The contamination problem appears to be worse in the lower core section (MGB3), including material derived from the entire length of the borehole, and penetrating deeper into the top of the core section. The age–depth model constructed from the radiocarbon dates (Fig. 6) indicates a hiatus at 250 cm depth which is supported by visual inspection of the core. The guano below a depth of 250 cm appears to be highly reworked, and could indicate a period of surface exposure and decomposition by coprovores, fungi, and bacteria. In addition, the presence of numerous brushite inclusions in the guano above 250 cm could indicate the return of bats, disturbing the roof, and causing carbonate fragments to drop into the guano. These carbonate inclusions are unlikely to be broken soda straw speleothems however due to a lack of decoration in the cave. It is probable that these fragments were derived from the cave roof that has deteriorated over time, possibly associated with the acidic conditions caused by the presence of the guano accumulation. The fragile carbonate would then be broken off by the returning bats and incorporated into the guano, until the roof became stable. Therefore, we postulate that there is a hiatus at 250 cm, coincident with a prominent change in the guano characteristics. 4.2. Environmental Interpretations Importantly, the presence of bats in the cave appears to coincide with periods of warmer European climate. The older episode of guano deposition at this roost is concurrent to the Medieval Climate

V.E. Johnston et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227

223

Fig. 6. Age–depth model constructed for the bat guano core from radiocarbon dating. The upper section of core was calibrated using the bomb pulse (CALIBomb, Reimer et al., 2004b), represented by black crosses. The lower samples were calibrated using conventional radiocarbon techniques (Stuiver and Reimer, 1993). The probability density functions of the resulting calibrations are shown in the lower panel, along with a greyscale bar which represents the probability intensity. These bars are used on the age–depth graph to show the calibration results for each sample, with the 95% uncertainty shown as a solid grey shading. The hiatus is shown as a horizontal dashed line. The solid curve shows the preferred age model (see text for details) with the associated deposition rate in the right hand panel. The dashed curve shows the alternative age model for the upper section of core. The temporal durations of the Little Ice Age (LIA) and Medieval Climate Anomaly (MCA) (Mann et al., 2009; Trouet et al., 2009), and LIA in the E. Carpathians (Popa and Kern, 2009) are shown as black bars, dashed where the temporal limits are uncertain.

Anomaly (MCA; 1049–1298 AD), a relatively warm period over the European and North Atlantic sectors (Trouet et al., 2009). The bats appear to have left this roost at the end of the warm period, possibly migrating to warmer regions at the onset of the Little Ice Age (LIA; 1400–1700 AD (Mann et al., 2009)). The bats return to the roost during the LIA, at 1647 AD (modelled calibrated age). Importantly however, a recent temperature reconstruction from tree-ring records from the Eastern Carpathians (130 km east of Măgurici Cave) shows a decoupling of the climate in this region from Alpine and other northern hemisphere temperature reconstructions between 1630 and 1740 AD (Popa and Kern, 2009). This period is typically characterised by cool and wet summers, but on the basis of the tree-ring data, Popa and Kern (2009) see no evidence for any significant temperature decline in the Eastern Carpathians. In fact, this tree-ring reconstruction shows a temperature peak in the year 1646 AD of +2.31 °C with respect to the reference period 1961–1990, exactly concurrent with the return of the bats in 1647 AD (modelled here). Popa and Kern (2009) demonstrate that the LIA in this region extends from 1370 to

1630 AD, and according to our age model for the guano this spans the period that this roost site in Măgurici Cave was devoid of bats (Fig. 6). Furthermore, when temperatures start to increase further around 1900 AD, the rate of guano deposition appears to increase, possibly indicating improved conditions for bat populations. From the guano deposit studied here, it is unclear whether the bats migrated entirely away from the cave site or simply moved to a different roost site within the cave. Migration of bat populations is typically triggered by environmental conditions, and migration is likely only if the benefits outweigh the costs (e.g. adaptation and unknown predation). Two potential environmental drivers for seasonal migration are roost temperatures and ambient temperature in the foraging area, likely to affect insect populations. In a study of Miniopterus schreibersii in Portugal, roost temperature was found to be the key driver of regional migration (Rodrigues and Palmeirim, 2008). However, ambient temperatures in the foraging area may become more important away from this Mediterranean climate. In Măgurici Cave, it is conceivable that the bats could have moved marginally

224

V.E. Johnston et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227

Table 2 Moisture and organic matter content, leachable anion concentrations and

MGB1 MGB1 MGB1 MGB1 MGB1 MGB1 MGB1 MGB2 MGB2 MGB2 MGB2 MGB2 MGB2 MGB3 MGB3 MGB3 MGB3 a b c

0–5 cm 15–20 cm 30–35 cm 45–50 cm 60–65 cm 75–80 cm 90–95 cm 10–15 cm 25–30 cm 40–45 cm 55–60 cm 70–75 cm 85–90 cm 20–25 cm 35–39 cm 50–55 cm 65–71 cm

36

Cl analyses on bat guano samples.

Depth (cm)

Model Age (Cal. yrs AD)

Guano wt. (dry g)

Moisture contenta (%)

Organic contentb (%)

Cl conc.c (ppm)

NO3 conc.c (×102 ppm)

PO4 conc.c (×102 ppm)

SO4 conc.c (×102 ppm)

36

Cl/Cl ratio (×10−15)

36

Cl conc.c (×1010 atoms/g)

2.5 ± 2.5 17.5 ± 2.5 32.5 ± 2.5 47.5 ± 2.5 62.5 ± 2.5 77.5 ± 2.5 93.0 ± 3.0 112.5 ± 2.5 127.5 ± 2.5 142.5 ± 2.5 157.5 ± 2.5 172.5 ± 2.5 187.5 ± 2.5 222.5 ± 2.5 237.0 ± 2.0 252.5 ± 2.5 268.0 ± 3.0

1998 1979 1962 1955 1951 1929 1907 1875 1850 1825 1801 1776 1751 1693 1669 1285 1195

1.77 1.68 1.88 1.82 1.58 1.54 1.71 1.57 1.97 2.23 1.96 1.90 2.05 1.92 1.90 1.58 1.66

60.5 56.9 59.0 63.6 63.1 61.8 60.3 62.4 57.6 54.0 57.3 59.0 53.5 58.5 56.1 61.0 57.9

87.7 89.4 87.9 84.6 79.7 77.6 78.4 82.9 75.0 68.1 65.3 78.1 74.5 75.7 63.1 67.2 79.1

4653 ± 233 5023 ± 251 5286 ± 264 4904 ± 245 5284 ± 264 4836 ± 242 4554 ± 228 4982 ± 249 3654 ± 183 3156 ± 158 3438 ± 172 3623 ± 181 2833 ± 142 3815 ± 191 3119 ± 156 4334 ± 217 4049 ± 202

598 ± 30 633 ± 32 728 ± 36 754 ± 38 895 ± 45 900 ± 45 911 ± 46 932 ± 47 815 ± 41 730 ± 37 805 ± 40 865 ± 43 704 ± 35 789 ± 39 769 ± 38 1027 ± 51 984 ± 49

297 ± 15 271 ± 14 323 ± 16 302 ± 15 318 ± 16 324 ± 16 300 ± 15 313 ± 16 253 ± 13 213 ± 11 182 ± 9 186 ± 9 149 ± 7 228 ± 11 155 ± 8 207 ± 10 160 ± 8

198 ± 10 195 ± 10 197 ± 10 207 ± 10 326 ± 16 248 ± 12 237 ± 12 250 ± 13 145 ± 7 111 ± 6 253 ± 13 264 ± 13 225 ± 11 253 ± 13 251 ± 13 200 ± 10 149 ± 7

2253 ± 53 2598 ± 77 3192 ± 80 3819 ± 162 4853 ± 126 5578 ± 110 6234 ± 146 5313 ± 110 6433 ± 153 5977 ± 156 5442 ± 139 5192 ± 162 4701 ± 162 4788 ± 137 4006 ± 110 3174 ± 139 2631 ± 78

17.54 ± 0.35 21.83 ± 0.65 28.23 ± 0.56 31.33 ± 1.25 42.90 ± 1.29 45.13 ± 0.90 47.50 ± 0.95 44.29 ± 0.89 39.33 ± 0.79 31.56 ± 0.95 31.30 ± 0.94 31.47 ± 0.94 22.28 ± 0.67 30.56 ± 0.92 20.90 ± 0.63 23.01 ± 0.92 17.82 ± 0.53

Moisture content is calculated as the mass loss from the wet guano after drying at 105 °C overnight. Organic content is calculated as the mass loss from dry guano after burning at 550 °C for 2 h. Based on dry guano weight. Note that the high concentrations of NO3, PO4 and SO4 are shown in the unit × 02 ppm (e.g. 59800 ± 3000 ppm).

deeper into the cave during the LIA, roosting within the Guano Gallery (see Fig. 1), shown to be slightly warmer than the Circular room at present (Borda et al., 2004). Further studies, including 14C dating of the guano deposits from deeper in the cave are required to identify whether the bats migrated out of or moved within the cave as a longterm ecological response to environmental changes during the LIA. If the bats did leave the cave site for a period of ∼350 years this would be an important ecological finding and would have wider implications for bat migration and the ability of bats to locate roost sites rapidly following regional climate changes. 4.3.

36

Cl in bat guano as a proxy for solar variability

Critically, the data show that the radiocarbon peak and the 36Cl concentration peak do not coincide (Fig. 8). The large peaks are caused by nuclear weapons testing during the 1950–1960. However, the two peaks should not necessarily match up exactly because of the difference in the formation mechanisms of 14C and 36Cl (see the Appendix). The radiocarbon bomb pulse is observed within the first 50 cm of the core, and appears as a rapid increase to a peak of 144.1 ± 0.39 pMC. The bomb peak measured in the atmosphere reached a

maximum value of 199 pMC (Levin and Kromer, 2004). However, when a 5 yr smoothing is applied (equivalent to the 5 cm core sample with a deposition rate of 1 cm year−1, as exhibited lower in the core), the bomb peak reaches 170 pMC, and therefore the radiocarbon bomb pulse is well represented in this bat guano deposit. In contrast to radiocarbon, the peaks in both 36Cl/Cl ratio and 36Cl concentration are broader and occur lower in the core (Fig. 8), with maximum values of 6433 × 10−15 and 48 × 1010 atoms/g, respectively. The 36Cl/Cl ratio peak value in the guano is substantially lower than that exhibited in the Dye 3 ice core (Greenland), the latter peaking at 28,600 × 10−15 in 1957 (Synal et al., 1990). Although recent atmospheric modelling of Heikkila et al. (2009) demonstrate that atmospheric transport patterns can affect the timing and magnitude of regional 36Cl bomb fallout (see the Appendix), the broad peak measured in the bat guano is shifted substantially downward in the profile with respect to the 14C peak (Fig. 8), and in particular with respect to the bomb calibrated radiocarbon date at 1955 (just prior to the expected 36Cl peak in 1956, see the Appendix). Therefore, we postulate that the chloride within the core was affected by post-depositional mobility that broadened and smoothed the bomb signal. This is unlikely to have occurred by bioturbation because such a process would also redistribute the bomb

Fig. 7. Moisture and organic matter content, and water leachable anion concentrations against depth within the guano core. The moisture content and chloride concentration are positively correlated (R2 = 0.62), consistent with the well-established hydrophilic nature of chloride.

V.E. Johnston et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227

225

Fig. 8. 36Cl/Cl ratio, 36Cl concentration, and 14C activity against depth within the guano core. The calibrated radiocarbon ages are also indicated (Table 1). The vertical dashed lines indicate the division between different sections of core. MGB1 is the upper core, MGB2 is the middle core section, and MGB3 is the lowest core section which penetrates the clay cave fill. The horizontal dashed line in the 14C activity represents modern 14C activity. The large peaks, which are associated with nuclear weapons testing, in the 36Cl/Cl ratio and 36Cl concentration do not coincide with the bomb pulse in 14C activity, indicating post-depositional mobilisation of 36Cl within the guano.

radiocarbon signal through the guano. Due to the hydrophilic nature of chloride, it is likely that fluid movements within the guano deposit are responsible for the broadening and shifting downward of the 36Cl bomb signal. This is likely to have occurred through the flushing by gravity of fluid bat excretions from above, through the relatively loose structure of the guano. Additional evidence for movement of fluids in the guano comes from the formation of taranakite within the silty clays underlying the guano deposit. For the phosphoric acid derived from the guano to react with the clay on the cave floor, the leachable phosphoric acid must be mobilised in fluids and transported downward through the guano, into the sediments. Although such 36 Cl signals may be useful over longer (millennial) time-scales, more robust solar proxy records from bat guano could be sought through the extraction of organically bound chloride or the use of an alternative particle reactive cosmogenic isotope, 10Be, which is likely to be retained within the guano with minimal post-depositional mobility (see the Appendix). 4.4. Core top

36

Cl/Cl; comparison with modern rainfall

The bat guano 36Cl/Cl ratio at the top of the core is around 2000 × 10−15 (Table 2), substantially higher than values obtained for present-day rainfall in the region, of around 200 × 10−15 (Johnston and McDermott, 2008). A number of mechanisms could be responsible for this high 36Cl/Cl ratio at the top of the core: (1) postdepositional upward migration of bomb-derived 36Cl, (2) higher local meteoric 36Cl/Cl ratio following the Chernobyl disaster, or (3) recycling of bomb 36Cl within vegetation. It is important to evaluate the reasons for elevated 36Cl/Cl because this could have serious implications for the

use of chloride as a conservative tracer through biological systems, or indicate additional atmospheric or post-depositional processes. The upward movement of water-rich fluids through capillary motion is unlikely since only very small changes in humidity and temperature are documented (Borda and Racovita, 2000–2001), indicating little air movement or connectivity to the external environment. In addition, urine from bats is likely to flush downwards through the guano, counteracting any upward fluid movements. Alternatively, chloride could exist also in the gaseous form HCl(g) due to the acidic nature of guano. Although the main movement of bombpulse 36Cl is downwards by fluid leaching, a small proportion of the chloride (including 36Cl from the bomb pulse) could migrate upwards as HCl(g) through the loose structure of the guano, causing the high 36 Cl/Cl ratio exhibited at the top of the core. The Chernobyl disaster in 1986 released 36Cl into the environment following neutron activation on 35Cl within reactor materials (Chant et al., 1996). This however, did not penetrate the stratosphere, and it is likely that 36Cl fallout only occurred locally (rather than globally), and strongly depended on wind direction and local precipitation events. In a study of 36Cl in lichens from three regions around Chernobyl, Chant et al. (1996) found elevated 36Cl/Cl ratios up to 24,000 × 10−15 in the region closest to Chernobyl, and 3100 × 10−15 in a region c.180 km away. A small elevation in 36Cl is possible in the Măgurici Cave area of N.W. Romania (c.650 km away from Chernobyl). However, it is unlikely that the 36Cl persisting in the atmosphere following the Chernobyl accident is sufficient to cause the high 36Cl/Cl ratio observed in the top of the guano core. Alternatively, the 36Cl fallout from the global nuclear weapons testing (with small additions from the Chernobyl disaster) may be

226

V.E. Johnston et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227

recycling within local vegetation giving rise to the higher than expected 36Cl/Cl ratios in the top of the core. This bomb-derived 36Cl would become incorporated into the bat guano through the food chain. Through a chloride mass balance calculation, Cornett et al. (1997) proposed retention of chlorine and 36Cl for 18 years in vegetation and 20 years within soil. This observation was consistent with measurements of 36Cl in Canadian conifer seeds of various ages (1950–1980), which exhibited a slower decline in the bomb pulse than measured in Arctic ice cores (Milton et al., 2003). More recently, 36 Cl tracer experiments in soil samples under various environmental conditions, showed that inorganic chloride could be retained in a forest soil by at least two different processes acting on different timescales (Bastviken et al., 2007). They showed that 11–24% of the added 36 Cl was retained by microbial uptake for around three weeks before being released back into the soil. In addition, they found a netformation of 4% organic chloride relative to inorganic chloride within soil organic matter, which appeared to be retained on decadal to centennial time-scales, leading to a slow turnover of organically bound chloride in the soil. Therefore, both retention in the soil and within vegetation could retain 36Cl from the bomb pulse, before incorporation into the diet of the bats through the food chain. This 36 Cl signal within the biosphere (bat guano) would decay more slowly than the rapid fallout experienced in the atmosphere, as recorded in modern rainfall and ice cores (Synal et al., 1990). This could then explain the high 36Cl/Cl ratio in the bat guano at the top of the core relative to present-day rainfall (Johnston and McDermott, 2008). However, we have not completely ruled out post-depositional upward migration of the bomb-derived 36Cl signal as HCl(g). Therefore, these data are not a conclusive argument for the recycling of chloride in vegetation and the soil; which, if proved, would have serious ramifications for the use of chloride as a conservative tracer in the field of hydrology. Further analyses of modern bat guano that is not associated with material deposited during the nuclear weapons testing, may allow a better understanding of the processes occurring in the soil, vegetation, and food chain. 5. Conclusions This study has assessed the feasibility of using a large bat guano deposit from Măgurici Cave, N.W. Romania as an archive of atmospheric 36Cl/Cl. Due to the strongly hydrophilic nature and hence mobility of chloride, downward leaching of the chloride through the guano deposit has occurred. This is likely associated with percolating bat excretions. We have demonstrated that peak (bomb) 36Cl/Cl ratios within the guano deposit do not coincide with the bomb 14C peak in the same material. The recycling of bombderived 36Cl in vegetation may have caused elevated 36Cl/Cl in the top of the core in comparison with local rainwater. Future attempts should consider the use of organically bound chloride, rather than the mobile leachable fraction used here, or the particle reactive cosmogenic isotope, 10Be, as solar proxies within bat guano. Radiocarbon dating shows that bats were present in Măgurici Cave since 1195 AD, but not continuously. Bat habitation between 1195 and 1285 AD appears to coincide with the Medieval Climate Anomaly, where European temperatures were relatively warm. The bats vacated the roost site at the onset of a regional cold period, linked to the Little Ice Age, returning at approximately 1647 AD, when local temperatures increased. These results support a recent, local temperature reconstruction using tree-ring widths (Popa and Kern, 2009), which indicate the climate of the Eastern Carpathians is decoupled from Alpine and other northern hemisphere temperature reconstructions during this period. Furthermore, the apparent depositional rate of guano increases at 1900 AD, when temperatures rise, associated with anthropogenic warming. However, this could be due in part to compaction of the guano at the base of the deposit. Remaining sample material is currently under examination for pollen

and stable isotopes, to corroborate the interpretation that environmental conditions have affected the occupation of the cave by bats. Acknowledgements This research was funded by the Science Foundation Ireland (SFI, 05/RFP/GEO 008). We thank the PRIME Laboratory, Purdue University, IN, USA, and Poznań Radiocarbon Laboratory, Poland, for carrying out AMS analyses. Lorenzo Cociani (University College Dublin) is thanked for field and sub-sampling assistance, the students at the Faculty of Geology, Babeş-Bolyai University, Cluj-Napoca helped with sampling, and cavers from Montana Caving Club, Baia Mare undertook cave surveying. Sylvia Dolan (University College Dublin) provided the muffle furnace. Comments from two anonymous reviewers and editor Thierry Corrège improved the manuscript greatly. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.palaeo.2010.02.031. References Bard, E., Raisbeck, G., Yiou, F., Jouzel, J., 2000. Solar irradiance during the last 1200 years based on cosmogenic nuclides. Tellus B 52, 985–992. Bastviken, D., et al., 2007. Chloride retention in forest soil by microbial uptake and by natural chlorination of organic matter. Geochim. Cosmochim. Acta 71, 3182–3192. Baudinette, R.V., Churchill, S.K., Christian, K.A., Nelson, J.E., Hudson, P.J., 2000. Energy, water balance and the roost microenvironment in three Australian cave-dwelling bats (Microchiroptera). J. Comp. Physiol. B 170, 439–446. Beer, J., et al., 2002. Cosmogenic nuclides during isotope stages 2 and 3. Quat. Sci. Rev. 21, 1129–1139. Bird, M.I., et al., 2007. A long record of environmental change from bat guano deposits in Makangit Cave, Palawan, Philippines. T. Roy. Soc. Edin.—Earth 98, 59–69. Borda, D., Borda, C., Tămas, T., 2004. Bats, climate, and air microorganisms in a Romanian cave. Mammalia 68, 337–343. Borda, D., Racoviţă, G., 2000–2001. Données thermo-hygrométriques sur la Grotte de Ciungi et al Grotte de Măgurici (Plateau du Someş, Transylvanie). Travaux de l'Institut de Spéologie “Émile Racovitza”. XXXIX-XL, 207–226. Carbonnel, J.P., Olive, P., Decu, V.G., Klein, D., 1999. Bat guano deposit Holocene datings in the south Carpathian Mountains (Romania). Tectonic Implications. Comptes Rendus de l'Academie des Sciences Serie II Fascicule A—Sciences de la Terre et des Planetes 328, 367–370. Chant, L.A., et al., 1996. I-129 and Cl-36 concentrations in lichens collected in 1990 from three regions around Chernobyl. Appl. Radiat. Isotopes 47, 933–937. Cornett, R.J., et al., 1997. Is Cl-36 from weapons' test fallout still cycling in the atmosphere? Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 123, 378–381. Feurdean, A., Mosbrugger, V., Onac, B.P., Polyak, V., Veres, D., 2007. Younger Dryas to mid-Holocene environmental history of the lowlands of NW Transylvania, Romania. Quaternary Res. 68, 364–378. Goslar, T., Czernik, J., Goslar, E., 2004. Low-energy C-14 AMS in Poznan Radiocarbon Laboratory, Poland. Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 223-24, 5–11. Heikkila, U., et al., 2009. Cl-36 bomb peak: comparison of modeled and measured data. Atmos. Chem. Phys. 9, 4145–4156. Johnston, V.E., McDermott, F., 2008. The distribution of meteoric Cl-36 in precipitation across Europe in spring 2007. Earth Planet. Sci. Lett., 275, 154–164. doi:10.1016/j. epsl.2008.08.021. Leroy, S.A.G., Simms, M.J., 2006. Iron age to medieval entomogamous vegetation and Rhinolophus hipposideros roost in South-Eastern Wales (UK). Palaeogeogr. Palaeoclimatol. Palaeoecol. 237, 4–18. Levin, I., Hammer, S., Kromer, B., Meinhardt, F., 2008. Radiocarbon observations in atmospheric CO2: determining fossil fuel CO2 over Europe using Jungfraujoch observations as background. Sci. Total Environ. 391, 211–216. Levin, I., Kromer, B., 2004. The tropospheric (CO2)-C-14 level in mid-latitudes of the Northern Hemisphere (1959–2003). Radiocarbon 46, 1261–1272. Maher, L.J., 2006. Environmental information from guano palynology of insectivorous bats of the central part of the United States of America. Palaeogeogr. Palaeoclimatol. Palaeoecol. 237, 19–31. Mann, M.E., et al., 2009. Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly. Science 326, 1256–1260. doi:10.1126/science.1177303. Milton, G.M., et al., 2003. Evidence for chlorine recycling—hydrosphere, biosphere, atmosphere—in a forested wet zone on the Canadian Shield. Appl. Geochem. 18, 1027–1042. Muscheler, R., et al., 2007. Solar activity during the last 1000 yr inferred from radionuclide records. Quat. Sci. Rev. 26, 82–97. Onac, B.P., Veres, D.S., 2003. Sequence of secondary phosphates deposition in a karst environment: evidence from Măgurici Cave (Romania). Eur. J. Mineral. 15, 741–745.

V.E. Johnston et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227 Plummer, M.A., et al., 1997. Chlorine-36 in fossil rat urine: An archive of cosmogenic nuclide deposition during the past 40,000 years. Science 277, 538–541. Popa, I., Kern, Z., 2009. Long-term summer temperature reconstruction inferred from tree-ring records from the Eastern Carpathians. Clim. Dynam. 32, 1107–1117. Reimer, P.J., et al., 2004a. IntCal04 terrestrial radiocarbon age calibration, 0–26 cal kyr BP. Radiocarbon 46, 1029–1058. Reimer, P.J., Brown, T.A., Reimer, R.W., 2004b. Discussion: reporting and calibration of post-bomb C-14 data. Radiocarbon 46, 1299–1304. Rodrigues, L., Palmeirim, J.M., 2008. Migratory behaviour of the Schreiber's bat: when, where and why do cave bats migrate in a Mediterranean region? J. Zool. (Lond.) 274, 116–125. Sharma, P., et al., 2000. PRIME lab AMS performance, upgrades and research applications. Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 172, 112–123.

227

Stuiver, M., Reimer, P.J., 1993. Extended C-14 data-base and revised Calib 3.0 C-14 age calibration program. Radiocarbon 35, 215–230. Synal, H.A., Beer, J., Bonani, G., Suter, M., Wolfli, W., 1990. Atmospheric transport of bomb-produced Cl-36. Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 52, 483–488. Trouet, V., et al., 2009. Persistent positive North Atlantic Oscillation mode dominated the Medieval Climate Anomaly. Science 324, 78–80. Wurster, C.M., McFarlane, D.A., Bird, M.I., 2007. Spatial and temporal expression of vegetation and atmospheric variability from stable carbon and nitrogen isotope analysis of bat guano in the southern United States. Geochim. Cosmochim. Acta 71, 3302–3310. Wurster, C.M., et al., 2008. Stable carbon and hydrogen isotopes from bat guano in the Grand Canyon, USA, reveal Younger Dryas and 8.2 ka events. Geology 36, 683–686.