Journal of Archaeological Sciencemarsci.haifa.ac.il/images/CiviLab/RuthGrross/Dunseth_and_Shahack-Gross_2018_JAS...spectroscopy analysis (cf. Weiner, 2010). During the analysis of

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Journal of Archaeological Science

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Calcitic dung spherulites and the potential for rapid identification ofdegraded animal dung at archaeological sites using FTIR spectroscopy

Zachary C. Dunsetha,b,∗, Ruth Shahack-Grossb,∗∗

a Institute of Archaeology, Tel Aviv University, Tel Aviv 6997801, Israelb Laboratory for Sedimentary Archaeology, Department of Maritime Civilizations, Recanati Institute of Maritime Studies, Leon H. Charney School of Marine Sciences,University of Haifa, Haifa 3498838, Israel

A R T I C L E I N F O

Keywords:Dung spherulitesGrinding curveFTIR

A B S T R A C T

Animal dung is increasingly a valuable resource for reconstructing ancient activity in archaeology. One of themost common archaeological indicators of dung in caves and arid environments are calcitic dung spherulites thatform in the digestive system of a variety of animals. Although many aspects of their formation and taphonomyare understood, details of their mineralogy remain poorly-defined. Using the Fourier transform infrared (FTIR)grinding curve method, we report here that archaeological sediments containing large amounts of dungspherulites can be differentiated from sediments composed of other forms of geogenic and pyrogenic calcites. Wepropose that this attribute can be used to rapidly identify well-preserved degraded dung deposits at archae-ological sites with routine laboratory or on-site field FTIR analysis. This observation at a 5000-year-old open airsite suggests that the grinding curve method also has potential to be used for assessing preservation of dungspherulites for future radiocarbon or stable isotope investigations.

1. Introduction

Three decades of ethno- and geo-ethnoarchaeological work hasshown the importance of animal dung to archaeological reconstruc-tions. The analysis of the organic and inorganic constituents of dung hasbeen used to inform various aspects of ancient animal and human ac-tivities, including identification of species (e.g., Shillito et al., 2011;Prost et al., 2017), fuel use (e.g., Miller and Smart, 1984; Sillar, 2000;Portillo et al., 2012; Gur-Arieh et al., 2013, 2014), and archaeology ofspace, human subsistence and foddering strategies (e.g., Brochier et al.,1992; Reddy, 1999; Shahack-Gross et al., 2003, 2014; Valamoti andCharles, 2005; Portillo et al., 2014; Polo-Diaz et al., 2016; Dunsethet al., 2016, 2018).

A variety of geoarchaeological methods are currently utilized toidentify dung at archaeological sites (see Shahack-Gross, 2011 and re-ferences therein). One of the most established is the identification andquantification of inorganic calcareous dung spherulites (and phytolithsin tandem) using optical microscopy (Shahack-Gross, 2011; Lancelottiand Madella, 2012).

Dung spherulites are microscopic (5–25 μm) roughly-spherical par-ticles made of radially-oriented acicular crystallites (Canti, 1997). Theyare known to form in the digestive system of a variety of animal species,

most abundantly in ruminants (e.g., Canti, 1999; Korstanje, 2005;Shahack-Gross and Finkelstein, 2008; Portillo et al., 2014), but boththeir formation mechanism and exact composition are still unclear. Ithas been shown experimentally that they form in the middle of thesmall intestine in sheep (Canti, 1999: 252–253). There has been somesuggestion that these precipitate directly from free calcium (Ca2+) andbicarbonate (HCO3

−) ions in the increasingly alkaline lower intestine(Canti, 1999), or as bacterially-mediated monohydrocalcite (Ca-CO3*H2O, Shahack-Gross, 2011: 208; cf. Rodriguez-Navarro et al.,2007; Zhang et al., 2017). In practice, sediments composed of largeamounts of dung spherulites are rich in calcite (see more below), im-plying that this is their composition in archaeological deposits.

As calcareous microremains that possess large surface areas relativeto bulk volume, they are prone to dissolution (cf. Gur-Arieh et al.,2014). They are mainly found at cave sites, rock shelters and sites inarid or semi-arid environments (e.g., Brochier et al., 1992; Matthewset al., 1996; Karkanas, 2006; Shahack-Gross et al., 2014; Portillo et al.,2014; Polo-Diaz et al., 2016; Dunseth et al., 2016, 2018). Because oftheir susceptibility to dissolution, it has so far been difficult to separatethem from associated organic and/or mineral components by estab-lished extraction procedures (Canti, 1997, 1998; Canti and Nicosia,2018; Shahack-Gross personal observations). Therefore, the infrared

https://doi.org/10.1016/j.jas.2018.07.005Received 27 April 2018; Received in revised form 29 June 2018; Accepted 13 July 2018

∗ Corresponding author. Institute of Archaeology, Tel Aviv University, Tel Aviv 6997801, Israel.∗∗ Corresponding author.E-mail addresses: [email protected] (Z.C. Dunseth), [email protected] (R. Shahack-Gross).

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signature of these archaeologically important calcitic micro-remains isrelatively little explored.

Fourier Transform Infrared (FTIR) spectroscopy has been used tostudy an extensive suite of archaeological materials (see recent reviewin Monnier, 2018). Calcite (the mineral polymorph of archaeologically-preserved dung spherulites) has three important characteristic vibra-tional peaks in the mid-IR range: 712 cm−1 (ν4, in-plane CO3 bend),874 cm−1 (ν2, out-of-plane CO3 bend) and 1420 cm−1 (ν3, asymmetricCO3 stretch) (White, 1974). Several archaeologically-relevant calcites,including geogenic (spar, limestone, flowstones, and chalk) and pyro-genic specimens (wood ash and lime plaster) have recently been studied(Chu et al., 2008; Regev et al., 2010, 2011; Poduska et al., 2011, 2012;Xu et al., 2015, 2016; Goshen et al., 2017). Although particle size,distribution and porosity (Duyckaerts, 1959; Lane, 1999; Surovell andStiner, 2001) and crystal morphologies (Ruppin and Englman, 1970;Koike et al., 2010) are known to affect peak height and shape in IRspectra, studies showed that plotting the ν2 and ν4 peak heights nor-malized to the ν3 height decouples these effects and provides in-formation on local atomic disorder (Regev et al., 2010; Poduska et al.,2011). Although more recent publications have shown limitations tospecifying the underlying causes of crystalline disorder (Xu et al.,2015), the method reliably shows variation among calcites of diverseorigins and is useful for rapidly differentiating pyrogenic from geogenicarchaeological materials during routine on-site or lab-based FTIRspectroscopy analysis (cf. Weiner, 2010).

During the analysis of archaeological sediments at Nahal Boqer 66,Negev Highlands (modern Israel), it was noted that all sediment sam-ples plotted in a range that corresponds to Regev et al.'s (2010) modernwood ash and lime plaster grinding curves (Fig. 1). However, none ofthese samples were, or contained, lime plaster or large amounts ofwood ash. At first approximation, it was noted that the samples thatplotted near and above the lime plaster grinding curve included highconcentrations of dung spherulites. In order to test whether this ob-servation can be developed into a screening method for identification ofdung-containing archaeological sediments, we present here an appli-cation of the FTIR spectroscopy grinding curve method. In this paper we

show: one, sediments dominated by dung spherulites can be differ-entiated from other calcite-containing sediments, and two, sedimentsthat plot closer to geogenic calcite as well as wood ash grinding curvesmay be the result of post-depositional mixing between dung and geo-genic deposits. In both cases, heat has no effect on where dung-derivedsediments plot. Furthermore, we posit that this observation can be usedto rapidly identify the presence of degraded dung in archaeologicalsediments.

2. Materials and methods

2.1. Archaeological and control samples

Archaeological sediments were collected from Nahal Boqer 66(WGS84: 30.9086° N, 34.7906° E, 521m a.s.l.), an open-air site locatedin the Negev Highlands (∼90mm annual precipitation; IsraelMeteorological Service) on a small saddle between two Turonianlimestone ridges (Avni and Weiler, 2013). The ridges in the study areaare mantled by calcite-containing aeolian dust deposits. Samples (c. 10g of sediment from various loci) were collected during excavations thatwere carried out in 2016 by the authors and served for microremainanalyses to address questions related to subsistence practices (Dunsethet al., 2018). For the current study we selected 4 unequivocal degradeddung samples that according to the previous study originate from li-vestock enclosures and had different concentrations of dung spher-ulites, ranging between 32 and 195 million per 1 gr of sediment(Table 1 and Appendix 1). Note that the analytical error in determiningconcentrations of dung spherulites can be around±30% of the calcu-lated value (Table 1; for more details see Gur-Arieh et al., 2013). Allfour samples studied here have very high concentrations of spherulitesthat correspond to those recorded in modern dung of sheep/goats,animals typically herded in the study region (Shahack-Gross andFinkelstein, 2008). Samples NB-1.2 and NB-2.6 are associated with adegraded dung deposit dated by four radiocarbon determinationsspanning c. 3300–2900 BCE (2 σ, Dunseth et al., 2017: Table 3).Samples NB-3.3 and NB-9.2 are from excavated loci associated with

Fig. 1. Data points from all sediments studied from the Early and Intermediate Bronze Age site of Nahal Boqer 66 (Negev Highlands, Israel) plotted against thereference trends reported in Regev et al. (2010). Note location of aeolian dust controls in this study (green triangles) near geogenic trendlines, while all archae-ological sediments (brown circles) plot in the range between chalk/ash and lime plaster trendlines. Following infrared and optical analysis, none of the archaeologicalsamples are composed of wood ash or lime plaster, or showed indicators of being severely affected by heat (Dunseth et al., 2018: Appendix 2). (n.a.u.= normalizedabsorbance units). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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pottery from the Early Bronze (c. 3000–2500 BCE) and IntermediateBronze Ages (c. 2500–1950 BCE) (Cohen, 1999: 60–61). As the site islocated on a calcareous terrane, aeolian dust and limestone sampleswere collected from the immediate landscape beyond the limits of thesite as geogenic controls (Table 1).

As dung was a common fuel source in antiquity, we used ashedmodern ovicaprine dung as a control (collected from Wadi Zeitan,Western Negev, December 2015). Fresh dung was not used as control toavoid interference due to organic matter in the spectra. As one of thearchaeological degraded dung samples also contained calcitic pseudo-morphs indicative of wood ash, we used modern wood ash of oak(Quercus calliprinos) and carob (Ceratonia siliqua) as further controls. Allmodern controls were ashed at 500 °C for 4 h in a muffle furnace(Thermo Scientific Thermolyne F6000) and allowed to cool at roomtemperature for at least 48 h before FTIR analysis.

To evaluate mixing effects on FTIR spectra and grinding curve data,we created mixtures of pre-determined weight ratios from sample NB-3.3 (the archaeological degraded dung sediment with the highest con-centration of dung spherulites) and sample NB-C2 (aeolian dust withoutdung spherulites) (Table 1). Before mixing, degraded dung sedimentsand aeolian dust were lightly homogenized with an agate mortar andpestle so that all particles passed through a 500 μm sieve to standardizethe range of particle sizes used in these experiments.

2.2. Determination of microremain concentrations

Calcitic dung spherulites and ash pseudomorphs were extractedfrom controls, ash and degraded dung sediment samples and a mea-sured aliquot was evenly dispersed on a microscope slide following thesodium polytungstate (SPT) method outlined by Gur-Arieh et al.(2013). Calcitic microremains were counted systematically in 16random fields of view at 400× in plane-polarized (PPL) and cross-po-larized light (XPL) using a Nikon Eclipse 50i POL petrographic micro-scope. Microremain concentrations from the archaeological and controlsediments were previously quantified and reported in millions per 1 g ofsediment in Dunseth et al., 2018: Appendix 2).

2.3. Fourier transform infrared (FTIR) spectroscopy

FTIR analysis of all samples followed the conventional KBr pelletmethod, utilizing about 0.3 g of the sediment/ash sample (Weiner et al.,

1993). Samples were analyzed in mid-IR range between 4000 and400 cm−1 at 4 cm−1 resolution using a Thermo Scientific Nicolet iS5with Omnic 9.3 software. Grinding curves were made following theprocedures outlined in previous studies of calcite, with the same KBrpellet ground to different extents using an agate mortar and pestle(Regev et al., 2010; Poduska et al., 2011). A minimum of five grinds ofthe same pellet were made per sample. Baselines for height measure-ments were determined following Chu et al. (2008: 907), examples ofwhich are shown in Fig. 2.

3. Results

Fig. 2 shows representative spectra from the degraded archae-ological dung, local aeolian dust, local limestone and experimentalmodern wood and dung ashes. Calcite peaks dominate all spectra. Notethe similarity in composition between the degraded dung and aeoliandust samples, containing calcite, clay and quartz. The limestone andmodern wood ash samples are composed entirely of calcite. Narrowingof absorbance bands and a slight shift of the ν3 carbonate peak(+1 cm−1 over 5 grinds) occurs with successive grinding of degradeddung samples (Fig. 3).

Grinding curve trendlines of the materials studied here are shown incomparison to trendlines reported in Regev et al. (2010) (Fig. 4). Thetrendlines of all four archaeological degraded dung sediments and themodern ashed dung control are clearly separated from those of aeoliandust and limestones from the vicinity of the site of Nahal Boqer. Theyhave a similar trend and position as modern and archaeological limeplasters reported earlier (Regev et al., 2010; Regev, 2011: Fig. 11;Poduska et al., 2012; Xu et al., 2015, 2016; Goshen et al., 2017). Theaeolian dust and limestone from the Negev Highlands studied hereoverlap with Regev et al.'s (2010) geogenic limestones. The trendlinesof both modern wood ash samples also overlap with those of Regevet al. (2010).

Fig. 5 shows the trendlines obtained from grinding mixtures ofknown ratios of degraded archaeological dung and aeolian dust. Withdilution of the degraded dung component, the position of the trendlinesof the mixtures shift towards the pure aeolian dust trendline, reflectingthe changes in relative proportions of the two end members.

There is no trend between archaeological spherulite concentrationsand position on the grinding curve plot (Fig. 4). Results of our mixingexperiments (Fig. 5) rule out the possibility of post-depositional mixing

Table 1List of samples, location, date and calcitic microremain concentrations (reported in millions per 1 g of sediment/ash). Error of microremain concentrations is reportedfor two samples that were prepared in duplicate. (* denotes radiocarbon-dated contexts, Dunseth et al., 2017: Table 3).

Sample Description Site Date (* = 14C age) Dung Spherulites (millions/1 gsediment)

Ash Pseudomorphs (millions/1 gsediment)

Modern Dung SampleW.ZEIT-15.3 ashed modern ovicaprine pellet Wadi Zeitan, Israel December 2015 616 1.7Degraded Dung SedimentsNB-1.2 degraded dung sediment Nahal Boqer 66,

Israel3300-2900 BCE* 75 0

NB-2.6 degraded dung sediment Nahal Boqer 66,Israel

3300-2900 BCE* 132 0

NB-3.3 degraded dung sediment + minor ashcomponent (?)

Nahal Boqer 66,Israel

c. 3000-1950 BCE 164 ± 45 0.3 ± 0.4

NB-9.2 degraded dung sediment Nahal Boqer 66,Israel

c. 3000-1950 BCE 43 ± 15 0

Modern Ash SamplesASH-4 carob (Ceratonia siliqua) 2013 0 218ASH-5 oak (Quercus calliprinos) 2013 0 279Control SamplesNB-C2 aeolian dust Nahal Boqer 66,

Israel– 0 0

NB-C7 limestone from wadi east of site Nahal Boqer 66,Israel

– – –

NB-C8 limestone from ridge immediately south ofsite

Nahal Boqer 66,Israel

– – –

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(i.e., dilution of dung spherulites by aeolian dust) as such an instancewould have resulted in change of trendline position. Additionally, allspectra reported here (including the aeolian dust controls) are domi-nated by calcite, implying that there is no relationship between calciteconcentration in the samples and their position on the grinding curveplot. We therefore suspect that this lack of correspondence betweendung spherulite concentrations and position on the plot reflects anoriginal archaeological signal, possibly related to differences in spaceuse at the site (e.g., different animal density at certain localities in thesite or different types of stabled animals).

4. Discussion

We show here that archaeological sediments and modern asheddung containing abundant dung spherulites exhibit characteristic

infrared spectral attributes. As seen in the trendlines of their calcitegrinding curves, it is easy to differentiate sediments enriched in dungspherulites from other calcareous sediments and rocks. The corre-spondence between our geogenic controls and previously publishedtrendlines of geogenic materials (i.e., Regev et al., 2010, 2015) in-dicates that our observations regarding the trendlines of normalized ν2to ν4 peak heights are not artifacts of the studied site's local environ-ment but reflect the infrared signal of calcitic dung spherulites.

The trendline of spherulite-rich deposits studied here is similar toand in the range of those previously reported for modern lime plaster,known to be composed of micritic calcite (Regev et al., 2010; Poduskaet al., 2012). Lime plaster was recently shown through X-Ray Dif-fractometry (XRD) to be composed of highly disordered calcite (Xuet al., 2016). We tentatively suggest the similarity in trendlines betweenlime plaster and degraded dung deposits may mean that dung spher-ulites are also composed of calcite that is highly disordered. This mightbe related to the biogenic formation pathway of dung spherulites froman amorphous or another disordered precursor (cf. amorphous calciumcarbonate: Addadi et al., 2003; Politi et al., 2008; Bots et al., 2012;other metastable calcium carbonates: Rodriguez-Blanco et al., 2014).Interestingly, the trendlines of deposits rich in dung spherulites ishigher in our study than trendlines of other biogenic calcites of marineorigin (sea urchin spines and tests: Regev et al., 2010: Fig. 2). We alsosuggest considering the contribution of the acicular crystal habit ofcalcite forming dung spherulites (Canti, 1997; Canti and Nicosia, 2018),and/or the spherulitic arrangement itself, as possible causes for thetrendlines observed here. While these observations may merit furtherresearch, in this article, however, we would like to focus on the ar-chaeological implications of this study.

The coincidence of sediments rich in dung spherulites and the areawhere they plot on the calcite grinding curve shows that this char-acteristic can be used as a tool to effectively and rapidly differentiategeogenic calcite-containing aeolian deposits and degraded dung de-posits during routine field or laboratory FTIR analysis of sediments.This is obvious in the observations at the site studied here—NahalBoqer 66—(Fig. 4), and we propose that it may also be a useful in-dicator in other contexts where dung spherulites are abundant.

The mixing experiments reported on here show that the signalstrength for dung spherulites decreases with increased mixing withaeolian dust. The resultant trendlines of mixed sediments plot in an areawhere chalk, flowstones, ash and aeolian dust also coincide. Site-spe-cific reference curves should be produced to utilize grinding curveseffectively in the study of archaeological sediments. As other studies

Fig. 2. Selected FTIR spectra of the materials used in this study: archaeologicaldegraded dung sediments, ashed modern dung, ashed modern wood, limestoneand aeolian dust from the study area. Indicative calcite absorbance bands arelabeled. Thin red lines indicate position of baselines used for calculation of peakheights. Note similarities between degraded dung, the dung control and aeoliandust. The degraded dung sediments show a heterogeneous composition,dominated by calcite and also including unheated clay (absorption peaks in-dicating structural water between 3600 and 3700 cm−1), quartz (peaks at 1083,797, 778, and 695 cm−1), and carbonated hydroxylapatite (shoulders at 605and 565 cm−1). (a.u.= arbitrary units). (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of thisarticle.)

Fig. 3. Effect of grinding on spectra of a representative degraded dung sediment(NB-3.3). Note absorbance bands resolve after the second grind and narrowwith subsequent grinds. The ν2 peak location shifts from 875 to 876 cm−1 withcontinued grinding.

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have noted, final characterization of sediments—and especially thosewhose grinding curves plot in the area where chalk, ash and aeoliandust overlap (Fig. 5)—should be confirmed with other methods, such asoptical microscopy.

Previous research has shown that FTIR can be an effective tool forassessing the state of preservation of disordered pyrogenic carbonateminerals to prescreen specimens for radiocarbon dating (Regev et al.,2011 and Poduska et al., 2012 for calcite in ash and plaster; Toffoloet al., 2017 for aragonite in ash). If our suggestion regarding the highdisorder of calcite forming archaeological dung spherulites can be va-lidated, then this attribute may be used to study the effect of diagenesison dung spherulites, which may then be utilized as another potentialarchaeological resource for radiocarbon dating. Additionally, thecarbon, nitrogen and oxygen in well-preserved calcitic dung spheruliteshave potential to be used for stable isotope paleoclimatic reconstructionor investigations into ancient animal diet (cf. Shahack-Gross et al.,2008).

5. Conclusion

We show here that the application of the calcite FTIR grinding curvemethod can be extended in archaeological contexts from looking intopyrogenic materials to also rapidly identifying sediments with highconcentrations of dung spherulites. While future application of thisobservation in other contexts and regions has yet to be carried out, itmay open up new vistas for in-field identification of dung depositswhich may aid reframing research questions and excavation strategieswhile excavation is on-going. The potential of dung spherulites as anindicator of preservation, a resource for radiocarbon dating, a resourcefor environmental reconstruction using their stable carbon, oxygen andnitrogen isotopes, as well as their formation pathways and taphonomy,are several open questions that may benefit archaeological investiga-tions with further research.

Fig. 4. Grinding curves of all samples studied here in comparison to curves published in Regev et al. (2010). Dung spherulite (DS) and ash pseudomorph (AP)microremain concentrations are included for reference; all values are in millions per 1 g of sediment/ash. Note that archaeological dung sediments and ashed moderndung plot together and separately from pyrogenic wood ashes and geogenic sediments and rocks. Note also that dung-rich sediments and the modern dung controlplot where lime plaster trendlines have been previously reported. (n.a.u.= normalized absorbance units).

Fig. 5. Grinding curves of quantitative mixtures (by weight % ratios) of degraded dung and aeolian dust. Note trend from 'pure dung' towards 'pure aeolian dust' withincreasing ratio of dust relative to dung. This mixing experiment indicates that the utility of the grinding curve method for the identification of dung deposits isexpected to diminish with increased post-depositional mixing of dung with geogenic deposits. (n.a.u.= normalized absorbance units).

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Acknowledgements

This work was supported by the German-Israeli Foundation forScientific Research and Development (GIF; Grant no. I-1244-107.4/2014) to R.S.-G. and Markus Fuchs (Justus-Liebig-University Gieβen) asprinciple investigators and Israel Finkelstein (Tel Aviv University) asco-investigator, and internal funds available at the Laboratory forSedimentary Archaeology, The Leon H. Charney School of MarinesScience, University of Haifa. In addition, Z.C.D. was supported by the

Dan David Scholarship for Archaeology and the Natural Sciences (2017-2018). We thank all the volunteers that participated in the excavationof Nahal Boqer 66 in 2013 and 2016, and Guy Bar-Oz and YotamTepper for the modern dung sample fromWadi Zeitan. Special thanks toLior Regev, David Friesem and Yotam Asscher for training and guidanceon FTIR and the grinding curve method in the early stages of Z.C.D.’sresearch. Thanks also to Don Butler for insightful discussion on spher-ulites and the methods described here. Finally, we would like to thankthe two anonymous reviewers who helped us improve the manuscript.

Appendix 1

Comparison between dung-derived sediments of the same sample weight (10.5 mg) with different dung spherulite concentrations. A) Focus-stacked crossed-polarized light (XPL) image of a representative field of view of sample NB-3.3 with the highest dung spherulite concentrations in thisstudy; dung spherulites can be seen well-dispersed throughout the field of view, a result of the extraction and dispersion method (Gur-Arieh et al.2013). Note that the image only captures 50% of the optical field of view. B) Focus-stacked XPL image of a representative field of view of sample NB-9.2, with the lowest dung spherulite concentration in this study. C) Box plots of the number of spherulites counted in each of the 16 fields of view forboth samples. Note the correspondence between the visual and statistical representations of the counting method, which must be based onknowledge of the weight of sediment taken for this type of analysis.

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