!"#

DOI: 10.5876_9781607322801.c011

11Oxygen Isotopes from Maya Archaeological Deer RemainsExperiments in Tracing Droughts Using Bones

Antoine Repussard, Henry P. Schwarcz, Kitty F. Emery, and Erin Kennedy Thornton

An increasing number of studies support the concor-dance between dry episodes in the Circum-Caribbean Basin and the political disruption of Maya society at the end of the Terminal Classic, during the period often called the “Classic Collapse” (e.g., Haug et al. 2003; Hodell, Brenner, and Curtis 2007; Ne; et al. 2006). However, the exact sequence of events and their impacts on the Maya people are still largely debated (see Iannone [Chapter 1], Aimers and Iannone [Chapter 2], Iannone, Yaeger, and Hodell [Chapter 3], this volume). <e aim of this chapter is not to dis-cuss these issues, but rather to examine the potential of using archaeological bone from white-tailed deer (Odocoileus virginianus) as an additional climatic proxy to track droughts in the Maya area. <e archaeologi-cal remains are especially promising for this purpose because (1) they are relatively abundant in the Maya archaeological record; (2) they are associated with cul-tural remains, allowing them to be easily and accurately dated; and, most important, (3) they are considered as a very local recorder of past meteorological conditions.

MATERIAL<e studied material consists of a total of eighty-

three bones of white-tailed deer (Odocoileus virginia-nus sp.) that were excavated from the archaeological sites of Piedras Negras (PN) by the Piedras Negras Archaeological Project directed by Stephen Houston and Hector Escobedo (71 bones), and Motul de San Jose (MSJ) by the Motul de San Jose Regional Archaeo-Ecological Project directed by Antonia Foias and Kitty Emery (12 bones). Both sites are located in

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F$%&'( ##.#. Topographical map of the Maya region, showing the southern cordillera of the Sierra Madre de Chiapas (corresponding to the Maya highlands), the relatively !at lowlands in the northern part of the peninsula, and the low-altitude Maya Mountains of southern Belize. "e location of the studied sites, the major modern city of Flores, as well as the direction of the prevailing easterly winds, are shown. Background: SRTM shaded relief map of Central America (Courtesy NASA/JPL-Caltech).

the Departamento del Peten, Guatemala; Piedras Negras along the shores of the Usumacinta River in western Peten; and Motul de San Jose a few kilome-ters from the Peten Itza Lake in Central Peten (Figure 11.1).

Bones were identi=ed to taxon, element, side, and age by <ornton and Emery (project zooarchaeologists for both the Piedras Negras and Motul de San Jose projects) using the comparative collections of the Florida Museum of Natural History (Environmental Archaeology Program). Only samples from separate adult individuals were selected by Emery and <ornton for this study. <is determination was based on contextual and chronological separa-tion between specimens or overlapping unique elements (e.g., two adult right deer femurs) within the same context. Context information was derived from published archaeological reports from the archaeological sites with assistance from archaeological project personnel. All bones are from assemblages well dated by stratigraphic and ceramic analysis by archaeologists associated with the two projects, and range from Preclassic (Piedras Negras) to Postclassic periods (Motul de San Jose and Piedras Negras; Table 11.1).

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OXYGEN ISOTOPES FROM MAYA ARCHAEOLOGICAL DEER REMAINS !""

BACKGROUNDBioapatite, the mineral fraction of bone and teeth, has been described as a

distinctive type of hydroxylapatite (Ca10(PO4)6(OH)2), in which phosphate groups (PO4

3-) are frequently (~7%) substituted by carbonate groups (CO32-;

T)*+( ##.# Origin, age, type, and number of bones analyzed in this study.

Cultural phase Years (AD)~Major period

Num-ber of bones Bone types

Motul de San JoseLate Classic 600–800 6 FemurTerminal Classic

800–1000 5 Coxa, humerus, rib, tibia

Early Postclassic

1000–1250 1 Metatarsal

All phases 12Piedras Negras

Abal/Pom 300 BC–350 2 Ilium, metatarsalNaba 350–560 Early

Classic4 Humerus, ilium,

mandibleBalche 560–620 Early

Classic12 Acetabulum, calcaneus,

femur, mandible, scapula, tibia, vertebrae

Yaxche 620–750 Late Classic

26 Astragalus, calcaneus, femur, Humerus, Ilium, ischium, mandible, phalanx, pubis, radius, scapula

Chacalhaaz 750–850 Late Classic

24 Astragalus, femur, humerus, ilium, ischium, mandible, metacarpal, metapodial, metatarsus, phalanx, radius, scapula, tibia

Kumche 850–900 Terminal Classic

3 Metatarsal, tibia

All phases 71

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Wright and Schwarcz 1996). Analyses of bioapatite oxygen isotopic compo-sition (į18O) can thus be conducted either on atoms carried by phosphate groups (į18OP) or on those carried by carbonate groups (į18OC).

Sources of Oxygen in Deer Bone BioapatiteSeveral studies on modern mammals have shown that, in the case of oxy-

gen, there is an isotopic equilibrium between mineralized tissues (bones and teeth) and body water (e.g., Fricke and O’Neil 1996; Longinelli 1984; Luz, Kolodny, and Horowitz 1984), this balance being species speci=c (e.g., Bryant and Froelich 1995; Fricke and O’Neil 1996). <e oxygen present in a mam-mal’s body water comes from three possible sources: the drinking water, the atmospheric oxygen, and the oxygen present in the food (e.g., Luz, Cormie, and Schwarcz 1990). When species-speci=c relations are known, it has been established that mammalian homeothermy allows the direct reconstitution of these parameters from the isotopic ratio of oxygen measured on mineralized tissues, for animals weighing more than 1 kg (e.g., Bryant et al. 1996; Kohn 1996).

White-tailed deer are nonmigrating browsers, widely distributed through-out North and Central America (Cormie, Luz, and Schwarcz 1994). <ey are also forage concentrators and opportunistic edge browsers. <ey subsist almost entirely on energy-rich leafy forbs or C3 Dicotyledonae (e.g., shrubs, wheat) and reject =brous tropical C4 plants (e.g., crabgrass, sorghum), unless driven to starvation (Bello et al. 2001; Brown 1994; Harlow and Guynn 1994; Kroll 1994), while opportunistically foraging on the domesticated C4 maize (Cormie and Schwarcz 1996; Emery, Wright, and Schwarcz 2000). White-tailed deer have the ability to thrive in semiarid regions because of their low drinking-water requirements; their water needs can be mostly satis=ed by the water contained in the plants ingested (i.e., by leaf water; Villarreal Espino-Barros and Marín Fuentes 2005). <ose plants generally have shallow root systems that directly absorb rainwater, without depending on groundwater (Cormie, Luz, and Schwarcz 1994). As a consequence of their low drinking-water requirements, it has been established that the major source of oxygen of deer body water (and ultimately bioapatite phosphate and carbonate-bounded oxygen) is the water contained in the leaves ingested by the animal (Cormie, Luz, and Schwarcz 1994; Luz, Cormie, and Schwarcz 1990). <e oxygen isotopic composition measured on both bioapatite carbonate groups and phosphate groups thus reJects that of ingested leaf water at a very local scale.

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OXYGEN ISOTOPES FROM MAYA ARCHAEOLOGICAL DEER REMAINS !"-

Oxygen Isotopic Composition of Leaf WaterNo fractionation of oxygen is observed in terrestrial plants during water

uptake and transport of crude sap in xylem (e.g., Barbour 2007; Landais et al. 2006), so the oxygen isotopic composition of leaf water (į18OLeaf) ingested by the deer is primarily related to the į18O of soil water (į18OSoil). <e į18OLeaf remains unchanged from that of soil water until a;ected by water loss via evapotranspiration mechanisms, which results in the residual leaf water being enriched in 18O, induced by the selective evaporation of water mol-ecules containing light oxygen atoms 16O. <us, evapotranspiration increases the į18OLeaf (Barbour et al. 2004; Webb and Longsta;e 2006). Given that changes in the local atmosphere’s relative humidity (RH) control the plant’s transpiration, leaf water becomes enriched in 18O (i.e., reaches higher val-ues of 18OLeaf) with decreasing RH, as shown on Figure 11.2. In the present study, we will assume that the overall processes of enrichment of leaf water in 18O are mainly driven by the humidity gradient between the inside of

F$%&'( ##.!. Enrichment of leaf water versus relative humidity in #ve C$ grass species. All grasses were grown in chambers where the isotope ratio of source water was held constant, and only relative humidity varied across treatments. Note that relative humidity was calculated based on leaf temperature (based on Farquhar, Cernusak, and Barnes 2007).

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the leaf and the surrounding air. Based on the measurements of Graham D. Farquhar, Lucas A. Cernusak, and Belinda Barnes (Farquhar, Cernusak, and Barnes 2007), it appears that a 10 percent decrease in relative humidity results in a ~3 ‰ increase of į18OLeaf.

Oxygen Isotopic Composition of Rainwater in the Maya Region<e isotopic composition of rainwater depends on the isotopic composi-

tion of the vapor source (principally ocean water), as well as climatic param-eters such as air temperature and/or precipitation amounts (e.g., Bowen 1991; Dansgaard 1964). In the tropics, the low monthly and annual thermal amplitudes make the e;ects of air temperature variations on į18O values of the local rainwater (į18ORain) negligible. <e Rayleigh distillation pro-cess occurring during the condensation of the vapor mass (clouds) thus becomes the predominant e;ect on the Juctuations of į18ORain, leading to the amount e%ect, in which į18ORain decreases with increasing amount of rain-fall (Dansgaard 1964).

<e Global Network of Isotopes in Precipitation (GNIP; IAEA/WMO 2009) is a convenient data source for seasonal Juctuation in į18O of rain. <e only data set from GNIP that was collected close to the Maya region has been recorded at the port of Veracruz (Mexico). <is gives temperature, vapor pres-sure, and precipitation, as well as į18ORain, on a monthly basis from 1962 to 1989 (n = 130). Figure 11.3 shows that the highest values of į18ORain are seen during winter (dry season), when the Inter-Tropical Convergence Zone (ITCZ) is moving southward away from the Maya region, leading to reduced amounts of rainfall (< 50 mm.month–1; Haug et al. 2003). Conversely, lowest į18ORain values are seen during summer (rainy season), when the ITCZ lies precisely on the Maya region, providing heavy rainfall (~300 mm.month–1).

<e following equation gives monthly averages of į18ORain as a function of monthly average precipitation amount (pp) at Veracruz:

[Equation 1] (n = 12; r2 = 0.79)<e strong correlation between monthly averages of į18ORain and precipi-

tation amounts clearly shows the primacy of the amount e;ect on the iso-topic composition of rainwater in the Maya region. Finally, one must note that į18OSoil may vary from that of į18ORain at a given site, because of direct evaporation from the soil surface, the latter leading to a gradient of į18O in soil moisture, from high į18OSoil values near the surface to lower ones deeper in the soil (Barbour 2007).

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EXPECTED CONSEQUENCES OF DROUGHTS ON DEER BONE b!"O<e use of oxygen isotopic composition of deer bones as humidity indicator

was originally proposed in 1990 (Luz, Cormie, and Schwarcz 1990). At the same time a similar strategy to that used in our research was applied to kan-garoo bones, because of their low drinking-water requirements (Ayli;e and Chivas 1990). In the Maya context, we consider the deer bone oxygen isotopic ratios to reJect changes in the average į18O of water in the leaves consumed by the deer. <is is in turn controlled by two combined e;ects: (1) changes in į18ORain, principally reJecting the amount e;ect causing į18ORain to increase with decreasing amount of rainfall (Figure 11.3); and (2) variations in relative humidity causing both leaf water į18O and soil į18O to increase with lowered humidity levels (Figure 11.2). Both of these e;ects would lead to a shift of bioapatite į18O (į18OP and į18OC) to higher values during droughts.

Deer Bone’s Temporal ResolutionIn vivo, bone is continuously resorbed by bone-destroying cells (osteoclasts),

and redeposited by bone-forming cells (osteoblasts), obliterating and then

F$%&'( ##.". į&'ORain values plotted as a function of precipitation amounts, recorded at Veracruz (monthly averages). Linear correlation is shown, as well as corresponding equation and correlation coe(cient r ) (source: GNIP [IAEA/WMO 2009]).

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!"0 REPUSSARD, SCHWARCZ, EMERY, AND THORNTON

depositing osteons, the structural units of cortical (dense) bone mineral (e.g., Van Oers et al. 2008). Little is known about deer bone turnover rate, but some studies have shown that this process is correlated with metabolic rate (Darr and Hewitt 2008) and seems to be strongly bone dependent (Baxter, Andrews, and Barrell 1999; Hillman, Davis, and Abdelbaki 1973). As a consequence of constant bone remodeling, the sampling of dense bone material consists of a complex mesh of osteons, whose age spans the life of the deer. It is thus con-sidered that each analyzed bone sample is likely to represent a “time-average” of biomineralization processes that took place within the span of a deer’s life.

All į18OP and į18OC values were obtained on archaeological deer bones iden-ti=ed as coming from separate adult individuals, so it is assumed that each deer bone averages roughly 2.5 years of the environmental record. <is period of time corresponds to what is believed to be an appropriate age for culling using sustainable “resource management” under high hunting impact. <is approach would allow a suMcient amount of time for deer to reach their sexual maturity (~1.5 years) and to breed (Brunjes et al. 2006; Smith 1991).

<e slow rate of bone turnover makes brief (< 2.5 years) and/or moder-ate droughts (i.e., causing no signi=cant change in į18OLeaf over 2.5 years) hardly detectable by isotopic analyses. <ese events would correspond to Palmer Drought Indexes superior to –3 (Palmer 1965) and corresponding to

“abnormally dry” and “moderate drought” categories, as described by Me-Bar and Valdez (2003). On the other hand, “severe,” “extreme” and “exceptional” droughts (longer than 2.5 years, and having a signi=cant e;ect on į18OLeaf over that interval), corresponding to PDIs inferior to –3 (Me-Bar and Valdez 2003), could be detected with con=dence.

Magnitude of Expected Deer Bone Bioapatite į18O ShiftBoaz Luz, Allison Cormie, and Henry Schwarcz (Luz, Cormie, and Schwarcz

1990) have measured deer bone į18OP on many specimens with a wide geo-graphic distribution over North America, and showed that deer bone į18OP is strongly dependent on yearly averages of į18ORain and relative humidity:

[Equation 2] (n = 44; r2 = 0.95)where RH is relative humidity (between 0 and 100). <e high correlation coef-=cient of Equation 2 con=rms the control of į18ORain on į18OP values, and its modulation by relative humidity.

<e yearly average values of į18ORain at Piedras Negras (PN) and Motul de San Jose (MSJ) today are not known, but can be inferred from the

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existing meteorological data. <e detailed climatic records of monthly aver-ages of temperature, relative humidity and precipitation amounts were recorded on a monthly basis from 1990 to 2006 at Flores (Peten, Guatemala; INSIVUMEH 2009). <e location of Flores, on the southern shore of Lake Peten-Itza, is relevant to this study because: (1) it is a few kilometers away (~10 km) from the archaeological site of Motul de San Jose; and, (2) it is situated ~150 km east of Piedras Negras, on the path of the prevailing east-erly trade winds that are not blocked by any orographic feature (Williams 1976; Figure 11.1). Even though Flores presumably receives somewhat higher rain amounts than sites located further away from the lake (due to the direct recondensation of evaporated lake water), this study focuses on the relative di;erences in rain amounts, and consequent changes in į18ORain values (and ultimately deer bone į18O values) between dry and wet periods, at a given site.

Total precipitation and yearly average humidity for the years 1994, 1997, 1998, and 2006 are presented in Table 11.2. It can be seen that yearly average pre-cipitation can be highly variable (from ~1,200 mm in 1994 to more than 2,400 mm in 2006; average 1990–2006 is ~1,700 mm.year–1). <is high variability in annual rainfall is not surprising: several meteorological studies in the Central American / Caribbean region have revealed recurrent years of “abnormal” pre-cipitation, related to the Juctuations in the intensity of the annual dry/wet seasonality cycle, a distinctive feature of the region (e.g., Curtis, Hodell, and Brenner 1996). <e variability of yearly average precipitation on a decadal basis is also con=rmed by an older climatic data set from Flores available in Edward Deevey and Don Rice (Deevey and Rice 1980), presenting average monthly precipitation records for the periods 1934–42 and 1971–76. It shows that the period from 1934 to 1942 was relatively wet (average precipitation ~2,000 mm.year–1) compared with the period from 1971 to 1976 (average precipitation

~1,400 mm.year–1).When Equations 1 and 2 are applied to the available climatic data set from

Flores, it becomes possible to calculate how deer bone į18OP would change in response to changes in annual precipitation and yearly average relative humid-ity, as presented in Table 11.2. <ose numbers are to be taken with caution, given the uncertainties related to the inferred values and especially the abso-lute į18ORain values (see above). However, it can be seen that rain distribution, as well the Juctuations of relative humidity through the year, can impact both į18ORain and the expected deer bone į18OP values.

We conclude that we would expect a shift of deer bone į18OP toward higher values by about 1.5 ‰ for an animal that has lived during conditions of severe

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drought. One can notice that the magnitude of this change lies in the range calculated by Matthew Kohn (1996) using the formula and data set of Luz, Cormie, and Schwarcz (1990), that is, a į18OP increase of 1.7(± 0.4) ‰ per 10 percent decrease in yearly averaged relative humidity, corrected for į18ORain variations.

METHODSampling Procedure

In each archaeological bone sample, we extracted a small piece (~0.25 cm3) of cortical bone in zones where bone layer was the thickest. <e external and inter-nal layers of cortical bone were manually removed to a depth of about 2 mm, using a handheld electric drill operated at low speed under a trickle of water (to prevent any local heating), to completely remove the periosteum, endosteum, and beyond. Each bone fragment was then ground in a liquid nitrogen cooled mill. Among the 83 bones analyzed in this study, 24 of them were large enough to permit sampling at two di;erent locations on the same bone.

Isotopic Measurements<e sample preparation for į18Op analyses followed the protocol from Damon

Bassett et al. (2007). <e sample preparation for į18OC analyses has been adapted from the oNine method outlined by Paul L. Koch, Noreen Tuross, and Marylin L. Fogel (Koch, Tuross, and Fogel 1997). <e į18OC measurements were

T)*+( ##.! Expected deer bone bioapatite į18Op in relation to average climatic conditions that have occurred at Flores during relatively dry (1994 and 1998) and wet years (1997 and 2006).

YearTotal precipitation

(in mm)aYearly average

relative humidityaEstimated b yearly average į&'ORain

Expected c deerbone į&'OP

(in *)1994 1,160 76 –1.9 20.41998 1,456 74 –2.2 20.51997 1,600 83 –2.3 18.92006d 2,395 77 –3.6 19.1

a Available in Flores climatic data set (INSIVUMEH 2009).b Using Equation 1.c Using Equation 2.d November 2006 values missing and were not taken into account.

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conducted on an Optima Isotope Ratio Mass Spectrometer (IRMS) and į18OP measurements on a <ermo Finnigan DELTAPlus GC-IRMS, attached to a <ermo Finnigan high-temperature elemental analyzer (TC-EA). Oxygen isotopic results are presented relatively to Vienna Standard Mean Ocean Water (V-SMOW), using the standard delta (į) notation in parts per thou-sand (Coplen 1996). All isotopic analyses were replicated three times and were conducted in the stable isotope laboratory at McMaster University (Ontario, Canada).

Diagenesis TestingStructure and chemical composition of archaeological bone can be altered

after an animal’s death by diagenetic processes (disintegration, dissolution, precipitation, bacterial remodeling, mineral replacement, ionic substitutions or recrystallization; e.g., Hedges 2002; Munro, Longsta;e, and White 2007). Ensuring that bone mineral has kept its original isotopic signature is then crucial.

Bone appearance Color, morphology, and histology can give information on possible post

mortem chemical alterations (Hedges 2002; Munro, Longsta;e, and White 2007; Shipman, Foster, and Schoeninger et al. 1984). To minimize the use of altered bones, charred and crumbly bones were automatically screened out, and only cortical bone has been sampled.

Infrared spectrometryBioapatite integrity has been checked by Fourier-Transform Infra-Red

(FTIR) spectrometry (e.g., Shemesh 1990; Sponheimer and Lee-<orp 1999; Termine and Posner 1966; Trueman, Privat, and Field 2008; Wright and Schwarcz 1996; Wopenka and Pasteris 2005). Samples showing inclu-sions of nonbiogenic minerals in the bone matrix such as calcite or francolite (betraying partial post mortem replacement of bioapatite) were screened out (Sponheimer and Lee-<orp 1999).

Comparison b�+,OC - b�+,OP Note that (1) phosphate- and carbonate-bound oxygen in bioapatite are

composed of oxygen atoms from the same body water source (cogenesis), and (2) metabolic reactions among mammals occur at a constant temperature (~37°C), in vivo. <erefore we expect that CO3 and PO4 are in isotopic equilib-rium (e.g., Lee-<orp 2000). <e relation between į18OC and į18OP has been

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calculated by empirical measurement and is considered to be (Iacumin et al. 1996):

[Equation 3] (n = 31; r2 = 0.98)<e 8.5 ‰ di;erence reJects the di;erent global fractionation factors

between the two metabolic routes from the body water to bioapatite phos-phate and carbonate groups (Figure 11.4; Bryant et al. 1996; Iacumin et al. 1996; Lee-<orp 2000). Given that phosphate-bound oxygen is less sensitive to chemical weathering than carbonate-bound oxygen (Kohn, Schoeninger, and Barker 1999; Longinelli 1984), it has been suggested that this linear relation-ship between į18OC and į18OP could be used as a test of diagenetic alteration of skeletal remains (e.g., Bryant et al. 1996; Kolodny and Luz 1991; Iacumin et al. 1996). Diagenetically altered bones are expected to show deviation from the theoretical line de=ned by Equation 3. In the present study, it has been decided arbitrarily to set the limits for a maximum accepted range of devia-tion from the expected equilibrium line, by following Bryant et al. (1996). <e corresponding correlation equation (represented by the shaded area shown on Figure 11.4) is[Equation 4]  (n = 42, r2 = 0.99)

It will be thus considered that values falling outside this range are diage-netically altered, and bones showing such relation between į18OC and į18OP were screened out.

RESULTSPrecision of Isotopic Measurements

A total of 107 samples were processed for į18OC and į18OP measurements. Each measurement has been replicated three times for each sampled bone powder. Table 11.3 summarizes the reproducibilities obtained on standards and samples for each set of isotopic measurement.

į+,OP measurements Replicated measurements of standards give a reproducibility comparable

to other studies using a similar method (e.g., Dettman et al. 2001; O’Neil et al. 1994; Vennemann et al. 2002; Wiedemann-Bidlack, Colman, and Fogel 2008). As for deer bone į18OP reproducibility, the most striking feature of the data set is the very high variability of standard deviations from triplicated

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F$%&'( ##.,. Relationship between į&'OP and į&'OC in several samples of modern mammal bioapatite (teeth and bone). "e solid line is the best #t indicated by Equation -. "e į&'O values of modern apatitic shells are also shown (Longinelli and Nuti 1973), along with the calculated straight line from geological calcite-apatite equation at -.° calibrated by Shemesh, Kolodny, and Luz (+/,,). "e shaded area corresponds to the accepted range of deviation from the equilibrium line in this study to consider deer bone bioapatite as unaltered (refer to Equation 0; modi#ed from Iacumin et al. 1996).

measurements, ranging from ~0‰ to ~1.4 ‰. <e reasons underlying such variability remain unclear. Although organic matter contamination of the samples is the most probable explanation, no signs of it were detected dur-ing sample preparation (O’Neil et al. 1994) or data acquisition (Wiedemann-Bidlack, Colman, and Fogel 2008).

į&'OC measurements<e reproducibility we obtained for į18OC is somewhat poor compared to

other studies using the same oNine reaction method (i.e., standard devia-tions ı close to 0.1 ‰; e.g., Wright and Schwarcz 1996; Koch, Tuross, and Fogel 1997; Garvie-Lok, Varney, and Katzenberg 2004). Although the oNine method is tedious, it is recognized to be the one giving the best reproducible results, compared to more recent and quicker online methods (e.g., Gasbench,

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Multiprep; Metcalfe, Longsta;e, and White 2009). <e average reproduc-ibility of į18OC measurements on deer bone bioapatite is comparable to the standard reproducibility (average ı at Piedras Negras is 0.4 ‰, n = 93), but some samples exhibit a high degree of scattering (maximum ı of 1.2 ‰ at Piedras Negras). No correlations between FTIR indexes and į18OC reproduc-ibility have been found (data not shown). However, a recent study (Metcalfe, Longsta;e, and White 2009) conducted on human bones from Maya sites of the Paci=c coast of Guatemala (dated from AD 250 to 400) obtained several results that support the validity of the method we used for į18OC measure-ments. Interestingly, the same authors highlighted “abnormal” low and highly variable į18OC values on some of their bones that are believed to be related to the production of an unidenti=ed contaminant gas during CO2 liberation. Similar abnormally low and inexplicable į18OC values were also observed by Lori Wright and Henry Schwarcz (Wright and Schwarcz 1996) on human bones from another Maya site of the Peten region, in Guatemala (dated from AD 600 to 950). <e high degree of scatter of į18OC we obtained on some bones is thus possibly related to either (1) the incomplete removal of organic matter, as in the case of į18OP measurements (see above); or (2) the diagenetic context of Maya sites (i.e., high levels of soil moisture).

In the end, only į18OP values are considered to be reliable to reconstruct past climate variations in the Maya region, so the poor reproducibility of į18OC measurements, although a;ecting the accuracy of the į18OP-į18OC diagenesis test, should have no signi=cant impact on the reliability of the following cli-mate reconstruction.

T)*+( ##." Summary of the reproducibilities obtained during the di;erent isotopic measurements.

Measure

Standards Deer bone samples

Name ıRepli-cates Location min ı

max ı

Aver-age ı Samples

į18OC NBS–18 0.4 17 MSJ 0.10 1.0 0.5 14NBS–19 0.3 17 PN 0.02 1.2 0.4 93

į18OP Benzoic acid 0.2 42 MSJ 0.10 1.0 0.5 14USGS–35 1.0 10 PN 0.03 1.4 0.4 93

Notes: For deer bones samples, standard deviations ı were calculated from the three replications of each isotopic measurement. All standard deviations are given in ‰. MSJ: Motul de San Jose; PN: Piedras Negras

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Diagenesis ScreeningAmong the twenty-four bones sampled twice, three con=gurations are

observed while comparing į18OP to į18OC values: (1) both the duplicates fall outside the stated range (1 case), so the bone from which the duplicates are from has to be eliminated for further use; (2) both the duplicates fall inside the range (18 cases out of 24), so the average į18OP and į18OC values from the duplicates are then considered for further use; and (3) one of the duplicate falls outside the range (5 cases), so that only the sample from which the couple į18OP-į18OC falls inside the range is considered for further use. In the end, =ve bones were rejected by į18OC-į18OP comparison (one from Motul de San Jose and four from Piedras Negras) and one because of the presence of calcite detected by FTIR spectrometry (from Motul de San Jose). A total of 77 bones (Table 11.4) were then =nally selected for reconstructing past climate varia-tions at Motul de San Jose and Piedras Negras.

Representativeness<e duration of archaeological ceramic phases may contribute to uncer-

tainty in the climatic representativeness of the data, as well as in comparing records between sites (Iannone, Yaeger, and Hodell, Chapter 3 in this volume). Individual cultural phases are generally long (50 to 210 years). One must there-fore be sure to analyze a suMcient number of individual deer bones to maxi-mize the chances of analyzing a broad range of specimens from across the phase. <erefore, we consider that Piedras Negras provided us with a reasonable num-ber of diagenetically tested samples for the Balche, Yaxche, and Chacalhaaz phases (see Table 11.4). <e density of sampling was signi=cantly less for the other cultural phases, as well as for Motul de San Jose, so the data are poorly supported and less defensible.

Variations of Deer Bone ǻ18OP through TimeFigure 11.5 presents the variations of į18OP through time at Motul de San

Jose and Piedras Negras. Obviously, the spread of the data in each cultural phase, represented by vertical whiskers, does not highlight any signi=cant cli-mate change at either Piedras Negras or Motul de San Jose. However, accord-ing to our model, the decrease seen in average į18OP isotopic signatures at both Motul de San Jose and Piedras Negras indicates a progressive trend toward overall wetter climate throughout the Late Classic phases. In particular, when

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T)*+( ##., Summary of all the į18op and į18oc values measured on samples of archaeological deer bones from Motul de San Jose and Piedras Negras.

Cultu

ral ph

ase

Years (

AD)

Bones

samp

led

Bones

accep

ted(*)

Avera

ge į+,

O P

Min į+,

O P

Max

į+,O P

St. de

v.(+ ı

)

Avera

ge į+,

O C

Min į+,

O C

Max

į+,O C

St. de

v.(+ ı

)

Motul de San JoseLate Classic 600–800 6 5 21.9 20.2 23.4 1.2 30.2 29.6 31.8 0.9Terminal Classic

800–1000 5 4 21.1 20.2 22.4 1.0 30.1 29.5 30.8 0.5

Early Post Classic

1000–1250 1 1 22.0 22.0 22.0 N/A 29.8 29.8 29.8 N/A

All phases 12 10 21.6 20.2 23.4 1.1 30.1 29.5 31.8 0.7

Piedras NegrasAbal/Pom 300 BC–

3502 1 19.6 19.6 19.6 N/A 30.5 30.5 30.5 N/A

Naba 350–560 4 4 20.4 20.1 21.1 0.5 29.8 28.6 30.6 0.9Balche 560–620 12 11 20.9 19.8 22.7 1.0 30.3 28.6 31.5 0.9Yaxche 620–750 26 26 20.6 18.7 22.2 0.9 30.0 28.0 31.6 1.0Chacalhaaz 750–850 24 22 20.3 18.5 22.4 1.1 29.8 28.3 32.2 0.9Kumche 850–900 3 3 20.7 19.3 22.9 1.9 29.9 28.4 31.5 1.5

All phases 71 67 20.5 18.5 22.9 1.0 30.0 28.0 32.2 0.9Notes: All isotopic values are given in ‰, relative to V-SMOW. <e table indicates the number of bones analyzed per cultural phase and the number of bones =nally accepted after diagenesis screen-ing. All standard deviations ı are given in ‰. N/A: not applicable; (*) bones for which no calcite was detected by FTIR spectroscopy and the comparison į18OC /į18OP falls in the range į18OC(± 1.3) = 1.02(± 0.04) × į18OP + 8.3(± 0.7).

compared to Yaxche and Balche, the Chacalhaaz phase shows no sign of any severe and/or prolonged dry period at Piedras Negras at that time. Given the magnitude of isotopic change expected to be found in deer bioapatite and the representativeness of those periods, we believe that extreme and/or severe droughts would have been detected. Surprisingly, we found no indica-tion of (1) the =rst phase of the Terminal Classic drought detected by Hodell, Brenner, and Curtis (2007) at ~AD 760–70, based on core analysis from Lake Chichancanab (Mexico); or (2) the intense nine-year drought centered on

~AD 810, inferred from low titanium content in Cariaco Basin sediments

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(Haug et al. 2003). Values in the Kumche phase were slightly higher suggest-ing a drying phase during this Terminal Classic period.

DISCUSSIONOnce again, the absence of signi=cant change in į18OP values from Balche

to Chacalhaaz cultural phases at Piedras Negras indicates no perceptible change in climate throughout the 290 years covered, but some comments can be made on the results we obtained.

Intersite į18OP ComparisonOn Figure 11.5, it can be seen that į18OP values from Motul de San Jose are

on average higher than those from Piedras Negras, in spite of the low number of samples. <is observation is not surprising. As stated earlier, the Rayleigh distillation process is the predominant control on į18ORain in the Maya region, resulting in two coupled isotope e;ects: the amount e;ect and the continental

F$%&'( ##.-. Summary plot of į&'OP values measured on archaeological deer bones from Motul De San Jose and Piedras Negras. Upper and lower time lines, as well as horizontal whisker caps, indicate the boundaries of the cultural phases distinguished at MSJ and PN, respectively (Kum: Kumche). Each phase is represented by its average value (ż: Motul De San Jose, Ŷ: Piedras Negras), plus or minus standard deviation (vertical whisker caps).

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!,0 REPUSSARD, SCHWARCZ, EMERY, AND THORNTON

e%ect (Gat 1996; Merlivat and Jouzel 1979). <e latter refers to the tendency for į18O of precipitation to decrease with distance from the sea. In Belize and northern Guatemala, a recent study has shown that (1) the amount e;ect pro-vokes a decrease of į18ORain values by ~1.2‰ per 100 mm increase of monthly average rainfall; and (2) the continental e;ect is of the order of ~0.7‰ decrease in į18ORain per 100 km distance from the Caribbean coast, once corrected for altitude e;ects (Lachniet and Patterson 2009). Today, the annual precipitation is close to 3,000 mm at Piedras Negras, and slightly less than 2,000 mm at Motul de San Jose (INSIVUMEH 2009).

Given that Motul de San Jose and Piedras Negras are fed with the same water vapor derived from seawater evaporated on the Caribbean Basin, and Piedras Negras being located downwind relatively to Motul de San Jose (Figure 11.1), the combination of the amount e;ect and the continental e;ect would cause the average į18ORain at Piedras Negras to be ~1.5 ‰ lower than the į18ORain at Motul de San Jose (Lachniet and Patterson, 2009). If it is assumed that other climatic Juctuations (relative humidity and temperature) were similar in the past between Motul de San Jose and Piedras Negras, we would expect a ~1.5 ‰ di;erence in deer bone į18OP between Piedras Negras and Motul de San Jose. When the Late Classic į18OP data set from Motul de San Jose (AD 600–800, n = 5) is compared to the į18OP data set from Piedras Negras—merging Balche, Yaxche, and Chacalhaaz phases (i.e., AD 560–850, n = 62)— the average į18OP at Motul de San Jose is ~1.4 ‰ higher than the one at Piedras Negras (21.9 against 20.5 ‰), with similar standard deviations of the two data sets (1.2 against 1.0 ‰), as expected. <is observation helps to con=rm the reliability of į18OP as a recorder of past climate in spite of the dif-ference in the numbers of bone samples included in each data set (5 against 62).

Expected versus Obtained į18OP ValuesWe estimated the average expected į18OP values for Piedras Negras and

Motul de San Jose from known average RH values and calculated į18ORain values for these localities using Equations 1 and 2. <e expected į18OP values presented in Table 11.2 are to be taken with caution for several reasons:

1. Estimate of į18ORain. <e yearly averaged į18ORain at Piedras Negras could be estimated based on the relation found between precipita-tion amount and measured į18ORain at Veracruz (Equation 1). Even if dominant winds are blowing from east to west, Veracruz is located on the coast of the Gulf of Mexico, so the į18ORain measured there may

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be inJuenced by water evaporated from the Gulf of Mexico, unlike at Piedras Negras and Motul de San Jose. <is inJuence of “foreign” water vapor could a;ect the accuracy of the estimated į18ORain values at Piedras Negras and Motul de San Jose.

2. Estimate of RH. We assume that the range of RH was not dissimilar to that obtained from RH excursions seen today at Flores. In the ancient past, however, even larger changes in RH may have occurred over decadal time ranges (Hodell, Brenner, and Curtis 2007), which would have led to higher shifts in į18OP (i.e., > 1.5 ‰).

3. Validity of Equation 1. Equation 2 was based on a į18OP data set obtained on deer bones coming from temperate regions (across the United States and southern Canada), where į18ORain is principally controlled by variations in air temperature. In those regions (espe-cially the northernmost ones), Luz, Cormie, and Schwarcz (1990) considered that bone growth tends to be more rapid in the spring and summer, when food supplies are more abundant. At Piedras Negras and Motul de San Jose, on the other hand, the climate is tropical and changes in į18ORain are mainly controlled by the amount e;ect. <erefore the true relationship between į18OP, į18ORain, and RH may be slightly di;erent from what is expressed in Equation 2.

In spite of these uncertainties, the į18OP values measured on deer bones from Piedras Negras and Motul de San Jose are very similar to those predicted using Equation 2 for a climate range like that at Flores (compare Tables 11.2 and 11.4). However, the variation in į18OP at both Motul de San Jose and Piedras Negras is small (ı ~1‰) and suggests that within each period, there were not large excursions in RH or precipitation. Finally, one should note that at Flores, average annual RH does not correlate with average annual precipi-tation, and yet we would expect that would be generally true; these data are therefore perhaps not the best analogue to real variable RH conditions.

Sensitivity of Deer to Climate ChangeCorrelating the climatic patterns inferred from our į18OP data to their

impacts on Maya populations would require an extensive study of their agri-cultural procedures. <e number of consecutive dry years needed to durably a;ect the Maya people is uncertain, and depends on the strength, the dura-tion, and the spread of dry episodes among a single year. Me-Bar and Valdez

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!-2 REPUSSARD, SCHWARCZ, EMERY, AND THORNTON

(2003) noted that the de=nition of a dry period can be variable, with respect to strength and duration. For example, it is now recognized that drought phases in the Maya region (ranging from 3 to 10 years) can be separated by periods of quite high precipitation (Haug et al. 2003).

As a whole, this region displays stochastic climatic events, as modeled by Barry Hunt and Tracey Elliott (Hunt and Elliott 2005). For example, some researchers found no correlation between droughts and El-Niño climatic events (Messenger 2002), whereas others state that supraregional climatol-ogy (linking California, Mesoamerica, and South America) is driven by solar cycles with a ~200 years periodicity (Schimmelmann, Lange, and Meggers 2003). At a smaller scale, precipitation is highly variable, even in the di;erent regions of Guatemala itself (INSIVUMEH 2009). <is high variability can either reduce or increase the impact of a regional dry period on a given site, and makes generalization diMcult. <e limited migratory range of deer makes it an ideal recorder of local climatic conditions. <is limitation suggests that the contradiction between our results and the =ndings of both Gerald Haug et al. (2003) and Hodell, Brenner, and Curtis (2007) could possibly be explained by the spatial resolution of our method; that is, more intense drought could have occurred elsewhere in the region. <is would raise questions about the ubiquity of the Great Maya Drought. Note however that Piedras Negras is located in one of the wettest zones of the Maya region (~3,000 mm of rain per year). <is position may have bu;ered Piedras Negras from drought e;ects being experience elsewhere.

Environmental FactorsFinally, the environment surrounding each archaeological site may also

have impacted į18OP. For example, Motul de San Jose was in the deforested Peten Lake region (e.g., Leyden 2002) whereas Piedras Negras was likely never deforested (Beach, Dunning, et al. 2006). Even though the evolution of forest cover at a certain site should be taken into account, given that leaf water enrichment mechanisms would be lower in dense forests (holding more atmospheric moisture under the canopy) than in deforested areas, we have no evidence of such e;ect. Intersite į18OP comparison shows the predominance of both continental and amount e;ects (see above).

Linking Climate and Piedras Negras History?Archaeological evidence shows that Piedras Negras was a powerful and

inJuential regional center in the Maya Lowlands, continuously occupied from

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the Middle Preclassic (~500 BC) to the Terminal Classic (until ~AD 900; Muñoz 2004). <e end of the Chacalhaaz ceramic complex (AD 750–850) is de=ned by the introduction of foreign ceramic styles in the archaeological record, marking the end of the regional inJuence of Piedras Negras (Houston et al. 2003). A population estimate of Piedras Negras highlighted a gradual abandonment of the site at around AD 850, leading to a dramatic decrease in population between the Chacalhaaz (AD 750–850) and Kumche (AD 850–900) ceramic phases (Nelson 2005). Further investigations have shown that the population started to decrease at around AD 810, at a time the ruler of the site had been captured, implying deep sociopolitical changes at Piedras Negras, but other sites located near Piedras Negras remained inhabited for decades after the royal court imploded (Scherer and Golden, Chapter 10 in this volume).

<e great intrinsic value of deer bone comes precisely from its source. <e samples we analyzed are from the same archaeological deposits used by (1) ceramicists, to provide period assignments; (2) settlement analysts, to derive population estimates; and (3) iconographers, to decipher site history. For these reasons, we can correlate with con=dence our data with site-speci=c settle-ment and political information derived from corresponding archaeological studies.

From į18OP analyses (Figure 11.5), it is believed that “average” climatic conditions were relatively drier during Balche phase (AD 560–620) com-pared to Yaxche (AD 620–750) and Chacalhaaz (AD 750–850) phases. <e minimum and maximum į18OP values during the Balche phase, considered to represent the extreme climatic events, are both higher than during Yaxche and Chacalhaaz phases (Table 11.4). In particular, wet periods during Balche were probably less intense (in terms of yearly averages precipitation amounts and/or relative humidity) than during the next two phases (minimum į18OP during Balche is ~1 ‰ lower than the minimum į18OP during Yaxche and Chacalhaaz phases). Dry periods seemed to have been slightly more intense (lower precipitation amounts and/or lower relative humidity) than during the next two phases (maximum į18OP during Balche is ~0.5 ‰ higher than maximum į18OP during Yaxche and Chacalhaaz phases). No signi=cant climate change seemed to have occurred between Yaxche and Chacalhaaz phases. <e average į18OP is slightly lower during Chacalhaaz phase, pos-sibly indicating climatic conditions on average slightly wetter than during Yaxche phase. <e broader range of į18OP values during Chacalhaaz phase (~3.9 against 3.5 ‰) indicates that more “extreme” events (wet and dry) were likely to have occurred compared to during Yaxche phase. It is suggested

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!-! REPUSSARD, SCHWARCZ, EMERY, AND THORNTON

that the relative stability of the climatic conditions between AD 620 and 850 could partly be responsible for the Jourishing of Piedras Negras during that time.

Unfortunately, conclusions about climatic conditions during the Kumche phase (AD 850–900) are less well supported by the poor representativeness of the data set (three bones were analyzed only). However, the maximum į18OP value during the Kumche phase is comparable to the maximum one during Balche (Table 11.4). It is thus likely that during the =fty years of the Kumche phase, an episode of drought took place, long enough to be detect-able by the poor temporal resolution of the data set. <is =nding would be in accordance with the dry phase detected by Hodell, Brenner, and Curtis (2007) at ~AD 850–60. If this assertion is true, it is likely that the drought detected during the Kumche phase would have probably hastened the fall of a city already fading due to internal sociopolitical causes.

In the end, our data are in overall agreement with the general patterns described by other biomarkers (e.g., Hodell, Brenner, and Curtis 2007), with a relatively dry Early Classic (corresponding here to the Balche cultural period), followed by wetter conditions during the Late Classic (correspond-ing to Yaxche and Chacalhaaz phases), and relatively dry conditions during the Terminal Classic (corresponding to Kumche phase).

CONCLUSIONSWe have shown that the oxygen isotopic composition of deer bone can

serve as a climatic recorder to track changes of precipitation amounts and relative humidity during the Maya era. However, the most important factor limiting the interpretation of the results is the time resolution of the material, both in respect to the resolution of the signal from individual bones, as well as resolution of site chronology.

First, because deer bone records roughly 2.5 years of local climatic condi-tions, only long-term climatic Juctuations (i.e., pluri-annual) are detectable. However, we see that during the entire period that Piedras Negras was Jour-ishing, the climate did not consistently deviate from a pattern consistent with average modern conditions at the site. It is only individual samples that sug-gest large o;sets toward both wetter and drier conditions for as long as the lifetime of a single deer.

Second, the number of bones available and the precision with which they are dated both a;ect the meaning of the results. Even if the exceptional archaeological context in the Maya region allows good time resolution, the

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time covered by each cultural phase (from 50 to 130 years in the case of Piedras Negras), in addition to the high variability of Central American climate at a decadal scale, both make the signi=cation of “average” climatic conditions during each cultural phase less meaningful (see Iannone, Yaeger, and Hodell, Chapter 3 in this volume). In the end, climatic extremes should be considered as indicative of past climate Juctuations in the Maya region, only if the num-ber of bones analyzed is suMcient to provide an acceptable sampling density within a given cultural phase.

If we focus on the average isotopic record for each period, it has not been possible to identify any particularly intense and/or prolonged period of dry climatic conditions at Piedras Negras before and while the city started to be abandoned. One must keep in mind that the beginning of the decline pro-cess took place at a time of the city faced major sociopolitical changes. <is highlights the need of a precise understanding of the history of each site to correlate the impacts of climate change to the decline of studied sites. We have shown that deer bone is truly a good material to ful=l this role. We would then conclude, on the basis of these observations, that drought was probably not the cause of the decline at Piedras Negras, but may have hastened the fall of a city already in a decline process due to internal sociopolitical causes (see Scherer and Golden, Chapter 10 in this volume).

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