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Journal of Archaeological Science 38 (2011) 2925e2938

Contents lists avai

Journal of Archaeological Science

journal homepage: http : / /www.elsevier .com/locate/ jas

Evidence for volcanic ash fall in the Maya Lowlands from a reservoirat Tikal, Guatemala

Kenneth B. Tankersley a,c,*, Vernon L. Scarborough a, Nicholas Dunning b, Warren Huff c, Barry Maynard c,Tammie L. Gerke c

aDepartment of Anthropology, University of Cincinnati, Cincinnati, OH 45221, USAbDepartment of Geography, University of Cincinnati, Cincinnati, OH 45221, USAcDepartment of Geology, University of Cincinnati, Cincinnati, OH 45221, USA

a r t i c l e i n f o

Article history:Received 21 March 2011Received in revised form30 May 2011Accepted 31 May 2011

Keywords:Powder X-ray diffractionX-ray fluorescenceMaya LowlandsTikalTephra

* Corresponding author. Department of AnthropolCincinnati, OH 45221, USA. Tel.: þ1 513 556 2772; fax

E-mail address: tankerkh@uc.edu (K.B. Tankersley

0305-4403/$ e see front matter Published by Elseviedoi:10.1016/j.jas.2011.05.025

a b s t r a c t

Powder X-ray diffraction and petrographic analyses of reservoir sediments from Tikal, Guatemala haveidentified significant quantities of decomposed volcanic ash in the form of smectite and euhedralbipyramidal quartz crystals. X-ray fluorescence trace element content analysis was used to eliminatedistant Sahara-Sahel and Antilles sources. The Zr/Y and Ni/Cr ratios of reservoir sediment from Tikal areconsistent with a source from Central American volcanism (e.g., Guatemalan and Salvadoran). AMSradiocarbon dating of the smectite and crystalline quartz-rich reservoir sediments show that volcanic ashfell during the Preclassic, Classic, and Postclassic Maya cultural periods. It may now be possible todevelop an effective chronology of ash fall at Tikal and the greater Peten.

Published by Elsevier Ltd.

1. Introduction

While volcanic ash has been well documented as temper inancient Maya ceramics, its sources remain in question (Ford, 1991;Ford and Fedick, 1992; Ford and Glicken, 1987; West, 2002). Theplethora of ash temper in ceramics recovered from the limestonelowlands of Guatemala has led some investigators to suggest thatash fell during the pre-Hispanic period of Maya occupation (Fordand Rose, 1995:149). This theory has significant implications forunderstanding the prehistoric exploitation of volcanic resources,landscape modification, and sustainability in the Maya Lowlands.

If significant quantities of volcanic ash fell on the limestonelowlands of Guatemala during the pre-Hispanic occupation of theregion, thenwe should expect to find direct positive evidence in thenumerous large reservoirs constructed in the Maya city of Tikal(Fig. 1). The Maya constructed reservoirs at Tikal to conserve waterduring the annual dry season and to control and contain floodwa-ters during the rainy months. Six major reservoir catchment areasdrained the elevated precincts of Tikal (Fig. 2), which covered anarea of approximately 300 ha with a total maximum reservoir

ogy, University of Cincinnati,: þ1 513 556 2778.).

r Ltd.

capacity of more than 570,000 m3 (Scarborough and Gallopin,2003:661). Surface water drained into the reservoirs and cultur-allymodified aguadas (i.e., natural depressions) and bajos (i.e., hugesolutional dolines) (Scarborough, 1993; 1994:116; 2003:51). Unlikedeeply inundated deposits from lake basins and ocean floors,abandoned and in-filling reservoir sediments can be easily sampledwith solid sediment drill cores.

While it is possible for ash to survive in deep lakes, such as Yojoain Honduras, this is not the case in smaller and shallower reservoirs.In these settings, a significant problem in sourcing ash from theMaya Lowlands is the fact that volcanic glass quickly weathers (i.e.,chemically decomposes) into smectite clay in moist, tropical andalkaline environments, all of which are characteristic to the region.This phenomenon is exemplified by an ash fall from the El ChichónVolcano, which lasted from 28 March to 4 April 1982 (Robock,2002). Although Tikal was blanketed by several centimeters ofash during this event, there is no visible evidence of the event in thesoils today, and local residents report that much of the ash hadalready been incorporated into the soil within a few months.

Smectite is a group of expanding-lattice clay minerals thatinclude beidellite, hectorite, montmorillonite, nantronite, saponite,and sauconite, and is the principal component of bentonite claydeposits (Laird et al., 1991). Smectite originates from the decom-position of eruptive igneous rocks (e.g., tuff) and volcanic ash (i.e.,

Fig. 1. Regional setting of Tikal and Central American volcanoes.

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e29382926

glass). Favorable physical and chemical conditions for the forma-tion of smectite include magnesium-rich environments with poordrainage, which are characteristic of the reservoirs of Tikal. Theexpandable nature of clays of this type is revealed by x-raydiffraction analysis of the separated clay fraction (Moore andReynolds, 1997). Likely sources for the weathered ash can beidentified using bulk X-ray fluorescence (XRF) and electronmicroprobe data as documented by Huff (2008) and Huff et al.(1999, 2000) for multiple Ordovician bentonites.

2. Archaeological context

Pre-Hispanic human occupation of the Maya Lowlands is oftendivided into the following chronological periods: PaleoIndian (pre-7000 BC), Archaic (7000e2200 BC), Preclassic (2200 BCe250 AD),Early Preclassic (2200e1000 BC), Middle Preclassic (1000e400 BC),Late Preclassic (400 BCeAD100), Terminal Preclassic (AD 100e250),Classic (AD 250e900), Early Classic (AD 250e600), Late Classic (AD600e770), Terminal Classic (AD 770e900), Postclassic (AD900e1500), Early Postclassic (AD 900e1250), and Late Postclassic(AD 1250e1500). While PaleoIndian, Archaic, and Early Preclassicmaterials and occupations have been found widely distributedwithin the Maya Lowlands, no materials from these periods havethus far been recovered at Tikal. Sometime prior to about 700 BC,small populations began to reside at Tikal, and by 600 BC the firstmonumental architectural constructions appeared (Laporte, 2003).By the Late Preclassic period (ca. 350 BC), Tikal had developed intoa significant “player” in the emerging political landscape of theMaya Lowlands. Unlike larger centers in the nearby Mirador Basin,Tikal survived the turmoil of the 2nd century AD and emerged asa major center of the Early Classic period (Dunning et al., in press).Tikal enjoyed variable prosperity during the Classic era, includinga notable downturn in its fortune or “hiatus” in the 6th century AD,but emerged as a paramount center in the Late Classic period witha population estimated to have been around 60,000 in the 8thcentury (Martin and Grube, 2008). Tikal’s affluence declined

dramatically in the 9th century and by AD 900 was largely aban-doned except for a small residual population that persisted foranother 200 or so years.

3. Corriental reservoir

Corriental is one of the largest reservoirs at Tikal (Fig. 3). It isstrategically positioned to collect most of the surface water runofffrom the southeastern margins of the central city promontory(Scarborough and Gallopin, 2003:251 and 252). The reservoir waslikely used as a water source for drinking, cooking, and probablybathing as indicated by water jar sherds. Although there are noprepared steps or walkways associated with the reservoir, thescarcity of ancient debris in the sediments and the apparent formerexistence of carbonate sand filters suggest that it was constructedfor public drinking water. The inferred presence of ancient sandfiltration berms positioned at the ingress of the reservoirdnodoubt occasionally “blown out” by capricious seasonal floodingeventsdis suggested by the repeated stratigraphic sand lensingreported in most excavation profiles; carbonate sand sources havenot been identified within 10 km of Tikal, suggesting an anthro-pogenic origin. The catchment area for the Corriental Reservoir isabout 40 ha, with a surface area of more than 15,000 m2 and anestimated capacity of more than 57,000 m3 of water (Gallopin,1990:60). It was constructed in a pre-existing localized depres-sion at the junction of several small seasonal streams. The reservoirfeatured two separate ingress gates at different elevations,approximately 205m amsl on the south side and 208m amsl on thenorthwestern side (Gallopin, 1990:32e38).

In 2009, ten pits and trenches were hand excavated from thecenter and margins of the Corriental Reservoir, the northwesternand southern ingress gates, and the eastern egress (see Fig. 3).Additionally, a hand-operated Environmental Subsoil Probe (ESP)was used to extract twenty-six 2-cm diameter cores fromCorrientald16 from the center of the reservoir, 7 from the earthenberms, 2 from the northwestern ingress gate, and 1 from the

Fig. 2. Main catchment areas and reservoirs of Tikal.

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e2938 2927

eastern egress (see Fig. 3). Twenty-three soil horizons were definedin the field by Dunning during excavations on the basis of color,texture, structure, and pedogenic features. Tankersley confirmedthe reservoir-wide stratigraphy in the lab during analysis andcorrelation of the cores with particle size analysis, magneticsusceptibility, and Munsell soil color.

3.1. Stratigraphy and geochronology

The stratigraphy for the interior sediments and buried soilswithin the Corriental Reservoir, as defined in Operation 1C, areoutlined in Table 1. Operation 1C was a 1 � 1.5 m trench excavatednear the center point of the Corriental Reservoir. The pit reacheda final depth of 3.15 m, at which point weathered limestonebedrock was encountered. Dr. Pat Culbert analyzed ceramics fromthis operation at Tikal.

Radiocarbon dating at Corriental was performed in a series ofAMS radiocarbonmeasurementsmade on carbonizedplant remains

and soil organic matter (SOM) (Table 2)dthe former abundantlyassociatedwith airbornewood ash from themany cooking fires andthe like from within the low-density urban setting. This workestablished that the stratigraphy (i.e., soil horizons) extended fromthe earlyHolocene, 8960� 60 14C yr BP (Beta-270566) to at least theEarly Postclassic 990� 40 14C yr BP (Beta-258720). It also suggestedthat the Corriental Reservoir was in use by the Early Classic,1560 � 40 14C yr BP (Beta-274990), but may have been initiallyconstructed toward the end of the preceding Late Preclassic period.

The 1560 � 40 14C yr BP (Beta-274990) AMS radiocarbon datewas obtained from core 17 on charred plant remains beneath theearthen berm (i.e., wall) 3 m above the northwest gate (Fig. 4). Thisdate demonstrates that this feature was built during or subsequentto the Early Classic. Stable carbon isotope values (d 13C�19.1,�19.5,and �20.3&) on the AMS radiocarbon dated soil organic mattersuggests that C4 photosynthetic pathway plants such as maizewere present in the Corriental catchment area during the LatePreclassic and Middle Preclassic cultural periods. Consequently,

Fig. 3. Location of Corriental Reservoir core samples and excavation units.

Table 1Age and relative percent composition of sediments identified in the Corrientalreservoir based on XRD.

Depth Horizon Measured14C yr BP

2 sCalibratedAge

Calcite(%)

Smectite(%)

Quartz(%)

Void(%)

10 A1 12.84 37.91 35.86 12.8420 A2 88.72 1.93 4.32 6.9730 ACss 80.51 0.00 0.88 18.6140 C2ss 70.75 14.70 5.41 12.4050 C2ss 67.51 13.13 2.87 16.4960 C2ss 51.77 8.89 3.48 38.2370 C3 990 � 40 AD 1010e1170 73.72 9.30 3.12 15.5380 C4ss 71.10 13.98 7.02 12.9690 C4ss 47.01 37.91 35.86 12.84100 C4ss 88.72 1.93 4.32 6.97110 C4ss 80.51 0.00 0.88 18.61120 C4ss 70.75 14.70 5.41 12.40130 C5 67.51 13.13 2.87 16.49140 C6ss 51.77 8.89 3.48 38.23150 C6ss 2010 � 40 340e30 BC 73.72 9.30 3.12 15.53160 C6ss 71.10 13.98 7.02 12.96170 C7 19.56 73.59 2.68 5.40180 C7 2110 � 40 190e80 BC 17.73 77.77 1.46 3.88190 C7 2120 � 40 380e170 BC 52.31 39.91 6.32 5.48200 C8 78.70 11.59 9.81 6.31210 C9 68.33 21.62 0.00 10.05220 C10 82.41 4.92 3.46 11.30230 C11ss 80.27 10.02 4.68 8.22240 C12 69.76 18.11 11.14 9.78250 2Abss 84.16 19.86 12.30 13.21260 2ACss 83.61 4.43 4.43 31.56270 2C1bss 2340 � 40 760e400 BC 79.43 9.65 2.02 33.01280 2C1bss 92.20 6.56 4.76 11.29290 2C2bss 76.03 9.86 11.48 10.69300 2C2bss 63.70 18.84 12.04 13.78310 3Abss 8960 � 60 8290e7970 BC 90.48 15.74 3.97 19.78

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e29382928

open environments may have led to increased erosion rates in thecatchment area (Anselmetti et al., 2007), the latter influenced byincreased precipitation with the onset of the Classic period assuggested by some climatic models for the greater Maya Lowlands(Dunning and Beach, 2010). The Early Classic berm additions mayhave been built to strengthen the northwestern ingress againstheavy runoff during strong seasonal rains and hurricanes, whichare powerful geomorphic agents at any time throughout the region(Dunning and Houston, 2011), as well as to increase the carryingcapacity of the reservoir. The lowermost horizons exposed inOperation 1C (i.e., 3Ab and 3AC horizons) represented a highlycompacted, skeletal soil overlying limestone bedrock. A radio-carbon date on soil organic matter from the analogous soil horizonin Core 8 produced a date of 8960 � 60 14C yr B.P or early Holoceneage (see below, Fig. 4 and Table 2). Nevertheless, based on simi-larities to other deeply buried soils in small depressions elsewherein the northeast Peten and northwest Belize, this soil may date asfar back as the late Pleistocene (ca. 11,000e13,000 BP) (Dunninget al., 2006; Beach et al., 2008).

With the onset of wetter conditions in themid-Holocene period,the small depression filled with sediment eroded from upslopeareas (2C1 and 2C2 horizons) onwhich a new soil surface (2Ab and2AC horizons) gradually developed. Radiocarbon dating of organicmatter within the 2C1 horizon place the age of this soil surface atapproximately 2340 � 40 14C yr BP (Middle Preclassic). However,this date is based on soil organic matter (organic matter whichaccumulated over hundreds of years, given the lack of significantoccupation and associated cooking fires at this time), hence the soilsurface was probably last exposed sometime in the Late Preclassic.This soil is a Vertisol, typical of small seasonally wet/dry depres-sions in the Peten. Four weathered sherds were recovered from thislevel, but were chronologically non-diagnostic.

At some point within the next 500 years, drainage within thedepression appears to have been substantially modified and sedi-ment began to accumulate much more rapidly. This modificationlikely corresponds to the initial construction of the Corriental

Table 2Radiocarbon dates for the Corriental reservoir.

Lab Number Sample d 13C& Horizon Measured 14C yr BP 2 s Calibrated Age Cultural Period

Beta-258720 Charcoal �26.9 C3 990 � 40 AD 1010e1170 Early PostclassicBeta-274990 Charcoal �23.7 Anthrosol 1560 � 40 AD 400e570 Early ClassicBeta-280839 SOM �19.5 C6 2010 � 40 340e30 BC Late PreclassicBeta-266124 SOM �20.2 C7 2110 � 40 190e80 BC Late PreclassicBeta-280837 SOM �20.3 C7 2120 � 40 380e170 BC Late PreclassicBeta-258721 SOM �19.1 2C1 2340 � 40 760e400 BC Middle PreclassicBeta-270566 SOM �23.1 3Ab 8960 � 60 8290e7970 BC Pre-habitation?

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e2938 2929

Reservoir, probably accomplished by a combination of quarrying(widening the natural depression), building up of the encirclingberm, and diversion of inflowing seasonal stream runoff. Subse-quently, the floor of the reservoir began to aggrade (fill with sedi-ment over time). Alternating strata of organic clay and laminatedcarbonate sand and small, rounded gravel are found between 65and 253 cm depth within Operation 1C (C3 to C12 horizons). Thelaminated or stratified nature of the deposits indicates that thesands were deposited by running water (i.e., a fluvial process).Notably, there are no known natural sand sources upstream from

Fig. 4. Profile drawing of the east wall of Op. 1C in the floor of Corriental Reservoir (after DunSee Table 2 for additional chronological information. Dates in brackets were obtained from

the reservoir. One possible explanation is that sand was used tofilter water entering the reservoir and that sand from the filters wasoccasionally flushed into the reservoir proper by storm-relatedflooding. On the other hand, the organic clay layers are typical ofstill water deposits and likely accumulated slowly while thereservoir was in active use. There are no obvious signs of dredgingwithin the sediments revealed in Operation 1C or other excavationsand cores in the reservoir.

Three radiocarbondateswere obtained fromorganicmatter in theC6 and C7 horizons: 2010 � 40 14C yr BP for C6, and 2110 � 40 and

ning et al., 2009). Radiocarbon dates to the left of the profile are in measured 14C years.analogous soil horizons in Core 8.

TK1 0-10

0 10 20 30 40 50 60 70

2Theta

TK1 30-40

2.6

14.3

90.8

15.3

11.2

39.4

10.8

TK1 50-60

TK1 60-70

TK1 10-20

TK1 20-30

- 1.5

Å- 1

.53Å

- 1.6

1Å

- 1.8

8Å- 1

.92Å

- 2.1

Å

- 2.2

9Å- 2

.35Å- 2

.5Å

- 2.5

8Å

- 3.0

6Å

- 3.3

7Å

- 3.8

8Å

- 4.5

Å

- 5.9

8Å- 15.

63Å

CAL

CIT

E

CAL

CIT

E

CAL

CIT

E

CAL

CIT

EC

ALC

ITE

CAL

CIT

E

CAL

CIT

E

QU

ARTZ

SMEC

TITE

, ZEO

LITE

S

SMEC

TITE

, ZEO

LITE

S, Q

UAR

TZ

SMEC

TITE

, ZEO

LITE

S

17.2

TK1 70-80

TK1 40-50

Fig. 5. X-ray diffractograms of sediments extracted from core 8 located at the center of the Corriental Reservoir (first core extracted from 0 to 80 cm below the surface).

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e29382930

TK2 30-40

TK2 60-70

TK2 50-60

TK2 40-50

TK2 0-10

0 10 20 30 40 50 60 70

TK2 20-30

TK2 70-78 - 3

. 05 Å

- 14

-17 Å

- 2 3

5 Å

- 2 . 0

3 Å

- 5 . 8

2 Å

- 4 . 4

8 Å

- 3 . 8

8 Å

- 3 . 3

7 Å

- 2 . 8

6 Å

- 2 . 5

8 Å - 2

. 5 Å

- 2 . 2

9 Å

- 2 . 1

Å

- 1 . 8

8 Å

- 1 . 6

3 Å

- 1 . 6

1 Å .

- 1 9

2 Å

CAL

CIT

E

CAL

CIT

E

CAL

CIT

E

CAL

CIT

E C

ALC

ITE

CAL

CIT

E

CAL

CIT

E

TK2 10-20

QU

ARTZ

SMEC

TITE

, ZEO

LITE

S

SMEC

TITE

, ZEO

LITE

S, Q

UAR

TZ

SMEC

TITE

, ZEO

LITE

S

2.3

20.8

76.0

78.5

31.9

3.1

6.0

80.4

2Theta

Fig. 6. X-ray diffractograms of sediments extracted from core 8 located at the center of the Corriental Reservoir (second core extracted from 80 to 160 cm below the surface).

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e2938 2931

2120 � 40 14C yr BP for C7. The dates correspond to the Maya LatePreclassic period and are appreciably older than the ceramic sherdsfound in the underlying C8 through C12 horizons. This inversionstrongly suggests that theorganicmatterbeingdated in theC6andC7horizons is derived from older soil eroded from surfaces in thewatershed above the reservoir. Analysis of the d 13C and d 15N content

of this organic matter also indicates that it is derived from C3terrestrial plants and not aquatics such as algae. Notably, soil surfacesdating from the Late Preclassic period are typical of the MayaLowlands, but they were subject to pulses of erosion resulting in thedeposition of sediments derived from these soils in countless localdepressions (Anselmetti et al., 2007; Dunning and Beach, 2000).

TK3 30-40

TK3 60-70

TK3 70-80

TK3 50-60

TK3 40-50

-

TK3 20-30

TK3 0-10

0 10 20 30 40 50 60 70

2Theta

- 1.5

3Å

- 1.6

1Å

- 1.8

8Å- 1

.92Å

- 2.1

Å- 2.2

9Å

- 2.5

Å

- 3.0

5Å

- 3.3

6Å

- 3.8

8Å

- 4.5

3Å

- 6Å - 1

.63Å- 1

4-1

7Å QU

ARTZ

SMEC

TITE

, ZEO

LITE

S

SMEC

TITE

, ZEO

LITE

S, Q

UAR

TZ

SMEC

TITE

, ZEO

LITE

S

CAL

CIT

E

CAL

CIT

E

CAL

CIT

E

CAL

CIT

E

CAL

CIT

EC

ALC

ITE

CAL

CIT

E

18.2

11.9

39.4

2116.2

37.3

903.0

39.2

353.5

TK3 10-20

Fig. 7. X-ray diffractograms of sediments extracted from core 8 located at the center of the Corriental Reservoir (third core extracted from 160 to 240 cm below the surface).

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e29382932

Ceramic sherds were recovered in the C3, C4, C5, C7, C8, C10, andC12 horizons principally in the sandy strata. Small and large waterjar forms predominate in all strata. The large majority of sherdswere too weathered to be chronologically diagnostic. C12, thedeepest alluvial stratum, and C9 included identifiable Early Classictypes. C10 had no diagnostic sherds. C8 contained amix of Early andLate Classic types. C5eC7 included only Late Classic types. C3 hadno diagnostic sherds. Charcoal within the C3 horizon (65 cm)produced an AMS radiocarbon date of 990 � 40 14C yr BP (2 sigmacorrelated range: AD 1010e1170), suggesting that the reservoir may

have continued to be in use to some extent as late as the EarlyPostclassic period.

Subsequently, there is no evidence of reservoir use, though ithas naturally continued to seasonally collect water. Themodern soilthat has developed within the reservoir (Oi through C2 horizons) isa Terric Fibrist, an organic soil with mineral subsoil typical ofregional depressions that remain partially moist year-round.

In summary, the Corriental Reservoir, on the southern flank ofcentral Tikal, was likely constructed sometime toward the end ofthe Late Preclassic or very early in the Early Classic period. The

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e2938 2933

reservoir was constructed by widening a small pre-existing naturaldepression by quarrying and mounding earth to form an encirclingberm and filled by diverting water flow from a local seasonalstream. Thick sediment deposits within the reservoir includedalternating deposits of stratified carbonate sands and organic clays.The clays indicate periods of stability during which clay and organicmatters gradually settled onto the reservoir floor. The sandy strataare indicative of running water, perhaps deposited during higher-energy storm runoff events. The origin of the sand is uncertain,but it may have been used to filter water as it entered the reservoir,and then was occasionally flushed into the reservoir proper duringflooding. Ceramics recovered from within the reservoir sedimentswere generally very weathered, but contained a mixture of EarlyClassic and Late Classic types. Most notable were the quantities ofhuge jar fragments.

0 10 20 30

2Thet

- 14

-20Å

- 4.4

1Å- 4

.22Å

- 3.8

3Å

- 3.3

1Å

- 2.8

2Å

- 3.0

1Å

- 2.5

5Å

CAL

CIT

E

CAL

CIT

E

QU

ARTZ

SMEC

TITE

, ZEO

LITE

S

SMEC

TITE

, ZEO

LITE

S, Q

UAR

TZ

14.9

9.5

0.0

50.3

22.1

38.7

10.2

Fig. 8. X-ray diffractograms of sediments extracted from core 8 located at the center of th

4. Powder X-ray diffraction

Powder XRD was used to identify the relative percent mineralcomposition of the reservoir sediments of Tikal. For this study, solidsediment core samples were extracted from the center of Corrientalto obtain the deepest and most complete stratigraphic samplespossible. Initially, the cores were cut into 10 cm subsamples. Soilstrata were defined on the basis of particle size analysis, sedi-mentary boundaries, and changes in Munsell soil color, thencorrelated with the horizons established in Operation 1C.

4.1. XRD methods

Following themethodology of Tankersley and Ballantyne (2010),XRD samples were taken at 10-cm intervals and sieved through

TK4 40-50

TK4 60-70

TK4 10-20

TK4 50-60

TK4 30-40

TK4 0-10

40 50 60 70

a

TK4 20-30

- 2.4

9Å

- 2.3

4Å- 2

.27Å

- 2.0

8Å- 2

.03Å

- 1.9

Å

- 1.6

2Å- 1

.6Å

- 1.5

4Å

- 1.8

7Å

CAL

CIT

E

CAL

CIT

E

CAL

CIT

EC

ALC

ITE

CAL

CIT

E

e Corriental Reservoir (third core extracted from 240 to 320 cm below the surface).

olcanic

sources

(wt%

orppm).

Fe2O3t

MnO

MgO

CaO

Na 2O

K2O

TiO2

P 2O5

Ba

Cr

Nb

Ni

Rb

SrV

YZr

4.20

0.24

0.15

17.35

1.29

0.09

0.20

0.12

217

3810

4117

4046

3814

63.55

0.25

0.15

23.10

1.12

0.08

0.18

0.14

350

3710

3817

4141

3513

14.99

0.16

0.15

7.07

6.23

0.09

0.40

0.16

340

339

3519

4235

3312

32.41

0.06

0.14

17.39

2.01

0.01

0.27

0.16

132

339

2816

4132

3011

03.94

0.09

0.13

9.19

2.83

0.02

0.33

0.13

265

3110

3216

4239

3112

66.62

0.34

1.53

9.68

0.04

0.17

0.45

0.01

289

7915

6932

2277

3321

16.70

0.34

1.51

9.26

0.05

0.17

0.45

0.01

264

7816

7432

2282

3621

47.21

0.28

1.54

8.73

0.04

0.17

0.43

0.01

290

8715

7631

2487

3821

47.34

0.31

1.58

7.63

0.05

0.18

0.40

0.01

193

7614

6130

2783

3620

27.09

0.38

1.56

7.86

0.05

0.18

0.40

0.01

217

7715

6730

2579

3719

83.37

0.13

1.32

3.01

3.19

2.16

0.46

0.11

813

124

1755

160

4620

158

1.25

0.07

0.20

1.15

3.99

2.77

0.18

na

na

na

na

na

na

na

na

na

na

4.90

0.07

1.56

5.78

0.94

1.76

1.03

0.24

449

7130

2361

.417

988

.325

.131

07.74

0.16

3.90

8.44

3.10

0.94

0.71

0.12

246

197

5.13

62.3

27.9

383

173

24.1

107

6.48

0.15

2.70

6.58

3.96

2.02

0.68

0.24

600

4312

.723

.360

.866

213

423

.915

35.97

0.18

2.45

6.03

3.91

2.07

0.68

0.30

654

197.3

12.1

41.5

485

136

17.9

132

7.98

0.16

2.90

7.15

3.60

1.54

0.79

0.21

695

122.32

9.74

31.2

420

168

21.3

108

8.75

9.63

0.17

4.32

8.61

3.56

1.40

1.36

502

5625

3721

568

211

4715

4

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e29382934

a 2 mm mesh. Approximately 20 g of clay from each stratum wasmixed with deionized water to make slurry in 100 ml beakers. Claywas dispersed using a high-speed stirrer and gravity settling wasused to obtain a fraction of <2 mm. A 5 ml pipette was used toobtain a clay sample from the top of the slurry and transferred toa glass slide and air-dried. A second oriented glass slide wasprepared from each sample and equilibrated overnight withethylene glycol vapor.

XRD patterns were obtained for both the air-dried and glyco-lated samples. All slides were initially scanned from 2� to 32� 2q at0.5 increments and then broadened to 60� 2q on a Siemens D-500X-ray diffractometer using a Cu-Ka radiation source. The intensitythreshold was set at 1.6 andminerals were identified on the basis ofpeak position and peak intensity as described by Chen (1977).

Glycolated samples were prepared to test for the presence ofexpandable clay minerals. Relative mineral percentages werecalculated from the total counts per second (cps), which weretotaled for each 10-cm sample. The sum of all peaks cps for eachmineral was divided by the total cps for each 10-cm sample. Therelative percent of each mineral was calculated from the total cpsper mineral divided by the total cps in the 10-cm sample.

4.2. XRD results

The minerals calcite, smectite, and quartz were found in theCorriental Reservoir sediments (Figures 5e8). Calcite was the mostabundant mineral (66%) and characterized by glycolated XRD peaksat 3.88 Å, 3.38 Å, 3.06 Å, 2.50 Å, 2.25 Å, 2.29 Å,1.92 Å, and 1.88 Å. Theremainder of the sediments consisted of smectite (27%), charac-terized by XRD peaks at 15.63 Å, 5.98 Å, and 4.50 Å, and quartz (6%),characterized by an XRD peak at 3.37 Å. Quartz and calcite wereunaffected by glycolation.

The abundance of calcite is undoubtedly related to the Creta-ceous and Tertiary limestone bedrock, which determines the terrainin the Maya Lowlands (Dunning et al., 1998). Because quartz isknown to occur in carbonate rock (Chafetz and Zhang, 1998),samples of local limestone bedrock were subjected to powder XRDanalysis. XRD peaks at 3.85 Å, 3.03 Å, 2.84 Å, 2.49 Å, 2.28 Å, 2.09 Å,1.91Å,1.62Å,1.60Å, and1.52Å characterized calcite. Peaks typical ofquartz and smectite were completely absent from the bedrocksamples. This finding suggests that smectite and quartz entered thereservoirs as aeolian minerals rather than dissolution of thesurrounding bedrock and subsequent water transport (contraCowgill and Hutchinson, 1963:41). Grim and Güven (1978:128)

Table

3Traceelem

entco

ntentof

Tika

lreservoirsedim

ents

andco

mparativev

Location

Reservo

irDep

thSiO2

Al 2O3

Tika

lCorrien

tal

180e

190

32.0

14.9

Tika

lCorrien

tal

190e

200

28.4

12.3

Tika

lCorrien

tal

200e

210

40.1

13.0

Tika

lCorrien

tal

210e

220

36.2

10.8

Tika

lCorrien

tal

220e

230

39.4

12.6

Tika

lPe

rdido

180e

190

42.4

17.8

Tika

lPe

rdido

190e

200

42.0

17.9

Tika

lPe

rdido

200e

210

42.9

18.5

Tika

lPe

rdido

210e

220

43.9

18.6

Tika

lPe

rdido

220e

230

43.6

18.6

Ilop

ango

Tephra

68.5

15.3

Tierra

Blanca

Jove

nav

erag

e77

.513

.0Te

phraSa

haran

Dust

averag

e60

.711

.0Antille

sTe

phra

averag

e57

.417

.3Mex

icoTe

phra

averag

e58

.417

.8Guatem

alaTe

phra

averag

e59

.517

.5El

Salvad

orTe

phra

averag

e57

.417

.3Hon

durasTe

prh

aav

erag

e52

.218

.3

a Dullet

al.,20

01bMoren

oet

al.,20

06.

c Carret

al.,20

07.

na¼

not

analyz

ed.

Fig. 9. Photomicrograph of a representative euhedral bipyramidal quartz crystal fromthe C7 horizon of Corriental reservoir sediment (see Table 1).

Fig. 10. Trace element Ni/Cr and Zr/Y ratios of Tikal reservoir sediments, Sahara-Shaheldust, Antilles volcanoes, and Central American volcanoes.

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e2938 2935

demonstrate that smectite forms from weathering volcanic ashvarying in composition from rhyolitic (quartz-rich) to basaltic(quartz-poor). However, most bentonites have formed from ashranging from rhyolitic to dacitic in composition. In other words, theassociation of quartz and smectite in altered volcanic ashes is quitecommon.

Grains isolated from all soil horizons were identified as quartzbearing with XRD (see Table 1). Under high magnification (up to

Fig. 11. Trace element Ni/Cr and Zr/Y ratios of Tikal reservoir sediments and Central AmericaES5 Izalco, ES6 Conchagua Peninsula, ES7 San Miguel, G1 Santa Maria, G2 Tacana, G3 AguaCarranza, M2 Santoton).

640�, Leica MZ12 stereomicroscope), the quartz grains appear asdispersed slivers, fragments of euhedral crystals, and completebipyramidal crystals with scarce inclusions (Fig. 9). Together, thesecharacteristics suggest they represent “first quartz” or an extrusiveigneous or volcanic origin.

While smectite, like the euhedral bipyramidal quartz crystals,suggests a volcanic ash source, Tikal is located in the Sahara-SahelDust Corridor (Moreno et al., 2006) and African dust has beenidentified as a major component of soils overlying other carbonateland masses in the Caribbean Basin including Florida, the Bahamas,and Barbados (Muhs et al., 2007). To determine if the smectite fromthe reservoirs of Tikal originated as airborne Sahara-Sahel dust orvolcanic ash, the trace element composition of the reservoir sedi-ments from Tikal was analyzed using XRF.

5. X-ray fluoresence

XRF was used to determine the trace element concentration ofsmectite-bearing reservoir sediments of Tikal and compare it withthe trace element content of airborne Sahara-Sahel dust (Morenoet al., 2006).

5.1. XRF methods

Core samples, which contained high relative percentages ofsmectite were selected for XRF analysis from the C7 and C8 soilhorizons of Corriental (i.e., Preclassic, Classic, and Postclassicstrata). As a control, samples from the pre-Maya strata of thenearby Perdido reservoir (see Fig. 2), AMS radiocarbon dated15,110 � 60 14C yr BP (Beta-289286), 15,480 � 60 14C yr BP (Beta-289285), and 15,310 � 60 14C yr BP (Beta-289284), were alsoanalyzed and compared. A Rigaku 3070 X-ray Fluorescence

n volcanoes (CR1 Arenal, ES1 Santa Ana, ES2 Cerro Verde, ES3 Apaneca, ES4 Meanguera,, G4 Pacaya, G5 Tecuamburro, G6 Moyuta, H1 Cerro de Hule, H2 Yojoa, M1 Venustaubi

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e29382936

spectrometer was used to determine the intensity of the traceelements; Mo, Ba, Co, Cr, Cu, Nb, Pb, Rb, Sr, Th, U, V, Y, and Zn.Following the methodology of Gerke et al. (2006:140), samplepowders were pressed into briquettes at 2000 psi. A separatealiquot was heated to 1000 �C for 1 h to measure volatile content.Intensity data were converted to parts per million (ppm) usingbivariate and multiple variable regressions applied to United StatesGeological Survey, National Institute of Standards, and JapanGeological Survey rock standards (Table 3).

5.2. XRF results

Significant differences in the trace element content of sedimentfrom the reservoirs of Tikal and Sahara-Sahel dust were found inthe ratios of Ni, Cr, Zr, and Y. These discriminations can be illus-trated in a plot of the ratio of Cr to Ni against the ratio of Zr to Y(Fig. 10). Ratios are used rather than absolute amounts to eliminatethe effect of the large amounts of locally derived calcite. The traceelement content of Sahara-Sahel dust has a significantly lowerrange of Cr/Ni ratio and a higher Zr/Y ratio than do the samplesfrom Tikal. Using the same comparison for volcanic materials fromthe Lesser Antilles, Tikal has comparable ratios of Zr/Y, but a greaterratio of Ni/Cr (see Fig. 10). The Ni/Cr trace element content of Tikalsediment is, however, comparable to volcanic rocks and tephrafrom Guatemalan or Salvadoran volcanoes (Fig. 11).

The Ilopango TBJ eruption is the largest and best-documentedHolocene volcanic event in Central America (Hart and Steen-McIntyre, 1983; Sheets, 2002). It occurred during the Early ClassicPeriod, sometime between A.D. 408 and 536 and its ecological andcultural impact would have been felt throughout the Maya region(Dull et al., 2001). Professor Payson Sheets of the University ofColorado kindly provided our team with samples of the TBJ tephrafor comparison. Data for other volcanic components are available inthe database developed by Carr (Carr et al., 2007). XRD analysisdemonstrated that the TBJ tephra is composed of plagioclase feld-spar with lesser amounts of quartz and a large amount of glass. Inother words, there are no clays in the TBJ tephra, because the glassis still intact, unlike the Tikal sediments where all the glass hasconverted to smectite. It is possible that the abundance of calcite inthe Tikal reservoirs compared to the slopes of the Ilopango volcanoaccounts for this difference (Cowgill and Hutchinson, 1963).

Weathering, however, should not affect the trace element ratiosfor high-field strength elements like Cr, Nb, Ti, Y, and Zr (Winchesterand Floyd,1977; Floyd andWinchester,1978;Maynard,1992).Nickelis more mobile than Cr in acidic soils under tropical conditions(Maynard, 1983), and accumulates lower in the profile, which is themechanism of genesis for lateritic nickel deposits. However, itshould remain fixed in the high carbonate environment of thesedeposits. TheNi/Cr ratios in the individual cores at Tikal are constantwith depth, indicating that vertical migration of Ni has beenminimal.

Our XRF analysis of the TBJ tephra sample found a higher ratio ofZr/Y than the sediments from the reservoirs of Tikal. Similar resultswere obtained using the analyses in the Carr database. Note that theposition for Ilopango in Fig. 11 is an average of our sample and theCarr data.

6. Implications

Several insights can be gleaned from this study, somewhatindependently of the surprising amounts of weathered volcanic ashnow revealed from Tikal. First, the pedological and stratigraphiccontrols suggest the presence of a sandfiltration systempresumablyto purify the water source in the Corriental Reservoir. Such a tech-nology was previously unknown, but it is not altogether surprising

given the difficulties associated with waterborne contaminantsaffecting any tropical setting. Secondly, the timing of the initial LatePreclassic construction and harvesting of water from the reservoircoincides with the posited drought-like conditions recently arguedfor this period (Dunning et al., in press). And the Classic period buildupof thenorthwesterngate bermmay suggest the return of awetterperiod, with the necessity of securing the principal ingress into thereservoir to prevent excess waters from eroding the gate. At thispoint, our understanding of regional paleoclimate does not allow usto determine if any of the volcanic eruptions that dumped ash onTikal also affected local, regional, or global climate.

Nevertheless, the most revealing data sets come from the XRDand XRF assessments of weathered volcanic ash over Tikal and byextension over much of the Maya Lowlands through time. Theimpact of volcanism went far beyond the immediacy of the Guate-malan or Chiapanhighlands into the limestone rich soils of theMayaLowlands. The study indicates the wealth of information sealed andpotentially extractable fromany abandoned reservoir setting.Whilereservoir sediments can be compromised by human activities suchas dredging, this was not the case at Corriental. Such reservoirs canprovide significant paleoenvironmental data because (1) we canobtain many more controlled cores, and (2) the diminutive size ofa tankallowsamuchmoremeaningfuldandaccuratedsampleof anentire basin than a lake. Reservoirs can act as a corrective for the“randomness” of lake core extraction. Ideally, lake and ocean floorcores should be used in combination with reservoir sediment data.

7. Conclusion

Corriental is an ancient Maya reservoir at Tikal, Guatemala. Itprovides the first evidence of volcanic ash fall in theMaya Lowlandsduring pre-Hispanic times. Corriental is a unique test location toexamine the chronology and stratigraphy in the reservoir, theaeolian minerals, and their potential source regions. Unlike largelakes, Maya reservoirs are restricted areas with a known constric-tion point over which volcanic minerals are deposited.

Powder XRD and petrography show that volcanic-derivedminerals, smectite and euhedral bipyramidal quartz, are presentin significant quantities in the sediments of Corriental. AMSradiocarbon dating of the Corriental reservoir demonstrates thatglass (now smectite) and euhedral quartz were deposited duringthe Preclassic, Classic, and Postclassic cultural periods. Althoughsmectite is known to occur in Sahara-Sahel dust, XRF trace elementanalysis indicates a different origin, one comparable to tephra fromGuatemalan or Salvadoran volcanoes (Cabadas-Baez et al., 2010).Fingerprinting the exact sources of these minerals will, however,require a comprehensive survey of tephra deposits from the MayaHighlands, Mexico, and the greater Caribbean region.

The presence of airborne volcanic minerals in the reservoirsediments of Tikal supports a model of centuries-long periods ofvolcanism during the prehistoric Maya occupation of the limestonelowlands (Ford and Glicken, 1987). As suggested by Ford and Rose(1995:159), the region likely experienced regular and periodictephra eruptions and ash falls into the lowlands. Ash temper andassemblage of crystals in Maya pottery include biotite, hornblende,hypersthene, and zircon, which all are minerals consistent withGuatemala Highland tephra (Drexler et al., 1980; Rose et al., 1981).The presence of biotite suggests El Chichon, Tajamulco, Acatenango,and Atitlán as possible volcanic sources, which were active in theperiod of AD 600e900 and Cerro Quemado, which was active atabout AD 800 and for some time there after (Conway et al., 1992;Ford and Rose, 1995). Because each reservoir stratum has its ownunique trace element composition, different volcanoes were likelyerupting at different moments in time. Montmorillonite (i.e.,smectite) in Lake Péten Itzá sediments radiocarbon dated A.D. 880

K.B. Tankersley et al. / Journal of Archaeological Science 38 (2011) 2925e2938 2937

to 1140 and El Bajo de Santa Fé may also correlate with thesevolcanic events (Cowgill and Hutchinson, 1963:41; Mueller et al.,2010:525).

The presence of airborne volcanic minerals also supports theposition that the Maya living in the limestone lowland hada dependable source of volcanic ash for the production of ceramics.Indeed, tephra is superior to the temper used as early as the Pre-classic and Early Classic as a result of its angularity (West, 2002).Also given the reverence of the Maya for sacred volcanoes, thetephra likely had religious implications. It may have been collectedat Tikal and the greater Peten from air-fall deposits and acquired bytrade from sources in the Maya Highlands.

Given the accounts of the 1982 ash fall from El Chichón in thePeten it is clear that it would have been very easy for the Maya tocollect and stockpile large quantities of ash. Although it is beyondthe scope of this paper, if we are able to develop an effectivechronology of ash fall at Tikal and the greater Peten, then it mightbe possible to compare this record to chronological patterns intemperingdto say nothing about chronological control of strati-graphic deposits. In addition to the use of ash for temper, smectite isan important raw material for ceramic production because of itsplasticity.

It has long been assumed that tropical soils in the Peten andsurrounding areas were the result of weathering of the localcarbonate bedrock. Ash enrichment of the soils would have alsoenhanced soil fertility by increasing the porosity, permeability, andnutrients needed for maize agriculture, thus helping support pop-ulation increases at Tikal and elsewhere (Ford and Rose, 1995). It islikewise possible that a short-term problem generated by heavy fallwould have been the clogging of water catchment and storagesystems. Additionally, ash falls would have directly affected localflora and fauna proportional to depth. For example, it would haveclogged the spiracles of insects and suffocated them, thus, insectpollinated plants would have suffered. Additionally, volcanic ashwould have been deleterious to aquatic food resources such asshellfish, fish, and manatees.

Although difficult to quantify, several of our stratigraphiccolumns examined in 10 cm arbitrary levels yielded volcanic ashand quartz ejecta in amounts as high as 70% by volume (seeTable 1). It is important to further note that preliminary analysis ofother soils and reservoir sediments from Tikal suggest that smectiteclay derived from weathered volcanic ash is a major component ofthe inorganic fraction of these soils. Future investigationmay revealthat aeolian volcanic material is, indeed, a dominant component ofthe mineral portion of the soils across wide areas of the MayaLowlands.

Acknowledgments

This study was made possible with funding from the CourtFamily Foundation, the Charles Phelps Taft Foundation, theAlphawood Foundation, the Wenner-Gren Foundation, and a grantfrom the National Science Foundation (BCS0810118) to David Lentz,Vernon Scarborough, and Nicholas Dunning. Fieldwork wasundertaken in 2009 as part of the University of CincinnatidIDAEHTikal Project co-directed by David Lentz and Liwy Grazioso Sierra.The laboratory assistance of Meredith Coates, Nathan Marshall, JonPaul McCool, Jim Milawski, Andras Nagy, Leslie Neal, and TonyTamberino were invaluable. Chris Carr led digitization and rectifi-cation of the original University of Pennsylvania map of centralTikal and produced Fig. 3. Eric Weaver led field mapping of theCorriental Reservoir in 2009. Liwy Grazioso Sierra directed many ofthe excavations in Corriental Reservoir assisted by Raquel Macario,Sheryl Carcruz, Ana Arriola, and Marielos Corado. Pat Culbertanalyzed the ceramics recovered from these excavations. Brian

Lane, Benjamin Thomas, and Eric Weaver accomplished the coringof Corriental Reservoir. Payson Sheets provided a tephra samplefrom Ilopango Volcano.

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