Carbon isotopic ratios of wetland and terrace soil sequences in the Maya Lowlands of Belize and Guatemala

Carbon isotopic ratios of wetland and terrace soil sequences in the Maya Lowlands of Belize and Guatemala

Catena 85 (2011) 109–118 Contents lists available at ScienceDirect Catena j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e...

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Catena 85 (2011) 109–118

Contents lists available at ScienceDirect

Catena j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t e n a

Carbon isotopic ratios of wetland and terrace soil sequences in the Maya Lowlands of Belize and Guatemala Timothy Beach a,⁎, Sheryl Luzzadder-Beach b, Richard Terry c, Nicholas Dunning d, Stephen Houston e, Thomas Garrison e a

Georgetown University, United States George Mason University, United States Brigham Young University, United States d University of Cincinnati, United States e Brown University, United States b c

a r t i c l e

i n f o

Article history: Received 24 November 2009 Received in revised form 13 August 2010 Accepted 29 August 2010 Keywords: Carbon isotopes Soil organic matter C4 and C3 plants Maya Lowlands Geoarchaeology

a b s t r a c t This article provides new data and synthesizes earlier findings on the carbon isotope ratios of the humin part of soil organic matter from a range of sites in the central Maya Lowlands. Changes down the soil profile in carbon isotope ratios can provide an important line of evidence for vegetation change and erosion over time, especially in well dated aggrading profiles. Research thus far has provided substantial evidence for significant inputs from C4 vegetation in buried layers from the Ancient Maya periods in depositional soils but equivocal evidence from sloping soils. We present new findings from soil profiles through ancient Maya wetland fields, upland karst wetlands, ancient Maya aguadas (reservoirs), and ancient Maya terraces. Most of the profiles exhibited δ13C enrichment greater than the 2.5–3‰ typical from bacterial fractionation. Seven of nine ancient Maya wetland profiles showed δ13C enrichment ranging from 4.25 to 8.56‰ in ancient Maya-dated sediments that also contained phytolith and pollen evidence of grass (C4 species) dominance. Upland karst sinks and ancient reservoirs produced more modest results for δ13C enrichment. These seasonal wetland profiles exhibited δ13C enrichment ranging from 1 to 7.3‰ from the surface to ancient Maya-period sediments. Agricultural terraces produced mixed results, with two terraces having substantial δ13C enrichment of 5.34 and 5.66‰ and two producing only equivocal results of 1.88 and 3.03‰ from modern topsoils to Maya Classicperiod buried soils. Altogether, these findings indicate that C4 plants made up c. 25% of the vegetation at our sites in the Maya Classic period and only a few percent today. These findings advance the small corpus of studies from ancient terraces, karst sinks, and ancient wetland fields by demonstrating substantial δ13C and thus C4 plant enrichment in soil profile sections dated to ancient Maya times. These studies are also providing a new line of evidence about local and regional soil and ecological change in this region of widespread environmental change in the Late Holocene. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Many studies have now used carbon isotopic ratios of organic carbon from soils and sediments to study environmental changes around the world (Runge, 2002; Webb et al., 2004; Pessenda et al., 2005; Beach et al., 2006; Webb et al., 2007; Beach et al., 2008, 2009; Wright et al., 2009). Over the last two decades this body of research has demonstrated that vegetation can influence the ratio of carbon isotopes (13C/12C) in soil organic carbon (e.g., Schwartz et al., 1986; Balesdent and Mariotti, 1987; Balesdent and Balabane, 1992; Boutton, 1996; Molina et al., 2001). This occurs because many tree and broadleaf evergreen species and some grasses use the C3 photosynthetic pathway, which reduces the 13C

⁎ Corresponding author. Fax: + 1 202 687 8399. E-mail address: [email protected] (T. Beach). 0341-8162/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2010.08.014

in soil organic matter (SOM) derived from C3 plants. In contrast, C4 plants, often grasses like Zea mays and land-cover disturbance related species, increase the ratio of 13C (Runge, 2002; Webb et al., 2004). In the Maya Lowlands, Webb et al. (2004) studied terrace soils around the ancient Maya site of Caracol in Belize (Fig. 1), where they hypothesized that Maya farmers had cultivated maize for many years on the largest and most extensive ancient Maya terrace systems yet studied. Profiles of soil carbon ratios at Caracol revealed a prominent increase of 13C enrichment between 10 and 50 cm, which paralleled the depth of maize root decomposition and rhizodeposition and bioturbation (Molina et al., 2001; Webb et al., 2004). Following this study, other reports have identified a similar 13C enrichment in the Maya region around the ancient sites of Piedras Negras and Motul de San José (Fernandez et al., 2005; Johnson et al., 2007a,b; Webb et al., 2007), the Petexbatún (Wright et al., 2009), and areas of wetlands and terraces in the Petén Department of Guatemala and adjacent Belize (Beach et al., 2006, 2008,

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Fig. 1. Map of the Maya Lowlands with sites in the article.

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2009). These studies focused on many depositional soil environments and found that the zone of 13C enrichment was often much deeper than the upper 50-cm root zone and in many cases the enrichment cooccurred with paleosols, ecofacts and artifacts, and soil horizons that contain radiocarbon dates linked to ancient Maya occupation periods. Since the δ13C composition of suspended sediments reflects the topsoil organic carbon from which they eroded (Bellanger et al., 2004), buried sediment sequences of a dated period should reflect the topsoil δ13C composition from approximately that period. Several past studies hypothesized that buried δ13C enrichment zones and dating evidence reflected depths and rates of deposition in these sinks (Beach et al., 2008; Wright et al., 2009). Carbon isotopic ratio changes could thus both provide evidence for vegetation changes as well as changes in erosion history. This article tests and reviews this hypothesis with a new series of data and a new synthesis from sites in Belize and Guatemala. 1.1. Background A number of overlapping fractions make up the organic matter of soils. These range from readily decomposable, labile, and short-lived organic matter to stable and long-lasting humic substances (Hsieh, 1992, 1996). The decomposable soil organic matter (SOM) fractions may last up to 20 or 30 years, but the stable parts of SOM resist decomposition for much longer times (Balesdent et al., 1988). These stable SOM components occur in association with non-crystalline clays and metal–humus complexes, which help resist microbial breakdown (Veldkamp, 1994; Sollins et al., 1996; Powers and Schlesinger, 2002). The most stable and oldest of SOM is the humin fraction, which persists longer than humic and fulvic acids. In temperate regions the stable fractions are often thousands of years old, though usually younger in the tropics (Jenkinson and Rayner, 1977; Janssen, 1984; Balesdent et al., 1988; Hsieh, 1992, 1996; Pessenda et al., 2001). A growing number of studies have used the C isotopic ratios of SOM as a proxy for broad scale vegetation change, with dated layers of δ13C enrichment indicating relative increases in C4 plants (Schwartz et al., 1986; Kelly et al., 1993; Martinelli et al., 1996; Huang, et al., 2001; Nordt, 2001; Sedov et al., 2003; Webb et al., 2004). Soils from the Maya Lowlands tend to have high amounts of organic matter (Beach, 1998a), and, despite forming under the rapid decomposition of tropical forests, have sufficient quantities for stable isotopic analysis (Sweetwood et al., 2009; Wright et al., 2009). Recent studies have focused on humin, the most stable part of SOM, to detect vegetation change in soils over time, though studies have also found that bulk organic carbon produced similar results. For example, in the terrace soils at Caracol, Belize both

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the humin fraction and bulk carbon showed a significant δ13C value enrichment, though humin was a more sensitive proxy of vegetation change (Webb et al., 2004). The vegetation that makes up SOM does not cause all the changes in δ13C of SOM. For example, the metabolic pathway of soil microbes enhances δ13C values somewhat (Blair et al., 1985). This increase in δ13C values is usually in the range of 1–2.5‰ and a maximum of 3‰ (Blair et al., 1985; Cerri et al., 1985; Balesdent and Mariotti, 1987; Ågren et al., 1996; Boutton, 1996; Martinelli et al., 1996). Thus, in this paper we ascribe δ13C increases greater than 3‰ to vegetation changes instead of microbial isotopic discrimination (Cerri et al., 1985; Boutton, 1996; Wright et al., 2009). Moreover, many profiles from the Maya Lowlands (e.g., Fig. 2) exhibit a bulge of δ13C enrichment in the root zone, with low δ13C of c. −28‰ in topsoils, increases by 5–12‰ at depths of 40 to 120 cm, and a decline in δ13C to c. −28‰ once again below into subsoils (Webb et al., 2004; Fernandez et al., 2005; Webb et al., 2007; Johnson et al., 2007a,b; Sweetwood et al., 2009; Wright et al., 2009). Thus, δ13C measurements of regional soil profiles can provide a proxy of historical vegetation change, and if the bulge of δ13C enrichment is buried below the zone of root decomposition, an independent proxy for deposition and erosion from surrounding watersheds. In the Petexbatún region of Guatemala's Petén, Wright et al. (2009) applied carbon isotope analysis and a mass balance estimate of the percentage of C4 vegetation in SOM to soil profiles from a typology of soil environments around the sites of Aguateca and Punta de Chimino. These included control areas (unlikely cultivated sites including steep slopes and ancient Maya ball courts), cultivable upland areas, defensive enclosures, rejolladas (intermittently wet karst sinkholes floored with cumulic soils), and seasonal and perennial wetlands. Upland samples produced mixed results and estimated 15–21% of SOM from C4 vegetation, indicative of varied or less intensive agricultural uses or erosion of ancient soil profiles. One feature, rejolladas, had the greatest amount of δ13C enrichment (38– 45%) of SOM derived from C4 vegetation, and these seasonally wet karst sinks can conserve soil moisture through the dry seasons. Indeed, because of greater soil moisture and depth, Dunning and Beach (1994) hypothesized that these were zones of ancient intensive agriculture, and the δ13C enrichment and burial at these sinks corroborate that C4 vegetation, such as Zea mays and disturbance species, and erosion and deposition increased during ancient Maya times. Maize and disturbance species can explain both the δ13C enrichment and the accelerated erosion. Johnson et al. (2007a,b) and Wright et al. (2009) also analyzed nine soils taken from seasonal and perennial wetland contexts. They collected perennial wetland samples at spring-fed locations near the

Fig. 2. Carbon isotope graphs from Cancuén, Guatemala, and Mahogany Ridge and La Milpa, Belize. These graphs include radiocarbon dates on charcoal or organic sediments or broader dates based on artifacts (after Beach et al., 2006, 2008).

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site of Aguateca that exhibited no evidence of δ13C enrichment. But samples from riparian wetland locations, which remain moist throughout the dry season, included buried anthropogenic soils at depths of 1.1 and 0.9 m. One of these paleosol sequences near Aguateca dated to 2540 to 1820 calibrated BP and had strong δ13C enrichment down the profile to two paleosols indicative of 30.5% C4 vegetation (Johnson et al., 2007b). In wider regional studies, Beach et al. (2006, 2008, 2009) reported carbon isotopic findings from wetland and upland soils near Cancuén, Guatemala and around La Milpa and Blue Creek, Belize (Fig. 1). The wetland field sites of Blue Creek all demonstrated δ13C enrichment at a depth of 40 to more than 120 cm, radiocarbon dated to the Classic period (1700–1100 BP). Likewise, δ13C enrichment occurred along with aggradation and buried soils in aguadas (karst sinks that are naturally wetter or Maya-altered to be wetter) at Cancuén and in the fills behind a Classic terrace at Mahogany Ridge, Belize. These sites also showed an increase of δ13C by 5‰ or more, from −27.3 to −21.7‰ at Mahogany Ridge, though it was smaller and near the range of natural fractionation in three other sites close to La Milpa: an ancient terrace, an aguada, and a soil in a karst depression (Fig. 2). The greater depths where δ13C enrichment occurs and in association with buried soil horizons also point to aggradation of the Classic and earlier Maya soils. Most of these sedimentary soil sequences were deposited in Classic period-dated levels and thus a product of erosion from the period when vegetation was altered. Indeed, these Classic era sediments are enriched by 4–5‰ over typical tropical forest ratios (−28‰) that have dominated the region for the last millennium, and often return back to −28‰ in underlying pre-Maya soil layers from before 3000 BP.

swamps (Brokaw and Mallory, 1993; Bridgewater et al., 2002; Ellison, 2004).

2. Area descriptions

Following Wright et al. (2009), we assumed −27‰ for δ13CC3 and −12‰ for δ13CC4 in calculations based on this equation.

The sites in this study include a range of wet lowland and well drained upland environments. These include two areas of ancient Maya wetland field complexes around Blue Creek, Belize, ancient Maya terraces near La Milpa, Belize, natural ‘bajo’, seasonal wetlands, sinks between La Milpa and Blue Creek, Belize, and ancient Maya aguadas, or reservoirs, at the sites of La Milpa and El Zotz, Guatemala (Fig. 1). The first two sites lie in the Coastal Plain of Belize, with the wetland fields about 10 masl (meters above sea level), whereas the ancient Maya terraces and upland bajos and reservoirs lie on uplifted structural blocks, elevated about 100 m above the Coastal Plain. The lowland wetlands are perennially moist, the upland bajos and aguadas are seasonally inundated, and the terraces are well drained (Beach et al., 2009; Luzzadder-Beach and Beach, 2009). The migrating Intertropical Convergence Zone (ITCZ) produces a tropical wet and dry climate that receives about 1500 mm of annual rainfall. The dry season and a substantial rainfall deficit occur as the ITCZ resides southward from January through May. As the ITCZ influences this zone from May through December the wet season occurs, with the highest rainfall in June and September and often a several-week-long cannicular drought in August. Frequent hurricanes and tropical storms influence the whole region (Boose et al., 2003; Beach et al., 2008, 2009; McCloskey and Keller, 2009; Dunning and Houston, in press). On the upland block above the Coastal Plain, broadleaf, evergreen forest (with a few deciduous species) is the major vegetation type that was not altered by modern clearance. These tropical forests merge into swamp forests and herbaceous marshes in the perennial wetlands (Brokaw and Mallory, 1993). The research area includes an array of wetlands, from perennially wet Coastal Plain depressions to seasonally wet floodplains, and to seasonally wet upland and intermittent bajos. The herbaceous and forested wetlands of the region include sawgrass (Cladium jamaicense) marsh, sparsely vegetated marl flats with a few Cyperus spp., calabash marsh with high occurrence of Eleocharis interstincta, and red mangrove (Rhizophora mangle) and hardwood

3. Methods In the field we described soil sequences in standard terms, including color, texture, structure, carbonate content (by HCl reaction), and other soil terms as outlined by the Soil Survey Manual (USDA, 1993; Beach et al., 2008). For the C isotopic analysis of soil horizon samples, we first collected all samples in plastic bags and began to dry the samples in the field laboratories. After further drying at the Brigham Young University Lab, we removed gravel by sieving the samples to 2 mm. Further, we crushed and sieved subsamples (5 g) to 0.25 mm prior to acidification to remove carbonates. We acidified the samples with 1 M HCl and heated them in a water bath to 70 °C for at least 2 h to remove calcium and magnesium carbonates. The alkaline pyrophosphate extraction method (Webb et al., 2004, 2007) removed the humic and fulvic acid fractions of the SOM. After removal of carbonates and extraction of humic and fulvic acids, we determined stable carbon isotope ratios, δ13C, of the residual soil humin with the Finnigan Delta Plus isotope-ratio mass spectrometer coupled with a Costech elemental analyzer (EAIRMS) (Wright et al., 2009). Several recent articles provide a further discussion of laboratory methods (Webb et al., 2004, 2007; Johnson et al., 2007a,b; Sweetwood et al., 2009; Wright et al., 2009). We used the equation below to estimate the percentage of soil organic carbon (SOC) obtained from C4 plants (Nordt, 2001: 423; Wright et al., 2009): % SOC obtained from C4 vegetation ðCC4 Þ     13 13 13 13 = 100⁎ δ Csoc –δ CC3 = δ CC4 −δ CC3

4. Results and analysis 4.1. Wetland fields: Birds of Paradise Fields This subgroup of δ13C findings originates from soil profiles in two main areas of ancient Maya wetland fields and canals, the Birds of Paradise (BOP) and Chan Cahal Fields near Blue Creek, Belize (Fig. 1) (Beach et al., 2009). First, as a pilot test to compare vegetation sources with the δ13C of the soil carbon, we sampled the 44 most prominent plant types from the wetland field areas to identify carbon isotopic ratios of the current plants that contribute organic matter to current soil surfaces (Fig. 3). The mean δ13C for all plants was −29.29‰ and the range was −14.92 to −33.02‰, but only two of the plant samples exhibited δ13C greater than −27.2‰, an epiphyte (−17.8‰) and a grass (−14.9‰). Hence, 95% of the modern plants were in the carbon isotopic range of C3 plants, typical of tropical forest species. Many of

Fig. 3. Carbon isotope graph of 44 prominent plants in the Birds of Paradise Fields.

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these wetland plants are very negative, which may be caused by the wet conditions and high humidity that allow the stomata to remain open longer and to discriminate against 13CO2. Second, the surface soils of all wetland profiles produced a mean δ13C of −27.57‰ with a narrow range of −27.3 to −28.34‰, again a signature typical of the C3 vegetation that exists today and slightly elevated (1.78‰) above the δ13C mean of −29.29‰ contained in the plant tissues. The site of BOP 1 provided a deep soil sequence through surface, canal, and sub-canal sediments. The surface soil's carbon isotopic ratio was a typical −27.31‰, which increased steadily downward to −19.6‰ at the Late Classic-dated base of the ancient Maya canal sediments before decreasing back to −24.07‰ in the sub-canal sediments, dated to the earlier Late Classic based on the regional rate of deposition and some artifacts (Beach et al., 2009) (Fig. 4). The mass balance equation above estimates the surface soil SOM to be entirely derived from C3 vegetation but 49.4% of the vegetation in the Late Classic lower canals' SOM (Table 1) and 19.53% of the canal subsoils' SOM were derived from C4 vegetation. The Late Classic canal bottoms also had macrobotanical, pollen, and phytolith evidence that confirmed the dominance by grasses and disturbance taxa, as well as high quantities of charcoal (Beach et al., 2009). BOP 3 and 7 provide well dated canal and field sequences. The BOP 3 field sequence (Fig. 4) abruptly changes from the topical forest signature (−27.4 to −29.6‰) through the top 60 cm to a mixed signature (−23.3 to −22.5‰) from 90 to 140 cm in Late Classic sediments (dated by AMS dates and artifacts). The BOP 3 canal through the top 120 cm increases slightly from −27.3 to −25.96‰ and then increases abruptly in Late Classic-dated sediments to −19.6‰ at 150 cm, the canal's base, before

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declining back below the canal sediments to −24.07‰ at 180 cm. Similarly, BOP 7 increases in the field sequence from −27.31 to −22.6‰ at 120 cm in Classic-dated sediments, and the canal abruptly increases from −26 to −17.4‰ from 120 to 140 cm in the Late Classic canal bottom sediments. The mass balance equation again estimates the SOM of surface soils to be entirely derived from C3 vegetation but up to 64% of the Late Classic canal bottom SOM in BOP 3 to be derived from C4 vegetation (Table 1). The BOP 3 sequences also became dominated by grass pollen and phytoliths and increased charcoal in the zones with elevated δ13C (Beach et al., 2009). The canal sequence at BOP 10 provides another well dated sequence of carbon isotopes, phytoliths, and radiocarbon dates. The top 45 cm had a tropical forest C3 plant signature (−28.03 to −29.06‰) that abruptly rose to −23.25‰ in a bulge dated to the Late Classic from 75 to 110 cm in the lower canal, before dropping to −28.6‰ at 135 cm and −29.2‰ at 230 cm in the Early Classic-dated sediments (Fig. 4). Again the Classic era bulge of δ13C (up to 25.1% derived from C4 vegetation (Table 1)) coincided with the highest levels of charcoal and grass phytoliths, whereas phytoliths of C3 taxa such as Marantacaea, a broadleaf evergreen family, dominate the top 50 cm (Beach et al., 2009). 4.2. Wetland fields: Chan Cahal The excavation of one canal sequence at Chan Cahal (66T, Fig. 5), displays a similar pattern to those described for BOP, with its typical accretion of δ13C from a surface low of −28.3‰, like the tropical forest of today, to a Late Classic canal fill of −24.1‰ and back to −28.2‰ in the Preclassic sub-canal sediments. The canal fill rises to 19.4% C4

Fig. 4. Carbon isotope graphs from wetland field excavations at the Birds of Paradise Fields. These include radiocarbon dates on charcoal or organic sediments from these excavations or from adjacent excavations. Note that BOP 7 graphs both the field profile and two points from the adjacent canal.

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Table 1 Topsoil, Maya Classic, Preclassic, and Pleistocene carbon isotopic ratios (δ13C ‰) and estimates of C4 plant inputs into soil organic matter. Topsoil

Maximum Estimated Maya Classic Preclassic maximum C4 soila and earlier increase soils vegetationb

δ13C ‰

δ13C ‰

δ13C ‰

δ13C ‰

%

Aguadas Cancuen La Milpa Zotz Diablo Tamarandito

−25.05 −27.46 −30.25 −29.40 −23.89

−17.75 −24.23 −25.41 −26.27 −22.89

−25.83 −26.45 −25.25

7.3 3.23 5 3.13 1

61.7 18.5 11.7 4.9 27.4

Bajos D05 Dumbbell Guijarral Palmar

−26.65 −23.10 −29.54 −26.10 −29.52 −25.45 −28.74

−25.43 −24.64c −26.43 −25.80

3.55 4.9 4.07 2.96

26 15.7 3.8 8

Terraces D17 Guijarral Medicinal Trail Mahogany Ridge

−26.43 −28.70 −28.82 −27.33

−24.55 −23.36 −25.79 −21.67

1.88 5.34 3.03 5.66

16.3 24.3 8.1 35.5

Wetlands 66J field 66J canal 66T BOP 1 BOP 3 canal BOP 3 field BOP 7 field BOP 7 canal BOP 10 Mean

−26.45 −26.68 −28.34 −27.30 −27.31 −27.41 −27.31 −25.96 −28.03 −27.57

−25.37 −25.13 −24.09 −19.59 −19.61 −22.50 −22.62 −17.40 −23.24 −23.15

1.08 1.55 4.25 7.71 7.7 4.91 4.69 8.56 4.79

10.9 12.5 19.4 49.4 49.3 30 29.2 64 25.1 25.7d

Sites

a b c d

−25.62 −27.22

Numbers in italics are the highest levels of δ13C in each profile. % SOC obtained from C4 vegetation (CC4) = 100 ⁎ (δ13Csoc − δ13CC3) / (δ13CC4 − δ13CC3). Pleistocene-dated soil. For Classic-dated levels.

vegetation between the uniformly C3 vegetation in the surface and the sub-canal soils (Table 1). Pollen and phytoliths from 66T coincided with the C isotopic evidence, indicating tropical forest taxa associated with the −28‰ levels and a strong mix of C4 species including Zea mays coinciding with the −24.1‰ level (Beach et al., 2009). Another sequence, 66J, 2 km north of 66T provides a comparison through well dated Preclassic to Postclassic canal fills. But, 66J indicates almost no change in its carbon isotope ratio with depth/ time, ranging from −26.7 to −25.1‰ through 220 cm sequences in both a canal and a field. Pollen and phytoliths are not available for 66J, but the carbon isotope ratios would indicate a mixed but mostly C3 signature (12.5% C4 vegetation at maximum) throughout its well preserved stratigraphy dated from modern to the Early Preclassic. Both 66J and 66T indicated more C3 vegetation than the Classic-dated sediments at BOP, except for the notable bulge of δ13C that coincides with the influx of Zea mays in the Late Classic 66T canal, which implies that these wetland fields focused more on arboriculture or other C3 crop species. In sum, the soil sequences for the wetland field areas (Figs. 4 and 5), except for the site of 66J, had consistent patterns, with tropical forest C isotopic signatures (c. −28‰) from surface soils downward to 40 to 80 cm, and an enrichment of δ13C from 60 to 140 cm by 4.25 to 8.56‰ (mean = 6.44‰) before decreasing back to c. −28‰ in lower depths in all the profiles deep enough to precede the periods of Maya impacts. Three of four ancient Maya canals gave parallel finding as well, because these had abrupt and substantial δ13C enrichments in the lowest portions of the canals from depths of 120 to 150 cm that date to the last occupation of the fields in the Maya Late Classic. The canals overall had more abrupt patterns of δ13C enrichment than the fields possibly

Fig. 5. Carbon isotope graphs from wetland field excavations at the Chan Cahal Wetland Fields. These include radiocarbon dates on charcoal or organic sediments from these excavations or from adjacent excavations.

because of their more rapid aggradation rates than fields (Beach et al., 2009), leaving less time for bioturbation of aggrading organic matter to mix δ13C levels. Given the strong microfossil and charcoal evidence together with the δ13C enrichment, the lowest canal sediments probably reflect field abandonment with direct deposition of burned vegetation and the highest inputs of C4 organic matter from the fields. 4.3. Seasonal ‘bajo’ wetlands and terraces Three soil profiles in seasonal ‘bajo’ wetlands with dated sequences of aggraded sediments provided both comparable and contrasting patterns (Fig. 6; Table 1). First, the large karst sink called the Dumbbell Bajo, Belize produced δ13C enrichment of −29.54 to −26.1‰ from the surface to Classic period-dated sediments at 80 cm, depletion to −28.6‰ at 180 cm, and another enrichment to −24.64‰ at 280 cm in a buried Pleistocene-dated Oxisol topsoil (Dunning et al., 2006). The 3.44‰ enrichment of Classical sediments is marginally above the δ13C enrichment by microbial decomposition of 1–3‰ and represents a 6% increase in C4 vegetation at most, but the 4.9‰ enrichment to the Pleistocene Oxisol is more significant and reflects 15.7% more C4 vegetation during this region's dryer Pleistocene (Leyden, 1984). Second, in a small sink, D05, near La Milpa, Belize, the δ13C SOM increased from the surface to Classic era deposits from −26.65 to −23.10 (a 3.55‰ enrichment and an estimated increase to 26% C4 vegetation). At the Guijarral bajo, the δ13C enrichment bulge from the surface into Classic-aged sediments ran from −29.52 to −25.45‰ (an enrichment of 4.07‰ and an estimated increase to 10.5% C4 vegetation). These bajo profiles showed Classic-period and Pleistocene enrichment but less overall than the perennial wetlands. The bajos have many Classic era sites and agricultural features with and around them (Beach et al., 2002, 2003; Dunning et al., 2006). Four agricultural terraces with buried soil sequences in the region provided variable results. Earlier, we reviewed that two Classic-period

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Fig. 6. Carbon isotope graphs from bajo wetland and terrace excavations in Guatemala and adjoining Belize. These include radiocarbon dates on charcoal or organic sediments from these excavations. The graphs titled Guijarral Terrace and Medicine Trail are from soils behind ancient Maya terraces.

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terrace sites at Caracol and Mahogany Ridge, Belize produced significant 13 C enrichment from modern topsoils to ancient Maya-period buried horizons (Fig. 2; Table 1). The terrace at Guijarral, one of a system adjacent to the Guijarral bajo (Fig. 6), produced a δ13C enrichment from −28.7 to −23.36‰ from the surface into Classic-period sediments, which indicates a 24.3% increase in C4 species (Table 1). Likewise, at Mahogany Ridge a deeper sequence produced an even greater C4 plant enrichment of 35.5%. Two additional sequences to buried soils associated with Classic period terraces near La Milpa, Belize (Fig. 6) produced δ13C enrichment of 3.03 and 1.88‰, which are within the 1–3‰ range of microbial decomposition. These well-drained terrace sediments did not produce pollen that would have allowed comparison with the C isotopic evidence. 4.4. ‘Aguadas’ at El Zotz (Petén, Guatemala) Two aguadas within the ancient site of El Zotz, Guatemala provided dated sedimentary sequences with carbon isotopic profiles to depths of 105 and 230 cm (Fig. 6). The El Diablo profile rises from −29.4‰ at its modern surface to −26.3‰ in Early Classic-dated sediments at depth c. 100 cm (Cal BP 1690 to 1520). The Diablo site was a locus of royal ritual and residence and lies on a high precipice that was active only in the Early Classic (BP 1700–1400). The main aguada at the site of El Zotz, a community that existed from BP 2200 to 800 (Pérez Robles et al., 2009), also showed a rise in the carbon isotopic ratio from −30.2‰ in its modern surface to −25.4‰ in Late Classic-dated sediments at c. 100 cm (Cal BP 1290 to 1140). The El Zotz profile declined to −26.2‰ through undated sediments at 150 cm before rising to −25.2‰ in Preclassic sediments at c. 230 cm (at 200 cm, Cal BP 2680 to 2340). The rise in δ13C by 4.8‰ and 5‰ into Late Classic and Preclassic levels through the El Zotz Aguada indicates a maximum input of 11.7% C4 vegetation, and thus still dominated by C3 plant-derived organic matter. The El Diablo profile's increase of 3.1‰ in δ13C from modern to Early Classic levels indicates even less input from C4 plants (4.9%). This high and steep site may not have had as much vegetation alteration (and little input from Zea mays) as the lower and less steep, main site of El Zotz, though based on deposition rates, erosion was high (and vegetation low) in the Early Classic when construction was occurring. One additional profile, grouped under bajos (Table 1), within the nearby Preclassic settlement of Palmar, provides comparable results to those of El Diablo. At Palmar, the carbon isotopic ratio rose from −28.7‰ at its modern surface to −25.8‰ in its Preclassic (Cal BP 2440 to 2320) dated sediments at c. 100 cm, which had a similar quantity of change as at El Diablo, and both occur in locations unlikely to have been influenced by agriculture. The Middle Preclassic isotopic ratio of −25.8‰ is comparable to similarly aged sediments at El Zotz (−25.25‰). 5. Discussion Upland soils in the Maya Lowlands display equivocal δ13C enrichment with soil depth. In the Petexbatún region, Guatemala (Wright et al., 2009) upland profiles exhibited mostly insignificant change with depth. At Chunchucmil, Yucatán (Sweetwood et al., 2009) the native mixture of C3/C4 plants of the savanna prevented identification of a C4 agriculture signature. Because of the large ancient Maya populations and long occupancies of these sites, most upland sites must also have had inputs from C4 plants like Zea mays. At least five factors, however, may have altered the C isotopic patterns through these upland soil profiles. First, soil erosion on sloping ground often truncated ancient agricultural surfaces that may have had elevated δ13C, and second, deposition in basins would have buried soils and evidence of elevated δ13C. Third, bioturbation would mix soils and their δ13C levels. Fourth, many surfaces may have had arboriculture or other types of C3 plant-based economic production. And lastly, the long Holocene inputs from tropical forest C3 species to SOM should have influenced carbon isotopic ratios of

soil profiles more, but in diminishing amounts in converted landscapes over time. Perennial wetland sites provide the strongest evidence of δ13C enrichment with soil depth. All of these well dated sequences, except 66J, indicated substantial increases in δ13C from the topsoils downward to ancient Maya Classic levels, and five cases decreased δ13C into earlier soil levels. Seven of these sites also have pollen and phytoliths (and some have macrobotanicals) (Beach et al., 2009) that correspond with the δ13C figures. As a whole, these δ13C data indicate wetlands contained 10.9 to 64% C4 species in the Late Classic. We need additional proxies to test the outlier case of 66J, which had small δ13C increases perhaps due to high bioturbation or dominance by C3 crop species. The ancient agricultural terraces and upland sinks did indeed demonstrate stronger patterns of δ13C isotopic enrichment. The thin terrace soils at Caracol tended to have enriched δ13C from 10 to 50 cm (Webb et al., 2004), which is similar to the thin terrace soils at La Milpa and Guijarral. The aggrading cross-channel terrace at Mahogany Ridge (Beach et al., 2006, 2008), however, experienced greater and deeper δ13C enrichment from 10 to 100 cm. Webb et al. (2004) hypothesized maize enrichment as a factor of the C isotope ratios at Caracol, and Guijarral and the Mahogany Ridge terrace have similar δ13C enrichment of 5.34–5.66‰. Although these well-drained upland soils have yet to produce maize phytoliths or pollen, another terrace, and like Guijarral above a bajo sequence, next to Bajo Donato Guatemala, did produce a carbonized maize kernel dated to the Late Preclassic period (c. 2000 BP) (Beach et al., 2009). This confirms, perhaps unsurprisingly, that the ancient Maya used terraces to grow maize and, in this case, as early as c. 2000 BP. Localized karst depositional basins such as rejolladas and aguadas, which Dunning and Beach (1994) suggested as sites of agricultural intensification, had a distinct pattern. Seven of eleven such sites in the Petexbatún had significant δ13C enrichment in depths up to 210 cm, ranging from surface lows of c. −28‰ to subsoil highs of c. −22‰. Similarly, aguadas, natural or engineered wetter sinks, also revealed δ13C enrichment through the ancient Maya periods. One well dated aguada at Cancuén in Guatemala's Petén increased from −25.05‰ at the surface to −17.74‰ in Late Classic sediments at 110 cm, where an estimated 64% of SOM was derived from C4 plants. Most other depositional basins also demonstrated some increase in δ13C enrichment through ancient Maya-dated sediments (Beach et al., 2008). One inconclusive site was the Classic period reservoir at the Petexbatún site of Tamarandito (Table 1) (Beach and Dunning, 1997; Beach, 1998b). The ratios of carbon isotopes were −23.9‰ at 14 cm and −22.9‰ at 107 cm, a small δ13C enrichment of 1‰ from the A2 horizon to just above the Preclassic paleosol, but one that had a near surface of dense clays, already δ13C enriched in this region of tropical forest. Tamarandito's reservoir had a mixed C3 and C4 signature (with up to 27.33% derived from C4 plants), similar to the other Classic period buried sequences elsewhere in this region (Wright et al., 2009). The upland sites and non depositional terraces from Caracol provide insight into quantities and timing of deposition as well (Webb et al., 2004). These terraces have δ13C enrichment with C4 grasses from 10 to 50 cm. But many of the depositional sites in our studies have δ13C enrichment down to 200 cm or more, indicating c. 150 cm of deposition, and much of it during the period of C4 plant inputs because the C4 signatures often extend up to 25 cm from the present surface. At this time there are insufficient data to trace the change in carbon isotopic ratios over time through sediments, but our present findings provide some outlines for future comparisons. The one Pleistocenedated paleosol produced a δ13C of −24.64‰, which is approximately 5‰ more than the surface soil and represents an estimated 15.7% C4 vegetation. This runs counter to the higher Pleistocene C3 evidence in regional lake cores (Huang et al., 2001), but generally corresponds to the pollen evidence (Leyden, 2002). There is a large time gap in our soil sequences to the later Holocene (Archaic and early to middle Preclassic).

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Again the samples are few, but there is a range from −28.2 to −27.2‰ in the Preclassic through Archaic-dated sediments at Chan Cahal to −27.22 to −25.25‰ in the Preclassic sediments at the other sites. Most of the well dated Classic Period sediments indicate δ13C enrichment from the modern surfaces (c. −28‰) into earlier Classic era sediments (−26.27 to −17.4‰), and the mean of the Classic era sediments indicates about 25.7% C4 vegetation. This corresponds with regional pollen and lake-core δ13C evidence. We cannot rule out Late Holocene drying as a contributing factor to land use (Huang et al., 2001), though the largest C4 plant increases occurred in perennial wetlands. Overall, carbon isotopic ratios add to the lines of evidence that point toward the most intensive land uses in the Classic period, but Beach et al. (2003) and Anselmetti et al. (2007) demonstrated greater localized soil erosion in the Preclassic with high but diminished rates in the Classic. 6. Conclusions Enrichment of δ13C occurs through most of the sediment profiles in the wetland fields and canals that date to ancient Maya times. Indeed, all but two of the wetland field profiles are enriched by more than the 2.5 to 3‰ typical of bacterial fractionation. Seven of nine profiles also exhibited δ13C enrichment (ranging from 4.24 to 8.56‰) into specifically Late Classic-dated sediments, and the highest enrichment occurred from recent upper sediments downward into layered, Late Classic canal sediments, which also produced phytolith and pollen evidence of grass dominance (Beach et al., 2009). Both the δ13C isotopic increases and microfossil evidence indicate the greatest quantity of vegetation change, from modern C3 tropical forest dominance downward to mixed C3 and C4 vegetation, in the Maya Late Classic. Moreover, the significant increases in C4 species evident through the Maya period as well as the abundance of charcoal in perennial wetlands indicate that fire must have been widespread in this region of modern C3 species dominance (Bond et al., 2005). We also found δ13C enrichment in upland karst sinks and ancient reservoirs. Two seasonal wetland bajos exhibited δ13C enrichment of 3.44 and 4.07‰ from the surface to Classic-period sediments, which were somewhat greater than the 2.5 to 3‰ range of bacterial fractionation. The first bajo also produced 4.9‰ δ13C enrichment from the surface to Late Pleistocene-period sediments. The three sites around El Zotz in the northern Petén of Guatemala had increases of 2.9, 3.1, and 5‰ in δ13C from surface to Maya-aged sediments. The first two were borderline findings in places that were unlikely sites of agriculture, but the latter indicates that significant enrichment occurred in the main reservoir of the main site of El Zotz. This was, nonetheless, an increase to −25.4‰ in the Late Classic and −25.2‰ in the Preclassic, which in both cases indicate much more C3 plant inputs into the SOM than at sites such as Cancuén and BOP 7 that attained δ13C ratios of −17.74 and −17.4‰ in Classic period sediments. El Zotz was the one site, however, that had the earliest dated Maya era δ13C enrichment (in the Preclassic), which was even higher than its Late Classic sediments. Since El Zotz became an urban space in the Classic period, the higher Preclassic C4 signature may indicate more intensive agricultural uses before the city rose around it. Agricultural terraces produced mixed results, with two terraces showing substantial δ13C enrichment and two only equivocal results from modern topsoils to Classic-period buried soils. Guijarral and Mahogany Ridge had δ13C enrichment of 5.34 and 5.66‰ into Classicperiod sediments, whereas terraces at La Milpa, Belize produced 13C enrichment of 3.03 and 1.88‰, largely within the 1–3‰ range of microbial fractionation. These findings and the growing body of research on δ13C through sediment profiles indicate that much of the Maya Lowlands was significantly enriched in organic matter from C4 plants, including Zea mays and other grasses, especially during the Late Classic period. The mean δ13C from our soil profiles that dates to the Classic period is −23.15‰, which indicates C4 plants made up 25.7% of the vegetation

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in our sites. This number is even higher in mainly Classic period sediments of wetland field systems, which were largely Late Classic era sites. Evidence from δ13C soil profiles is beginning to add to the microfossil and erosion and sedimentation evidence to show how widespread this vegetation change was over the Maya period and the Late Holocene. Moreover, an advantage of δ13C sediment profiles is that they can provide more local scale evidence from numerous depositional environments to provide wider coverage of environmental change over time. Lastly, this study has demonstrated the increase of δ13C down soil profiles and compared this with several lines of proxy evidence that help to elucidate these changes. In itself, carbon isotopic evidence provides another perspective on the significant change in the Maya Lowlands from pre-Maya times to the present. Nevertheless, we need additional studies in soil-archaeological contexts that use carbon isotope ratios alongside multiple lines of evidence to reveal more about soil–human interactions over millennia.

Acknowledgements We thank several organizations for supporting this research: the Georgetown University's School of Foreign Service, the Cinco Hermanos Chair in Environment and International Affairs; grants from the National Geographic Society (CRE-7506-03, CRE-7861-05; T. Beach and S. Luzzadder-Beach PIs), the Guggenheim Foundation, Dumbarton Oaks, and the National Science Foundation (Nos. BCS-0924510, T. Beach, PI; BCS-0924501, S. Luzzadder-Beach, PI; BCS-0241757 and BCS-0650393, N. Dunning PI; and BCS-0840930, S. Houston, PI); the George Mason University's Center for Global Studies and Provost's Office; and the University of Cincinnati. In Belize we worked within the Maya Research Program with Dr. T. Guderjan, Director, the Programme for Belize Archaeological Project, Dr. F. Valdez Jr., Director, and with the gracious cooperation of the Department of Archaeology, Ministry of Tourism and the Environment, the Programme for Belize, and the communities of Blue Creek and San Felipe. We also thank Drs. M. Benedetti and C. Cordova, K. Cox, Esq., anonymous reviewers, and many graduate and undergraduate students of our institutions. Findings and interpretations are the responsibility of the authors, not of the supporting agencies and institutions.

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