Mineral magnetic properties of an alluvial paleosol sequence in the Maya Lowlands: Late Pleistocene–Holocene paleoclimatic implications

Quaternary International xxx (2016) 1e12

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Mineral magnetic properties of an alluvial paleosol sequence in the Maya Lowlands: Late PleistoceneeHolocene paleoclimatic implications
zquez C. a, *, Berenice Solís C. b, Elizabeth Solleiro-Rebolledo c, Gabriel Va Avto Goguitchaichvili d, Juan J. Morales C. d
tzcuaro No.8701, Ex Hacienda de Sa
n Jos
Escuela Nacional de Estudios Superiores, Unidad Morelia-UNAM, Antigua Carretera a Pa e de la Huerta, 58190
n, Mexico Morelia, Michoaca
n, Mexico Centro de Investigaciones en Geografía Ambiental, UNAM, Campus Morelia, Michoaca c Instituto de Geología, UNAM, Mexico d
n, UNAM, Campus Morelia, Mexico Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica, Unidad Michoaca a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Available online xxx

In the Maya Lowlands region, along the Usumacinta River, Late PleistoceneeHolocene profiles exhibit sedimentary characteristics and soil formation processes, as a result of environmental conditions and humaneenvironmental interactions. Here, we report a comprehensive environmental magnetic investigation on paleosols and sediments at the Tierra Blanca archaeological site which shows clear evidences of human occupation. The pedogenetical magnetic properties were characterized by rock magnetism studies to determine the composition, concentration and domain size distribution of magnetic minerals. The Late Pleistocene paleosols were formed under the influence of a humid climate, as shown by the presence of recent fine hematite formation, directly related to reductioneoxidation processes during Gleysol development. The transition from early to middle Holocene was obtained from a paleovertisol, dated as 2.34e2.3 ka, which showed a marked heterogeneity in its magnetic parameters due to continuous argilloturbation processes. The environmental conditions retrieved, indicate the presence of a drought period around 5.5 ka. The upper two paleosols were less developed and seems to be strongly affected by human activities during the Maya Classic period, dated between 1.14 and 0.97 ka. These soils were formed under more humid conditions, and yielded a magnetic mineralogy (hematite-magnetitemaghemite) due to incipient weathering and anthropic disturbance. © 2016 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Soil magnetism Environmental conditions Human impact

1. Introduction One of the techniques used to analyze paleosol sequences is environmental magnetism, which is used as paleoenvironmental proxy and as an indicator of pedogenetic processes in relation with lithogenic magnetic components. The interrelation between magnetic properties and pedogenesis has been widely studied in loesspaleosol sequences but in lesser proportion in alluvial and volcanic sequences.

* Corresponding author.
zquez C.), bsolisc. E-mail addresses: [email protected] (G. Va [email protected] (B. Solís C.), [email protected] (E. Solleiro-Rebolledo), [email protected] (A. Goguitchaichvili), jmorales@geofisica.unam.mx (J.J. Morales C.).

Numerous mineral magnetic studies of loessepaleosols have shown the potential of magnetic methods to provide reconstructions on palaeoenvironmental and paleoclimatic changes during the Quaternary (Dearing et al., 2001; Maher et al., 2003; Geiss et al., 2004; Quinton and Dahms, 2011; Lu et al., 2012; Moron et al., 2013; Lyons et al., 2014). Some of them have been applied to characterize pedogenic processes that occurred during soil development under specific climatic conditions (Maher, 1998; Maher et al., 2002; Ortega et al., 2004; Alekseeva et al., 2007; Bartel et al., 2011). Other works have been focused on understanding the mineralogy, the concentration of magnetic minerals, and the relationship between pedogenesis and climatic fluctuations through time; for example, in China (Vidic et al., 2004; Liu et al., 2007a, 2008), United States (Singer et al., 1992; Grimley et al., 2003; Geiss et al., 2008; Quinton and Dahms, 2011), Europe

http://dx.doi.org/10.1016/j.quaint.2015.09.094 1040-6182/© 2016 Elsevier Ltd and INQUA. All rights reserved.


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(Jordanova et al., 2003, 2004; Maher et al., 2003; Alekseeva et al., 2007; Liu et al., 2010), Argentina (Orgeira et al., 2003; Bidegain et al., 2009), and recently in volcanic paleosols of Central Mexico (Ortega et al., 2004; Rivas et al., 2006). Magnetic rock properties usually include magnetic mineralogy, magnetic concentration, and magnetic grain size. Full understanding of these factors is crucial to clearly establish their significance into the environmental context. These techniques are usually easy to measure, obtaining potential paleoenvironmental records. The influence of pedogenic processes is also reflected on magnetic parameters. For instance, changes in redox conditions due to plant decomposition, or by repeated wet and dry periods during soil development, undergo in situ formation of (ultra) fine-grained ferromagnetic minerals (>0.1 mm) (Maher, 1988; Geiss et al., 2004). Also gleyic conditions are associated with a minimum concentration of magnetic minerals due to the chemically reducing environment that is destructive to magnetic minerals (Ding et al., 2000; Liu et al., 2007b; Hao et al., 2008). More interest has been shown in the formation of antiferromagnetic minerals (hematite-goethite) because their presence directly results from pedogenesis, and they are correlated to the degree of soil development (Liu et al., 2002, 2007a; Ji et al., 2004; Hao et al., 2008). Those magnetic minerals are the result of cyclic moisture changes (i.e. alternation of redox and oxidizing conditions). Hematite production can be favored by warm climates with dry periods and goethite by precipitation events (Ji et al., 2001; Balsam et al., 2004; Orgeira et al., 2011). On the other hand, pedogenic hematite can be formed by the transformation of ferrihydrite due to chemical weathering during soil development
n and Torrent, 2002; Torrent et al., 2006, 2010). (Barro Studies on paleosols have suggested that magnetic properties can be used as a “proxy” for the estimation of the chemical weathering degree during pedogenesis (Maher, 1998; Torrent et al., 2007; Liu et al., 2012). Strong pedogenesis enhances the transformation of magnetic maghemite to hematite, while relative concentrations of those magnetic minerals correlate with the weathering degree (Liu et al., 2012; Wang et al., 2013). The magnetic properties develop with time. Singer et al. (1992) and Torrent et al. (2010) suggest that several thousands of years are necessary to have significant magnetic enhancement on soils. On the contrary, Maher et al. (2003) suggest that magnetic minerals are acquired within a few centuries, as is shown in many modern and Holocene soils. Human activities can modify the trends of pedogenic processes and in consequence, the soil magnetic minerals. Changes in magnetic susceptibility have been detected as a consequence of tillage (Jordanova et al., 2011), fires (LeBorgne, 1960; Kletetschka and Banerjee, 1995), deforestation and erosion (Mokhtari-Karchegani et al., 2011), and pollution (Karimi et al., 2011), among others. The Maya Lowlands have been affected by regional environmental changes during the Late Pleistocene and Holocene, as recorded in lacustrine sediments (Rosenmeier et al., 2002; Carleton et al., 2014), and paleopedological records (Sedov et al., 2007; Fedick et al., 2008; Cabadas et al., 2010; Beach et al., 2011). Some of the significant climatic changes coincide with disruptions of archaeological events such as the collapse of the Maya civilization (Haug et al., 2003). The alluvial-paleosol sequences of the Usumacinta River in the Northwest Maya Lowlands have documented Late PleistoceneeHolocene environmental changes and a long history of human occupation (Solís et al., 2013a, 2013b). The anthropogenic activities have altered the climatic record through agricultural practices which produced changes in the erosion and sedimentation rates from middle to late Holocene.

In this study, we analyze the rock magnetic parameters of a paleopedological sequence of the Maya Lowlands, which showed a different pedogenesis degree according to past environmental conditions and landscape stability (Solís et al., 2013a). The magnetic parameters were linked to the pedogenic signal, and used as paleoenvironment proxies. Additionally, we show the potential use of magnetic data in geoarchaeological studies. 2. Regional setting The Usumacinta River has been formed since the PlioPleistocene. This river shows different phases of accumulation and erosion that produced a complex system of terraces and an extensive alluvial plain where many Maya archaeological sites can be found (Solís et al., 2014a). It runs from south to north, starting in Guatemala and crossing through the Chiapas Range into the Gulf of Mexico (Fig. 1). The regional climate is warm and humid with a mean annual temperature of 27
C. Major precipitation generally occurs during the summer, reaching 1800 mm per year (García, 1988). The vegetation in the floodplain area is characterized mainly by grasses and aquatic species, while the evergreen tropical rainforest is present in the Chiapas Range (Bueno et al., 2005; Rzedowski, 2006). The alluvial architecture is controlled by erosion, sedimentation and pedogenesis, related to changes in environmental conditions since Late Pleistocene, and human disturbance during the Middle Holocene (Solís et al., 2013b). Detailed paleopedological studies have been done and showed the differences between the pedogenesis types through time (Solís et al., 2013a, 2013b, 2014a). 3. Material and methods For this research, we have selected a paleopedological section constructed during the 2010 field trip. This section is constituted by three profiles: Tierra Blanca I (TBI), Tierra Blanca II (TBII), and Tierra Blanca III (TBIII). The complete Tierra Blanca composite sequence consists of 9.7 m of paleosols and sediments (Fig. 2). The upper part of the sequence has three organic paleosols with a thickness of 3.6 m; the middle part corresponds to 2.6 m of silty sediments and finally, the lower part has a 3.5 m in thickness and is formed by four paleosols, classified as Gleysols (6, 7, 8, and 9). These paleosols are better expressed as TBI. They are clayey (40e70% of clay) and have grayish brown colorations and reddish-yellowish mottles; also they have a subangular blocky structure, Fe concretions and dendritic Mn. These features decrease with depth (Solís et al., 2013a). Additionally, they show strong weathering, which is shown by the presence of high concentrations of stable minerals (Solís et al., 2013b). The Gleysols rest over a sandy alluvial sediment, which has an age of 65e123 ka dated by OSL. The age differences are due to a poor bleaching of the alluvial sediments after deposition (Solís et al., 2013b). One of the most remarkable features on these soils is the presence of hard pedogenic carbonate concretions that are very young (5.45e5.38 ka). The interpretation of these concretions is that they were formed in the upper part of the section during the paleosol development (Solís et al., 2013a). Above the Gleysols, there is alluvial silty sediment that was registered in TBII and dated by OSL on 9 ka (Solís et al., 2013b). This sediment has a white coloration, and is mainly composed by volcanic glass, although it is not homogeneous. At the top, it is laminated with vertical cracks cutting through the sediment layers. In the middle, it has a sandy texture, reductomorphic features, secondary carbonate concretions, and cross stratification that alternates with laminations. Also, it contains a pedosediment between 5.16 and 5.51 m, with some structure and vertic features. The lower


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Fig. 1. Location of Usumacinta River in the southeastern region of Mexico and the paleopedological section at Tierra Blanca. The map also shows the archaeological sites of Classical Mayan civilization.

sediment is laminated and shows a wavy boundary (Solís et al., 2013b). The chronology of the upper paleosols in section TBIII (labeled as paleosols 2, 3, 4, and 5) was established by 14C and OSL dating and corroborated by the archaeological context, which usually may supply only relative ages (Solís et al., 2013a, 2013b). These paleosols are associated with each of the cultural periods of the region: Formative (800 BC- AD 150), Classic (AD 150e1000) and Postclassic (AD 1000e1500). The higher soil development occurs on Formative paleosols at horizons 5Ass, 5Bss and 5BC. Horizon 5Ass is dark, and has an organic matter content of 1.5%, a high content of clay (>50%) and possess vertic properties (i.e. cutan stress, angular aggregates). This paleosol was classificated as a Vertisol. The paleosol of the Classic period rests on a pedosediment (4Bk), that reflects an erosion phase, deposition, and a low pedogenesis. The paleosol consists of horizons 3A/3C with a 30% of clay and shows less development than the Formative paleosol. The OSL age of its C horizon is 2100 years and the 14C dating of the A horizon is 1140e970 B.P. The PostClassic paleosol is located above the Classic ones and it is also less developed than the Formative paleosol. There is not instrumental dating for this paleosol, although the abundant archeological material indicates that it corresponds to the interval between 1000 and 1500 years AD. The top of the sequence has a very weakly developed soil, with an incipient C horizon, which corresponds to the modern soil cover. This soil as well as the Classic and Post-Classic paleosols, are classified as Fluvisols.

Samples were collected every 10 cm along the profiles TBI, TBII, and TBIII, and then they were correlated with the pedogenic horizons designated by Solís et al., 2013a. The material from the three profiles was packed into weakly diamagnetic plastic cubes (8 cm3) for magnetic analyses on 102 samples from the composite sequence. Mass-specific low-field magnetic susceptibility (clf) was measured in a Bartington MS3 system, it is a measure of the concentration of magnetic minerals (Butler, 1998), it represents the total contribution of Fe-bearing minerals on materials and is controlled by the total ferrimagnetic concentration (Thompson and Oldfield, 1986). Anhysteretic Remanent Magnetization (ARM) was acquired in a 50 mT bias field, superimposed on a peak alternating field of 100 mT in a Schonstedt GSD-1 demagnetizer. ARM is highly sensitive to grain size and concentration of ferrimagnetic minerals of single domain (SD) and pseudo-single domain (PSD) grains (~0.1e1 mm) (Hunt et al., 1995). The Isothermal Remanent Magnetization (IRM) acquisition was carried out in a pulse magnetizer in fields of 100, 1000, 100 and 300 mT, and then measured in a Molspin fluxgate magnetometer, and it also reflects the concentration of magnetic minerals (Butler, 1998). ARM/SIRM ratio is used to estimate the concentration of SD grains contribution (Geiss, 1999). Frequency-dependent susceptibility cfd (%) is a sensitive parameter of the concentration of ultrafine grained (<30 nm in magnetite) superparamagnetic (SP) particles (Dearing et al., 1996; Jordanova et al., 1997; Maher, 1998), but it is dependent on titanium present in Ti-magnetites (Wall and Worm, 2000).


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Fig. 2. Exposed paleosols at profiles over the Usumacinta River bank. a) Tierra Blanca I, b) Tierra Balnca II and c) Tierra Blanca III.

High coercivity IRM was measured in backfields of 300 mT (HIRM ¼ IRM1000 mT þ IRM-300 mT/2). It is a tool to estimate the concentration of high coercivity minerals, or very fine-grained ferrimagnetic grains (as hematite or goethite) (Opdyke and Chanell, 1999; Geiss et al., 2004; Inoue et al., 2004; Rivas et al., 2006; Yong-Xiang et al., 2006). S ratio was calculated as Sx ¼ IRMx mT/SIRM) (Evans and Heller, 2003; Ortega et al., 2006; Rivas et al., 2012). S300 ratios are commonly used to interpret the presence of high coercivity minerals, especially when S300 ratios are <80%, it generally indicates the presence of hematite or goetithe (Opdyke and Chanell, 1999). S100 ratios display grain size variations in low coercivity components among coarse-grained ferromagnetic particles, although it cannot be distinguished from mixtures of a high-coercivity and a fine-grained (>0.05 mm) low-coercivity fraction (Robinson, 1986). Saturation Magnetization (MS and Mr) and coercivity parameters (Hcr and Hc), were obtained from hysteresis loops (at a maximum field of 1 T) measured with an Advanced Variable Field Translation Balance equipment (AVFTB) at room temperature. Both Hcr and Hc are used to indicate the magnetic hardness of minerals, and are used to show the presence of high coercivity minerals (i.e. hematite); and Ms and Mr measurements reflect the magnetic concentration. To correct the presence of paramagnetic minerals, ferrimagnetic susceptibility (cf) was calculated by subtracting the paramagnetic contribution (cp) estimated from the high field slope of the hysteresis loops. The magnetic susceptibility vs. temperature (so-called continuous thermomagnetic curves or Curie temperature [Tc] determination) is one of the most commonly approaches used in the estimation of magnetic mineralogy (Butler, 1998; Opdyke and Chanell, 1999; Evans and Heller, 2003). These measurements were carried out on the bulk of soil samples using atmospheric air (neither an inert gas atmosphere was applied, nor sealed capsules were used); the changes in c during heating and cooling to know the Curie temperature, were measured on a Bartington MS2WF furnace system, from 35 to 650
C. Pure Ti-free magnetite has a

Curie temperature close to 580
C; hematite, has a Curie temperature of 675
C, and goethite are difficult to observe in thermal demagnetization and susceptibility curves due to their low magnetic signal that is overshadowed by the stronger ferromagnetic Timagnetites. 4. Results The paleosols of Usumacinta River are grouped into four units: (i) Gleysols, (ii) sediments, (iii) Vertisols, and (iv) Fluvisols. The main features of each unit are described as follows: (i) Gleysols are characterized by their reductomorphic features; (ii) sediment composition corresponds to 90% of volcanic glass and 10% of stable minerals with a carbonate matrix; (iii) Vertisols have a high organic matter accumulation, angular structure and hard consistence; finally, (iv) Fluvisols are characterized by their lower development, where sedimentation and pedogenesis are present. 4.1. Magnetic concentration Magnetic concentration can be estimated from de c, ARM, SIRM, Ms and Mr measurements. Depth plots of c, SIRM (Fig. 3), ARM, Ms, and Mr, show the same variation pattern along to the sequence. Based on a high field slope of measured hysteresis curves, diamagnetic contribution to c is considered negligible in all samples, and the signal of c is mainly carried out by the ferrimagnetic mineral enrichment. Magnetic susceptibility values range from 0.07  105 mm3/kg in the middle and lower part of the section, to 7  105 mm3/kg in the upper part 40 cm, decreasing an order of magnitude below this depth. Thus, the highest magnetic concentration (c z 7  105 mm3/kg) is found in the C horizon of the modern soil, which is characterized by the presence of alluvial sandy sediments. The rest of the sequence has a very low concentration of magnetic minerals, although with some differences. Gleysols (9BCgk, 9Bkg, 9G, 8Gk, 8G and 6G horizons) have very low


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Fig. 3. Stratigraphy and selected magnetic parameters measured on the pedocomplex: magnetic susceptibility c and SIRM as indicators of concentration of magnetic minerals, S ratios as an indicator of coercivity on materials, HIRM300 as an indicator of high coercivity mineral concentration, cfd and ARM/IRM100, as indicators of fine grain magnetic particles.

values with the exception of the 7Bg horizon, where c is one order of magnitude higher (with maximum values of c < 0.8  105 mm3/ kg). The silty sediment, located in the middle part of the sequence, has values of c < 0.3  105 mm3/kg. The lowest values are present in the Vertisols (5BC, 5Bss and 5Ass), which are similar to those found in the Gleysols. The pedo-sediment 4Bk and the Fluvisols in the upper part of sequence have values of c < 0.6  105 mm3/kg (2A, 2AC, 3A, and 3C horizons). The HIRM300 relation in our data suggest the presence of minerals of high coercivity in the upper part of the sequence, principally on sediments, Fluvisols (2A, 2AC and 3A) and horizon 7Bg on the Gleysols (Fig. 3). Most of the samples show a positive correlation of SIRM vs. c (Fig. 4), evidencing a dependence on the concentration of magnetic minerals. Only the three top sediment samples present a clearly separation of the main path, suggesting changes in magnetic mineralogy in relation to concentration, but other causes such as grain size and paramagnetic content can be responsible of this behavior (Thompson and Oldfield, 1986). Those sediments possibly reflects the primary detrital material deposited by the river, on which the pedogenic processes are incipient.

important contribution of fine-grained SD magnetite particles along the sequence. However, some differences are clear in the 6G horizon and in the middle part of the silty sediment, where ratios are the highest. In contrast, the lower part of the silty sediment and the 4Bk horizon has the lowest ratios (close to cero). A low ARM/ SIRM ratio can result from coarse MD grains (>10 mm) and hematite content. The observed differences between magnetic parameters in the sequence are related no solely to changes in grain size distribution, but also to changes in magnetic mineralogy.

4.2. Grain size Frequency-dependent susceptibility (cfd) values on the lower Gleysols (Fig. 3), have a very heterogeneous pattern with values ranging from 0 to 7.5%; the highest percentages are found in the 9BCgk, 7Bg, and 6G horizons. Above the Gleysols, the silty sediments and the 5C horizon have values of <5%, in contrast with the Vertisol (5BC, 5Bss, and 5Ass) who shows < 3% values. At the top of the sequence, the pedo-sediment 4Bk and the 3C horizon have the highest cfd values. The uppermost Fluvisol has lower percentages (Fig. 3). The ARM/SIRM ratio (Fig. 3) is relatively uniform along all the profile with values around 0.5  101, possibly indicating an

Fig. 4. SIRM vs. Susceptibility (c) biplot. The positive correlation indicates that larger variations depend on the concentration of magnetic minerals.


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Hysteresis parameters in the Day diagram (Day et al., 1977), allow estimating the size of magnetic domains in a material. Dunlop (2002) suggests some specific trends to SP, SD and MD grains in soil samples. In our case, a sample of sediments and Vertisols, plot closely to the multi-domain (MD) area. The rest of the samples show a dispersion behavior between MD and SP curves, but Vertisols and Gleysols fall closely into the SP domain (Fig. 5). 4.3. Magnetic mineralogy Most of the thermomagnetic curves display an irreversible behavior, suggesting that magnetic phases transform during heating. The plots show two ferrimagnetic phases during the heating, with an increment between 250 and 325
C, followed by a gradual decrease which is more pronounced near 580
C, and some of them continue until 630e650
C. This observed behavior on the silty sediment, points to maghemite-magnetite as the dominant magnetic mineral (Fig. 6A). The loss of c (T) between 630 and 650
C, was found in the Vertisol and the Gleysols, and suggests the occurrence of little quantities of hematite along the sequence; but it is not so clear in other measurements, such as magnetic susceptibility, ARM and IRM because the higher magnetization of the magnetite masks the signal (Fig. 6B). In the lower part of the sequence, the Tc experiments on Gleysols show a noisy behavior due to their magnetic weakness; but on gleyic horizons, it is possible to see a gradual decrease on c in all the experiment until it reaches the 630e650
C, pointing to a hematite phase as the dominant magnetic mineral (or some end members of titanohematites).

S ratios on Fig. 3 show that high coercivity minerals (hematite) are abundant in the sequence. Particularly, in Vertisols (5Ass and 5Bss) and part of the Gleysols (6G, 7Bg, 8G, 8Gk, 9G and 9Bkg), the S300 ratios suffer a drop nearly to 0.7, which suggest an increment of high coercivity minerals. In particular the base of the 6G horizon, the S300 ratios are between 0.1 and 0.3, denoting a high concentration of those minerals. On the other hand, the high-field hysteresis parameters (Fig. 5), coercivity force (Hc) and coercivity of remanence (Hcr) indices, indicate high values in the same horizons of Vertisol and Gleysols, consistently characterized by lower S300 ratios. 5. Discussion 5.1. Pedogenesis and rock magnetic properties 5.1.1. Gleysols The oldest paleosols of the studied sequence, correspond to Gleysols, which according to the OSL date (123e65 ka), obtained from the basal alluvial sediment, were developed during the Late Pleistocene (Solís et al., 2013b). Judging from the c vs. T curves, their magnetic properties suggest that hematite is the dominant component. The presence of hematite in this case is mainly due to weathering of primary minerals on soils, particularly of the ferromagnesian group, as occurred in the pyroxene alteration: the FeSiO3 breaks into SiO2 and Fe2þ; when Fe2þ reacts with O2, it becomes Fe3þ and then reacts with water (H2O); finally hematite precipitates (Fe2O3). Solís et al. (2013b) have documented the weathering degree of these Gleysols based on the stable mineral content, such as garnet, titanite, and zircon. The relatively low

Fig. 5. Plot of Mr/Ms vs. Hcr/Hc proposed by Day et al. (1977) and modified by Dunlop (2002). Some samples tend to be of multi-domain (MD) grain size and others have dispersion between the curves of MD and SP þ SD grain sizes. Example of hysteresis cycle of a representative sample, and IRM acquisition curves of representative samples showing different behavior.


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Fig. 6. (A)Thermomagnetic curves showing the magnetization increase between 250 and 325
C and subsequent decrease suggesting the presence of magnetite; and (B) some of them continue until 630e650
C, pointing to the occurrence of small quantities of hematite along the sequence.

values of the S ratios indicate the presence of high coercivity minerals (Fig. 3), and the concentration curve of high coercivity minerals (HIRM) is in good agreement; although the values are low, they show a weak concentration of antiferromagnetics (Fig. 3). In Gleysols, the oxide-reduction processes are predominant and they occurred under water-logging conditions, where Feþ2 become a mobile element. Considering the aforementioned, minor pro-

The increase in cfd and ARM/SIRM ratios indicates the presence of some fine (SP) particles on these Gleysols. In addition, secondary carbonates (up to 16%, Table 1), contribute to the presence of a low magnetic susceptibility signal in this sequence interval. In general, the concentration of magnetic minerals is very low (low c and SIRM), due to the presence of antiferromagnetic particles that little contribute to the magnetic signal.

Table 1 Chemical and physic properties of soils and sediments of the Tierra Blanca section. Soil age determination and cultural archaeological periods were taken from Solís et al. (2013a,b). Soil profile horizon

Depth (cm)

TBII-C TBII-2A

0e100 100e130

Cultural period

Age

CaCO3 (%)

Features

Modern soil

11.16 4.58

Alluvial sediment poorly affected by pedogenesis Dark gray, silty, with well developed, fine subangular blocks. PostClassic ceramics More sandy and less structured. More clayey. Brownish gray with compact subangular blocks. Classic Maya ceramic More sandy and less structured Yellowish brown, silty material with a lot of reworked concretions, showing little pedogenesis. Dark brown to black. Clayey. Structure in angular blocks, slikensides, and carbonate concentrations. Clayey. Grayish brown, slickensides. Concentrations of carbonates in the ped surfaces along fractures. More silty. Here, there are also abundant ceramic fragments. The overlying sediment is laminated and a wavy boundary, in the middle part alternates with lamination to crossed stratification, sandy texture, reductomorphic features and carbonates concretions. Clayey, greenish gray, with reddish mottles; structure in subangular blocks very friable. The underlying horizon is ondulated with the highest concentration of Mn. In the base of this soils have dendritic Mn and Mn spots. Yellowish brown with subangular blocky structure. Clayey, strong gleyic feature and slickensides are frequent. Clayey, the slickensides are less frequent, but here there are carbonates. Prismatic structure, clayey and abundant carbonates concentrations. Clayey, mottled, with Mn films. Presence of very hard carbonate concretions 5e7 cm diameter, which dated is 5450e5380 yr. BP.
nesis. Concentrations along the ped Alluvial sediment with low pedoge surfaces of carbonates. Alluvial brownish sand, with lamination.

TBII-2AC TBII-3A

130e195 195e210

TBII-3C TBII-4Bk

210e270 270e290

TBII-5Ass

290e315

TBII-5Bss

315e340

TBII-5BC TBIII-Silty Sediment

340e365 365e665

TBI

6G

TBI TBI TBI TBI TBI

7Bg 8G 8Gk 9G 9Bkg

2.03 1.93 2.81 1.82 29.82

TBI

9BCgk

23.45

TBI

9C

Post-classic (A.D. 1000e1500) Classic (A.D. 150e830)

1140e970 yr BP 2.1 ± 5 ky

Middle-Formative (800e300 DC)

2340e2300 yr BP

12.62 14.86 16.18 16.39 4.33 3.90

Early Holocene 9.0 ± 2 ky

1.47

Late Pleistocene

2.21

125e65 ka

portions of magnetic Fe-bearing minerals can be produced on the soils; nevertheless, during oxidation phases when dry conditions are present, low proportions of recent formed hematite may precipitate.

Particularly, the lower paleosol (9BCgk), that corresponds to a colluvio-alluvial sediment, has a low genesis degree of pedogenic processes, possibly related to its quartz and feldspar composition; also it has suffered redox conditions, and the presence of


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carbonates could be responsible for a low signal, although it has been reported that magnetic susceptibility in Gleysols is in general low, possibly related to gleyzation processes (Solleiro-Rebolledo et al., 2011; Rivas et al., 2012; Solis et al., 2012; Tovar et al., 2013). Unfortunately in this case, it was not clear if it could be related to the magnetic mineralogy. Horizon 7Bg shows a special behavior. Although it is Gleysol and shows all the related gleyic features, such as a high clay proportion (~80%), grayish colors, and a little concentration of secondary carbonate, it suffers an increment in the magnetic susceptibility (Fig. 3). This horizon also contains soft magnetic minerals (particularly magnetite) indicated by high values of S ratios, c vs. T curves, and the IRM acquisition curves. This magnetic pattern is related to SP particles (>cfd and ARM/SIRM). Another characteristic of horizon 7Bg is the presence of high coercivity minerals, like hematite [HIRM]. What kind of components makes the 7Gb horizon different to the rest of the Gleysols? Solís et al. (2013b) indicate that it has a higher mineral content of pyroxene, olivine, and amphiboles, which suggests a change in the parental material and the sediment source. It is clear from the mineral assemblage that the magnetic enhancement is produced by the enrichment in volcanic materials which can also contribute with magnetite. These minerals can be altered in the soil environment under water saturation conditions, which result in the formation of the magnetic components such as hematite and fine particles. 5.1.2. Silty sediments The middle section of the profile is composed by a silty sediment (TBIII), dated on 9 ka (Solís et al., 2013b). The major component of this sediment corresponds to acid volcanic glass (Solís et al., 2013b), and an important proportion of secondary carbonates; both materials have low magnetic susceptibility values. The observed changes in the magnetic parameters of this silty sediment, coincide with variations in the volcanic mineral content, ranging from 12% close to the base, 2.5% in the middle, and 23.5% at the top (Solís et al., 2013b). The cfd% and ARM/SIRM have a better contrast. Particularly, the uppermost part of the sediment exhibits a slight increment in c and the presence of magnetite, although in very low proportions. The relatively high values of the S ratios indicate the presence of low coercivity minerals in the whole sediment interval (Fig. 3). Another cause for the low concentration of magnetic minerals (c and SIRM), is the presence of antiferromagnetic particles (hematite), that little contribute to the magnetic signal (Geiss et al., 2004). The presence of magnetite and hematite in the sediments is suggested by the c vs. T curves. The hematite can be related to oxide-reduction processes, which are common in sub-aerial environments (alluvial terraces) and water saturated soils with repeated oxygen exposition (Gleysols). The presence of some FeeMn mottling and reductomorphic features produced by water saturation supports this idea. The increment in cfd and ARM/SIRM ratios, show the presence of some new fine particles (SP), suggesting the creation of fine hematite on these sediments. A rise in the ARM/SIRM ratios at the interval top reveals a higher increase, indicating that the oxide reduction conditions are more intense. These conditions can be related to a prolonged air-exposure of the sediment, reflected as an increment of hematite in the top of the interval (S300 ratios close to 80% suggest an increment of high coercivity minerals). The uppermost sediment shows mudcracks that are evidence of a change in the environmental conditions. 5.1.3. Vertisols The radiocarbon ages of the carbonate concretions found in the Gleysol section (5.45e5.38 ka) and from the organic matter from

5Ass (2.34e2.3 ka), helped to constrain the age of the Vertisol found in TBIII (Solís et al., 2013a). This temporal interval allows establishing the period of soil formation. The Vertisol has also formed during the Formative period, and consequently records the human occupation during that time. The magnetic parameters found in this paleosol, such as the c vs. T curves, the IRM acquisition curves, the low values of the S ratios, and the concentration curve of high coercivity minerals (HIRM), indicate the presence of hard magnetic minerals (i.e. hematite). In general, the concentration of magnetic minerals is low (c and SIRM, Fig. 3), because of the little contribution of antiferromagnetic minerals to the magnetic signal. This paleosol has a different pedogenesis pathway in comparison to the Gleysols. It started with a high humification, followed by the development of angular blocky aggregates, clay illuviation, and argilloturbation. Magnetic susceptibility values from the Vertisol are slightly higher than in Gleysols. The 5Ass horizon observes the typical enhancement for organic horizons found in loess-paleosol sequences (Maher et al., 2003). In such paleosols, the enhancement is due to pedogenetic processes which produce magnetic particles. In this study, the increase in Xfd and particularly on ARM/SIRM ratios, indicate the presence of fine (SP-SD) particles. We consider that these finer particles are the result of weathering, with the new formation of fine hematite minerals within the soils. Another important process observed in Vertisols is the clay illuviation due to the water flow through the pores, observed in the thin sections of 5Ass and 5Bss horizons. Clay illuviation produces a very specific feature: clay cutans, which result of the migration of clay from the 5Ass horizon through the 5Bss, and finally into the 5BC horizon. In this way, fine clay particles are mainly concentrated in the lower horizons. This feature is also related to the presence of fine high coercivity minerals (low S ratios and high ARM/SIRM ratios, Fig. 3). As a consequence of the illuviation in Vertisols, MD particles have a relative increase in the upper horizon (5Ass), as observed in the Day's diagram (Dunlop, 2002) (Fig. 5). A similar process is described by Rivas et al. (2012), who relate the resultant net size increase of magnetic domains with the clay illuviation process. Concerning argilloturbation, where the soil is affected by constant expansionecontraction processes (vertic features), the materials of the upper horizons (5Ass) are displaced into the fractures, where they get mixed with the lower horizon material (5Bss). Rivas et al. (2012), suggest that the presence of MD particles in the B horizons is a consequence of this mixture, but we suggest that the mixture is not enough to have such an increase in MD particles, as was observed in our soils. In consequence, the rise in the concentration of these particles is mainly attributed to illuviation. 5.1.4. Fluvisols The youngest soils of the section correspond to Fluvisols and are located in the TBII profile. These soils were developed during the Holocene and include several evidences from the human occupations periods. According to the c vs. T curves, they are composed by magnetite; however, the IRM acquisition curves and S ratios also indicate the presence of soft magnetic minerals as hematite. This mineral is also represented in the low c values of the c vs. T curves around 630
C, which is in agreement with the concentration curve of high coercivity minerals (HIRM300, Fig. 3) although in lower concentrations. A particular case on these Fluvisols is the 3A horizon, which exhibits a slight increment in fine and ultra fine (SP) magnetic particles (indicated by the increase in cfd and especially in the ARM/ SIRM ratios), suggesting the formation of fine hematite. In general, the low hematite content in Fluvisols can be related to a low soil


zquez C., G., et al., Mineral magnetic properties of an alluvial paleosol sequence in the Maya Lowlands: Late Please cite this article in press as: Va PleistoceneeHolocene paleoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2015.09.094


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development degree, which in turn results in a weak weathering process, so the primary mineralogy remains unaltered. Another fact that affects the low concentration of magnetic minerals (c and SIRM, Fig. 3) is the presence of secondary carbonates (~16%) and volcanic glass. Bartel et al. (2011), explain the relation of low c values related to the presence of carbonates in Fluvisols, even with low carbonate proportions (3%). In general, the concentration of magnetic minerals in these soils is low and similar to the one reported for the silty sediment. A possible explanation for this low concentration can be due to the little development of Fluvisols that produce a similar behavior like parental sediments. The magnetic parameters of some Fluvisols in central Mexico have shown, commonly elevated c values associated to high amounts of magnetite found in the volcanic minerals of the parental material (Rivas et al., 2012; S
anchez et al., 2013). The biggest difference between central Mexico Fluvisols and those found in the Maya Lowlands is the kind of parental material. Usumacinta Fluvisols mainly contain metamorphic-plutonic minerals with a high ferromagnesian component (48%), but the magnetite proportions are low. In consequence, the magnetic parameters reflect low concentrations of magnetic minerals (c and SIRM). 5.2. Environmental implications Paleosols and sediments integrate the Tierra Blanca profile (TBI, TBII, and TBIII sections), which originate during the late PleistoceneeHolocene. Both magnetic and non-magnetic properties reflect the past environmental conditions in this Maya lowland area. Magnetic rock studies of the TB alluvial-paleosol profile, show a close relation between environmental conditions, pedogenic processes and human activities; all these factors in combination, changed the composition and distribution of the pedogenic magnetic minerals. In accordance to the chronology proposed by Solís et al. (2013a, 2013b), the alluvial-paleosol sequence can be divided into four periods: late Pleistocene, early Holocene, middle Holocene, and late Holocene. Each period contains characteristic paleosols and sediments. The two last ones reflect not only the natural environmental conditions but also the interaction between the environment and human activities. 5.2.1. Late Pleistocene (ca. 65e9 ka) This period is characterized by the development of the Gleysols. The basal alluvial sand has an age of 123 to 65 ka (Solís et al., 2013b). This marks the beginning of the alluvial terrace system formation in the region, which was also documented by Solís et al. (2014b). Overlying the Gleysols, the 9 ka silty sediment indicates the establishment of a time interval of 56 ky, during which these paleosols are developed. This extensive time of soil formation, allows the pedogenesis evolution and weathering action, the clay formation, and the development of reductomorphic processes (Solís et al., 2013b). During these processes, fine hematite grains are produced, related directly to the weathering and gleyzation processes. Gleyzation is known to occur under repeatedly water saturation cycles. In consequence, paleoenvironmental conditions include high humidity, necessary to weather primary minerals into clay, and to generate gleyic features. Nevertheless, these conditions are not homogeneous during the whole period; some differences in the Gleysols are found, revealed by the magnetic parameters. The lowermost paleosols (paleosols 9 and 8) follow a similar pattern in all the measured magnetic properties. Paleosol 7 (7Bg) shows the highest values in all the parameters, which are associated to the contribution of volcanic materials from the parental material. Its magnetic signal is so intense that gleyzation is capable of delete it. In this way, we deduce that this

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paleosol is the less developed in the Gleysol sequence. The upper Gleysol (paleosol 6) also exhibits some differences, particularly in the size parameters, as they contain more SP and SD particles (Fig. 3). This paleosol is characterized by the presence of vertic features influence by dry-seasonal environmental conditions (Solis et al., 2013a). The climate records for the area document humid conditions during the Marine Isotope Stages (MIS) 5a, 4, and early 3 (Hodell et al., 2008). Lowermost Gleysols may form during such periods. MIS 3 is characterized by climatic oscillations, where cooler conditions are also present (Correa-Metrio et al., 2012). The Late Glacial is characterized by drier conditions, and paleosol 6 documents it. 5.2.2. Early Holocene (ca. 9 to <6 ka) The Early Holocene period (9 ka) is represented by the white silty alluvial sediment (TBIII) (Solís et al., 2013b). This sediment indicates a strong instability phase on the alluvial system. Because of the high sedimentation rate present on this time interval, the pedogenesis stopped, associated to the increment in sediment discharge and the production of MD particles (Fig. 5). The discharge frequency of sediments was more or less continuous along the time, producing the aggradation of the sedimentary sequence within the valley (Solís et al., 2014b). These changes in the sedimentation rates as well as in the source of materials are associated to wetter conditions, although there exist some intervals of sediment airexposure, which produce an increment in the oxideereduction processes. This is suggested by the hematite increment at the top of the interval, indicating the presence of drier conditions. Rock magnetism suggests the formation of new fine hematite along the sediment interval, which may be related to repeated cycles of wet and dry conditions along the early Holocene. The wetter conditions are also registered in other localities. For example, a tropical forest expansion is recorded at 10 ka in the Cariaco Basin (e.g. Peterson et al., 2000; Hodell et al., 2008) and in
n area (Correa-Metrio et al., 2012). the Pete 5.2.3. Middle Holocene (ca. <6e2.3 ka) This period is represented by the formation of Vertisols which in part corresponds to the early - middle Formative archaeological period (1800e300 BC). Vertisol properties are product of weathering, illuviation, carbonate leaching, and argilloturbation that occurred as a consequence of a stability period on the alluvial landscape. This Vertisol has been characterized as polygenetic because it was formed under different environmental conditions through the time. Weathering and illuviation occurred first, under humid conditions, presumably during the Early Holocene (<6 ka); carbonate leaching and precipitation, and the development of vertic features were produced later, under drier conditions. The age of the carbonate concretions (5.45 ka) constrains the beginning of such drier conditions. This tendency to aridization is well documented in other records (Gill, 2000; Rosenmeier et al., 2002; Haug et al., 2003). Magnetic properties are in good agreement to the Vertisol characteristics. Weathering and clay illuviation produce changes in the particle size. The coarser MD particles are found in the 5Ass, while the SP-SD particles are mainly concentrated in horizon 5Bss (Fig. 5), as a consequence of the clay translocation. Argilloturbation, related to the contractioneexpansion processes, probably generated a mixture of MD particles in the lower part of the soil (Rivas et al., 2012). In the TB Vertisol, this mixture is not enough to have an increment of that type of particles. The last stage in the Vertisol formation (2.34e2.3 ka), is documented by the abundance of archaeological material. It corresponds to the oldest occupation period for the Maya Lowlands during the Formative (800e300 BC). In this study, we suggest that the changes in the magnetic particle


zquez C., G., et al., Mineral magnetic properties of an alluvial paleosol sequence in the Maya Lowlands: Late Please cite this article in press as: Va PleistoceneeHolocene paleoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2015.09.094

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distribution can be more associated to the human activities than to argilloturbation. Human activities can modify the magnetic properties through cultivation, deforestation, and erosion (Jordanova et al., 2011; Mokhtari-Karchegani et al., 2011). The presence of horizon 4Bk in TBIII, characterized as a pedosediment, documents an erosional phase after the site abandonment.

conditions comes from the disseminated carbonates that are found in the TB profiles, who have an age of A.D 1230e1290 (Solís et al., 2013a), although they do not contribute significantly to the observed magnetic signal.

5.2.4. Late Holocene (<2.3 ka) The absolute ages for Fluvisols are established by an OSL dating of the 3C horizon (2.1 ka), and by a radiocarbon date of charcoal found in the 3A horizon (1.14e0.97 ka; A.D. 810e980) (Table 1). The relative chronology, based on the ceramic fragments, establishes their correspondence with the Classic and Post-Classic archaeological Maya periods (150 BCeAD 1500). Fluvisols have been associated with the more humid environmental conditions that occurred during the Formative. This is indicated by d13C values (20‰) and phytolite content, indicatives of a predominance in C3 vegetation, which is typical of humid climates (Solís et al., 2014a). However, the lower fine hematite content observed in these paleosols indicates weak weathering processes, and thus the primary mineralogy remains poorly altered. What kinds of processes are responsible for the weak development of these paleosols? First of all, the parent material content reveals an assemblage of volcanic and methamorphic-plutonic minerals (Solís et al., 2013b). Particularly, volcanic minerals are represented by pyroxenes and amphiboles that can be easily weathered (Rai and Kittrick, 1989). The presence of these minerals evidences a relative lower degree of development in the soils (Jackson and Sherman, 1953). Secondly, time of soil formation also becomes an important factor for the weak paleosol development. After 1000 years of pedogenesis of the paleosol 3 in a humid environment (this age is calculated from the difference between the age of the 3C horizon and the 3A horizon), it is expected that the pedogenetic processes of this paleosol will be better expressed. For instance, Young paleosols of the Chichon Volcano, located 170 km southwest of the Usumacinta River, are well developed and it is estimated that their pedogenesis processes occurred in a time span of 550 y (Solleiro-Rebolledo et al., 2015). In this study, we suggest that the weak development of the Fluvisols is due to landscape instability which causes continuous phases of sedimentation and soil rejuvenation. From the aforementioned, the following question arises: Is this instability caused by a change in the environmental conditions? or by human activities through intensive land use? At the moment, there are not enough evidences to establish a direct influence of human activities on the soil magnetic properties. However, some indicators of human influence can be detected. The 3C horizon has abundant SP particles, which are also present in horizon 3A along with SD grains (Fig. 3). The uppermost paleosol, related to the PostClassic period, is strongly perturbed by the abundant burials and ceramic content (Solís et al., 2013a). The magnetic signal also shows a combination of SD and SP particles. In consequence, we suggest that these coarser particles are related to land degradation (i.e. erosion), which causes a loss of finer materials. In addition, Jordanova et al. (2011) have documented changes in the magnetic properties as a tillage consequence. The higher X signal obtained for the horizon 3A, can be associated with fire occurrence due to the presence of charcoal particles inside this horizon. It is well known that “slash and burn practices” have been common in the Mesoamerican agriculture system (Rojas, 2001). On the other hand, the erosion intensification during the Classic period has been well documented in the Mayan area (e.g. Beach, 1998). However, drier conditions have also been reported for the Terminal Classic period inside this area (e.g. Gill, 2000; Haug et al., 2003; Hodell et al., 2008). The best evidence of these

(1) The Maya lowland region is controlled by a dynamic and complex river system that responds to environmental changes. This response has been recorded at different stages of sedimentation and pedogenesis. The floodplain of late Pleistocene-Recent exhibit a system of terraces in which the paleosols are well recorded. (2) The paleosol sequence that has been developed since the Late Pleistocene, offers a good record of the past environmental conditions in the area. The type and intensity of the pedogenic processes (gleyzation, weathering, illuviation, argilloturbation, and humus and carbonate accumulation) are different through time. (3) The magnetic rock properties of the Usumacinta alluvialpaleosol sequence represent clearly the different types of pedogenic processes. In consequence, each type of paleosol (Gleysols, Vertisols, and Fluvisols) has a specific set of magnetic parameters which differs from the sediment behavior. (4) The Late Pleistocene has humid environmental conditions, which in consequence produced a set of Gleysols. The reductomorphic processes generate new Fe-bearing mineralogy (fine hematite). At the end of the Pleistocene, the Gleysols recorded a change in the environmental conditions, which in turn produced vertic features (argilloturbation) that are well expressed in the magnetic signal with a mixture of SD and SP particles. (5) In the Early Holocene, the pedogenesis stopped and was followed by the deposition of high sedimentation rates, which produce silty sediment mainly composed of volcanic minerals. (6) The middle Holocene is characterized by a transition into drier conditions, indicated by the precipitation of pedogenic carbonates and low magnetic susceptibility values. Afterwards, a more seasonal stage is indicated by the presence of fine hematite precipitation as a product of a strong weathering stage. Vertisols are characteristic of this period and contained evidence of Formative archaeological materials. (7) The Late Holocene is associated to more humid environmental conditions than the Formative period, but it has been related to landscape instability which produces weaker developed soils (Fluvisols). We suggest that some of the magnetic properties can be correlated to human activities that produced deforestation, fires, and erosion.

6. Conclusions

Acknowledgments This work was supported by projects PAPPIT (IN117709, IN110710; E. Solleiro-Rebolledo, IN108714; S. Sedov, and IA104815; G. Vazquez-Castro), CONACYT (166878; Elizabeth Solleiro). We also thank Hermenegildo Barceinas, Jaime Díaz Ortega, and Jorge Rivas for technical assistance. We appreciate the valuable comments or the reviewers who contributed to improve the document. References Alekseeva, T., Alekseev, A., Maher, B., Demkin, V., 2007. Late Holocene climate reconstructions for the Russian steppe, based on mineralogical and magnetic properties of buried palaeosols. Palaeogeography Palaeoclimatology Palaeoecology 249, 103e127.


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zquez C., G., et al., Mineral magnetic properties of an alluvial paleosol sequence in the Maya Lowlands: Late Please cite this article in press as: Va PleistoceneeHolocene paleoclimatic implications, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2015.09.094