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Ocho mil años de la Cuenca de Zacapú

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Pierre Pétrequin

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1The Project Michoacán I (1983-1986) focussed on a broad zone in the north west of the western part of the Zacapu Basin and, in particular, on the peninsula of the ʺLomasʺ of Zacapu, a raised, undulating area lying several meters above the extensive marshes of the Ciénega de Zacapu. Archaeological work in the Lomas revealed widespread occupation during the first nine centuries A.D. (oldest radiocarbon date obtained 1961 ± 34 B.P.—inah 331— about 86 B.C. - 68 A.D. after calibration). The present landscape, however, is very different from that of Prehispanic and Colonial times as the marsh was artificially drained at the beginning of the 20th century. The archaeologist, therefore, must try to reconstruct this earlier environment. According to the reports of the last Tarascans, before the drainage of the Ciénega de Zacapu, people travelled around the basin by canoe using channels kept open through very dense aquatic vegetation. In the marsh, fish and wildfowl were abundant. On the shores of the Lomas, reeds were exploited for woven mats and deer, wild boar and duck were hunted. A description of a hamlet in the middle of the Lomas in 1590 indicates a few houses in the middle of fallow land and fields of maize, with many trees and agave plants. We cannot, however, assume that these conditions and ways of life were current in the Prehispanic period. At he end of the first phase of archaeological research three questions led to the focussing of a new research programme on the palaeoenvironment of the Lomas (Michoacán Project, stage II):

  • How were the Lomas formed?
  • Where was the shore of the lake located and how did this change over the long term?
  • Is it possible to understand the timing and changes in agricultural exploitation of the shoreline and the Lomas?

2The only information available to contribute to this study came from a 14 m core from Lake Zacapu, a perennial water body in a sub-basin feeding the main marsh. The rate of sedimentation in the core is very high due to very high diatom productivity. Two periods of low lake level are indicated between 2800-2400 B.P. and about 1100 B.P. (dates obtained using AMS methods on samples of very low organic matter content giving rise to some uncertainties). However, the results from the sub-basin (the lake) cannot be readily applied to the rest of the Zacapu basin, the two areas being separated by a ridge of old diatomites whose date of formation is unknown.

CHOICE OF STRATIGRAPHIC COLUMNS

3To try to resolve these palaeoenvironmental problems in a context of possible severe human impact, we selected four sampling points:

  • Point 1, located 900 m as the crow flies from the outer edge of the Lomas, well in to the interior of the marsh. It was hoped that here, in the marsh, conditions would be favorable for rapid and continuous sedimentation, with little chance of discontinuities in the record; it was also hoped that basin-wide changes would be detectable here.
  • Point 2 lying close to the present edge of the marsh in a former embayment where the marsh once extended in to the interior of the Lomas. Under these topographical conditions, where a transect of 13 cores allowed the evolution of the shore to be traced through time, basin-wide indicators should be less important than local, shoreline influences, especially if agriculture was practiced on the Lomas themselves.
  • Point 4 located at the head of a broad, internal marsh in the Lomas with their potential for cultivation and close to known archaeological sites dated to the first millennium A.D. This sample site, in an area known to be affected by human impact since at least the year 0, should allow some assessment of the impact of people on the later evolution of the Lomas.
  • Point 5 samples taken in a trench at the archaeological site of Loma Alta.

4Eighteen samples were radiocarbon dated to estimate the timespan represented by the stratigraphic sequences (8 samples from Point 1, and 5 each from Points 2 and 4). These dates revealed a major problem: the most recent date was 2170 ± 60 B.P., inah 890, (zac-79), about 398 - 18 B.C., from the top of Point 2; this is older than the oldest date obtained from the archaeological site of Loma Alta (see above). Thus we can be certain: the palaeoenvironmental study only indicates conditions prior to the first occupation of the Lomas for which there is archaeological evidence.

  • * For the figures cf. Resumen sintético.

5Unfortunately, the sediment profiles chosen did not fulfill our expectations and it is, therefore, impossible to answer directly the archaeologist's questions concerning the nature of the environment during the first centuries A.D. It appears that only Lake Zacapu, with its sub-basin separated from the marsh by a ridge, has retained sediments from this period. The sampled sequences do, however, provide a long and quite detailed record beginning about 8220 ± 160 B.P. inah 890 (zac-133 and 134), with a maximum depth of 3.85 m at Point 1. The radiocarbon dates, stratigraphy, pollen and diatom diagrams were used to correlate the 3 cores. These four methods have allowed reliable correlations to be made for the major sedimentary episodes; contradictions are minor and are only points of detail. By using continuous core samples from 3 sites we have, in theory, a transect from the internal marsh in the Lomas out to the floor of the former great marsh of the ciénega (Figs. 62 and 63)*.

PHASE A OF THE GREAT MARSH OF ZACAPU, BEFORE THE 6TH MILLENNIUM B.C.

6During the first phase of sedimentation in the Zacapu marsh, evidence from the cores indicates a temperate, humid to subhumid climate. A vegetation cover of forests of pine, oak and alder with an herbaceous stratum, alternated with grasslands. These changes in vegetation in the basin could reflect the partial destruction of tree cover by volcanic eruptions and ash falls recorded in the samples. In the marsh itself, the water was shallow, with aquatic vegetation near the shores. Pools of water underwent minor variations, in a slightly acidic environment, with influxes of eroded material early in the period. The similarity of the sediments, diatoms and pollen at the base of the 3 cores can only be explained by the same marsh environment occurring at all 3 sampling locations, which places us chronologically before the creation of the Lomas. At that time, the marsh, the shore and the Lomas could not be differentiated on the basis of topography; the pollen and diatoms just indicate that the floor of the basin rose gradually from Point 1 (near the center of the marsh) in the direction of Point 4 (close to the former shore).

PHASE B, THE DEVELOPMENT OF THE LOMAS DURING THE 6TH MILLENNIUM B.C.

7In the stratigraphic sequence, a major event interrupts the evolution of the Zacapu marsh. This episode is recorded in the stratigraphy (by pitch inversion of the beds from Phase A, successive layers of volcanic ashes, major erosion of ancient diatomites), in the magnetic susceptibility (a series of high peaks), in the diatoms (a complete change in the flora) and in the pollen. This upheaval in the topography of the marsh floor occurred between about 290-258 cm in profile 1; it is dated to about 6300 to 6000 B.C.

8This period of vulcanism and the tectonics which followed had enormous consequences for the form and functioning of the marsh of Zacapu. Before this episode all three sampling Points (1, 2 and 4) lay within the same large marsh, with its shore well away from the study area. By the end of this episode, however, we see the uplift of the Lomas, breaking up the original marsh area and cutting off the internal marsh (Point 4). The history of this process can be broken down as follows:

  • a series of eruptions with fires in at least some of the forests in the Zacapu basin. The source of the volcanic ashes was quite close by.
  • tectonic instability and uplift of the Lomas through deformation of ancient diatomite beds along the lava flows perhaps from Capaxtiro.
  • intensified erosion as the Lomas emerged to about their present elevation.

9The fact that the assemblages of diatoms are completely different before and after the period of volcanic and tectonic activity suggests effects on the whole of the basin and probably on the exit at Villa Jimenez, lying on the same fault line as the cone of Capaxtiro.

10It is tempting to attribute these changes in the Basin of Zacapu to eruptions of Capaxtiro to the west, but the level of dating of these nearby volcanic features does not allow this.

PHASE C THE REESTABLISHMENT OF STABILITY IN THE 5th MILLENNIUM B.C.

11During the major event (see above) the diatoms show that the marsh was shallow. The assemblages are dominated by Fragilaria which can live in a minerogenic environment. The increase in the number of diatoms could have been a response to the silica brought in by the volcanic ashes. In addition, severe erosion of the Lomas and the accumulation of reworked ancient diatomite at the foot of the Lomas in both the main marsh and the internal marsh might have favoured Fragilaria, which become more and more abundant, while taxa associated with aquatic vegetation decrease in abundance.

12The episode of erosion of the Lomas, with deposition at the edge of the marsh, continued until about the middle of the 5th millennium B.C. The debris deposited in shallow water, along the shoreline and in the internal marsh, confirm that the Lomas were by now in place with their present topography and that the vegetation cover was incomplete.

13Pollen and diatoms indicate a possible drying, with the gradual development of a temperate semi-arid climate, certainly by the end of the period (mid 5th millennium). The area of the Zacapu marsh reduced, with shallow, more eutrophic water and the development of abundant aquatic vegetation. On dry land, herbaceous plants became increasingly important.

PHASE D LOWEST WATER LEVEL IN THE 4th AND 3rd MILLENNIA B.C.

14Deposits of peats and black clays are common to both the low lying marsh and the internal marsh. Sharp variations in osmotic pressure indicated by the diatoms, the abundance of Botryococcus and evidence of stagnant, eutrophic water in the pollen are significant signs of a drier climate and of a shallow, alkaline marsh.

15If one looks at events outside the Zacapu Basin one finds, at about the same time, an episode of dry climate in the Upper Lerma basin near Toluca (Metcalfe et al. 1991), in the Basin of Mexico (Gonzalez Quintero 1986; Bradbury 1989) and possibly, in Pátzcuaro (Watts and Bradbury 1982).

16The beginning of the 2nd millennium B.C. (the end of Phase D) was marked by a renewed expansion of the marsh and the return of a temperate, sub-humid climate.

PHASE E AND HUMAN IMPACT ON THE ENVIRONMENT IN THE 1st AND 2nd MILLENNIA B.C.

17With the return of a slightly more humid climate, open but shallow pools of water again developed on the marsh, although with aerophilous diatom taxa. The internal marsh was, however, still muddy, with elevated salinities. Pollen diagrams show clearly that after 2069-1660 B.C. the forest in the basin suffered a clear reduction, associated with a decline in Chenopodiaceae-Amaranthaceae pollen and an increase in grass pollen. In the marshes themselves, exploitation by man is indicated by a shard, some deposits of allochthonous pebbles and by a basalt tool which could have been used to cut agave leaves or reeds. A cactus, Opuntia, was found in the seeds at Point 4. However, the techniques used provide no evidence for agriculture on the Lomas themselves, even at the end of the 1st millennium B.C. just before the establishment of the first archaeological sites.

18The study of the sequences from the marsh of Zacapu provides evidence of human impact in the basin at the basin scale occurring 2000 years before the oldest archaeological remains found so far (Loma Alta), but at the same time as the first indications of maize cultivation in the Pátzcuaro basin.

THE ENVIRONMENT OF THE LOMAS IN THE LAST 1000 YEARS B.C.

19This study has no answer to the last of the archaeologists questions ʺwhat was the environmental setting of the archaeological sites of the Lomas?ʺ as the timespan of the sequences studied does not overlap with the archaeological sequence.

20Pollen analyses from Loma Alta, quite close to Point 4, yielded sparse and poorly preserved pollen, with no pollen of cultivated plants. However, in view of the poor preservation this conclusion is somewhat tentative.

There are no clear grounds for accepting or rejecting the archaeological hypothesis that water levels were particularly high during the period of occupation of the Lomas, because chronologically the sites excavated on the Lomas fall exactly between the top of the sediment sequences studied and the first evidence of the Tarascans. Only the type of sediment (largely unaltered black clays) in the upper parts of Points 1, 2 and 4 (not analyzed because of root penetration) suggests some stability in the Zacapu marsh during the first centuries A.D. This unsubstantiated observation contradicts the suggestion that high lake levels affected conditions for occupation of the Lomas at the end of the Lupe period (about A.D. 850) when they were largely abandoned. Image

Bibliographie

BIBLIOGRAFIA

Metcalfe, S.E., F.A. Street-Perrott, R.A. Perrott y D.D. Harkness 1991 - Paleolimnology of the Upper Lerma Basin, Central Mexico: a Record of Climatic and Anthropogenic Disturbance since 11600 B.P. journal of Paleolimnology: 197-218.

González Quintero, L. 1986 - Análisis polínicos de los sedimentos. In J.L. Lorenzo y L. Mirambel (eds.). Tlapaccya: 35 000 años de historia del lago de Chalco. Colección Científica, Serie Prehistoria: 157-166. inah, México.

Bradbury, J.P. 1989 - Late Quaternary Lacustrine Paleoenvironments in the Cuenca de México. Quaternary Science Reviews 8: 75-100.

Watts, W.A. y J.P. Bradbury 1982 - Paleoecological Studies at Lake Pátzcuaro on the West-Central Mexican Plateau and at Chalco in the Basin of México. Quaternary Research 17: 56-70.

Notes de fin

* For the figures cf. Resumen sintético.

© Centro de estudios mexicanos y centroamericanos, 1994

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