BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL 83, PP. 241-264, 4 FIGS. MARCH 1962 PALYNOLOGY IN SOUTHERN NORTH AMERICA. I: ARCHEO- LOGICAL HORIZONS IN THE BASINS OF MEXICO BY PAUL B. SEARS ABSTRACT Profiles of sediments in the Basin of Mexico show an inverse relation between the percentages of fossil pine and oak pollen. Vegetation studies suggest that any marked increase in oak indicates an increase in available moisture, while consistently low percentages of oak indicate moisture deficiency. On this basis prolonged moist and dry periods have alternated. The more recent of these climatic shifts can be correlated stratigraphically with known phases of human occupation (Sears, 1951b). The Archaic Culture, 2000 or 1500 to 500 or 400 B.C., and the Nahua, 800 or 900 to 1521 A.D., both flourished within the Basin during relatively moist conditions. These two humid periods were separated by a prolonged dry interval which overtook the Late Archaic during a time of very low lake level. The Late Archaic level was then covered by a volcanic ash fall, and the culture shifted northward to higher ground. There it became the Teotihuacan which lasted from 500 or 400 B.C. to 800 or 900 A.D. Since this highly developed culture existed during a dry period, it must have made use of ground water. If so, the sources should have been unfavorably affected by heavy soil erosion known to have occurred, and by any deforestation, as Valliant (1941) suggests in explanation of the ultimate Teotihuacan collapse. Conclusions based on pollen analysis are consistent with what is known of soil profiles and former lake levels. CONTENTS TEXT Figure Page Page 3. Improved light-weight extension rods for Introduction .......................................................... 242 Swedish Hiller borer. ...... 249 4. Ten profiles showing relation between Acknowledgments ................................................. 242 Statement of problem .......................................... 242 pollen frequencies of climatic indicators and known archeological levels The environment .................................................. 242 ............................ 253 Archeology and history ........................................ 245 1. Pre-Archaic man .......................................... 245 TABLES 2. Archaic or Middle Culture ........................ 245 3. Teotihuacan Culture ................................... 245 Table Page 4. Nahua Culture ............................................ 246 5. European Occupation ................................. 246 1. Pollen profile, Lake Chalco, about 200 Résumé ............................................................... 247 yards west of school at Xico ........................ 250 Field work ............................................................. 247 2. Pollen profile, Lake Chalco, about 5 meters 1948 .................................................................... 247 north of profile in Table 1 ............................ 250 1949 .................................................................... 248 3. Pollen profile, Culhuacan, beginning just 1950 .................................................................... 249 below zone of Nahua artifacts ...................... 250 Procedure ............................................................... 250 4. Pollen profile, Santiago Tlatelolco, begin- Results ................................................................... 250 ning just above base of oldest pyramid ........ 251 Discussion .............................................................. 253 5. Pollen profile, El Tepalcate, rush turf near References cited .................................................... 254 edge of water .................................................. 251 6. Pollen profile, Chimalhuacan, 1 kilometer south of El Tepalcate ................................... 251 ILLUSTRATIONS 7. Pollen profile, about 90 meters northeast of profile in Table 6 ........................................... 251 Figure Page 8. Pollen profile, near site in Table 6 but con- 1. Sketch guide to the lacustrine history of the siderably deeper 252 Basin of Mexico ............................................. 243 9. Pollen profile, Lake Texcoco, dry bed 3.75 2. Indicator significance of the four principal kilometers southwest of Chapingo. ............. 252 upland forest genera of the Mexico City 10. Pollen profile, Chapingo, peaty cultivated area. 244 field 3.75 kilometers west. ............................ 252 241 242 P. B. SEARS—PALYNOLOGY IN SOUTHERN NORTH AMERICA INTRODUCTION agave pollen. A dry period within recent, but probably pre-historic, times was postulated. Pollen analysis or palynology has been chiefly used in tracing climatic change, although its ACKNOWLEDGMENTS value in stratigraphic correlation is increasingly recognized. Most of the sediments employed In 1948 Dr. Pablo Martinez del Rio invited for climatic studies have been within glaciated my attention to the problem of past climates areas, hence their record applies only to times in relation to human activity within the Basin when the basins containing them have been of Mexico and has most generously assisted ice-free. me throughout. The basins considered in the present series For invaluable guidance in the field, I am of studies lie 1500-3000 international air km indebted to Ing. A. R. V. Arellano; Drs. beyond the border of continental Pleistocene Faustino Miranda, Manuel Maldonado-Koer- glaciation. Fossil pollen has already been re- dell, and Helmut de Terra; Senores Arturo covered from them at depths of more than 60 Romano and Luis Aveleyra Arroya de Anda; meters, giving a continuous record extending Senora Antonieta Espejo, and Senor Pina-Chan. back into the Pleistocene. Such a record has the For generous provision of facilities, acknowledg- added value of showing the degree to which ments are due to Dr. Rubin de Borbolla, climatic changes associated with known ad- Director of the Museo Nacional, to the Escuela vances and retreats of the ice were effective de Agricultura at Chapingo, and to Dr. Sanchez at these distances beyond the glacial limits Marroquen of the Instituto Politecnico. (Sears, 1951a). Many other individuals have, in one way or This initial paper, however, deals only with another, given help and encouragement. Aside the analysis of relatively shallow sediments from those named in table captions, I must which, in the Basin of Mexico, can be cor- mention Mr. Philip Clisby, Professor Victor related with certain known phases of human Lytle, Mrs. Jean Galloway McNab, and the culture in that area. late Mr. Ralph Smith, a North American long The principle involved is presented in Dee- resident in Mexico City who first informed me vey's (1944) pioneer paper on pollen analysis of the late Archaic site, El Tepalcate, which has in Mexico. Samples taken at suitable intervals provided invaluable stratigraphic evidence. in the sedimentary column are treated so as Acknowledgments are also due The Geological to free and concentrate pollen and other spores Society of America, The Viking (Wenner-Gren) and prepare them for microscopic examination. Foundation, and the United Fruit Company for Counts are then made, and percentages cal- generous aid, and to Oberlin College for a leave culated for forest-tree pollen; all other forms of absence in 1948 as well as for laboratory are reckoned as so many per hundred of forest- space. tree pollen. By this method major changes in forest STATEMENT Or PROBLEM composition are revealed, and with them sig- nificant changes in climate. The non-forest Two questions are involved in the present pollen in turn is useful not only for confirma- paper. Is there evidence of climatic change within the Basin of Mexico during the time of tion as to climate, but as an index to local known human occupancy? If so, can such changes, such as vulcanism, cultivation, or shift- change be correlated with known cultural hori- ing water levels. zons? Both questions seem answerable in the While Deevey's (1944) study was frankly affirmative. exploratory, it revealed certain essential facts, here amply confirmed. Upland forest pollen was THE ENVIRONMENT represented by fir, pine, alder, and oak, her- baceous flora by grasses, chenopods, and com- The general setting is given by the map posites. Evidence of agriculture was afforded (Fig. 1), kindly furnished, along with the by the finding of a few grains of maize and levels here used, by Ing. Arenano. Essentially THE ENVIRONMENT 243 INSTITUTO GEOLOGIC° OE MFXICO SKETCH — GUIDE to the LACUSTRINE HISTORY of the BASIN of MEXICO AAKA lkòdt.VSF, .9iik* PL I FIGURE 1.—SKETCH GUIDE TO THE LACUSTRINE HISTORY OF THE BASIN OF MEXICO the map shows a large basin—not a valley— From the remnant of Lake Texcoco on the enclosed by mountains and displaying the evi- present basin floor at 2235-2236 meters, traces dence of a long volcanic and lacustrine history. of ancient beaches occur up to an elevation of 244 P. B. SEARS—PALYNOLOGY IN SOUTHERN NORTH AMERICA 2263 meters, while sediments occur under the Since this is true, and there is a striking in- present City of Mexico to a depth of several verse relation between oak and pine in our hundred meters. data, we have assumed that oak indicates POPOCATEPETL 5448 METERS ELEVATION IN METERS 4000 PINE 3500 (P. Hartwegii) DECREASING-•••• FIR-PINE PINE (Abies religiose (P. Montezuma.) P. petula) TEMPERATURE ca 2800 PINE cr OAK-ALDER ( P. Isiophyllo ) (Osercus spp. Minas spp.) 2500 2300 -.a—it/CREASING DECREASING— MOISTURE MEXICO CITY 2236