Climate, agriculture, and cycles of human occupation over the last 4000 yr in southern Zacatecas, Mexico

Scholars attribute the growth and decline of Classic period (AD 200-900) settlements in the semi-arid northern frontier zone of Mesoamerica to rainfall cycles that controlled the extent of arable land. However, there is little empirical evidence to support this claim. We present phytolith, organic carbon, and magnetic susceptibility analyses of a 4000-yr alluvial record of climate and human land use from the Malpaso Valley, the site of one such Classic frontier community. The earliest farming occupation is detected around 500 BC and appears related to a slight increase of aridity, similar to the level of the modern day valley. By AD 500, the valley's Classic period Mesoamerican settlements were founded under these same dry conditions, which continued into the Postclassic period. This indicates that the La Quemada occupation did not develop during a period of increased rainfall, but rather an arid phase. The most dramatic changes detected in the valley resulted from the erosion associated with Spanish Colonial grazing and deforestation that began in the 16th century. The landscape of the modern Malpaso Valley is thus primarily the product of a series of intense and rapid transformations that were concentrated within the last 400 yr.     

Selected geochemical factors influencing diagenesis of eocene carbonate rocks,peninsular Florida,U.S.A.

The geochemical significance of three selected ions (Mg²⁺, Na⁺, and Sr²⁺) supports a model of dolomitization by brackish groundwater. This groundwater zone contains sufficient quantities of Mg²⁺ to facilitate dolomitization ($\text{Mg}\text{Ca}\text{ratios 1}$). Rising and falling of sea level and fluctuations of the phreatic zone related to climatic variations account for the thickness of the dolomite layers and the chemical distributions within these layers. Sodium concentrations in the calcite are 70–185 ppm, indicating formation in brackish water. Dolomite has sodium concentrations between 50–1400 ppm, suggesting formation in waters of similar salinity.Strontium in calcite ranges from 320–600 ppm, suggesting diagenesis in slightly saline waters in an open system. Dolomite contains 241 ppm Sr²⁺ on the average and calcite has 418 ppm Sr²⁺. The Sr²⁺ concentrations of the dolomite are characteristic of diagenesis in water less saline than sea water. Average strontium concentrations in the dolomite occur in two distinct groups, 260 ppm for dolomite with 39–43 mole-% MgCo₃ and 195 ppm for the dolomite with 44–50 mole-% MgCO₃. The difference in the Sr²⁺ concentrations of the two dolomite groups indicates the higher mole-% MgCO₃ dolomite recrystallized in a less saline environment than the lower mole-% MgCO₃ dolomite. These different environments are attributed to a relatively more saline coastal environment and a less saline inland environment.The more nearly stoichiometric dolomite (44–50 mole-% MgCO₃) has less scatter when mole-% MgCO₃ is plotted against Sr²⁺ and Na⁺. This suggests a greater approach to equilibrium with the dolomitizing fluid than the lower mole-% MgCO₃ (39–43) dolomite. The more saline environment has higher Mg/Ca ratios and promotes more calcium-rich dolomite during diagenesis because of the inhibition from competing foreign ions and because it is thermodynamically a more favorable environment which causes more rapid crystallization. The less saline waters allow recrystallization to proceed more slowly, producing better ordering in the dolomites, textural preservation and development of subhedral to euhedral rhombic crystals.