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From the first millennium B.C. through the 9th-century A.D. Classic Maya collapse, nonurban populations grew exponentially, doubling every 408 years, in the twin-lake (Yaxha-Sacnab) basin that contained the Classic urban center of Yaxha. Pollen data show that forests were essentially cleared by Early Classic time. Sharply accelerated slopewash and colluviation, amplified in the Yaxha subbasin by urban construction, transferred nutrients plus calcareous, silty clay to both lakes. Except for the urban silt, colluvium appearing as lake sediments has a mean total phosphorus concentration close to that of basin soils. From this fact, from abundance and distribution of soil phosphorus, and from continuing post-Maya influxes (80 to 86 milligrams of phosphorus per square meter each year), which have no other apparent source, we conclude that riparian soils are anthrosols and that the mechanism of long-term phosphorus loading in lakes is mass transport of soil. Per capita deliveries of phosphorus match physiological outputs, approximately 0.5 kilogram of phosphorus per capita per year. Smaller apparent deliveries reflect the nonphosphatic composition of urban silt; larger societal outputs, expressing excess phosphorus from deforestation and from food waste and mortuary disposal, are probable but cannot be evaluated from our data. Eutrophication is not demonstrable and was probably impeded, even in less-impacted lakes, by suspended Maya silt. Environmental strain, the product of accelerating agroengineering demand and sequestering of nutrients in colluvium, developed too slowly to act as a servomechanism, damping population growth, at least until Late Classic time.

16 M. Stem, unpublished data, Corrosion Laboratory, M.I.T.17 G. Gemmell found that the corrosion rate for mild steel (0.06 per cent C) in deaerated 4 per cent NaCl of pH 5.6 was zero within the experimental variations of such measurements, whereas at pH 3.8 the rate averaged 24 mg/dm2/day (mdd), in agreement with Stern's value for iron. This means that the rate at pH 5.6 must be less than perhaps the order of 0.5 mdd. Pryor and Cohen, J. Electrochem. Soc., 100, 206 (1953), report a value of 0.3 mdd for the corrosion rate of mild steel in deaerated water. . 21 Based on atomic percentages and atomic radii of Cd and Hg, the fraction of surface composed of Cd atoms for 10-13 per cent Cd amalgam is equal to about 0.2. However, in measurements of anodic polarization, this fraction remains constant up to fairly high current densities because the potential for formation of Hg2++ is not approached so long as the surface concentration of Cd is appreciable. Therefore, measured values of b are for an anodic area similar to that which applies to the corrosion process, and, hence, the apparent value of A in eq. (2) under these conditions is equal to unity. Data by Wagner and Traud, op. cit., for zinc amalgam confirm this conclusion. For iron, on the other hand, the anodic surface becomes larger, the higher the value of current density externally applied for anodic polarization, and, hence, A is usually less than unity.

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