Season of rainfall can be a major influence on the relative proportions of C3 and C4 grasses in a landscape, since their relative distribution is essentially constrained by temperatures during the growing season; cool growing seasons favor C3 grasses, whereas warm ones favor C4 grasses. 13C/12C ratios in tooth enamel carbonate of grazers from the site of Equus Cave, northern Cape Province, South Africa, were used to determine vegetation and rainfall seasonality shifts during the late Quaternary. The results show shifts in the relative proportion of C3 and C4 grasses during the latter stages of the Pleistocene, and domination by C4 grasses in the Holocene. The data show that the northwestern Cape did not fall within a winter rainfall zone during this period as has been postulated in recent climate models. Instead, a more complex situation is suggested in which at least two periods of enhanced winter rainfall occurred within a predominantly summer rainfall regime.

A 12,500-yr pollen record from Loon Lake, Wyoming provides information on the climate history of the southwestern margin of Yellowstone National Park. The environmental reconstruction was used to evaluate hypotheses that address spatial variations in the Holocene climate of mountainous regions. Loon Lake lies within the summer-dry/winter-wet climate regime. An increase in xerophytic pollen taxa suggests drier-than-present conditions between ca. 9500 and 5500 14C yr B.P. This response is consistent with the hypothesis that increased summer radiation and the expansion of the east Pacific subtropical high-pressure system in the early Holocene intensified summer drought at locations within the summer-dry/winter-wet regime. This climate history contrasts with that of nearby sites in the summer-wet/winter-dry region, which were under the influence of stronger summer monsoonal circulation in the early Holocene. The Loon Lake record implies that the location of contrasting climate regimes did not change in the Yellowstone region during the Holocene. The amplitude of the regimes, however, was determined by the intensity of circulation features and these varied with temporal changes in the seasonal distribution of solar radiation.

Plant and vertebrate macrofossils in an early Holocene fossil packrat (Neotoma sp.) midden with a radiocarbon age of 8640 ± 100 14C yr B.P. are reported from the Chocolate Mountains, near the Salton Sea, Riverside County, California. An inventory of the midden has permitted a comparison of the modern flora and fauna of the site with that extant during the early Holocene. Whereas the biota had assumed most aspects of its modern Sonoran desert aspect by this date, statistically significant evidence of differences is attributed to an increased flow of surface water in Salt Creek, a high-standing, low-salinity Lake LeConte, and the late arrival of some characteristic Sonoran desert plants. These observations are consistent with models of significant fall-winter precipitation in the Sonoran Desert, although we cannot exclude alternative explanations.