Vegetation history along the eastern, desert escarpment of the Sierra San Pedro, Baja California, Mexico

摘要

Plant macrofossils from 38 packrat middens spanning the last ~33,000cal yr BP record vegetation between ~650 and 900m elevation along the eastern escarpment of the Sierra San Pedro, northern Baja California. The middens span most of the Holocene, with a gap between ~4600 and 1800cal yr BP, but coverage in the Pleistocene is uneven with a larger hiatus between 23,100 and 14,400cal yr BP. The midden flora is relatively stable from the Pleistocene to Holocene. Exceptions include Pinus californiarum, Juniperus californica and other chaparral elements that were most abundant >23,100cal yr BP and declined after 14,400cal yr BP. Despite being near the chaparral/woodland-desertscrub ecotone during glacial times, the midden assemblages reflect none of the climatic reversals evident in the glacial or marine record, and this is corroborated by a nearby semi-continuous pollen stratigraphy from lake sediments. Regular appearance of C_4 grasses and summer-flowering annuals since 13,600cal yr BP indicates occurrence of summer rainfall equivalent to modern (JAS average of ~80-90mm). This casts doubt on the claim, based on temperature proxies from marine sediments in the Guaymas Basin, that monsoonal development in the northern Gulf and Arizona was delayed until after 6200cal yr BP.