EVIDENCE IN THE BIOTIC RECORD OF LATE QUATERNARY MILLENNIAL CYCLES OF CLIMATE

Periods of wetter climate, their frequencies, magnitudes, rates of onset and decline, and shifts in their seasonal distribution were identified for the late Quaternary of the southern Great Basin and northern Mojave Desert. One hundred and ninety-five radiocarbon dates conducted upon the plant remains from 122 ancient woodrat nest (midden) strata collected in southern Nevada provide proxy data to directly determine these changes in past atmospheric input into the hydrologic system. Using the modern mean annual precipitation (MAP) requirements of key plant species, changes in plant species distribution during the Pleistocene as revealed in the plant macrofossil data from southern Nevada (and northern Nevada) woodrat middens indicate rapid shifts toward century- to millennia-long wetter, winter precipitation patterns with a recurrence of ~3,500 to 5,000 years. Using the MAP ranges of modern analogue plant species we suggest a Pleistocene MAP of from at least 380 mm to 635 mm at elevations between 1,500 and 2,000 m. A shift towards dominance of Utah juniper during the Pleistocene elevations from 1,500 to ~700 m suggests precipitation increases of ~33% to 66% (a gain of 50 to 100 mm over current values). Whereas today's MAP is only 150 mm, the Pleistocene MAP was at least 200 mm or even up to ~255 mm. However, most of the Pleistocene between 35 and 12 k was dominated by more continental conditions characterized by cold-dry climate ~1.33 times wetter than today. The shift from limber pine dominance during the LGM to white fir during the onset and decline of the LGM suggests MAT at most 6oC warmer. Precipitation gradients increased significantly during the Pleistocene. A comparison of reconstructed precipitation and current precipitation at two elevations during the Pleistocene reveals that precipitation was ~50% greater at 1,500 m than it was at 750 m. This gradient indicates more dramatically enhanced orographic precipitation effect at intermediate elevations than at lower or higher elevations during the late Pleistocene.