Soil-geomorphology and “wet” cycles in the Holocene record of North-Central Mexico

The distinction between the impact of climatic periodicities or land-use practices on soil erosion is an important issue for Pre-Hispanic and Colonial Mexico. That question can best be addressed by first documenting the dynamics of changing “wet” cycles during the Holocene in the central Mexican region between the northern limits of Pre-Hispanic agriculture and its southern margins in northwestern Chihuahua. Consequently the Laguna Project targeted a 125,000 km2 sector of North-Central Mexico, 250 km from north to south and 500 km from east to west, from Saltillo to Durango. Some 40 sedimentary profiles with multiple cumulic soils were studied in the field and laboratory, supported by 163 conventional 14C and AMS dates on charcoal and soil humates. We distinguish: (1) wet floodplains (with humic paleosols, redox phenomena reflecting high water tables, channel-ponding sequences, and interbedded tufas) that imply aquifer recharge, sustained base flow, and mainly low-energy conditions; and (2) high-energy pulses of discharge that mobilized cobble gravels or forced channel entrenchment (“gullying”) and were tied to episodic, excessive rains that promoted valley and slope instability. In between such “wet” cycles and recurrent disequilibrium events, climate was similar to today, probably less humid, with limited geomorphologic change or slow soil formation.“Wet” cycles were rare at the end of the Pleistocene, but prominent during the Holocene. Disequilibrium proxies became common and dramatic after 2500 BP. The drainages from the Eastern and Western Sierra Madres responded in phase, but varied in detail. Around AD 1050–1200 “natural” erosion led to loss of soil organic carbon, as alternating severe droughts and heavy rains destroyed the ground cover and led to ecological aridification, well before arrival of Spanish miners and settlers. The evidence that human activity triggered Pre-Hispanic or Colonial erosion in Central Mexico should therefore be re-evaluated. Global comparisons and interpretations are discussed, but with caution, since no single theory can explain the whole of the record. The soil-geomorphology geoarchive of North-Central Mexico primarily is an environmental history of alternating “wet” cycles, rather than of sustained wet or dry climates. The critical differences between “soaking” and “excessive” rains, with their respective impacts, may be due to switching between winter and summer storm categories.