Human change was probably the driving factor of synchronous ecological change between both sedimentary sequences. These features present in both cores probably indicate forest clearance in the area. Starting from 3.0 ka, maize pollen was present in the record along with increases in herbaceous pollen, magnetic susceptibility and charcoal particles (Fig. Human impact characterized the late Holocene in the sequences of Zirahuen. In Zacapu basin, evidences of deforestation suggest land clearance associated with agricultural activity from 4.0 to 3.6 ka, although no maize pollen has been found (Arnauld and Faugère Kalfon, 1998). However, evidence of agriculture as indicated by the presence of maize pollen in the record dates to 3.6 ka (Watts and Bradbury, 1982), postdating the reduction of Alnus.
Bradbury (2000) pointed to human activity as a probable cause for the demise of Alnus pollen in the Patzcuaro basin. This change in arboreal assemblages was also recorded in the pollen sequences from neighboring Patzcuaro, Zacapu and Cuitzeo (Watts and Bradbury, 1982 Xelhuantzi-López, 1994 Velázquez-Durán et al., 2004). 3.9 ka, when Alnus percentages decreased. In Zirahuen, a trend towards drier climates was also the cause of pollen spectra variability at ca. Late Holocene sedimentary sequences from several locations in central Mexico contain fi ngerprints of past human activities (maize pollen, deforestation, changes in sediment accumulation) mixed with climatic and environmental changes (Caballero et al., 2002 Lozano-García et al., 2007 Metcalfe and Davies, 2007 Sedov et al., 2010). Water bodies in the Mexican central highlands have provided plenty of resources for early humans in America, with large pre-Hispanic settlements and societies developing in the region. A more variable rainfall regime during the late Holocene with the increase of ENSO type variability could explain these episodes of erosion but also, at this time, human impact could have been an important factor driving the environmental change. 6) and high percentages of titanium and low abundance of diatoms (Ortega et al., 2010), indicating events of high erosion (marked in Fig. The Zirahuen record shows, starting at 4 ka, periods of instability with more clastic sedimentation, high values of magnetic susceptibility (Fig.
For the central highlands reduction in moisture availability in the period between 19 correlates with strong El Niño events (Mosiño and Morales, 1988) whereas moderate El Niño favors normal to abundant rains. 1A), southwestern Mexico (Bernal et al., 2011). In southern Mexico negative rainfall anomalies cor- relate with the warm phase of ENSO (El Niño) (Magaña et al., 2003), and this climatic signal is evident in the high values of δ 18 O at the El Diablo cave (Fig. It has been suggested that a shift in climate associated with an increase in ENSO activity took place across the tropics at ca 4.3 ka (Moy et al., 2002 Conroy et al., 2008 Bernal et al., 2011). This mid-Holocene dry episode has been reported as one of the major rapid climate change events for the Holocene at a global scale (Mayewski et al., 2004), probably related to the end of the orbital controlled early Holocene warm period ( Fig. In the lacustrine system, the main change was the increase of Isoetes, evidencing lower lake levels, and reduction of Botryococcus concentration, indicating changing environments. 4.5 and 4.2 ka, a short period of dry conditions is evidenced by a rapid expansion of Pinus before T3 tephra deposition, with high charcoal concentrations in core MOLE-ZIR03 re fl ecting fi res associated with this drought.
Ecological change showed synchronism in the response of the central and littoral cores from ca. of arboreal taxa has been reported as characteristic of late Holocene pollen assemblages from several sequences in the highlands of central Mexico (Watts and Bradbury, 1982 Lozano-García and Ortega-Guerrero, 1998 Lozano-García et al., 2005 Correa-Metrio et al., 2012b).
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