Climate changes during the Holocene

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Metcalfe (1997) has studied the sediments of the lakes in the Zacapu Basin of central Mexico west to the Basin of Mexico City. No sediments were found for the period of transition from the Pleistocene to the Holocene. During the Holocene, only minor changes occurred in the levels of the lakes, except at c. 5.0 ka and 1.1 kaBP, when the lakes dried up.

Lozano-Garcia and Xelhuantzi-Lopez (1997) have studied the pollen, diatoms, macrofossil plant remains and tephras from cores in the sediments of this lake. They observed three main climatic stages. From c. 8.1 ka to 6.7 ka BP, a temperate sub-humid climate prevailed, which brought into existence a shallow lake with a low rate of sediment accumulation. From c. 6.7ka to 5kaBP, there was a reduction in humidity, and a temperate semi-arid climate prevailed. This caused the lake level to drop. During the last 5000 years, a temperate sub-humid climate prevailed.

Arnauld et al. (1997) made a synthesis of the various investigations of this basin and came up with the following division into Holocene phases.

Phase A: before c. 8 kaBP. The climate was humid to sub-humid and the lake was quite shallow. Phase B: c. 8ka to c. 7kaBP. This was characterized by major volcanic activity, which interfered with the topography of the lake and the pollen record. Phase C: c. 7ka to c. 6kaBP. In this period, the region stabilized and pollen and diatoms indicate a reduction in humidity with a very gradual change towards a semi-arid temperate climate. Phase D: c. 6ka to c. 4kaBP. It was mainly dry and a shallow alkaline marsh existed. At about 4 ka BP, sub-humid climatic conditions returned and the marsh expanded again. Phase E: c. 4ka to c. 2kaBP. There was a marked human impact on the environment. The climate became a little wetter and open water areas developed in the marsh.

The Aztecs settled to the east in the Basin of Mexico at c. 1345 AD, most probably driven from the north by particularly dry conditions, which were connected with a severe El Nino event (Manzanilla, 1997). After their settlement the climate became more humid, and at c. 1382 AD, the level of Lake Texcoco rose, flooding Tenochtitlan (now Mexico City) for 4 years. Extreme wet conditions prevailed until 1450 AD, after which a few years of extremely cold and dry conditions prevailed. During these dry years, which were also a period of famine, the temperatures were below normal levels. Later, and prior to the conquest by the Spaniards (around 1500 AD), the climate was very wet and the Aztecs had to build dams to protect Tenochtitlan from being flooded. A shift to slightly drier conditions, and droughts, occurred after the conquest by the Spaniards, during the period 1521-1640 AD, which suggests that the country was influenced by the sub-tropical pressure belt. Still during this period the climate was rather variable, and cold and dry years alternated with warm and humid periods. However, from c. 1640 to 1915 AD, the climate was dominated by droughts, which intensified during the mid to late 1700s, andlate 1800s. Since 1915, there has been a shift towards wetter conditions. Therefore, it can be concluded that the period which coincided with the Little Ice Age was also a period of droughts and famine. This was caused by the southward displacement of the ITCZ, suppressing the monsoons. During periods of El Nino, however, north Mexico may get higher rates of precipitation coming from the Pacific during winter, while the central part of Mexico still remains dry (O'Hara and Metcalfe, 1997).

Jauregui (1997) gave evidence for a strong causal link between cold periods, drought, famine and social unrest in Mexico. The climax was the drought years that preceded the 1810 and 1910 revolutions.

Hodell et al. (1995) have investigated the oxygen isotope composition and chemistry of a sediment core taken from the bottom of Lake Chichananab in the Yucatan peninsula. They found that the interval between 1.3 ka and 1.1 kaBP (800-1000 AD) was the driest of the Middle to Late Holocene and coincided with the collapse of classic Maya civilization. Whitmore et al. (1996) traced climatic and human influences on the nature of the sediments of three lakes. The climate impact started at c. 8 ka BP as groundwater levels rose as a result of a rise in sea level and increased precipitation. In its initial stages, Lake Coba, the most eastern lake and in the more humid part of the peninsula, was shallow and brackish but at c. 2.6 ka BP, the lake was at a high level and freshwater was present. This was followed by a decline in the water salinity and later an increase in salinity that continued to the present time. In Lake Sayaucil, in the intermediate precipitation zone, total salinity was high from 3.05 ka to 2kaBP, followed by consistently higher lake levels. In San Jose Chulcaha, a lake in the most western and drier zone, salinity was high at 1.86 ka BP. This was followed by a gradual freshening of the water to the present time.

Gunn etal. (1995) have constructed a model that links global climate changes with the hydrology of the Candelaria river drainage basin and with agricultural production in the Yucatan peninsula. Highest levels of discharge and longest wet seasons were associated with global high temperatures, while lesser amounts of discharge and longer dry seasons were linked with low global temperatures. They came also to the conclusion that the general Maya collapse was caused by a severe drought period.

In northern Guatemala, variations in the pollen assemblage in a core in Lake Peten-Itza records the impact of climate as well as humans on the local environment. From 8.6ka to 5.6kaBP, the tropical forest dominated. The decline that followed was either a result of a climate change towards drying or caused by human impact. The clearance of the forest by the Mayan people and its substitution by agricultural plants was clearly evidenced beginning at about 2 kaBP (106BC to 122 AD (cal.)). The forest returned after the Mayan collapse at c. 900 AD (Islebe et al., 1996).

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  • eemil
    What are the phases of the hydrologic system?
    8 years ago

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