LATE PLEISTOCENE-HOLOCENE ENVIRONMENTAL MAGNETISM FROM LAKE ZIRAHUEN SEDIMENTS, MICHOACAN, MEXICO

Magnetic and geochemical analyses were made in a sedimentary sequence form Lake Zirahuen (west-central Mexico), at the Trans Mexican Volcanic Belt (19° 26´ N, 101° 45´ W, 2,075 m asl). The aim of this study is to interpret the past climatic conditions since Pleistocene-Holocene transition up to the present. Zirahuen is a tropical oligotrophic-monomictic lake, with maximum depth of 40 m. Two lacustrine sequences were analyzed. One 4.5 m long from the deepest, central part of the lake, that represents ca. 17 kyr BP. The second, 6.6 m long, from the shallow littoral zone spans the last 11.6 kyr. The base of the sequence is characterized by massive sediments, the middle zone is composed by laminated sediments, and the upper part has very thin bed stratification. Three main components in the lake sediments have been identified 1) clastic, 2) volcaniclastic and 3) biogenic components (mainly from diatoms). Mineral magnetic, TIC-TOC and FRX geochemical measurements were performed on both sequences. Twenty ¹⁴C AMS dates were used to establish the time scale.

Curie temperature measurements, low-temperature susceptibility and low-temperature remanence indicate that the magnetic fraction is characterized by Ti-magnetite and maghemite. Low S₃₀₀ ratios reflect the existence of high coercivity iron oxide phases (hematite). Both minerals have been transported from rocks and soils from the surrounding area. SIRM/c ratios and coercivity indicators, along with geochemical data, suggest the presence of autigenic ferrimagnetic sulfides (greigite). The comparison of magnetic and geochemical properties led to complement the knowledge of terrigenous input, biogenic components, and other characteristics as carbonate precipitation and oxide-reducing conditions at the water-sediment interface. Results suggest that sediments record dry conditions for the end of the Last Glacial Maximum period (late Pleistocene). A hiatus is recorded for the Pleistocene-Holocene transition (12.1–11.6 kyr BP). Early to mid Holocene has little moist but dry conditions persisted until 4 kyr BP. Late Holocene suggest alternated wet and dry climatic conditions related to high precipitation; and accelerated erosion is related to human activities, possibly related to Purepechas occupation.