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Carbon isotopes (δ13Corg) in biomass, carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotopes in authigenic calcite and estimations of chemical weathering of siliciclastic minerals (chemical index of alteration [CIA] and plagioclase index of alteration [PIA]) in lacustrine sediments of the ephemeral Babicora Basin, located within the core region of the North American Monsoon at northwest Mexico, helped to evaluate the source of biomass, variations in hydrological balance and sediment–water interactions during three different marine isotope stages (late MIS 4: ≥65.4–59.5 cal ka BP, MIS 3: ~59.5–28 cal ka BP and MIS 2: ~28–15 cal ka BP) of the last glacial. Comparisons of δ13Corg with previously published records of C/N, diatom abundance and fossil pollen indicate that biomass of the late MIS 4 was sourced from C3 aquatic and land plants. During the MIS 3, the C3 plants continued to contribute biomass and the influence of lacustrine algae, however, increased during its later part. The water column experienced more evaporation compared to two other MIS stages and sediment–water interactions varied between minimal to moderate (CIA: 49–68). Sediments were transported for longer distances during the wetter intervals and eroded from nearby lithologies during the drier intervals. Interactions between sediments and inflowing water (CIA: 59–65) remained homogeneous during the MIS 2. Source of the biomass, however, was variable. Lacustrine algae and C3 land plants contributed biomass during the early MIS 2 (~28–19 cal ka BP) and contributions from the C4 and CAM vegetation slightly increased towards the deglaciation (~19–15 cal ka BP). [ABSTRACT FROM AUTHOR]

A new geochemical record from the paaleolake Santiaguillo documents the hydrological variability of sub-tropical northern Mexico over the last ∼14 cal. ka. Summer-season runoff, lake water salinity and deposition of sediments by aeolian activity were reconstructed from concentrations of K, Ca and Zr/ K in bulk sediments. More-than-average runoff during c. 12.3 -9.3 cal. ka BP represented an interval of enhanced summer precipitation. Arid intervals of c. 14 -12.3 cal. ka BP and c. 6-4.3 cal. ka BP were characterized by average and more-than-average aeolian activity. Comparison with proxy records of summer as well as winter precipitation from tropical and sub-tropical North America and sea surface temperatures from the Atlantic and Pacific provides insight into the source of moisture and possible forcing. The wet Pleistocene− Holocene transition and early Holocene was contemporary with warmer conditions in the Gulf of California. We suggest that the Atlantic had minimal influence on the summer precipitation of the western part of sub-tropical northern Mexico and that the source of moisture was dominantly Pacific. [ABSTRACT FROM AUTHOR]

We present elemental concentrations and magnetic susceptibility data from a new 270-cm-long sediment core collected from the western part of palaeolake Babicora and infer millennial-scale hydrological variations over the last 27 cal. ka in the western Chihuahua Desert. Variations in the available water content at the sediment-air interface of the watershed, lake salinity and lake productivity are inferred from values of the chemical index of alteration ( CIA), CaCO3 and Corg, respectively. An abrupt increase in runoff at c. 24 cal. ka BP appears correlative with the Heinrich 2 ( H2) event. Except for this event, diminished runoff between c. 27 and 19 cal. ka BP indicates lower annual precipitation (weak summer rainfall) during the Last Glacial Maximum. The deposition of chemically altered sediments between c. 25 and 22 cal. ka BP results from the higher sediment-water interaction in the watershed owing to lower evaporation, cooler conditions and higher precipitation during the H2 event. Since 19 cal. ka BP the runoff has been characterized by high-amplitude fluctuations with intervals of reduced precipitation identified at c. 19, 18, 17.5, 13-14, 11.5, 10, 7.5 and 3 cal. ka BP. [ABSTRACT FROM AUTHOR]