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The hydrogen isotopic composition (δ²H) of leaf waxes, especially of n-alkanes (δ²Hn-alkanes), is increasingly used for paleohydrological and paleoclimate reconstructions. However, it is challenging to disentangle past changes in the isotopic composition of precipitation and changes in evapotranspirative enrichment of leaf water, which are both recorded in leaf wax δ²H values. In order to overcome this limitation, Zech M. et al. (2013) proposed a coupled δ²Hn-alkaneδ18Osugar biomarker approach. This coupled approach allows for calculating (i) biomarker-based "reconstructed" δ²H=δ18O values of leaf water (δ²H=δ18Oleaf water), (ii) biomarker-based reconstructed deuterium excess (d- excess) of leaf water, which mainly reflects evapotranspirative enrichment and which can be used to reconstruct relative air humidity (RH) and (iii) biomarker-based reconstructed δ²H/δ18Oprecipitation values. Here we present a climate transect validation study by coupling new results from δ²H analyses of n-alkanes and fatty acids in topsoils along a climate transect in Argentina with previously measured δ²18O results obtained for plant-derived sugars. Accordingly, both the reconstructed RH and δ²H=δ;Oprecipitation values correlate highly significantly with actual RH and δ²H=δ18Oprecipitation values.We conclude that compared to single δ²Hn-alkane or δ18Osugar records, the proposed coupled δ²Hn-alkane-δ18Osugar biomarker approach will allow more robust δ²H=δ18Oprecipitation reconstructions in future paleoclimate research. Additionally, the proposed coupled δ²Hn-alkane-δ18Osugar biomarker approach allows for the establishment of a "paleohygrometer", more specifically, the reconstruction of mean summer daytime RH changes/history. [ABSTRACT FROM AUTHOR]

Paleoenvironmental and paleoclimate reconstructions based on molecular proxies, such as those derived from leaf wax biomarkers, in loess-paleosols sequences represent a promising line of investigation in Quaternary research. The main premise of such reconstructions is the synsedimentary deposition of biomarkers and dust, which has become a debated subject in recent years. This study uses two independent approaches to test the stratigraphic integrity of leaf-wax biomarkers: (i) Long-chain nalkanes and fatty acids are quantified in two soil profiles in till on the Swiss Plateau. Since glacial sediments are extremely poor in organic matter, significant amounts of leaf-wax biomarkers in the lower part of those profiles would reflect post-sedimentary root-derived or microbial contributions. (ii) Compound-specific radiocarbon measurements are conducted on n-alkanes and fatty acids from several depth intervals in the loess section "Crvenka", Serbia, and the results are compared to independent estimates of sediment age. We find extremely low concentrations of plant wax n-alkanes and fatty acids below the topsoils in the soil profiles. Moreover, compound-specific radiocarbon analysis yields plant wax 14C ages that agree well with published luminescence ages and stratigraphy of the Serbian loess deposit. Both approaches confirm that post-sedimentary, root-derived or microbial contributions are negligible in the two systems investigated. The good agreement between n-alkane and fatty acid ages, as well as between odd and even homologues, further indicates that reworking and incorporation of fossil leaf-waxes is not particularly relevant either. [ABSTRACT FROM AUTHOR]

The latitudinal position of the southern westerlies has been suggested to be a key parameter for the climate on Earth. According to the general notion, the southern westerlies were shifted equatorward during the global Last Glacial Maximum (LGM: ~24-18 ka), resulting in reduced deep ocean ventilation, accumulation of old dissolved carbon, and low atmospheric CO2 concentrations. In order to test this notion, we applied surface exposure dating on moraines in the southern Central Andes, where glacial mass balances are particularly sensitive to changes in precipitation, i.e. to the latitudinal position of the westerlies. Our results provide robust evidence that the maximum glaciation occurred already at ~39 ka, significantly predating the global LGM. This questions the role of the westerlies for atmospheric CO2, and it highlights our limited understanding of the forcings of atmospheric circulation. [ABSTRACT FROM AUTHOR]

The authors respond to comments made on their article "LGM and Late Glacial Glacier Advances in the Cordillera Real and Cochabamba (Bolivia) Deduced From 10Be Surface Exposure Dating." The introduction of the manuscript highlighted the significance of the glacial chronology in the Andes for the global climate system. The small dataset used in the article did not limit the ability of the researchers to reach paleoclimate interpretations, and it is the systematic uncertainties that may limit the interpretations.

Despite the important role of the Central Andes (15-30° S) for climate reconstruction, knowledge about the Quaternary glaciation is very limited due to the scarcity of organic material for radiocarbon dating. We applied 10Be surface exposure dating (SED) on 22 boulders from moraines in the Cordon de Doña Rosa, Northern/Central Chile (∼31° S). The results show that several glacial advances in the southern Central Andes occurred during the Late Glacial between ∼14.7±1.5 and 11.6±1.2 ka. A much more extensive glaciation is dated to ∼32±3 ka, predating the temperature minimum of the global LGM (Last Glacial Maximum: ∼20 ka). Reviewing these results in the paleoclimatic context, we conclude that the Late Glacial advances were most likely caused by an intensification of the tropical circulation and a corresponding increase in summer precipitation. High-latitude temperatures minima, e.g. the Younger Dryas (YD) and the Antarctic Cold Reversal (ACR) may have triggered individual advances, but current systematic exposure age uncertainties limit precise correlations. The absence of LGM moraines indicates that moisture advection was too limited to allow significant glacial advances at ∼20 ka. The tropical circulation was less intensive despite the maximum in austral summer insolation. Winter precipitation was apparently also insufficient, although pollen and marine studies indicate northward shift of the westerlies at that time. The dominant pre-LGM glacial advances in Northern/Central Chile at ∼32 ka required lower temperatures and increased precipitation than today. We conclude that the westerlies were more intense and/or shifted equatorward, possibly due to increased snow and ice cover at higher southern latitudes coinciding with a minimum of insolation. [ABSTRACT FROM AUTHOR]

Despite the important role of the Central Andes (15-30° S) for climate reconstruction, knowledge about the Quaternary glaciation is very limited due to the scarcity of organic material for radiocarbon dating. We applied Be surface exposure dating (SED) on 22 boulders from moraines in the Cordon de Doña Rosa, Northern/Central Chile 5 (∼31° S). The results show that several glacial advances in the southern Central Andes occurred during the Late Glacial between ∼14.7±1.5 and 11.6±1.2 ka BP. A much more extensive glaciation is dated to ∼32±3 ka BP, predating the temperature minimum of the global LGM (Last Glacial Maximum: ∼20 ka BP). Reviewing these results in the paleoclimatic context, we note that the Late Glacial advances coincide with (i) lower temperatures during the Younger Dryas (YD) and the Antarctic Cold Reversal (ACR), (ii) the intensification of the tropical circulation and a corresponding increase in summer precipitation and (iii) a minimum in austral summer insolation favouring reduced ablation. The absence of LGM moraines indicates that moisture advection was too limited to allow significant glacial advances at ∼20 ka BP. The tropical circulation was much less intensive despite the maximum in austral summer insolation. Winter precipitation was apparently also insufficient, although pollen and marine studies indicate a northward shift of the westerlies at that time. The dominant pre-LGM glacial advances in Northern/Central Chile at ∼32 ka BP required lower temperatures and increased precipitation than today. They coincide with (i) a minimum of southern high-latitude insolation suggesting an equatorward shift of the westerlies due to increased snow and ice cover, (ii) a maximum winter insolation resulting in ocean-continental temperature and pressure gradients favouring moisture advection, (iii) minimum summer insolation suggesting lower temperatures and reduced ablation and (iv) low high-latitude temperatures corroborating that they affect subtropical and tropical temperatures. More glacier-climate modelling is necessary to quantify the influence of the various forcings on the dated glacial advances. [ABSTRACT FROM AUTHOR]