Abrupt Climate changes Recorded in Loess Sequences

The terrestrial sediments known as "loess" represent an important archive of paleoclimatic variability. Mineral material mobilized by wind at the ground surface was transported over distances from a few hundred to thousands of kilometers and deposited back on the ground where it underwent a gradual transformation into loess sediment. The largest loess deposits are found in the Northern Hemisphere, primarily because the continental surface is much larger than the Southern Hemisphere. Depending on their location on the globe, the deposits have formed under the influence of different climate factors, and contain more or less detailed records of regional climate change. here we review the current knowledge about the three main loess regions of the Northern Hemisphere. The many underlying references are provided in an extensive online archive of references, listed by regions

Modeling oxygen isotopes in ice sheets linked to Quaternary ice-volume variations

Climate change through the Quaternary was dominated by repeated build-up and retreat of large ice sheets, especially in the Northern Hemisphere. The best indicator for past variations of global ice volume is the oxygen-isotopic composition (d18O) of seawater (dw) recorded in foraminiferal calcite in marine sediments. But the interpretation calcite-d18O (dc) records is not straightforward, mainly because dc depends on both dw and seawater temperature. Furthermore, in common paleoceanographic practice the relationship between dw and ice volume is assumed linear, but two main factors may induce considerable nonlinearity: the mean isotopic composition of ice, which varies during a glacial cycle, and the ocean circulation. In the present study, these two factors were investigated at glacial-interglacial timescale using a 2.5-dimensional thermomechanical ice-sheet model including oxygen-isotope transport, combined with different ocean models. First, using a well-mixed ocean, it was found that the effect of mean-ice d18O variations can be neglected in reconstructing ice volume from marine d18O records. Second, the effect of ocean circulation was investigated qualitatively by replacing the well-mixed ocean by a spatially-resolved ocean model. It was concluded that the ocean circulation is an important element to be taken into account in the climatic and stratigraphic interpretation of dc records and that leads/lags inferred on the basis of proxies depending on more than one physical variable, such as dc, can be misleading in interpreting causal relationships. Finally, the interaction between climate variability at orbital and millennial timescales was investigated in order to gain further understanding of the origin of d18O variations in marine sediments and ice cores. It was found that YD-type events may occur during any deglaciation of the past 800 kyr, so that YD is probably not a one-time event, but an intrinsic feature of the climate change at millennial timescales

Eurasian contribution to the last glacial dust cycle:how are loess sequences built?

International audienceThe last 130 000 years have been marked by pronounced millennial-scale climate variability, which strongly impacted the terrestrial environments of the Northern Hemisphere, especially at middle latitudes. Identifying the trigger of these variations, which are most likely associated with strong couplings between the ocean and the atmosphere, still remains a key question. Here, we show that the analysis of δ 18 O and dust in the Greenland ice cores, and a critical study of their source variations, reconciles these records with those observed on the Eurasian continent. We demonstrate the link between European and Chinese loess sequences, dust records in Greenland, and variations in the North Atlantic sea ice extent. The sources of the emitted and transported dust material are variable and relate to different environments corresponding to present desert areas, but also hidden regions related to lower sea level stands, dry rivers, or zones close to the frontal moraines of the main Northern Hemisphere ice sheets. We anticipate our study to be at the origin of more sophisticated and elaborated investigations of millennial and sub-millennial continental climate variability in the Northern Hemisphere

Neanderthal Extinction by Competitive Exclusion

International audienceBackground: Despite a long history of investigation, considerable debate revolves around whether Neanderthals became extinct because of climate change or competition with anatomically modern humans (AMH). Methodology/Principal Findings: We apply a new methodology integrating archaeological and chronological data with high-resolution paleoclimatic simulations to define eco-cultural niches associated with Neanderthal and AMH adaptive systems during alternating cold and mild phases of Marine Isotope Stage 3. Our results indicate that Neanderthals and AMH exploited similar niches, and may have continued to do so in the absence of contact. Conclusions/Significance: The southerly contraction of Neanderthal range in southwestern Europe during Greenland Interstadial 8 was not due to climate change or a change in adaptation, but rather concurrent AMH geographic expansion appears to have produced competition that led to Neanderthal extinction

Prácticas socioculturales, turismo e identidad entre los hijos de migrantes yucatecos en Cancún

Southern Mexico’s commercialized tourist hub Cancun draws many working migrant Mayan Indian families whose origins are based in the Eastern corn-growing region of Yucatán, an area wh. In their new social scenario, the transmission of social y cultural practices, norms and values to new generations is in constant tension with the surrounding touristic social context. From the perspective of children of migrant families, this article focuses on the roll that Mayan Culture plays in signifying practices and constructs, which offer congruency and a sense of place to their lives. Among young people there are new ways of understanding, interpreting and appropriating their "Yucatecan Maya heritage".El centro turístico de Cancún en el sur de México atrae a muchas familias migrantes mayas originarias de la región maicera de la península yucateca, donde las tradiciones ceremoniales y rituales se conservan. En el nuevo escenario, la transmisión de prácticas y valores indígenas a las nuevas generaciones está en tensión constante con el contexto turístico de la ciudad. Desde la perspectiva de los hijos de las familias migrantes, nos enfocamos en el rol de la cultura maya para dar significado a las prácticas y construcciones sociales que ofrecen un sentido de congruencia a las vidas.&nbsp

Dynamics and sources of last glacial aeolian deposition in southwest France derived from dune patterns, grain-size gradients and geochemistry, and reconstruction of efficient wind directions

Dune pattern, grain-size gradients and geochemistry were used to investigate the sources and dynamics of aeolian deposition during the last glacial in southwest France. The coversands form widespread fields of low-amplitude ridges (zibars), whereas Younger Dryas parabolic dunes mainly concentrate in corridors and along rivers. Spatial modelling of grain-size gradients combined with geochemical analysis points to a genetic relationship between coversands and loess, the latter resulting primarily from dust produced by aeolian abrasion of the coversands. The alluvium of the Garonne river provided also significant amounts of dust at a more local scale. The geochemical composition of loess shows much lower scattering than that of coversands, due to stronger homogenisation during transport in the atmosphere. Overall, sandy loess and loess deposits decrease in thickness away from the coversands. Dune orientation and grain-size gradients suggest that the efficient winds blew respectively from the W to the NW during the glacial, and the W-SW during the Younger Dryas. A comparison between the wind directions derived from the proxy data and those provided by palaeoclimatic simulations suggests a change of the main transport season. Ground surface conditions and their evolution throughout the year, i.e. the length of the season with snow and frozen or moist topsoil, and the seasonal distribution of wind speeds able to cause deflation are thought to have been the main factors that controlled the transport season in the study area

Effective radiative forcing and adjustments in CMIP6 models

The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmosphere and surface, has emerged as the key metric of evaluating human and natural influence on the climate. We evaluate effective radiative forcing and adjustments in 17 contemporary climate models that are participating in the Coupled Model Intercomparison Project (CMIP6) and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global-mean anthropogenic forcing relative to pre-industrial (1850) levels from climate models stands at 2.00 (&plusmn;0.23) W&thinsp;m&minus;2, comprised of 1.81 (&plusmn;0.09) W&thinsp;m&minus;2 from CO2, 1.08 (&plusmn; 0.21) W&thinsp;m&minus;2 from other well-mixed greenhouse gases, &minus;1.01 (&plusmn; 0.23) W&thinsp;m&minus;2 from aerosols and &minus;0.09 (&plusmn;0.13) W&thinsp;m&minus;2 from land use change. Quoted uncertainties are 1&nbsp;standard deviation across model best estimates, and 90&thinsp;% confidence in the reported forcings, due to internal variability, is typically within 0.1 W&thinsp;m&minus;2. The majority of the remaining 0.21&thinsp;W&thinsp;m&minus;2 is likely to be from ozone. In most cases, the largest contributors to the spread in effective radiative forcing (ERF) is from the instantaneous radiative forcing (IRF) and from cloud responses, particularly aerosol&ndash;cloud interactions to aerosol forcing. As determined in previous studies, cancellation of tropospheric and surface adjustments means that the stratospherically adjusted radiative forcing is approximately equal to ERF for greenhouse gas forcing but not for aerosols, and consequentially, not for the anthropogenic total. The spread of aerosol forcing ranges from &minus;0.63 to &minus;1.37&thinsp;W&thinsp;m&minus;2, exhibiting a less negative mean and narrower range compared to 10 CMIP5 models. The spread in 4&times;CO2 forcing has also narrowed in CMIP6 compared to 13 CMIP5 models. Aerosol forcing is uncorrelated with climate sensitivity. Therefore, there is no evidence to suggest that the increasing spread in climate sensitivity in CMIP6 models, particularly related to high-sensitivity models, is a consequence of a stronger negative present-day aerosol forcing and little evidence that modelling groups are systematically tuning climate sensitivity or aerosol forcing to recreate observed historical warming. </div