Climatic and anthropogenic impacts on δ13C variations in a stalagmite from central China.

In this paper, we present a delta C-13 record that covers the past 750 years at a resolution of 2 - 3 years which was preserved in a precisely dated stalagmite (DY-1) obtained from the Dayu Cave on the south flank of the Qinling Mountains in central China. Between 1249 AD and 1800 AD, climate-induced vegetation changes appear to have been the primary control on delta C-13 values at a centennial scale. Variations in precipitation amounts control the residence time of seepage water and may have affected the dissolution of bedrock, prior carbonate precipitation in the unsaturated zone above the cave, and the degassing of CO2 within the cave. These hydrogeochemical processes are likely to have been the most important controls on delta C-13 levels over annual to decadal scales, and may also have influenced centennial-scale variations. The reduced delta C-13 value of atmospheric CO2 since the Industrial Revolution may have caused the decreasing trend in delta C-13 values seen in stalagmite DY-1 after 1800 AD. Increased visitor numbers in the unventilated Dayu Cave over time produced a large amount of CO2, and maintained a raised level of pCO(2) in the cave air. This artificially enhanced pCO(2) may have decreased the fraction of CO2 degassing, and hence carbonate precipitation, which could partly cause the decreasing trend in the stalagmite delta C-13 seen over the past 200 years.</p

Leaf wax n-alkane distributions across plant types in the central Chinese Loess Plateau

Terrestrial ecosystems generally contain various plant types (e.g. dicots, monocots, gymnosperms), and an evaluation of the dominant plant type in an ecosystem is the key to understanding geological records in paleoenvironmental research. In this study, we examined n-alkane chain length distributions in terrestrial higher plants in the central Chinese Loess Plateau, and found that average chain length (ACL) could be utilized as an indicator for differentiating gymnosperms from angiosperms. ACL21&ndash;33 was less than 27 for gymnosperms, but more than 27 for angiosperms. Moreover, a derived Pv index from a selected n-alkane ratio, (i.e. (C31 + C33)/RCn(n=27&ndash;33)), provided a relative criterion for distinguishing between dicots and monocots within the angiosperms. When Pv &lt; 0.1, the ecosystem was predominated by dicots, but those with Pv &gt; 0.1, were dominated by monocots. Discrimination of the dominant plant type in an ecosystem is important when leaf wax n-alkanes are used as a proxy for paleoenvironmental reconstructions.</p

Changing color of Chinese loess: Geochemical constraint and paleoclimatic significance

Two loess-paleosol sequences on the central Chinese Loess Plateau were investigated to understand spatial and temporal variations in the soil color (e.g., lightness and redness) and factors that control those variations. Color difference between the original samples and pretreated samples suggests that loess lightness is influenced by the pedogenic matter (e.g., iron oxides, organic matter, and calcium carbonate) to varying degrees, depending on the concentrations. Iron oxides and organic matter darken the loess lightness, whereas carbonate lightens the loess lightness. By contrast, the redness is dominantly controlled by the types and concentrations of iron oxides. Variations in magnetic susceptibility and redness are associated with different magnetic minerals that formed mainly during post-depositional processes, and therefore both proxies can be employed to reconstruct the history and variability of the East Asian summer monsoon. Since the sensitivity of these two proxies to the summer monsoon variation is different, we generate a stacked summer monsoon index by combining these two records and interpret the stacked index as most indicative of precipitation variability. The fidelity of this new index is supported by an independent ratio, estimated from the first-derivative values of the color reflectance spectra. Our loess-based proxies provide a new understanding of the East Asian summer monsoon variability as a two-phase strengthening of summer monsoon intensity during the penultimate deglaciation.</p

Timing and Spatial Distribution of Loess inXinjiang, NW China

Central Asia is one of the most significant loess regions on Earth, with an important role in understanding Quaternary climate and environmental change. However, in contrast to the widely investigated loess deposits in the Chinese Loess Plateau, the Central Asian loess– paleosol sequences are still insufficiently known and poorly understood. Through field investigation and review of the previous literature, the authors have investigated the distribution, thickness and age of the Xinjiang loess, and analyzed factors that control these parameters in the Xinjiang in northwest China, Central Asia. The loess sediments cover river terraces, low uplands, the margins of deserts and the slopes of the Tianshan Mountains and Kunlun Mountains and are also present in the Ili Basin. The thickness of the Xinjiang loess deposits varies from several meters to 670 m. The variation trend of the sand fraction (>63 μm) grain-size contour can indicate the local major wind directions, so we conclude that the NW and NE winds are the main wind directions in the North and South Xinjiang, and the westerly wind mainly transport dust into the Ili basin. We consider persistent drying, adequate regional wind energy and well-developed river terraces to be the main factors controlling the distribution, thickness and formation age of the Xinjiang loess. The well-outcropped loess sections have mainly developed since the middle Pleistocene in Xinjiang, reflecting the appearance of the persistent drying and the present air circulation system. However, the oldest loess deposits are as old as the beginning of the Pliocene in the Tarim Basin, which suggests that earlier aridification occurred in the Tarim Basin rather than in the Ili Basin and the Junggar Basin

Clay mineral records of the Erlangjian drill core sediments from the Lake Qinghai Basin, China

Located at the northeastern margin of the Qinghai-Tibet Plateau (QTP) in the Asian interior, the Lake Qinghai is sensitive to environmental change and thus an outstanding site for studying paleoenvironmental changes. Thick deposits in the Lake Qinghai provide important geological archives for obtaining high-resolution records of continental environmental history. The longest drilling core obtained from the Lake Qinghai, named Erlangjian (ELJ), reached about 1109 m and was investigated to determine its clay mineral assemblage and grain size distributions. Clay mineralogical proxies, including type, composition, and their ratios, as well as the illite crystallinity (KI) and chemical index (CI), in combination with grain size data, were used for reconstructing the history of paleoenvironmental evolution since the late Miocene in the Lake Qinghai Basin. The clay mineral records indicate that the clay mainly comprise detritus originating from peripheral material and has experienced little or no diagenesis. The proportion of authigenic origin was minor. Illite was the most abundant clay mineral, followed by chlorite, kaolinite, and smectite. Variations of clay mineral indexes reflect the cooling and drying trends in the Lake Qinghai region, and the grain size distribution is coincided with the clay minerals indexes. The paleoclimatic evolution of the Lake Qinghai Basin since the late Miocene can be divided into five intervals. The climate was relatively warm and wet in the early of late Miocene, then long-term trends in climate change character display cooling and drying; later in the late Miocene until early Pliocene the climate was in a short relatively warm and humid period; since then the climate was relatively colder and drier. These results also suggest multiple tectonic uplift events in the northeastern QTP.</p

Wet climate during the 'Little Ice Age' in the arid Tarim Basin, northwestern China

A relatively cold period, the &#39;Little Ice Age&#39; (LIA), just prior to the current warm period, has been documented from many regions of the globe. However, hydrological changes (wet/dry conditions) during the LIA appear to be very different across different climatic regimes, suggesting that the position, strength and/or pattern of atmospheric circulation could have changed significantly in the past. Therefore, paleohydrological studies may have some implications for potential future hydrological changes in an anticipated warming world. In this study, we investigated hydrological changes over the last similar to 800 years in the arid Tarim Basin, northwestern China. We used carbon isotopes of plant remains (mainly leaves) from a 10.5 m long aeolian sand sediment section to evaluate changes in dry/wet conditions. The average value of carbon isotopes of plant remains during the LIA period (similar to AD 1490 to 1890) is -26.0%, ranging from -28.2% to -23.6%, much lower than values before (average of -24.3%, from -25.5% to -22.4%) and after this period (average of -22.8%, -24.6% to -21.6%). Although other environmental factors could also affect carbon isotopes to some degree, such consistent, large negative isotopic excursions of up to 6% strongly suggest a wetter climatic condition in the study area at that time, which is also consistent with two silty clay layers, indicating a wet depositional environment, found around ad 1560 and ad 1625 during the LIA period. This inference is consistent with previous studies suggesting a relatively wet period during the LIA in the Tarim Basin and possibly extending to the western central Asia region, but appears to be opposite to hydrological changes in regions that are today dominated by Asian monsoon systems. This pattern of regional hydrological changes may be related to possible changes in the trajectory or strength of the westerlies and/or the orographic effect in this region, as previous studies suggested.</p

Magnetostratigraphic and paleoenvironmental records for a Late Cenozoic sedimentary sequence drilled from Lop Nor in the eastern Tarim Basinsedimentary sequence drilled from Lop Nor in the eastern Tarim Basin

The Tarim Basin, one of the largest inland basins in the world, is situated in the northwestern China and to the north of the Tibetan Plateau. Continuous Cenozoic deposits have accumulated in this basin, which are crucial for investigating the growth of the Tibetan Plateau and the paleoclimatic evolution in Asian interior. Here we report the magnetostratigraphic and paleoenvironmental records for a Late Cenozoic sedimentary sequence drilled from Lop Nor in the eastern Tarim Basin. Magnetostratigraphic results show that this core has recorded a magnetic polarity sequence from C3Bn to C1n, covering an age range from ca. 7 Ma to the present. Decreased magnetic susceptibility occurred after ca. 5.6&ndash;5.1 Ma, which was interpreted to indicate an enhancement of aridity in the Tarim Basin since this period. We attribute this aridification to the combined effect of global climate cooling and the uplift of the Northern Tibetan Plateau since the late Miocene.</p

Stable isotope composition alteration produced by the aragonite-to-calcite transformation in speleothems and implications for paleoclimate reconstructions

Aragonite, a mineralogical constituent of speleothems in cave environments, is unstable and susceptible to inversion to calcite, a diagenetic process that involves changes in the mineralogy, texture and geochemistry of speleothems. However, the exact alterations of stable isotope compositions during such diagenesis have not been fully investigated. In this study, two aragonite stalagmites (SN3 and SN15) from the Shennong Cave, southeast China, were found partially inverted to calcite, as determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses, and thin-section inspections under microscope. The fiber relics and textural ghosts of aragonite preserved in coarse and equant mosaic calcite crystals clearly indicate that the calcite in these two stalagmites were inverted from aragonite. The stable isotope compositions (delta C-13 and delta O-18, given in per mil versus VPDB standard) of primary aragonite and secondary calcite were analyzed and compared, along both growth layers and growth axes. The results show that, along growth layers, differences of delta C-13 values between aragonite and calcite are negligible (0.1 parts per thousand-0.2%.), whereas differences of delta O-18 values between aragonite and calcite are significant (0.63 parts per thousand-0.87%0). Comparisons along growth axes show similar results: i.e., differences of delta 13C values are negligible (0.06% +/- 0.22%.) whereas differences of delta O-18 values are significant (0.85 parts per thousand +/- 0.29%.). Most likely, the aragonite in SN3 and SN15 were internally inverted by interactions of trace calcite crystallites and pore water within intercrystalline pore spaces, by a dissolution-reprecipitation process occurring in trapped pore water. In the case of the inversion of aragonite to calcite in speleothems, such as that observed in SN3 and SN15, the delta 13C values could be used in paleoclimate and paleoenvironment reconstructions because they are inherited from those of primary aragonite. Although the delta O-18 values might be cross-calibrated to those of primary aragonite if the aragonite-calcite fractionation offset is known (e.g., 0.85 parts per thousand +/- 0.29%0 in this study), however, the delta O-18 values of secondary calcite should be used with caution in such reconstructions as the delta O-18 offset value is not consistently invariable.</p