Lithium distribution and isotopic fractionation during chemical weathering and soil formation in a loess profile

Lithium (Li) is a fluid-mobile element and delta Li-7 in secondary deposits represents an excellent proxy for silicate weathering and authigenic mineral formation. The soil samples from 1205 to 1295 cm in the Weinan profile, one of the best developed loess-paleosol sequences covering the last glacial-interglacial climatic cycle, were collected and chemically separated into detritus and carbonate fractions for subsequent analyses of Li, delta Li-7, major and trace elements. Other desert specimens (i.e., Qaidam Desert, Tengger Desert, Badain Juran Desert and Taklimakan Desert) near the Chinese Loess Plateau (CLP) and various standard clays were analyzed for assisting provenance determination. The Li and delta Li-7 distributions in the detritus are rather homogeneous, 1.4-2.0 mu g/g and +2.5 parts per thousand to +4.7 parts per thousand, respectively, compared with the carbonate fraction. The detrital delta Li-7 varies systematically with magnetic susceptibility and grain size changes, reflecting significant Li isotopic variation associated with sources and mineralogy of detrital material. On the other hand, Li and delta(7) Li in carbonates show large changes, 781-963 ng/g and -4.1 parts per thousand to +10.2 parts per thousand, respectively. These carbonate Ski correlated well with the estimated index of chemical weathering, as a result of Li mobilization and soil formation during chemical weathering.</p

The dominance of loess weathering on water and sediment chemistry within the Daihai Lake catchment, northeastern Chinese Loess Plateau

This study investigated modern loess weathering and its control on the chemistry of surface water and sediment within the Daihai Lake catchment. The mineral types and the abundances of major and trace elements in loess, sediments and bedrocks were determined to ascertain the provenance of river sediment. The major cation compositions and Sr isotopic ratios of surface and subsurface waters were measured to distinguish the contributions of dissolved loads from various parent materials. The data show that mineralogical characteristics and elemental abundances of the river sediments are almost identical with those of the loess, but are different from the bedrocks, indicating that river sediments are predominantly derived from loess. River waters feeding Daihai Lake show a similar range in Sr-87/Sr-86 ratios as those of HOAc-soluble carbonate minerals in loess from the Chinese Loess Plateau. The slightly lower Sr-87/Sr-86 of river waters in the southern catchment relative to other rivers reflect potential weathering of large areas of outcropping basalt. These results imply that (1) surface processes are dominated by weathering of loess which only accounts for 18% of the total catchment area, and (2) loess weathering but not basalt controls the river Sr isotopic signature, although the latter covers a larger catchment area. For groundwater, Sr-87/Sr-86 ratios indicate that subsurface processes might be controlled by interactions with ambient lithology and hydrological flowpaths. Comparing the rivers draining the Chinese Loess Plateau with global rivers, both Mg/Ca and Sr-87/Sr-86 in the Daihai surprisingly agree well with those in the upper and downstream Huanghe (Yellow River), as well as HOAc-soluble loess, but differ significantly from other global rivers. This result reinforces the argument that loess weathering plays the most important role in controlling the sediment and water chemistry in the loess-covered areas, whereas the influence of bedrock weathering is minor. This study on modern processes might provide baselines to decipher down core records for paleoclimate reconstructions, especially for lake/river sediments in (semi-)arid areas.</p

Seasonal contributions of catchment weathering and eolian dust to river water chemistry, northeastern Tibetan Plateau: Chemical and Sr isotopic constraints

River waters collected weekly over the whole year of 2007 from the Buha River draining to Lake Qinghai on the northeastern Tibetan Plateau were analyzed for major ions and Sr isotopes. Dissolved loads in the river exhibit distinct seasonal variability in major cation ratios and Sr isotopes over the 1 year period, reflecting seasonal differences in relative inputs from various sources and weathering reactions in the catchment. Distinct geochemical signatures suggest that eolian dust may affect river water chemistry significantly, resulting in a twofold increase influx of dissolved loads during spring relative to winter. It is noticeable that both the lowest and the highest (87)Sr/(86)Sr values of the Buha River waters occurred in the monsoon season, indicating a sensitive response of carbonate versus silicate weathering sources to hydrological forcing on a seasonal basis. A significant decrease in Na/cation, together with lower Sr isotope ratios, is consistent with a greater proportion of carbonate weathering relative to silicate weathering in the early monsoon season. High temperature and increased rainfall during the peak of the monsoon facilitate an increased proportion of ions derived from silicates, partly from groundwaters, to river water. In other seasons, elemental and 87Sr/86Sr ratios vary much less, indicating a constant ratio of silicate to carbonate weathering, consistent with limited variation in discharge. Our results highlight that in a semiarid region where climatic conditions vary seasonally, in addition to silicate and carbonate contributions, supply from eolian dust may also play a significant role in controlling seasonal variations in chemistry of river waters.</p

Rare earth element patterns in a Chinese stalagmite controlled by sources and scavenging from karst groundwater

The rare earth elements and yttrium (REY) in a Chinese stalagmite (SJ3) have been studied to delineate their possible sources and controlling mechanisms. The following results are obtained: (1) The REY in karst groundwater at the study site have very low concentrations, e. g. ranging from 59 x 10(-6) to 614 x 10(-6) mu mol/L for La, and are transported predominantly in particle/colloidal phases. These elements show a high linear positive correlation with Fe, Mn and Al (r(2) = 0.98, 0.95 and 0.91, respectively), which are also transported largely in particle/colloidal phases, suggesting a close association of REY with Fe, Mn and Al in colloidal/particle phases; (2) REY mobilization from the sources, i.e. the limestone host rock and the overlying soil layer, may not have exerted a significant influence on the REY patterns for SJ3 including anomalies of La, Ce and Y and fractionation between light and heavy REY; (3) The REY patterns of SJ3 display significant variations which are controlled in part by change in contributions from the limestone host rock and the overlying soil layer and can be associated with stadial-interstadial changes in past climate and environment. The REY contribution from the overlying soil layer is higher under a warm-humid climate phase than a cold-dry one and the REY contribution from the limestone host rock is reversed; (4) Some of the variations of the SJ3 REY patterns could not be explained with a simple mixing of different REY sources. Removal of REY from the groundwater in association with precipitation of colloidal and particle materials, an analogue to REY scavenging from seawater, is the most important mechanism influencing the SJ3 REY patterns. Strong REY &quot;scavenging&quot;, which may be caused by both less dynamic hydrology, longer travel-time and increased ionic strength of karst groundwater, is suggested to be responsible for the large positive Y anomalies and negative Ce anomalies and depletion of light REE relative to heavy REE during the cold-dry climate phases (e. g. the periods corresponding to the last glacial maximum and Heinrich event 1).</p

Timing and structure of the Younger Dryas event in northern China

A high-resolution and absolute-dated stalagmite record from Kulishu Cave, Beijing characterizes Asian Monsoon (AM) history in northern China between ca 14 and 10.5 ka BP (thousand yrs before present, present = 1950), including the entire Younger Dryas (YD) event. Using Th-230 dates and counting of annual-layers, the shift into the YD began at 12,850 +/- 40 yr BP and took similar to 340 yrs and the shift out of the YD began at 11,560 +/- 40 yr BP and took &lt;38 yrs (best estimate similar to 20 yrs), broadly similar to previously reported AM records from central and southeastern China. The more gradual nature of the start of the YD event as observed in the AM records appears to contrast with the more abrupt beginning observed in the Greenland ice records. The total amplitude of the AM YD event is also smaller than the amplitude of the AM Heinrich Stadial 1 event. In addition, the general rising trend of the AM during the Bolling-Allerod period contrasts with the general cooling trend in Greenland temperature during that time. The influence of rising insolation on the AM may explain this observation.</p

Acceleration of modern acidification in the South China Sea driven by anthropogenic CO2

Modern acidification by the uptake of anthropogenic CO2 can profoundly affect the physiology of marine organisms and the structure of ocean ecosystems. Centennial-scale global and regional influences of anthropogenic CO2 remain largely unknown due to limited instrumental pH records. Here we present coral boron isotope-inferred pH records for two periods from the South China Sea: AD 1048-1079 and AD 1838-2001. There are no significant pH differences between the first period at the Medieval Warm Period and AD 1830-1870. However, we find anomalous and unprecedented acidification during the 20th century, pacing the observed increase in atmospheric CO2. Moreover, pH value also varies in phase with inter-decadal changes in Asian Winter Monsoon intensity. As the level of atmospheric CO2 keeps rising, the coupling global warming via weakening the winter monsoon intensity could exacerbate acidification of the South China Sea and threaten this expansive shallow water marine ecosystem.</p