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

Hydrogeochemical processes between surface and groundwaters on thenortheastern Chinese Loess Plateau: Implications for water chemistryand environmental evolutions in semi-arid regions

A large area of the continents is covered by loess that is subject to fast erosion, but detailed research is lacking about processes influencing the hydrogeochemistry in loess-covered regions. This study presents the first &delta;18O and &delta;D, and major ion contents of various waters (rain, rivers, lake, springs and wells) from Daihai Lake catchment on the Chinese Loess Plateau (CLP). In combination with historical hydrological and meteorological data during the past 60 years, we investigate factors affecting water chemistry and lake evolution on the CLP, and thereby provide insight into hydrogeochemical processes under semi-arid climatic conditions. On the northeast CLP, river and groundwaters show elevated TDS (450 and 461 mg/L, respectively), about ~4 times higher than the global river mean value. Their water chemistry is dominantly influenced by carbonate weathering. Lake waters show even higher TDS at 5758 mg/L, ~50 times of the global mean, which is attributed to strong evaporation and associated with precipitation of calcite and dolomite. The order of carbonate (calcite and dolomite) saturation indexes follows lake water N river water N groundwater. Downstream rivers to the west of Daihai Lake are characterized by elevated SO4 2&minus;, indicating high lake levels in the past. Comparison of &delta;18O, &delta;D, TDS and ion contents of river and groundwaters implies that shallow groundwaters are derived from surface runoff via fast infiltration, a hydrological process different from limited infiltration of groundwater on the Tibetan Plateau. Water quality assessment indicates that all river and 79% of well waters belong to moderately hard to hard-fresh waters, suitable for drinking and irrigation of plants with moderate salt tolerance. In contrast, all lake and spring waters and 21% of well waters belong to very bad water quality, and hence are not suitable for irrigation or drinking. Owing to intensified human activities under drying and warming climate, the lake water level of Daihai declined by 4.83 m from 1955 to 2003, and started to accelerate since 1980. As a result, TDS and Cl&minus; increased by 1.8 times and Mg/Ca rose by 156 times from 1953 to 2010. Our data show that environment degradation poses a great threat to human occupation on the CLP. Considering the uniformity of loess, hydrogeochemical processes between surface and groundwaters on the loess regions may represent a widespread status of the CLP.</p

Dilution of Be-10 in detrital quartz by earthquake-induced landslides: Implications for determining denudation rates and potential to provide insights into landslide sediment dynamics

The concentration of Be-10 in detrital quartz (Be-10(qtz)) from river sediments is now widely used to quantify catchment-wide denudation rates but may also be sensitive to inputs from bedrock landslides that deliver sediment with low Be-10(qtz). Major landslide-triggering events can provide large amounts of low-concentration material to rivers in mountain catchments, but changes in river sediment Be-10(qtz) due to such events have not yet been measured directly. Here we examine the impact of widespread landslides triggered by the 2008 Wenchuan earthquake on Be-10(qtz) in sediment samples from the Min Jiang river basin, in Sichuan, China. Landslide deposit material associated with the Wenchuan earthquake has consistently lower Be-10(qtz) than in river sediment prior to the earthquake. River sediment Be-10(qtz) decreased significantly following the earthquake downstream of areas of high coseismic landslide occurrence (i.e., with greater than similar to 0.3% of the upstream catchment area affected by landslides), because of input of the Be-10-depleted landslide material, but showed no systematic changes where landslide occurrence was low. Changes in river sediment Be-10(qtz) concentration were largest in small first-order catchments but were still significant in large river basins with areas of 10(4)-10(5) km(2). Spatial and temporal variability in river sediment Be-10(qtz) has important implications for inferring representative denudation rates in tectonically active, landslide-dominated environments, even in large basins. Although the dilution of Be-10(qtz) in river sediment by landslide inputs may complicate interpretation of denudation rates, it also may provide a possible opportunity to track the transport of landslide sediment. The associated uncertainties are large, but in the Wenchuan case, calculations based on Be-10 mixing proportions suggest that river sediment fluxes in the 2-3 years following the earthquake increased by a similar order of magnitude in the 0.25-1 mm and the &lt;0.25 mm size fractions, as determined from Be-10(qtz) mixing calculations and hydrological gauging, respectively. Such information could provide new insight into sediment transfer, with implications for secondary sediment-related hazards and for understanding the removal of mass from mountains.</p

Connectivity of earthquake-triggered landslideswith the fluvial network: Implicationsfor landslide sediment transport afterthe 2008 Wenchuan earthquake

Evaluating the influence of earthquakes on erosion, landscape evolution, and sediment-related hazards requires understanding fluvial transport of material liberated in earthquake-triggered landslides. The location of landslides relative to river channels is expected to play an important role in postearthquake sediment dynamics. In this study, we assess the position of landslides triggered by the Mw 7.9 Wenchuan earthquake, aiming to understand the relationship between landslides and the fluvial network of the steep Longmen Shan mountain range. Combining a landslide inventory map and geomorphic analysis, we quantify landslide-channel connectivity in terms of the number of landslides, landslide area, and landslide volume estimated from scaling relationships. We observe a strong spatial variability in landslide-channel connectivity, with volumetric connectivity (&xi;) ranging from ~20% to ~90% for different catchments. This variability is linked to topographic effects that set local channel densities, seismic effects (including seismogenic faulting) that regulate landslide size, and substrate effects that may influence both channelization and landslide size. Altogether, we estimate that the volume of landslides connected to channels comprises 43 + 9/ 7% of the total coseismic landslide volume. Following the Wenchuan earthquake, fine-grained (&lt;~0.25 mm) suspended sediment yield across the Longmen Shan catchments is positively correlated to catchment-wide landslide density, but this correlation is statistically indistinguishable whether or not connectivity is considered. The weaker-than-expected influence of connectivity on suspended sediment yield may be related to mobilization of fine-grained landslide material that resides in hillslope domains, i.e., not directly connected to river channels. In contrast, transport of the coarser fraction (which makes up &gt;90% of the total landslide volume) may be more significantly affected by landslide locations.</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

Accurate and Precise Determination of Boron Isotopic Ratios at Low Concentration by Positive Thermal Ionization Mass Spectrometry Using Static Multicollection of Cs2BO2+ Ions

A static double-collector system for accurate, precise, and rapid boron isotope analysis has been established by employing a newly fixed Faraday H3 and H4 cup enabling simultaneously collected Cs2BO2+ ion beams (m/z = 308 and 309) on a Finnigan-MAT Triton thermal ionization mass spectrometer of boron (Triton B). The experimental result indicated that Cs2BO2+ ion beams (m/z = 308 and 309) were simultaneously collected using a fixed Faraday H3 and H4 cup without using the &quot;Zoom Quad&quot; function and reduced accelerating voltage. Furthermore, the method enabled the measurement of samples containing as little as 20 ng of boron. An analysis of the National Institute of Standards and Technology standard reference material (NIST SRM) 951 standard showed external reproducibility (2RSD) of +/- 0.013 parts per thousand, +/- 0.013 parts per thousand, and +/- 0.019 parts per thousand for 100, 50, and 20 ng of boron, respectively. The present method of static multicollection of Cs2BO2+ ions is applicable to a wide field of boron isotopic research that requires high precision and accuracy to analyze samples with low boron concentrations, including pore fluids, foraminifera, rivers, rainwater, and other natural samples.</p

Deep South Atlantic carbonate chemistry and increased interocean deep water exchange during last deglaciation

Carbon release from the deep ocean at glacial terminations is a critical component of past climate change, but the underlying mechanisms remain poorly understood. We present a 28,000-year high-resolution record of carbonate ion concentration, a key parameter of the global carbon cycle, at 5-km water depth in the South Atlantic. We observe similar carbonate ion concentrations between the Last Glacial Maximum and the late Holocene, despite elevated concentrations in the glacial surface ocean. This strongly supports the importance of respiratory carbon accumulation in a stratified deep ocean for atmospheric CO2 reduction during the last ice age. After similar to 9 mu mol/kg decline during Heinrich Stadial 1, deep South Atlantic carbonate ion concentration rose by similar to 24 mu mol/kg from the onset of Bolling to Preboreal, likely caused by strengthening North Atlantic Deep Water formation (Bolling) or increased ventilation in the Southern Ocean (Younger Drays) or both (Pre-boreal). The similar to 15 mu mol/kg decline in deep water carbonate ion since similar to 10 ka is consistent with extraction of alkalinity from seawater by deepsea CaCO3 compensation and coral reef growth on continental shelves during the Holocene. Between 16,600 and 15,000 years ago, deep South Atlantic carbonate ion values converged with those at 3.4-km water depth in the western equatorial Pacific, as did carbon isotope and radiocarbon values. These observations suggest a period of enhanced lateral exchange of carbon between the deep South Atlantic and Pacific Oceans, probably due to an increased transfer of momentum from southern westerlies to the Southern Ocean. By spreading carbon-rich deep Pacific waters around Antarctica for up-welling, invigorated interocean deep water exchange would lead to more efficient CO2 degassing from the Southern Ocean, and thus to an atmospheric CO2 rise, during the early deglaciation.</p