Controls on seasonal variations of silicate weathering and CO2 consumption in, two river catchments on the NE Tibetan Plateau

Water samples from the Buha and Shaliu Rivers, located on the semi-arid northeastern Tibetan Plateau, were collected weekly over a one year period. The major ionic compositions of water samples were measured and the daily contents of suspended particulate material (SPM) were monitored in both rivers in order to investigate the influence of lithology, climate and physical erosion on seasonal silicate weathering. In the Shaliu River, weathering of trace amounts of calcite contributes more than 50% of the ca(2+) and HCO3- to the river water. Through high-resolution variations of Ca2+ concentrations and elemental ratios, the signal of carbonate precipitation is captured at the end of monsoon in this river. The measured physical erosion rate is only 8.7-16.0 mm/kyr in this semi-arid region, which is 2-3 orders of magnitude lower than that in the Himalaya and nearby regions. In contrast with several orders of magnitude in seasonal variations of silicate weathering rates in both catchments, the distinct lithology between the catchments only leads to a 15 times difference of annual net CO2 consumption. The correlation analysis shows that seasonal silicate weathering is strongly dependent on water discharge in the semi-arid area. The most important observation is that, unrecognized by the previous studies, both physical erosion rate and air temperature exhibit two distinct trends with silicate weathering rates (and net CO2 consumption) during the years. The two trends might suggest that temperature plays a more important role on the CO2 consumption rate before the mid-monsoon under a condition of low water discharge than that after the monsoon with a high water discharge. During the period before the mid-monsoon, the relationship between temperature and silicate weathering rate exhibits higher activation energy than after the mid-monsoon, suggesting a greater dissolution of uneasily weatherable minerals from groundwater, frozen soil, and/or dust input. The relationship between erosion and weathering during the period before the mid-monsoon indicates a faster increase of silicate weathering rate, because freezing erosion produces large amount of high surface area minerals.</p

Geochemical records in Holocene lake sediments of northern China: Implication for natural and anthropogenic inputs

Daihai Lake is located in a hydrologically closed basin within the transitional zone of the East Asian monsoon, which has experienced significant lake-level fluctuations. The sedimentary sequence of a 12.08 m core was analyzed for mobile (Ca, Mg, and Sr) and immobile elements (Al and Fe) and trace metals (e.g., Co, Ni, Cu, Zn, and Pb) in order to study the changes of natural chemical compositions and the potential influences of the historical mining and use of metals during the Holocene period. Climate changes have a significant influence on the concentrations of mobile elements in the Holocene lake sediment; high concentrations occurred during the times with high lake level, resulting from enhanced catchment weathering due to strong monsoon effects. Different from these mobile elements, the variation of immobile elements and trace metals in Daihai Lake sediment shows clear anthropogenic impact of the mining and use of metals in the last several millenniums. A gradual increase in the concentrations and fluxes of metals from similar to 5000 cal. a B.P. is correlated well with the emergence of Chinese civilization. The concentrations and fluxes of these metals and immobile elements in the sediments increased rapidly between 2100 and 1250 cal. a B.P., indicating the extensive use of metals during the Warring States Period (475-221 B.C.), and the early Han Dynasty (206 B.C.-220 A.D.). Further increase of trace metals, such as Cu, Ni, Co, and Pb, after the Medieval Warm Period (1200-800 a B.P.) likely reflects the increased metal emissions associated with extensive mining and utilization activities. Similar patterns of sedimentary metals between Daihai Lake in northern China and Liangzhi Lake in central China further indicate significant environmental impacts of the mining and utilization of metals in the progress of Chinese civilization in the past several thousand years.</p

A validated analytical procedure for boron isotope analysis in plants by MC-ICP-MS

Boron (B) is an essential micronutrient for plant growth. Lack of valid methods for pretreatment and measurement of delta B-11 in plant restrict applications of it in the biosphere. Dry ashing, one step cation exchange and micro-sublimation were combined to separate and purify boron (B) in plant tissues. The low procedure blank, high B recovery and the accurate delta B-11 values of the plant reference materials demonstrate that this method is suitable and valid for B pretreatment and delta B-11 measurement in plant samples by MC-ICP-MS. Based on this method, the delta B-11 in different plants (Brassica napes, Chenopodium album L, moss, lichen, and Nostoc commune) was analyzed. For Brassica napus, delta B-11 increased gradually from root to leaf, and then decreased to rapeseed. For the same parts, the delta B-11 increased from the lower parts to the higher parts. This variation may be due to the B(OH)(3) transporter of NIP6;1 and the incorporation of B into the cell. The reason for lower delta B-11 values in shell and rapeseed compared to those in leaves presumably is to the preferred transport of borate in the phloem. The largest delta B-11 fractionation between leaf and root in Brassica napus and Chenopodium album L was + 24.2 parts per thousand and + 26.6 parts per thousand, respectively. The large variation and fractionation of delta B-11 within plants indicates that 81113 is a good tracer to study the B translocation mechanisms and metabolism within plants. The delta B-11 in Nostoc commune, lichen, and moss showed variations of -4.1 parts per thousand to + 21.5 parts per thousand, - 9.4 parts per thousand to + 7.3 parts per thousand, and - 18.3 parts per thousand to + 11. 9 parts per thousand, respectively. In the same site, delta B-11 in different plants ranked Nostoc commune > moss > lichen and delta B-11 in mosses growing in different environment ranked soil > tree > rock. Rain and soil available B are the main B sources for these plants. The delta B-11 in Nostoc commune, lichen, and moss may be a useful tracer to study the atmospheric B input. In the future, plants culture experiments under certain environments and studies from molecular level are necessary to decipher the variation of delta B-11 and fractionation mechanisms within plants

Controls on Sr/Ca in benthic foraminifera and implications for seawater Sr/Ca during the late Pleistocene

Changes in the Sr to Ca ratio of sea water have important implications for the interpretation of past climate. It has proven difficult to interpret Sr/Ca of foraminiferal calcite as a measure of seawater Sr/Ca or as reflecting the influence of deep water carbonate ion saturation (Delta[CO32-]) on the incorporation of Sr into benthic foraminiferal carbonate. Here, we address this issue by measurements of paired benthic foraminiferal Sr/Ca and B/Ca (a proxy for deep water A[Delta[CO32-]) for core-tops from the global ocean and three down cores at different settings during the Last Glacial-interglacial cycle. These new data suggest a significant control of deep water Delta[CO32-] on benthic foraminiferal Sr/Ca, and that down-core shell Sr/Ca variations can be largely accounted for by past deep water Delta[CO32-] changes. We conclude that seawater Sr/Ca has likely remained near-constant on glacial-interglacial timescales during the late Pleistocene, in agreement with model results. With due caution, benthic Sr/Ca may be used as an auxiliary proxy for deep water Delta[CO32-] if seawater Sr/Ca is constant.</p

Morphology and ultrastructure of Hippodonta qinghainensis sp nov. (Bacillariophyceae), a new diatom from Lake Qinghai, China

A new medium-sized species of Hippodonta (Bacillariophyceae) is described from Lake Qinghai, China. The morphology and ultrastructure of Hippodonta qinghainensis sp. nov. are described using light and scanning electron microscopy. This new species is compared with similar species of Hippodonta using conventional and geometric morphometric analyses. Hippodonta qinghainensis can be separated from the other species of Hippodonta by a unique combination of characters that include an elliptic-lanceolate to rhombic-lanceolate valve shape, non protracted apices, the absence of fascia, relatively coarse, uniseriate striae and the presence of two rows of lineolae around the valves apices.</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

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

Citrobacter sp strain GW-M Mediates the Coexistence of Carbonate Minerals with Various Morphologies

To better understand the formation mechanism of carbonate minerals by microbes, culture experiments with a duration of 70days were performed under the mediation of strain GW-M isolated from soil using modified Lagoa Vermelha (LV, a hypersaline coastal Lagoon, Rio de Janeiro, Brazil) medium with 6:1 Mg/Ca molar ratio. The results demonstrated that strain GW-M can mediate the formations of both high-Mg calcite and aragonite and that dumbbell-, cauliflower-, rhombohedra-shaped, and irregular minerals coexist in the modified LV medium. The amount of rhombohedra-shaped crystals increased significantly with culture time. A proposed mechanism for these formations is the following. Heterogeneous nucleation on the surface of the extracellular polymeric substances (EPS) always occurred, and carbonates with irregular shape existed in experimental products at any stages. The morphologies evolved from rod to dumbbell and finally to cauliflower. At the initial stage (till day 20), hydrogen ions and EPS secreted by the bacteria only influenced the microenvironment around the cells, and carbonates were precipitated on the surface of bacterial cells. At the middle and late stages (on days 45 and 70), microbes and their secretions influenced the whole medium. Under these conditions, rhombohedra-shaped crystals were formed when homogeneous nucleation occurred. In addition, the results of energy-dispersive spectrometry (EDS) showed that Mg contents in the synthesized carbonate minerals with rhombohedra-shaped were significantly lower than those of carbonates with other shapes, though relationship between morphology and species of mineral cannot be obtained by this phenomenon alone. These results shed further light on the mechanism of carbonate precipitation in the presence of microbes.</p

Seismic mountain building: Landslides associated with the 2008 Wenchuan earthquake in the context of a generalized model for earthquake volume balance

Here we assess earthquake volume balance and the growth of mountains in the context of a new landslide inventory for the M-w 7.9 Wenchuan earthquake in central China. Coseismic landslides were mapped from high-resolution remote imagery using an automated algorithm and manual delineation, which allow us to distinguish clustered landslides that can bias landslide volume calculations. Employing a power-law landslide area-volume relation, we find that the volume of landslide-associated mass wasting (approximate to 2.8+0.9/-0.7 km(3)) is lower than previously estimated (approximate to 5.7-15.2 km(3)) and comparable to the volume of rock uplift (approximate to 2.61.2 km(3)) during the Wenchuan earthquake. If fluvial evacuation removes landslide debris within the earthquake cycle, then the volume addition from coseismic uplift will be effectively offset by landslide erosion. If all earthquakes in the region followed this volume budget pattern, the efficient counteraction of coseismic rock uplift raises a fundamental question about how earthquakes build mountainous topography. To provide a framework for addressing this question, we explore a group of scaling relations to assess earthquake volume balance. We predict coseismic uplift volumes for thrust-fault earthquakes based on geophysical models for coseismic surface deformation and relations between fault rupture parameters and moment magnitude, M-w. By coupling this scaling relation with landslide volume-M-w scaling, we obtain an earthquake volume balance relation in terms of moment magnitude M-w, which is consistent with the revised Wenchuan landslide volumes and observations from the 1999 Chi-Chi earthquake in Taiwan. Incorporating the Gutenburg-Richter frequency-M-w relation, we use this volume balance to derive an analytical expression for crustal thickening from coseismic deformation based on an index of seismic intensity over a defined area. This model yields reasonable rates of crustal thickening from coseismic deformation (e.g., approximate to 0.1-0.5 km Ma(-1) in tectonically active convergent settings), and implies that moderate magnitude earthquakes (M-w approximate to 6-7) are likely responsible for most of the coseismic contribution to rock uplift because of their smaller landslide-associated volume reduction. Our first-order model does not consider a range of factors (e.g., lithology, climate conditions, epicentral depth, and tectonic setting), nor does it account for viscoelastic effects or isostatic responses to erosion, and there are important large uncertainties on the scaling relationships used to quantify coseismic deformation. Nevertheless, our study provides a conceptual framework and invites more rigorous modeling of seismic mountain building.</p