Geomorphic processes of a dammed palaeo-lake in the middle Yarlung Tsangpo River, Tibet.

The failure of a natural dam is an extreme geological event. Palaeo-lake sediments were discovered in the broad Xigazê valley and Dazhuka-Yueju gorge in the middle reach of the Yarlung Tsangpo River in Tibet. However, the sedimentary processes, dam failure, and peak flood of the Xigazê dammed palaeo-lake are poorly understood. Hence, we conducted a field survey of eight lacustrine sedimentary terraces in the area. We divided the sedimentary processes of the palaeo-lake into five stages and deposit types: pre-palaeo-lake sediments (fluvial or aeolian deposits); early stage sediments of the palaeo-lake (coarse sand); main stage palaeo-lake sediments (clayey silt and sand), sediments following the discharge of the palaeo-lake (sand and gravel-cobbles); and cover deposits (aeolian sediments and colluvium). Additionally, the water level along the palaeo-lake was almost constant (3811 m a.s.l.). The dam was likely located at the eastern end of the Dazhuka-Yueju gorge. Based on the water level, dam location and 30-m ASTER GDEM2 data, the capacity of the palaeo-lake was estimated as 22.55 km3. To separate the water volume and sediment volume, the sediment surface elevation along the palaeo-lake was simulated based on the elevations of the six lacustrine sedimentary terraces. The volume of the sediment was ~11.56 km3, which was calculated from the dam location, sediment surface elevation, and the ASTER GDEM2 data. Finally, subtraction of the sediment volume from the capacity of the palaeo-lake gave a backwater volume of 10.99 km3. The peak flood possibly exceeded 3.4 × 105 m3/s as a moraine dam joined the discharge during the dam failure. However, the dammed event probably had a limited effect on the landforms at downstream because of the presence of another dammed palaeo-lake in the broad Zetang valley; moreover, the bedrock upstream of the dam was protected from erosion.

Metal release from contaminated coastal sediments under changing pH conditions: Implications for metal mobilization in acidified oceans.

To investigate the impacts and processes of CO2-induced acidification on metal mobilization, laboratory-scale experiments were performed, simulating the scenarios where carbon dioxide was injected into sediment-seawater layers inside non-pressurized chambers. Coastal sediments were sampled from two sites with different contamination levels and subjected to pre-determined pH conditions. Sediment samples and overlying water were collected for metal analysis after 10-days. The results indicated that CO2-induced ocean acidification would provoke increased metal mobilization causing adverse side-effects on water quality. The mobility of metals from sediment to the overlying seawater was correlated with the reduction in pH. Results of sequential extractions of sediments illustrated that exchangeable metal forms were the dominant source of mobile metals. Collectively, our data revealed that high metal concentrations in overlying seawater released from contaminated sediments under acidic conditions may strengthen the existing contamination gradients in Maluan Bay and represent a potential risk to ecosystem health in coastal environments.

Assessment of metal contamination in coastal sediments of the Maluan Bay (China) using geochemical indices and multivariate statistical approaches.

Total concentrations and chemical forms of heavy metals in surface sediments of Maluan Bay were determined and multiple geochemical indices and guidelines were applied to assess potential contamination and environmental risks. Metal concentrations exhibited significant spatial variation and the speciation of Cr was presented dominantly in the residual fraction, while Cd was found mostly in the non-residual fraction and thus of high potential bioavailability. Cluster analysis separated four subgroups of sampling sites with different levels of contamination. Further, a multivariate method offered the specific interpretation of possible contaminant sources and/or pathways. Factor scores characterized the sampling locations and elucidated the pollution status, pointing out the impact of multiple "hidden hotspots" of contaminants and providing further evidence of the existence of clear pollution-risk gradients in lagoon areas. The study supports the integrative approach as powerful tool to diagnose the pollution status scientifically for management decisions in coastal sediment of complex environment.