No abstract

No abstract

Waterfall tufa is widely distributed around the world, especially in tropical and subtropical karst areas. In these areas river water is generally supersaturated with respect to calcite, and the precipitation occurs mainly at waterfall and cascade sites. Development of waterfall tufa has been described as simply being the result of water turbulence. We believe, however, that three physical effects can lead to tufa deposition at waterfall sites: aeration, jet-flow, and low-pressure effects. The three physical effects are induced by two basic changes in the water: an accelerated flow velocity, and enlargement of the airwater interface area. These two changes increase the rate of CO 2 outgassing and the SI c , so that a high degree of supersaturation is achieved, which then induces calcite precipitation. These ''waterfall effects'' have been simulated in laboratory and field experiments, and each of them can accelerate, or trigger, calcite precipitation. Field measurements of river water chemistry also show that tufa deposition occurred only at waterfall sites. In these experiments and observations, waterfall effects play the most important role in triggering and accelerating CO 2 outgassing rates.Field and laboratory observations indicate that plants and evaporation also play important roles in tufa formation. Growth of algae and mosses on tufa surfaces can provide substrates for calcite nucleation and can trap detrital calcite, accelerating tufa deposition. However, the prerequisite for such deposition at waterfall sites is a high degree of supersaturation in river water, which is mainly caused by waterfall effects. Evaporation can lead to supersaturation in sprays and thin water films at a waterfall site and cause the precipitation of dissolved CaCO 3 , but the amount of such deposition is relatively small.

On the basis of concentrations of 21 major and minor elements in a well-dated sediment core from the deepwater part of Lake Erhai, principal component analysis (PCA) reveals three main factors controlling the inorganic chemical composition of lake sediments. These are physical erosion in the watershed (Component 1), autochthonous calcite precipitation in lake water (Component 2) and early diagenesis in sediments (Component 3). Variations of factor scores of Component I and Component 3 may reflect fluctuations in rainfall and temperature, respectively in the lake region. High factor scores of Component 3 correlate with low factor scores of Component 1 in sediments within the intervals AD 1340-1550 and AD 1890-1950, indicating two warm-dry episodes. Low factor scores of Component 3 correlate with high factor scores of Component 1 from AD 1550-1890, indicating a cold-wet climate corresponding to the ‘Little Ice Age’ in Europe. Our study provides a new approach to reconstructing palaeoclimate, and adds to a growing body of evidence for a widespread cooling during the ‘Little Ice Age’. The alternation between warm-dry and cool-humid conditions possibly implies different climate change characteristics between the southwest monsoon zone and the southeast monsoon zone where the climate pattern is warm-humid and cool-dry.

Fluctuations in climatic proxies of the Milanggouwan section in the Salawusu River valley of the Ordos Plateau (Inner Mongolia, China) during Marine Isotope stage 3 (MIS 3) coincide well with sedimentary cycles for palaeo-mobile dune sands alternating with fluvial-lacustrine facies and palaeosols. We compared the palaeo-mobile dune sands with modern mobile dune sands (products of a cold and dry climate dominated by the East Asian winter monsoon), whereas the fluvial-lacustrine facies and palaeosols were controlled by a wet-warm climate similar to that of the East Asian summer monsoon. The MIS 3 climate of the Salawusu River valley appears to have experienced at least nine wet-warm and ten cold-dry fluctuations, divided into five stages: MIS 3e (58 900-49500 yr BP), MIS 3d (49 500-40 700 yr BP), MIS 3c (40 700-36 900 yr BP), MIS 3b (36 900-27 000 yr BP) and MIS 3a (27 000-22 300 yr BP). The 19 cold-warm climatic fluctuations corresponded roughly to the GRIP and Guliyan records, and with fluctuations in the North Atlantic climate. Notable peaks in the spectral analysis occurred at 19 500 yr, 1020 yr, 640 yr and 500 yr. Our results show that the millennial-centennial climate was closely related to the relative strengths of East Asian monsoons, which are controlled by the North Atlantic thermohaline circulation, and which is also closely linked to the Sun's precession period.