Shrinking lakes in Tibet linked to the weakening Asian monsoon in the past 8.2 ka

Expansion or shrinkage of closed lakes is a natural response to fluctuations in precipitation and evaporation, linked closely to changes in strength or position of atmospheric circulation. In Tibet, there are many such lakes with paleo-shorelines that can be used for reconstructions of climate history. Despite the fact that many paleo-shorelines are well preserved in Tibet, dating them has been seriously hindered by various difficulties. Here we present the first optical dating chronology for a series of paleo-shorelines in Zhari Namco, the third-largest inland lake in central Tibet. Our results indicate that the lake level has dropped 128 m over the past 8.2 ka. Younger shorelines are found at lower altitudes, indicating that the shorelines follow a geomorphic-chronological order and a broadly continuous trend of stepwise shrinkage. The surface area of Zhari Namco has shrunk in size from 4605 km² at 8.2 ka ago to 996 km² at present; 300 km³ of water has been lost from this lake. Such a loss in water implies a significant reduction in precipitation over the past 8.2 ka, a likely result of a weakening Asian monsoon. Following the decreasing precipitation since the early Holocene, this area has become increasingly arid.     

Geochemical evolution within a Devonian intra-oceanic island arc: The Gamilaroi terrane, southern New England orogen, Australia

Abstract Geochemical analyses of volcanic rocks in the Gamilaroi terrane reveal several phases of arc activity within an intra-oceanic island-arc terrane. Felsic volcanic rocks at the base of the section have rare earth element (REE) and trace element compositions which indicate that they were derived from an island-arc source. Basalts immediately overlying the felsic volcanic rocks have a distinctive geochemical signature with low levels of Ti and Y and high levels of Ni, Cr and Mg. Low concentrations of REE and trace elements relative to mid-ocean-ridge basalts (MORB) indicate that they were also derived from an intra-oceanic island-arc source. Extensive basalts and basaltic andesites among the youngest rocks of the terrane have typically flat to enriched REE and trace element compositions, indicating a transitional arc-back-arc source. The change in basalt compositions indicates that rifting had occurred by this stage in the evolution of the arc. Confirmation of an intra-oceanic setting for this terrane enables a more detailed comparison with similar intra-oceanic rocks in the northern New England orogen. This study of the Gamilaroi terrane is an example of the potential use of geochemical data to identify other ancient intra-oceanic island-arc-rift suites.