Since 2006,we have conducted a geological exploration of the Khammuane area in southeastern Laos.Several thick sequences of potash deposits containing sylvite and carnallite were discovered from drilling Paleocene strata of the Thangon Formation(E1tg).To constrain the origin of the brine that formed thick sequences of salt mineral deposits,we measured the strontium and boron isotope composition of some salt minerals in the potash deposit.Boron isotope composition of halite-and potash-associated minerals in the Khammuane potash deposit varied from 19.91%to 31.01‰,which differs significantly from continental evaporates(-6.83‰--5.79‰)but is very close to that of salts precipitated from evapo-concentrated seawater(10‰-36‰).The 87Sr/86Sr ratios(0.707542 to 0.709461)of salts were also close to the values of Paleocene seawater(0.70772 to 0.707830).The content of Br-(0.01%-0.25%)and Br×103/Cl ratios(0.10-0.52)of halite were>10 times higher than that of common continental salts.These characteristics indicate clearly that the Laos potash deposits were deposited from seawater.The small variation in boron and strontium isotopes in all salt sedimentary sequences(gypsum-halite-potash-halite)also suggests that the recharge source should be stable and the effects of other continental water mixing are negligible.Large-scale potash deposits are commonly precipitated from seawater.Hence,the results of recent regional drilling work and geochemical research suggest extensive potash resources could occur in the Thangon Formation of southeastern Laos.
TAN HongBingMA HaiZhouLI BinKaiZHANG XiYingXIAO YingKai
Traditionally, the mid-Holocene in most parts of China was thought to be warmer with higher precipitation, resulting from a strong Asian summer monsoon. However, some recent researches have proposed a mid-Holocene drought interval of millennial-scale in East Asian monsoon margin areas. Thus whether mid-Holocene was dry or humid remains an open issue. Here, Zhuyeze palaeolake, the terminal lake of the Shiyang River Drainage lying in Asian monsoon marginal areas, was selected for reconstructing the details of climate variations during the Holocene, especially mid-Holocene, on the basis of a sedimentological analysis. Qingtu Lake (QTL) section of 6.92m depth was taken from Zhuyeze palaeolake. Multi-proxy analysis of QTL section, including grain size, carbonate, TOC, C/N and δ13C of organic matter, was used to document regional climatic changes during 9-3 cal ka B.P. The record shows a major environmental change at 9.0-7.8 cal ka B.P., attributed to a climate trend towards warmth and humidity. This event was followed by a typical regional drought event which occurred during 7.8-7.5 cal ka B.P. And a warm and humid climate prevailed from 7.5 to 5.0 cal ka B.P., attributed to the warm/humid Holocene Optimum in this region. After that, the climate gradually became drier. Moreover, comparison of the climate record from this paper with the summer insolation at 30°N indicates that the climate pattern reflecting the Asian monsoon changes was caused by insolation change.
