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Abstract: Carbonate minerals, oxygen and carbon isotopes, and Mg/Ca from surface sediments and soils were examined in Nam Co Lake, Central Tibet. This paper provides information of the irregular distribution and formation of monohydrocalcite, a metastable mineral. Carbonates of soils surrounding Nam Co Lake consist of calcite with minor dolomite. Carbonates in surface sediments in Nam Co Lake consist mainly of calcite and monohydrocalcite. All of the monohydrocalcite and most of the calcite in surface sediments are authigenic. The monohydrocalcite is distributed irregularly in Nam Co Lake because of inhomogeneous features of lake water, the variable micro-environment, high precipitation rate, and/or nucleation. Mg ions could play a greater role in the precipitation of monohydrocalcite than that of phosphate ions, because the amount of phosphate ions could be much lower than Mg ions in the 4781 m-altitude altiplano lake with poor nutrients. The presence of monohydrocalcite occurs at 2.1 ka BP, corresponding with the sharp increase of precipitation rate from 0.134 mm/y to 1.639 mm/y. High precipitation rate and/or nucleation is necessary for water molecule infiltration into the crystal lattice of monohydrocalcite. The δ18OV-PDB and δ13CV-PDB of carbonate minerals mixed with calcite and monohydrocalcite range from −5.3‰ to −8.0‰ with an average of −6.5‰, and from 2.8‰ to 4.5‰ with an average of 3.73‰, respectively. δ18OV-PDB and δ13CV-PDB of calcite range from −8.0‰ to −10.6‰ with an average of −8.09‰, and from 2.8‰ to 4.2‰ with an average of 3.25‰, respectively. Monohydrocalcite possibly has more positive values of δ18O and δ13C than those of calcite. [Copyright &y& Elsevier]

Different from the ITCZ-driven global tropical monsoon, the development of a monsoon-dominated climate within previously subtropical arid Asia has conventionally been regarded as marking the onset of a modern-like monsoon over East Asia. This monsoon has increasingly been reported as having appeared in the Eocene or older and linking with different forcings. Here we present robust climatic records from well-dated fluviolacustrine sediments from the eastern central Tibetan Plateau which demonstrate that such a monsoon was not established in the northern subtropical area (∼32-33oN) until at least ∼35 Ma, rather a Mediterranean-type-like climate toward drier prevailed. Numerical modelling supports that neither the existence of an Andes-like Gangdese Mts. along Tibet southern margin, nor an elevated south-central Tibet, nor an increased atmospheric CO 2 concentration, were sufficient to drive the expansion of the Asian tropical monsoon northward to ∼32oN in Asia in the Eocene. • The Paleogene succession on the eastern central Tibetan Plateau was determined at ∼53–35 Ma. • Asian monsoon did not expand to northern subtropical East Asia before 35 Ma. • Forcing factor for late Eocene monsoon could not enhance monsoon to expand to northern subtropics. [ABSTRACT FROM AUTHOR]