The occurrence of hydromagnesite in bottom sediments from Lake Siling, central Tibet: implications for the correlation among δ18O, δ13C and particle density

A core (305 cm long) taken from Lake Siling in central Tibet is subdivided on the basis of carbonate composition into a dolomite zone (275–255 cm depth), a dolomite–aragonite zone (255–245 cm depth), an aragonite zone (245–147.5 cm depth) and a hydromagnesite–aragonite zone (147.5–30 cm depth). Most core samples contain clay minerals, quartz and low-Mg calcite. Six samples on terraces near the lakeshore consist of hydromagnesite with heavy O and C stable isotopic ratios and low density. The hydromagnesite–aragonite zone is characterized by a strong correlation among δ18O (−4.9–2.3‰), δ13C (5.5–8.0‰) and particle density (2.40–2.68 g/cm3). This occurs when carbonates from the underlying aragonite zone, characterized by lighter O and C isotopic ratios and high density, are plotted against terrace samples of hydromagnesite. The correlation in the hydromagnesite–aragonite zone is attributed to the mixing of hydromagnesite with the aragonite zone sediments in varying proportions. When compared with available 14C TAMS ages, it appears that hydromagnesite was added to bottom sediments at about 5000 years bp. At that time, the lake began to shrink and formed lacustrine terraces because evaporation dominated over precipitation around the lake. The depositional rate of the hydromagnesite–aragonite zone (0.6 mm/year) was about six times faster than that of the aragonite zone (0.1 mm/year) due to formation of hydromagnesite within an arid climate. The plausible sources of four carbonate minerals are as follows: (1) aragonite is an inorganic carbonate from the lake water, (2) hydromagnesite is an evaporitic carbonate on terraces, (3) dolomite is a diagenetic carbonate and (4) low-Mg calcite is a detrital carbonate. [Copyright &y& Elsevier]