Stratigraphy and otolith microchemistry of the naked carp Gymnocypris przewalskii (Kessler) and their indication for water level of Lake Qinghai during the Ming Dynasty of China

Abstract

Otoliths are biogenic carbonate minerals in the inner ear of teleost fish, whose compositions can record the physical and chemical conditions of the ambient water environment inhabited by individual fish. In this research, the fishbones and otoliths of naked carp sampled near the Bird Island, offshore Lake Qinghai, were dated and analyzed for mineralogy and microchemical compositions. Comparing the microchemical compositions of ancient otoliths with those of modern otoliths, we conclude that the ancient naked carps inhabited a relict lake formed when the lake shrank from a high lake level, by combining with the AMS-14C ages of fishbones and otoliths, the stratigraphy and surrounding topography of the sample site. AMS-14C dating results of ancient fishbones and otoliths show that these naked carps lived from 680 to 300 years ago, i.e. during the Ming Dynasty of China. The X-ray diffraction (XRD) patterns demonstrate that the ancient lapillus is composed of pure aragonite, identical to modern one, indicating that the mineral of lapillus didn’t change after a long time burial and that the ancient lapillus is suitable for comparative analysis thereafter. Microchemical results show that both ratios of Mg/Ca ((70.12±18.50)×10−5) and δ18O ((1.76±1.03)‰) of ancient lapilli are significantly higher than those of modern lapilli (average Mg/Ca=(3.11±0.41)× 10−5 and δ18O=(−4.82±0.96)‰). This reflects that the relict water body in which the ancient naked carp lived during the Ming Dynasty was characterized by higher Mg/Ca and δ18O ratios than modern Lake Qinghai, resulting from strong evaporation after being isolated from the main lake, similar to today’s Lake Gahai. Based upon the stratigraphy and altitude of naked carp remains, it can be inferred that the altitude of lake level of Lake Qinghai reached at least 3202 m with a lake area of 4480 km2 during the Ming Dynasty, approximately ∼5% larger than it is today.