| 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 experienced by individual fish. In this research, the fishbones and otoliths of ancient naked carps 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 concluded that the ancient naked carps inhabited in 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 OSL (optically stimulated luminescence) ages of the sediments overlaying the fishbone layers, and with the stratigraphy and surrounding topography of the sample site.The AMS-14C dating results of the ancient fishbones and otoliths showed that these naked carps lived from680to300years ago. This age duration was confirmed by consistent OSL ages of the overlain sediments dated to about230to420calendar years. Combining the results from two independent dating methods, the ages extending across the Ming Dynasty when the ancient naked carp lived were reliable.The X-ray diffraction (XRD) patterns demonstrated that the mineral of ancient lapillus was pure aragonite as same as the modern one, indicating that the mineral of lapillus didn’t change after a long time burial and that the ancient lapillus was suitable to comparative analysis thereafter. Microchemical results showed that the Mg/Ca ratio of the ancient lapillus (70.12±18.49×10-5) was significantly higher than that of modern lapillus (3.11±0.41×10-5). This reflected that the Mg/Ca ratios of the water inhabited by ancient naked carp were obviously higher than that of modern Lake Qinghai, which resulted from strong evaporation. The nearly equal Sr/Ca ratios of ancient and modern lapillus (average values were2.34±0.17×10-3and2.27±0.29×10-3, respectively) might reflect the similar Sr/Ca ratios of waters in the relict and modern lakes caused by the precipitation of authigenic carbonates. Meanwhile, both ratios of Li/Ca and B/Ca of the ancient lapillus (average values were3.88±0.47×10-6and1.99±0.37×10-5, respectively) were higher than those of modern one (average ratios of Li/Ca and B/Ca were3.03±0.49×10-6and1.35±0.33×10-5, respectively), which might reflect that the water in the relict lake where the ancient naked carp lived was enriched in both Li and B after its isolating from the main lake, or might relate to the low water temperature. The Ba/Ca ratios of ancient lapillus were somewhat higher than those of the modern one on the whole (average values were7.77±2.67×10-5and4.99±1.18×10-5, respectively), and the small difference could be related to the similar uptake of Ba by fish or to the somewhat enrichment of Ba in the relict lake. The (513C value of the ancient lapillus (-10.06±0.41‰) was significantly lower than that of the modern one (-4.05±1.35‰), probably because the water where the ancient naked carp lived had a small quantity of organisms or the water was shallow. The δ18O values of the ancient lapillus (1.76±1.03‰) was significantly higher than that of modern lapillus (1.76±1.03‰), which reflected the relict lake in which the ancient naked carp lived experienced strong evaporation resulting in high δ18O values water. The chemical compositions of the ancient lapillus reflected that the relict water body in which the ancient naked carps lived during the Ming Dynasty experienced strong evaporation after isolating from the main lake, similar to today’s Lake Gahai. Based upon the stratigraphy and altitude of the naked carp remains, we inferred that the altitude of lake level of Lake Qinghai reached at least3202m with a lake area of4480km2during the Ming Dynasty, which was～5%larger than that of today. |
PDF Full Text Request