试论利用青海湖介形虫体长定量恢复湖水古盐度的可靠性

Discussions on Feasibility of Paleo-salinity Reconstruction of Lake Qinghai Water by Variations in Body Length of Ostracod Valves

以青海湖东南部湖盆钻取的长475 cm沉积岩芯(QH-2005)为材料,基于6个层位14C年代(4个总有机碳(BOC)样品年代和2个木质素样品年代)建立了晚冰期以来QH-2005钻孔的"深度—年代"模型,并对BOC样品14C年代的硬水效应(732～2475年)进行了校正。挑选沉积物中成年介形虫(Limnocythere inopinata或Eucypris inflata)壳体,对其测定了δ18O值;同时测定了L.inopinata壳体体长,在此基础上利用经验公式定量恢复了青海湖湖水古盐度;此外还对QH-2005钻孔沉积物进行了红度指标的测定。结果表明:QH-2005孔沉积物红度和介形虫壳体δ18O指标所揭示的晚冰期以来气候演化模式与前人研究结果基本一致,而L.inopinata壳体体长定量重建的湖水古盐度只在较长时间尺度上(对应较大的湖水盐度变化)与气候演化模式较为一致,但在千/百年尺度上并不能很好指示气候突变事件,可见青海湖湖水盐度是L.inopinata壳体体长变化的一个重要影响因素,但并非唯一的决定性因素。

Lake Qinghai, the largest closed-basin brackish lake in China, is located in the northeastern margin of Ti- betan Plateau. The authors studied a 475 cm long sediment core （ QH-2005 ） retrieved from the southeastern part of Lake Qinghai The six chronological constraints for core QH-2005 include： four 14C dates on bulk organic carbon （BOC） andtwo on lignin. The inferred changing ＂lake reservoir effect＂ was used to correct the older BOC dates. The corrected BOC dates, together with the lignin dates were converted into calendar ages to yield the ＂depth-age＂ model for core QH-2005. We randomly collected adult ostracod （Limnocythere inopinata or Eucypris inflata ） valves from the sediment and measured the body length for 30 L. inopinata valves, which were later statistically processed to yield its average val- ue. For sections without enough L. inopinata valves in core QH-2005, E. inflata valves were used to instead. In ad- dition, sediment redness was also determined for Core QH-2005. Ostracod δ18O values for Core QH-2005 were revealed to be mainly related to dilution and evaporative enrichment of the lake water and hence can be further used to indicate enhanced monsoonal precipitation of Lake Qinghai area when negative ostracod δ18O values were observed. High redness values indicate increased riverine supply to Lake Qinghai associated with increased monsoon rainfall. The results of ostracod δl8O and redness show that： climate dur- ing 13,500 - 10,900 cal a BP was relatively cold and dry with frequent short-term fluctuations; warm and wet climate began at about 10 900 cal a BP and culminated around 6 500 cal a BP as a result of monsoon strengthening; climate became cold and dry afterwards and has remained relatively stable since 3 400 cal a BP. The reconstructed salinity based on ostracod body length coincides well with the above changing pattern. Generally, the reconstructed lake water salinity shows evident decreasing trend with considerable fluctuations in late glacial, synchronous to the amelioration of monsoon climate; the salinity displays overall increasing trend since late Holocene, coinciding with the deduced dete- rioration of monsoon climate. However, on millennial/centennial timescales, the paleo-salinity based on ostrocad body length is not in accord with the short-term climatic fluctuations （ including ： ice-rafting events 4, 3 and 2 （ by ~ 5 900 cal a BP, -4 300 cal a BP and -3 100 cal a BP respectively） and Little Ice Age （by -500 cal a BP） ） deduced by ostracod δ18O and redness results. We speculate that lake water salinity was one of the most important factors influ- encing the L. inopinata body length, but not necessarily the only one factor. Other factors including water tempera- ture, water depth and water chemical composition could as well exert certain influences on the micro-habitat of ostra- cod and hereby in turn give rise to changes in ostracod body length. Therefore more studies are needed to make clear the mechanism of environmental factors influencing ostracod body length before it can be used to reconstruct environ- mental variables.