The Holocene salinity history of Lake Lop Nur (Tarim Basin, NW China) inferred from ostracods, foraminifera, ooids and stable isotope data

Terminal lakes without outlet respond directly to climate change and human impact, and provide important evidence for environmental conditions prior to times of instrumental monitoring. Lop Nur in northwestern China is a terminal lake which was still one of the world's largest lakes in historical times. Sediments from the excavated section YKDO301 in its presently dry basin were investigated to reconstruct the Holocene salinity history of the lake. Ostracod shells of Cyprideis torosa and Eucypris mareotica, tests of the foraminifer Ammonia tepida, and fruits of Ruppia maritima are abundant in the early Holocene part of the section. Two stratigraphic units in the middle Holocene part of the section contain ostracod shells typical of fresh to mesohaline waters such as Limnocythere inopinata, Cypridopsis vidua, Ilyocypris sp. and Darwinula stevensoni. Three units do not contain fossils or only few remains regarded as allochthonous. Ooids are abundant in the upper half of the section, and are here reported for the sediments of Lop Nur or sub-recent sediments from China for the first time. Ooids from YKDO301 are aragonitic and either subspherical around a detrital nucleus, or elongate and probably formed around fecal pellets of brine shrimps. The recorded fossils from Lop Nur indicate that lake waters were poly- to hyperhaline (20-100 parts per thousand) at ca. 9.0 ka, oligo- to mesohaline (0.5-18 parts per thousand) between ca. 8.7-7.5 ka and probably mesohaline (5-18 parts per thousand) from 6.0-5.0 ka. In the intervening periods and after 5.0 ka, Lop Nur was a hyperhaline (> 100 parts per thousand) lake. The period of freshest conditions of Lop Nur (ca. 8.7-7.5 ka) coincides with wettest Holocene conditions reconstructed from other lake records in the region although a uniform temporal pattern of wettest Holocene climate conditions in NW China and adjacent regions cannot be inferred.