Moisture availability in the Tangra Yumco region during the Late Quaternary and the implications for the precipitation regime on the southern Tibetan Plateau

It is generally accepted that the Asian summer monsoon systems as well as the westerlies affect the precipitation regime on the Tibetan Plateau. The spatial and temporal variability of the monsoon has, however, been under strong discussion in recent years. The question arose whether the spatial and temporal monsoonal dynamics show a generally homogenous or heterogeneous pattern. To investigate the temporal and spatial variability of the monsoonal pattern three lakes along an east-west-transect on the southern Tibetan Plateau including Nam Co, Tangra Yumco, and Taro Co were targeted within the framework of the DFG priority program 1372 “TiP – Tibetan Plateau: Formation–Climate–Ecosystems”. This thesis comprises three independent studies, examining the central lake Tangra Yumco and its catchment revealing that the monsoonal dynamics on the southern Tibetan Plateau show a generally homogenous pattern

High‐resolution paleomagnetic and sedimentological investigations on the Tibetan Plateau for the past 16 ka cal B.P.—The Tangra Yumco record

The spatial distribution of paleomagnetic secular variation (PSV) records on the Tibetan Plateau and adjacent areas covering the Holocene and Late Glacial is sparse. In order to reconstruct PSV in this area, a piston core covering the past 17.5 ka cal B.P. retrieved from Lake Tangra Yumco, southern‐central Tibetan Plateau, was analyzed. In the laminated sediments, several event layers are intercalated. Those were identified by sedimentological analysis and excluded for age‐depth modeling and interpretation. Paleomagnetic measurements on u‐channels reveal two contrasting core sections. The lower section (dated to 17.5–15.9 ka cal B.P.) is very coarse grained (up to 220 µm) and characterized by low intensities (0.8 mA m−1) and high maximum angular deviation values (mean 25°), making it unsuitable for PSV reconstruction. In contrast, the upper unit (dated to <15.9 ka cal B.P.) yields ideal properties with a well‐defined magnetization carried by low‐coercivity minerals in the pseudo single domain state making those younger sediments a proper record for PSV studies. The robustness of the PSV reconstruction for the past 3000 years is highlighted by a comparable inclination and declination pattern of three short sediment cores (2 m) from Tangra Yumco. On a regional scale, the obtained inclination signal for the past 15.9 ka cal B.P. is in good agreement with the Lake Baikal record (3000 km to the North), PSV stack for East Asia, as well as with predictions of geomagnetic field models. This study is a step forward in constructing a PSV reference curve for central Asia. -- Keywords : paleomagnetic secular variations (PSV) ; magnetostratigraphy ; Tibetan Plateau ; lake sediments ; Tangra Yumco ; inclination

Geochemistry and grain size analysis of the Tangra Yumco lake sediment record

A possible asynchronicity of the spatial and temporal moisture availability on the Tibetan Plateau was controversially discussed in recent years. Here we present the first attempt to systematically investigate possible spatial and temporal variations of moisture availability by examining two lakes, Tangra Yumco and Nam Co, on an east-west-transect on the southern Tibetan Plateau using identical proxies for palaeoenvironmental reconstruction. In this study, an independent record from Tangra Yumco was analyzed applying a multi-proxy approach to reconstruct variations in moisture availability since the Lateglacial. Results were subsequently compared to previously published records from Nam Co and additional records from Tso Moriri (northwestern Himalaya) and Naleng Co (south-eastern Tibetan Plateau). Our results show that Tangra Yumco was at least partially ice-covered prior to 17.1 cal. ka BP. A temperature rise after 17.1 cal. ka BP probably resulted in thawing of the permafrost. At 16.0 cal. ka BP moisture availability increased representing an initial monsoonal intensification. Warmer conditions between 13.0 and 12.4 cal. ka BP and cooler conditions between 12.4 cal. ka BP and the onset of the Holocene reflect the Bølling-Allerød and Younger Dryas. At the onset of the Holocene moisture availability rapidly increased, while moisture was highest prior to 8.5 cal. ka BP concurrently with highest temperatures. After 8.5 cal. ka BP the moisture availability gradually decreased and showed only minor amplitude variations. These findings are well in phase with the records from other large lakes likes Nam Co, Tso Moriri, and Naleng Co revealing a synchronous pattern of moisture availability on the southern Tibetan Plateau

Age determination of Tangra Yumco lake sediments

Massive carbonate banks representing ancient lacustrine deposits are exposed in the catchment of the lake Tangra Yumco (southern-central Tibetan Plateau) and nearby lake Xuru Co. Nine sediment samples were taken below and above these lacustrine deposits to determine periods of changing lake level using optically stimulated luminescence (OSL) applying a multiple aliquot regeneration (MAR) protocol. According to facies and stratigraphy samples below the carbonate banks indicate a rising and samples from above a falling lake level. The results indicate that the rising lake level of Tangra Yumco passed the sampling location at 10.5 ka and 2.1 ka while a falling lake level passed the sampling location at 0.9 ka. The rising lake level of Xuru Co passed the sampling location 7.9 ka and 1.7 ka and a falling lake level passed the sampling location at 0.5 ka showing a similar trend. Combining these results with recalculated cosmogenic nuclide ages and previously published feldspar luminescence data allow the establishment of an improved Holocene lake level reconstruction for Tangra Yumco, which is unique for the southern-central Tibetan Plateau. Within the last 10.5 ka the lake level of Tangra Yumco crested a lake level highstand of 181-183 m above the recent lake level at 8.5 ka and has generally fallen since, with a minor lake level rise at 2.1 ka. Lake level variations at Tangra Yumco occur simultaneously with other lakes on the Tibetan Plateau indicating that variations were controlled by monsoonal dynamics with a moist early Holocene and a successive reduction of available moisture thereafter. The precipitation directly reaching the lake via runoff that was required for the lake level rise between 10.5-8.5 ka is 9 mm a-1 (compared to a modern value of 35 mm a-1), providing valuable insights in the magnitude of monsoonal variations on the southern central Tibetan Plateau