Lake Nam Co (Tibet, China) – a suitable target for a deep drilling project as confirmed by a preliminary airgun seismic survey

Abstract:

Lake Nam Co, located on the central Tibetan Plateau at the intersection of the Westerlies and the Indian Ocean Summer Monsoon, is well suited to study the monsoonal regime over different time scales. High-resolution and continuous sedimentary records from the Tibetan Plateau are still rare and only few reach back to the Last Glacial Maximum. For Nam Co, numerous multiproxy studies unravel the regional paleoclimate and paleoenvironmental history for the past 24,000 years. These promising results demonstrate the potential of Lake Nam Co as a geoarchive, but nature, thickness and geologic time of the sediment fill have not yet been determined.

Therefore the Institute of Tibetan Plateau Research (Chinese Academy of Sciences) and the Universities of Bremen and Jena jointly carried out an airgun multichannel seismic survey at Nam Co in June/July 2014. As main equipment, a micro GI Gun(2 x 0.1 L) was used in conjunction with a 64 m long seismic streamer (32 channels/2 m spacing) to achieve deep signal penetration, to confirm a thick sediment infill and to prove the suitability for deep coring of several hundred meters. Although only few lines could be shot due to technical and weather issues, several lines particularly from the deepest part of the lake provide new insight.

Preliminary data processing and interpretation reveal a well layered sediment cover of >700 m in the center of the lake. Seismic facies appears to vary in a cyclic manner, indicating a coupling to climatically-driven changes in lake level and sediment delivery. From a comparison with the Holocene/Late Glacial sedimentary and seismic record, several similar units could be imaged. Furthermore, rapid sedimentation is confirmed from the continuous cover of growth faults and doming, and continuous sedimentation throughout glacial/interglacial cycles appears likely due to the absence of erosional unconformities. By tentatively assigning these units to marine isotope stages, different seismostratigraphies can be developed assuming average sedimentation rates of 0.4 mm a^-1 to 1.7 mm a^-1, resp.. This compares well with results determined on a sediment core showing sedimentation rates between 0.1 and 2.4 mm a^-1. Accordingly, geologic ages of >400 or >1660 ka, respectively, may be potentially reached by drilling.