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Abstract:
Grain-size distributions offer powerful proxies of
past environmental conditions that are related to sediment
sorting processes. However, they are often of multimodal
character because sediments can get mixed during deposition.
To facilitate the use of grain size as palaeoenvironmental
proxy, this study aims to distinguish the main
detrital processes that contribute to lacustrine sedimentation
across the Tibetan Plateau using grain-size end-member
modelling analysis. Between three and five robust grain-size
end-member subpopulations were distinguished at different
sites from similarly–likely end-member model runs. Their
main modes were grouped and linked to common sediment
transport and depositional processes that can be associated
with contemporary Tibetan climate (precipitation patterns
and lake ice phenology, gridded wind and shear stress data
from the High Asia Reanalysis) and local catchment configurations.
The coarse sands and clays with grain-size modes
> 250 μm and < 2 μm were probably transported by fluvial
processes. Aeolian sands ( 200 μm) and coarse local dust
( 60 μm), transported by saltation and in near-surface suspension
clouds, are probably related to occasional westerly
storms in winter and spring. Coarse regional dust with modes
25 μm may derive from near-by sources that keep in longer
term suspension. The continuous background dust is differentiated
into two robust end members (modes: 5–10 and 2–
5 μm) that may represent different sources, wind directions
and/or sediment trapping dynamics from long-range, upperlevel
westerly and episodic northerly wind transport. According
to this study grain-size end members of only fluvial
origin contribute small amounts to mean Tibetan lake
sedimentation (19±5 %), whereas local to regional aeolian
transport and background dust deposition dominate the clastic
sedimentation in Tibetan lakes (contributions: 42±14%
and 51±11 %). However, fluvial and alluvial reworking of
aeolian material from nearby slopes during summer seems
to limit end-member interpretation and should be crosschecked
with other proxy information. If not considered as
a stand-alone proxy, a high transferability to other regions
and sediment archives allows helpful reconstructions of past
sedimentation history.
