Summary: Continent‐scale biogeography has been extensively studied in soils and marine systems, but little is known about biogeographical patterns in non‐marine sediments. We used barcode pyrosequencing to quantify the effects of local geochemical properties and geographic distance for bacterial community structure and membership, using sediment samples from 15 lakes on the Tibetan Plateau (4–1670 km apart). Bacterial communities were surprisingly diverse, and distinct from soil communities. Four of 26 phyla detected were dominant: Proteobacteria , Bacteroidetes , Firmicutes and Actinobacteria , albeit 20.2% of sequences were unclassified at the phylum level. As previously observed in acidic soil, pH was the dominant factor influencing alkaline sediment community structure, phylotype richness and phylogenetic diversity. In contrast, archaeal communities were less affected by pH. More geographically distant sites had more dissimilar communities ( r = 0.443, P = 0.030). Variance partitioning analysis showed that geographic distance (historical contingencies) contributed more to bacterial community variation (12.2%) than any other factor, although the environmental factors explained more variance when combined (28.9%). Together, our results show that pH is the best predictor of bacterial community structure in alkaline sediments, and confirm that both geographic distance and chemical factors govern bacterial biogeography in lake sediments.

Summer temperatures on the Tibetan Plateau (TP) significantly affect stability of glaciers that provide steady water resources to nearly half of the world population. However, lack of reliable, long‐term proxy records greatly impedes understanding of regional temperature sensitivity to climate forcings. Here we present a 16 ka long, alkenone‐based summer temperature record from Lake Qinghai, northeastern TP that demonstrates major regional temperature response to changes in summer insolation and Atlantic Meridional Overturning Circulation during the Holocene and late glacial. Importantly, we find a period of sustained summer temperature decline (>4°C) between 5 and 3.5 ka, which coincides with expansion of Barents Sea ice coverage and is likely driven by intensification of the Westerlies. This unusually long and pronounced regional cooling event likely delayed permanent human settlements on the high‐altitude regions (>3000 m) of the TP by at least 500 years.