Sedimentary brGDGTs in China: An overview of modern observations and proposed land Holocene paleotemperature records

Results from globally distributed surface soils and surface lake and peat sediments indicate that brGDGTs (branched glycerol dialkyl glycerol tetraethers), derived from bacteria, have the potential to reconstruct the history of land temperature changes. Studies of brGDGTs in China have been conducted on a large spatial scale, but the proposed brGDGTs-based land Holocene temperature records show contrasting temporal patterns. In this study, we used modern observations of brGDGTs combined with the analysis of other modern climate data and paleoclimate records to re-evaluate the proposed brGDGTs-based land Holocene paleotemperature records from China. Our findings indicate that: (i) the proposed brGDGTs-based Holocene paleotemperature records from Chinese loess deposits are more possibly indicative of humidity changes. (ii) brGDGTs in lake sediments are significantly influenced not only by air/water temperatures but also by water depth, salinity, seasonal freezing and so on, and therefore the paleoclimatic significance of individual brGDGTs-based Holocene climate records from Chinese lakes needs to be carefully evaluated. The long-term Holocene warming trend indicated by brGDGTs-based records from most shallow, freshwater Chinese lakes without surface freezing are relatively dependable. (iii) Given modern observations of brGDGTs are rarely conducted in peatlands in Northeast China, we therefore suggest the long-term Holocene warming trend indicated by brGDGTs-based records from the 5 peatlands along the transect from Southeast to Northwest China is more reliable than the long-term Holocene cooling trend indicated by brGDGTs-based records from the two peatlands in Northeast China. Therefore, currently, we are roughly incline to believe the long-term Holocene warming trend in some of the Chinese lake and peat brGDGTs-based records. As our knowledge of the environmental significance of brGDGTs continues to increase, we anticipate that they can make a substantial contribution to resolving the "Holocene temperature conundrum".