Unusually low TEX86 values in the transitional zone between Pearl River estuary and coastal South China Sea: Impact of changing archaeal community composition

• GDGTs and 16S rRNA gene were examined in SPM and surface sediments.
• Unusually low TEX86-derived temperatures were observed in the transitional zone.
• TEX86 was positively correlated with GDGT-2 and GDGT-3 in the transitional zone.
• Archaeal community composition may be the key factor to affect TEX86.

TEX86, calculated based on the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), is used worldwide for paleotemperature reconstruction in marine and lacustrine environments. Recently, however, increasing evidence showed that TEX86 could be affected by multiple environmental variables. In this study, TEX86 was calculated for core and polar GDGTs obtained from water column and surface sediments in the lower Pearl River and its estuary. Together with previously published core GDGT data from the coastal and open South China Sea (SCS), a comparison was made between TEX86-derived and satellite-based surface water temperature, which showed a TEX86-temperature minimum that is considerably lower than winter satellite temperature in the transitional zone between the Pearl River estuary and the coastal SCS. TEX86 showed significantly positive correlation with GDGT-2 and GDGT-3 in this transitional zone, indicating that they are the primary compounds for the TEX86 bias toward cooler temperature. Lipid and molecular DNA data both indicate that the variation in archaeal community composition rather than water depth or seasonality is likely the crucial factor causing the deviation of TEX86 in the transitional area. Our study has implications for studies in ancient continental margins where unusually low TEX86 temperatures may be observed in the geological record.