“…These discrepancies have led to independent modern calibrations for different sample types [e.g., soils and peats (36), lacustrine sediments (37) or SPM (38), bones (39), speleothems (40), and marine sediments (23)] and regions [e.g., East Africa (41) and China (42)] and have necessitated much effort to disentangle the allochthonous versus autochthonous sources of brGDGTs to sedimentary archives (2,23,(43)(44)(45)(46)(47)(48). Further complicating the matter, brGDGT distributions are affected by a wide array of other environmental parameters, including oxygen levels (26,27,29,(49)(50)(51)(52)(53)(54), salinity/electrical conductivity (55,56), seasonality (27,36,46,57), nutrient availability (58,59), and soil chemistry (28), which can mask or override relationships with temperature or pH. Last, an observed bias in brGDGT-derived temperatures toward warmer seasons has proven difficult to quantify, with various studies finding summer air temperature (60), the mean air temperature of months above freezing (MAF) (36,37,55), growing degree days above freezing (61), or other temperature indices to provide the strongest correlations in modern training sets.…”