Bacterial tetraether lipids in ancient bones record past climate conditions at the time of disposal

Assessing impacts of climate change on ancient human societies requires accurate reconstructions of regional climate variations. However, due to the scarcity of in situ climate indicators in archaeological sites, climate interpretation often relies on indirect, geographically distant data from geological archives such as lake or ocean sediments, ice cores and speleothems. Because many cultural changes occurred abruptly over periods of years to decades, and are regional or even local in scale, correlating societal changes with climate reconstructions from geological archives induces significant uncertainties: factors such as chronological dating inconsistencies and geographic heterogeneity of climate can severely undermine interpretation. Here we show, for the first time, that it is possible to determine past climate change by analyzing bacteria-derived 'branched glycerol dialkyl glycerol tetraethers' (br-GDGTs) in ancient bones from archaeological sites. To the best of our knowledge this proxy has never been applied before to bones, nor with the intention of developing the method for application in archaeological research. We demonstrate that these compounds are likely derived from bacterial growth within bones following deposition in the ground, and the potential for their distributions to reflect climate and environmental conditions during the years immediately following deposition when bacteria consume internal substrates. Our preliminary results show that bone samples from different climate zones display distinct br-GDGT distributions. Well-dated late Pleistocene and Holocene bones from Alaska yield reconstructed temperatures consistent with existing climate reconstructions. While further work is necessary to determine how quickly the signal stabilizes in the bones, and to continue ongoing refinement of calibrations for temperature, precipitation, and other influences on br-GDGTs, we propose that br-GDGTs from ancient bones in archaeological sites may be taken as a new, in situ archive for reconstructing past climate conditions. This opens new perspectives for assessing connections between climate variations and social transformations in the past.