DIAT-17

Relative atmospheric humidity (RH) is a major climate parameter that is poorly estimated by global climate models. Comparisons between models and data for the pre-instrumental period are necessary to improve the models but are hampered by the lack of truly quantitative markers of past RH. Over the past 15 years, an international effort has been made to develop the use of the triple oxygen isotopic composition of the water molecule (expressed as d¹⁸O, d¹⁷O and ¹⁷O-excess) as a marker for evaporative fractionation, where relative atmospheric humidity is the main driver.

In the DIAT-17 proof-of-concept project, we aim to determine whether the triple oxygen isotopic composition of diatom frustules can be used to record the triple oxygen isotopic composition of lake water, inform the extent of evaporation in the lake's hydrological balance, and have the potential to complete the toolbox of quantitative proxies of past atmospheric relative humidity.

To this end, the triple oxygen isotopic composition of modern diatoms and lake water samples from four West and Central African lakes will be studied and the fractionation (in the triple oxygen isotopic system) between water and diatoms will be defined. One of the lakes has been equipped for full hydro-meteorological monitoring which will allow us to use the Craig & Gordon model to estimate the evolution of the excess d¹⁸O vs ¹⁷O of the lake water with evaporation and relative humidity. The data and estimates will be compared.

The result will be used to discuss the¹⁸O and ¹⁷O-excess diatoms from a sediment core covering the African wet period (11000-5000 cal BP yr).