A 3.5–YEAR RAINFALL ISOTOPE RECORDS FROM NORTHWESTERN MADAGASCAR FEATURING 17OEXCESS AND IMPLICATION FOR PALEOCLIMATE RESEARCH (Invited Presentation)

Mahajanga, in northwestern Madagascar, has become a location of interest to study paleoclimate using isotope geochemistry from cave deposits like stalagmites. However, the lack of regional rainfall isotope data leaves a gap in the paleorecords calibration and limits our understanding of the transfer function between these climatic signals to the cave archivers. Doing such kind of work is expensive and often fails due to the remoteness of the location and the lack of expert to lead and to follow up on the research. This paper reports the first 3.5–year rainfall stable isotopes of hydrogen (d²H and d-excess) and oxygen, (d¹⁷O and d¹⁸O), highlighting a novel ¹⁷O_excess proxy from Mahajanga to paleoclimate interpretation in this region. The results are cross-checked with model simulations using ERA5T reanalysis and HYSPLIT back trajectory models. The Mahajanga data was compared with data from Antananarivo (located in the central highlands, about 553 km driving distance from Mahajanga), the only prior location with available rainfall isotope dataset in Madagascar, for a regional context.

While the amount effect is widely known to explain the decrease in d¹⁸O and d²H with increased rainfall amount during the austral visit of the Inter-Tropical Convergence Zone (ITCZ) in Madagascar, the conventional d-excess and the novel ¹⁷O_excess provide additional insights on hydrological processes, with a possibility to better constrain the effects of the latitudinal migration of the ITCZ, such as regional aridification or a change in the source of moisture. Combining themeasured and simulations dataset, these rainfall isotopic records suggest strong local processes that includes evaporation and condensation, specifically at the onset and termination of a rainy summer season, and changes in moisture source.

With Mahajanga storing a great sources of paleoclimate archives, such as stalagmites, knowledge from studying local rainfall stable isotope signals, specifically ¹⁷O_excess, is valuable to further our understanding of their transfer function into caves, where paleoclimate data are retrieved. This could be an important step to better interpret paleoclimate and hydrological process over longer time scale in the past.