A 3.5-year rainfall isotope records from northwestern Madagascar featuring 17Oexcess and implication for paleoclimate research

Abstract

The lack of modern rainfall stable isotope datasets can leave a knowledge gap in paleoclimate reconstructions using climate sensitive archivers like stalagmites. This is the case of the growing number of paleoclimate records from Anjohibe Cave, where no local rainfall data has been available. This paper reports the first modern rainfall stable isotopes of hydrogen (δ²H and d-excess) and oxygen (δ¹⁷O and δ¹⁸O), highlighting a novel ¹⁷O_excess proxy, spanning 3.5 years of collection from Mahajanga, the closest city of Anjohibe. These data are compared to the only prior existing rainfall isotope data from Madagascar, which was collected from the capital, Antananarivo. Interpretation of the results was helped with model simulations using ERA5T reanalysis and HYSPLIT back trajectory models. Overall, the rainfall data provide better insights on hydrological processes in Mahajanga, including a better constrain on the latitudinal migration of the Inter-Tropical Convergence Zone (ITCZ) and its impact on the kinetic and equilibrium isotopic effect that is best observed in both d-excess and ¹⁷O_excess.

Results show that rainfall δ¹⁸O (ranging between −7.9 and + 3.9‰, vs. VSMOW) and δ²H (ranging between −49.8 and + 19.7 ‰, vs. VSMOW) from Mahajanga fit within the Global Meteoric Water Line, and they are best explained by the amount effect. However, both d-excess and ¹⁷O_excess are kinetically affected by evaporation for months receiving <1000 mm of rainfall. These months represents the onset and termination of a rainy summer season, when the ITCZ is positioned further north. Above 1000 mm, both values plateaued at ∼15 ‰ and at ∼20 per meg, respectively, regardless of the amount. These values seem to represent the regional equilibrium value for d-excess and ¹⁷O_excess in Mahajanga, when the air is moist during the southward visit of the ITCZ. It is also noted that Mahajanga ¹⁷O_excess values are at the lower spectrum relative to the global ¹⁷O_excess values (20–30 per meg), which are currently biased toward more northern hemisphere datasets. Anjohibe Cave drip waters (varying between −5.6 and −2.1‰ vs. VSMOW for δ¹⁸O and between −33.9 and −8.3 ‰ vs. VSMOW for δ²H), on the other hand, reflect more mixing of the rainfall values due to epikarst storage effects. Calculation of the isotopic fractionation factor between stalagmite and the drip waters in Anjohibe Cave also suggests a good fit with the global fractionation value, 1000ln¹⁸α = 16.89 ± 0.62 × 10³/T(K) – 27.41 ± 2.14, and serves as an excellent motivation to develop future ¹⁷O_excess records from stalagmites in the region.