A multimillennial climatic context for the megafaunal extinctions in Madagascar and Mascarene Islands

Abstract

Madagascar and the Mascarene Islands of Mauritius and Rodrigues underwent catastrophic ecological and landscape transformations, which virtually eliminated their entire endemic vertebrate megafauna during the past millennium. These ecosystem changes have been alternately attributed to either human activities, climate change, or both, but parsing their relative importance, particularly in the case of Madagascar, has proven difficult. Here, we present a multimillennial (approximately the past 8000 years) reconstruction of the southwest Indian Ocean hydroclimate variability using speleothems from the island of Rodrigues, located ∼1600 km east of Madagascar. The record shows a recurring pattern of hydroclimate variability characterized by submillennial-scale drying trends, which were punctuated by decadal-to-multidecadal megadroughts, including during the late Holocene. Our data imply that the megafauna of the Mascarenes and Madagascar were resilient, enduring repeated past episodes of severe climate stress, but collapsed when a major increase in human activity occurred in the context of a prominent drying trend.

Drought Coincided with, but Does Not Explain, Late Holocene Megafauna Extinctions in SW Madagascar

“…Aside from ostracods that date to~400-200 cal yr BP, ostracods relatively enriched in 18 O come from layers that tend to have low and less variable organic content. Application of BCPA to the Lake Ranobe ostracod δ 18 O and sediment Ca/Zr values revealed change points that coincide approximately with change points in other Indian Ocean records (Figure 7, Dataset S6, [16,59]).…”

“…That record probably reflects, in part, a drop in relative sea level and coastal water tables [14,15]. However, a speleothem from Rodrigues Island in the western Indian Ocean recorded at least four megadroughts over the past 2000 years that could have also affected southern Madagascar [16], and a speleothem from SW Madagascar recorded a prolonged drought 1000 years ago [9]. Lake Ranobe is a shallow (<3 m deep), closed basin, and its geochemistry was sensitive to regional aridification during the Late Holocene.…”

“…Although average rainfall δ 18 O values can differ by as much as 8‰ between wet and dry months in the central highlands of Madagascar (Antananarivo, IAEA GNIP Database), the arid SW part of the country receives rainfall almost entirely during a single wet season. Because the vapor pressure of H 2 16 O is greater than that of H 2 18 O, evaporation causes preferential loss of H 2 16 O from the lake, with consequent 18 O enrichment in lake water and associated ostracod carbonates. High E/P thus results in higher δ 18 O values in ostracod shells, and, in extreme cases, promotes authigenic carbonate precipitation, which leads to higher CaCO 3 concentration and Ca/Zr ratios in lake sediment [18,19].…”

“…Records of Lake Ranobe sediment core Ca/Zr and ostracod δ 18 O, with overall means marked by vertical lines, means between BCPA-inferred change points in red, and relatively high and low values shaded with red and blue, respectively. Regional context comes from records of diatom-inferred lake-water conductivity from Lac Ihotry, SW Madagascar (with locally weighted smoothing, [10]), a record of speleothem δ 18 O values from Asafora, SW Madagascar [9], speleothem δ 18 O values from Anjohibe, NW Madagascar [57], a variety of records from Lake Challa, Tanzania [60], speleothem δ 18 O values from Rodrigues Island [16], and a record of E Indian Ocean sea surface temperature inferred from a geochemical record from the Makassar Strait [59]. Time periods that included the Medieval Warm Period (MWP) and Little Ice Age (LIA) are shaded in gray.…”

StalGrowth—A Program to Estimate Speleothem Growth Rates and Seasonal Growth Variations

“…Highly reliable growthage models are based on accurate and precise absolute 230 Th/U-dating [4][5][6][7]. A wide range of geochemical proxies, e.g., stable oxygen isotope ratios and trace element ratios, have been interpreted as paleoclimate proxies in many settings [8][9][10][11][12][13][14].…”