The African Humid Period, rapid climate change events, the timing of human colonization, and megafaunal extinctions in Madagascar during the Holocene: Evidence from a 2m Anjohibe Cave stalagmite

Abstract

Stalagmite ANJ94-5 from Anjohibe Cave in northwest Madagascar suggest six distinct climate periods from 9.1 to 0.94 ka. Periods I and II (9.1-4.9 ka) were wetter and punctuated by a series of prominent droughts. Periods IV-VI (4-0.94 ka) were much drier and less variable. Period III (4.9-4 ka) marks the transition between wetter and drier conditions and consists of two significant droughts: the first (4.8-4.6 ka) coincides approximately with the end of the African Humid Period and the second (4.3-4.0 ka) may be the expression of the Northern Hemisphere 4.2 ka dry event in northwest Madagascar. Strong positive correlations between δ 13 C and δ 18 O values in Periods I-IV (r = 0.63-0.91) suggest that both isotopes were influenced by natural climate changes indicating that humans may not have been present in the area. In contrast, during Periods V (r = 0.07) and VI (r = −0.12) the "decoupling" of δ 13 C and δ 18 O might signal an impact from human activities starting around 2.5 ka. Rapid changes in climate during the early and middle Holocene, with prominent droughts lasting up to 800 years, did not kill off Madagascar's megafauna, and neither did a human population, present since the early Holocene if evidence from south Madagascar is reliable. However, many extinctions occurred under the more stable climatic conditions of the late Holocene, despite an antiphase climate relationship between northern and southcentral Madagascar. This suggests that initial human colonization, or significant increase in human population, triggered the megafaunal extinctions by hunting and destruction of megafaunal habitats.

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

“…In summary, although no consistent picture of hydroclimate variability over the past one to two millennia has yet emerged from the available suite of speleothem  18 O records from Madagascar, we tentatively assign higher confidence to our ABC-1  18 O record because it shows a higher degree of replication not only with the AB2 (26) but also with the LAVI-15-7  18 O records from Rodrigues, despite the latter being located 1600 km east of Madagascar. We also note that nearly all speleothem records from Madagascar covering this period are marked by the presence of aragonite layers (26)(27)(28)(29), which typically form under drier conditions (Fig. 5).…”

“…4 and figs. S1, S2B, and S3A) (10,(26)(27)(28)(29)(30). These records span various portions of the past 2 ka with highly variable temporal resolution (~1.5 to 30 years) and age constraints.…”

“…However, a considerable spread in the timing, magnitude, rate, and even in the sign of  13 C shifts in speleothems from the same and a nearby cave (figs. S2A and S3B) (10,(26)(27)(28)(29)(30) suggests complex interplays of human activities (e.g., vegetation burning/clearing above the cave), karstic processes (e.g., changes in drip water discharge, feeding routes, and ventilation), and climate (e.g., changes in precipitation and evaporation) in producing the observed changes. Critically, some of these processes such as biomass burning above the cave, either human or climate-induced, can also affect the karst vadose zone and plausibly overprint the  18 O of meteoric water in both directions (and consequently in the speleothem) by inducing changes in soil properties, water infiltration, cave ventilation, and CO 2 degassing (31,32).…”

“…Several speleothem records from northwest Madagascar ( fig. S1) spanning portions of the past 2000 years (10,(26)(27)(28)(29)(30) are characterized by very large increases [~10 per mil (‰)] in  13 C but no concomitant shifts in  18 O during the late Holocene (figs. S2 and S3).…”

Archaeology, environmental justice, and climate change on islands of the Caribbean and southwestern Indian Ocean

“…Wetter conditions during the Early Holocene likely drove social and ecological changes in the broader Indian Ocean region (49,50). Despite an overall shift toward wetter conditions, successive dry and wet phases characterize the SWIO Holocene record (51)(52)(53)(54)(55).…”