Disentangling interglacial sea level and global dynamic topography: Analysis of Madagascar

Abstract

Global inventories of stable sea-level markers for the peak of the last interglacial period, Marine Isotopic Stage (MIS) 5e, play a pivotal role in determining sea-level changes and in testing models of glacial isostatic adjustment. Here, we present surveying and radiometric dating results for emergent terraces from northern Madagascar, regarded as a stable equatorial site. Fossil coral specimens were dated using conventional and open-system corrected uranium series methods. Elevation of the upper (undated) terrace decreases from 33.8 m to 29.5 m over a distance of 35 km. An intermediate terrace has an average radiometric age of 130.7 ± 13.2 ka (i.e. MIS 5e). Its elevation decreases from 9.3 m to 2.8 m over a distance of 80 km. The record of the lower terrace is fragmentary and consists of beach rock containing rare corals with ages of 1.6-3.8 ka. The spatial gradient of the MIS 5e marker is inconsistent with glacio-isostatic adjustment calculations. Instead, we propose that variable elevations of this marker around Madagascar, and possibly throughout the Indian Ocean, reflect spatial patterns of dynamic topography generated by sub-plate mantle convection.

Last Interglacial changes in sea level on Aldabra, western Indian Ocean

“…The 8 m and 4 m terraces imply erosion, potentially of several metres, of lithified material, and unless they formed after the Last Glacial Maximum their diagenesis would have required a substantial retreat of sea level before a return to a position about +8 m above the present datum. The correlation of these features to similar elevations in Kenya and the granitic Seychelles argues that these changes reflect eustatic variation in sea level and, notwithstanding data from Madagascar (Stephenson et al ., ) do not reflect tectonically induced uplift.…”

“…(), is that differences in the sequences reflect the marginal influence of the crustal movements on Madagascar described by Stephenson et al . ().…”

“…Deposits of MIS 7 have been reported on the Kenya coast (Accordi et al ., ) where two distinct marine transgressions have been identified in late Pleistocene deposits (Caswell, ; Braithwaite, ) and on Madagascar [Battistini (); with new dates in Stephenson et al . ()]. However, MIS 7 deposits do not appear to be common although they have been described at +4.5 to +6.0 m above present sea level in Bermuda [Rowe et al .…”

“…However, recent work by Stephenson et al . () indicates that Madagascar, 420 km to the south‐east, has been on the northern margin of a Neogene crustal flexure, active since the Last Interglacial, with the elevations of deposits from this interval decreasing from 9.3 to 2.8 m over a distance of only 80 km. Aldabra provides one of the best known Late Pleistocene sequences, dominated by deposits of the Last Interglacial interval.…”

Last Interglacial sea-level proxies in East Africa and the Western Indian Ocean

“…These ages mirror the younger age of Baie des Dunes (130 ± 40 ka) by Battistini (1977) and are significantly younger than the older 160 ± 30 ka age. The elevation reported by Stephenson et al (2019) is also higher than that described by Battistini (1977), 6.8 8 https://doi.org/10.5194/essd-2020-349 This translates to a PRSL of + 5.3 ± 1.56 m. Finally, further south on the shore of Irdo Bay, the coral reef terrace is observed again, this time at an elevation of +2.8 ± 1.2 m MLWS and an age of between 126.6 ± 2 and 141.8 ± 1.9 ka (ST18-005-001 to ST18-009-001, WALIS ID# 162; Table 4). This correlates to a PRSL of +3.8 ± 1.56 MSL.…”