Last interglacial sea level in the western Indian Ocean: multifaceted approach to paleo relative sea level indicator interpretation and analysis

Abstract

The global population living within low lying coastal zones face significant challenges in the coming century as sea levels rise. Unfortunately, the acceleration and magnitude of the rise is still a subject of debate, leaving coastal communities often paralyzed in the decision whether to mitigate or abandon. In order to better elucidate future sea-level rise, process analogues from the geological record are used to constrain model predictions. The Last Interglacial (LIG, ca. 128–116 ka), represents the most recent period when Earth’s climate was warmer and sea levels were higher than today. Such insights are derived from the global accumulation of geological sea-level proxies, called paleo relative sea level (PRSL) indicators that are comprised of accurate chronological and elevation constraints as well as considerations for post-depositional processes. However, the spatial and temporal coverage of PRSL indicators is uneven, especially within Eastern Africa and the Western Indian Ocean (EAWIO). In order to establish the current understanding of LIG sea-level in the EAWIO, this dissertation compiles a sea-level database using published LIG geological sea-level proxies. The EAWIO comprises of extensive tropical coastlines and coralline islands with many occurrences of well preserved LIG sequences. These LIG sequences vary in form from islands entirely composed of Pleistocene reefs (i.e., Aldabra, the Seychelles) to uplifted marine terraces in the north of Somalia to Marine Isotope Stage (MIS) 5 tidal notches in Mozambique. This effort uses theWALIS interface and contains 58 sea-level indicators and 2 terrestrial limiting data points. Studies within this database stretch from the 1960s until present, with varying degrees of PRSL precision. Taking this into account, this dissertation explores the advantages of applying modern chronological constraints and advanced surveying techniques to a classic Pleistocene site along the southwestern shores of Madagascar. Here, new investigative techniques provide an updated late Pleistocene geomorphological understanding of southwestern Madagascar. Through the use of Structure from Motion / Multi-View Stereo in combination with differential Global Navigation Satellite System surveys, 3D outcrop reconstructions of sequential geomorphologcial zones and their intrinsic facies are presented. This sequence is chronologically constrained using U-series ages from both in situ and reworked coral samples. From this chronologically sequence, the LIG in Lembetabe was initiated by the deposition of a fringing reef that reaches approximately 2 m above mean sea level. This fringing reef environment persisted throughout the LIG, before a combination of slow regression and changes in the sedimentation regime covered the fringing reef with intertidal and beach sediment. A final close-out of the system occurred as sea level fell further and dune fields began to migrate offshore before. This interpretation of the Lembetabe LIG sequence is then examined through the use of Forward Stratigraphic Models (FSMs). Currently, FSMs are commonly deployed within large-scale basinwide characterizations for industry. By scaling-down the FSM DIONISOS, a suite of synthetic Quaternary subtropical fringing reefs in southwestern Madagascar are produced. Through this approach, each reef is subjected to a distinct Greenland Ice-Sheet (GrIS) and Antarctica Ice-Sheet (AIS) melt scenarios produced by a coupled ANICE-SELEN glacial isostatic adjustment (GIA) model. These scenarios match LIG sea-level curves previously hypothesized within the Indo-Pacific Basin. The resulting collection of synthetic reef sequences so dramatic shifts in both sedimentation and preservation regimes along the Lembetabe coast. By establishing a broad compilation of PRSL indicators at a basin scale, the reexamination of a previously described PRSL indicator, and finally the evaluation of outcrop interpretation, this dissertation provides a new PRSL benchmark for the EAWIO and a new workflow for the evaluation of PRSL indicators along carbonate coasts.