Abstract. The warmest peak of the Last Interglacial (ca. 128–116 ka) is considered a process analogue and is often studied to better understand the effects of a future warmer climate on the Earth's system. In particular, significant efforts have been made to better constrain ice sheet contributions to the peak Last Interglacial sea level through field observation of paleo relative sea level indicators. Along tropical coastal margins, these observations are predominantly based on fossil shallow coral reef sequences, which also provide the possibility of gathering reliable U-series chronological constraints. However, the preservation of many Pleistocene reef sequences is often limited to a series of discrete relative sea level positions within the interglacial, where corals suitable for dating were preserved. This, in turn, limits our ability to understand the continuous evolution of paleo relative sea level through an entire interglacial, also affecting the possibility of unraveling the existence and pattern of sub-stadial sea level oscillations. While the interpretation of lithostratigraphic and geomorphologic properties is often used to overcome this hurdle, geological interpretation may present issues related to subjectivity when dealing with missing facies or incomplete sequences. In this study, we try to step back from a conventional approach, generating a spectrum of synthetic Quaternary subtropical fringing reefs for a site in southwestern Madagascar (Indian Ocean). We use the Dionisos forward stratigraphic model (from Beicip-Franlab) to build a fossil reef at this location. In each model run, we use distinct Greenland and Antarctica ice sheet melt scenarios produced by a coupled ANICE–SELEN glacial isostatic adjustment model. The resulting synthetic reef sequences are then used test these melt scenarios against the stratigraphic record. We propose that this sort of stratigraphic modeling may provide further quantitative control when interpreting Last Interglacial reef sequences. 

pdf/refining-patterns-of-melt-with-forward-stratigraphic-models-20tmn5b6sy.pdf

Refining patterns of melt with forward stratigraphic models of stable Pleistocene coastlines

Abstract. The warmest peak of the Last Interglacial (ca. 128–116 ka) is considered a process analogue, and is often studied to better understand the effects of a future warmer climate on the Earth's system. In particular, significant effort has been made to better constrain ice sheet contributions to peak Last Interglacial sea level through field observation of paleo relative sea level indicators. Along tropical coastal margins, these observations are predominantly based on fossil shallow coral reef sequences, also thanks to the possibility of gathering reliable U-series chronological constraints. However, the preservation of many Pleistocene reef sequences is often limited to a series of discrete relative sea-level positions within the interglacial, where corals suitable for dating were preserved. This in turn, limits our ability to understand the continuous evolution of paleo relative sea-level through an entire interglacial, also affecting the possibility to unravel the existence and pattern of sub-stadial sea level oscillations. While the interpretation of lithostratigraphic and geomorphologic properties is often used to overcome this hurdle, geological interpretation may present issues related to subjectivity when dealing with missing facies or incomplete sequences. In this study, we try to step back from a conventional approach generating a spectrum of synthetic Quaternary subtropical fringing reefs for a site in southwestern Madagascar (Indian Ocean). We use the DIONISOS forward (Beicip Franlab) stratigraphic model to build a fossil reef at this location. In each model run, we use distinct Greenland and Antarctica Ice Sheet melt scenarios produced by a coupled ANICE-SELEN glacial isostatic adjustment model. The resulting synthetic reef sequences are then used test these melt scenarios against the stratigraphic record. We propose that this sort of stratigraphic modelling may provide further quantitative control when interpreting Last Interglacial reef sequences. 

Refining patterns of melt with forward stratigraphic models on stable Pleistocene coastlines

New datings and elevations of a fossil reef in Lembetabe, southwest Madagascar: eustatic and tectonic implications

Understanding past sea-level variations is essential to constrain future patterns of sea-level rise in response to warmer climate conditions. Due to good preservation and the possibility to use various geochemical methods to date fossil sea-level index points, the Last Interglacial (Marine Isotope Stage, MIS, 5e, 130-116 ka) is often regarded as one of the best climate analogs for a future warmer climate. MIS 5e coastal stratigraphic sequences, such as fossil coral reefs in tectonically stable areas, are characterized by abrupt shifts in their geological facies, steps within the reef topography or backstepped fossil reefs, which have been often interpreted as proxies for abrupt sea-level fluctuations within the interglacial. However, the observational evidence and magnitude of such abrupt changes are controversial. Here, we run nearly 50 thousand simulations of a 2D kinematic reef model that can reproduce coral reef growth and demise through time. Our aim is to investigate the parameters of space, the sea-level scenarios, and the processes which multiple-stepped MIS 5e fossil reefs form. As inputs to the model, we use both published and synthetic sea-level histories (17 sea-level curves ranging from one to several sea-level peaks), and a wide range of reef growth rates, marine erosion rates and bedrock foundation slopes. Our results show that the only sea-level history that could explain the generation of an emerged MIS 5e backstepped reef is a first sea-level peak followed by an abrupt rise in sea level and a second short-term peak. Any other multiple-stepped stratigraphy can be explained by the interplay between reef growth, marine erosion, and bedrock slope. 

pdf/sea-level-oscillations-within-the-last-interglacial-insights-598tuavjko.pdf

Sea-level oscillations within the Last Interglacial: insights from coral reef stratigraphic forward modelling

Abstract. The warmest peak of the Last Interglacial (ca. 128&ndash;116 ka) is considered a process analogue, and is often studied to better understand the effects of a future warmer climate on the Earth's system. In particular, significant effort has been made to better constrain ice sheet contributions to peak Last Interglacial sea level through field observation of paleo relative sea level indicators. Along tropical coastal margins, these observations are predominantly based on fossil shallow coral reef sequences, also thanks to the possibility of gathering reliable U-series chronological constraints. However, the preservation of many Pleistocene reef sequences is often limited to a series of discrete relative sea-level positions within the interglacial, where corals suitable for dating were preserved. This in turn, limits our ability to understand the continuous evolution of paleo relative sea-level through an entire interglacial, also affecting the possibility to unravel the existence and pattern of sub-stadial sea level oscillations. While the interpretation of lithostratigraphic and geomorphologic properties is often used to overcome this hurdle, geological interpretation may present issues related to subjectivity when dealing with missing facies or incomplete sequences. In this study, we try to step back from a conventional approach generating a spectrum of synthetic Quaternary subtropical fringing reefs for a site in southwestern Madagascar (Indian Ocean). We use the DIONISOS forward (Beicip Franlab) stratigraphic model to build a fossil reef at this location. In each model run, we use distinct Greenland and Antarctica Ice Sheet melt scenarios produced by a coupled ANICE-SELEN glacial isostatic adjustment model. The resulting synthetic reef sequences are then used test these melt scenarios against the stratigraphic record. We propose that this sort of stratigraphic modelling may provide further quantitative control when interpreting Last Interglacial reef sequences.

/pdf/comment-on-egusphere-2023-95-2p4628ra.pdf

Comment on egusphere-2023-95

The study of paleo shorelines, particularly of those formed during the late Quaternary, provides robust insights into past climate variability. Advances in surveying techniques and chronological methodologies have dramatically improved the inter-comparability of regional and basin-wide paleo shoreline surveys. However, these advances have been applied unevenly across the globe. This is especially true in southwestern Madagascar, where, in the 1960s and 1970s, emerged Pleistocene beach and reef facies were first described in detail and dated to Marine Isotope Stage (MIS) 5a using U–Th alpha activity counting by french geologist René Battistini. Now, 50 years on, no further analysis of the coastal sequence has been made. In this study, we present an updated late Pleistocene coastal evolution model for the southwestern Madagascar coast. Utilizing a combination of Structure-from-Motion/Multi-View Stereo techniques and differential Global Navigation Satellite System surveys, we have created five high-resolution 3D outcrop reconstructions that have, in turn, been chronologically constrained using 10 U-series ages from both in situ and reworked coral samples. Our data suggest that the emerged reef was deposited during MIS 5e (∼125 ka), then was covered by intertidal and beach sediment (including redeposited coral clasts of MIS 5e age), and finally capped off by thick eolianites. This sequence would suggest that the local sea level must have remained stable throughout MIS 5e in order to allow for the progradation of both the beach and reef environments. 

Revisiting Battistini: Pleistocene Coastal Evolution of Southwestern Madagascar

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Revisiting Battistini: Pleistocene Coastal Evolution of Southwestern Madagascar