B. A. Thomassin

Mayotte coral reveals hydrological changes in the western Indian Ocean between 1881 and 1994

We reconstruct the hydrologic history of the tropical western Indian Ocean by calculating the δ18Oseawater from coupled coral Sr/Ca and δ18O measurements in a massive Porites coral from Mayotte (Comoros) between 1881 and 1994. We found that the precipitation-evaporation balance varies naturally on time scales of 5–6 years and 18–25 years. High (low) SSTs are associated with positive (negative) δ18Oseawater implying that atmospheric variability is linked with remote climate modes in the Indian Ocean and the tropical/extratropical Pacific Ocean. Warm El Nino-Southern Oscillation events are associated with a negative freshwater balance at Mayotte. This case study demonstrates that a much denser network of δ18Oseawater reconstructions is crucial for understanding the spatial patterns of hydrological conditions.

Postglacial flooding history of Mayotte Lagoon (Comoro Archipelago, southwest Indian Ocean)

Abstract Four cores from the fringing reefs and five sediment cores from Mayotte Lagoon, Comoro Archipelago, southwest Indian Ocean, which reached the Pleistocene/Holocene boundary, form the database of this study. They offer the opportunity to reexamine and complete the postglacial sea-level curve, especially for the time interval between 11.6 to 8 kyr cal BP. Between 11.6 kyr cal BP until present the history of sea-level rise showed the following steps: (1) by about 19 mm/yr between 11.6 and 9.6 kyr cal BP, (2) by 9 mm/yr between 9.6 and 8 kyr cal BP, (3) by 3 mm/yr between 8 and 7 kyr cal BP, and (4) by 0.9 mm/yr after 7 kyr cal BP until stabilisation at present level at 2.5 kyr cal BP. In addition, a decline in the rates of sea-level rise or even a stillstand is observed between 13 to 11.6 kyr cal BP. The flooding of the lagoon of Mayotte was controlled by the depth of the reefal passages, which were cut by rivers and/or due to erosion during the time of emergence since the last interglacial. The differences in the shape of the sea-level curve from Mayotte compared to those from other sites located far from the former glaciated regions are related to: (1) the small size of the island, (2) the rapid downward movement of this small volcanic island with the oceanic plate into the mantle due to hydro–isostatic compensation after addition of meltwater, and (3) the location between large continents.

Seismic architecture and sediment distribution within the Holocene barrier reef-lagoon complex of Mayotte (Comoro archipelago, SW Indian Ocean)

Twenty gravity cores and a large set of high-resolution seismic profiles from various lagoonal settings were studied to determine the Holocene sediment distribution and sequence architecture within the Mayotte barrier reef–lagoon complex. The Holocene seismic sequence comprises a type 1 sequence with lowstand, transgressive and highstand systems tracts. The lowstand systems tract consists of a paleosoil horizon formed during subaerial exposure. The transgressive systems tract is composed of four depositional systems: (1) inner transgressive layer, (2) proximal and distal incised valley fills, (3) mid-lagoonal layer and (4) keep-up or catch-up fringing and barrier reef sequence. The highstand systems tract comprises three depositional systems: (1) of a proximal terrigenous wedge, (2) mid-lagoonal and distal carbonate sands or muds and (3) reefal carbonates. Our studies show that the nature of the Holocene sequence is controlled by the rate and amplitude of sea-level rise and environmental changes, which are expressed by changes in clastic sediment supply and carbonate production. The pre-Holocene topography and water dynamics steer the vertical and spatial sediment thickness distribution of the Holocene. Additional important parameters are the proximity to a source area (carbonate or terrigenous) and the width of the depositional area. Climate dynamics are also of great importance while they determine carbonate production and terrigenous runoff. Sedimentation rates in the subtidal settings always lacked behind sea-level rates. Thus, a steep relief was created keeping most lagoonal parts within the deep subtidal realm in which sediment production was not efficient enough to fill up accommodation space. In addition, wave and/or current energy might prevent the fill up of the lagoon. This ultimately resulted in a typical empty bucket morphology. Only a high amplitude sea-level fall would allow the subtidal lagoon to build up to base-level. Unfilled accommodation space, therefore, must be a very common feature in the geologic record.

Systems tracts sedimentology in the lagoon of Mayotte associated with the Holocene transgression

Twelve gravity cores from various settings within the Mayotte barrier reef–lagoon complex were studied to determine the sedimentology of the sequence stratigraphic systems tracts that formed during the Holocene transgression. Our studies focussed on the determination of physical, chemical, mineralogical and biological parameters of the sediments from specific systems tracts. These parameters determine the thickness and facies of each systems tracts and are controlled by the rate and amplitude of sea-level rise, lagoonal topography and environmental changes. The lowstand systems tract (LST) (before 11.5 ka BP) comprises ferralitic or organic-rich paleosoils in the proximal and middle lagoon and karstified Pleistocene reefal carbonates in the distal lagoon. The transgressive systems tract (TST) (11.5–7 ka BP) consists of a lower terrigenous and an upper mixed terrigenous–carbonate or carbonate-dominated unit. Locally, mangrove muds were deposited. The highstand systems tract (HST) can be divided into an early highstand (eHST) (7–1 ka BP) and a late highstand systems tract (lHST) (after 1 ka BP). In the proximal lagoonal wedge, the early highstand systems tract consists of terrigenous or mixed terrigenous–carbonate muds to sandy muds. In the middle lagoon, it shows carbonate mud to sandy mud and carbonate gravel to reefal carbonates in the distal lagoons. Terrigenous muds dominate the late highstand systems tract in the proximal lagoonal wedge. In the mid-lagoonal plain, mixed terrigenous–carbonate or carbonate mud to sandy mud dominates, while carbonate gravel to reefal carbonate prevails in the distal lagoon. For the last 9 ka, sedimentation in the lagoon of Mayotte has been spatially divided into a proximal terrigenous and a distal, carbonate-dominated province. Maximum carbonate concentrations between 4 and 1 ka BP coincide with the time of maximum solar insolation. After 1 ka BP, a general decrease in carbonate concentrations can be observed. This coincides with increased terrigenous sediment input, which results from a reduction in accommodation space and to some extent is of anthropogenic origin.