Description: The Ifaty coral record from off SW Madagascar provide a 336-year coral oxygen isotope record that is used to investigate the natural variability of the western Indian Ocean subtropical SST dipole and ENSO. The coral oxygen isotope record primarily reflects past sea-surface temperature (SST) variability on seasonal to multidecadal scales. To validate the SST reconstructions derived from oxygen isotopes, Sr/Ca ratios were obtained for selected time windows (1973–1995, 1863–1910, 1784–1809, 1688–1710). The period 1675–1760 was found to be the coolest period of the entire record with anomalies of 0.3–0.5 °C that includes the Late Maunder Minimum (1675–1710). The warmest periods, as indicated by our data, occur between 1880 and 1900 and the upper part of the Ifaty record (1973–1995). We generated a time series of coral δ18O for different seasons of the year to investigate austral winter and summer SST variability that influences rainfall intensity over southern Africa. Winter coral δ18O is coherent with winter SST on decadal and multidecadal time scales between 1854 and 1995. We suggest that the Ifaty winter time series provides a record of winter SST variability over the Mozambique Channel/Agulhas Current region over 336 years. Strong Indian Ocean subtropical dipole events, occurring during austral summer, are displayed in the Ifaty record. The austral summer coral δ18O is coherent and in phase with ENSO indices on interannual time scales (2–4 years) between 1880–1920, 1930–1940 and after 1970. Our data indicate that the impact of ENSO on SW Indian Ocean SST and atmospheric circulation was also strong between 1680–1720 and 1760–1790, in agreement with other studies. We show evidence that these variations are caused by changes in the regional hydrologic balance. The results demonstrate that the impact of ENSO cycles in the region of the SW Indian Ocean has changed significantly since 1970 and relate to a warming of southwestern Indian Ocean surface waters altering the spatial signature of ENSO.

Description: It is highly debated whether global warming contributed to the strong hurricane activity observed during the last decade. The crux of the recent debate is the limited length of the reliable instrumental record that exacerbates the detection of possible long-term changes in hurricane activity, which naturally exhibits strong multidecadal variations that are associated with the Atlantic Multidecadal Oscillation (AMO). The AMO, itself a major mode of climate variability, remains also poorly understood because of limited data. Here, we present the first coral-based proxy record (δ18O) that clearly captures multidecadal variations in the AMO and the hurricane activity. Our record, obtained from a brain coral situated in the Atlantic hurricane domain, is equally sensitive to variations in sea surface temperature (SST) and seawater δ18O, with the latter being strongly linked to precipitation, by this means amplifying large-scale climate signals in coral δ18O. The SST and precipitation signals in the coral provide the longest, thus far, continuous proxy-based record of hurricane activity that interestingly exhibits a long-term increase over the last century. As multidecadal SST variations in this region are closely related to the AMO, this study raises new possibilities to extend the limited observations and to gain new insights into the mechanisms underlying the AMO and long-term hurricane variations.

Description: 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.

Description: 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.

Description: 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.