ALBERT

Description: Holocene reef growth, present date bioerosion .and recorded carbonate production were studied in the fringing reef at Aqaba, Red Sea. Water depth, wave impact as well as nutrient availability were considered. The carbonate production was measured for several coral samples. Samples of Porites-colonies were collected from several depths and sites near the Marine Science Station at Aqaba. Growth rate depends on water depth, size and age of colonies. Within the coral optimum of water depth growth rates vary between 5 and 16 mm/yr. Coral carbonate production was calculated on the base of annual growth increments and skeletal density using transects from shallow subtidal down to 40 m water depth. High resolution stable isotope data were measured to prove the origin of growth increments. Long-term trends of sea surface temperature and carbon isotope shift (1800-today) fit to the known global deviations. Bioerosion rates were determined using standard dead coral substrates exposed in different water depths and environmental settings. Rates vary between 0.6 and 1.4 kg/m2yr. Sediment export evaluated by means of simple sediment traps ranges between 0.3 and 0.7 kg/m2yr. Gross carbonate production, mainly built up by scleractinian corals, amounts to ca. 1.57 kg/m2yr. Bioerosion alters approx. 1.3 kg/m2yr of hard substrates into sediment. Sediment export is estimated to be ca. 0.4-0.6 kg/m2yr. Thus a net production of ca. 0.7 to 0.9 kg/m2yr should remain in the present reef, which is proved by the recorded carbonate production (reef drillings). Net production preserved in the reef can be given with ca. 800 kg/m2kyr (=0.8 kg/m2yr).

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.