Patterns of reef coral calcification and preservation in an upwelling zone (Oman)
Prof. Dr. Thomas Brachert
Modern coral reefs form a continuous belt on both sides of the equator in the low latitudes. The belt is limited towards the mid latitudes by the winter minimum temperature of the ambient, oligotrophic water masses (18 °C) and a saturation state of seawater with respect to aragonite < 3.1. The modern ocean is likely not a good analogue for reef environments of the geological past and the near future, however, because present-day water temperatures of the warm pools are lower and pH values higher than during the interglacials of the Quaternary and upon future ocean warming and acidification. During the interglacials, higher-than-present temperatures caused low reef coral diversity near the equator and poleward range shifts of many coral taxa. Hence, in addition to minimum temperatures limiting coral distributions, high temperatures associated with future ocean warming will also be a factor controlling reef coral growth within the warm pools near the equator. It has been suggested previously, that regions of upwelling may represent refugia for coral reefs, however, very little is known with regard to the effects of high nutrients and low aragonite saturation on reef coral skeletal precipitation and preservation. From our previous research on the Plio-Pleistocene Florida platform, we hypothesize upwelling to have a negative net effect on skeletal calcification rates in reef corals, essentially due to deficits in density and in spite of enhanced linear extension rates. This inference has far reaching implications on coral colonization of space in reefs, survival and preservation over time. The proposed research is to study reef coral calcification along the Arabian Sea and Gulf of Oman coasts of the Arabian Peninsula (Oman). Reef growth within this region faces the open ocean and is nowhere affected by "inimical bankwaters" or freshwater plumes. Rather, a spatial gradient of environmental stress exists through variable upwelling (high and seasonally variable temperature, variable aragonite saturation, eutrophy). We will investigate modern and Quaternary reef corals at oceanographically distinct sites along depth transects using classical methods of sclerochronology (stable isotope and geochemical proxy analyses, epifluorescence). In a novel approach, this data will be combined with measurements of skeletal density and size growth of the colonies (linear extension rate) in order to infer skeletal calcification rates at a given set of environmental variables. Detailed microscopic investigation (SEM) will be necessary to describe and quantify effects of bioerosion and early marine and freshwater diagenesis (cementation and dissolution) on density and preservation potential. The proposed research will provide new clues as to the recognition of the effects of low aragonite saturation and high water temperature on skeletal calcification rates and contribute to a poorly understood missing link in our understanding of the past and future worlds.