U-Th dating of calcitic corals from the Gulf of Aqaba indicates freshwater activity during sapropels S5 and S4

Wednesday, 17 December 2014
Maayan Yehudai1,2, Boaz Lazar1, Neta BAR (KOHN)1, Amotz Agnon1, Yonathan Shaked3 and Mordechai Stein4, (1)Hebrew University of Jerusalem, Jerusalem, Israel, (2)Lamont -Doherty Earth Observatory, Palisades, NY, United States, (3)Interuniversity Institute for Marine Sciences, Eilat, Israel, (4)Geological Survey of Israel, Jerusalem, Israel
Most of the fossil corals in the uplifted reef terraces along the Gulf of Aqaba (GOA, which is currently one of the most hyper-arid regions on Earth) were extensively altered to calcite. This observation indicates extensive interaction with freshwater, possibly when the terraces were uplifted through a coastal aquifer that existed along the shores of the GOA, implying a wetter climate during time of recrystallization from aragonite to calcite. Thus, dating of the recrystallization events should yield timing of past wet conditions in the current hyper-arid area of the GOA. In the current study 16 samples of aragonitic and calcitic corals were collected from several uplifted coral terraces off the coast south to the city of Aqaba. While aragonite corals were dated with the conventional closed system (with no initial Th) age equation, the dating of the calcitic corals required the development of adequate equations that allow the calculation of both the initial cystallization age of the coral and time of recrystallization to calcite. The two age calculations were based on the assumptions that each reef terrace went through a single major recrystallization event and that the pristine aragonite corals had an initial U concentration in the range of 2.5-3 ppm (typical for pristine modern corals). Two recrystallization events were identified: at 124±30 ka BP and 106±7 ka BP, which coincide with the timing of sapropel events S5 and S4 when the African monsoon induced enhanced wetness in the desert area (e.g. Torfstein et al., 2014). The original reef deposition ages were calculated to be: ~ 129-127±8 ka BP, 112±3 ka BP and 5.7±0.3 ka BP, matching the interglacial peaks of global sea level high stands MIS5e, MIS5c and the mid-Holocene high stand.