STRONTIUM-BASED NERITIC CARBONATE SHEDDING RECORD AT IODP 325 HOLE 58A ON THE CENTRAL GREAT BARRIER REEF UPPER SLOPE: EVIDENCE FOR A DROWNED GBR DURING THE PEAK PENULTIMATE SEA-LEVEL HIGHSTAND (MIS-5E)

High resolution stratigraphy at IODP Exp. 325 Hole-M0058A (58A) shows that the 41 m-long mixed sedimentary section, drilled at 167 mbsl on the upper GBR slope SE of Cairns, corresponds to almost two full late Quaternary glacial-interglacial cycles (or the last 220 Ky). Sr count (XRF) variations down Hole 58A clearly demonstrate to be an excellent proxy for aragonite rich reef-derived carbonate shedding toward the GBR upper slope. The substitution of Sr for Ca within the orthorhombic aragonite lattice is well established and, as is the case at Hole 58A, plotted Sr counts and discrete bulk percent aragonite display a statistically significant fit with R²= 0.85. During Termination II (TII), early peak interglacial (MIS 5e), and end of MIS 5d-a interglacial, Sr count variations at Hole 58A are unexpectedly out of phase with the well-established highstand shedding model often used to describe neritic carbonate export toward adjacent slopes during periods of high sea level when the neritic carbonate factory is in full production.

Because of Hole 58A’s close proximity to the modern GBR and its shallow water depth, glacial-interglacial 100-120 m sea level fluctuations trigger dramatic sedimentation and energy changes. Globigerinoides ruber (white) were picked and analyzed to produce a high resolution oxygen isotope record that clearly exhibits cyclic δ¹⁸O variations corresponding to MIS 1-7 or the last 220Ky. The disappearance of G. ruber (pink) at 28.5 m confirms that the interval between 30-28 m, characterized by a sharp δ¹⁸O decrease, corresponds to TII. During the early TII, Sr counts slightly peak in response to an early intermediate highstand and then reach the lowest levels between 130-123 Ka, MIS 5e maximum sea level. On the other hand, highest Sr count levels are observed as sea level fell from MIS 5e to 5d at intermediate levels (20-40 mbsl). This is uncharacteristic compared with dozens of shallow cores along the GBR upper slope that record the Last Deglaciation and Holocene, suggesting that the central GBR was drowned during MIS-5e following the TII rapid sea level rise and recovered during relatively stable but lower sea levels of MIS 5d-5a. Carbonate sediment trends at Hole 58A compare well with records at Holes 820A and 819, also from the Central GBR upper slopes and, therefore, strengthen our interpretation and conclusions.