OUT OF PHASE FORE REEF UPPER SLOPE MIXED SEDIMENTATION RESPONSE TO GLACIAL-INTERGLACIAL SEA LEVEL FLUCTUATIONS: GREAT BARRIER REEF (NORTH EAST AUSTRALIA)

High resolution stratigraphy in IODP Expedition 325 Hole-M0058A(58A) shows that the mixed carbonate siliciclastic sedimentary section, cored on the upper slope of the Great Barrier Reef (GBR) SE of Cairns, corresponds to the last 200 ky or almost two full late Quaternary glacial-interglacial cycles. Contrary to observations in similar settings, strontium (Sr) and silica (Si) X-ray Fluorescence counts – proxies for reef-derived carbonate and terrigenous exports to the upper slope, respectively – appear to be out of phase with well-established models such as highstand shedding for periplatform sediment and lowstand shedding for siliciclastics systems.

Hole 58A, drilled at 167 mbsl, recovered a 41 m-long sedimentary sequence composed of three unconsolidated mud sections intercalated with two distinct sandy intervals. Close proximity to the modern GBR and shallow water depth of this hole allows for dramatic changes in sedimentation and energy as a result of the glacial-interglacial high amplitude sea level fluctuations. Planktic foraminifer tests, Globigerinoides ruber (white), were picked and analyzed every 10 cm to produce a high resolution oxygen isotope record. This record clearly exhibits cyclic δ¹⁸O variations interpreted to correspond with Marine Isotope Stages (MIS) 1-7 or the last 200ky. The disappearance of G. ruber (pink) at 28.5 m and uranium/thorium dates confirm that the interval between 30 and 28 m, characterized by a sharp δ¹⁸O decrease, corresponds to Termination II.

Throughout Hole 58A, Sr and Si count variations appear to be out of phase suggesting periods of alternating neritic and terrigenous input dominance to the upper slope. The pattern, however, does not follow the typical reciprocal mixed system model of highstand carbonates and lowstand siliciclastics. During the early Termination II, Si counts increase as δ¹⁸O values decrease and remain high during MIS 5e interglacial peak while Sr counts remain low. Sr counts increase during the MIS 5e-5d interglacial-glacial transition and remain high until Sr counts drop off before MIS 5a, while Si counts decrease and stay low. This is uncharacteristic when compared to similar holes on the upper slopes of the GBR and Gulf of Papua, suggesting a local phenomenon where neritic carbonate material is diluted by highstand siliciclastic shelf bypass.