Depositional environments beneath the shelf-edge slopes of the Great Barrier Reef, inferred from foraminiferal assemblages: IODP Expedition 325

摘要

To understand sea-level changes since the Last Glacial Maximum (LGM) and their effects on coral reef systems, the shelf-edge slopes of the Great Barrier Reef (GBR) were cored during the Integrated Ocean Drilling Program (IODP) Expedition 325. Recovered unconsolidated sediments beneath the submerged shelf edge reefs contain abundant foraminiferal tests, which record changes in depositional environments and paleo-water depths. A total of 177 sediment samples were collected from 17 holes along three transects located within two geo-graphical areas (Noggins Pass and Hydrographers Passage), and were analyzed to determine stratigraphic changes in foraminiferal assemblages (2-0.5 mm size fraction). Results show that four foraminiferal assemblages (A, B, C and D) are delineated by multivariate analyses (Q-mode cluster analysis and non-metric multi-dimensional scaling: NMDS), and these assemblages correspond to a back-reef to reef margin zone (0-10 m deep; Assemblage A), an upper photic zone (10-30 m deep) associated with hard substrates (Assemblage B), an intermediate to lower photic zone (30-90 m deep) characterized by soft substrates (Assemblage C), and a lower photic zone (90-130 m deep) only found in modern shelf slopes (Assemblage D). Gradual shifts in these four foraminiferal assemblages superimposed on a two-dimensional NMDS ordination mainly reflect water-depth gradients and the relative dominance of two substrate types (hard and soft substrates). Pre-LGM (older than Marine Isotope Stage 3: = MIS3) sediments along transects at Hydrographers Passage were deposited at intermediate to lower photic zones. In contrast, relatively shallow-water sequences found in = MIS3 deposits at Noggins Pass likely owe their origin to either turbidite and/or land slide processes. The lack of Assemblage D in = MIS3 deposits from all transects could be related to lowering temperature and/or increasing terrestrial influences (i.e. more light attenuation). Shallowing upward seq