Bioerosion traces in the Great Barrier Reef over the past 10 to 30 kyr

Abstract

Bioerosion traces preserved in coral reef framework provide insight into past environmental conditions and reef health. However, few studies have explored the relationship between bioerosion, reef growth history and environmental changes. The International Ocean Drilling Program (IODP) Expedition 325 sampled late glacial to deglacial reef sequences that developed from 30 to 10 ka on the shelf-edge adjacent to the modern Great Barrier Reef (GBR). This study quantifies spatiotemporal patterns of macrobioerosion intensity (macrobioerosion intensity referred to as BI) of trace ichnogenera: Entobia (sponges), Gastrochaenolites (bivalves) and Maeandropolydora/Trypanites (worms) based on detailed observations of 13 drill holes from two cross-shelf transects in two regions: Hydrographers Passage in the southern central GBR and Noggin Pass in the north. Our high-resolution stratigraphic and temporal records of bioerosion activity are defined and compared to published multi-proxy paleoenvironmental interpretations from the Exp. 325 cores. Coralgal assemblages, geochronology and lithology are used to understand the controls on the multi-scale spatiotemporal patterns of bioerosion across and along the GBR shelf and within individual holes. Noggin Pass is characterized by significantly higher bioerosion activity than Hydrographers. At each transect changes in bioerosion intensity are observed to be coeval with some major reef sequence boundaries, however due to the unconsolidated nature and limited recovery many boundaries have inconclusive trends. Statistical analysis is used to interpret the relationship of bioerosion intensity to various factors including paleo-water depth, lithology, coral and algal assemblages, transect location, geomorphic feature, vertical accretion and reef sequences. Terrigenous sediment flushing, depositional regime and a pronounced latitudinal gradient in sea surface temperature are likely to have contributed to the significant regional differences in bioerosion intensity between northern and southern transects and also during reef demise. Bioerosion at both transects is highest in the ‘proto-barrier’ (Reef 4) sites when compared to all fringing terrace reefs (Reef 3a, 3b and 4). Higher bioerosion tends to occur when vertical accretion is at its extremes (its lowest and highest limits). The enhanced bioerosion alongside lowest reef accretion in a sea level rise scenario (Reef 3a, 3b and 4) might have contributed to reef degradation prior to its demise.