Evolution of the Great Barrier Reef shelf-edge system in response to major environmental changes: A synthesis of IODP expedition 325

We review the scientific highlights and contributions of IODP Expedition 325 (Great Barrier Reef Environmental Changes) to address fundamental questions about global sea-level changes, paleoclimate variability and coral reef responses to these perturbations over the past 30 kyrs. Located in the tropics and far from ice sheets, the Great Barrier Reef (GBR) is a prime locality to achieve these objectives. Expedition 325 (Exp. 325) used a Mission Specific Platform (MSP) (RV Greatship Maya ) to drill on the shallow shelf edge and upper slope, recovering a diverse suite of reef framework and detrital facies, to capture a robust record of relative sea-level (RSL) changes, paleoclimate proxies and paleoecological reef data. Expedition 325 overcame mechanical issues, challenging borehole conditions and weather, recovering a total of 225 m of core at four transects (26% recovery) between 46 and 170 m water depth. Despite these challenges, the Exp. 325 record provides new insights into the patterns of sea-level fall from marine isotope stage 3 (MIS 3) into MIS 2 and the deglacial sea-level rise. This includes the precise structure, timing and full extent of the Last Glacial Maximum (LGM) and new constraints on deglacial meltwater pulses (i.e., MWP-1B). Paleoclimate proxy data (Sr/Ca, δ 18 O) from fossil Isopora constrain past sea surface temperatures (SSTs) and provide new insights into the deglacial reef response, with a near-modern warming during the Younger Dryas (YD),since the LGM. These environmental proxy data, along with a pronounced decrease in coral-skeleton δ 13 C, suggest major variations in regional oceanographic phenomena, including reductions in the strength of the East Australian Current (EAC) during the LGM, and also changes in carbon cycling due to shelf flooding, and initiation of the extensive “proto-barrier” reef system at 13 ka. We also document the evolution of the GBR in response to these and other environmental changes over the past 30 ka through the analysis of geochronological (U/Th, 14 C accelerator mass spectrometry (AMS) dating), petrophysical (physical properties, downhole gamma ray), sedimentological (microfacies, microbialites), paleoecological (corals, algae and larger benthic foraminifera) and geophysical records from the Exp. 325 cores. Reefs migrated seaward as sea-level fell to its lowest level during the last glaciation (ca. 20.5–20.7 ka), then landward as the shelf flooded and SSTs increased during the subsequent deglacial (ca. 20–10 ka). Growth was interrupted by five reef demise events, caused by either: (1) subaerial exposure during the falling stage or (2) reef drowning as rapid sea-level rise (combined with poor paleowater quality) outpaced reef growth, increasing reef stress that was expressed as major changes in reef assemblages, vertical accretion and macro-bioerosion intensity. Around 10 ka the reef drowned as the sea level continued to rise, flooding more of the shelf, and causing higher sediment flux and the demise of the “proto-barrier” reef system. The high degree of spatio-temporal connectivity between shallow and deep (mesophotic) reef communities on the shelf edge allowed rapid and significant lateral migrations of the reef ecosystem – a mechanism that likely occurred during previous glacial-interglacial cycles. The sedimentary record from one borehole on the upper slope extends the record of fluctuations in neritic carbonate production driven by sea-level changes, climatic variability and sedimentary processes over the last two glacial-interglacial cycles. Together, Exp. 325 has revealed a wealth of exciting information about past sea-level, climate and environmental changes, but also crucial new insights into how the GBR responded to these perturbations over the past 30,000 years.