The eastern Australian coastline features significant coastal landforms, including Fraser Island (K'gari), the world's largest sand island, and the Great Barrier Reef (GBR), a globally important biodiversity hotspot and carbon sink. Despite their prominence, the precise timing and mechanisms behind their formation remain unclear. Previous research suggests that the GBR's earliest geological development phases date back to the early-middle Miocene, but age control from in situ coral suggests that the extant GBR was established much later, during the Middle Pleistocene. The long-term tectonic quiescence and stable coastal configuration along eastern Australia since the onset of the Antarctic Circumpolar Current (25 Ma) further complicates the understanding of the GBR's relatively recent establishment. This study aims to address these uncertainties by providing a detailed chronology for Fraser Island and the Cooloola Sand Mass, demonstrating their crucial role as a precondition for the initiation of the southern and central GBR.

Existing literature on the GBR's initiation points to various hypotheses, including sea-surface warming due to plate movement, the warm MIS 11 interglacial, and widening/deepening of continental shelves during the Middle Pleistocene Transition (MPT). However, the timing of these factors doesn't fully align with the known age of the GBR's establishment. Previous chronologies for the Cooloola Sand Mass suggest sand accumulation since at least 730 ka. This research builds upon these existing studies, aiming to refine the chronology of dune formation and demonstrate its direct link to GBR initiation.

The study employed a multi-method approach, combining high-resolution drone imagery to map the stratigraphy of the Rainbow Beach cliffs, detailed field descriptions and sampling for sedimentological and pedological analysis, optically stimulated luminescence (OSL) dating of quartz sand to determine the last exposure to light, and palaeomagnetic analysis of ferricretes to identify the primary palaeomagnetic polarity. OSL dating utilized the single-aliquot regenerative-dose procedure, while palaeomagnetic analysis involved progressive alternating-field (AF) and thermal demagnetization to identify the characteristic remanent magnetization (ChRM) direction. The accuracy of the basal OSL ages was supported by the reversed or transitional-polarity palaeomagnetic signals obtained from basal aeolian units. Detailed methods for drone image acquisition, field descriptions and sampling, OSL sampling and analysis, and palaeomagnetic sampling and analysis are provided in the supplementary material.

The OSL dating revealed that the earliest dune-building phase on Fraser Island and the Cooloola Sand Mass dates back to ~1.2 Ma. The oldest basal dune packages indicate initial deposition at ~0.8–1.2 Ma. These ages are supported by reversed-polarity sediments found in the basal aeolian units, predating the Matuyama/Brunhes boundary (773 ka). The study identified two major dune morphologies: modern parabolic dunes and older, thicker tabular dune sands, representing distinct dune-forming events during Middle Pleistocene sea-level transgressions. These findings align with the fundamental climate system changes that occurred during the MPT, including a shift from 40 kyr obliquity-driven glacial cycles to 100 kyr paced glacial cycles and a corresponding increase in the amplitude of sea-level fluctuations. The increased magnitude of sea-level change during the MPT is interpreted as the driver for the elevated coastal sediment supply that created Fraser Island and the Cooloola Sand Mass. The emplacement of Fraser Island redirected northward sediment transport, leading to a decrease in terrestrial sediment reaching the GBR and allowing carbonate sedimentation and reef growth to dominate.

The findings directly link the formation of Fraser Island and the Cooloola Sand Mass to the increased magnitude of sea-level change during the MPT. The resulting increase in sediment supply created the dune fields. Once established, Fraser Island acted as a barrier, deflecting sediment away from the GBR's southern and central regions, thus creating conditions favorable for coral reef development. This explains the relatively young age of the GBR's initiation despite suitable climatic conditions being present for much longer. The absence of major reefs south of Fraser Island is attributed to the high volume of sand in the longshore drift system. The study suggests that the formation of Fraser Island was a necessary precondition for the initiation of the southern and central GBR, which is supported by distal sediment records showing a marked decrease in terrestrial sediment and an increase in carbonate content at around 700 ka.

This study significantly advances our understanding of the Great Barrier Reef and Fraser Island's formation. It demonstrates that the increased amplitude of sea-level fluctuations during the Middle Pleistocene Transition directly led to the formation of Fraser Island and the Cooloola Sand Mass, and subsequently, to the initiation of the southern and central Great Barrier Reef. The formation of Fraser Island acted as a crucial control on sediment transport, preventing the influx of terrestrial sediment into the GBR and thereby facilitating reef development. Future research should investigate similar coastal sedimentary system reorganizations in other passive-margin coastlines.

While the study provides strong evidence linking Fraser Island's formation to GBR initiation, it relies primarily on data from the southern and central GBR. Further investigation is needed to fully assess the impact of Fraser Island on the northern GBR. The accuracy of the palaeomagnetic dating hinges on the assumption that iron mobilization events in the ferricretes are closely timed with dune deposition, although the evidence suggests this is a valid assumption. The OSL age scatter in the oldest dune units, though within the expected range for this dating method, might impact the precise timing of the initiation of the process.