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Several theories have been proposed to explain atoll formation. While karst dissolution during glacial periods and preferential coral reef accretion along raised bank margins during deglaciations and interglacials have been invoked to explain atoll formation, the respective roles of karst dissolution and reef margin construction in atoll formation have not been adequately evaluated by simulations. In this study, we conducted three‐dimensional numerical simulations of the Quaternary development of Meiji Atoll in the southern South China Sea based on interpreted data from a 2020‐m‐deep borehole drilled on its northeast rim in 2018. Our results suggest that the origin of atolls is more likely due to spatially differential dissolution across margin and interior areas (i.e., minimal along margins and maximal in bank centers) rather than preferential reef accretion along margins of flat‐topped banks. Preferential reef accretion along margins of flat‐topped banks that can result in central lagoons and atoll morphology can hardly result in the formation of lagoonal patch reefs that reach mean sea level. Preferential reef accretion along margins is mainly predicated on the karst‐induced morphology that has a central depression surrounded by raised rims, that is, using antecedent karst morphologies. If topographic highs in the lagoon are similar in elevation to the margins, reef accretion on these topographic highs can be similar to that observed on the margins, resulting in lagoonal patch reefs that reach mean sea level. Our simulation shows that spatially differential dissolution across margin and interior areas is a critical driver of worldwide central lagoons and atoll formation. Several theories have been proposed to explain how atolls form. In these theories, different processes are involved, such as sea‐level changes, subsidence, karst dissolution, and preferential reef accretion along margins. However, we still do not fully know if any of these processes surpasses others in importance. Here, we conducted three‐dimensional numerical simulations of the formation of Meiji Atoll in response to a range of different forcing conditions and, in particular, evaluated the respective roles of karst dissolution and reef margin construction in atoll formation based on well‐established sea‐level changes during the last 2.65 million years. Our simulations suggest that the origin of atolls is mainly due to maximum dissolution in the center of the bank and minimum dissolution along its margins, creating a karstic central depression surrounded by raised rims, rather than preferential reef accretion along margins of flat‐topped banks. Preferential reef accretion along margins of flat‐topped banks that can create central lagoons and atoll morphology can hardly result in lagoonal patch reefs that reach mean sea level. Preferential reef accretion along margins is mainly predicated on the antecedent karst morphologies. Our modeling highlights spatially differential dissolution across margin and interior areas as the primary factor for worldwide atoll formation. The origin of atolls is due to spatially differential dissolution rather than preferential reef growth along margins of a flat‐topped bankPreferential reef accretion along margins is mainly predicated on karstic morphology that has a central depression surrounded by raised rimsReef accretion on topographic highs in the lagoon can be similar to that on the margins when they are similar in elevation to the margins The origin of atolls is due to spatially differential dissolution rather than preferential reef growth along margins of a flat‐topped bank Preferential reef accretion along margins is mainly predicated on karstic morphology that has a central depression surrounded by raised rims Reef accretion on topographic highs in the lagoon can be similar to that on the margins when they are similar in elevation to the margins

The carbon isotope 13C is commonly used to attribute the last deglacial atmospheric CO2rise to various processes. Here we show that the growth of the world's largest reef system, the Great Barrier Reef (GBR), is marked by a pronounced decrease in δ13C in absolutely dated fossil coral skeletons between 12.8 and 11.7 ka, which coincides with a prominent minimum in atmospheric δ13CO2and the Younger Dryas. The event follows the flooding of a large shelf platform and initiation of an extensive barrier reef system at 13 ka. Carbon cycle simulations show the coral δ13C decrease was mainly caused by the combination of isotopic fractionation during reef carbonate production and the decomposition of organic land carbon on the newly flooded shallow‐water platform. The impacts of these processes on atmospheric CO2and δ13CO2, however, are marginal. Thus, the GBR was not contributing to the last deglacial δ13CO2minimum at ∼12.4 ka. An outstanding problem in our understanding of the global carbon cycle is unraveling the processes that were responsible for the rise of atmospheric CO2during the last deglaciation (19,000–11,000 years ago). The carbon isotope 13C is commonly used to attribute the last deglacial atmospheric CO2rise to various processes. The growth of tropical coral reefs has been controversially discussed in this context. To test this, well constrained reef carbonate records that span the last deglaciation are necessary, but such records are generally not available. Here we make use of a multi‐proxy coral reef record obtained at the Great Barrier Reef (GRB). The δ13C signal in the carbonate skeletons of fossil corals indicates a pronounced minimum that precisely coincides with a prominent minimum in atmospheric δ13CO2as indicated by ice core records for the Younger Dryas cold period. We show, by carbon cycle simulations, that the GBR coral δ13C signal can be explained by changes in reef carbonate production and decomposition of organic land carbon on a newly flooded wide area. However, the simulations indicate that that the world's largest reef system in existence appears to have little effect on the last deglacial atmospheric CO2and δ13CO2changes. Great Barrier Reef corals indicate pronounced decrease in skeletal stable carbon isotopes between 12.8 and 11.7 ka during Younger DryasEvent follows shelf flooding and barrier reef initiation at 13 ka and coincides with prominent atmospheric stable carbon isotope minimumCarbon cycle modeling reveals marginal impact of changes in reef carbonate production and land carbon decomposition on atmospheric carbon Great Barrier Reef corals indicate pronounced decrease in skeletal stable carbon isotopes between 12.8 and 11.7 ka during Younger Dryas Event follows shelf flooding and barrier reef initiation at 13 ka and coincides with prominent atmospheric stable carbon isotope minimum Carbon cycle modeling reveals marginal impact of changes in reef carbonate production and land carbon decomposition on atmospheric carbon

Corals do not all grow near the sea surface. Different species favour different ranges in depth and can survive at different depths from 0 to 5 m to over 0–100 m. Fossil corals, as markers of sea level, are the preferred choice in radiometric dating. Timing and magnitude of sea-level variations are determined from the location and age of a set of in-situ coral samples, and mark the lower bound in sea level. In addition, they have to be assigned an uncertainty to define the upper bound in sea level. This is referred to as the paleowater depth (PWD) and creates a major ambiguity in estimating the appropriate span of habitable depth in sea level. This uncertainty also has an impact on constraining the timing and magnitude of rapid changes in sea level. However, there are approaches that may minimize PWD uncertainties. These include assessing sea level data for groups of coral, during relatively stable sea levels, rather than individually. If samples were recovered in multiple drill-cores, along a sea-bed transect, tighter PWD constraints may apply for samples, in each core, depending on the magnitude of the slope of the sea bed. This method is particularly useful in cases of rapid sea level change over shallow continental shelves when the shift in the shoreline, with sea level change, is large. In these circumstances, the presence of a shallow-water reef established up or down-slope is indicative of a rapid change in sea level.

During the last interglacial period, 127–116?kyr ago, global mean sea level reached a peak of 5–9 ?m above present-day sea level. However, the exact timing and magnitude of ice sheet collapse that contributed to the sea-level highstand is unclear. Here we explore this timing using stratigraphic and geomorphic mapping and uranium-series geochronology of fossil coral reefs and geophysical modelling of sea-level records from Western Australia. We show that between 127 and 119?kyr ago, eustatic sea level remained relatively stable at about 3–4?m above present sea level. However, stratigraphically younger fossil corals with U-series ages of 118.1±1.4?kyr are observed at elevations of up to 9.5?m above present mean sea level. Accounting for glacial isostatic adjustment and localized tectonics, we conclude that eustatic sea level rose to about 9?m above present at the end of the last interglacial. We suggest that in the last few thousand years of the interglacial, a critical ice sheet stability threshold was crossed, resulting in the catastrophic collapse of polar ice sheets and substantial sea-level rise.

The northern Great Barrier Reef (GBR) Halimedabioherms have accumulated on the outer continental shelf from calcium carbonate algal sediments over the past ∼10,000 years and cover >6,000 km2of shelf area. As such, Halimedabioherms play a key role in the shallow marine carbon cycle over millennial timescales. The main source of nitrogen (N) to these bioherms is thought to be westward transport of upwelled NO3‐‐rich water from the Coral Sea. However, the primary N source has not been traced geochemically, and we have no understanding of any temporal variation. Here, we reconstruct patterns of N supply to Halimedabioherms in the GBR since the mid‐Holocene using the 15N/14N ratio of skeletal‐bound organic N (δ15N‐skeletal organic material [SOM]) in modern and fossil Halimedasediment cores. Average Halimedaskeletal δ15N‐SOM was 6.28 ± 0.26‰, consistent with δ15N‐NO3‐ from western tropical South Pacific (WTSP) thermocline waters. Thus, geochemically validating shelf‐break upwelling of an oceanic N source that regulates bioherm spatial distribution. Halimedaδ15N‐SOM decreased by 1‰–2‰ from 5,000 to 2,000 cal. yr BP, reaching a minima of 5.5‰ that persisted for almost 1,000 years. The Halimedaδ15N‐SOM variation reflects mid‐ to late Holocene changes in regional climate and intensified El Niño activity that likely facilitated elevated N2fixation in the WTSP, thereby lowering thermocline δ15N‐NO3‐. Thus, Halimedaskeletal material provides a valuable high‐resolution geochemical archive of past oceanographic and climatic processes over centennial to millennial timescales, complementing existing paleoclimate proxy records. Well‐preserved fossil Halimedais a valuable geochemical proxy archive of millennial‐scale oceanographic and climatic processesNitrogen (N) supply to Halimedabioherms originates from western tropical South Pacific thermocline waters since at least the past 5,000 yearsHalimeda15N signature records a 1‰–2‰ change in δ15N‐skeletal organic material (SOM) concurrent with regional climate variation and the El Niño Southern Oscillation Well‐preserved fossil Halimedais a valuable geochemical proxy archive of millennial‐scale oceanographic and climatic processes Nitrogen (N) supply to Halimedabioherms originates from western tropical South Pacific thermocline waters since at least the past 5,000 years Halimeda15N signature records a 1‰–2‰ change in δ15N‐skeletal organic material (SOM) concurrent with regional climate variation and the El Niño Southern Oscillation

The Great Barrier Reef (GBR) is an internationally recognized and widely studied ecosystem, yet little is known about its sea surface temperature (SST) evolution since the Last Glacial Maximum (LGM) (~20 kyr BP). Here, we present the first paleo‐application of Isoporacoral‐derived SST calibrations to a suite of 25 previously published fossil Isoporafrom the central GBR spanning ~25–11 kyr BP. The resultant multicoral Sr/Ca‐ and δ18O‐derived SST anomaly (SSTA) histories are placed within the context of published relative sea level, reef sequence, and coralgal reef assemblage evolution. Our new calculations indicate SSTs were cooler on average by ~5–5.5°C at Noggin Pass (~17°S) and ~7–8°C at Hydrographer's Passage (~20°S) (Sr/Ca‐derived) during the LGM, in line with previous estimates (Felis et al.,  2014, https://doi.org/10.1038/ncomms5102). We focus on contextualizing the Younger Dryas Chronozone (YDC, ~12.9–11.7 kyr BP), whose Southern Hemisphere expression, in particular in Australia, is elusive and poorly constrained. Our record does not indicate cooling during the YDC with near‐modern temperatures reached during this interval on the GBR, supporting an asymmetric hemispheric presentation of this climate event. Building on a previous study (Felis et al., 2014, https://doi.org10.1038/ncomms5102), these fossil IsoporaSSTA data from the GBR provide new insights into the deglacial reef response, with near‐modern warming during the YDC, since the LGM. Near‐modern sea surface temperatures were reached during the Younger Dryas Chronozone on the Great Barrier ReefSea surface temperature, reef assemblage, and sea level data provide an overview of the evolution of the Great Barrier Reef since the LGMIsopora‐based sea surface temperature calibrations can be applied to fossil Isoporato understand environmental change on coral reefs