New constraints on the postglacial shallow-water carbonate accumulation in the Great Barrier Reef

More accurate global volumetric estimations of shallow-water reef deposits are needed to better inform climate and carbon cycle models. Using recently acquired datasets and International Ocean Discovery Program (IODP) Expedition 325 cores, we calculated shallow-water CaCO3 volumetrics and mass for the Great Barrier Reef region and extrapolated these results globally. In our estimates, we include deposits that have been neglected in global carbonate budgets: Holocene Halimeda bioherms located on the shelf, and postglacial pre-Holocene (now) drowned coral reefs located on the shelf edge. Our results show that in the Great Barrier Reef alone, these drowned reef deposits represent ca. 135 Gt CaCO3, comparatively representing 16-20% of the younger Holocene reef deposits. Globally, under plausible assumptions, we estimate the presence of ca. 8100 Gt CaCO3 of Holocene reef deposits, ca. 1500 Gt CaCO3 of drowned reef deposits and ca. 590 Gt CaCO3 of Halimeda shelf bioherms. Significantly, we found that in our scenarios the periods of pronounced reefal mass accumulation broadly encompass the occurrence of the Younger Dryas and periods of CO2 surge (14.9-14.4 ka, 13.0-11.5 ka) observed in Antarctic ice cores. Our estimations are consistent with reef accretion episodes inferred from previous global carbon cycle models and with the chronology from reef cores from the shelf edge of the Great Barrier Reef

Role of the deglacial buildup of the Great Barrier Reef for the global carbon cycle

An outstanding problem in our understanding of the global carbon cycle is unravelling the processes that were responsible for the rise of atmospheric CO2 during the last deglaciation (~19 to 11 ka). The carbon isotope 13C is commonly used to attribute the last deglacial atmospheric CO2 rise 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 by IODP Expedition 325. We show that the growth of the world’s largest reef system, the Great Barrier Reef, 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 δ13CO2 and the Younger Dryas cold period of the Northern Hemisphere. The event follows the flooding of a large shelf platform and initiation of an extensive barrier reef system at 13 ka. We show, by carbon cycle simulations, that the Great Barrier Reef 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 CO2 and δ13CO2, however, are marginal. Thus, the Great Barrier Reef was not contributing to the last deglacial δ13CO2 minimum at ~12.4 ka, and the world’s largest reef system in existence appears to have little effect on the last deglacial atmospheric CO2 and δ13CO2 changes

Role of the Deglacial Buildup of the Great Barrier Reef for the Global Carbon Cycle

The carbon isotope 13C is commonly used to attribute the last deglacial atmospheric CO2 rise 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 δ13CO2 and 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 CO2 and δ13CO2, however, are marginal. Thus, the GBR was not contributing to the last deglacial δ13CO2 minimum at ∼12.4 ka

Response of the Great Barrier Reef to sea level and environmental changes over the past 30,000 years

Previous drilling through submerged fossil coral reefs has greatly improved our understanding of the general pattern of sea-level change since the Last Glacial Maximum, however, how reefs responded to these changes remains uncertain. Here we document the evolution of the Great Barrier Reef (GBR), the world\u27s largest reef system, to major, abrupt environmental changes over the past 30 thousand years based on comprehensive sedimentological, biological and geochronological records from fossil reef cores. We show that reefs migrated seaward as sea level fell to its lowest level during the most recent glaciation (~20.5-20.7 thousand years ago (ka)), then landward as the shelf flooded and ocean temperatures increased during the subsequent deglacial period (~20-10 ka). Growth was interrupted by five reef-death events caused by subaerial exposure or sea-level rise outpacing reef growth. Around 10 ka, the reef drowned as the sea level continued to rise, flooding more of the shelf and causing a higher sediment flux. The GBR\u27s capacity for rapid lateral migration at rates of 0.2-1.5 m yr−1 (and the ability to recruit locally) suggest that, as an ecosystem, the GBR has been more resilient to past sea-level and temperature fluctuations than previously thought, but it has been highly sensitive to increased sediment input over centennial-millennial timescales

Rationale, study design, and analysis plan of the Alveolar Recruitment for ARDS Trial (ART): Study protocol for a randomized controlled trial

Background: Acute respiratory distress syndrome (ARDS) is associated with high in-hospital mortality. Alveolar recruitment followed by ventilation at optimal titrated PEEP may reduce ventilator-induced lung injury and improve oxygenation in patients with ARDS, but the effects on mortality and other clinical outcomes remain unknown. This article reports the rationale, study design, and analysis plan of the Alveolar Recruitment for ARDS Trial (ART). Methods/Design: ART is a pragmatic, multicenter, randomized (concealed), controlled trial, which aims to determine if maximum stepwise alveolar recruitment associated with PEEP titration is able to increase 28-day survival in patients with ARDS compared to conventional treatment (ARDSNet strategy). We will enroll adult patients with ARDS of less than 72 h duration. The intervention group will receive an alveolar recruitment maneuver, with stepwise increases of PEEP achieving 45 cmH(2)O and peak pressure of 60 cmH2O, followed by ventilation with optimal PEEP titrated according to the static compliance of the respiratory system. In the control group, mechanical ventilation will follow a conventional protocol (ARDSNet). In both groups, we will use controlled volume mode with low tidal volumes (4 to 6 mL/kg of predicted body weight) and targeting plateau pressure <= 30 cmH2O. The primary outcome is 28-day survival, and the secondary outcomes are: length of ICU stay; length of hospital stay; pneumothorax requiring chest tube during first 7 days; barotrauma during first 7 days; mechanical ventilation-free days from days 1 to 28; ICU, in-hospital, and 6-month survival. ART is an event-guided trial planned to last until 520 events (deaths within 28 days) are observed. These events allow detection of a hazard ratio of 0.75, with 90% power and two-tailed type I error of 5%. All analysis will follow the intention-to-treat principle. Discussion: If the ART strategy with maximum recruitment and PEEP titration improves 28-day survival, this will represent a notable advance to the care of ARDS patients. Conversely, if the ART strategy is similar or inferior to the current evidence-based strategy (ARDSNet), this should also change current practice as many institutions routinely employ recruitment maneuvers and set PEEP levels according to some titration method.Hospital do Coracao (HCor) as part of the Program 'Hospitais de Excelencia a Servico do SUS (PROADI-SUS)'Brazilian Ministry of Healt

Shelf-edge reefs of the Great Barrier Reef: A time-capsule from the last glaciation

A detailed investigation of the internal and external architecture of the shelf-edge reefs (SERs) of the Great Barrier Reef (GBR), Australia is presented here, constituting the most comprehensive seismic stratigraphy study of these drowned reefs. In two sites of the central GBR, seismic reflectors and facies were identified, ground-truthed against core and downhole data from the Integrated Ocean Drilling Program, Expedition 325. Marked depositional differences between the two sites were found and linked to local and regional physiographic and environmental contrasts. A sequential stratigraphy framework was established for these sites, which exhibit a complete depositional sequence dominated by transgressive reefs, bounded by two flooding surfaces. The postglacial flooding of the GBR shelf was also simulated. The measured parameters suggest a strong influence of the local antecedent substrate and of the interplay of regional physiographic variations and sea level change in the development of the SERs. Supported in these new interpretations, local and regional SERs CaCO3 accumulation were estimated. It was found that the Pleistocene SERs of the GBR are equivalent to ca. 20 % of the GBR's Holocene reef mass. Both the magnitude and the timing of the shelf-edge reef accumulation suggest that the drowned reefs in the GBR (and globally) had the potential to influence postglacial climate change. In addition, forward stratigraphic simulations were run on models based on this dataset, which suggest that the ensemble of conditions for reef growth deteriorated as the transgression advanced, resulting in shelf-edge reef demise. The role of the basement substrate was significant, but limited. All together, the multidisciplinary reconstructions in this study represent a useful framework to constrain the development of these under-studied formations, which according to the findings had a significant role in shaping the Quaternary GBR and, possibly, in postglacial climate change

Postglacial sediment deposition along a mixed carbonate-siliciclastic margin: new constraints from the drowned shelf-edge reefs of the Great Barrier Reef, Australia

A seismic stratigraphy analysis was conducted at two sites, Hydrographers and Noggin passages, separated by about 540 km on the shelf-edge of the central Great Barrier Reef (GBR), Australia. We used recently available seismic and bathymetry data and a new synthesis of downhole logs and lithological, petrophysical and radiometric data from cores recovered by the Integrated Ocean Drilling Program Expedition 325 (Great Barrier Reef Environmental Changes). We compared the stratigraphy of both sites, identifying a full depositional sequence with deposits from at least 28 ka BP to ~ 7 ka BP, bounded by two marine flooding surfaces. Within this sequence, each systems tract is represented by unique depositional features characteristic of the shelf-edge systems. Despite the broad environmental and geomorphic similarities between the two sites, differences in postglacial reef development were remarkable. These contrasts can be explained as a result of: (1) local antecedent substrate variations and (2) the interplay of shelf physiography with Late Quaternary sea level fluctuations, which favoured changes in biological production and sediment flux as the palaeo-shoreline evolved from linear to complex during intermediate sea levels. During these intermediate sea levels, the northern estuarine coast and its steep substrate at shelf-edge locations contrasted strongly with the protected palaeo-lagoons and the extensive, gentle marginal terraces found at the southern central GBR. This setting enhanced the regional differences in sediment transport and reef development through the last transgression. The conceptual model presented here provides a broader depositional framework and improves the understanding of the main processes controlling the spatial and temporal depositional patterns on the shelf-edge of mixed siliciclastic-carbonate margins

Simulation results for the role of the deglacial buildup of the Great Barrier Reef for the global carbon cycle

The carbon isotope 13C is commonly used to attribute the rise of atmospheric CO2 during the last deglaciation to various processes. Here we discuss the role of the growth of the Great Barrier Reef (GBR), the world's largest reef system, in the global carbon cycle marked by a pronounced decrease in d13C of 1.25‰ in absolutely dated fossil coral skeletons between 12.8 and 11.7 ka, which coincides with a minimum in atmospheric d13CO2. The abrupt start of this event follows the flooding of a large platform along the shelf off northeastern Australia and initiation of an extensive barrier reef system at 13 ka. We show the d13C decrease recorded by the corals to be mainly caused by a combination of reef carbonate production rates and rapid oxidation of organic land carbon on the newly flooded shallow-water platform. Both processes contributed together less than 1 ppm to the ongoing deglacial rise of atmospheric CO2 and, rather counterintuitively, to a small rise in atmospheric d13CO2 of less than 0.001‰. We can thus exclude that the GBR was even partially responsible for the minimum in atmospheric d13CO2 centred at ~12.4 ka. The measured d13C from the GBC are already arichved under https://doi.org/10.1594/PANGAEA.833408. The simulation results of a 3-box model performed with MATLAB, including the MATLAB code are archived. Additionally, simulation results for atmospheric CO2 and d13CO using the BICYCLE-SE model (and its forcing in terms of CaCO3 accumulation rate and flooded area) are included here

Seismic stratigraphy and development of the shelf-edge reefs of the Great Barrier Reef, Australia

Extensive shelf-edge reefs (SERs) and terraces are common along the seaward margin of the Great Barrier Reef (GBR). We aim to better understand the architecture of the SERs, deposited during a poorly constrained period: the Last Glacial Maximum (LGM) and early postglacial. A dense array of Topas seismic lines was interpreted at Hydrographers Passage in the southern central GBR following a seismic stratigraphic approach ground-truthed against Integrated Ocean Drilling Program Expedition 325 (IODP Exp. 325) sediment core data. We interpreted two prominent sub-bottom reflectors, an upper R1 and a lower R2, that separate three seismic units. According to our borehole-to-seismic correlation in a representative location, R1 corresponds to a downward increase of impedance which is also reflected in lithological and diagenetical changes. R2 is poorly constrained, but likely also represents a lithological contrast. Seismic velocities were also estimated in the same borehole location for inter-reef deposits and reef framework. Together with the geomorphic interpretation, we reconstructed velocity maps for the study area, leading to the generation of a 3D geomorphic model of the antecedent topography or substrate, and volumetric calculations. Geomorphic and thickness trends suggest a strong relationship between the antecedent topography and the morphology of the SERs. The distribution of seismic facies identified in the dataset, coupled to geomorphic trends and published sea level curves, allowed us to reconstruct the LGM to postglacial depositional history of these deposits. Our interpretation suggests significant regressive reefal accumulations formed on top of a marine flooding surface represented by reflector R2 during the sea level fall to the LGM. R1 is the distal equivalent of an unconformity previously identified in the GBR inner- and mid-shelf, widely affected by subaerial exposure, on top of which a postglacial, transgressive, mainly reefal Unit 1 was deposited. Two zones are distinguished: (1) a distal zone (below 80 mbsl) with thick deposits displaying fringing-reef morphologies, influenced by the late-LGM fall and subsequent early-postglacial rise in sea level and by the updip availability of substrate; and (2) a proximal zone (above 80 mbsl) of enhanced vertical growth with unfilled lagoons, resulting from the lack of lateral substrate and the rapid sea level rise. Our seismic stratigraphic model of the development of the SERs, sediment volumetrics, and 3D reconstruction of the antecedent topography provides a foundation for future forward modeling stratigraphic studies

Relative sea level response to mixed carbonate-siliciclastic sediment loading along the Great Barrier Reef margin

The continental shelf along northeastern Australia is the world’s largest mixed carbonate-siliciclastic passive margin and the location of the Great Barrier Reef (GBR). Following sea-level transgression during the last deglaciation, extensive sediment was deposited along the GBR due to neritic carbonate deposition (including shelf edge reefs, Holocene reefs and Halimeda bioherms) and fluvial discharge of terrigenous siliciclastic sediments. Such sediment loading can alter local relative sea level (RSL) by several metres through the sediment isostatic adjustment (SIA) process, a signal that is poorly constrained at the GBR. In this study, we used a glacial isostatic adjustment (GIA) model to develop an ensemble-based sediment loading history for the GBR since Marine Isotope Stage 2 (MIS 2). A Bayesian style framework is adopted to calibrate the sediment history ensemble and GIA model parameters using a sea-level database. According to our results, 1853.7 Gt (1613.1-2078.7 Gt, 95% confidence interval) of sediment have been deposited across the GBR since MIS 2 (28 ka BP), causing spatially variable relative sea-level change with the highest magnitude (0.9-1.1 m) found in the outer shelf of the southern central GBR (18.4-21.6◦ S). Because the SIA-induced RSL rise is unrelated to ice mass loss, failing to correct for this signal will lead to systematic overestimation of grounded ice volume by up to ∼4.3 × 105 km3 during the Last Glacial Maximum. Additionally, we found that spatial variation in sediment loading and coastal environment may explain the different RSL history documented by published fossil coral reef records from Noggin Pass and Hydrographer’s Passage. These results highlight the importance of considering SIA for any postglacial sea-level studies adjacent to large sediment systems. Lastly, by quantifying both the GIA and SIA signals, we provide a spatially and temporally complete RSL reconstruction that is well-suited to be used as a boundary condition to study the evolution of the GBR shelf and slope sedimentary system