William G. Thompson

An open-system model for U-series age determinations of fossil corals

The source of excess 234U in fossil corals and its relationship to U-series age determinations has been an outstanding problem in geochronology for more than 20 years. With increasing numbers of U-series isotope measurements in corals, and significant improvements in analytical precision through mass spectrometry, it is increasingly apparent that a substantial fraction of observed isotope ratios cannot be reasonably explained by closed-system decay. Moreover, observations of a positive correlation between 234U/238U and 230Th/238U ratios in corals from the same terrace are difficult to explain. However, the decay of dissolved uranium and α-recoil mobilization of uranium daughters produce particle-reactive 234Th and 230Th, and the coupled addition of these Th isotopes could simultaneously increase coral 234U/238U and 230Th/238U. Here we present a quantitative model, based on decay-dependent redistribution of 234Th and 230Th, permitting calculation of open-system coral ages. These equations provide a general solution to the α-recoil redistribution problem, applicable to any alpha decay series. While measured isotope ratios of corals from the three youngest stratigraphically defined Barbados terraces are inconsistent with closed-system decay, they fall in broadly linear arrays agreeing with model predictions. Isotopic arrays of older Barbados corals, and corals from terraces around the world, are also consistent with model predictions suggesting the open-system model is generally applicable. Corals with extreme isotopic compositions that are impossible to produce by closed-system decay are consistent with the limited range of isotopic compositions predicted by the model at ages older than 600 ka. For corals from a single terrace, 234Th and 230Th redistribution appears to be a source of systematic conventional age error, even for corals with slightly elevated 234U. However, open-system ages are consistent, even for corals with extremely elevated 234U. For the youngest three Barbados terraces, mean open-system terrace ages are consistent with mean conventional terrace ages calculated from pristine samples. If the most accurate conventional ages are from corals with an initial 234U/238U identical to modern seawater, then the open-system model will improve the accuracy of coral U-series age determinations and dramatically increase the number of reliable ages.

Intensification of the meridional temperature gradient in the Great Barrier Reef following the Last Glacial Maximum

Tropical south-western Pacific temperatures are of vital importance to the Great Barrier Reef (GBR), but the role of sea surface temperatures (SSTs) in the growth of the GBR since the Last Glacial Maximum remains largely unknown. Here we present records of Sr/Ca and δ18O for Last Glacial Maximum and deglacial corals that show a considerably steeper meridional SST gradient than the present day in the central GBR. We find a 1–2 °C larger temperature decrease between 17° and 20°S about 20,000 to 13,000 years ago. The result is best explained by the northward expansion of cooler subtropical waters due to a weakening of the South Pacific gyre and East Australian Current. Our findings indicate that the GBR experienced substantial meridional temperature change during the last deglaciation, and serve to explain anomalous deglacial drying of northeastern Australia. Overall, the GBR developed through significant SST change and may be more resilient than previously thought.

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’s 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’s 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.The Great Barrier Reef has migrated rapidly in response to sea-level changes since the last glacial period, suggesting resilience to environmental stress over this interval, according to a reconstruction of reef accretion.

Erratum: Intensification of the meridional temperature gradient in the Great Barrier Reef following the Last Glacial Maximum (Nature Communications (2014) 5 (4102) DOI: 10.1038/ncomms5102)

Intensification of the meridional temperature gradient in the Great Barrier Reef following the Last Glacial Maximum Thomas Felis1, Helen V. McGregor2, Braddock K. Linsley3, Alexander W. Tudhope4, Michael K. Gagan2, Atsushi Suzuki5, Mayuri Inoue6, Alexander L. Thomas4,7, Tezer M. Esat2,8,9, William G. Thompson10, Manish Tiwari11, Donald C. Potts12, Manfred Mudelsee13,14, Yusuke Yokoyama6 & Jody M. Webster15