Search

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 δ(18)O 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.

The approximately 10,000-year-long Last Glacial Maximum, before the termination of the last ice age, was the coldest period in Earth's recent climate history.sup.1. Relative to the Holocene epoch, atmospheric carbon dioxide was about 100 parts per million lower and tropical sea surface temperatures were about 3 to 5 degrees Celsius lower.sup.2,3. The Last Glacial Maximum began when global mean sea level (GMSL) abruptly dropped by about 40 metres around 31,000 years ago.sup.4 and was followed by about 10,000 years of rapid deglaciation into the Holocene.sup.1. The masses of the melting polar ice sheets and the change in ocean volume, and hence in GMSL, are primary constraints for climate models constructed to describe the transition between the Last Glacial Maximum and the Holocene, and future changes; but the rate, timing and magnitude of this transition remain uncertain. Here we show that sea level at the shelf edge of the Great Barrier Reef dropped by around 20 metres between 21,900 and 20,500 years ago, to -118 metres relative to the modern level. Our findings are based on recovered and radiometrically dated fossil corals and coralline algae assemblages, and represent relative sea level at the Great Barrier Reef, rather than GMSL. Subsequently, relative sea level rose at a rate of about 3.5 millimetres per year for around 4,000 years. The rise is consistent with the warming previously observed at 19,000 years ago.sup.1,5, but we now show that it occurred just after the 20-metre drop in relative sea level and the related increase in global ice volumes. The detailed structure of our record is robust because the Great Barrier Reef is remote from former ice sheets and tectonic activity. Relative sea level can be influenced by Earth's response to regional changes in ice and water loadings and may differ greatly from GMSL. Consequently, we used glacio-isostatic models to derive GMSL, and find that the Last Glacial Maximum culminated 20,500 years ago in a GMSL low of about -125 to -130 metres.Details of the drops in sea level associated with glaciation during the Last Glacial Maximum are revealed using a coral proxy from the Great Barrier Reef., Author(s): Yusuke Yokoyama [sup.1] [sup.2] [sup.3] , Tezer M. Esat [sup.4] [sup.5] , William G. Thompson [sup.6] , Alexander L. Thomas [sup.7] , Jody M. Webster [sup.8] , Yosuke Miyairi [...]

Past sea-level records provide invaluable information about the response of ice sheets to climate forcing. Some such records suggest that the last deglaciation was punctuated by a dramatic period of sea-level rise, of about 20 metres, in less than 500 years. Controversy about the amplitude and timing of this meltwater pulse (MWP-1A) has, however, led to uncertainty about the source of the melt water and its temporal and causal relationships with the abrupt climate changes of the deglaciation. Here we show that MWP-1A started no earlier than 14,650 years ago and ended before 14,310 years ago, making it coeval with the Belling warming. Our results, based on corals drilled offshore from Tahiti during Integrated Ocean Drilling Project Expedition 310, reveal that the increase in sea level at Tahiti was between 12 and 22 metres, with a most probable value between 14 and 18 metres, establishing a significant meltwater contribution from the Southern Hemisphere. This implies that the rate of eustatic sea-level rise exceeded 40 millimetres per year during MWP-1A., Although dynamic responses of the Greenland and Antarctic ice sheets to climate forcing may already be contributing to present-day sea-level rise (1), projections of sea-level change for the twenty-first century [...]

The meridional overturning circulation (MOC) of the Atlantic Ocean is considered to be one of the most important components of the climate system. This is because its warm surface currents, such as the Gulf Stream, redistribute huge amounts of energy from tropical to high latitudes and influence regional weather and climate patterns, whereas its lower limb ventilates the deep ocean and affects the storage of carbon in the abyss, away from the atmosphere. Despite its significance for future climate, the operation of the MOC under contrasting climates of the past remains controversial. Nutrient-based proxies (1,2) and recent model simulations (3) indicate that during the Last Glacial Maximum the convective activity in the North Atlantic Ocean was much weaker than at present. In contrast, rate-sensitive radiogenic [sup.231]Pa/[sup.230]Th isotope ratios from the North Atlantic have been interpreted to indicate only minor changes in MOC strength (4-6). Here we show that the basin-scale abyssal circulation of the Atlantic Ocean was probably reversed during the Last Glacial Maximum and was dominated by northward water flow from the Southern Ocean. These conclusions are based on new high-resolution data from the South Atlantic Ocean that establish the basin-scale north to south gradient in [sup.231]Pa/[sup.230]Th, and thus the direction of the deep ocean circulation. Our findings are consistent with nutrient-based proxies and argue that further analysis of [sup.231]Pa/[sup.230]Th outside the North Atlantic basin will enhance our understanding of past ocean circulation, provided that spatial gradients are carefully considered. This broader perspective suggests that the modern pattern of the Atlantic MOC--with a prominent southerly flow of deep waters originating in the North Atlantic--arose only during the Holocene epoch., A characteristic feature of our present ocean circulation is the deep convection that occurs in the North Atlantic and spreads North Atlantic Deep Water (NADW) to the world's abyssal oceans. [...]

This manuscript describes a new protocol for determination of Pa/Th/U in marine sediments. It is based on microwave-assisted digestion and represents an important reduction of working time over conventional hot-plate digestion methods, and the use of HCl[O.sub.4] is avoided. Although Th and U are completely dissolved with a first microwave step, around 40% of [sup.231]Pa remains undissolved, and a short hot-plate step with reverse aqua regia is required to achieve total digestion and spike equilibration. Next, the method involves a separation of these elements and a further purification of the Pa fraction using Dowex AG1-X8 resin. Separation with Bio-Rad and Sigma-Aldrich resins was compared; although both perform similarly for Th and U, Pa yields are higher with Bio-Rad. Finally, samples are measured using a Nu instruments multiple collector inductively coupled plasma mass spectrometer (MC-ICPMS). Overall chemical yields range around 50% for Pa, 60% for Th, and 70% for U.

Shallow-water tropical corals can be used to calibrate the radiocarbon timescale. In this paper, we present a new data set based on the comparison between 14C ages and U-Th ages measured in fossil corals collected offshore the island of Tahiti during the Integrated Oceanic Drilling Program (IODP) Expedition 310. After applying strict mineralogical and geochemical screening criteria, the Tahiti record provides new data for 2 distinct time windows: 7 data for the interval between 29 and 37 cal kyr BP and 58 for the last deglaciation period, notably a higher resolution for the 14-16 cal kyr BP time interval. There are 3 main outcomes of this study. First, it extends the previous Tahiti record beyond 13.9 cal kyr BP, the oldest U-Th age obtained on cores drilled onshore in the modern Tahiti barrier reef. Second, it strengthens the data set of the 14-15 cal kyr BP period, allowing for better documentation of the 14C age plateau in this time range. This age plateau corresponds to a drop of the atmospheric 14C synchronous with an abrupt period of sea-level rise (Melt Water Pulse 1A, MWP-1A). The Tahiti 14C record documents complex changes in the global carbon cycle due to variations in the exchange rates between its different reservoirs. Third, during the Heinrich event 1, the Tahiti record disagrees with the Cariaco record, but is in broad agreement with other marine and continental data. [ABSTRACT FROM AUTHOR]

Past sea-level records provide invaluable information about the response of ice sheets to climate forcing. Some such records suggest that the last deglaciation was punctuated by a dramatic period of sea-level rise, of about 20?metres, in less than 500?years. Controversy about the amplitude and timing of this meltwater pulse (MWP-1A) has, however, led to uncertainty about the source of the melt water and its temporal and causal relationships with the abrupt climate changes of the deglaciation. Here we show that MWP-1A started no earlier than 14,650 years ago and ended before 14,310 years ago, making it coeval with the Bølling warming. Our results, based on corals drilled offshore from Tahiti during Integrated Ocean Drilling Project Expedition 310, reveal that the increase in sea level at Tahiti was between 12 and 22 metres, with a most probable value between 14 and 18 metres, establishing a significant meltwater contribution from the Southern Hemisphere. This implies that the rate of eustatic sea-level rise exceeded 40 millimetres per year during MWP-1A. [ABSTRACT FROM AUTHOR]

Abstract: Aeolian dust is a significant source of iron and other nutrients to the surface ocean, but constraining the size of this flux is difficult. Chemical tracers, such as Al, are useful, but are limited by lack of information about their rate of removal from surface seawater. In this study, we test the use of Th isotopes as an improved tracer of dust input by measuring 232Th and 230Th concentrations of upper-ocean seawater at six stations along a meridional Atlantic section that spans the Saharan dust plume. Open ocean 232Th, like Al, is dominantly derived from dust dissolution, while 230Th is produced in seawater by U decay and provides an assessment of Th removal rates. The highest 232Th concentrations and 232Th/230Th ratios, particularly in the surface waters, are observed at stations expected to be influenced by input of Saharan dust. The pattern of 232Th concentration is in agreement with dissolved Al distributions and SeaWiFS aerosol imagery, supporting the use of 232Th as a proxy for short-term dust addition. Deeper in the water column, 232Th concentration may also reflect longer-term dust inputs. In addition, lateral input of 232Th from Mediterranean Outflow Water is detected at 1000m in the north Atlantic. Surface-water residence times are calculated separately from 230Th and 234Th using a simple scavenging model and values from the two isotopes are generally consistent with one another. Th-230 residence times appear to reflect the longer-term value, however, rather than exhibiting the strong seasonal influence seen for 234Th. These Th residence times allow dust flux to the surface ocean to be calculated using the known concentration of Th in dust, and assumed values for the solubility of dust Th in seawater. Resulting dust fluxes show good agreements with models of dust deposition if Th solubility is 1 to 5% in the north Atlantic (yielding fluxes of 0.4–19.2gm−2 yr−1) and 5 to 10% in the south Atlantic (fluxes of 0.2–0.7gm−2 yr−1). Despite the need for better constraints on the fractional solubility of dust Th, this study demonstrates the power of combining Th isotope measurements to constrain dust fluxes to the surface ocean. [Copyright &y& Elsevier]

The last deglaciation is characterized by a rapid sea-level rise and coeval abrupt environmental changes. The Barbados coral reef record suggests that this period has been punctuated by two brief intervals of accelerated melting (meltwater pulses, MWP), occurring at 14.08-13.61 ka and 11.4-11.1 ka (calendar years before present), that are superimposed on a smooth and continuous rise of sea level. Although their timing, magnitude, and even existence have been debated, those catastrophic sea-level rises are thought to have induced distinct reef drowning events. The reef response to sea-level and environmental changes during the last deglacial sea-level rise at Tahiti is reconstructed based on a chronological, sedimentological, and paleobiological study of cores drilled through the relict reef features on the modern forereef slopes during the Integrated Ocean Drilling Program Expedition 310, complemented by results on previous cores drilled through the Papeete reef. Reefs accreted continuously between 16 and 10 ka, mostly through aggradational processes, at growth rates averaging 10 mm yr-1. No cessation of reef growth, even temporary, has been evidenced during this period at Tahiti. Changes in the composition of coralgal assemblages coincide with abrupt variations in reef growth rates and characterize the response of the upward-growing reef pile to nonmonotonous sea-level rise and coeval environmental changes. The sea-level jump during MWP 1A, 16 ± 2 m of magnitude in ~350 yr, induced the retrogradation of shallow-water coral assemblages, gradual deepening, and incipient reef drowning. The Tahiti reef record does not support the occurrence of an abrupt reef drowning event coinciding with a sea-level pulse of ~15 m, and implies an apparent rise of 40 mm yr-1 during the time interval corresponding to MWP 1B at Barbados. [ABSTRACT FROM AUTHOR]