Jacques Récy

Sea-Surface Temperature from Coral Skeletal Strontium/Calcium Ratios

Seasonal records of tropical sea-surface temperature (SST) over the past 105 years can be recovered from high-precision measurements of coral strontium/calcium ratios with the use of thermal ionization mass spectrometry. The temperature dependence of these ratios was calibrated with corals collected at SST recording stations and by 18O/16O thermometry. The results suggest that mean monthly SST may be determined with an apparent accuracy of better than 0.5�C. Measurements on a fossil coral indicate that 10,200 years ago mean annual SSTs near Vanuatu in the southwestern Pacific Ocean were about 5�C colder than today and that seasonal variations in SST were larger. These data suggest that tropical climate zones were compressed toward the equator during deglaciation.

Annual cycles of U Ca in coral skeletons and U Ca thermometry

Abstract We have discovered annual variations in the 238 U 40 Ca ratio of Porites coral skeletons. Measurements were made using thermal ionization mass spectrometric techniques, yielding precisions of ±2ℵ. (2σ) for 238 U 40 Ca and ±l‰ (2σ) for 88 Sr 40 Ca . Coralline aragonite subsamples weighed ∼2 mg, enabling submillimeter sampling resolution corresponding to ∼monthly temporal resolution. The annual nature of the cycles was confirmed by comparison with annual banding observable in X-radiographs. For two modern and one fossil sample, the amplitude of the U Ca variation ranges from 6 to 23%, well outside of analytical error. As annual U Ca cycles appear to be a general feature of primary coralline aragonite, the preservation of such features will be important in identifying unaltered coral for U-series dating studies. U Ca variations mimic and are in phase with annual variations in 88 Sr 40 Ca . For a given fractional shift in Sr Ca , the fractional shift in U Ca is about 6 times larger. For the two modern corals, 238 U 40 Ca is strongly anticorrelated with measured temperature, suggesting that 238 U 40 Ca has potential as a paleothermometer. If temperature is the only significant control on coralline 238 U 40 Ca , we reach the following conclusions from analyses of fossil samples: a Vanuatu sample, which grew halfway through the last deglaciation, gives U Ca temperatures 4 to 5°C below modern values. U Ca thermometry applied to published data for Barbados corals indicates that (1) temperature generally correlates with sea level, (2) glacial temperatures (stages 2 and 4) were 4 to 6°C lower than interglacial temperatures (stages 7a, 5e, and 1), and (3) temperatures rose from glacial to interglacial values early in the last deglaciation. Thermometry applied to Papua New Guinea corals indicates that (1) temperatures were 5 to 6°C lower than interglacial temperatures from ∼13 to ∼10 Ky bp , then rose to present values ∼9 Ky bp , (2) the temperature depression between ∼13 and ∼10 Ky bp is consistent with low temperatures observed in Vanuatu during the same time interval, and (3) the Papua New Guinea deglacial temperature history differs from that of Barbados. The results generally support estimates of tropical temperatures obtained from Sr Ca thermometry and snow line elevation data, but disagree with those based on foram transfer functions. A thermodynamic model suggests that coralline 238 U 40 Ca may also be sensitive to marine carbonate ion concentration, raising the possibility that some of the observed glacial-interglacial 238 U 40 Ca variation may result from glacial-interglacial carbonate ion changes. However, the key experiments that might establish a coralline 238 U 40 Ca-carbonate ion relationship have yet to be performed.

Evidence for stronger El Niño‐Southern Oscillation (ENSO) Events in a Mid‐Holocene massive coral

We present a 47-year-long record of sea surface temperature (SST) derived from Sr/Ca and U/Ca analysis of a massive Porites coral which grew at ∼ 4150 calendar years before present (B.P.) in Vanuatu (southwest tropical Pacific Ocean). Mean SST is similar in both the modern instrumental record and paleorecord, and both exhibit El Nino-Southern Oscillation (ENSO) frequency SST oscillations. However, several strong decadal-frequency cooling events and a marked modulation of the seasonal SST cycle, with power at both ENSO and decadal frequencies, are observed in the paleorecord, which are unprecedented in the modern record.

A high-resolution radiocarbon calibration between 11,700 and 12,400 calendar years BP derived from 230Th ages of corals from Espiritu Santo Island, Vanuatu

This paper presents radiocarbon results from a single Diploastrea heliopora coral from Vanuatu that lived during the Younger Dryas climatic episode, between ca. 11,700 and 12,400 calendar yr BP. The specimen has been independently dated with multiple 230Th measurements to permit calibration of the 14C time scale. Growth bands in the coral were used to identify individual years of growth. 14C measurements were made on each year. These values were averaged to achieve decadal resolution for the 14C calibration. The relative uncertainty of the decadal 14C data was below 1% (2σ). The data are in good agreement with the existing dendrochronology and allow for high-resolution calibration for most years. Variations in the fine structure of the 14C time series preserved in this specimen demonstrate sporadic rapid increases in the Δ14C content of the surface ocean and atmosphere. Certain sharp rises in Δ14C are coincident with gaps in coral growth evidenced by several hiatuses. These may be related to rapid climatic changes that occurred during the Younger Dryas. This is the first coral calibration with decadal resolution and the only such data set to extend beyond the dendrochronology-based 14C calibration.

Annual cycles of UCa in coral skeletons and UCa thermometry

Abstract We have discovered annual variations in the 238 U 40 Ca ratio of Porites coral skeletons. Measurements were made using thermal ionization mass spectrometric techniques, yielding precisions of ±2ℵ. (2σ) for 238 U 40 Ca and ±l‰ (2σ) for 88 Sr 40 Ca . Coralline aragonite subsamples weighed ∼2 mg, enabling submillimeter sampling resolution corresponding to ∼monthly temporal resolution. The annual nature of the cycles was confirmed by comparison with annual banding observable in X-radiographs. For two modern and one fossil sample, the amplitude of the U Ca variation ranges from 6 to 23%, well outside of analytical error. As annual U Ca cycles appear to be a general feature of primary coralline aragonite, the preservation of such features will be important in identifying unaltered coral for U-series dating studies. U Ca variations mimic and are in phase with annual variations in 88 Sr 40 Ca . For a given fractional shift in Sr Ca , the fractional shift in U Ca is about 6 times larger. For the two modern corals, 238 U 40 Ca is strongly anticorrelated with measured temperature, suggesting that 238 U 40 Ca has potential as a paleothermometer. If temperature is the only significant control on coralline 238 U 40 Ca , we reach the following conclusions from analyses of fossil samples: a Vanuatu sample, which grew halfway through the last deglaciation, gives U Ca temperatures 4 to 5°C below modern values. U Ca thermometry applied to published data for Barbados corals indicates that (1) temperature generally correlates with sea level, (2) glacial temperatures (stages 2 and 4) were 4 to 6°C lower than interglacial temperatures (stages 7a, 5e, and 1), and (3) temperatures rose from glacial to interglacial values early in the last deglaciation. Thermometry applied to Papua New Guinea corals indicates that (1) temperatures were 5 to 6°C lower than interglacial temperatures from ∼13 to ∼10 Ky bp , then rose to present values ∼9 Ky bp , (2) the temperature depression between ∼13 and ∼10 Ky bp is consistent with low temperatures observed in Vanuatu during the same time interval, and (3) the Papua New Guinea deglacial temperature history differs from that of Barbados. The results generally support estimates of tropical temperatures obtained from Sr Ca thermometry and snow line elevation data, but disagree with those based on foram transfer functions. A thermodynamic model suggests that coralline 238 U 40 Ca may also be sensitive to marine carbonate ion concentration, raising the possibility that some of the observed glacial-interglacial 238 U 40 Ca variation may result from glacial-interglacial carbonate ion changes. However, the key experiments that might establish a coralline 238 U 40 Ca-carbonate ion relationship have yet to be performed.

Little Ice Age sea surface temperature variability in the southwest tropical Pacific

We present a 60-year near-monthly record of tropical sea surface temperature (SST) during the Little Ice Age derived from coupled Sr/Ca and U/Ca analysis of a massive coral from New Caledonia (southwest tropical Pacific). The record indicates that, from 1701 to 1761, surface temperatures were on average 1.4°C cooler than during the past 30 years. This cooling was accompanied by strong interannual to interdecadal oscillations that changed the background state. Correlations between SST changes and the Southern Oscillation and the Pacific Decadal Oscillation are evolutive and appear to depend on the background state.

Geodetic measurements of convergence across the New Hebrides subduction zone

Between 1990 and 1994, geodetic measurements (GPS observations) have been conducted across the New Hebrides subduction zone where the Australia plate subducts under the New Hebrides Arc. This paper establishes convergence rate variations along the trench. In the South, at Tanna, the relative motion is oriented N244 ±4 and has a uniform rate of 11.7±0.8 cm/yr. The rate at Efate is 10.3±0.9 cm/yr, oriented N242 ±4. Both azimuths very well compare with slip vectors of the last major earthquakes. In the North, the rate at Santo is only 3.6 ±1.2 cm/yr, oriented N253 ± 26. The difference in the convergence rates between Santo on the one hand and Efate and Tanna in the other hand points to a right lateral shear zone between Santo and Efate. At Santo where the plate coupling is very high, the very low convergence rate might be related to the absence of recent strong earthquakes. No significant variations are detected for the baselines within the Australia plate.

Development patterns of fringing and barrier reefs in New Caledonia (southwest Pacific)

In New Caledonia, core data indicate that the fringing reefs grew during the last interglacial and the Holocene, while the barrier reefs developed during several high sea level stands of the Quaternary. These growth periods are archived in a 128-m-long core from Ilot Amedee, offshore of Noumea. Directly upon the peridotitic substrate (reached at 126.50 m), a basal unit comprising abundant rhodoliths, molluscs and rare corals is characteristic of the pioneer stage of barrier reef development. Above it, a severely calcitized unit contains corals, molluscs and abundant micritic levels. Then, a sequence punctuated by several minor discontinuities and by a downward increase in diagenetic alteration is found. At 47 m core depth, a unit, which is thought to be related to isotopic stages 7 and 9, contains thin beds of coralgal frameworks and muddy detritus. The 125 ka-old reef, from 14 to 37 m core deep, is predominantly composed of biocalcarenites and rare coral colonies. Finally, the Holocene reef is composed of sands and scarce coral buildups. Drilling results indicate that the distributional patterns of the 125-ka-old reef bodies around New Caledonia express an increasing tendency of island subsidence northward, southwestward and more markedly seaward, mainly controlled by isostatic readjustments and margin collapse.

Interseismic and coseismic motions in GPS series related to the Ms 7.3 July 13, 1994, Malekula Earthquake, Central New Hebrides Subduction Zone

On July 13, 1994, an earthquake Ms=7.3 occurred at Malekula, in the New Hebrides archipelago. The GPS data collected across the New Hebrides trench between 1990 and 1996 were processed in order to separate the interseismic and coseismic motions from the drifts related to the convergence with the Australian plate. The GPS-derived coseismic displacements at the GPS site in Malekula are 49 ± 15 mm southward, 230 ± 30 mm westward and 170 ± 37 mm downward, when the CMT-derived displacements are 50 mm southward, 210 mm westward and 150 mm downward. Taking into account the interseismic strain accumulation (25 mm/yr at the source established from historical seismicity, 7.5 mm/yr at the GPS site), the strain-free convergence rate at Malekula is 49 ± 3 mm/yr. Other GPS-derived convergence rates are 95 ± 1 mm/yr at Efate and 37 ± 2 mm/yr at Santo. These rates imply a regional right-lateral motion between the Efate and the Santo-Malekula segments. In contrast, the focal mechanism of the earthquake mostly indicates a left-lateral motion. Therefore, we hypothesize that the earthquake is related to variations in the interplate coupling along the converging boundary of the Santo-Malekula segment.

Annual cycles of in coral skeletons and thermometry

Abstract We have discovered annual variations in the 238 U 40 Ca ratio of Porites coral skeletons. Measurements were made using thermal ionization mass spectrometric techniques, yielding precisions of ±2ℵ. (2σ) for 238 U 40 Ca and ±l‰ (2σ) for 88 Sr 40 Ca . Coralline aragonite subsamples weighed ∼2 mg, enabling submillimeter sampling resolution corresponding to ∼monthly temporal resolution. The annual nature of the cycles was confirmed by comparison with annual banding observable in X-radiographs. For two modern and one fossil sample, the amplitude of the U Ca variation ranges from 6 to 23%, well outside of analytical error. As annual U Ca cycles appear to be a general feature of primary coralline aragonite, the preservation of such features will be important in identifying unaltered coral for U-series dating studies. U Ca variations mimic and are in phase with annual variations in 88 Sr 40 Ca . For a given fractional shift in Sr Ca , the fractional shift in U Ca is about 6 times larger. For the two modern corals, 238 U 40 Ca is strongly anticorrelated with measured temperature, suggesting that 238 U 40 Ca has potential as a paleothermometer. If temperature is the only significant control on coralline 238 U 40 Ca , we reach the following conclusions from analyses of fossil samples: a Vanuatu sample, which grew halfway through the last deglaciation, gives U Ca temperatures 4 to 5°C below modern values. U Ca thermometry applied to published data for Barbados corals indicates that (1) temperature generally correlates with sea level, (2) glacial temperatures (stages 2 and 4) were 4 to 6°C lower than interglacial temperatures (stages 7a, 5e, and 1), and (3) temperatures rose from glacial to interglacial values early in the last deglaciation. Thermometry applied to Papua New Guinea corals indicates that (1) temperatures were 5 to 6°C lower than interglacial temperatures from ∼13 to ∼10 Ky bp , then rose to present values ∼9 Ky bp , (2) the temperature depression between ∼13 and ∼10 Ky bp is consistent with low temperatures observed in Vanuatu during the same time interval, and (3) the Papua New Guinea deglacial temperature history differs from that of Barbados. The results generally support estimates of tropical temperatures obtained from Sr Ca thermometry and snow line elevation data, but disagree with those based on foram transfer functions. A thermodynamic model suggests that coralline 238 U 40 Ca may also be sensitive to marine carbonate ion concentration, raising the possibility that some of the observed glacial-interglacial 238 U 40 Ca variation may result from glacial-interglacial carbonate ion changes. However, the key experiments that might establish a coralline 238 U 40 Ca-carbonate ion relationship have yet to be performed.