Guy Cabioch

Radiocarbon calibration by means of mass spectrometric 230Th/234U and 14C ages of corals: an updated database including samples from Barbados, Mururoa and Tahiti

As first shown by Bard et al. (1990a), high-precision 230Th-234U ages can be used successfully to calibrate the radiocarbon time scale beyond the high-precision tree-ring calibration that now reaches 11,900 cal BP (Kromer and Spurk 1998). Using mass spectrometric techniques, we measured 14C and 230Th ages on new samples collected from boreholes drilled off the islands of Tahiti and Mururoa (French Polynesia) in order to complement the database previously obtained on Barbados corals (Bard et al. 1990a, 1993).

Nature and environmental significance of microbialites in Quaternary reefs: the Tahiti paradox

Drill cores from the bamer reef-edge of Tahiti exhibit 85-93-m-thick coralgal sequences recording at least 13,500 years of continuous reef growth in optimal environmental conditions. The cored reef sequences form an overall shallowingupward succession with assemblages of branching or massive colonies of Porires at the base overlain by a robust-branching community (Acropom gr. dmmi/robirstn) heavily encrusted by coralline algae, sessile vermetid gastropods and arborescent foraminifers, which grew at depths less than 6 m. Microbialites generally form the last stage of encrustation of coral colonies, or more commonly, of related encrusting organisms, thus appearing as a major structural component of the reef sequence where they may locally form 809 of the rock by volume. They developed in an open cavity system of the reef framework with freely circulating normal-marine water. Microbialites include laminated crusts and clotted micritic masses, commonly associated in compound crusts, probably feflecting differences in the composition of the involved biological communities and in biomineralization processes that controlled the accretion of the crusts. The isotopic composition of the microbialites (+2.05 to +3.92% I they underwent also indivitlual processes of biominrlali2ation. Besicles the overall decrease in lizht and enqy conditions rttlecting progressive burial by conil growth, the widespread development of microbialites within the reef framework may be related to increased alkalinity and niitrient availability in interstitial waters due to temstrial groundwwr seepage and puiodic runoffs. The tlevelopment of micrubialites in the cryptic niches of the reef framework ceased about 6000 years

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.

Interdecadal variation in the extent of South Pacific tropical waters during the Younger Dryas event

During the Younger Dryas event, about 12,000 years ago, the Northern Hemisphere cooled by between 2 and 10 °C (refs 1, 2) whereas East Antarctica experienced warming. But the spatial signature of the event in the southern mid-latitudes and tropics is less well known, as records are sparse and inconclusive. Here we present high-resolution analyses of skeletal Sr/Ca and 18O/16O ratios for a giant fossil Diploastrea heliopora coral that was preserved in growth position on the raised reef terraces of Espiritu Santo Island, Vanuatu, in the southwestern tropical Pacific Ocean. Our data indicate that sea surface temperatures in Vanuatu were on average 4.5 ± 1.3 °C cooler during the Younger Dryas event than today, with a significant interdecadal modulation. The amplified annual cycle of sea surface temperatures, relative to today, indicates that cooling was caused by the compression of tropical waters towards the Equator. The positive correlation in our record between the oxygen isotope ratios of sea water and sea surface temperatures suggests that the South Pacific convergence zone, which brings 18O-depleted precipitation to the area today, was not active during the Younger Dryas period.

Holocene initiation and development of New Caledonian fringing reefs, SW Pacific

The analysis of 8 selected cores through fringing reefs in New Caledonia reveals that accretion in the Holocene has been less than 6 m. The cores exhibit three main facies: branching coral (Acropora, dominantly), massive coral heads (Porites, mainly) and coral sand/ rubble, principally made up of acroporid fragments. Subordinate facies are composed of coralline algae and alcyonarian spiculite. The initiation of growth varies according to location. The southern reefs (i.e. early settled reefs) generally began to grow first, prior to 5000 y BP. The northern structures (i.e. more recently settled reefs) are younger, occurring after 4200 y BP. This retardation could be ascribed to differences in local physical conditions (nature of substrate, wave energy). Vertical accretion rates were generally higher in areas of lower energy (3.25–6.4 mm·y-1) versus those exposed to higher energy conditions (1.4–3.1 mm·y-1). Vertical development through time was accompanied by changes in composition of biological assemblages which reflect changes in hydrodynamics. The basal Acropora-dominated facies was replaced upwards by a Porites-dominated framework. The New Caledonian fringing reefs reached the sea surface generally between 5000 and 2500 y BP after the stabilization of sea level. Hence all of these reefs can be classified as catch-up reefs.

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.

Radiocarbon calibration and comparison to 50 kyr BP with paired 14C and 230Th dating of corals from Vanuatu and Papua New Guinea

We calibrated portions of the radiocarbon time scale with combined (super 230) Th, (super 231) Pa, (super 14) C measurements of corals collected from Espiritu Santo, Vanuatu and the Huon Peninsula, Papua New Guinea. The new data map (super 14) C variations ranging from the current limit of the tree-ring calibration (11,900 calendar years before present [cal BP], Kromer and Spurk 1998, now updated to 12,400 cal BP, see Kromer et al., this issue), to the (super 14) C-dating limit of 50,000 cal BP, with detailed structure between 14 to 16 cal kyr BP and 19 to 24 cal kyr BP. Samples older than 25,000 cal BP were analyzed with high-precision (super 231) Pa dating methods (Pickett et al. 1994; Edwards et al. 1997) as a rigorous second check on the accuracy of the (super 230) Th ages. These are the first coral calibration data to receive this additional check, adding confidence to the age data forming the older portion of the calibration. Our results, in general, show that the offset between calibrated and (super 14) C ages generally increases with age until about 28,000 cal BP, when the recorded (super 14) C age is nearly 6800 yr too young. The gap between ages before this time is less; at 50,000 cal BP, the recorded (super 14) C age is 4600 yr too young. Two major (super 14) C-age plateaus result from a 13 ppm drop in Delta (super 14) C between 14-15 cal kyr BP and a 700 ppm drop in Delta (super 14) C between 22-25 cal kyr BP. In addition, a large atmospheric Delta (super 14) C excursion to values over 1000 ppm occurs at 28 cal kyr BP. Between 20 and 10 cal kyr BP, a component of atmospheric Delta (super 14) C anti-correlates with Greenland ice Delta (super 18) O, indicating that some portion of the variability in atmospheric Delta (super 14) C is related to climate change, most likely through climate-related changes in the carbon cycle. Furthermore, the 28-kyr excursion occurs at about the time of significant climate shifts. Taken as a whole, our data indicate that in addition to a terrestrial magnetic field, factors related to climate change have affected the history of atmospheric (super 14) C.

Pronounced interannual variability in tropical South Pacific temperatures during Heinrich Stadial 1

The early last glacial termination was characterized by intense North Atlantic cooling and weak overturning circulation. This interval between ~18,000 and 14,600 years ago, known as Heinrich Stadial 1, was accompanied by a disruption of global climate and has been suggested as a key factor for the termination. However, the response of interannual climate variability in the tropical Pacific (El Niño-Southern Oscillation) to Heinrich Stadial 1 is poorly understood. Here we use Sr/Ca in a fossil Tahiti coral to reconstruct tropical South Pacific sea surface temperature around 15,000 years ago at monthly resolution. Unlike today, interannual South Pacific sea surface temperature variability at typical El Niño-Southern Oscillation periods was pronounced at Tahiti. Our results indicate that the El Niño-Southern Oscillation was active during Heinrich Stadial 1, consistent with climate model simulations of enhanced El Niño-Southern Oscillation variability at that time. Furthermore, a greater El Niño-Southern Oscillation influence in the South Pacific during Heinrich Stadial 1 is suggested, resulting from a southward expansion or shift of El Niño-Southern Oscillation sea surface temperature anomalies.

Newly identified segments of the Pacific–Australia plate boundary along the North Fiji transform zone

Abstract The North Fiji transform zone, a 1500 km long and 200 km wide transform segment of the Pacific–Australia plate boundary, is one of the major transform fault systems of the Earth. New data collected during the ALAUFI cruise (March 2000) on board the R/V L’Atalante make it possible to define more accurately the geometry and kinematics of this transform plate boundary. Three spreading centers or extensional zones (the North Cikobia spreading center, the Futuna spreading center and the southeast Futuna volcanic zone) and a strike-slip fault zone (the Futuna transform fault) have been discovered over a distance of 500 km along the eastern North Fiji transform zone, from the north of the Fiji platform to the east of the Futuna archipelago. The Futuna transform fault oriented 100° has been mapped over a distance of 250 km. It must be considered to be an important tectonic element of the transform plate boundary. Pure strike-slip as well as transpression and transtension motions are responsible for the complex morphology of this feature. The uplifted Futuna–Alofi ridge represents a major compressional relay along the Futuna transform fault. The Futuna spreading center trending 20–30° is composed of a series of en echelon left-stepping spreading segments. It represents a 200 km long extensional relay between the Futuna transform fault and the western part of the North Fiji transform zone, the Fiji transform fault, which bounds the Fiji platform to the north. The opening rate at the Futuna spreading center is estimated at 4 cm/yr. Although the North Cikobia spreading center and the southeast Futuna volcanic zone have been only partly mapped, bathymetric and reflectivity data clearly reveal that active extension also takes place along these two features. A spreading rate of 2 cm/yr is inferred at the North Cikobia spreading center. Therefore, the North Fiji transform zone appears to be composed of two main overlapping transform segments relayed by parallel extensional zones. The three active extensional zones have an ENE–WSW to NNE–SSW orientation, while compressive features along the Futuna transform fault are NW–SE to NNW–SSE oriented, in accordance with the present-day left-lateral transform motion along this part of the Pacific–Australia plate boundary.

Lithium in the aragonite skeletons of massive Porites corals: A new tool to reconstruct tropical sea surface temperatures

Previous studies have demonstrated the potential for the Li content of coral aragonite to record information about environmental conditions, but no detailed study of tropical corals exists. Here we present the Li and Mg to Ca ratios at a bimonthly to monthly resolution over 25 years in two modern Porites corals, the genus most often used for paleoclimate reconstructions in the tropical Indo-Pacific. A strong relationship exists between coral Li/Ca and locally measured SST, indicating that coral Li/Ca can be used to reconstruct tropical SST variations. However, Li/Ca ratios of the skeleton deposited during 1979-1980 do not track local SST well and are anomalously high in places. The Mg/Ca ratios of this interval are also anomalously high, and we suggest Li/Ca can be used to reconstruct tropical SST only when Mg/Ca data are used to carefully screen for relatively rare biological effects. Mg/Li or Li/Mg ratios provide little advantage over Li/Ca ratios, except that the slope of the Li/Mg temperature relationship is more similar between the two corals. The Mg/Li temperature relationship for the coral that experienced a large temperature range is similar to that found for cold water corals and aragonitic benthic foraminifera in previous studies. The comparison with data from other biogenic aragonites suggests the relationship between Li/Mg and water temperature can be described by a single exponential relationship. Despite this hint at an overarching control, it is clear that biological processes strongly influence coral Li/Ca, and more calibration work is required before widely applying the proxy