Chantal Alibert

Strontium/calcium ratios in modern porites corals From the Great Barrier Reef as a proxy for sea surface temperature: Calibration of the thermometer and monitoring of ENSO

High-precision Sr/Ca ratios are reported for Porites corals from Davies Reef, at ∼19°S on the midshelf of the Great Barrier Reef, Australia. Skeletal growth parameters were also determined from coral slabs using X ray photographs and γ-densitometry measurements. A calibration of the Sr/Ca thermometer has been obtained, using in situ measured sea surface temperatures, which gives Sr/Ca × 10³ = 10.48 (±0.01) − 0.0615 (±0.0004) × T, whereT is in °C. This calibration applies to different species of Porites and a large range of annual extension and calcification rates, provided sampling follows a major growth axis. Biases of 1°–2°C have been observed in the low-density margins where growth is hampered by interfering fans of corallites. The analytical and the long-term reproducibility of Sr/Ca-derived temperatures are both better than ±0.3°C (2σ). Coral growth parameters such as the extension rate or density generally appear to respond to environmental changes such as rainfall but do not affect the Sr/Ca partitioning between coralline aragonite and seawater. No correlation has been found between calcification rate and Sr/Ca variations, consistent with Sr/Ca being mainly controlled by temperature. In the Great Barrier Reef, the interannual variability in SST is primarily related to the El Nino-Southern Oscillation phenomenon and is typically marked by a cold anomaly during the winter preceding the mature warm phase. Instrumental records show that the surface waters of the Coral Sea around 19°S have experienced a significant cooling associated with the El Ninos of 1965, 1972, and 1982-1983 which the Porites corals record faithfully. The coral-derived temperatures for the period from 1965 to 1993 show an increase of 1.3°C, which has occurred mainly since 1979. This warming is larger in magnitude but consistent with the trend for the southern hemisphere land and marine temperature records, which shows an increase of around 0.4°C.

Rare earth element and neodymium isotopic compositions of the banded iron-formations and associated shales from Hamersley, western Australia

Abstract Major, trace element, and Nd isotopic compositions are reported for the early Proterozoic iron-formations from the Hamersley Basin, western Australia. These data enable new constraints to be placed on the relative roles of continental weathering and submarine hydrothermal systems in the REE budget of the Hamersley banded-iron formations (IF). The lower part of the Hamersley Group (Jeerinah shales/cherts, Marra Mamba IF, Wittenoom Dolomite and McRae Shale units) have an average ϵ Nd = −0.6 ± 0.9 at 2.6 Ga. This uniformity suggests that Nd was mostly supplied by erosion products coming from the nearby upper Fortescue Group and also that these sediments were deposited in a restricted basin. The overlying Dales Gorge and Joffre banded iron-formations at Wittenoom record a major metamorphic event at 2.14 ± 0.03 Ga. Initial ϵ Nd values are estimated to be +1.0 ± 0.5 at the time of deposition (2.5 Ga), after correction for metamorphic effects. This more positive ϵ Nd value is consistent with the increased influence of submarine hydrothermal sources, probably in response to exchange with the open ocean. The Hamersley IF are therefore viewed as shelf deposits in a newly formed ocean, possibly of an evolving rift. The shaley bands of the Dales Gorge-Joffre formations containing around 30–60% detrital material give ϵ Nd = −0.9 ± 0.6. An ϵ Nd ≈ −2 is therefore inferred for river waters/aeolian sources in the Hamersley basin. The Nd mass balance obtained from the Dales Gorge-Joffre IF indicates a proportion of Nd derived from hydrothermal sources associated with mid-ocean ridge volcanism of 50 ± 10%.

Source of trace element variability in Great Barrier Reef corals affected by the Burdekin flood plumes

Abstract Massive corals in the Great Barrier Reef, analyzed at high-resolution for Sr/Ca (thermal ionization mass spectrometry) and trace elements such as Ba and Mn (laser ablation inductively coupled plasma mass spectrometry), can provide continuous proxy records of dissolved seawater concentrations, as well as sea surface temperature (SST). A 10-yr record (1989 to 1998) from Pandora Reef, an inshore reef regularly impacted by the freshwater plumes of the Burdekin River, is compared with an overlapping record from a midshelf reef, away from runoff influences. Surface seawater samples, taken away from river plumes, show little variability for Sr/Ca (8484 ± 10 μmol/mol) and Ba (33.7 ± 0.7 nmol/kg). Discrete Ba/Ca peaks in the inshore coral coincide with flood events. The magnitude of this Ba/Ca enrichment is most likely controlled by the amount of suspended sediments delivered to the estuary, which remains difficult to monitor. The maximum flow rate at peak river discharge is used here as a proxy for the sediment load and is shown to be strongly correlated with coral Ba/Ca (r = 0.97). After the wet summer of 1991, the coral Ba/Ca flood peak is followed by a plateau that lingers for several months after dissipation of plume waters, signifying an additional flux of Ba that may originate from submarine groundwater seeps and/or mangrove reservoirs. Both Mn and Y are enriched by a factor of ∼5 in inshore relative to midshelf corals. Mn/Ca ratios show a seasonal cycle that follows SST (r = 0.7), not river discharge, with an additional high variability in summer suggesting a link with biological activity. P and Cd show no significant seasonal variation and are at a low level at both inshore and midreef locations. However, leaching experiments suggest that part of the coral P is not lattice bound.

An ancient Sm-Nd age for a ferroan noritic anorthosite clast from lunar breccia 67016

Strontium and neodymium systematics have been examined in a clast of ferroan noritic anorthosite from Apollo 16 breccia 67016. Two splits (,328 and ,326) of the same clast give different Sm-Nd results. Split ,328 gives a well defined internal isochron age of 4.562 ± 0.068 Ga and an initial 143Nd/144Nd ratio of 0.50673 ± 10 corresponding to ϵNd = 0.1 ± 0.2 (2σ optimized error) relative to the Murchinson carbonaceous chondrite. The pyroxene separate from split ,326 lies on the same isochron. In contrast, the plagioclase and whole-rock from split ,326 fall below this line, indicating a small-scale disturbance of the Sm-Nd system. This may reflect either an isotopic exchange between the plagioclase and a low Sm/Nd mineral or a loss of radiogenic 143Nd from the plagioclase, possibly during the period of major impacts at ~3.9 Ga. The preservation of an extremely old age for the noritic ferroan anorthosite 67016,328 suggests a rapid cooling of this rock at an early stage in the evolution of the lunar magma ocean. This old age is also consistent with giant impact models for the formation of the Moon but implies a relatively early event (pre 4.50 Ga) and, therefore, rapid accretion and differentiation of the terrestrial planets.

Peridotite xenoliths from western Grand Canyon and The Thumb: A probe into the subcontinental mantle of the Colorado Plateau

Rare earth elements (REE) and Sr and Nd isotopic compositions are presented for spinel peridotite xenoliths from western Grand Canyon, on the margin of the Colorado Plateau (CP), and for garnet peridotite inclusions from The Thumb, central CP. The Grand Canyon spinel peridotites show original textures and geochemical characteristics which do not have counterparts in the xenoliths from the nearby Basin and Range. The restricted range of mineral compositions in these Iherzolites (Mg #Ol = 0.90 ± 0.03; Cr/(Cr+Al)Sp = 0.18 ± 0.02) suggests that they represent a limited series of residues of partial melting. Pyroxene equilibration temperatures are in the range 800–1000°C. The 143Nd/144Nd ratios in clinopyroxene (cpx) are extremely variable (0.5127–0.5202). Sample TRU 2, with eNd = +147, records an ancient depletion event around 1.2 Ga. More than half of the cpx separates show concave downward REE profiles between Gd and Yb, indicating that partial melting occurred in the garnet stability field before final recrystallization of the residues in the spinel peridotite field, probably during decompressional upwelling. The strong light REE (LREE) depletion in some of the cpx cannot be modeled by batch melting of a LREE-depleted source but requires 5–10% melting with segregation in ∼1% increments. These peridotites could be associated with the production of picritic melts either in a rift environment or beneath a mid-ocean ridge or oceanic island. A second group of xenoliths, characterized by cpx with strong LREE enrichment, includes amphibole-bearing peridotites and has a more limited range of 143Nd/144Nd ratios (0.51269–0.51285). Interactions with melts of variable composition (alkali basalt or carbonatite) are inferred on the basis of mineralogical and geochemical evidence. Contrasting with the extreme isotopic heterogeneity of Nd, a high, uniform 87Sr/86Sr ratio around 0.7045 is associated with high Sr/Nd ratios (up to 540). Aqueous fluids derived from a subducted slab could account for this preferential Sr enrichment. Six porphyroclastic garnet Iherzolites from The Thumb sample a deeper part of the subcontinental mantie (120–150 km). One xenolith has a refractory chemistry, low temperature of equilibration (T = 1220°C, P = 3.8 GPa) and Sr-Nd isotopic compositions (87Sr/86Sr = 0.7037,143Nd/144Nd = 0.51285), suggesting a lithospheric origin. The other xenoliths have a fertile chemistry, higher temperature of equilibration (T = 1260–1280°C, P = 4.5–4.7 GPa), and homogeneous isotopic compositions (87Sr/86Sr = 0.70306–0.70313,143Nd/144Nd = 0.51281–0.51284) which fall slightly to the left of the mantle array in a similar way as St. Helena oceanic basalts. The garnet shows an enrichment in Ti and Fe in the outer 300 μm. Three garnet-cpx pairs give Sm-Nd apparent ages between 60 and 100 Ma, which are older than the host minette (∼25 Ma). These observations together show that the cpx has readjusted its composition more completely than garnet during an interaction with melts having a St. Helena-type isotopic signature. As this signature has not been identified in the CP volcanic rocks, it is inferred that the high-temperature Iherzolites from The Thumb record the presence of a mantle plume at ∼25 Ma beneath the central CP.

A Reconstruction of Subtropical Western North Pacific SST Variability Back to 1578, Based on a Porites Coral Sr/Ca Record from the Northern Ryukyus, Japan

We present a seasonal reconstruction of sea surface temperature (SST) from 1578 to 2008, based on a Porites coral Sr/Ca record from the northern Ryukyus, within the Kuroshio southern recirculation gyre. Interannual SST anomalies are generally ~0.5°C, making Sr/Ca-derived SST reconstructions a challenging task. Replicate measurements along adjacent coral growth axes, enabled by the laser ablation inductively coupled plasma mass spectrometry technique used here, give evidence of rather large uncertainties. Nonetheless, derived winter SST anomalies are significantly correlated with the Western Pacific atmospheric pattern which has a dominant influence on winter temperature in East Asia. Annual mean SSTs show interdecadal variations, notably cold intervals between 1670 and 1700 during the Maunder Minimum (MM) and between 1766 and 1788 characterized by a negative phase of the North Atlantic Oscillation. Cold summers in 1783 and 1784 coincide with the long-lasting Laki eruption that had a profound impact on the Northern Hemisphere climate, including the severe “Tenmei” famine in Japan. The decades between 1855 and 1900 are significantly cooler than the first half of the twentieth century, while those between 1700 and 1765, following the MM, are warmer than average. SST variability in the Ryukyus is only marginally influenced by the Pacific Decadal Oscillation, so that external forcing remains the main driver of low-frequency temperature changes. However, the close connection between the Kuroshio extension (KE) and its recirculation gyre suggests that decadal SST anomalies associated with the KE front also impact the Ryukyus, and there is a possible additional role for feedback of the Kuroshio-Oyashio variability to the large-scale atmosphere at decadal timescale.