Daniel J. Sinclair

High resolution analysis of trace elements in corals by laser ablation ICP-MS

Abstract A method has been developed using laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) for rapid high resolution analysis of B, Mg, Sr, Ba, and U in corals. Corals represent a challenge for a microbeam technique due to their compositional and structural heterogeneity, their nonsilicate matrix, and their unusual range of trace element compositions relative to available standards. The method employs an argon-fluoride excimer laser (λ = 193 nm), masked to produce a beam 600 μm wide by 20 μm across to average ablation sampling over a range of structural features. Coral sections are scanned at a constant rate beneath the laser to produce a continuous sampling of the coral surface. Sensitivity drift is controlled by careful preconditioning of the ICP-MS to carbonate material, and standardisation is carried out by bracketing each traverse down the coral sample by analyses of a CaSiO3 glass synthesised from coral powder. The method demonstrates excellent reproducibility of both the shape and magnitude of coralline trace element profiles, with typical precisions of between 1.0 and 3.7% based on analysis of the synthetic standard. Accuracy varies between 3.8% for B and 31% for U. Discrepancies are attributed to heterogeneities in the synthetic standard, and matrix differences between the silicate standard and carbonate sample. The method is demonstrated by analysis of a coral collected from Australia’s Great Barrier Reef near a weather station recording in-situ sea-surface-temperature (SST). The elements B, Mg, Sr, and U show seasonal compositional cycles, and tentative calibrations against SST have been derived. Using independent ICP-MS solution estimates of the coral composition to correct for standardisation uncertainties, the following calibrations have been derived: B/Ca (μmol/mol) = 1000 (±20) − 20.6 (±0.8) × SST Mg/Ca (mmol/mol) = 0.0 (±0.3) + 0.16 (±0.01) × SST Sr/Ca (mmol/mol) = 10.8 (±0.1) − 0.070 (±0.004) × SST U/Ca (μmol/mol) = 2.24 (±0.07) − 0.046 (±0.003) × SSTl These calibrations agree with literature within experimental errors, except for Mg which displays a 35% greater temperature dependence than reported previously. None of the elements in the coral appear to be sensitive to decreases in salinity associated with heavy rainfall in the summer of 1991/1992.

Corals at their latitudinal limits: laser ablation trace element systematics in Porites from Shirigai Bay, Japan

The rapid analytical technique of laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) was used to measure the trace elements B, Mg, Sr, Ba and U in a high-latitude coral colony (Porites lobata) taken from Shirigai Bay, Japan (32°N). A wide range of sea surface temperatures (SSTs 14.5–28°C) and upwelling events influenced this coral. Cold winter SSTs caused a decrease and/or cessation of skeletal extension. Measurements of U/Ca and Sr/Ca indicate an approximately linear response to SSTs above 18°C and a non-linear response below 18°C. Mg/Ca and B/Ca measurements both showed annual cycles broadly consistent with SST variations, but also exhibited intra-annual fluctuations not associated with temperature, suggesting that the incorporation of Mg and B into the coral skeleton was not simply regulated by temperature. It is shown that Ba/Ca ratios provide a proxy for wind-induced seasonal upwelling. This is inferred from the strong correlations between the strength of zonal winds ∼1 month prior to the SST minimum and the Ba/Ca maximum. Secondary upwelling events occurred during the summers of 1982, 1987, 1991 and 1992. These summers were cooler than average and were associated with El Nino Southern Oscillation events.

A review of nitrogen isotopic alteration in marine sediments

Key Points: Use of sedimentary nitrogen isotopes is examined; On average, sediment 15N/14N increases approx. 2 per mil during early burial; Isotopic alteration scales with water depth Abstract: Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes and detailed mechanisms of isotopic alteration are still under investigation. Case studies of the Mediterranean and South China Seas underscore the complexities of investigating isotopic alteration. In an effort to evaluate the evidence for alteration of the sedimentary N isotopic signal and try to quantify the net effect, we have compiled and compared data demonstrating alteration from the published literature. A >100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that, at sites located off of the continental margins, an increase in sediment 15N/14N occurs during early burial, likely at the seafloor. The extent of isotopic alteration appears to be a function of water depth. Depth-related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of N isotopic alteration. Moreover, the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global N cycle.

Multidecadal rainfall variability in South Pacific Convergence Zone as revealed by stalagmite geochemistry

Pacifi c decadal variability (PDV) causes widespread, persistent fl uctuations that affect climate, water resources, and fi sheries throughout the Pacifi c basin, yet the magnitude, frequency, and causes of PDV remain poorly constrained. Here we present an absolutely dated, subannually resolved, 446 yr stable oxygen isotope (δ 18 O) cave record of rainfall variability in Vanuatu (southern Pacifi c Ocean), a location that has a climate heavily infl uenced by the South Pacifi c Convergence Zone (SPCZ). The δ 18 O-based proxy rainfall record is dominated by changes in stalagmite δ 18 O that are large (~1‰), quasi-periodic (~50 yr period), and generally abrupt (within 5‐10 yr). These isotopic changes imply abrupt rainfall changes of as much as ~1.8 m per wet season, changes that can be ~2.5◊ larger than the 1976 C.E. shift in rainfall amount associated with a PDV phase switch. The Vanuatu record also shares little commonality with previously documented changes in the Intertropical Convergence Zone during the Little Ice Age or solar forcing. We conclude that multidecadal SPCZ variability is likely of an endogenous nature. Large, spontaneous, and low-frequency changes in SPCZ rainfall during the past 500 yr have important implications for the relative magnitude of natural PDV possible in the coming century.

Skeletal Mg/Ca in Primnoa resedaeformis: relationship to temperature?

It has been suggested that the deep-sea gorgonian coral Primnoa resedaeformis may be an important paleoceanographic archive. Seventeen colonies collected from the upper slope of the NW Atlantic margin (229 – 447 m) were analyzed to see if skeletal Mg/Ca is related to temperature. Analyses were focused on the calcite cortex region of skeletal sections to avoid interference from organic Mg in the horny layers found closer to the center of sections. Comparison of bulk skeletal Mg/Ca with hydrographic temperature yielded the relationship Mg/Ca (mmol/mol)=5 (+/− 1.4) T (°C)+64 (+/− 10). This relationship was used to calibrate profiles of Mg/Ca measured across the annual rings of one large, well-dated colony, over the period 1950–2002. Mg/Ca profiles were broadly consistent among three sections spaced 10 cm apart along the main trunk of the colony. These profiles were in general agreement with the local instrumental record of temperature at 375 – 450 m. Some discrepancies between the coral and instrumental records of temperature may be a result of chronological error, poor sampling density, or additional factors influencing Mg partitioning in the coral. Overall, these preliminary results support the hypothesis that temperature drives Mg/Ca in the skeletal calcite of this species. It appears that environmentally meaningful records from Primnoa resedaeformis will be found at decadal scales or longer.

Testing the reproducibility of Mg/Ca profiles in the deep-water coral Primnoa resedaeformis: putting the proxy through its paces

Two samples of the calcitic deep-sea coral Primnoa resedaeformis have been analysed for Mg/Ca ratios by micro-beam methods ( laser-ablation ICP-MS and electron microprobe). Continuous profiles of Mg/Ca have been studied with the aim of establishing the reproducibility of the variations in different parts of the coral, and therefore the potential use of Mg/Ca as a paleoceanographic or paleoclimatic tracer.