Kuo-Fang Huang

In situ calibration of Mg/Ca ratio in planktonic foraminiferal shell using time series sediment trap: A case study of intense dissolution artifact in the South China Sea

This study examines Mg/Ca-temperature equations through the use of planktonic foraminifers collected from continuous time series sediment traps at four water depths in the South China Sea. This deployment provides an opportunity to refine partial dissolution artifacts and to identify responses of shell chemistry to changes in ambient seawater conditions. Paired Mg/Ca and δ18O measurements on shells are strongly correlated with seasonal variability in terms of temperature at specific habitat depths. However, partial dissolution can significantly alter Mg/Ca ratios even at depths well above the calcite lysocline or carbonate compensation depth. Of the three species studied, Neogloboquadrina dutertrei is the most sensitive species to dissolution relative to the other surface-dwelling species (Globigerinoides ruber and Globigerinoides sacculifer). Differences between the Mg-derived temperature equation here and those from previously established core top or sediment trap calibrations can be attributed to various degrees of dissolution and lateral advection, thus highlighting the need for in situ empirical calibration for different ocean basins.

High-precision and accurate determinations of neodymium isotopic compositions at nanogram levels in natural materials by MC-ICP-MS

A comparatively simple, rapid, low-blank, high-precision and reproducible technique for Nd isotope analysis (143Nd/144Nd) of trace amounts (1–5 ng) of Nd by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) was developed and is reported here. The technique utilizes a combination of a high-sensitivity desolvator (Aridus II or APEX-IR) and high-efficiency Jet-sample/X-skimmer cones, together with high-yield pre-concentration and column chemistry, to achieve optimal conditions for Nd isotope determination in natural materials with a variety of Nd-depleted matrices. This new method was validated by analyzing a series of international reference materials (RMs), which span a wide range of the Nd isotopic composition and cover different sample matrices in order to provide a useful benchmark for future studies. The long-term precision of 143Nd/144Nd (±0.000016, 2SD) for a sample size of ca. 1.25 ng Nd is a factor of 5 better than the existing high-sensitivity MC-ICP-MS methods using <2 ng of Nd and is comparable to the best TIMS methods. Replicate analyses of the RMs, including JNdi-1 (0.512116 ± 0.000016, 2SD), AGV-1 (0.512794 ± 0.000013, 2SD), BHVO-1 (0.512992 ± 0.000015, 2SD), Nod-P-1 (0.512430 ± 0.000015, 2SD) and two sets of synthetic solutions for seawater and calcium carbonate matrices, are in excellent agreement with the published values, demonstrating the robustness of the present protocol for natural samples with different matrices. A depth profile of the seawater Nd isotopes near the Bermuda Atlantic Time-series Study station was also analyzed for comparison, and is identical to the reported seawater profile in this region. Applying this improved technique, we are able to generate high-precision and reproducible Nd isotope data for Nd-depleted natural materials, such as seawater, continental runoff, benthic fluxes at continental margins, and marine biogenic carbonate, which can provide pertinent and complementary information about the marine Nd cycle and past ocean circulation.

Precise determination of triple Sr isotopes (δ87Sr and δ88Sr) using MC-ICP-MS

The non-traditional stable strontium (Sr) isotopes have received increasing attention recently as new geochemical tracers for studying Sr isotopic fractionation and source identification. This has been attributed to the advancement in multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), allows to determine precisely and simultaneously of the triple Sr isotopes. In this study, we applied a modified empirical external normalization (EEN) MC-ICPMS procedure for mass bias correction in Sr isotopic measurement using (92)Zr/(90)Zr. High-purity Zr Standard was spiked into sample solutions and the degree of fractionation was calculated off-line using an exponential law. The long-term external reproducibility for NIST SRM 987 δ(87)Sr and δ(88)Sr was better than 0.040‰ and 0.018‰ (2SD), respectively. The IAPSO standard seawater was used as a secondary standard to validate the analytical protocol and the absolute ratios measured were 0.709161±0.000018 for (87)Sr/(86)Sr, 0.177±0.021‰ for δ(87)Sr, and 0.370±0.026‰ for δ(88)Sr (2SD, n=7). These values are in good agreement with the literature data analyzed by thermal ionization mass spectrometry (TIMS) double spike technique. Rock standards, BHVO-2, BCR-2 and AGV-2 were also analyzed to validate the robustness of the methodology and showed identical results with literature data. Compared to previous (91)Zr/(90)Zr correction, we obtained improved results based on (92)Zr/(90)Zr, probably due to similar mass difference between (92)Zr/(90)Zr and measured Sr isotopes. The new analytical protocol presented in this study not only improves the analytical precision but also increases sample efficiency by omitting the use of the standard-sample bracketing (SSB) procedure.

How well can we quantify dust deposition to the ocean

Deposition of continental mineral aerosols (dust) in the Eastern Tropical North Atlantic Ocean, between the coast of Africa and the Mid-Atlantic Ridge, was estimated using several strategies based on the measurement of aerosols, trace metals dissolved in seawater, particulate material filtered from the water column, particles collected by sediment traps and sediments. Most of the data used in this synthesis involve samples collected during US GEOTRACES expeditions in 2010 and 2011, although some results from the literature are also used. Dust deposition generated by a global model serves as a reference against which the results from each observational strategy are compared. Observation-based dust fluxes disagree with one another by as much as two orders of magnitude, although most of the methods produce results that are consistent with the reference model to within a factor of 5. The large range of estimates indicates that further work is needed to reduce uncertainties associated with each method before it can be applied routinely to map dust deposition to the ocean. Calculated dust deposition using observational strategies thought to have the smallest uncertainties is lower than the reference model by a factor of 2–5, suggesting that the model may overestimate dust deposition in our study area. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Nonhomogeneous seawater Sr isotopic composition in the coastal oceans: A novel tool for tracing water masses and submarine groundwater discharge

Here we present high-precision (2σ = ±3 ppm) 87Sr/86Sr measurements in coastal waters, together with salinity, to evaluate water mass mixing and the influence of submarine groundwater discharge (SGD) in coastal waters and marginal seas. Nonhomogeneous Sr isotopic variations in water columns were documented in the Southern Okinawa Trough (SOT), South China Sea, and Kao-ping Canyon (KPC), where seawater 87Sr/86Sr varied up to 70 ppm. Seawater Sr isotopic composition changes only slightly in the upper 200 m of the SOT but was detectable and highly correlated with salinity, indicating a mixing between radiogenic North Pacific Tropical Water (high 87Sr/86Sr and high salinity) at 100–150 m and a less radiogenic component with low 87Sr/86Sr and low salinity at ∼200 m. Vertical profiles of seawater 87Sr/86Sr along the KPC show significant variations, suggesting dynamic mixing affected by continental inputs (i.e., river runoff and SGD) in this region. These results highlight the potential use of seawater Sr isotopes as a powerful tracer for determining mixing ratios and the dynamic mixing of oceanic water masses, especially in coastal and marginal seas.

Antarctic intermediate water circulation in the South Atlantic over the past 25,000 years

Antarctic Intermediate Water is an essential limb of the Atlantic meridional overturning circulation that redistributes heat and nutrients within the Atlantic Ocean. Existing reconstructions have yielded conflicting results on the history of Antarctic Intermediate Water penetration into the Atlantic across the most recent glacial termination. In this study we present leachate, foraminiferal, and detrital neodymium isotope data from three intermediate-depth cores collected from the southern Brazil margin in the South Atlantic covering the past 25 kyr. These results reveal that strong chemical leaching following decarbonation does not extract past seawater neodymium composition in this location. The new foraminiferal records reveal no changes in seawater Nd isotopes during abrupt Northern Hemisphere cold events at these sites. We therefore conclude that there is no evidence for greater incursion of Antarctic Intermediate Water into the South Atlantic during either the Younger Dryas or Heinrich Stadial 1. We do, however, observe more radiogenic Nd isotope values in the intermediate-depth South Atlantic during the mid-Holocene. This radiogenic excursion coincides with evidence for a southward shift in the Southern Hemisphere westerlies that may have resulted in a greater entrainment of radiogenic Pacific-sourced water during intermediate water production in the Atlantic sector of the Southern Ocean. Our intermediate-depth records show similar values to a deglacial foraminiferal Nd isotope record from the deep South Atlantic during the Younger Dryas but are clearly distinct during the Last Glacial Maximum and Heinrich Stadial 1, demonstrating that the South Atlantic remained chemically stratified during Heinrich Stadial 1.

Precise determination of seawater calcium using isotope dilution inductively coupled plasma mass spectrometry

We describe a method for rapid, precise and accurate determination of calcium ion (Ca(2+)) concentration in seawater using isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS). A 10 μL aliquot of seawater was spiked with an appropriate (43)Ca enriched solution for (44)Ca/(43)Ca ID-ICP-MS analyses, using an Element XR (Thermo Fisher Scientific), operated at low resolution in E-scan acquisition mode. A standard-sample bracketing technique was applied to correct for potential mass discrimination and ratio drift at every 5 samples. A precision of better than 0.05% for within-run and 0.10% for duplicate measurements of the IAPSO seawater standard was achieved using 10 μL solutions with a measuring time less than 3 minutes. Depth profiles of seawater samples collected from the Arctic Ocean basin were processed and compared with results obtained by the classic ethylene glycol tetra-acetic acid (EGTA) titration. Our new ID-ICP-MS data agreed closely with the conventional EGTA data, with the latter consistently displaying 1.5% excess Ca(2+) values, possibly due to a contribution of interference from Mg(2+) and Sr(2+) in the EGTA titration. The newly obtained Sr/Ca profiles reveal sensitive water mass mixing in the upper oceanic column to reflect ice melting in the Arctic region. This novel technique provides a tool for seawater Ca(2+) determination with small sample size, high throughput, excellent internal precision and external reproducibility.

Riverine lithium isotope systematic during continental weathering from an active orogenic belt, Taiwan

HCl molecules emitted from volcanoes breakdown to form chlorine free radicals via heterogeneous chemical reactions and photolysis, which act as catalysts to the breakdown of ozone in the stratosphere. Ozone depletion of up to 2-7% was estimated following the Pinatubo 1991 eruption [1]. However, only stratospheric HCl is dangerous to ozone, and the amount of HCl that reaches these levels is often lower than expected [2]. This suggests that HCl is removed from the eruption column at tropospheric levels. Previously suggested mechanisms include inclusion of HCl into supercooled droplets or ice crystals [3]. In order to investigate the removal of HCl from the atmosphere by adsorption onto ash in volcanic plumes, glass with the composition of the Pinatubo 1991 dacite [4] was synthesised and ground to ash-sized particles using a planetary mill. The ash was then placed in a simple volumetric vacuum device, which was purged with HCl gas to a desired pressure. The ash was connected to the system and the adsorption of HCl onto the ash surface recorded by the resulting pressure drop until an equilibrium pressure was reached. Preliminary results from experimental runs beginning with an HCl gas pressure of 31 mbar, 100 mbar, 250 mbar, 504 mbar and 975 mbar indicate that adsorption on the order of 0.5 mgm-2 occurs even at low partial pressures of HCl. [1] Robock (2000) Rev. Geophys. 38, 191-219. [2] Oppenheimer (2003) In Treatise on Geochemistry. [3] Textor et al. (2003) Geol Soc Lon Spec Pub 213, 307-328. [4] Scaillet & Evans (1999) J. Petr. 40, 381-411