GEOCHEMICAL JOURNAL Vol. 51(2017) No. 4

Secondary ion mass spectrometry (SIMS) is a powerful technique for measuring low-concentration volatile elements such as H, C, F, Cl, and S in micrometer-sized samples. The number of studies on volatiles in terrestrial and extraterrestrial materials using SIMS, particularly in volcanic glasses, has been rapidly increasing in the recent decade. Although SIMS has the capability of measuring volatile abundances at the level of parts per million (ppm) in samples, standard materials are required to obtain reliable datasets. We therefore prepared fourteen glass standards, including nine synthetic glasses and five natural glasses (0.02–4.8 wt% for H₂O, 85–9400 ppm for CO₂, 55–2957 ppm for F, 55–2833 ppm for Cl, and 51–1372 ppm for S), which covered the broad ranges of volatile contents in volcanic silicate glasses reported to date. H₂O and CO₂ concentrations of the glasses were determined by Fourier transform infrared spectroscopy (FTIR) and F, Cl, and S concentrations were determined by ion chromatography following pyrohydrolysis. Two SIMS instruments, NanoSIMS 50L and IMS-1280HR, were used to measure the volatile contents including H₂O, CO₂, F, Cl, and S in addition to the P₂O₅ content in a series of volatile standard silicate glasses together with four basaltic glass standards distributed by the United States Geological Survey for the P₂O₅ content. The ³⁰Si-normalized intensities of volatile elements and ³¹P showed good correlations with volatile and P₂O₅ abundances in standard glasses in all ranges with the squared correlation coefficient (R²) > 0.996 except for CO₂, where R² = 0.992. We confirmed that most synthetic and natural glasses are of good quality for volatile standards of silicate glasses.

Plumbonacrite [Pb₁₀(OH)₆O(CO₃)₆] is a lead(II) hydroxycarbonate mineral that has received scarce attention regarding its geochemical formation and behavior. Previous methods to synthesize it were based on complex experimental conditions that frequently yielded this mineral mixed with the more common hydrocerussite [Pb₃(CO₃)₂(OH)₂] and Pb(II) oxide, preventing its study in pure form. In this work, we present very simple and novel methods to separately synthesize pure plumbonacrite and hydrocerussite in an aqueous medium under ambient conditions starting from Pb(ClO₄)₂ and Pb(NO₃)₂ salts, respectively. We studied their long-term stability and determined their thermodynamic solubility product constants. This yielded values that are more than 24 orders of magnitude lower (log K_sp = –65.92) than the latest report for plumbonacrite (and 57 orders of magnitude lower than the value used in current geochemical speciation programs). This new value predicts a much higher geochemical formation potential for plumbonacrite under lower than ambient CO₂ partial pressures. The K_sp value for hydrocerussite was also found to be 5–7 orders of magnitude lower than previous reports. The improved Pb(II) solubility measurements were achieved by effectively dialyzing out all solids formed from the aqueous phase analyzed as a significant proportion of small Pb(II) hydroxycarbonate nanoparticles are formed during precipitation. The considerably lower solubilities of the Pb(II) hydroxycarbonates obtained will help correct geochemical speciation databases and predict their potential formation more accurately under varying geochemical conditions. Further, the synthesis methods presented will be very useful for geochemists attempting to study the details of their formation, structure, and environmental behavior.

A 70 kyr record of δ¹⁵N from the eastern Arabian Sea, Core AAS9/21 was investigated. High δ¹⁵N values during interglacials and low δ¹⁵N values during glacials are documented, which mimics the global δ¹⁵N records in the marginal ocean basins. However, during the Holocene, when the world’s Oxygen Minimum Zone (OMZ) gradually retreated as seen from a decreasing trend in δ¹⁵N, we see a δ¹⁵N increase through the entire Holocene which is also seen in other northeastern Arabian Sea records. Higher δ¹⁵N values recorded in the core (AAS9/21) from beyond the OMZ depth than the core (AAS9/19) from within the OMZ, during the Holocene, suggests that intense denitrification occurred in the eastern Arabian Sea within the OMZ. Furthermore, U/Th ratios also suggest that the shallower core was bathed by anoxic waters during the entire Holocene. Conversely, the deeper core was bathed by oxic waters during Holocene as well as during MIS4. These observations suggest that the vertical extent of the OMZ was nearly the same throughout the Holocene.

Abnormal increase in radon (²²²Rn, half-life = 3.82 days) concentration has frequently been observed in soil gas environments before major earthquakes. This observation suggests that radon emission may be suitable for use as an earthquake precursor. The objective of this study is to evaluate the use of radon emission as an earthquake precursor in and around northern Pakistan. For this purpose, radon was monitored in soil gas at the foothills of Maragalla, Islamabad. The data presented in this work was obtained using a radon monitor (RTM2200) ion-implanted silicon detector and covers a period from autumn 2014 to spring 2015. Our results suggest that anomalous radon concentrations were observed before five earthquake events, which had magnitudes (m_b) of 4.2, 4.2, 4.7, 4.8 and 5.4. The values of earthquake preparation zone (R_E/R_D) for these events were found to be 1.5, 1.5, 0.49, 0.89 and 1.26, respectively. Furthermore, temporal investigation of radon concentration with respect to temperature, pressure and relative humidity suggests no significant correlation, which indicates that abnormal increase was not associated with the meteorological parameters but with precursory signals before earthquake events.

Enhanced ocean stratification during glacial periods is the main factor depressing the biogenic opal productivity in the subarctic North Pacific and its marginal seas. However, there are no quantitative estimations of glacial nutrient reduction despite the importance of the glacial-interglacial nutrient cycle. In this study, δ³⁰Si of diatom frustules (δ³⁰Si_diatom) was measured in order to provide a record of silicic acid utilization over the last 600 ka at IODP Site U1343. The δ³⁰Si_diatom record revealed distinct orbital-scale changes: high δ³⁰Si_diatom (i.e., high silicic acid utilization) during the interglacial periods and low δ³⁰Si_diatom (i.e., low silicic acid utilization) during the glacial periods. These changes are explained by the glacial-interglacial differences of Fe-concentration and sea-ice influence on surface-water productivity. This study estimated the glacial reduction of silicic acid supply and utilization in the Bering slope area. Based on the glacial-interglacial average δ³⁰Si_diatom values using the Rayleigh closed model, the glacial silicic acid level was estimated to be >63% of the interglacial level and the glacial silicic acid utilization was significantly lower (<37%) than during interglacial periods (~64%).

High latitude winter soil CO₂ emission is an important component of the annual carbon budget at regional and global scales. Here, continuous monitoring of winter CO₂ flux-measurement in black spruce forest soil of interior Alaska was performed using non-destructive infrared (NDIR) CO₂ sensors at 10, 20, and 30 cm above the surface during the snow-covered period of 2006/7. To analyze the effects of environmental factors in the CO₂ flux, the dataset was clustered based on major meteorological patterns. Periods were selected based on atmospheric pressure corresponding to well-identified synoptic large scale patterns: ambient pressure larger than 1000 hPa (HP: high pressure), atmospheric pressure is in the range between 985 and 1000 (IP: intermediate pressure), and cases in which the atmospheric pressure was below 986 hPa (LP: low pressure). Furthermore, the dataset corresponding to the snowmelt period (MP) was treated independently for all values of ambient pressure. Winter CO₂ fluxes were 0.20 ± 0.02, 0.23 ± 0.02, 0.29 ± 0.03, and 0.17 ± 0.02 gC m^–2 d^–1 for the HP, IP, LP, and MP phases, respectively. Atmospheric and soil temperature at 5 cm depth, modulated by atmospheric pressure, were significant factors in regulating winter soil-originated CO₂ emission and fluctuation. We found that changes in CO₂ fluxes during the snow-covered period can be as much as 35% on the average. These results are significant, as wintertime CO₂ emissions represent ~20% of annual soil-originated emissions.

The improper discharge has brought many environment problems in Pearl River Delta, China. As important reserve resources, groundwater resources have been affected by acidification. It is urgent to clarify the mechanism of groundwater acidification and take measures to alleviate this situation. The purposes of this study were to determine the main factor causing groundwater acidification in shallow aquifers, to quantify and analyze the hazard of acid precipitation on groundwater, and to analyze the characteristics of groundwater chemistry under acid precipitation. Acid rain hazard index (AHI) was defined and introduced to characterize the effects of the actual input of hydrogen ion on groundwater. The techniques of geographic information system (GIS) and principal components analysis (PCA) were used in this new analysis method. By analyzing the groundwater chemical data of 407 samples and meteorological data, the results showed that acid rain was the dominant factor leading to groundwater acidification rather than pyrite oxidation. Four principle components were derived. PC1 represented seawater intrusion effect and was consisted of Cl^–, Na⁺, Mg²⁺ and TDS. PC2 demonstrated the water-rock interactions occurred in acidic groundwater environment, which was consisting of pH, Ca²⁺, HCO₃^–, PO₄^3– and SO₄^2–. PC3 represented agricultural activities with the variables of K⁺ and NO₃^–. PC4 was associated with Mn²⁺, Al and AHI and represented the directly influence resulting from the input of acid deposition. Therefore, to strengthen supervision of discharge and improve the industrial desulfurization technology could alleviate the deterioration of acidification.

Understanding seasonal to interannual characteristics of the climate during the transition from the “Holocene Optimum” (7.0–5.0 kyr BP) to mid-Holocene cold and dry period (4.6–4.0 kyr BP) is important since it is related to the evolution and collapse of human civilization in East Asia. To investigate those characteristics, we reconstructed a seasonal scale sea surface temperature (SST) and measured oxygen isotope ratios (δ¹⁸O) in seawater (δ¹⁸O_seawater) using modern and fossil (4.9 kyr) corals from Kikai Island. Larger seasonal amplitudes observed among the reconstructed SST values and δ¹⁸O_seawater change at 4.9 kyr suggest that the East Asian Monsoon (EAM) circulation might be stronger than the present-day. Our compiled coral records, along with the previous studies from Kikai Island, also suggest that the largest SST amplitude during the Holocene Optimum was recorded at 4.9 kyr and an abrupt cold climate shift occurred during the Holocene Optimum and the Pulleniatina Minimum Event (PME) in the north-western Pacific.