Huei-Ting Lin

A new analytical method has been developed to determine atomic ²³⁶U/²³⁸U ratios in samples with only femtograms of ²³⁶U using a secondary electron multiplier (SEM) on a multicollector high-resolution inductively coupled plasma mass spectrometer (MC-ICPMS). The abundance sensitivity of the ²³⁸U tail at 236 atomic mass unit is reduced from 10^–6 to 10^–10 with the deployment of a retarding potential quadrupole lens. This method features the reduction of polyatomic interferences from hydride, nitride, lead, and plutonium and the evaluation of nonlinear SEM behavior. The instrument sensitivity is 1–2%, and the estimated methodological detection limit of the ²³⁶U/²³⁸U atomic ratio is as low as 2 × 10^–10. Measurements on reference materials with ²³⁶U/²³⁸U ratios of 10^–7–10^–9, including the IRMM-075 series and the ETH Zurich in-house standard ZUTRI, demonstrate the accuracy of our MC-ICPMS technique. The analytical precisions (2σ) are ±4% for 5 fg of ²³⁶U at a ²³⁶U/²³⁸U of 1 × 10^–8 and ±8% for 2 fg of ²³⁶U at a ²³⁶U/²³⁸U of 4 × 10^–9 level. Compared to state-of-the-art accelerator mass spectrometry techniques and triple quadrupole-based ICPMS, our detection limit is not as low, but the required sample size is 3–40 times lower, and the throughput is as high as 3–4 samples per hour. The new MC-ICPMS–SEM technique is sensitive enough for determining ²³⁶U/²³⁸U in various small natural samples, such as marine carbonates and seawater.

The impact of submarine hydrothermal systems on organic carbon in the ocean—one of the largest fixed carbon reservoirs on Earth—could be profound. Yet, different vent sites show diverse fluid chemical compositions and the subsequent biological responses. Observations from various vent sites are to evaluate hydrothermal systems' impact on the ocean carbon cycle. A response cruise in May 2009 to an on‐going submarine eruption at West Mata Volcano, northeast Lau Basin, provided an opportunity to quantify the organic matter production in a back‐arc spreading hydrothermal system. Hydrothermal vent fluids contained elevated dissolved organic carbon, particulate organic carbon (POC), and particulate nitrogen (PN) relative to background seawater. The δ¹³C‐POC values for suspended particles in the diffuse vent fluids (−15.5‰ and −12.3‰) are distinct from those in background seawater (−23 ± 1‰), indicative of unique carbon synthesis pathways of the vent microbes from the seawater counterparts. The first dissolved organic nitrogen concentrations reported for diffuse vents were similar to or higher than those for background seawater. Enhanced nitrogen fixation and denitrification removed 37%–89% of the total dissolved nitrogen in the recharging background seawater in the hydrothermal vent flow paths. The hydrothermal plume samples were enriched in POC and PN, indicating enhanced biological production. The total “dark” organic carbon production within the plume matches the thermodynamic prediction based on available reducing chemical substances supplied to the plume. This research combines the measured organic carbon contents with thermodynamic modeled results and demonstrates the importance of hydrothermal activities on the water column carbon production in the deep ocean.

Diagenesis in the Southern Okinawa Trough was examined using pore-water data The total carbon remineralization rate is 1.68 ± 0.21 mmol C m-2 d-1 The...