Yama Tomonaga

Noble Gases as Environmental Tracers in Sediment Porewaters and Stalagmite Fluid Inclusions

In well-studied aquatic systems such as surface waters and groundwater, noble gases are used extensively as natural tracers to reconstruct palaeoenvironmental conditions, to study transport and mixing, and to identify the geochemical origin of geogenic fluids. It has been suggested that less well-studied aquatic systems such as the porewaters of lacustrine and oceanic sediments and the fluid inclusions present in stalagmites might also be suitable as noble gas archives for environmental studies, but until recently the lack of adequate experimental techniques had hindered the development of noble gas geochemistry in these systems. This chapter reviews recent technical advances in this field and describes the scientific applications that these advances have made possible. The porewaters of lacustrine and oceanic sediments are now well established as noble gas archives in studies of temperature, salinity and mixing conditions that prevailed in the overlying water body in the past, as well as in studies of the transport and origin of solutes and pore fluids in the sediment. The geochemistry of noble gases in stalagmite fluid inclusions is still in the early stages of development. However, the results available to date suggest that stalagmite fluid inclusions have great potential as a noble gas archive in reconstructing palaeoclimatic conditions near caves with suitable stalagmites.

Improved Method for the Quantification of Methane Concentrations in Unconsolidated Lake Sediments

There is conclusive evidence that the methods most commonly used to sample methane (CH4) dissolved in the pore water of lake sediments produce results that are likely to be affected by gas loss or gas exchange with the atmosphere. To determine the in situ amount of CH4 per unit mass of pore water in sediments, we developed and validated a new method that combines techniques developed for noble-gas analysis in pore waters with a standard headspace technique to quantify the CH4 present in the pore space in dissolved and gaseous form. The method was tested at two sites: Lake Lungern, where CH4 concentrations were close to saturation; and Lake Rotsee, where CH4 concentrations are known to exceed saturation and where CH4 bubble formation and gas ebullition are commonly observed. We demonstrate that the new method, in contrast to the available methods, more reliably captures the total amount of CH4 per unit mass of pore water consisting of both dissolved and free CH4 (i.e., gas bubbles) in the pore space of the sediment.

Porewater salinity reveals past lake-level changes in Lake Van, the Earth’s largest soda lake

In closed-basin lakes, sediment porewater salinity can potentially be used as a conservative tracer to reconstruct past fluctuations in lake level. However, until now, porewater salinity profiles did not allow quantitative estimates of past lake-level changes because, in contrast to the oceans, significant salinity changes (e.g., local concentration minima and maxima) had never been observed in lacustrine sediments. Here we show that the salinity measured in the sediment pore water of Lake Van (Turkey) allows straightforward reconstruction of two major transgressions and a major regression that occurred during the last 250 ka. We observed strong changes in the vertical salinity profiles of the pore water of the uppermost 100 m of the sediments in Lake Van. As the salinity balance of Lake Van is almost at steady-state, these salinity changes indicate major lake-level changes in the past. In line with previous studies on lake terraces and with seismic and sedimentological surveys, we identify two major transgressions of up to +105 m with respect to the current lake level at about 135 ka BP and 248 ka BP starting at the onset of the two previous interglacials (MIS5e and MIS7), and a major regression of about −200 m at about 30 ka BP during the last ice age.

Helium and methane sources and fluxes of shallow submarine hydrothermal plumes near the Tokara Islands, Southern Japan

Shallow submarine volcanoes have been newly discovered near the Tokara Islands, which are situated at the volcanic front of the northern Ryukyu Arc in southern Japan. Here, we report for the first time the volatile geochemistry of shallow hydrothermal plumes, which were sampled using a CTD-RMS system after analyzing water column images collected by multi-beam echo sounder surveys. These surveys were performed during the research cruise KS-14-10 of the R/V Shinsei Maru in a region stretching from the Wakamiko Crater to the Tokara Islands. The 3He flux and methane flux in the investigated area are estimated to be (0.99–2.6) × 104 atoms/cm2/sec and 6–60 t/yr, respectively. The methane in the region of the Tokara Islands is a mix between abiotic methane similar to that found in the East Pacific Rise and thermogenic one. Methane at the Wakamiko Crater is of abiotic origin but affected by isotopic fractionation through rapid microbial oxidation. The helium isotopes suggest the presence of subduction-type mantle helium at the Wakamiko Crater, while a larger crustal component is found close to the Tokara Islands. This suggests that the Tokara Islands submarine volcanoes are a key feature of the transition zone between the volcanic front and the spreading back-arc basin.

Determination of Natural In Vivo Noble-Gas Concentrations in Human Blood

Although the naturally occurring atmospheric noble gases He, Ne, Ar, Kr, and Xe possess great potential as tracers for studying gas exchange in living beings, no direct analytical technique exists for simultaneously determining the absolute concentrations of these noble gases in body fluids in vivo. In this study, using human blood as an example, the absolute concentrations of all stable atmospheric noble gases were measured simultaneously by combining and adapting two analytical methods recently developed for geochemical research purposes. The partition coefficients determined between blood and air, and between blood plasma and red blood cells, agree with values from the literature. While the noble-gas concentrations in the plasma agree rather well with the expected solubility equilibrium concentrations for air-saturated water, the red blood cells are characterized by a distinct supersaturation pattern, in which the gas excess increases in proportion to the atomic mass of the noble-gas species, indicating adsorption on to the red blood cells. This study shows that the absolute concentrations of noble gases in body fluids can be easily measured using geochemical techniques that rely only on standard materials and equipment, and for which the underlying concepts are already well established in the field of noble-gas geochemistry.

Origin of methane-rich natural gas at the West Pacific convergent plate boundary

Methane emission from the geosphere is generally characterized by a radiocarbon-free signature and might preserve information on the deep carbon cycle on Earth. Here we report a clear relationship between the origin of methane-rich natural gases and the geodynamic setting of the West Pacific convergent plate boundary. Natural gases in the frontal arc basin (South Kanto gas fields, Northeast Japan) show a typical microbial signature with light carbon isotopes, high CH4/C2H6 and CH4/3He ratios. In the Akita-Niigata region – which corresponds to the slope stretching from the volcanic-arc to the back-arc –a thermogenic signature characterize the gases, with prevalence of heavy carbon isotopes, low CH4/C2H6 and CH4/3He ratios. Natural gases from mud volcanoes in South Taiwan at the collision zone show heavy carbon isotopes, middle CH4/C2H6 ratios and low CH4/3He ratios. On the other hand, those from the Tokara Islands situated on the volcanic front of Southwest Japan show the heaviest carbon isotopes, middle CH4/C2H6 ratios and the lowest CH4/3He ratios. The observed geochemical signatures of natural gases are clearly explained by a mixing of microbial, thermogenic and abiotic methane. An increasing contribution of abiotic methane towards more tectonically active regions of the plate boundary is suggested.