Atmospheric helium isotope ratio: Possible temporal and spatial variations

摘要

The atmospheric ~3He/~4He ratio has been considered to be constant on a global scale, because the residence time of helium is significantly longer than the mixing time in the atmosphere. However, this ratio may be decreasing with time owing to the anthropogenic release of crustal helium from oil and natural gas wells, although this observation has been disputed. Here, we present the ~3He/~4He ratios of old air trapped in historical slags in Japan and of modern surface air samples collected at various sites around the world, measured with a newly developed analytical system. In air helium extracted from metallurgical slag found at refineries in operation between AD 1603 and 1907 in Japan, we determined a mean ~3He/~4He ratio of (5106±108)×10~(-5)R_(HESJ) (where R_(HESJ) is the ~3He/~4He ratio of the Helium Standard of Japan), which is consistent with the previously reported value of (5077±59)×10~(-5)R_(HESJ) for historical slags in France and United Arab Emirates and about 4% higher than that of average modern air, (4901±4)×10~(-5)R_(HESJ). This result implies that the air ~3He/~4He ratio has decreased with time as expected by anthropogenic causes. Our modern surface air samples revealed that the ~3He/~4He ratio increases from north to south at a rate of (0.16±0.08)×10~(-5)R_(HESJ)/degree of latitude, suggesting that the low ~3He/~4He ratio originates in high-latitude regions of the northern hemisphere, which is consistent with the fact that most fossil fuel is extracted and consumed in the northern hemisphere.