死海の塩の巨人、地球の地質学的過去を垣間見る貴重な機会(Dead Sea’s salt giants offer rare glimpse into Earth’s geological past)

2025-07-21 カリフォルニア大学サンタバーバラ校(UCSB)

死海は極端な蒸発と塩分濃縮により、世界で唯一現在も成長を続ける巨大塩鉱床「ソルトジャイアンツ」が形成されている場所です。UCSBの研究チームは、この現象を数値モデルと観測で解析し、多重拡散による層構造の変化や、塩の「雪」が沈殿・堆積に寄与する仕組みを解明しました。この研究は、過去の地質時代の塩層形成を理解し、現代の気候変動や干ばつへの洞察を提供します。

＜関連情報＞

死海の流体力学 Fluid Mechanics of the Dead Sea

Eckart Meiburg and Nadav G. Lensky
Annual Review of Fluid Mechanics Published:September 11, 2024
DOI:https://doi.org/10.1146/annurev-fluid-031424-101119

ABSTRACT

The environmental setting of the Dead Sea combines several aspects whose interplay creates flow phenomena and transport processes that cannot be observed anywhere else on Earth. As a terminal lake with a rapidly declining surface level, the Dead Sea has a salinity that is close to saturation, so that the buoyancy-driven flows common in lakes are coupled to precipitation and dissolution, and large amounts of salt are being deposited year-round. The Dead Sea is the only hypersaline lake deep enough to form a thermohaline stratification during the summer, which gives rise to descending supersaturated dissolved-salt fingers that precipitate halite particles. In contrast, during the winter the entire supersaturated, well-mixed water column produces halite. The rapid lake level decline of O(1 m/year) exposes vast areas of newly formed beach every year, which exhibit deep incisions from streams. Taken together, these phenomena provide insight into the enigmatic salt giants observed in the Earth’s geological record and offer lessons regarding the stability, erosion, and protection of arid coastlines under sea level change.

死海における二重拡散性塩指からのハライト析出：数値シミュレーション Halite Precipitation From Double-Diffusive Salt Fingers in the Dead Sea: Numerical Simulations

Raphael Ouillon, Nadav G. Lensky, Vladimir Lyakhovsky, Ali Arnon, Eckart Meiburg
Water Resources Research Published: 03 May 2019
DOI:https://doi.org/10.1029/2019WR024818

Abstract

We employ direct numerical simulations in order to analyze the role of double-diffusive salt fingering in halite precipitation from hypersaline lakes. Guided by field observations from the Dead Sea, which represents the only modern deep stratified lake that precipitates halite under hydrological crisis, we consider a saturated layer of warm, salty brine (epilimnion) overlying a layer of colder, less salty brine (hypolimnion) that is also saturated. The double-diffusive instability originating in the metalimnion gives rise to an asymmetrical pattern of less salty ascending fingers, accompanied by descending salt fingers that lose heat as they propagate through the metalimnion. The net result is a strong, downward salinity flux that leads to the undersaturation of the epilimnion, while the hypolimnion becomes oversaturated and precipitates halite. These interfacial processes within deep, hypersaline water columns in warm and dry regions suggest a potential route toward the formation of thick halite layers found in the geological record.

Key Points

Guided by Dead Sea observations, we simulate double-diffusive salt fingering in saturated, hypersaline water columns
Salt fingering results in the undersaturation of the epilimnion, as well as halite precipitation in the hypolimnion
The observed interfacial processes suggest a route toward the formation of thick halite layers found in the geological record