サハラ砂漠の塵が海洋の繁栄を助ける(Sahara Desert dust is helping oceans thrive)

2024-09-27

2024-09-26 カリフォルニア大学リバーサイド校(UCR)

サハラ砂漠から吹き飛ばされる鉄を含む塵が、大気中の化学反応を通じて生物に利用可能な「バイオリアクティブ」な鉄に変わり、遠くの海洋で生命を支えることが確認されました。研究は、サハラ砂漠から大西洋に広がる塵の中の鉄が、距離が増すほど生物に利用しやすくなることを示し、海洋のプランクトンによる炭素固定を促進し、地球の気候にも影響を与える可能性があることを示唆しています。

＜関連情報＞

塵の長距離輸送が海洋での鉄の生物学的利用能を高める
Long-range transport of dust enhances oceanic iron bioavailability

Bridget Kenlee,Jeremy D. Owens,Robert Raiswell,Simon W. Poulton,Silke Severmann,Peter M. Sadler,Timothy W. Lyons
Frontiers in Marine Science Published:20 September 2024
DOI:https://doi.org/10.3389/fmars.2024.1428621

Wind-borne dust supply of iron (Fe) to the oceans plays a crucial role in Earth’s biogeochemical cycles. Iron, a limiting micronutrient for phytoplankton growth, is fundamental in regulating ocean primary productivity and in turn the global carbon cycle. The flux of bioavailable Fe to the open ocean affects oscillations in atmospheric CO₂ due to its control on inorganic carbon fixation into organic matter that is eventually exported to the sediments. However, the nature of dust-delivered Fe to the ocean and controls on its bioavailability remain poorly constrained. To evaluate the supply of wind-borne bioavailable Fe and its potential impact on Fe-based climate feedbacks over the last 120,000 years, we examine sediment profiles from four localities that define a proximal to distal transect relative to Saharan dust inputs. Bulk δ⁵⁶Fe isotope compositions (average = -0.05‰) and Fe_T/Al ratios suggest crustal values, thus pointing to a dominant dust origin for the sediments at all four sites. We observed no variability in grain size distribution or in bioavailable Fe supply at individual sites as a function of glacial-versus-interglacial deposition. Importantly, there is no correlation between sediment grain size and Fe bioavailability. Spatial trends do, however, suggest increasing Fe bioavailability with increasing distance of atmospheric transport, and our sediments also indicate the loss of this Fe and thus potential bioavailability utilization once deposited in the ocean. Our study underscores the significance of Fe dynamics in oceanic environments using refined speciation techniques to elucidate patterns in Fe reactivity. Such insights are crucial for understanding nutrient availability and productivity in various ocean regions, including the Southern Ocean, where wind-delivered Fe may play a pivotal role. It is expected that dust delivery on glacial-interglacial timescales would be more pronounced in these high-latitude regions. Our findings suggest that studies linking Fe availability to marine productivity should benefit significantly from refined Fe speciation approaches, which provide insights into the patterns and controls on Fe reactivity, including atmospheric processing. These insights are essential for understanding the impacts on primary production and thus carbon cycling in the oceans and consequences for the atmosphere.