冰河期的南冰洋並未發生施肥效應

 原文網址：https://www.uni-bonn.de/en/news/074-2022

波昂大學主持的國際研究從岩芯當中得到了150萬年的氣候紀錄

富含鐵的沙塵是否可以對海洋施肥，刺激藻類生長，進而捕捉大氣裡的二氧化碳？波昂大學主持的國際研究團隊透過採自斯科細亞海的深海沉積物岩芯，探討這種可以吸收溫室氣體的假說是否在冰河期有所作用。雖然冰河期有很多沙塵飄到南冰洋，卻沒有發現施肥效應的證據。反之在沙塵輸入量較少的暖期，才發生了藻類生產量提高等現象，因此可以封存更多二氧化碳。研究結果最近發表在《自然通訊》(Nature Communications)。

大氣二氧化碳濃度的變化被認為是過去與未來氣候變遷的主因。過往冰河期的大氣二氧化碳含量比現在低了大約30%，原因是否為鐵施肥作用(iron fertilization)？長久以來對此有相當多的討論。有人主張富含鐵的沙塵經由風和水進入海洋之後，可以刺激藻類生長而吸收更多二氧化碳。當藻類死亡並且永遠沉入海底之後，這些二氧化碳也會待在那裏，就像是被埋起來一樣。雖然證據清楚指出冰河期的時候有更多沙塵進到海洋，但是施肥效應是否發生的證據卻有所爭議，至少就南冰洋來說是如此。

由波昂大學地球科學研究所的Michael Weber博士主持，包含13個國家38名研究人員的國際團隊，在最近一項研究當中探討了這項問題。做為綜合大洋鑽探計畫的一部份，團隊在2019年搭上鑽探船「聯合果敢號」前往斯科細亞海，花費兩個月的時間從水深3000到4000公尺處採集了數根岩芯。Weber說：「在南極以及它的沙塵主要來源——巴塔哥尼亞附近的氣候紀錄中，我們採集到的是歷來時間最長、解析度也最高的紀錄。」

150萬年的氣候史

一根200公尺長的深海岩芯U1537詳細記錄了最近150萬年的氣候史。由於之前南極的冰芯只涵蓋了最近80萬年的歷史，因此U1537可以讓沙塵輸入量的重建結果再延長一倍。目前的深海紀錄顯示冰河期的沙塵堆積量實際上比現在高出5到15倍，而冰芯也反映出同樣的訊息。

然而，研究人員發現在冰河期的南冰洋，沒有任何證據顯示沙塵曾經引發施肥效應。反之，在沒有太多沙塵進入斯科細亞海的暖期，才看到藻類生產量變高等現象，代表有更多二氧化碳被封存。這意味著在冷期，有其他作用防止被海洋捕捉的二氧化碳進到大氣當中而引發暖化。主要的原因包括海冰的覆蓋面積擴大、海洋的分層現象變得更加明顯、洋流系統減緩，這些都可以降低冷期大氣中的二氧化碳含量。

更新世冰河期與間冰期的沙塵堆積量和海洋生產力呈現相反的趨勢，也伴隨著南極地區氣候系統的逐漸變化。在最近40萬年的間冰期中，生物生產力有特別高的現象；但是在70萬年到120萬年前的中更新世轉型期，暖期與冷期卻幾乎沒有差別。隨著轉型期進展，沙塵輸入到南半球的面積也變得越來越大。這種急遽的變化一直持續到90萬年前，代表了南極的冰河期變得更加劇烈。

「在南極地區以外的岩芯，確實有證據顯示冰河期發生了施肥效應，」Weber總結。「但是我們的研究顯示大氣二氧化碳波動的原因不僅和沙塵堆積造成的鐵施肥作用有關，而是西風系統、生物生產力、海冰產生的回饋作用三者之間複雜的交互關係。在最近150萬年都能看到這種關係。」

 

No glacial fertilization effect in the Antarctic Ocean

International study led by the University of Bonn records 1.5 million years of climate in the drill core

Can iron-rich dust fertilize the ocean, stimulate algae growth there, and thereby capture carbon dioxide from the atmosphere? An international research team led by the University of Bonn used deep-sea sediment cores from the Scotia Sea to investigate whether this hypothetical greenhouse gas sink had an effect during ice ages. Although dust input was high during ice ages, no evidence of a fertilization effect could be found in the Antarctic Ocean. Rather, the production of algae, for example, and thus carbon dioxide sequestration, was high only during warm periods when dust input was low. The study has now been published in "Nature Communications."

Changes in the concentration of atmospheric carbon dioxide (CO₂) are considered to be the main cause of past and future climate change. A long-standing debate centers on whether the roughly 30 percent lower CO₂ content of the ice-age atmosphere was caused by iron fertilization. It is argued that iron-rich dust is carried into the ocean by wind and water, where it stimulates the growth of algae that absorb more CO₂. As the algae die and then sink permanently into the depths of the ocean, the CO₂ also remains there like in a trap. Although there is clear evidence that dust input increased during the ice ages, the fertilization effect is controversial, at least for the Antarctic Ocean.

In a recent study, an international team of 38 researchers from 13 countries led by Dr. Michael Weber from the Institute for Geosciences at the University of Bonn investigated this question. As part of the Integrated Ocean Discovery Program (IODP), the team traveled to the Scotia Sea on the drillship "JOIDES Resolution" and spent two months in 2019 bringing up cores from the seafloor at depths of 3,000 to 4,000 meters. Weber: "We collected the highest-resolution and longest climate archive ever obtained near Antarctica and its main dust source, Patagonia."

1.5 million years of climate history

In the 200-meter-long deep-sea core U1537, the climate history of the last 1.5 million years was recorded in detail. This allows the reconstruction of the dust input to be nearly doubled, since Antarctic ice cores only cover the last 800,000 years. Current records from the deep ocean show that dust deposition during the ice ages was actually five to 15 times higher. This is also reflected in the ice cores.

However, the researchers found no evidence of a fertilization effect from dust in the Antarctic Ocean during the ice ages. Rather, the production of algae, for example, and thus carbon CO2 sequestration, was high only during warm periods when dust input into the Scotia Sea was low. This means that during cold periods, other processes prevented the CO2 captured in the ocean from escaping into the atmosphere and triggering warming. The main factors here are much more extensive sea ice cover, more intense stratification in the ocean, and reduced dynamics of the current systems, which contributed to a reduction in the CO2 content of the atmosphere during cold periods.

The opposing trends in dust deposition and oceanic productivity during the ice ages and interglacial periods of the Pleistocene are accompanied by long-term, gradual changes in the climate system in the southern polar region. Bioproductivity was particularly high during the interglacial periods of the last 400,000 years, but during the mid-Pleistocene transition 1.2 million to 700,000 years ago, it differed little from that during cold periods. As the transition progressed, the dust input covered larger and larger areas in the Southern Hemisphere. Abrupt changes continued to occur 900,000 years ago, indicating greater glaciation of Antarctica.

"There is indeed evidence of a fertilization effect during the ice ages in cores outside the Antarctic zone," Weber concludes. "However, our study shows that atmospheric CO2 fluctuations do not depend solely on iron fertilization from dust deposition. In the Antarctic Ocean, it is rather a complex interplay of a westerly wind system, productivity, and feedback with sea ice. This relationship has been consistent over the last 1.5 million years."

原始論文：Michael E. Weber, Ian Bailey, Sidney R. Hemming, Yasmina M. Martos, Brendan T. Reilly, Thomas A. Ronge, Stefanie Brachfeld, Trevor Williams, Maureen Raymo, Simon T. Belt, Lukas Smik, Hendrik Vogel, Victoria L. Peck, Linda Armbrecht, Alix Cage, Fabricio G. Cardillo, Zhiheng Du, Gerson Fauth, Christopher J. Fogwill, Marga Garcia, Marlo Garnsworthy, Anna Glüder, Michelle Guitard, Marcus Gutjahr, Iván Hernández-Almeida, Frida S. Hoem, Ji-Hwan Hwang, Mutsumi Iizuka, Yuji Kato, Bridget Kenlee, Suzanne OConnell, Lara F. Pérez, Osamu Seki, Lee Stevens, Lisa Tauxe, Shubham Tripathi, Jonathan Warnock, Xufeng Zheng. Antiphased dust deposition and productivity in the Antarctic Zone over 1.5 million years. Nature Communications, 2022; 13 (1) DOI: 10.1038/s41467-022-29642-5

引用自：University of Bonn. "No glacial fertilization effect in the Antarctic Ocean."