“Birth” of the modern ocean twilight zone

doi:10.1126/science.abg5994

GSTDTAP > 气候变化

DOI	10.1126/science.abg5994
	“Birth” of the modern ocean twilight zone
	Laurent Bopp
	2021-03-12
发表期刊	Science
出版年	2021
英文摘要	The ocean “ twilight zone,” where sunlight hardly penetrates, is located at depths of 200 to 1000 m from the sea surface. Inhabited by iconic species such as lantern fish and giant squid, it has stimulated our collective imagination for generations. To this day it remains largely unexplored and has revealed few of its secrets. Also known as the mesopelagic, this zone occupies 20% of the volume of the world's oceans. It plays a key role in the global carbon cycle ([ 1 ][1], [ 2 ][2]) and may contain both biomass and biodiversity that have been largely underestimated ([ 3 ][3], [ 4 ][4]). On page 1148 of this issue, Boscolo-Gallazzo et al. ([ 5 ][5]) provide evidence that the establishment of the modern twilight zone is, on planetary time scales, a relatively recent phenomenon that has taken place gradually over the past 15 million years. Using an approach that couples ocean biogeochemical simulations and a compilation of sedimentary isotopic data, Boscolo-Galazzo et al. show the role of the gradual cooling of the climate system (by 4° to 6°C for the global surface ocean) in establishing the modern twilight zone. This global decrease in temperature has reduced the decomposition of organic matter that is formed by upper-ocean ecosystems and then sinks to depth. As such, the export of organic carbon to the deeper ocean has become more efficient and has increased the amount of food available to mesopelagic ecosystems. These long-term paleoclimatic changes are likely responsible for the establishment of relatively new deep ecological niches and the modern twilight zone. The role of the twilight zone in the ocean carbon cycle has been recognized for more than 30 years, with the identification of the importance of the biological carbon pump in transporting carbon out of the ocean's surface layers ([ 1 ][1]). Indeed, it is in the twilight zone that most of the organic carbon that is exported from the surface ocean is remineralized back to carbon dioxide through a series of biological, physical, and chemical processes that are not yet well understood. However, what is known is that this input of organic matter is critical to the survival of most organisms living at these depths and to the ecosystems they inhabit. Hence, although a few billion tons of carbon leave the surface ocean every year in the form of particulate organic carbon, only a tiny fraction reaches the bottom of the twilight zone at 1000 m. This fraction nevertheless represents a long-term carbon sink, with this deep carbon remaining isolated from the atmosphere for centuries to millennia. The mean depth at which organic carbon is converted back to CO2, known as the remineralization depth, has a substantial impact on atmospheric CO2. Kwon et al. ([ 6 ][6]) have shown that a deepening or shoaling of the mean remineralization depth by just a few tens of meters could decrease or increase atmospheric CO2 by a few tens of parts per million over long time scales. It is therefore crucial to determine as accurately as possible the mechanisms by which organic matter is used by biota in the twilight zone, so as to better project the response of these mechanisms under changing ocean conditions and their role in altering the capacity of the ocean to hold carbon in the coming decades and centuries. Some of the growing interest in the twilight zone stems from the recognition that it could harbor considerably more fish biomass than previously estimated ([ 3 ][3])—potentially 20 times as much as in the surface ocean. This discovery has led to new prospects for industrial fisheries, which hope to overcome the limitations posed by depleted or overfished surface fish stocks ([ 7 ][7]). Increasing interest in commercial exploitation of mesopelagic stocks for human consumption, fishmeal, and nutraceuticals is exemplified by the recent European Union Blue Growth Strategy position paper ([ 8 ][8], [ 9 ][9]), which is open to the exploration and exploitation of mesopelagic fish resources. However, there are large uncertainties about the extent of these stocks, which are estimated at 1000 to 20,000 million tons ([ 3 ][3], [ 10 ][10]), and little is known about their vulnerability to fishing and other human pressures. Taking advantage of several oceanographic campaigns and many innovative technological developments (autonomous floats, video profilers), several large-scale scientific projects grouped within the JETZON consortium ([ 10 ][10]) aim to explore the ocean twilight zone and unravel some of its mysteries. The questions are numerous: For example, it is not known what species are present at these depths and what the biomass of these organisms is. Also unclear is the role of processes in the mesopelagic zone in the transfer of carbon from the surface ocean to the deep ocean where it is sequestered for long periods. Another question concerns the sensitivity of these mesopelagic ecosystems to the input of matter from the surface, and to variations in local environmental conditions, in particular temperature, oxygenation, and acidification. The work of Boscolo-Galazzo et al. on the “birth” of the current twilight zone is particularly timely and relevant to these questions. Indeed, it shows how mesopelagic ecosystems may be vulnerable to future ocean warming through its impact on the delivery of organic matter from surface waters. Other ocean conditions, such as oxygen concentrations and ocean acidity, may also alter the viability of these specific ecosystems. The climate projections carried out in the framework of the recent Coupled Model Intercomparison Project Phase 6 (CMIP6) confirm that future warming, deoxygenation, and acidification may concurrently alter the deep-ocean environment, with unknown consequences for mesopelagic ecosystems ([ 11 ][11]). In Twenty Thousand Leagues Under the Sea ([ 12 ][12]), Jules Verne used his legendary intuition to describe the depths of the ocean, evoking their mysteries and the fascination they exert on humankind: “The sea is the vast reservoir of Nature. The globe began with sea, so to speak; and who knows if it will not end with it? In it is supreme tranquility. The sea does not belong to despots. Upon its surface men can still exercise unjust laws, fight, tear one another to pieces, and be carried away with terrestrial horrors. But at thirty feet below its level, their reign ceases, their influence is quenched, and their power disappears.” Even the prescient Jules Verne had not foreseen how far-reaching human influence may become. More than 150 years after the publication of his classic novel, we are confronted with the realization that the ocean twilight zone now faces several human threats. 1. [↵][13]1. E. T. Sundquist, 2. W. S. Broecker 1. T. Volk, 2. M. I. Hoffert , in The Carbon Cycle and Atmospheric CO2: Natural Variations, Archean to Present, E. T. Sundquist, W. S. Broecker, Eds. (American Geophysical Union, 1985), pp. 99–110. 2. [↵][14]1. C. M. Marsay et al ., Proc. Natl. Acad. Sci. U.S.A. 112, 1089 (2015). [OpenUrl][15][Abstract/FREE Full Text][16] 3. [↵][17]1. X. Irigoien et al ., Nat. Commun. 5, 3271 (2014). [OpenUrl][18][CrossRef][19][PubMed][20] 4. [↵][21]1. C. Robinson et al ., Deep Sea Res. II 57, 1504 (2010). [OpenUrl][22][CrossRef][23] 5. [↵][24]1. F. Boscolo-Galazzo et al ., Science 371, 1148 (2021). [OpenUrl][25][Abstract/FREE Full Text][26] 6. [↵][27]1. E. Y. Kwon, 2. F. Primeau, 3. J. L. Sarmiento , Nat. Geosci. 2, 630 (2009). [OpenUrl][28][CrossRef][29] 7. [↵][30]Food and Agriculture Organization of the United Nations, The State of World Fisheries and Aquaculture 2020: Sustainability in Action (2020). 8. [↵][31]European Union Directorate-General for Maritime Affairs and Fisheries, Blue Bioeconomy Situation Report and Perspectives (2018); [www.eumofa.eu/documents/20178/84590/Blue+bioeconomy_Final.pdf][32]. 9. [↵][33]European Commission, Blue Growth (2020); [https://ec.europa.eu/maritimeaffairs/policy/blue\_growth\_en][34]. 10. [↵][35]1. A. Martin et al ., Nature 580, 26 (2020). [OpenUrl][36][CrossRef][37][PubMed][38] 11. [↵][39]1. L. Kwiatkowski et al ., Biogeosciences 17, 3439 (2020). [OpenUrl][40] 12. [↵][41]1. J. Verne , Vingt mille lieues sous les mers (Hetzel, 1869–1870). 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/318679
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Laurent Bopp. “Birth” of the modern ocean twilight zone[J]. Science,2021.
APA	Laurent Bopp.(2021).“Birth” of the modern ocean twilight zone.Science.
MLA	Laurent Bopp."“Birth” of the modern ocean twilight zone".Science (2021).