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Water Takes a Deep Dive Into the Mariana Trench

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Water Takes a Deep Dive Into the Mariana Trench NEWS & VIEWS RESEARCH related antibody) can produce similar results University of Melbourne and Royal Melbourne 6. Bruel, T. et al. Nature Commun. 7, 10844 (2016). in people. These clinical trials are being Hospital, Melbourne, Victoria 3000, Australia. 7. Okoye, A. A. et al. Nature Med. 24, 1430–1440 (2018). planned, and the results are eagerly awaited. In e-mail: [email protected] 8. Nishimura, Y. & Martin, M. A. Cell Host Microbe 22, the meantime, antiretrovirals remain the best 1. Colby, D. J. et al. Nature Med. 24, 923–926 (2018). 207–216 (2017). and only option for the long-term treatment of 2. Borducchi, E. N. et al. Nature 563, 360–364 (2018). 9. Henrich, T. J. et al. PLoS Med. 14, e1002417 HIV infection. ■ 3. Pitman, M. C., Lau, J. S. Y., McMahon, J. H. & (2017). Lewin, S. R. Lancet HIV 5, e317–e328 (2018). 10. Luzuriaga, K. et al. N. Engl. J. Med. 372, 786–788 4. Lim, S.-Y. et al. Sci. Transl. Med. 10, eaao4521 (2018). (2015). Sharon R. Lewin is at The Peter Doherty 5. Kim, Y., Anderson, J. L. & Lewin, S. R. Cell Host Institute for Infection and Immunity, Microbe 23, 14–26 (2018). This article was published online on 3 October 2018. EARTH SCIENCE oceanic plate. Most constraints on estimates come from controlled-source seismic data, which are produced by measuring seismic waves generated by artificial sources using Water takes a deep dive dense arrays of recording instruments. These data provide excellent constraints on hydra- tion of the crust and shallow mantle, but, with into the Mariana Trench one notable exception7, do not constrain the full depth of hydration. It is clearly not possible A tectonic plate descending into the Mariana Trench carries sea water deep into to tally the total volume of subducted water Earth’s interior. It seems that much more water enters Earth at this location than without knowing the full hydration depth. was thought — with implications for the global water budget. See Letter p.389 Another important challenge is to untangle all the factors that alter the speed at which seismic waves travel through different parts of DONNA J. SHILLINGTON ocean, but seamounts (underwater mountains) the plate; measurements of such seismic waves and fracture zones provide pathways for are one of the primary means of estimating the he subduction zones at which the further water input and output, so that circula- amount of water in the subducting oceanic tectonic plates beneath the sea thrust tion continues away from the mid-ocean ridge3. plate. Most estimates assume that any reduc- into the deep Earth act as gigantic con- The final infiltration occurs at the ‘outer rise’ of a tion in wave speed results from the replace- Tveyer belts, carrying water, fluids and volatile subduction zone, where the oceanic plate bends ment of olivine (the main mineral found in compounds into our planet. Water in Earth’s before entering the trench. Here, extensional the mantle) by serpentine. However, in the interior is released back into the oceans and faults form in response to bending, and are crust and shallow mantle, water-filled cracks atmosphere by volcanoes. These inputs and thought to enable pervasive, deeply penetrat- can also contribute to velocity reductions8. To outputs constitute a global deep-Earth water ing hydration of the crust and upper mantle4–6. complicate things further, seismic-wave speeds cycle, but quantifying the total water input The evidence for water entering subduction in the upper oceanic mantle are aniso tropic — from oceanic plates has proved difficult. On zones is clear, but several knowledge gaps have they depend on the direction of propagation. page 389, Cai et al.1 report that the Pacific hindered attempts to quantify the total volume This is because olivine crystals align in the plate, which subducts in the Mariana Trench, of water going down these hatches, even at direction in which the sea floor spreads when contains much more water than was previ- individual subduction zones. One unknown new oceanic plates are created at mid-ocean ously supposed — a finding that has major is the depth to which water penetrates the ridges. Further aniso tropy can result from frac- ramifications for Earth’s water budgets. tures formed at the outer rise. Water is as crucial to the workings Cai et al. tackle all of these issues by of Earth’s interior as it is to Earth’s sur- Mariana Oceanic presenting constraints on the hydra- face processes: among other things, it Trench crust tion of the approximately 150-million- triggers magma generation beneath year-old Pacific plate as it subducts volcanoes, lubricates deep fault zones, at the Mariana Trench. The authors and funda mentally alters the strength analysed seismic waves from distant and behaviour of Earth’s mantle. Sea Philippine Hydrated earthquakes, recorded by an array of 30 km water seeps into the oceanic lithosphere plate mantle seismometers on the sea floor. This through fractures and pores, and reacts Hydrated mantle allowed them to model seismic-wave with minerals in the crust and mantle with cracks speeds to much greater depths (albeit at to form hydrous minerals (such as ser- Mantle lower resolution) than is possible using pentine) that store water in their crystal controlled-source seismic data. structures. Pacic The researchers find that, impres- plate Water infiltration occurs at a couple sively, the full hydration depth of the of key stages of an oceanic plate’s life lithosphere extends to approximately cycle. The first is at mid-ocean ridges, 30 kilometres below the sea floor when water circulates through hot, Figure 1 | Hydration of the Pacific tectonic plate at the Mariana (Fig. 1). They were also able to examine newly formed oceanic plates2. But at subduction zone. At the Mariana Trench in the Pacific Ocean, velocity reductions in deep regions at fast-spreading ridges (which are the the Pacific plate slips (subducts) beneath the adjacent Philippine which the pressure would be sufficiently primary ‘diet’ of the subduction zones plate, transporting sea water into the deep Earth. The water high to close all cracks, thus allowing seeps through cracks and pores in the plate, and reacts with that ring the Pacific Ocean), circulation minerals in the crust and mantle to form hydrated regions them to eliminate the possible contribu- and hydration are mainly restricted to consisting of minerals that store water in their crystal structures. tion of such cracks to velocities in these the plate’s upper crust. The accumula- Cai et al.1 have used seismic measurements to show that water regions. Finally, because the authors tion of sediments subsequently seals penetrates to depths of about 30 kilometres below the ocean floor. recorded waves travelling in all direc- off most of the oceanic plate from the (Approximated from Fig. 2d of ref. 1.) tions across their array, they were able ©2018 Spri nger Nature Li mited. All ri ghts reserved. ©2018 Spri nger Nature Li mited. All ri ghts reserv15ed. NOVEMBER 2018 | VOL 563 | NATURE | 335 RESEARCH NEWS & VIEWS to account for the contribution of anisotropy 5. Ivandic, M., Grevemeyer, I., Berhorst, A., Flueh, E. R. Abers, G. A. J. Geophys. Res. 116, B01401 to wave speed. & McIntosh, K. J. Geophys. Res. 113, B05410 (2011). (2008). 10. Parai, R. & Mukhopadhyay, S. Earth Planet. Sci Lett. Cai et al. report that more than four times as 6. Lefeldt, M., Grevemeyer, I., Goßler, J. & Bialas, J. 317–318, 396–406 (2012). much water is entering the Mariana subduc- Geophys. J. Int. 178, 742–752 (2009). 11. Shillington, D. J. et al. Nature Geosci. 8, 961–964 tion zone than was previously estimated9. Old, 7. Van Avendonk, H. J. A., Holbrook, W. S., (2015). cold subducting plates such as that entering Lizarralde, D. & Denyer, P. Geochem. Geophys. 12. Fujie, G. et al. Nature Commun. 9, 3844 (2018). Geosys. 12, Q06009 (2011). 13. Bayrakci, G. et al. Nature Geosci. 9, 384–388 the Mariana Trench are particularly effec- 8. Miller, N. C. & Lizarralde, D. Geophys. Res. Lett. 43, (2016). tive conveyors of water into the deep Earth 7982–7990 (2016). 14. Wada, I., Behn, M. D. & Shaw, A. M. Earth Planet. Sci. because hydrous minerals in such plates are 9. van Keken, P. E., Hacker, B. R., Syracuse, E. M. & Lett. 353–354, 60–71 (2012). stable to greater depths than in younger, hotter plates. If extrapolated globally to other places where old, cold plates subduct, the authors’ ORGANIC CHEMISTRY result implies that the amount of water enter- ing Earth’s interior greatly exceeds current estimates10 of the amount being emitted by volcanoes, and thus requires a rethink of the From hydrocarbons to global water budget. Several questions still need to be answered to better constrain estimates of the inputs to precious chemicals Earth’s deep-water cycle and evaluate the impli- cations of the new results. First, how variable is Operationally simple chemistry enables aliphatic carbon–carbon bonds — the the water content in the oceanic plate at a range ‘girders’ in the framework of many organic molecules — to be prepared from of depths and scale lengths along subduction widely available hydrocarbons known as alkenes. See Letter p.379 zones? Many studies have reported changes in extensional faulting and in crustal and upper- mantle hydration along subduction zones, and JAMES P. MORKEN materials, pharmaceuticals or agrochemicals. several competing factors have been proposed The reactions reported by Fu and co-workers1 to contribute to these changes7,11,12. Characteriz- any valued reagents and catalysts on page 379 are a case in point. Not only are ing this variability throughout the hydrated part used in the preparation of organic they simple to carry out, but they also deliver of the plate and understanding its causes will be compounds are highly reactive and a variety of useful products that are otherwise essential for us to tally water inputs and com- Mare therefore incompatible with exposure to much more difficult to make.
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