Walter Maren

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  • Article
    Overview of the MOSAiC expedition: physical oceanography
    (University of California Press, 2022-02-07) Rabe, Benjamin ; Heuzé, Céline ; Regnery, Julia ; Aksenov, Yevgeny ; Allerholt, Jacob ; Athanase, Marylou ; Bai, Youcheng ; Basque, Chris R. ; Bauch, Dorothea ; Baumann, Till M. ; Chen, Dake ; Cole, Sylvia T. ; Craw, Lisa ; Davies, Andrew ; Damm, Ellen ; Dethloff, Klaus ; Divine, Dmitry V. ; Doglioni, Francesca ; Ebert, Falk ; Fang, Ying-Chih ; Fer, Ilker ; Fong, Allison A. ; Gradinger, Rolf ; Granskog, Mats A. ; Graupner, Rainer ; Haas, Christian ; He, Hailun ; Hoppmann, Mario ; Janout, Markus A. ; Kadko, David ; Kanzow, Torsten C. ; Karam, Salar ; Kawaguchi, Yusuke ; Koenig, Zoe ; Kong, Bin ; Krishfield, Richard A. ; Krumpen, Thomas ; Kuhlmey, David ; Kuznetsov, Ivan ; Lan, Musheng ; Laukert, Georgi ; Lei, Ruibo ; Li, Tao ; Torres-Valdes, Sinhue ; Lin, Lina ; Lin, Long ; Liu, Hailong ; Liu, Na ; Loose, Brice ; Ma, Xiaobing ; McKay, Rosalie ; Mallet, Maria ; Mallett, Robbie ; Maslowski, Wieslaw ; Mertens, Christian ; Mohrholz, Volker ; Muilwijk, Morven ; Nicolaus, Marcel ; O’Brien, Jeffrey K. ; Perovich, Donald K. ; Ren, Jian ; Rex, Markus ; Ribeiro, Natalia ; Rinke, Annette ; Schaffer, Janin ; Schuffenhauer, Ingo ; Schulz, Kirstin ; Shupe, Matthew ; Shaw, William J. ; Sokolov, Vladimir T. ; Sommerfeld, Anja ; Spreen, Gunnar ; Stanton, Timothy P. ; Stephens, Mark ; Su, Jie ; Sukhikh, Natalia ; Sundfjord, Arild ; Thomisch, Karolin ; Tippenhauer, Sandra ; Toole, John M. ; Vredenborg, Myriel ; Walter, Maren ; Wang, Hangzhou ; Wang, Lei ; Wang, Yuntao ; Wendisch, Manfred ; Zhao, Jinping ; Zhou, Meng ; Zhu, Jialiang
    Arctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. An international team designed and implemented the program to characterize the Arctic Ocean system in unprecedented detail, from the seafloor to the air-sea ice-ocean interface, from sub-mesoscales to pan-Arctic. The oceanographic measurements were coordinated with the other teams to explore the ocean physics and linkages to the climate and ecosystem. This paper introduces the major components of the physical oceanography program and complements the other team overviews of the MOSAiC observational program. Team OCEAN’s sampling strategy was designed around hydrographic ship-, ice- and autonomous platform-based measurements to improve the understanding of regional circulation and mixing processes. Measurements were carried out both routinely, with a regular schedule, and in response to storms or opening leads. Here we present along-drift time series of hydrographic properties, allowing insights into the seasonal and regional evolution of the water column from winter in the Laptev Sea to early summer in Fram Strait: freshening of the surface, deepening of the mixed layer, increase in temperature and salinity of the Atlantic Water. We also highlight the presence of Canada Basin deep water intrusions and a surface meltwater layer in leads. MOSAiC most likely was the most comprehensive program ever conducted over the ice-covered Arctic Ocean. While data analysis and interpretation are ongoing, the acquired datasets will support a wide range of physical oceanography and multi-disciplinary research. They will provide a significant foundation for assessing and advancing modeling capabilities in the Arctic Ocean.
  • Preprint
    Rapid dispersal of a hydrothermal plume by turbulent mixing
    ( 2010-08-23) Walter, Maren ; Mertens, Christian ; Stober, Uwe ; German, Christopher R. ; Yoerger, Dana R. ; Sultenfuß, Jurgen ; Rhein, Monika ; Melchert, Bernd ; Baker, Edward T.
    The water column imprint of the hydrothermal plume observed at the Nibelungen field (8°18' S 13°30' W) is highly variable in space and time. The off-axis location of the site, along the southern boundary of a non-transform ridge offset at the joint between two segments of the southern Mid-Atlantic Ridge, is characterized by complex, rugged topography, and thus favorable for the generation of internal tides, subsequent internal wave breaking, and associated vertical mixing in the water column. We have used towed transects and vertical profiles of stratification, turbidity, and direct current measurements to investigate the strength of turbulent mixing in the vicinity of the vent site and the adjacent rift valley, and its temporal and spatial variability in relation to the plume dispersal. Turbulent diffusivities Kp were calculated from temperature inversions via Thorpe scales. Heightened mixing (compared to open ocean values) was observed in the whole rift valley within an order of Kp around 10-3 m2 s-1. The mixing close to the vent site was even more elevated, with an average of Kp = 4 x 10-2 m2 s-1. The mixing, as well as the flow field, exhibited a strong tidal cycle, with strong currents and mixing at the non-buoyant plume level during ebb flow. Periods of strong mixing were associated with increased internal wave activity and frequent occurrence of turbulent overturns. Additional effects of mixing on plume dispersal include bifurcation of the particle plume, likely as a result of the interplay between the modulated mixing strength and current speed, as well as high frequency internal waves in the effluent plume layer, possibly triggered by the buoyant plume via nonlinear interaction with the elevated background turbulence or penetrative convection.
  • Article
    Volcanically hosted venting with indications of ultramafic influence at Aurora hydrothermal field on Gakkel Ridge
    (Nature Communications, 2022-10-31) German, Christopher R ; Reeves, Eoghan P ; Türke, Andreas ; Diehl, Alexander ; Albers, Elmar ; Bach, Wolfgang ; Purser, Autun ; Ramalho, Sofia P ; Suman, Stefano ; Mertens, Christian ; Walter, Maren ; Ramirez-Llodra, Eva ; Schlindwein, Vera ; Bünz, Stefan ; Boetius, Antje
    The Aurora hydrothermal system, Arctic Ocean, hosts active submarine venting within an extensive field of relict mineral deposits. Here we show the site is associated with a neovolcanic mound located within the Gakkel Ridge rift-valley floor, but deep-tow camera and sidescan surveys reveal the site to be ≥100 m across-unusually large for a volcanically hosted vent on a slow-spreading ridge and more comparable to tectonically hosted systems that require large time-integrated heat-fluxes to form. The hydrothermal plume emanating from Aurora exhibits much higher dissolved CH/Mn values than typical basalt-hosted hydrothermal systems and, instead, closely resembles those of high-temperature ultramafic-influenced vents at slow-spreading ridges. We hypothesize that deep-penetrating fluid circulation may have sustained the prolonged venting evident at the Aurora hydrothermal field with a hydrothermal convection cell that can access ultramafic lithologies underlying anomalously thin ocean crust at this ultraslow spreading ridge setting. Our findings have implications for ultra-slow ridge cooling, global marine mineral distributions, and the diversity of geologic settings that can host abiotic organic synthesis - pertinent to the search for life beyond Earth.