Beyond the Abyss—Scientists Investigate the Virtually Unexplored Mariana Trench
In November and December of 2014, Jill Bourque of the U.S. Geological Survey (USGS) Southeast Ecological Science Center’s Benthic Ecology Group (Gainesville, Florida) joined an international team of scientists exploring the biology and geology of the Mariana Trench, the deepest known part of the ocean. The Mariana Trench, approximately 7 miles deep, lies in the western Pacific Ocean off the coast of Guam. Its deepest point, the Challenger Deep, has received increased attention from deep-sea explorers as technology advances, yet there is still much to learn about the biology, ecology, and geology of trench ecosystems. The deepest parts of the oceans, trenches remain a largely unstudied ocean frontier.
Led by co-chief scientists from the University of Hawai‘i, Jeff Drazen and Patty Fryer, the Hadal Ecosystem Studies (HADES) Mariana expedition set out to understand the basic environmental conditions within the trench, how animals in the trench have adapted to withstand pressure at such depths, how they are distributed across depth ranges, and the processes that control earthquake and tsunami generation.
Trenches typically form where two tectonic plates collide and one plate slides beneath the other (known as subduction). These subduction zones are geologically active areas that produce complex topography and some of the world’s largest earthquakes. The Mariana Trench was created where the Pacific plate subducts beneath the Mariana plate. The expedition targeted multiple locations within the trench, on both tectonic plates and at multiple depths ranging from approximately 5,000 to almost 11,000 meters. Investigations included sampling from the second-deepest spot in the trench, the Sirena Deep (10,731 meters). The broad spectrum of sampled environments enabled scientists to directly address hypotheses concerning the relationships between biological and geological processes across the abyssal (less than 6,000 meters deep) and hadal (greater than 6,000 meters deep) zones.
Aboard Schmidt Ocean Institute’s research vessel (R/V) Falkor, the researchers relied on five deep-sea landers for insight into life on the seafloor (“Anatomy of A Lander”). Landers are free-falling vehicles that are deployed by the ship and sink to the seafloor. Outfitted with various types of instruments, they are deep-sea observatories, collecting data and sampling the seafloor. Once sampling is complete, an acoustic signal is sent from the ship to the lander to release weights, and the lander rises to the surface. Landers fitted with coring respirometers measured the change in sediment oxygen content over time to help scientists understand the activity or pace of life of organisms residing in the seafloor sediment. In addition, the same sediment was returned to the surface for further analysis of organic content (proxy for food), sediment characteristics, and species diversity. The landers also collected water samples, sediments, and rocks for chemical, microbial, and geological analyses. Camera landers were baited with fish, providing the researchers an opportunity to observe and record scavenger species, including larger-than-life amphipods and hadal fish. Researchers used specimens collected by baited fish and amphipod traps to conduct morphological studies and tests of physiological adaptations to pressure, to assess population dynamics via genetic analysis, and to study feeding ecology through stable isotope analysis.
USGS benthic ecologist Bourque collected sediment samples to help determine population abundance and diversity of macrofauna (organisms larger than 0.3 millimeter) and meiofauna (less than 0.3 millimeter). These organisms are important to ecosystem functioning because they provide a mechanism for energy transfer between the water column and the sediment. Meiofauna and macrofauna play a major role in the breakdown and burial of organic carbon raining down from surface waters, and they serve as food for larger invertebrates and fish.
Samples from the Mariana Trench will help researchers build upon previous work done with HADES in the Kermadec Trench, which lies off of New Zealand. In contrast to the Mariana Trench, the Kermadec Trench occurs in an area where there is higher productivity in the water column, potentially providing more food for animals at the bottom of the trench. Led by Tim Shank of the Woods Hole Oceanographic Institution, the May 2014 Kermadec expedition used landers and the hybrid remotely operated vehicle (HROV) Nereus (see “Hybrid Remotely Operated Vehicle Nereus: Exploring the oceans' deepest depths”) to understand trench environments, enabling scientists to directly compare these two different trench systems. The collaborating researchers aim to assess how food supply, depth, and sediment characteristics structure communities in the two trenches.
New species were discovered on both HADES expeditions, including an unknown variety of snailfish (Family: Liparidae) in the Mariana Trench that broke the record for the deepest living fish caught or seen on video (“FVCR 2014 11 18 new species”). Found as deep as 8,143 meters, the fish was first spotted on a lander camera while sediment samples were being collected. These rare sightings provide unique insight into how organisms adapt to extreme pressure. Rocks collected from the inner slope of the trench broke depth records for previous rock collections, and their analysis will provide a unique opportunity to improve scientists’ understanding of the mechanisms involved in tectonic plate subduction.
The scientific discoveries following the Kermadec and Mariana Trench Expeditions have been extensive, and the researchers have only begun to analyze their data. As data are analyzed and samples processed, scientists will gain insight into the ecological functioning of each trench, as well as assess what patterns are consistent among trench systems as a whole.
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