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A Chemotrophic Ecosystem Found Beneath Antarctic Ice Shelf

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A Chemotrophic Ecosystem Found Beneath Antarctic Ice Shelf Eos, Vol. 86, No. 29, 19 July 2005 VOLUME 86 NUMBER 29 19 JULY 2005 EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 269–276 the mat, infi lling as ponds of silt and clay and A Chemotrophic Ecosystem patches of diatomaceous algal “fl uff” (Figure 2c). Fresh dropstones also have fallen on the mat (Figure 2b). Found Beneath Antarctic These stratigraphic relationships, clearly ob- served in the imagery, indicate that the mat was, at one time, forming across most of the bottom Ice Shelf within the survey region but is now being cov- ered by a fallout of ice-rafted debris and phyto- PAGES 269, 271–272 if not nonexistent [Lipps et al., 1979]. The re- detritus, refl ective of the ice shelf collapse and cent collapse of the Larsen Ice Shelf along the subsequent open marine conditions [Gilbert A new habitat for chemotrophic ecosystems eastern coast of the Antarctic Peninsula (Fig- and Domack, 2003]. has been found beneath the former extent of ure 1) has provided an unprecedented op- In addition, small mounds (1.5–2 m across the Larsen Ice Shelf in Antarctica. This is the portunity to examine the deep confi nes of a and 0.75 m high) are found in places, and these fi rst report of such ecosystems in the Antarctic. sub-ice-shelf setting from a marine perspective. are encircled by conspicuous, large bivalves (Chemotrophic ecosystems derive their prima- This article reports on observations of a (20–30 cm across) which cluster around a cen- ry metabolic energy from chemical reactions unique seafl oor habitat that developed far tral orifi ce a few decimeters in diameter (Figure other than those of photosynthetic origin.) beneath the edge of the fl oating Larsen B Ice 2d). Approximately 37 individual clams can be An association of microbial mats and cold Shelf, and which is now undergoing dramatic observed around the largest of these features seep clam communities, is described, that change in response to increased sediment (Figure 2d). Radiating from such vents are clear thrived within an 850-m-deep glacial trough fl ux to the seafl oor. These observations are indications of suspended particulate matter and some 100 km, or more, from the ice shelf front. based upon bathymetric mapping, bottom fl uidized mudfl ows, the latter marked by super- However, the continued existence of this sampling, and bottom photo/video coverage imposed channels with levees that terminate at unique ecosystem is uncertain, given the in- obtained during a March 2005 U. S. Antarctic the base of the mound in fan-shaped mudfl ats. creased loading of sediment to the seafl oor as Program cruise to the northwestern Weddell Several episodes of channel activity (eruptive a result of the ice shelf’s collapse in early 2002. Sea (http://www.hamilton.edu/news/exp/ant- events) are marked by clear patterns of cross- The vent-related ecology could have a meth- arctica/2005/). cutting relations between abandoned and more ane source, based upon the vent’s similarity recently active channels. with other cold seeps located along continen- Bathymetric Setting Remnant vents are marked by less orga- tal margins. nized groupings of large mollusks that are These results have implications for the dis- The bathymetry of the Larsen B embay- partly buried and lack clear channel systems. covery of life in extreme environments, includ- ment is refl ective of glacial erosion super- Two remnant vents were observed across the ing those found beneath the enormous extent imposed upon a bedrock transition from survey track. Noticeably absent from the bot- of existing ice shelves and large lakes that lie the crystalline metamorphic/igneous rocks tom images are established communities of beneath the Antarctic Ice Sheet. Because of of the Antarctic Peninsula to offshore mud benthic organisms typically associated with its restricted conditions, the seafl oor beneath rocks of Mesozoic age [Sloan et al., 1995]. Antarctic systems. ice shelves may provide a suitable, widespread An over-deepened inner shelf is transected habitat for chemotrophic systems; given this, by a deep trough that is silled by a narrow there may be many more such habitats wait- bank (Figure 1c). Within the deep (850 m) Affinity of Vent Community ing to be discovered beneath existing ice axis of the trough, photo imagery was taken shelves. across approximately 5540 m2 of the bottom The observations support the interpretation The seafl oor beneath Antarctica’s fl oating via a towed video sled (Figure 1c) and small of a chemotrophic benthic community estab- ice shelves covers more than 1.54 million scenes were captured via a still camera. lished on the seafl oor beneath the Larsen B km2 [Drewry, 1983], an area of the same order Across the survey area, 50–70% of the bot- Ice Shelf. The white mats are similar to Begiat- of magnitude as the Amazon basin of Brazil tom is draped by a white mat covering sea- toa, a genus of hydrogen sulfi de (H2S) oxidiz- or the Sahara desert. Yet, this expansive ma- fl oor irregularities (Figure 2a). The thickness ing chemoautotrophs, common at the interface rine setting remains largely unexplored, even of the mat varies from a few millimeters to between sulfi de-rich sediments and oxygen-rich though it represents an important coupling perhaps a centimeter or more. Across large waters. Chemoautotrophs are those organisms zone among ice sheets, the seafl oor, and the areas (as much as several hundred square that derive their metabolic energy and fi x ocean water column [Rignot and Jacobs, meters), the mat takes on a pustular morphol- their own carbon from the chemical transfor- 2002] (Figure 1). ogy that is independent of the underlying relief. mations associated with non-photosynthetic Even more poorly understood are marine In places, gas bubbles can be seen emanating processes. habitats beneath ice shelves where sources of from patches of mat (Figure 2a). Also, irregular The widespread occurrence of the mats, and primary phyto-production are severely limited, branching networks are common and appear their association with the observed seep com- to refl ect discontinuous horizontal burrows munities, suggest a source of carbon fi xation or places where brittle stars have disrupted that is not related to phyto-primary produc- BY E. DOMACK, S. I SHMAN, A. LEVENTER, S. S YLVA, the sediment surface. Recent sediment can be tion. Indeed, the sub-ice-shelf setting of the V. W ILLMOTT, AND B. HUBER found in most available depressions covering Larsen B trough (Figure 1) is an environment Eos, Vol. 86, No. 29, 19 July 2005 by currents into the trough (prior to ice shelf breakup) may have allowed a methanotrophic ecosystem to have fl ourished, despite moderate- to-low levels of methane seepage to the surface. Under normal marine conditions, such levels may be insuffi cient to maintain a chemotrophic ecology. This is because the metabolic pathway would need to compete with benthic systems that are maintained by a fallout of phyto-detritus and macroalgal particulate matter derived from the surface layer and littoral zone, respectively. Therefore, ice shelves may play a critical role in allowing chemotrophism across the seafl oor, when normally it might not establish itself. These hypotheses remain to be tested by a more focused program of benthic study across the Larsen B embayment. Existing ice shelves should also receive renewed scrutiny in regard to their potential for such unusual benthic ecology. Further, these results demon- strate that chemoauto- trophy can exist in extreme settings beneath ice shelves, and suggest that given suffi cient chemical pathways such systems are quite possible beneath even more restricted envi- ronments, such as beneath Antarctica’s sub- glacial Lake Vostok [Priscu et al., 1999]. Ecosystem Changes The profound changes now affecting the benthic habitat are equally intriguing. As shown by the observations, the microbial mats are rapidly (within three years) being reduced in extent by either burial from ice- rafted sediment or phytoplankton detritus. The latter product alters the ecosystem in Fig. 1. (a) Area of the Larsen B Ice Shelf, northwestern Weddell Sea showing generalized bathym- fundamental ways by shifting the carbon etry of the seafloor and the location of the study area. Inset shows the location of the study area source to material more readily available to in relation to Antarctica and large ice shelves (blue). (b) Landsat image of the Larsen Ice Shelf “normal” benthic grazers and consequent illustrates calving line positions since March 1986 (dates indicated; after Skvarca and DeAngelis higher-level predators; colonization is already [2003]) and the region encompassed by the 650 nm of single-beam acoustic soundings (pink taking place. polygon). (c) Oblique view of the seafloor morphology within the survey region depicting deep The impact of glacial detritus, including large glacial troughs that emanate from the Crane and Hektoria/Evans glaciers. Contour intervals are dropstones found displacing microbial lamina- 20 m, and the location and direction of the bottom video survey are indicated by the pink arrow. tion, also has intriguing analogies with ancient Color code is depth in meters with an interval of 20 m. glacial/carbonate (microbialaminite) associa- tions of the Neoproterozoic “snowball Earth” that has been restricted, if not isolated, from et al., 2005]. Hence, unlike cold seep systems [Hoffman and Schrag, 2002]. These ancient sources of phytoplankton detritus for more reported from other regions, the methane may rocks are believed to have been deposited dur- than 10,000 years [Domack et al., 2005]. In this not be derived from methanogenesis within ing periods of global ice cover and widespread case, chemoautotrophy might be expected to the near-surface sedimentary section via mi- ice shelf extent and are interstratifi ed with mi- dominate a benthic habitat, if appropriate el- crobial activity.
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