Biological Response to Catastrophic Disturbance on the Aleutian Margin, Gulf of Alaska
R. Rathburn, L. Levin, J. Gieskes and W. Ziebis
Earthquakes and tsunamis are major sources of catastrophic disturbance in the deep sea, particularly on the Pacific Rim. The accompanying effects on seabed geology, geochemistry and physics should have a profound influence on biological communities. We propose to examine biological responses to an enormous 1946 earthquake and tsunami centered in the Unimak sector of the Aleutian Trench. Our objectives are to (1) relate the existing geological and chemical manifestations of the seabed disturbance to properties of the protozoan (foraminiferal) and metazoan communities, and (2) compare ecological features of the benthos on the disturbed Unimak margin to those of undisturbed (or less disturbed) habitats at the nearby Shumagin margin and other parts of the Alaska margin. Disturbance responses will be documented as a function of disturbance type (erosion, burial, seepage), taxonomic guilds (foraminifera, metazoan meiofauna, macrofauna, epibenthic megafauna), and substrate type (hard, sediment). This research will reveal the multidecadal biological 'fingerprint' of large-scale disturbance for modern and fossil (foraminifera) continental margin faunas.
The proposed research will address three key information needs: (1) the
recovery potential of deep-sea biota following large-scale disturbance
(2) baseline biological data for management of the margin ecosystem and
(3) a foraminifera-based 'disturbance fingerprint' that can be used to
recognize disturbance and the resulting successional events in the historical
sediment record. This research will be among the first to characterize
the Gulf of Alaska upper and lower slope benthic communities, recognizing
the likely mosaic of habitat patches (in different stages of succession)
formed by slumping, turbidity flows, seepage and other forms of disturbance.
Development of mineral, energy, and fisheries resources on the margin
will require an understanding of the potential impacts and recovery processes.
Fig. 1. Bathymetric chart showing location of earthquake epicenter and probable slides. Proposed working area is shown as a rectangular box.
Abstract: Third International Hydrothermal Vent and Seep Symposium, La Jolla, California, September 2005
Comparison of deep (3200-3300 m) methane seep macrofauna on the Aleutian margin and FL Escarpment, Gulf of Mexico.
Lisa Levin1, Guillermo Mendoza1, Wiebke Ziebis2, Melissa Cheung1, Jennifer
Gonzalez1, Carlos Neira1, Tony Rathburn3, Robert Carney4
Sediment-dwelling macrofauna (> 300 microns) were studied at depths of 3200-3000 m at seeps on the Aleutian margin off Unimak Island, Alaska in the Pacific Ocean and near the Florida Escarpment in the Gulf of Mexico. Off Alaska macrofaunal densities were on average 6390 ind/m2) in the clam bed and 4995 ind/m2 in the pogonophoran field, and did not differ from total densities in surrounding non-seep sediments. However, on the Florida Escarpment, sediments in microbial mats yielded densities (24,323 ind/m2) over 50x higher than in background sediments. Pogonophoran field densities (1243 ind/m2) were only twice those of background sediments (414 ind/m2). Number of species per core in seep and non-seep sediments were similar in Alaska but in the Gulf of Mexico, species richness was higher in seep sediments. Overall macrofaunal species richness was lower in Gulf of Mexico than Alaska seep. Composition differed in seep sediments in both settings. Alaska seep macrofauna were comprised largely of gastropods, nemerteans, tanaids, ampharetid polychaetes, scaphopods and bivalves. In contrast, non-seep Alaska macrofauna included cumaceans, tanaids and polychaetes (cossurids, spionids, paraonids and ampharetids). Florida Escarpment macrofauna were heavily dominated by hesionid polychetes (Orseis sp.: 67-82% of total fauna), with ampharetid polychaetes (Sabellides sp.) also abundant. Dorvilleid polychaetes were dominant in the Gulf of Mexico pogonophoran fields.
Sulfide concentrations in Alaska seep sediments were low (5-10 micromolar and sulfide was apparently located deep, favoring infaunal pogonophorans with deep 'roots' for sulfide uptake. In the Gulf of Mexico – sulfide concentrations reached 1-2 mM in microbial mat sediments, but was barely detectable < 1 micromolar in the pogonophoran field and 1-2 micromolar in background sediments.d13C signatures of heterotrophic seep macrofauna reflected a broad range of nutritional sources at both sites, but were not clearly correlated with sulfide concentrations. At Alaska seeps, d13C ranged from -12 to -66 ‰ in the clambed and -18 to –39 ‰ d13C in the pogonophoran bed. d15N signatures were also variable, with values of -6.6 to 13.8 in the clambed and -3.1 to 14.5 in the pogo field. The pogonophorans themselves had consistently light d15N signatures (-1.8 to -4.8). In the Gulf of Mexico macrofauna d13C ranged from –17 to –96 ‰; 15N from –6.3 to +6.5‰. In both regions we observed very light C and N signatures in some heterotrophic taxa, and strong patchiness in macrofaunal nutritional sources over very small scales, both within and among species. The factors responsible for animal distributions and isotopic signatures in deep methane seep sediments remain something of a mystery, but present numerous hypotheses for future testing.
September 30, 2010
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