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Relationships among trophic structure, bioturbation and organic-matter preservation across the oxygen minimum zone on the Indus Margin.

OMZs are extreme and widely distributed environments where biological and geochemical processes differ fundamentally from those in oxygenated zones. In particular, benthic food-web structure and animal-geochemical interactions may change dramatically at the core of OMZ’s, producing vast habitats dominated by chemoautotrophy and high organic-carbon burial.

In collaborative with British and Dutch colleagues, we propose to directly study the impacts of low oxygen on benthic food-web structure, particle processing, and organic–carbon burial. We propose to test (in brief) the following hypotheses along a transect crossing the OMZ on the Indus margin off Pakistan:

(1) Movement along a bottom-water oxygen gradient into an intense OMZ yields a fundamental benthic shift from reliance on detrital phototrophic production to in situ chemotrophic production.
(2) This “trophic shift” involves novel animal-microbe associations that facilitate metazoan persistence at very low oxygen concentrations (< 0.12 ml l-1 or 5.4 µmol l-1).
(3) This “trophic shift” also fundamentally alters rates and patterns of biogenic particle mixing and bioirrigation to directly influence the burial efficiency of organic matter in underlying sediments.

We will use stable-isotopic, radiochemical, biogeochemical and faunal measurements as well as incubation experiments to characterize shifts in ecosystem functions (food sources, nutritional modes, particle mixing) and faunal adaptations (e.g., symbioses) along a seafloor transect running across the OMZ. We will sample 6 stations (100 – 2000 m depth) ranging from fully oxic to permanently hypoxic conditions during both monsoon and inter-monsoon periods. Mensurative studies will address links between redox conditions, microbial activity, faunal composition, and related changes in particle mixing and carbon preservation. In situ and shipboard 13C-tracer experiments with labeled algae will be used to explore rates and mechanisms of organic-matter utilization and the roles of particular infauna under different oxygen regimes. British collaborators (G. Cowie, G. Wolff, D. Pond) will make numerous complimentary geochemical and biological measurements and provide the shiptime under a program entitled "Benthic processes in the Arabian Sea: Interrelationships between the benthos, sediment biogeochemistry and organic matter cycling" (G. Cowie - lead PI, NERC 2002-2005).

This research program will be the first fully integrated study of biological and geochemical processes in OMZ sediments. Our results will fundamentally advance our understanding of ecological processes (e.g., food-web dynamics) in OMZ ecosystems, and the impacts of these processes on sediment geochemistry and carbon burial. This improved understanding of ecological-geochemical coupling in the vast areas impacted by OMZs will substantially enhance our ability to (1) predict the impacts of global climate change on continental margins and (2) to interpret the nature of paleoceanographic and paleoclimatic changes from the sediment record.

INTERACTIONS OF BENTHIC COMMUNITIES AND SEDIMENT GEOCHEMISTRY ACROSS THE PAKISTAN MARGIN (ARABIAN SEA) OXYGEN MINIMUM ZONE

COWIE, G. and the CD145/146 shipboard scientific parties

Grant Institute, School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland, glcowie@glg.ed.ac.uk

Benthic organisms strongly influence sediment biogeochemistry but processes remain poorly quantified. We are conducting parallel assessments of benthic communities and sediment geochemistry, combined with rate determinations for microbial processes and sediment accumulation and mixing, during monsoon and inter-monsoon periods at sites across the oxygen minimum zone on the Pakistan margin (Arabian Sea). The roles of benthic communities in particle mixing, organic matter cycling and benthic fluxes are being addressed with shipboard and in situ incubation experiments. Preliminary biological results indicate negligible macrofauna (> 300 µm) within the OMZ core [300-700 m, O2 = 0.09-0.15 ml/l], where sediments were fully laminated and calcareous foraminiferans dominant. Microelectrode profiling and incubations indicated clear differences in O2 penetration and consumption rates across the OMZ. Visual inspection of 2- and 5-day experiments revealed rapid ingestion of 13C labeled algae by Uvigerina and Cassidulina (epifaunal foraminiferans). Uptake was focused at the sediment surface with no vertical mixing, although infaunal Globobulimina ingested some tracer. The lower portion of the OMZ, where O2 increases from 0.15 to 0.3 ml/l (800-1100 m), exhibited sharp gradients in macrofaunal communities and bioturbation. Low-diversity assemblages of larger organisms, several exhibiting evidence of 13C phytodetritus ingestion or chemosynthetic symbionts, occurred at 800-950 m. Foraminiferal communities changed more gradually, some species spanning substantial depth ranges (e.g. 950-1200 m). Macrofauna were sparse but deep burrowers occurred and sediments were fully bioturbated at the lower OMZ boundary (1200 m) and below the OMZ (1850 m) where large epifaunal agglutinated foraminiferans (e.g. komokiaceans, Rhizammina) were dominant.

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