Impact of spatial heterogeneities on oxygen consumption in sediments: Experimental observations and 2D numerical modeling

• Published in: Journal of geochemical exploration 2012, Vol.112, p.76-83
• Main Authors: Mügler, Claude, Rabouille, Christophe, Bombled, Bruno, Montarnal, Philippe
• Format: Article
• Language: English
• Subjects: Continental interfaces, environment; Heterogeneity; Microelectrode; Ocean, Atmosphere; Organic-carbon mineralization; Reactive transport modeling; Sciences of the Universe
• ISSN: 0375-6742; 1879-1689
• DOI: 10.1016/j.gexplo.2011.07.008

In situ measurements of the sediment geochemical composition performed with high-resolution techniques often exhibit the heterogeneous character of this shallow water environment. Many processes can be responsible for this small-scale variability. Here, we investigate two of these processes: (i) a local deposition of organic matter related to organic aggregates; (ii) a horizontal diffusion of oxygen due to the presence of biological structures, such as burrows or galleries, which result from the activity of macrofauna. In this paper, laboratory experiments and associated 2D numerical simulations are performed together to confirm and to quantify the impact of such small-scale heterogeneities on the dynamics and spatio-temporal variability of biogeochemical cycles in sediments. Both experiments and calculations confirm that a local deposit of organic matter involves an increase in oxygen demand of the sediment as a result of labile organic matter deposit and of its degradation by microorganisms. On the contrary, the presence of biological structures involves lateral diffusion and deeper penetration of oxygen. Burrows act as preferential pathways for fluids and lead to spatially heterogeneous distributions of chemical species. The combination of the results from oxygen microelectrode profiles measured at high lateral resolution in sediments with those from modeling allows to quantify the characteristic distance on which the presence of aggregate or burrow influences the oxygen spatial distribution in the sediment. ► High-resolution oxygen microelectrode profiles are combined with multi-dimensional transport of reactive tracer. ► Two processes responsible for small-scale heterogeneities are investigated. ► The increase in oxygen demand due to a local deposit of organic matter is explicitly simulated. ► The deeper penetration of oxygen into the sediment due to a biological structure is quantified. ► The induced lateral spatial scale of oxygen perturbation is of the same order as the aggregate's or burrow's size.