AN EXPERIMENTAL AND MODELING STUDY OF PH AND RELATED SOLUTES IN AN IRRIGATED ANOXIC COASTAL SEDIMENT

Macrofaunal irrigation is an important process in nearshore sediments, facilitating greater exchange between sediments and seawater and impa rting significant lateral heterogeneity to the porewater profiles of m any constituents. Like many macrofaunal activities, irrigation is a tr ansient behavior, i.e. tubes and burrows are flushed periodically, at frequencies that generally are species-specific. As a result, transien t concentrations within the dwelling arise, potentially impacting grad ients, fluxes and reaction rates in the vicinity of the dwelling. We i nvestigated the impact of periodic burrow irrigation on the distributi on of several diagenetically important porewater constituents. Laborat ory experiments evaluated irrigation periodicity using artificially ir rigated tubes embedded in nearshore organic-rich sediments, and microd istributions of oxygen and pH in laboratory experiments were measured with microelectrodes. To help interpret our results, we also construct ed a simplified time and space-dependent transport-reaction model for oxygen, pH and sulfide in irrigated sediments. Laboratory results show substantial differences in the pH field of sediments surrounding an i rrigated tube as a function of irrigation frequency. Higher pH values, indicative of an overlying water signature, were observed in the vici nity of the tube wall with increasing duration of irrigation. Converse ly, oxygen concentrations did not vary significantly with the amount o f irrigation, most likely a result of extremely high sediment oxygen d emand. Model results are consistent with laboratory findings in predic ting differences in the measured variables as a function of irrigation frequency. However, the nature and extent of the model-predicted diff erences are often at variance with the experimental data. Overall, exp erimental and modeling results both suggest irrigation periodicity can substantially influence porewater distributions and diagenetic proces ses in sediments. Future studies should examine the influence of irrig ation periodicity on the types and rates of reactions, and the attenda nt biological features, in the environment encompassing the tube or bu rrow wall.