BIOGEOCHEMICAL HETEROGENEITY AND SUBOXIC DIAGENESIS IN HEMIPELAGIC SEDIMENTS OF THE PANAMA BASIN

Early diagenetic processes in hemipelagic sediments of the Panama Basi n(similar to 3900 m) are strongly influenced by an active benthic comm unity and intense bioturbation comparable to organic-rich, shallow wat er deposits. Rapid reworking enhances the role of suboxic decompositio n, particularly Mn cycling, and burrow formation increases biogeochemi cal heterogeneity as reflected in pore-water profiles. Apparent regula r patterns in the distribution of some solutes, such as NH4+ and Mn2+, around identifiable burrow structures are consistent with the active irrigation of burrows and a biogenic origin for heterogeneity. The eff ects of infauna are sufficiently pervasive, relative to the size of no rmal samples, that similar variations in solute patterns between cores can be observed when samples are separated over wide-ranging spatial scales, from a few centimeters to hundreds of meters. The spatial vari ation in pore-water solute distributions in samples (cores, passive-eq uilibration ''peepers'') taken during a single cruise is comparable to the interannual variability observed between cruises over a 4 yr peri od. Mineralized tubes and associated burrow complexes are direct visua l evidence for biogenic heterogeneity in these deposits and for a sign ificant interaction between macroinfauna and suboxic microbial communi ties. Comparison of in situ peeper arrays with cores processed on boar d ship confirms a variety of retrieval and handling artifacts also inf erred in other studies. These appear to be largely related to core war ming, promoting lysis of biomass, dissolution of biogenic debris, and irreversibly producing subsurface NH4+ maxima and increasing concentra tion gradients of NO;, NO;, and Si(OH)4. Agreements and mismatches bet ween solute fluxes predicted from cores, in situ peeper arrays, and a benthic flux incubation chamber are consistent with various retrieval artifacts and with the enhancement of NO, (nitrification) and SI(OH), fluxes by burrow irrigation. Unlike organic-poor, deep-sea deposits fr om oligotrophic regions, where the sediment-water interface can be the site of most benthic decomposition, subsurface processes are signific ant in the Panama Basin, and average reaction rate attenuation with se diment depth, is similar to that in bioturbated shallow-water deposits . (C) 1998 Elsevier Science Ltd. All rights reserved.