C S RATIOS IN ESTUARINE SEDIMENTS OF THE ODIEL RIVER-MOUTH, SW SPAIN/

iron-sulfide formation in estuarine environments is controlled by sulf ur availability, which is distinct in each subenvironment. Along the m esotidal, mixed-energy Huelva coast, s.w. Spain, appreciable quantites of iron sulfide appear in all the estuarine sediments. Formation of i ron sulfides in the Odiel River estuary is produced under different cr ystallization habits in the various subenvironments. There are: (1) oc tahedral bipyramidal crystals, (2) concentrated crystals in the tests of foraminifers, and (3) framboidal aggregates. Sediments display high er organic carbon/sulfur ratios in the subtidal channel subenvironment than in the intertidal channel-margin sectors. This is because the ch annel is always submerged under water, thereby producing the sulfur av ailable for bacterial activity. In contrast, the channel-margin sector s are only periodically submerged. Sediments of the salt marsh sectors show C/S ratio values near to those observed in fresh-water environme nts, since the organic carbon content is highest with only minor sulfu r content because the flood water enters it only periodically. The ste rile marsh sectors show a higher C/S ratio than those from the marine environments because they are reached only by astronomic tides, occurr ing every 60 or 183 days. The water introduced there remains for a suf ficient time to allow for the sulfur to be completely comsumed. All th ese differences found in each estuarine subenvironment are clearly rec orded in the direct correlations with distinct slopes corresponding to different C/S ratios.