During the latter part of January through about 26 February we have a cruise scheduled aboard the R/V New Horizon to study diagenetic processes in sediments near Mazatlan. The specific questions we want to address are: 1) What are the carbon oxidation rates, how are they partitioned between oxygen respiration, denitrification and sulfate reduction and how are they effected by the oxygen minimum zone? 2)What is the magnitude of sedimentary denitrification in this area? and 3)What is the effect of denitrification on the 15N/14N ratio of nitrate and nitrogen gas from both water column dentirification and sedimentary denitrification? We have chosen the Mazatlan area because it is within the northern extent of the water column oxygen minimum zone. Due to the lack of oxygen in the overlying water, the anaerobic respiratory processes of denitrification and sulfate reduction should be very important. We are especially interested to determine the role of denitrification in these types of sediments in the global nitrogen cycle. Also, we have done a similar cruise off the coast of Washington State (Devol, 1991, Nature 349:319-321) and we have found that the carbon oxidation rate in the sediments there decreases as would be predicted by empirical relationships between sinking carbon flux and depth (See enclosed figure; empirical relationships, e.g. Betzer et al., Deep-Sea Res. 1984, 31:1-11; Martin et al., Deep- Sea Res. 1987, 34:267-285.) We do not think that this will be the case off Mexico, because the oxygen minimum zone will restrict zooplankton and this will probably alter the sinking carbon flux.

To address these questions we will use a benthic flux tripod to measure benthic fluxes and we will compare these measurements with estimates derived from sediment core samples and pore water gradients. The tripod is an instrument that we put on the bottom that encloses a volume of water overlying and samples that water as a function of time. The samples are analyzed and fluxes are determined from changes in concentration with time.

We plan to occupy about 10 to 15 stations near Mazatlan with stations located between 100 and 1500 m water depth. At each station we will deploy the tripod, take cores for sediment analysis and do water column hydrography. From the tripod we will determine the fluxes of 02, NO, TC02, P03 , NH and Si. Using the sediment core samples we will measure the sulfate reduction rate, as well as determine the vertical profiles of the solutes listed above. We will also take samples for bulk carbon and nitrogen analyses, porosity and 210Pb sedimentation rate. I have enclosed a chart showing the locations of stations that we occupied during a similar cruise in 1993 and we will sample in the same general area again this year. The benthic tripod remains on the bottom for 18 to 36 hours, depending on the expected rates which are determined primarily by water depth. Consequently, I envision a significant amount of free wire time. I would like to invite 1 or 2 Mexican scientists to participate on our cruise. We will enter the port of Mazatlan varios times during the cruise.