Show these notes with Cruise Track included

Date:  7 June 1995 19:19:42 GMT

Cruise Summary--Mooring/Coring   Process Cruise #3  
Final Cruise Summary--JGOFS Second Sediment Trap-Coring Cruise

The pre-monsoon (May 3 to May 21) sediment trap-sediment coring cruise to the Arabian Sea was an outstanding success that featured 100% performance of our time-series sediment traps. The cruise was designed to recover and redeploy sediment trap moorings and collect sediment cores at the four JGOFS mooring locations nearest to the continent. We successfully recovered and redeployed moorings at sediment trap mooring sites 1, 2, 3, and 4. These moorings contained 13 WHOI/OSU time-series sediment traps and 8 UW/Skidaway/SUNY sediment traps, which use swimmer-exclusion valves. The sample changers of all the WHOI/OSU time-series traps functioned perfectly to provide us with 273 samples (average 8.5 days each) that will greatly add to our understanding of the factors controlling the fluxes of biogenic and lithogenous materials to this part of the Arabian Sea. The UW/Skidaway/SUNY sediment traps, which were prepared, tested, deployed, and recovered by Mike Peterson (UW) and Stuart Wakeham (Skidaway), also worked very well and collected 189 additional samples. We are looking forward to detailed comparisons between the fluxes of the traps with and without swimmer exclusion valves to determine the significance of swimmers both in traps deployed within the oxygen minimum zone and the deep ocean. Based on shipboard measurements of the volume of sediment trap material in each of the WHOI/OSU sample cups, we made qualitative estimates of the temporal and spatial variability of the sediment trap fluxes. We are somewhat surprised to find that the average flux during the past six months was similar at all three mooring sites over this 410 km long transect across the western margin of the Arabian Sea. This situation will probably not occur during the SW monsoon that will be sampled during the second half of mooring deployments. During this season we expect to see strong spatial and temporal gradients in the flux. During the first six months (November to May) of our sediment trap experiment, however, we found significant differences in the temporal variability of flux at the four sites. At the three sites closest to shore (mooring sites 1, 2, and 3), peak flux events occurred several times between December and March, with much lower fluxes prior to and following these periods. The timing of the flux events at each of these sites matched rather closely. At mooring site 4, however, which is adjacent to the physical oceanographic moorings, the pattern of temporal variability in flux differs from that observed in the three sites adjacent to the continental margin. A modest pulse in flux occurred in late December-early January and a much larger pulse (nearly 10 times that of the low flux periods) occurred in late February to early March. Preliminary discussions with Bob Weller and examinations of his data suggest that the high flux events correspond to times when the mixed layer deepened (late December) and shoaled (late February) at this site (Bob Weller, pers. comm.). We expect more complete comparisons between the sediment trap data and the data collected by the weather buoy and the other moored instruments over the time of the entire experiment will enable us to define the factors which control the particle fluxes to the deep ocean. An extensive sediment coring program (19 stations and 38 coring attempts) successfully collected one large-diameter gravity core and one multicore deployment (eight 4-in diameter cores) at each JGOFS Sediment Trap mooring site. We also collected a transect of cores within the oxygen minimum zone from two areas of the Oman margin. The Siquirah Bay transect (5 sites) ranged from 1477 m to 302 m, and the Masirah Bay transect (9 sites) ranged from 1257 to 71 m. The so-called "fluff layer" that represents the most recent deposition of biogenic-rich materials was much thinner and more degraded during this pre-monsoon period compared to the cores collected during the November post-monsoon trap deployment leg. This comparison probably reflects the relative strength of biological production and export to the seafloor during the NW and SE monsoons. Suspended particles samples were collected using an in situ pumping system that typically filtered 3000 liters of sea water at each station. Glass fiber filters for measurement of lipid and alkenone contents (Wakeham and Prahl) and Nucleopore filters for measurement of trace element contents of suspended particles (Dymond) were taken at each mooring site and several sites along the continental margin. Typically large volume filtrations were done within the chlorophyll maximum, within the O2 minimum, and below the O2 minimum. We occupied 31 filtration stations during the cruise. CTD casts and water sampling were conducted at each mooring site and at several sites along the continental margin. We occupied nine CTD stations and collected 64 water samples to characterize the nutrient and stable isotopic content of the water column. As we have experienced in the past, the crew of the R/V Thompson was outstanding. We especially appreciated the excellent ship control during our mooring work, the well-maintained ship, and the great food. We look forward our final cruise to the Arabian Sea in January 1996 when we will recover the four moorings deployed during this leg as well as mooring #5 at 10 degrees N, 65 degrees E. Jack Dymond co-chief scientist Warren Prell co-chief scientist