Date: Tue, 11 Nov 1997 11:27:10 -0500 AESOPS Polar Front Survey - RV Revelle - Work Week 2
Cruise Report - November 10, 1997 Antarctic Environment Southern Ocean Process Study (AESOPS) Polar Front Survey I R/V Roger Revelle, Expedition KIWI, Leg 06 As described in our first report last week, the primary objective of this leg of the Southern Ocean JGOFS program is to provide mesoscale resolution of the physical, chemical, and biological properties in and around the Polar Front. We are working toward that objective through the deployment of a mooring array within the frontal boundary, grid surveys with an instrumented SeaSoar vehicle, and station work around the front. The primary activities of the past week have been mooring deployments and station work, both of which have been interrupted by periods of heavy seas and high winds. Our transits of the frontal region during mooring deployments (Richman, Abbott, Root) have extended our picture of the shape of the frontal boundaries in the region between 59 S, 62 S, 169 W and 170 W. The sea surface temperature (SST) signature of the front shows a meandering southwest - northeast boundary, with flows to the NE at 30- 40 cm per sec down to 200m (based on ADCP analysis by Pierce). Station Work CTD casts at all stations in and around the Polar Front reveal well-mixed upper layers, with mixed layer depths of 80 - 200 meters. The CTD/hydro deck crew (Wilson, Realander, Cook, Williams, Willis, O'Malley, Wingard) have helped us get the CTD/hydro rosette and Trace Metal rosette in and out of the water under difficult situations. Bottle samples from both rosettes show that chlorophyll values (Polk) have shown a modest increase south of the 0.5-0.0 degree C Polar Front boundary. We have observed chl values in the mixed layer between 0.3-0.4 micrograms per liter north of the front, nearly 0.5 just south of the front, and down to 0.3 well south of the front in -1.5 degree C water. Nitrate and silicate values remain high throughout the region (20 and 15 micromoles per liter, respectively) with a steep increase in silicate to 45 micromoles within and south of the front (Morgan, Mordy, Franck). Primary production measurements within the front reveal a daily integrated production of about 600 mgC per square meter per day (Kulak, Meadows, Polk). Total organic carbon values are about 40-45 micromoles per liter in the upper 50m, decreasing to about 35 through the rest of the water column (Goyet and Kirchlechner). Surface pCO2 values of 380-400 ppm (above equilibrium) were found throughout the region (Zhu and Degler). Casts for trace metal analysis have been made at locations south, north, and within the front (Coale, Gordon, Tanner). The Moss Landing group has also initiated iron and zinc enrichment experiments with water from north and south of the front. Preliminary results from the southern station (62.3S, 170W) indicate iron limitation of phytoplankton growth under ambient conditions. In addition, it appears that zinc may enhance silicate uptake by diatoms (Franck). The Moss Landing trace metal group also is conducting an intercomparison of trace metal sampling and analytical methods using samples from hydro rosette bottles, TM rosette bottles, Moss Landing GoFlo bottles, and underway samples. Thorium profiles of the water column (Ball and Crossian) north of the front show a 40% deficit at 300m compared to a 10% deficit near the surface, suggesting significant particle export from the upper water column. Additional thorium pump casts have been made within and south of the front and the samples are being analyzed on board. Microplankton growth rates are low (0.1 - 0.2 per day) at stations within and north of the front (Landry, Brown, Cassiotti). The Landry team also is photographing the taxonomic composition of microplankton at all stations and along our underway transit lines. Other activities at stations included bio-optical profiles (Cowles and Wingard), a MOCNESS tow (Gonzales), primary productivity experiments and P vs E experiments (Kulak and Meadows), and characterization of particulate matter (Stillman, Polk, and others). Mooring Deployments We have deployed 9 current meter/optical sensor moorings within the Polar Front region between 60S and 61S. Two of the mooring deployments were conducted in very rough weather. Their safe completion is a reflection of the expertise and experience of Dennis Root, Bob Wilson, Marc Willis, Jim Richman, and Linda Fayler, with considerable credit due to the excellent ship handling by Captain Desjardins, Chief Mate Dave Murline, Second Mate Eric Wakeman and Third Mate Rob Widdrington. We broke off the mooring deployment work on Nov 7 due to 25-30' seas, but will deploy the remaining three moorings within the next week. Mesoscale mapping We began a 5-day SeaSoar survey grid in the early morning hours of Nov 8. The 50 knot winds of Nov 7 had dropped to 10 knots, making for an easy deployment (Willis, Fayler, Barth, O'Malley, Pierce, Wingard, Cowles). The survey grid includes the area covered by the optical/current meter mooring array. As of Nov 10, we have completed three north-south lines across the Polar Front and all instruments are working well. The frontal boundaries have moved south about 10' of latitude from their position one week ago. We have noted a number of intrusive features along the SeaSoar track, suggesting significant secondary circulation. There is enhancement of phytoplankton biomass, based on pigment fluorescence and spectral absorption, just south of the two major temperature steps across the Polar Front. Optical plankton counter data (Gonzalez) is also part of the SeaSoar data set. Ten WOCE GPS drifters, four of which had optical sensors, were deployed along the first two grid lines (Abbott and Richman), upstream of the mooring array. A natural drifter, a 1.6 mile wide iceberg, was found to be in the core of the Polar Front eastward flow, and has been moving eastward at approximately 30 cm per sec. Many other groups on the ship are conducting underway sampling of surface waters during the SeaSoar tow, including the trace metal analyses (Vink and McDonough), pCO2, total carbon (Zhu and Degler), organic carbon (Goyet and Kirchlechner), chlorophylls (Polk and Tanner), microplankton (Landry and Brown), nutrients (Morgan, Mordy, Williams, Franck). We will complete the SeaSoar grid survey on Nov 13 and return to station sampling and mooring deployments for a few days. We have had a productive week, even with some time lost to bad weather. We still have a few CTD deployment problems to solve, but we are making progress. Captain Desjardins, the ship's officers and crew, and the resident technical staff continue to provide solid support for our science operations. Respectfully submitted, Tim Cowles Chief Scientist