Deployment: TN376

See more information at R2R: https://www.rvdata.us/search/cruise/TN376

Description of Cruise (provided by Chief Scientist Barney Balch):
Due to the ship breakdown early into the cruise and the need to divert to Durban, SA, for engine repairs, we divided the cruise into five legs, as defined by our revised cruise plan, then pre- and post-diversion engine repairs in Durban, SA. We present a summary below of each of the legs and associated measurements.

Leg 1: Transit from Cape Town, South Africa (S.A.), zonal transect through Agulhas meander system, and sampling of a coccolith-rich filament; CTD stations 1-17, VPR tows 1-7; trace metal casts 1, 3, 5, 6, 7, 8, 12 and 17; 0800h, 25 January to 0222h, 4 February, 2020.

For this leg, we transited across the Agulhas Meander system, beginning with a station in the Agulhas Retroflection eddy (station 2), criss-crossing the Agulhas, Southern Subtropical Fronts with Video Plankton Recorder (VPR) and underway bio-optical systems running, and performed full CTD water casts (stations 2-4). This line of stations crossed into the end of our filament of interest, which showed (with the Acoustic Doppler Current Profiler (ADCP), cyclonic circulation around a zero-velocity core of this frontally-embedded eddy. Station 35 was situated in the western interior side of this eddy. This was where we collected water for our first carboy experiment and also performed a trace metal cast consisting of nine Niskin X samplers deployed on a Kevlar line. After collecting seawater for the carboy experiment, the VPR was deployed and towed for the entire west-to-east section, then north-to-south section through the center of the eddy. The same sections were then visited (in reverse) for CTD casts. Daily productivity casts to measure photosynthesis and calcification, plus trace metal casts were run at stations 1, 3, 4, 5, 6, 7, 8, 12, and 17. The carboy experiment for this feature was run from surface water taken at station five. Measurements of photosynthetic photophysiological variables were made underway and at stations 1-17. These included photosynthetic efficiency and rapid light curve data. An imaging PAM system (PSI, Cz) was also used to obtain cell type-specific photosynthetic efficiency data. Filter/freeze/transfer (FFT) preparations were made for qualitative viewing of surface and fluorescence maxima phytoplankton assemblages (400x magnification bright-field, polarized microscopy, and epi-fluorescence using 480nm and 530nm excitation) viewing at stations 5,6,7,8, 12, and 17. Barite precipitation measurements were performed at station 5 in this feature.

Leg 2: Transit to eddy feature and its survey; CTD stations 18-25; VPR tows 8-9; trace metal casts 18, 20, and 23; 0222h, 4 Feb. to 1400h, 8 Feb., 2020.

This leg of the cruise involved sampling a cyclonic eddy roughly centered at 35° 53'S and 37° 38'E. We first did a full 195-kilometer east-to-west VPR survey, and towed it from the east end of the eddy to the northern end of the eddy followed by a complete VPR section (163 kilometers) from north to south. The area of this PIC-enhanced, elliptical eddy was about 25,000 km². Productivity and trace metal casts were performed at stations 18, 20, and 23 and the water for a second carboy experiment was collected from station 18 (eddy interior). Measurements of photosynthetic variables were made underway and at stations 18-25. FTF preparations were made for semi-quantitative viewing of surface and fluorescence maxima phytoplankton assemblages (400x magnification bright-field, polarized microscopy, and epi-fluorescence microscopy using 480nm and 530nm excitation wavelengths) viewing at stations 18, 20, and 23. Barite precipitation measurements were performed at station 18 in this feature. A 10m-sock drogue equipped with a satellite Argos transmitter was deployed in the eddy center prior to our departure for Durban as a means to track the feature in our absence.

Leg 3: science stopped and ship diverted for engine repair; 1400h, Feb. 8, with science sampling resumed at 1726h, 16 February.

All overboard sampling at Leg 2 stopped on 8 February for the steam back to the port of Durban for engine repairs. Only the carboy experiments were sampled during the two-day transit to the port but given that we had a temperature-controlled seawater incubator, the carboy experiments could be maintained at their in situ temperatures for the duration of the multi-day experiment. The engine repair work in Durban was completed by the evening of 13 February, after which the ship sailed for station 26 to resample the first filament that we had sampled in Leg 1.

Leg 4: Re-sampling the meander filament and transit to first deep CTD; CTD Stations 26-53; VPR tows 10-12; trace metal casts 28 and 39; 0347h,16 Feb. to 0418h, Feb. 20, 2020.

The ship proceeded to re-sample the meander filament by performing three east-to-west, VPR sections across the feature, followed by three CTD sections made immediately afterward across the same lines, from west-to-east. Those sections were made zonally at 41°30', 40°30'S and 39°30'S and had lengths of 222km, 222km, and 167km, respectively, such that they adequately sampled the cross-section of the feature. Beginning with station 27, we alternated each CTD full-water cast with a "trip on the fly" water cast. These later casts were used only to sample DIC and nutrients and served to provide greater resolution sections across the features. This pattern of CTD sampling was continued for the remaining feature surveys. Following the completion of each VPR and CTD zonal leg, the VPR was towed to the next zonal leg. Productivity/TM casts were made at stations 28, 39, and 50, near the mid-points of the filament. The carboy experiment in this feature was run using water from station 28. Measurements of photosynthetic variables were made underway and at stations 26, 28, 30, 32, 34, 35, 37, 39, 41, 43, 44, 46, 48, 50, and 52. Filter-Transfer-Freeze (FTF) preparations were made for semi-quantitative microscopy viewing at stations 28, 29, 30, 39, 42, and 50. Barite precipitation measurements were performed at station 28 in this feature.

Leg 5: Re-sampling Eddy 3, Deepwater casts, transit to Mauritius; CTD Stations 54-73; VPR tows 13-14; trace metal casts 50, 56, and 70; 0418h, Feb. 20 to 0800h, March 3, 2020.

From leg 4, we proceeded to re-sample the cyclonic eddy, originally sampled in leg 2. On the way, we made the first deep CTD cast to sample for nutrients, oxygen, and carbonate chemistry down to the sea floor (4500m). The eddy re-sampling consisted of a 163km west-to-east VPR tow followed by a 203km east-to-west CTD section. Heavy seas forced us to cancel the west-most CTD station. The ship then proceeded to the north eddy station with all weather decks secured. Again, heavy sea states made deployment of the VPR impossible, so we performed the north-to-south CTD section but had to call off some of the middle CTDs from that section due to heavy seas. The drogue had spiraled about 100km from the eddy center by this point, so the ship broke from the N-S line to recover it, after which the interior eddy stations (that had been skipped due to weather) were re-sampled under safer sea states, finally arriving at the southern eddy station, #71 at 1853h on 2/24/20. At this point, the VPR could finally be redeployed to tow the entire south-to-north eddy survey transect. Two productivity/trace-metal stations were run in the eddy at stations 56 and 70. (The carboy experiment was sampled at station 56. Measurements of photosynthetic variables were made underway and at stations 54, 56, 58, 60, 62, 64, 66, 68, 70 and 71. FTF preparations were made for semi-quantitative microscopy viewing at stations 56 (east eddy interior) and 70 (eddy center). Barite precipitation measurements were performed at station 56 in this feature. We performed a deep, 24-bottle, cast for nutrients, oxygen, and dissolved inorganic carbon chemistry 183km NE of the eddy (34.42°S x 38.04°E; depth 5217m), sampled to 5200m. The last station of the cruise was a 24-bottle deep cast at 27° 24.5'S 049°, 49.33'E for freons, nutrients, temperature, salinity, PIC, POC, biogenic silica, coccolithophore and coccolith abundance, dissolved oxygen and dissolved inorganic carbon chemistry. The purpose of this cast was to examine water ages of SAMW, examine the stoichiometry of the changes in the chemistry from assumed preformed levels, and to provide comparative values for the meridional transect to be performed in the following cruise on R/V Revelle.