Voyage Plans and Summaries

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Franklin Voyage Summary No. FR05/2001

Title

Monitoring ocean climate change around Australia: the Deep-Ocean Time-Series Sections (DOTSS).

Itinerary

Leg 1:
Departed Wellington (New Zealand) 0915hrs Thursday 24 May 2001
Arrived Nuku’alofa (Tonga) 0830hrs Saturday 16 June 2001

Leg 2:
Departed Nuku’alofa (Tonga) 2200hrs Saturday 16 June 2001
Arrived Apia (Western Samoa) 1000hrs Saturday 7 July 2001 local (8 July 2001 AEST)

Principal Investigators

Susan E. Wijffels (Chief Scientist)
CSIRO Marine Research
GPO 1538
Hobart, Tasmania 7000 Australia
Phone: 03 6232 5450 Fax: 03 6232 5123
e-mail: Susan.Wijffels@marine.csiro.au

John Church, Bronte Tilbrook and Steve Rintoul
Antarctic CRC and CSIRO Marine Research

Nathan Bindoff
Antarctic Co-operative Research Centre, University of Tasmania

Mark Warner and Chris Sabine
University of Washington, Seattle, USA

John Bullister
NOAA-PMEL, Seattle, USA

Scientific Objectives

• Establish a time series of full-depth repeat ocean measurements capable of resolving decadal and longer time-scale changes in the structure and carbon storage of the oceans around Australia, from Antarctica to the equator.

• Use these data to test climate model predictions and to determine whether and how fast climate is changing due to the Greenhouse Effect and/or natural decadal variability.

Cruise Objectives

The proposed work involves standard full depth CTD and Niskin bottle casts that measure salinity, temperature, dissolved oxygen and pressure continuously and the major nutrients discretely. We will be relying on achieving WOCE accuracy in order to measure what might be small but significant changes at depth. Samples will be analysed on board for dissolved inorganic carbon concentrations (DIC) and alkalinity. Through collaboration with the Pacific Marine Environmental Laboratories and the University of Washington in the US we will be able to measure concentrations of freons, which are an important indicator of ventilation rates and are particularly useful for testing ocean circulation models.

Cruise Track

Starting in Wellington, Franklin steamed into deep water south of the Chatham Rise and a test station was completed. Franklin then steamed directly to 170° W, 50° S where the meridional survey began. From there, Franklin worked northwards and westwards to near Chatham Island crossing a deep western boundary current (see Figure 1). From there, the track was northeastward recrossing the boundary current back to 170° W, then along 170° W, until interrupted for an exchange of personnel in Tonga.

On Leg 2, near 17° S, the meridional line was interrupted in order to complete an additional crossing of the deep boundary currents found between 170° W and the Tonga-Kermadec Ridge. After completing this short zonal line, the 170° W meridional line was resumed until interrupted again near 10° S for a section across the deep Samoa Passage. From here the meridional line was completed to the equator along 168° 45'W.

Results

A total of 129 CTD casts were completed. Four of these were test casts of various types but the rest of the casts were mostly to within 15m (or more usually 10m) of the bottom. The casts were made along 3 sections (Figures 1, 2 and 3): along roughly 170° W from 50° S to the equator (a partial repeat of WOCE section P15S), along 17.5° S from 170° W across the deep western boundary current east of the Tonga-Kermadec Ridge (a partial repeat of WOCE section P21) and across the Samoa Passage (a partial repeat of WOCE section P31) which is the main pathway of deep water from the South to the North Pacific Ocean.

On all casts a 24-bottle rosette system (10 litre bottles) was used to collect samples throughout the water column. Samples were collected for salinity, oxygen and nutrients (nitrate, phosphate and silicate) on all casts. On about half of the casts samples were also collected for dissolved inorganic carbon, alkalinity and CFCs (Freon 11, Freon 12) and on some casts carbon tetrachloride. The ship mounted acoustic Doppler current profiler, precision depth recorder and other underway instrumentation were run throughout the cruise. Continuous measurements of the fugacity of carbon dioxide (fCO₂) in surface waters were also made along the cruise track.

At 6 stations, samples were also collected for John Lupton (NOAA-PMEL, Seattle, USA) for helium analysis.

Initial analysis on board indicate the data are mostly of high quality. A post CTD calibration comparison indicates an rms difference between the bottle and CTD salinity of 0.0012 for bottles deeper than 1000m (over 1300 comparisons). The bottle oxygen data also appears to be of high quality. Initial calibrations of the CTD oxygen sensor look promising with an rms difference between the bottle and CTD oxygen data of 5-6 m mole/litre. However, even after the calibration, the times when the CTD oxygen sensor was changed can be seen, implying a need for further work on the calibrations. The nutrient profiles look promising but there is some station to station noise in the data and some apparent jumps in deep nutrients. Some of the later stations are being rerun because of contamination, the result of growth in the auto-analyser sample line. The station to station noise and the apparent jumps should decrease after the samples have been rerun and corrections from the standard reference material have been applied.

On the last part of the first leg of the cruise and all of the second leg of the cruise, there was a contamination problem with the CFC samples, in particular with CFC-12. This was finally tracked down to the eucalyptus oil injected into the air-conditioning system. It appears that the oil settles on the Niskin bottles in the wet lab and then absorbs CFCs from the air before releasing it into the water samples. The CFC signals in the upper part of the water column and in the deep boundary currents are clear but the ability to determine CFC ages may be compromised (at least for depths of 1000db to 4000db). A full report on the CFC data collection and analysis in included as an Appendix.

The sections clearly show the major features expected — the northward penetration of Southern Ocean water masses (Sub-Antarctic Mode Water, Antarctic Intermediate Water, Circumpolar Deep Water and Antarctic Bottom Water) and the southward penetration of North Pacific water masses. The temperature and salinity sections along 170° W are shown in Figures 2 and 3. In the deep zonal sections at 17.5° S and across the Samoa Passage, the northward flowing boundary currents are clear. At both sections, there has been an increase in CFC concentration since these sections were last occupied. However, no quantitative comparison with previous data has yet been undertaken.

Overview of sampling/analyses for dissolved inorganic carbon (DIC) and sea water alkalinity (TA)

Samples were analysed for dissolved inorganic carbon (DIC) and seawater alkalinity (TA). The DIC values were measured by coulometry using a SOMMA system. TA values were measured by potentiometric titration.

Full profiles (24 Niskin bottles) were taken every other CTD station along the cruise track, with surface and some fill-in samples (up to 14) collected at other CTD stations. Where possible, carbon analyses were made at stations that coincided with locations that had been analysed for carbon during WOCE on sections P15S, P15N, P21 and P6.

Data quality for both DIC and TA was monitored during the cruise using duplicate samples and by analysing Batch 52 Certified Reference Material (CRM) provided by Dr. Andrew G. Dickson, Scripps Institution of Oceanography. At each 24 bottle cast for carbon, three depths were sampled in duplicate. The duplicates were interspersed with the other samples from the cast and analysed. Differences between the duplicates and the values of the CRM’s were plotted versus station number. On Leg II, a preliminary analysis of the results for DIC, show the standard deviation of 94 duplicates as +/- 1.08 umol/kg and for 31 bottles of CRM, as +/- 0.62 umol/kg. For alkalinity, the results for 68 duplicates and 17 bottles of CRM were +/- 1.57 and +/- 1.09 m moles/kg respectively. Similar results were obtained for duplicates and CRM’s during Leg I for both DIC and TA. Post-cruise processing of the data is required, but preliminary quality control indicate both DIC and TA analyses are within WOCE specifications for precision and accuracy.

Surface DIC values followed expected trends with gradually decreasing concentrations to the north, a minimum occurring at station 73, at ~17.5° S, before increasing again. The bottom water, below 5000db, showed a very consistent value of 2257 +/- 2 m moles/kg until station 112, at ~8° S, when the concentration started to increase reaching values of 2277 m moles/kg at station 128, at the equator. A mid-depth maximum was very apparent on Leg II and increased in concentration as the track proceeded north.

Continuous measurements of the fugacity of carbon dioxide (fCO₂) in surface waters were also made along the cruise track. The fCO₂ measuring system is based on a "Weiss" type equilibrator and a LICOR 6252 Infrared Gas Analyser (IR). During a six hour cycle three CO₂-in-air standards, clean outside air, and air equilibrated with surface waters are analysed. The three standards and the air sample are analysed for eight minutes each at the beginning of the six hour cycle. Measurements are made in surface waters for the remainder of the cycle. Data are recorded as one minute averages of readings taken every second. The CO₂-in-air standards are referenced to the WMO molar scale and were prepared and calibrated by Dr. P. Steele, CSIRO Atmospheric Research, Melbourne. During leg 1, the underway seawater line does not appear to have been flushed sufficiently rapidly, resulting in warming of about 1° C between the seawater intake and equilibrator. The large warming and the low flushing rates through the water lines are likely to result in poor fCO₂ data quality for leg 1. The flushing problem was corrected but not eliminated on leg 2, and the data quality for this leg is expected to have improved.

Cruise Narrative

Leg 1

We left Wellington at 0915 on Thursday the 24th of May, 2001 and started heading South-East as soon as we were clear of the harbour. The first station, a bottle test station was done on the afternoon of the 25th of May at 44° 26'S, 179° 57'E. We then proceeded to the first station on the WOCE P15 section at 49° 30'S, 170° 00'W. We reached this station in the early afternoon of the 27th of May. We had had moderate following conditions all the way from Wellington. As we approached this station the weather obligingly swung around to the South, giving us moderate following conditions for the first part of the section.

When we reached the station at 45° 57'S it seemed that our luck with the weather had run out - the station had to be abandoned with the CTD at 4,000 metres due to rapidly worsening conditions. Once conditions improved we were able to start work again, having lost about 12 hours (29 and 30 May).

The weather then continued to moderate and we were once again able to make good progress until early on the 3rd of June, when we were at 39° S, 172° W. Conditions worsened rapidly, culminating in winds gusting above 60 knots. We only lost about a day here as, after about 12 hours, conditions started to moderate and we were able to begin work again in another 12 hours or so.

After that we continued to work in conditions varying between very light (winds less than 5 knots) and moderate (average winds in the low twenties, gusting to the high twenties) for several days. Mostly we had following conditions, enabling us to make good time between stations.

Late on the 12th the weather degenerated again, resulting in the loss of nearly half a day. Once we were able to start CTDs again we continued, completing the CTD at 20° 30'S late on the 14th of June before heading for Tonga.

We reached Tonga at 0830 on the 16th of June, having completed 66 CTD stations, two further along the section than had been planned. At three places along the section we had replaced two stations with one half-way between to help make up time lost for bad weather.

Leg 2

We departed Tonga at 10 PM Saturday June 16 and steamed east to recommence the 170° W CTD section. En-route we had a Muster, a safety briefing and a cruise briefing on Sunday June 17. We also did a test CTD station. After reaching 170° W, we continued the CTD section northward. At 17° 30'S, we completed a CTD section westward across the deep western boundary current near the Tonga-Kermadec Trench. Through much of this period, winds were light. After completing the section across the Tonga-Kermadec Trench, we steamed eastward to return to the 170° W section. By this time, the winds had increased to 25 kts from the southeast and the eastward steam was slow. We completed an additional trial CTD station, firing all bottles at 2500 m to do a blank test for CFC-12. We then continued the 170° W CTD section northward in decreasing winds, completed a section across the Samoa Passage, and then continued the CTD section north along 168° W to the equator. Winds remained light to moderate for the rest of the cruise. A final test CTD station, firing all bottles at 2000 m, for a blank test for CFC-12 was completed. We then steamed to Samoa, docking at Apia at 1000 hours Saturday July 7, 2001. Enroute, the ship lost power on Wednesday July 4. Because the main UPS was not functional, many of the instruments in the labs had to be restarted to complete the analyses of samples and the processing of data.

Bottom Depth

One unexpected feature observed on the PDR was an uncharted sea-mount at about 2° 3.36'S, 168° 45.098'W. The sea-mount rose from the ocean floor at about 5300m to 1400m over a distance of about 10km. The bottom topography data bases derived from satellite altimeter data were checked after the cruise. They also show this seamount although it is not as shallow in this data set as observed during the cruise.

Summary

There were many more samples collected than is usual for the small number of scientists that can be accommodated on board Franklin. As a result, the cruise was a heavy work load for all the scientific party on board. The available 12 berths limits the National Facility’s capabilities. However, the cruise was successful. Virtually all of the stations were completed and an excellent data set collected. This data set should be a sound basis for detection of changes compared with previous observations.

Problems and Recommendations

The main UPS did not work for the whole of this cruise. As a result many of the instruments in the labs had to be restarted when the ship lost power on Wednesday July 4 and once on Leg 1. Fortunately, this was after completion of all of the CTD stations. However, many of the analyses were still being undertaken and some samples were lost. The UPS should be fixed as soon as possible.

There are a limited number of spares on board. We would have liked the ability to change the oxygen sensor but there were no further spares. Also there was no spare sounder display.

The 10 litre Niskin bottles, provided by John Bullister, University of Washington, were used throughout much of the cruise in an effort to minimise CFC contamination problems. While these bottles gave good salinity results, the valves and spigots were difficult to operate and there were numerous comments on the CTD log sheets about leaks from the end caps.

The bow thruster caused some problems as it can drop out when used at 100% power. This was not a major problem for this cruise, but could lead to losing more time to bad weather on a cruise where the weather was generally worse.

The Franklin should have a second MATLAB licence. This software is used extensively for user analysis functions, and is now also used heavily by ORV personnel for processing of data. The single licence leads to inefficiency in processing and analysing of cruise results.

On all of the deep CTD casts wire tension was very high and some casts were stopped short of the bottom. The high tensions necessitated hauling of the deep portion of casts at speeds as low as 20 m/minute to stay within recommended working tensions of the wire. Similarly, winch speeds were low at the start of the casts when lowering the CTD in moderate and rough weather. This is a result of not being able to put enough weight on the rosette package (so as not to load the wire excessively during the deep portion of the casts) to make it sink more rapidly. If winch speeds could be kept at 60 m/minute then the order of two days of ship time could have been saved. It is recommended that as soon as an opportunity arises that a thicker wire should be used. This would have the advantages of increasing the safety margin, saving ship time, allowing casts to 6000db, and increasing the payload.

Personnel

Scientific participants on Leg 1

Scientific participants on Leg 2

Franklin officers and crew members

Acknowledgements

We received excellent support from the Ship’s officers and crew, the scientific staff and Franklin Operations officers. We thank them and the shore based support staff for ensuring the success of the cruise.

This cruise is a contribution to CSIRO’s Climate Change Research Program, with partial funding provided by Australia’s National Greenhouse Research Program. The CFC and US CO₂ programs were supported by funds from the National Science Foundation, award OCE-0095960.

Neil White,
Chief Scientist Leg 1,

John Church
Chief Scientist Leg 2

Figures

Figure 1. Cruise Track from Wellington to Tonga to Apia, Samoa. The CTD station locations are indicated by the dots.

Figure 2. CTD temperatures in the upper 1000 dbar and from 1000 dbar to the bottom along the meridional transect from 50° S north to the equator. The station numbers are indicated at the top of the plot. The contour interval is 1° C.

Figure 3. CTD salinities in the upper 1000 dbar and from 1000 dbar to the bottom along the meridional transect from 50° S north to the equator. The station numbers are indicated at the top of the plot. The contour interval is 0.01 (PSU).

Appendix

CHLOROFLUOROCARBONS (CFC)

Principal Investigators

Mark Warner and John Bullister

Sample Collection and Analysis

Frederick Menzia and Regina Cesario

Summary

Specially designed 10L water sample bottles were used on the cruise to help reduce CFC contamination during R/V Franklin cruise FR05/2001 between 50° S and the equator nominally along 170° W.

Samples for the analysis of dissolved CFC-11, CFC-12 and CFC-113 were drawn from approximately 1900 of the water samples collected during the expedition. Samples for carbon tetrachloride (CCl4 or CFC-10) analysis were drawn from approximately one quarter of the samples. When taken, water samples for CFC analysis were usually the first samples drawn from the 10L bottles. Care was taken to co-ordinate the sampling of CFCs with other samples to minimize the time between the initial opening of each bottle and the completion of sample drawing. In most cases, dissolved oxygen, DIC, and alkalinity were collected within several minutes of the initial opening of each bottle. To minimize contact with air, the CFC samples were drawn directly through the stopcocks of the 10L bottles into 100ml precision glass syringes equipped with 2-way metal stopcocks. The syringes were immersed in a holding tank of clean seawater until analyzed.

To reduce the possibility of contamination from high levels of CFCs frequently present in the air inside research vessels, the CFC extraction/analysis system and syringe holding tank were housed in a modified 20' laboratory van on the aft deck of the ship.

For air sampling, a 45 meter length of 3/8" OD Dekaron tubing was run from the CFC lab van to the bow of the ship. A flow of air was drawn through this line into the CFC van using an Air Cadet® pump. The air was compressed in the pump, with the downstream pressure held at 1.5 atm using a back-pressure regulator. A tee allowed a flow (100 cc min^-1 ) of the compressed air to be directed to the gas sample valves, while the bulk flow of the air (>7 l min^-1) was vented through the back pressure regulator. Air samples were only analyzed when the relative wind direction was within 60 degrees of the bow of the ship to reduce the possibility of shipboard contamination. The Air Cadet pump was run for at least 60 minutes prior to analyzing each batch of air samples to insure that the air inlet lines and pump were thoroughly flushed

Concentrations of CFC-11, CFC-12 and CFC-113 in air samples, seawater and gas standards on the cruise were measured by shipboard electron capture gas chromatography (EC-GC), using techniques similar to those described by Bullister and Weiss (1988). For seawater analyses, a 35ml aliquot of seawater from the glass syringe was transferred into the glass sparging chamber. The dissolved CFCs in the seawater sample were extracted by passing a supply of CFC-free purge gas through the sparging chamber for a period of 4 minutes at 70 cc min^-1. Water vapor was removed from the purge gas during passage through an 18 cm long x 3/8 inch diameter glass tube packed with the desiccant magnesium perchlorate. The sample gases were concentrated on a cold-trap consisting of a 1/8 inch OD stainless steel tube with an ~7 cm section packed tightly with Porapak N (60-80 mesh). To cool the trap, isopropanol cooled by a Neslab Cryocool® refrigeration system was forced from a reservoir beneath the trap to a level above the packing with a stream of compressed nitrogen. After quickly bringing the isopropanol to the top of the trap, a low flow of nitrogen was bubbled through the bath to reduce gradients and maintained a temperature of -20° C. After 4 minutes of purging the seawater sample, the sparging chamber was closed and the trap was held open for an additional 1 minute to allow nitrous oxide (N20) to pass through the trap and thereby minimize its interference with CFC-12. The trap was isolated, the cold isopropanol in the bath was drained, and the trap was heated electrically to 125° C. The sample gases held in the trap were then injected onto a precolumn (30 cm of 1/8 inch O.D. stainless steel tubing packed with 80-100 mesh Porasil C, held at 90 C), for the initial separation of the CFCs and other rapidly eluting gases from the more slowly eluting compounds. The CFCs then passed into the main analytical column (~183 cm of 1/8 inch OD stainless steel tubing packed with Carbograph 1AC, 80-100 mesh, held at 90° C) for final separation, and into the EC detector for quantification.

The analysis of carbon tetrachloride was made on a separate, but nearly identical apparatus to the electron capture-gas chromatography system used in the analysis of CFC-11, CFC-12 and CFC-113 (Bullister and Weiss, 1988). Samples were drawn in the same type of syringes used for the CFC analysis. In the CCl4 system, the sample injection port was flushed with 30-40ml of sample before injecting sample into a calibrated loop (~30ml). After filling, an additional 30ml of water was pushed through the loop and allowed to overflow. For analysis, a valve was switched and the water sample held in the loop was pushed into the stripper with the same CCl4 free nitrogen that was used to strip the sample. The gases removed from the sample were dried while passing through an ~18cm x 3/8 inch OD tube of magnesium perchlorate and concentrated on a trap packed with four inches of Porapak N and held at —30° C during trapping. At the conclusion of stripping, the trap was heated electrically and the contents swept onto the precolumn (0.53mm I.D. x 30 meters, DB624 capillary column, 45° C)) with clean nitrogen. The desired gases passed on to the main analytical column (0.53mm I. D. x 30 meters, DB624 capillary column, 45° C), before the precolumn vented the later peaks. All other aspects of the analysis were the same as the CFC analysis.

Both of the analytical systems were calibrated frequently using a standard gas of known CFC composition. Gas sample loops of known volume were thoroughly flushed with standard gas and injected into the system. The temperature and pressure were recorded so that the amount of gas injected could be calculated. The procedures used to transfer the standard gas to the trap, precolumn, main chromatographic column and EC detector were similar to those used for analyzing water samples. Two sizes of gas sample loops were present in the CFC analytical system, while four calibrated sample loops were used in the CCl4 system. Multiple injections of these loop volumes could be made to allow the system to be calibrated over a relatively wide range of concentrations. Air samples and system blanks (injections of loops of CFC-free gas) were injected and analyzed in a similar manner. The typical analysis time for a seawater, air, standard or blank sample was 15 minutes on the CFC system and 20 minutes on the CCl4 system.

Concentrations of the CFCs and CCl4 in air, seawater samples and gas standards are reported relative to the SIO93 calibration scale (Cunnold, et. al., 1994). Concentrations in air and standard gas are reported in units of mole fraction CFC in dry gas, and are typically in the parts-per-trillion (ppt) range. Dissolved CFC and CCl4 concentrations are given in units of picomoles per kg seawater (pmol kg^-1). CFC and CCl4 concentrations in air and seawater samples were determined by fitting their chromatographic peak areas to multi-point calibration curves, generated by injecting multiple sample loops of gas from a working standard (PMEL cylinder 33790 for CFC-11, CFC-12 and CFC-113; PMEL cylinder 33780 for CCl4) into the analytical instrument. The concentrations of CFC-11 and CFC-12 in this working standard were calibrated before and after the cruise versus a primary standard (36743) (Bullister, 1984). No measurable drift in the concentrations of CFC-11 and CFC-12 in the working standard could be detected during this interval. Full range calibration curves were run at intervals of 3 days during the cruise. Single injections of a fixed volume of standard gas at one atmosphere were run at intervals of 1 to 2 hours to monitor short term changes in detector sensitivity.

Extremely low (<0.01 pmol kg^-1) CFC concentrations were measured in water between 2000 and 3000 meters at the Northern end of the section between 15° S and 45° S along this section. Based on the median of CFC concentration measurements at these depths, which is believed to be nearly CFC-free, blank corrections will be applied to the data set. If the measured CFC concentration for a sample is very low, subtracting a blank can result in a very small negative number reported. Blank corrections will be applied to the CCl4 data if necessary.

On this expedition, we estimate precision (1 standard deviation) of 1-2% or 0.005 pmol kg^-1 (whichever is greater) for dissolved CFC-11, 2% or 0.005 pmol kg^-1 (whichever is greater) for dissolved CFC-12 measurements. F-113 and CCl4 precision is yet to be determined as there was F113 contamination for most of the cruise.

A number of water samples had clearly anomalous concentrations relative to adjacent samples for one or more of the trace gases. These anomalous samples appeared to occur more or less randomly during the cruise although more frequently for F12 and F-113, and were not clearly associated with other features in the water column (e.g. elevated oxygen concentrations, salinity or temperature features, etc.). This suggests that the high values were due to individual, isolated low to moderate level CFC contamination events. The source of the contamination was eventually tracked down to eucalyptus oil that is regularly injected into the ships air conditioning unit. It appears that some of the oil was collecting on the bottles and absorbing CFCs. Measured concentrations for all samples will be included in subsequent reports, but those showing contamination will be given a quality flag of either 3 (questionable measurement) or 4 (bad measurement).

Updated: 31/01/03