Sampling the ocean piling up against the largest surviving giant floating glacier


this comes from a blog ....  The Adventures of K132* 2012

sciblogs.co.nz/guestwork/2012/01/16/antarctic-report-measuring-the-melt/

1          Arrival   - 9 Jan

This is a new blog describing the day-to-day work for an experiment looking at a coastal ocean flow in Antarctica.  Unlike most oceanography the work is conducted from camps on sea ice.  This is the frozen skin of the ocean perhaps a few m thick, sufficient for us to set up tents, drill through the ice, and then sample.  This water is quite special in that it is extremely cold but relatively fresh.  Indications are that it plays a big role in how sea ice forms in the wider southern ocean.  The growth and decay of sea ice is one of the key elements in global climate science.   

Over the past decade our group (NIWA, IRL, U. Otago) has done quite a bit of work in Southern McMurdo Sound looking at how the ocean beneath the Ross Ice Shelf finds its way into the local ocean.  My specialty being the rate at which it mixes with the surrounding waters.   Part of this work has been looking at the evolution of the outflow as it (presumably) hugs the coast moving north.  A logical extension of this is to repeat our southern sampling in the far north of the region.  To this end, in the present field season we’re planning work with Italian colleagues out of their base in the Terra Nova Bay area some 200 km to the north of Scott Base.

After a brief delay we got away from Christchurch at around 11 pm on January the 8th.  The US C-17 transport was quite full as a flight had been cancelled a few days previous so there wasn’t a spare seat on our flight.  The views as we crossed the Victoria Land coast well to the north of McMurdo Sound were fantastic but they quickly gave way to cloud cover.  This had me a little nervous as it’s not unheard of for the flight to turn back even once it has arrived in the McMurdo area due to poor weather and visibility.  Ten hours flying to be where you started doesn’t make for a great feeling of achievement.  Of course compared to 2 weeks battling the southern ocean in a ship this seems a little soft.

The late departure meant we didn’t arrive until 4:30 a.m. – the landing was followed by an hour long crawl into Scott Base on a big transporter bus.  It was actually snowing quite heavily by the time we made it in to base.  This is unusual in itself – most of the time snow in Antarctica is actually just wind-blown material.  While we are trying to deploy more gear that automatically samples year-round, we typically actually work in the field in the October-December period as this is when sea ice operations are more feasible.  So it is strange for me to be here so late in the field season with lots of exposed rock and dirt around Scott Base and lots of melt ponds on the sea ice out the front of Scott Base.  This melting will continue to weaken the sea ice making it more susceptible to breaking up.  In fact last year was the biggest break up in the last 12 or so years.

(*K132?  It’s our experiment reference number and forms the basis of any communications and decisions here.  It’s specifically associated with a grant from the prestigious Marsden Fund at the Royal Society of New Zealand to look at how melting ice influences ocean circulation.).


2          Preparations  - 11 jan

The first few days of any work down here are taken up with preparations, training, meetings and checklists.  This seems a chore but is pretty important for keeping experiments running and having as few surprises as possible.  Then the next task was to finalize our freight and make last minute decisions on what to take up north on the last leg of our journey to the Italian Base.  As the flight up to the Italian Base is being operated by an America company all the weights are in pounds.  Our entire experiment has to come under 2400 pounds.  As is the way, we were way over our allowance so all those nice-to-have items dropped off the list.  Then like a late Christmas present we were told that our allowance was about 200 kg more than we thought.  So we got to put a few sampling luxuries back in – well when I say luxuries I mean extra batteries, a backup temperature sensor and more duct tape.

The freight is to be shipped out of Scott Base to Mario Zuchelli Base this afternoon, a day earlier than we originally thought so it was a bit of a scramble and gear didn’t have time to be re-checked.  We’re hoping we won’t regret that later on, but these decisions need to be made all the time.  Besides, it’s easier to spread out at our final destination; just a long way from spares!  The one bit of serious testing we did do was to see how good we are at putting up the field tent.  Pretty good it turns out – just not so good at putting it away.  The tent is an excellent compromise between size, portability and resilience, regardless in the past we’ve had some challenging times putting these tents up in arduous conditions.

We’re not overly concerned about the lack of re-checking of the instrumentation as Brett Grant, my technical whizz in this endeavour, went through all the gear before it shipped out.  We’ve worked together quite a bit over the past five years, both in Antarctica, and at sea in Cook Strait and further afield aboard the Tangaroa.  Brett’s an expert in making the various profilers do their thing in an orderly fashion.  I can usually get them to make sense, but in a much more chaotic fashion.

We went for a walk around the pressure ridges near Scott Base after dinner.  These are ripples and deformations in the sea ice as it is squeezed by the land, the ice shelf and the wider sea ice itself.  Earlier in the season these ridges form a sharp jagged alien landscape.  But now with the heat (-0.3 deg C! plus sunshine) the lumps of ice are all melting and all the dips between the ridges are filling with… liquid!  Rather strange really.  So these beautiful ponds of greeny-blue are forming almost before our eyes.  An example of the rapidity of this development is the flagged trail set out for hikers to safely go through the ridges actually went straight into a pond.

 

3          Hurry Up and Wait 12Jan

The flight up to Mario Zuchelli Station in Terra Nova Bay was brought forward by two days so we had to pack early.  Now it’s been dropped back a day due to aircraft commitments for weather balloon experiment recovery.  This is all OK and normal.  It doesn’t pay to get too precious about timing.  It is a good time to tuck away somewhere in the nooks and crannies of Scott Base and get some work done on previous data sets.   I’m looking at data from the preceding experiment in this series conducted 14 months previously.  It’s interesting to reflect that if we’d be doing that experiment this season it wouldn’t have gone ahead in anything nearly like the form it did.  The same week as the February 2011 Christchurch earthquake the largest sea ice breakout in McMurdo Sound for 12 years occurred exposing the ocean as far south as Scott Base.  The subsequent sea ice evolution resulted in serious cracks around our 2010 field site at the tip of the Erebus Glacier Tongue.

Part of this breakout opened up water in Haskell Strait – the stretch of water between Cape Spencer-Smith on White Island and Cape Armitage near Scott Base.  In otherwords the view out the window from Scott Base.  This massive oceanic valve is normally hidden beneath ice shelf and sea ice.  We were immensely proud to have been able to name this after Tim Haskell, a long time mentor and collaborator who essentially has made sea ice research in the region possible.  Tim’s science career epitomises a successful approach to science in New Zealand where one has to be flexible to changing initiatives and funding models while at the same time keeping an eye on what makes for demonstrably good science.  To have played a role in officially naming a piece of water the size and magnitude of the Straits of Gibraltar or Cook Strait after him is pretty satisfying.  The Strait acts as a valve between the literally supercool (we’ll come to this later) water beneath the vast Ross Ice Shelf and the waters of the western Ross Sea.  The flow rate through the valve influences the sea ice growth in the region.  Sea ice being the largest annual geophysical change on the planet – millions of square km of the globe literally changes reflectivity every six months.  This dramatically affects how heat and gases exchange between atmosphere and ocean, provides a vast habitat for growth of algae – the basis of the ocean food chain, and drives the thousand year global oceanic thermohaline conveyor belt.  Natalie Robinson in our group neatly describes this annual fluctuation as the global heartbeat.

Anyway, the preceding experiment involved collaborative work with some pioneers of sea ice research from the US – Miles McPhee and Tim Stanton (actually a Kiwi but long-since based in California).  We setup one of Tim Haskell’s field camps around 50 m away from the Erebus Glacier Tongue in October 2010 looking at the effect of the glacier on local flow.  This is driving us to look at how the glacier melting might relate to the flow itself as well as the influence of the glacier on the local ocean and sea ice growth.  One interesting feature is the supercool nature of the ocean in the area mean that ice crystals were forming on the underside of the ice driven by the coldness of the ocean rather than the atmosphere.  These crystals change the roughness of the ice and so change the heat exchange.  This is the underpinning motivation for the present experiment – does this effect persist further to the north?

 

4          Buongiorno! Jan 14

Right on schedule we flew in to Terra Nova Bay to the Mario Zuchelli Station run by the Italian Antarctic Programme.  It is comparable in size to Scott Base, but located a bit further north is called the Caribbean of Antarctica – by some!  We had perfect views on the way up as we crossed over Ross Island and directly over the Erebus Glacier Tongue – the scene of last season’s big experiment.  The sea ice has been far less stable this season to the extent that we probably couldn’t have conducted the 2010 experiment this season.  As we headed further north we crossed open-ocean with diminishing pack ice in patches here and there.  Finally after around an hour or so of flying the Drygalski came into view.  This stunning feature flows some 80 odd km out into the Ross Sea and is part of the reason we’re here.  There’s substantial ice packed up on the south side but beautiful open water to the north.  This beauty belies its normal function most times of the year – one of the coldest windiest bits of ocean in the world.

            As ever, anyone not used to dealing with oceanographers is surprised by the amounts of gear we travel with.  The Italian team that greeted us at the airfield (a smooth bit of glacier) piled all our gear into the back of a truck and we were delivered to the base.  The base has a whole new skyline to familiarize oneself with – along with the language and cultural differences.  We met our Italian collaborator for the first time, Giannetta Fusco, who has worked in the region both at the base and aboard the Italian ship the Italica.  She is proving a great host and showed us around the base, which has quite a different feel when compared to Scott Base or McMurdo Station.  The Programme Director greeted us and wished us well in our scientific endeavours.  We returned the favour with some fine NZ wines.

With focused process-based experiments where we look at how a specific set of mechanics works it is sometimes difficult to see how they fit into the big picture.  From my perspective they contribute in two ways.  First, the data in specific situations can be combined to build up a quantitative picture of behaviour.  This directly gets included in models maybe as a new coefficient.  The second way that this work proves useful is that it exposes mechanics that hitherto had not been considered relevant.  Thus, extra aspects might be added to a model or a different model altogether might be used.  When I say models here I mean both snazzy computer simulations through to things that as not much more that collections of thoughts on the back of an envelope.

Tomorrow starts with some survey work south of the ice tongue to look for a suitable site.  Anyway must run – it’s pizza night at Mario Zuchelli Station.

           

5          Survey  Jan 15

We completed our first helicopter survey of the field location this morning.  It was around a 30 minute flight south of the Station.  In that time we stopped off for some brief tourism at Inexpressible Island where a number of Scott’s team were forced to spend the winter of 1912 in a snow cave surviving on seals before walking back to Cape Evans very early in the spring.  This brought home the changes of the last century as we had barely finished our breakfast that would have been a week’s nutrients for those fellows - and quite a bit tastier than freeze-dried seal.

I must admit to some (much!) trepidation before the flight.  There are competing needs here – we need ice that’s in a useful location, not too thick, not too thin and not all broken up and melting.  The only ice likely to fit this bill is held in place by the seaward extension of the massive David Glacier – the Drygalski Ice Tongue.  This icy behemoth some 80 km long, 15 km wide, 200 m thick and travelling at a couple of km a year is the last giant floating glacier left – for the time being.  If features like this or its close relative the ice shelf, break up and allow increased flow of ice off the polar ice cap then sea level will likely increase rapidly.  So fast might this increase be that it will not only become detectable in a human lifetime, it may actually dominate the activities of the human species for centuries to come.

Erich von Drygalski led a substantial German Antarctic voyage in the first years of the 1900s.  They worked over in the regions south of the Indian Ocean and Australia.  They intentionally froze their ship, The Gauss, into the sea ice and wintered successfully.  Unlike in the arctic, where the ice breaks up and refreezes in a constant grinding jumble of floes, they found the sea ice around their ship remained in one contiguous piece which would have been some comfort as they had to deal with some appalling weather.  This was similar to the piece of ice we landed on south of the Ice Tongue – nice and flat.

My viewing of the satellite imagery suggested there was quite a bit of fast ice real estate just south of the Drygalski but this gave little indication of its thickness or condition. We flew about 5 km south of the southern edge of the massive ice tongue.  Upon landing we shovelled off the small layer of crusty snow and started drilling using a kovacs.  This thin drill is simply designed to measure ice thickness.  I couldn’t watch as they drilled away – fully expecting them to get to 4 m and still not have broken through – well beyond the capabilities of our lightweight gear.  But good news came soon enough when our Italian Navy Seal guide Davide broke through the bottom of the ice at around 2.6 m!  This was a little thicker than I’d worked through in the past but possible.  We drilled a few more holes in a 10 m vicinity to confirm this thickness – took a GPS location and departed. 

I felt that the ice north of us towards the ice tongue would likely be thicker but we didn’t see any obvious rifting or steps so we flew a kilometre north and drilled again as this might be slightly more be protected from breakout in the coming weeks.  Ice breakout can happen in a matter of hours – ocean swell from storms sometimes 1000s of km away literally flexes the sea ice to pieces.

A little surprisingly it turned out to be thinner – just a shade under 2 m, again confirmed with a few repeat holes nearby.   This should be well within our drilling capabilities.  Essentially, I think the region is made up of a jumble of large sea ice pieces that have broken out and refrozen.  We took a wide sweeping departure with the helicopter as we left, showing fresh cracks seaward of our site and clear steps up to much thicker multi-year ice to the north and west.  So I think we’ve chosen as well as we could hope.

 

 

6          Camp   Jan 18

We got the all-clear for heading out to the site this morning and set up our camp site.  It was a stunning day and we had all day to drill a hole, set up the tent and do some sampling – easy!  Step one was delivery of ourselves and our gear in helicopter sling-loads to the field site some 30 minutes helicopter flying south of the Italian station.  These sling loads wrap up our boxes in big nets and chain them to the underside of the helicopter.  When the load is on the ground at its destination point it is gently released.  Hundreds of kilograms of gear can be moved in this way.

The setup proceeded slowly as it always does with the usual sequence of breakages and challenges that is essential in fieldwork and never more so than polar oceanography where everything takes three times as long as you expect.  For these shallow “hydro-holes” in a remote location we use a combination of drilling techniques that gives us a not very pretty, but functional, hydro-hole.  The ice is very soft and wet and almost immediately the holes we were drilling filled with slush which makes it hard going.  Right as we broke through the last little piece the drill fell apart so that was good timing! 

We then moved our tent over the hole and setup the gear inside.  The first thing we did was get the acoustic current profiler in place – this is called ADCP for short-hand with the D standing for Doppler.  It sends out pings like a fish finder but instead of simply listening to the amount of return signal as it bounces off small particles in the water it listens to the Doppler-shift in the signal. This way, after some geometry, we arrive at the water speed over a hundred or so metres of water depth.  This unit will stay in for the duration hopefully giving a nice picture of the flow beneath the ice.

First thing we did was use a depth sounder to measure the depth.  The charts put the depth in the vicinity of 600 m here.  However, the sounder struggled to find something to lock on to so the results were inconclusive unfortunately.  This meant we didn’t have the depth and would have to do the first profile slowly – essentially by feel.

            Finally after all day setting up and with time getting away from us before pickup.  I turned on the turbulence profiler - our reason for being here.  And it didn’t work!  Your worst nightmare!  It was clear no power was getting to the unit so I bypassed the power conditioner and it worked but not very well as spikes from the generator corrupt the signal.  The power was sufficient for us to profile slowly to get the depth and basic properties of the water column.

At the same time the winch was putting out a funny burning smell which is never good.  Either it’s a problem with the fan in which case we’re probably ok as the air will help cool the unit.  Or it’s something more sinister in which case there could be challenges ahead.  The trouble is the winch is 130 kg and not easily moved around back to base for repair.  Of course it’s just gone 100 years since Scott and his team reached the pole… second.  So we’re not complaining.

And the data!  Even the one profile recorded so far is interesting and probably the first such data from this complicated piece of the ocean where ice shelf water meets polynya water.  In other words two radically different waters - but both associated with the birth of sea ice.   The profile captured a warm (well… -1.9 oC) fresh layer just beneath the ice, then deep down there is a clear layer that is a little bit colder and saltier than the water around it.    We hope to get a few more profiles over the next few days, especially in high resolution mode where we can really begin to unravel things.

 

7          Data!  Jan 19-23

The weather was too challenging for us to get to our field site for a few days which was frustrating as we were all set to go.  On the plus side it’s not often I get to celebrate a birthday in Antarctica - or Italy - and a few days ago I sort of managed both!  Dinner was followed by a huge cake, champagne, happy birthday in two languages and then a lesson in table soccer. 

We did have some modest scientific achievement in that we recovered four dust traps for a research team at Victoria University.  The dust traps are designed to capture wind blown material to determine the rate of injection of this material into the nearby ice and ocean.  This same wind is also responsible for driving the massive cooling effect that causes the Terra Nova Bay polynya.  This recovery involved flying out to the Nansen Ice Sheet and Hell’s Gate – serious pieces of ice ten minutes helicopter ride south of here.  It was rather exciting actually standing on the type of ice that it driving lots of the processes we are studying.  Huge seams of sediment, melt ponds and even rivulets of running water combine at a massive scale.  And this is just what you can see – beneath it all lies the ocean.

The next day’s waiting for weather involved a helicopter flight to within 5 km of the field site before we turned back by a wall of cloud over our field camp.  This brought home to us how localised the good and bad weather can be around here. Finally, proving that patience is a huge virtue with Antarctic weather, we had a windless, sunny day at the camp and captured a serious amount of data.  We took full advantage of the opportunity and had the equipment running pretty continuously through the day.  It’s not a full tidal cycle but it is a substantial part of one. 

Our main profiler is a Canadian-built instrument that can measure temperature and salinity down at perhaps the cm scale and to hundredths of degree accuracy.  It also captures to tiny velocity variations in the ocean that act to stir up material and also represent energy lost from the main ocean currents.  Equipment this sophisticated is rather delicate and needs a good deal of care and attention to detail to keep it running.  However, give the harshness of the journey and conditions with which it must deal it’s a miracle it works at all, let alone consistently for hours on end.

The profile data show a myriad of slowly changing layers in the top 50 or so meters, sitting above a core fluid that remained pretty consistent in terms of its constituents.  Just below the ice, the seawater was cooled down to near-freezing and suggested a pretty stable situation with regard to the condition and survival of this sea ice in the remainder of the season.  Deeper, near the base of our profile at around 300 m depth (we can’t get right to the seafloor here as we couldn’t fly in a big enough winch) there was another layer with a clear signature that looks like it might have come from the nearby polynya – although with the warm weather the polynya itself is not obviously active.  This we will need to tease apart when other data from other sources like ships and satellites come in.  

We were able to get out to the field camp again the following afternoon for some follow-up profiles to provide us with a comparison.  Things had changed dramatically in the top 50 m and now there was a warm fresh layer that had slid in just beneath the ice and was almost certainly causing melting.  Also, with this change came a good deal more turbulence in the upper part of the ocean.   If we can get out in the next day or two we hope to confirm the persistence of this warm layer, because if it remains it will have a dramatic effect on the sea ice structure.

A couple of emperor penguins came visiting during this most recent field day.  They seemed a little less curious than previous encounters we’ve had as they didn’t come too close to camp.  Other than that there have been a seal or two visiting the hydro-hole but nothing too disruptive.  There are plenty of other holes in the area so I think they quickly decide they don’t need to share the space with oceanographic instruments.  Also the bird life has a little more variety – i.e. more than just skua.

It is always tricky drawing global conclusions from such local finescale (– both in terms of duration and spatial extent) observations.  However, the results give us a new perspective of some of the oceanographic processes in a basically unsampled bit of the ocean and give us a bit more certainty as to how to build ocean models.  A quick look at the marine chart for the Victoria Land coast along the western Ross Sea shows a myriad of glacier tongues all jutting out into the ocean to varying degrees.  While the Drygalski is far and away the biggest glacier tongue, they all certainly influence the local circulation.  This in turn plays roles in regional aspects of both climate and ecosystem processes in what is a rather important part of the ocean (i.e. cold salty water for ocean thermohaline circulation and sea ice for habitat for algae that starts a massive food chain). Our new data give us some clues with regard to the way layers of ocean water collect and build up along the sidewall of the glacier.  Also by sampling so late in the summer season we’re seeing some of the warmer waters and how they bathe the underside of the sea ice – reducing its resilience.

 

8          Stuck!             26feb

Despite being stuck on base due to weather for what is now the fourth day we feel incredibly lucky.  The station went from being the Caribbean of the South (or so the locals say) to blowing snow and dare I say it – a little bit cold.  Really, we are in Antarctica.  An onshore wind picked up and built up a decent ocean swell.  Of course this happened right when the Italian ship, The Italica, came in to harbour to offload equipment and food.  This eventually had to be abandoned and the ship temporarily departed to deploy some observational moorings nearby with a plan to return in a few days and try again.

This same swell started to eat away at the first year ice breaking it up into bite-sized floes – maybe 20-30 m across – that stretched away into the distance as far as the eye could see.  The wind continued to force the floes in against the coast providing a spectacular vista.  Brett and I headed down to shore to watch the waves and ice do their thing.  The quality of environmental documentaries these days is so high we are familiar with the look of such displays.  But nothing really compares with the reality happening right in front of you. Massive chunks of ice are tossed around as if they are weightless which indeed, whilst floating, they are in a way.  Penguins hopped on and off floes without a care in the world.  I snuck off early to bed.  Brett, powered by youth, stayed up watching the display and claimed to have seen an Orca blowing in the distance.  He may have been having me on, but seeing as Orca are a regular occurrence for us in Wellington, maybe not.  Either way, the penguins clearly do have some concerns.

The wind changed direction overnight, picking up now to a not-inconsequential 40 knots.  Hence we are stuck on base despite it being a bright sunny day.  The helicopters (and me) don’t want to be bouncing around in the wake of the mountains to the west of here.  The wind made the sea of ice disappear literally overnight.  We now have a sunny, wavy coastal vista that could be anywhere in the world if it wasn’t for the frozen volcano (Mount Melbourne) in the distance and the water temperature being around -1.9 degrees Celsius.  This means the floes don’t melt, they aggregate in huge bands which are moved around by the currents and the wind. 

The ice floes end up in the marginal ice zone which surrounds Antarctica during this season, filled with broken ice.  I think it must have been cartoons from childhood that made me think the floes would be round-ish in shape but they’re not.  They’re close on square most of the time and sharp-edged and sharp-cornered.  Like tiles on a roman mosaic, except in monochrome.  This marginal ice zone (MIZ for short) can extend for hundred of kilometres.  The ability of waves to pass through the MIZ is quite an important issue.  If the waves do get through then they can eat away at the fast ice along the shore and speed up coastal fast ice break-up.  However, only a precious few experiments have ever managed to provide information on this process.  A colleague, Alison Kohout, has an ambitious plan to deploy wave sensors on floes from a ship next summer as it slices through this marginal ice zone.  This will be an incredibly exciting opportunity to get at some key numbers to help models come closer to representing the myriad processes that ultimately control this area of the ocean.

So now we are down to a scheduling squeeze.  We have three more days to finish up here before our flight south.  In that time we actually have to be able to get to our field camp to retrieve equipment that has been recording background properties, as well as the camp itself.  We have also entertained the possibility that the ice upon which our camp is set up is now a floe too.  We’ll deal with that if it happens.  There’s a rule in oceanography – never expect anything you put in the ocean to come back.  Certainly it’s nice when it does, but don’t assume it’ll happen.  Sea ice oceanography, at least this time of year, is the same.  I would say this, but from a distance people naturally focus on the cost of the equipment.  The reality is, in the face of the operational costs of getting and staying (safely) here, the cost of the replaceable gear is far outweighed by the value of totally new observations (and ideas they spawn), in a new and important location and season. 

 

9 Recovery    Jan 26-

Finally came a break in the weather.  The wind eased off and we got out to the sea ice camp.  It was a nervous trip out as I thought open water on the horizon might be all that remained of our field camp.  Fortunately, the water turned out to be much further south and our field area was as solid as it had ever been.   Of course this is a slightly false sense of security as the ice can breakup in chunks at a moments notice.  Satellite imagery from a few years back shows a 10x30 km piece of ice from the area we are on simply disconnecting from the coast over the space of a day.  Something that big though stays together for a while and so with close helicopter support we’d be fine.

A reasonable pile of snow had accreted around the tent indicating that the local wind was from the South.  This must’ve helped with the stability of the ice as it held the sea ice against the ice tongue rather than pushing it offshore as I’d pictured in my broken sleep during the previous night.  We recorded some underwater video using our trusty camera-on-a-flag-pole technique.  Not pretty, but effective.  This showed some hallmarks of ice shelf influence with platelets aggregated into the ice and a generally lumpy underside to the ice.  These are aspects our group, especially the two winter teams led by Pat Langhorne at the University of Otago, have explored in some detail over the last decade in work some 300 km to the south around the edge of Ross Ice Shelf.  It’s really exciting to see this sort of structure this far north.

Because of the difficulty getting out to our site and the looming return-deadline I decided to pull the camp as early as possible.  Inevitably, I felt sure the weather would turn perfect for the remaining days.  As it was, after some early success with the profiler, the decision was made for us by having a cable failure.  This is readily fixable - but not in a tent.  The problem occurred, we think, because the flow is so slow the profiler goes almost directly downwards making line tangle difficult to avoid.  One of the tangles must’ve pulling so tight it broke wires within the cable - maybe. 

We needn’t have worried about finishing early though as it took a couple of hours just to chip the tent out of the ice that had frozen around its edges.  It all came flooding back to me that the downside of this tent is it takes forever to take down.  This is a perfectly acceptable chore though as it also means it stays up in any weather.  Besides, it only feels like proper sea ice science if you are scrabbling around on your hands and knees with an ice axe.

During the day Giannetta, our Italian collaborator, deployed a sea ice thermistor string.  Developed in Scotland, this setup is designed to send a profile of temperature, from within and beneath the sea ice, back to her office via Iridium link.  Our New Zealand group uses a different setup to make the same sorts of measurements.  There are pros and cons with both approaches so it’ll be good to see how well the Italian/Scottish gear does.  It’ll be interesting to see how the Scottish unit deals with ice accretion – something not seen in the Arctic.

The other bit of gear that we recovered was an acoustic Doppler current profiler (ADCP for short).  This sends out pings like a fish-finder but, as well as simply listening for the strength of the backscatter from the pings, it listens to their Doppler shift (i.e. like a passing ambulance siren) to gauge the speed of the water.  The instrument was still pinging away when we recovered it so we knew it was still working - always a good thing.  This will help us to determine the speed of the water, the strength of the tides and also to give us an idea of the amount of tiny suspended material in the water – beit ice crystals or biology.

The helicopters transport our 1500 kg of gear using “sling loads” – big nets.  It’s a pretty effective way of quickly getting heavy gear from place to place and two loads will be sufficient for all our gear.  One sling load came back mid-afternoon while we cleaned up camp.  With the low wind speeds we again took the opportunity to skim past the face of the Nansen Ice Sheet, making clear the myriad of patterns in the ice due to changes in deposition and pressure. 

The final sling load came back the next day and so began the laborious but important task of cleaning up the gear.  Cleaning and rinsing everything as much as possible, from delicate instruments through to ropes and tents, makes them more likely to work next time.

 

10 Berg-tastic    Jan 31-

Our last full day at Mario Zuchelli Station took sublime to new levels.  We had the opportunity to use their coastal vessel, The Skua, for a few hours.  It was great to look over the boat.  There are no small boat operations out of Scott Base, nor have there needed to be for there has been a distinct lack of open water for the last decade or more.  However this is changing and such operations may one day become part of the science support requirements.  The Skua gets used for a range of sampling tasks as ice conditions allow.  We took the vessel out to a modest grounded iceberg and sampled the ocean stratification and turbulence right by the berg in order to get a better understanding of how they melt when surrounded by open water rather than ocean capped by sea ice. 

Rather than take our large velocity-based turbulence profiler I opted to bring a much more portable device, The SCAMP, that determines the amount of mixing from tiny temperature variations over distances as small as a mm.  I’ve struggled to get good results with this device in Antarctica before because there has simply not been enough temperature signal around – basically everything was the same temperature – the freezing point of seawater.  However, being further north and with open seas, there is ample information to work with here.  There are two main advantages with this profiler.  First, it’s relatively small – shaped sort of like a dandelion seed if it were 60 cm high.  The second big advantage is it can start sampling almost as soon as it’s released.  The big profiler we were using south of the Drygalski Ice Tongue needs quite a few metres to get up to good sampling speed.

One of the interesting things (to me!) about this sort of sampling is that it is actually doing quite simple things, albeit to incredibly high precision and with high signal to noise ratios.  And also that these things were readily achievable to a degree decades ago.  Where the challenge comes in is getting to enough places of importance and then getting enough data at the right times to represent the various conditions.  There have been precious few attempts at directly measuring ocean mixing in Antarctic waters.  Certainly, much of what goes on is as you would expect anywhere in the world’s oceans.  It’s the departures from this, especially relating to unique features like the presence of massive ice shelves and glaciers, that need to be considered.

Being a small berg and seemingly well-grounded there was not much problem with the ice itself.  However, even the slow, modest swell was enough to make holding the vessel off from the ice a challenge.  The skipper did nicely bringing in the boat right over the spot I wanted and allowing me enough time to get the profiler into the water, the boat would then reverse out and stand off a few tens of meters whilst I paid out the thin line as the profiler slowly sank recording as it went.  The thin kevlar line is only a few mm in diameter but is able to carry several hundred kilograms. Still, it takes some getting used to, throwing expensive equipment over the side of ships with nothing but a thin line for recovery.  It could be tougher. There are deep ocean versions of our main profiler that have no line.  When they return to the surface the operator has to rely on satellite communications, lights, keen eyes and nervous energy to get the gear and the data back.

One of the curious features of oceanography that a melting berg highlights is a set of processes relating to the combined and separate effects of heat and salt.  If you were to throw some “seawater” into the ocean it would sink until it found a depth where the ocean density was the same as that of the injected water.  The interesting thing is that the density of seawater mainly comprises the effects of temperature and salt concentration – and these two diffuse at different rates.  So our injected patch of seawater might have quite different temperature and salt signatures to the water it found as its neighbour, even though it has matching density.  The result then is the heat and/or salt start to diffuse and equilibrate such that the local density conditions might become unstable.  Weird things happens – thin layers of constant density form- thermohaline staircases.  If ocean models don’t capture this sort of effect they get the resulting circulation wrong.

 

11 Wrap-up

A busy last morning at Mario Zuchelli Station involved packing our pile of gear onto the back of a tractor trailer for the trip up the hill to the local airfield to be picked up by a Twin Otter aircraft.  These planes work in remote conditions throughout the world.  Our flight was the crew’s last flight of the season and they were taking the aeroplane to northern Canada via the South Pole, Chile and upwards.  It puts the concept of a “long commute” into perspective!  Anyway our freight is nicely trimmed to basically fill the plane. 

We had excellent views on the way south.  As we flew over our sea ice site the aerial view revealed a large crack a little to the south of the camp that must’ve appeared in only the last four days.  Beyond that we passed over open water as well as vast areas of pack ice broken into a huge variety of scales.  These will either drift north and melt or be reintegrated into multi year ice contributing to a highly complex ocean skin that challenges us constantly in terms of mapping its thickness, resilience and suitability for biological habitat.   The challenges around predicting future possibilities for the earth system and its climate has placed a strong spotlight on this aspect of Antarctica and realistically we are only scratching the surface.

Now begins the task of collating, verifying and archiving the data.  Actually much of this we’ve been doing as we go in the down-time between sampling.  Once we’ve settled on good quality realisation of the results I have to tease it into some form of scientific story whereby the data fit into a context of need, questions, critique and  implications.  I’ll do this in conjunction with my various colleagues in the field, many of whom are vastly more knowledgeable than me on the topic. 

And after all this I’m not going to tell you the punch line as to what we found!  Before that happens I have to run my conclusions past my “peers” – essentially it gets torn apart and restructured by anonymous reviewers. One develops a very thick skin in this game.  I have had papers that have had more space devoted to the reviews and response than the paper itself.  This is a hugely critical part of science and a real challenge for field science where one simply cannot control every (any!) aspect.  Certainly the process is not perfect.

What I can say is we’ve seen some initially pretty intriguing conditions in the waters near the ice tongue.  If we can follow through and justify their apparent uniqueness and the associated response in the sea ice then we’ll have a strong story.  It the baseline level there’s a good chance the end results will help improve modelling of sea ice processes or at the very least suggest that they need to focus down to small scales in order to make sense at large scales.   We’ve also come away with a new appreciation for the different environments that go to make up Antarctic coastal waters.  And finally, probably most importantly, we have made a start on a new collaborative link with scientists who have new ideas and different perspectives. 

The next mission is likely to take place in a years time and in many ways is even more challenging.  We plan to sail to the Mertz Polynya area which until a few years ago was also home to a giant glacier tongue.  This however was broken apart whn a large iceberg collided with it.

Thanks… MZS + PNRA, Helicopters NZ,Brett Grant, Giannetta Fusco, Giogio Budillon, Tim Haskell, AntNZ, Scott Base, Marsden Fund.