“…[samples were] reduced to graphite before AMS 14C analysis at the SUERC AMS Facility.”

To determine the rate of ice sheet retreat it is necessary to determine WHERE the ice sheet margin was WHEN. The WHERE is determined by studying landforms characteristic of glacier margins, such as moraines, erratic boulders, and sediment deposited both onshore and offshore by ice and glacial meltwater near the margins of the ice sheet. The WHEN is determined using different dating techniques depending on the material sampled. For BRITICE-CHRONO we are applying three different dating techniques, (1) optically stimulated luminescence dating (OSL) for sands, (2) radiocarbon dating (¹⁴C) for organic material like shell fragments picked out of sediment cores collected from the sea floor during the research cruises, and (3) surface exposure dating of rock using the terrestrial cosmogenic isotope beryllium-10 (¹⁰Be) in the mineral quartz.

Radiocarbon dating and surface exposure dating rely on being able to measure the abundance of extremely rare radioisotopes in the sample material using a technique called accelerator mass spectrometry (AMS) performed at the Scottish Universities Environmental Research Centre (SUERC) AMS Laboratory. What is extremely rare? Well, the natural abundance of ¹⁴C in modern carbon is 1 part per trillion (10^-12) which gets smaller with the age of the material being dated because ¹⁴C decays after the death of the organism. The ratio between the radioisotope ¹⁰Be and stable beryllium is roughly 10^-13 to 10^-14 for surface exposure dating of BRITICE-CHRONO samples. To put this in perspective, if you counted at one number per second it would take you about 3.25 million years to count to 10¹⁴. In other words, radiocarbon and surface exposure dating is only possible because we are able to count individual radioisotopes among trillions of almost identical stable isotopes.

In scientific papers these  measurements are often summarised in a few words, such as “…[samples were] reduced to graphite before AMS ¹⁴C analysis at the SUERC AMS Facility” or “¹⁰Be/⁹Be ratios were derived from measurements at the SUERC AMS Laboratory”. Neither sentence does justice to the complexity of the method and the efforts of a dedicated team of scientists and technicians. Here I will try to provide some insight into how the concentrations of the extremely low quantities of ¹⁴C are determined using accelerator mass spectrometry (AMS). For ¹⁰Be the process is similar but explaining the differences would add unnecessary complexity in what follows.

Accelerator mass spectrometry (AMS) is an ultra-sensitive technique for isotopic analysis in which atoms extracted from a sample are ionized (the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons to form ions); accelerated to high energies; separated according to their momentum, charge, and energy; and then individually counted after identification as having the correct atomic number and mass. The principle difference between AMS and conventional mass spectrometry (MS) lies in the energies to which the ions are accelerated. In MS the energies are thousands of electron volts (1 keV = 1.6 x 10^-16 J), whereas in AMS they are millions of electron volts (MeV). The practical consequences of having higher energies is that ambiguities in identification of atomic and molecular ions with the same mass are removed.

This is how we make the measurements at SUERC

Fig 1. Cathodes in sample wheel after measurement. The hole in the cathodes contains the sample and is 1mm wide.

Samples come to the AMS Laboratory in the form of pressed cathodes containing a few milligrams of graphite (for radiocarbon) and BeO mixed with Niobium for ¹⁰Be surface exposure dating. It takes a lot of time and effort to get from a sample collected in the field to the cathode stage, but that is another story.

The cathodes are loaded into a 134-position sample wheel together with standard materials that have known isotope ratios.

Fig 2. Sample wheel loaded in ion source.

The wheel is loaded into the ion source, the ion source is closed and pumped down to the same very high vacuum as the remainder of the beam line (steel tube) through which the ions produced in the ion source are going to travel (the correct term is drift, but it does not really convey the speed at which the ions move). We need a very high vacuum (comparable to conditions found in outer space) because we do not want our ions to collide with neutral atoms or molecules drifting anywhere along the 32 metres of beam line.

Fig 3. Section through ionizer

We are now ready to perform some ion sorcery. We heat up a caesium reservoir to generate a Cs vapour in the space between the cathode holding our sample and a heated ionizing surface. Some of the vapour condenses onto the cooled cathode, some is ionised by the ionizer creating Cs⁺ ions that are accelerated and focused towards the sample because the sample cathode is at -5kV compared to the ionizer (Fig 4). The impact of the Cs+ ions on the sample surface causes sputtering of particles from the sample surface.

Fig 4. Cs+ focus lens at front of ioniser (Fig. 3). The sample cathode being ionised is located 1 mm in front of the hole in the lens

Some materials will preferentially sputter negative ions. Other materials will preferentially sputter neutral or positive particles, which pick up electrons as they pass through the condensed caesium layer, producing negative ions. All of this happens within 1 mm of the sample surface. The negative ions are removed from the sample surface and focused into the beam line by an extraction electrode (extractor) set at 15 kV. The extracted ion beam (shown in grey in Fig. 3) is spreading, just like the spreading of light from a torch (which is just another type of particle beam). This is known as beam emittance and it must be kept small to ensure high transmission to the detector. But I am getting way ahead of myself safe to say that we cannot get the emittance of the beam back to the original 1mm diameter at the sample surface and much of the experimental apparatus of the AMS is designed to focus the beam to specific places along the beam line.

Fig 5. Closed ion source. The metal rings around the beam line are part of the 45 kV injector. The apparatus to the left of the rings are for pumping the beam line and and focussing the beam.

So now we have extracted a negative ion beam from our sample material, and the ions are starting to move along the beam line. We accelerate the particles to 66 keV by exposing them to an additional 45kV in the injector (Fig. 5 and Fig 6). The particles pass through a spherical electrostatic analyser (ESA, Fig. 6), where particles with the incorrect energy over charge (E/q) are removed from the beam, before being injected into the accelerator via the injection magnet.

Fig 6. Schematic of SUERC 5MV tandem spectrometer.

The injection magnet separates particles based on momentum (= mass x velocity). For radiocarbon we set the magnetic field to allow particles with mass 14 through the magnet and into the accelerator, but we also need to put carbon-12 and carbon-13 into the accelerator to get the ratio ¹⁴C/¹²C and ¹⁴C/¹³C. Since both ¹²C and ¹³C have lower mass than ¹⁴C we use a magnet bouncing system (MBS; Fig. 6) to give the ion beam more energy so that ¹²C and ¹³C temporarily behave like ¹⁴C. Because ¹²C and ¹³C are much more abundant than ¹⁴C we inject the former two for only a few microseconds per second. While ¹⁴C enters the accelerator we measure the ¹³C current in a low energy Faraday cup and the same is the case for ¹²C when ¹³C is injected (Fig. 6. Inset A). ¹⁴C cannot be measured in a Faraday cup because there are far too few atoms to generate a current.

Unfortunately sample materials are not pure and therefore ion sources do not only produce the ions we want. However the next stage in the process takes care of many of the molecular isobars (= same atomic mass number) such as the hydrocarbons ¹²CH₂^– and ¹³CH^–, which have the same atomic mass number as ¹⁴C and are therefore also injected into the accelerator.

So far the system described is similar to conventional mass spectrometry. The next stage in the ion transport is what sets AMS apart. Up to now the ions have been energised by the ion source and injector to 66keV (the low energy end of the spectrometer), which means they are travelling at roughly 1000 km/s. Next they are accelerated to a speed of roughly 7500 km/s by exposing the negatively charged ions to a 4.5 MV positive charge at the terminal in the centre of the 8 m long accelerator tank (Fig. 6 & 7).

Fig 7. Accelerator pressure vessel known as the tank. It is filled with an insulating gas.

When the negatively charged ions reach the terminal they pass through a gas stripper (Fig. 6). The collisions between the ions and the gas removes electrons from the ions thereby changing them from being negatively charged to being positively charged. The terminal voltage is set to remove at least three electrons because by this process molecular isobars of ¹⁴C (such as ¹²CH₂^– or ¹³CH^–) are destroyed due to the high instability of their positively charged forms, and atomic C⁺ ions such as ¹²C⁺, ¹³C⁺, and ¹⁴C⁺ can be separated due to their different mass to charge ratios. Once the now positive atomic ions emerge from the stripper they find themselves next to a very high positive charge (4.5 MV in the case of radiocarbon) and they accelerate away from this, hence the term tandem accelerator (two acceleration steps). The particles emerge from the tank at a velocity of greater than 17000 km/s (that’s equivalent to traveling around the Earth at the equator in just over 2 seconds), the high-energy part of the AMS.

The particles now enter another mass spectrometer, the analysing magnet (Fig. 6), where the ¹²C⁺, ¹³C⁺, and ¹⁴C⁺ are separated according to momentum. ¹²C⁺ and ¹³C⁺ currents are bent more than ¹⁴C⁺ and are collected and measured in Faraday cups, while ¹⁴C⁺ is allowed to continue on towards the gas ionisation detector. Even after all of this the ion beam still contains ions with incorrect mass, energy, or charge as a result of energy- or charge-changing collisions with system components or residual gas. Unfortunately these interferences mimic the path of the ions of interest. Some of them are removed by another electrostatic analyser (ECA) before the remaining particles arrive at the gas ionisation detector where the final identification and counting of atoms takes place (Fig. 8).

Fig 8. Detector (foreground) and electrostatic analyser (background).

As the ions enter the detector they are slowed down and stopped by passing through a gas. Each atom ionises some of the gas and the resulting electrons are collected, amplified, and digitised. In this way the path and location of each atom arriving in the detector can be determined and each arrival counted. The gas ionisation detector allows us to determine the atom species because heavier atoms travel further and deposit more energy than lighter atoms. This capability allows us to set the electronics to separately count ¹⁴C atoms and different interferences arriving in the detector (Fig 9).

Fig 9. Detector spectrum of one 6 minute measurement. Red dots (n=40933) are counted 14C events. Black dots (n=430) are scattered 14C and Lithium atoms (labelled) that have made it into the detector.

To make all of this possible requires every component of the accelerator mass spectrometer to operate together within very tight tolerances. Even when everything is working well, each time a new sample wheel is introduced into the ion source the conditions inside the ion source vary slightly and the machine has to be very carefully tuned to these new conditions prior to commencing the measurement of the precious samples. Once satisfied the AMS is operating within the necessary limits each sample is measured until measurement statistics are met (usually within 8 measurements), or the sample is exhausted. Each individual measurement lasts about 6 minutes. For high precision measurements it is not unusual to measure a sample for a total of 1 to 1.5 hours. Thus to complete the measurement of a sample wheel is a multi-day undertaking during which the AMS has to be continually monitored for any changes in the condition of multiple machine components that could compromise measurement integrity.

The ¹⁴C/¹³C ratio for each measurement is derived from the counted ¹⁴C atoms in the sample divided by the number of ¹³C atoms calculated from the ¹³C current in the high energy Faraday cup. The final ¹⁴C/¹³C ratio for each sample is the average of the combined ¹⁴C/¹³C ratio measurement for the sample, normalised to known standard materials that were measured in the same wheel. It is this final ¹⁴C/¹³C ratio that is used to calculate the radiocarbon age for the sample.

I hope the above summary provides a little bit of insight into what is meant when you come across sentences like “…AMS ¹⁴C analysis were made at the SUERC AMS Facility” in the literature.

Derek

End of an Era ……….for the mighty British-Irish Ice sheet and our mammoth fieldwork campaign.

By Chris Clark (with photography by Alex Ingle)

Voyages around a former ice sheet…..

After a decade of dreaming and years of planning our team of 40 data-hungry geoscientists were given the scent and released from their cages (~desks) with the audacious task of blitzing the whole ice sheet to find samples for dating its retreat. This started in November 2012 in a grey drizzle at Seisdon sand and gravel quarry near Stourport and finished 09:30am 1st August 2015 in bright sunshine when we extracted our last sample, a seafloor core, from the Cleaver Bank in the southern North Sea. It really has been an epic two and half years witnessing the Terrestrial Team with sun-cream in the Scilly Isles to shivers in Shetland, and with dressing gowns in Donegal to JCBs in Norfolk. We really did covered the ground from south to north and east to west and snuck in 28 – yes 28 – different islands of Britain and Ireland, including Scilly Rock and Foula. When samples were not easy to spot and grab, we used radar, seismics and some occasional guesses to work out where to dig with shovel or digger or to core the hidden sediments. It is not quite true that no stone was left unturned, but I have been amazed at how close we got to that, thanks to some amazing levels of energy and motivation; it is indeed lucky that our team displayed traits of obsessiveness and kleptomania when it came to sampling. Bloody well done to all.

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So our very last sample (core 179-VC) on BRITICE-CHRONO has now been collected, marking the end of Cruise Two on RRS James Cook. Even though we never got to shout ‘One hundred and …eighty’ it is more than we had planned. We have sailed, steamed, or dieseled 8971.65 kilometres, taking in Skye, Rona, Shetland, and more North Sea banks including (the infamous Dogger) that you could shake a stick at. We have sampled deep (525 m) and very shallow (19 m), and calm and troubled (force 7). Our ship-track might look erratic to some but, as they say in marketing non-speak, it comprises a subtle blend of caution and well-planned targets with a hint of adventure and wild abandon yielding a truly inspiring collection of mud and sand to sate the yearnings of the most inquisitive discerners of ice sheet curios.

The loot under the care of Team Marine (Lou and Margot)

The haul, now sat in our refrigerated lorry-container and packed in plastic tubes was obtained by lowering our vibro- and piston corers through 18,891.4 metres of seawater and extracting over half a kilometre of sediment (Rich says 542.4 m). As well-known, of course, it is not the length that counts, but the quality. It will be some time however before we know which cores, places and transects yield the best shells and forams for dating, but Margot and Lou have already bagged, sifted and labelled the celebrity shells which we think have the best stories to tell….’well there was this bloomin’ huge great wall of ice that kept crashing down, and would you believe what happened next….’.

Science crew of the RRS James Cook cruise JC123

Thanks to Colm and his science team, the Captain and crew and the geological survey coring teams, and the weather, some good planning, crazy hunches and some luck, this scientific cruise has been a great and enjoyable success. We have a mammoth payload that we hope will provide a legacy of new information for decades. It has been a pleasure having Alex, the ever-present black ninja-photographer on-board, – he stalks, clicks and then runs – in his quest to document our highs, lows and silly moments. Hopefully you have already seen much of his work.

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We set out to do 50 years work in five. Taking this cruise with last year’s, which circumnavigated Ireland, along with our >300 person-days of terrestrial fieldwork we have bagged around 15 tonnes of samples for dating and I hope you agree that we have been around a bit. Sorry if we missed your patch, why don’t you have a go? It is an end of an era for our sampling effort. As project leader, I now breathe a large sigh of relief that it is over and has gone so well, phew and phew again. There is a tinge of sadness though, that we all feel as the fun, bonhomie and making of new friends on hard-won field exploits has now ended. No more pie shops or sneaky pints. Team Terrestrial (Rich and his gang) and Colm’s Marine Crew, can now stand-down to great applause. Derek’s Geochron Team have their work cut out to carefully analyse all the samples and then our Transect Leaders (Tom, Dave, Rich, James, Colm, and Sara) will rise to the challenge of making sense of it all and telling us the story that the shell started to blurt out.

Taking things one day at a time

Chris Clark, signing off on behalf of BRITICE-CHRONO, currently steaming 11 knots, homeward bound, over the Tea Kettle Bank of the southern North Sea. All cores logged and packed and the pinging geophysics finally turned off.

A Perfect Core……..

By Margot Saher, Dave Roberts and Rich Chiverrell (Photography by Alex Ingle)

Darkness. A great mass of ice overhead. The eerie rumbling of a large, uncompromising mass, slowly but steadily on the move. Below a thick layer of stiff red sediment, ground off the red bedrock, crushed and churned into a lumpy, sticky blanket of glacial till.

Dark coasts

What would later be called Cape Wrath was only miles to the south, but there was no cape yet. Just the grinding of slow and unforgiving ice moving north into the North Atlantic. But the times were changing. The sun gained in strength, atmosphere and ocean started to warm and the gigantic ice mass, later to be known as the British-Irish Ice Sheet, was in decline. As its surface melted, more water reached its bed, and it began to slide helplessly over its own sediments. Slowly it thinned, and retreated in the direction of the Scottish mountains with the ocean lapping relentlessly at its edges.

There seemed to be no hope, but the ice sheet made one last bold dash towards the edge of the continental shelf before it faltered. The recently deglaciated seabed and freshly deposited grey ocean sediments were bulldozed and overrun again by ice on the move, and buried once more in a blanket of red till. Linear ridges (moraines) marked the limit of this temporary re-advance. But it was only a death throw; the re-advance didn’t get far. The ice sheet’s days were numbered. The advance stopped, and turned into irreversible retreat.

A geophysical search for the perfect core…….

Against a backdrop of rumbling, calving icebergs, station JC123-048VC slowly became ice free, as the snout of the ice sheet moved back over the site. A cold, shallow sea took its place; first, still close to the snout of the ice sheet, where streams of meltwater rushing into the waiting sea water lay down a blanket of coarse sand. As the ice retreated further, taking the meltwater streams with it, the sea fell silent. Only fine sediments spat out into suspension by the dying ice sheet made it to our site, slowly covering it in a thick, grey blanket.

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The ice sheet sent a final message as the ice margins retreated south towards the land; a message from an iceberg. As it passed, melting, overhead of station JC123-048VC, pebbles slipped from its icy grip. They plummeted into the depths, impacting into the soft fine clay sea bed. As soon as this excitement started it was over, and the pebbles were slowly covered by more of the same grey clay.

With the great weight of the ice gone, the Earth’s crust rose like an ancient giant from its slumbers, pushing the Scottish continental shelf closer to the sea surface. Over time, the waters shallowed, and the seabed currents became stronger. The last vestiges of the glacial seafloor were scoured by contour currents, which deposited the spoils of an energetic coast on the eroded sediment below. Millennia later coarse sand and shell debris formed a layer of several inches thick. And then on Sunday the 12th July 2015 all changed.

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There was an unfamiliar thud, and then the uncanny sensation of a vibrating tube burrowing into the sediment from above. It cut through the sand in a jiffy, passed the pebbles, and into the soft clays. The tube slid through it like a hot knife through butter. No struggle with the coarse sands lain down by meltwater streams either, only slowing on reaching the stiff, red till. It battled its way into it for a meter and a half. Then the friction became too much. The vibrocorer stopped, and then the whole tube, now full of sediment, was pulled back up to the sea surface, and hoisted back up onto the deck of the RRS James Cook, the ship it had come from. Peace returned once again on to the sea floor, at core site VC123-048VC, a few miles north of Cape Wrath, on the northwestern edge of Scotland; a land mass now devoid of ice sheets and glaciers.

The core came on board and was cut into sections, labelled, scanned, and split. Finally, we, the scientists who had planned the project, planned the cruise, sailed all the way from Southampton to Cape Wrath, and waited for the British Geological Survey (BGS) to deliver the core, first laid eyes on the sediment. The story was there: a stiff basal till deposited beneath the ice sheet; fines marking the first incursion of the sea; further glacial till documenting the ice re-advance, meltwater stream sediments deposited in front of the retreating ice margin; the fine clays deposited when the ice began to recede southwards containing drop-stones from the icebergs, and the marine sand of the modern seafloor. That was what we had come for. And this was the 48th core; none of the previous 47 had told the story of the vanishing British ice quite this clearly.

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Hopefully we’ll be getting more cores like this in the coming three weeks of the cruise. We need this story told in every sector of the British-Irish continental shelf. Only then will we have what we set out for: the complete saga of the Last British-Irish Ice Sheet.

1 o’clock, 2 o’clock, 3 o’clock rock (well…compressed mud), you can rock around the clock….

By Margot Saher and Lou Callard

Take some of the ‘finest’ brains in the country. Put them on a state-of-the-art research vessel which is filled to the brim with geophysical equipment, and has its own core scanning lab. Imagine what one could do with that! And what do we do with it? What is the scientific treasure we hunt? Mud. Six weeks at sea for mud (occasionally sand…)? We’ll be scrutinising it for years to come! Without mud the whole endeavour would be a failure; the mud must be treasured, cared for. It is the sedimentary archive that could answer the question of when and how the British-Irish Ice Sheet vanished. It is the wet lab coring teams that handle, care for and love the mud. There are two shifts: the Night-watch from midnight to noon, and the Day-team from noon to midnight. Whilst in the lab and, more importantly, out on deck, these have to wear armour: PPE (Personnel Protective Equipment) – a hard hat, hobnailed boots and some rather unflattering (generally oversized) overalls. The overalls are optional, but a sensible option at that; the job is a messy one, so unless you have an endless supply of clothes…. As the British Geological Survey (BGS) core team recover the mud to deck we have to wait – impatiently. How much have we recovered, and is it the right stuff? Even before the barrel is laid down we swarm expectantly around its end to get the first glimpse (and touch) of the treasure. The strength of the mud gives us so much information; we prod it, taste it…. Does it feel like silt, sand, clay; is it stiffened, reflecting the weight of former ice sheet bearing down on it? We recover everything from the core shoe, the core catcher; whatever sticks or falls out of the liner gets bagged, labelled, photographed and stored cool. But what is inside the liner is what we really want, it contains the story of the ice coming and going from the waters around these islands.

Heave

Ho

The liners are not easily released from the barrel; muscles are needed to get it out, and a tug-of-war ensues of scientists, BGS engineers, crew, random passers-by, anyone versus the barrel. But once the liner is out, it’s ours. The muds we desire are only useful if we know exactly where they are from, so labelling is everything. Every single core section has its own unique label, which will end up on its liner, caps, wrapping material, and the box it’s stored in. There are yellow caps for the tops and black for the base of each segment; which way is up matters! And that is only the beginning; there is no such thing as over-labelling, and that holds for cores sections, record sheets, scanned records, spreadsheets, photographs……

Lou: “The day shift consists of Steve, Zoe, Catriona, Kevin and me. Whilst Colm and Katrien spend the day planning where we will core next, we collect and process the cores. Generally our day starts at 11:20 with breakfast, which also happens to be lunch for the other crew members. Breakfast can be anything from a curry to fish and chips. Today’s option was Thai fish cakes, with noodles and sweet chilli sauce. Although having such a large meal first thing was rather odd to begin with, six weeks in it seems quite normal and a bowl of cereal would now disappoint. Shift begins with the midday handover meeting and our goodnights to the night team.

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Then work begins! We spend most of our shift either out on deck collecting and cutting the cores, or in the wet lab splitting, logging and packing the cores away with a constant dual stream of material either from the deep or from the MSCL cave. Frequently we split over 20 sections during our shift, and often it’s more than 25. Each section is 1 m long with each 1 m weighing between 10-12 kg, so after 12 hours of carrying, splitting, logging and packing it is a good workout. So the coring job may seem rather unglamorous and exceptionally mucky, and involves hard manual labour; it is also an exciting and rewarding part of the cruise. The sub-bottom profiles and bathymetry data provides a tantalising look at what might lie beneath, but it is only when the core is taken and the material viewed that you know whether or not we have captured the right material needed for the project, and whether there is something we can use for dating. Sometimes we are disappointed normally if we fail to guess correctly in the ‘guess-the-core-length’ sweepstake (Steve is slightly in the lead at the moment), but when a good core is opened, it changes the mood of everyone involved.

Our work still isn’t quite complete, cleaning and maintaining the lab ready for the night team, labelling, cropping and archiving all photographs, and Zoe dutifully scans all of the deck sheets. At midnight the night team relieves us and then we head either straight to bed or take a detour past the kitchen to get a post shift snack. A day shift favourite is Nutella (somewhere in the multi-verse other nutty spreads might exist) on toast. It is hungry work, coring!”

Margot: “the Nightwatch consists of Kasper, Riccardo, Jenny, myself and occasionally Richard (if he can drag himself away from the Geophysics, picking core sites and mostly chatting on deck). As we start our shift, we tend to find ourselves in the middle of a coring transect that has been planned before, so we of the night often start our shift on station, vibrocoring. We’ve discovered that Riccardo has a special talent for working hard but still staying clean, while Richard has the useful talent for removing almost any sediment from an unwilling core catcher. Kasper’s Danish (or Viking) muscles come in handy for removing the liner from the barrel, and Jenny has useful BGS contacts (which saves us, for instance, from running out of sample bags). I myself have developed the modest knack of writing upside down, for liner labelling purposes.

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Splitting and description has to wait until we receive cores from Elke and the core scanner cave, and she works 6AM to 6PM. The early part of the shift can be quite calm, if the core stations we have picked are far apart. Quite often nature festoons such a quiet early part of the shift with an amazing sunrise. The pace picks up dramatically as the cores start to emerge from the MSCL cave. It can get a bit hectic if we’re busily splitting and describing, interrupted by receiving new cores from the seabed. Core sections everywhere! But the splitting is exciting, as we get to see the whole sedimentary sequence for the first time. Do we have the ideal core, which consists of a subglacial till at the bottom, conformably overlain by marine sediments deposited after the ice retreated? Are there perhaps some nice shells in it for radiocarbon dating? When we see something we could use for 14C dating, we take it out. As we are coring, we have a competition running: guess the core length. It is very tight at the moment; Riccardo is in the lead, closely followed by Jenny, and Richard is trailing miles behind, but it all can still change, even with only two days to go. Eventually noon arrives, when we hand over to the day team, and then plonk down tiredly for lunch, which, for us, is more like late supper. After lunch and a cup of tea it’s bedtime! And then at around 11PM (ish) we get up again, and the sequence repeats.”

Treasure…..

As we both write this, the 212th core has been recovered from the large moraine in outer Galway Bay. There is some 6 tonnes of mostly mud in our refrigerated container, and we have picked more than 100 shells for dating. But we know exactly where every kilo came from, what it looks like, and which ones we want to target for further research. When we get back on land, we can hit the ground running…..

Photography mostly by Alex Ingle, except where it isn’t….

Sun setting on the Celtic Sea and B-C Transect 4

By James Scourse

A wonderful place…….

The first of the BRITICE-CHRONO marine transects (transect 4, Celtic Sea) was completed late on Saturday evening. It has been hugely successful – the result of unbelievably excellent weather and sea state, detailed planning and effective delivery by a great team. This has been a controversial and enigmatic part of the British-Irish Ice Sheet for decades with generations of Quaternary geologists attempting to reconstruct glacial events from meagre and sporadic sequences. It was the focus of my PhD back in the early 80’s. A lot of this was spent onshore on the Scillies where the evidence suggested that the Late Devensian maximum advance straddled the northern islands – a conclusion that caused me not inconsiderable grief at the time because large and influential parts of the UK Quaternary community could not accept that the last ice sheet reached this far south. Subsequent work with colleagues using new techniques has supported this original interpretation. I also analysed a series of 12 or so BGS vibrocore samples recovered in the 70’s from the central and southwestern Celtic Sea containing “glacigenic” facies. A northern suite resembling the Scilly Till I interpreted as basal till facies, whereas a southern group – containing spectacular microfossil assemblages – appeared to be glacimarine. On the basis of this available evidence I suggested a mid-shelf grounding line and marine terminus to the Irish Sea Ice Stream. I was unable to explain the origin of some apparently “basal” type diamictons very close to the shelf break; they might possibly be iceberg turbates. More recently I suggested – with additional information from palaeotidal simulations – that the huge Celtic Sea linear ridge bedforms are tidal features reworking the sediments of the terminal ice stream and the Channel River.

Then, starting in the late 2000’s, I became aware that Daniel Praeg from Italy and Steve McCarron from Ireland had become interested in these ridges and were suggesting in conference presentations (e.g. INQUA 2011) that the ridges might actually be subglacial “ giant eskerine” bedforms which, if it were true, would mean that the ice sheet reached right to the shelf break. In Daniel’s model the shelf break diamictons are just that – evidence for shelf edge glaciation. One of the original BGS cores – site 44 – recovered till from the flank of a sand ridge which might suggest that the ridges at least partly pre-dated the glacial event; Daniel, following Pantin & Evans (1984) suggested that the ridges have a carapace of glacigenic sediment and were therefore overridden by ice. But, alternatively, do the glacigenic sediments dive through and under the ridges? A major unanswered question was/is the stratigraphic relationship of the glacigenic sediments to the ridges. There was something faintly ironic in all this: I’d had a lot of grief having argued for an advanced southerly position for the ice sheet, and now here was another team arguing for an even more spectacularly extended southerly limit.

Daniel, with great persistence and motivation, has organised a series of geophysical and coring campaigns with Italian, Irish and BGS colleagues – the last in February-March this year – to attempt to resolve the two models. Spectacularly their last cruise recovered overconsolidated diamicton and normally consolidated glacimarine sediments close to the shelf edge at the southern end of Cockburn Bank (for further details). I won’t steal their thunder because their work is being prepared for publication, but it is fascinating and has injected energy into our researches in this area. Daniel and Steve and colleagues Dayton Dove and my former research student Gill Scott, are now working alongside BRITICE-CHRONO colleagues to help address these questions. Having Daniel as a participant on this James Cook cruise has been a delight and the two hypotheses have been constructively batted to and fro, day and night, with lots of jocular repartee on the nature of things emerging on the sub-bottom profiler; “that’s clearly a buried drumlin”, “no, it’s a proto tidal sand ridge” etc etc.! Were that all scientific controversies were discussed in such a friendly, stimulating and constructive way.

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So, what have we found? Searching for glacigenic sediments in this area is like looking for a needle in a haystick, so aggressive was the subsequent transgressive episode. Much of the sedimentary evidence has simply been eroded (incorporated into sand ridges??!) or buried. The BGS only found glacigenic sediments in 12 cores of the hundreds that were taken. Well, about a third of all our coring deployments recovered glacial or glacimarine sediments, from sites extending from the shelf edge to the Celtic Deep, a total of 14 vibrocores and 5 piston cores. This success is a testament to painstaking preparation – including a reconnaissance geophysical cruise – led by Katrien Van Landeghem, Sara Benetti, Lou Callard and colleagues – so that our targets were well defined. Excellent onboard sub-bottom data has also been crucial, pored over night and day by Daniel, Katrien, Colm, Richard and myself, and the expertise of the BGS and NOC coring teams. There is no doubt that these samples and their contextual geophysical data will transform our understanding of the LGM in the Celtic Sea, a topic that continues to fascinate, bemuse and, occasionally, infuriate. One of our key targets, site 44, stubbornly refused to yield anything but sand – dubbed the “sands of woe” by Lou Callard – that left Daniel, head in hands, muttering “Oh bloody, bloody, hell”!

What about the two hypotheses…well, I already have some modified interpretations emerging – new working hypotheses if you like – but I’m not going to be pushed on these until we have the data analysed from the cores. Having said that, I think Daniel might be partly right and partly wrong, and that I, too, might have been partly right and partly wrong. Such is science!

Cruise 1: Days 1-6 trials, tribulations and triumphs

By Rich Chiverrell and co from the edge of the shelf

Developed as a concept 3-4 years ago, and planned over the last 2 years with massive input from across the Britice-Chrono team and Colm Ó Cofaigh (Marine Theme Leader), on Monday 14th July it finally began to happen, Cruise 1 (JC106) of the NERC Consortium Project Britice-Chrono. The vessel, the RRS James Cook was waiting for us moored at the wharfside of the National Oceanography Centre (NOC) in Southampton as the various team members mobilised. For me this would be a first, after running the terrestrial field programme for the past 18 months, and now for something completely different – having visited numerous boulders, quarries and cliff sections, the chance to see and sample the extensive offshore sediment and landform record of the decline/collapse for the former British-Irish Ice Sheet (BIIS).

The leaving of Southampton

Individual preparations for an undertaking like this began months ago; spending 3-6 weeks living offshore on a state-of-the-art research vessel does not happen overnight. In June marine survey or personal survival training qualifications were needed. This involved 7/8 hours of training and tests at the Fleetwood Nautical Campus, which covered survival equipment, how to abandon ship from a 5m platform and in the appropriate survival gear (immersion suites, life jackets, entering life-rafts, management of life rafts, individual and group mobility in the water). All this in a state-of-the-art 8-9m deep wave simulator, where for the finale we abandoned ship from 5m, in the dark, smoke everywhere, rainfall and spray, into a wave churning pool, after 5-10 mins in the life-raft mal-de-mer was looming! Medical certification testifying fitness to work was also needed. And then on to other tasks, helping with permissions for geophysical survey and coring in Irish, English, Welsh, Manx and Scottish waters all were required; remarkably all interpret EU law differently and this resulted in a major undertaking for Colm Ó Cofaigh amongst many others.

It was with a little trepidation and expectation that I first visited the RRS James Cook on Monday to drop of personal luggage and cameras (on board duties for me included amateur film maker and outreach obsessive). First impressions, a big and well equipped ship, and my cabin is more spacious than I expected. Second impression there was still a great deal to do before our scheduled departure noon Tuesday 15th July, with an impressive set of additional equipment being loaded as I arrived; the British Geological Survey waited on replacement cranes to load the 6m vibro-corer. The NOC 12m length piston corer was also working its way on board. A freight container that housed the University of Leicester Multi-Sensor Core Logger was also being loaded. The science team added research consumables to allow the sampling of ~200 core profiles and then on Tuesday ourselves for familiarisation and preparations to depart. It perhaps was more of a surprise than it should have been when our eventually departure lunchtime on Friday came around, because mobilising this scale of operation is challenging, and we did have a few issues with the vibrocorer that the BGS team worked largely round the clock to fix including extra and replacement equipment from Edinburgh. The wider BGS team got to know the route from Southampton – Edinburgh well…..

After some final repairs and tests of the vibrocorer and we met our Friday departure time, and headed for a date with the English Channel and a test location identified to the south of the Isle of Wight. Casting off and the journey to the Solent was in calm seas and glorious sunshine, within 3-4 hours we reached the test area, and the BGS and JC106 Science teams readied themselves for the fray. Using the RRS James Cook’s dynamic positioning system the crew manoeuvred the ship into position in 30-40m of water. The BGS team, thoroughly checking the physical operation of the equipment, lowered, sampled and recovered a vibrocore. The equipment was functioning fine, we were ready for the Celtic Sea and the Science team had materials on which to practice our procedures e.g. core cutting, splitting and description.

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On finishing the test core location, the geophysical equipment was powered up, so that the watch teams could gain experience and practice the 24 hr geophysics monitoring duties. My first watch was 12 midnight through to 4am! Actually pretty good fun, not sure how quickly my body clock will adjust to a warmed up dinner for breakfast, breakfast for lunch and lunch for dinner, and for that matter sleeping 4pm til 11pm……

Saturday through to Sunday was spent in transit; the shelf edge of the Celtic Sea is quite a long way ~ 36 hours at ~10 knots. In the mid-morning our safety skills were put through the paces, with a muster drill, the alarms sound we secure warm clothing and life jackets and convene in the muster point, from which we are led to and board the two life boats. Very spacious, well kind of, the each can take ~50 people and we are a crew of around 50. You can imagine they would get very warm and pretty unpleasant if full of people for a long time.

On waking 11pm after not much sleep, still adjusting to the new life cycle… Taking over from the end of my day watch partner, Catriona, my watch was good fun acquiring the data for the next 5-6 hours involving scouting for core sites as we began our target geophysics transect on the shelf edge, with some success finding some promising targets in between problems with the Sub-Bottom Profiler. Riccardo and then Kasper followed, with ever present input from night coring lead Sara, night geophysics lead Fabio and Margot. On this watch we completed an acoustic velocity profile as a calibration for the multibeam survey systems, and worked the geophysics transect. Once complete we arrived at the destination for our shelf edge piston core. The piston coring team from NOC made quick work of the 459 meter, recovering ~ 4m (JC106-002PC). Fabio and the RRS James Cook computation team carried out a further calibration of the multibeam survey using the sea floor topography. The core awaits acclimatising to the MSCL container and whole core analysis of the physical properties with Elke.

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At noon, the baton passed to day watch (Katrien, Lou, James, Catriona, Daniel, Zoe) and the challenges of obtaining five vibrocores along the geophysics transect in search of that Holy Grail, a contact between subglacial diamicts and glaciomarine deposits in 280m of water. JC106-003VC, the first stop, was on the flank of the western side of Little Sole Bank near an earlier BGS vibrocore. The materials were very tough penetrating ~1.6m, with a much consolidated stony diamict at the base; admittedly a little/lot early to say it looks a lot like what we were hoping for, and there are four more sites to follow, but….

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Minching about on a sunny Isle of Lewis

By Rich Chiverrell

Port Skigersta delta

One of the smaller ice-masses draining the former British-Irish Ice Sheet, the Minch palaeo ice stream drained much of the NW sector of the British–Irish ice sheet (∼15,000 km2) feeding sediments to the large Sula Sgeir fan fronting the continental shelf. But if this is small, standing on the east coast of Lewis (Outer Hebrides) looking across the sunlit, blue seas and skies east to the feeder fjords and mountains of the Summer Isles and Wester Ross helps one visualise how large this former ice sheet really was. Our aim for this ongoing Briticechrono Transect 8 fieldwork was to secure a series of targets for Optically Stimulated Luminescence (OSL) dating from outwash sands from Lewis, one of the outermost land-masses on the western flank of the Minch Ice Stream. This work will support previous sampling efforts targeting boulders on the Scottish Mainland and the Hebrides for cosmogenic nuclide dating. Previous OSL sampling had targeted the inner sector of the Minch on Skye and north of Ullapool. The team (Rich Chiverrell, Matt Burke, OSL Postdocs Rachel Smedley and Alicia Medialdea) set off first thing on Tuesday morning to join Transect Leader (Tom Bradwell) on Lewis. Departing a cloudy Manchester via Glasgow Airport we landed before lunchtime to blue skies, sunshine and searing temperatures at Stornoway Airport.

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Inspired by our surroundings, the weather and the prospect of excellent sediments the four newcomers sped off to meet up with advance team Tom Bradwell, Adrian Hall and Maarten Krabbendam (from the Netherlands) at Port Skigersta in the far north of Lewis. The site an embayment on the western flank of the Minch Ice Stream gave stunning views across the water back to the ice source areas in western Scotland and beautiful turquoise seas. The sediments were very impressive with the sequence a stacked delta sequence with steeply dipping fore-set sands capped by top-set horizontally stratified gravel. Intriguingly the basal delta is buried by laminated bottom-set muds, in turn buried by a second delta fore-set and top-set couplet. The repeating delta suggests changes in water level probably lake level, dammed between the ice stream and bedrock rise into Lewis. We sampled both deltas close to the fore-set – top-set contact. And then for some geological tourism, the raised beach at Galsom guided by Adrian Hall, stunning and confusing sediments, all contributing to produce a plethora of hypotheses. Difficult to address under the banner of Briticechrono, the beach deposits (guess the isotope stage) appear altered by over-ride by ice, locally there is a surface diamicton and the beach pea/rounded gravels are probably thrust or stacked. We have targeted an outwash (ish!) deposit above a glacial diamicton, fingers-crossed for contributing to the debate. Excellent food followed at the Cabarfeidh Hotel our home for the next few days (well some of us!).

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Day 2, still warm, still still (no winds) and the sunshine popped in later! After an epic breakfast (Lewis did us proud) back to the north and just south of the Butt of Lewis the west coast Swainbost Sands offered much promise. The sections were epic more glaciotectonics, tills, shells than you can shake a stick at, and the beach!!!! One of the best beaches I have seen in the British – Irish Isles…. Selecting targets was challenging, much of the outwash deposit was rich with shells, thrust, tectonised and not where it was deposited! How? Well by marginal movements and override by ice and at a substantial scale. Three sample locations were found and in the back, along with crucial in gamma detector comparisons, duplication with different detectors at some of the samples. The sampling completed our targets after ~36 hours on the islands, and so we racked our brains for other targets. After a quick visit ~5/6km south down the west coast of Lewis where we encountered convincing striae in steeply foliated Lewisian gneiss, where the glacial lineation trends cross obliquely the metamorphic structure heading northwest. We also prospected for sites further up-ice around Stornoway; another fine meal at the hotel and some gin-assisted colour-by-numbers approaches to former ice geometry and let’s see what tomorrow brings for our last 3-4 hours on this eye-opening island (hopefully a final sample)….

BRITICE-CHRONO: sampling bedrock in Cheshire/Shropshire for CN dating

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By Matthew Burke and Richard Chiverrell

Taking advantage of the improving weather and as the snows subside Rich Chiverrell and Matt Burke were joined by Derek Fabel and David Small (University of Glasgow) in getting the terrestrial sampling programme underway for BRITICE-CHRONO. The project a NERC-funded consortium that aims to constrain the rate of collapse of the last British-Irish Ice Sheet. The project will employ a number of dating techniques – Optically Stimulated Luminescence (OSL), Radiocarbon (¹⁴C), and Terrestrial Cosmogenic Nuclide (TCN) dating – in order to document the retreat of all major ice streams that drained the largely marine-based ice sheet. Terrestrial sampling along the Irish Sea East transect is now well underway after three days of intensive rock removal along the Mid-Cheshire Ridge for TCN dating. Although renowned for its flatness, the rolling farmland of Cheshire/Shropshire is broken by the enormous Mid-Cheshire ridge that reaches a whopping 227 m asl and is, conveniently, aligned roughly perpendicular to the retreating ice margin. As the ridge has been eroded by overriding ice and is cut by deeply incised meltwater channels it presents a great opportunity for us glacial geologists to do our thing: clamber around, stare at, chisel away at, and rest upon various rocks!

Field Personnel: Derek Fabel (rock sculpting), David Small (rock sculpting & note taker), Matt Burke (dogsbody), Rich Chiverrell (Driver & lookout), Geoff Thomas (Local geological expert), Pat Alexander (Local gastronomic expert)

DAY 1: Northern sector

Whilst Rich was enthusing year 2 undergraduates about the joys of glacial geomorphology, Matt, David and Derek began the day by braving the wilds of the Wirral with a stroll around Thurstaston Common in order to take a look at the ‘controversial’ Thor’s rock: a site of much debate as to the origin of the numerous erosion marks that cover its surfaces. Some believe the marks record scalloping by glacial meltwater, a plausible argument given the rock sits within a meltwater channel, yet the obvious steps and chutes down its flanks are probably testament to the alternate hypothesis that these marks simply record 100+ years of children climbing over and sliding down the rock!

Meltwater or scallies: which was the greater erosive agent at Thor’s Rock?

After careful consideration we decided to sit on the fence, concluding that many of the marks were originally scoured by meltwater, but have now been enhanced by the locals (us included) and so we decided to sample at a less controversial site: Thurstaston Hill. From here the team drove a couple of miles to Barnston Dale where we were joined by Rich, resisted the temptation of a 10 am pick-me-up at the Fox and Hounds, and hacked a piece of rock from the Barnston meltwater channel.Next stop Urchin’s  Kitchen at Primrose Hill Wood. Following what seems to be a tradition for this transect, Rich and  David did an excellent job of navigation (Rich: “we don’t need a map to navigate…”).

C3W film crew capturing Derek’s activities

Although  we did eventually arrive at the site, we seemed to take the scenic route as we zigzagged our way across the countryside, which was all part of the plan, of course. On site we met up with Saskia Pagella and Vince Jones from C3W who filmed the sampling procedure and interviewed Rich and Derek with the eerie backdrop of the Urchin’s Kitchen.

Unfortunately, although Derek gave us a great recital of a poem commemorating the discoverer of cosmic rays, it was not captured on film and it  was a one-off performance. Lesson learned; always have a video camera at the ready in case of any future Britice-Chrono poetry recitals….. After a rather late lunch at Delamere Forest, the team finished off northern sector sampling by bagging (literally) rock at Manley Knoll and Helsby Channel. All in all, a very successful day of sampling, but could we cope without Derek on day 2?

DAY 2: Central sector

Afraid to look down: David measures the shielding at Raw Head.

As Derek whizzed off to Sheffield to give a talk, Rich, David and Matt were left to pick up the pieces and continue with another round of TCN sampling. This time the central part of the ridge was the target, where surprisingly little bedrock was exposed. However, after several hours of rock hunting, David stepped into Derek’s shoes admirably and showed his skills at Raw Head and Bickerton Hill. Raw Head proved particularly challenging (not including having to avoid the local fox hunt on route) given David and Matt had to clamber atop a block that had slumped from the main outcrop and was hanging precariously above the valley below. After a long and cold morning of walking the ridge, we were met for lunch by Geoff Thomas at the aptly-named pub “The Sandstone”. Along with a rather large lunch, Rich crumbled under the slightest of peer-pressure to enjoy a pint of “Scrum  Down” (BRITICE-CHRONO approved beer 1) with the rest of us.Luckily, after lunch the fully-awake and motivated team were able to expend their excess energy with a couple of steep hikes. The first proved fruitless as we discovered on arrival at the outcrop that it had been heavily quarried. Thankfully, the second hike to the head of the very deeply incised meltwater channel that is Peckforton Gap produced something we could sample. After another successful day, Matt was finally able to break the habit and navigate us home without any major detours.

Excellent lunch stop

Deeply incised meltwater channel at Hawkstone Park.

Cautious bedrock sampling at Hawkstone Park.

DAY 3: Southern sector

Ahead only lowland Shropshire and the Severn basin

Day 3 began with a long drive to the most southern extent of the ridge in Shropshire where the team were given unrestricted and free access to Hawkstone Park, despite it being closed to the public. After Geoff successfully found his way to the chute-like meltwater channels, the question was asked; have these actually been eroded by meltwater or were they excavated to produce nice walking routes for all those Victorian tourists? After careful consideration, we decided they were in fact meltwater channels, but unfortunately their steep walls and the resulting shielding meant TCN sampling was not possible. Instead, more precarious sites were chosen right at the top, and at the edge, of the escarpment. With bushes on one side and the prospect of quite drop a long fall on the other, Derek and David held their nerves to bag another two good samples. No fieldwork thus far appears complete without without a visit to the pub, the team stopped for another well-deserved lunch at “The Inn at Grinshill” where, alongside very nice fish and chips, a further sample was taken (“Six Nations” –BRITICE-CHRONO approved beer 2). After lunch the sun was shining and we were able to bag the final sample of the day (and the trip) from The Cliff at Corbett Wood. This proved to be a fitting conclusion to the trip: all TCN samples for the transect have now been collected, and the final site at the most southern tip of the ridge overlooked the outwash plains that will be now targeted for OSL dating over the coming weeks…

T3 sampling sites

Rapid retreat of the Irish Sea Ice Stream – just out in the Journal of Quaternary Science

A new paper has just been published by Richard Chiverrell and a hefty team of Britice-Chrono co-workers (James Scourse, Katrien van Landeghem, Chris Clark, Colm O Cofaigh, Dave Evans, Danny Mccarroll, Colin Ballantyne) presenting the first Bayesian integration and modelling of all the dating control for the marine sectors of the largest ice stream that the last British-Irish Ice Sheet ~ 24,000 years ago. The modelling shows very rapid retreat for this marine-terminating ice stream over greater distances (650 km) and timescales (8000 years) than is available from short term (decadal) observations of present day ice stream margins. The modelling shows this retreat 24,000 years ago was rapid and linked with climatic warming, sea-level rise, mega-tidal amplitudes and reactivation of meridional circulation in the North Atlantic. But, significantly the pattern of retreat appears uneven with a pulsed pattern of retreat attributed to the passage of the ice stream between normal (sloping away from the ice margin) and adverse (sloping towards) ice bed gradients and changes in the geometry or marginal constriction of the ice stream. To read more click here.

The methodology and application kind of formed an important test case for Britice-Chrono as we attempt to constrain rates of and controls on marine ice stream retreat over millennial timescales for eight ice stream radiating out from the last British-Irish Ice Sheet. The methodology outlined in the paper will underpin and be used as a guide for our data collection for the wider British-Irish Ice Sheet. It would be quite good fun to play around with some of the available chronology for other ice streams…..

For news and updates on Britice-Chrono see our Twitter site and everyone please get tweeting or twittering!.

ice flowing from off to onshore on west coast of Ireland?

This is a recurrent theme. Was there enough nourishment of ice on the shelf such that flow could be directed shorewards? I have come across this a number of times and have wondered.

James raised it recently in an email and with a Colm reply, which i reproduce below;

”On another but related issue, Roland Gehrels was also at the meeting. He interestingly mentioned that years ago, when he was a student in Maine, Harold Borns gave a seminar on ice flow directions in Connemara, suggesting that the striae had been misinterpreted and that the ice was flowing from the shelf onto land. I’ve found an abstract on his website:

Borns H.W., Jr., and Warren, W.P. , 1990, New Evidence for Ice-Flow Directions over Connemara, Ireland (Abst.) Abstract Volume of †Connemara, 1990″ conf., Unin. College, Galway, Ireland.

This may well be a load of bollocks but I thought it might be worth mentioning. I’d be interested in Colm’s response to this.

Cheers,

James”

Hi James,
Pretty grey literature this as the conference that the abstract relates to is not properly listed! I have had a quick search but did not find it. I see Willie Warren is the co-author. I remember being at the Drumlin Symposium in Coleraine in 1992 and Willie giving a talk which mentioned this idea of ice flow out of clew bay and on shore – so that may be what the abstract is referring to. It did not go down well with the audience as I remember although admittedely it is a long time ago now. I have seen nothing further to the south of Clew Bay in Connemara or Clare that shows ice flow onshore but of course that is not to say it is not possible.
Colm.

I just went trawling through Sarah Greenwood’s thesis and found some more support by Warren and also by Jordan. see below (Sarah maybe you want to comment some more, I seem to remember us puzzling over some flow sets?);
”Warren’s reprisal (1992; Warren and Ashley, 1994) of Close’s ice sheet model (Figure 2.1c)
goes further than simply suggesting Connemara was a primary ice centre, but rather depicts an
extensive Connemara ice divide stretching from the Slieve Aughtys, across Connemara, Clew
Bay and into the Nephin Beg range in North Mayo. Under this model, Warren describes
onshore ice movement through Clew Bay, an idea which has received some limited support
from Jordan (2002).” p13-14

This keeps coming up and if I add it to Sara Benneti’s comments about glacial evidence way out on the Porcubine sea bight, it all sounds rather interesting. I think we should keep an open mind and also take some active looking

Chris