Guide for the Collection And Analysis Of ITASE Snow And Firn Samples
By: Mark Twickler
and Sallie Whitlow
Climate Change Research Center
University of New Hampshire
Durham, New Hampshire USA
In order to assure collection of uncontaminated snow and firn samples from Antarctica it is necessary to follow strict procedures. The following discussion based on the procedures used by the Climate Change Research Center (CCRC) to assure high quality uncontaminated samples, should be used as a guide for ITASE traverses. One of the primary ITASE measurements, major ion chemistry, is used as a base for discussions in this document. Other primary measurements (stable isotopes, microparticles and other chemical measurements) will have similar collection procedures but detailed laboratory analysis will vary from what is described here for major ion chemistry.
Sampling resolution for the major ion chemistry and any annual layer counting method should be sub-annual (8 to 10 samples per year) in order to allow counting. However, at low accumulation sites sub-annual (more than 8 samples per year) resolution may not be possible. In this case sample resolution should be as detailed as possible provided the chosen sample length will generate enough melted sample volume for the desired analysis. This will vary with each individual lab, typically CCRC uses 4-8 ml for major anions and cations. Major ion chemistry and stable isotope chemistry sampling requires discrete samples, which are continuous down the core. Annual layer counting is also possible using a "continuous melter" if the appropriate chemistry can be analysed using a detector that can accommodate continuous sample input . At each site discrete spatial sampling of each chemical species should be made to determine whether they are suitable annual layer indicators.
A spatially comprehensive surface snow sampling programme is required to enable the spatial variability and seasonality of stable isotope and chemical species to be determined. Surface snow sampling should be conducted on linear, grid or radial patterns at intervals of 5 to 20 km from a prospective deep drill site or at ~100 km intervals along traverse routes.
Thorough planning and reconnaissance surveys are required in conjunction with ITASE shallow ice core drilling projects, to ensure that the most regionally significant and highest resolution proxy climate and environmental records are obtained.
Locally, snow pits
and ice core drilling sites should be selected on the basis of the following:
* at least several km from exposed mountains, scientific stations and transportation corridors in order to minimise inputs of locally derived crustal debris or anthropogenic emissions;
* at least 400-500 m upwind from local camp; and
* local snow surface should be relatively flat and have minimal sastrugi with a non recently eroded surface.
Sample Containers and Scrapers
Containers for the samples should yield chemical blanks below the detection limit for all species being analysed. In addition, the lids should have a water tight (leak proof) seal. High density polyethylene, wide mouth Nalgene jars (60 ml) have been used with great success. Glass containers should be avoided since cations such as Na leaches out of the glass. Sample containers are cleaned by rinsing three (3) times with 18 mega-ohm water (eg. Milli-Q, Nano-Pure) , filling with 18 mega-ohm water and soaking for at least 24 hours. After soaking, the bottles are rinsed three more times. The cleaned, empty, and labelled containers are then packed for shipping. If samples for stable isotopes are also being generated in these containers, the containers must be dry prior to shipping. Drying the containers for stable isotopes is best done in a clean room or in a clean hood. Containers should not be acid washed if the samples from those containers are to be analysed for Cl, NO3 or SO4.
Clean equipment is critical in the collection of samples. All bottles and scrapers must be pre-cleaned. Scrapers can be made out of polycarbonate (Lexan) or plexiglass (Lexan is stronger and works better in the cold). Three different sizes (15 x 15 cm, 10 x 10 cm, 10 x 2) make for a good system. Edges can be bevelled on several sides to facilitate the scraping of the snow. At least one edge should not be bevelled so that it can serve as the "holding" end. To clean the scrapers, methanol and a toothbrush can be used to scrub the scrapers. Scrapers should be well rinsed with purified water then placed in a pre-cleaned bin, covered and allowed to soak for 48 hours. Scrapers should be set in the bin so that all sides are in contact with the water. After soaking, water should be drained, scrapers rinsed 3 times and the bin refilled to soak again for at least 24 hours. Ideally the second rinsed water would be analysed for the chemical species being collected. Once the scrapers have completed their final rinse they should be rinsed 3 times and place in a clean hood for drying. After drying scrapers can be placed in a polyethylene bag or cleaned "Tupperware" container then bagged again in a polyethylene bag. The double bagged scraper then can be bagged together with other scrapers in a larger polyethylene bag. Clean polyethylene gloves should be worn during all stages of cleaning.
Templates help with sample collection and depth control. They can be made out of machined Lexan. Depending on accumulation rate several sizes may be necessary (ie., 1 cm, 2 cm, 3 cm,...)( see Figure 1).
Polyethylene gloves (disposable type) must be used in the sampling process. Make sure they are large enough to fit over gloves that will be worn while sampling.
Clean suits may be made of polyester or tyvek. It is important to pack them in triple bags in a clean area before shipping (packing technique similar to final scraper packaging). Clean suits may be used several times if kept clean during and after sampling.
Particle masks are used to keep your breath from contaminating the samples and must be worn during all sampling. To eliminate fogging of glasses, goggles can be used in the snowpit or core collection area.
Plastic (Poly) Shovel
A cleaned plastic shovel should be used in the initial cleaning of the sampling wall once the pit is dug. Plastic shovels can be found in most safety supply catalogues. Shovels should be cleaned similar to the scrapers and stored in plastic bags. This must be a dedicated shovel for snowpit work.
There must be a dedicated shovel for the snowpit to minimise contamination. A large scoop type works well in removing snow from the pit. A large pointed or square shovel may be needed under some conditions.
Snow saws can be used to cut blocks from the snowpit. A large toothed tree pruning saw or "drywall saw " works well under most conditions. As with the metal shovel this should be dedicated to snowpit work.
It is important to keep accurate depth registration while sampling snowpits. A fibreglass tape measure works well for this.
Snow Density Kit
Accurate density measurements are needed to calculate accumulation rates. There are several types of kits available.
Concise notes are essential when doing snowpit work. Make sure to keep track of containers numbers, descriptions of snow made during the sampling, note when dropped lids or open containers, etc.
Snowpits are sampled for a variety of reasons. For example, the upper several meters of the core will usually be in poor condition due to the softness of the snow and larger samples can be taken from the snow pit allowing for sufficient sample volume and sample recovery of fragile hoar layers.
After selecting the site for a snowpit it is best to have all sampling equipment at the site to allow for immediate sampling of the snowpit upon completion of the excavation. Determine the direction of the wind and using a clean, dedicated metal shovel define an edge perpendicular to the wind direction about 1.5-2 meters long. This will be the sampling wall. Nothing should be placed on the upwind part of this line (see below for exception, building wind block) at any time. Also it is imperative that while digging nothing touches the sampling wall except the shovel or saw (ie., hands, body, etc.). The upper 10-50 cm will probably be most easily removed using the shovel. The top of the snowpit should be ~2 x 1.5 meters. This will give sufficient room for working in the pit. Once the loose snow is removed from the upper 10-50 cm the snow saw can be usually be used to cut blocks and using the pointed or square shovel break the blocks free. All spoils (excavated snow) from the pit should be thrown downwind, away from the sampling wall. If there is blowing snow a snow wall may be built on the upwind side of the pit. This should be done using the blocks cut out of the pit. The wall should be at least 1 meter from the sampling wall and the person building the wall must not step between the snow wall and sampling wall. Once the wall is built no one should be upwind of the pit.
Ideally the sampling wall of the snowpit will not be in direct sunlight. It may be necessary to plan the snowpit excavation and sampling according to wind and sun direction. It is possible to shift the sampling wall angle to the wind up to 45 degrees to keep the direct sun off the sampling wall. Obviously this planning must be done before excavation is started. Final depth of the snowpit should be ~2.1 meters (for a 2 meter snowpit). To help facilitate the sampling of the pit a large, firn block can be left in the bottom of the pit to stand on while sampling the top of the pit.
In order to efficiently sample the snowpit two people are needed. One person (Sampler) will do the actual sampling and the other (Helper) assist with bottles and note taking. Upon completion of the excavation of the snowpit the Sampler will put on the cleansuit, particle mask and poly gloves. The Helper should be dressed in clean non-particulating clothing. Since this person will be fairly inactive during the sampling period it may be best to have a dedicated 1 piece insulated nylon suit for warmth. The Helper should also wear a particle mask and clean dedicated gloves. Using the clean shovel the Sampler will remove a ~50 cm wide section of the sampling wall and at least 15 cm into the wall from top to bottom of the sampling wall. Once this is done the spoils should be removed from the pit and the plastic shovel should be placed back into a clean bag for use in the next snowpit.
The Sampler should replace the poly gloves worn while using the shovel. Next the Sampler should take one of the 15 x 15 cm scrapers and remove ~5 more centimetres from the cleaned shovel surface. This scraper can then be inserted into the corner of the cleaned sampling wall and act as a shelf to place the sample cup lid while sampling. Next the Sampler should take one of the 10 x 10 cm scrapers and clean the sampling surface back several centimetres down (about 50 cm from the top of the pit). This procedure will be repeated as samples are collected down the wall. The 10 x 10 cm scraper can be inserted into a cleaned section of the wall to keep clean for the next 50 cm cleaning. A measuring tape can be hung down the wall to allow for good sample depth registration. A scraper at the surface will help alleviate the tape cutting into the surface.
Select the sampling template corresponding to the desired sample interval (ideal sample interval is 8-10 samples/year). By keeping the index marker parallel to the surface mark the wall using the template. Several passes may be necessary to get good markings on the snow wall. Place the template into a cleaned section of the wall. Now the wall is ready for sampling. The Helper should then hand the Sampler a sample cup with the lid pre-loosened. Remove the lid and place on the "shelf" made by the 15 x 15 scraper with the inside of the lid down. Take one of the 20 x 2 cm scrapers and remove ~1 cm just above the line made by the template. Then place the sample cup lightly against the wall and using the scraper fill the cup with snow. Place the scraper into a clean section of the wall and place lid back on cup. Exchange cups with the Helper and continue collection until you are about 3-4 samples from the marks from the template. Using the previous marks as a guide make additional marks down the cleaned snow wall. You will need to use the 10 x 10 scraper to pre-clean the wall once you reach the ~50 cm cleaning. Using the measuring tape you can cross correlate true depth with sample depth. The template usually has excellent depth correlation. As you increase in sampling depth in the snowpit the snow hardness will increase. It may be necessary to use the 20 x 2 scraper like a chisel in order to break up the snow.
The Helper plays an important role in the collection process. Sample cups must be checked each time to assure they are in order. The Helper should pre loosen the lids to the sample cups before handing to the Sampler. Once the sample is collected and cup returned to Helper it is necessary to tighten the lid as tight as possible. Samples must be placed out of the direct sun to prevent melting. The Helper must record notes about the snowpit which include description of snow from the Sampler, container numbers, general weather and sampling conditions and miscellaneous notes related to sampling (eg. stratigraphy, grain size, etc..).
* - while sampling check poly gloves often for rips or tears and replace as necessary
* - 2 pairs of poly gloves work better than one pair
* - record good notes on what you see and happens in the snowpit
* - you should have an additional "clean" section on the pit wall. This section can be used to clean tools that may drop to the pit floor. The snow is a clean, abrasive cleaner.
* - keep your hands away from your particle mask. If you touch it with your poly gloved hands, replace your poly gloves. The mask concentrates your breathe which has very high concentration of ions measured in the snowpack.
Stratigraphy and Physical Studies
Density and physical stratigraphy should be completed after the sampling of the snowpit. Parameters such as layer thickness, hardness, colour, crystal size and shape, location and thickness of ice or debris bands should be recorded. CRREL (1962) should be used as the guide for the physical study on the snowpits. Photographs of the pit wall should be taken to record the visible structure and layering.
Firn Core Drilling
The following is general description on how to collect firn cores for chemical analysis. Firn cores must be carefully collected in order to assure contamination free samples. At no time should the core be touched with anything but clean poly gloves or clean tools. It is imperative that if the cores are handled any other way than described here that good notes be made and used to determine any possible contamination.
The core site and the sampling site, should be located upwind from the camp and any vehicle tracks. The core should be collected as close to the snowpit as possible. Any power source should be at least 50 meters from the drill site and constant checks made to assure wind has not shifted direction.
Due to logistic concerns most drills used in ITASE will probably recover 7.6 cm (3 inch) diameter core. The limited core diameter requires great caution be taken to minimise any possible contamination. This is extremely important due to the porous nature of the firn. Drill barrels should be cleaned initially at home institutions if possible. Although it is usually important to ship the barrels, if they are metal, with rust protector this should be removed before cores are collected. This can possibly be done at the base station or at the first sampling site. Methanol should work well to remove most of the protector if it is oil based. At the first drilling site the drill should be set up and several drill runs made to check the cores recovered. There should not be any noticeable coloring on the exterior of the core and the drill chips should be snow white. It may be necessary to drill 15-20 meters before the core barrel is sufficiently cleaned. If many meters were drilled in cleaning the barrel it would be best to move the drill 10-15 meters away from the initial drill spot to assure a pristine drilling site.
While the drill is being cleaned a compact processing line can be set up. This should be kept to minimum processing in the field but should include at least logging and physical stratigraphy. It is essential that the setup be done the same way each time so the top portion of the core is received on the processing line in the same orientation to avoid reversal of core sections. Some groups may decide to conduct ECM measurements on the core in the field.
An example of a typical procedure is:
A surface with clean polyethylene sheeting cover can serve as an area to extract the core from the core barrel. The push stick used to extract the core from the core barrel should have a polyethylene bag over the end. Any core that is sticking out of the bottom of the barrel should not be pushed back in except with a clean poly gloved hand.
The core should be extruded into a 1/2 round holder, preferably made from a clean plastic material. This holder should be cleaned the same way as the scrapers for the snowpit work. Once the core is extruded the length should be recorded in the field book, and the core fitted to the adjacent core section, to determine if there has been any core loss. The extruded core section can then be transferred to the cutting set up and sectioned into storage lengths. A mitre box type saw and trays with metre rulers attached help make for accurate and efficient sectioning. Once the core is sectioned into desired lengths (typically 1 meter) initial stratigraphy should be done on the core. Careful logging of hoar layers, wind crust and other features should be recorded. It may be necessary to construct an enclosed light box for better stratigraphy due to the bright surface light. After stratigraphy is completed the core is ready for either ECM or storage. Cores should be placed in polyethylene layflat tubing that have labels with core name, depth and an arrow pointing to the top of the core. The core ID card should be stapled to the top of the core bag. Cores should then be packed into insulated containers. Cores should be packed in order to eliminate movement in the storage box. Packing snow on the ends of the core keeps the cores from shifting. Boxes must be stored out of direct sunlight. This may necessitate the burial of boxes at stops along the traverse. Ideally, each box would have a thermistor inside so temperatures can be monitored.
Ideally 2 people would be needed to properly collect firn cores. This number does not include the drillers. Both people should be dressed in cleansuits, particle masks and poly gloves. Two people are needed due to the rate core is recovered from the upper sections of the ice sheet. At no time should the core be touched with anything that has not been pre-cleaned or covered with clean polyethylene.
Careful record keeping of the cores is essential and a suggested logging card is presented (See Figure 2). The essential parameters to have on any logging card are: top, bottom, location, date, breaks and core quality.
Major Ion Laboratory Analysis
Anions are readily determined using suppressed ion chromatography. It is possible to determine chloride, nitrite, bromide, nitrate, sulfate, and others on one sample injection. Some of the columns currently on the market also allow determination of fluoride, acetate, formate, and methylsulfonic acid in addition to the major anions listed above all in one injection run. Necessary sensitivity will vary with geographic location, chloride values will be maximum close to the coast where nitrate values will probably be minimal (<50 ug/kg). In general the analytical system should be able to quantify at least 10 ug/kg for the major anions (0.28 uM Cl; 0.16 uM NO3; 0.10 uM SO4).
Cations are readily determined using suppressed ion chromatography or by flameless atomic absorption spectrophotometry. Na, NH4, K, Mg, and Ca can be quantified with one injection using ion chromatography (AAS can determine the above with the exception of NH4). For the Antarctic plateau region where Na is the dominant cation, Na concentrations less than 5 ug/kg (0.22 uM) have been observed in sub-annually collected samples. In coastal areas Na concentrations, still the dominant cation, are much higher. In general the analytical system should be able to quantify at least 1 ug/kg for the major cations (0.04 uM Na; 0.025 uM K; 0.042 uM Mg and 0.025 uM Ca).
Analysis in the lab should include a suite of at least 4 standards that encompass the concentration range expected for the samples and with the species in proportions similar to the samples. Standards should be analysed at the start and end of each analytical session. Two aliquots from one sample out of every 20 samples should be analysed to keep track of processing and the accuracy of the instrument to reproduce measurements.
The Climate Change Research Center uses Dionex columns with electronic suppression, the AS11 for the anions and the CS12 for the cations. A 500 uL loop is used for cation samples and a 250 uL loop for the anions from the plateau region. Smaller sample loops are used for samples from coastal regions.
A comparison between laboratories is highly desirable if the logistics can be managed. Samples for this could be a large sample of snow that is split into aliquots and then distributed to all the interested laboratories, or it could be a stock solution. In either case, it is important that the sample/stock standard remains frozen until analysed.
CRREL (1962). Instructions for Making and Recording Snow Observations. U. S. Army Cold Regions Research and Engineering Laboratory, Corps of Engineers, Hanover, New Hampshire. June 1962