Limnological Research Center Core Facility Draft v.1.0 SOP series 3/11/09 freezecoring.pdf A. Myrbo Freeze coring and freeze core handling Freeze coring is used to collect the watery (95-99% water), frequently gassy uppermost sediments from all types of lakes. It is a particularly useful technique in laminated sequences, where collection and preservation of undisturbed stratigraphy is essential. The LRC freeze corer is a wedge-shaped box that is filled with dry ice and alcohol and lowered by rope, cable, or drive rods, penetrating about 1m into the sediment. Sediment freezes to the outside of the box; when the corer is retrieved the dry ice-alcohol slush is replaced with warm water, loosening a slab of frozen sediment from either side of the corer. Slabs are stored frozen, and can be polished, imaged, and sectioned while frozen, by working with the slab on a bed of dry ice pellets (i.e., you do not need to work in a coldroom). Other types of freeze corer include the “frozen finger,” which operates on the same principle but is cylindrical, and freeze corers that use liquid nitrogen circulated from a vessel or surface above to freeze sediment to the inside or outside of the corer. Procedure: in the field The freeze corer can be deployed from the ice surface in winter or from a boat. If you are dropping the corer on a rope, the boat does not need to be stably anchored (i.e., fixed in one location) but if using drive rods to lower the corer the boat must be anchored at three or four corners so that your position does not move while the corer is embedded in the sediment. If you are using drive rods, you must also use a safety rope tied onto the off-center eye bolt on the top of the corer: the shaking of the corer that occurs as the CO2 bubbles off can rotate the corer and unscrew the drive rods. If coring from the ice, you will need to auger two adjacent holes and chop out between them with an ice chipper to make a large enough hole for the corer. Please mark your ice hole with a stick or small branch when you are finished, for the safety of others.

• Measure water depth to datum (datum=ice surface, boat platform surface, etc.) • Determine how far you want the corer to penetrate into the sediment (max~1m) • Add this length to the depth to datum • This is the total depth to the bottom of your core • Subtract the length of the corer, measured from the bottom of the corer to either

the top of the ring bolt or the top of the unthreaded portion of the drive rod connector

• This is the total length of rope or drive rods you will need. Mark this length on the rope or drive rods before you begin coring.

• Fill the freeze corer chamber with dry ice pellets or pieces chipped off a block. The corer will make a hellacious noise as the pellets sublimate and jump around.

• Slowly add alcohol, pausing if the slush mixture bubbles up to the top of the corer. Continue to add alcohol and dry ice until the mixture is close to the top of

Limnological Research Center Core Facility Draft v.1.0 SOP series 3/11/09 freezecoring.pdf A. Myrbo

the corer, stirring periodically (up and down, not around and around) with a stick to break up clumps and uniformly cool the alcohol. The volume of the slush will be large at first but will decrease as the alcohol cools down and the dry ice sublimation slows. Add more dry ice if the mixture does not become viscous.

• Clamp the lid onto the corer and immediately, quickly, lower it to your mark. If using rope, let the corer sink into the sediment under its own weight (do not drop it from more than a few tens of cm above the sediment-water interface, as it can go in at an angle) but stop it at the mark so that it doesn’t overpenetrate.

• Leave the corer embedded in the sediment for 10-15 minutes to allow mud to freeze to the outside of the corer.

• Raise the corer to the deck. It’ll be considerably heavier than when you lowered it, with a bunch of mud stuck to it.

• Meanwhile, heat water on the camp stove. (If coring in summer, surface lake water is usually warm enough to use directly from the lake)

• Scrape unfrozen mud off the frozen mud using a spatula (you can also dunk the corer up and down in surface water to wash off some of the mud)

• Pour slush mixture out of corer into a bucket • Using a bucket with spout, a watering can, or a kettle (to avoid spilling water onto

the outside of the corer), pour warm water into corer to begin the process of thawing the slabs off the corer. It’s a good idea to keep the level of the water below the level of the top of the ice slab: the top of the slab will melt off most quickly, as it’s the thinnest and mose exposed, and the extra heat from the water may make it completely melt before you can get the bottom part off. If you are particularly interested in capturing the sediment-water interface, you should be especially careful of this.

• Begin to insert a piece of aluminum (?) roof flashing between the corer and the slab to help it come off. Someone should be ready with a large piece of aluminum foil to catch the slab (or part of it if it breaks) as it comes off. Wrap each slab thoroughly in foil (do not use plastic wrap: it can freeze into the sediment), mark “up” with an arrow and core designation, and store on dry ice (blocks) in cooler.

Materials and equipment: Freeze corer Drive rods or rope Safety rope Dry ice pellets (dry ice is available in most places where hunting or fishing occurs; you

will need 10-20 lb for each freeze core plus blocks for your cooler for transport) Dry ice blocks Cooler Work gloves for handling corer with dry ice slush, dry ice, etc. Alcohol (isopropyl or whatever is cheapest is fine) Stick (e.g., broom handle) for stirring slush Large lab spatula

Limnological Research Center Core Facility Draft v.1.0 SOP series 3/11/09 freezecoring.pdf A. Myrbo Bucket large enough to contain 2x alcohol volume, preferably with lid Camp stove and ~1 gal pan/kettle Flashing, 6”(?) width, as short a piece as you can find (6’?) Aluminum foil, large (restaurant supply) roll if possible Sharpies or other permanent markers Safety: Dry ice can give you frostbite/nip relatively quickly, so always wear gloves when handling it. Accumulation of CO2 in closed areas can be lethal, so never work in (cold)rooms with active dry ice storage, and keep your vehicle well-ventilated when carrying dry ice. Procedure: working with freeze core slabs in the lab Build a tray for your slabs. The simplest is a low (~1” tall sides) cardboard box several inches longer and wider than your largest slab. This configuration keeps the cold air (from dry ice) surrounding your core from dissipating, while still allowing easy access. Place the slab in this box and surround it with dry ice pellets or chips from block dry ice. Add dry ice as pellets sublimate. Slabs need to be polished for imaging and sectioning (to expose visual stratigraphy) and the ice that froze to the corer on its way up and down in the lake needs to be removed, for correct water content measurement and because the lake water chemistry is likely different from the sediment pore water chemistry. A power planer (e.g., Ryobi HPL51K) is an inexpensive and excellent tool for removing layers of ice and smoothing the core surface. The blade depth should be adjusted to its smallest increment, at least until you get used to the process. Keep a supply of towels at hand to wipe melted mud off the planer between passes (or every 2-3 passes) – the melt will freeze back to the slab in streaks if the planer blade becomes too muddy. Stop the planer before wiping the blade, of course. At the end of the day you should also dismantle the planer and thoroughly clean the blade and other parts, to prevent mud buildup and rust. The planer is not really meant for such a delicate task as shaving ice, and it is possible to crack the slab if you push too hard, or if your bottom surface is uneven (causing part of the slab to be unsupported as you push down on it). Be gentle and go slowly. You can sometimes “glue” the parts back together with a little water, but the stratigraphy will be disrupted. If you plan to image the core, we suggest you work on the top side (outside of the slab relative to the corer) first, image the core, and then work on the bottom side. The bottom side (toward the corer) typically has a lot of lake water ice that needs to be removed, and it is more likely that you will break the slab while doing this more violent task, so get an image of the slab intact first.

Limnological Research Center Core Facility Draft v.1.0 SOP series 3/11/09 freezecoring.pdf A. Myrbo After planing off as much of the top surface as you can without losing too much mud, finish surface preparation using a razor blade (held in a window paint scraper can be good) or utility knife blade, held as flat as possible against the slab surface. Scrape the surface and brush away the scrapings so that they don’t refreeze to the slab surface. This technique is a bit awkward but it is a very good way of removing the last smears and imperfections from the surface. Just be glad you are using the planer and don’t have to do the whole thing with a blade. Imaging on a bed of dry ice can be tricky, especially in the summer, as humidity in the air readily freezes to the cold slab surface and obscures the nice stratigraphy with puffy white frost. We have found that a quick treatment of the surface with a heat gun (or hair dryer) just before imaging melts the frost (but not the slab) for long enough to get an image. Use of polarizing film on the light source and a polarizing filter on the camera lens (as at LacCore) eliminates all glare caused by light reflection off the wet slab surface. Sectioning the core can be done with a bandsaw (returning slab subsamples to a freezer or dry ice-filled cooler in between each cut) or with a microtome. Digital photos taken during sectioning are helpful as a supplement to notes on depths sampled. Some photos of the LRC freeze corer are included below.

Figure 1. Top of the freeze corer showing clamps, vent for evolved gas, and ring for attachment of rope. Scale in cm.

Limnological Research Center Core Facility Draft v.1.0 SOP series 3/11/09 freezecoring.pdf A. Myrbo

Figure 2. Top of the freeze corer open to show gasket and safety cable for lid with quick release. Scale in cm.

Figure 3. Whole corer viewed from bottom.