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Sampling your marker horizons

The marker horizons that are established at the beginning of a study will need to be sampled to determine how much deposition is occurring. This needs to be accomplished without disturbing a large area of the sampling plot. We have developed a method which uses a small self-pressurized liquid nitrogen dewar to take a small diameter frozen core from the marker horizon plots (Cahoon et al. 1996). The below page provides detailed instructions on sampling marker horizons.

1) Supplies needed for Cryogenic Coring.
2) Sampling marker horizons using Cryogenic Coring.
3) Sampling marker horizons by Cutting plugs.


Supplies needed for Cryogenic Coring:

1) 15L Self pressurized dewar - Cost is about $1200-1500 (US).

15L DEWAR:
Here are some of the specifications you should include on the dewar:

a: stainless steel construction (better with salt environments)
b: Self-Pressurized (low pressure unit, 22 psi)
c: Halo ring on top for handling and protection
d: handles on the side for easy transportation in the field (no wheels)
e: Top mounted fill port

The liquid fill and vent valves that come with the dewar use screw type valves which require about 3 or more turns to close or open the valve. We typically replace these valves with inexpensive 90 degree ball valves when we get a new tank. These allow for better control over the flow of liquid nitrogen.

15 L Liquid Nitrogen Dewar

2) Stainless Steel flexible hose (purchase locally) - Cost about $75 each

The hose is constructed of 1/2" diameter x 6'-6.5' long stainless steel flexible hose with 1/2" female hydraulic connections at each end. The hose is covered with a SS mesh to protect the hose. SS flexible hose

3) Fittings - The LN2 tank and hose typically use 1/2" hydraulic fittings for connections. The ball valve on the tank and the connection for the copper bullet use pipe fittings. You will need to make sure you have get the appropriatey fittings to connect the flexible hose to your tank and the copper bullet. These fittings can be found at a local hose and fitting company found in most larger towns and cities.

Liquid Valve on Dewar -- Adapter (a) -- Stainless Steel Hose -- Adapter (b) -- Bullet

a) Adapter for Dewar valve - 1/2" mail pipe thread to 1/2" male JIC fitting.
b) Adapter for bullet - 1/4" female pipe thread to 1/2" male JIC fitting.

The below drawing lists the fittings to use. Note that the descriptions of these fittings may differ from what your local hose company may use. It may be best to use the above descriptions.

4) Copper bullets for taking accretion cores. - cheap, make yourself.
Click here to download instructions (PDF 110k).

Bullet - 2 parts, Outer Sleeve and Inner Sleeve
Bullets are made out of copper tubing. The Outer sleeve is 3/8" copper tubing and, has a 30 caliber bullet soldered to the end. The Inner sleeve is 1/8" copper tubing with holes drilled at the bottom for aiding the flow of LN2. It has a 1/4" compression adapter at the top.
Closeup of the bullet tips
Close-up of Bullet

dewar diagram

Image from: Cahoon, D. R., J. C. Lynch, and R. M. Knaus. 1996. Improved cryogenic coring device for sampling wetland soils.
Journal of Sedimentary Research 66:1025-1027.
Sampling Instructions: Cryogenic coring

 

Dewar Facts:
The 15L cryo tank weighs 35 lb. empty and about 55-60 lb. full. A 25L tanks weighs about 100 lb. full.

A full 15L tank can take from 15-40 cryo cores before you will need to refill it. The actual number of cores will depend on the temperature (both air and water), the depth of the core and how long you leave the valve open. It will take trial and error to determine the best procedure for a given situation.

Dewars are self pressurized tanks which will build an internal pressure of about 22 psi. A full dewar sitting in your lab will eventually go empty after 2 or 3 days (this is normal). Fill your dewar just before your field trips to minimize the loss that occurs.

A LN2 Dewar will always emit a slight hissing noise when at pressure. This is the excess pressure bleeding off and is completely normal.

These dewars also have a pressure building coil which will allow them to build pressure quickly after they are filled.

15 L Dewar
15L dewar with hose and copper bullet assembly attached.

SAFETY: You need to be very careful when working with Liquid nitrogen. It is very cold
(-320° F, -196° C) and can easily burn your skin if you are not careful. Wear gloves and glasses/goggles to protect your skin from getting in contact with the liquid. Keep the dewar vertical at all times. If you have it in a vehicle, make sure there is proper ventilation for the occupants. Keep a window open.

A) Filling the dewar -

You will first need to fill your dewar with liquid nitrogen. NOTE: The dewars we use are low pressure vessels. They only build 22 psi of pressure. Make sure that you are filling your dewar from another low pressure dewar. Some of the larger dewars are set to build a pressure of 250 psi. We do not recommend using a tank with this pressure to fill a dewar. It could be very dangerous.

Your options are to;

1) Take the tank to a compressed gas company and have them fill it. This is the easiest solution but also the most expensive. You will more than likely pay $2-4 per liter to fill the tank. The costs to fill a 15 liter tank can vary widely. We have paid from $20 to $60 to fill them. You will also need to call ahead and make sure they have LN2 in stock. Some smaller companies only carry a small amount at their office.

2) Order a large tank from the gas company and have them deliver this tank to your facility or lab. These tanks are typically 160 liters and can last for over a week. You would use this tank to fill your smaller tank as needed. Do not order a large tank if you are not taking many cores. This can be the most economical method for obtaining LN2 if you need a lot of it. Cost for the large tank are usually around $100 (plus a monthly rental fee).

How can I get Liquid Nitrogen at a remote study site? You could try to find a gas company that can deliver a large dewar to a facility near your sampling sites. This is not always feasible. The best way to solve this is to have multiple smaller tanks which you take with you. For example, we have numerous 15L L tanks which we would fill at our laboratory and bring with us in the field. The 15L tank is the most versatile size. We would not recommend using a larger 25L tank because of the weight.

Mississippe Delta - Dewars
Cryo-coring at Delta NWR, LA USA

Steps to fill the a 15L dewar from a larger "low pressure" dewar:

These instructions assume you have the correct fittings to connect your SS hose from the larger dewar to the smaller dewar. Check with the gas company to find out the exact type of fittings you will need.

a) Open both valves (Liquid and Vent) on an empty 15L dewar to remove any internal pressure. Leave the valves open!
b) Connect SS hose to "Liquid" side valve on the 15L dewar
c) Connect the other end of the SS hose to the valve coming from the large dewar. The smaller dewar should now be directly coupled to the larger dewar via the hose. NOTE: Depending on your equipment, you may need some additional fittings to make this connection.
e) Open the valve on the larger tank and start the flow of LN2. If it's a low pressure tank (about 22 psi) you can pretty much open the valve wide open.
f) The dewar will get heavier as it fills. This is the only way to easily gauge how fast it is filling up. Gas will be continually venting from the Vent valve. This is normal.
g) When the 15L tank is full you will start to see liquid spurting from the vent valve.
h) Shut off LN2 from big tank.
i) Shut off both valves on the 15L dewar.
j) Using a wrench, crack the connection in the hose so the pressure inside can be released. You can wait till it warms up before removing the hose entirely.

 

B) Collecting a Cryocore-

1) Connect the hose and bullet to "Liquid" Valve on the 15L dewar.

2) Push bullet into the marsh. Unless you believe the maker horizon is very deep, try not to push the bullet too far into the substrate. It may be difficult to remove. Three to four inches is usually deep enough.

3) Completely open the valve on the dewar and start the flow of LN2. You may have to steady the bullet with your hands (wear gloves) until it starts freezing. You will hear the nitrogen exhaust coming from the top of the bullet.

4) Your first core may take a while since the entire hose needs to cool down before the bullet gets cold enough to begin freezing the marsh. Subsequent cores shouldn't take as long to freeze.

Continue freezing the core until you start seeing a white cloud of gas coming out of the top of the bullet. This usually indicates that the entire hose and bullet are frozen so you can stop pumping the LN2 soon after this cloud forms.

Problems - If you freeze for too long, the core is very large and could be difficult to remove from the substrate. If you don't freeze the core long enough, it may come off the bullet when you pull it out or try to clean it up with the knife.

Cryo Coring
Old Oyster Bayou, LA USA

5) Turn off the LN2 valve.

6) Grab the top of the bullet (wear gloves!) and pull the core and hose out of the marsh.

You have made what we call a "Cryo-core" or "Marshsicle".

cryo coring
Removing a very deep cryo core from the sediment. Note white feldspar layer at the bottom of the core.

7) Remove the outer bullet sleeve from the inner sleeve (the part with the frozen core on it is attached to outer sleeve). Be sure to put an empty bullet (outer sleeve) on the inner sleeve to keep it from getting dirty.

8) Scrape excess soil and roots off the core with a knife.

OOB coring
Feldspar Marker Horizon
25L dewar - Old Oyster Bayou, LA USA

9) Read the distance from the feldspar to the top of the soil surface in millimeters. This distance will vary in a single core, so we try to take 3 or 4 readings, if possible. Some parts of the core may not produce a reading so you will normally get from 1 to 4 numbers from a single core

Recording the Data:

It is helpful to describe the quality of the marker horizon in the data book. We will sometimes write how many cores it took to find the feldspar and mention the quality of the layer if found. We will typically write "Poor Layer", "Good Layer", or something similar when recording the data from cores. This will help to determine the quality of the marker horizon and may help in determining when to establish new markers in the future.

measure a cryo core
Measuring a "cryo core"
Sample Data book entries - Date: 8/2/2009 Site: Platform 2
Plot Readings(mm) #cores Quality/Notes
Core 1a 4, 2, 2, 3 1 good layer
Core 1b 1, 1, 2, 1 1 good layer
Core 1c 4, 3, 4, 6 2 poor layer
Core 2a 0, 0, 1, 0 1 feldspar at surface
Core 2b no data 3 could not find marker
Core 2c 1, 2, 2, 2 1 good layer

If the feldspar is visible on the surface it may not be necessary to core the plot. You can enter zero's for the value. Do NOT enter zero's in the data book if you can't find the marker horizon. In this situation you do not record any values at all though you should make a note that you didn't find the layer.

 

10) If you didn't find any of the marker horizon, take another core in a different part of the plot. Before continuing to the next plot, make notes of core quality, number of misses, quality of marker, etc.

Sampling Instructions: Cutting Plugs

 

Cut Plug
Cut Plug - Tijuana Slough NWR, CA USA

In some situations you may be able to use a sharp knife and cut small soil plugs from the marker horizon plot. This will work only if:

1) the sediments are quite firm and maintain their structure when cut and removed from the soil.
2) The surface is dry and free from standing water.
3) The marker horizon is not too deep.

In general, tidal, saline marshes with more mineral sediments work very well for cutting plugs.

1) Cut a small four sided plug (about 3 cm x 3 cm) from within the marker horizon plot. If there are extensive roots, it helps to have a sharp knife. A long serated kitchen knife works well.

2) Pry the plug out of the ground from one of the sides. The plug will be about 3 cm x 3 cm x 6 cm in size. You need to make sure the core is deep enough to include the marker horizon. The depth is also related to the length of your knife blade.

3) You should be able to see a marker layer right away. It's difficult to scrape a core that's not frozen, but you can give it a try to improve the visibility of the layer.

4) We try to get 4 readings from a single plug, one representative reading from each side of the plug.

5) Put the plug back into the ground.

6) Move on to your next marker plot and repeat the above steps.

 

U.S. Department of the Interior, U.S. Geological Survey,
Patuxent Wildlife Research Center | URL: http://www.pwrc.usgs.gov/
Last Updated: 26 January 2010
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