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GEMAXThe ultimate corer

forsoft sediments

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THE GEMAX CORER FOR SOFT SEDIMENTS

by

Boris Winterhalter*)

ABSTRACT

The GEMAX gravity corer is an improvement of the very successful Niemistö-corer originally designed by Dr. Lauri Niemistö in the late sixties. Thehydrodynamically balanced corer is quick and easy to use from any sized vesselequipped with a light-weight winch with a lifting capability of a few hundredkilograms and with a wire speed of around 1 m/s. The corer is ideally suited forundisturbed sampling of soft muds for environmental monitoring purposes. Theextruder unit permits very precise slicing of the sediment core for detailed analysiseither directly on the wet sample using various probe technologies or for lateranalysis with more conventional methods.

*) Boris Winterhalter, senior marine geologistGeological Survey of Finland02150 Espoo, Finland

The photo on the front page shows the the designer,Dr. Lauri Niemistö and the original GEMINI corer inits cocked setup ready to be lowered down for easyand controlled sampling of unconsolidated muddysediments on the continental shelf of the Weddell Seaoff Antarctica. Picture was taken in 1996 on boardthe Finnish r/v Aranda.

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INTRODUCTION

Dr. Lauri Niemistö designed in the late sixties a highly successful gravity corer forenvironmental sampling of muds (Niemistö, 1974). The ease and reliability of handling the corerand the effective procedure for sub-sampling of the sediment core made the Niemistö-corerquickly the Baltic Sea “standard” for marine sampling of recently deposited sediments. The needfor larger sub-samples led to further development work and the GEMINI-corer evolved (coverphoto). The GEMINI-corer, true to its name consists of two independent core barrels for moresample material in thinner slices. The special extruder unit for sub-sampling (slicing) thesediment core was an integral part of the system.

In both the Niemistö-corer and the GEMINI, in very watery sediments, material from the “fluffy”top surface had a tendency to flow down between the inside wall of the plastic core liner and thesediment core. In more sticky sediments (e.g. glacial clay) the disturbed contact surface betweensediment and core barrel was also a source of minor contamination of sub-samples duringslicing. Although the amount of mixing can in many cases be accepted and is anyhow a realityin bioturbated sediments, the mixing can be a critical source of error when trying to determinethe exact depth of e.g. an environmentally significant change in sediment character (chemicalpollution).

The development in analytical equipment hasled to increased requirements for preventingcontamination of the sub-samples duringslicing. This led to the construction of theultimate corer, the GEMAX. The main changeis the increase of the internal diameter form 80mm to 90 mm. The slicer unit has seen themajor change in the design. The larger core-barrel diameter of GEMAX together with thenew slicer head with the peeler facility, limitssample contamination to a minimum withoutloss in sample volume. This is a veryimportant feature when collecting sedimentsamples for environmental monitoring andassessment of anthropogenic pollution.

Figure 1. The GEMAX corer with large diameter core

barrels in ready-to-go configuration.

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Figure 2. The GEMAX twin barrel corerfor environmental sampling of soft seabedsediments. The figure shows the maincomponents of the corer unit.

CONSTRUCTION

The GEMAX corer consists of twoidentical and independently functioningstainless steel core barrels attachedsymmetrically to the main body andrelease mechanism (Figure 2). The plastic(methylmeta-acrylate or polycarbonate)core liner with the stainless steel corecutter are attached to the corer body witha simple screw-locked bayonet fitting.

The aluminum fins attached to a pivoted PVC frame have a dual function. During lowering theyimprove the hydrodynamics of the corer helping to steer it vertically down into the bottom. Uponretrieval the fins help to close and lock the core catcher thus preventing premature extrusion ofthe sediment material.

The rubber flanged plastic (polyamid) valves at the top of each barrel help to retain the sedimentsample during initial pullout from the sea bed. They have been designed to cause minimumrestriction to water flowing thru the barrels during lowering, thus ensuring good sedimentpenetration.

The sediment core extruder consists of a wide, shallow PVC box which helps contain spilled outsediment from spreading on deck. The central pylon (column) is set in an aluminum base whichitself is fixed inside the PVC box. The extruder piston, with o-ring seal at the end and screwedto the extender rod, fits exactly into the core liner. The core liner clamped to the slide and movedvertically under control of the threaded shaft is used to extrude the sediment through the top ofthe liner. (Figure 3).

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Figure 3. The sediment extruder used for sub-sampling of the sediment core. Clockwiserotation of the knob at the top of the threadedshaft causes the clamp fixture to move downalong the pylon pressing the sediment out ofthe core liner for sectioning.

Sub-sampling of the sediment core iscontrolled with the help of the slicer unit(Figure 4). The unit is fixed to the top of theliner with the aid of three screw knobs. Thediameter of the hole thru the peeling slicerbase is smaller than the inner diameter of thecore liner and the core itself. When extrudinga 5 mm thick peel (side wall contamination) isremoved from the core and discarded thru thespoil orifices.

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Figure 4. The peeling slicer. Asimpler slicer with an extrusiondiameter equal to the core linerdiameter is also available

THANKS TO SOUND DESIGN AND THE USE OF PROPER RAW MATERIALS, THEGEMAX SYSTEM WILL GIVE YEARS OF ALMOST MAINTENANCE FREE SERVICEPROVIDED THE UNIT IS PROPERLY CLEANED AFTER EACH DEPLOYMENT.

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OPERATION

Figure 5. Detail of thetrigger unit with the stringloops secured in a cockedposition by the spring loadedpins.

The winch wire is attached to the trigger unit (Figure 5), which consists of two individual springloaded cocking pins. The looped strings from the valves and the fins are locked by these pinswhile the weight of the entire corer hangs from the wire. When the corer hits the sea bottom aminor slack on the wire triggers off the release mechanism. The weight on the trigger unit forcesthe cocking pins against their springs to release the strings attached to the fins and the valves.

It is important that prior to deployment, the core catcher and the check valves be properlyattached to the trigger device. This ensures verticality during lowering and completelyunobstructed flow of water through the corer and subsequent sediment penetration. It isadvantageous to use a swivel, as seen in the cover photo, between the corer and the lifting wire.

The penetration of corer into the seabed is a function of the amount of attached lead weight,winch speed, and sediment compaction. In soft muddy sediments, typical of Baltic Sea anoxicbasins, a wire speed of 1 m/s and two standard weights have generally given optimumpenetration of ca. 50-60 cm. Upon end of penetration into the seabed, sufficient slight slack hasto be given the lifting wire to trigger the release mechanism. This allows the upper check valvesto close thus preventing sediment loss during the initial retrieval process. Once free of the seabedthe core catchers swing into place by hydrodynamic force exerted by the fins and finally lockedin place by dual permanent magnets.

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The core catcher is gently swung out while capping the core cutter (lower end of core liner) bythe palm of a hand. The plastic liner with the sediment inside is removed downwards from thecorer and transferred in a vertical position to the fixed piston on the extruder unit (Figure 3). Theslider unit and clamp, uncoupled from the threaded shaft, is brought up around the core cutterand screwed tight. Also the threaded yolk is closed around the shaft.

Figure 5. The slicer unit with peeler mounted on top of the core liner. During the extraction andsub-sampling the contaminated outer part of the core is removed. The spoil (black ooze) can beseen flowing out thru the side orifices on the slicer unit. The sub-sampler with centimetre scalingcan be used to measure the amount of sediment to be sliced. Alternatively special slice holderswith a hole equal to the inner diameter of the extruded sample and with thin plastic slidingcovers can be used in the sub-sampling process (see Figure 6) .

The slicer unit is attached to the top of the plastic liner. The sediment extrusion process is startedby rotating the shaft in a clockwise movement. Each full turn of the shaft amounts to twomillimetres of extrusion. Thus two and a half turns extrudes 10 millimetres of sediment. In theinitial phase the thin plastic slide covering the opening in the slicer unit is kept slightly open toallow removal of excess water above the sediment either to be wasted or sampled as “bottom-near-water”. When almost all water is removed from the top of the sediment, slicing can

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commence. The plastic cover is fully opened, the sub-sampler measuring ring (Figure 5) isplaced so as to allow sediment to enter. When the required amount of sediment is extruded, theplastic slide is closed cutting the sediment. The sediment in the measuring ring is then gently slidoff the slicer into a plastic bag or sample container. The plastic slide is opened and the nextlength of sediment is extruded and transferred into a sub-sample container. The thickness of thesub-sampled slice can be controlled either with the measuring ring or just by counting therevolutions of the shaft.

For alternative sub-sampling without the measuring ring, special sample holders of variousthickness with thin plastic slides above and below are available (Figure 6). The covers on theseholders fit tightly enough to keep the sample from contact with air for a short time, beingnormally sufficient for in situ Ph, Eh and other probe measurements.

Figure 6. Special plastic sub-sample holder with top and bottom sliding covers can also be usedin sample handling. The sliding covers fit snugly to protect the sample from air contaminationfor some time. The drawing depicts the holder used in the original Niemistö corer with a 5 cmcore barrel.

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After coring and sub-sampling the core liners are rinsed and inserted back into the GEMAXcorer (Figure 7), the unit is cocked and ready for the next sampling station.

Figure 7. When preparing the corer for deployment, the core liner with attached stainless steelcore cutter are inserted into the core barrel and locked with a twist. The wing screw is tightenedto prevent loss of cutter and liner.

Reference:

Niemistö, L., 1974: A Gravity Corer for Studies of Soft Sediments. Merentutkimuslait.Julk/Havsforskningsinst. Skr. 238, 33-38.