Boxcore Collection

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Introduction

Boxcoring is an easily executed method for obtaining shallow sediment cores. These boxcorers are modified Klovan style boxcorers (Greenwood et al., 1984) and are useful both on land and in water. First, the boxcorer is described. Next, logistics and equipment that greatly increase diver safety while obtaining cores in shallow waters are presented.

Diver-operated Boxcorer

The stainless steel boxcorers used in this work are 15 cm wide x 10 cm deep x 30 cm long and have a removable slide hammer and angled sliding door (Fig. A.1). Grooves along the open side of the corer guide the removable sliding door down the open face once the corer is in place in the sediment. This angled sliding door eliminates the need to excavate and expose the lower surface of the corer to install a lower plate as was required in previous versions of diver-collected ‘box’ cores.

One diver easily manipulates this corer. The primary component is the wedge shaped box that has a hollow pipe handle that can be threaded to the top of the corer. The hammer is comprised of a sliding sleeve on the pipe handle. The sliding door has a handle of wood, synthetic polymer, or metal to enable pushing the door into place.

Figure A.1. Stainless steel boxcorer with slide hammer and removable door.

Nearshore Boxcoring

To commence coring in waters deeper than 2 m, a diver descends along a down line with a lift box containing 1-3 corers and a 50 lb. lift bag (Fig. A.2).The second diver carries the slide hammer attached to an additional corer should 4 cores be planned.The use of a lift box equipped with a lift bag allows 2 divers to descend and ascend along a down line with minimal equipment.Our lift box is a plastic container with ¼ inch holes drilled in the base and lid to allow the closed box to fill with water.Lines attached to each side of the box provide a bridle to attach to a lift bag.

Figure A.2. Boxcoring equipment: (left to right) lift box on its side with 50 lb. lift bag clipped to bridle, tag line with clips on each end, boxcorer with removable sliding door and hammer, filled boxcore with door removed, clear tray, and metal slide used to extract core from boxcorer.

Once on the seabed, divers proceed to the coring site (Fig. A.3) along a known bearing and a set distance from the instrument frame. In low visibility conditions, a tag line (Fig. A.2) is clipped to the instrument frame.The lift box can also be secured to the instrument frame during coring activity.

Figure A.3. Diver sliding door into second of an orthogonal pair of boxcores.

A boxcorer with slide hammer attached is held perpendicular to the sediment surface and oriented with the diver’s compass. Light to moderate force with the sliding hammer is used to drive the corer into the sediment.Forceful pounding of the hammer can drive target sediment away from the corer.In very fine sands, the corer can be driven level with the sediment surface in 1-2 minutes.In fine to medium sands, especially sediments compacted by wave action, the coring may take 3-5 minutes per corer.Once the corer top is flush with the sediment surface, the corer is full. The sliding door is aligned with the guide grooves and slid into the sediment until it reaches the base of the corer (Fig. A.3).

The second corer is then placed orthogonal to or in series with the first corer. The process of hammering in the corer, inserting the sliding door, removing the hammer, and readying a new corer is continued until finished at a particular location. The cores can be extracted by pulling on the slide hammer or pulling the corer from the sediment without the hammer in place.

To minimally disturb the cores, the extracted cores are placed in the lift box. Elevating the base of the filled corers in the lift box helps to prevent sediment draining from the bottom of the corer. Once the cores are secured in the lift box, a 50 lb. lift bag is attached to the lift box bridle, inflated, and facilitates transport of cores to the water surface. If the cores are not transported in a lift box, tilting the base of the core slightly above horizontal prevents loss of sediment from the base of the core.

Figure A.4. Boxcorer filled with sediment. Core top is to the left. Scale is in cm.

In the lab, the boxcorer door is removed (Fig. A.4), and the sediment is allowed to dry for 4-24 hours. Methods for extracting the core, making sediment relief peels, and developing xrays of core sediments are contained in the following appendix.

Applications

The unique application of this method of core collection near deployed acoustic altimeters has allowed us to document individual storm event beds in the sediment record. Depth downcore to changes in sediment structures or type corresponds remarkably well with erosional maxima documented during storm events by sonar altimeters (Beavers et al., 1997b). While conditions prevent diving during storms, a deployed altimeter and post-storm diver-collected boxcores can be combined to document the sediment record from storm events.

This boxcorer can be effectively used in muddy to gravely sands. Although diver-collected boxcores work well in numerous sediment types, they cannot penetrate large rocks or shells. These boxcorers have been used to collect a transect of cores extending below the waterline to subaerial portions of a sandy spit near Beaufort, NC. This transect of cores was able to document hurricane overwash deposits. With an operational range that includes subaerial and subaqueous sediments, instruction on the coring procedure can be demonstrated to scientific divers on land before they enter the water to collect boxcores. When fundamentals of this technique, including core orientation, use of the slide hammer, and aligning the sliding door, are practiced and mastered on land, in-water core collection can proceed more efficiently.


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The content for this web page was taken from the Dissertation Storm Sedimentation on the Surf Zone and Inner Continental Shelfof Rebecca Lenel Beavers in the Department of Geology of Duke University

This web site was created by Doug Call (Contract Student, University of Virginia) on July 31, 2001.