Sequential extraction procedure to obtain the composition of terrigenous detritus in marine sediments

The geochemical and isotopic composition of terrigenous clays from marine sediments can provide important information on the sources and pathways of sediments. In order to extract the detrital signal from bulk marine sediments, standard sediment leaching methods are commonly applied to remove carbonate and ferromanganese oxides. In comparison to most previous studies that aimed to extract the terrestrial signal from marine sediments we additionally applied a CsCl wash throughout the sample preparation Simon et al. [1]. The motivation behind that extra step, not frequently applied, is to remove ions that are gained on the clay surface due to re-adsorption of authigenic trace metals in the ocean or during the leaching procedure and thus could alter the original composition of the detrital fraction if no cation exchange was applied. Here we present an improved and detailed step-by-step leaching protocol for the extraction of the detrital fraction of bulk deep-sea sediments including commonly used buffered acetic acid and acid-reductive mix solutions including a final cation exchange wash.• standard method to remove carbonate and ferromanganese oxides and Stokes settling to isolate the clay fractions• additional application of cation cation exchange wash (CsCl)• removal of ions that are gained on the clay surface due to adsorption of authigenic trace metals in the ocean or during the leaching procedure


a b s t r a c t
The geochemical and isotopic composition of terrigenous clays from marine sediments can provide important information on the sources and pathways of sediments. In order to extract the detrital signal from bulk marine sediments, standard sediment leaching methods are commonly applied to remove carbonate and ferromanganese oxides. In comparison to most previous studies that aimed to extract the terrestrial signal from marine sediments we additionally applied a CsCl wash throughout the sample preparation Simon et al. [1] . The motivation behind that extra step, not frequently applied, is to remove ions that are gained on the clay surface due to re-adsorption of authigenic trace metals in the ocean or during the leaching procedure and thus could alter the original composition of the detrital fraction if no cation exchange was applied. Here we present an improved and detailed step-by-step leaching protocol for the extraction of the detrital fraction of bulk deep-sea sediments including commonly used buffered acetic acid and acid-reductive mix solutions including a final cation exchange wash.

Method details
Following is the outline and detailed protocol ( Fig. 1 ) for the preparation of equipment, labware, reagents and solution used in the protocol for sediment sample leaching in order to extract the detrital signal from marine sediments. Because this is not a method of analysis, subsections normally found relating to risks of contaminations and quality control of the chemical analyses have been omitted. For more information on these subjects the reader is referred to the main publication [3] where these details are published and standard treatment is described.
Step : CsCl cation exchange wash 7 • 0.1N Cesium chloride solution • double rinse with ethyl alcohol

Extraction procedure to obtain the composition of terrigenous detritus in marine sediments
Step : Step 1: Initial Weighing & Disaggregating of Samples 1. Weigh 2-3g of the sample to be sieved in a dry, tared 50mL centrifuge tube and record the mass. 2. Also weigh a clean, labeled 63μm sieve for each sample and record the mass. 3. Fill each tube to ~30mL with deionized water to disaggregate the samples. Disaggregating may be facilitated by vortexing and sonicating (the better the samples are disaggregated, the less water will be necessary to sieve the samples, thus saving time later).
Step 2: Sediment Sieving 1. Place a sieve in a 400mL beaker with a corresponding label, and pour the sample onto the sieve; use a squirt bottle with deionized water to ensure the whole sample is transferred. 2. Use the spray bottle to wash the fine fraction through the sieve. This is complete when the water coming out of the sieve is clear and the coarse fraction does not appear to contain any clumps when viewed under a microscope. 3. Place the sieves with the coarse fraction in the oven at 50 °C, and weigh them when dry.
Transfer the coarse fractions to small glass vials for storage. This is best done by emptying the sieve onto creased weighing paper and using the weighing paper to pour the coarse fraction into the vial. 4. Allow the fine fractions to settle in the beakers overnight; if they are still cloudy the next day (or longer if needed), a few drops of buffered acetic acid (recipe see Step 4 above) may be added to facilitate settling. 5. After the fine fractions have settled, remove as much water as possible and recombine multiple beakers if necessary. 6. Return the fine fraction to the original centrifuge tube; it may be necessary to fill the tubes to 50mL, centrifuge for 30min at 2400rpm, decant the water, add the rest of the water and fine fraction, and centrifuge and decant again.

Repeat
Steps 1-4 until the sediment no longer appears to react with the acid; there will be no more carbon dioxide formed, and no bubbles will form when the acid is added (however, bubbles may form as the sediment shifts when the acid is poured over it, and the acid itself may foam when vortexed, even if no reaction is taking place).

Step 5: Leach of Fe-Mn oxyhydroxide fraction with a buffered Acetic acid/ Sodium acetate solution (pH ~4)
1. Fill each tube to ~20mL with buffered acetic acid/ sodium acetate solution and place on rocking Step 6: Settling of clay fraction Separations of grains smaller than 20 microns are carried out by settling in a column of water in glass cylinders. Particles will settle in the water according to Stokes Law: V = 2/9( ρg -ρf) g r 2 / η where: V = the settling velocity in cm/sec ρg = density of the mineral grains (2.6 -2.8 g/cm 3 for clay minerals) ρf = density of fluid (1g/cm 3 for water) g = acceleration due to gravity (980 cm/sec 2 ) r = radius of the mineral particle (10 −4 cm for clays) η = viscosity of water (10 −2 g cm/sec2) 1. Label as many clean, 100mL graduated cylinders as you have samples with a piece of tape placed 5cm down from the 100mL mark. Also label a set of clean centrifuge tubes with the sample names and size fraction (for our purposes, < 2μm). 2. Add ~30mL MQ water and ~2mL 0.5% sodium metaphosphate solution, i.e calgon ((NaPO3)6, Fischer Chemical, Laboratory Grade Powder, Catalogue #: S333-500) to each tube after decanting the 3rd MQ rinse. Sodium metaphosphate, a detergent, helps disperse the clays and prevent flocculation. 3. Vortex until samples are disaggregated. 4. Pour the contents of each tube into the appropriate cylinder, using the MQ squirt bottle to make sure that all sediment is transferred (and save the original tube for the 2-63μm fraction). 5. Fill the cylinders to the 100mL mark with MQ water and cover with a square of parafilm. 6. Sonicate the cylinders for about 5 min, then shake and invert them a few times to evenly disperse the sediment throughout the cylinder. At this point, it is extremely important to make sure the sample is fully disaggregated. If heavy clumping is observed, sonicate and shake the cylinder until the clumps disappear -this may take some effort and multiple sonicating rounds. 7. Place the cylinders in a safe place on the counter where they won't be bumped or otherwise agitated, and start a timer for 3hr 52min. Be sure to leave enough room between the cylinders so you can reach around them a little more easily later. Staggering them is also a good idea. 8. After time is up, use a turkey baster to transfer the top 5cm of water (25mL) to the clean centrifuge tubes labelled earlier. Clods (for Ar) may be made at this point if desired. 9. To reset the cylinders for another round of settling, add ~2mL calgon (for one or two settling rounds only) to each and fill to 100mL with MQ and cover with parafilm, sonicate for about five minutes, shake and invert, and reset the timer. Only add more calgon prior to the second round of settling. 10. Centrifuge the tubes at 40 0 0rpm for 30min to settle the < 2μm fraction. The resulting supernatant should be translucent, and it will likely be tinted. If the solution is still cloudy when removed from the centrifuge, return it to the centrifuge for a longer length of time. 11. Repeat Steps 8-10 until settling is complete or until enough 2μm fraction has been collected to perform desired analyses. If bringing settling to completion, a good way to tell if it is complete is to hold up a paper with black and white type on it. The black and white should contrast sharply and the letters shouldn't look clouded. 12. Transfer the 2-63μm fraction to the original centrifuge tubes (if settling is complete or close to complete, allowing the remaining sediment to settle for a while and then pipetting clear water off will save time when transferring). Use the MQ squirt bottle to ensure all sediment is transferred. This can be done by pouring the sediment directly into the tubes from the cylinders, and then centrifuging for 30min, decanting, and continuing to add to the tubes until the cylinder is empty. Alternatively, if you are in a hurry and need the cylinders immediately, transferring the sediment to a beaker first and then the tubes (following the same procedure) is also an option. Additionally, if settling was stopped after a particular number of rounds and was not fully completed, be sure to note on the tubes that there is still some < 2μm fraction present).
Step 7: CsCl Cation Exchange Wash (Adapted from [3] ) 1. Fill each 50mL centrifuge tube up to the 15mL line with MQ water. 2. Label new 15mL centrifuge tubes with the sample name + "w/ CsCl" 3. Vortex to completely disaggregate sample. 4. Quickly, while sample is still completely suspended, pipette 5mL of sample from the 50mL tube to the new 15mL tube.