Turning Sediment into Rock

Fluid Displacive Drying and Resin Embedding of wet sediment

Elemental analysis of sediments is yet another proxy assessment which is essential for paleoclimate reconstructions. We use elemental analysis along with macroscopic charcoal data, Loss On Ignition Organic Content data and grain size to infer conclusions about the past climate of the Amazon. Prior to using the iXRF machine for elemental analysis we cut and prepare sediment slabs into polished thin sections called pucks. Making pucks takes days of drying and embedding them with a resin epoxy. The technique used here is called fluid displacive drying and resin embedding of wet sediment.

1. Cutting of sediment slabs from a larger core

We use aluminium trays  to cut out small sediment slabs from the bigger core. This ensures sediment samples are of the same size. The slabs are then placed on a mesh and then set into little boats, making sure to keep the soft slices of sediment undisturbed.

A slab of sediment is being scooped out of a larger core and placed into boats.

2. Dehydration or the fluid displacive drying by acetone
The next step is important, we use acetone to remove porewater whilst still leaving them intact. This is called fluid displacive drying. Acetone serves as an ideal dehydrating agent because it evaporates very fast, taking with its some of the water and leaving the sediment ready for epoxy resin embedding.

Sediment is left seating in acetone as it undergoes fluid displacive drying.

3. Resin Embedding
Using epoxy resin is dangerous because the chemicals are carcinogenic and so the procedure  must be performed under a fume hood. Mixing the resin recipe happens under the hood too and then small amounts of the resin solution are discharged into the boats, making sure not to pour it over the sediment. The sediment is then left to set for 24 hours.

Once the acetone solution has cleared up the sediment is left embedded in resin epoxy.

4. Resin Hardens
After multiple epoxy exchanges (8-10) over about a week, the concentration being increased after every exchange, we place the boats containing sediment slabs into an oven at 30 and then 60 degree celsius over two days. Once the resin has hardened and allowed to cool, the pucks are popped out of the boats and prepared for sawing and polishing.

5. Sawing and Polishing

Pucks often come out in odd shapes and sizes so we use a diamond encrusted saw to resize the pucks. The saw also helps to cut open the pucks to reveal the interior side of the sediment, so that fresh laminations can be exposed for iXRF scanning. Polishing takes place soon afterwards, to smooth and level the surface of the pucks.

Sediment slabs are sliced by a diamond band saw to reveal their interior side. 

6. Use of iXRF
Polished pucks are then inserted into the iXRF and scanned for elemental composition and
laminations. iXRF imaging also provide high resolution photographs that are perfect for varve counting.

Fluid displacive drying and resin embedding though time intensive are ideal for preserving the true structure of sediment samples. Resin embedded pucks are also hard wearing and stay useable for years. The useful data gathered from iXRF scanning provides insights on the elemental composition of the core. Consequently elemental data is then used to infer local natural processes and climatic conditions that might have led to the deposition of identified elements. Next week we will delve more into elemental data analysis and the usefulness of iXRF scanning in paleoclimate research.

I hope you'll keep reading!

Kopo