January 9, 2018

Into the mud of the Great Australian Bight

Posted by larryohanlon

Here we are in the operations room! From left to right: Joe, Rashid, & Helen

By Joe Moraga, Rashid Saleem & Helen Truscott

We are privileged to join the first CAPSTAN (Collaborative Australian Post Graduate Sea Training Alliance Network) voyage from Freemantle, Western Australia to Hobart Tasmania. The voyage brings together 20 students from universities all over Australia aboard the RV Investigator to learn and gain hands on experience in marine sciences. From a range of universities, we share our experience with grab sediment sampling.

Why the grab sample? From being on board the RV Investigator and participating in all the various sample retrieval and analysis operations, we learnt the importance of the grab sample in determining not only the characteristics of sea floor sediments but also how successful other equipment will be at collecting samples at a specific site.

The Smith Mac Grab

We watched the crew lower the Smith Mac Grab machine over the side of the vessel with a crane and winch assembly to the sea floor. Once at the sea floor the grab mechanism was triggered and a section of sediment retained in the jaws of the grab and hoisted back to the deck. Based on the grab samples we collected, a gravity core was only favourable at two out of our six sites.

Crane and winch assembly off the starboard side used to deploy the Smith Mac Grab

Continental margin consists of three different features, continental rise, slope, and shelf. We collected ocean floor sediment samples from continental shelf and continental slope areas in the Bremer Basin region of the western Great Australian Bight. We made four transects along the shelf break, having one site above shelf break and one below on each transect.

With a grab onboard, we headed to the appropriately named ‘wet dirty’ lab to identify and describe the sample as a mixed group of students, those with sediments experience and those completely new to the field. With beginners quickly learning the ‘hands on’ approach to sedimentology, the first step was a handful of mud. For some of us that never grew up playing with mud is always fun, but when that mud is from the ocean floor 1100 m below the surface it is an incredible experience! We felt privileged to be part of it.

Trainer and sedimentologist, Leah Moore, demonstrates grain size descriptions with a grab sample collected at 1100 m water depth. It begins with a handful of mud!

Working with samples obtained from grabs we then described grain size, roundness, sphericity, sorting using the Geoscience Australia grain size comparator charts, and whether matrix or clast supported. Next was colour using the Munsell colour chart, where we learnt just how many shades of brown actually exist!

Tools of the trade! We used the Munsell Soil Colour Chart (blue notebook), containers of size-sorted sediments, and the Geoscience Australia grain size chart to identify, classify, and describe the sediments we collected.

Sediment samples where sieved and separated to understand proportions of the different clast sizes. Then the sediments were onto the microscope for further examination to understand composition. Earlier in the voyage a talk from trainer Kelsie Dadd introduced the basics of sedimentology to those new to the field. We were all given an appreciation and understanding of what we were finding in our analysis of samples and the immense processes involved in formation of these sediments.

Some of our sediment samples! From left to right: gravel sized sediments from 80 m water depth, sand sized sediments from 80 m water depth, and muddy sand sediments from 700 m water depth.

The sediments collected on the continental shelf at the depths around 80m contained mostly coarse sands and gravels, and carbonate dominated sand in a clay matrix. The sediments collected at the deeper slope sites are muddy sand with calcareous fossils. These finer sediments are typical of the deeper regions due to lower energy environments. Whereas the coarser sand and gravels are typical of high energy environments. A vertical drop camera deployed on the shelf not only entertained all of the students and trainers, but confirmed the findings from the grab samples.

Joe Moraga, University of Canberra

Rashid Saleem, University of Western Australia

Helen Truscott, Charles Darwin University

Want to know more about the CAPSTAN program? Check out our official website at: https://goto.mq/6g

Read more from the student participants in the CAPSTAN blog at https://voyage9181.wordpress.com or from our CAPSTAN director at https://aprilabbott.wordpress.com