Initial data and sample collection

By Nathan Teder, Flinders University

The first two days at sea were mainly used to steam ahead to our study area off the coast of Portland, and due to this, the main thing that had occurred was seafloor mapping. We used a single beam sonar system to take data ranging between 5 m per sample to 50 m per sample depending on the depth of the location, and how flat the sea floor is, with topographic structures (i.e. canyons) being taken at a smaller resolution. This method of data acquisition does require some manual cleaning however (figure 1) due to the sonar system being susceptible to noise, especially on the edges of its pulses. This will be running throughout the voyage, but will be especially focused on a set of four canyons in the Otway Basin as these canyons could either be funnelling cold water turbidites to the submarine fan, or potentially playing a role in upwelling depending on if a low or high pressure system is present in the bight.

Bathymetric data displayed with a rainbow color scale.  Red represents the shallowest depths and blue indicates deeper waters.  CAPSTAN students on board RV Investigator learned how to do quality control to process this data as it was collected as part of their at-sea marine science training
Figure 1: The output of a small section of recently measured bathymetry in a 2D wave (top) and a 3D model (bottom). This screen allows the user to manually delete data points that are anomalous (noise).

Day three saw the first deployment of the CTD, plankton nets and coring samples from the sea floor. The plankton nets, and CTD both had samples which could be used to count marine life present at that depth. For the CTD, samples of 10 m, 40 m and 100 m were used with the amount of life decreasing as the depth increased, to the point the 100 m sample didn’t have any life present that was above 100 μm. This was an expected result, due to being at a depth which is deeper than the photic zone which would reduce the amount of life present, due to insufficient light. That said one of the more unsettling parts of this observational work was the amount of plastic present as the 100 m sample had ~ 49 blue fibres of >100 μm present in it, which was from 6 L of water. Switching to a horizontal tow of a phytoplankton net ended up getting a much better result life wise, with ~ 150 various forms of copepods, massive clumps of biomass, as well as a crustacean larvae at a ~ size of 1-2 μm (figure 2).

CAPSTAN students collected plankton from the Great Australian Bight/Bonney Upwelling Region using Bongo nets as part of their at-sea marine science training.  Here is a dead crustacean larvae under the microscope from one of the tow collections.
Figure 2: A dead crustacean larvae present in the towed net sample.

The first core was taken from a ~ depth of 1727 m, and from that, a 2.16 m section of the sea floor was obtained. This whole section was a homogenous olive coloured mud (figure 3) which was firstly split up into an archived, and a working core. The working core was then sampled at a rate of 1 per 10 cm, and each 10 cm block was sampled four time. There was also a point around the 1.73 m to 1.75 m section which was also sampled 3 times due to the presence of a broken shell at the surface. These samples will be analysed later on during the trip, once we move away from our study area.

Figure 3: The kasten core after initial sampling in the wet-dirty lab on RV Investigator

Once that was completed, smear slides were created using mud from the 1 m mark of that sample. These smear slides showed up some air pockets, and biotite in this sample (figure 4). We also saw foraminifera within the smear slides, which are a component of the cool carbonates we are focused on this trip. Our goals include trying to measure if they do descend down to the abyssal plane, and if a canyon system influences the amount that flows down.

Microscope view of sediments from a kasten core collected by CAPSTAN students on RV Investigator as part of their at-sea marine science training.  A variety of micro-organisms can be seen along with record or donut shaped air bubbles.
A smear slide made from sediment from 1 m depth in the Kasten core. Air bubbles are the ‘record’ looking circular objects and the black dots are biotite.
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