Although it would be hard to imagine, you couldn’t have seen a more excited group of adults than when a three-metre rectangular block of muddy sediment was pulled onto the ship. This surreal moment is when you realise you’ve fallen into the rabbit hole and entered a whole new world; the world of a sedimentologist.
This block of muddy sediment is a sediment core taken from
the bottom of the ocean and reveals a whole plethora of wonderful and strange
stories from Earth’s history. These stories relate to how our planet’s
environment, climate, and ocean currents have changed over time. What is truly
amazing is that we know so much about the long and dramatic history of our
planet despite the fact that we have not been part of that history for very
long. This amazement is humbling and is a reminder of the capabilities of the
human race, and the responsibilities we have as stewards of the planet.
Once the core is brought into the lab there is a flurry of activity to open the metal casing, which holds the sediment core, and to see what strange and mysterious tales from the ocean depths have been brought to the surface. With the casing removed heads are bent over to observe the colour, structure and composition of the sediments. Quick, sharp remarks are exchanged between the various parties involved before the processing of the core is started without delay. First, the core is logged which involves documenting the major characteristics of the core. This is important because these observations will underpin the majority of the interpretations which brings the whole story together. From here smear slides and small sediment samples are taken along the core to examine the changes which occur from top to bottom.
Hours will be spent analysing these slides and samples, with more sampling done along areas of interest until the sediment core looks less than pristine. Not to worry however since before the sediment core was scooped, poked and prodded an archive core was taken and stored in the fridge. This archive core is kept with all its structures and features intact as an original record for safekeeping.
There is a certain amount of chaos and untidiness in the lab which may be disconcerting to the casual viewer, but there is a method to the madness with great care being taken to systematically record and sample the sediment core. Furthermore, there are efforts to limit contamination across the core (i.e. avoid mixing sediment from one area of the core to another). In fact, it is quite liberating to be able to conduct science in a lab where things are more practical, and improvisation is encouraged. A day in a life of a sedimentologist will surely shake up the perception of the typical scientist in a lab coat conducting experiments in a clean and well organised laboratory.
By Mardi McNeil, Queensland University of Technology
Every marine science voyage has a research plan and specific aims and objectives that the science party wants to achieve. Months, or sometimes (usually) years, goes into planning the voyage and targeted survey site selection in order to achieve aims, test a hypothesis, or answer questions which will fill a knowledge gap in our understanding of the marine system we are studying. This is how science works!
The science objectives for our CAPSTAN voyage have been planned out by our Chief Scientist Dr Leah Moore, and the educational objectives by our CAPSTAN Director Dr April Abbott. On this research cruise we are targeting a submarine canyon system which connects the continental shelf margin off Portland Victoria, to the Otway Basin at 5,500 m water depth in the Southern Ocean. We are literally sailing across the abyss!
Our primary geological objective is the search for a cool-water carbonate turbidites, resulting from the funnelling of sediment down the submarine canyon until it is deposited in a submarine fan at the base of the canyon. Cool-water carbonate systems are not as well studied as their sub-tropical and tropical counterparts as there are fewer places in the world where they occur, and they’re typically in deeper water.
The term “Carbonates” refers to sediment grains which are comprised of calcium carbonate minerals, commonly calcite and aragonite. Over geological time these sediments lithify to form limestone rock. Most carbonate sediments are biogenic in origin, which means they are produced by biological organisms. The classic example is a coral reef, where the soft coral polyps precipitate their hard skeletons, and coralline algae produces the calcite cement which glues it all together, resulting in hard limestone.
In a cool-water carbonate
system there are definitely no reef building corals. In southern Australia, the
main carbonate producers are bryozoans and foraminifera. Bryozoans are
colonial, meaning hundreds to thousands of tiny animals called zooids, live
together in a colony and collectively produce a hard carbonate skeleton. This
skeleton can take many forms, like delicate fan-like nets, or robust upright
Foraminifera (or just “forams”) are single celled organisms similar to an amoeba, but they secrete a calcite “test”, or shell. Foram tests come in an almost endless variety of shapes and sizes, and can be benthic (bottom dwelling) or planktic, meaning they live freely in the water column. Forams have evolved rapidly throughout geological time (hundreds of millions of years), so geologists and micropalaeontologists use foram test shapes to determine the age of the sediments we are looking at. This helps us to quickly “date” our cores in the field, where we don’t have the capacity to use isotope mass-spectroscopy analysis to determine an absolute age. One reason we want to know the age of our cores is to determine whether the sediments we’re looking at were produced during a glacial cold period, or an inter-glacial warm period like today.
On this CAPSTAN voyage we have collected three cores from different water depths within the Portland Canyon, and one from the bottom of the canyon in the fan. We hope to capture evidence of glacial-interglacial cycles, and a cool-water carbonate turbidite system.
In geological speak, a turbidite is a characteristic sedimentary deposit which forms when sediment is transported down-slope in a fluidised (watery) plume under the influence of gravity. Because different sediment grains have different densities and shapes, they settle out of suspension in a characteristic way. The most dense sediments settle first, and the lighter less dense sediments are the last to fall out of suspension. This cycle repeats over and over every time there is a gravity driven turbid flow, resulting in a characteristic cyclical pattern of deposition which we call a turbidite.
Onboard RV Investigator we have now finished our coring and are working through sampling the cores at 10 cm intervals, looking at the sediments under the microscope to see what carbonate grains we have. Our preliminary results are in, and there is some excitement coming from the Sedimentology lab! We have picked up a glacial – interglacial cycle, and managed to estimate an oldest date based on a nanno-fossil called a coccolith, which we know from the geological record was abundant from about 80,000 years ago, so we now know that our cores cannot be older than 80,000 years.
So the big heroes of the Sedimentology Lab are the tiniest carbonate grains which allow us to read our cores like a history book, and interpret biological and physical processes through geological time. And it turns out that we have indeed, found our cool-water carbonate turbidites, and glacial-interglacial cycles. Science mission accomplished!
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.
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).
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.
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.