The hunt for cool-water carbonate turbidites

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!

Map of the ship track for CAPSTAN's 2019 voyage.  The map shows the eastern part of the Great Australian Bight showing the ship track from Hobart to the canyons off of Portland and then the planned track as the ship begins its transit from the study site to Fremantle.
The CAPSTAN voyage survey area of the Portland Canyons and Otway basin off southern Australia (green and red dots). RV Investigator will then transit across the Great Australian Bight to Fremantle.

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.

Photos of the sediment collected in the third kasten core with a full core image on the left and a inset with a zoomed in photo of the biological hash visible between 165 cm and 182 cm in the core. The core was collected as part of the hands-on marine science training on CAPSTAN's 2019 voyage.
Photomosaic of Core #3 (left) and inset of the 165 to 182 cm section. This close up shows coarse carbonate bioclasts (grains) of bryozoans and forams, referred to as a bryomol/foramol assemblage and considered typical of the cool-water carbonate factory. The coarser grains are embedded in a muddy matrix comprised almost entirely of planktonic forams (visible only under the microscope). Photo Credit: Matt Jeromson and Mardi McNeil.

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 branching sticks.

Microscope images showing species of calcareous plankton that are being used in the description of the cores collected on the 2019 CAPSTAN marine science training voyage and an image of a smear slide with these species present from one of the cores collected from RV Investigator.
Examples of the micro and nanno fossils we have been using as stratigraphic markers in our sediment cores: A) Scanning Electron Microscope image of planktonic foraminifera Neogloboquadrina pachyderma typical of polar or glacial assemblages (Almond et al., 1993), B) Scanning Electron Microscope image of the coccolithophore Emiliani huxleyi indicating sediments are younger than 80,000 years (www.mikrotax.org), and C) transmitted light microscope image of a sediment smear slide from the current voyage that shows abundant E. huxleyi (photo: Annabel Payne). A and B are not to scale.

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.

Schematic from Passlow 1997 showing a classic turbidite sequence with the coarser grains settling out first (lower in the sediment column) and fining upwards.
A ‘classic’ turbidite sequence showing how sediments are deposited out of suspension after gravitational and hydrodynamical flows (Credit: Passlow, 1997)

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.

Four of the sedimentology team sit around the laboratory bench excited about the preliminary results from the sediment cores and grabs taken as part on marine science training on CAPSTAN's 2019 voyage.
The other heroes of the Sedimentology Lab feeling triumphantly satisfied at the results coming out of the canyon cores. From left to right: Kaycee, Stephen, Jin Sol, and Matthew (missing: Bella and Mikala)

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!

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