Explorers marvel at 'Brittlestar City' on seamount in powerful current swirling around Antarctica

Oceans worldwide contain an estimated 100,000 seamounts rising at least one km above the seabed; fewer than 200 have been sampled in any detail.

Undersea mountains can be highly productive and biodiverse, sometimes host unique species and serve as feeding grounds for fishes, marine mammals and seabirds. They also may serve as important way stations for marine migrations. The scientists’ work sheds light on factors underpinning seamount biodiversity, suggesting ways to improve their environmental management.

The odd shape and circumstances of Brittlestar City seamount

Tangaroa’s acoustic “multibeam” technology mapped the sizes, shapes and depths of the Macquarie Ridge.

The Brittlestar City seamount displayed several geological faults affecting its shape and geomorphology. The odd rectangular edge of its southern peak was formed by the intersection of two perpendicular faults. Because the upper surface is relatively flat, experts believe it was once at sea level, or slightly submerged. The flat topography suggests wave erosion occurred during the last ice age 18,000 years ago, when sea level was low. Although the base of Brittlestar City seamount is 850 meters below surface, its peak is just 90 meters underwater.

Intrigued by the dynamics of the Antarctic Circumpolar Current as it passed over and through ridge gaps, physicists aboard the Tangaroa calculated its speed over the top of the seamount at a “rattling” 2 knots (about 4 km per hour).

“This current is estimated to be 110 to 150 times larger than all the water flowing in all the rivers of the world,” says Dr. Mike Williams of NIWA. “In terms of the world’s oceans, New Zealand sits right beside the motorway.”

The Macquarie Ridge creates a strategic marine junction. “Understanding this current will shed light on how much water flows into the Pacific as opposed to continuing to circumnavigate Antarctica. This is important for understanding, and ultimately predicting, the impact of potential changes in the current on climate throughout the Southwest Pacific.”

Data was collected from nine strings of metering instruments, anchored a year earlier in two gaps or "choke points" in the Ridge, through which the current squeezes. Experts were astonished to find the current had pushed the top instruments on some strings to a depth of one kilometer below the surface.

Scientists also took this ocean area’s temperature and salinity readings for the first time since the 1960s, looking for climate-related changes, and obtained water samples to measure and compare levels of marine life nutrients.

April’s Tangaroa voyage continues the recent collaboration of scientists in New Zealand (NIWA, TePapa and other institutes) and Australia (CSIRO, Museum Victoria and other institutes) to document invertebrate and fish life on seamounts that range between the two countries.

The Census of Marine Life CenSeam programme

Seamount biology research over the years has been uncoordinated, causing lost opportunities for collaborative studies, inefficient sampling that left major gaps, and data that can’t be compared across regions.