|Some of the new data acquired for Tropic Seamount, including|
ship-board multibeam swath bathymetry and geoacoustic
sub-bottom profiler, AUV surveying, ROV sampling and
CTD water information
We have taken about 35 CTDs to measure conductivity, temperature and depth (pressure), and collect water samples at a range of depths over and around the seamount. We have completed 14 AUV missions to acquire higher resolution swath bathymetry, sidescan, sub-bottom profiles and still images of the seafloor in specific areas of interest. This data has been ground truthed with numerous ROV dive (18 to date), which have used for geological mapping, rock sampling (we have collected more than 250 rock samples, many insitu), core drilling (a total of 46 cores have been acquired) and biological surveying and sampling. This is probably now one of the most thoroughly surveyed seamounts globally, and the new datasets will form a basis for geological and oceanographic research for years to come.
The new rock cores have been acquired using a core drill designed at the National Oceanography Centre specifically for this project and mounted on the front of the ROV. The cores are potentially the most valuable samples for studying the lateral variations in ferromanganese crust composition, texture and thickness as a result of local-scale processes, such as such as micro-topography, currents and sedimentation rates. These cores form the basis for the research BGS is leading on, which will be delivered by a two year post-doc position, held by the newly appointed Pierre Josso. Pierre undertook his PhD with the University of Southampton on the potential for recovery of rare earth elements from metalliferous sediments on Cyprus. He will be joining BGS/NIGL in February to work on these cores and other samples, with the objective of establishing a litho-chemo-stratigraphy across the seamount.
Anyway we were back on the science programme by Saturday 26th November. Improved physical and chemical characterisation of ferromanganese crusts is crucial to understanding local-scale processes controlling deposit formation and predicting their occurrence. One of the first missions once back at Tropic Seamount was a 17 hour ROV drilling dive, with the aim of acquiring 12 drill cores (the maximum number the ROV can collect in a single deployment) over a lateral distance of some 100 m of ferromanganese crust pavement. These cores will principally be used by BGS to investigate lateral variations in crust thickness and composition, using a combination of high resolution trace element geochemistry and isotope analysis. The ROV drill requires a flat surface for the ROV to land on, so its use is restricted to the large areas of rock pavement, mainly occurring on the summit of the seamount. However, as water depth is likely to have a strong influence on crust composition and thickness it is important to sample over the full depth range of the seamount, from the summit plateau at about 950 m down to where the flanks meet the abyssal plain in water depths >4000 m. This type of sampling relies on picking up loose rock with one of the manipulator arms on the ROV Isis or preferably snapping off insitu samples from the seabed exposures. The manipulator arms are very powerful and capable of lifting about 200 kg, but it still challenging to break pieces off the highly encrusted rock slabs whilst attempting to keep the ROV static. The crust samples we are obtaining typically have very nice sub millimetre-scale layering, representing millions of years of gradual deposition and reflecting paleooceanographic changes (e.g. water mass provenance, depth of the oxygen minimum zone, biological productivity, current velocity and upwelling patterns) in the north-east Atlantic. To compliment the scanning electron microscope, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and isotope studies planned at BGS, high-resolution x-ray computer tomography scanning will be done at the University of Leicester. This quick and entirely non-destructive technique allows the reconstruction and visualisation of the internal structure and textures of materials in three dimensions. It is ideally suited to imaging materials such as crusts, which have well-defined, compositionally distinct layers. Combined with compositional data provided by LA-ICP-MS, this technique provides a powerful tool to explore the heterogeneity (or lack of continuity) of litho-chemo-stratigraphy within crusts and nodules at the centimetre scale.
The potential role of microbes in Fe-Mn deposit formation remains unclear. To study this process colleagues at NOC are collecting small pieces of ferromanganese crust to study. In an attempt to transport these from the seabed sampling site to the surface in their ambient environment we have adapted a niskin bottle mounted on the front of the ROV. Once a piece of rock has been placed in the niskin it is closed to seal it from the wall column as the ROV is recovered back to deck. The uncontaminated material will be used to study any microorganisms colonising the crust and which may have role in concentrating E-tech elements in these deposits.
Co-Chief Scientist JC142