Wednesday, 10 February 2016

First direct evidence of a deep-water cold-seep ecosystem within UK waters...by Heather Stewart

The 'Scotia Seep' Pirates offshore on board the Marine Scotland Science
Vessel M/V Scotia in July 2015 
In 2012 Marine Scotland Science discovered two new species of chemosynthetic bivalve (those that don't rely on sunlight as a source of energy but oxidation of molecules such as methane) and a polychaete (a type of segmented worm) during a biological survey of Rockall Bank located 500km west of Scotland. The discovery of these species suggested for the first time the presence of a deep-sea cold-seep (an area of the sea floor where the seepage of fluids such as hydrogen sulphide and methane occurs) within UK waters. However, the precise location and size of the seep remained unknown as these new species (amounting to a handful of shells) were collected in 1200m of water during a 3 mile long trawl. After two failed attempts to secure funding from Europe to go out and find the seep (“it can’t be done, you’ll never find it”), the BGS along with researchers from Marine Scotland Science, Oceanlab (University of Aberdeen), National Museum of Wales and the Scottish Association for Marine Science went out to Rockall last July with the aim of finding this ‘needle in a haystack’.

Frozen core packed in dry ice
waiting for transport to BGS
The search and discover expedition was a resounding success and the ‘Scotia Seep’ was located within a deep-sea furrow that forms an enclosed depression approximately 10km long and 3km wide in water depths between 1100m and 1200m. The first observational evidence revealed bacterial mats and extensive areas of remobilised sediment that form positive topographic features on the seafloor. Excitingly, evidence of active fluid expulsion at the seabed was also observed, recorded in all its glory in HD.

The surrounding seabed habitats include areas of burrowing animals and anemones in churned-up soft sediment, sponge aggregations on soft sediment and cold-water corals. Several chemosynthetic bivalves (those that feed on chemicals from the sea water) were also recovered and are being identified by collaborators at the National Museum of Wales.

Six megacore samples were recovered from the seep and were frozen offshore to preserve unusual layering observed within the sediment and overlying water. One of those cores was packed in dry ice and transported to BGS for analysis including scanning electron microscopy (SEM) and geochemical analyses.  Some of these analyses have already yielded some interesting results including a marked increase in concentration of base and heavy metals in particular copper, nickel, cobalt, arsenic and perhaps most interestingly uranium within sediments immediately below the sediment/water interface.

These analyses thus far indicate that upwelling, sulphurous and methane-rich fluids are being expelled at seabed on the western flank of Rockall Bank which supports a chemosynthetic community. This cold-seep is the first documented within the UK deep sea and the combination of unusual geochemistry and species suggests that it is quite different from the other cold-seep ecosystems in the north-east Atlantic such as those offshore Norway and Spain.

The exciting and unique results from this cruise are currently being written up for publication.

Photos a-i: a. remobilised sediment, b. bacterial mats, c. active fluid expulsion and soft-sediment remobilisation at seabed, d. burrowing animals and anemones in bioturbated soft sediment, e. sponge aggregations on soft sediment, f. cold water corals, g. gypsum/anhydrite crystals that crystallised following thawing of the core indicating sulphate-rich pore-waters near the sediment/water interface, h. vase-shaped coccosphere (a microscopic marine organism) with more typical round coccosphere on the right, and i. authigenic, magnesium-rich calcium carbonate precipitated on the surface of a foraminifera shell (photos g. to i. seen using a SEM). 
Scientific Team and Acknowledgements

The Offshore Science Party
Francis Neat (Marine Scotland Science (MSS)) led the offshore expedition to find the elusive seep with Alan Jamieson (Oceanlab), Heather Stewart (BGS), Jim Drewery (MSS), Neil Collie (MSS), Mike Stewart (MSS), Mike Robertson (MSS), Graham Oliver (National Museum of Wales), Dave Hughes (Scottish Association for Marine Science (SAMS)), Amy Scott-Murray (Oceanlab), Thom Linley (Oceanlab) and Chris Welch (SAMS).

Post-Cruise Work
Geological interpretation has been undertaken by Thomas Barlow, Aurelie Devez, Lorraine Field, Andrew Marriott, Antony Milodowski and Heather Stewart at the British Geological Survey.

Biological interpretation has been undertaken by Jim Drewery (MSS), Brodie Fischbacher (Oceanlab), Martin Foley (SAMS), Dave Hughes (SAMS), Alan Jamieson (Oceanlab), Bhavani Narayanaswamy (SAMS), Francis Neat (MSS), Graham Oliver (National Museum of Wales), and Matthew Snape (SAMS).

Thanks to the officers and crew of the MRV Scotia. Offshore data acquisition was supported by the Marine Alliance for Technology Scotland Deep-Sea Forum.

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