Shelf Sea Biogeochemistry blog

Saturday, 7 March 2015

What is happening in the benthos?

Louis Byrne, British Oceanographic Data Centre, NOC

The seas picked up again on Saturday and unfortunately again a few members of the science crew have been feeling a bit green, however the strong winds left us on Sunday and we had our first days’ proper sunshine of the cruise, complete with the obligatory sunset photograph, but not the fabled green flash!

The focus on this cruise is on processes that are happening in the benthos – meaning the environment above and within the seabed – and how these processes change as the seabed moves from sandy sediment to muddy sediment. To do this we are investigating four sites which are characterised as sand, sandy mud, muddy sand and mud. We managed to complete the NIOZ coring of the sandy site (Site G) over Sunday night, giving our sedimentologists some sandy samples to analyse along with the muddy sediment collected from site A.


A sample of sandy sediment ready to be sliced

Natalie Hicks (SAMS) is using the samples collected by the NIOZ corer to investigate the dynamics of benthic carbon cycling, including how deep and for how long carbon is stored in the different types of marine sediments, and how much is released back into the water column. She is collecting sediment samples from the seabed to a depth of 25 cm and then slicing it into cross sections, with each slice containing sediment from a different depth. These will then be stored in a freezer until the end of the cruise, when they are taken back to the laboratories in the Scottish Association for Marine Science (SAMS) to be analysed.

 


Natalie slicing her core into cross sections

Back at SAMS, the sediments will be analysed for their porosity, which refers to how much space there is for water between the grains in the sediment. Muddy sediment has smaller grains which fit together more tightly than sandy sediment, leaving less space for water between them. This makes it easier for pockets of water deeper in the sediment to be cut off from the sea water above.

Once this happens the water in the pocket will quickly run out of oxygen (there will be more about this later in the blog), making it impossible for aerobic bacteria (they are the ones requiring oxygen for respiration) to consume the organic matter in the water. This organic matter will then be stored in the sediments, unless it is resuspended through physical water movement or animal activity.
 



A NIOZ core about to be dropped into the Celtic Sea

Apart from the porosity and grain size, the carbon (both inorganic and organic) is measured as well as the amount of the lead isotope,
210Pb. The carbon is measured so that we can have an idea of how much biological material is buried within the different sediment types.  210Pb is measured as it can be used as an indicator for how often the sediment at each depth is being resuspended or accumulated. Putting all of these measurements together, can give you a better understanding of benthic carbon cycling, and how this differs between the different sediment types

All of this is important so that we can determine whether each sediment type is a source or a sink of carbon. If we can understand better how deep carbon needs to be buried in the different sediments before it is sequestered (stored permanently in the seabed sediments) and how long it will stay the sediments for, then we can know how much carbon they will absorb over time. This will help us predict how much atmospheric CO2 may be buried in our marine sediments over a certain timescale.

The rains and rough weather returned on Sunday, however I think most of us have our sea legs now!
 



Sunset on Saturday

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