Friday, 4 July 2014

Mud, Silt and Sand

So, when I said to a couple of people that I was heading off on a research cruise to the Norwegian Sea, I got responses of 'Oh how lovely, the fjords are supposed to be beautiful'.  I am sure that the fjords are exceedingly picturesque, but the likelihood of seeing them is a prospect that might only happen from a helicopter, and hey, let's not go there!  We, the team of ten scientists from the National Oceanography Centre, Southampton, Swansea University, BGS Nottingham and BGS Edinburgh are going to do something even more exciting... We are going to come face to face with mud from the sea floor that is thousands of years old, and which flowed down the continental slope (the rise between the deep abyssal plain and the shallow shelves, the ancient relict of palaeo ice margins) as mighty bulldozers in the deep – large submarine debris flows and turbidites.  But why is this exciting?, I hear you ask. Well, let me tell you why.

Mud, silt and sand (aka. sediment) are deposited in layers on the sea-floor.  They form from admixtures of organic and inorganic matter (soft and hard parts of living organisms), and sediments that either settle through the water column that could have been derived from rivers, or remobilised by deep ocean currents.  These layers of mud build up over time and often contain fossilised remains of past environments.  Protists (animal-like, single celled organisms) called foraminifera have a hard calcitic (the same substance that limestone is made from) test (shell: see below) and can be found today pretty much in every ocean basin.  These live either at the surface or and the bottom of the ocean.  Their tests take on the geochemistry of the ambient sea water as they grow and different species, which are distinguished by their different shapes are fussy about what temperature of water they live in.  Information on the ocean environment can also be obtained from the physical characteristics of the sediments themselves, which may give clues to the ice-sheets that abutted the ocean basins.  Therefore if we core through this layer-cake of sediment, we can obtain a record of environmental change in the ocean through time (see next blog post for more details). 

Scanning Electron Micrograph of a Planktonic Foraminifera (Photo J. Stanford)

Occasionally, these layers of sediment can become very thick due to high rates of sediment being delivered to that particular area. One such site was just west of Norway, not now, but around 18 – 14 thousand years ago.  During this time, the ice-sheet that covered Norway during the peak of the last ice-age was melting rapidly as the air temperatures in the Northern Hemisphere started to warm.  Ice contains large amounts of sediment, which have been worn away from the bedrock that the glacier once flowed over.  Today in the Arctic, ice melt tends to happen during the relatively short summer season, and in the past, mixtures of meltwater and sediment would have been injected into the Norwegian Sea as highly sediment laden plumes.  The sediment may have entered at the surface, or may have been injected as periodic, highly dense flows.  These high density flows which form from a steady pulse of sediment laden waters are called hyper- (super) pyncal (density) flows.  Sediment from the last deglaciation (18-14 ka BP or so) accumulated in piles, which today is seen as a <25 m thick sediment drape over much of the Vøring Plateau.  These sediments became unstable over time due to one, or a combination of three key factors (1)  due to gravity (just like when you are digging a hole in the garden and eventually the sides become so steep the mud collapses back in), (2) the shear weight of the sediments causing loading of the thin rigid crust that covers the Earth, triggering earthquakes and/or (3) organic matter, trapped within these sediments started to decompose over time within these thick deposits, releasing gas that escaped through the sediment pile.  Eventually, a crack would have propagated from deep within the sediment pile, all the way to the sea-floor, spawning what is known as a submarine 'gravity flow'.  Depending on whether the sediment flowed downslope as one block, or whether it disintegrated into a much more fluid flow, defines whether these flows as a debris flow or turbidity flow, respectively (see for a visual demonstration of what a turbidity flow looks like and future blog post).

As these sediments fail and rush down the submarine continental slope, they displace the sea-water around them, giving rise to the possibility of powerful and destructive tsunamis (a recent example of this can be seen in Lituya Bay).  One such failure on the Norwegian margin, is known as the Storegga Slide, which mobilised around 900 km3 of sediment and occurred around 8 thousand years ago. It is thought that there are tell-tale traces of this large tsunami that resulted from this failure as far afield as the Shetland Islands.  However, the exact timing and nature of this event is still unsure, despite decades of research. Other slides include Andøya and Traenadjupet (~4 thousand years ago), and Nyk (~16 thousand years ago), precise ages are also still unsure for these.

There is a need to know what caused these large failures in order to mitigate against future catastrophic events, since large accumulations of these sediments still exist on the Norwegian margin today, as a relict legacy of the past cold climate that persisted between throughout the last ice age.  What makes Storegga even more interesting is that fact that its failure roughly coincided with an extreme cold snap in Northern Hemisphere temperatures around 8.2 thousand years ago and therefore, we need to untangle whether abrupt climatic change has a role in destabilising the sediments.  Given that global temperatures have on average risen by 0.72 degrees Celsius since 1951 (IPCC, 2013), and that this change is not uniformly distributed, with enhanced warming in the polar regions (a process known as polar amplification), increased urgency surrounds this need to discover the mechanism behind these potentially catastrophic events.  A series of other, much smaller, but still large failure scars can be seen on the Norwegian sea floor and previously recovered cores of sediment have dated these events as having occurred at intervals during the current warm period, the Holocene that followed the last ice age.  So, we are heading to these sites to try to discover (a) how and why these large failures occurred, (b) when these failures occurred and (c) what were their impacts.  That is why we are excited about going to the Norwegian Sea for some very hard graft, but some really rewarding returns!


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