Relative sea-level

 

Orkney Riddle 

Relative Sea Level

Relative sea-level describes the change in sea level relative to the shorelines of the world's continents. Many countries with a shoreline have undertaken offshore surveys to establish a graph that relates present sealevel to those at prehistoric dates in the last 10,000 to 20,000 years.

For Britain this work was carried out by a team headed by Ian Shennan.

The results for their work are published as:- Relative sea-level changes and crustal movements in Britain and Ireland since the Last Glacial Maximum by Ian Shennan, Sarah L. Bradley, and Robin Edwards

They introduce their results as follows:-

The new sea-level database for Britain and Ireland contains >2100 data points from 86 regions and records relative sea-level (RSL) changes over the last 20 ka and across elevations ranging from ~þ40 to -55 m. It reveals radically different patterns of RSL as we move from regions near the centre of the Celtic ice sheet at the last glacial maximum to regions near and beyond the ice limits. Validated sea-level index points and limiting data show good agreement with the broad patterns of RSL change predicted by current glacial isostatic adjustment (GIA) models. 

Their graphic presentation of the dates and heights of the 2100 individual data points is presented below:-

 

A more detailed series of graphs has been modelled by the authors to present change in sea level over time as a linear process.

In this interpretation, in some places sea-level has risen more or less consistently over the last 20,000 years.

In others it has fallen, and in some it has remained somewhat the same.

Further modelling attempts to demonstrate how the presence of ice on Britain forced the mountains down into earth's crust, and how the ice sheets were then compressed to force them to flow from the highlands of Scotland, Wales, Ireland, and Northern England out to the Atlantic Ocean and the North Sea 

 

From "Flow pattern evolution of the last British Ice Sheet by Anna L.C. Hughes, Chris D. Clark, and Colm J. Jordan

This is a projection based on the idea hat snow laid a thick ice sheet on Britain, forcing glaciers to cut glacial valleys out towards the coasts.

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The graphic of individual terrestrial and freshwater data points separated out from the graphic above is shown here:-

These red data points are at locations close to the coast. The most extreme top limit for freshwater water at a coastal location is at 15,000BP when a sample appears at well over 100 metres above the level of the sea at that time. [Sea level here is marked by the lower edge of the group of points, beginning here at 40 metres below sea level at 10,000BP, and rising to zero today]. In fact it is likely that all the data points for terrestrial freshwater are in effect either meltwater or, more recently, river water.

At 10,000BP, the last blast of deglaciation, the depth of meltwater draining offshore at some locations exceeded 50 metres. That is 50 metres above sea level where it was at that date.

This is all freshwater traversing down stream, in one direction, away from land.

Further though, the rate of meltwater flowing off the mountains and towards the sea will have changed through an annual cycle, peaking at midsummer and falling back to a trickle in winter.

 

The blue symbols in the graphic above represent locations where evidence of marine organisms have been found, and the dates and elevations of these suggest that sea level relative to land here rose significantly above global sea-level at that time. 

The above chart marks the dates and levels of marine and terrestrial/freshwater samples in the Clyde Valley in Scotland, without the modelled guidelines that suggest how sealevel in that location changed over time, in a linear manner. 

Global sea levels for some of the data points are measured and calculated as follows, and the distances above sealevel, of the samples, are added to these values. 

17,000BP 100+ metres below

14,000BP 80 metres below 

11,000BP 50 metres below 

7,000BP 10 metres below 

5000BP 5 metres below 

The prehistory of this area of coastal land began at 17,000BP when marine water reached 130 (100+30) metres above sealevel as it was at that date.

At 14,000BP seawater reached land at 95 (80+15) metres above the sealevel as it was at that date.

At 11,000BP freshwater rose to 80 (50+30) metres above prehistoric sealevel. At the same time marine water was lapping shores at 50 metres above prehistoric sea level. 

At 7000BP marine organisms are registered at 15 (10+5) metres above sea level at 7000BP. 

At 5000BP freshwater organics are found at 10 (5+5) metres above sea level at 5000BP. 

Some explanation is required, for what is happening at around 11,000BP when there are both, freshwater sample locations, and marine sample locations present in the same area at the same time. 

(Given a lack of detailed information i have to assume that both marine and terrestrial samples were located on verifiable ancient shorelines)

The freshwater sample is 80 metres above prehistoric sealevel and the marine is 45 metres above prehistoric sealevel. 

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The blue symbols, representing marine transgressions, are present on a select few of the charts, in locations in Scotland and Ireland. These are the data points that scientists use to model prehistoric sealevel as a gradual linear event.

Some of those charts are presented here, the munber refers to Shennan et al, and the date is that of marine transgression in that location:-

The locations are:-

Scotland 

13 north east Scotland 18,000 BP

15 Montrose 14,000BP

16 Tay Valley 15,000BP 

17 Forth Valley 15,000 BP

19 Ardyne 15,000 to 11,000BP 

20 Clyde 15,000BP to 7000BP

22 SE Scotland 6000BP

Ireland (not considered in detail below)

59 Solway Firth 10,000BP

84 West Cork global

88 Shannon Inner, global

91 N Mayo 20,000 BP

95 Lough Swilly 20,000 to 17,000BP

96 Derry 8000BP 6000BP 

97 N Antrim 8000BP

98 N Down 15,000BP 2000BP

99 S Down 17,000BP 7000BP 3000BP

100 Dundalk 20,000 to 17,000 BP 

The locations of most of the sample locations are visible in the map above. Only those that are circled are considered in detail. 

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Aberdeenshire 

Source location 13 is on the Northeast coast of Scotland in the county of Aberdeenshire. The samples that were found and dated were at a location that was about 12 metres above our sea level.

The date for the samples is around 18,000BP, and sealevel at that date was more than 100 metres below present sea level. 

At this time ice-sheets were still settled on Scotland and in the middle of the North Sea, but Shetland had probably lost its ice sheet after 50,000BP. [4]

A large embayment separated Shetland from a promontory ridge along the east coast of the Norwegian channel. This embayment extended south as far as Orkney and the North Coast of Scotland 

Owing to this topography it would have been possible for the instability of sediments in the Atlantic Ocean north of Shetland to cause a tsunami, like the Storegga Submarine Landslide to dump material on the Aberdeenshire coast. 

Location 13 is close to another site that was studied by David Long where geological deposits were defined by him as caused by the Storegga Submarine Landslide.

The Storegga Slide happened in 8150BP but was preceded by the Nyegga Slide, from the same undersea source, possibly in 18,000BP

The Nyegga Slide is recorded in "Revised Storegga Slide reconstruction reveals two major submarine landslides 12,000 years apart" by Jens Karstens et al

Based on their observations of the Storegga area, the authors suggest an updated reconstruction of the present-day slide scar. At ~20,000 BP, more than 35 m of rapidly deposited and presumably poorly consolidated glacial sediments failed during the Nyegga Slide. The Nyegga slope failure mobilized sediments deposited between >30,000 and ~20,000 BP, forming the Nyegga discontinuity (ND) that likely corresponds to the internal boundary in the deposits in the northern sidewall and the top of the intact block in the slide scar infill.

At ~8150 BP, parts of the sediments deposited after the LGM failed during the first phase of the Storegga Slide

It is difficult to constrain whether the Nyegga Slide was a local event limited to the northern margin of the slide scar, or a regional event. However, at least three sediment cores from the southern slide scar segment also show indications for LGM slope failure, with C49 showing ~18,000 BP sediments beneath Storegga Slide deposits and C75 even including LGM mass-transport deposits (Fig. 4f, g). While this may indicate that the Nyegga Slide affected the entire Storegga Slide scar area, this is far from certain due to a lack of conclusive evidence for large stretches of slide scar. Based on the available data, we observe that ~70% of the 50 m that failed at the northern margin of the scar failed during the Nyegga Slide, and not during the Storegga Slide, indicating that the Nyegga Slide was a major failure event.

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The Great Glen 

 The following group of locations are in the Great Glen that joins Glasgow, across Scotland to Edinburgh. They are:-

Locations 15, 16,17,19,20 are sampled along the valley which will have been the main drain route for the release of meltwater from the Scottish Highlands at the peak of deglaciation. 

The amount of freshwater entering this valley from the mountains will have been measurable in cubic kilometres per day at the peak of the hot period which saw the most destructive ice-sheet retreat. [1]

This amount of meltwater would have drained into the valley during the hot summers, and drained away, into neighbouring seas in autumn and winter.

The level of water flowing through the valley will have risen and fallen in phase with the seasons. 

In winter, as reduced flows of meltwater were entering coastal waters from the mountains Atlantic storms were generating huge waves and subsea currents that shredded reed beds in inshore waters creating huge floating rafts of uprooted seaweed. 

As sealevel rose and fell these rafts were dragged into sheltered embayments and left stranded on raised beaches. They carried with them the shells of crustaceans and the bones of fish and birds, anything that was caught up in the seafloor environment. 

Winter still brings these floating rafts, and often the bladder wrack, for instance, is actually attached to the shells that are caught up in it.

This phenomenon is likely to have led to the deposition of shells and organic matter at heights well above the local tide line at times when the amount of meltwater escaping to the sea raised water level, not only above global sea-level in prehistory but also above our own present day sealevel. 

 

This is a huge dump of uprooted seaweed on an Orkney beach after a severe winter storm. (My Yorkshire Terrier for scale!)

A fresh tideline, with higher tide lines up the beach

 Mussel shells caught up in the uprooted seaweed of the tideline. 

Mussel shell with strands attaching it to the uprooted tideline debris.

East Lothian 

Location 22 is in East Lothian, near Edinburgh, Scotland. The date of the sample is comparable with the Neolithic period of human prehistory, and the altitude is just a metre above present day sea-level, but also perhaps 10 metres above prehistoric global sea-level.

I cannot hazard a guess that suggests a cause for this deposit. The time of the event though is the period in which the North Sea was developing and when the Dover Strait was being eroded through, joining the English Channel with the North Sea. 

Ireland 

The sample locations that follow the Irish coast, are at present outside the scope of this enquiry for detailed study. 

It may be sufficient to suggest that the high relative positions of marine samples around the North Channel of the Irish Sea is because this is and was a relatively enclosed area of water with meltwater draining from the Great Glen, and possibly also from Northern Ireland. 

The volumes of meltwater arriving at this area would have been huge, and would have taken significant periods of time to flow out to the Irish Sea and Atlantic Ocean. 

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This research follows a radically altered perspective towards Quaternary history which is outlined in a series of blogs, some of which are detailed here.

The general theory of the glaciation and deglaciation of Britain is recorded in Ice Sheet Britain. [1]

The extent of Doggerland is examined in Archaeology in the North Sea. ]2]

Walkable land in the North Sea describes the land bridge that joined Holland to Norfolk in England [3]

Quaternary Addendum quotes sources for the detailed analysis of events in the North Sea. [4]

Jeffery Nicholls 

South Ronaldsay 

Orkney 

Jiffynorm@yahoo.co.uk