Statistically Shaky Sea Shells

Statistically Shaky Sea Shells - I

Field trips are part of the game when it comes to the Earth Sciences and Greenland offers a fine array of natural wonders to tempt even the most jaded palate during the summer months.

Eighty percent of Greenland is covered by ice and, in places, this is up to 3.4 km thick.

So, Greenland might not immediately spring to mind as a place to go to observe rocks.

However, it is a huge country and the ice-free area, at 410,000 km2, is nearly twice the size of the UK.

This is generally sparsely vegetated, leaving the rocks beautifully exposed and the geology incredibly easy to see.

The area of East Greenland around Scoresby Sund, Kong Oscar and Kejser Franz Joseph Fjords is the largest ice-free area in Greenland.

It also has incredible geodiversity, with basement rocks as old as three billion years, an almost complete sedimentary record of the last 1.6 byrs and huge volumes of flood basalts from the splitting of the Atlantic.

If you were an alien and wanted to try to piece together the geological story of Planet Earth – but could only visit one area – East Greenland would be the place to go.

The Geology of East Greenland - James Cresswell

The Geology of East Greenland – James Cresswell

The Geological Survey of Denmark and Greenland (GEUS) has more than 50 years of experience in field work in Greenland.

As a natural consequence of this expertise, we are able to arrange customised geological field trips everywhere in Greenland.

The geology in Greenland includes some of the most spectacular analogues to the offshore areas in the North Atlantic.

The geology can be studied either in detail or in true “seismic scale”.

Geological Survey of Denmark and Greenland (GEUS)

Summer field trips also provide the participants with something interesting to write-up during the long winter nights.

From 1989 to 1994, more than 200 m2 were excavated at the Saqqaq site of Nipisat, situated on a small island 15 km south of Sisimiut.

The excellent preservation conditions for organic material, and the fact that some of the stone artefacts were not previously known from the Saqqaq Culture, were the main reasons for the excavation.

More than 70,000 bone fragments, 20,000 flakes and 1,000 artefacts were recovered.

A total of 33 dates, making this site one of the best dated in the entire Arctic, reveal that Nipisat was occupied continuously for nearly 1,500 years.

Although protruding bedrock disturbed the stratigraphy and several lenses of crushed shells interrupted the layers, three different chronological phases could be identified.

Phase 1 is dated by eight 14C dates ranging from 2020 to 1740 BC (cal).

Phase 2 partly overlaps, but is mainly younger than phase 1 and dated by five 14C dates to 1860-1325 BC (cal).

Phase 3 is dated by 16 14C dates to 1310-810 BC (cal).

One date was very young (520 BC (cal)) and problematic because of extreme oscillations of the 14C curve.

Nipisat – a Saqqaq Culture Site in Sisimiut - Map

Map of the Sisimiut District with the three Saqqaq sites: Asummiut, Akia and Nipisat (marked with large black dots) and the additional sample sites (marked with small red dots)

Nipisat – a Saqqaq Culture Site in Sisimiut, Central West Greenland – 2004
Anne Birgitte Gotfredsen, Tinna Møbjerg, Ella Hoch and Kaj Strand Petersen
Meddelelser om Grønland – Man and Society 31

The write-up includes a description of the ancient sea shells that were recovered for carbon dating from the ancient raised beaches of Greenland.

The molluscan assemblages – recent and subfossil

The sampling activities were concentrated to six localities, however some localities were sampled at several levels.

On Nipisat, faunas as well as 14C dates were recorded at five levels: 3, 10-13, 20, 30 and 35 m (Table 57).

Nipisat must be considered the main locality as sampling here included specimens of marine molluscs from the 12 m terraces upon which the Saqqaq people settled (Ni93a-d, Table 57).

The settlement activities are documented, among other things, by the AMS dated bones of Rangifer tarandus and charcoal from the 10-13 m level (Table 57).

The dates span 1500 years, from 3950 to 2470 BP, i.e. 2000 to 520 BC, covering the major part of the Saqqaq Culture period, according to the chronology of the Sisimiut area.

The oldest dated mollusc is Mya truncata, 9450 BP, from the locality “Vejsende”, which at 80 m a.s.l. also represents the highest level of the six localities.

However, the lowermost, found at a level of 3 m, as at Nipisat, is fairly old with ages up to 7660 years BP (Table 57, no. 9 and 19).

Such old dates at low levels can be explained by looking into the state of preservation of the dated shells which, in this case, must be regarded as reworked.

Nipisat – a Saqqaq Culture Site in Sisimiut, Central West Greenland – 2004
Anne Birgitte Gotfredsen, Tinna Møbjerg, Ella Hoch and Kaj Strand Petersen
Meddelelser om Grønland – Man and Society 31

Mya truncata, common name the “blunt gaper”, is a species of edible saltwater clam, a marine bivalve mollusk in the family Myidae.

Mya truncata

Mya truncata: a) foot b) siphon sheath c) exhalant siphon d) inhalant siphon e) umbones or beaks f) anterior g) posterior end of the shell.

The grand finale of the write-up is the “construction of the sea-level curve”.

Repeated measurements were most kindly taken the following summer by a group of students from the Technical University of Denmark.

It was shown, that the measurements taken by us were of the same order of magnitude, except the one from “Sandy Beach”, which was corrected by no less than +19 m.

All elevations are used together with 14C dates in the construction of the sea-level curve.

Nipisat – a Saqqaq Culture Site in Sisimiut, Central West Greenland – 2004
Anne Birgitte Gotfredsen, Tinna Møbjerg, Ella Hoch and Kaj Strand Petersen
Meddelelser om Grønland – Man and Society 31

However, construction of the sea-level curve is no easy task because careful checking is required to ensure the final product is internally consistent and comprehensible within the wider context of other published sea-level curves.

Nipisat – a Saqqaq Culture Site in Sisimiut - Published

The sea-level curve, or rather zone, is drawn through the dated molluscan assemblages with affinity to the eulittoral zone. In this way, assemblages of an infralittoral affinity will appear under this zone, and their depths of living at the time of dating are given in the vertical range from their elevation to the curve.

Taking a closer look at the data between 4000 and 2000 BP covering most of the Saqqaq and Dorset Culture periods, it appears that the shoreline “fell” more than 10 meters during this time.

However, the Saqqaq people stayed on the 10 m terraces at Nipisat as well as at Asummiut, see Table 57 and Fig. 220.

According to Kuijpers et al. (1999) “various indications show that after mid-Holocene times the initial glacio isostatic rebound of Greenland was followed by increased subsidence”.

It seems that the younger part of the sea-level curve from the Sisimiut area may be explained in this way, resulting in a relative sea-level rise up towards present time.

It has long been known that part of the Norse settlements in Southwest Greenland were submerged by the rising sea.

Also in the Disko area north of Sisimiut similar situations have been recorded for some Inuit sites (Rasch and Jensen 1997).

The question arises as to whether this submergence is entirely or partly caused by eustatic rise of sea-level.

During the Holocene the outer fjord region of West Greenland was far from the ice margin.

Therefore the area is more likely to have been influenced by upheaval as it lies within the forebulge area.

Another possibility is an eustatic rise of sea-level.

It has been argued that a sea-level rise in the Mediterranean, from at depth of ~2 m up to the present sea level, is taking place up to the present time, as concluded from a substantial bioerosion (Petersen 2000:fig. 54).

Kuijpers et al. (1999:64) state “that the average late Holocene relative sea level rise is in the order of about (at least) 3 m pr. 1000 years” in Southwest Greenland.

Such a high rise of sea level is not recorded elsewhere around the globe, which would be the case if this rise was based on eustatic movement.

New studies on a model of glacio isostatic movements from West Greenland (Kelly 1980), show that the forebulge mentioned above is situated further away from the ice, so that the Sisimiut region should be also regarded as part of the down pressed area.

All in all there seems to be a general sea-level rise on the west coast of Greenland, although the main cause is difficult to ascertain.

However, we might say that glacio-isostatic movements could be the main part varying from place to place along the coast.

This could be influenced by a minor eustatic sea level rise of a magnitude like the one observed in the Mediterranean.

Nipisat – a Saqqaq Culture Site in Sisimiut, Central West Greenland – 2004
Anne Birgitte Gotfredsen, Tinna Møbjerg, Ella Hoch and Kaj Strand Petersen
Meddelelser om Grønland – Man and Society 31

It is important to note that this sea-level curve [covering 8,500 years] is solely based upon ten selected specimens [out of an original population of 31].

Seven of these specimens are Mytilus Edulis [the blue or common mussel] whose habitat ranges down to 40 metres.

Mytilus edulis

Nipisat – a Saqqaq Culture Site in Sisimiut, Central West Greenland – 2004
Anne Birgitte Gotfredsen, Tinna Møbjerg, Ella Hoch and Kaj Strand Petersen
Meddelelser om Grønland – Man and Society 31

Their selection criteria focussed upon “dated molluscan assemblages with affinity to the eulittoral zone” [foreshore] but it wasn’t rigorously enforced e.g two Mytilus Edulis specimens were excluded.

Interestingly, the highest elevation included in the study [and the sea-level curve] was from Vejsende at 80 metres whilst the highest raised beach in Isortok Fjord is at about 115 metres.

In the Isortok Fjord (67° 11′ N.) the highest raised beach is 380 ft. above sea-level.

Project Gutenberg EBook of Encyclopaedia Britannica, 11th Edition, Volume 12, Slice 5

Isortok Fjord

Resolving that tantalising discrepancy between Vejsende and Isortok Fjord could help clarify whether the maximum raised beach level was:
a) The same in both locations i.e. 115 metres
b) Whether the fjord topography of Isortok Fjord focussed one [or more] tidal surges [or tidal waves] so that an additional raised beach was created at 115 metres in Isortok Fjord.

Additionally, it should be noted that 60% of the specimens included in this sea-level curve were obtained from the low lying island of Nipisat.

Nipisat; 35 m; 00617; No. 10
Only two species were identified in this random collection.
However, Mytilus edulis was so common on the surface that we feel safe to say that this is a former littoral zone.
Later it will be shown that the dating of this level to c. 6250 BP fits well with the 5 m lower level dated to c. 5380 BP, when both of these levels are taken as littoral zones, reflecting isostatic rebound.

Nipisat; 20 m; 00619; No. 8
From this bulk sample only four species in a fragmentary state of preservation are recorded, three Barnacles dominate, and all shell parts are well rounded.
Considering the 15 m higher terrace here on Nipisat, mentioned above, a hundred years older, and this sample lacking in typically infralittoral species, the dated shell material seems to be reworked/downwashed from higher level.

The archipelago surrounding Nipisat

Nipisat – a Saqqaq Culture Site in Sisimiut, Central West Greenland – 2004
Anne Birgitte Gotfredsen, Tinna Møbjerg, Ella Hoch and Kaj Strand Petersen
Meddelelser om Grønland – Man and Society 31

Coordinates: 66°48′50″N 53°30′30″W

Nipisat Island is situated 15 km (9.3 mi) south of Sisimiut, on the shores of Davis Strait.

It belongs to the group of small islands and skerries located at the mouth of Ikertooq Fjord, immediately to the west of Sarfannguit Island.

Dwarf scrub heath, dwarf birch, arctic willow, well-drained lichens, and herb vegetation dominate the flora.

Therefore, basing 60% of the sea-level curve upon specimens of Mytilus Edulis from Nipisat Island seems to be an extremely bad idea when:
1) The mainland specimens indicate that Nipisat Island was submerged during the chronology.
2) The habit of Mytilus Edulis extends down to 40 metres below sea level.
3) Some Nipisat specimens appear to have been “reworked / down washed from higher level”.

However, at the end of the day, all this hard work transforms raw data into published data.

Nipisat – a Saqqaq Culture Site in Sisimiut

Sadly, it seems unlikely that any sea-level curve based upon this data could be meaningful.

UPDATE 24 April 2014
Insight into the “tantalising discrepancy between Vejsende and Isortok Fjord” is provided by an intriguing paper from 2009 which provides two topographical cross sections from the Sisimiut area.

The cross sections clearly quantify the “marine limit” which [in other words] is the maximum raised beach altitude.

Intriguingly, both cross sections show that the “marine limit” decreases progressively as you move away from the coast.

Sisimiut - Cross Section

Ice sheet extent and early deglacial history of the southwestern sector of the Greenland Ice Sheet
David H. Roberts, Antony J. Long, Christoph Schnabel, Bethan J. Davies, Sheng Xu,Matthew J.R. Simpson, Philipe Huybrechts
Quaternary Science Reviews 28 (2009)

Clearly, the observations cannot be explained by a general rise in sea level because the marine limits are at different altitudes.

The mainstream interpretation is that post-glacial uplift [rebound] has occurred and this is demonstrated by the increased level of uplift the further you move away from the edge of the Greenland Ice Sheet.

However, the observed pattern can [alternately] be viewed as the steady dissipation of a tidal wave [or waves] that moved inland towards the Greenland Ice Sheet.

Gallery | This entry was posted in Earth, Greenland. Bookmark the permalink.

1 Response to Statistically Shaky Sea Shells

  1. malagabay says:

    Posting updated regarding the “tantalising discrepancy between Vejsende and Isortok Fjord”

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