Transect 1 – Shetland

Transect Leader:

Tom Bradwell (University of Stirling)

Transect Personnel:

T Bradwell, D Fabel, D Small, C Clark, R Chiverrell, M Burke, A Medialdea, M Saher, D Dove, S Morgan, C O’Cofiagh

Fieldwork and activities:

  • 2-9 September 2013 – reconnaissance fieldwork undertaken

  • 5-16 May 2014 – fieldwork season

  • Summer 2015 – Cruise JC 123 – collection of 40 VCs and 3 PCs.  Votal core lenth of 116m of sediment.  Total Geophysical data acquisition 1436km.

 

Click here to view Gallery photos

Background

A: Olex (bathymetric) image of northern North Sea Basin showing the main bathymetric features and glaciological context. Study site (transect) area defined by transparent grey polygon. White lines are inferred time-transgressive ice sheet margins between Shetland and Norway, reconstructed from seabed moraine patterns (Bradwell et al., 2008). Note the large embayment that opens along a N-S corridor of deeper water from the Tampen Ridge to the Witch Ground Basin. Black arrows show the inferrred direction of ice sheet retreat east and west, adjacent to this unzipping corridor (after Bradwell et al., 2008; Clark et al., 2011). B: Simplified Quaternary geology of the continental shelf in the northernmost UK sector and northern North Sea Basin (data from BGS, 2011). Territorial median line is also shown. Note how the Witch Ground Basin is largely mud dominated, whereas the unzipping corridor is characterized by interbedded sands. [‘Undivided’ category may include mud, sand and diamicton of Quaternary or Pre-Quaternary age]. C: Detail of bathymetric surface model showing the complex, but well preserved, geomorphology on the shelf east of Shetland. Large arcuate moraines and lobate till aprons mark the limit of a retreating (oscillating) ice sheet margin. Preliminary mapping shows a number of key moraine complexes: ESR1-3 = East Shetland Readvance(s) 1-3; R4-5 = Major retreat positions 4-5; Numerous intermediate/minor stillstands and retreat positions occur between these. All limits are currently undated. Map also shows potential sampling sites for TCN (onshore), and marine cores along a NE transect from Whalsay (Shetland) to the Tampen Ridge; a second transect broadly N-S would help to constrain the timing of ice sheet unzipping/separation. [Seabed surface – Olex database; land surface – NEXTMap Britain hillshaded DSM].

A: Olex (bathymetric) image of northern North Sea Basin showing the main bathymetric features and glaciological context. Study site (transect) area defined by transparent grey polygon. White lines are inferred time-transgressive ice sheet margins between Shetland and Norway, reconstructed from seabed moraine patterns (Bradwell et al., 2008). Note the large embayment that opens along a N-S corridor of deeper water from the Tampen Ridge to the Witch Ground Basin. Black arrows show the inferrred direction of ice sheet retreat east and west, adjacent to this unzipping corridor (after Bradwell et al., 2008; Clark et al., 2011).
B: Simplified Quaternary geology of the continental shelf in the northernmost UK sector and northern North Sea Basin (data from BGS, 2011). Territorial median line is also shown. Note how the Witch Ground Basin is largely mud dominated, whereas the unzipping corridor is characterized by interbedded sands. [‘Undivided’ category may include mud, sand and diamicton of Quaternary or Pre-Quaternary age].
C: Detail of bathymetric surface model showing the complex, but well preserved, geomorphology on the shelf east of Shetland. Large arcuate moraines and lobate till aprons mark the limit of a retreating (oscillating) ice sheet margin. Preliminary mapping shows a number of key moraine complexes: ESR1-3 = East Shetland Readvance(s) 1-3; R4-5 = Major retreat positions 4-5; Numerous intermediate/minor stillstands and retreat positions occur between these. All limits are currently undated. Map also shows potential sampling sites for TCN (onshore), and marine cores along a NE transect from Whalsay (Shetland) to the Tampen Ridge; a second transect broadly N-S would help to constrain the timing of ice sheet unzipping/separation. [Seabed surface – Olex database; land surface – NEXTMap Britain hillshaded DSM].

The interplay and separation of glaciologically distinct ice sheets is a rarely studied but crucially important phenomenon. Following dynamic separation, ice sheets can rapidly re-organize – as their geometries, internal structures and flow patterns adjust to new equilibria conditions – dramatically changing the way they behave. During the last glacial cycle, the British-Irish Ice Sheet (BIIS) extended into the northern North Sea Basin and during maxima coalesced with the much larger Fennoscandian Ice Sheet (FIS) to the east (Carr et al., 2006; Graham et al., 2007; Bradwell et al., 2008; Chiverrell and Thomas, 2010). Suites of Late Weichselian moraines on the continental shelf edge, west of Orkney and Shetland, relate to this maximum stage c. 25-30 ka BP (Fig. A).

At this time (LGM) the Norwegian Channel Ice Stream may not have operated, or fed into a larger ice stream to the west. Although the geometry and ice flow directions in this sector at LGM are still not firmly understood, there is now – since the advent of shelf-wide bathymetric data (Olex) – a much clearer understanding of the patterns and dynamics of the BIIS and FIS during retreat (Bradwell et al., 2008; Clark et al., 2011).

The clearly defined, but complex, glacial geology and geomorphology on the shelf east of Shetland display the unique footprint of a dynamically retreating ice sheet (Figs B, C). The retreat pattern describes the opening of a large corridor or deep embayment in the ice sheet margin, first reported by Bradwell et al (2008). This is believed to represent the initial separation or ‘unzipping’ of the BIIS from the larger FIS to the east. Detailed mapping shows how this corridor progressively widened, possibly as the result of marine processes playing a greater part in ice-sheet break up (Bradwell et al., 2008; Clark et al., 2011) (Fig. D). The rate and timing of this break-up is still unknown; however it was a major tipping point and probably resulted in widespread re-organization of the northern sector of the BIIS. This undoubtedly represents one of the key events within the decay of the last BIIS.

This transect will allow a detailed understanding of the rate and style of ice sheet separation, along with the internal and external forces driving this phenomenon. Straddling both the BIIS and FIS margins, this transect affords a unique examination of marine ice sheet break-up and re-organization in a climatically sensitive setting. These research questions could be ideally tested east of Shetland using a combination of existing offshore geophysical data (multibeam bathymetry, seismic profiling), coupled with limited onshore sampling (TCN) and more extensive offshore sampling of Quaternary sediments for dateable material (see Fig C).

Sampling programme

TCN (Be-10 in quartz-bearing rocks)

  • 6 key locations around the terrestrial periphery of east Shetland (including the eastern headlands of Unst, Fetlar, Whalsay and Out Skerries).
  • Minimum of 3 samples (glacially deposited boulders) at each site.

C-14 (marine fauna)

  • 18 marine cores (vibrocores) through glacial, glaciomarine and postglacial sediment (unlithified).
  • 4 x AMS analysis of marine fauna (minimum) per core to determine age of seismo-stratigraphically resolved units of importance; constrain timing of ice sheet retreat (+readvances) along main NE-SW flow line and along N-S unzipping corridor.

OSL (terrestrial sands)

  • Field sampling of glaciofluvial / glacio-lacustrine sediments for OSL analysis
  • 2 sites in total; both on Unst (Shetland)