Unit lithology of gravity core sample BGS CS 56/-10/239!

5.4.2.1. Base of core to 2.03 m

As shown in Figure 5.2.B, this unit is dominated by deformed clays, at times within a sandy matrix. Lithology appears similar to the folded mud and mud-clast conglomerate facies described by Jenner et al. (2007) on the Nova Scotia margin, ascribed to sliding or cohesive debris flow deposits. Selby (1989) and Armishaw (1999) suggest that rapid deposition from overturning icebergs could result in chaotic clay deposits, citing Pickering et al.ʼs (1989) description of dropstone facies, based on ODP (Ocean Drilling Program) site 645. However, the facies described by Srivastava et al. (1987) from Baffin Bay appear appreciably different, lacking folding and deformed clay clasts, to those visible in gravity core sample 56/-10/239.

High terrigenous metal content, as indicated by XRF results, signifies a terrestrial sediment source. N. pachyderma (s.) δ18_{O values demonstrate large variation, but}

reach 4.5‰ VPDB, indicative of full glacial conditions (Knutz et al., 2007). These values are 0.5‰ heavier than equivalent δ18_{O values acquired from nearby core}

sample 56/-10/36, which extends back to 14.6 ka cal BP, by Kroon et al. (1997). G. bulloides are scarce or absent.

On the basis of this evidence the unit is classified as a cohesive debris flow, or slide, deposit consisting of glacial sediments.

5.4.2.2. 2.03 m to 2.00 m

The coarse sand layer is a striking feature of the core sample and is identifiable in all datasets. Containing large shell fragments, this unit appears to erode into the underlying clays and is very poorly sorted.

The unit is classified as a turbidite.

It is also possible that the units between base of core and 2.00 m consist of one large mud intraclast within a coarse sandy matrix (sampled between 2.03 and 2.00 m) of a debris flow. Without more core samples and sub-bottom data it is impossible to determine for certain which interpretation is correct, but this does not affect the central work presented in this thesis.

5.4.2.3. 2.00 m to 1.58 m

Alternating layers of sandy clay and silty clay; silt is the dominate particle size fraction between 1.82 m and 1.62 m. XRF data show a significant reduction in metal concentration and mean grain size increases to top of unit. N. Pachyderma (s.) is

absent or scarce above 1.84 m, as opposed to G. bulloides, which becomes more abundant and displays δ18_{O values of 3.4‰ - 2.5‰ VPDB, indicative of the glacial}

transition (Peck et al., 2008).

The unit contains two AMS 14_{C dated samples, which yield dates of 10,688 ± 44} 14_C

BP between 1.98 m to 1.96 m; and 11,134 ± 41 14_{C BP between 1.98 m and 1.94 m.}

The sample that includes the shallowest material yields the oldest date. There are a number of possible explanations for this.

Bioturbation can lead to under- and over-estimation of ages, as Figure 2.2 demonstrates. If a sample overlies a carbonate poor unit, as in this case, there will be unequal mixing of carbonate leading to incorporation of younger material and an age that is too young. However, as the two samples have a very similar depth range this is thought unlikely to cause the 450 14_{C year age difference observed.}

A second option is that older sediment is emplaced above younger material by downslope transport; in this case, as there is no visual sign of the event, probably a turbidite.

Consideration was also given to whether these differences could have been caused by reworking via bottom currents. It is well documented that silts are transported in this manner (McCave et al., 1995) and the transport of foraminifera has been documented, particularly due to storm action in shallow water (Hohenegger and Yordanova, 2001). The key parameters effecting the ease of entraining foraminifera into transport are the testʼs degree of projection above the seabed, coupled with relatively low weight (Kontrovitz et al., 1978). N. pachyderma have relatively small tests that have been shown to be relatively dense, particularly when <250 μm diameter (Oehmig, 1993).

In this study the foraminifera picked for analysis were well preserved and showed no sign of abrasion or staining. Additionally, their glacial transition δ18_{O values are}

consistent with the 14_{C ages. Therefore, it is considered unlikely that foraminifera in this}

unit are reworked by bottom currents.

These dates occur during the late glacial 14_{C plateau (Kitagawa and van der Plicht,}

1998; Beck et al., 2001), during a period when the marine reservoir effect may have changed (Austin et al., 1995), as such it is difficult to justify treating them as being significantly different. Therefore, this thesis considers the mean of the two 14_{C dates}

(10,911 14_{C BP) as the age of the base of this lithological unit.}

The unit is classified as turbidites grading upwards into contourites, as the sortable silt fraction coarsens, possibly associated with the Younger Dryas to Preboreal transition.

5.4.2.4. 1.58 m to 1.00 m

Particle size data shows this unit to consist of a succession of upwards fining sequences and alternation between less sorted (coarser grained) and more sorted (finer grained) sediments. XRF data show an increase in metal concentrations and N. pachyderma (s.) return above 1.38 m. Planktonic foraminiferal δ18_{O values decline.}

An AMS 14_{C date between 1.58 m and 1.56 m yields an age of 12,889 ± 44} 14_{C BP,}

some 2000 years older than underlying dated material.

This unit is classified as a turbidite, on the basis upwards fining sequences and the stratigraphic discontinuity. A lack of other visible mass movement morphologies make more coherent mass movement processes less likely. Additionally, the likely anoxic conditions discussed in section 5.4.1 provide further support to a turbidite interpretation as rapid deposition of fine particles via turbidity current could result in sedimentary anoxia (Raiswell et al., 2008).

The sand layer visible in the core log (Figure 5.2.A), between 1.01 m and 1.00 m was not sampled. However, in conjunction with the striking changes observed in N. pachyderma (s.) δ18_{O (which decreases by 0.5‰ VPDB), chlorine, sulphur and the}

large increase in, more sorted, clay this layer is interpreted as the boundary between turbidite and unit above.

5.4.2.5. 1.00 m to 0.16 m

This unit is characterised by silty sediments of relatively consistent sorting and particle size. Sediment metal content is relatively high and there is a slight decrease in N. pachyderma (s.) δ18_{O, G. bulloides is present from 0.70 m to top of unit.}

An AMS 14_{C date between 0.18 m and 0.16 m yields an age of 11,098 ± 41} 14_{C BP at}

the top of unit.

This unit is interpreted as muddy contourite, however, the stratigraphic discontinuity, between 1.94 m and 1.58 m, indicates that the whole section has undergone downslope transport.

5.4.2.6. 0.16 m to core top

The uppermost unit in the core consists of sandy clay with reduced terrestrial input and increased CaCO3, as indicated by the XRF data. As there is a large increase in CaCO3

detritus particle size data may be less reliable for this unit.

N. pachyderma (s.) are absent from this unit and G. bulloides displays an increase in δ18_{O of 0.3‰ VPDB.}

This unit is classified as a sandy contourite, increased abundance of foraminiferal species and a decline in terrigenous input may signify a transition to Pre-Boreal or Holocene conditions, however this is not reflected in the δ18_{O values, which are}

heavier. This may be due to increased salinity resulting from reduced freshwater ice input. The presence of a rounded pebble indicates potential dropstone or downslope input and the complicated nature of sedimentation in the region.

Figure 5.12 presents the lithological interpretation of CS 56/-10/239.

5.5. Chapter summary

Sedimentological, geochemical and isotopic analysis have demonstrated that CS 56/-10/239 samples material from a mass movement deposit.

Foraminiferal δ18_{O values indicate that material below 2.00 m is of glacial age and the}

lithofacies appears to be that of a debris flow deposit. A further unit, located above an age reversal, appears to be turbiditic. Sediments between 2.00 m and 1.00m appear to have undergone diagenetic modification, probably via methanogenic pyrite formation.

The stratigraphy of the core is complicated, with a number of discontinuities that indicate a debris flow or slide mass, consisting of glacial material, prior to 10,911 14_C

BP and a further failure, containing material dated at 12,889 ± 43 14_{C BP, after 11,098 ±}

41 14_{C BP.}

Foraminifera are not thought to have been reworked by bottom currents, due to a lack of abrasion and the consistency of δ18_{O and} 14_{C dates.}

Chapter six will discuss the sedimentology of the region in relation to process and chronology.

6. Morphology and timing of sedimentary processes on the northwest British