Hydrographic reconstructions

The percentage abundance of the planktonic foraminifera N. pachyderma (sinistral) has been used extensively for the reconstruction of glacial-interglacial conditions within the North Atlantic (Ericson 1959, CLIMAP 1976, Bond et al 1993) utilising the reciprocal faunal abundance variation with temperature/water mass variations (e.g. Ericson 1959, CLIMAP 1976, Bé and Tolderlund 1971). The left coiling N.

pachyderma (sinistral) morphospecies is dominant in the modern cold northern

latitudes (< 7ºC) and is used as an indicator of the position of the polar front i.e. the transition between polar and subpolar water masses (Bé and Tolderlund 1971, Johannessen et al 1994, Pflaumann et al 1996, 2003).

The relative abundance of N. pachyderma (sinistral) was used to construct the chronology of the core (via correlation to the Greenland and Antarctic ice cores; chapter 3) as well as a proxy for sea surface temperatures (i.e. high %N. pachyderma

(sinistral) is associated with polar conditions, low % N. pachyderma (sinistral) with sub-polar conditions) and the movement of the polar front. IRD counts were converted to IRD flux and used as a proxy of ice sheet dynamics.

The relative abundance of the planktonic, polar species of foraminifera

Neogloboquadrina pachyderma (sinistral), a semi-quantitative proxy for sea surface

temperatures (SST) in the North Atlantic region (Bond et al., 1993; Johannessen et al.,1994) and total IRD concentration, was determined from the >150 μm size fraction. Counts were a minimum of 300 for both total planktonic foraminifera and IRD (all analyses undertaken by F. Hibbert).

Samples of very high abundances of foraminifera and/or IRD were progressively split in half until sufficient to spread evenly across a picking tray. Squares within the picking tray were sampled at random and all foraminifera and/or IRD grains in each square were counted until a minimum of 300 was achieved.

Total number of grains in split = total number grains counted * 42 number of squares counted

(42 is the total number of squares within the picking tray)

Total number of grains within sample = total number grains in split * 2number of splits

Concentration (grains or foraminifera g-1) = Total number of grains in sample dry weight of sediment processed

Materials and Methods 6.2.Stable Isotope measurements (δ18O and δ13C)

For oxygen isotopes, there are three principle influences upon the δ18

O composition of the mineral precipitated:

1. The water temperature at which the mineral is precipitated (δT) (Emiliani 1955, 1971, Shackleton 1967, 1974).

2. The δ18O of the water in which the mineral is precipitated (δ18Owater) and is

determined by both global (variation in global ice volumes (ΔδGIV); Shackleton

1967, Fairbanks 1989, Waelbroeck et al 2002) and local influences (ΔδLocal).

These local influences include: the balance between evaporation and precipitation; inputs of low δ18

O freshwater (e.g. meltwater of river); sea ice3 with resulting seasonal fluctuations (Strain and Tan 1993, Dokken and Jansen 1999) and; advection and mixing of waters from different source areas (Weiss et al 1979, Fairbanks 1982, Parrein and Potter 1984, Kipphut 1990, Frew et al 1995).

3. Vital effects including: ontogenic effects (e.g. Spero and Lee 1996, Kroon and Darling 1995), symbiotic photosynthesis effects (e.g. Spero and Lee 1993, Spero et al 1997), respiration effects (e.g. Lane and Doyle 1956, Grossman 1987), gametogenic effects (e.g. Bé 1980, Bemis et al 1998) and the effect of changes in [CO22-] (e.g. Spero et al 1997, Bemis et al 1998, Bijma et al 1999). Therefore: T O O_Mineral  _Water      18 18 (1) Vital T

O_Mineral   _GIV  _Local  

18 (   )  (2)

The δ18

O of the benthic foraminifera C. wuellerstorfi was used to construct the age model for MD04-2822 (chapter 3) as well as a first order approximation of global ice volumes. The δ13

C of the same species may be used to construct bottom water properties. This taxonomic group reliably records deep water properties (Curry and Oppo 2005) although the epibenthic C. wuellerstorfi does not calcify in isotopic equilibrium with bottom water but in a constant 1:1 relationship (Mackensen and Bickert 1999). Accordingly C. wuellerstorfi values are often adjusted by 0.64 ‰ to place them on a Uvigerina equivalent scale (Shackleton and Opdyke 1973). It has also, been suggested thatC. wuellerstorfisecretes calcite close to isotopic equilibrium of seawater δ18

O (Bemis et al 1998). No correction has been applied to the values obtained for MD04-2822 samples. The reader is directed to chapter 3,

Chronostratigraphy, for a full discussion of influences upon the benthic δ18O isotope

signal.

3 _{Newly formed sea ice is enriched by ~2.57 ± 0.10 ‰ relative to sea water δ}18_{O (Macdonald et al}

1995) and presents seasonal fluctuations in local δ18_{O of sea water with melting and formation of sea}

ice (cf. Strain and Tan 1993). Increases in surface salinity with brine rejection during sea ice formation may lead to convection and the transport of surface waters to the ocean interior (Rohling and Bigg 1998, Dokken and Jansen 1999).

CHAPTER 2:

Materials and Methods Benthic δ13

C carries an imprint of the nutrient content of deep water masses (Shackleton 1977, Duplessy et al 1984) and may be used for the reconstruction of bottom water circulation through time (e.g. Curry et al 1998, Mackensen et al 2001, Curry and Oppo 2005). Species specific vital effects and microhabitat effects may produce deviations from the δ13

CDIC of bottom waters (e.g. review by Mackensen 2008).

The δ18_{O of planktonic foraminifera may be used to reconstruct the δ}18

O of the water in which they calcified and thus allow temperature and salinity changes of the surface ocean to be discerned.

N. pachyderma(sinistral) calcifies below the surface at depths up to 200 m (Bauch et

al 1997) during summer within its ecological range (Bé and Tolderlund 1971, Reynolds and Thunell 1986) but may calcify outside of the summer at the upper extent of its range (Schmidt and Mulitza 2002, King and Howard 2005). N.

pachyderma(sinistral) is commonly used for isotopic measurements of surface waters

of high and mid-latitudes during glacial intervals (e.g. Bond et al 1993, Fronval et al 1995, Lackschewitz et al 1998, Elliot et al 1998, van Kreveld et al 2000).

G. bulloidesis a shallow dwelling planktonic foraminifera with calcification at ~ 30 m

depth (Schiebel et al 1997, Barker and Elderfield 2002, Hillaire-Marcel and Bilodeau 2000) and is associated with the spring bloom (Ganssen and Kroon 2000, Chapman 2010).G. bulloidesis found in modern subpolar waters of the North Atlantic (Bé and Tolderlund 1971, Johannessen et al 1994, Carstens et al 1997). When sufficient numbers of this species was present, they were also picked for stable isotope analyses in order to provide, when compared with δ18

O of N. pachyderma (sinistral),

information upon surface water stratification.

A conventional light microscope was used to select foraminifera for stable isotope analysis at the NERC Isotope Geosciences Laboratory (NIGL), BGS Keyworth and Facility for Earth and Environmental Analysis (FEEA), University of St Andrews. The samples were dry sieved at >250 to 350 μm and 30 monospecific individuals of the planktonic foraminiferaN. pachyderma(sinistral) andGlobigerina bulloideswere selected for each depth interval. Where present at >250 μm, the epibenthic species

Cibicidoides wuellerstorfiwas also picked for stable isotope analysis. In addition, 20

G. bulloides individuals from the 300 to 355 μm size fraction were removed for stable

isotope analysis from the same intervals as samples for Mg/Ca analysis (i.e. paired measurements). All analyses were undertaken by F. Hibbert.

Analytical precision of δ18_{O and δ}13_{C (1σ) is <0.08‰ (NIGL), 0.07 ‰ (FEEA), with} respect to both ratios and is reported on the Vienna Pee Dee Belemnite (VPDB) scale through the NBS standards and working laboratory standard (Carrara Marble for both laboratories). The values obtained for C. wuellerstorfi have not been corrected for disequilibrium effects (e.g. Shackleton and Opdyke, 1973).

Repeats of planktonic isotopes from the same interval, although not from a homogenised sample aliquot, show fairly close correspondence for both NIGL and FEEA. The percentage error in the values obtained are greater for δ13_{C than for δ}18

O (Appendix B1). This may be due to incomplete cleaning and/or the inadvertent inclusion of organic material within the chambers of the foraminifera. The average

Materials and Methods percentage error is similar for both planktonic (N. pachyderma (sinistral) and G.

bulloides) as for the benthicC. wuellerstorfi.

In order to investigate the main processes influencing δ18

O, specific analytical strategies have been adopted, e.g. using monospecific records picked from a narrow size range in order to minimise vital effects. The use of Mg/Ca analysis enables temperature effects to be constrained for selected paired isotope and Mg/Ca analyses (MIS 5e and Holocene samples; see chapter 7).

6.3. Palaeotemperature reconstruction (Mg/Ca analysis of planktonic foraminifera) The use of foraminiferal Mg/Ca ratio utilises the thermodynamic control upon the substitution of Mg2+ for Ca2+ within the shell matrix during calcification. This substitution is related to the temperature of the surrounding waters, with the Mg/Ca ratio increasing exponentially with increasing temperature (e.g. Lea et al 1999, Barker et al 2005). This is based upon the assumption that the Mg/Ca content of the open ocean is constant over glacial-interglacial timescales (Barker et al 2005). Mg/Ca ratios determinations have become a well established method for the reconstruction of past ocean temperatures (e.g. Barker and Elderfield 2002, Eggins et al 2003, Cléroux et al 2008, Thornalley et al 2009). A palaeotemperature record was generated for both MIS 5e (the last interglacial; chapter 7) and a portion of the Holocene within MD04-2822 (chapter 7). All analyses were undertaken by F. Hibbert.

For Mg/Ca determinations, 40G. bulloides individuals (300 to 355 μm) were picked

under a conventional light microscope for the depth intervals 2510.5 to 2560.5 cm (inclusive) at 2cm resolution, 2490.5 cm to 2510.5 cm (inclusive) at 5cm resolution and 2450.5 cm to 2490.5 cm (inclusive) at 10cm resolution. These were cleaned following the procedure of Barker et al (2003); foraminiferal tests were gently broken by crushing between two glass slides; clay removal via centrifuging and minimum settling technique using ultra pure water followed by methanol; boiling with alkali buffered 1% H2O2 removed any organic material; coarse grained silicates were removed manually under a microscope and finally a dilute acid leach (0.001 M HNO3).

Once cleaned, samples were dissolved in nitric acid (0.075 M) and diluted to a fixed Ca concentration of 60 to 100 ppm within 250 μl solution. Analyses were carried out on a Varion Vista ICP-AES at the University of East Anglia in February 2008 and February 2009. Analytical precision of 0.02 mmol/mol (relative standard deviation of 0.49 %) was determined from replicate runs of a standard solution. When this is calculated with respect to temperature, the mean standard deviation is 0.05m/ºC, however, the uncertainty associated with the conversion to temperature (conservative estimates of 1 ºC) are far in excess of the intra-sample variation.

Conversion of concentration to temperature estimates was undertaken using the palaeotemperature equation of Elderfield and Ganssen (2000):

) 1 . 0 ( 72 . 0 /Ca e T Mg    where, T is temperature in ºC

CHAPTER 2:

Materials and Methods

Analysis of procedural blanks indicates that Mg leaching from vials and/or laboratory contamination was minimal. Aluminium concentrations of each sample were monitored to ensure effective removal of clay contaminants had been achieved; samples with detectable Al concentrations were discarded.

Attempts were made to run replicate analyses, however, only a limited number were successful (Appendix B2).