Diatom Assemblage Data

The down-core concentration of diatom valves should provide a good approximation of the sedimentation of biogenic opal and broadly parallels the trends in MS (Howe and Pudsey, 1999). In TPC063, two extended periods of comparatively high diatom concentrations sandwich a period o f low biogenic production, roughly consistent with the MS curve (section 4.1.2.). The variability in total diatom concentrations and the associated diatom species

assemblages will now be described from the base of the core to the core top with reference to the age-depth model (Figure 4.1.)

4.1.3.1. MIS 3

The lower portion of the total diatom absolute abundance record, between 702 cm and 542 cm, has a mean diatom concentration o f 5.43 x 106 v/gds, with a standard deviation of 1.22 x 106 v/gds (n = 6). Concentrations at the base (702 cm) and top of the section (542 cm) are 4.87 x 106 v/gds and 5.09 x 106 v/gds respectively, with the only significant variation in between being a peak in concentrations o f 7.92 x 106 v/gds at 670 cm. The high, stable relative abundances of F. kerguelensis (mean = 55.3%, standard deviation = 5.08%, n = 6) and T. lentiginosa (mean = 13.4%, standard deviation = 1.73%, n = 6) and low relative abundance of Hyalochaete Chaetoceros resting spores (Chaetoceros rs) (mean = 10.4%, standard deviation = 3.13%, n = 6) are most evident over this lower portion (702 cm to 542 cm) of the species assemblage data. A significant degree o f variability is observed in A.

tabularis and Rhizosolenia spp., with relative abundances shifting between maxima of 2.8%

and -2.2% and minima of 0.2%, and -0.88%, respectively. Furthermore, relative abundance of the Fragilariopsis sea-ice group {Fragilariopsis curta and Fragilariopsis cylindrus) breach the 3% threshold for the first time in the record, recording a value o f 3.09% at 574 cm (Figure 4.1). Between 542 cm and 494 cm valve concentrations exhibit a gradual increase from 5.09 x 106 v/gds to 8.85 x 106 v/gds. This portion of the record also has distinct peaks (510 cm) in the relative abundances of the Fragilariopsis sea-ice group (2.4%), Thalassiothrix antarctica (5.1%), and A tabularis (4.1%) (Figure 4.1).

The gradual rise in absolute abundance between 542 cm and 494 cm, is followed by a rapid increase to 21.8 x 106 v/gds at 490 cm. Valve concentrations then drop to 13.4 x 106 v/gds followed by a second rapid increase to a peak concentration o f 138 x 106 v/gds at 478 cm.

These peak concentrations are brief because total diatom absolute abundance drops back to 5.04 x 106 v/gds at 474 cm. Despite the magnitude of these peaks a signature is not observed in the MS record (Figure 4.1). The large, double peak in total diatom concentration at 494 cm and 474 cm, is exclusively dominated by Chaetoceros rs, which has relative abundances of 72.7% and 92% at 490 cm and 478 cm, respectively. The rapid ending of this peak (478 cm to 474 cm) results in enhanced relative abundances across the diatom assemblage, most notably in F. kerguelensis, T. lentiginosa, Thalassiothrix antarctica and E. antarctica, which increase from 1%, 0.4%, 0.1% and 0% to 49.4%, 12.3%, 3.4% and 6.6%, respectively (Figure

4.1.3.2. MIS 2

Between 474 cm and 286 cm, the total diatom absolute abundance curve is characterised by high-frequency variability, with a succession of peaks and troughs of varying amplitude.

Initially, during the transition into MIS 2, between 474 cm and 382 cm, the record shows a gradual decline in values to 2.55 x 106 v/gds (382 cm), where concentrations oscillate between peaks o f -8.43 x 106 v/gds and troughs of -4.39 x 106 v/gds. Above this decline, valve concentrations recover to a peak of 9.06 x 106 v/gds at 362 cm, prior to a second decline culminating in low of 1.66 x 106 v/gds at a depth of 314 cm. Above this low, between 314 and 286 cm, the total diatom concentration curve describes two small peaks in valve concentration at 310 cm and 294 cm, with values o f 5.20 x 106 v/gds and 5.95 x 106 v/gds respectively, before returning to a lower concentration of 2.55 x 106 v/gds at 286 cm (Figure 4.1). The high-frequency variability in total diatom absolute abundance, between 474 cm and 286 cm is even more pronounced in the diatom species assemblage relative abundance plots. The variability is characterised by elevated relative abundances of the Fragilariopsis sea-ice group, F. obliquecostata, and Chaetoceros rs, which increase between the depths o f 474 cm and 366 cm, peak around 366 cm and steadily decrease thereafter to 286 cm. The Fragilariopsis sea-ice group relative abundance increases from 2.5% at 474 cm and peaks between 374 cm and 356 cm, where values vary between -4.6% and -1.7%, and subsequently declines to 0.25% at 286 cm. Chaetoceros rs relative abundance increases from 14.2% at 474 cm to a double peak of 65.8% and 60.2% at 370 cm and 362 cm, respectively.

Chaetoceros rs concentrations subsequently decline to a minimum of 8.6% at 286 cm. F.

obliquecostata relative abundance varies dramatically between maxima of -1.6% and minima of 0% between 374 cm and 314 cm and decline to trace values beyond 286 cm. The opposite of the Fragilariopsis sea-ice group and Chaetoceros rs relative abundance pattern is exhibited by T. lentiginosa, Thalassiothrix antarctica and A. tabularis. Their relative abundance curves show a gradual reduction from 12.3%, 3.4% and 2.8% at 474 cm, 474 cm and 458 cm, to minima of 1.8%, 0% and 0.24% at 370 cm, 362 cm and 378 cm, and subsequently increase to peaks of 15.8%, 5.2% and 4.5% at 286 cm, 286 and 302 cm, respectively. The Thalassiosira antarctica group (T. antarctica (cold) and T. antarctica (warm)) increases to maximum relative abundances of 2.9% at 458 cm, declining thereafter to 0.49% at 286 cm. P. glacialis relative abundance gradually climbs from 0% at 474 cm to peak abundance of 3.2% at 350 cm and declines to trace values above 314 cm. The major taxon F. kerguelensis shows a fairly constant relative abundance throughout the period (474 cm to 286 cm), with a mean

relative abundance of 44.8% (standard deviation = 8.67%, n = 48), with the exception of a decline to 1% and 17.2% at 450 cm and -366 cm, respectively. The drops in F. kerguelensis relative abundance at 450 cm and -366 cm are coeval with peaks in the Chaetoceros rs relative abundance curve (Figure 4.1). The high-frequency variability characteristic of the previous 188 cm (474 cm to 286 cm) is absent over the next 198 cm (286 cm to 90 cm) with the diatom absolute abundance curve increasing gradually from 2.55 x 106 v/gds at 286 cm to 4.54 x 106 v/gds at 90 cm. Between these depths (286 cm and 90 cm) the species assemblage shows elevated relative abundances in E. antarctica, T. trifulta, and Chaetoceros rs, with means of 9.6% (standard deviation = 2.86, n = 20), 1.59% (standard deviation = 0.82, n = 20) and 30.9% (standard deviation = 9.26, n = 20), respectively. Over the same period the T.

lentiginosa and Thalassiothrix antarctica relative abundance curves show a gradual reduction, declining from 15.8% and 5.2% to 9.8% and 3.3% between 286 cm and 90 cm, respectively (Figure 4.1).

4.1.3.3. MIS 1

The remaining 86 cm (90 cm to 4 cm) o f the total diatom absolute abundance curve, during MIS 1, is characterised by rapid fluctuations in valve concentrations, between low values (4.54 x 106 v/gds (90 cm), 4.56 x 106 v/gds (74 cm), 5.13 x 106 v/gds (38 cm)) and higher values (11.3 x 106 v/gds (82 cm), 17.2 x 106 v/gds (54 cm), 16.6 x 106 v/gds (24 cm), 12.0 x 106 v/gds (4 cm)). The transition observed in the diatom absolute abundance data is consistent with transition between MIS 2 and MIS 1 in the MS curve (Figure 4.1). Total diatom concentrations at the core top (4 cm) are 12.0 x 106 v/gds. This section of the core is characterised by a large increase in the relative abundance curves of Rhizosolenia spp.

(0.69% to 4.1%), F. separanda (0.67% to 2.6%), T. lentiginosa (9.8% to 15.7%) and A.

tabularis (0.92% to 1.38%) and a coeval decline in the relative abundance curves of Chaetoceros rs (57.1% to 14.5%), E. antarctica (10.5% to 3.6%), Thalassiothrix antarctica (3.3% to 0.76%) and the Fragilariopsis sea-ice group (1.8% to 0.69%) (Figure 4.1).