EN LOS BARRIOS URBANO MARGINALES DE LA CIUDAD DE PUNO ”

The DWAF in this fraction are again low in abundance and exhibit a varied distribution through the core. The dominant species, C. pusilla, is present from the base and exhibits a series of sharp peaks, in the lower section of the Matuyama Chron. These correspond with the MB2 complex, which lies just below a gap in abundance, probably corresponding with oxygen isotope Stage 22. This is the level at which the MPR occurred in the core. The first indication of interglacial-glacial cycles is given by the DWAF between Samples 599-601 cm and 557-559 cm (Figure 5.15a/b). This region of the core correlates with MBl (oxygen isotope Stage 21) in Poore's et al. (in prep.) lithostratigraphic model. The species has two more peaks, the first of which closely follows the MBl peak, and relates to BB9. The proposed model indicates that this corresponds with oxygen isotope Stage 19. Cyclammina pusilla is lastly observed between Samples 418-420 cm and 384-386 cm. This top (youngest) part of this band corresponds with BB8 in the extended Brunhes Chronozone represented by the core. The DWAF continue to fortify Poore's et al. (in prep.) argument that the position of the "Brown Beds" correlate with the interglacial periods, which are highlighted by peaks in planktonic foraminifera abundance.

A single peak is produced by Recurvoides contortus in Sample 255-257 cm (Figure 5.15a/b). This corresponds with part of BB5 and the presence of agglutinated foraminifera were observed by Poore et al. (in prep.) at this level. In addition, the species is present in Samples 84-86 cm and 1.0-3.0 cm, which correspond with BB2 and BBl. This is the first locality in the Canada Basin, which the DWAF are seen to correlate with these younger beds, with BB2 possibly representing oxygen isotope Stage 3 and BBl, oxygen isotope Stage 1. This is comparable with the ^^C-AMS dates obtained from analyses of Neogloboquadrina pachyderma in three Canada Basin sedimentary cores, PI-8 8-AR P3, 5 and 9 determined by Poore et al. (1994) and Core PI-8 8-AR P5 only by Poore et al. (1993). Their results indicated that this planktonic species inhabited sediments which were deposited during the current interglacial, i.e. oxygen isotope Stage 1.

Core PI-92-AR 27 125-2000 pm fraction

Cyclammina pusilla

V V

Number of S pecim ens 0 10 20 30 40 I ■ I ■ I . I depth 92-27 100 - 100- 200- BB6 3 0 0 - 4 0 0 5 0 0 ,700 complex Recurvoides contortus

Number of S pecim ens 0 10 20 8 0 0 - -90 0 9 0 Inclination 60 mT Trochammina lo m onosovensis

Number of Specim ens 0 2 4 6 8 10 I . 1. . J

200 -

A tveolophragm ium polarensis

N um ber of S pecim ens 0 1 2 3 4 _l 1----1---1

T o ta l A b u n d a n c e

N um ber of S pecim ens 0 10 20 30 40 50

I I 1 1 1 t 1 1 J

Figure 5.15a: Abundance of foraminifera plotted against magnetostratigraphy

'Lith = Clark e t a l . (1980) " after Poore e t a l . (unpublished) Lithological classifaction. Interpreted by Larry Phillips, U.S.G.S., Menlo Park, CA. Palaeomagnetic scale from Poore e ta l. (in prep )____________________________________________________________________________________

Core PI-92-AR 27 125-2000 ^jm fraction

Cyclammina pusilla R ecurvoides contortus Trochammina lo m o n o so ven sis Alveolophragm ium polarensis T o ta l A b u n d a n c e 5 0 0 - 6 0 0 - depth 9 2 - 2 7 100 - 200 -

J

Number of Specim ens/(g) N umber of Specim ens/(g) Number of S pecim ens/fg) Number of Specim ens/(g) N um ber of Specim ens/(g)

0 1 2 3 4 5 0,0 0.5 1.0 1.5 0.0 0.1 0.2 0 .3 0 .4 0.5 0.6 0.0 0.1 0 .2 0 .3 0 1 2 3 4 5 BBS 3 0 0 — BB7 IM B 2 I c om plex 8 0 0 i r . " -90 0 90 Inclination 60 mT 1 I J I . . . . I . . . . I .1 J. l .L 1 1 1 ■ 1 . 1 , 1 I I I 1 1 I 1 . I >

Figure 5 .1 5b: N um ber foraminifera sp e c im e n s/(g ) plotted again st m agn etostratigrap hy S a m p le s from 1 cm to 7 4 2 cm

'Lith = Clark e t a l . (1 9 8 0 ) " after P o o re e t a l . (in prep ) Lithological classifa ctio n . Interpreted by Larry Phillips, U S G .S ., M enlo Park, CA.

P a la eo m a g n e tic s c a le from P o o re e t a l . (in prep.)___________________________________________________________________________________________________________________

A less abundant species, but with longer distribution is T. lomonosovensis. As with C. pusilla, it is first observed with a series of peaks at the base of the core, within the Matuyama Chronozone. These correspond with the MB2 complex of "Brown Beds". Two further peaks correspond approximately with M Bl, just below the Brunhes/Matuyama boundary. These are followed by a gap, after which a further peak is produced in Sample 407-409 cm, representing BBS. In concordance with R. contortus, the species exhibits peaks corresponding with BB5, this is closely followed by BB4 and finally BB2 and BBl. The final species illustrated in Figure 5.15, A.

polarensis is represented by five abundance peaks and are not present between them. It is firstly observed as with C. pusilla and T. lomonosovensis, at the base of the Matuyama Chron correlating with the MB2 complex. This is followed by a large gap, which may correspond with the MPR; until a peak is produced by the specimens in Sample 592-594 cm correlating with MBl (oxygen isotope Stage 21). The species appears to contribute towards the DWAF record, however, it only highlights a slice of the interglacial periods record. Only one further peak is observed in the core, at Sample 255-257 cm, which represents BB5 in the middle of the Brunhes Chron.

5.16 Core PI-93-AR 21 (63-125 |im) - Northwind Ridge

Poore et al. (in prep.) indicated that this core can be correlated with cores of a similar age, which illustrate features of the Pliocene to middle Pleistocene section from the Northwind Ridge. The egelida bed, MCU and palaeomagnetic analyses were used to generate the stratigraphie framework of this core. It is not possible to use the "Brown Bed" lithostratigraphic model in this case as these features are often obscured because of the condensed Brunhes' sections.

The DWAF are present in low numbers in this fraction and are restricted to the Gilbert and Gauss Chrons. The two species illustrated in Figure 5.16, G. charoides

and Cystammina pauciloculata, are represented by a series of low abundance peaks.

Glomospira charoides is firstly observed between Sample 510-512 and 503-505 cm, which spans the Gilbert/Gauss boundary at 500 cm and indicates that the species lived on sediments 3.53 Ma ago. This is followed by a small peak (1 specimen) at the base of the Gauss Chron in Sample 475-477 cm. The other species present, C.

pauciloculata, has a wider distribution compared with G. charoides and is firstly observed near the base of the core in the Gilbert Chronozone, with abundance peaks representing single specimens Sample 587-589 cm to 552-554 cm. This is followed by a gap, followed by a similar pattern to that shown by G . charoides. The species spans the boundary between the Gilbert and Gauss Chronozones; the presence of both species at this point in the core suggests that the conditions for the DWAF were just favourable at this time.

C o r e P I -9 3 -A R 21 6 3 - 1 2 5 fraction d epth (cm) 9 3 - 2 1 Glom ospira charoides Glomospira charoides Cystammina pauciloculata C ystam m ina pauciloculata Total A b u n d a n c e T o tal A b u n d a n c e

Number of Specim ens Number of Specim ens/(g) Number of Specim ens Number of S pecim ens/(g) Number of Specim ens N umber of Specim ens/(g)

100 - 200-1 300 M 400 - 500 - 600 - 700 800 0.0 0.1 0.2 0 2 4 6 8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0 1 2 3 4 5 6 0.0 0.1 0.2 0.3 0.4 0 1 2 3 □ 100 — 1 0 0 - 1 0 0- 1 0 0- 1 0 0- 100 - 2 0 0- 200 - 200 — 200 — 2 0 0— 200 — bed 5. MCU n 300 - 300 — 3 0 0 - 3 0 0 - 3 0 0 - 300 - 4 0 0 - 400 - 400 — 400 — 4 0 0 - 400 - ■ 5 0 0 - ■500. 6 0 0 - 6 0 0 - 6 0 0 - 600 - ■ 600 - 600 7 0 0 - 700 - 7 0 0 - 700 - 7 0 0 - 700 — 8 0 0 -* 800- 800 —' 800 800 -90 0 90 Inclination 60 mT O - Olduvai subchron

Figure 5.1 6 : A b u n d a n ce and num ber o f foraminifera sp e c im e n s/(g ) plotted a g a in st m agnetostratigrapfiy S a m p le s 3 cm to 6 2 2 cm

Poore et al. (in prep.) observed benthic foraminifera just below the boundary, but planktonic foraminifera do not occur until the egelida bed, in the mid Matuyama Chron. In addition, they noted that the Gilbert and Gauss interval mostly contains <150 jim mineral and rock grains and that the grain sizes alters near the base of the Olduvai subchron to >2 pm. According to Hebbein & Wefer (1991) in a modem study of the Fram Strait and Spjeldamaes (1980), sea-ice transports silt and fine-medium sand grains, whereas sediment transported by glacial ice is typically coarser and poorly sorted. The change of sediment size during the time intervals, as Poore et al.

(in prep.) indicated, may signify the change from conditions where sea ice prevailed during the Gilbert and Gauss interval with little glacial ice, to conditions where glacial ice dominated in the western Arctic Ocean, and possibly a change in source area for the formation of sea ice. This may explain the low abundance of the DWAF in the core, suggesting that they are more suitable to an environment dominated by glacial- ice, rather than sea-ice. However, it is important to note that the DWAF and calcareous benthic foraminifera were able to sustain a living population during these time, whereas the planktonic foraminifera observed by Poore etal. (in prep.) appear to prefer glacial-ice conditions further up core after the MPR.