Holocene calcareous nannofossils and planktonic foraminifera assemblages in the western Magellan Strait (Chile)

INA8 8th International Nannoplankton Association Conference ABSTRACTS [Abstracts] [A] [B] [C] [D] [E] [F] [G] [H] [I] [J] [K] [L] [M] [N] [O] [P] [Q] [R] [S] [T] [U] [V] [W] [X] [Y] [Z] Filomena O. Amore, Mauro Caffau, Ester Colizza, Gianguido Salvi, Erika Tsakiridou: Holocene calcareous nannofossils and planktonic foraminifera assemblages in the Western Magellan Strait (Chile) (Poster) Two gravity cores, MB91/40 (53°04'S, 73°33'W; 451cm long) and MB91/54R (53°25'S, 72°54'W; 458.5cm long), have been recovered in the deeper and more inner basinal zone of the Western Magellan Strait in the framework of the marine geological research of the Programma Nazionale di Ricerche in Antartide. This work plans to identify the Late Quaternary changes through micropalaeontological data together with sedimentological data and their link to the climatic cycles. In particular, it highlights the value of the coccolithophore assemblages, never analysed before in the Strait of Magellan, and planktonic foraminifera in identifying the palaeoenvironmental and palaeoceanographic characteristics that influenced this area during the Holocene climatic evolution. The western sector of the Strait of Magellan is set up on a transform fault (Magellan Fault), mostly WNW-ESE oriented. A stratified and stable structure of the water-masses characterises this sector, where a colder and more dilute surficial water-mass overlies a deeper, warmer and saltier one. The deeper water-mass testifies to the Pacific influence and the inflow of sub-Antarctic water in this sector of the Strait. The cold, low-salinity surface-waters testify to the influence of continental runoff due to glaciofluvial input (Panella et al., 1991). In the western Magellan Strait, the surficial sediments are characterised by sand, silty sand, carbonate and biogenic carbonate in the threshold zone, whilst the processes of fine sedimentation of carbonate material compounded with terrigenous sediment dominate in the inner part of the Pacific branch (Brambati et al., 1994). The lowermost part (451-420cm) of Core MB91/40 is characterised by clayey silt, sparsely laminated at the bottom with sparse clasts >2mm. The overlying sediment is characterised by gravel and sand with pelitic matrix. The sediment between 400 and 230cm is mainly clayey silt, laminated in the lower part; the base is radiocarbon dated at 18250±60yr BP. Heavy bioturbation in the upper part of the core marks the passage to upper levels. Rare clasts >2mm are also reported. A sandy silt, graded upwards, characterises the interval between 230 and 210cm. The contact with the overlying deposit is bioturbated. The upper part (210-0cm) is clayey silt with low percentages of sand (from 3% to 0.5%). Radiocarbon dates are made at 200-203cm (8900±70yr BP) and 36-39cm (2740±60yr BP). The basal part of Core MB91/54R (458.5-435cm) is characterised by coarse, massive sediments overlain by a laminated, silty clay (435-361cm) with abundant clasts (>2mm). Radiocarbon dates were obtained at 432-435cm (29770±430yr BP) and 363-364cm (18380±130yr BP). Between 361 and 132cm, it is characterised by a silty sand, grading upward, which gradually passes into a sandy silt and clayey silt, with scattered clasts; bioturbation occurs near 170cm. The upper part (132-0cm) consists of sandy silt which gradually passes into clayey silt and silt, upcore. The sand is characterised by a 40-60% biogenic fraction. Radiocarbon dates are made at 129.5-131cm (7740±50yr BP) and 0-1cm (980±40yr BP). The sediments of the upper part of cores MB91/40 and 54R (from 200 and 140cm to the top of the core, respectively) show advanced percentages of CaCO3 to 10%, whilst the lower levels are represented by siliciclastic sediments. The beginning of the carbonate sedimentation may be related to improvement of the climatic conditions during the Holocene (Colizza & Salvi, in press). The most abundant coccolithophore species, in both cores, are Emiliania huxleyi, Gephyrocapsa muellerae, G. caribbeanica and small Gephyrocapsa group. Other species, such as Calcidiscus leptoporus, Helicosphaera carteri var. hyalina, H. carteri and Umbilicosphaera hulburtiana contribute to the assemblages. Rare in the assemblages are species such as Coccolithus pelagicus, Umbilicosphaera sibogae, Syracosphaera pulchra and Helicosphaera carteri var. wallichii. Such species have been found only in the carbonate part of both cores. This could indicate, as proposed by Melis et al. (in press), the beginning of Pacific circulation within the Strait of Magellan. The planktonic foraminifera assemblages, in both cores, are characterised by a lower part consisting of Globigerina bulloides and an upper one which presents G. bulloides, Neogloboquadrina pachyderma and Globigerina quinqueloba. Thus, they could record a similar environmental evolution, although a difference can be highlighted. Core MB91/54R shows a peak of planktonic foraminifera at 110-140cm, which also corresponds to the increase of the sand fraction and the occurrence of reworked/displaced specimens. The coccolithophores and the planktonic foraminifera assemblages in both cores are characterised by low diversity. The palaeoproductivity values (n° coccoliths/g) are higher in Core MB91/54R. Cores MB91/40 and 54R, to 120 and 60cm respectively, record the contemporary quantitative decrease of the species Gephyrocapsa muellerae with a correspondent increase of G. caribbeanica upward in both cores. Such variation may be due to the influence of Pacific waters already recorded, i.e. before in Core MB91/40 and subsequently in Core MB91/54R. The palaeoenvironmental and palaeoclimatic interpretations are refined by means of a preliminary statistical analysis of the various taxa (coccolithophores and planktonic foraminifera). It was performed in the uppermost part of each core. In Core MB91/40 (0-220cm), this analysis has extrapolated two factors (which account for 90% of the variance) corresponding to two parts: Factor 1 characterising the interval between 120-220cm; and Factor 2, which characterises the surficial levels. Thus, such separation could record a palaeoenvironmental and/or a palaeoclimatic change which affected the examined area during the Holocene. The MB 91/54 core, although with less accuracy, records the same variations.