Upper Miocene to Upper Pliocene in borehole ODP Site 987

ODP Site 987 (70.50ºN, 17.94ºW) was drilled at a water depth of approximately 1672 m on Anomaly 5 (10 Ma) oceanic crust at the mouth of Scoresby Sund (Map 2, Figs. 4 and 5). The total drilled depth was to 859.4 metres below sea floor (mbsf, Figs. 1-3). The hole did not reach the basement, but the basement, based on seismic data, is inferred to be only a few metres below the bottom of the drillhole (Jansen et al. 1996, Channel et al. 1999, Butt et al. 2001). The dating of the drilled section is primarily based on magnetic stratigraphy which is fairly unambiguous throughout the section and yields an age of approximately 7.5 Ma (Chron 4n) for the base of the hole (Fig. 3). Shipboard paleontology indicated that siliceous and calcareous microfossils would provide little in the way of biostratigraphic constraints and corroboration of the magnetostratigraphic interpretation. However, a shore-based study of dinoflagellate cysts (M. Smelror, unpublished data) provided general support, particularly at the base of the section. The age model of Channel et al. (1999) indicates relatively low sedimentation rates (approximately 5 cm/k.y.) at the base of the section with rates at least four to five times greater during intervals of debris flows at approximately 5-4.6 Ma and approximately 2.6 Ma (Figs. 1-3).

summary figure for borehole ODP Site 987, fig 1

summary figure for borehole ODP Site 987, fig 2

summary figure for borehole ODP Site 987, fig 3

summary figure for borehole ODP Site 987, fig 4

summary figure for borehole ODP Site 987, fig 5

We investigated the borehole from 851.89 (Upper Miocene) to 379.42 m (Upper Pliocene). The top of the Upper Pliocene was not investigated. The recovered cores were sampled at approximately 1.5 metre intervals. The spaces between the cores vary between 3 and 6 m in the upper part of the column to approximately 4-28 m in the lower part (Figs. 1-3). Our palaeontological investigation is based on analysis of calcareous benthic and planktonic foraminifera. Agglutinated foraminifera are quite common in parts of the column (probably both reworked and in situ forms), but these are being described by another author. Our investigation showed that the calcareous benthic and planktonic foraminiferal faunas are scarce in most parts of the sediment column, and the lower part of the Lower Pliocene is nearly barren. The Upper Miocene is also barren with respect to planktonic foraminifera, but in this part calcareous benthic forms occur in small numbers throughout most of the section. In the upper part of the Lower Pliocene and Upper Pliocene calcareous benthic foraminifera are quite common. Planktonic foraminifera are quite scarce in this part, but they occur throughout most of the section (Figs. 1-3). Although the foraminiferal fauna is depleted we were able to make some correlation with the calcareous benthic fauna on the Norwegian continental shelf and the planktonic fauna in the Norwegian Sea. The calcareous foraminiferal fauna at Site 987 is much poorer than in corresponding sediments in the North Sea and Norwegian Sea continental shelf and slope (Eidvin & Rundberg 2001 and 2007, Eidvin et al. 2007). However, similar depleted calcareous foraminiferal faunas are recorded in late Paleogene and Neogene deposits in the western Barents Sea (Eidvin et al. 1998b, Ryseth et al. 2003). The cause of the depletion of the faunas, in both areas, is probably that excess CO2 in the bottom water became corrosive to the very thin-walled CaCO3 tests of the foraminifera, and it seems that the planktonic foraminifera were most affected by this.


Upper Miocene (851.89 to approximately 715 m, lower part of Lithostratgraphic Unit V)

The benthic foraminiferal Globocassidulina subglobosa - Ehrenbergina variabilis - Cibicides dutemplei assemblage coincides nearly completely with the part of the borehole which is given a Late Miocene age by magnetic stratigraphy (Fig. 1). The nominate species are long-range late Paleogene to early Neogene forms, and they are very typical for Upper Miocene to Lower Pliocene deposits in the northern North Sea and on the Norwegian Sea continental shelf (Eidvin et al. 2007, Eidvin & Rundberg 2001 and 2007).

Lower Pliocene (approximately 715 to 508 m, upper part of Lithostratgraphic Unit V, Unit IV and lower part Unit IIIB)

In the upper part of the Lower Pliocene we recorded the lower part of the benthic calcareous Cassidulina teretis - Nonion affine - Elphidium excavatum assemblage and lower part of the planktonic Neogloboquadrina atlantica (sinistral) assemblage (Figs. 1-3). The nominate species of the benthic foraminiferal assemblage are long-range forms which occur throughout most of the Neogene of the North Sea and the Norwegian Sea continental shelf and slope (King 1989, Eidvin et al. 2007, Eidvin & Rundberg 2001 and 2007). M. pseudotepida, which is the index fossil for the Early Pliocene and early Late Pliocene in the North Sea and on the eastern part of the Norwegian Sea continental shelf, is not recorded, and that is natural since it is a shallow shelf form (King 1989, Eidvin et al. 2007, Eidvin & Rundberg 2007). It is noteworthy that the first appearance datum (FAD) of C. grossus is just below the Early/Late Pliocene boundary (Fig. 3). N. atlantica (sinistral) is known from the late Middle Miocene through the Late Pliocene to approximately 2.4 Ma on the Vøring Plateau in the Norwegian Sea (Spiegler & Jansen 1989).

Upper Pliocene (approximately 508 to 379.42 m, upper part of Lithostratigraphic Unit IIIB and Unit IIIA)

In the Upper Pliocene we recorded the upper part of the benthic calcareous Cassidulina teretis - Nonion affine - Elphidium excavatum assemblage, the upper part of the planktonic Neogloboquadrina atlantica (sinistral) assemblage and the Globigerina bulloides assemblage (Fig. 3). The magnetic stratigraphy indicates that the LAD of N. atlantica (sinistral) is close to 3.0 Ma (Fig. 3). This is considerably earlier than what is recorded from the Vøring Plateau (2.4 Ma, Spiegler & Jansen 1989). However, the reason that N. atlantica (sinistral) is not recorded further up in the borehole may be due to the scarcity of planktonic foraminifera in this part. G. bulloides is also very scarce, but is recorded throughout the Upper Pliocene up to the uppermost sample, which is given an age of slightly older than 2.58 by the magnetic stratigraphy (Fig. 3). On the Vøring Plateau, G. bulloides is common in the Late Miocene and Pliocene until approximately 2.4 Ma (Spiegler & Jansen 1989).

Lithology and depositional history

The lithology at Site 987 principally consists of silty clay with varying amounts of ice-rafted clasts and stones. The shipboard scientific party (Jansen et al. 1996) subdivided the succession at the site into five principal lithostratigraphic units (Figs. 1-3) based on visual core descriptions, smear slide examination, measurements of bulk calcium carbonate, spectral reflectance, magnetic susceptibility and natural gamma radiation and X-ray diffraction of bulk sediments (Butt et al. 2001).

The succession is seen to consist of three dominantly hemipelagic phases with varying input from ice-rafting (Units I, III and IV) and two debris-flow units (Units II and IV). We have not investigated samples from Units I and II. The existence of ice on continental Greenland is indicated since approximately 7.5 Ma (Unit V), but environments at this time were dominantly hemipelagic. Sediments in this lowermost unit were chiefly derived from a volcanic source, probably the Iceland Plateau and/or the Kolbeinsey Ridge. The first major advance across the Scoresby Sund took place at approximately 5 Ma (Unit IV). Sediment

input from Jameson Land (Fig. 4) in addition to volcanic sources is indicated by the presence of sedimentary clasts. A return to a dominantly hemipelagic environment took place at around 4.5 Ma (Unit III). Another major glacial advance across the shelf at 2.58 Ma is represented by another debris-flow unit (Unit II). This phase is related to the onset of major Northern Hemisphere glaciations at approximately 2.6 Ma (Butt et al. 2001).


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