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doi:10.2204/iodp.proc.303306.203.2009

Results and discussion

Approximately 320 samples from the upper 48 mcd of the corrected splice composite section at Sites U1302 and U1303 were studied. Following the North Atlantic diatom zonation proposed by Baldauf (1987), the event indicative of the change from the Fragilariopsis doliolus Zone (0–0.69 Ma) to the Fragilariopsis reinholdii Zone (0.69–1.88 Ma), which is the last occurrence of F. reinholdii, was not observed. F. reinholdii is present in a few samples, but because of the rarity of the specimens and sparsity of their occurrence biostratigraphically, these are thought to be reworked, which places the interval examined here entirely within the F. doliolus Zone (Baldauf, 1987) (Fig. F1).

Diatom concentration varies widely and ranges from 1.0 × 10³ to 2.1 × 10⁶ valves/g (average = 1.33 × 10⁵ valves/g) (Table T1). The highest diatom concentration is recorded in the lower part, mostly between 35 and 48 mcd (Fig. F1). The moderate diatom concentration corresponds well with moderate to low biogenic silica (opal) content at Sites U1302 and U1303 (O. Romero, unpubl. data) and responds to the dominant mixture of terrigenous components (primarily quartz and detrital carbonate) and calcareous debris (Shipboard Scientific Party, 2005)

The diatom community is highly diversified (see the “Appendix”). We identified ~110 species of diatoms in the upper 48 mcd of the corrected spliced composite section of Sites U1302 and U1303. On average, the most abundant diatoms are Thalassiosira angulata (Gregory) Hasle, Actinocyclus curvatulus Janisch and resting spores (RS) of Chaetoceros spp. (average relative contribution = 13.4%, 10.5%, and 9.6%, respectively).

The highly diversified diatom assemblage reflects the complex oceanographic setting of surface waters overlying Sites U1302 and U1303. The most abundant diatom group is the Arctic/​Subarctic group, composed of A. curvatulus, Rhizosolenia hebetata var. hiemalis Bailey, Thalassiosira antiqua (Grunow) Cleve-Euler, T. angulata, the vegetative cell of Thalassiosira gravida Cleve, and Thalassiosira trifulta G. Fryxell (Andersen et al., 2004). The influence of coastal waters is reflected by the occurrence of several species of Chaetoceros RS, Actinocyclus octonarius Ehrenberg, Coscinodiscus radiatus Ehrenberg, and Thalassionema nitzschioides var. nitzschioides (Grunow) Van Heurck (Romero et al., 2003). Transport from coastal shallow waters into the hemipelagic realm is mirrored by Delphineis karstenii (Boden) G. Fryxell and Paralia sulcata (Ehrenberg) Cleve (Romero et al., 2003, 2008), whereas Alveus marinus (Grunow) Kaczmarska and G. Fryxell and F. doliolus (Wallich) Medlin and Sims mainly represent a tropical/subtropical signal (Romero et al., 2005). The qualitative and quantitative variations of the fossil diatom assemblage at Sites U1302 and U1303 will provide insight into paleoclimatologic and paleoceanographic Pleistocene variability in the high-latitude northwestern Atlantic.

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