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Paleontology and biostratigraphy

Shipboard studies of calcareous microfossils (calcareous nannofossils and foraminifers) allow constraints on age and depth of the middle slope sediments of the Costa Rica subduction zone. Pleistocene calcareous nannofossils as well as foraminifers were observed in the marine sediment samples retrieved at Site U1378. The calcareous nannofossil record provides strong biostratigraphic control for this section (Fig. F9). The sediments throughout the core are estimated to be at least lower–middle Pleistocene in age. Approximate sediment accumulation rates are based on the last occurrence datums of the observed microfossils (Fig. F10).

Calcareous nannofossils

A particularly thick Pleistocene section is observed in the calcareous nannofossil record recovered from Site U1378B. Presence, relative abundance, and preservation are documented in Table T3. Sample abundances range from barren to abundant and in general show moderate to good preservation. For the most part, the assemblage abundances and the presence of key marker species were sufficient to delineate biozonal boundaries.

Samples 334-U1378B-1H-CC (5.36 mbsf) through 27X-CC (224.39 mbsf) are assigned to the undifferentiated Zones NN21-NN20 of the upper to middle Pleistocene based on the presence of Emiliania huxleyi in Sample 1H-CC (5.36 mbsf) and the absence of Pseudoemiliania lacunosa. Good preservation allows the tentative identification of E. huxleyi in Samples 1H-CC (5.36 mbsf) through 16H-CC (127.91 mbsf). However, because of the small size of this species, positive identification must be made on the basis of scanning electron microscopy.

Samples 334-U1378B-28X-CC (237.47 mbsf) through 63X-CC are assigned to Zone NN20 based on the last occurrence of P. lacunosa in Sample 28X-CC (237.47 mbsf) and the absence of Discoaster brouweri. Sample 57X-CC (476.18 mbsf) contains the first occurrence of early Pleistocene marker Helicosphaera sellii. Dominant species include Helicosphaera carterii, Gephryocapsa oceanica, Gephryocapsa caribbeanica, Gephryocapsa small, and Calcidiscus leptoporus.


Sample 334-U1378C-5H-CC (43.77 mbsf) contains the pink-colored Globigerinoides ruber (last appearance age of 0.12 Ma, located in marine isotope Stage 5e). In contrast, Sample 334-U1378C-4H-CC (34.62 mbsf) does not contain the pink specimen. Thus, the onset of marine isotope Stage 5e would be located in the interval between these two samples.

Coiling change of planktonic foraminiferal genus Pulleniatina from sinistral to dextral was observed from Sample 334-U1378C-42X-CC (351.71 mbsf) to 30X-CC (245.19 mbsf), but the precise horizon of the coiling change was not identified. Thus, at least the lower Sample 42X-CC would be older than 0.8 Ma in age.