Previous   |    Next

doi:10.2204/iodp.proc.334.104.2012

Paleontology and biostratigraphy

At Site U1379, Pleistocene calcareous nannofossils and foraminifers were observed in the thick sediments deposited on the upper slope/continental shelf of Costa Rica. Calcareous nannofossils provide excellent biostratigraphic control for most of the investigated section (Fig. F14). The deepest drilled basal core was tentatively estimated to be lowermost Pleistocene. Thus, the sediments throughout the core would be younger than 1.8 Ma. Based on the nannofossil record, the estimated average sedimentation rates are ~1035 m/m.y. for the uppermost ~566 m of the drilled sedimentary section and ~160 m/m.y. between ~566 and ~722 mbsf of the section. An accumulation rate for the lowermost part of the drilled sedimentary section could not be estimated because of missing zones (Fig. F15). Although, the boundaries of planktonic foraminiferal zones were not recognized because of the limited numbers of samples examined, a few datum events based on the present index species are consistent with the nannofossil zonation. Benthic foraminifers provide significant insights to paleobathymetric changes on the Costa Rica subduction margin. The faunal changes reflect continuous, alternating environmental changes from continental shelf to upper slope (middle bathyal, 500–1000 mbsl).

Calcareous nannofossils

A particularly thick Pleistocene section is observed in the calcareous nannofossil record recovered from Site U1379. Presence, relative abundance, and preservation of nannofossils are documented in Table T3. Sample abundances range from barren to abundant and, in general, show poor to moderate preservation. For the most part, the assemblage abundances were sufficient to delineate biozonal boundaries. However, in several intervals the recognition of key marker species has been restricted because of poor preservation caused by dissolution and dilution of the nannofossil signal caused by a large amount of terrigenous sediments. In this context, the boundaries established during the shipboard examination are tentative.

Samples 334-U1379C-1H-CC through 56X-CC (6.68–466.37 mbsf) are assigned to the undifferentiated Emiliania huxleyi–Gephryocapsa oceania Zone NN21-NN20 of the lower to middle Pleistocene. Dominant species include Helicosphaera carterii, G. oceanica, Gephryocapsa caribbeanica, Gephryocapsa small, and Calcidiscus leptoporus.

Sample 334-U1379C-57X-CC (476.20 mbsf) is very tentatively assigned as the top of the lower Pleistocene Zone NN19 based on the presence of Pseudoemiliania lacunosa and the absence of Discoaster brouweri. This consideration is due to the rare representation of Pseudoemiliania lacunosa in the nannofossil record at this site. Two additional biostratigraphic events that support this zonal assignment include the appearances of H. sellii in Sample 334-U1379C-73X-CC (622.28 mbsf) and Calcidiscus macintyrei in Sample 80X-CC. (679.99 mbsf). Samples 57X-CC through 85X-CC (476.20–722.48 mbsf) are tentatively assigned to the P. lacunosa Zone NN19 of the lower Pleistocene.

The presence of D. brouweri, which establishes the base of Zone NN19 and the top of Zone NN18, is not detected in the sample record. Based on the stratigraphic position, this event probably occurs in the interval between Samples 334-U1379C-86X-CC and 105X-CC (735.99–889.57 mbsf). The sediment in this sequence is either barren or shows low nannofossil abundances and contains no biostratigraphic zonal markers. Therefore, the placement of the Pleistocene/Pliocene boundary is undetermined.

The section between Sample 334-U1379C-106X-CC and Sample 118X-CC (907.14–947.25 mbsf), which contains the zonal marker Discoaster pentaradiatus, is assigned to the upper Pliocene Zone NN17 (Berggren 1995a, 1995b).

Foraminifers

The Pleistocene sediments cored at Site U1379 were deposited in shallow-water shelf environments (shallower than 200 mbsl). Hence, diverse benthic foraminifers abundantly occur, especially in the uppermost part of the drilled sedimentary section, and are well preserved. Conversely, the number of planktonic foraminifers is less than that of benthic foraminifers, as shown by a general trend in modern shallow-marine sediments (Ingle et al., 1980).

Planktonic foraminifers

Because of the time-consuming work required for a foraminiferal study, planktonic foraminifers were analyzed in limited horizons of 18 core catcher samples (Table T4). Planktonic foraminifers are common or rare in the uppermost part of Hole U1379C. The sediment investigated from Hole U1379C is mainly dominated by tropical fauna such as Globigerinoides quadrilobatus (Globigerinoides sacculifer), Globigerinoides ruber, Orbulina universa, Globorotalia menardii, Globorotalia tumida, and Neogloboquadrina dutertrei. However, because of the scarcity or absence of many of the species commonly used as makers for planktonic foraminiferal biostratigraphy, the zone boundaries of Wade et al. (2011) could not be established (Fig. F14). Only three index marker species of planktonic foraminifers were identified in limited horizons. Sample 334-U1379C-16H-CC (90.17 mbsf) contains Globorotalia truncatulinoides (first appearance at 1.93 Ma) and is assigned to foraminiferal Zone PT1. Coiling change of Pulleniatina from sinistral to dextral was observed in Samples 334-U1379C-53X-CC through 64X-CC (436.08–537.13 mbsf). Thus, this horizon would be older than 0.8 Ma. Sample 80X-CC (679.94 mbsf) contains Globigerinoides extremus and would be assigned to be older than 1.93 Ma, the last appearance age. Planktonic foraminiferal abundances are very rare to barren in the interval from Sample 90X-CC to 103X-CC (757.43–882.3 mbsf). Thus, the foraminiferal zones in the lower part of the hole are unclear. The preservation in basement Sample 112X-CC (937.16 mbsf) is poor.

Benthic foraminifers

Benthic foraminifers were analyzed in limited horizons of 16 core catcher samples (Table T5). Benthic foraminifers abundantly occur in the uppermost part of Hole U1379C, whereas their abundances in the lower parts decrease to common or rare. In general, many of the species of benthic foraminifers observed in the sediments of Hole U1379C are similar to modern assemblages, being characteristic of the oxygen minimum zone of the seafloor (100–800 mbsf) in this region.

Sample 334-U1379C-1H-CC (6.63 mbsf) contains Bulimina marginata, Uvigerina peregrina, small Cassidulina spp., and Pseudoparrella spp., indicating an outer continental shelf paleobathymetry. Samples 3H-CC to 68X-CC (24.45–575.36 mbsf) contain Bolivina interjuncta bicostata and Bolivina acuminata, indicating an outer continental shelf to upper bathyal paleobathymetry. Samples 16H-CC and 20X-CC (90.17 and 117.11 mbsf) contain a few shallow-water specimens of euryhaline or inner shelf fauna such as Ammonia beccarii and Buliminella elegantissima. Exceptionally, Sample 10H-CC (63.5 mbsf) contains some abundant species such as Elphidium crispum, indicative of a beach environment, and thus reflects reworked sediments from coastal shallow-marine or temporal shallowing paleobathymetry.

Samples 334-U1379C-58X-CC to 90X-CC (482.33–757.43 mbsf) contain rare deepwater species such as Oridorsalis and Gyroidina, which do not occur at depths shallower than 400 mbsf. In addition, Samples 80X-CC and 90X-CC include Bolivina argentea, which is found in modern slope sediments from 400 to 900 mbsf. Thus, the paleobathymetry of these horizons is estimated to be the deepest depths of upper bathyal or middle bathyal in the Pleistocene. In Samples 69X-CC and 70X-CC, foraminifers are very rare. Half the specimens observed in Sample 90X-CC (757.43 mbsf) are brown, indicating diagenetic overgrowth with carbonate.

Sample 334-U1379C-103X-CC (882.3 mbsf), just above the basement, contains a few shallow-marine species such as Cibicides reflugens, indicative of a beach environment. Basement Sample 112X-CC (937.16 mbsf) includes Cassidulina limbata and Planulina ornata. Although the most dominant species are different from the other samples, the benthic foraminiferal fauna in this sample is composed of several species similar to those in the other horizons. Paleobathymetry of this sample is tentatively estimated to be upper or middle bathyal.

Top of page   |    Previous   |    Next