Previous   |    Next

doi:10.2204/iodp.proc.344.107.2013

Paleontology and biostratigraphy

We examined the microfossil content of core catcher samples from Holes U1413A–U1413C. Biostratigraphy was primarily based on calcareous nannofossils, except for the uppermost section of Hole U1413A, where radiolarians were present. Benthic foraminifers were used to characterize the paleoenvironmental changes at this site.

Calcareous nannofossils

Calcareous nannofossil abundance and preservation varied from common and good in Hole U1413A to rare and moderate in Hole U1413C. Samples 344-U1413A-1H-CC through 14H-CC are assigned to Zone NN21, based on the presence of Emiliania huxleyi. For Samples 344-U1413A-15H-CC through 19X-CC, the age is less well constrained and is assigned to Zones NN20–NN21 (1.89 Ma to present). The first appearance of Pseudoemiliania lacunosa, which defines the top of Zone NN19, is found in Sample 344-U1413A-20X-CC.

Nannofossil assemblages from the top of Hole U1413C overlap with those from the base of Hole U1413A and are assigned to Zone NN19. The last occurrence (LO) of Helicosphaera sellii is observed in Sample 344-U1413C-15R-CC (Table T3). Raffi et al. (2006) assigned this event to 1.34 Ma, although this biohorizon has a moderate to low degree of reliability.

Sedimentation rates for this horizon were calculated using the lower limit of Zone NN19 (1.89 Ma), the LO of H. sellii (1.34 Ma), and the first occurrence of P. lacunosa (0.44 Ma). The sediment accumulation rate of the uppermost interval (Cores 344-U1413A-20X through 344-U1413C-14R, with a thickness of 149.67 m) is estimated to be 166 m/m.y., although this estimate is preliminary and should be considered conservative.

For the lower interval, between Samples 344-U1413C-15R-CC and 43R-CC (a thickness of ~274.74 m) the estimated sediment accumulation rate is 590 m/m.y. The average sediment accumulation rate from Sample 344-U1413A-20X-CC to the bottom of the hole is 383 m/m.y.

Radiolarians

The abundance of radiolarians at Site U1413 varied between few in the upper sections of Hole U1413A (Samples 344-U1413A-1H-CC through 10H-CC) (Table T4) to rare and barren at the bottom of Hole U1413A and the top of Hole U1413C. Preservation ranged from moderate to poor. Spicules were more abundant than radiolarians but were also characterized as few.

Biostratigraphic markers were not abundant in the upper sections of Hole U1413A, with only a few species present, including Anthocyrtidium ophirense, Acrosphaera trepanata, and Sphaerozoum punctatum aff. The presence of S. punctatum aff. in Samples 344-U1413A-2H-CC and 3H-CC can be used to refine this age to <0.2 Ma (Goll, 1980). However, the definitive taxonomic assignment of this species will be confirmed by scanning electron microscopy.

Benthic foraminifers

Benthic foraminifers were observed in 48 core catcher samples (25 from Hole U1413A and 23 from Hole U1413C). An additional sample was taken from interval 344-U1413A-1H-1W, 115–124 cm (Table T5; Fig. F13).

Benthic foraminifer abundances and preservation are dominant and good in Samples 344-U1413A-1H-1W through 19X-CC. However, they range from common to few in the lower part of Hole U1413A (Samples 344-U1413A-20X-CC through 25X-CC) and throughout Hole U1413C, with preservation decreasing from good to moderate. Most of the foraminifer samples show signs of mechanical breakage.

Planktonic benthic ratios display large ranges that generally decrease downhole (Table T5). Benthic foraminifer assemblage changes downhole are remarkable for the presence of only some species in particular intervals. Brizalina bicostata, Cassidulina tumida, Cancris inflatus, and Uvigerina cf. juncea are only present in the uppermost 149 m of sediment (between Samples 344-U1413A-1H-1W and 19X-CC). Other species such as Brizalina spissa, Epistominella smithi, Uvigerina cf. excellens, and Hansenisca altiformis characterize the assemblages between 160 mbsf (Sample 344-U1413A-20X-CC) and 478 mbsf (Sample 344-U1413C-32X-CC). Further downhole (Samples 344-U1413C-36R-CC through 42R-CC), another substantial change in the benthic assemblages is marked by the appearance of Brizalina cf. dilatata.

Two intervals are characterized by the dominance (>40%) of a few species of benthic foraminifers. The first interval, between Samples 344-U1413A-13H-CC and 19X-CC, is dominated by B. bicostata and U. cf. juncea, comprising 70%–90% of the benthic foraminifer assemblage. The second interval is at the bottom of Hole U1413C (between Samples 344-U1413C-32R-CC and 42R-CC) and is characterized by large relative abundances of Uvigerina peregrina and B. cf. dilatata, which comprise 70%–90% of the assemblage. The high abundance and dominance of these species is rather unusual in the fossil record.

Top of page   |    Previous   |    Next