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doi:10.2204/iodp.proc.329.202.2015

Results

Semiquantitative relative abundance data for radiolarians have been produced for the uppermost 108 m of the sedimentary section drilled in Hole U1371D (Fig. F2; Table T1). Because of the geographic location of the drill site (Fig. F1) at the interface between subtropical and subantarctic surface water masses, the observed taxa comprise marker species present in both middle- (Nigrini and Sanfilippo, 2001) and high-latitude (Lazarus, 1990, 1992; Abelmann, 1992) radiolarian biozonation schemes (shown in SMF1.JPG and SMT1.XLS in RADIOLAR in “Supplementary material.”)

The age calibration of the recognized bioevents has been performed in a series of previous studies (see the “Methods” chapter, [Expedition 329 Scientists, 2011b]), and it is reported, along with the occurrence of the events at Hole U1371D, in Table T2. This information has been used to derive a list of events (Fig. F4) that has allowed the establishment of two age models for Site U1371 (Fig. F3).

Although there are some slight differences between these two models, with the most obvious one occurring at ~90.4 mbsf (3.06 Ma according to middle-latitude markers or 4.6 Ma according to high-latitude markers), the two models are in good agreement. Diatom biostratigraphy (see fig. F3 in Suto and Uramoto [in press]) provides an age of ~7.25 Ma at 90.4 mbsf (Fig. F5) based on interpolation between the two diatom events at 80 mbsf (last appearance datum Hemidiscus triangularis; 6 Ma) and 100 mbsf (first appearance datum Actinocyclus ingens var. ovalis; 8.67 Ma). This result suggests that the high-latitude radiolarian zonation provides a better estimate of the actual age compared to its middle-latitude counterpart.

The general sedimentation history at this site seems to imply a relatively constant sediment accumulation rate (SR) downhole to 90.4 mbsf (mean SR = 29.5 m/m.y. when using the middle-latitude marker at this depth; mean SR = 19.7 m/m.y. when using the high-latitude marker instead), followed by a steep decrease downhole to 101.4 mbsf (mean SR = 2.14 m/m.y. or mean SR = 3.1 m/m.y., depending on the age assignment of the sample at 90.4 mbsf).

At ~108 mbsf (top part of Core 329-U1371D-12H), a sharp lithologic boundary between clay-bearing diatom ooze (lithologic Unit I) and zeolitic pelagic clay (Unit II) suggests the presence of a transition, possibly through an unconformity, to much older, preMiocene material (see the “Site U1371” chapter [Expedition 329 Scientists, 2011c]) (Fig. F3). Based on the age model for the upper biosiliceous ooze section and the estimated crustal age at this location, the estimated sediment accumulation rate for the underlying pelagic clay section is ~0.45 m/m.y.

The age interpretation of the radiolarian results presented here is also in good agreement (during the Pleistocene and the late to middle Miocene) with the diatom biostratigraphy from the same drilled section proposed by Suto and Uramoto (in press). The combined diatom, radiolarian, and paleomagnetic stratigraphies (Fig. F5), however, suggest how this agreement breaks down during the Pliocene. During this time, radiolarian markers are always younger than diatom and paleomagnetic events, with the high-latitude radiolarian zonation providing a better match to both diatom and paleomagnetic stratigraphies.

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