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doi:10.2204/iodp.proc.339.109.2013 BiostratigraphySediments at Site U1391 date from the Holocene to the late Pliocene (Fig. F18; Table T4), with the base age of Hole U1391C estimated to span between 3.31 and 3.5 Ma. Site U1391 shows a continuous record with some changes in sedimentation rates. For the Pleistocene, from the seafloor to ~1.5 Ma, a sedimentation rate of 27 cm/k.y. was estimated, whereas during the early Pleistocene (1.5–2.588 Ma), it is 17 cm/k.y. The Pliocene records the lowest values with an estimation of 13 cm/k.y. The microfossil content of sediment recovered at Site U1391 is usually high. The samples are rich in planktonic and benthic foraminifers as well as calcareous nannofossils (Tables T5, T6). Ostracods are present but were not studied in this site. Pollen and spores are abundant in the eight samples analyzed in Holes U1391A and U1391C. Total pollen and spore concentrations range from ~12,000 to 48,000 grains/cm3, excluding Sample 339-U1391A-35X-CC in which the amount is surprisingly an order of magnitude higher, exceeding 170,000 grains/cm3. The preservation of the grains is mostly good to moderate. The proportion of unidentifiable grains progressively increases downhole, as is the case at Sites U1387, U1389, and U1390 (see Fig. F26 in the “Site U1387” chapter, Fig. F22 in the “Site U1389” chapter, and Fig. F21 in the “Site U1390” chapter [Expedition 339 Scientists, 2013d, 2013e, 2013f]). We also observed microcharcoal particles and dinocysts. Calcareous nannofossilsWe examined all core catcher samples from Holes U1391A–U1391C for calcareous nannofossil biostratigraphy. Additionally, selected samples from Hole U1391A were analyzed in order to constrain biohorizons, paying attention only to marker species. Calcareous nannofossil assemblages are very abundant to common and diverse, and the preservation is good to moderate. Small placoliths (<3 µm) and occasionally medium placoliths (3–5 µm) dominate the assemblages. In total, 12 Pleistocene and Pliocene nannofossil datums defined and/or calibrated by Raffi et al. (2006 and references therein) and Flores et al. (2010) were identified in the holes (Table T4). Inorganic input and reworking of early Neogene and Paleogene species vary from few to common throughout all sections (Table T5). The change in abundance of large Emiliania huxleyi (>4 µm) that characterizes Termination 1 in mid- to low-latitude water masses in the Atlantic Ocean has been proven as a useful event by Flores et al. (2010). This change in abundance was recorded between Samples 339-U1391A-1H-2, 142 cm, and 1H-3, 60 cm (2.92–3.60 mbsf), and between the top of Hole U1391B to 1H-CC (0–9.61 mbsf), making it possible to distinguish the onset of the Holocene. The first occurrence (FO) of E. huxleyi (0.26 Ma), which marks the base of Zone NN21, was placed between Samples 339-U1391A-8H-CC and 9H-1, 75 cm (70.18–71.35 mbsf), and between 339-U1391B-7H-CC and 8H-CC (67.07–76.94 mbsf). However, this event should be taken with caution because of dissolution effects and the low proportion of this species. The last occurrence (LO) of Pseudoemiliania lacunosa (0.46 Ma), considered a globally synchronous event that defines the top of Zone NN19, occurs between Samples 339-U1391A-13H-5, 75 cm, and 13H-6, 75 cm (114.57–115.95 mbsf), and between 339-U1391B-12H-CC and 13H-CC (114.83–123.75 mbsf). A biohorizon considered useful in Pleistocene sediments is the LO of Reticulofenestra asanoi (0.90 Ma), which was placed between Samples 339-U1391A-25X-CC and 26X-3, 84 cm (227.46–231.11 mbsf), and between 339-U1391B-25X-CC and 26X-CC (231.21–240.53 mbsf). The FO of R. asanoi (1.07 Ma), another significant event for the Pleistocene, was recorded between Samples 339-U1391A-32X-5, 60 cm, and 32X-7, 60 cm (291.84–294.70 mbsf), and between 339-U1391B-32X-CC and 33X-CC (298.63–308.79 mbsf). To define these biohorizons, we considered specimens of R. asanoi ≥6 μm in size. The LO of large Gephyrocapsa spp. (>5.5 μm) (1.24 Ma) was recorded between Samples 339-U1391A-36X-CC and 37X-1, 75 cm (334.10–343.25 mbsf), and between 339-U1391B-35X-CC and 36X-CC (328.74–338.45). The LO of Helicosphaera sellii (1.25 Ma) was identified between Samples 339-U1391A-37X-CC and 38X-CC (343.25–353.10 cm) and between 339-U1391B-35X-CC and 36X-CC (328.74–338.45 mbsf). This event is considered diachronous (Raffi et al., 1993; Wei, 1993). However, the occurrence of H. sellii at this site is consistent with the ages provided by Raffi et al. (2006) for the Mediterranean Sea when compared with other calibrated events. The FO of large Gephyrocapsa spp. (>5.5 μm) (1.61 Ma) was identified between Samples 339-U1391C-7R-CC and 8R-CC (391.49–399.92). The LO of Calcidiscus macintyrei (1.66 Ma) was placed between Samples 8R-CC and 9R-CC (399.92–412.14 mbsf). The genus Discoaster is rare in Hole U1391C, however, it is possible to recognize some bioevents. The LO of Discoaster brouweri (1.95 Ma), defining the boundary of Zones NN18 and NN19, occurs between Samples 339-U1391C-15R-CC and 16R-CC (470.36–478.52 mbsf). The LO of Discoaster surculus (2.53 Ma) is an event useful to approximate the Pleistocene/Pliocene boundary. This bioevent was identified between Samples 339-U1391C-24R-CC and 25R-CC (556.62–566.10 mbsf). The LO of Discoaster tamalis (2.8 Ma), another species with low abundance at this site, was placed between Samples 28R-CC and 30R-CC (594.85–613.97 mbsf). These events should be taken with caution because of the scarcity of the species used for its definition. Planktonic foraminifersPlanktonic foraminifers were studied in all core catcher samples from Holes U1391A–U1391C. Planktonic foraminifers are abundant and well preserved in most samples (Table T6). Planktonic foraminifer assemblages dominated by Neogloboquadrina pachyderma (sinistral), Turborotalita quinqueloba, and Globigerina bulloides, typically living in polar to subpolar waters today, are replaced by those dominated by Globigerinodes ruber, including the pink morphotype, Globigerinoides trilobus, and Globigerinoides sacculifer, which are characteristic of warmer waters prevalent during interglacial or interstadial periods. Sample 339-U1391A-1H-CC, taken at 4.16 mbsf, contains abundant N. pachyderma (sinistral), and therefore we relate it to a Heinrich event, very likely Heinrich Event 1, constraining the depth of the Holocene to the upper part of Core 339-U1391A-1H. In many samples, especially in glacial periods, G. bulloides is very abundant, indicating high productivity conditions. Deep-dwelling foraminifers, typically Globorotalia truncatulinoides and/or Globorotalia crassaformis, were observed in many samples, especially in interglacial periods (Table T6). No evidently reworked foraminifers were found at this site. In Holes U1391A and U1391B, only one biostratigraphic event, the top of the paracme of N. pachyderma (sinistral), has been observed. This event, defined by the reappearance of the species in marine isotope stage (MIS) 36 (1.21 Ma; Lourens et al., 2004; Raymo et al., 1989; Sierro et al., 2009) (Table T6), was observed between Samples 339-U1391A-35X-CC and 36X-CC (324.44–334.10 mbsf) and between 339-U1391B-34X-CC and 35X-CC (318.90–328.74 mbsf). Large, heavily encrusted specimens of Neogloboquadrina atlantica (dextral) have been observed in other sites of this expedition occurring at ~1.3 Ma, always within the paracme of N. pachyderma (sinistral). In Holes U1391A and U1391B, such specimens are missing, indicating that the bases of these holes are probably younger than 1.3 Ma. However, this event was observed in Hole U1391C between Samples 339-U1391C-5R-CC and 15R-CC (374.53–470.36 mbsf). The FO of Globorotalia inflata, (2.09 Ma; Lourens et al., 2004), was placed between Samples 339-U1391C-18R-CC and 19R-7, 0–5 cm (498.75–507.95 mbsf) (Table T4). For the first time during this expedition, the LO of N. atlantica (sinistral) (2.41 Ma; Weaver and Clement, 1987) was observed between Samples 339-U1391C-23R-CC and 24R-CC (546.94–556.62 mbsf). The LO of Globorotalia puncticulata (2.41 Ma; Hilgen, 1991; Lourens et al., 2004), was observed between Samples 339-U1391C-24R-CC and 25R-CC (556.62–566.10 mbsf). The coiling change in G. crassaformis from sinistral to dextral (2.99 Ma; Zachariasse et al., 1989; Berggren et al., 1995; Lourens et al., 2004; L. Lourens, pers. comm., 2012) that was also observed at Site U1389 (see “Biostratigraphy” in the “Site U1389” chapter [Expedition 339 Scientists, 2013e]) was placed between Samples 339-U1391C-28R-CC and 30R-CC (594.85–613.97 mbsf) (Table T4). The LO of Sphaeroidinellopsis seminulina (3.19 Ma; Lourens et al., 2004) was found between Samples 339-U1391C-33R-5, 32–34 cm, and 33R-6 (639.55–641.6 mbsf), occurring, as in the Mediterranean and at Site U1389, during an interval of dextral coiling G. crassaformis (Zachariasse et al., 1989). The LO occurs below the dolostone (see “Lithostratigraphy”), confirming the basal age obtained for the dolostone and associated hiatus at Site U1387. The age of this event is well constrained (Lourens et al., 2004), and its offset from the general sedimentation rate line in Figure F18 points to a potentially lower sedimentation rate between the coiling change in G. crassaformis at 2.99 Ma and the LO of S. seminulina at 3.19 Ma. The top of the temporal disappearance of G. puncticulata (3.31 Ma; Lourens et al., 2004) was recorded between Samples 339-U1391C-34R-CC and 36R-CC (652.11–670.98 mbsf) and constrains the base age of Hole U1391C. Benthic foraminifersA total of 28 samples between Sections 339-U1391A-1H-CC and 38X-CC and 20 samples between Sections 339-U1391C-2R-CC and 36R-CC were analyzed for benthic foraminiferal assemblages (Table T7). Benthic foraminifers are well preserved and abundant. The benthic foraminiferal fauna is mainly composed of species of Brizalina, Bulimina, Cassidulina, Cibicidoides, Globobulimina, Melonis, Siphonodosaria, Sphaeroidina, and Uvigerina in varying proportions. Based on major fluctuations in Brizalina spp., Cibicides/Cibicidoides spp., and Siphonodaria spp., three assemblages can be distinguished that suggest variations in upwelling intensity, ventilation, and/or MOW current strength and that alternate throughout the succession:
The “epibenthos group,” which has been suggested as an indicator for MOW intensity in the area (Schönfeld, 1997, 2002; Schönfeld and Zahn, 2000), is generally rare at Site U1391, potentially resulting from the location in a distal portion of MOW. The “Stilostomella extinction” event (0.58–0.7 Ma; Hayward, 2002; Kawagata et al., 2005) was placed between Samples 339-U1391A-21X-CC and 22X-CC (189.27–199.40 mbsf) based on the last common occurrence of stilostomellids, nodosariids, and pleurostomellids. However, individual shells occur in Samples 20X-CC and 19X-CC. This datum agrees well with the age constraints by the nannoplankton assemblages. PalynologyEight samples, six from Hole U1391A (Samples 1H-CC, 7H-CC, 14H-CC, 21X-CC, 28X-CC, and 35X-CC) and two from Hole U1391C (Samples 5R-CC and 15R-CC), were analyzed (Table T8). A problem in the sample preparation could account for the high concentration value, 170,000 grains/cm3, found in Sample 339-U1391A-35X-CC. The sample is characterized by very fine clay, the dominance of semidesert plants, and one of the first occurrences of N. pachyderma (sinistral), can be assigned to MIS 36 or 34 (see above). This sample had to be sieved several times through the 10 µm sieve in order to concentrate the pollen grains. This methodology could have biased the known concentration of Lycopodium spores. However, the same protocol has been applied to proximate and similarly very fine clay Samples 339-U1391C-5R-CC and 15R-CC, which in contrast show similar concentration values to the other samples from this site. Therefore, other reasons that may relate to particular environmental and geomorphological conditions of one of these early Quaternary glacial periods in this region could explain the high pollen concentration of Sample 339-U1391A-35X-CC. Besides that, the very low resolution pollen record reflects (Fig. F19), as at the previous sites (see “Biostratigraphy” in the “Site U1385” chapter and “Biostratigraphy” in the “Site U1390” chapter [Expedition 339 Scientists, 2013c, 2013f]), the alternating dominance of the four main plant ecological groups which characterize this region, Pinus, Mediterranean forest, semidesert, and grasslands, during the last 1.5 m.y. In particular, the high abundance of semidesert plants in the uppermost sample (339-U1391A-1H-CC) at the expense of the Mediterranean forest, heathlands, and grasslands allows us to assign this level to one of the most recent Heinrich stadials, as identified in the nearby Core SU81-18 (Turon et al., 2003). |
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