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doi:10.2204/iodp.proc.344.105.2013

Paleontology and biostratigraphy

We examined the microfossil content of core catcher samples from Holes U1412A–U1412C. Calcareous nannofossils and radiolarians provided biostratigraphic age estimates, and benthic foraminifers were used to characterize paleoenvironmental changes at Site U1412.

Calcareous nannofossils

Calcareous nannofossil abundance and preservation varied considerably at Site U1412 (Table T3). In Hole U1412A, nannofossils ranged from abundant to rare, with preservation from good to poor. Samples 344-U1412A-1H-CC and 2H-CC are assigned to Zone NN21, based on the presence of Emiliania huxleyi. For Samples 344-U1412A-3H-CC through 19X-CC, the age is less well constrained and is assigned to Zones NN19–NN21 (1.89 Ma to present). The first appearance of Pseudoemiliania lacunosa, which defines the top of Zone NN19, occurs in Sample 344-U1412A-20X-CC. Sample 344-U1412A-25X-CC contains the first appearance of the genus Discoaster spp., which constrains the base of Hole U1412A to Zones NN19–NN18 (1.89–2.39 Ma).

Nannofossil assemblages from the top of Hole U1412B overlap with those from the base of Hole U1412A. Samples 344-U1412B-3X-CC and 4X-CC are indicative of Zones NN19–NN21. Sample 344-U1412B-5X-CC, containing P. lacunosa, places Samples 5X-CC through 7X-CC in Zone NN19. The presence of E. huxleyi in Samples 344-U1412B-6X-CC and 7X-CC might suggest contamination, potentially due to the difficult drilling conditions. A major change in assemblage occurs between Samples 344-U1412B-7X-CC and 8X-CC, coinciding with a change in lithology from green claystone to nannofossil ooze (see “Lithostratigraphy and petrology”). Samples 344-U1412B-8X-CC through 19X-CC contain a middle Miocene (Zone NN5) assemblage, including Sphenolithus moriformis, Sphenolithus heteromorphus, Coccolithus pelagicus, Discoaster spp., Cyclicargolithus floridanus, and Reticulofenestra pseudoumbilicus. This middle Miocene assemblage is also found in Samples 344-U1412C-2R-CC through 5R-CC. A change from Miocene to Pleistocene assemblages occurs at the top of Core 344-U1412C-6R and is coincident with a change in lithology from nannofossil ooze to green claystone.

Radiolarians

The siliceous fraction is dominated by radiolarians and diatoms, with a minor proportion of sponge spicules. Radiolarians at this site are generally common and well preserved, with the exception of Samples 344-U1412C-5R-CC through 7R-CC.

Radiolarian assemblages from Sample 344-U1412A-1H-CC through 344-U1412B-8X-CC (Unit I) are representative of the Pleistocene–Holocene. However, detailed biostratigraphic zonations could not be assigned to this interval because of the lack of diagnostically important species. The presence of Pterocorys minythorax and Amphirhopalum ypsilon suggests a depositional sequence <1.8 Ma (above Zone RN14; Sanfilippo and Nigrini, 1998). Species such as Acrosphaera trepanata, Sphaeropyle langii, Dictyophimus crisiae, Botryostrobus auritus, and Didymocyrtis tetrathalamus are characteristic of this assemblage.

Assemblages in Samples 344-U1412B-8X-CC through 344-U1412C-4R-CC (Unit II) are indicative of the early middle Miocene (Zones RN5–RN4). Cores 344-U1412B-8X through 18X are assigned to Zone RN5 (14.98–12.02 Ma; Kamikuri et al., 2009). Samples 344-U1412B-19X-CC through 344-U1412C-4R-CC are constrained to Zone RN4 (14.98–17.03 Ma; Kamikuri et al., 2009). Marker species for Zones RN4–RN5 include Didymocyrtis mammifera, Didymocyrtis laticonus, Cyrtocapsella cornuta, Cyrtocapsella tetrapera, Liriospyris parkerae, Tholospyris kantiana, Periphaena decora, Centrobotrys thermophila, and Giraffospyris sp. (Table T4). Because of poor preservation and low abundances, Samples 344-U1412C-5R-CC through 10R-CC (Unit III) were not assigned to a biostratigraphic zone.

Benthic foraminifers

Benthic foraminifers were studied in 41 core catcher samples: 21 from Hole U1412A, 15 from Hole U1412B, and 5 from Hole U1412C (Table T5). Three additional samples from working-half sections were taken at intervals 344-U1412A-19X-6W, 21–24 cm, and 344-U1412B-1H-1W, 0–2 cm, and 8X-1W, 54–56 cm. Benthic foraminifer abundances range from common to present and preservation from good to moderate in the upper and middle sections of Unit I (Samples 344-U1412A-1H-CC through 18X-CC). Abundance decreases significantly in the middle and lower sections of Unit I (Samples 344-U1412A-19X-CC through 344-U1412B-7X-CC). Foraminifers are abundant to common at the top of Unit II (Samples 344-U1412B-8X-1W, 54–56 cm, and 9X-CC). Preservation and abundance decrease downhole within Unit II, with foraminifers becoming rare and strongly recrystallized, especially in the uppermost section of Hole U1412C (Samples 344-U1412C-2R-CC through 6R-CC). Samples contained within Unit III (Samples 344-U1412C-7R-CC and 9R-CC), contain similar species to the ones in Unit I. Throughout Unit III, benthic foraminifers are rare or present.

Benthic foraminifer assemblages indicate two distinct paleoenvironments at Site U1412 that correspond closely with the lithostratigraphic units and biostratigraphic zones. The upper interval (Samples 344-U1412A-1H-CC through 344-U1412B-7X-CC) is characterized by Cibicidoides pachyderma, Uvigerina spp. (Uvigerina peregrina and Uvigerina auberiana), “Globobuliminids” (including the genera Globobulimina, Praeglobobulimina, and Chilostomella), Stainforthia complanata, Valvulineria glabra, Melonis affinis, and Cassidulina carinata (Fig. F12; Table T5). These species show substantial abundance variations downhole that are likely related to changes in the quantity and quality of the organic flux or variations in sediment oxygen concentrations (Fontainer et al., 2002).

The lower interval (Sections 344-U1412B-8X-1W through 344-U1412C-6R-CC) roughly coincides with Unit II. Benthic foraminifer assemblages are substantially different from Unit I and include Globocassidulina subglobosa, Hansenisca altiformis, and two species completely absent from Unit I, Planulina renzi and Cibicidoides pachyderma var. bathyalis. The last occurrence of P. renzi occurs in the middle Miocene, coinciding with the first appearance of Fontbotia wuellerstorfi (van Morkhoven et al., 1986). Although these two species, present in Unit II (Table T5), are not widely used biostratigraphic markers, they help to further constrain Unit II to the Miocene. Benthic foraminifer assemblages in Unit II are characteristic of mesotrophic to oligotrophic conditions and contain a relatively high proportion of epibenthic foraminifers (e.g., Planulina and Cibicidoides), suggesting low sedimentation rates and/or high bottom currents.

Unit III benthic foraminifers differ considerably from those in Unit II but are similar to those in Unit I. However, given the low abundance of individuals, a comparison with Unit I was not possible (Table T5).

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