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doi:10.2204/iodp.proc.318.104.2011 BiostratigraphySamples 318-U1356A-1R-CC through 106R-CC (0–1000.08 mbsf) and selected samples from cores were analyzed for microfossils. Siliceous microfossils (diatoms, radiolarians, silicoflagellates, ebridians, actiniscidians, chrysophyte cysts, and sponge spicules), palynomorphs (e.g., dinocysts, spores, and pollen), planktonic and benthic foraminifers, and calcareous nannofossils are present in various parts of cores from Hole U1356A. A lower Eocene through lower upper Miocene sedimentary succession was recovered from Hole U1356A. In addition, the upper two sections of Core 318-U1356A-1R represent a ~3 m thick uppermost Miocene–lowermost Pliocene sedimentary drape. The results are summarized in Tables T3, T4, T5, T6, T7, and T8. Magnetostratigraphy and biostratigraphy are in good agreement (see “Paleomagnetism”). All index events (biostratigraphic and magnetostratigraphic) are compiled in Table T9. An integrated age-depth model for Hole U1356A is presented in Figure F4. Siliceous microfossils are abundant and well preserved in the uppermost lower Miocene through lower upper Miocene interval (318-U1356A-1R-1, 3–5 cm, through 41R-CC; 0–387.00 mbsf) (Tables T3, T4). Although no biogenic opal is preserved below the uppermost lower Miocene in Hole U1356A (Cores 318-U1356A-42R through 106R; below 387.00 mbsf), it is commonly replaced by pyrite in the interval from Core 42R to the Eocene–Oligocene break between Cores 94R and 95R. Abundant to rare calcareous nannofossils occur throughout the Oligocene sediments (Cores 46R through 94R) (Tables T5, T6). Palynomorphs are recorded in nearly all samples investigated (Table T7). Their preservation varies between poor and good and is at least moderate in most samples. Planktonic and benthic foraminifers are absent above the uppermost lower Miocene (~422.06 mbsf). They occur sporadically in the lower Miocene to lowermost Oligocene interval, typically as low-abundance, low-diversity assemblages, with agglutinated forms dominating. Foraminifers are recorded in most lower and middle Eocene samples (below 886.94 mbsf) (Table T8). Agglutinated forms dominate (typically 90% or more of the foraminifer assemblage), whereas poorly preserved planktonic foraminifers are occasionally observed in extremely low abundances. The different microfossil groups resolve a stratigraphy nearly exclusive of one another by depth in Hole U1356A. Diatoms and radiolarians provide a high-resolution biostratigraphy for the upper 387.00 mbsf (Cores 318-U1356A-1R through 41R; uppermost lower Miocene through lower upper Miocene and uppermost Miocene–lowermost Pliocene) (Tables T3, T4, T9). Calcareous nannofossils, foraminifers, and, to a lesser extent, dinocysts resolve the interval between Cores 318-U1356A-46R and 92R (~434.50 and ~875.12 mbsf; Oligocene) (Tables T5, T6, T9). Dinocysts provide the only microfossil-based age control between ~895.50 and ~995.32 mbsf (Cores 91R through 105R; lower Eocene and Oligocene) (Tables T7, T9). Diatoms and radiolarians suggest that the Pleistocene and all but the lowermost of the Pliocene (i.e., 0 to ~4.74 Ma) is missing in Hole U1356A. Some core disturbance must be taken into account, making interpretation of the upper 2–3 mbsf of the core problematic. Diatoms and radiolarians also suggest a hiatus at ~3 mbsf (between Samples 318-U1356A-1R-2, 138–139 cm, and 1R-CC, coinciding with a sharp color change at 1R-2, 140 cm), which indicates that the latest Miocene (9.5–4.74/5.5 Ma) is also missing (Fig. F4; Table T9). A thick (~430 m) continuous uppermost lower Miocene and middle Miocene through lower upper Miocene (17.0 to ~9.5 Ma) biosiliceous sedimentary succession is defined at high resolution by diatoms and radiolarians to 387.00 mbsf. A hiatus or condensed interval from upper lower Miocene through uppermost Oligocene is suggested by calcareous nannofossils and magnetostratigraphy and appears to be associated with unconformity WL-U5 (~433–445 mbsf) (Fig. F4). The maximum duration of this hiatus or condensed interval is estimated as ~5.62 m.y. between 17.5 and 23.12 Ma. However, detailed study is required to resolve uncertainties resulting from reworking and limited biostratigraphic data. A thick (~460 m) Oligocene sequence is currently constrained by magnetostratigraphy (see “Paleomagnetism”) and partially by dinocyst and calcareous nannofossil data. The Oligocene/Miocene boundary is not preserved in Hole U1356A (Fig. F4). The early middle Eocene to earliest Oligocene is missing in a long hiatus at ~893 mbsf spanning 47.9 to 33.6 Ma based on dinocyst evidence. This ~14 m.y. hiatus is closely associated with unconformity WL-U3 (Fig. F4). The oldest break in sedimentation occurs in lower Eocene strata between Sample 318-U1356A-101R-1, 100 cm, and 100R-1, 100 cm (949.80–940.60 mbsf), based on magnetostratigraphy and the last occurrence (LO; highest stratigraphic occurrence) of Dracodinium waipawaense at 100R-1, 75–76 cm (see also “Paleomagnetism”). The base of Hole U1356A (1000.08 mbsf) is dated at ~53.8 Ma based on dinocyst and magnetostratigraphic data and shows that the Eocene succession recovered in Hole U1356A is ~107 m thick. Reworking of all microfossil groups occurs throughout the post-Eocene sedimentary succession in Hole U1356A. Conspicuously, palynomorphs derived from upper middle Eocene strata are abundantly reworked into lower Oligocene sediments. Siliceous microfossilsCores 318-U1356A-1R through 39R and 41R (387.00 mbsf) contain well-preserved and abundant biogenic silica (opal-A) dominated by diatoms and radiolarians, with variable abundances of silicoflagellates, ebridians, actiniscidians, chrysophyte cysts, and sponge spicules (Table T3). The diatom biostratigraphy provides primary age control in this biosiliceous sedimentary section. Radiolarians provide secondary age control that is in agreement with the diatom data. Opal-A is not preserved in Core 40R (375.82 mbsf) and between Core 42R and the bottom of Hole U1356A (Sample 318-U1356A-106R-CC; 1000.08 mbsf). However, pyritized whole and fragmented siliceous microfossils are consistently present throughout Hole U1356A (noted from analysis of microscope slides prepared for palynology) and are relatively more abundant below the level of opal preservation. DiatomsA characteristic open-water Neogene Southern Ocean diatom flora was encountered in all samples analyzed through the biosiliceous-rich sedimentary interval (i.e., above 387.00 mbsf). Diatoms are abundant to few (Table T3), according to the categories outlined in “Biostratigraphy” in the “Methods” chapter. Samples 318-U1356A-1R-1, 3–5 cm; 1R-2, 2–4 cm; and 1R-2, 138–139 cm (0.03 to 2.89 mbsf), contain a well-preserved, common to abundant latest Miocene to earliest Pliocene diatom assemblage with minor reworked middle Miocene diatoms. The presence of Thalassiosira inura, Thalassiosira torokina, and Thalassiosira oliverana in the absence of other middle to late Pliocene index species such as Fragilariopsis barronii and Thalassiosira complicata (Table T3) suggest that these three samples be assigned to the earliest Pliocene, older than the first occurrence (FO; lowest stratigraphic occurrence) of F. barronii (4.4 Ma) and younger than the FO for T. inura (4.74 Ma). However, the common occurrence of Denticulopsis simonsenii (Table T3) in these upper samples indicates that the LO of D. simonsenii (4.89 Ma) is lost from the core top, supporting an earlier FO of T. inura (e.g., 5.5 Ma as identified by Winter and Iwai, 2002) and a somewhat older age for the section. Further, the presence of Actinocyclus actinochilus (first appearance datum [FAD] 2.81–2.72 Ma; see “Biostratigraphy” in the “Methods” chapter) in Cores 318-U1356A-1R and 2R also indicates some core disturbance by rotary drilling that must be taken into account. A clear succession of index species (e.g., Cody et al., 2008) was recognized through the uppermost lower Miocene through lower upper Miocene interval (Fig. F4; Table T9). This allows assignment of Samples 318-U1356A-1R-CC through 7R-CC (4.60–57.04 mbsf) to the lower upper Miocene, Samples 8R-CC through 39R-CC (66.57–366.25 mbsf) to the middle Miocene, and Sample 41R-CC (387.00 mbsf) to the uppermost lower Miocene. Lower upper MioceneTwelve diatom datums were recognized between Samples 318-U1356A-1R-CC and 7R-CC (4.60–57.04 mbsf) (Table T9):
Diatoms suggest a hiatus at ~3 mbsf (between Samples 318-U1356A-1R-2, 138–139 cm, and 1R-CC, coinciding with sharp color change at 1R-2, 140 cm), which indicates that the upper upper Miocene (4.74/5.5–9.5 Ma) is missing (Fig. F4). A linear sedimentation rate of 37 m/m.y. is inferred for the mid-middle missing lower upper Miocene interval (Fig. F4). We use this sedimentation rate to infer the position of the middle/late Miocene boundary (11.61 Ma) between Samples 318-U1356A-7R-CC and 8R-CC (57.04–66.57 mbsf). Middle MioceneTen diatom datums were recognized between Samples 318-U1356A-8R-CC and 39R-CC (Table T9):
No major hiatuses are inferred in the mid-middle to upper lower Miocene interval with a linear sedimentation rate of 67 m/m.y. (Fig. F4). The early/middle Miocene boundary (15.97 Ma) is inferred between Cores 318-U1356A-38R and 41R (354.05–387.00 mbsf). Lower Miocene, Oligocene, and middle and lower EoceneThe base of opal preservation (~387.00 mbsf) is close to the FO of D. maccollumii (17.01 Ma) in the uppermost lower Miocene (Table T9). Below this, pyritized siliceous microfossils are generally abundant in the Oligocene through lower Eocene sedimentary interval. In five samples (Samples 318-U1356A-76R-5, 87 cm; 76R-CC; 79R-CC; 87R-CC; and 89R-CC, some of which are those prepared for palynology) (Table T7), pyritized specimens are very abundant and are useful for age control and paleoenvironmental reconstruction in conjunction with palynomorph data. Their presence indicates prolific production and synsedimentary diagenesis in a restricted circulation (low oxygen) environment. The most commonly pyritized diatoms are Pyxilla spp., Hemiaulus spp., and stephanopyxids. In some samples, index taxa are recognized, for example, the presence in Sample 318-U1356A-87R-CC of pyritized specimens of Cestodiscus spp. (common), Hemiaulus caracteristicus (trace), and Skeletonemopsis barbadense (trace) in association with abundant pyritized stephanopyxids and Pyxilla spp. indicates a late Eocene to early Oligocene age. This age assignment is partly in agreement with dinocyst and paleomagnetic data and is primarily based on unpublished siliceous microfossil range data from Ocean Drilling Program (ODP) Leg 189 Hole 1172A (C.E. Stickley, pers. comm., 2010), Leg 177 Hole 1090B (S.M. Bohaty and C.E. Stickley, pers. comm., 2010), and Leg 113 Hole 689B (S.M. Bohaty and D.M. Harwood, pers. comm., 2010). One complete opaline specimen of the upper Oligocene–lower Miocene index diatom Rocella gelida was recorded in Sample 318-U1356A-49R-CC (~460.00 mbsf). The current age model, as defined by calcareous microfossils and magnetostratigraphy (Fig. F4), indicate that this specimen is in situ. RadiolariansRadiolarians occur in variable abundances throughout Hole U1356A (Table T4). They are well preserved to moderately well preserved from Sample 318-U1356A-1R-1, 3–5 cm, through 39R-CC (0.03–366.25 mbsf). Sample 40R-CC (375.82 mbsf) contains only a few recrystallized radiolarians, whereas Sample 41R-CC (387.00 mbsf) contains common and comparatively well preserved radiolarians. Preservation is poor and abundance is rare (or barren) in all samples below this level. Samples 318-U1356A-1R-1, 3–5 cm (0.03 mbsf), and 1R-2, 2–4 cm (1.52 mbsf), contain Prunopyre titan (LO at ~3.5 Ma) and Eucyrtidium pseudoinflatum (LO at ~4.2 Ma) but lack Helotholus vema (2.3–4.2 Ma). The absence of Miocene radiolarians such as Acrosphaera australis (5.6–10.4 Ma) and Acrosphaera? labrata (~5 to 7.8 Ma) provides an additional maximum age constraint that agrees with the diatom data, indicating that these two samples must be earliest Pliocene in age. Acrosphaera australis is present in Sample 318-U1356A-1R-CC (4.60 mbsf) but absent in samples below this level. The evolutionary ancestor to A. australis, Acrosphaera murrayana, co-occurs with Actinomma golownini (LO at ~11.0 Ma) in Sample 318-U1356A-3R-CC (19.8 mbsf). The evolutionary transition of A. murrayana to A. australis (10.4 Ma) therefore occurs within the interval between Samples 1R-CC and 3R-CC (4.60–19.80 mbsf). Sample 11R-CC (98.66 mbsf) contains only few radiolarian specimens; therefore, the LO of Cycladophora spongothorax (12.6 Ma) likely falls between Samples 10R-CC and 12R-CC (85.89–109.33 mbsf). The FOs of A. golownini (13.5 Ma) and Cycladophora humerus (14.1 Ma) fall within the interval between Samples 318-U1356A-14R-CC and 15R-CC (130.70–141.75 mbsf) and Samples 21R-CC and 22R-CC (193.85–203.95 mbsf), respectively (Table T9). The FO of Eucyrtidium punctatum (17.0 Ma) is recognized within Sample 41R-CC (387.00 mbsf), indicating an age younger than 17.0 Ma. However, Sample 41R-CC marks the lowermost sample from which radiolarians were recovered (and identifiable to species level); thus, the lower part of its range may be truncated in the barren interval. Hence, the given age should be regarded as a minimum age only. Silicoflagellates, ebridians, chrysophyte cysts, actiniscidians, and sponge spiculesAbove 387.00 mbsf (bottom of Sample 318-U1356A-41R-CC), ebridians and sponge spicules occur in few to trace abundance, whereas silicoflagellates, actiniscidians, and chrysophyte cysts occur in trace abundance (Table T3). All groups exhibit good preservation. Calcareous nannofossilsCalcareous nannofossils are abundant to rare between Cores 318-U1356A-47R and 94R (444.04–886.99 mbsf). Because they are the primary fossil group that helps resolve the Oligocene and Oligocene/Miocene boundary intervals for Site U1356, a series of light microscope images that captured the characteristics of the assemblage in each sample were e-mailed to a shore-based nannofossil scientist during the expedition. Preliminary data acquired during the expedition are presented in Table T5. In addition, selected samples were taken during the expedition for the purpose of refining Oligocene calcareous nannofossil biostratigraphy. The new calcareous nannofossil data collected immediately postcruise are presented in Table T6. The LO of the index species Reticulofenestra bisecta occurs between Samples 318-U1356A-47R-3, 126–127 cm, and 47R-4, 31–32 cm (444.59 and 444.94 mbsf), and is placed in calcareous nannofossil Zone NP25 (Martini, 1971), correlating to magnetostratigraphic Chron C6Cn.2r (23.03–23.249 Ma). This is in agreement with magnetostratigraphy. Several specimens of R. bisecta occur in samples above this level but in low abundance and are therefore possibly reworked. According to Berggren et al. (1995), the LO of R. bisecta is 23.9 Ma, equivalent to ~23.12 Ma on the Gradstein et al. (2004) timescale, and approximating the Oligocene/Miocene boundary in the latest Oligocene. The LO of Chiasmolithus altus, a Zone NP25 (Martini, 1971) marker, is identified between Samples 318-U1356A-47R-CC and 48R-1, 6–7 cm (446.02–449.97 mbsf), correlating to Chron C8n (25.295–26.154 Ma). Berggren et al. (1995) assign an age of 26.1 Ma for this event, which corresponds to ~25.56 Ma on the Gradstein et al. (2004) timescale, within the late Oligocene. However, magnetostratigraphy disagrees with this assignment, suggesting further work is necessary. Magnetostratigraphy and the LO of R. bisecta define a latest Oligocene through late early Miocene hiatus or condensed interval ~5.62 m.y. in duration (17.5–23.12 Ma) in the 433–445 mbsf interval (Fig. F4). The LO of Cyclicargolithus abisectus is placed between Samples 318-U1356A-78R-CC and 79R-1, 12–13 cm (744.11–747.13 mbsf), identifying Zone NP23 of the early Oligocene, between the top of Chron C11n.2n and base of C12r (29.853–33.266 Ma). However, we note the sporadic presence of specimens of C. abisectus above this interval (Table T6) with a size close to 11 µm, within the boundary of its taxonomic definition. However, we cannot discard reworking. The LO of other biostratigraphically significant species such as Reticulofenestra umbilicus, Isthmolithus recurvus, and Chiasmolithus oamaruensis occurs between Samples 318-U1356A-93R-CC and 94R-4, 29–30 cm (880.72–885.93 mbsf). Particularly, the LO of R. umbilicus defines the limit between Zones NP22 and NP23 in the early Oligocene within late Chron C12r according to Berggren (1992) (from ~32 to 31.116 Ma on the Gradstein et al., 2004, timescale). In addition, the LO of I. recurvus occurs within early Chron C12r according to Berggren (1992) (~32–33.266 Ma on the Gradstein et al., 2004, timescale). The data suggest a thick Oligocene sequence of ~460 m with an average linear sedimentation rate of ~100 m/m.y. for the uppermost Oligocene and ~30 m/m.y. for the rest of the Oligocene (Fig. F4). PalynologyNinety-four samples from Hole U1356A were processed and analyzed for palynological content. The samples were predominantly taken from the core catcher material; from critical intervals, additional samples from the working half were examined. A list of all palynological samples from Hole U1356A is provided in Table T7. Nearly all samples from Hole U1356A yielded rich palynological associations, with palynomorph preservation ranging from poor to good. Generally, the palynological associations are dominated by marine palynomorphs (predominantly dinocysts). Sporomorphs are present in most samples, albeit in generally low abundance. Other palynomorphs (acritarchs and fungal spores) occur in trace amounts only. DinocystsDinocysts are present in all samples. Their abundances vary between trace amounts (e.g., Sample 318-U1356A-92R-CC) to abundant (e.g., Sample 93R-CC). Dinocyst preservation varies between poor and good; it is at least moderate in most samples. Selected dinocyst taxa are shown in Plates P1, P2, P3, P4, P5, P6, P7, P8, P9, P10, P11, and P12. Samples 318-U1356A-1R-CC through 93R-1, 15–17 cm (0–876.77 mbsf)From 0 to 876.77 mbsf, dinocyst assemblages are dominated by protoperidinioid taxa belonging to the genera Brigantedinium, Lejeunecysta, and Selenopemphix or related genera. At least six undescribed species of protoperidinioid dinocysts were identified, which will be formally described elsewhere. The predominance of the cysts of protoperidinioid dinoflagellates, which are likely all heterotrophic, co-occurs with an abundance of pyritized diatoms recorded in this part of the succession. It suggests high sea-surface productivity, probably connected to sea ice ecosystems, akin to the modern Southern Ocean (e.g., Esper and Zonneveld, 2007). Within two distinct intervals, one from Sample 318-U1356A-39R-CC to 43R-CC (366.25–404.89 mbsf) and one from Sample 57R-CC to 59R-CC (538.18–556.73 mbsf), the protoperidinioid dinocyst assemblages are replaced by indicators of more oceanic, oligotrophic surface water conditions, such as Impagidinium spp., Nematosphaeropsis labyrinthus, and Batiacasphaera spp. In addition, the occurrence of lower latitude Impagidinium species suggests warming of the surface waters during these times. Based on the age model presented here for Hole U1356A, the two pulses correspond to ages of 18–16.5 and 22–21 Ma, respectively. All samples contain reworked dinocysts characteristic of Eocene strata, such as Deflandrea antarctica, Deflandrea sp. A sensu Brinkhuis et al. (2003), Enneadocysta dictyostila, Vozzhennikovia apertura, and Vozzhennikovia stickleyae (Fig. F14; e.g., Brinkhuis et al., 2003). Specimens of these species are generally fragmented, bleached, and/or poorly preserved, which is in stark contrast to the good preservation of the protoperidinioid dinocysts within the same samples. Given the composition of the dinocyst assemblages and the fact that most protoperidinioid taxa are stratigraphically long-ranging, the age control based on dinocysts is limited. Biostratigraphic age control comes mainly from diatoms and, to a lesser extent, from calcareous nannofossils and foraminifers. They suggest an early Oligocene to late Miocene/early Pliocene age for the succession between 0 and 886.00 mbsf. The diatom-based age information is supported by the range of Selenopemphix cf. armageddonensis from Samples 318-U1356A-1R-CC through 14R-CC (4.60–130.7 mbsf), the FO of Impagidinium patulum in Sample 318-U1356A-43R-CC (404.89 mbsf), and an isolated finding of Filisphaera filifera in Sample 318-U1356A-58R-CC (565.21 mbsf). Selenopemphix armageddonensis sensu stricto ranges from 5.4 to 9.0 Ma in the lower latitudes. Impagidinium patulum has a low-latitude FO at 16.2 Ma, and F. filifera has a northern mid-latitude first occurrence at 23.95 Ma (see Tables T3, T4 in the “Methods” chapter for references on dinocyst event ages). Despite the lack of stratigraphic Oligocene–Neogene index species, different members of the protoperidinioid assemblages indicate distinct bioevents. Notably, the first consistent occurrence of Brigantedinium spp. in Sample 318-U1356A-85R-CC (811.34 mbsf) and the LO of a (previously undescribed) large Lejeunecysta species (“Lejeunecysta large” in Table T7) in Sample 86R-CC (822.54 mbsf) may be of stratigraphic significance and may merit a more detailed analysis during future onshore work. The in-depth morphological/stratigraphical evaluation of the protoperidinioid assemblages during postcruise research may eventually yield additional stratigraphically important datums. Samples 318-U1356A-94R-CC through 93R-1, 15–17 cm (886.94–876.77 mbsf)This interval, which is characterized by slump features based on sedimentological observations (see “Lithostratigraphy”), contains late Eocene to late middle Eocene dinocysts. Notably, Sample 318-U1356A-94R-CC (886.94 mbsf) contains abundant Lejeunecysta spp. and Selenopemphix spp. and is strongly reminiscent of the protoperidinioid-dominated assemblages characterizing the younger part of the succession. Typical markers for the Eocene/Oligocene boundary interval (such as Stoveracysta kakanuensis and Stoveracysta ornata) were not found. Of particular importance is the presence of Forma T sensu Goodman and Ford, 1983, in Samples 91R-7, 64–65 cm, through 95R-3, 25–26 cm (869.76–894.12 mbsf). Goodman and Ford (1983) reported this taxon from DSDP Leg 71 Site 511 on the Falkland Plateau. There, its range has been suggested to be restricted to the early Oligocene based on nannofossil and dinocyst age control. Unpublished high-resolution investigations on dinocyst samples from Site 511 have shown that Forma T sensu Goodman and Ford, 1983, has its FO at ~100 k.y. following Oligocene isotope Event 1 (Oi-1), at 33.6 Ma (A.J.P. Houben, pers. comm., 2010). Therefore, strata at Site U1356 above Sample 318-U1356A-95R-3, 25–26 cm (894.12 mbsf), are considered to be 33.6 Ma or younger in age. Samples 318-U1356A-100R-CC through 95R-CC (939.85–891.97 mbsf)Conspicuously, the abundances of protoperidinioid dinocysts decline dramatically from Sample 95R-1, 97–100 cm (892.00 mbsf), downhole (Figs. F14, F15). Brigantedinium spp. are absent in this interval altogether, and Lejeunecysta spp. and Selenopemphix spp. are present in low numbers only. Instead, the dinocyst assemblages are characterized by the dominance of D. antarctica, Phthanoperidinium spp., and Spinidinium spp., all of which are characteristic of Eocene strata (e.g., Brinkhuis et al., 2003). The absence of members of the typical protoperidinioid assemblage as encountered higher in the core suggests that these Eocene dinocysts are in situ. For the interval between Samples 318-U1356A-100R-1, 75–76 cm, and 95R-1, 79–100 cm (939.86–892.00 mbsf), a number of biostratigraphic index species for the early to early middle Eocene are recorded. In alphabetical order, these markers include Apectodinium homomorphum, Arachnodinium antarcticum, Charlesdowniea coleothrypta, Charlesdowniea edwardsii, Hystrichokolpoma spinosa, and Membranophoridium perforatum. The LO of M. perforatum between Samples 318-U1356A-95R-3, 25–26 cm, and 95R-CC (894.12–895.72 mbsf) suggests a minimum age of 47.9 Ma. The FO of C. edwardsii (in Sample 318-U1356A-95R-CC; 895.72 mbsf) is 52.2 Ma (see Tables T3, T4 in the “Methods” chapter for references). The LO of A. homomorphum (between Samples 318-U1356A-97R-CC and 96R-CC; 911.73–901.00 mbsf) has previously been reported to mark the early/middle Eocene boundary (48.6 Ma) in the South Pacific Ocean. The FO of A. antarcticum (between Samples 318-U1356A-97R-CC and 98R-1, 33–34 cm; 911.73–920.14 mbsf) is at ~51.5 Ma. The FO of H. spinosa between Samples 318-U1356A-99R-CC, 30–35 cm, and 100R-1, 75–76 cm, suggests a maximum age of 51.9 Ma for the former sample (see Tables T3, T4 in the “Methods” chapter for references on ages of individual dinocyst events). The LO of D. waipawaense between Samples 318-U1356A-101R-CC and 100R-1, 75–76 cm (951.92–939.85 mbsf), suggests an age older than 52.6 Ma. Samples 318-U1356A-101R-CC through 106R-CC (951.9–1000.08 mbsf)The general dinocyst assemblage of this interval differs from the overlying interval by lower abundances of D. antarctica and the dominance of gonyaulacoid dinocysts such as Areoligera spp., Hystrichosphaeridium tubiferum, and Hystrichosphaeridium truswelliae. Another common species within the dinocyst assemblage is Enneadocysta sp. A of Brinkhuis et al. (2003), which has been recorded in the mid-lower Eocene at Site 1172. The presence of D. waipawaense in Samples 318-U1356A-101R-CC through 106R-CC (951.97–1000.08 mbsf) confirms a mid-early Eocene age (53.3–52.6 Ma). Of particular interest is the dominance of A. homomorphum within Sample 318-U1356A-105R-CC (955.32 mbsf). This tropical species has previously been shown to invade high-latitude environments only during the Paleocene/Eocene Thermal Maximum (PETM; 55.8 Ma) (Sluijs et al., 2007). Because Sample 318-U1356A-105R-CC has a mid-early Eocene age (based on the presence of D. waipawaense), it must be younger than the PETM. We therefore tentatively correlate Sample 318-U1356A-105R-CC to another Eocene hyperthermal phase, possibly the Eocene Thermal Maximum 2 or 3 (Cramer et al., 2003; Lourens et al., 2005; Nicolo et al., 2007). In this context, it is interesting to note that mid-early Eocene sections in New Zealand have also yielded horizons with abundant Apectodinium spp. (Crouch and Brinkhuis, 2005). Thus, they might be correlative with the acme of Apectodinium spp. in Sample 318-U1356A-105R-CC (955.32 mbsf). Dinocyst data indicate that ~107 m of early Eocene and late early to earliest middle Eocene sediments are preserved in Hole U1356A, with an average sedimentation rate of ~24 m/m.y. for the early Eocene (Fig. F4). SporomorphsSamples 318-U1356A-1R-CC through 94R-CC (0–886.94 mbsf)From 0 to 892.00 mbsf, sporomorphs occur in nearly all samples investigated, albeit usually in low abundances. With the exception of pollen from Nothofagus, they are generally remarkably well preserved. The prevalence of thick-walled and/or particularly chemical-resistant taxa (as indicated by the strong representation of ferns and bisaccates, respectively), the degree of thermal alteration (as indicated by dark colors), and the previously established stratigraphic range of various taxa suggest that most, if not all, sporomorphs from the succession between 0 and 886.94 mbsf are reworked. Sporomorphs reworked from Mesozoic strata include Callialasporites turbatus. A Mesozoic to early Paleogene age can be ascribed to Aquilapollis, Cicatricosisporites, Classopollis/Corollina, and Ischyosporites. Paleozoic (Devonian? to Permian) taxa comprise Triancoraesporites and Taeniaesporites. Although Nothofagus is an extant taxon, the Nothofagus pollen grains found between 0 and 886 mbsf are likely reworked. The oldest pollen assemblages containing Nothofagus are known from the late Campanian (71 Ma). However, based on a comparison with the (poorly preserved) dinocysts of Eocene age and the (well preserved) sporomorphs from the Mesozoic, we tentatively suggest that the Nothofagus pollen grains observed in Samples 1R-CC through 94R-CC are reworked from Eocene strata. Selected sporomorphs are shown in Plate P12. Samples 318-U1356A-106R-CC to 95R-1, 97–100 cm (1000.08–891.97 mbsf)From 1000.08 to 891.97 mbsf, sporomorphs are present in all samples investigated (Cores 318-U1356A-106R through 95R), with generally higher abundances than further up in the succession. Sporomorph assemblages are characterized by well-preserved Nothofagus pollen grains, bisaccates, and spores. No indications of reworking have been found. Other palynofacies componentsAll samples contain palynofacies components other than dinocysts and sporomorphs. Acritarchs, foraminifer test linings, and fungal spores occur in trace amounts in various samples. Black and brown phytoclasts are recorded commonly in all samples. Amorphous organic matter, in contrast, is encountered in trace amounts in most samples; it increases in abundance downhole from Sample 318-U1356A-91R-1, 32–34 cm (862.64 mbsf). ForaminifersPlanktonic foraminifersShipboard examination of all core catcher samples yielded sporadic occurrences of planktonic foraminifers from Core 318-U1356A-34R downhole (Table T8). However, the assemblages are poorly preserved and low in abundance and diversity. This is likely a combination of thin shells and corrosive bottom waters. In addition, downslope processes also likely resulted in reworking of foraminifers in some samples. This includes the specimens identified in Samples 318-U1356A-46R-CC and 45R-CC (434.71–422.32 mbsf), in which poorly preserved and heavily recrystallized planktonic foraminifers of Oligocene age were identified (i.e., Globigerina euapertura and Globigerina labiacrassata). This is supported by the presence of abundant fine to medium quartz sands in these samples, interpreted to be turbidite deposits (see “Lithostratigraphy”). In contrast, a low-abundance, low-diversity assemblage identified in Sample 318-U1356A-50R-CC (469.92 mbsf) contains planktonic foraminifers interpreted to be in situ, based on the lack of coarser grained sediments, which would have suggested deposition by turbidity currents. The co-occurrence of Catapsydrax dissimilis and Globigerina connecta provides an age constraint for this interval of older than 17.15 Ma and younger than 21.5 Ma. Finally, the occurrence of Acarinina wilcoxensis in Sample 318-U1356A-106R-CC (1000.08 mbsf) constrains the age of these sediments to between 55.55 and 51.70 Ma. Benthic foraminifersBenthic foraminifers are absent above Sample 318-U1356A-45R-CC (422.32 mbsf), below which they occur sporadically downhole to Sample 318-U1356A-90R-CC (852.90 mbsf). From this sample to the top of the Eocene section, they are present in samples not associated with turbidite or debris flow deposition. Where present, benthic foraminifer assemblages are generally dominated by agglutinated forms, often with low abundances (generally <20 specimens). Twelve species were identified between Samples 318-U1356A-46R-CC and 45R-CC (434.71 and 422.32 mbsf), including species of the genera Cibicidoides, Fissurina, Gyroidinoides, Melonis, Oolina, Pyrulina, and Stilostomella as well as several agglutinated forms. One specimen of Melonis sphaeroides was present; this species indicates lower bathyal to abyssal depths (Loeblich and Tappan, 1988). Several apparently reworked planktonic foraminifers were observed in this sample, suggesting that the benthic foraminifers may have also been transported downward from higher on the rise. This is supported by the presence of fine to coarse sands in the sediments washed from this sample, which are interpreted as a turbidite deposit (see “Lithostratigraphy”). The upper Oligocene section is mainly barren of foraminifers, with only occasional occurrences, typically numbering <10 specimens per sample. The best-preserved benthic foraminifer assemblage in the Oligocene interval occurs in Sample 318-U1356A-73R-CC (695.81 mbsf) and is relatively diverse (16 species, 54 specimens). It is dominated by agglutinated types (62% of the total assemblage), such as Cyclammina pusilla, Bathysiphon discreta, and Jaculella acuata. Globocassidulina subglobosa, Dentalina advena, and Rectuvigerina stonei are calcareous types, suggesting a generally abyssal to lower bathyal environment. This assemblage is similar to those from other studies from abyssal sites off Antarctica, in which the first downhole occurrences (stratigraphic LOs) of in situ foraminifers are recorded from the early Miocene and late Oligocene (Rögl, 1976; Barker, Kennett, et al., 1988). Benthic foraminifers occur in most Eocene samples deeper than Sample 318-U1356A-90R-CC (deeper than 852.90 mbsf) not associated with slumping or turbidites. Agglutinated species dominate the assemblage, with Ammodiscus glabrans, Psammosphaera fusca, Rhabdammina linearis, Bathysiphon spp., J. acuata, and Reophax spp. Calcareous forms are generally rare, typically only a few specimens were observed per sample. These taxa have been found in abyssal water depths off Antarctica in Paleogene through lower Neogene cores (Rögl, 1976). Age model, sedimentation rates, and paleoenvironmental interpretationIn this section, we summarize the biostratigraphic and paleoenvironmental interpretation inferred from the individual microfossil groups described above. The integrated age-depth model presented in Table T9 and Figure F4 also incorporates the magnetostratigraphic age constraints (see “Paleomagnetism”). Age model and sedimentation ratesDiatoms resolve the biosiliceous sedimentary sequence in the upper 387.00 m of Hole U1356A (Cores 318-U1356A-1R through 38R and 41R) at high resolution. They indicate deposition of lower upper Miocene through uppermost lower Miocene sediments at relatively high linear sedimentation rates of 37 m/m.y. for the lower upper Miocene through mid-middle Miocene interval (~9.5–14 Ma), increasing to 67 m/m.y. for the mid-middle Miocene through uppermost lower Miocene (~14–17 Ma) (Fig. F4). The age model suggests that the Miocene interval is ~430 m thick in Hole U1356A. Based on linear sedimentation rates the middle/late Miocene boundary (11.61 Ma) occurs between 57.04 and 66.57 mbsf and the early/middle Miocene boundary (15.97 Ma) occurs between 354.05 and 387.00 mbsf. Reworking of early Paleogene, Mesozoic, and Paleozoic material is moderate to high. A sedimentary drape of earliest Pliocene to latest Miocene is inferred in the top few meters of the hole. Calcareous nannofossils, foraminifers, and dinocysts resolve the interval below 387.00 mbsf to the bottom of the hole (1000.08 mbsf), suggesting deposition of uppermost Oligocene sediments at a sedimentation rate of ~100 m/m.y. and lower to uppermost Oligocene sediments at a rate of ~30 m/m.y. A latest Oligocene to late early Miocene hiatus or condensed interval between 23.12 and 17.5 Ma is placed between 433 and 445 mbsf based on calcareous nannofossil and magnetostratigraphic data. A long hiatus from 47.9 to 33.6 Ma at ~890 mbsf indicates that the earliest stages of the Eocene–Oligocene transition are not recovered in Hole U1356A but that ~100 k.y. post-Oi-1 sediments are preserved. This information indicates that the Oligocene strata are ~460 m thick and the lower Eocene to lower middle Eocene is ~107 m thick in Hole U1356A. Lower Eocene deposition occurred at a sedimentation rate of on average ~24 m/m.y., although we acknowledge this carries a high error due to poor core recovery. The base of Hole U1356A is ~53.8 Ma based on magnetostratigraphic data (Fig. F4). Paleoenvironmental interpretationOrganic microfossil–rich sediments were deposited at Site U1356 during early to middle Eocene times. Based on the high dinocyst species diversity and notably the abundance of Spiniferites spp., the dinocyst assemblages typically reflect outer shelf facies. This is much shallower than that inferred from the Oligocene and Miocene microfossil assemblages, which typically reflect oceanic settings. This discrepancy implies that a deepening event, or events, occurred during the middle to late Eocene. The lack of siliceous microfossils in the Eocene in Hole U1356A also suggests a very shallow water, restricted environment. Dinocysts are associated with abundant pyritized siliceous microfossils and nannofossils in Oligocene and lower Miocene sediments, which indicates a nutrient-rich pelagic setting during the Oligocene to early Miocene. The abundance of pyrite and absence of opal suggests reduced oxygen conditions below the sediment/water interface. Latest early Miocene to early late Miocene (and Pliocene) siliceous microfossil assemblages are dominated by open-marine pelagic taxa with low abundances of benthic, neritic, or sea ice–associated taxa, which implies that during this time biosiliceous-rich sediments were deposited in an open, well-ventilated, nutrient-rich, pelagic-marine setting (at relatively high sedimentation rates of 37–67 m/m.y.). Considering this, the most likely explanation for the absence of latest Miocene and late early Pliocene to Holocene sediments at Site U1356 is strong current erosion and/or mass flow erosion. Further, the preservation of opal from the late early Miocene (~17 Ma) onward may indicate a switch in deepwater circulation patterns from a poorly oxygenated low-silica system (early to early middle Eocene to late early Miocene) to a well-ventilated, silica-enriched system akin to the modern Southern Ocean. |
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