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doi:10.2204/iodp.proc.342.109.2014 LithostratigraphyThe sedimentary succession recovered from Holes U1408A–U1408C consists of pelagic deep-sea sediment of Pleistocene to upper Paleocene age and is divided into four lithostratigraphic units (Figs. F4, F5, F6; Table T2). Unit I is ~13 m thick and composed of Pleistocene to Oligocene sediment. Brown silty clay is found in the core top, underlain by brownish foraminiferal nannofossil ooze and interbedded reddish brown clay. Manganese nodules and dropstones are common. The 10 m thick Unit II (Oligocene) consists of silty clay with nannofossils and nannofossil clay with silt. Unit III is a 202 m thick sequence of greenish gray nannofossil clays with cyclic color changes between greenish gray to dark green and very light gray intervals on a decimeter scale. The upper 3 m of Unit III is Oligocene in age followed by 199 m of middle Eocene aged sediments. Unit IV was recovered only in Hole U1408A and consists principally of lower Eocene whitish nannofossil chalk and upper Paleocene pinkish brown nannofossil chalk. Lithostratigraphic units and boundaries are defined by changes in lithology as identified by visual core description and smear slide observations (Figs. F7, F8, F9), physical properties including color reflectance (L*, a*, and b*), and biogenic content (calcium carbonate) (Fig. F4). Lithologic differences observed between units are primarily attributable to varying abundances of nannofossils, diatoms, radiolarians, and foraminifers. Lithologic descriptions are based on sediment recovered from Hole U1408A and supplemented with observations from the two shorter holes, U1408B and U1408C. Unit I
Unit I was encountered in all three holes at Site U1408 (Fig. F5A, F5B; Table T2) and comprises a 12–13 m thick succession of sediment of brown (7.5YR 5/3) and gray (2.5Y 5/1) clayey silt atop light brown (7.5YR 6/3) well-bioturbated foraminiferal nannofossil ooze (Fig. F6A; below 1.50 mbsf). Decimeter-scale alternations of reddish brown (5YR 5/3) clay intervals (Fig. F5B) with foraminiferal nannofossil ooze are observed throughout, likely representing Pleistocene glacial–interglacial cycles. The foraminiferal nannofossil oozes have layers and blebs of clayey brick-red sediment throughout as well as foraminiferal sand layers (Fig. F5A). Bioturbation is moderate to complete with rare discrete burrows. Core 342-U1408A-2H contains discrete manganese nodules as large as 6 cm in diameter. Patches of disseminated manganese oxide increase in abundance downhole. Reworking was documented on the basis of biostratigraphy in Sections 342-U1408A-2H-4 through 2H-5, where sediments containing Pliocene and Miocene calcareous nannofossils are both underlain and overlain by Pleistocene age sediment. Dropstones occur occasionally in the Pleistocene sequences. The most prominent example is a subrounded, metamorphic rock cobble ~6 cm in diameter occurring close to the base of Unit I in Hole U1408B. The basal boundary of Unit I is defined by a change from predominantly brownish (7.5YR 5/3) foraminiferal nannofossil ooze to yellow-brown (2.5Y 7/3) silty clays. In addition, underlying Unit II displays lower and less variable magnetic susceptibility values than Unit I. The boundary between the two units is characterized by a sharp drop in carbonate content into Unit II from ~40 to <10 wt% (Fig. F4). Unit II
Unit II is ~10 m thick and consists of silty clay with nannofossils (Figs. F5C, F6B) to nannofossil clay with silt. Color grades downhole from pale yellow (2.5Y 7/3) over light gray (2.5Y 7/3) to brown (10YR 5/3). Lightness (L*) also decreases downhole (Fig. F4). The upper pale yellow interval (Sections 342-U1408A-2H-6, 42 cm, through 2H-CC) has abundant layers and flecks of manganese oxide and high silt content relative to Unit I. The sediment is well bioturbated and homogenized. A distinct color change from brown to greenish gray nannofossil clay within nannofossil Zone NP23 (see “Biostratigraphy”) marks the basal boundary of Unit II. Unit II has the lowest carbonate values at this site (<10 wt%), and the transition toward Unit III is characterized by an increase to >40 wt% (Fig. F4). Unit III
Unit III is a 202 m thick succession of predominantly greenish gray (10Y 5/1 to 10Y 6/1 and 5GY 5/1 to 5GY 6/1) nannofossil clay (Figs. F4, F5D, F6C). Green mottles and layers as well as brownish blebs and burrows are observed throughout this unit. Commonly intercalated within this rather uniform lithology are light gray (10Y 8/1 and N 7) to white (N 8) nannofossil ooze (Fig. F5E). The successive occurrences of these beds are, in some cores, cyclically spaced, suggesting orbital forcing as a possible driving factor (Fig. F10). X-ray diffraction (XRD) analyses suggest that mineralogical composition varies only slightly between the three different lithologies shown in Figure F11, consisting of greenish gray nannofossil clay; a centimeter-thick greenish, possibly glauconite-chlorite–bearing layer; and light gray nannofossil ooze. The main change across color cycles is an increase in the intensity of the XRD calcite peak in the whitish layer compared to the greenish gray intervals. Other identified mineral phases include quartz, illite, chlorite, and kaolinite (in descending order of XRD peak area). Glauconite occurs in discrete bands. Bioturbation is mostly moderate, with common Zoophycos, Planolites, and Chondrites burrows (Fig. F12). Similar to other sites drilled during Expedition 342, sand-sized lithoclasts dominated by angular, clear quartz are found in the >63 µm size fraction as far downhole as Section 342-U1408A-4H-3 (nannofossil Zone NP23; Oligocene) (Fig. F13). The composition of the sand-sized lithics resembles that of lithics observed at Sites U1404 and U1406 over the same time interval. Unit III covers the Middle Eocene Climatic Optimum (MECO) (Fig. F10), as identified by planktonic foraminifer biostratigraphy and magnetostratigraphy in Cores 342-U1408A-6H and 7H. This interval is distinguished from the underlying and overlying sediment by well-developed cyclicity and higher carbonate content (Figs. F4, F10). The observed alternations are not unique in this unit, but they are quite distinct during the MECO interval. The transitions from greenish gray nannofossil clay to decimeter-thick, carbonate-rich whitish intervals are often characterized by the occurrence of 1–1.5 cm thick, green glauconite-chlorite–bearing layers at the top of the clay (Fig. F11) that generally have either a sharp contact at the top or bottom and occur nearly exclusively in the greenish gray nannofossil clay. Bundles of millimeter-thick greenish layers are observed in some places. Zoophycos burrows are common in the claystone but are crossed by the greenish layers (Fig. F12), suggesting that the thin glauconitic layers formed postdepositionaly. Additionally, in Section 342-U1408A-11H-2 we observed an ~40 cm thick interval of inclined bedding without bioturbation, suggesting reworking of material into a bedform. The Unit III/ IV boundary is defined by an abrupt change from greenish gray (5GY 6/1) nannofossil claystone to whitish (N 8) nannofossil chalk. Calcium carbonate content increases from ~50 wt% in Unit III to >80 wt% in Unit IV, accompanied by an increase in L* (Fig. F4). Biostratigraphically, this lithologic shift represents a hiatus between nannofossil Subzone NP15a and Zone NP12. Unit IV
Unit IV was recovered only from Hole U1408A and is ~22 m thick. The unit consists predominantly of moderately bioturbated nannofossil chalk (Figs. F5F, F6D) and chert layers 1–2 cm thick. Core 342-U1408A-26X is composed of white (N 8) to light greenish gray (10GY 8/1) nannofossil chalk with two prominent chert layers in intervals 342-U1408A-26X-1, 15–25 cm, and 26X-3, 144–150 cm, and a light greenish gray (10Y 7/1) nannofossil claystone in the upper half of Section 26X-3. Biostratigraphy constrains the Paleocene/Eocene boundary close to the base of interval 26X-2, 145–150 cm. However, the PETM was not identified and is possibly represented by a hiatus. Sediment in Core 342-U1408A-27X is pale brown (10YR 7/3 to 10YR 8/2) and composed of nannofossil chalk with radiolarians (Figs. F5F, F6D) of late Paleocene age. Pinkish brown (10R 8/3) blebs of montmorillonite occur occasionally in this core. Lithostratigraphic unit summaryThe lithostratigraphy at Site U1408 comprises a discontinuous sedimentary succession of Pleistocene to Paleocene age. All stratigraphic epoch boundaries (Paleocene/Eocene, Eocene/Oligocene, and Oligocene/Miocene) are represented by hiatuses. The division of the recovered lithology at Site U1408 into four lithostratigraphic units is associated with changes in sediment carbonate content (see “Geochemistry”; Fig. F4). The downhole transition from foraminiferal nannofossil ooze to silty clay in Unit II is characterized by a drop from 45 wt% calcium carbonate to values <10 wt%. Across the Unit II/III boundary, calcium carbonate increases from 20 to 40 wt%. The 202 m thick Unit III consists of sediment with ~40–50 wt% calcium carbonate, with cyclically intercalated whitish gray horizons that show substantially higher values of as much as 88 wt%. Unit IV is characterized by high calcium carbonate values between 55 and 91 wt%. All pre-Pleistocene samples show carbonate contents well above 10 wt%, indicating that the site was positioned above the CCD for much of its Cenozoic history. Middle Eocene Climatic OptimumThe MECO (~40 Ma) was a warming event that lasted ~500 k.y., with a main peak in temperatures for 100 k.y. (Bohaty et al., 2009). The MECO has been observed in most ocean basins and is proposed to be associated with elevated atmospheric pCO2 levels and ocean acidification inferred from shoaling of the CCD (Bohaty et al., 2009; Bijl et al., 2010). Biostratigraphically, the interval is well constrained by the occurrence of the planktonic foraminifer Orbulinoides beckmanni (planktonic foraminiferal Zone E12; Edgar et al., 2010). At Site U1408, the MECO is distinguished from the underlying and overlying sediment by well-developed cyclicity and higher carbonate content (Figs. F4, F10). Cyclic alternation on a decimeter scale between greenish gray nannofossil clay and whitish to light gray nannofossil ooze occurs throughout the MECO interval. These cycles are not restricted to the MECO itself; they occur nearly throughout all of Unit III, but they are more distinct than directly above or below this interval. Similar well-developed cyclicity is observed in Cores 342-U1408A-9H and 10H, probably representing carbonate accumulation Event 3, and in Cores 16H and 17H. Origin of green layersThe sediment of Unit II includes green layers that contain glauconite and chlorite. At Site U1406, we hypothesized that these layers represent times of no, or extremely slow, deposition. However, Zoophycos burrows observed in Core 342-U1408A-24H are crosscut by these layers (Fig. F12), suggesting that these thin greenish glauconitic layers formed postdeposition and therefore are diagenetic in origin. One possibility is that these green layers represent a reduction/oxidation boundary that formed syndepositionally with the carbonate-rich white layers. The green banding is rarely seen in the white layers. If we assume that the background carbonate flux is relatively constant and changes in sedimentation rate are driven by changes in the clay flux to the site, the occurrence of more numerous green layers within the greenish gray nannofossil clay beneath the white carbonate layers would be consistent with a slowdown in sedimentation rate during intervals of background sedimentation less diluted by clay (Fig. F11). The more common greenish gray, clay-rich layers would then represent higher sedimentation rates. Individual green bands are typically 1–1.5 cm thick but can grade in intensity in either direction. The green layers have a very sharply delineated top or bottom paired with a diffuse boundary on the opposite side and can appear thicker when multiple generations of the green bands are stacked together (Figs. F11, F12). |
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