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doi:10.2204/iodp.proc.303306.111.2006 LithostratigraphySite U1312 consists of two holes, both cored with the APC system. Hole U1312A reached 238 mbsf and Hole U1312B reached 232 mbsf. Sediments at Site U1312 are composed of varying mixtures of biogenic and detrital components, primarily nannofossils, foraminifers, and clay minerals (Fig. F4). Lithologies include nannofossil ooze, foraminifer nannofossil ooze, foraminifer nannofossil ooze with clay, nannofossil ooze with foraminifers, nannofossil ooze with clay, nannofossil ooze with clay and foraminifers, silty clay nannofossil ooze with foraminifers, silty clay nannofossil ooze, nannofossil silty clay, and silty clay calcareous ooze. Most contacts between these lithologies are bioturbated or gradational. Typical estimates from smear slides of the most abundant detrital components are clay (5%–30%), calcite (<10%), quartz (<10%), and opaques (<5%). Other detrital components only occur in trace amounts (see “Site U1312 smear slides” in “Core descriptions”). No discrete ash layers were observed. Dropstones are rare at Site U1312, with the majority occurring in the upper 23 m (Fig. F5; Table T2). Common smear slide estimates of biogenic components include nannofossils (30%–90%), foraminifers (5%–30%), diatoms (<5%), radiolarians (trace), silicoflagellates (trace), and sponge spicules (trace) (see “Site U1312 smear slides” in “Core descriptions”). Total carbonate contents range from 59 to 98 wt% in these cores and show a clear pattern of higher content and lower variability in the lowermost part of the sequence (Fig. F5; Table T24). Sediments at Site U1312 are divided into two lithologic units (Fig. F5). Unit I represents deposition during the Holocene to late Pliocene and is dominated by biogenic sediments with major to minor detrital components that produce alternating diffuse color bands through much of the unit. Unit II is composed of upper Pliocene–upper Miocene sediments, dominated by nannofossil ooze that exhibits little color change due to a decreased abundance of both detrital content and diffuse color bands downhole. Description of unitsUnit I
Lithologic Unit I is a Holocene–upper Pliocene sequence of sediment composed of nannofossil ooze, nannofossil ooze with clay, foraminifer nannofossil ooze, nannofossil ooze with foraminifers, clay nannofossil ooze with foraminifers, nannofossil ooze with clay and foraminifers, silty clay nannofossil ooze, and nannofossil silty clay. Sediment from the upper 1.50 m of Unit I in Hole U1312B varies in color from light gray (10YR 7/2), light yellowish brown (10YR 6/4), yellowish brown (10YR 5/4), very pale brown (10YR 7/4), to pale brown (10YR 6/3). At the very top of the section, the brownish colors reflect the surface oxidized equivalents of the underlying lithologies. The top of the section also has increased detrital clay content and, in places, a lower carbonate content relative to the remainder of Unit I (Figs. F5, F26; Table T24). Changes in color can be seen downhole from 1.50 mbsf to the bottom of Unit I (Fig. F6). The sediment is dominantly white (N9, 10YR 8/1, and 5Y 8/1) and very light gray (N8) and contains striking alternating bands of light gray (10YR 7/1, 10YR 7/2, 5Y 7/1, 5Y 7/2, 2.5Y 7/2, and N7), medium light gray (N6), gray (10YR 6/1 and 10YR 5/1), very dark gray (10YR 3/1), grayish brown (10YR 5/2), light brownish gray (10YR 6/2 and 2.5Y 6/2), very pale brown (10YR 8/3, 10YR 8/2, and 10YR 7/3), pale brown (10YR 6/3), and grayish brown (10YR 5/2 and 2.5Y 5/2). The presence of these decimeter-scale alternating color bands is one of the dominant lithologic characteristics of Unit I, suggesting fluctuating amounts of detrital input. Contacts between these color changes are often gradational and bioturbated. Through much of Unit I, bioturbation varies from rare to abundant. In darker lithologies, bioturbation is most evident by the presence of diffuse millimeter- to centimeter-scale burrows and the mottled appearance of the sediment. In the lighter lithologies, bioturbation is most easily defined by flecks of pyrite near the burrows. Unit I also shows millimeter- to centimeter-scale olive (5Y 5/3), bluish white (5B 9/1), light greenish gray (5G 7/1), and pale green (5G 7/2) bands. The origin of these thinner color bands is unclear, possibly related to either primary or postdepositional processes (Figs. F7, F8; Table T3). The range of carbonate contents varies from 59 to 96 wt% through the entire unit. According to smear slide estimates, biogenic components include nannofossils (40%–90%), foraminifers (0%–30%), diatoms (<5%), radiolarians (0%–trace), silicoflagellates (0%–trace), and sponge spicules (0%–trace). Detrital components include clay (5%–30%), calcite (<10%), quartz (<10%), opaques (<5%), and not more than trace amounts of volcanic glass, feldspar, chlorite, and glauconite (see “Site U1312 smear slides” in “Core descriptions”). Two distinct layers containing as much as 20% detrital calcite and 20% quartz were identified by smear slide analyses at 6.38 and 13.76 mbsf in Hole U1312B (Fig. F9). X-ray diffraction (XRD) data from the same levels (Sample 306-U1312B-2H-CC, 13–14 cm) illustrate the presence of detrital calcite, dolomite, and quartz (Fig. F10A). Both results suggest these levels may represent layers of ice-rafted debris (IRD). Sediment components >2 mm present in Unit I are inferred to be dropstones, and their occurrence is generally rare, except for the upper part (Table T2). Unit I is divided into two subunits: Subunits IA and IB. This division is mainly based on changes in the magnetic susceptibility record and total carbonate content. Subunit IA
Subunit IB
Subunit IA exhibits more variability in the magnetic susceptibility record, showing multiple excursions toward higher values (Fig. F29), and presents lower average and higher variability in carbonate contents than Subunit IB (Figs. F5, F26; Table T24). Furthermore, dropstones are more concentrated in Subunit IA than in Subunit IB, particularly in the upper 23 m of the sedimentary succession. Preliminary analyses of these dropstones show that they range in size from 2 to 15 mm, are subrounded to angular, and are of basic igneous and/or metamorphic (metabasalts) and sedimentary/metasedimentary (carbonates, sandstones, and mudstones) origins. Only two dropstones were identified in Subunit IB, at 77 mbsf in Hole U1312A and at 78 mbsf in Hole U1312B and are of unidentified origin. Unit II
Lithologic Unit II differs from Unit I in the loss of distinct color changes caused by cyclic terrigenous input and higher carbonate contents. The contact between the two units is gradational, and it is not easily identifiable by visual core description. This boundary mainly corresponds to reduced-amplitude fluctuations in the lightness record (Fig. F5). Unit II consists of predominantly white (N9) nannofossil oozes with minor amounts of foraminifers and clay. The uppermost part of the unit contains diffuse and very faint light gray bands (10YR 7/1, 10YR 7/2, 5Y 7/1, 5Y 7/2, 2.5Y 7/2, and N7), with millimeter- and centimeter-scale thickness. The number of color bands gradually decreases downcore, and the remainder of Unit II is typically white and homogeneous. Pale green (5G 7/2) and various shades of gray (N8 and N7) streaks and bands, similar to those observed in Unit I, are frequently present through the upper part of the sequence (Fig. F8; Tables T3, T4). Calcium carbonate contents are higher than in Unit I, varying between 81 and 98 wt%, with most of the unit having >90 wt% carbonate (see Table T24). This confirms that pure oozes with between 5% and 10% terrigenous detritus are present in this part of the sequence. XRD data from Sample 306-U1312A-14H-4, 55–56 cm, demonstrate that calcite is the predominant component in this interval, strengthening this argument (Fig. F10B). Dropstones are absent to rare in cores of Unit II. Only a total of six isolated gravels were found in Hole U1312B at 127.52, 166.33, 166.37, and 168.24 mbsf (Fig. F5; Table T2). These gravels occur near the top of individual cores in soupy intervals and consequently may not be in situ and may have fallen from the upper part of the sedimentary succession during drilling operations. Finally, an important feature found in the sedimentary sequence is a foraminiferal sand (foraminifer ooze) bed, which occurs from 118.10 to 118.90 mbsf in Hole U1312A and from 114.11 to 114.45 mbsf in Hole U1312B. This bed has a sharp erosional base and a gradational upper contact and is normally graded, probably representing a low-density turbidite (Fig. F11). This bed correlates with a graded foraminiferal sand described at Site 608 in depth intervals of 114.69 to 114.95 mbsf (Hole 608) and 119.9 to 120.4 mbsf (Hole 608A) (Shipboard Scientific Party, 1987). A 3 cm thick isolated layer with the same characteristics was also observed at 33.26–33.29 mbsf in Hole U1312B. In Hole U1312A, a corresponding coarse-grained foraminifer ooze was not found at a similar depth interval, probably due to drilling disturbance (flow-in interval) or very local occurrence. Toward the base of Unit I, a change in the degree of lithification of the sequence was observed; the sediments grade from ooze to firm ooze. The use of drillover and presence of drilling-induced fractures are evidence that the bottom of Unit II is close to the transition zone from nannofossil ooze to nannofossil chalk, similar to that described at Site 608 (Shipboard Scientific Party, 1987). DiscussionSediments at Site U1312 mainly represent pelagic deposition during late Neogene–Quaternary times, dominantly expressed as varying mixtures of biogenic and detrital components, primarily nannofossils, foraminifers, and clay minerals. The upper Miocene–upper Pliocene sediments of Unit II are indicative of uniform stable pelagic sedimentation processes interrupted only by one large isolated episode of a turbidity-current event near 4 Ma. In contrast, a preliminary age model based on variation in the abundance of biogenic and detrital sediment characterizes the deposition of Unit I. According to biostratigraphic and paleomagnetic results, this prominent change in deposition occurred between 3.3 and 3.5 Ma. This interval marks a critical point in Earth’s recent climate history. Benthic stable isotope data record a progressive but oscillatory deterioration of the northern hemisphere climate during this time interval, which gradually led to the onset of significant continental ice sheets at ~2.7 Ma (Raymo, 1992). The preliminary age model for Site U1312 indicates that the lithologic changes recorded in Unit I may be related to climate-controlled changes in detrital input during glacial–interglacial cycles, as interpreted from the lightness record (see Fig. F23). The growth of continental ice sheets in the northern hemisphere provided sources for the detrital sediment component in Unit I through ice rafting and eolian deposition associated with higher regional-hemispheric wind speeds (Stein and Sarnthein, 1984). The occurrence of different biostratigraphic events suggests that the subunit boundary is placed between 1.97 and 2.74 Ma. Although this boundary cannot be constrained with precision, data from Subunit IA support more distinct glacial–interglacial cyclicity, which corresponds to the timing of major ice sheets during the late Pliocene and Pleistocene. The distinctive occurrence of dropstones found in the upper 23 m representing the last ~1 m.y. may be of great importance in identifying IRD events and possible source areas at Site U1312. In particular, the data from Hole U1312B show that these dropstones occur during glacials, as suggested by the preliminary age model based on the lightness record (see Fig. F23). Finally, an increased abundance and size of the dropstones are present in the interval from 13.99 to 14.43 mbsf (Fig. F5; Table T2), possibly correlating with the prominent glacial Stage 16 (see “Stratigraphic correlation”). |
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