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doi:10.2204/iodp.proc.303306.113.2006 LithostratigraphyThree holes were drilled and cored at Site U1314, reaching a maximum depth of 279.91 mbsf in Hole U1314B. Sediments at Site U1314 are characterized by a strong presence of biogenic and terrigenous material, with varying proportions of nannofossils and detrital clay minerals that contain only minor amounts of biogenic silica and, to a lesser extent, foraminifers. Sediments are predominantly greenish gray with recurring variations in shade that occur in decimeter-scale bands. These color changes are primarily related to changes in the relative proportion of biogenic carbonate and detrital clay minerals in the sediments. Only one lithologic unit, spanning the Holocene–late Pliocene, was defined at Site U1314. Description of unitsUnit I
Unit I comprises nannofossil- and clay-rich sediments having varying proportions of diatoms and foraminifers (Fig. F5). In particular, two sets of lithologies can be identified: (1) predominantly nannofossil oozes enriched in biogenic (mainly diatoms and foraminifers) and terrigenous (principally clay minerals, quartz, opaque minerals, and calcite) components and (2) terrigenous silty clay with varying proportions of calcareous and siliceous organisms (see “Site U1314 smear slides” in “Core descriptions”). The sediment varies in color mainly from very dark gray (5Y 3/1) to light gray (5Y 7/1) to hues of greenish gray (5GY 6/1–4/1 and 5G 6/1–4/1). In contrast to these gray colors, the uppermost 9 cm of sediment in Hole U1314A and the uppermost 14 cm in Hole U1314B consist, respectively, of yellowish brown (10YR 5/4) and dark yellowish brown (10YR 4/4) silty clay biosiliceous-nannofossil ooze. The yellowish color of this zone is probably due to the sediment being above the redox boundary and reflects the circulation of oxygen-rich seawater through the sediments. Horizontal and parallel bedding planes and color contacts without erosional relief suggest that there is not visible evidence of significant sediment disturbance by natural processes. Most lithologic changes are gradual, spanning several decimeters, although more pronounced transitions sometimes occur within a decimeter. However, a very distinct, sharp contact between very dark gray (5Y 3/1) and gray (5Y 6/1) sediments was observed at 29.03 mbsf in Hole U1314B (Section 306-U1314B-4H-5, 3 cm) and 30.80 mbsf in Hole U1314C (Section 306-U1314C-4H-3, 60 cm). Here, a thick clay-rich interval with laminations at the base has a sharp basal contact with an underlying biogenic carbonate-rich layer (Fig. F6). This contact was not observed in Hole U1314A. Inferred depth by correlation with Hole U1314B and Hole U1314C suggests the contact might have been lost in the break between Cores 306-U1314A-4H and 5H. Micropaleontological analysis of core catcher 306-U1314A-4H-CC indicates a poor abundance of foraminifers and the presence of aggregates (see Table T8). This sharp contact, the scarcity of foraminifers, and the presence of aggregates suggest this abrupt change in lithologies may have been associated with distinct changes in current strength. Disseminated pyrite staining on millimeter to centimeter scale is present in most of the sediment below the uppermost section of the holes. Intervals 1–5 cm thick, with distinct green coloration (grayish green: 5G 4/2 and 5G 5/2) and sometimes coarser texture, are also distributed throughout the core (Fig. F7). Preliminary X-ray diffraction data suggest that some glauconite may contribute to the green coloration within these lenses (Fig. F8). Bioturbation is most obvious where there is a rapid change in lithology presenting contrasting colors, and it is mainly characterized by millimeter- to centimeter-scale mottling and blurred transitions (Fig. F9). Discrete pyritized worm burrows are also occasionally scattered through the sequence. Although bioturbation is often difficult to assess in the darkest sediments, the presence of faint mottling in most of the sections suggests that it is a quite pervasive process throughout the sedimentary succession. Smear slides show most of the sediments to be in the clay size range (see “Site U1314 smear slides” in “Core descriptions”). However, some centimeter- to decimeter-scale intervals contain lenses of silt- and sand-sized grains. These intervals are more pronounced in darker sediments. Calcium carbonate ranges from 7.48 to 70.43 wt%, with an average of 33.8 wt% throughout Unit I (see Fig. F26), reflecting varying proportions of clay and both biogenic and detrital carbonate. Calcareous nannofossils make up a significant proportion of the clay-sized fraction in carbonate-rich sediments (smear slides indicate up to 85%; see “Site U1314 smear slides” in “Core descriptions;” Fig. F10). Foraminifers also contribute a minor amount of calcium carbonate, generally up to 5%, as estimated from the smear slides. Slightly higher carbonate values are observed in the upper 50 m of the sedimentary succession (see Fig. F26). Biogenic silica in the sediments is represented by diatoms, sponge spicules, radiolarians, silicoflagellates, and ebridians. Generally, biogenic silica abundances are <25% (see “Site U1314 smear slides” in “Core descriptions;” Fig. F10). In an isolated interval, however, diatom levels comprise 80% of the sediments (interval 306-U1314B-4H-2, 18–30 cm; 24.68–24.80 mbsf). Sponge spicules, radiolarians, silicoflagellates, and ebridians usually occur only in trace amounts to a few percent. Terrigenous sediments are mainly represented by clay minerals, which range in abundance from 5% to 70% (see “Site U1314 smear slides” in “Core descriptions;” Fig. F10). Other terrigenous components, in decreasing order of abundance, include quartz, opaque minerals, detrital calcite, accessory minerals, glauconite, and feldspar. These components range in abundance from trace amounts to 55%. Volcanic glass occurs in trace and rare amounts throughout the sedimentary sequence of Site U1314, with some peak occurrences of 10%–80% (see “Site U1314 smear slides” in “Core descriptions”). Gravel-sized dropstones are common at Site U1314, occurring from 0 to 240 mbsf (Table T2; Fig. F11). Preliminary analyses of these dropstones show that they range in size from 2 to 45 mm, are rounded to angular, and are of felsic and mafic igneous, (granite and basalt), metamorphic (gneiss and quartzite), or sedimentary/metasedimentary (sandstone and mudstone) origins (Table T2; Figs. F12, F13). The magnetic susceptibility, color reflectance, and natural gamma ray records show an overall distinct short-term variability. In addition, long-term changes are centered between ~240 and 270 mcd and at ~70 mcd (Fig. F14). However, apart from darker sediment colors, no changes in lithologic characteristics and composition are clearly discernible from visual core description and smear slide examination. Therefore, at this preliminary stage, lithologic Unit I cannot be effectively divided into subunits. DiscussionThe sediments at Site U1314 consist of carbonate-rich and carbonate-poor intervals that alternate on decimeter to meter scale. Sediment types range from nannofossil oozes with biosilica and/or silty clay as minor components (the carbonate-rich intervals) to silty clay (the carbonate-poor intervals). Changes in the relative proportions of these two sediment types might reflect variable deposition rates rather than selective sediment removal, as suggested by the good preservation of calcareous and siliceous microfossils and by the visual absence of sedimentary structures related to erosion or winnowing. Distribution of fine-grained sediments in this region is probably controlled by deepwater circulation (McCave and Tucholke, 1986). In particular, the Iceland-Scotland Overflow Water (ISOW), which travels south of Iceland along the eastern flank of the Reykjanes Ridge, plays an important role in producing and shaping the Gardar Drift (Johnson and Schneider, 1969; Jones et al., 1970). Silt- and clay-sized terrigenous components from Site U1314 show the presence of volcanic products (see “Site U1314 smear slides” in “Core descriptions”). The finer fraction of volcanic material, mainly derived from the Icelandic ice sheet, was deposited on the continental slope south of Iceland and subsequently transported downslope by turbidity currents and further along slope by the southwest-flowing ISOW and Norwegian Sea Water (Davies and Laughton, 1972; Kissel et al., 1999). In addition, volcaniclastic sediments can also be transported in this region as tephra fallout (Lacasse et al., 1998). Although high sedimentation rates at this site are mainly related to deep-sea sediment focusing, the presence of dropstones throughout the sedimentary succession recovered at Site U1314 indicates that ice-rafting played an important role (Figs. F11, F12, F13). Sand- and gravel-sized sediments, observed either during visual core descriptions or smear slide estimates, provide direct evidence of ice rafting and document the influence of the late Pliocene–Pleistocene glaciation in this region. The presence of gravel-sized grains downcore is coupled with an increased abundance of peaks of quartz and sand-sized sediment (Fig. F11), indicating several possible pulses of IRD input. Preliminary age estimates based on biostratigraphic and paleomagnetic data show that higher abundances occurred from 2.6 to 2.2 Ma, 1.7 to 0.8 Ma, and during the last 0.23 m.y., suggesting that these time intervals were probably characterized by higher IRD input. Possible provenance areas for this terrigenous sediment include Iceland, Greenland, and the Canadian Shield (Bond and Lotti, 1995). More specifically, felsic igneous dropstones may be sourced from southeast Greenland, and mafic igneous dropstones may be derived from either Greenland flood basalts or Iceland’s volcanic province (Krissek and St. John, 2002). Additionally, the presence of sand-sized, hematite-stained quartz, which was identified in smear slide estimates, is well documented in previous IRD studies in this region and might be sourced from the east central coast of Greenland (Bond and Lotti, 1995; Bond et al., 1999). Sediment at Site U1314 is also supplied from surface biological production. Smear slide estimates suggest that the biogenic sediment component at this site is mainly composed of calcareous nannofossils. Fluctuations in the relative percentages of calcareous nannofossils may be related to dilution by high terrigenous sedimentation and also may reflect changes in sea-surface productivity. In general there is an inverse relationship between nannofossil abundance and the abundance of terrigenous components, such as clay minerals and quartz. This may suggest movement of the Polar Front to the east-southeast when nannofossil abundance decreased and terrigenous input increased (Andruleit and Baumann, 1998). |
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