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doi:10.2204/iodp.proc.323.103.2011 LithostratigraphyFour holes were drilled at Site U1339, with the deepest (Hole U1339D) reaching 200 mbsf. Overall, the sediments recovered at Site U1339 are a mixture of three components: biogenic (mainly diatom frustules with varying proportions of nannofossils, foraminifers, silicoflagellates, and radiolarians), volcaniclastic (mainly fine ash), and siliciclastic (clay- to pebble-sized clasts). Other accessory lithologies identified at this site include authigenic carbonate and pyrite. In general, the color of the sediment reflects its lithologic characteristics: sediment composed of mixed lithologies (diatom silt or diatom ashy silt) is dark greenish gray, whereas diatom ooze is olive-gray to olive. Most of the volcaniclastic ash layers are black, although a few massive light gray ash layers were also observed. One lithologic unit spanning the Pleistocene was defined at Site U1339 (Figs. F5, F6, F7, F8). Description of unitUnit I
Unit I is composed of mainly diatom-rich sediments mixed with varying amounts of volcaniclastic and siliciclastic material. In particular, three main types of lithologies were recognized. The first type is biogenic sediment, which is mainly diatom ooze with minor (<20%) amounts of other biogenics, including nannofossils, foraminifers, and trace (<5%) silicoflagellates and sponge spicules. The biogenics occur mixed with variable amounts (<5%–40%) of silt-sized volcaniclastic and siliciclastic material, including quartz, feldspar, clay minerals, and trace amounts of micas and zeolites. The color is usually olive-gray (5Y 4/2). The second type is sediment composed of subequal proportions of siliciclastic (silt-sized quartz, feldspar, and rock fragments and/or clay) and biogenic (mainly diatoms and secondarily nannofossils, foraminifers, silicoflagellates, and sponge spicules) material with only secondary volcaniclastic components. Depending on the proportion of siliciclastic to biogenic grains, the color varies from greenish gray (5GY 5/1) to dark greenish gray (5G 4/1 and 10Y 4/1). The third type is sediment composed of subequal proportions of volcaniclastic (mainly volcanic ash) and biogenic (mainly diatoms and secondarily nannofossils, foraminifers, silicoflagellates, and sponge spicules) material with only secondary siliciclastic components (see "Site U1339 smear slides" in "Core descriptions"). The color varies from very dark greenish gray (10Y 3/1) to dark gray (5Y 4/1.4). Volcanic ash layers are commonly black (5Y 2.5/1) and less commonly light gray (10Y 7/1 and 10YR 7/2). Distinct ash layers ranging in thickness from a few millimeters to 10 cm occur throughout the unit, and layers thicker than 2 cm are reported as distinct lithologies (Figs. F5, F6, F7, F8). Several prominent volcaniclastic layers show either parallel or undulating sharp contacts with the underlying sediment. Conversely, the top boundaries are gradational and the ash is mixed with the overlying diatom ooze, most likely from bioturbation and/or diffusion. Some black ash layers are also characterized by graded bedding, with the coarser clasts concentrated at the bottom (Fig. F9B). Apart from distinct ash layers, fine volcaniclastic material is often concentrated in mottles that are likely the result of bioturbation of thin ash layers. Volcanic ash is a common secondary or trace lithologic component in most of the recovered sediments, and most volcaniclastic layers are black. The largest scale sedimentary features at Site U1339 are decimeter- to meter-scale alternations of sediment color and texture that reflect alternations in lithology. The transitions between beds are commonly gradational, although sharp boundaries were also observed. Well-preserved laminations were primarily observed in six distinct intervals, each ranging between 10 and 40 cm in thickness (Figs. F5, F6, F7, F8). The laminations are alternations of millimeter-scale dark (mainly diatom ooze) and light (mainly nannofossil- and foraminifer-rich diatom ooze) laminae (see "Site U1339 smear slides" in "Core descriptions"). Laminated intervals always have ash layers either below or above them (Fig. F9). The thickest volcanic ash layer (~20 cm thick: interval 323-U1339B-3H-2, 108–128 cm) is overlain by ~20 cm of laminated sediment. The major component of the sediments recovered at Site U1339 is biogenic, predominantly pennate and centric diatoms. The preservation of diatom frustules is generally good (see "Biostratigraphy"); however, in several of the smear slides, broken pennates and girdles without valves indicate dissolution. Diatom frustules hosting pyrite framboids were also observed. Calcareous tests are relatively rare because of the low preservation potential in these sediments and/or low rates of primary productivity of marine organisms producing carbonate tests. Both benthic and planktonic foraminifers are present; however, they are never abundant, possibly because smear slides selectively sample smaller grain sizes (see "Lithostratigraphy" in the "Methods" chapter). Thin laminae dominated by foraminifer tests were also observed (always <20%; see "Site U1339 smear slides" in "Core descriptions"). These laminae are dominated by the benthic foraminifer Bulimina sp. Calcareous nannofossils, radiolarians, and sponge spicules are rare. Terrigenous particles are common in the sediments recovered at Site U1339. The most abundant terrigenous grain types are silt-sized quartz and feldspar, clay, mica, and rock fragments (mainly polycrystalline quartz). Gravel- to pebble-sized rounded to angular clasts are interpreted as dropstones delivered by melting sea ice or icebergs (see "Site U1339 thin sections" in "Core descriptions") (Figs. F5, F6, F7, F8, F10). Onboard petrographic thin section descriptions of two representative pebbles indicate that one of the main source lithologies for the ice-rafted material is metasandstone (see "Site U1339 smear slides" in "Core descriptions") (Fig. F11). Some pebbles are composed of pumice or obsidian, suggesting a volcanic source. Authigenic precipitates were also observed. Authigenic carbonate was found below 34 m core composite depth below seafloor (CCSF-A) in Holes U1339B–U1339D. Note that Hole U1339A extends only to 35.03 m CCSF-A. Authigenic carbonates occur more frequently below 145 m CCSF-A (Figs. F5, F6, F7, F8, F12), either as rhombohedra scattered in the sediment or as (semi-)lithified layers 5–10 cm thick. The disseminated dolomite rhombs range in size from 10 to 30 µm, and crystallization occurs as pore space infilling in diatom cavities and/or as replacement of biosiliceous tests (Fig. F12). The authigenic carbonates were determined to be dolomite because of the presence of characteristic dolomite rhombs (Fig. F12) by XRD analysis (Fig. F13) (see XRD in "Supplementary material"). Bioturbation is a common feature at Site U1339, as suggested by the rare occurrence of laminated intervals and the faint to pervasive mottling that characterizes most cores. The most commonly recognized trace fossils are Planolites, and a few examples of Skolithos were also observed. However, in most cases, bioturbation could not be related to specific ichnofacies types, and bioturbation intensity was mostly described as slight to moderate. Coring disturbances, mainly in the form of subhorizontal gas expansion cracks and voids, were frequently observed in cores collected from all Site U1339 holes. Cracks and voids are likely caused by the high concentrations of methane found below the sulfate–methane transition (8–10 mbsf; see "Geochemistry and microbiology"). Note that because of sediment loss from punctures, stratigraphic distortion may occur at puncture sites. Depth variations of color reflectance parameter b*, gamma ray attenuation (GRA) bulk density, and magnetic susceptibility were compared to lithologic variations at Site U1339 (Figs. F5, F6, F7, F8). Overall, these parameters show distinct short-term variability and longer term trends that can be correlated to lithologic variations at both short- and long-term scales. GRA bulk density in mixed biogenic and terrigenous sediments increases with higher amounts of siliciclastics. Hence, sediments rich in detrital material have higher bulk densities than siliciclastic-poor, mainly biogenic sediments. Color reflectance parameter b* reflects the yellowness of sediment, and these two parameters often run in tandem, with higher b* values corresponding to less dense, more biogenic-rich sediments. Changes in magnetic susceptibility with depth mainly reflect volcaniclastic content, and a very good correlation exists between the thickest volcanic ash layers and the highest magnetic susceptibility excursions (Figs. F5, F6, F7, F8). DiscussionThe lithostratigraphic analysis of the sediment cores collected at Site U1339 as well as the comparison between sediment characteristics and depth variations in sediment color and physical properties suggest that late Pleistocene sedimentation on Umnak Plateau was influenced by ice sheet variability and in particular by glacial–interglacial changes in sea level and sea ice extent (Figs. F5, F6, F7, F8). Glacial sediments are rich in silt-sized siliciclastic grains, probably reflecting greater terrigenous delivery to the site at times of low sea level than during interglacials, when much of the river sediment load would have been trapped on the wide continental shelf. Glacial sediments are also richer in granule- to pebble-sized clasts, which were probably delivered to the site by sea ice or glacial ice. Interglacial conditions are reflected by the predominance of biogenic components, mainly diatoms; however, it is unclear whether this predominance is due to higher productivity or less dilution by siliciclastic material. According to the Site U1339 age model (see "Biostratigraphy" and "Paleomagnetism"), this variability occurs at very different scales. Considering that biostratigraphic data suggest a minimum average sedimentation rate for this site of 4.5 k.y./m, decimeter- to meter-thick rhythmic bedding occurs in the sub-Milankovitch range, whereas larger alternations between lithologies dominated by the two sediment types occur within the Milankovitch range. This variability is well represented by depth changes in bulk density, where denser lithologies are richer in siliciclastics. Color reflectance parameter b* also shows clear changes on the same scales and is more negative in the more grayish silt-rich sediments. Lithologic changes are represented to a lesser extent by magnetic susceptibility. Laminated intervals are rare at Site U1339, indicating that bioturbation, and therefore oxygenated conditions, prevailed during most of the depositional history at this site. However, there is no clear relationship between the depths at which the laminated intervals occur, the deepest minima in bulk density recorded by GRA, and the other parameters shown in Figures F5, F6, F7, and F8, suggesting that low-oxygen conditions are not uniquely controlled by the same processes responsible for lithologic change. Lamination includes thin laminae dominated by foraminifer tests. Based on the analysis of several smear slides, these laminae appear to be dominated by the benthic foraminifer Bulimina sp., which is characteristic of low oxygen content (Kaiho, 1994). At this stage, it is not possible to conclude that the association between volcanic ash layers and laminated intervals is the result of a causal relationship between the two. Intermixed ash and ash-filled mottles are very common at this site, showing that thin ash layers are ubiquitous and with bioturbation are mixing into the surrounding sediment. The low-oxygen conditions that allowed the preservation of the laminated intervals could also have helped preserve adjacent volcanic ash layers, accounting for an apparent association between laminated sediments and ash (Fig. F9). However, the presence of thin, unbioturbated volcanic ash layers with no visible association with laminated intervals suggests that the absence of bioturbation at this site is not always made visible by changes in sediment composition or color. Several prominent volcaniclastic layers show either parallel or undulating sharp contacts with the underlying sediment. Conversely, the top boundary is gradual and the volcaniclastic material is mixed with the overlying diatom ooze, most likely from bioturbation. Some black volcaniclastic layers are also characterized by graded bedding, with coarser clasts concentrated at the bottom (Fig. F9B). This may indicate redeposition by vertical density currents (e.g., Carey, 1997). |
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