The section at Site 1145 mainly consists of clay, and only one sedimentary unit is assigned to the sediments at this site (Fig. F5). In addition to clay minerals, the upper part (0-75 mbsf; 0-85.6 mcd) is characterized by small amounts (<10%) of biogenic silica, mainly in the form of radiolarians, diatoms, sponge spicules, and silicoflagellates. Below 40 mbsf (42.9 mcd), the percentage of calcareous nannofossils increases slightly (<10%), although the abundance of planktonic foraminifers remains moderate.
A red-brown oxidized layer (mudline) is observed in interval 184-1145A-1H-1, 0-15 cm (0-0.15 mbsf; 0-0.15 mcd), and consists of clay with radiolarians, sponge spicules as long as 1 cm, and small amounts of nannofossils. This layer is intensely bioturbated, and a dark brown manganese-rich redox horizon is observed at its base. Soupy greenish gray clay with foraminifers, nannofossils, and radiolarians dominates below this horizon.
Carbonate content estimated from smear slides varies within the sediment but is rarely higher than 10%, except within some of the lighter gray intervals (Table T5). The visually identified light carbonate-rich layers seen in the cores were also detected with color reflectance data collected by spectrophotometer (Fig. F5). The latter data set was effective in showing that carbonate content varied over a broader zone than noted by the naked eye.
Slightly lighter intervals, with higher carbonate content, are observed throughout the entire interval. We systematically recorded these intervals only for Hole 1145C (Table T5). Internal sedimentary features in the light layers include abundant visible foraminifers, common green clay layers (typically 1-3 cm thick), and slightly yellowish gray patches, which probably represent traces of bioturbation. The higher carbonate content of the light layers results in a stiffer clay, which often shows a rough surface caused by the cutting wire. A typical example of these light gray carbonate-rich intervals is a mottled nannofossil ooze bed (more than 50% nannofossil observed in smear slides) with gradational top and basal contacts, occurring in interval 184-1145A-7H-4, 43-123 cm (59.43-60.23 mcd [Fig. F5]). This bed is not a turbidite deposit; instead, its character suggests a short-term increase in carbonate production or a decrease in carbonate dissolution. It may correspond to isotopic Stage 9.
Green layers and less distinct green mottles, which resemble the green clay layers observed at Site 1143, occur frequently in the upper part of the drilled sequence, principally above ~170 mcd. They are characterized by stiff clay and have a lower water content and a faster P-wave velocity.
Bioturbation is intense throughout the cored interval. The sediment is generally completely homogenized, and individual burrows are observed only rarely. Exceptions are the pyrite-filled burrows, which occur at a frequency of about one to two per section throughout the drilled intervals, commencing at ~28.5 mcd. Large pyrite-filled burrows reach several centimeters in length and up to 2 cm in diameter. Remineralization of former organic carbon-rich sediments is also expressed by pyrite layers. For example, a fecal pellet horizon is fossilized in interval 184-1145A-15X-1, 2-5 cm (136.3 mcd; Fig. F6).
Shell fragments and complete tests of pteropods are commonly observed in the upper part (above 73.6 mcd) of the recovered section. A unique feature of Site 1145 is the presence of several complete echinoderms, which were only cut and fragmented by core splitting. Some echinoderm fragments are remineralized by pyrite (e.g., Section 184-1145A-7H-CC, 43 cm; 64.43 mcd). Biogenic silica represented by radiolarians, sponge spicules, diatoms, and silicoflagellates is abundant and represents an important part of the biogenic fraction in the upper part of the holes. Biogenic silica progressively decreases in abundance and is seldom observed below 81 mcd.
Three wood fragments were found at Site 1145. Samples were taken for detailed shore-based analysis. Wood is preserved at intervals 184-1145A-13H-4, 4-6 cm (120.6 mcd), 184-1145B-1H-1, 10-11 cm (0.1 mcd), and 184-1145C-9H-2, 100-101 cm (85.5 mcd). The largest wood fragment is more than 4 cm long and almost 1 cm in diameter (Fig. F7).
Slightly darker intervals that have gradational contacts with the dominant sediment are noted at intervals 184-1145B-3H-5, 60-63 cm (23.7 mcd), and 3H-6, 33-37 cm (24.9 mcd). These beds contain increased amounts of pyrite and volcanic glass and may correspond to volcanic ash layers, homogenized by bioturbation. At Section 184-1145A-5H-2, 66 cm (34.9 mcd), a pyrite-stained volcanic pumice clast 1.2 cm long is observed. A small number of cores contain fresh angular volcanic glass shards within burrows, as well as dispersed ash within the sediment (Fig. F8). Volcanic ash represents a volumetrically insignificant proportion of the total sequence. Because of intense bioturbation, the correlation of ash layers from hole to hole is not always possible (Table T6).
An unusual nodular cemented silt layer with a sharp basal contact is observed in intervals 184-1145B-16X-6, 93-99 cm (156.13-156.19 mcd), and 184-1145C-16X-4, 25-29 cm (156.25-156.29 mcd). This layer may be a diagenetically altered silty turbidite layer or a strongly altered volcanic ash.
Thin foraminifer turbidites occur at Site 1145 but are rare. The best example is observed in interval 184-1145B-18X-7, 49 cm, through 18X-CC, 2 cm (175.19-175.24 mcd). This ~5-cm-thick, normally graded turbidite has a sharp bottom contact and consists almost entirely of planktonic foraminiferal tests.
X-ray diffraction of randomly packed bulk sediment samples demonstrates no long-term changes in the terrigenous mineral composition within the sediment at this site (Fig. F9). The most notable changes are the increase in carbonate and decrease in plagioclase below 160 mcd in the light gray carbonate-rich layers.
The most striking features of the sedimentary section recovered at Site 1145 are light carbonate-rich clay layers of ~0.5- to 4-m thickness. The contacts of these layers are generally gradational, and the transition zone is only a few centimeters long in most cases. These observations closely correspond to the spectrophotometer measurements that indicate changes of ~8% of the total lightness (L* parameter). These intervals are also characterized by more common layers, mottles, and patches of green clay and have higher contents of foraminifers and calcareous nannoplankton. The higher density of green layers and bioturbation (mottles and patches) within these intervals are an indication of increased biogenic activity in the sedimentary environment.
We interpret the light intervals as representing deposition during relative sea-level highstands, when Site 1145 was largely protected from terrigenous flux by broad flooded shelves. This interpretation agrees with the biostratigraphic data, which allows us to tentatively place the first five important light intervals in the interglacial isotope Stages 5, 7, 9, 11, and 13 (Fig. F5). Relationships between lightness, carbonate content, and relative sea level have also been reported in this area by Sarnthein et al. (1994). Changes in the lysocline depth could also explain some variations in the carbonate (lightness) content. The present-day lysocline is located between 3000 and 3200 m. Because Site 1145 was drilled at 3200 meters below sea level, this location is especially sensitive to any change of sea level, seawater properties, and primary productivity. Several additional light layers are observed below 84.5 mcd in Hole 1145C (Fig. F5; Table T5). However, the identification of these layers becomes more problematic with depth, as the average sediment composition becomes more carbonate rich and the contrast between dark and light layers decreases. Because correlation with the color reflectance curve becomes increasingly difficult, we do not attempt to correlate these layers with isotope stages.