and then a sharpening of the crystobalite peak in deeper samples. At Site 1225, the opal-A to opal-CT phase change occurs between ~250 and 300 mbsf.
The sedimentary sequence at Site 1225 consists mainly of nannofossil ooze with varying amounts of diatoms, radiolarians, and foraminifers, except one interval that was dominated by diatom ooze. One lithostratigraphic unit was recognized in all holes at Site 1225 (Fig. F1). Site 1225 had previously been drilled during Leg 138 as Site 851 (Shipboard Scientific Party, 1992). The age framework presented in this chapter was obtained by using the timescale of Berggren et al. (1995a, 1995b) to update the age model of Site 851 (Shipboard Scientific Party, 1992). This age model was based on magnetostratigraphy and planktonic foraminifer biostratigraphy for the uppermost 90 m and on nannofossil, radiolarian, and diatom biostratigraphy for the rest of the section.
The lithologic description is based on the visual description of sediment color and sedimentary structures as well as on other parameters such as smear slide analyses and color reflectance. Calcium carbonate content, expressed as weight percent CaCO3, and organic carbon analyses (see "Biogeochemistry"), X-ray diffractometry (XRD) (Fig. F2), and laboratory measurements of magnetic susceptibility (Fig. F3), density, and water content (see "Physical Properties") were also used to characterize lithologic changes.
The sediments at Site 1225 consist of middle Miocene to Pleistocene nannofossil ooze with varying amounts of both calcareous (foraminifers) and siliceous (diatoms, radiolarians, and sponge spicules) microfossils. These variations become apparent in the measurements of percentage of CaCO3 (see "Biogeochemistry") and color reflectance (Fig. F1). Calcium carbonate content is relatively high throughout the unit, ranging between 58 and 85 wt% (average = 74 wt%), except for a low value of 42 wt% CaCO3 measured near the top of Subunit ID (see "Biogeochemistry"). Smear slide estimates of carbonate content are generally in good agreement with the pattern indicated by the CaCO3 analyses.
The sediments of Unit I are either massive or bioturbated and exhibit no other notable primary sedimentary structures. The color of sediments varies from gray and pale green to pale gray and white. Secondary alteration features such as centimeter-scale greenish, purple, and pale gray banding as well as dark layers and spherical concentrations of pyrite are common. Based on these variations in sediment composition, degree of bioturbation, color reflectance data, and changes in the degree of postdepositional alteration, five subunits were distinguished.
Subunit IA consists of foraminifer-, diatom-, and radiolarian-rich nannofossil ooze of Pleistocene to late Pliocene age. The first 2.5 m of the sedimentary section in all holes contains two ~50-cm-thick layers of brown to brownish gray sediment containing iron and manganese oxides. Below the redox boundary between 2.5 and 2.6 mbsf, the sediment is characterized by a rhythmic alternation between darker-colored, 40- to 50-cm-thick foraminifer-, radiolarian-, and diatom-rich intervals and 70- to 80-cm-thick lighter-colored intervals characterized by higher nannofossil contents. The darker intervals commonly show centimeter-scale, pale green and gray alteration bands, whereas the lighter-colored layers are dominated by yellowish gray mottled zones, which are most likely the result of more intensive bioturbation. The bases of the darker intervals are commonly sharp, and burrows in the underlying lighter sections are filled with darker material from above. Dark sulfide-rich spots and thin bands of dark gray material (presumably pyrite) are common (Fig. F4F). The sediments contain 50%-90% nannofossils, 5%-25% foraminifers, 5%-30% radiolarians, and 5%-30% diatoms. This lithologic variability is also reflected in the color reflectance and CaCO3 data. The carbonate content of this subunit ranges from 64 to 85 wt% CaCO3, with the lower values presumably corresponding to intervals richer in siliceous microfossils. Water content is generally high and highly variable (see "Physical Properties").
Subunit IB consists of ~135 m of early Pliocene to late Miocene pale greenish gray to light gray diatom-bearing and diatom-rich nannofossil ooze (5%-20% diatoms) with minor amounts of radiolarians (>10%, decreasing toward the base of the subunit). In contrast to Subunit IA, foraminifers are present only in trace amounts. The sediments are characterized by a decimeter-scale interbedding of pale greenish gray to gray banded intervals with pale greenish gray mottled and bioturbated zones. Alteration bands usually show pale purple, greenish gray, and yellow colors (Fig. F4A). They are either oriented subparallel to the weakly developed depositional layering, are present as curvilinear features, or form ellipsoidal reaction fronts around burrows (Fig. F4B). The mottled zones are characterized by pale greenish gray nannofossil ooze with burrows filled with light yellowish gray sediment. Gray pyrite-rich spots, nodules, and streaks are common throughout the interval (e.g., Fig. F4G). These sulfide-rich zones are often accompanied by pale purple-gray halos around dark gray spots and thin layers. Diagenetic banding and the presence of pyrite-rich layers and spots is much more predominant in Subunit IB than in the overlying and underlying sedimentary subunits. Bioturbation is generally strong in this subunit (Fig. F3). Color reflectance data, magnetic susceptibility, water content, and many other physical properties of the sediment are significantly different from the properties in the overlying Subunit IA (Fig. F1). In particular, the water content of the sediments in Subunit IB is much lower and appears to vary systematically with depth. Magnetic susceptibility values are low throughout the section (see Fig. F3; "Physical Properties").
Subunit IC is a 60- to 65-m-thick section of pale greenish gray to gray diatom-bearing and diatom-rich nannofossil ooze (5%-40%) of Miocene age. Radiolarians are present in minor amounts (1%-10%). The main distinguishing feature of Subunit IC is the more homogeneous overall appearance of its sediments. Alteration bands and intensively bioturbated zones are far less common in the upper part of the subunit than in the lower part of the subunit. Pyrite-rich spots and pyritized burrows are common. Carbonate contents tend to be lower and show a higher degree of variability than in the other subunits (see "Biogeochemistry"). Color reflectance data are more variable than in Subunit IB, and magnetic susceptibility values are both higher and more variable compared to the 70- to 200-mbsf interval. Water content and porosity increase toward the base of Subunit IC (see "Physical Properties"). These increases might be related to the increase of biosilica (diatoms and radiolarians).
The lower ~2.5 m of Cores 201-1225A-30H and 201-1225C-29H consist of dark olive-green nannofossil-rich laminated diatom ooze. This section defines the top of Subunit ID, a ~30-m-thick sedimentary section of late Miocene age. Below the diatom ooze are sediments dominated by nannofossils (80%-90%) with subordinate amounts of diatoms and radiolarians (generally <10%). With the exception of the homogeneously dark green diatom layer, the sediments appear similar to the nannofossil oozes of Subunit IB. Pale greenish gray to gray and purple alteration bands are common and alternate with pale greenish gray and pale yellow bioturbated intervals. High concentrations of pyrite are common as dark gray nodules (Fig. F4G) and pyritized burrows, as well as in dark gray bands and spots throughout the section. Magnetic susceptibility data show a sharp decrease to values comparable to the susceptibility pattern in Subunit IB (Fig. F3). The first 5 cm of Section 201-1225C-30H-1 (274.8 mbsf) contains several pieces of gray laminated porcelanite, indicating the onset of silica diagenesis and the transformation of biogenic opal-A to more crystalline opal-CT in the lower part of Site 1225. This trend was confirmed by XRD analysis as well as by a sudden drop in porosity (Fig. F3).
Subunit IE near the base of Hole 1225A represents sediments directly overlying the oceanic basement. This interval consists of pale yellow diatom-rich nannofossil ooze, which grades with depth into a laminated nannofossil chalk between Sections 201-1225A-35X-2 and 35X-3 (Fig. F4E). These semi-indurated sediments contain several darker diatom-rich layers. In Core 201-1225A-35X, brown volcanic glass is disseminated in the sediment as well as concentrated in thin layers (Fig. F4D). The top of Section 201-1225A-34H-1 (~300 mbsf) contains pieces of gray opal-CT porcelanite (Fig. F4C).
The combination of spectrophotometric analysis with visual observations and other lithostratigraphic methods greatly improved the stratigraphic and lithologic framework of Site 1225. In particular, the main inflection points of the a*/b* time series show a good match with the boundaries of the five lithostratigraphic subunits (Fig. F1), as well as with the relative variations of the main biogenic components observed in smear slide analysis. In general, higher values of the a*/b* ratio are present in the white to pale gray nannofossil ooze layers (e.g., Subunit IB), whereas the a*/b* ratio is lower where darker diatom-rich layers predominate (e.g., Subunit ID). Figure F3 also shows that the average variations of the a*/b* ratio and magnetic susceptibility are in opposition of phase and suggests that darker layers (less reflectance) are probably richer in magnetic oxides.
XRD analyses were performed on 37 specimens from both Holes 1225A and 1225C. Overall, the mineralogic composition of Site 1225 samples reflects the dominant lithology, which is nannofossil ooze for most of the sedimentary section. The mineralogic assemblage is thus dominated by calcite, as also confirmed by calcium carbonate content (see "Biogeochemistry"). Even though the biogenic silica component (diatoms and radiolarians) is present in both Subunits IA and IB, silica minerals (Fig. F2) were detected only in the lower part of the section (Subunits IC and ID). In fact, the primary composition of diatom frustules and radiolarian tests is opal-A, an amorphous admixture of water and silica tetrahedrons undetectable by XRD. However, with increasing burial and temperature, opal-A tends to crystallize into crystobalite (opal-C) and trydimite, which together form the more stable opal-CT silica phase. As shown in Figure F2, such a silica phase change is detected by XRD analyses, which first show a progressive bulging between 15° and 28°2 and then a sharpening of the crystobalite peak in deeper samples. At Site 1225, the opal-A to opal-CT phase change occurs between ~250 and 300 mbsf.
In general, the opal-A to opal-CT phase change has important consequences for the physical and chemical properties of the sediment. Whereas an increase of the opal-A component has a positive effect on sediment porosity, the onset of the opal-A to opal-CT phase change and the resultant silica reprecipitation causes a porosity reduction and a release of water, as shown in Figure F2, where silica diagenesis and porosity variations are compared.
As reported in "Unit I" in "Description of Lithostratigraphic Units," millimeter-sized pyrite specks and streaks were observed in most of the subunits of Site 1225 (see the XRD results for Section 201-1225C-5H-3 in Fig. F2). However, larger pyrite nodules were observed only in the lowermost subunits (IC and ID) (Figs. F4F, F2: Section 201-1225A-33H-2).
Most of the sediments recovered at Site 1225 show burrows and mottled structures of probable biogenic origin. The most common type of bioturbation is ellipsoidal to irregularly shaped mottles of varying sizes (between 1 and 5 cm), which can generally be attributed to either Chondrites or Planolites. The mottled structures of Subunit IB are often surrounded by purple or green alteration halos.
Bioturbation is concentrated in the central part of Subunit IB, where it usually occurs at the boundary between the alternating white and darker banded layers.
Based on visual description of the cores, the density of bioturbation was estimated. In Figure F3, the thickness of bioturbated layers is plotted vs. depth and compared to magnetic susceptibility. Overall, variations in density of bioturbation and variations in magnetic susceptibility (see "Physical Properties") show a negative correlation. In particular, opposite variations are evident in Subunits IB, IC, and ID (Fig. F3). Thus, bioturbation may have been one of the factors that contributed to the degradation of magnetic minerals in the low magnetic susceptibility parts of the section. Alternatively, the density of bioturbating fauna may have been higher and the magnetic minerals may have been chemically reduced during early diagenesis, both due to a higher deposition rate of organic matter.
The variation in intensity of bioturbation reflects changes in the amount of biogenic activity that occurred at the water/sediment interface during or soon after deposition. Higher intensity of bioturbation may have resulted from either higher concentration of free oxygen in bottom waters or higher availability of food sources on the sea bottom (e.g., higher input of organic carbon). The latter hypothesis seems to be supported by the geochemical data presented in the Leg 138 Initial Reports volume for Site 851 (Shipboard Scientific Party, 1992, fig. 35, p. 941), which show that the highly bioturbated Subunit IB is characterized by relatively higher organic carbon accumulation rates (from 0.1% on top of Subunit IA to up to 0.4% in Subunit IB).
Light gray, pervasively altered basalt was recovered in the lowermost section of Core 201-1225A-35X. Most of this material was consumed by microbiological sampling, but a few small pieces were archived. During transport to the ODP repository, these pieces disaggregated and were unsuitable for thin section preparation or bulk geochemical analysis.
At Site 1225 a nearly complete Pleistocene to middle Miocene section of mainly pale gray nannofossil ooze with varying amounts of diatoms, radiolarians, and foraminifers was recovered. The maximum thickness of this section is 319.6 m. Site 1225 is within 100 m of Site 851, which was drilled during Leg 138 (Shipboard Scientific Party, 1992).
Because of the overall homogeneity of the observed lithologies, only one stratigraphic unit was recognized. However, minor changes in sedimentologic features, biogenic components, color, and other physical and mineralogic parameters allowed the subdivision of Unit I into five subunits.
Subunit IA (0-~71 mbsf) consists of a foraminifer-, diatom-, and radiolarian-rich nannofossil ooze of Pleistocene to Pliocene age characterized by rhythmic alternation of pale gray nannofossil-dominated layers and darker, often banded and/or bioturbated foraminifer-, radiolarian-, and diatom-rich nannofossil ooze layers.
Subunit IB (~71-205 mbsf) is an early Pliocene to late Miocene age pale greenish gray to light gray radiolarian- and diatom-bearing and diatom-rich nannofossil ooze section. Decimeter-scale interbedding of pale greenish gray to gray banded intervals with pale greenish gray mottled and bioturbated zones is common, as well as pale purple alteration bands that are present either in subhorizontal layers or as curvilinear features.
Subunit IC (~205-270 mbsf) is a pale greenish gray diatom-bearing and diatom-rich nannofossil ooze of Miocene age. This subunit is characterized by less-pronounced alteration bands and bioturbation.
The upper part of Subunit ID (~270-305 mbsf) consists of dark olive-green nannofossil-rich laminated diatom ooze and a few opal-CT-laminated porcelanite nodules. The lower part is dominated by nannofossils with subordinate amounts of diatoms and radiolarians. Pale greenish gray to purple alteration bands and pale yellow bioturbated intervals are common. High concentrations of pyrite are common in the form of dark gray nodules and pyritized burrows.
Subunit IE (~270-320 mbsf) includes the sediments that directly overlie the oceanic basement. The interval consists of pale yellow diatom-rich nannofossil ooze, which grades into a laminated nannofossil chalk.
The boundaries between the lithostratigraphic subunits match well with major inflection points of magnetic susceptibility and color reflectance curves. Moreover, the latter two curves show a negative correlation through most of the sedimentary section, suggesting that the darker layers (less reflectant) are probably richer in magnetic oxides (higher magnetic intensity).
XRD analyses allowed us to determine that the silica phase transition from amorphous opal-A to the mineral opal-CT occurs between ~250 and 300 mbsf. Such a silica phase change is probably the cause of a decrease in porosity, release of water, and important changes in other physical properties of the sediment. In Subunits IB and ID, the variations of the intensity of bioturbation (plotted as bioturbation index in Fig. F3) are mostly in opposition of phase with the variations of magnetic intensity, thus suggesting either attenuation of the primary magnetic signal by biogenic activity at the sediment/water interface or enhanced bioturbation and chemical alteration of magnetic minerals during early diagenesis of organic-rich sediments.
