Four holes were drilled at Site 1265. Holes 1265A and 1265B were continuously cored to 359.5 mcd (312.4 mbsf) and 288.8 mcd (251.9 mbsf), respectively. Hole 1265C was washed to 208.4 mcd (185.0 mbsf) and cored to 229.9 mcd (204.3 mbsf). Holes 1265B and 1265C were terminated prematurely because of mechanical problems (see "Operations"). Hole 1265D was washed to 285.9 mcd (248.0 mbsf) and cored to 315.9 mcd (274.9 mbsf) to obtain a second copy of the P/E boundary. Major lithologies include nannofossil ooze, foraminifer-bearing nannofossil ooze, foraminifer-nannofossil ooze, nannofossil-foraminifer ooze, clay-bearing nannofossil ooze, and foraminifer- and clay-bearing nannofossil ooze. Minor and accessory lithologies include ash-bearing nannofossil ooze, ashy nannofossil ooze, clay-bearing volcanic glass, zeolite- and nannofossil-bearing clay, clay, calcareous dinoflagellate cyst–bearing nannofossil ooze, and hematite-bearing nannofossil ooze. Evidence for moderate to extensive bioturbation is prevalent within sediment intervals of higher color contrast but is largely absent within sediment intervals of lower color contrast. Drilling disturbance was minimal in most cores; some intervals were affected by APC soft-sediment deformation or XCB biscuiting.
By combining measurements of MS, sediment L*, and smear slide–based estimates of foraminifer abundance, we have divided this sequence into two lithostratigraphic units, with the lower unit subdivided into three subunits (Table T4; Fig. F7). Stratigraphic variation for whole-core MST parameters is presented in Figure F8. Stratigraphic variation in sediment L*, chromaticity (a* and b*), and carbonate content is presented in Figure F9. Stratigraphic smear slide variation in major and minor lithologies is presented in Figure F10 and highlights the predominance of nannofossils throughout Site 1265 with relatively higher foraminifer abundances within Unit I. As also reflected in the carbonate content, Site 1265 clay abundance is commensurately low throughout and is too subtle for use in defining lithostratigraphic units.
Half-core physical property data show linear downcore trends punctuated by three horizons of change (Fig. F11). The first horizon (~52 mcd) is marked by a downcore decrease in compressional wave (P-wave) velocity values measured with the P-wave velocity sensor 3 (PWS3), a broad relative maximum in grain density, and less variation within bulk density and porosity. The second horizon (~185 mcd) is marked by a step decrease in grain density and porosity. The third horizon (~239 mcd) is marked by a step increase in grain density and P-wave velocity. The strong correlation and general downcore increase in both gamma ray attenuation (GRA)-based and moisture and density (MAD)-based estimates of bulk density coupled with the strong negative correlation between bulk density and porosity (Fig. F12) suggest increasing downhole sediment compaction. This interpretation is also supported by the weak correlation between bulk density and PWS3 (Fig. F12) measurements.
Unit I consists primarily of light gray to very pale brown foraminifer-bearing nannofossil ooze with smear slide–based foraminifer percentages averaging 22% (Fig. F7). Nannofossil oozes are less common, and the base of the unit is a clay-bearing nannofossil ooze. MS is consistently low except for a marked increase at the base of Unit I, whereas L* values show a broad and unit-centered relative maximum at ~35 mcd (Fig. F7). Carbonate content averages 96 ± 1 wt% (Fig. F9).
Irregularly shaped, centimeter-scale, pinkish to whitish blebs with gray halos of disseminated fine Mn oxides (Fig. F13) are common in the upper 30 m of Unit I but are absent in the lower part. These blebs show minimal vertical compression with relatively few noncircular cross sections and a large range in diameter (<2 cm to >17 mm). Two ~1-cm-diameter blebs carved from the working half did not extend through the core half. Smear slide comparisons show that the composition of the blebs differ from those of the surrounding sediments only in their Mn oxide content. In intervals without blebs, faint centimeter-scale dark bands are common and rarely bioturbated. Based on these limited observations, blebs may represent localized diagenetic effects related to oxidation-reduction reactions, although a burrow trace origin cannot be ruled out. Discrete centimeter- to decimeter-scale intervals of fining-upward nannofossil foraminifer oozes and nannofossil-bearing foraminifer oozes are present at ~27.4 mcd (interval 208-1265A-3H-4, 62–66 cm; 24.13–24.16 mbsf) and ~41.5 mcd (interval 208-1265A-4H-6, 70–72 cm; 36.55–36.57 mbsf) and are interpreted as turbidites or gravity flows (Fig. F14).
In the middle to upper parts of Unit I, decimeter-scale oscillations in sediment color, marked by an alternation between light gray brown and pale brown lithologies, are well expressed in both L* and MS values (Fig. F15). These color alternations may reflect oscillations in carbonate production and preservation during the glacial–interglacial cycles of the Pliocene–Pleistocene.
Unit II consists predominantly of light gray to very pale brown nannofossil ooze with rare intervals of darker brown foraminifer-bearing nannofossil ooze. Three subunits (IIA, IIB, and IIC) are defined by markedly higher MS and slightly lower L* within Unit IIB (Fig. F7). Carbonate content averages 94 ± 3 wt% within Unit II, excluding the occasional clay-rich horizons present within the Paleocene and Eocene part of the sequence.
Subunit IIA consists predominantly of very pale brown to light gray nannofossil ooze. Rare intervals of slightly darker foraminifer-bearing nannofossil ooze are present between ~90 and ~120 mcd, and the basal portion of the unit is a clay-bearing nannofossil ooze. Carbonate content averages 94 ± 2 wt%. The contact between Unit I and Subunit IIA is marked by a downcore shift in predominance from foraminifer-bearing nannofossil ooze to nannofossil ooze, a step increase in the value and variability of MS, and a marked decrease in L* (Fig. F7). Cyclicity in both L* and MS values is present throughout Subunit IIA, with these values generally showing an inverse relationship (Fig. F16A).
Blebs are rare to absent. Discrete centimeter to decimeter intervals of fining-upward nannofossil foraminifer oozes and nannofossil-bearing foraminifer oozes are present at ~63 mcd (Sample 208-1265A-6H-6, 17 cm; 55.17 mbsf) and ~85.2 mcd (interval 208-1265A-8H-7, 10–35 cm; 75.30–75.55 mbsf) and are interpreted as turbidites or gravity flows. Centimeter-thick horizons of Braarudosphaera-dominated nannofossil ooze are present at the centimeter to decimeter scale from ~152.7 to ~162.4 mcd (Core 208-1265A-15H; 133.00–142.64 mbsf). Centimeter-scale mottling produced by finely disseminated to concentrated Mn oxide grains first appears at ~173.2 mcd (Core 208-1265A-17H; 152.00–161.51 mbsf) and increases in abundance markedly downcore.
At the boundary between Subunits IIA and IIB, which is near the Eocene–Oligocene transition at ~190 ± 4 mcd (see "Biostratigraphy"), MS increases and L* decreases (Fig. F17). The clay mineral assemblage was examined for sediments in the lowermost part of Subunit IIA at ~191.60 mcd (Sample 208-1265A-18H-7, 63 cm; 169.60 mbsf). X-ray diffraction (XRD) diffractogram results are presented in Figure F18. The loss of the 7-Å peak in the heated sample compared with the untreated and glycolated samples and the lack of a discernable 14-Å peak indicate an absence of chlorite. All 12- to 15-Å peaks in the untreated sample shift to ~10 Å in the heated sample and to 16–20 Å in the glycolated sample, indicating an abundance of mixed-layer smectite. The clay mineral assemblage, therefore, mainly contains illite, kaolinite, smectite, and smectite-relative mixed-layered clays associated with minor quartz and feldspar.
Subunit IIB is a light gray to very pale brown nannofossil ooze with occasional intervals of foraminifer-bearing nannofossil ooze. The boundary between Subunits IIA and IIB coincides with the interval containing the E/O boundary and is marked by a downhole increase in MS and in a* and b* chromaticity values and by a decrease in L* and red-green-blue values (Figs. F7, F9, F17). Note that because of differential preservation and core recovery, the Subunit IIA/IIB boundary is ~1.75 mcd shallower in Hole 1265A (190.98 mcd) than in Hole 1265B (192.73 mcd) (see "Biostratigraphy" and "Composite Depth"). Subunit IIB is characterized by higher and more variable MS and lower L* relative to Subunits IIA and IIC. These differences may reflect slightly greater clay, ash, and oxide content, although the average carbonate content (94 ± 2 wt%) of Subunit IIB is similar to that of Subunits IIA and IIC. Pronounced oscillations in both sediment L* and MS are present in the upper part of this subunit (Fig. F16B). Relatively large MS variations are generally correlated with higher L* and appear related to the volcanic ash layers in this subunit.
Blebs are rare in the upper ~20 m of Subunit IIB and common in the lower ~30 m. Centimeter-scale mottling, produced by disseminated to highly concentrated Mn oxide nodules, is more abundant than in Subunit IIA. Calcareous dinoflagellate cysts are unusually abundant from ~194.3 to ~204.1 mcd (Core 208-1265A-19H; 171.00–180.82 mbsf) and merit minor modifier status; dinoflagellate cysts are rare in deep-ocean environments and may reflect downslope transport. Distinct centimeter-scale ash-rich layers are common from ~221.3 to 248.7 mcd (Cores 208-1265A-21H through 23H; 190.00–218.96 mbsf) and are indicated by prominent spikes in MS (Figs. F7, F19).
Subunit IIC consists of light gray to white nannofossil ooze and is differentiated from Subunit IIB by lower MS and a step increase in L* (Fig. F7). Subunit IIC is characterized by an initial downcore increase in MS that peaks at ~316 mcd and then decreases through ~327.2 mcd (Sample 208-1265A-32H-5, 55 cm). L* shows a similar downcore increase to ~300 mcd and is relatively high through the remainder of the subunit. Carbonate content averages 93 ± 2 wt% when excluding the clay-rich horizon of the P/E boundary interval. Cyclicity represented by generally positive covariation in L* and MS is well developed in Subunit IIC (Fig. F16C).
Blebs and mottling, along with diffuse darker laminae at a centimeter scale, decrease in abundance downcore and are absent below ~294.9 mcd (Section 208-1265A-27H-CC; 257.02 mbsf). To determine the composition of bleb halos and sediment mottling, a millimeter-scale black mottle from ~294.5 mcd (Sample 208-1265A-D-27H-7, 47 cm; 256.47 mbsf) was analyzed by XRD (Fig. F13). The XRD diffractogram indicates calcite as the dominant mineral component but also shows the presence of the Mn oxide lithiophorite ([Al,Li]MnO2[OH]2), which suggests that bleb halos and sediment mottling comprise submillimeter-scale Mn oxide grains. We note that the prevalence of these Mn oxides generally coincides with intervals of higher interstitial water Mn concentrations (Fig. F20).
Chalky centimeter-scale layers of nannofossil ooze, as well as siliceous intervals and chert nodules, are first present at ~285 mcd (interval 208-1265A-27H-3, 130–145 cm; 251.3–251.5 mbsf) and are particularly abundant from ~289 to ~299 mcd (Core 208-1265A-27H; 247.00–257.02 mbsf). A partially silicified limestone layer is present at 354.13–354.23 mcd (interval 208-1265A-35X-5, 65–75 cm; 308.35–308.45 mbsf). Hole 1265A ends at 359.93 mcd (Section 208-1265A-36X-CC; 312.41 mbsf) with a chert gravel to coarse sand, an artifact produced by XCB drilling. From this, we infer an increase in chert abundance in the lowermost part of Hole 1265A. A porcellanite nodule from 292.65 mcd (Section 208-1265A-27H-12; 254.62 mbsf) was examined to determine its mineral composition using optical microscopy and bulk XRD analysis. Optical microscopy revealed siliceous material characterized by calcite crystal habits, whereas the bulk XRD diffractogram shows quartz and opal-CT as major mineral components with a minor calcite component (Fig. F21). Thus, the presence of opal-CT pseudomorphs suggests that this porcellanite formed by diagenetic replacement of calcium carbonate rather than as a cement. Given the paucity of biosiliceous components in Leg 208 sediments, a likely source of silica for this porcellanite and chert could be volcanic ash dissolution or silicate weathering in underlying basement rock. This interpretation is consistent with frequent volcanic ash layers in Subunits IIB and IIC (Fig. F7) and increased downcore silicon contents in interstitial waters (see "Geochemistry").
Peaks in MS and chromaticity a* coincide with a ~5-cm interval of reddish nannofossil clay at 277.4 mcd (interval 208-1265A-26H-3, 53–58 cm), which likely corresponds to a similar clay-rich layer (Chron C24n clay layer) recovered in early Eocene sediments from Sites 1262, 1263, 1266, and 1267 (Fig. F22). The presence of this horizon, if truly time coincident and depth variable, may indicate a rapid but short-lived event of carbonate dissolution or nondeposition.
The P/E boundary interval was recovered in Hole 1265A at ~315.87 mcd (Sample 208-1265A-29H-7, 71 cm; 275.09 mbsf), whereas only the lowermost Eocene was recovered from the base of Hole 1265D at ~315.88 mcd (Sample 208-1265D-5H-CC, 10 cm; 274.90 mbsf). The P/E boundary interval occurs at the contact between underlying Paleocene nannofossil ooze and a reddish brown nannofossil clay of the lowermost Eocene (Fig. F23). This is accompanied by a dramatic upcore increase in MS and chromaticity a* and decrease in carbonate content. Nannofossil ooze of the underlying Paleocene averages 85–90 wt% carbonate and decreases abruptly to ~30 wt% at the boundary. Following the sharp increase in values above the boundary, MS gradually returns to lower baseline values in a stepped fashion over a 50-cm interval. A similar progression is seen in both carbonate content and chromaticity a* as the sediment composition changes from nannofossil clay at the boundary, upward into clay-bearing nannofossil ooze, and finally to very pale brown nannofossil ooze. This marked decline in carbonate content across the P/E boundary likely represents a basin-wide dissolution event, developing in response to a shoaling of the CCD. Given that carbonate content decreases to ~0 wt% at Site 1263, a site of shallower paleodepths, we would expect similarly low carbonate contents at Site 1265. Therefore, it is probable that the lowermost Eocene may not have been completely recovered in either Hole 1265A or Hole 1265D.