The 297 m of sediment cored in three holes at Site 1209 consists largely of nannofossil ooze and nannofossil ooze with clay (Fig. F6). Several discrete clay-rich units representing condensed intervals are also present. Minor components throughout the sequence include foraminifers, diatoms, and radiolarians. Other minor to trace components include pyrite, Fe-oxides, and zeolites. Volcanic glass is a disseminated trace component concentrated in a few discrete ash layers in the Neogene portion of the sequence. Both the Neogene and Paleogene are characterized by pervasive color/lithologic cycles on decimeter to meter scales.
The sequence has been subdivided into three major lithologic units. In Hole 1209A, Unit I extends from 0 to 111.2 mbsf, the base of an upper lower Miocene unconformity. The unit consists primarily of nannofossil ooze with clay interbedded with clayey nannofossil ooze and nannofossil ooze. These variations are expressed as decimeter-scale light-dark color cycles. Unit II begins at the base of a condensed interval and/or unconformity at 111.2 mbsf representing the Oligocene/Miocene boundary and extends to 235.2 mbsf, the base of the Cenozoic. It consists of alternating nannofossil ooze and ooze with clay and contains several discrete centimeter-thick clay-rich horizons. Unit II is distinguished from Unit I by higher carbonate contents and more oxidized, reddish colored sediments, as indicated by higher red/blue ratios (Fig. F7). Unit III extends from the K/T boundary at 235.2 mbsf to the lower Maastrichtian at the bottom of the drilled section at 297 mbsf and consists primarily of very pale orange (10YR 8/2) to white (N9) nannofossil ooze.
Unit I has been subdivided into three subunits (Fig. F6). Subunit IA extends from 0 to 37 mbsf in Hole 1209A (Holocene to upper Pliocene). It consists mostly of alternating nannofossil ooze with clay and clayey nannofossil ooze, and is characterized by relatively high amplitude, high-frequency variance in color reflectance (Fig. F6). Subunit IB extends from 37 to 84.2 mbsf in Hole 1209A (upper Pliocene to upper Miocene) and consists of alternating nannofossil ooze with clay and nannofossil ooze. It is distinguished from Subunit IA primarily on the basis of carbonate content, color, and cycle amplitude. Subunit IC extends from 85.0 to 111.2 mbsf (middle to upper lower Miocene). It is characterized by more variable carbonate content (5-90 wt%; Table T10) and high red/blue ratios relative to Subunits IA and IB.
The periodic lithologic variations of Subunit IA are expressed as decimeter-scale light-dark color cycles ranging in wavelength between 40 and 150 cm (Figs. F6, F7). The longer wavelength cycles are most pronounced in the upper 10-m, upper Pleistocene portion of the unit. The thinner (30-50 cm) dark beds are light olive gray (5Y 6/1) to olive gray (5Y 4/1) in color, whereas the thicker (~50-150 cm) light beds range from very light gray (N8) to light gray (N7). The contacts between the interbeds tend to be gradational, although some contacts from light to dark are sharp. Millimeter-scale pyrite laminae and blebs are common throughout Subunit IA. Dark gray to pale green millimeter-scale bands and laminae are randomly distributed, although they appear to be more frequent in the light intervals and around some lithologic contacts. Disseminated volcanic glass and discrete centimeter-thick ash bands are frequent in this subunit. As many as three to four ash layers occur in some cores. Common sedimentary structures include burrows and indistinct mottling. Bioturbation is rare to moderate throughout Subunit IA, and with the exception of Core 198-1209A-1H, there is no indication of significant deformation by drilling. Several contacts present in Core 198-1209A-2H are inclined at high angles and thus appear to be erosional in nature. Section 198-1209A-2H-6 also contains a fault with a normal offset.
Subunit IB extends from 37 to 84.2 mbsf in Hole 1209A and 37 to 86.06 mbsf in Hole 1209B (intervals 198-1209A-5H-1, 99 cm, through 10H-1, 0 cm, and 198-1209B-5H-3, 40 cm, through 10H-4, 46 cm) and consists primarily of very light gray (N8) nannofossil ooze and light gray (N7) nannofossil ooze with clay. The concentration of biosiliceous components reaches 20% at ~51 mbsf, the highest value recorded at this site (Fig. F8). The transition from Subunit IA is marked by an increase in the overall reflectance as well as a distinct decrease in the amplitude of the high-frequency color cycles and an increase in the amplitude of long (5-8 m) wavelength color cycles (Fig. F7). Note that similar transitions at other sites are not distinguished with a subunit boundary with the exception of Site 1212. Carbonate content increases slightly downhole in this unit to Core 198-1209A-10H (Fig. F6). Trace pyrite is present as blebs, in burrows, and in millimeter-scale faint black bands or laminae. Clusters of pale green laminae with traces of pyrite are present as well. Siliceous microfossils, primarily radiolarians, and volcanic glass are present in minor to trace quantities. Although this subunit is moderately bioturbated, distinct, identifiable ichnofossils are very rare.
Subunit IC extends from 84.2 to 111.2 mbsf in Hole 1209A, 86.06 to 110.6 mbsf in Hole 1209B, and 98 to 110.7 mbsf in Hole 1209C (Sections 198-1209A-10H-1, 0 cm, through 12H-6, 50 cm; 198-1209B-10H-4, 46 cm, through 13H-1, 130 cm; and 198-1209C-1H-1 through 2H-3, 23 cm). The lithology is predominantly clayey nannofossil ooze and nannofossil ooze with clay. Sediment hues vary from grayish orange (10YR 7/4), to pale yellowish brown (10YR 6/2) and dark yellowish brown (10YR 4/2), to very pale orange (10YR 8/2). The transition from the overlying subunit is marked by a very distinct rise in the red/blue ratios (Fig. F9). Note that this transition is analogous to the Subunit IA/IB boundary at the other sites (except for Site 1212). The unit also contains several clay layers, the most prominent of which is a 30-cm-thick dusky yellowish brown (10YR 2/2) clay in Section 198-1209A-12H-4. This clay represents a condensed interval that includes much of the upper lower Miocene. Well-developed ichnofossils, primarily Zoophycos burrows, are common at the contacts between the dark and light beds in this unit. Bioturbation is rare to moderate throughout the unit, and there is virtually no drilling disturbance.
Unit II at Site 1209 has been subdivided into two subunits. The contact with Unit I, which occurs in Section 198-1209A-12H-6, is marked by a major lower Miocene/upper Oligocene unconformity (Fig. F6). Moreover, the contact with Unit I is marked by an abrupt shift in the apparent stiffness of the sediment. In Unit I, the nannofossil ooze is firm and stiff, whereas in Unit II the ooze tends to be much softer with a pastelike consistency. The latter property, which contributed to some flow-in and stretching during coring, persists to the bottom of the hole.
Subunit IIA extends from 111.2 to 198.0 mbsf in Hole 1209A (lower Oligocene to Paleocene/Eocene [P/E] boundary). This unit contains primarily nannofossil ooze with occasional intervals of nannofossil ooze with clay. Color in the Oligocene and upper Eocene portion of the Subunit (112-130 mbsf in Hole 1209A) ranges from grayish orange (10YR 7/4) to very pale orange (10YR 8/2). The Eocene-Oligocene transition (127-128 mbsf in Hole 1209A) is marked by an upcore increase in carbonate content as inferred from reflectance (Fig. F10). The lithology of the lower and middle Eocene (157-217 mbsf in Hole 1209A) is relatively uniform with nannofossil ooze and nannofossil ooze with clay. With the exception of disseminated white burrows with dark halos, sedimentary structures are relatively rare in this subunit. Decimeter-scale (50-80 cm) lithologic cycles occur throughout the unit (Fig. F7). The cycle amplitudes as expressed in color reflectance are lower than in overlying units, suggesting reduced amplitude variations in carbonate/clay content. The exceptions are several distinct darker, more clay-rich intervals, which are easily distinguished in all holes as peaks in reflectance or magnetic susceptibility. One of these clay-rich intervals occurs at the base of Subunit IIA at 198.0 mbsf (Sections 198-1209A-21H-7, 25 cm; 198-1209B-22H-1, 132 cm; and 198-1209C-11H-3, 130 cm; Figs. F11, F12). Biostratigraphy indicates that this horizon lies approximately at the Paleocene/Eocene boundary and is thus coincident with the PETM. In the sediment just a few centimeters below the clay-rich layer, we find moderate amounts of inorganic calcite needles (Fig. F13). Note that the Paleocene/Eocene boundary is not distinguished by a subunit boundary at the other sites with the exception of Site 1212.
Subunit IIB extends from 198.0 mbsf to the K/T boundary at 235.2 mbsf in Hole 1209A. It primarily comprises very pale orange (10YR 8/2) nannofossil ooze and pale yellowish brown (10YR 6/2) nannofossil ooze with clay. These two major lithologies tend to alternate on a meter scale with very gradational contacts. There are also several centimeter-scale horizons characterized by darker more clay-rich lithologies with more abrupt contacts. The most prominent of these occurs in Section 198-1209A-23H-5. Disseminated white burrows and blebs occur throughout the unit. Minor amounts of pyrite are also present, primarily as foraminiferal infill. The base of Unit II, the K/T boundary, is marked by a thin nondistinct horizon. The sediments both above and below this horizon are white (N9) to very pale orange (10YR 8/2) nannofossil ooze. Above the boundary, this light foraminiferal nannofossil ooze gradually grades into a darker, grayish orange (10YR 7/4) nannofossil ooze. A similar trend in total reflectance indicates that this gradation represents an uphole transition into sediment with lower carbonate content (Fig. F14).
Lithologic Unit III consists predominantly of a uniform white (N9) to very pale orange (10YR 8/2) nannofossil ooze with carbonate content in excess of 96 wt%. The top of Unit III is defined by the K/T boundary transition. The base of the unit is marked by a chert layer of unknown thickness that prevented further coring. All sediments are moderately bioturbated, and mild coring disturbance is common. A thin (3 cm) bed of Inoceramid shell fragments is present in Section 198-1209B-31H-1 and large fragments of Inoceramus extend from Section 198-1209C-21H-1, 125 cm, to Section 21H-3, 128 cm. Three chert layers were encountered in Hole 1209C. The absence of these layers in Hole 1209B, however, suggests that the layers are discontinuous or nodular or were not recovered.
Cyclic variations in the position of the lysocline and CCD have exerted a strong influence on the composition of sediment in the Pacific through the Cenozoic. At present, Site 1209 is situated well above the lysocline and CCD, which are at 3.5 and 4.1 km, respectively, in the region. As such, the uppermost Holocene sediments at Site 1209 have a relatively higher carbonate content than the Holocene of Sites 1207 (3.5 km) and 1208 (3.3 km). Because the CCD generally deepens in the Pacific during glacials (Farrell and Prell, 1989), its role in driving the Pleistocene lithologic cycles recorded in Unit I is probably minor. Instead, productivity and sediment transport may be more important. As with the deeper-water Sites 1207 and 1208, the darker-colored intervals generally contain higher amounts of biosiliceous material and clay and probably represent intervals of higher surface water productivity and increased in situ carbonate dissolution. The frequency of cycles in color reflectance through the Pleistocene suggest that they are related to glacial/interglacial cycles. The cycle amplitudes are smaller than the deeper site, owing to a lower contribution of clay and siliceous microfossils to the sediment. Moreover, despite the shallower water depth, the sedimentation rate is significantly lower at Site 1209, ~13-14 m/m.y. over the Pleistocene compared to 42.4 m/m.y. at Site 1208 (Fig. F23). This indicates that carbonate production and preservation may have played a more important role in driving the Pleistocene lithologic cycles. As at the other sites, the dominant period of the cycles corresponds to eccentricity (100 k.y.) subsequent to 0.6 Ma, and obliquity (41 k.y.) for the period from 0.6 to 2.5 Ma. The cycle wavelength at Site 1209, however, is much more irregular, suggesting that accumulation rates were highly variable through time.
Below 37 m (Subunit IB) in Hole 1209A, which roughly corresponds to the onset of Northern Hemisphere glaciation at 2.6 Ma, the high-frequency (~41 k.y.) cycle amplitude decreases. The decrease results primarily from the absence of the low-carbonate "glacial" end-member of the cycles, despite a relative increase in silica content in the mid-Pliocene. Moreover, there is an apparent shift in power to a lower-frequency, longer-wavelength (9-13 m) oscillation. Assuming an average sedimentation rate of 13.4 m/m.y., these cycles would have periods close to those of the long obliquity (1.25 m.y.) cycles. The peak in siliceous microfossil deposition at Site 1209 occurred in the mid-Pliocene, at roughly the same time as in regions of the North Pacific (Rea et al., 1995).
The sediment of Subunit IC shows evidence of greater oxidation as reflected by the predominance of shades of pale orange and gray orange. This is also reflected in the red/blue reflectance values, which are three times those of Subunit IB, and magnetic susceptibility (Fig. F7), which is also higher owing to the greater Fe oxide content (see "Physical Properties"). The decrease in carbonate content within Subunit IC is accompanied by the presence of several more clay-rich intervals, most notably in Section 198-1209A-12H-3. This pattern is similar to those observed at the deeper Sites 1207 and 1208. The biostratigraphy demonstrates that these clay horizons are condensed intervals, most likely produced by dissolution of carbonate at the seafloor during the middle and late early Miocene. Such a phenomenon would be consistent with the reported shoaling of the global lysocline and CCD through the early and middle Miocene (Rea and Leinen, 1986). Studies of previously drilled sites suggest that the lysocline was as shallow as 3 km during the early Miocene. The data collected here indicate that the lysocline might have been even shallower (<2.4 km) during much of this period.
On average, Unit II sediment shows evidence of greater oxidation than Unit I. This is illustrated by the red/blue reflectance ratios, which are three times more than the average for Unit I. Carbonate content on average is also much higher than that in Unit I (Core 198-1209A-15H; Fig. F6). As observed at the deeper sites on Shatsky Rise, a low rate of sedimentation (<6 m/m.y.) in the Paleogene indicates that the oxidized character is partially a product of lower surface water productivity. Another potential contributing factor is global deep-sea circulation, which for most of the period prior to the late Miocene lacked deepwater formation in the North Atlantic (Wright et al., 1992). The net result was that Pacific deep waters were relatively less nutrient rich and corrosive, and more oxygenated, than prior to the Miocene. An increase in carbonate accumulation in the early Oligocene is consistent with a global deepening of the CCD coincident with the Eocene/Oligocene boundary. At Site 1208, the deepening trend appears to have been nonlinear, or punctuated by periodic reversals or oscillations (Fig. F9). The cycle wavelengths (~1.6 m) and sedimentation rate (~3.9 m/m.y.) suggest concentration of power at a frequency close to that of the long eccentricity (400 k.y.) cycle. These may be related to fluctuations in the global carbon cycle, as similar cycle frequencies have also been recognized in late Eocene and early Oligocene deep-sea carbon isotope time series (Diester-Haass and Zahn, 1996; Zachos et al., 1996).
The style of early to middle Eocene sediment accumulation in the lower portion of Subunit IIA indicates deposition under oxic conditions and low productivity. The subtle color cycles also hint at some influence of orbital oscillations on regional sedimentation. For example, in the middle Eocene, the dominant cycles have 50- to 60-cm wavelengths and sedimentation rates average 0.52 cm/k.y., thus indicating a primary response to 100-k.y. eccentricity forcing (Fig. F15). The most prominent feature of this subunit, however, is the clay-rich layer at the P/E boundary. This layer most likely represents an episode of enhanced dissolution related to a hypothesized rapid shoaling of the global CCD (Dickens et al., 1995, 1997). The CCD rise is attributed to the effects of dissolving ~2000 Gt of carbon (as CO2) on the pH and carbonate ion content of deep waters. The Site 1209 P/E boundary horizon is characterized by a sharp basal contact rich in clay, which then grades upward into more carbonate-rich sediment (Fig. F12). This carbonate dissolution and recovery pattern is similar to that observed in Atlantic pelagic and hemipelagic sequences (Kennett and Stott, 1991; Bralower et al., 1997; Thomas et al., 1999). Because this represents the first documentation of this pattern in the Pacific, it lends substantial support to the methane outflux model, which calls for shoaling of the CCD in all ocean basins.
The base of Unit II is marked by the K/T boundary. Because this appears to be one of the more complete K/T boundary sequences for the Pacific (see "Biostratigraphy"), the primary lithologic changes observed across this interval should reflect on the regional ecological and water column chemical variations associated with this biotic crisis. For example, the thin, basal clay most likely formed in the immediate aftermath of the impact, a period of widespread extinction of nanno- and zooplankton, and collapse in global primary and net production (Thierstein, 1981; Zachos and Arthur, 1986; Zachos et al., 1989).
The Maastrichtian is known for the widespread deposition of calcareous nannofossil-rich sediments. The Maastrichtian at Site 1209 is no exception. It is characterized by the accumulation of exceptionally pure calcareous nannofossil ooze. The primary difference is that the Site 1209 Maastrichtian sediments are poised at the ooze-chalk transition, whereas elsewhere the vast majority are chalks or limestones. The high carbonate content and pale orange color indicate oxidizing conditions and low rates of eolian and other dilution in the central Pacific. The presence of Inoceramus in the mid-Maastrichtian may be an important finding, as Inoceramid debris are generally restricted to sediments older than mid-Maastrichtian elsewhere (MacLeod et al., 1996). More detailed examination of the Site 1209 cores should provide insight as to the nature of the environmental conditions in the Pacific at the time of this extinction event.