Site 1212 is located in the approximate vicinity of DSDP Site 47.2 and within 1.5 km of DSDP Site 577. The site was drilled in a water depth of 2681 m. The 207.6 m of sediment recovered from Holes 1212A and 1212B is primarily Upper Cretaceous and Cenozoic nannofossil ooze and clayey nannofossil ooze that are entirely pelagic in character. Nannofossils are the major constituent, with variable amounts of clay and foraminifers. In general, there are subtle downhole trends of an increase in the carbonate content and oxidation of sediment. Despite the relatively uniform character of the sediment, the sequence is subdivided into three major lithologic units on the basis of variations in physical properties and sediment composition (Fig. F4). Unit I is light gray (N7) nannofossil ooze with clay that is interbedded with light olive-gray (5Y 6/1) to olive-gray (5Y 4/1) clayey nannofossil ooze. The unit extends from the sediment-water interface to a major Paleogene-Neogene unconformity at 61.3 mbsf in Hole 1212A. Unit II consists of pale yellowish brown (10YR 6/2) nannofossil ooze with clay to grayish orange (10YR 7/4) and very pale orange (10YR 8/2) nannofossil ooze that extends from 61.3 mbsf to the K/T boundary at 101.4 mbsf in Hole 1212A. Unit III encompasses uniform white (N9) to very pale orange (10YR 8/2) nannofossil ooze from 101.4 mbsf to the base of the drilled section at 207.6 mbsf in Hole 1212B. Units I and II are further subdivided on the basis of minor variations in sediment composition and color. The Site 1212 unit and subunit boundaries are nearly identical to those selected for DSDP Site 577 (Heath, Burckle, et al., 1985).
Unit I is subdivided into two subunits (Fig. F4). Subunit IA (0 to 29.78 mbsf [Hole 1212A] and 0 to 35.2 mbsf [Hole 1212B]; Sections 198-1212A-1H-1 to 4H-3, 133 cm, and Sections 198-1212B-1H-1 to 5H-1, 0 cm; Holocene to upper Pliocene) is light gray (N7) nannofossil ooze with clay that is interbedded with a light olive-gray (5Y 6/1) and olive-gray (5Y 4/1) clayey nannofossil ooze with foraminifers. In addition to nannofossils (50%-80%), the sediment contains subordinate amounts of clay (5%-35%) and foraminifers (0%-32%) (see "Site 1212 Smear Slides"). The percentage of nannofossils in this subunit generally increases downhole, whereas the percentage of foraminifers decreases. Minor biogenic components include diatoms (0%-5%), radiolarians (0%-5%), silicoflagellates (0%-2%), and sponge spicules (0%-2%) (Fig. F5). Nonbiogenic components include Fe oxides, quartz, volcanic glass, feldspar, and mica. Carbonate content averages 72 wt% and varies from 53 to 89 wt% (Fig. F4). Cycles are expressed as decimeter-scale light/dark alternations with high-amplitude and high-frequency variation in color reflectance that correspond with similar frequency rhythms in magnetic susceptibility and bulk density (see "Physical Properties"). In general, total color reflectance (L*) increases downhole as carbonate content increases (Fig. F4). The red/blue ratio, however, is low (~1.3) and relatively constant with depth (Fig. F6). Lithologic contacts are mostly gradational and highly bioturbated. Bioturbation within beds is rare to moderate. Pyritized burrow fills and blebs are rare to common throughout the subunit as are green diagenetic bands/laminae. Volcanic ash layers are restricted to Subunit IA. There are few distinct ash beds in this unit, and Section 198-1212A-4H-7 contains a single large (~1.5 cm) pumice fragment.
Subunit IB (29.78 to 61.3 mbsf [Hole 1212A] and 35.2 to 63.26 mbsf [Hole 1212B]; Sections 198-1212A-4H-3, 133 cm, through 8H-3, 150 cm, and 198-1212B-5H-1, 0 cm, through 7H-7, 6 cm; upper Pliocene to middle Miocene) consists of alternating very light gray (N8) nannofossil ooze with clay and light olive-gray (5Y 6/1) clayey nannofossil ooze. This subunit is distinguished from Subunit IA by higher average carbonate content and less pronounced cycles. In addition to nannofossils (69%-85%), sediment constituents include clay (8%-25%), foraminifers (1%-10%), radiolarians (0%-5%), diatoms (0%-2%), and sponge spicules (0%-1%). Other minor components include Fe oxides, quartz, volcanic glass, feldspar, mica, and opaque minerals. The average carbonate content is 82 wt%, with variations from to 73 to 89 wt%. The light/dark alternations (decimeter scale) are more subtle than in Subunit IA (Fig. F7). Total color reflectance averages 75%. The red/blue color reflectance ratio is relatively constant at 1.3 until the lower part of the subunit (~55 mbsf), where it rapidly increases to 2.5. This increase in the red/blue ratio is analogous to the Subunit IA/IB boundary at Sites 1207, 1208, 1210, and 1211. The amplitude of the high-frequency cycles in total reflectance is lower than it is in Subunit IA. Bioturbation is rare to moderate. Rare pyritized burrow fills and green diagenetic bands are restricted to the upper portion of Subunit IB. Some burrows are filled with foraminifer-rich sediment.
The boundary between Units I and II was placed at the base of a middle Eocene/middle Miocene unconformity in Section 198-1212B-7H-6. This unconformity coincides with an abrupt shift in both bulk density and magnetic susceptibility values (Figs. F26, F33). Unit II is further distinguished from Unit I by a higher average carbonate content. Unit II has been subdivided into two subunits.
Subunit IIA (61.3 to 76.54 mbsf [Hole 1212A] and 62.3 to 81.43 mbsf [Hole 1212B]; Sections 198-1212A-8H-3, 150 cm, through 10H-1, 64 cm, and 198-1212B-7H-7, 6 cm, through 9H-5, 73 cm; lower middle Eocene to P/E boundary) consists predominantly of pale yellowish brown (10YR 6/2) nannofossil ooze with clay alternating with pale orange to grayish orange (10YR 7/4) nannofossil ooze. There are a few 1-cm-thick layers of moderate yellowish brown (10YR 5/4) clayey nannofossil ooze. The average carbonate content in Subunit IIA is 82 wt%. Minor components include clay (~3%-10%), foraminifers (0%-7%), and radiolarians (0%-1%). Other minor components (0%-4%) include feldspar, volcanic glass, and opaque minerals. A prominent clay-rich layer (~35%) is present in Section 198-1212A-8H-3, 120-150 cm. This clay layer contains minor amounts of Fe oxides (4%), phillipsite (3%), and barite (2%). The subunit exhibits decimeter-scale, light/dark alternations, and contacts between lithologies that are heavily bioturbated. Abundant well-preserved burrows including Zoophycos and large (2-3 cm) unidentified burrows with white halos are present in Sections 198-1212B-8H-2 through 8H-3. A relatively high abundance of pyrite blebs are present in lower Eocene Sections 198-1212A-9H-4 through 9H-CC and Sections 198-1212B-8H-6 through 9H-3 (Fig. F8). Total color reflectance averages 67.5%. Red/blue color reflectance ratios, which average 2.1, are markedly higher in comparison with Unit I. At the base of the subunit there is a sharp contact between a moderate yellowish brown (10YR 5/4) clayey nannofossil ooze in Subunit IIA and a very pale orange (10YR 8/2) nannofossil ooze of Subunit IIB (Fig. F8). The P/E boundary occurs at the contact (see "Biostratigraphy"). Sediments just below and within the clay-rich layer contain up to 5% microscopic inorganic calcite needles. In Hole 1212B, this clayey nannofossil ooze unit contains an unusual feature—a 1.5-cm burrow filled with opaline fragments, interpreted initially as remains of a sponge.
Subunit IIB (76.54 to 101.4 mbsf [Hole 1212A] and 81.43 to 101.44 mbsf [Hole 1212B]; Sections 198-1212A-10H-1, 64 cm, to 12H-CC, 6 cm, and 198-1212B-9H-5, 73 cm, to 11H-7, 24 cm; P/E boundary to K/T boundary) consists of predominantly very pale yellowish brown nannofossil ooze with clay interbedded with very pale orange (10YR 8/2) nannofossil ooze. The carbonate content is slightly higher (96.3 wt%) and is less variable (94.9%-96.4%) than in the overlying subunits. In addition to nannofossils, major constituents include clay (0%-25%) and foraminifers (0%-15%). Minor nonbiogenic components (0%-1%) include Fe oxides, volcanic glass, feldspar, quartz, and pyrite. Inorganic/organic calcite makes up 1%-3% of the sediment. Bedding is on a decimeter scale, and contacts are heavily to moderately bioturbated. Total color reflectance increases in Subunit IIB to 74.1%. The red/blue color reflectance ratio averages 1.5. The base of this unit lies at the K/T boundary and is marked by a sharp contact at which a pinkish gray (5YR 8/1) clayey unit overlies white nannofossil ooze.
Unit III is predominantly homogeneous, soft, white (N9) nannofossil ooze and nannofossil ooze with foraminifers interbedded with chert. In addition to nannofossils (83%-99%), constituents include foraminifers (0%-10%), clay (1%-5%), and a trace of pyrite. Inorganic calcite, mostly rhombs and blades, makes up to 7% of the sediment in some intervals. The upper few centimeters of most cores in Unit III commonly contains very pale orange (10YR 8/2) chert fragments. Overall, the sediment is compositionally homogeneous and featureless. Carbonate content in the unit averages 96.7 wt% and varies within a narrow range (96.1-97.4 wt%). As a result, total color reflectance is high, averaging 92.3%, and the red/blue ratio is low, averaging 1.2%. Cores 198-1212B-17H and 24H are highly disturbed by coring. There are fragments of Inoceramus in Section 198-1212B-23H-2, 129 cm. At the base of Section 198-1212B-24H-7, two large unconformities from the Campanian to lower Santonian-Coniacian and from the Coniacian to the Albian are present. Recovery through the upper Campanian and lower Maastrichtian exceeded 90% in some intervals. The lower Santonian-Coniacian is represented by a pink, clayey layer in interval 198-1212B-24H-7, 13-24 cm, that contains glauconite.
In general, the Unit I lithology is similar to Unit I at all of the Leg 198 sites on the southern Shatsky Rise, except Sites 1213 and 1214. The pervasive light-dark bedding cycles indicate the strong influence of glacial-interglacial climate cycles on local sedimentation. The dominant periodicity corresponds to eccentricity (100 k.y.) from 0 to 0.6 Ma, and obliquity (41 k.y.) from 0.6 to 2.5 Ma. These depositional cycles possibly reflect regional shoaling and deepening of the lysocline/CCD and/or climatically controlled variations in clay fluxes and carbonate production. Site 1212, however, differs in that the sediment accumulation rate through the Pliocene-Pleistocene was two to three times lower than at Sites 1209 and 1210, and slightly higher than at Site 1211 (Fig. F15). Although Site 1212 is roughly 300 m deeper than Site 1209, it is just 108 m deeper than Site 1210, suggesting that differential preservation was not responsible for the relative offsets in long-term accumulation rates. This is reflected in the nearly identical average Neogene carbonate contents between sites. Thus, current activity may be continuously or episodically sweeping sediment away from Site 1212, as well as Site 577, which has a similar unconformity. The location of Site 1212 on the western flank of the rise near an apparent canyon would support this supposition. Current-mediated erosion would also account for the extent of the middle Miocene-middle Eocene unconformity at the base of the unit.
In terms of overall general lithologic characteristics, Site 1212 is very similar to DSDP Site 577 with one notable exception: the color hues are darker on average throughout Unit I of Site 1212. Rather than the light gray (N7) and light olive-gray (5Y 6/1) hues observed at Site 1212, the Site 577 sediments were described as white (N9) to light gray (N7) (Heath, Burckle, et al., 1985). Although this difference is relatively subtle, it may be significant as there was little to no pyrite identified at Site 577.
As observed at the other Leg 198 sites, Unit II sediment has a distinctly higher ratio of red to blue hues than sediment in Unit I. This change in color is partially indicative of a shift to more oxidized sediment pore water conditions, which at the other Southern High sites (1209 and 1210) has been attributed to the low Paleogene sedimentation rates and the low clay contents. One exception, however, is a 10-m interval at the base of Subunit IIA characterized by numerous pyrite blebs and burrows. Pyrite was not observed in such high concentrations at the other Leg 198 sites for the time-equivalent interval. Given the low sedimentation rate (2 m/m.y.) at this site, it is clear that some other factor was controlling pyrite formation. One possibility is that the pyrite is the result of locally high Corg or reactive Fe-mineral contents. However, nothing in the sediment composition as determined by smear slide examination currently supports this. For example, magnetic susceptibility levels are low, similar to other sites (see "Physical Properties").
The clay layer at the P/E boundary is very likely associated with the PETM event. During the PETM, the lysocline and CCD are thought to have risen rapidly as a result of massive dissociation of methane hydrate and its subsequent oxidation to CO2 in bottom waters (Dickens et al., 1997). The added CO2 lowers the pH and carbonate ion content of the seawater, thereby increasing dissolution of carbonate. Hence, the event is usually represented by a condensed interval. Highly fragmented foraminifers and high concentrations of fish debris in this layer at Site 1212 support the dissolution hypothesis.
In Subunit IIB in Hole 1212B, the red/blue color reflectance ratio abruptly increases upcore at 85.1 mbsf, whereas for the same time interval in Hole 1212A, the ratio increases at 94.7 mbsf. The contrasting colors between the two holes is difficult to account for because the magnetic susceptibility, bulk density, and carbonate content are identical. One possibility is that the color difference is an artifact of redox changes associated with the exposure of the sediment to the atmosphere. During coring at Site 1212, a delay in core processing necessitated storage of some cores on the catwalk for an extended period of time, whereas other cores were stored inside. A consequence of this, unfortunately, is that some reactions may have occurred in some cores and not in others.
The K/T boundary marks the base of Unit II. Here, a thin basal foraminiferal clay overlies white (N9) nannofossil ooze as observed at Sites 1209 and 1210. Previous drilling in the vicinity of Site 1212 at Sites 577 (Heath, Burckle, et al., 1985) and 47.2 (Fischer, Heezen, et al., 1971) also recovered K/T boundary intervals. The completeness of the boundary at DSDP Site 47.2, however, was difficult to ascertain because of severe coring disturbance. The boundary sequence at Site 577, on the other hand, was recovered intact and has provided essentially all that is known about the regional paleoenvironmental changes associated with that event. The carbonate record from Site 577 exhibits a subtle reduction (~2-3 wt%) in carbonate content across the boundary. This and a reduction in vertical carbon isotope gradients have been interpreted to represent a collapse in primary carbonate production as a result of the plankton extinction (Zachos et al., 1985). Carbonate content eventually recovers further upsection, indicating a recovery in production. Although carbonate content has not yet been measured in similar detail at Site 1212, the abrupt shift to lower reflectance across the boundary and subsequent recovery several meters above suggest a similar change in carbonate deposition.
The Unit I and II nannofossil oozes show little to no signs of carbonate recrystallization. The exception is the occurrence of micrometer-scale inorganic calcite needles in the interval within and below the clay-rich layer at the P/E boundary (Sections 198-1212A-10H-1, 64 cm, and 198-1212B-9H-5, 73 cm). These needles appear to be a common feature of the P/E boundary clay-layers at all sites on Shatsky Rise (see Fig. F13 in the "Site 1209" chapter). Their origin remains uncertain, however. They may be related to the initial dissolution pulse associated with CO2 injection, or they may have formed in response to the rapid rise in oceanic carbonate and Ca2+ ion content in the immediate aftermath of the dissolution pulse (Dickens, 2000).
In Unit III, inorganic calcite crystals (i.e., needles) become a more common constituent (1%-10%), indicating onset of incipient calcite solution and recrystallization in these Upper Cretaceous oozes. Still, as observed elsewhere on the rise, the sediment texture exhibits no obvious signs of lithification.