SPECIALTY SYNTHESES

Cretaceous Chert and Goo

The Cretaceous sedimentary sequence on Shatsky Rise is characterized by undercompacted nannofossil ooze to semilithified chalk that is locally partly to wholly silicified, forming porcellanite and chert, respectively (Fig. F34). The presence of chert hampered both the drilling and recovery of this section, particularly when rotary cored. The chert effectively reduced pre-Campanian recovery rates to <10%. Given the preferential recovery of chert and somewhat harder porcellanite over softer chalk/ooze interbeds, electric, caliper, and FMS logs through these intervals provide the best estimate of actual lithological proportions (see "Downhole Measurements, Physical Properties, and Core Logging").

The broadest temporal history of Cretaceous carbonate and biosiliceous sedimentation across the Shatsky Rise is provided by the combination of drilling results from Sites 1207 (Northern High) and 1213 (Southern High). Sediment (ooze/chalk/chert) from the Maastrichtian through the Berriasian was recovered, albeit in piecemeal fashion, with the exception of a few gaps interpreted as erosional unconformities, primarily during the Turonian-Coniacian and Barremian. Campanian and Maastrichtian ooze was best recovered at Sites 1208, 1209, 1210, 1211, and 1212. The latter four sites record the mid-Maastrichtian Inoceramus extinction event.

High carbonate contents (up to 99 wt%) of the Cretaceous ooze/chalk suggest deposition above the CCD except during OAE1a (see "Geochemistry"). Where recoveries were higher at Site 1213, there is some indication of decimeter-scale cyclicity likely tied to variations in carbonate dissolution or production. Bioturbation was ubiquitous, with minor laminated intervals (radiolarites) associated with periodic reworking and winnowing by currents.

Prior to silicification, the Cretaceous sediment consisted of nannofossil ooze with variable amounts of radiolarians and foraminifers. The higher proportion and preservation of radiolarians down through the Cretaceous section is likely linked to the equatorial position of the Shatsky Rise during the Early Cretaceous. At the equator, high productivity may have resulted in higher concentrations of biosiliceous material. These radiolarians are considered to be the major source of diagenetic silica in the section. Thin sections of chalk, porcellanite, and chert from Sites 1207 and 1213 verify the stages of silica diagenesis from opal-A (unaltered radiolarian tests), to opal-CT (lepispheres), to chert (microquartz and chalcedony).

Chert and porcellanite colors are generally either reddish brown or grayish to black. These color groupings are interpreted to reflect the prevailing redox conditions at the time of chert formation, with reddish brown indicative of more oxidizing conditions and grayish black reflecting a more reducing environment. Comparison of Sites 1207, 1213, and 1214, as well as Leg 32 Sites 305 and 306 reveals a very generalized secular trend of chert color (Fig. F37). It is likely that secular changes in prevailing redox conditions were driven by a combination of organic matter flux, sedimentation rate, and deepwater oxygen level.

Notions of Stinking Oceans: Early Aptian OAE1a

Coring at three sites recovered parts of sequences deposited during early Aptian OAE1a (Fig. F38). Very C_org-rich radiolarian claystones were recovered at Sites 1207 and 1213, whereas it appears that intervals with significant enrichment of organic carbon were either not recovered or were absent from this interval at Site 1214. The latter possibility seems unlikely given the intermediate-water depth of Site 1214 with respect to the other two sites. The lithology of the lower Aptian differs somewhat from site to site, but characteristic is the general absence of carbonate and the presence of abundant radiolarians. The C_org-rich, finely laminated radiolarian claystone at Site 1207, which is the shallowest of the three sites, was bracketed above and below by limestones. Cores at Sites 1213 and 1214 contained only radiolarites, radiolarian porcellanites, and claystones. The absence of carbonate indicates that the CCD rose to levels shallower than the paleodepth of Site 1207 but that the duration of the episode of severe carbonate dissolution was shorter there than at the two deeper sites.

An additional intriguing feature of the rocks at Site 1214 is the presence of at least four thin, gray layers of altered tuff interbedded with radiolarian claystones. Altered tuffaceous material was present in association with the C_org-rich layers recovered at Sites 1207 and 1213, but no discrete layers were noted. The presence of altered volcanic ash on Shatsky Rise reinforces the association of volcanism with OAE1a in the Pacific Basin (e.g., Larson, 1991a; Larson and Erba, 1999).

Ups and Downs of Carbonate Dissolution

Sediment deposition for much of the Cenozoic occurred above the CCD and the lysocline. During the Paleocene and Eocene, the CCD maintained a position below Sites 1209, 1210, 1211, and 1212 resulting in the deposition of carbonate-rich (>95%) oozes at those localities, and clays at deeper localities (i.e., Site 1208). Periodic lysoclinal shoaling, however, produced low-frequency and low-amplitude oscillations in carbonate deposition with a dominant cycle frequency close to that of the 100-k.y. eccentricity cycle. Carbonate deposition was further perturbed by episodic dissolution "events." One of the most prominent lies at the P/E boundary (~55.5 Ma) as represented by a sharp, thin basal clay-rich layer at each site. These dissolution horizons probably resulted from a rapid shoaling of the lysocline and CCD brought about by the massive dissociation of methane hydrate and its subsequent oxidation to CO₂ in bottom waters (Dickens et al., 1997). From the basal clays, carbonate content gradually recovers to levels that may exceed pre-event carbonate levels (Fig. F39). This suggests the lysocline overshot its original depth, consistent with the predicted silicate weathering feedback as the primary sink for the CO₂ (Dickens, 2000).

The middle Eocene is marked by a shoaling of the lysocline over the entire rise as inferred by a systematic decrease in carbonate content at Sites 1209, 1210, and 1211. Recovery occurs at the Eocene to Oligocene transition, which shows a gradual rise in carbonate content that reflects a deepening of the lysocline and CCD over the Shatsky Rise (Fig. F28). This CCD transition, which was global in extent (van Andel, 1975), initiated in the latest Eocene and peaked just above the boundary in the earliest Oligocene. At its peak, the CCD was sufficiently deep to allow carbonate to accumulate at Site 1208 (today at a depth of 3.3 km). In the mid-Oligocene, the lysocline and CCD began to shoal and, for extended intervals in the early and middle Miocene, were located above the Shatsky Rise, resulting in regional deposition of several prominent clay-rich condensed intervals. By the late middle Miocene, carbonate deposition resumed across the entire Shatsky Rise as the CCD deepened close to its present-day level. From that point to the present, orbitally driven variations in the lysocline together with carbonate production and clay fluxes have created distinct lithologic cycles across the entire rise.

Neogene-Quaternary Sediment Drifts and Biosiliceous Sedimentation

Based on the seismic character of the upper Neogene-Quaternary sections across the Shatsky Rise, thick sections at Sites 1207 (Northern High) and 1208 (Central High) are drift deposits, whereas those at the Southern High are thinner pelagic drape deposits. Although the drift deposits at Sites 1207 and 1208 show no distinct sedimentological evidence for current reworking, the higher than expected sediment accumulation rates at these sites are in part attributable to drift deposition. We believe that current reworking served to amplify regional variations in input of wind-borne volcanic ash and eolian terrigenous material, as well as production of biosiliceous material at these sites. The greater abundance of diatoms, radiolarians, and silicoflagellates in the upper Neogene-Quaternary sections at Sites 1207 and 1208, illustrated in Figure F40, can be attributed, in part, to a northerly increase in sea-surface productivity.

Clay Mineral Authigenesis

Green diagenetic laminae are prevalent in Quaternary and Neogene sediment at Sites 1207-1211. Similar features have been described in sections from the Ontong Java Plateau and Lord Howe Rise and were interpreted as altered layers of volcanic ash (Gardner et al., 1986; Lind et al., 1993). Our observations have led us to conclude that the green laminae encountered at Shatsky Rise are diagenetic features composed primarily of saponite, a smectite group clay (see "Lithostratigraphy" in the "Site 1210" chapter). Clay was observed in discrete bands, along the edges of burrows filled with volcanic glass grains, but also as an authigenic product infilling foraminiferal tests (Fig. F41). Alteration of disseminated grains of volcanic glass in the matrix and background saponite in x-ray diffraction scans of bulk sediment suggest that saponite may be forming throughout the bulk sediment. Formation of this Mg and Ca bearing clay is also consistent with pore water profiles (see "Geochemistry").

Eight sites and 16 holes cored on Shatsky Rise recovered a virtually complete composite section ranging from upper Pleistocene to lowermost Berriasian representing ~140 m.y. of geological history. A number of short intervals of lower and middle Miocene stratigraphy were not identified shipboard but are part of a fascinating regional sedimentation history which, grossly simplified, saw rapid deposition in the Pliocene-Pleistocene, reduced sedimentation rates in the late Miocene, condensation and hiatuses through the middle Miocene to Oligocene, complete but relatively slow deposition in the Paleogene (almost entirely on the Southern High), and rapid deposition for much of Maastrichtian to earliest Berriasian time (Table T3). Chert-rich sediments dominate from the mid-Campanian to lowest recovered sediments (Figs. F21, F34, F42).

With the exception of Site 1214, all sites include complete and rapidly deposited Pliocene-Pleistocene sections (8-14 m/m.y. sedimentation rates [SRs] and >1.0 g/cm²/k.y. mass accumulation rates [MARs]) in which the noncarbonate fraction forms a significant proportion (Fig. F43). A particularly expanded drift-deposit succession (200 m and 42.4 m/m.y. SR) was recovered at Site 1208, and this section should prove to be invaluable for timescale studies allowing the integration of calcareous and siliceous biostratigraphies, cyclostratigraphy, and paleomagnetic polarity and intensity records. The Pliocene-Pleistocene at Site 1207 on the Northern High was particularly rich in siliceous microfossils most likely due to cool, productive waters, which were also responsible for the paucity of tropical-subtropical species of planktonic foraminifers at this location. The influx of warmer water planktonic foraminifers and reduction of biosiliceous plankton to the south at Site 1209 shows the paleobiogeographic and paleoceanographic importance of the Shatsky Rise drill sites near the path of the Kuroshio Current.

The lowermost Pliocene and uppermost Miocene sections show reduced sedimentation and mass accumulation rates (<1.0 g/cm²/k.y. MAR) and overlie intervals of condensed section and unconformities that include parts of the upper, middle, and lower Miocene, the Miocene/Oligocene boundary interval as well as the upper Oligocene at most sites (Figs. F21, F42, and F43). The amount of missing section is variably better expressed across Shatsky Rise and, for example, at Sites 1209 and 1211, much of the Oligocene is present. Northern and Central High Sites 1207 and 1208, and Sites 1213 and 1214 on the Southern High are also missing all or most of the Paleogene and uppermost Cretaceous. However, Southern High Sites 1209, 1210, and 1211 recovered apparently complete lower Oligocene to Paleocene sections that were deposited relatively slowly (0.2-7.6 m/m.y. SR and <1.0 g/cm²/k.y. MAR) and dominated by carbonate. These sections include complete and lithologically well-expressed PETM and E/O boundary intervals and provided excellent records of biotic response to these periods of rapid environmental change. Preliminary shipboard analysis of calcareous nannofossils through the PETM interval reveals striking changes in assemblages, in particular the decline and extinction of Fasciculithus within the event and the subsequent increase in Zygrhablithus bijugatus.

Four Southern High sites (1209-1212) include complete and relatively expanded K/T boundary sections characterized by a lithologic switch from white Maastrichtian ooze to a thin, pale orange basal Paleocene ooze (~8-12 cm) with pyrite specks overlain by a distinctive pure white ooze (Table T3). The boundary is bioturbated but otherwise undisturbed. The paleontological succession across the boundary interval is well preserved and apparently complete with the recognition of nannofossil Zones CC26 and lowermost CP1 and foraminiferal Zones uppermost KS31, P0 (within burrows) and P. Clay spherules were observed at all sites. The calcareous plankton record above the boundary is exceptionally good, and in particular, there is only limited Cretaceous nannofossil reworking and foraminiferal mixing. Both calcareous nannofossils and planktonic foraminifers reveal the classic succession of abrupt extinction, followed by assemblages dominated by "disaster" taxa and rare "survivor" taxa, succeeded by assemblages that record the gradual introduction of new Danian "recovery" taxa. These sections represent some of the best-preserved and least-disrupted deep-sea records of this major extinction event and the following recovery and diversification of calcareous plankton.

At the same four sites (1209-1212), we also recovered thick, carbonate-dominated, rapidly deposited sections of pure white Maastrichtian to Campanian ooze (6.5-31.6 m/m.y. SR and 1.0 g/cm²/k.y. MAR). Calcareous plankton preservation was generally good throughout, and the sections will allow the integration of calcareous plankton biostratigraphies and the study of the response of these groups to the mid-Maastrichtian and other events that include environmental changes.

Below the upper Campanian, the occurrence and stratigraphy of the Cretaceous section is variable (Fig. F21). Good lower Campanian sections were recovered below major unconformities on the Northern and Central Highs but below this interval recovery was poor and dominated by chert. Paleontological material was often limited to scraping chalk and limestone from chert pieces. However, a relatively complete Campanian to Barremian section was cored at Site 1207 with remarkably uniform and high sedimentation rates (8.7 m/m.y. SR) and moderate recovery through the Aptian and Barremian. An outstanding record of OAE1a is well constrained by the plankton biostratigraphy.

Site 1213 yielded another thick Cretaceous section including a rapidly deposited Cenomanian to Albian succession and an Aptian section that includes OAE1a. Below this was an expanded section of Hauterivian to basal Berriasian. This Neocomian section yielded only few planktonic foraminifers but includes the FO in the Pacific of the tiny Hauterivian plankton foraminiferal ancestors (hedbergellids and globigerinelloidids) of the diverse Aptian assemblages. The section is rich in calcareous nannofossils and radiolarians, and the Berriasian in particular represents one of the best records of this time interval in the deep sea.

The consistent occurrence of pervasive chert lithologies from the Campanian to Tithonian across the range of sites drilled on Shatsky Rise is strongly suggestive of a long-term location beneath high-productivity surface waters related to a near paleoequatorial position during much of the Cretaceous. Preliminary paleontological observations support this hypothesis (e.g., very low diversity mid-Cretaceous planktonic foraminiferal assemblages, indicative of high-productivity regimes, contrast strongly with peak diversities in the Campanian-Maastrichtian as Shatsky Rise drifted out of the equatorial zone). There are also intriguing and conspicuous absences within the Lower Cretaceous nannofossil assemblages, particularly the almost total lack of Nannoconus and Micrantholithus nannoliths, which are such important, often rock-forming components in coeval Tethyan and Atlantic sites. Again, the location beneath highly productive surface waters may prove to have been the controlling factor.

Magnetostratigraphic interpretations from Leg 198 sediments were restricted to upper Miocene and younger sections at all sites because cores from older sediments gave erratic magnetization directions. Despite making tens of thousands of measurements on hundreds of APC cores and using alternating field demagnetization up to 20-mT peak fields, the Oligocene to Cretaceous cores produced inclinations and declinations with poor correlation between different holes at any given site. The problem is thought to be the highly deformable nature of these sediments, which allowed disturbance and distortion during coring, recovery, and/or splitting.

Measurements from Neogene sediments generally yielded an interpretable polarity record, typically showing polarity chrons from late Miocene to Pleistocene time (Fig. F44). Of these records, Sites 1209 to 1212 on the southern Shatsky Rise yielded a magnetostratigraphic sections of 50-100 m, going back in time to C3An (~6 Ma). The outstanding record is that from Hole 1208A, on the Central High, where a drift deposit with high sedimentation rates was cored. This hole gave an extraordinarily expanded magnetic stratigraphy extending back to C5An in the mid-Miocene (Fig. F44).

Shipboard measurements were also made on archive-half sections from the igneous sill section drilled at the bottom of Hole 1213B. The results are unusual because two of the sills gave high positive inclinations (50°-70°) and the third yielded negative inclinations (~-30°). If it is assumed that all of the sills were formed within a relatively short geologic time span, these results imply that a mixture of normal and reversed polarity. The steep positive values are inconsistent with the low predicted paleolatitude of Shatsky Rise during Late Jurassic and Cretaceous time and may represent a transitional field between normal and reversed.

Downhole Measurements

Two holes were logged during Leg 198: Hole 1207B with the triple combo and FMS-sonic tool strings, and Hole 1213B with the triple combo. Collecting data from the OAE1a interval near the base of the Aptian was one of the major objectives of the leg. This black shale interval is clearly represented as peaks in the natural gamma radiation logs at both of the logged sites (Fig. F45). Most of the gamma radiation comes from uranium adsorbed onto the organic matter, and potassium and thorium are also high, indicating the presence of clay in the sedimentary rocks. The gamma radiation peak at OAE1a is much stronger than the other peaks in the log; other OAEs are either absent or are weaker than OAE1a on Shatsky Rise.

The FMS resistivity images from Hole 1207B reveal the form, thickness, and depths of the chert horizons that constitute the bulk of the recovered core below 230 mbsf (Fig. F46). Between 210 and 375 mbsf the chert layers occur on average every 83 cm and have an average thickness of 9 cm. The cherts typically appear as layers rather than nodules. Low core recovery in chalk/chert alternating sequences has been the subject of much discussion, and the image logs from Hole 1207B provide important data to develop better strategies for core recovery in chalk/chert sequences.

Synthetic seismograms were constructed from density and velocity data from both logs and core physical properties measurements. These reconstructions enabled the core and logs to be correlated with the seismic section and, hence, enabled ages to be assigned to the seismic reflectors.

Physical Properties

Physical properties data show variation both with depth below the seafloor and with location. The Northern and Central Highs of Shatsky Rise (Sites 1207 and 1208) have similar sedimentary histories dominated by major unconformities that are of tens of millions of years in duration. The unconformities are manifested in the physical properties data as large peaks in magnetic susceptibility, P-wave velocity, and a downhole increase in bulk density (both GRA and discrete measurements) (Figs. F12, F16). At Site 1207 a peak in natural gamma radiation is also apparent because of the presence of a manganese nodule proximal to the level of the major unconformity.

The Southern High of Shatsky Rise is characterized by more continuous sedimentation and shorter-duration hiatuses relative to the Northern and Central Highs. The physical properties data from Sites 1209, 1210, 1211, and 1212 suggest that compaction and dewatering, although important processes in the upper part of the sedimentary column, cannot explain all the trends observed (Figs. F20, F26, F30, F35). The Eocene-Cretaceous sediments are notable in that they appear to be underconsolidated with respect to their age and burial depth, suggesting that primary conditions, such as sediment composition, may be controlling the degree of lithification.

At Sites 1207, 1213, and 1214, a variety of lithologies of Cretaceous age, including chert, porcellanite, radiolarite, chalk, and limestone, were recovered. These sedimentary rocks exhibit a wide range of average P-wave velocities, from ~2200 m/s in chalk and limestone to ~4760 m/s in chert (Fig. F12). Porcellanite and radiolarite have average P-wave velocities between this range. The high degree of variability of P-wave velocities in the Lower Cretaceous has significant implications for seismic data interpretation on Shatsky Rise.

Composite Depths and Cycle Stratigraphy

The recovery of complete sediment sections of APC-cored intervals was crucial to fulfilling the primary paleoceanographic objectives of Leg 198. Coring of multiple holes at Sites 1209, 1210, 1211, and 1212 ensured the recovery of the complete (except unconformities) Cenozoic sedimentary record. Consequently, composite records of MST-derived physical properties and color reflectance data were produced for this time interval. These data compilations are unique in that previous drilling of Paleogene and Cretaceous sediments in the western Pacific failed to recover the complete and undisturbed sequences that are necessary to identify and characterize high frequency sedimentary cycles. As a result, little was understood about the influence of orbital and other periodic forcing on pre-Neogene sedimentation in the Pacific. The high-quality double and triple APC cores have the potential to remedy this situation. Most such sedimentary intervals exhibit pervasive lithologic cycles throughout the Cenozoic including those identified across several key transitions (Figs. F47, F48). Systematic changes in cycle amplitude and frequency are consistent from site to site, suggesting that these changes reflect regional paleoceanographic processes. The cycle packages (in all physical properties) are sufficiently distinct to allow for detailed correlation between sites (Fig. F48).

The most distinct cycles in terms of color variation and other physical properties occur in the upper Neogene. These are best represented by the total color reflectance (L*) records from Sites 1208 and 1209 as plotted along with orbital obliquity and eccentricity for the intervals 0-2 and 3-5 Ma (Fig. F49). The Pleistocene-Holocene color data at both sites exhibit the "classic" asymmetric glacial to interglacial cycle pattern. The interglacials are characterized by carbonate-rich, light-colored nannofossil ooze with clay, whereas the glacials are characterized by clay- and diatom-rich, dark-colored clayey nannofossil ooze or nannofossil clay. The transitions are mostly gradational, although several glacial to interglacial contacts are sharp. Preliminary biostratigraphic age constraints suggest that the dominant cycle frequency over the last 0.6 m.y. is near that of the 100-k.y. eccentricity cycle (Fig. F49). From 0.6 to 2.6 Ma, the dominant period shifts toward a higher frequency close to that associated with the 40-k.y. obliquity cycle. Throughout the last 2.6 m.y., the cycle amplitudes in reflectance remain remarkably similar between the Southern and Central Highs, although the mean total reflectance is higher on the Southern High. Climate-driven variations in opal and carbonate production and preservation, and in clay fluxes are responsible for these changes.

The lower Pliocene prior to the onset of Northern Hemisphere glaciation at 2.6 Ma is also characterized by regular lithologic cycles. As in the upper Pliocene and lower Pleistocene, the wavelengths indicate a dominant period close to 40 k.y. However, the cycle amplitude in this period is noticeably reduced, particularly at sites on the Southern High. The reduction in the high-frequency cycle amplitude is accompanied by an apparent increase in a low-frequency cycle amplitude. In the Site 1209 color reflectance record (over the period 3 to 5 Ma), for example, there appears to be a long wavelength oscillation with a period of roughly 1.0 to 1.25 m.y. Comparison with the derived orbital curves (Laskar, 1990) suggests that this cycle may be in phase with the long period 1.25-m.y. cycle of obliquity (Fig. F49).

In the Paleogene, the sedimentation rates are sufficiently low (~3 to 5 m/m.y.) over most of the Southern High that it is difficult, if not impossible, to identify cycles associated with obliquity and precessional-scale forcing. Nevertheless, prominent periodic to quasi-periodic color (light-dark) and magnetic susceptibility cycles occur throughout the upper Paleocene to upper Eocene at Sites 1209, 1210, 1211, and 1212. The variations represent subtle changes in carbonate and/or Fe oxide content. The mean wavelength of the highest amplitude oscillations indicates a cycle frequency in the approximate range of the 100-k.y. eccentricity cycle. These cycles in turn exhibit an amplitude modulation with a frequency range suggestive of 400-k.y. eccentricity forcing. If so, these data would be consistent with observations elsewhere of a dominant response to precession and eccentricity forcing during the ice-free Paleogene prior to the late Eocene appearance of ice sheets on Antarctica.

Interstitial Water Geochemistry

Interstitial water samples were collected from Neogene-Campanian sediments at seven sites. The older sedimentary sequences, marked by poor recovery associated with the presence of chert, yielded no samples suitable for pore water extraction. Site 1213 samples were affected by drilling disturbance associated with rotary coring and are not considered in this synthesis. No samples were collected from Site 1214, where <1 m of unlithified sediment was cored.

Reactions involving the degradation of organic matter occur primarily in the Neogene sections. Although the sediments contain little organic matter (e.g., <0.3 wt% at Site 1207 and <0.2% at Site 1210), there is sufficient variation among the sites to lead to significant differences in the degree of sulfate (SO₄^2-) reduction and ammonium (NH₄⁺) production (Fig. F50). These differences correspond with contrasts in MAR on the Northern, Central, and Southern Highs of Shatsky Rise. The SO₄^2- profile is most depleted at Site 1208, located within a drift deposit atop the Central High, where the highest Neogene MARs are recorded (Fig. F50). Site 1208 is also marked by higher NH₄⁺ production relative to other sites. By contrast, relatively minor amounts of SO₄^2- reduction and NH₄⁺ production were observed at Site 1211 (Southern High), where Neogene MARs are lowest (Fig. F50). The profiles from the other sites lie between the two end-members defined by Sites 1208 and 1211. These relationships suggest that organic matter preservation is enhanced slightly in areas characterized by higher MARs. In areas with relatively low MARs, it appears that most organic matter is degraded before it can be incorporated into the sediment column.

One of the most unusual aspects of the sections cored during Leg 198 is the lack of significant alteration (i.e., compaction, recrystallization, and cementation) in the Campanian-Paleogene section relative to overlying sediments. The convex-upward shape of the Sr²⁺ and Sr/Ca pore water profiles through the uppermost sediments at Sites 1207-1212 suggests that carbonate recrystallization is occurring largely within the Neogene section. By contrast, Sr²⁺ and Sr/Ca profiles through the underlying Campanian-Paleogene sediments are invariant, suggesting that carbonate dissolution and reprecipitation processes are not significant through this stratigraphic interval. This unusual downcore trend in carbonate diagenesis may be a consequence of the foraminifer-dominated ooze of the Campanian-Paleogene interval. The relatively low surface area and associated excess surface free energy of the sediment particles may have imparted a low diagenetic potential (i.e., Schlanger and Douglas, 1974) on these sediments as compared with the overlying Neogene section, in which nannofossils (with higher surface area and excess free energy) predominate (e.g., Baker et al., 1982; Walter and Morse, 1984). A second factor may be related to the clay-rich, condensed intervals that at all sites mark the transition between the Paleogene and middle to upper Miocene sections. The condensed horizons could have acted as crude barriers, preventing significant expulsion of pore fluids as the Campanian-Paleogene sediments were buried. The high water content of these sediments could have significantly reduced their compressibility, such that the load at grain-to-grain contacts remained insufficient to initiate pressure solution of carbonate. No significant inflections in pore water profiles were observed across the Neogene-Paleogene transition. This pattern suggests that although the clay-rich horizons may have played a role in controlling processes of compaction, they have not acted as an effective barrier to diffusion.

Excursions to higher Mn²⁺ concentrations in the lower parts of the pore water profiles coincide with a series of condensed intervals and unconformities in the middle-lower Miocene and Oligocene sections, which contain inferred Mn-rich phases. Excursions to higher Mn²⁺ concentrations reflect the dissolution of Mn minerals and diffusion of Mn²⁺ away from Mn-rich horizons.

Pore water profiles are also affected by reactions involving the alteration of silicate minerals and exchange with basaltic basement. Reactions associated with the formation of clays and the alteration of ash and biogenic silica are prevalent in the Neogene sediments and are reflected in the pore water profiles as sharp decreases in Mg²⁺ and elevated concentrations of K⁺ and Ca²⁺ in the upper parts of profiles. These trends are overprinted on diffusion trends related to exchange with basaltic basement, including gradual downcore decreases in Mg²⁺ and K⁺ and increases in Ca²⁺.

Organic-Rich Intervals in the Lower Aptian and Valanginian

Evaluation of the abundance, character, and origin of the organic matter in the Cretaceous organic-rich intervals at Sites 1207, 1213, and 1214 was the primary focus of shipboard organic geochemical investigations employing a combination of analysis of elemental compositions, pyrolysis products, and biomarker distributions.

Remarkable amounts of organic carbon are present within the intervals corresponding to early Aptian OAE1a at Sites 1207 and 1213. Four samples possess C_org that exceeds 10 wt%; one is almost 35 wt% (Table T4). These values are among the highest ever recorded for Cretaceous marine sequences and attest to the extraordinary nature of the depositional conditions at this time. Moreover, this enrichment is restricted to specific intervals; other samples within the lower Aptian contain significant, but less exceptional amounts of C_org (1.7 and 2.9 wt% C_org). The highest value at Site 1214 is 1.4 wt% (Table T4). Two C_org-rich calcareous intervals were also recovered from the Valanginian at Site 1213 with C_org contents of 2.5 and 3.1 wt% (Table T4). Their anomalously high C/N ratios suggest that nitrogen cycling in these intervals did not follow normal marine trends, perhaps associated with bacterial communities or hierarchies that include denitrifiers or with organic matter enriched in N-poor lipidic components, such as cyanobacterial sheaths.

Rock-Eval pyrolysis (Fig. F51) provides further measures of the abundance and character of the organic matter and of its thermal maturity. The low T_max values demonstrate well the immaturity of the samples with regard to petroleum generation. The lower Aptian samples from Sites 1207 and 1213 with high C_org contents (>3.5 wt%) possess high hydrogen indices (>420) and low oxygen indices (25) (Table T4), which corresponds to type I organic matter (Tissot et al., 1974) rich in algal and bacterial debris. Their position on a modified van Krevelen diagram matches closely with organic-rich samples from the OAE1a interval at Sites 866 and 463 (Fig. F51). The apparent comparability of the organic matter associated with the early Aptian OAE1a event may reflect some uniformity in conditions exerting controls on the character of ocean production and biomass and its preservation. Sample 198-1213B-8R-1, 96-97 cm, contains C₃₇ and C₃₈ alkadienones, components only biosynthesized by haptophyte algae. The discovery of alkenones extends the record of the occurrence of these paleotemperature proxies by 15 m.y. (cf. Farrimond et al., 1986).

The biomarker composition of the organic-rich intervals provides clear testimony of the extraordinary preservation of organic matter, not only in terms of its abundance, but also with regard to its unaltered character, especially at Site 1213 where the abundance of ketones significantly exceeded that of hydrocarbons. The distribution patterns in all of the lower Aptian and Valanginian samples show many similarities, especially the prominence of steroidal ketones and hydrocarbons derived from eukaryotes (see "Organic Geochemistry" in the "Site 1207" chapter and "Organic Geochemistry" in the "Site 1213" chapter). Thus, there appears to be a broad uniformity in the nature of the organic matter attributable to algal primary producers at Shatsky Rise. However, this observation stands in stark contrast to the restricted occurrence of critical bacterial markers, notably extended (C₃₂) and methylhopanoids, which record contributions of organic matter from cyanobacteria and from methylotrophic and methanogenic bacteria (Ourisson et al., 1979, 1987; Rohmer et al., 1989, 1992). They are prominent in several of the lower Aptian organic-rich intervals and imply a significant role for bacteria associated with processing of organic matter at higher trophic levels during OAE1a. These compounds record the development of bacterial communities, perhaps as microbial mats, in the depositional environment that may reflect the establishment of dysoxic or anoxic conditions. Thus, the extraordinary sequestration of organic matter (up to 34.7%) in the lower Aptian at Shatsky Rise, even by comparison with other organic-rich intervals within the mid-Cretaceous, records a response to a radical change in the processes controlling biogeochemical cycling, which in turn reflects a perturbation of the ocean-climate system.