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Interstitial Water Geochemistry

Interstitial water samples were collected from the Neogene–Campanian sediments at seven of eight sites drilled during Leg 198. 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 degrees of sulfate (SO42–) reduction, ammonium (NH4+) production, and methane (CH4) generation (Fig. F49). These differences correspond with contrasts in mass accumulation rates (MARs) on the Northern, Central, and Southern Highs of Shatsky Rise. The SO42– profile is most depleted at Site 1208, located within a drift deposit atop the Central High, where the highest Neogene MARs are recorded (Fig. F49). Site 1208 is also marked by higher NH4+ and CH4 production relative to other sites. By contrast, relatively minor amounts of SO42– reduction, NH4+ production, and CH4 generation were observed at Site 1211 (Southern High), where Neogene MARs are lowest (Fig. F49). 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 nature of the Campanian–Paleogene ooze on Shatsky Rise. 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 (i.e., 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 underwent burial. The high water content of these sediments could have significantly reduced their compressibility, such that the load at grain-to-grain contacts remains 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 the diffusion of chemical elements.

Excursions to higher Mn2+ concentrations in the lower parts of the pore water profiles coincide with a series of condensed intervals and hiatuses within 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 occur within the intervals corresponding to early Aptian OAE1a at Sites 1207 and 1213. Four samples possess Corg that exceeds 10 wt%; one is almost 35 wt% (Table T3). These values are among the highest ever recorded for Cretaceous marine sequences and attest to the extraordinary nature of the depositional conditions that led to enhanced sequestration of organic matter at this time. Moreover, this enrichment is restricted to specific intervals; other samples within the lower Aptian contain significant, but less exceptional amounts of Corg (1.7 and 2.9 wt% Corg). The highest value at Site 1214 is 1.4 wt% (Table T3). Two Corg-rich calcareous intervals were also recovered from the Valanginian at Site 1213 with Corg contents of 2.5 and 3.1 wt% (Table T3). The anomalously high C/N ratios for these samples suggest that nitrogen cycling within 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. F50) provides further measures of the abundance and character of the organic matter and of its thermal maturity. The low Tmax values demonstrate well the immaturity of the samples with regard to petroleum generation. The lower Aptian samples from Sites 1207 and 1213 with high Corg contents (>3.5 wt%) plus Sample 198-1207A-44R-1, 103–104 cm (1.7 wt%), all possess high hydrogen indices (>420) and low oxygen indices (25; Table T3), 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 an organic-rich sample (3.6 wt% Corg) from the interval at Site 866 that directly overlies the occurrence of a negative 13C excursion in carbonate associated with OAE1a (Jenkyns, 1995) and with lower Aptian samples from Site 463 with >2.5 wt% Corg plot (Fig. F50). 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.

The exception to this pattern at Site 1213 is Sample 198-1213B-8R-1, 96–97 cm (2.9 wt% Corg), which has a markedly higher oxygen index (OI) and plots closer to the type II kerogen path (Fig. F50). Its hydrogen index (HI) value is similar to that of the Valanginian organic-rich intervals, although their position on the modified van Krevelen diagram is compromised by anomalous OI values related to their high carbonate contents. Sample 198-1213B-8R-1, 96–97 cm, therefore, might be expected to contain enhanced proportions of marine organic matter compared to the other lower Aptian samples. Remarkably it is the only sample determined to contain alkenones, components only biosynthesized by haptophyte algae. The discovery of C37 and C38 alkadienones (Fig. F51) in this sample extends the record of the occurrence of these paleotemperature proxies by 15 m.y. (cf. Farrimond et al., 1986). The presence of these components in high abundance provides convincing evidence that alkatrienones are likely to have survived if they had been produced in the original depositional setting. Thus, the absence of alkatrienones may reflect warm waters (>28°C; cf. Brassell et al., 1986) or, alternatively, these sediments may predate the evolutionary development of alkatrienones.

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 fact that alkenones were only detected in one sample implies that contributions of organic matter from their haptophyte source were less important during deposition of the other intervals. The distribution patterns of the hydrocarbons and ketones in all of the lower Aptian samples show many similarities (Fig. F51), especially the prominence of steroidal ketones and hydrocarbons derived from eukaryotes. The same suites of compounds are also observed in the Valanginian organic-rich intervals, augmented in Sample 198-1213B-15R-1, 9–10 cm, by the presence of sterol ethers (cf. Boon and de Leeuw, 1979; Brassell et al, 1980). 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 (Fig. F51). These compounds, in concert with other diagnostic components, 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 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.

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