October 22, 2001

This site summary was received on 21 October from Drs. Tim Bralower and Isabella Premoli Silva, Co-Chief Scientists aboard JOIDES Resolution, Leg 198.

Shipboard Scientific Party, Leg 198

SITE SUMMARY, SITE 1213

Hole 1213A: Latitude 31 degrees 34.6402'N, Longitude 157 degrees 17.8605'E,

Water depth = 3882.8 m

Hole 1213B: Latitude 31 degrees 34.6576'N, Longitude 157 degrees 17.8621'E,

Water depth = 3882.6 m

Time on site: 7.28 days (1215 hr, 2 October to 1900 hr, 9 October)

Total depth: 494.4 mbsf

BACKGROUND

Site 1213 is the southernmost and deepest site on the Shatsky Rise depth transect, located in lower bathyal (3883 m) water depth on the southern flank of the Southern High of Shatsky Rise. The site is close to the oldest part of the rise with underlying basement formed in the Kimmeridgian (Late Jurassic). The major goals of Site 1213 drilling were to core a relatively deep-water mid- (Barremian-Cenomanian) and Lower Cretaceous sequence as well as to obtain fresh volcanic rocks from the underlying basement. The correlation of seismic lines down the southern flank of Shatsky Rise suggested that a relatively thick mid- and Lower Cretaceous section exists at relatively shallow burial depth. Basement was thought to lie between 400 and 600 mbsf depending on velocity estimates. Thus both the lower part of the Cretaceous section and basement can be recovered without time consuming drilling through extensive chalk/ooze-chert sequences.

Holes 1213A and 1213B were cored with the RCB. Hole 1213A was terminated at 198.9 mbsf after the coring wireline parted. Lithology at the base of this hole consists of chert, porcellanite, and radiolarite of middle Albian age. A total of 447.8 m of sedimentary rock was penetrated in Hole 1213B with claystones near the base of the hole containing nannofossils and radiolarians of earliest Berriasian age. Core recovery in chert-bearing units was low. Underlying the sedimentary package, plutonic or volcanic rocks totaling 46.6 m in thickness were penetrated and higher recovery was obtained than in the sedimentary sequence. Hole 1213B was terminated when the rate of penetration decreased significantly and the recovered material was considered sufficiently unaltered for detailed geochemical and radiometric analyses. Finally a full suite of logs were to be collected through the whole sequence. The triple combo tool reached 431 mbsf but experienced several tight passes in the hole. With the approach of severe tropical storm Krosa combined with the poor hole conditions, it was considered too risky to deploy the FMS-sonic logging tool.

SUMMARY OF RESULTS

Coring at Site 1213 recovered four lithologic units. Sedimentary units are divided based on composition and color. Lithologic Unit I (0-54.6 mbsf) consists of olive gray to pale yellowish brown nannofossil ooze, clayey nannofossil ooze, and nannofossil clay that ranges from Holocene to lower Pliocene (0-5.0 Ma). This unit contains minor amounts of diatoms, foraminifers, and ash. Subunit IIA (54.6-66.1 mbsf) comprises pale orange nannofossil ooze and brown chert of Santonian age (84.8-85.5 Ma). Moderate to pale yellow brown chert, and light brown and pale orange porcellanite of middle to late Cenomanian age (94.1-96.8 Ma) are grouped in Subunit IIB (66.1-85.4 mbsf).

Subunit IIIA (85.4-179.6 mbsf) of early Cenomanian to late Albian age (96.8-100.5 Ma) contains medium dark gray chert, gray, greenish gray, and pale orange porcellanite, and light greenish gray limestone. A mixture of chert, shades of brown and red in color, and pale orange to gray orange and pale yellow brown porcellanite and radiolarite with an age range from early middle Albian to late Aptian (119-106 Ma) are grouped in Subunit IIIB (179.6-256.8 mbsf). Subunit IIIC (256.8-266.4 mbsf) corresponds to olive black to greenish black organic-carbon rich clayey porcellanite, dusky green radiolarian porcellanite, and minor altered tuff. This unit is early Aptian in age (119.5-120.5 Ma). Gray chert, white to yellowish gray porcellanite, and light greenish gray and olive gray nannofossil chalk to clayey nannofossil chalk of Hauterivian to late Berriasian age comprise Subunit IIID (266.4-410.3 mbsf). Subunit IIIE (410.3-447.8 mbsf) of late to early Berriasian age contains brown, gray, grayish red, and pale to moderate brown chert, yellowish gray and light pale orange porcellanite, and brown claystone with nannofossils. A limonitic claystone breccia is found at the base of this subunit. Throughout the sedimentary section, porcellanite, limestone, and claystone have variable amounts of radiolarians and nannofossils. In many intervals nannofossil biostratigraphy was conducted on chalk adhered to the sides of chert nodules. This sediment indicates the nature of the unrecovered intervals. Finally, Unit IV (447.8-494.4 mbsf) contains at least three separate units of dark greenish gray, sparsely phyric or fine-grained diabase with chilled basaltic margins and thin interbedded pieces of altered chert.

Preliminary investigations of the almost 500-m cored sequence at Site 1213 yields a detailed 145 million year history of the deep south flank of Shatsky Rise. The record from this site provides a new perspective to the evolution of the plateau, from its formation by volcanic and plutonic activity, through its depositional history, to the erosional episodes that have removed sediment and sculpted its morphology. True basaltic basement was not recovered at this site, instead we cored a sequence of diabase units with chilled margins and interbedded sediment that are interpreted as sills intruded in a late-stage, widespread plutonic event. Overlying these intrusives are limonitic breccias, evidence for hydrothermal activity associated with sill intrusion. The earliest sediments of pelagic origin are radiolarian-rich horizons that were deposited under high productivity surface-water conditions in well-aerated deep waters that were swept by currents. These sediments have been transformed during burial to porcellanite and chert. At times, lower productivity conditions led to deposition of nannofossil ooze, now chalk in the deep record.

The early history of Shatsky Rise was interrupted by separate intervals of dysoxia/anoxia in the Valangianian and early Aptian that led to deposition of organic-carbon rich sedimentary rocks. Evidence for the Valanginian event is found in southern Europe. To our knowledge, however, this is the first record of this event in the Pacific Ocean as well as in the pelagic realm. Organic-carbon rich rocks deposited during the well-known early Aptian event, Oceanic Anoxic Event (OAE1a), were also found at Site 1207 on the Northern High. These unusual deposits with organic carbon contents from 2.9 to 25.2 wt.% at Site 1213 (up to 34.6 wt. % at Site 1207) show up clearly on gamma ray and U logs. The organic matter is almost exclusively of bacterial and algal origin.

The Barremian is only represented by one sample and clearly most of the stage corresponds to an unconformity in the Site 1213 record. This interval was also unconformable at Site 1214. The CCD in the Barremian is thought to have been relatively deep, certainly no shallower than the paleodepths of these sites at this time (2-2.5 km). Thus the hiatus was most likely a result of a deep-water erosional event that scoured away the southern margin of Shatsky Rise.

The remainder of the mid-Cretaceous after the early Aptian was a return to better-oxygenated conditions with variation in productivity leading to deposition of radiolarian oozes (porcellanites and chert in the record) and nannofossil ooze (limestone). A hiatus occurred in the late Cenomanian to Santonian interval, the result of a widespread erosional episode that affected a wide area on the Southern High. After a short depositional episode in the Santonian, a long hiatus in sedimentation lasted until the late Miocene. This episode was likely a result of a combination of erosion of the exposed, deep flank of the Southern High and carbonate dissolution at frequent times when the CCD in the Pacific shoaled above the depth of the site.

HIGHLIGHTS

Anoxic events in the early Aptian and Valanginian

Lower Aptian organic-carbon rich horizons were recovered in Core 1213B-8R. The horizons include olive black to greenish black, clayey porcellanites, and radiolarian porcellanites with minor tuff. Three organic carbon analyses yielded contents of 2.9, 10.2 and 25.2 wt. %. The sample with the highest organic carbon content is a clayey porcellanite. Gamma ray and uranium logs show that the lower Aptian organic-carbon rich units are about 3-m thick. Total recovery of carbonaceous and non-carbonaceous in this interval is just over one meter. Thus the recovery of organic-rich rocks is less than 30%.

Preliminary organic geochemistry provides information on the origin of the organic matter in lower Aptian samples from Site 1213. Characterization of the organic matter from the most organic-carbon rich samples indicates it is algal and bacterial in origin, including production of some of this organic matter by haptophyes and some by cyanobacteria. The haptophyte alkenones identified represent the oldest records of these compounds, extending their first occurrence by 15 Ma. The character and preservation of the bacterial material are also evidence for the existence of microbial mats at the time of deposition. Finally, the excellent preservation of organic compounds indicates deposition in highly dysaerobic conditions.

Core 1213B-8H was mostly non-calcareous, however a sample at the top, Sample 1213B-8R-1, 3 cm contains the nannofossil Eprolithus floralis, but lacks Rhagodiscus achlyostaurion and Prediscosphaera columnata, and thus correlates to late Aptian Zone NC7. Sample 1213B-9R-1, 17 cm lacks Hayesites irregularis, Calcicalathina oblongata, and Cruciellipsis cuvillieri, and thus correlates to upper Hauterivian-Barremian Zone NC5. These two samples bracket the age of the carbonaceous horizons and provide a firm correlation with organic-carbon rich units of OAE1a from central Italy (the "Selli" level), the Italian and Swiss Alps, Sicily, France, (the Goguel level), Site 641 in the eastern North Atlantic, and northern Mexico. In the Pacific, organic-carbon rich horizons of OAE1a age are also found at Sites 463 (Mid Pacific Mountains) and 866 (Resolution Guyot), and elsewhere at Shatsky Rise at Sites 305 and 1207. Organic-carbon rich, lower Aptian sedimentary rocks from Sites 1207 and 1213 provide evidence for the nature of environmental changes in a truly pelagic regime during OAE1a.

Two organic-carbon rich levels in the Valanginian have considerably lower organic carbon contents than the lower Aptian horizons. A level in the lowermost Valanginian has 3.1 wt.% Corg, one in the "mid" Valanginian contains 2.5 wt.%. These horizons also contain clear evidence of bioturbation indicating that deep waters were sufficiently oxygenated to support a benthic community. The horizons do not show up on gamma-ray logs suggesting that they are thin. The "mid" Valanginan horizon is significant as it appears to be broadly similar in age to organic-carbon rich horizons that had not previously been observed outside Tethys. The presence of a positive carbon isotope excursion that correlates to this event suggested that it was more widespread or global in extent. Although recovery at Site 1213 does not allow us to characterize the temporal distribution of organic-carbon rich units, this record extends the known geographic distribution of organic-rich sediments considerably.

The chert record

Although poorly recovered, pieces of chert in the Site 1213 section yield valuable information on variations in redox conditions through the Cretaceous. Nodules with orange, red, and brown hues indicate deposition and diagenesis in oxidizing environments, whereas those with olive green to black hues indicate more reducing conditions during deposition and burial. The color stratigraphy suggests that oxygenated conditions prevailed in the early Berriasian, late Aptian through middle Albian, and, using data from other Leg 198 sites, from the late Cenomanian through the Maastrichtian. Reducing conditions prevailed in the Berriasian through the early Aptian and in the late Albian to middle Cenomanian. Similar trends were observed at Sites 1207, 305, 306, and 1214 for coeval portions of the sequence, suggesting that for much of the Cretaceous, the entire Shatsky Rise experienced similar redox conditions at bathyal depths. Minor variations exist, however. At Site 1207, generally reducing conditions seem to persist until the late Aptian, with oxidizing conditions characterizing the early-middle Albian, and from late Albian through the Turonian.

An important Neocomian reference section in the Pacific

Site 1213 contains one of a handful of Neocomian pelagic sections in the Pacific Ocean. Although the section is generally poorly recovered, nannofossils are well preserved, and radiolarians are diverse and moderately well preserved. These groups should provide precise correlations with Neocomian sections worldwide. Planktonic foraminiferal occurrence is very rare. However, shore-based investigations have the potential to map out some of the earliest evolution of this group.

Shipboard nannofossil biostratigraphy has proven challenging due to the absence of important Neocomian marker taxa, especially the nannoconids. This group, which has a poorly understood affinity, are centerpieces of the biostratigraphy of the Jurassic/Cretaceous boundary interval in particular. Their absence at Site 1213 is intriguing as they are abundant in samples from Site 463 in the Mid-Pacific Mountains. At another site, Site 167 on Magellan Rise, nannoconids are absent except in the lowermost three cores. This group is also absent in assemblages that we interpret as Berriasian in Holes 49 and 50 on the deep western flank of Shatsky Rise. What are the major controls on the distribution of nannoconids in the Pacific? Nannoconids are generally most common in continental-margin and epicontinental locations. In the Atlantic Ocean, this group is common in sites along the margins, but absent in the deep sea except where transported by turbidity currents. Nannoconids are thought to be oligotrophic, and to possibly represent calcareous dinoflagellate cysts.

The record of nannoconids in the Pacific is complex. Their abundance at relatively shallow Site 463, at Site 167 in its early (shallow) history, and absence at deeper Sites 1213, suggests that water depth may be a factor controlling their distribution. Alternatively, at Site 1213 and other sites, the abundance of radiolarians throughout the section might indicate high productivity conditions hostile to oligotrophic organisms, including the nannoconids. However, at Site 463, radiolarians are often abundant along with nannoconids. Most likely a variety of environmental parameters influenced the distribution of this group.

Recovery of shallow "basement" of Shatsky Rise

Coring in Hole 1213B terminated in mafic igneous rocks on the flanks of southern Shatsky Rise. In all, 46.4 m of igneous section was drilled, with 33.4 m of recovered core (72%). Six cores recovered mostly massive basalt and diabase from three subunits (IVA to IVC), with each subunit thought to be a separate sill. The igneous rocks are predominantly hypocrystalline, fine-grained diabase (97.6%) with a small amount of sparsely phyric, aphanitic basalt (2.0%) at contacts. The diabase groundmass consists mainly of euhedral to subhedral plagioclase and intervening subhedral pyroxene and olivine, with minor glass. Alteration in the igneous section ranges from minor to moderate. Plagioclase and pyroxene crystals are locally altered to clay and in thin section, glassy groundmass has been ubiquitously devitrified and/or altered to clay minerals. Basaltic material in the section occurs at subunit contacts, symmetrically disposed around pieces of metasediment that mark the subunit boundaries. From the chilled contacts, the basalt grades toward more coarse-grained diabase in the unit centers, where the groundmass approaches medium grained.

The sills must be early Berriasian age or younger, since this is the age of the host sediment. Paleomagnetic data show two subunits have positive magnetic inclinations, whereas the third, basal subunit has a negative inclination, implying both normal and reversed magnetic polarities are recorded in the igneous section. This mixture indicates that the sills must have formed either before or after the Cretaceous Long Normal Superchron (i.e., the Cretaceous Quiet Period, 121-83 Ma). On the seismic profile along which Hole 1213B was drilled, the seismic "basement" has an odd character that may be related to the presence of intrusive, rather than extrusive, igneous rock at the sediment-igneous rock contact. The "basement" reflector, that being the deepest continuous seismic horizon, is weaker than elsewhere on Shatsky Rise and other, stronger reflectors occur beneath it. These deeper reflectors were not considered "basement" because they are not continuous all along the line, as is the weaker, shallower horizon. The cored section suggests the weak "basement" horizon denotes the top of the sills, whereas the deeper reflectors may be the top of the extrusive lava pile.