Stratigraphic correlation of ODP Site 199-1218 and 199-1219

During Ocean Drilling Program Leg 199 a high-resolution (~1-2 cm/k.y.) biogenic sediment record from the late Paleocene to the early Miocene was recovered, containing an uninterrupted set of geomagnetic chrons as well as a detailed record of calcareous and siliceous biostratigraphic datum events. Shipboard lithologic proxy measurements and shore-based determinations of CaCO3 revealed regular cycles that can be attributed to climatic forcing. Discovering drill sites with well defined magneto- and biostratigraphic records that also show clear lithologic cycles is rare and valuable and creates the opportunity to develop a detailed stratigraphic intersite correlation, providing the basis to study paleoceanographic processes and mass accumulation rates at high resolution. Here we present extensive postcruise work that extends the shipboard composite depth stratigraphy by providing a high-resolution revised meters composite depth (rmcd) scale to compensate for depth distortion within individual cores. The depth-aligned data were then used to generate stacked records of lithologic proxy measurements. Making use of the increased signal-to-noise ratio in the stacked records, we then proceeded to generate a detailed site-to-site correlation between Sites 1218 and 1219 in order to decrease the depth uncertainty for magneto- and biostratigraphic datums. Stacked lithologic proxy records in combination with discrete measurements of CaCO3 were then exploited to calculate high-resolution carbonate concentration curves by regression of the multisensor track data with discrete measurements. By matching correlative features between the cores and wireline logging data, we also rescaled our core rmcd back to in situ depths. Our study identifies lithology-dependent core expansion due to unloading as the mechanism of varying stratigraphic thicknesses between cores.

Core correlation data and revised composite depth scale of Exp302 (ACEX) cores

Expedition 302 of the Integrated Ocean Drilling Program (IODP), also known as the Arctic Coring Expedition (ACEX), successfully penetrated a sequence of Cenozoic sediments draping the crest of the Lomonosov Ridge in the central Arctic Ocean. The cumulative sedimentary record spans the last 57 m.y. and was recovered from three sites located within 15 km of each other. Merging the recovered cores onto a common depth scale that accurately reflects their stratigraphic placement below the seafloor is a fundamental step toward interpreting this unique sedimentary record. However, the lack of overlapping recovery in adjacent holes and intervals of high core disturbance complicated traditional methods of stratigraphic correlation. Here we present a revised composite depth scale for the ACEX sediments, generated in part by performing a regional stratigraphic correlation with sediments recovered from previous expeditions to the Lomonosov Ridge. The revised depth scale also reassesses the offsets for cores in the upper 55 meters below seafloor, where no overlapping recovery was acquired, and proposes modifications to these depths.

Tab. 1: Rb-Sr data on metasediments in the Shackleton Range

Summary: The stratigraphy of the Shackleton Range established by Stephenson (1966) and Clarkson (1972) was revised by results of the German Expedition GEISHA 1987/88. The "Turnpike Bluff Group" does not form a stratigraphic unit. The stratigraphic correlation of its formations is still a matter of discussion. The following four formations are presumed to belong to different units: The Stephenson Bastion Formation and Wyeth Heights Formation are probably of Late Precambrian age. The Late Precambrian Watts Needle Formation, which lies unconformably on the Read Group, is an independant unit which has to be separated from the "Turnpike Bluff Group". The Mount Wegener Formation has been thrusted over the Watts Needle Formation. Early Cambrian fossils (Oldhamia sp., Epiphyton sp., Botomaella (?) sp. and echinoderms) were found in the Mt. Wegener Formation in the Read Mountains. The Middle Cambrian trilobite shales on Mount Provender, which form the Haskard Highlands Formation, are possibly in faulted contact with the basement complex (Pioneers and Stratton Groups). They are overlain by the Blaiklock Glacier Group, for which an Ordovician age is indicated by trilobite tracks and trails, low inclination of the paleomagnetic field and the similarity to the basal units of the Table Mountain Quartzite in South Africa. The Watts Needle Formation represents epicontinental shelf sediments, the Mount Wegener Formation was deposited in a (continental) back-arc environment, and the Blaiklock Glacier Group is a typical molasse sediment of the Ross Orogen.

(Table 1) Planktonic foraminifer abundance and stable isotope ratios of a sediment core near Baddeckenstedt, north Germany

A refined sample processing technique using glacial acetic acid has been applied to Upper Cenomanian and Lower Turonian limestones from Baddeckenstedt (Lower Saxony) enabeling the first quantitative analysis of planktonic foraminiferal populations through the Stage boundary succession in northwestern Germany. Measurements of carbonate contents, organic carbon and stable carbon and oxygen isotopes were also reported. These data allow a correlation to be made of the Baddeckenstedt section with those at Misburg (basinal facies, northwestern Germany) and Dover (Plenus Marls, southern England). Significant maxima of the organic carbon content at Baddeckenstedt correspond to prominent black shale couplets at Misburg. The planktonic foraminiferal generic groups show at Baddeckenstedt similar fluctuations as reported from Dover. Their correlation reveals details of a complex paleoceanographic regime in the NW-German Basin during the Cenomanian/Turonian Oceanic Anoxic Event.

Stable isotope record of benthic and planktonic foraminifera in sediment cores of ODP Leg 177, Southern Ocean

While onboard ship during Leg 177, we used variations in sediment physical properties (mainly percent color reflectance) in conjunction with biomagnetostratigraphy to correlate among sites and predict the position of marine isotope stages (MISs) (e.g., see fig. F11 in Shipboard Scientific Party, 1999, p. 45). Our working assumption was that physical properties of Leg 177 sediments are controlled mainly by variations in carbonate content. Previous studies of Southern Ocean sediment cores have shown that carbonate concentrations are relatively high during interglacial stages and low during glacial stages at sites located within the Polar Frontal Zone (PFZ). Today, the PFZ marks a lithologic boundary in underlying sediment separating calcareous oozes to the north and silica-rich facies to the south (Hays et al., 1976). Although there is debate whether the position of the "physical" PFZ actually moved during glacial-interglacial cycles (Charles and Fairbanks, 1990; Matsumoto et al., 2001), the "biochemical" PFZ, as expressed by the CaCO3/opal boundary in sediments, certainly migrated north during glacials and south during interglacials. This gave rise to lithologic variations that are useful for stratigraphic correlation. At Leg 177 sites located north of the PFZ and at sublysoclinal depths, we expected the same pattern of carbonate variation because cores in the Atlantic basin are marked by increased carbonate dissolution during glacial periods and increased preservation during interglacials (Crowley, 1985).

Stable carbon and oxygen isotope ratios of bulk sediment from the Weddell Sea

Carbon isotope measurements were made on bulk sediments from the well preserved calcareous sequences recovered at ODP Sites 689 and 690 on the Maud Rise, Weddell Sea, Antarctica. The very positive delta13C values that characterize the late Paleocene and the rapid trend toward lighter values in the early Eocene established in other sites are clearly recorded here and may be of value for long-distance stratigraphic correlation. However, values in the late Eocene are significantly more positive than have been reported from other areas. The general pattern of the records from Sites 689 and 690 is sufficiently unlike those previously reported from lower latitudes that we suggest that carbon isotope data should be used only with considerable caution for correlating sequences from such high latitudes with lower latitude records.

Strontium isotopic ratios of evaporites of the Tyrrhenian Sea

Strontium isotopic ratios of gypsums recovered from upper Miocene (Messinian) evaporites at ODP Leg 107 Holes 652A, 653B, and 654A (Tyrrhenian Sea) are lower than expected. The values for the Messinian balatino-like gypsum, single gypsum crystals, and anhydrites range from 0.70861 to 0.70886 and are approximately 25 * 10**-5 less than would be expected for evaporites precipitated from Messinian seawater (0.70891-0.70902). Pre-evaporitic planktonic foraminifers from Hole 654A show variable degrees of dolomitization and 87Sr/86Sr values that irregularly decrease upward from normal marine values approximately 81m below the lowest evaporite occurrence. This suggests diagenetic alteration by advecting interstitial water with a low 87Sr/86Sr ratio or that the lower Sr isotopic ratios for the Messinian evaporites could have resulted from a greater influence of fresh water on the Sr isotopic composition of the desiccating Tyrrhenian Sea. Fluctuations of the 87Sr/86Sr-ratio for evaporites in the sedimentary cycles recognized for Holes 653B and 654A, the generally low Sr isotopic ratio of river water entering the Mediterranean Sea, and the presence of dwarf marine microfossils suggest that the 87Sr/86Sr ratio of the evaporites responded to hydrologic variations in a very restricted basin with variable rates of marine and fresh water input. The strontium isotopic ratios of the Messinian anhydrites from the proposed lacustrine sequence at Hole 652A fall in the same range as the marine evaporites from Holes 654A and 653B. This suggests a common or similar origin of the brines at the three locations. The complex depositional and hydrologic conditions in the Mediterranean during the Messinian salinity crisis preclude the use of Sr isotopic values from the evaporites for stratigraphic correlation and dating. They are, however, very useful in the interpretation of the depositional history of the basin. General calculations assuming a closed system suggest that the 87Sr/86Sr ratio of Messinian seawater (-0.7090) could be reduced to that of the evaporites (-0.7087) by mixing with fresh water (e.g., Nile River) in times of 10**4 to 10**5 yr.

Age-depth correlations for Ontong Java Plateau sites

We used well logs, in some cases combined with shipboard physical properties measurements to make more complete profiles and to correlate between sites on the Ontong Java Plateau. By comparing sediment bulk density, velocity, and resistivity logs from adjacent holes at the same site, we showed that even subtle features of the well logs are reproducible and are caused by variations in sedimentation. With only minor amounts of biostratigraphic information, we could readily correlate these sedimentary features across the entire top of the Ontong Java Plateau, demonstrating that for most of the Neogene the top of the plateau is a single sedimentary province. We found it more difficult, but still possible, to correlate in detail sites from the top of the plateau to those drilled on the flanks. The pattern of sedimentation rate variation down the flank of the plateau cannot be interpreted as simply controlled by dissolution. Site 805, in particular, oscillates between accumulating sediment at roughly the same rate as cores on top of the Ontong Java Plateau, and accumulating sediment as slowly as Site 803, 200 m deeper in the water column. These oscillations do not match earlier reconstructions of central Pacific carbonate compensation depth variations.

Neogen carbonate and stable isotope record of ODP Hole 120-751A

Lower Miocene through upper Pleistocene benthic foraminifer assemblage records from Ocean Drilling Program Site 751 on the Southern Kerguelen Plateau (57°44'S, water depth 1634 m) were combined with benthic and planktonic foraminifer oxygen and carbon isotope records and high-resolution CaCO3 data from the same site. Implications for the Neogene productivity and paleoceanography of the southern Indian Ocean are discussed. We used distinctive features of the Miocene d18O and d13C curves for stratigraphic correlation. Coinciding with a lower middle Miocene hiatus from 14.2 to 13.4 Ma, there was a rapid increase in benthic d18O values by 1.2 per mil. This distinct increase occurs in middle Miocene benthic foraminifer oxygen isotope curves from all oceans. No major change, however, in benthic foraminifer faunal composition occurred in this period of growth of the Antarctic ice cap and cooling of deep ocean waters (14.9-14.2 Ma). A drastic change in benthic foraminifer faunas coincided with a hiatus from 8.4 to 5.9 Ma. Shortly after this hiatus, in the latest Miocene, the CaCO3 content of the sediments dropped from 75% to 0%. From that time ( 5.8 Ma) through the early Pliocene, Site 751 has been situated beneath a high biogenic siliceous productivity zone. Carbonate contents of upper Pliocene and Pleistocene sediments vary between 20% and 70%. The benthic foraminifer faunas in the uppermost Pliocene and lower Pleistocene reflect strong bottom current conditions, in contrast to those in the upper Pleistocene, which indicate calm sedimentation and high food supply. High d13C values of planktonic foraminifers compared with low values of benthic foraminifers suggest high primary productivity in the late Pleistocene. The changes in productivity were probably a result of latitudinal migration and meandering of the Polar Frontal Zone.

Sediment accumulation rates of sediment cores from the North-West African continental margin

Six soft sediment cores, up to and over 9 m in length, and additional surface samples were selected for study of their planktonic foraminifera to provide information on the Holocene and Pleistocene stratigraphy of the West African continental margin south of the present boundary of the Sahara. The material was collected by the German research vessel "Meteor" during Cruise 25 in 1971. The residues larger than 160 microns determined, counted and statistically evaluated. Stratigraphical correlations with trans- Antlantic regions are given by occurrence of Truncorotalidoides hexagonus and Globorotalia tumidula flexuosa which mark the last interglacial stage. According to the climatic record the two deep-sea cores extend down to the V-zone, considered here as equivalent to the Mindel-Riss-interglacial time, as there are three distinctly warm and two cold periods indicated in the cores by planktonic foraminiferal faunas. Z-zone = Holocene is present in all cores, Y-zone = Wuermian glacial can be divided into five section, three cold and two warm stages; the X-zone can be divided into three warm stages, separated into two cool periods. The earliest warm stage is indicated to be the warmest one. There are excellent correlations to the Camp century ice core from Greenland, to the Mediterranean, to the Carribean and to the tropical Atlantic as well as to the Barnados stage. The W-zone was correlated to the Riss-glacial. V-zone is a warm period, the upper limit of which being not sufficiently defined, which contains also some cool sections. Increasing sedimentation rates from the deep-sea to the upper slope reveal climatic and regional details in Holocene and Late Pleistocene history of the continental margin. These were based mainly on different parameters of planktonic foraminiferal thanatocoenoses which are the main components of the size fraction >160 microns of the pelagic core. They become incerasingly diluted by other faunal and terrigenous components with decreasing slope depths. Estimates of absolute abundances, ranging from 25000 specimens/gm of sediment in the deep sea to less than 100, indicate various sedimentary processes at the continental margin. An ecological correlation by dominant species is possible. Readily computed temperature indices of different scales are presented which indicate, for instance, three distinctly cold sections within the last glacial and seven warm sections within the last interglacial lime. These are used for estimates of sedimentation rates. During cold periods sedimentation rates are higher than during warmer periods. Stratigraphic correlation and faunal record, combined with absolute abundances and sedimentation rates, indicated that in the deep sea turbidity currents not only cause high sedimentation rates for short periods of time, but also that material is occasionally eroded. Effects of upwelling may be detected in the surfacc sediment samples as well as in late Pleistocene and early Holocene samples of the slope by planktonic foraminiferal data which are not influenced by sedimentary processes.