Radioisotope stratigraphy, sedimentology and geochemistry of Late Quaternary sediments from the Eastern Arctic Ocean

To reconstruct Recent and past sedimentary environments, marine sediments of Upper Pleistocene and Holocene ages from the eastern Arctic Ocean and especially from the Nansen-Gakkel Ridge (NGR) were investigated by means of radioisotopic, geochemical and sedimentological methods. In combination with mass physical property data and lithological analysis these investigations allow clearly to characterize the depositional environments. Age dating by using the radioisotope 230Th gives evidence that the investigated sediments from the NGR are younger than 250,000 years. Identical lithological sediment sequences within and between sediment cores from the NGR can be related to sedimentary processes which are clearly controlled by palaeoclimate. The sediments consist predominantly of siliciclastic, terrigenous ice-rafted detritus (IRD) deriving from assorted and redeposited sediments from the Siberian shelfs. By their geochemical composition the sediments are similar to mudstone, graywacke and arcose. Sea-ice as well as icebergs play a major roll in marine arctic sedimentation. In the NGR area rapid change in sedimentary conditions can be detected 128,000 years ago. This was due to drastic change in the kind of ice cover, resulting from rapid climatic change within only hundreds of years. So icebergs, deriving mostly from Siberian shelfs, vanished and sea-ice became dominant in the eastern Arctic Ocean. At least three short-period retreats of the shelf ice between 186,000 and 128,000 years are responsible for the change of coarse to fine-grained sediments in the NGR area. These warmer stages lasted between 1,000 and 3,000 years. By monitoring and comparing the distribution patterns of sedimentologic, mass physical and geochemical properties with 230Th ex activity distribution patterns in the sediment cores from the NGR, there is clear evidence that sediment dilution is responsible for high 230Th ex activity variations. Thus sedimentation rate is the controlling factor of 230Th ex activity variations. The 230Th flux density in sediments from the NGR seems to be highly dependent On topographic Position. The distribution patterns of chemical elements in sediment cores are in general governed by lithology. The derivation of a method for dry bulk density determination gave the opportunity to establish a high resolution stratigraphy on sediment cores from the eastern Arctic Ocean, based on 230Thex activity analyses. For the first time sedimentation and accumulation rates were determined for recent sediments in the eastern Arctic Ocean by 230Th ex analyses. Bulk accumulation rates are highly variable in space and time, ranging between 0.2 and 30 g/cm**2/ka. In the sediments from the NGR highly variable accumulation rates are related to the kind of ice cover. There is evidence for hydrothermal input into the sediments of the NGR. Hydrothermal activity probably also influences surficial sediments in the Sofia Basin. High contents of As are typical for surficial sediments from the NGR. In particular SL 370-20 from the bottom of the rift valley has As contents exceeding in parts 300 ppm. Hydrothermal activity can be traced back to at least 130,000 years. Recent to subrecent tectonic activity is documented by the rock debris in KAL 370 from the NGR. In four other sediment cores from the NGR rift valley area tectonically induced movements can be dated to about 130,000 years ago, related most probably to the rapid climate change. Processes of early diagenesis in sediments from the NGR caused the aobilization and redeposition of Fe, Mn and Mo. These diagenetic processes probably took place during the last 130,000 years. In sediment cores from the NGR high amounts of kaolinite are related to coarse grained siliciclastic material, probably indicating reworking and redeposition of siberian sandstones with kaolinitic binding material. In contrast to kaolinite, illite is correlated to total clay and 232Th contents. Aragonite, associated with serpentinites in the rift valley area of the NGR, was precipitated under cold bottom-water conditions. Preliminary data result in a time of formation about 60 - 80 ka ago. Manganese precipitates with high Ni contents, which can be related to the ultrabasic rocks, are of similar age.

Calcareous nannofossil stratigraphy of ODP Holes 173-1065A and 173-1069A

Upper Jurassic calcareous nannofossil assemblages have been studied from strata cored over basement blocks now buried under the Iberia Abyssal Plain (Ocean Drilling Program Leg 173 Sites 1065 and 1069). The youngest Jurassic assemblages at each site are Tithonian in age, the same as those at nearby Leg 149 Site 901, an age that predates the breakup of the Iberia continental margin. The paucity of the assemblages, the prevalence of coccospheres, and the relatively high organic contents of the fine clastic sediments in which they occur are characteristic of a restricted interior basin that had little communication with the open ocean. During the major rifting episode (a Berriasian event), the Jurassic sequences were dispersed along with their underlying blocks of presumed continental crust across the ocean-continent transition of the Iberia Abyssal Plain, probably as a result of detachment faulting.

Magnetic stratigraphy of ODP Leg 133 sites

This study is a synthesis of paleomagnetic and mineral magnetic results for Sites 819 through 823 of Ocean Drilling Program (ODP) Leg 133, which lie on a transect from the outer edge of the Great Barrier Reef (GBR) down the continental slope to the bottom of the Queensland Trough. Because of viscous remagnetization and pervasive overprinting, few reversal boundaries can be identified in these extremely high-resolution Quaternary sequences. Some of the magnetic instability, and the differences in the quality of the paleomagnetic signal among sites, can be explained in terms of the dissolution of primary iron oxides in the high near-surface geochemical gradients. Well-defined changes in magnetic properties, notably susceptibility, reflect responses to glacio-eustatic sea-level fluctuations and changes in slope sedimentation processes resulting from formation of the GBR. Susceptibility can be used to correlate between adjacent holes at a given site to an accuracy of about 20 cm. Among-site correlation of susceptibility is also possible for certain parts of the sequences and permits (tentative) extension of the reversal chronology. The reversal boundaries that can be identified are generally compatible with the calcareous nannofossil biostratigraphy and demonstrate a high level of biostratigraphic consistency among sites. A revised chronology based on an optimum match with the susceptibility stratigraphy is presented. Throughout most of the sequences there is a strong inverse correlation both between magnetic susceptibility and calcium carbonate content, and between susceptibility and d18O. In the upper, post-GBR, sections a more complicated type of magnetic response occurs during glacial maxima and subsequent transgressions, resulting in a positive correlation between susceptibility and d18O. Prior to and during formation of the outer-reef barrier, the sediments have relatively uniform magnetic properties showing multidomain behavior and displaying cyclic variations in susceptibility related to sea-level change. The susceptibility oscillations are controlled more by carbonate dilution than by variation in terrigenous influx. Establishment of the outer reef between 1.01 and 0.76 Ma restricted the supply of sediment to the slope, causing a four-fold reduction in sedimentation rates and a transition from prograding to aggrading seismic geometries (see other chapters in this volume). The Brunhes/Matuyama boundary and the end of the transition period mark a change to lower and more subdued susceptibility oscillations with higher carbonate contents. The major change in magnetic properties comes at about 0.4 Ma in the aggrading sequence, which contains prominent sharp susceptibility peaks associated with glacial cycles, with distinctive single-domain magnetite and mixed single-domain/superparamagnetic characteristics. Bacterial magnetite has been found in the sediments, particularly where there are high susceptibility peaks, but its importance has not yet been assessed. A possible explanation for the characteristic pattern of magnetic properties in the post-GBR glacial cycles can be found in terms of fluvio-deltaic processes and inter-reefal lagoonal reservoirs that develop when the shelf becomes exposed at low sea-level.

Stable isotope record and age model for sapropel stratigraphy of ODP Site 160-967

Stable oxygen isotope data from four holes drilled at the Ocean Drilling Program Site 967, which is located on the lower northern slope of the Eratosthenes Seamount, provide a continuous record of Eastern Mediterranean surface-water conditions during the last 3.2 Ma. A high-resolution stratigraphy for the Pliocene-Pleistocene sequence was established by using a combination of astronomical calibration of sedimentary cycles, nannofossil stratigraphy, and stable oxygen isotope fluctuations. Sapropels and color cycles are present throughout the last 3.2 Ma at Site 967, and their ages, as determined by calibration against the precessional component of the astronomical record, are consistent with those estimated for the sapropels of the classical land-based marine sequences of the Punta Piccola, San Nicola, Singa, and Vrica sections (southern Italy). The Site 967 oxygen isotope record shows large amplitude fluctuations mainly caused by variations in surface water salinity throughout the entire period. Spectral analysis shows that fluctuations in the d18O record were predominantly influenced by orbital obliquity and precessional forcing from 3.2 to 1 Ma, and all main orbital frequencies characterize the d18O record for the last million years. The start of sapropel formation at 3.2 Ma indicates a possible link between sapropel formation and the build up of northern hemisphere ice sheets. The dominance of the obliquity cycle in the interval from 3.2-1 Ma further points to the sensitivity of Eastern Mediterranean climate to the fluctuations in the volume of Arctic ice sheets. An intensification of negative isotope anomalies at Site 967, relative to the open ocean, supports a link between high run-off (during warm periods) and sapropel formation. freshwater input would have inhibited deep-water formation, which led to stagnation of deeper waters. Comparison with the land sections also confirms that differential preservation and diagenesis play a key role in sapropel occurrence.

Upper Cretaceous-Paleogene stratigraphy of ODP Sites 113-689 and 113-690

This contribution summarizes the biostratigraphy of planktonic foraminifers, calcareous nannofossils, and benthic foraminifers, in combination with the magnetostratigraphy, carbon and oxygen isotope stratigraphy of benthic foraminifers, and CaCO3 stratigraphy for the Maestrichtian through Paleogene calcareous sequences recovered at Sites 689 and 690 on Maud Rise (at about 65°S, eastern Weddell Sea, Antarctica). These data represent the southernmost calciumcarbonate record available for that interval, and thus extend the biostratigraphic and isotopic database to higher latitudes. Sites 689 and 690 form the southernmost anchor of a north-south transect through the Atlantic Ocean for Paleogene biostratigraphy and chemostratigraphy.

Calcareous nannofossil stratigraphy and key biohorizons of ODP Leg 173 sites

Six sites were drilled on the southern Iberia Abyssal Plain during Ocean Drilling Program (ODP) Leg 173. Three holes (1067A, 1068A, and 1069A) recovered Eocene sediments consisting of thinly bedded turbidite deposits with interbedded hemipelagic sediments (Bouma sequence Te) deposited near the calcite compensation depth. The hemipelagic sediments are barren of nannofossils, necessitating the use of the turbidite deposits to erect an Eocene biostratigraphy for these holes. Moderately preserved, diverse assemblages of nannofossils were recovered from silty clays (Bouma sequence Td) and poorly preserved, less diverse assemblages were recovered from sandy/silty clays (Bouma sequence Tc). Hole 1067A has a continuous record of sedimentation (Subzones CP9a-CP14a) and Holes 1068A and 1069A have similar continuous records (Subzones CP9a-CP12a), although all holes contain barren intervals. Holes 1067A, 1068A, 1069A, 900A (ODP Leg 149), and 398D (Deep Sea Drilling Project Leg 47B) display a similar increase in mass accumulation rates in the lowermost middle Eocene. A reliable Eocene biostratigraphy has been erected using nannofossil data from turbidite sequences, allowing for correlation between Iberia Abyssal Plain sites.

Oxygen isotope and paleomagnetic stratigraphy of sediment core V28-239

Core Vema 28-238 preserves an excellent oxygen isotope and magnetic stratigraphy and is shown to contain undisturbed sediments deposited continuously through the past 870,000 yr. Detailed correlation with sequences described by Emiliani in the Caribbean and Atlantic Ocean is demonstrated. The boundaries of 22 stages representing alternating times of high and low Northern Hemisphere ice volume are recognized and dated. The record is interpreted in terms of Northern Hemisphere ice accumulation, and is used to estimate the range of temperature variation in the Caribbean.

Sequence stratigraphy and sea-level history of Kangan formation in south pars field

Dehram group includes Faraghan, Dalan and Kangan formations. Kangan formation ages lower terias. That is one of the important reservoir rocks of southern Iran and Persian Gulf. In this research Kangan formation is studied in two A and B wells. Based on 75 studies on thin section, four carbonate litho acies association A, B, C, D with 12 subfacies are identified. A lithofacies association includes 4 subfacies: A1, A2, A3 and A4. B lithofacies association consists of 3 subfacies: B1, B2 and B3. C lithofacies association consists of 3 subfacies: C1, C2, C3 and D lithofacies association includes 2 subfacies: D1 and D2. On the base of studies lithofacies association of Kangan formations are formed in 3 environments of: Tidal Flat, Lagoon and Barrier Shore Complex in a Carbonated Platform Ramp type. Diagenetic processes have effected this formation. The most important Diagenetic processes are: Cementation, Anhydritization, Micrization, Neomorphism, Bioturbation, Dissolution, Compaction, Dolomitization and Porosity. Sequence staratigraphy studies were performed base on the vertical and horizontal relationship of lithofacies association and well logging in gamma ray and sonic type that causes the identification of two sedimentary sequences: First sedimentary sequence includes: Transgressive System Tract (TST) and High Stand System Tract (HST). The lower boundary of this sequence is in Sequence Boundary 1 (SB1) which shows unconformities of Dalan and Kangan that are Permian-terias unconformities. The upper boundary is in Sequence Boundary 2 (SB2) type that is identified by carbonate facies associated by anhydrite nodular. Second sedimentary sequence includes: TST and HST. Lower and upper boundaries of these sequences are both in SB2 type. The lower and upper boundary is made of carbonate facies with anhydrite nodular.