Paleomagnetic reconstructions of sediments at ODP Sites 135-840 and 135-841

The geometry of the Tonga Arc implies that it has rotated approximately 17° clockwise away from the Lau Ridge as the Lau Basin formed in between. Questions have arisen about the timing of the opening, whether the arc behaved rigidly, and whether the opening occurred instead from motion of the Lau Ridge, the remanent arc. We undertook to address these questions by taking paleomagnetic samples from sediment cores drilled on the Tonga Arc at Sites 840 and 841, orienting the samples in azimuth, and comparing the paleodeclinations to expected directions. Advanced hydraulic piston corer (APC) cores from Holes 840C and 841A were oriented during drilling with a tool based on a magnetic compass and attached to the core barrel. Samples from Hole 841B were drilled with a rotary core barrel (RCB) and therefore are azimuthally unoriented. They were oriented by identifying faults and dipping beds in the core and aligning them with the same features in the Formation MicroScanner (FMS) wireline logs, which were themselves oriented with a three-axis magnetometer in the FMS tool. The best results came from the APC cores, which yielded a mean pole at -69.0°S, 112.2°E for an age of 4 Ma. This pole implies a declination anomaly of 20.8° ± 12.6° (95% confidence limit), which appears to have occurred by tectonic rotation of the Tonga Arc. This value is almost exactly that expected from the geometry of the arc and implies that it did indeed rotate clockwise as a rigid body. The large uncertainty in azimuth results from core orientation errors, which have an average standard deviation of 18.6°. The youngest cores used to calculate the APC pole contain sediments deposited during Subchron 2A (2.48-3.40 Ma), and their declinations are indistinguishable from the others. This observation suggests that most of the rotation occurred after their deposition; this conclusion must be treated with caution, however, because of the large azimuthal orientation errors. Poles from late and early Miocene sediments of Hole 841B are more difficult to interpret. Samples from this hole are mostly normal in polarity, fail a reversal test, and yield poles that suggest that the normal-polarity directions may be a recent overprint. Late Miocene reversed-polarity samples may be unaffected by this overprint; if so, they imply a declination anomaly of 51.1° ± 11.5°. This observation may indicate that, for older sediments, Tonga forearc rotations are larger than expected.

Depth ranges of calcareous nannofossil datums from ODP Holes 113-689B and 113-690B

Magnetostratigraphic studies of Paleogene sediments piston-cored on Maud Rise, Weddell Sea (ODP Sites 689 and 690), are a cornerstone of Southern Ocean Paleogene and Neogene chronostratigraphy. However, parts of previous magnetostratigraphic interpretations have been called into question, and recent reinvestigation of the upper Paleocene-middle Eocene portion of Site 690 suggested that the records might be contaminated by spurious magnetizations, which raises doubts about the reliability of these important records. We undertook a high-resolution magnetostratigraphic study of Eocene-Oligocene u-channel samples from ODP Holes 689B, 689D, 690B, and 690C in order to address these concerns. A pervasive overprint appears to be present below the middle Eocene, which compromises magnetobiostratigraphic interpretations for the upper Cretaceous and lower Paleogene. Nevertheless, our new results provide a robust record of geomagnetic field behavior from 38.5 to 25 Ma and confirm the reliability of these sediments for calibration of biostratigraphic datum events during a crucial phase of earth history when major Antarctic ice sheets developed. Also, comparison of magnetozone thicknesses in multiple holes at the same site indicates that ~1.2-1.8 m of the stratigraphic record is missing at each core break, which corresponds to time breaks of 120-360 k.y. Lack of a continuous record within a single hole renders useless spectral analyses for investigating long geomagnetic and paleoclimatic time series. This observation reinforces the need for coring of multiple offset holes to obtain continuous paleoceanographic records. Sedimentary hiatuses have been identified only at the deeper of the two investigated sites (Site 690), which could mark a local response to the onset of the Antarctic Circumpolar Current.

Integrated Paleocene calcareous plankton magnetobiochronology of DSDP Hole 43-384 in the Northwest Atlantic Ocean

At Deep Sea Drilling Site 384 (J-Anomaly Ridge, Grand Banks Continental Rise, NW Atlantic Ocean) Paleocene nannofossil chalks and oozes (~70 m thick) are unconformably/disconformably underlain (~168 m; upper Maastrichtian) and overlain (~98.7 m; upper lower Eocene) by sediments of comparable lithologies. The chalks are more indurated in stratigraphically higher levels of the Paleocene reflecting increasing amounts of biosiliceous (radiolarians and diatoms) components. This site serves as an excellent location for an integrated calcareous and siliceous microfossil zonal stratigraphy and stable isotope stratigraphy. We report the results of a magnetostratigraphic study which, when incorporated with published magnetostratigraphic results, reveals an essentially complete magnetostratigraphic record spanning the interval from Magnetochron C31n (late Maastrichtian) to C25n (partim) (late Paleocene, Thanetian). Integrated magnetobiochronology and stable isotope stratigraphy support the interpretation of, and constrain the estimated duration of, a short hiatus (~0.9 my) within the younger part of Chron C29r (including the K/P boundary) and an ~6 my hiatus separating upper Paleocene (Magnetozone C25n) and upper lower Eocene (Magnetozone C22r) sediments. Some 30 planktonic foraminiferal datum levels [including the criteria used to denote the Paleocene planktonic foraminiferal (sub)tropical zonal scheme of Berggren and Miller, Micropaleontology 34 (4) (1988) 362-380 and Berggren et al., SEPM Spec. Publ. 54 (1995) 129-212, Geol. Soc. Am. Bull. 107 (11) (1995) 1272-1287], and nearly two dozen calcareous nannoplankton datum levels have been recognized and calibrated to the magnetochronology. Planktonic foraminiferal Subzones P4a and P4b of (upper Paleocene) Zone P4 are emended/redefined based on the discovery of a longer stratigraphic extension of Acarinina subsphaerica (into at last Magnetozone C25n). Stable isotope stratigraphies from benthic foraminifera and fine fraction (<38 µm) carbonate have been calibrated to the biochronology and magnetostratigraphy. A minimum in benthic foraminifer delta13C was reached near the Danian/Selandian boundary (within Chron C26r, planktonic foraminiferal Zone P3a and calcareous nannoplankton Zone NP4) and is followed by the rise to maximum delta13C values in the late Thanetian (near the base of C25n, in Zone P4c and NP9a, respectively) that can be used for global correlation in the Paleocene.

Sedimentary and subsidence history of ODP Sites 135-840 and 135-841

Sedimentary sections recovered from the Tonga platform and forearc during Ocean Drilling Program Leg 135 provide a record of the sedimentary evolution of the active margin of the Indo-Australian Plate from late Eocene time to the Present. Facies analyses of the sediments, coupled with interpretations of downhole Formation MicroScanner logs, allow the complete sedimentary and subsidence history of each site to be reconstructed. After taking into account the water depths in which the sediments were deposited and their subsequent compaction, the forearc region of the Tofua Arc (Site 841) can be seen to have experienced an initial period of tectonic subsidence dating from 35.5 Ma. Subsidence has probably been gradual since that time, with possible phases of accelerated subsidence, starting at 16.2 and 10.0 Ma. The Tonga Platform (Site 840) records only the last 7.0 Ma of arc evolution. However, the increased accuracy of paleowater depth determinations possible with shallow-water platform sediments allows the resolution of a distinct increase in subsidence rates at 5.30 Ma. Thus, sedimentology and subsidence analyses show the existence of at least two, and possibly four, separate subsidence events in the forearc region. Subsidence dating from 35.5 Ma is linked to rifting of the South Fiji Basin. Any subsidence dating from 16.2 Ma at Site 841 does not correlate with another known tectonic event and is perhaps linked to localized extensional faulting related to slab roll back during steady-state subduction. Subsidence from 10.0 Ma coincides with the breakup of the early Tertiary Vitiaz Arc because of the subduction polarity reversal in the New Hebrides and the subsequent readjustment of the plate boundary geometry. More recently, rapid subsidence and deposition of a upward-fining cycle from 5.30 Ma to the Present at Site 840 is thought to relate to rifting of the Lau Basin. Sedimentation is principally controlled by tectonic activity, with variations in eustatic sea level playing a significant, but subordinate role. Subduction of the Louisville Seamount Chain seems to have disrupted the forearc region locally, although it had only a modest effect on the subsidence history and sedimentation of the Tonga Platform as a whole.

Pliocene-Pleistocene calcareous nannofossils from the Iberian Abyssal Plain

During Ocean Drilling Program Leg 149, five sites were drilled on the Iberia Abyssal Plain, west of the Iberian Peninsula. Five holes (Holes 897A, 897C, 898A, 899A, and 900A) yielded Pliocene-Pleistocene sediments, which consist mainly of turbidites. Among these, Holes 897C and 898A yielded significant Pliocene-Pleistocene sediments that provided a high-resolution nannofossil biostratigraphy essential for locating paleomagnetic polarity events and for interpreting the age and frequency of turbidite sedimentation in the Iberia Abyssal Plain. Pliocene-Pleistocene nannofossils recovered during Leg 149 are generally abundant and well to moderately preserved. Although reworking is evident in most samples, the Pliocene-Pleistocene nannofossils proved quite reliable for dating the sediments. Most Pleistocene zonal boundaries proposed by S. Gartner in 1977 and the Pliocene standard zonal boundaries proposed by E. Martini in 1971 were easily recognized. In addition, several other nannofossil events proposed by D. Rio et al. in 1990 and by T. Sato and T. Takayama in 1992 were recognized and proved valuable for improving the resolution of Pliocene-Pleistocene nannofossil biostratigraphy. The Pliocene-Pleistocene nannofossil biostratigraphic results of Holes 897C and 900A coincide rather well with the discerned paleomagnetic polarity events. As a result, the combination of nannofossil biostratigraphic and paleomagnetic studies provides important information for fulfilling the second objective of this leg: to determine the history of turbidite sedimentation in the Iberia Abyssal Plain. The general trend of sedimentation rates inferred by nannofossil biostratigraphy indicates that sedimentation rates increase from the continental margin to the deep sea along with increasing water depth.

Paleogene planktonic foraminifera from the southern Kerguelen Plateau

An essentially complete Paleogene record was recovered on the Central and Southern Kerguelen plateaus (55°-59°S) in a calcareous biofacies. Recovery deteriorated in the middle Eocene and down to the upper Paleocene because of the presence of interbedded cherts and chalks. The stratigraphic distribution of about 70 taxa of planktonic foraminifers recovered at Sites 747-749 is reported in this paper. Faunas exhibited fairly high diversity (approximately 20-25 species) in the early Eocene, followed by a gradual reduction in diversity in the middle Eocene. A brief incursion of tropical keeled morozovellids occurred near the Paleocene/Eocene boundary, similar to that recorded on the Maud Rise (ODP Sites 689 and 690). The high-latitude Paleogene zonal scheme developed for ODP Leg 113 sites has been adopted (with minor modifications) for the lower Eocene-Oligocene part of the Kerguelen Plateau record. A representative Oligocene (polarity chronozones 7-13) and late Eocene-late middle Eocene (questionably polarity chronozones 16-18) magnetostratigraphic record has allowed the calibration of several biostratigraphic datum levels to the standard Global Polarity Time Scale (GPTS) and established their essential synchrony between low and high latitudes.

Magnetostratigraphy of ODP Leg 138 sites

The aims of this study are twofold. First, the study tries to provide the most reliable chronology possible for two critical sections by correlating the magnetic polarity stratigraphy measured in these sediments with a newly revised geomagnetic polarity time scale. Second, this study attempts to examine in detail the nature of seven short events not included in the shipboard standard time scale, but for which abundant magnetostratigraphic evidence was obtained during the Leg. Data presented here force some modifications of the shipboard interpretations of the magnetostratigraphy of Sites 845 and 844 on the basis of new data generated using discrete samples and from a greater appreciation of the magnetostratigraphic signature of Miocene-age short events. Those short events can be classified into two groups: those that probably reflect short, full-polarity intervals and those that more likely represent an interval of diminished geomagnetic intensity. Three of the seven events documented here correspond well with three subtle features, as seen in marine magnetic profiles, that have been newly included in the geomagnetic polarity time scale as short, full-polarity chrons. One of the seven events corresponds to a poorly defined feature of the marine magnetic record that has also been newly included in the geomagnetic polarity time scale, but which was considered of enigmatic origin. The three remaining events investigated here, although they have not been identified with features in the seafloor magnetic record, are suggested to be events of a similar nature, most likely times of anomalously low geomagnetic intensity. In addition to the Miocene magnetostratigraphic results given, several sets of averaged paleomagnetic inclinations are presented. Although these results clearly show the effects of a residual coring overprint, they demonstrate that paleomagnetic estimates of paleolatitudes can be made which are in good general agreement with ancient site positions calculated using hot spot-based plate reconstructions.

Ocean bottom seismometer sgy-files of refraction seismic profiles from Professor Logachev cruise TTR16/3

Over the last decade pockmarks have proven to be important seabed features that provide information about fluid flow on continental margins. Their formation and dynamics are still poorly constrained due to the lack of proper three dimensional imaging of their internal structure. Numerous fluid escape features provide evidence for an active fluid-flow system on the Norwegian margin, specifically in the Nyegga region. In June-July 2006 a high-resolution seismic experiment using Ocean Bottom Seismometers (OBS) was carried out to investigate the detailed 3D structure of a pockmark named G11 in the region. An array of 14 OBS was deployed across the pockmark with 1 m location accuracy. Shots fired from surface towed mini GI guns were also recorded on a near surface hydrophone streamer. Several reflectors of high amplitude and reverse polarity are observed on the profiles indicating the presence of gas. Gas hydrates were recovered with gravity cores from less than a meter below the seafloor during the cruise. Indications of gas at shallow depths in the hydrate stability field show that methane is able to escape through the water-saturated sediments in the chimney without being entirely converted into gas hydrate. An initial 2D raytraced forward model of some of the P wave data along a line running NE-SW across the G11 pockmark shows, a gradual increase in velocity between the seafloor and a gas charged zone lying at ~300 m depth below the seabed. The traveltime fit is improved if the pockmark is underlain by velocities higher than in the surrounding layer corresponding to a pipe which ascends from the gas zone, to where it terminates in the pockmark as seen in the reflection profiles. This could be due to the presence of hydrates or carbonates within the sediments.

Microtekties in sediments from the Brunhes-Matuyama transition

A mechanism had been recently proposed to show how an impact event can trigger a geomagnetic polarity reversal by means of rapid climate cooling. We test the proposed mechanism by examining the record from two high sedimentation rate (8-11 cm/kyr) deep-sea sediment cores (ODP Sites 767 and 769) from marginal seas of the Indonesian archipelago, which record the Australasian impact with well-defined microtektite layers, the Brunhes-Matuyama polarity reversal with strong and stable remanent magnetizations, and global climate with oxygen isotope variations in planktonic foraminifera. Both ODP cores show the impact to have preceded the reversal of magnetic field directions by about 12 kyr. Both records indicate that the field intensity was increasing near the time of impact and that it continued to increase for about 4 kyr afterwards. Furthermore, the oxygen isotope record available from sediments at ODP Site 769 shows no indication of discernible climate cooling following the impact: the microtektite event occurred in the later part of glacial Stage 20 and was followed by a smooth warming trend to interglacial Stage 19. Thus the detailed chronology does not support the previously proposed model which would predict that a decrease in geomagnetic field intensity resulted from a minor glaciation following the impact event. We conclude that the evidence for a causal link between impacts and geomagnetic reversals remains insufficient to demonstrate a physical connection.

Magnetostratigraphy of ODP Leg 117 sites

Magnetic measurements were made on discrete samples from the Neogene pelagic and hemipelagic sediments recovered during ODP Leg 117. Polarity zones, usually identified for uppermost 200 m, were correlated to the geomagnetic polarity time scale referring to biostratigraphic datums. Quality and resolution of the magnetostratigraphy was partly limited by the weak and relatively soft magnetic character of almost all intervals, and core disturbance by gas expansion at some Oman Margin sites. Clear polarity records of the Brunhes and Matuyama chrons (C1 to C2r) were observed at Sites 724 and 727 on the Oman continental margin. Extended reversal records of the Pliocene were found at Site 722 on the Owen Ridge and Site 728 on the Oman Margin, and correlated to the Gauss to Gilbert chrons (C2A to C3).

Strontium isotope ratios of middle Eocene to Oligocene sediments from Maud Rise, Antarctica

A 87Sr/86Sr isotope curve of the middle Eocene to Oligocene was produced from analysis of foraminifera in Ocean Drilling Program Hole 689B, Maud Rise, near the coast of Antarctica. Sediments from the hole are well preserved with no evidence of diagenetic alteration. The sequence is nearly complete from 46.3 to 24.8 Ma, with an average sampling interval of 166 kyr. Excellent magnetostratigraphy in Hole 689B allows calibration to the geomagnetic polarity time scale of Cande and Kent (1992). Marine strontium isotopic ratios were nearly stable from 46.3 to 35.5 Ma, averaging near 0.70773, after which they began to increase. A slow increase began after 40.4 Ma, rising at a rate of only about 8*10**-6/m.y. from base values of 0.707707. From 35.5 Ma to 24.8 Ma the average slope increased to 40*10**-6/m.y. The slope remained constant at least until 24.8 Ma, when the record becomes discontinuous owing to unconformities. We evaluate several possible controls on the marine strontium isotope curve that could have led to the observed growth in 87Sr/86Sr ratios near the Eocene/Oligocene boundary. Three mechanisms are considered, including the onset of Antarctic glaciation, increased mountain building in the Himalayan-Tibetan region, and decreased hydrothermal activity. None of the mechanisms alone seems to adequately explain the increased 87Sr/86Sr ratios during the Oligocene. Glaciation as a weathering agent was too episodic and probably began too late to explain the upturn in marine 87Sr/86Sr ratios. There is evidence that uplift in the Himalayan-Tibetan region began in the Miocene, much too late to control Oligocene strontium isotope ratios. Lastly, hydrothermal flux changes since the Eocene were apparently not great enough alone to account for the rise in marine 87Sr/86Sr ratios. We suggest that a combination of causes, such as decreased hydrothermal activity perhaps followed by increased glaciation and mountain building, might best explain the growth of the marine 87Sr/86Sr curve during the Oligocene.

Chemical composition and description of a ferromanganese crust, sample 89D, obtained during R/V Capricorne cruise ARCANTE 1

A manganese oxide encrustation (2.5 kg) was dredged, in an island arc setting, downslope of Bertrand bank, a seamount culminating at 70-m depth and located NNE of Grande-Terre, Guadeloupe, and SE of Antigua, West Indies. A thorough texturai analysis indicated a rhythmic precipitation and growth polarity as well as mineralogical ( 10 A tektomanganate) and geochemical (low concentrations of Ni, Cu, Co, Zn, Pb and REE) criteria, point to a submarine hydrothermal origin for most of the sample. The crust was coated with a fine ferromanganese oxide cortex deposited iii a "normal" oceanic environment; it also included micritic fillings, a main pyroclastic zone near the top of the crust, and a Mg-Al sulphate deposit. Planktonic foraminifera coeval with the precipitation of the manganese oxide indicate an age of ca. 3 m. y. (upper Pliocene); i.e., more than 20 m. y. after the cessation of the volcanic activity of the Lesser Antilles outer arc that was responsible for the buildup of the Bertrand seamount. Furthermore, the genesis of the crust is not linked to the activity of the contemporaneous inner arc (Miocene to Present), particularly of its nearmost segment (Basse Terre, Guadeloupe-Montserrat) located about 50 km to the West. The authors suggest that the manganese oxide is the result of convective circulation of sea water through a faulted system occurring in an area of intense seismic activity. The remobilization of chemical elements (Mn, S, etc.) within the seamount volcanic core bas probably affected a substratum that was still hydrothermally altered during the previous volcanic activity of the outer arc. The authors insist on the interest in using texturai analysis for Fe/Mn oxide investigations.

Maestrichtian planktonic foraminifera of the Maud Rise

The southernmost record of Maestrichtian pelagic carbonate sedimentation was recovered from ODP Leg 113 Holes 689B and 690C, drilled on the Maud Rise in the eastern Weddell Sea sector of the Southern Ocean (65°S). Well preserved and abundant planktonic foraminifers occur throughout Maestrichtian cores from both holes, providing a nearly complete biogeographic and biostratigraphic history of this region. Diversity is low compared to tropical and subtropical assemblages, with a maximum within sample diversity of 16 planktonic foraminifer species and a diversity total for the Maestrichtian of 24 species. The assemblages are dominated throughout by Heterohelix, Globigerinelloides, and a new species of Archaeoglobigerina, whereas keeled taxa are completely absent from the lower Maestrichtian and rare in the middle through upper Maestrichtian sediments. Three planktonic foraminifer species are described as new and are recognized as being endemic to the Austral Province. These include Archaeoglobigerina australis n. sp., Hedbergella sliteri n. sp., and Archaeoglobigerina mateola n. sp. The former two species were previously illustrated in reports on Late Cretaceous foraminifers from the Falkland Plateau and the northern Antarctic Peninsula. Two keeled and five non-keeled planktonic foraminifers, previously not found in high latitude Maestrichtian sediments, first appeared at the Maud Rise during the late early and late Maestrichtian. Correlation with their stratigraphic ranges in low latitude sequences shows that their first appearance datums are considerably younger at the Maud Rise than in the lower latitudes. The most likely explanation for this observation is that there was a warming in the south polar region during the late early and late Maestrichtian and a concomitant poleward migration of stenothermal taxa. However, oxygen isotopic paleotemperature results from Sites 689 and 690 (Barrera and Huber, 1990, doi:10.2973/odp.proc.sr.113.137.1990) show a long-term cooling trend throughout the Maestrichtian, indicating that other factors may have played a more important role than temperature in the distribution of Maestrichtian planktonic foraminifers. A new biostratigraphic scheme is proposed for the Antarctic because of the absence of thermophilic planktonic foraminifers used to identify existing low to middle latitude zones. The Globigerinelloides impensus Partial Range Zone is defined for the late Campanian-Maestrichtian, the Globotruncanita havanensis Partial Range Zone is redefined for the early to late early Maestrichtian, and the Abathomphalus mayaroensis Total Range Zone is recognized. Good quality magnetic polarity data obtained from both Maud Rise sites (Hamilton, 1990, doi:10.2973/odp.proc.sr.113.179.1990) enables magnetobiostratigraphic correlation of twelve foraminifer datums with the geomagnetic polarity time scale of Haq et al. (1987). The geochronology thus obtained is crucial for accurate cross-latitudinal correlation and interpretation of the paleoceanographic history of the Antarctic region during the Maestrichtian time period.

Paleomagnetic of ODP Leg 125 sites

During Leg 125, scientists drilled Sites 782, 783, 784, and 786 across a transect of the Izu-Bonin forearc near 31°N. Magnetostratigraphy for whole-core and discrete specimens has been integrated with biostratigraphic data and correlated to the geomagnetic polarity time scale. These correlations are good back to the middle Miocene at Sites 783, 784, and 786 and to the late Oligocene at Site 782, but become more tentative in older sediments because of poor recovery and complex magnetizations.

Rock magentic investigations of ODP Site 162-983

We report natural remanent magnetization (NRM) directions and geomagnetic paleointensity proxies for part of the Matuyama Chron (0.9-2.2 Ma interval) from two sites located on sediment drifts in the Iceland Basin. At Ocean Drilling Program Sites 983 and 984, mean sedimentation rates in the late Matuyama Chron are 15.9 and 11.5 cm/kyr, respectively. For the older part of the record (>1.2 Ma), oxygen isotope data are too sparse to provide the sole basis for age model construction. The resemblance of the volume susceptibility record and a reference d18O record led us to match the two records to derive the age models. This match, based on Site 983/984 susceptibility, is consistent with available Site 983/984 benthic d18O data. Paleointensity proxies were derived from the slope of the NRM versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30-60 mT peak field range. Paleointensity lows correspond to polarity reversals at the limits of the Jaramillo, Olduvai, Cobb Mountain, and Réunion Subchrons and to seven excursions in NRM component directions. Magnetic excursions (defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator) are observed at 932, 1048, 1115, 1190-1215 (Cobb Mountain Subchron), 1255, 1472-1480, 1567-1575 (Gilsa Subchron), and 1977 ka. The results indicate that geomagnetic directional excursions, associated with paleointensity minima, are a characteristic of the Matuyama Chron and probably of polarity chrons in general.