Paleomagnetism at DSDP Sites 86-576 and 86-578

The upper sections of Deep Sea Drilling Project Sites 576 (32°21.4'N, 164°16.5'E) and 578 (33°55.6'N, 151°37.7'E) both have stable detrital remanence that can be correlated with the standard reversal stratigraphy. Site 576 contains all reversals above the base of the Gilbert Epoch (5 m.y.) at about 25 m, whereas Site 578 contains a remarkable section of about 60 reversals extending to Anomaly 5B (15 m.y.) at about 150 m sub-bottom depth. In both cases, the paleomagnetic stratigraphy breaks down when accumulation rates drop below 2 m/m.y. At both sites, authigenic manganiferous clays deposited from 70 to 16 m.y. ago accumulated at about 0.4 m/m.y. Similarly, at both sites, the Pleistocene pulse of eolian debris increased accumulation rates by about 6 m/m.y.**2. From 16 to 2 m.y. ago, however, sediment accumulated at Site 578 about five times as rapidly as at Site 576, apparently because of augmented input to the western site by bottom currents.

Magnetostratigraphy of ODP Leg 104 holes

Results of a detailed paleomagnetic study on largely undisturbed sedimentary sequences recovered in the Voring Plateau region of the Norwegian Sea during Ocean Drilling Program Leg 104 are presented. At each drill site an essentially continuous downhole magnetic reversal pattern could be defined to depths between 200 and 300 m below seafloor allowing correlations with a calibrated geomagnetic time scale and establishing almost complete magnetostratigraphic records for the core material analyzed. A composite section of the drill holes represents the first high-quality chronostratigraphic framework from the lower Miocene through Holocene obtained in the Norwegian Sea. It should provide a basis for first-order correlations with calcareous and siliceous microfossil events and contribute to a further elaboration of the regional paleoceanographic history. A series of major hiatuses in the upper and middle Miocene accounts for about 4 million yr of missing stratigraphic record.

Late Neogene carbonate sedimentation and stable isotope record of ODP Site 121-758

The evolution of oceanic and climatic conditions the northeast Indian Ocean during the last 7 m.y. is revealed in the sediments from Site 758. We present detailed and continuous records of d18O and d13C from planktonic foraminifers, weight percent calcium carbonate, weight percent coarse fraction, magnetic susceptibility, and geomagnetic reversals. Sample spacing of the records ranges from 3 to 10 cm and is equivalent to an average time interval of 2000 to 6000 yr. Despite the fact that core recovery ranged between 100% and 105%, recovery gaps as large as 2.7 m occurred at nearly every break between advanced hydraulic piston cores. Approximately 12% of the late Neogene sequence was not recovered in each of the two holes drilled at Site 758. To circumvent the discontinuity introduced by the gaps, a composite depth section was constructed from multiple cores taken from offset holes at Site 758. The resulting composite depth section extends continuously from 0 to 116 mbsf, from the Holocene to the upper Miocene. A detailed chronostratigraphy is based on geomagnetic reversals which extend from the Brunhes Chron to Chron 6, and on d18O stages 1 through 105, which span from 0 to 2.5 Ma. The d18O record is dominated by a ~40-k.y. cycle in the late Pliocene and early Pleistocene, and is followed by a change to a ~100-k.y. cycle in the late Pleistocene. The mid-Pleistocene transition between these two modes of variability occurs between d18O stages 25 and 22 (between 860 and 800 Ka). Thirteen major volcanic ash horizons from the Indonesian arc are observed throughout the sedimentary section and are dated by their relative position within the geomagnetic reversals and the d18O chronostratigraphy. Since 5 Ma, there has been a long-term decline in weight percent CaCO3 and CaCO3 mass accumulation rates, and an associated rise in non-CaCO3 mass accumulation rates. We attribute these changes to a decrease in CaCO3 productivity and an increase in terrigenous sedimentation through enhanced riverine input. Such input may be linked to rapid tectonic uplift of the Himalayas and the Tibetan Plateau via mechanisms such as the intensification of the monsoonal rains, increased fluvial erosion, and regional glaciation. The long-term increase in percent coarse fraction since 5 Ma suggests a gradual increase in CaCO3 preservation. Higher frequency fluctuations in CaCO3 preservation are superimposed on the long-term trend and are related to climate fluctuations. The abrupt drop (-50%) in CaCO3 accumulation at 3.4 Ma signals a dramatic decrease in CaCO3 production that occurred over much of the Indian Ocean.

(Table 22-30) Revised composite depth scales of IODP Sites 320-U1331, U1332, U1333, U1334 and ODP Sites 199-1218, 199-1219, 199-1220

Integrated Ocean Drilling Program (IODP) Expedition 320 recovered high-quality paleomagnetic records with over 800 dated reversals and decimeter-scale cyclic sediments which provide an outstanding framework to inter-calibrate major fossil groups and refine magnetic polarity chrons for the early Miocene, the entire Oligocene and the late Eocene Epoch. In order to reconstruct the climate history of the Equatorial Pacific one of the major objectives of the Pacific Equatorial Age Transect (PEAT) is the compilation of a Cenozoic Megasplice which integrates all available bio-, chemo-, and magnetostratigraphic data including key records from Ocean Drilling Program (ODP) Leg 199. Here we present extended post-cruise refinements of the shipboard composite depth scales and composite records of IODP Expedition 320 Sites U1331, U1332, U1333, U1334 as well as ODP Leg 199 Sites 1218, 1219 and 1220. The revised composite records were used to perform a site-to-site correlation and integration of Leg 199 and Exp. 320 sites. Based on this decimeter scale correlation a high resolution integrated paleomagnetic and biostratigraphic framework for the Equatorial Pacific is established covering the time from 20 to 40 Ma. This unprecedented sedimentary compendium from the Equatorial Pacific will be the backbone for paleoceanographic reconstructions for the late Paleogene.

Chemistry and age determination of basement fluids from Juan de Fuca Ridge

The geochemical implications of thermally driven flow of seawater through oceanic crust on the mid-ocean ridge flank have been examined on a well-studied 80 km transect across the eastern flank of the Juan de Fuca Ridge at 48°N, using porewater and basement fluid samples obtained on ODP Leg 168. Fluid flow is recognised by near-basement reversals in porewater concentration gradients from altered values in the sediment section to seawater-like values in basaltic basement. In general, the basement fluids become more geochemically evolved with distance from the ridge and broadly follow basement temperature which ranges from not, vert, similar16° to 63°C. Although thermal effects of advective heat exchange are only seen within 20 km east of where basement is exposed near the ridge crest, chemical reactivity extends to all sites. Seawater passing through oceanic crust has reacted with basement rocks leading to increases in Ca2+ and decreases in alkalinity, Mg2+, Na+, K+, SO42- and delta18O. Sr isotope exchange between seawater and oceanic crust off axis is unequivocally demonstrated with endmember 87Sr/86Sr ~ 0.707. Evidence of more evolved fluids is seen at sites where rapid upwelling of fluids through sediments occurs. Chlorinities of the basement fluids are consistent with post-glacial seawater and thus a short residence time in the crust. Rates of lateral flow have been by estimated by modelling porewater sulphate gradients, using Cl as a glacial chronometer, and from radiocarbon dating of basal fluids. All three methods reveal fluid flow with 14C ages less than 10,000 yr and particle velocities of ~1-5 m/yr, in agreement with thermally constrained volumetric flow rates through a ~600 m thick permeable layer of ~10% porosity. Delta(element)/Delta(heat) extraction ratios are similar to values for ridge-crest hydrothermal systems.

Oligocene magnetostratigraphy for equatorial Pacific ODP Sites

An Oligocene magnetostratigraphy from ODP Sites 1218 and 1219 (Equatorial Pacific) has been obtained by measurements made on u-channel samples, augmented by about 221 discrete samples. U-channel samples were measured at 1 cm intervals and were stepwise demagnetized in alternating fields (AF) up to a maximum peak field of 80 mT. The magnetization directions were determined at 1 cm intervals by principal component analysis of demagnetization steps in the 20 to 60 mT peak field range. A similar treatment was carried out on the discrete samples, which confirmed the results obtained with u-channel measurements. Sites 1218 and 1219 were precisely correlated based on multisensor track, paleontological and shipboard magnetostratigraphic data; this correlation is substantiated by u-channel measurements. Although the magnetostratigraphy obtained from the u-channels is similar to the interpretation deduced from shipboard measurements based on blanket demagnetization at peak AF of 20 mT, the u-channel results are substantially more robust since many interpretative uncertainties are resolved by the stepwise demagnetization and higher stratigraphic resolution. The temporal resolution of u-channel-based magnetic stratigraphy in the Oligocene section of Sites 1218 and 1219 is better than 5 kyr, and it is therefore suitable for detection of brief polarity subchrons. However, in spite of the high resolution, we did not find any reversals corresponding to the numerous cryptochrons identified in this time span by Cande and Kent (1995, doi:10.1029/94JB03098).

Paleointensity record of Site 320-U1336

We present a high-resolution magnetostratigraphy and relative paleointensity (RPI) record derived from the upper 85 meters of IODP Site U1336, an equatorial Pacific early to middle Miocene succession recovered during Expedition 320/321. The magnetostratigraphy is well resolved with reversals typically located to within a few centimeters resulting in a well-constrained age model. The lowest normal polarity interval, from 85 to 74.87 meters, is interpreted as the upper part of Chron C6n (18.614-19.599 Ma). Another 33 magnetozones occur from 74.87 to 0.85 m, which are interpret to represent the continuous sequence of chrons from Chron C5Er (18.431-18.614 Ma) up to the top of Chron C5An.1n (12.014 Ma). We identify three new possible subchrons within Chron C5Cn.1n, Chron 5Bn.1r, and C5ABn. Sedimentation rates vary from about 7 to 15 m/Myr with a mean of about 10 m/Myr. We observe rapid, apparent changes in the sedimentation rate at geomagnetic reversals between ~16 and 19 Ma that indicate a calibration error in geomagnetic polarity timescale (ATNTS2004). The remanence is carried mainly by non-interacting particles of fine-grained magnetite, which have FORC distributions characteristic of biogenic magnetite. Given the relative homogeneity of the remanence carriers throughout the 85-m-thick succession and the quality with which the remanence is recorded, we have constructed a relative paleointensity (RPI) record that provides new insights into middle Miocene geomagnetic field behavior. The RPI record indicates a gradual decline in field strength between 18.5 Ma and 14.5 Ma, and indicates no discernible link between RPI and either chron duration or polarity state.

Strontium isotopes in pore waters of east equatorial Pacific sediments

Pore-water samples from the equatorial sedimentary bulge area show reversals in depth profiles of 87Sr/86Sr ratios at the sediment/basement interface. Results of this work support inferences made from previous pore-water data (from DSDP drilling in the area) that large-scale horizontal advection of seawater has occurred through the basement underlying the thick sedimentary sequence in this region. The area of apparent advection includes the eastern part of the equatorial high-productivity zone and part of the Guatemala Basin. We attempted to find links between the observed near-basement reversals in pore-water chemistry and sedimentary thickness, age, and topography of the area. Most of the sites that show horizontal advection have disturbed basement topography or outcrops within 10 to 20 km, suggesting that the cooling effects of outcrops may extend for at least 20 km horizontally. Heat-flow data from the area were compared to determine whether sites showing near-bottom chemistry reversals were consistent with areas of low conductive heat flow. This was generally true for the area of the sedimentary bulge and Guatemala Basin. Not enough pore-water data from the Nazca Plate were available to establish any reliable systematics. Because the high-productivity area is well-sealed from hydrothermal circulation, the missing heat must be lost by horizontal advective heat transport. From profiles of strontium isotopes and other elements that show departure from seawater values with increasing depth in the sediments, but return to seawater values near the basement, it appears that water flows relatively freely through much of the oceanic crust, even when sealed by considerable sedimentary cover.

Chemistry of interstitial waters and sediments at DSDP Leg 64 Holes

Studies of interstitial waters obtained from DSDP Leg 64 drill sites in the Gulf of California have revealed information both on early diagenetic processes in the sediments resulting from the breakdown of organic matter and on hydrothermal interactions between sediments and hot doleritic sill intrusions into the sediments. In all the sites drilled sulfate reduction occurred as a result of rapid sediment accumulation rates and of relatively high organic carbon contents; in most sites methane production occurred after sulfate depletion. Associated with this methane production are high values of alkalinity and high concentrations of dissolved ammonia, which causes ion exchange processes with the solid phases leading to intermediate maxima in Mg++, K+, Rb+, and Sr++(?). Though this phenomenon is common in Leg 64 drill sites, these concentration reversals had been noticed previously only in Site 262 (Timor Trough) and Site 440 (Japan Trench). Penetrating, hot dolerite sills have led to substantial hydrothermal alteration in sediments at sites drilled in the Guaymas Basin. Site 477 is an active hydrothermal system in which the pore-water chemistry typically shows depletions in sulfate and magnesium and large increases in lithium, potassium, rubidium, calcium, strontium, and chloride. Strontium isotope data also indicate large contributions of volcanic matter and basalt to the pore-water strontium concentrations. At Sites 478 and 481 dolerite sill intrusions have cooled to ambient temperatures but interstitial water concentrations of Li+, Rb+, Sr++ , and Cl- show the gradual decay of a hydrothermal signal that must have been similar to the interstitial water chemistry at Site 477 at the time of sill intrusion. Studies of oxygen isotopes of the interstitial waters at Site 481 indicate positive values of d18O (SMOW) as a result of high-temperature alteration reactions occurring in the sills and the surrounding sediments. A minimum in dissolved chloride at about 100-125 meters sub-bottom at Sites 478, 481, and particularly Site 479 records a possible paleosalinity signal, associated with an event that substantially lowered salinities in the inner parts of the Gulf of California during Quaternary time.

Fossil datums of Late Neogene sediments from the Southwest Pacific Ocean

The assumption of synchrony of first and last occurrences of fossil taxa can be tested using graphic correlation procedures which, by allowing measured stratigraphic sections to be compared on a common depth scale, make it possible to develop a correlation model which integrates information from a number of cores. The strategy of the test presented here is to use a graphic correlation model that is based on data from the Atlantic (Deep Sea Drilling Project (DSDP) sites 502, 516A) and north Pacific (DSDP site 577A) as a basis for determining to what extent fossil datums in the southwest Pacific are synchronous. First and last occurrences of Pliocene calcareous nannofossils and planktonic foraminifers have been compared in five DSDP cores from the southwest Pacific ocean (sites 586, 587, 588, 590A, and 592). All cores were recovered using hydraulic piston coring technology, which assures the best recovery and minimal disturbance. Most of these cores contain abundant, well-preserved foraminifers and nannofossils, as well as a partial record of many of the expected magnetic polarity reversals in this part of the section. To assure taxonomic consistency, all taxonomic identifications were made by the author. Graphic correlation of this data set suggests that several important biostratigraphic markers are highly diachronous. For example, this study confirms that Globorotalia truncatulinoides first occurs at approximately 2.4 Ma between 20° and 35° south latitude in the southwest Pacific, approximately 0.5 m.y. earlier than it is found elsewhere in the Atlantic and Pacific. Other datums, such as the last occurrence of Discoaster brouweri, are essentially synchronous. These findings suggest that biostratigraphic models based on the assumption of synchrony of first and last occurrences of fossil taxa may be incorrect. Biostratigraphic models created with the Graphic Correlation method offer an opportunity to examine the biogeographic dimensions of origination, migration, and extinction of planktonic taxa.

Stratigraphy of ODP Leg 117 sites

During the late early Miocene to early middle Miocene, the Owen Ridge was uplifted to a sufficient height as to be above the realm of turbidite deposition. Monsoonal-induced upwelling appears to have been initiated during the Miocene. On the Oman Margin, the effect of upwelling on the microplankton was established by the middle Miocene. However, the effects of upwelling on the Owen Ridge region were not realized until later, in the early late Miocene. A transition in the upwelling regime took place between the Pliocene and Pleistocene. While the Miocene and Pliocene sediments are dominated by the siliceous component, the Pleistocene sediments seem to be dominated by the calcareous component.

Paleomagnetic of ODP Holes 122-762B and 122-763A

The magnetostratigraphy of Neogene sediments from Holes 762B and 763A are presented in this paper. Hole 762B contains 17 reversals. All reversals above the base of the Gilbert are correlated with the magnetic polarity time scale (Haq et al., 1988). Hole 763A yields a record of about 20 reversals that can be correlated to the magnetic polarity time scale, documenting all reversals to the base of Chron 4A. Based on the correlation, the sediment accumulation vs. time for Holes 762B and 763A are determined. The age-depth curves obtained show a similar pattern of sedimentation rate since 6.8 Ma. The study also indicates a correlation between the fluctuations in the magnetic parameters (natural remanent magnetization intensity and susceptibility), the lithologic changes, and changes in iron content at both holes. This correlation suggests that the natural remanent magnetization intensity and susceptibility changes observed in Holes 762B and 763A are controlled by changes in depositional processes probably associated with climatic variations.

Magnetostratigraphy of DSDP Leg 73 in the South Atlantic

The main objective of DSDP Leg 73 was to obtain high-quality records of major paleooceanographic events in the South Atlantic. This was achieved by coring six sites on the African plate. The sediments thus recovered span the Cenozoic and five of the six sites proved ideally suited for magnetostratigraphic analysis. The results presented in this paper and elsewhere in this volume constitute the first opportunity to extend the direct correlation of the magnetostratigraphic and biostratigraphic time-scales into the Paleogene in deep-sea cores. The magnetostratigraphic analyses from DSDP Leg 73 sediments are presented in this paper. The correlation of the magnetostratigraphy to the magnetic polarity time-scale provides tight age-depth control for the five sites analyzed, allowing the accurate calculation of sediment accumulation rates. The data presented here represent a remarkable record of the fine-scale polarity history of the Earth's magnetic field. These data place constraints on the interpretation of smallscale marine magnetic anomalies which are modelled equally effectively by field intensity fluctuations as polarity reversals. At least some of the "tiny wiggles" correspond to very short polarity units in the magnetostratigraphic record. By assuming an axial geocentric dipole, the inclination of the time-averaged magnetic field recorded in the sediments can be used to calculate the paleolatitude at which the sediments were deposited. Combining the age and average inclination information available from the magnetostratigraphy, we present paleolatitudes versus time for the Leg 73 drill sites.

Age models and foraminifer isotopes of four ODP Leg 202 sites

We present benthic isotope stratigraphies for Sites 1236, 1237, 1239, and 1241 that span the late Miocene-Pliocene time interval from 6 to 2.4 Ma. Orbitally tuned timescales were generated for Sites 1237 and 1241 by correlating the high-frequency variations in gamma ray attenuation density, percent sand of the carbonate fraction, and benthic d13C to variations in Earth's orbital parameters. The astronomical timescales for Sites 1237 and 1241 are in agreement with the one from Atlantic Site 925/926 (Ocean Drilling Program Leg 154). The comparison of benthic d18O and d13C records from the east Pacific sites and Atlantic Site 925/926 revealed a surprising clarity of the "41-k.y. signal" in d13C records and a remarkably good correlation between their d13C records. This suggests that the late Miocene-Pliocene amplitudes of obliquity-related d13C cycles reflect a magnitude of global response often larger than that provided by obliquity-related d18O cycles. At Site 1237, the orbitally derived ages of Pliocene magnetic reversal boundaries between the base of Réunion and the top of Thvera confirm astronomical datings of the generally accepted ATNTS2004 timescale, except for the top of Kaena and the base of Sidufjall. Our astronomical age for the top of Kaena is about one obliquity cycle older. The base of Sidufjall appears to be about one precession cycle younger. The age models of Sites 1236 and 1239 were established by correlating their benthic d18O and d13C records directly to the orbitally tuned isotope record of Site 1241.

Strontium isotope data from DSDP Sites 32-305 and 62-463 and ODP Sites 113-689 and 113-690

Fluctuations in oxygen (d18O) and carbon (d13C) isotope values of benthic foraminiferal calcite from the tropical Pacific and Southern Oceans indicate rapid reversals in the dominant mode and direction of the thermohaline circulation during a 1 m.y. interval (71-70 Ma) in the Maastrichtian. At the onset of this change, benthic foraminiferal d18O values increased and were highest in low-latitude Pacific Ocean waters, whereas benthic and planktic foraminiferal d13C values decreased and benthic values were lowest in the Southern Ocean. Subsequently, benthic foraminiferal d18O values in the Indo-Pacific decreased, and benthic and planktic d13C values increased globally. These isotopic patterns suggest that cool intermediate-depth waters, derived from high-latitude regions, penetrated temporarily to the tropics. The low benthic d13C values at the Southern Ocean sites, however, suggest that these cool waters may have been derived from high northern rather than high southern latitudes. Correlation with eustatic sea-level curves suggests that sea-level change was the most likely mechanism to change the circulation and/or source(s) of intermediate-depth waters. We thus propose that oceanic circulation during the latest Cretaceous was vigorous and that competing sources of intermediate- and deep-water formation, linked to changes in climate and sea level, may have alternated in importance.