On Ocean Drilling Program Research Articles

On the duration of the Paleocene‐Eocene thermal maximum (PETM)

The Paleocene‐Eocene thermal maximum (PETM) is one of the best known examples of a transient climate perturbation, associated with a brief, but intense, interval of global warming and a massive perturbation of the global carbon cycle from injection of isotopically light carbon into the ocean‐atmosphere system. One key to quantifying the mass of carbon released, identifying the source(s), and understanding the ultimate fate of this carbon is to develop high‐resolution age models. Two independent strategies have been employed, cycle stratigraphy and analysis of extraterrestrial helium (HeET), both of which were first tested on Ocean Drilling Program (ODP) Site 690. These two methods are in agreement for the onset of the PETM and initial recovery, or the clay layer (“main body”), but seem to differ in the final recovery phase of the event above the clay layer, where the carbonate contents rise and carbon isotope values return toward background values. Here we present a state‐of‐the‐art age model for the PETM derived from a new orbital chronology developed with cycle stratigraphic records from sites drilled during ODP Leg 208 (Walvis Ridge, Southeastern Atlantic) integrated with published records from Site 690 (Weddell Sea, Southern Ocean, ODP Leg 113). During Leg 208, five Paleocene‐Eocene (P‐E) boundary sections (Sites 1262 to 1267) were recovered in multiple holes over a depth transect of more than 2200 m at the Walvis Ridge, yielding the first stratigraphically complete P‐E deep‐sea sequence with moderate to relatively high sedimentation rates (1 to 3 cm/ka, where “a” is years). A detailed chronology was developed with nondestructive X‐ray fluorescence (XRF) core scanning records on the scale of precession cycles, with a total duration of the PETM now estimated to be ∼170 ka. The revised cycle stratigraphic record confirms original estimates for the duration of the onset and initial recovery but suggests a new duration for the final recovery that is intermediate to the previous estimates by cycle stratigraphy and HeET.

Using neural networks to predict thermal conductivity from geophysical well logs

SUMMARY We present a new approach, based on neural networks, to predict the thermal conductivity of sedimentary rocks from a set of geophysical well logs. This method is calibrated on Ocean Drilling Program (ODP) data, which provide several thousands of conductivity measurements combined with five geophysical well logs (sonic, density, neutron porosity, resistivity and gamma ray). This data set is used to train multilayer perceptrons (MLP) and to find an empirical relationship between well logs (MLP inputs) and thermal conductivity (MLP output). Validation tests suggest that MLP provide better estimates of thermal conductivity (within ∼15 per cent confidence level) than classical linear models, and still give satisfying results with sets of only four well logs if neutron porosity is included. In two ODP sites (863B and 1109D), MLP’s predictions are compared to conventional ‘mixing’ methods. Although this latter technique gives reliable results provided that rocks description is precise enough, the MLP is more straightforward, does not need any extra parameter and makes predictions in good agreement with the experimental trends. This method will be useful in the estimation of heat flow from data acquired in scientific and industrial boreholes.

Seismic stratigraphy of Detroit Seamount, Hawaiian‐Emperor seamount chain: Post‐hot‐spot shield‐building volcanism and deposition of the Meiji drift

Detroit Seamount, one of the northernmost seamounts of the Hawaiian‐Emperor seamount chain, was formed at ca. 76 Ma. New seismic data suggest renewed volcanism as late as 25 m.y. after initial seamount formation. We use high‐resolution single‐channel seismic (SCS) data acquired over the summit of Detroit Seamount in 2001 on Ocean Drilling Program (ODP) Leg 197, supplemented by older SCS data acquired as part of the GLORIA mapping program of the U.S. Geological Survey, to characterize the seismic stratigraphy of Detroit Seamount. Volcanic edifices occur on the summit of the seamount and are older than the oldest beds of the Meiji drift (early Oligocene: ca. 34 Ma). On the basis of ash layers in ODP drill holes, we suggest the edifices were active throughout much of the Eocene (ca. 52–34 Ma), with activity possibly extending into the early Oligocene (<34 Ma). Hence the age difference between the shield‐building lavas and the postshield cones on Detroit is far greater than the shield/postshield age differences observed on the Hawaiian Islands, suggesting that renewed volcanic activity and tectonic collapse may be possible on any of the Hawaiian Islands. We confirm earlier assertions that the thick sediment cap, Oligocene and younger in age, was deposited by an ocean‐bottom current with a southeastward flow direction, along the northeast facing flank of the Emperor Seamount chain. This sediment cap, the Meiji drift, was deposited by a lower‐velocity current than many other sediment drifts. A low‐angle normal fault, dipping ∼19°, suggests topographic collapse of Detroit seamount sometime during the Eocene or late Cretaceous.

Co-existence of gas hydrate, free gas, and brine within the regional gas hydrate stability zone at Hydrate Ridge (Oregon margin): evidence from prolonged degassing of a pressurized core

Standard scientific operations on Ocean Drilling Program (ODP) Leg 204 documented a horizon of massive gas hydrate and highly saline pore water ∼0–20 m below the southern summit of Hydrate Ridge offshore Oregon. The sediment zone lies near active seafloor gas venting, raising the possibility that free gas co-exists with gas hydrate in shallow subsurface layers where pore waters have become too saline to precipitate additional gas hydrate. Here we discuss a unique experiment that addresses this important concept. A 1-m-long pressurized core was retrieved from ∼14 m below sea floor at Site 1249 and slowly degassed at ∼0 °C in the laboratory over ∼178 h to determine in situ salinity and gas concentrations in the interval of massive gas hydrate. The core released ∼95 l of gas (predominantly methane), by far the greatest gas volume ever measured for a 1 m core at ambient shipboard pressure and temperature conditions. Geochemical mass-balance calculations and the pressure of initial gas release (4.2 MPa) both imply that pore waters had an in situ salinity approaching or exceeding 105 g kg −1, the approximate salinity required for a gas hydrate–free gas–brine system. Relatively high concentrations of propane and higher hydrocarbon gases at the start of core degassing also suggest the presence of in situ free gas. Gas hydrate, free gas and brine likely co-exist in shallow sediment of Hydrate Ridge. Near-seafloor brines, produced when rapid gas hydrate crystallization extracts large quantities of water, impact the distribution and cycling of gas and gas hydrate in this region and perhaps elsewhere.

Co-existence of gas hydrate, free gas, and brine within the regional gas hydrate stability zone at Hydrate Ridge (Oregon margin): evidence from prolonged degassing of a pressurized core

Standard scientific operations on Ocean Drilling Program (ODP) Leg 204 documented a horizon of massive gas hydrate and highly saline pore water ∼0–20 m below the southern summit of Hydrate Ridge offshore Oregon. The sediment zone lies near active seafloor gas venting, raising the possibility that free gas co-exists with gas hydrate in shallow subsurface layers where pore waters have become too saline to precipitate additional gas hydrate. Here we discuss a unique experiment that addresses this important concept. A 1-m-long pressurized core was retrieved from ∼14 m below sea floor at Site 1249 and slowly degassed at ∼0 °C in the laboratory over ∼178 h to determine in situ salinity and gas concentrations in the interval of massive gas hydrate. The core released ∼95 l of gas (predominantly methane), by far the greatest gas volume ever measured for a 1 m core at ambient shipboard pressure and temperature conditions. Geochemical mass-balance calculations and the pressure of initial gas release (4.2 MPa) both imply that pore waters had an in situ salinity approaching or exceeding 105 g kg−1, the approximate salinity required for a gas hydrate–free gas–brine system. Relatively high concentrations of propane and higher hydrocarbon gases at the start of core degassing also suggest the presence of in situ free gas. Gas hydrate, free gas and brine likely co-exist in shallow sediment of Hydrate Ridge. Near-seafloor brines, produced when rapid gas hydrate crystallization extracts large quantities of water, impact the distribution and cycling of gas and gas hydrate in this region and perhaps elsewhere.

Nitrogen geochemistry of subducting sediments: New results from the Izu‐Bonin‐Mariana margin and insights regarding global nitrogen subduction

Toward understanding of the subduction mass balance in the Izu‐Bonin‐Mariana (IBM) convergent margin, we present an inventory of N and C concentrations and isotopic compositions in sediments obtained on Ocean Drilling Program (ODP) Legs 129 and 185. Samples from Sites 1149, 800, 801, and 802 contain 5 to 661 ppm total N (organic, inorganic combined) with δ15NAir of −0.2 to +8.2‰ (all δ15N values <+2.5‰ from Site 800). At Site 1149, N content is higher in clay‐rich layers and lower in chert and carbonate layers, and δ15N shows a distinct down‐section decrease from 0 to 120 mbsf (near +8.0 at shallow levels to near +4.0‰). Reduced‐C concentration ranges from 0.02 to 0.5 wt.%, with δ13CVPDB of −28.1 to −21.7‰. The down‐section decreases in δ15N and N concentration (and variations in concentrations and δ13C of reduced C, and Creduced/N) at Site 1149 could help reconcile differences between δ15N values of modern deep‐sea sediments from near the sediment‐water interface and values for forearc metasedimentary rocks. At Site 1149, negative shifts in δ15N, from marine organic values (up to ∼+8‰) toward lower values approaching those for the metasedimentary rocks (+1 to +3‰), are most likely caused by complex diagenetic processes, conceivably with minor effects of changes in productivity and differing proportions of marine and terrestrial organic matter. However, the forearc metamorphic suites (e.g., Franciscan Complex) are known to have been deposited nearer continents, and their lower δ15N at least partly reflects larger proportions of lower‐δ15N terrestrial organic matter. Subduction at the Izu‐Bonin (IB) margin, of a sediment section like that at Site 1149, would deliver an approximate annual subduction flux of 2.5 × 106 g of N and 1.4 × 107 g of reduced C per linear kilometer of trench, with average δ15N of +5.0‰ and δ13C of −24‰. Incorporating the larger C flux of 9.2 × 108 g/yr/linear‐km in carbonate‐rich layers of 1149B (average δ13C = +2.3‰) provides a total C flux of 9.3 × 108 g/yr/linear‐km (δ13C = +1.9‰). Once subducted, sediments are shifted to higher δ15N by N loss during devolatilization, with magnitudes of the shifts depending on the thermal evolution of the margin.

The last occurrence of Proboscia curvirostris in the North Atlantic marine isotope stages 9-8

Well-preserved diatoms are present in high sedimentation rate Pleistocene cores retrieved on Ocean Drilling Program (ODP) Legs 151, 152, 162 and IMAGES cruises of R/V Marion Dufresne from the North Atlantic. Investigation of the stratigraphic occurrence of diatom species shows that the youngest diatom event observed in the area is the last occurrence (LO) of Proboscia curvirostris (Jousé) Jordan and Priddle. P. curvirostris is a robust species that can easily be identified in the sediments, and therefore can be a practical biostratigraphic tool. We have mapped its areal distribution, and found that it stretches from 40°N to 80°N in the North Atlantic. Further, we have correlated the LO P. curvirostris to the oxygen isotope records of six cores to refine the age of this biostratigraphic event. The extinction of P. curvirostris is latitudinally diachronous through Marine Isotope Stages (MIS) 9 to 8 within the North Atlantic. This is closely related to the paleoceanography of the area. P. curvirostris first disappeared within interglacial MIS 9 (324 ka) from the northern areas that are most sensitive to climatic forcing, like the East Greenland current and the sea–ice margin. It survived in mid-North Atlantic until the conditions of the MIS 8 (glaciation) became too severe (260 ka). In the North Pacific at ODP Site 883 the LO P. curvirostris falls within MIS 8. The observed overlap in age between the North Atlantic and the North Pacific strongly suggests that the extinction of P. curvirostris is synchronous between these oceans.

Inferred pore pressures at the Costa Rica subduction zone: implications for dewatering processes

Drilling on Ocean Drilling Program (ODP) Leg 170, offshore Costa Rica indicates that the entire incoming sedimentary section is underthrust. Thus, observed changes in the thickness of underthrust sediments as they are progressively buried beneath the margin wedge provide a direct measure of the rate and magnitude of sediment dewatering. Laboratory consolidation tests indicate that in situ excess pore-fluid pressures within the underthrust section range from 1.3 MPa at the top of the section to 3.1 MPa near the base. The inferred pore pressure profile implies that fluids escape the uppermost sediments most rapidly, whereas the basal sediments remain essentially undrained. This interpretation suggests that the sedimentary and underlying ocean crustal hydrologic systems are decoupled. We use a simple model of fluid flow to demonstrate that dewatering of the underthrust sediments can occur via lateral flow only if sediment permeability is strongly anisotropic, or if flow is focused along permeable stratigraphic layers. If significant dewatering occurs by vertical fluid flow, it must occur within closely spaced, high-permeability conduits.

Research in Great Australian Bight yields exciting early results

Confirmation that Pleistocene mound features underlying the outer shelf and shelf edge are bryozoan buildups—effectively coolwater reefs—was one of several exciting discoveries during recent research earlier this year on the Great Australian Bight. Also found on Ocean Drilling Program (ODP) Leg 182 was startling evidence of high salinity fluid circulation within the shelf substrate, producing extremely high methane and hydrogen sulfide gas levels and accelerated diagenesis.In addition, an expanded middle Eocene to early Oligocene record of Southern Ocean initiation was recovered by the shipboard scientific party, aboard the JOIDES Resolution. The group also calculated unexpectedly high rates of shelf edge cool‐water carbonate accumulation (up to 40 cm/kyr).

Velocity structure in upper ocean crust at Hole 504B from vertical seismic profiles

Hole 504B provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust, providing a baseline for comparison with seismic measurements elsewhere. We determine P and S velocities from vertical seismic profiles (VSPs) obtained on Ocean Drilling Program (ODP) Legs 111 and 148. Four issues are considered: the location of the seismic layer 2/3 boundary, P to S wave conversion by scattering, transverse isotropy, and Poisson's ratio as an indicator of lithology, porosity, and structure. (1) In the P velocity profile, the change in slope marking the layer 2/3 boundary coincides with the top of the sheeted dike unit. Seismic layer 2 is composed of the extrusives and the lithologic transition zone, the layer in which flows and dikes interfinger. (2) Even in these normal incident VSPs, several second arrivals with velocities indicative of vertically polarized shear energy are observed. P to S wave conversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on angle of incidence as predicted by plane boundary transmission coefficient analysis. (3) Vertical velocities determined from the VSP differ by <10% from horizontal velocities obtained from the oblique seismic experiment (OSE) on Deep Sea Drilling Project (DSDP) Leg 92. The P wave velocity structure is determined by small and intermediate (<1 cm) pore structure with no measurable anisotropy. The large‐scale, well‐oriented vertical fractures, which are formed tectonically, do not have a detectable effect on compressional wave velocities. (4) High Poisson's ratio in the upper 300 m of basement coincides with an extrusive layer composed of pillows and thin flows. Low Poisson's ratio at 850–1150 m below seafloor (mbsf) coincides with the downhole decrease in bulk porosity caused by the transition from extrusives to dikes. Relatively large‐aspect ratio cracks are required to produce such low values of Poisson's ratio. The cracks were likely created by hydraulic fracturing when hot dikes encountered low‐temperature seawater.

Downhole Logging as a Paeoceanographic Tool on Ocean Drilling Program Leg 138: Interface Between High‐Resolution Stratigraphy and Regional Syntheses

On Ocean Drilling Program (ODP) Leg 138, standard shipboard procedures were modified to allow for the real‐time monitoring of several laboratory core‐scanning systems that provide centimeter‐scale measurements of saturated bulk density, magnetic susceptibility and digital color reflectance. These continuous, high‐resolution data sets were used to ensure the proper offset of multiple holes and to splice together complete sedimentary sections. Typically, the spliced, continuous sediment sections were found to be about 10% longer than the section drilled, as measured by the length of the drill string. While the source of this elongation is not yet fully understood, it must be compensated for in order to property determine sediment fluxes and mass accumulation rates. Downhole logging, in conjunction with inverse correlation techniques provided a means to determine where the distortion occurred and to correct back to true in situ depths.Downhole logging also provides a means, through the generation of synthetic seismograms, of precisely relating the paleoceanographic events found in the core record to the high‐resolution seismic record. Once correlated to the seismic record, the spatial and temporal extent of paleoceanographic events can be traced well beyond the borehole. Most seismic events in the equatorial Pacific are related to rapid changes in carbonate content that, in turn, are related to both productivity events (often expressed as monospecific laminated diatom oozes) and times of enhanced dissolution. While many of these events may have oceanwide extent, others, like the absence of carbonate in the late‐Miocene to Recent in the Guatemala Basin have been shown to be regional and confined to only the deeper portions of the Guatemala Basin. As we identify and trace specific paleoceanographic events in the seismic record, we can begin to explore the response of the ocean through gradients of latitude, productivity, and depth.

Passive, off‐axis convection through the southern flank of the Costa Rica Rift

Pore fluids are passively convecting through young oceanic sediments and crust around Deep Sea Drilling Project (DSDP) site 504 on the southern flank of the Costa Rica Rift, as inferred from a variety of geological, geochemical, and geothermal observations. The presence of a fluid circulation system is supported by new data collected on Ocean Drilling Program (ODP) leg 111 and a predrilling survey cruise over the heavily sedimented, 5.9 Ma site; during the latter, elongated heat flow anomalies were mapped subparallel to structural strike, with individual measurements of twice the regional mean value, and strong lateral and vertical geochemical gradients were detected in pore waters squeezed from sediment cores. Also, there is a strong correlation between heat flow, bathymetry, sediment thickness, and inferred fluid velocities up through the sediments. On an earlier DSDP leg, an 8‐bar underpressure was measured in the upper 200 m of basement beneath thick sediment cover. Although the forces which drive passive circulation are not well understood, it has generally been thought that the length scale of heat flow variations provides a good indication of the depth of hydrothermal circulation within the oceanic crust. This assumption was based on analytical and numerical analyses of relatively simple porous media models. Deep crustal convection had been inferred near site 504 based on the geometry of surface heat flow anomalies with a wavelength of 4–7 km but appears to be precluded by low crustal permeability, as measured in DSDP hole 504B. The widely varied geothermal and hydrogeological observations near site 504 are readily explained by a model which combines (1) basement relief, (2) irregular sediment drape, (3) largely conductive heat transfer through the sediments overlying the crust, and (4) thermal and geochemical homogenization of pore fluids at the sediment/basement interface, which results from (5) topographically induced, passive hydrothermal circulation with large aspect ratio, convection cells. This convection involves mainly the permeable, upper 200–300 m of crust; the deeper crust is not involved. This convection is induced through a combination of buoyancy fluxes, due to heating from below, and topographic variations on the seafloor and at the basement‐sediment interface. This model was designed to incorporate data from both near the sediment surface and deep within boreholes; it is successful in duplicating numerous field observations.

Transient fluid flow through the toe of the Barbados Accretionary Complex: Constraints from ocean Drilling Program Leg 110 heat row studies and simple models

Thirty‐four sediment and mud line temperatures were collected from six drill holes on Ocean Drilling Program (ODP) leg 110 near the toe of the Barbados accretionary complex. When combined with thermal conductivity measurements from sediment cores and results from earlier surveys, these data delineate the complicated thermal structure on the edge of this convergent margin. Heat flow values at the seafloor of 92–192 mW m−2 are 80–300% higher than those predicted by standard heat flow versus age models for oceanic crust but are compatible with earlier seafloor measurements made in this area at the same latitude. Heat flow tends to decrease downhole at four sites, suggesting the presence of heat sources within the sediments. These results are consistent with the flow of warm fluids through the complex along high permeability conduits, including thrust faults, the major décollement zone, and sandy intervals. Simple calculations suggest that this fluid flow is transient, occurring on time scales of tens to tens of thousands of years. Fluid flow velocities along the décollement zone, estimated with a simple thermal model, are about 10−7 m s−1, 100 times faster than predicted by numerical, steady state analyses. The estimated maximum sediment permeability within the décollement zone, which was based on the above fluid flow velocity, is about 10−12 m2, also 100 times higher than that calculated numerically. High heat flow in the vicinity of 15°20′N and not elsewhere along the deformation front suggests that the leg 110 drill sites may be situated over a prism‐water discharge zone, with dewatering more active here than elsewhere along the accretionary complex.

The Hydrogeology of the Costa Rica Rift as Constrained by Results of Ocean Drilling Program Downhole Experiments: ABSTRACT

Pore fluids are passively convecting through young oceanic sediments and crust around Deep Sea Drilling Project (DSDP) site 504 on the southern flank of the Costa Rica Rift, as inferred from a variety of geological, geochemical, and geothermal observations. The presence of a fluid circulation system is supported by new data collected on Ocean Drilling Program (ODP) Leg 111 and a predrilling survey cruise over the heavily sedimented, 5.9 Ma site; during the latter, elongated heat flow anomalies were mapped subparallel to structural strike, with individual measurements of twice the regional mean value, and large lateral and vertical geochemical gradients were detected in pore waters squeezed from sediment cores. Also, there is a strong correlation between heat flow, bathymetry, sediment thickness, and inferred fluid velocities up through the sediments. On an earlier DSDP leg, an 8-bar underpressure was measured in the upper 200 m of basement beneath thick sediment cover. The widely varied geothermal and hydrogeological observations near site 504 are readily explained by a model that combines (1) basement relief, (2) irregular sediment drape, (3) largely conductive heat transfer through the sediments overlying the crust, and (4) thermal and geochemical homogenization of pore fluids at the sediment/basement interface, whichmore » results from (5) topographically induced, passive hydrothermal circulation with large aspect ratio, convection cells. This convection involves mainly the permeable, upper 200-300 m of crust; the deeper crust is not involved. This convection is induced through a combination of buoyancy fluxes, owing to heating from below, and topographic variations on the seafloor and at the basement-sediment interface.« less