SITE 1039

We successfully obtained a complete and undeformed stratigraphic section of the incoming sediments on the Cocos Plate at Site 1039, which will be used for comparison with sites on the Costa Rica margin. Three sedimentary units and one intrusive unit were recognized at Site 1039 (Fig. 6). Unit U1 consists of dark olive green diatomaceous ooze with ash layers. Subunit U1A (Holocene to Pleistocene, 0-5 mbsf) is distinguished by abundant graded sand layers interpreted as turbidites, and grades downward into Subunit U1B (Pleistocene, 5-84 mbsf), in which graded sand layers are sparse to absent. Below a sharp contact at a thick gray ash layer, Unit U2 is distinguished by a sharp decrease in biogenic sediment. Subunit U2A (early Pleistocene to late Pliocene, 84-133 mbsf) consists of dark olive-green silty clay, and grades downward into Subunit U2B (late to early Pliocene, 134-152 mbsf). Subunit U2B is a dark olive-green silty clay interbedded with light olive-green calcareous clay; ash layers are common throughout Unit U2.

Unit U3 records a dramatic increase in biogenic sedimentation, changing sharply from the nearly barren clays of Unit U2 to calcareous and siliceous oozes. Subunit U3A (early Pliocene to late Miocene, 152-180 mbsf) is ivory-colored siliceous nannofossil ooze interbedded with calcareous clay; ash layers are sparse. Subunit U3B (late to middle Miocene, 180-280 mbsf) consists of ivory to light green and mottled siliceous nannofossil ooze with minor ash layers. Subunit U3B grades downward into Subunit U3C (middle Miocene, 280 to 378 mbsf in Hole B and 363 to 445 mbsf in Hole C), and consists of a mottled ivory-colored calcareous ooze, interbedded in the lower part with siliceous ooze and matrix-supported breccia of calcareous ooze clasts. Preliminary shipboard X-ray fluorescence (XRF) results show that the basal oozes of Subunit U3C are metalliferous, being enriched in nickel, copper, and zinc.

Unit U4 was encountered at depths of 378-381 mbsf in Hole 1039B and 423-445 mbsf in Hole 1039C. It consists of fine- to medium-grained glassy pyroxene gabbro with plagioclase glomerocrysts. Multiple chill zones were recovered within the 22 m of gabbro. The reason for the difference in the depth to the top of the intrusion in Holes 1039B and C is not yet known.

A complete or nearly complete late Pleistocene through early middle Miocene (approximately 16.4 Ma) section cored at Site 1039 is recognized in the combined calcareous nannofossil, diatom, and planktonic foraminifer record. An age-depth model calculated from the combined last and/or first occurrence datums of index microfossils yields average rates of about 46 m/m.y. (0-120 mbsf) for the Pleistocene in the upper part of the section, 6 m/m.y. for the upper Miocene and Pliocene interval (120-200 mbsf), and 47 m/my for the middle Miocene interval (200-448 mbsf). Calcareous nannofossil, diatom, and planktonic foraminifer biostratigraphic zones are easily resolved in the intervals of higher interval/age rates. However, in the upper Miocene and Pliocene section, a few of the late Miocene and Pliocene zones apparently cannot be resolved due to low rates and widely spaced (9.6 m) sampling intervals. All microfossils generally exhibit good preservation and sufficient abundances for reliable biostratigraphic analysis. The distribution of diatoms with depth shows clear relationships with the boundaries between Units 1 and 2 and Units 2 and 3.

Demagnetization of natural remanence in both split cores and discrete samples was successful in defining portions of magnetostratigraphy at Site 1039. The uppermost sequence (0-132 mbsf) has reversals ranging in age from the Blake (0.105 Ma) and Jamaica (0.200 Ma) events to the termination of Chron C2An.3n (3.33 Ma). The lower sequence of well-defined reversals (302-387 mbsf) spans Chrons C5Ar.1n (12.678 Ma) to C5Cn.2n (16.488 Ma). A large interval of predominantly normal polarity, coinciding with a zone of increased intensity of remanence (120 180 mbsf) occurs within sediment ranging in age from 3.5 to 8.2 Ma. Calculated depth-age rates are 105 m/m.y. for Subunit U1A, and 30 m/m.y. for the interval 11 to 133 mbsf. Within Unit U3, rates decrease from 20 m/m.y. (305-370 mbsf) to 5 m/m.y. (370-380 mbsf) downhole.

The geochemical objectives at Site 1039, similar to those for lithostratigraphy and biostratigraphy, were to obtain a reference section for the distribution of chemical components in the incoming sedimentary section. Here again our objectives were met. Concentrations of methane were low throughout Site 1039. Only in the interval between 25 and 110 mbsf did methane contents exceed the background concentration of 4-8 ppm. In this sequence, slightly enhanced methane concentrations ranging from 11 to 110 ppm were detected. The strongly reducing conditions needed for bacterial methane generation evidently never were achieved in the sediments at Site 1039, and sulfate concentrations remain high throughout the recovered sedimentary sequence.

The carbonate content varies from 0.5 to 87.8% CaCO3, assuming that all of the carbonates are present as pure calcite. Low CaCO3 concentrations (0.3 to 4.6%) are found in the upper part of the sediment column (Unit U1). At depths between 113 and 152 mbsf, alternating high and low carbonate contents were measured (Unit U2). Below this zone, CaCO3 increases rapidly (up to 87.8%) and remains high throughout the whole sequence of Unit U3.

Organic carbon contents range from 0.1% to 1.89%. The highest concentrations occur in the turbidite sequence, suggesting a downslope transport of sediments enriched in organic compounds. The pelagic, calcareous sediments are characterized by low total organic carbon (TOC) concentrations ranging from 0.1% to 0.6%. Sulfur concentrations-depth profile parallels that of organic carbon contents. Only the hemipelagic sediments contain significant amounts of sulfur, whereas in the sequence dominated by carbonates no sulfur was detected.

In the organic-matter-rich hemipelagic lithologic Unit U1, with TOC content of 0.6-1.9 wt%, and also Unit U2 with 0.4-0.6 wt% TOC, bacterially mediated organic matter diagenesis, dissolution of diatoms, and volcanic ash alteration reactions control the chemistry of the pore waters. These reactions affect the physical, magnetic, and chemical properties of the sediments via carbonate, sulfide(s), and probably magnetite precipitation, as well as ion exchange reactions between ammonia, K, and Na in the clay minerals. In the pelagic calcareous Unit U3, where the TOC content is mostly <0.3% and where dissolved sulfate concentration is close to that of seawater, the chemical signatures of the pore waters are controlled by volcanic ash alteration, diatom dissolution, and to a lesser extent carbonate recrystallization. Dissolved silica concentrations are at equilibrium with amorphous silica (opal A) solubility, and Ca concentrations are twice as high as in seawater. The concentration-depth profiles of Ca, Mg, Si, and Cl faithfully reflect the lithologic units and even the subunits at this site. In the lowermost variegated, somewhat metalliferous sedimentary Subunit U3C, diffusion of dissolved metals, such as Mn, influence the sediment color and its distribution. Except for a fluid conduit at 95-130 mbsf, the pore-water chemical-depth profiles do not support vertical or horizontal fluid advection within the sediment section. In the basal section, however, the pore-water concentration profiles of Cl, K, Ca, Mg, and Si indicate seawater flow in the upper oceanic basement. This hydrologic regime may be responsible for the unusually low heat flow at this site.

Physical properties objectives included obtaining complete distributions of in-situ density and porosity, magnetic susceptibility, thermal conductivity, and changes in original lithology, consolidation state, and diagenesis in the sedimentary section. These objectives were met with abundant core and downhole measurements at Site 1039. Lithostratigraphic units are mapped with the color spectrometer, mainly because of the drastic differences in color between the diatomaceous ooze and the nannofossil ooze, as well as the ash-rich layers. Magnetic susceptibility also mapped the transitions from hemipelagic to pelagic carbonate intervals, as well as intervals with abundant ash. Porosities are high and show a general decrease downsection from about 75% at the seafloor to 65-70% at the base of Unit U3. A marked drop in porosity and increase in bulk density and thermal conductivity occur at about 180 mbsf, marking the region where the sediments become dominated by calcareous oozes. P-wave velocities are exceptionally low, attaining maximum values of 1650 m/s at 350 mbsf.

In situ density and porosity measurements were collected by the Compensated Density Neutron tool (CDN) as part of the LWD downhole assembly. Downhole measurements correlate closely with core measurements, although downhole densities are a little higher than the core-based values, particularly for carbonate Subunit U3B below 200 mbsf. Major features occur at slightly greater depths (a few meters) on the downhole profiles than on the core-based profiles. This effect may be caused by a small angle deviation of Hole 1039D relative to Hole 1039B. Below 395 mbsf, exceptionally high density values (up to 2.8 g/cm3) are consistent with the gabbro intrusions in this interval.

Porosities calculated directly from the downhole neutron log fluctuate widely throughout the logged interval. The filtered neutron porosity profile correlates well with porosity measurements on core specimens. The large decrease in porosity at 185-195 mbsf corresponds with a marked increase in downhole bulk density at 188 mbsf. Cyclical changes in porosity occur over the interval 195-285 mbsf, with rapid downward decreases between more gradual downward increases.

In situ resistivity measurements were collected using the Compensated Dual Resistivity tool (CDR) of the LWD assembly in Hole 1039D. The deep and shallow resistivity logs show very similar trends and amplitudes, indicating excellent hole conditions. Computed formation factors based on constant pore-water salinity of 0.035 range from 1.7 to 3 and show the same trends as the resistivity variations. Resistivity trends largely mimic the bulk-density log. They vary between 0.6 to 0.8 ohm-m over the sedimentary interval. The gabbro intrusion at 395-398 mbsf shows exceptionally high resistivities (up to 20 ohm-m).

In situ natural gamma-ray measurements were collected by CDR, and the photoelectric effect (PEF) was measured by CDN in Hole 1039D. The profile of the total spectral gamma ray shows a similar trend to the magnetic susceptibility profile from cores throughout the section. Uranium yields a distinctive decrease at 127 mbsf. Most of the strong peaks of the gamma-ray profile can be correlated to ash layers. The PEF profile shows a similar trend to that of carbonate and colorimetry. Between 138-188 mbsf, PEF increases from 2 to 4, which coincides with the increasing content of carbonate in lithologic Unit U3A. In the intervals between 210 and 227 mbsf and 267 and 292 mbsf, PEF decreases to values as low as 1.5, which is best explained by an increased abundance of biogenic opal.

To better understand the anomalously low heat flows reported from this region, one drilling objective was to determine heat-flow measurements though the sedimentary section. Our efforts were successful. Temperature measurements at Site 1039, using the Adara and Davis-Villinger Temperature Probe (DVTP) tools, reveal a geothermal gradient of 9.8 K/km. Measured thermal conductivity of 0.85 W/(m*K) gives a heat flow of 8.4 mW/m2, which is very close to values obtained from surface heat-flow measurements. This very low heat flow suggests that processes act to refrigerate the uppermost igneous crust in this location.

In summary, all lithostratigraphic and biostratigraphic, as well as all geochemical and physical properties objectives were reached in the sedimentary section of Site 1039. We were not completely successful with the basement section because the basal units were gabbroic intrusive rocks, which did not represent true basement. A seismic reflection model of the intrusives suggests a thickness of about 70 m. We concluded that, while useful, finding extrusive basalt did not justify the potential time expenditure at this site. Lack of a 50-m hole into true basement limited our ability to understand the recycling of deeper crustal rocks. An additional goal of obtaining an unequivocal maximum age of the downgoing crust was not achieved for the same reasons. Future drilling in this area should plan to obtain 100-200 m of oceanic basement for geochemical recycling studies.

To 170 Results - Site 1040

To 170 Table of Contents

Publications Homepage

ODP Homepage