Paleomagnetic and rock magnetic investigations aboard the JOIDES Resolution during Leg 194 included routine measurements of natural remanent magnetization (NRM). Whole cores were measured before and after alternating-field (AF) demagnetization to 30 mT. Low-field magnetic susceptibility (k) measurements were made with the MST. NRM and a limited set of rock magnetic observations were made with discrete samples. A nonmagnetic APC core-barrel assembly was used for alternate cores in selected holes and the magnetic overprints in core recovered with this assembly were compared with those obtained with standard assemblies.
The remanence measurements made during Leg 194 were conducted using the shipboard pass-through cryogenic magnetometer. The standard ODP magnetic coordinate system was used (+x = vertical upward from the split surface of archive halves, +y = left along split surface when looking upcore, and +z = downcore; see Fig. F9).
The output of the 2G magnetometer is given in magnetic moment in electromagnetic units with a background noise of ~10-7 gauss cm (SI 10-10 A/m2). This gives an approximate limit for the intensity of magnetization that is required to reliably measure standard half- or whole-core samples, being ~10-6 A/m, or 0.001 mA/m. However, it should be noted that the magnetization of core liners can exceed this intensity. For standard discrete samples, the weakest measurable intensity is more than an order of magnitude greater than the long cores because of the smaller volume of material in the pickup coils.
NRM was initially measured on all archive-half sections. However, after comparisons between half-core and whole-core measurements revealed that core splitting sometimes induced significant overprints, whole cores were measured for the remainder of the leg. Core flow through the laboratory was modified with the long-core measurement being made before the MST run, which sped up processing significantly.
Long-core measurements were made at 5-cm intervals with 15-cm-long headers and trailers. Measurements at core and section ends and within intervals of drilling-related core deformation were removed during data processing. AF demagnetization was applied to cores at 30 mT and when time permitted, a 10-mT step was also measured.
Using the extrusion tool, discrete samples were collected from the working halves in standard 8-cm3 plastic cubes. The discrete samples were analyzed with the shipboard pass-through cryogenic magnetometer using a tray designed for measuring six discrete samples at a time. To maintain ODP convention, the cube face with the arrow was placed downward on the tray. Samples were demagnetized by AF using the in-line demagnetizer installed on the pass-through cryogenic magnetometer. They were given anhysteretic remanent magnetizations and isothermal remanent magnetizations, and these were demagnetized to establish the magnetic characteristics of the recovered core. Magnetic susceptibility was measured for each whole-core section as part of the MST analysis (see "Core Physical Properties"). Susceptibility was measured on the MST using a Bartington MS2 meter coupled with a MS2C sensor coil with a diameter of 88 cm operating at 0.565 kHz. The sensor was set on SI units, and the data were stored in the Janus database in raw meter units. The sensor coil is sensitive over an interval of ~4 cm (half-power width of the response curve), and the width of the sensing region corresponds to a volume of 166 cm3 of cored material. To convert to true SI volume susceptibilities, these values should be multiplied by 10-5 and then multiplied by a correction factor to take into account the volume of material that passed through the susceptibility coils. Except for measurements near the end of each section, the correction factor for a standard full ODP core is ~0.66 (= 1/1.5). The magnetic effect induced at the end of each core section was not corrected.
During APC coring, full orientation was achieved with the Tensor multishot tool rigidly mounted onto a nonmagnetic sinker bar. The Tensor tool consists of three mutually perpendicular magnetic-field sensors and two perpendicular gravity sensors. The information from both sets of sensors allows the azimuth and dip of the hole to be measured, as well as the azimuth of the APC core double orientation line.
Where magnetic cleaning successfully isolated the characteristic remanent magnetization, paleomagnetic inclinations were used to define magnetic polarity zones. On some occasions, it was possible to recover a satisfactory magnetic stratigraphy even when the inclination was of a single polarity because of a persistent overprint. On such occasions, the intensity and associated minor differences in inclination were used to extract the magnetostratigraphic signal. To recover the magnetostratigraphy, the persistent bias of the z-component was removed so that the alternating magnetic polarities required to define the magnetostratigraphic signal were made clearer. Interpretations of the magnetic polarity stratigraphy, with constraints from the biostratigraphic data, are presented in each of the site chapters. The revised timescale of Cande and Kent, as presented in Berggren et al. (1995a, 1995b), was used as a reference for the ages of Cenozoic polarity chrons.
Age models, or continuous age-depth relationships, were constructed for Leg 194 sites to provide age estimates at any depth interval, particularly those of lithologic and seismic sequence boundaries. Age models also define significant changes in sedimentation rates. Calcareous nannofossil and planktonic foraminifer datums and ranges provided the age-depth control. Magnetostratigraphic control points, which were calibrated using the biostratigraphic results, were also considered. However, technical difficulties, poor core recovery, and abundant depositional hiatuses often prevented successful magnetostratigraphic interpretation.
The most common biostratigraphic datums are first occurrences (FO) or last occurrences (LO) of specific genera or species (see "Biostratigraphy"). An FO or LO datum cannot be observed in a particular sample, but must be interpreted to occur between two samples that define the interval of uncertainty for the true occurrence of the datum. Age controls are therefore plotted as depth intervals (error bars) extending from the top sample to the bottom sample. The symbol for the fossil type (calcareous nannoplankton or foraminifer) is plotted at the midpoint of the depth interval for graphical identification; however, the true LO or FO may occur anywhere in that interval in the absence of additional sampling.
In some cases, particularly at the top or bottom of a cored sediment sequence, an LO or FO datum cannot be defined, but the sample or samples contain age-diagnostic assemblages. In such cases, the known age range for the assemblage can be plotted as an age uncertainty interval for a particular sample or as an age-depth box for a series of samples. Age error bars (or age-depth boxes) should generally be plotted when the age uncertainty of a reference datum exceeds the time interval represented by the sample spacing. For instance, average sedimentation rates at Leg 194 sites range from 10 to 100 m/m.y., and the time interval represented by core catcher sample spacing (~10 m) is thus 1-10 m.y. This means that the core catcher sampling error is much larger (1-2 orders of magnitude) than the age errors of typical Cenozoic FO and LO datums (0.1-0.2 m.y.) In this case, plotting depth error bars is appropriate. However, for larger benthic foraminifer age control, used in carbonate platform sequences drilled on Leg 194, the age range for assemblages is substantial (millions of years) and age-depth boxes are used to plot the relatively loose but significant age-depth constraints.
When constructing the age model for a site, the depth error range for a datum also includes the uncertainty of where the sample came from in the drilled interval of a core. Recovered core segments are curated at the top of the cored interval and the nominal depth of a sample is defined in the curated space (see "Introduction"). If the recovered interval is shorter than the cored interval, the sample may originate from farther down in the section than implied by the nominal sample depth. In such cases, the depths of the bottom (downsection) samples of the datum depth intervals are corrected to the "bottom of cored interval" (BCI) (see datum tables in "Age Model" sections of the site chapters). This increases the depth interval of uncertainty to a more realistic interval, which may be up to 100% larger than the uncorrected one if recovery is limited to core catcher samples.
Age models were constructed by fitting straight line segments through the datum depth intervals. Rather than connecting midpoints of these depth intervals, straight lines were extended through as many control intervals as possible until a change in slope was necessary. Subsequent segments were fit so that changes in slope were minimal. This averaging procedure avoids introduction of artificial kinks (changes in sedimentation rates) in the age model that are not actually defined by the data but would result from low sampling resolution. The procedure was also used to define the duration of hiatuses by extrapolation of straight slope segments to the depth indicated by the physical expression of the hiatus, such as a hardground. The model thus becomes as simple, yet as accurate, as possible, reflecting the limitations of shipboard sampling.
Finally, sedimentation rates were computed and plotted, when appropriate, from the slopes of the straight line segments defining the age models. These rates are averages of actual rates that may vary significantly over short, unresolved time intervals.