Core archive-halves from Holes 1122A and 1122C were measured on the shipboard pass-through cryogenic magnetometer. Declination, inclination, and intensity of natural remanent magnetization (NRM) and 20-mT alternating field (AF) demagnetization steps were measured at 5-cm intervals. The first few cores of each hole were also measured at a 10-mT demagnetization step; this step added little extra information and, because of time constraints, only the 20-mT step was continued. In situ tensor tool data were collected for Cores 181-1122A-3H through 8H and 181-1122C-3H through 13H. Tensor tool data were good for APC cores from Hole 1122C, but a problem with the shipboard pass-through cryogenic magnetometer prevented the use of declination for polarity determination in the APC cores. Therefore, only inclination could be used to determine magnetic polarity of Holes 1122A and 1122C. At least two discrete oriented samples were collected from the working half of each core interval for progressive AF and thermal demagnetization and rock magnetic studies. Whole-core magnetic susceptibility was measured on all cores using a Bartington susceptibility loop on the automated multisensor track (MST). For the purposes of this initial report, only Hole 1122C is discussed in detail below.
Magnetic susceptibility and intensity of magnetic remanence define several zones of magnetic behavior in Hole 1122C (Fig. F20). The upper 110 mbsf have high susceptibility (0.5-1 x 10-3 SI) and high intensity of magnetic remanence. NRM values averaged 0.1-0.2 A/m and dropped only slightly to 0.05-0.1 A/m after 20 mT of AF demagnetization. Between 110 and 260 mbsf, average susceptibility values dropped to 2 x 10-4 SI, but average remanence values remained high for both NRM and 20-mT levels. Beneath 260 mbsf, susceptibility and remanence values were both low (2 x 10-4 SI and 10-4 A/m, respectively), except for the interval between 320 and 360 mbsf in the vicinity of two tephra horizons, where susceptibility and NRM values increased to 5 x 10-4 SI and 8 x 10-2 A/m respectively. These zones defined by magnetic susceptibility and intensity of remanence are roughly equivalent to lithologic changes in the core (see "Lithostratigraphy").
NRM measurements displayed consistent, steeply positive (downcore) inclinations ranging between +70° and +80° , consistent with a drill-string overprint induced during coring. The single 20-mT AF demagnetization step proved very effective in removing the overprint and elucidating a polarity reversal stratigraphy (Fig. F21). Intensity of magnetization was strong enough that discrete samples could be subjected to stepwise AF demagnetization up to 80 mT in the shipboard pass-through cryogenic magnetometer (Fig. F22). The drilling-induced overprint was only observed in reversed polarity samples (e.g., Fig. F22A, F22B, F22G, F22H, F22I), and it only accounted for a very small percentage of the NRM intensity. Further stepwise AF demagnetization demonstrated that all samples had reached a stable primary remanence direction by 20 mT of demagnetization (Fig. F22). Several samples from Hole 1122C demonstrate radial remagnetization from demagnetization attempts with AF fields above 50 mT (e.g., Fig. F22B, F22C). Figure F23 shows acquired isothermal remanence magnetizations (IRM) to saturation (SIRM) and backfield SIRM. Most of the samples subjected to SIRM are magnetically "soft" and were saturated by an applied field of 300 mT. However, samples from 450-580 mbsf did not saturate fully until applied fields were above 700 mT (Fig. F23C). Despite this difference in SIRM, coercivity of remanence (Bcr) was between 25 and 75 mT in all cases. Samples from between 300 and 400 mbsf are particularly uniform in rock magnetic character, with an SIRM of 300 mT and Bcr of 25 mT. AF and thermal demagnetization of the SIRM demonstrates two types of behavior caused by variations in magnetic mineralogy and magnetic grain-size. Samples from the upper 200 mbsf of Hole 1122C have very low unblocking temperatures (350° -400° C) and "soft" but distributed coercivity spectra (Fig. F24A, F24B). Beneath 200 mbsf, unblocking temperatures are mostly higher (~600° C) (Fig. F24C, F24E, F24F).
The "soft" SIRM, low Bcr, distributed unblocking temperatures up to ~600° C, and distributed coercivity spectra demonstrate that magnetite of distributed grain size is the main carrier of remanence in samples from beneath 200 mbsf in Hole 1122C. In each case a secondary component of magnetization is clear, from a variable but small loss of intensity between 320° and 360° C in the unblocking temperature spectra (Fig. F24C, F24E, F24F). This may be a result of the presence of sulfide minerals, which, in turn, may also explain the slight "hardness" of SIRM acquisition in some cases (Fig. F23). Above 200 mbsf, however, the carrier of magnetic remanence is more difficult to isolate. The low unblocking temperatures rule out magnetite as a magnetic carrier and suggest that a sulfide mineral might be the sole carrier of remanence. However, the IRM acquisition curve is saturated at low applied fields (Fig. F23A), and Bcr values are between 50 and 75 mT.
After 20 mT of demagnetization, the inclination record from Hole 1122C is well defined and allows a clear pattern of magnetic polarity to be established (Fig. F21). Two zones of characteristic polarity behavior are identified: above 309 mbsf, polarity is normal except for short excursions that do not have fully reversed inclinations. Beneath 309 mbsf, polarity alternates between normal and reversed intervals but is dominantly reversed. In the XCB cores (beneath 110 mbsf), polarity interpretation is complicated by the low recovery. It was not possible to determine a characteristic polarity pattern beneath 520 mbsf. Despite poor recovery, most of the polarity transitions occur in recovered intervals rather than in "core breaks." Furthermore, polarity transitions occur within intervals of continuous sedimentation rather than at stratigraphic breaks identified in the core (see "Lithostratigraphy"). Several key biostratigraphic datum events (see "Biostratigraphy") define a unique correlation between the magnetic polarity record of Hole 1122C and the Geomagnetic Polarity Time Scale (GPTS) (Berggren et al., 1995; Cande and Kent, 1995) (Fig. F25).
The uppermost 309 mbsf of Hole 1122C is of normal polarity and the last occurrences (LO) of Globorotalia puncticulinoides (F2, 0.7-0.8 Ma), Actinocyclus ingens (D3, 0.64 Ma), and Thalassiosira elliptipora (0.65-0.7 Ma) at 213, 265, and 300 mbsf, respectively, suggest that this interval is entirely within the Brunhes normal chron (C1n). Several short excursions are recognized above 309 mbsf (Fig. F21). These may correlate with intervals of depressed NRM and 20-mT intensity (Fig. F20). Further paleomagnetic and dating work may define them well enough to correlate with polarity excursions known from high-resolution terrestrial records (e.g., Worm, 1997) and high-resolution marine intensity records from the equatorial regions (e.g., Meynadier et al., 1995).
Between 309 and 487 mbsf, nannofossils provide the key datums for correlation (Fig. F25). The FO of Gephyrocapsa parallela (0.95 Ma, N4) and Reticulofenestra asanoi (1.06 Ma, N5) confine the mainly reversed polarity with short normal polarity intervals, between 309 and 350 mbsf to the upper part of the Matuyama Chron (C1r). The normal polarity intervals between 325 and 331 mbsf are correlated with the Jaramillo Subchron (C1r.1n) and the lower normal polarity event (339-342 mbsf) is correlated with the Cobb Mountain Event. Between 342 and 442 mbsf the normal-reversed-normal-reversed (N-R-N-R) polarity pattern is correlated with the lower part of the Matuyama Chron. The occurrence of Gephyrocapsa (large) (N6-N7, 1.1-1.36 Ma) between 356 and 388 mbsf and the FO of Gephyrocapsa (medium) (N8, 1.66 Ma) at 410 mbsf confines the uppermost reversed polarity interval of the N-R-N-R pattern to Subchron C1r.2r. A short excursion at ~370 mbsf may correlate with the Vrica or younger Olduvai Subchron (Baksi, 1995). Two radiolarian datums (LO Lithelius nautiloides, 1.93 Ma and FO Eucyrtidium calvertense, 1.92 Ma) confine the remaining N-R-N pattern to Subchrons C2r.1n (Reunion Subchron), C2r.1r, and Chron C2n (Olduvai Chron), respectively.
Foraminifer (F3) and nannofossil (N9) faunas beneath 461 mbsf suggest that a disconformity at this level separates the underlying lower Gauss Chron-age strata from the overlying Matuyama Chron-age strata. At 487 mbsf, all microfauna and microflora identify a major stratigraphic disconformity with underlying strata of middle Miocene age (see "Biostratigraphy"). Magnetic polarity stratigraphy in this interval (442-487 mbsf) suggests an even more complex situation, with additional stratigraphic discontinuities, as the Gauss Chron (C2An) is of mostly normal polarity, yet in Hole 1122C the magnetic polarity at this level is mostly reversed. Beneath the disconformity at 487 mbsf, foraminifers (Neogloboquadrina continuosa, N. pachyderma, and LO of Gr. praemenardii, F5), nannofossils (LO Coccolithus miopelagicus, N10, FO Calcidiscus macintyrei, N11, and LO Sphenolithus heteromorphus, N12), and diatoms (LO of Denticulopsis dimorpha, D6, and FO Simonseniella barboi, D7) all suggest that the interval between 487 and ~575 mbsf ranges in age between 10 and 12.5 Ma. A characteristic magnetic polarity reversal pattern of two short normal events within an interval of reversed polarity allows better definition of age as Chron C5r is the only distinct interval of reversed polarity in this section of the GPTS. Chron C5r contains two short normal polarity subchrons (C5r.1n and C52.2r) that are correlated with the two short normal polarity intervals at 494-496 mbsf and 519-522 mbsf, respectively. Foraminifers (co-occurrence of Gr. zealandica and Gr. miozea, F6) and the LO of the nannofossil C. premacintyrei (N13) suggest that the base of Hole 1122C is older again (~16-17 Ma). The basal 20 m of the hole is of reversed polarity and may correlate with Chron C5Br. However, poor recovery makes this correlation uncertain.
From the age model presented in Figures F21 and F25, average sedimentation rates are on the order of 400 m/m.y. for Subunits IA, IB, and IC (see "Lithostratigraphy") and 100 m/m.y. for Subunits ID and IIA. Average sedimentation rates could not be determined accurately for the lowermost part of Hole 1122C (lithostratigraphic Subunits IIB, IIIA, and IIIB) although they are likely to be an order of magnitude lower (~20 m/m.y.; Fig. F25). Tephras were identified at seven horizons in Hole 1122C (Figs. F21, F25; see "Lithostratigraphy"). Dating of these will provide important additional information to help refine the magnetobiostratigraphic age model presented here. The tephra at 12 mbsf has already been identified as the Kawakawa Tephra (Carter et al., 1995). Using the average sedimentation rates from above and the stratigraphic position of the remaining tephra horizons, the following correlations with tephras reported from onland studies in New Zealand are possible: the tephra at 137 mbsf may correlate with the Rangitawa Tephra (Pillans et al., 1996); and the tephras at 317, 328, and 390 mbsf with the Kaukatea, Potaka, and Pakihikura Tephras, respectively (Pillans et al., 1994). Dating of the two tephras at 454 and 457 mbsf will provide important constraints on this interval of Hole 1122C, as the age is, as yet, poorly constrained.
Differences in susceptibility and intensity of magnetic remanence (NRM and 20 mT), combined with magnetic mineralogical variations (identified by IRM, SIRM, and AF and thermal demagnetization behavior), demonstrate changes in the magnetic character at depth in Hole 1122C from changes in sedimentologic processes and sources. It is clear that in Unit I the susceptibility record is driven primarily by the sandier interbeds of the turbidite sequence, whereas the intensity of magnetic remanence is held by similar minerals of similar grain size in both the sandy and muddy interbeds. Such a record suggests a preferential loss of the coarser sediments in the XCB coring process as compared with APC coring where both coarse- and fine-grained interbeds are recovered. A change in intensity of remanence (at 20 mT of demagnetization) is noted beneath ~260 mbsf. This is coincident with the boundary between lithostratigraphic Subunits IC and ID. Thermal demagnetization experiments show that this is most likely because of a change in magnetic mineralogy. An increase in fine-grained magnetite beneath 260 mbsf suggests a possible change in sediment source. Variations in intensity of magnetic remanence in the upper 100 mbsf of Hole 1122C (Fig. F20) suggests fluctuations in sedimentation rates not recognized in the lithostratigraphic record and/or variations in intensity of the geomagnetic field at time of deposition.