Core archive halves from Holes 1124A, 1124B, 1124C, and 1124D 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 after a 10-mT demagnetization step; this step added little extra information and, because of time constraints, only the 20-mT step was continued. Because of a complex and hard overprint, XCB cores between 175 and 276 mbsf from Hole 1124C were also demagnetized at 40-mT, 60-mT, and 80-mT AF steps. In situ tensor tool data were collected for all APC cores, but a problem with the shipboard pass-through cryogenic magnetometer prevented the use of the declination for polarity determinations in these cores. Therefore, only inclination could be used to determine magnetic polarity in Holes 1124C and 1124D. At least two discrete oriented samples were collected from the working half of each core interval, but time constraints prevented any shipboard measurement of these. Whole-core magnetic susceptibility (MS) was measured on all cores using a Bartington susceptibility loop on the automated multisensor track (MST).
A composite paleomagnetic record was constructed for Site 1124 using data from Holes 1124C and 1124D. Holes 1124A and 1124B were not used in constructing a composite paleomagnetic record because of APC refusal immediately beneath the seafloor in a tephra layer (see "Lithostratigraphy"). XCB coring was used to penetrate this layer and then coring with the APC recommenced beneath. Paleomagnetic results for the uppermost 50 meters composite depth (mcd) in Holes 1124A and 1124B were not easy to interpret because recovery was poor and complicated by possible overlap and flow-in. For these reasons, a composite paleomagnetic record was constructed from below ~16 mcd in Holes 1124C and 1124D. Intensity of magnetic remanence varied markedly with depth in Hole 1124C (Fig. F18). The upper 280 mbsf have average NRM intensities that vary cyclically between 4 × 10-3 and 8 × 10-2 A/m. Between 290 and 420 mbsf, average NRM intensity is reduced to less than 4 × 10-3 A/m. Beneath 420 mbsf, NRM intensity increases to a range of 8 × 10-2 to 0.2 A/m and at 428 mbsf is similar to the upper 280 mbsf of the core (average NRM intensity = 2 × 10-2 A/m).
NRM measurements displayed consistent, steeply positive (down-core) inclinations ranging between +70° and +80° , consistent with a drill string overprint induced during coring. The 20-mT AF demagnetization step effectively removed this overprint above 185 mbsf and between 290 and 475 mbsf (base of Hole 1124C). However, the single 20-mT demagnetization step was not effective in removing the steep positive overprint from the core between 185 and 290 mbsf, and the overprint persisted to demagnetization steps of 40-60 mT. An 80-mT AF demagnetization step was therefore used to elucidate the primary remanence between 185 and 290 mbsf (Fig. F19B).
Interpretation of magnetic polarity from the composite inclination record for Site 1124 (Fig. F19) is well constrained by key foraminiferal and nannofossil datums for each of the core-catcher samples and for additional samples either side of the Cretaceous boundary in the lowermost part of the core (see "Biostratigraphy" and "Biostratigraphy" of the "Explanatory Notes" chapter). The inclination record for the upper 170 mcd of the composite record is taken from Holes 1124C and 1124D and, when compared with the Geomagnetic Polarity Time Scale (GPTS) (Cande and Kent, 1995; Berggren et al. 1995), it provides a complete record of the Brunhes (C1n), Matuyama (C1r-C2r), Gauss (C2An), Gilbert (C2Ar-C3r), C3A, and C3B magnetochrons (Fig. F19A). The interval between 19 and 28.85 mcd is of normal polarity and is inferred to represent the lower part of the Brunhes (C1n) Chron. The inclination record between 28.85 and 33.75 mcd is noisy and prevents an exact placing of the Brunhes (C1n)-Matuyama (C1r.1r) boundary. The characteristic normal-reversed-normal-reversed-normal pattern of the Gauss Chron (C2An) occurs between 81.60 and 97.30 mcd, constrained by the LOs of nannofossils Discoaster surculus (2.6 Ma) and D. pentaradiatus (2.3 Ma) and of foraminifers Globorotalia puncticulata (3.8 Ma), G. pliozea (3.7 Ma), and G. crassaconica (3.2 Ma), the FOs of the nannofossil Pseudoemiliania lacunosa (4.0 Ma), and foraminifers G. puncticuloides (3.4 Ma) and G. crassula (2.6 Ma). Between 33.75 and 81.60 mcd, the polarity is mostly reversed with four short normal polarity events. Two of these short normal polarity events immediately underlie the Brunhes (C1n) Chron and are assigned to the Jaramillo (C1r.1n) and Cobb Mountain (C1r.2r-1n) Subchrons, respectively. The other two short normal events occur between 64.50 and 71.60 mcd and, constrained by nannofossils D. pentaradiatus (LO, 2.3 Ma), D. brouweri (LO, 2.0 Ma), and Calcidiscus macintyrei (LO, 1.6 Ma) and foraminifer G. caribbeanica (FO, 1.7 Ma), are assigned to the Olduvai (C2n) Chron and Reunion (C2r.1n) Subchron, respectively.
The inclination record between 97.30 and 170 mcd (Fig. F19A) is very stable and well behaved after the single 20-mT cleaning step, and the overlap between Holes 1124C and 1124D contain a complete polarity record of Chrons C2Ar-C4n. Between 97.30 and 135.10 mcd, the polarity pattern is distinctive of the Gilbert Chron (mostly reversed with four short normal subchrons centered between two longer reversed subchrons, C2Ar-C3r) (Fig. F19A). Many nannofossil and foraminifer events confirm that this interval represents the Gilbert Chron (C2Ar-C3r). Nannofossil events include the LO of Reticulofenestra pseudoumbilicus (3.8 Ma) and the FO of P. lacunosa (4.0 Ma), and key foraminifer datums include the FO of G. inflata (3.8 Ma), the ranges of G. puncticulata (3.8-5.2 m.y.) and G. sphericomiozea (5.2-5.6 m.y.), and the FO of G. pliozea (5.4 Ma) and G. mons (5.5 Ma). Between 135.10 and 170 mcd, correlation with the polarity time scale is constrained only by the LO of the nannofossil Minylitha convallis (7.7 Ma) at ~165 mcd. However, the polarity is very distinctive and the normal-reversed-normal pattern of Chron C3An can be clearly recognized underlain by Chrons C3Ar, C3Bn, C3Br, and C4n (Fig. F19A).
The remaining polarity record is constructed only from Hole 1124C and is complicated by several large hiatuses that truncate foraminifer and nannofossil zones (see "Biostratigraphy") and further short hiatuses defined by missing and truncated polarity chrons or subchrons. A thick interval of normal polarity between 187.40 and 223.10 mbsf (Fig. F19B) contains the FO of the nannofossil Catinaster coalitus (10.8 Ma) and the LOs of the nannofossil Coccolithus miopelagicus (10.9 Ma) and foraminifer G. dehiscens (10.1 Ma) and is correlated with the characteristic long normal Chron C5n (9.74-10.95 m.y.) of the magnetic polarity time scale of Cande and Kent (1995). An interval of reversed polarity directly overlying C5n contains the nannofossils M. convallis (FO, 9.3 Ma) and C. calyculus (LO, 9.6 Ma) and is correlated with C4Ar. A very thin normal polarity excursion is recognized in the upper part of this reversed polarity interval and is correlated with Subchron C4Ar.1n. Subchrons C4Ar.2n and C5n.1r of the polarity time scale are not recognized in Hole 1124C. They could be missing in core breaks or short unconformities that are not resolvable here. Above 173.80 mbsf in Hole 1124C, however, the polarity record is complete and Chrons C4An, C4r, and C4n complete the upper Miocene record.
Combined nannofossil events (see "Biostratigraphy") and magnetostratigraphy define a short hiatus at ~226 mbsf in Hole 1124 (Fig. F19B). The LO of Sphenolithus heteromorphus (13.5 Ma) and C. premacintyrei (12.7 Ma) suggest that the reversed-normal-reversed-normal pattern above 226 mbsf correlates with C5Br-C5ADn and that ~3 m.y. is missing in the hiatus that juxtaposes the upper part of C5r (above) with C5ACr (below). A coring gap between 276 and 293 mbsf prevents any definitive correlation of the lower Miocene record in Hole 1124C with the polarity time scale. Some constraints are afforded, though; the acme of the nannofossil D. deflandrei (16.2 Ma) occurs at ~260 mbsf and the polarity between 257.60 and 276.40 mbsf, although noisy, is reversed and probably correlates with Chron C5Cr of the polarity time scale. The thin normal polarity interval between 256.96 and 257.60 mbsf most likely correlates with the base of Chron C5Cn. The upper part of C5Cn is truncated in a hiatus that is likely ~1.5 m.y. in duration (Fig. F19B).
Beneath the coring gap (276-293 mbsf), the inclination record, after 20 mT demagnetization, again defines a clear polarity pattern (Fig. F19C). The LOs of the nannofossils S. umbrellus (23.6 Ma), Dictyococcites bisectus (23.9 Ma), S. delphix (24.3 Ma), and Chiasmolithus altus (26.1 Ma) constrain the interval between 276 and 344 mbsf to the early Miocene and late Oligocene, but correlation with the polarity time scale is ambiguous. The interval between 305 and 344 mbsf most likely correlates with Chron C6Cn. The location of the Oligocene/Miocene boundary is also ambiguous. A thick interval of reversed polarity between 343.90 and 370.00 mbsf affords more control and is correlated with Chron C6Cr (Fig. F19C). The FO of the nannofossil S. distentus (27.5 Ma), within normal polarity at 408.90-411.05 mbsf, is correlated with Chron C9n of the polarity time scale. With these constraints there is a direct correlation of the polarity reversals between 370.00 and 408.90 mbsf with Chrons C7n through C8r.
Foraminifer and nannofossil events define a very complex stratigraphy beneath 411 mbsf (see "Biostratigraphy") with several long hiatuses accounting for most of the lower Oligocene, upper Eocene, lower Eocene, and upper Paleocene record in Hole 1124C. The first hiatus occurs at 411.05 mbsf, has a duration of ~6 m.y., and is marked by a polarity transition separating Chron C9n (above) from Chrons C12r and C13n (below). Chrons C12r and C13n are defined by several nannofossil datums, including the LO of R. umbilicus (32.3 Ma) and Ericsonia formosa (32.8 Ma), and the acme of Clausicoccus subdisticus (33.3 Ma). The second hiatus (419.35 mbsf) is ~3 m.y. in duration and separates Chron C13n (above) from Chrons C18r, C19n, and C19r (below). The FO of the foraminifers G. index (42.9 Ma) and Ponticulosphaera semiinvoluta (38.4 Ma) constrain the interval below 419.35 mbsf to the middle Eocene and Chrons C18 and C19. A major hiatus of ~21 m.y. at 429.10 mbsf separates middle Eocene strata from upper Paleocene strata. It is defined by the LO of the nannofossil Hornibrookina teuriensis (58.3 Ma) and the LO of the foraminifer Gavelinella beccariiformis (55 Ma). Several nannofossil and foraminifer datums define the 430-463 mbsf interval as early-late Paleocene in age. At ~465 mbsf, a 3-m coring gap contains the K/T boundary (see "Biostratigraphy"). The inclination record is too poorly defined to allow unambiguous correlation of the lower 45 m of Hole 1124C with the polarity time scale, but the lower 43 m of Hole 1124C is loosely correlated with Chrons C26-C30.
A complete list of depths and ages of magnetic polarity events for Site 1124 is given in Table T11. An age-depth model for Site 1124, using the GPTS reversal polarity ages of Berggren et al. (1995), is given in Figure F20. The average sedimentation rate for lithostratigraphic Subunits IA and IB (greenish gray clay-bearing nannofossil ooze and silty clay, see "Lithostratigraphy") is 21 m/m.y. However, the sedimentation rate is 44 m/m.y. down to 53 mcd, 21 m/m.y below an inflection point at 1.2 Ma, and 16 m/m.y. below a second inflection point at 2.6 Ma (Fig. F21). Despite large changes in the sedimentation rate, the composite record of the upper 200 mcd of Site 1124 appears complete and contains all magnetic polarity chrons down to the base of Chron C5n. Some 139 discrete tephra horizons (see "Lithostratigraphy") are preserved in the composite 0-200 mcd record for Site 1124. A few tephra horizons are also preserved below 200 mcd. The age model presented in Figure F21 affords very good constraints on the ages of these horizons. While the tephra horizons are distributed throughout, several phases of volcanism are noted (see "Sedimentation Rates").
Lithostratigraphic Subunit IC (a clay-bearing nannofossil chalk and mudstone; see "Lithostratigraphy") is less complete and contains three hiatuses (Fig. F20): a middle Miocene hiatus at 238 mcd representing 3 m.y., a lower-middle Miocene hiatus at 269 mcd occupying 1.5 m.y., and a lower Miocene hiatus at 290 mcd occupying ~5 m.y. of missing time. Again in contrast, lithostratigraphic Unit II (a light greenish gray nannofossil chalk; see "Lithostratigraphy") represents a near complete record of the upper Oligocene-lower Miocene with an average sedimentation rate of 28 m/m.y. Lithostratigraphic Units II and III (a white nannofossil chalk; see "Lithostratigraphy") are separated by a hiatus representing ~6 m.y. of missing time (27-33 Ma) and is presumably correlative in part to the hiatus between lithostratigraphic Units III and IV at Site 1123 (see "Paleomagnetism" in the "Site 1123" chapter) and with the Marshall Paraconformity (Carter, 1985; Fulthorpe et al., 1996). Two further hiatuses separate Miocene to lower Oligocene strata from upper Paleocene strata: a ~3-m.y.-long upper Eocene hiatus at 432 mcd within lithostratigraphic Unit IV (a reddish brown to dark brown mudstone, see "Lithostratigraphy"), and a ~21-m.y.-long Paleocene-Eocene hiatus at 441 mcd, which separates lithostratigraphic Unit IV from Unit V (a white nannofossil chalk and mudstone; see "Lithostratigraphy"). Sedimentation below this second unconformity is relatively continuous across the K/T boundary at ~472 mcd, which separates lithostratigraphic Unit V from Unit VI (a pinkish white to pale brown nannofossil mudstone and chert; see "Lithostratigraphy"). Unfortunately, the K/T boundary itself was lost in a break between coring intervals.