Four types of magnetic behavior have been recognized within sediments of the top six cores at Site 1007. They reflect differences in magnetic mineralogy and grain size of magnetic minerals within the sediment. A steep initial increase in the IRM acquisition indicates dominance of low-coercivity magnetic phases like magnetite and maghemite. A gradual rise in IRM acquisition seen in some samples indicates that a high-coercivity phase such as hematite is also present. The lower coercivity of IRM compared to ARM suggests a predominance of finer grained magnetic minerals like single domain or pseudo-single domain (Lowrie and Fuller, 1971; Johnson et al., 1975). The present results indicate that sediment of magnetic Type 2 is dominated by high-coercivity magnetic minerals, whereas those of magnetic Types 1, 3, and 4 are dominated by a low-coercivity phases. The grain size of magnetic minerals varies from one type to another. The dominant magnetic minerals of Type 1 are estimated to be of single- or pseudo-single-domain size. Types 3 and 4 mainly contain multidomain states with slight variations between them.
The interval below 12 mbsf is characterized by meter-scale cyclic changes of light-colored nannofossil ooze to dark-colored unlithified mudstone and wackestone. The top of Site 1006 (0-128 mbsf), ~17 km offshore, consists of a lithology with similar cyclic changes. The clay-rich intervals reflect redeposition of eroded siliciclastics during lowstands of sea level. Light-colored nannofossil ooze lithofacies reflect pelagic sedimentation during highstands of sea level (see Eberli, Swart, Malone, et al., 1997). Relatively high magnetic remanent intensities are observed in this clay-rich interval.
Magnetic mineralogy changes precipitously to a high-coercivity phase (Type 2) with the facies change at 12 mbsf. This conversion in magnetic properties agrees well with the intervals in which sedimentation rate begins to change. We believe that below 12 mbsf, Site 1007 is characterized by pelagic sedimentation, as is the offshore Site 1006 (at present). Progradation of the Bahamas carbonate platform had not developed to the sampling site since this interval (~1.2 Ma). The sedimentation rate of this interval is relatively constant before ~1.2 Ma (Fig. 2). After progradation developed at Site 1007, the current of the Straits of Florida (Santaren Channel) washed out the drift sediment, including the low-coercivity magnetic minerals. An accumulation of mass-gravity flow from the bank is seen within sediments between 2 and 12 mbsf.
An abrupt change from magnetic Types 1 and 2 occurs between Samples 166-1007B-1H-2, 39 cm, and 1H-2, 59 cm (~2 mbsf), comparable to the same intervals of a hiatus between 0.08 and 0.41 Ma. Magnetic minerals above 2 mbsf change completely to a low-coercivity phase with remanent intensity differing by an order of 102-103. White to light gray nannofossil ooze facies gradually change downhole into a white to pale yellow unlithified foraminifer wackestone to packstone lithofacies in Section 166-1007B-1H-1 (top of core) through Sample 166-1007B-1H-2, 40 cm. The first recorded turbidite occurs at the base of this interval. This turbidite is marked by a sharp erosional basal contact that is overlain by graded unlithified bioclastic grainstone. The basal unlithified grainstone contains fine to medium sand-sized neritic carbonate grains. The average sedimentation rate in this interval is estimated to be >2.5 cm/k.y.
McNeill et al. (1993) relates a decrease in magnetic intensity with depth in the carbonate sediment at the core top to the oxidation of magnetite to maghemite. More detailed study is necessary in order to trace the origin of the magnetic mineral after 0.08 Ma. Many means to obtain magnetic minerals from sediments ranging from siliciclastic, authigenic, diagenetic, and biogenic can be considered. The presence of a hiatus in the same interval reflects different processes of accumulation below and above 2 mbsf.
Remanent intensities of ARM and IRM (applied field = 2 T) plotted against the age of sediments based on the calcareous nannofossil biostratigraphy are shown in Figure 6. High intensities appear above the hiatus. Relatively low intensities of remanent magnetization characterize sediments accumulated from before 1.2 Ma to the hiatus. The variability curve of the magnetic remanent intensities shows cyclic changes, especially after 1.2 Ma, within an interval of relatively constant sedimentation rate (Fig. 6). A high-amplitude variability curve is observed between 1.2 and 1.5 Ma, with a tendency to decrease downward in the core. Some studies of magnetic remanent intensities suggest that high concentrations of magnetic minerals are found during glacial periods, and their record is controlled by glacial-interglacial cycles (e.g., Robinson, 1986; Thouveny et al., 1994; Arai et al, 1997). We compared our variability curve with the oxygen isotope curve of Wei (1993). The amplitude of the variability curve is similar to the oxygen isotope curve (Fig. 6). We need more substantiation and a comparison with the oxygen isotope curve from the present site, however.
In the cores at Site 1003, relatively high intensities of remanent magnetization appear within coarse-grained floatstone-packstone layers. The magnetic signals also show the decrease of finer grained magnetic minerals in the coarse-grained layers, based on the ratio of ARM to SIRM (see Fig. 5, right panel). The sediments of coarse-grained layers are characterized by high amounts of neritic grain (e.g., Halimeda debris) export from the platform and are distinguished by a high concentration of relatively coarse-grained magnetic minerals. The fact that variations of magnetic remanent intensity are well correlated with sedimentary lithofacies suggests that the cyclic variability curve is dependent on the change of sedimentary processes of the margin of the Great Bahama Bank. The present magnetic studies seem to be useful for revealing the enhancement of effects from the platform. The variability curves at both Sites 1003 and 1007 seem to depend on the amounts of the same origin of magnetic minerals within the sediments. More extensive analyses are necessary to substantiate this contention.