Sediment composition and sedimentation rates of four Holes in ODP Leg 166

To date, work on the Great Bahama Bank's western, leeward margin has centred chiefly on seismic-scale expressions of carbonate sequences and systems tracts. However, periplatform, slope sediments also exhibit very well developed cyclicity on scales of decimetres to several metres. It is these small-scale, high-frequency cycles within the larger-scale facies successions of the Quaternary which form the main topic of this paper. Previous studies have shown that the small-scale cycles correlate to the orbitally forced, high-frequency sea-level changes. Therefore these cycles should indicate how sea level has affected the slope development and thus platform-margin evolution during this period. Through detailed, high-resolution sequence stratigraphy of the Great Bahama Bank's leeward margin, obtained via delta18O isotope and mineralogical (XRD) analyses, confined by U/Th dating and nannofossil bioevents, a greater understanding of the bedding geometries within the Pleistocene-Holocene seismic sequences and clues as to the nature of the slope development has been achieved. The high-resolution seismic profiles indicate that since the Plio-Pleistocene change in geometry, in which the Great Bahama Bank developed into a rimmed platform, continued steepening and subsequent progradation of the leeward margin has typified slope development during the Quaternary, which is described as an accretionary slope. However, on the basis of our observations we conclude that only the early to lower middle Pleistocene section (isotope stages 45-20) and the Holocene (isotope stage 1) of the leeward margin is accretionary. This indicates that a degree of erosion and/or by-passing has occurred on the leeward margin since the lower middle Pleistocene (isotope stage 19). During the first part of this period (isotope stages 19-12) erosion and/or by-passing occurred in the middle to lower slope regions and toe-of-slope. By the end of the upper middle to late Pleistocene phase (isotope stages 11-2) erosion also occurred on the upper slope. This erosion by currents at the toe-of-slope and oversteepening of the upper and middle slopes have led to back-cutting upslope and resulted in the progressive retreat of the toe-of-slope towards the platform to the east. However, the rise in sea level since the Last Glacial Maximum to its present-day level has allowed high productivity on the platform top during the Holocene and the deposition of a thick sediment wedge on the slope and sedimentation across the entire leeward flanks. This has led to the redevelopment of an accretionary slope and continued westward progradation of the Great Bahama Bank's western, leeward margin.

Magnetic measurements on ODP holes 178-1095A, 178-1095B and 178-1101A

We have determined the azimuth of bottom-current flow in drift deposit sediments recovered at ODP Sites 1095 and 1101, Antarctic Peninsula, using paleomagnetic reorientation of anisotropy of magnetic susceptibility (AMS) ellipsoids. A total of 38 cores from the two ODP sites have been measured, providing spatial and directional information on the physical record of the ACC (Antarctic Circumpolar Current) in the Plio-Pleistocene. Declination and inclination of the paleomagnetic vector of each core segment were used to reorient the AMS principal axes to the geographic coordinates. The cores were reoriented using the measured direction of the characteristic remanent magnetization (ChRM) with respect to a common reference line for the core, from which we are able to determine the orientation of the paleocurrent flow for Sites 1095 (Drift 7) and 1101 (Drift 4) relative to the geographic coordinates. Both sites have paleocurrent directions trending ~NW-SE, which in the former locality are parallel to a sediment wave field. Our study shows that a combination of magnetic fabric analysis and paleomagnetism allows deep-sea sedimentary fabric to be used as a long-term proxy of bottom-current flow history.

Geochemical and mineralogical analyses of clay-rich intervals in ODP Sites 166-1006 and 166-1007

The western flank of the Great Bahama Bank, drilled during ODP Leg 166 at seven sites, represents a prograding carbonate sequence from late Oligocene to Holocene [Eberli et al., Proc. ODP Init. Reports 166 (1997)]. The signatures of the detrital input and of diagenetic alteration are evident in clay enriched intervals from the most distal Sites 1006 and 1007 in the Straits of Florida. Mineralogical and chemical investigations (XRD, TEM, SEM, ICP-MS) run on bulk rocks and on the clay fractions enable the origin and evolution of silicate parageneses to be characterized. Plio-Pleistocene silt and clay interbeds contain detrital clay assemblages comprising chlorite, illite, interstratified illite smectite, smectite, kaolinite and palygorskite. The greater smectite input within late Pliocene units than in Pleistocene oozes may relate either varying source areas or change in paleoclimatic conditions and weathering intensity. The clay intervals from Miocene-upper Oligocene wackestone sections are fairly different, with prevalent smectite in the fine fraction, whose high crystallinity and Mg contents that point towards an authigenic origin. The lower Miocene section, below 1104 mbsf, at depths where compaction features are well developed, is particularly characterized by abundant authigenic Na-K-clinoptilolite filling foraminifer tests. The authigenic smectite and clinoptilolite paragenesis is recorded by the chemical trends, both of the sediment and the interstitial fluid. This diagenetic evolution implies Si- and Mg rich fluids circulating in deeper and older sequences. For lack of any local volcaniclastic input, the genesis of zeolite and the terms of water rock interaction are discussed. The location of the diagenetic front correlates with that of the seismic sequence boundary P2 dated as 23.2 Ma. This correspondence may allow the chronostratigraphic significance of some specific seismic reflections to be reassessed.

(Table 2) Correlation of Mi zones with hiatuses in ODP Site 181-1124

Results from Ocean Drilling Program sites 1121-1124 show the Eastern New Zealand Oceanic Sedimentary System (ENZOSS) evolved in response to: (1) the inception of the circum-Antarctic circulation, (2) orbital and nonorbital regulation of the global thermohaline flow, and (3) development of the New Zealand plate boundary. ENZOSS began in the early Oligocene following opening of the Tasmanian gateway and inception of the ancestral Antarctic Circumpolar Current (ACC) and SW Pacific Deep Western Boundary Current (DWBC). Widespread erosion, marked by the Marshall Paraconformity, was followed by extensive drift formation in the late Oligocene- early Miocene. Alternating nannofossil chalk and nannofossil-rich mud deposited in response to 41-kyr orbital regulation of the abyssal circulation, with the mudstones representing times of increased inflow of corrosive southernsource waters. Drift deposition at the deepest sites was interrupted by bouts of erosion coincident with Mi 1-5 isotopic events signifying expansions of the East Antarctic Ice Sheet and enhanced bottom water formation. By late Miocene times, the basic ENZOSS was established. South of Bounty Trough, the energetic ACC instigated an erosional/low depositional regime. To the north, where the DWBC prevailed, orbitally regulated drift deposition continued. Increased convergence at the New Zealand plate boundary enhanced the terrigenous supply, but little of this sediment reached the deep ENZOSS as the three main sediment conduits - Solander, Bounty and Hikurangi channels - had not fully developed. The Plio-Pleistocene heralded a change from a carbonate- to terrigenous-dominant supply caused by interception of the DWBC by the three channels (~1.6 Ma for Bounty and Hikurangi, time of Solander interception unknown). The Solander and Bounty fans, and Hikurangi Fan-drift systems formed, and drifts downstream of those systems, received terrigenous detritus. Supply increased with accelerating uplift along the plate boundary, but delivery to the DWBC was regulated by eustatic fluctuations of sea level. Times of maximum supply to all three channels was during glacial lowstands whereas the supply either ceased (Bounty, Solander), or reduced (Hikurangi) in highstands. In glacial times, sediment was entrained by a DWBC invigorated by an increased input of Antarctic bottom water. The ACC also accelerated under strengthened glacial winds. Thus, glacials were times of optimum sediment supply to ENZOSS depocentres where depositional rates were 2-3 times more than interglacial rates.

Late Pliocene stable oxygen isotope record of the Mediterranean Sea

Detailed pollen analyses and oxygen isotope records of three foraminiferal species, Globigerina bulloides, Uvigerina peregrina and Cibicides pachyderma, from the Semaforo and Vrica composite sections (Crotone, southern Italy) have been compared to the global climatic changes depicted by late Pliocene-early Pleistocene foraminiferal d18O records of Site 607 in the North Atlantic, and Hole 653A in the Tyrrhenian basin, West Mediterranean. Major overturns in the mid-altitude vegetation are shown near isotopic stages 82, 60, 58 and 50, at about 2.03 Ma, 1.6 Ma and 1.37 Ma according to the Raymo et al. (1989, doi:10.1029/PA004i004p00413) and Ruddiman et al. (1989, doi:10.1029/PA004i004p00353) timescales. At the same dates, glacial 18O maxima either became higher or display step increases in the western Mediterranean or in the open ocean as well. This suggests that size increases of Northern Hemisphere ice sheets were the driving factor for regional or local marine and continental environmental changes within the Mediterranean basin. Near isotopic stages 62-60, close to the conventional Plio-Pleistocene boundary, the climatic conditions severed enough within the Mediterranean basin to modify the continental environment, as depicted by a sudden increase of Artemisia percentages, while the first significant southward migration of the North Polar Front may have been recorded by an influx of left coiling Neogloboquadrina pachyderma in the central Mediterranean. It also appears that 'Boreal Guests' entered the Mediterranean during phases of 18O enrichment of foraminiferal calcite. There does not seem to be any discrepancy between the climatic concept of the Pliocene-Pleistocene boundary and its chronostratigraphic definition.

Paleomagnetic of ODP Leg 101 sites

Paleomagnetic measurements were made on 913 samples from 11 holes (626B, 626C, 627B, 628A, 630A, 631A, 632A, 632B, 633A, 634A, and 635B) drilled in and around the Bahamas carbonate bank during Ocean Drilling Program Leg 101. These samples displayed a wide range of magnetic intensities (from about 1.0 A/m to 1.6 * 10**- 6 A/m) and magnetic behavior. Most samples were weakly magnetized and had low mean destructive fields; however, sediments from sections of several holes were strongly magnetic with stable magnetizations. Magnetic-polarity interpretations were made on a Campanian unit from Hole 627B, a mid-Oligocene unit from Hole 628A, and a Plio-Pleistocene section from Hole 633A. Sediments in the upper parts of Holes 627B, 632A, and 633A have high magnetic intensities that decay 2 to 3 orders of magnitude over depths of 5 to 18 mbsf. The pattern of decline of the magnetism and the change in mean destructive fields and geochemical conditions in these holes are consistent with diagenetic dissolution of the magnetic minerals in a suboxic or anoxic-sulfidic environment. Paleolatitudes were calculated from samples from 16 time units in 7 holes and compared to the apparent polar wander path of the North American plate.