To some extent, whether because of dissolution and precipitation or oxidation-reduction changes, all Leg 172 sedimentological patterns have a diagenetic signal that must be kept in mind. Çagatay et al. (in press) evaluated the broad diagenetic patterns across the Leg 172 depth transect based on their synthesis of interstitial water chemistry and sediment composition. They found that, in general, shallow water sites have greater sulfate reduction than deeper water sites because of higher iron content. From pore water analyses from eight of the sites along the depth transect, Borowski et al. (Chap. 2, this volume) found that dissolved carbon dioxide and methane pools are geochemically coupled through anaerobic methane oxidation and CO2 reduction. They documented large 13C depletions at the sulfate/methane interface in dissolved CO2 and methane. The similarity in levels of depletion of dissolved CO2 and methane continue into the shallow methanogenic zone below the sulfate/methane interface and are inferred to result from the dissolved CO2 being used as the substrate in the production of methane.
König et al. (Chap. 1, this volume) examined changes in the bulk sediment Fe(II)/Fe(III) ratio and in the distribution of iron among different minerals that result from storage after collection. Focusing their analysis on samples from Site 1062, they found that 24%-45% of the initial Fe(II) was oxidized to Fe(III) within only 6 months of refrigerated storage. Nearly the entire Fe(II) fraction was found to reside in structural iron in silicate mineral lattices. Thus, oxidation took place within the lattices. From these results, they suggested that modification of the magnetic signal during storage would be unlikely for Site 1062 sediments. They warned, however, that the reverse reactions (reduction diagenesis) likely occurred near the seafloor, which could have resulted in some chemical overprinting of the original magnetic signal as magnetite was progressively dissolved below the iron redox boundary. Diagenetic dissolution and redistribution of iron at and just below the seafloor could also affect the lithostratigraphic position of red lutites.
Giosan et al. (Chap. 6, this volume, submitted [N1]) and Giosan (2001) integrated reflectance spectra (color) with sediment chemistry for post-late Pliocene sediments. Part of their work illustrates how color reflectance can be used to estimate geochemical variations, particularly carbonate content. Their regression analysis provides a rapid and nondestructive means of estimating carbonate content that would otherwise require labor-intensive sampling and laboratory analysis. They also identified several key events in ocean climate history. First, a pulse of sediment delivery from the St. Lawrence system at the Pliocene/Pleistocene boundary indicates a Laurentide Ice Sheet influence on climate and ocean circulation at that time. For the past 900 k.y., sedimentation patterns in the western North Atlantic were dominated by the combination of sediment delivery and deep ocean circulation. Second, they infer that increased strength and deepening of the Western Boundary Undercurrent began between about 500 and 400 ka. Since that time, sediment delivery has increased during glacial stages, and in particular since marine isotope Stage (MIS) 6.
Complementing the color and sediment chemistry approach is the more geological methodology of Yokokawa (Chap. 7, this volume) and Yokokawa and Franz (in press). Based on an analysis of X-ray images, Yokokawa (Chap. 7, this volume) described sedimentary structures associated with episodic currents (turbidity currents and benthic storms) and more subtle sedimentary features associated with changes in color or magnetic susceptibility but not associated with distinct sedimentary structures in visual inspection of the core surfaces. Yokokawa and Franz (in press) analyzed sedimentary features, grain size variations, and anisotropy of magnetic susceptibility on sediments deposited during MIS 8 and 9. They found that the Deep Western Boundary Current (DWBC) must have strengthened and moved downward during interglacial 9.3 and interstadial 8.5. In contrast to Blake Outer Ridge sites, patterns of sedimentation were found to differ at the mud wave cored at Site 1062.
Two studies investigated the relationship between physical properties, sedimentary processes, and paleoceanography. Haskell (Chap. 3, this volume) found that the sediments from Sites 1057 and 1061 have nearly isotropic magnetic susceptibility. He suggested this possibly indicates a circulation regime that was diffuse or at least currents that were too weak to effectively align oblate or elongate magnetic particles in the sediments at these sites. Dunbar (Chap. 5, this volume) examined why bulk density and P-wave velocity are in phase in some intervals but not in others within four Dansgaard-Oeschger (D-O) cycles that straddle the MIS 4/5 boundary (65-95 ka) in Hole 1063D (25-30 meters below seafloor [mbsf]) at the Bermuda Rise. Within this interval, density highs and lows are clearly associated with interstadial and stadial periods, respectively. In contrast, P-wave velocity can be high or low in the interstadials or stadials. Using X-ray diffraction, X-ray fluorescence, and particle size analyses, Dunbar showed that, as expected, calcite concentrations are highest in interstadials and aluminosilicate concentrations are highest in stadials, which explains much of the relationship between D-O cycles and density. More interestingly, he found that the composition of the aluminosilicates has subtle changes that correspond with the D-O cycles, with the ratios K2O/Al2O3 and Al2O3/TiO2 being low during interstadials and high during stadials. The P-wave velocity record does not, however, reflect simple fluctuations in the amount of calcite and aluminosilicates or in the composition of clay minerals. Instead, biogenic silica seems to play an important role, with high concentrations of biogenic silica correlating with intervals of high P-wave velocity, lower density, and increased particle sizes (increased volumes of silt and fine sand). Apparently, the biogenic silica forms a rigid framework that decreases density while being conducive to the transmission of acoustic waves. Hence, Dunbar concluded that the variable mixtures of calcite, aluminosilicates, and biogenic silica generate the distinct variations in physical properties as follows: intervals with high concentrations of calcite are characterized by high P-wave velocity and high density, intervals with high concentrations of aluminosilicates are characterized by low P-wave velocity and low density, and intervals with high concentrations of biogenic silica are characterized by high P-wave velocity and low density.
Winter (Chap. 8, this volume) cataloged the types and abundances of diatoms in the upper 30 m of Hole 1063D. Like Dunbar (Chap. 5, this volume), she found that high diatom abundance correlates with density lows in the 25-30 mbsf range and she confirmed that this relationship continues further uphole. She also observed diatom species that are restricted ecologically to coastal or littoral waters, requiring that they have been transported to Site 1063 by currents. She suggested the uphole decrease in abundance of these species may indicate changes in currents or in the water masses that reach the site over time.
During Leg 172, we were puzzled by the presence of a waxy, yellowish brown amorphous material, which occupied vertical fractures in drilling biscuits in two intervals (265.5-270.0 and 347.44-349.09 mbsf) in Hole 1063B. Eglinton and Whelan (Chap. 4, this volume) solved this puzzle by analyzing samples of the material with flash pyrolysis gas chromatography mass spectrometry. They concluded that the material was neither derived from petroleum nor geochemically related to thermally mature hydrocarbons. Instead, they suggested it is possibly a fragment of an algal mat or some other immature biologically derived material.
Flood and Giosan (submitted [N2]) focused exclusively on the multiple holes at Site 1062 along the crest of a mud wave near the Bahama Outer Ridge. They combined lithostratigraphy and acoustic stratigraphy to determine the history of wave migration and, hence, current speed. Wave migration was pronounced during the interval 5 to 1 Ma, but it has been slower since 1 Ma. During deglacial episodes of the past 0.5 m.y., there may have been no wave migration at all. This could reflect sluggish deep ocean circulation as ice sheets melted and suppressed ocean convection.