ALBERT

Description: We have generated approx. 300 Kyr records of biogenic opal, calcite, and organic carbon (Corg) for three cores in the eastern and central equatorial Pacific Ocean and have compared the records to determine whether common periods of biogenic sedimentation have occurred throughout the region. We find that Corg has been deposited in common pulses throughout the area, while opal has a much more local pattern of variation. Calcite varies regionally, but the record is shaped by superimposed dissolution and productivity processes. The most intense Corg peak occurs at 18 ka and can have greater than 2 times the Holocene Corg content. Other major Corg peaks occur 150 ka and perhaps at 280 ka. We have compared the Corg record in one of the cores, V19-28, to a model deepwater oxygen record developed from d13C data in the nearby V19-30 to test whether the Corg record has been mostly shaped by degradation or by the rain of organic matter from the euphotic zone. We found no coherence between the two records, implying that the Corg record is primarily a measure of productivity. By comparing the opal, calcite, and Corg records in V19-28, a core which is at or above the lysocline, we found that both increased calcite and opal deposition matches high Corg accumulation. We also found, however, that the calcite and opal records were uncorrelated, so that episodes of high opal deposition do not necessarily accumulate calcite rapidly. We hypothesize that at least two different plankton communities have been dominant in the waters above this site, one rich in opal-secreting plankton and one more dominated by calcite producers. The opal-rich plankton community was dominant during the intervals 10-15 ka and 35-60 ka.

Description: Site 639, drilled during Leg 103 of the Ocean Drilling Program, penetrated an Upper Jurassic to Lower Cretaceous carbonate platform on a tilted fault block along the Galicia margin off the northwest Iberian Peninsula. The carbonate platform is composed primarily of a sequence of dolomite overlying limestone. Samples were analyzed for mineral chemistry, stable isotope geochemistry, fluid inclusion microthermometry, and volatile contents and by dolomite pyrolysis mass spectrometry for trace sulfate minerals. The dolomite recovered from the Galicia margin at Site 639 formed during shallow burial from sulfate-bearing, hypersaline brines at slightly elevated temperatures. The light oxygen isotopic signatures of the dolomite are interpreted as the result of the evaporative loop and slightly elevated temperatures during dolomite formation or from reequilibration at higher temperatures during deeper burial. The hypersalinity is interpreted to be associated with a nearby, shallow restricted basin that formed during rifting of the Iberian margin from Newfoundland. The dolomitization of the platform is therefore a by-product of the rifting.

Description: Along the slopes and in adjacent basinal areas of Bahamian platforms, periplatform ooze forms the host sediment for variable proportions of carbonate turbidites. In unlithified sections, these turbidites appear as unconsolidated layers intercalated with stiffer ooze. Within an individual turbidite, differences in grain size result in variations in consolidation and physical properties. With decreasing grain size, water content and porosity decrease, and two distinct surfaces develop at the lower and upper boundaries of the turbidite. These surfaces are potential instability horizons where mass-wasting can occur. Therefore, a relation between turbidites and slumping frequency is proposed. The higher proportion of turbidites in sediments deposited on low-angle, accretionary terrains, such as the toe of the northern slope of Little Bahama Bank, probably facilitates frequent, small-scale slumping and creeping, as seen in seismic profiles. In contrast, slumping is less frequent along the steeper (12°) bypass slope in Exuma Sound, where turbidites are rarely found. Where the background sediment was initially a pelagic nannofossil ooze rather than a periplatform ooze, mineralogical composition results in lithification differences. In lithified sections having chalk as the background sediment, turbidites display a higher sonic velocity, indicating that they are the more competent beds. This lithification variation is the result of differential diagenesis between the platform-derived turbidites, enriched in metastable carbonates, and the calcitic nannofossil ooze of the background sediment. The different lithification owing to dissimilar mineralogical composition could also influence fluid migration in carbonate sequences. In a periplatform sequence, the more porous turbidite might be the fluid conduit, whereas in a pelagic sequence the chalky background sediment allows for fluid migration.