Mineralogy of the clay fraction from Cenozoic Strata at DSDP Leg 94

Leg 94 Sites are located in a large geographic area of the northeastern Atlantic. Clay mineral analyses of the sediments recovered on Leg 94 (Eocene to the present), together with results obtained from previous DSDP legs (47B, 48, 80, 81, 82), provide greater insight into the paleoenvironmental evolution of the northeastern Atlantic. The characteristics of Eocene clay sediments are regional, reflecting, in the absence of strong bottom currents, the influence of neighboring petrographic environments: basic volcanic rocks (Sites 403-406, 552, and 608) and acid volcanic rocks (Sites 508 to 510). During the Oligocene, atmospheric circulation patterns left their mineralogical signatures in the southern part of the area investigated (Sites 558 and 608), whereas during the Miocene the intrusion of northern water masses led to the gradual homogenization of the clay sedimentation throughout the North Atlantic. In the late Pliocene, input from glacial sources became widespread.

Geochemistry, SiO2 minerals and isotopes at DSDP Hole 72-516F

Cherts recovered during DSDP Leg 72 from Rio Grande Rise sediments (Site 516) consist of both cristobalite and quartz, and contain ghosts of foraminifers and (more rare) radiolarians. Porcelanite made of disordered cristobalite is found in most old enclosing sediments. Local dissolution of siliceous microfossils during diagenesis is the most likely source of the silica required for the chert formation. As sediment age increases, the proportion of biogenic silica decreases and authigenic silica increases.

Results of bulk sediment X-ray diffraction analysis and quantification of mineral phases based on the RockJock and on the QUAX quantitative analysis

We have performed quantitative X-ray diffraction (qXRD) analysis of 157 grab or core-top samples from the western Nordic Seas between (WNS) ~57°-75°N and 5° to 45° W. The RockJock Vs6 analysis includes non-clay (20) and clay (10) mineral species in the <2 mm size fraction that sum to 100 weight %. The data matrix was reduced to 9 and 6 variables respectively by excluding minerals with low weight% and by grouping into larger groups, such as the alkali and plagioclase feldspars. Because of its potential dual origins calcite was placed outside of the sum. We initially hypothesized that a combination of regional bedrock outcrops and transport associated with drift-ice, meltwater plumes, and bottom currents would result in 6 clusters defined by "similar" mineral compositions. The hypothesis was tested by use of a fuzzy k-mean clustering algorithm and key minerals were identified by step-wise Discriminant Function Analysis. Key minerals in defining the clusters include quartz, pyroxene, muscovite, and amphibole. With 5 clusters, 87.5% of the observations are correctly classified. The geographic distributions of the five k-mean clusters compares reasonably well with the original hypothesis. The close spatial relationship between bedrock geology and discrete cluster membership stresses the importance of this variable at both the WNS-scale and at a more local scale in NE Greenland.

Biogenic silica in ODP Hole 105-647A

Eocene to Holocene sediments from Ocean Drilling Program (ODP) Site 647 (Leg 105) in the southern Labrador Sea, approximately 200 km south of the Gloria Drift deposits, were investigated for their biogenic silica composition. Three sections of different diagenetic alteration products of primary siliceous components could be distinguished: (1) opal-A was recorded in the Miocene and the early Oligocene time intervals with strongly corroded siliceous skeletons in the Miocene and mostly well preserved biogenic opal in the early Oligocene; (2) opal-CT precipitation occurs between 250-440 meters below seafloor (mbsf) (earliest Oligocene to late Eocene); (3) between 620-650 mbsf (early/middle Eocene), biogenic opal was transformed to clay minerals by authigenesis of smectites. Using accumulation rates of biogenic opal, paleoproductivity was estimated for the early Oligocene to late Eocene interval. A maximum productivity of biogenic silica probably occurred between 35.5 and 34.5 Ma (early Oligocene). No evidence for opal sedimentation during most of middle Eocene was found. However, at the early/middle Eocene boundary (around 52 Ma), increased opal fluxes were documented by diagenetic alteration products of siliceous skeletons.