Chemical analyses of ODP Hole 105-647A basalts (Table 2)

Basalt samples recovered from the lowermost 37 m of Leg 105 Hole 647A in the Labrador Sea are fine- to medium grained, have microphenocrysts of clinopyroxene, and show little evidence of alteration. Chemically, these rocks are low potassium (0.01-0.09 wt% K20), olivine- to quartz-normative tholeiites that are also depleted in other incompatible elements. In terms of many of the incompatible trace elements, the Labrador Sea samples are similar both to iV-type midocean ridge basalts (MORBs) and to the terrestrial Paleocene volcanic rocks in the Davis Strait region of Baffin Island and West Greenland. However, significant differences are found in their strontium and neodymium isotope systematics. Hole 647A samples are more depleted in epsilon-Nd (+9.3) and are anomalously rich in 87Sr/86Sr (0.7040) relative to the Davis Strait basalts (epsilon-Nd +2.54 to + 8.97; mean 87Sr/86Sr, 0.7034). We conclude that the Hole 647A and Davis Strait basalts may have been derived from a similar depleted mantle source composition. In addition, the Davis Strait magmas were generated from mantle of more than one composition. We also suggest that there is no geochemical evidence from the Hole 647A samples to support or to refute the existence of foundered continental crust in the Labrador Sea.

Chiasmolithus species in middle Eocene and Oligocene sediments of ODP Holes 105-647A and 120-748B

In this preliminary biometric study of the calcareous nannofossil species Chiasmolithus expansus, Chiasmolithus oamaruensis, and Chiasmolithus altus from the upper middle Eocene to lower Oligocene of Sites 647 and 748, we document a complete gradation of forms among all three species. Chiasmolithus oamaruensis has significantly higher morphologic variance than the other species. The Chiasmolithus population at each site changes from C. expansus to C. oamaruensis and then to C. altus. This may not reflect a true evolutionary sequence because a major reversal in shape change of the central cross-bar structure accompanies this sequence, and because C. altus is morphologically closer to C. expansus than it is to C. oamaruensis. The change in the width of the cross-bar structure is primarily a result of changes in the alignment of the central connecting bar, rather than of changes in the cross-bar angle. At Site 748, two fluctuations in morphology produce sample populations intermediate between all three species. In addition, reported stratigraphic and paleogeographic occurrences of C. oamaruensis and C. altus show different latitudinal distributions. These morphological and distributional patterns may be explained by a continuous morphologic gradient between C. oamaruensis and C. altus, with C. oamaruensis occurring more commonly in cool-water paleoenvironments, and C. altus occurring more commonly in cold-water paleoenvironments. Thus, paleoenvironmental fluctuations at Site 748 may be the cause of the morphologic fluctuations in Chiasmolithus. This hypothesis can be tested against previously proposed evolutionary models by more detailed sampling of sections along a latitudinal transect.

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.