Carbonate and Aluminium accumulation in the central equatorial Pacific Ocean

We examined the flux of Al to sediment accumulating beneath the zone of elevated productivity in the central equatorial Pacific Ocean, along a surface sediment transect at 135°W as well as downcore for a 650 kyr record at 1.3°N, 133.6°W. Across the surface transect, a pronounced, broadly equatorially symmetric increase in Al accumulation is observed, relative to Ti, with Al/Ti ratios reaching values 3-4 times that of potential detrital sources. The profile parallels biogenic accumulation and the modeled flux of particulate 234Th, suggesting rapid and preferential adsorptive removal of Al from seawater by settling biogenic particles. Normative calculations confirm that most Al is unsupported by the terrigenous fraction. The observed distributions are consistent with previous observations of the relative and absolute behavior of Al and Ti in seawater, and we can construct a reasonable mass balance between the amount of seawater-sourced Al retained in the sediment and the amount of seawater Al available in the overlying column. The close tie between Al/Ti and biogenic accumulation (as opposed to concentration) emphasizes that biogenic sedimentary Al/Ti responds to removal-transport phenomena and not bulk sediment composition. Thus, in these sediments dominated by the biogenic component, the bulk Al/Ti ratio reflects biogenic particle flux, and by extension, productivity of the overlying seawater. The downcore profile of Al/Ti at 1.3°N displays marked increases during glacial episodes, similar to that observed across the surface transect, from a background value near Al/Ti of average upper crust. The excursions in Al/Ti are stratigraphically coincident with maxima in both bulk and CaCO3 accumulation and the excess Al appears to not be preferentially affiliated with opaline or organic phases. Consistent with the similar behavioral removal of Al and 234Th, the latter of which responds to the total particle flux, the Al flux reflects carbonate accumulation only because carbonate comprises the dominant flux in these particular deposits. These results collectively indicate that (1) Al in biogenic sediment and settling biogenic particles is strongly affected by a component adsorbed from seawater. Therefore, the common tenet that Al is dominantly associated with terrestrial particulate matter, and the subsequent use of Al distributions to calculate the abundance and flux of terrestrial material in settling particles and sediment, needs to be reevaluated. (2) The Al/Ti ratio in biogenic sediment can be used to trace the productivity of the overlying water, providing a powerful new paleochemical tool to investigate oceanic response to climatic variation. (3) The close correlation between the Al/Ti productivity signal and carbonate maxima downcore at 1.3°N suggests that the sedimentary carbonate maxima in the central equatorial Pacific Ocean record increased productivity during glacial episodes.

Chemical and isotopic compositions of zircons from an eclogitized gabbronorite dike at the Gridino village, Northern Karelia

A comprehensive (mineralogical, geochronological, and geochemical) study of zircons from an eclogitized gabbronorite dike was carried out in order to identify reliable indicators (mineralogical and geochronological) of genesis of the zircons in their various populations and, correspondingly, ages of certain geological events (magmatic crystallization of the gabbroids, their eclogitization, and overprinted retrograde metamorphism). Three populations of zircons separated from two rock samples comprised xenogenic, magmatic (gabbroic), and metamorphic zircons, with the latter found exclusively in the sample of retrograded eclogitized gabbroids. Group I zircons are xenogenic and have a Meso- to Neoarchean age. Mineral inclusions in them (quartz, apatite, biotite, and chlorite) are atypical of gabbroids, and geochemistry of these zircons is very diverse. Group II zircons contain mineral inclusions of ortho- and clinopyroxene and are distinguished for their very high U, Th, Pb, and REE concentrations and Th/U ratios. These zircons formed during the late magmatic crystallization of the gabbroids at temperatures of 1150-1160°C, and their U-Pb age 2389±25 Ma corresponds to this process. Eclogite mineral assemblages crystallized shortly after the magmatic process, as follows from the fact that marginal portions of prismatic zircons contain clinopyroxene inclusions with elevated contents of the jadeite end-member. Group III zircons contain rare amphibole and biotite inclusions and have low Ti, Y, and REE concentrations, low Th/U ratios, high Hf concentrations, contain more HREE than LREE, and have U-Pb age 1911±9.5 Ma, which corresponds to age of overprinted amphibolite-facies metamorphism.

Geochemistry of ODP Leg 129 igneous rocks

This report presents all the available major and trace elemental analyses and Sr, Nd, and Pb isotopic compositions of basaltic rocks recovered from Ocean Drilling Program Sites 800, 801, and 802 during Leg 129 (Table 1). Its main purpose is to provide other investigators a complete summary of geochemical data for Leg 129 basement basalts that they can use for later work. Detailed discussions of the data are presented elsewhere in the volume by Floyd and Castillo (Site 801 geochemistry and petrogenesis, dataset: doi:10.1594/PANGAEA.779154) Floyd et al. (Sites 800 and 802 geochemistry and petrography, dataset: doi:10.1594/PANGAEA.779129), Alt et al. (Site 801 alteration, dataset: doi:10.1594/PANGAEA.779207), and Castillo et al. (Sr, Nd, and Pb isotope geochemistry of Leg 129 basalts, dataset: doi:10.1594/PANGAEA.779191).