Age marks in bottom sediments of the Pacific Ocean

Distribution of K-Ar age marks in Pacific bottom sediments is shown on the base of large analytical material. Dependence of K-Ar age marks on abundance of potassium-bearing authigenic diagenetic minerals (glauconite, phillipsite) widely spread in the studied part of the World Oceans.

Major, trace element and XRF data for sediments and fish tooth samples, from a shallow marine setting with varying redox conditions (Site U1356, Wilkes Land Margin, E.Antarctica)

Fossil fish teeth from pelagic open ocean settings are considered a robust archive for preserving the neodymium (Nd) isotopic composition of ancient seawater. However, using fossil fish teeth as an archive to reconstruct seawater Nd isotopic compositions in different sedimentary redox environments and in terrigenous-dominated, shallow marine settings is less proven. To address these uncertainties, fish tooth and sediment samples from a middle Eocene section deposited proximal to the East Antarctic margin at Integrated Ocean Drilling Program Site U1356 were analyzed for major and trace element geochemistry, and Nd isotopes. Major and trace element analyses of the sediments reveal changing redox conditions throughout deposition in a shallow marine environment. However, variations in the Nd isotopic composition and rare earth element (REE) patterns of the associated fish teeth do not correspond to redox changes in the sediments. REE patterns in fish teeth at Site U1356 carry a typical mid-REE-enriched signature. However, a consistently positive Ce anomaly marks a deviation from a pure authigenic origin of REEs to the fish tooth. Neodymium isotopic compositions of cleaned and uncleaned fish teeth fall between modern seawater and local sediments and hence could be authigenic in nature, but could also be influenced by sedimentary fluxes. We conclude that the fossil fish tooth Nd isotope proxy is not sensitive to moderate changes in pore water oxygenation. However, combined studies on sediments, pore waters, fish teeth and seawater are needed to fully understand processes driving the reconstructed signature from shallow marine sections in proximity to continental sources. This article is protected by copyright. All rights reserved.

Rare earth element contents in deposits and minerals of the ocean floor

Current understanding of rare earth element (REE) geochemistry in the ocean is given in the book. Chemical properties determining REE migration ability in natural processes, sources of REE in the ocean, behavior of REE in river-sea mixing zones, fractionation of dissolved and particulate REE in ocean waters under aerobic and anaerobic conditions, distribution of REE in terrigenous, authigenic, hydrothermal and biogenic sediment components (clay, bone detritus, barite, phillipsite, Fe- and Mn-oxyhydroxides, Fe-Ca hydroxophosphate, diatoms and foraminiferas) are under consideration.

(Table 1) Quantitative mineralogy at DSDP Holes 59-447A and 59-450

The authigenic minerals contained in the altered basal intervals of volcaniclastic sediments from Sites 447 and 450 of Deep Sea Drilling Project Leg 59 are dioctahedral smectite (with variable crystallinity), phillipsite, and sanidine. Sanidine seems the most widespread and common product of basal alteration in the Philippine Sea marginal basins. The neomorphic mineral suites may have been produced by (1) halmyrolisis of the volcaniclastic sediments; (2) halmyrolisis of the underlying basalts; or (3) hydrothermalism associated with basaltic intrusions. At Site 450, other authigenic minerals occur (carbonates, analcime, clinoptilolite, Fe-Mn oxides), and the basal paragenesis is consistent with a hydro thermal origin. Such a process could have produced temperatures up to 200 °C in the tuffs lying as much as 2 meters above the contact with a basaltic intrusion. Products of low-temperature alteration, however, are also present in the altered interval of this site.

Chemical composition of layer silicates from the Sado Ridge, Sea of Japan

Detailed mineralogical investigations of high-Fe layer silicates from loose sediments (glauconite sands) of the Sado Ridge revealed that green aggregates found on submarine rises of the Japan Sea floor have different genesis. It was demonstrated that round dark green grains approximate micas in composition. Primary volcanic rocks presumably have undergone extensive secondary alterations and then were disintegrated. Their disintegration products (protoceladonite) filling pores were redeposited and buried in sediments for a long time. Angular green grains mainly represented by smectite also formed at lower temperatures during disintegration of altered volcanosedimentary rocks. These younger grains had no prolonged exposure. Pseudomorphs of siliceous microplankton consist of both hydromica and smectites. They are presumably authigenic products formed with participation of microorganisms or electrostatic processes (spherical shape), or their combination. The formation mechanism of minerals filling cavities in pyroclastics is not entirely clear.

Geochemistry of tropical Atlantic sediments of Marine Isotope Stage 4

Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are the main conduits for the supply of dissolved silicon (silicic acid) from the deep Southern Ocean (SO) to the low-latitude surface ocean and therefore have an important control on low-latitude diatom productivity. Enhanced supply of silicic acid by AAIW (and SAMW) during glacial periods may have enabled tropical diatoms to outcompete carbonate-producing phytoplankton, decreasing the relative export of inorganic to organic carbon to the deep ocean and lowering atmospheric pCO2. This mechanism is known as the "silicic acid leakage hypothesis" (SALH). Here we present records of neodymium and silicon isotopes from the western tropical Atlantic that provide the first direct evidence of increased silicic acid leakage from the Southern Ocean to the tropical Atlantic within AAIW during glacial Marine Isotope Stage 4 (~60-70 ka). This leakage was approximately coeval with enhanced diatom export in the NW Atlantic and across the eastern equatorial Atlantic and provides support for the SALH as a contributor to CO2 drawdown during full glacial development.

(Table 1) Mineralogical composition of sediments from DSDP Sites 69-504 and 69-505

We discuss the provenance of minerals detected by X-ray-diffraction analyses of sediments of Sites 504 and 505 of Deep Sea Drilling Project Leg 69. These are X-ray-amorphous material, opal-CT, calcite, quartz, feldspar, apatite, smectite, illite, kaolinite, magnetite, maghemite, pyrite, marcasite, barite, sepiolite, and clinoptilolite. Authigenic marcasite and clinoptilolite together with opal-CT are restricted to Site 504, indicating the special diagenetic conditions related to relatively high sediment temperatures at this site. Marcasite formation is likely dependent on the relatively low pH values of <7.1 found in interstitial waters of Site 504 sediments below 50 meters sub-bottom. Clinoptilolite evidently was formed by diagenetic alteration of rhyolitic volcanic glass or smectite plus biogenic silica within the chalk-limestone-chert sequence of Site 504, where opal-CT also reflects a high degree of silica dissolution and reprecipitation. This was a consequence of high temperatures (50-55 °C) at the base of the sediment column.

Chemical composition and dated ages of detrital smectites in sediments at DSDP Legs 43, 48 and 50

Mineralogical, morphological and isotopic (Rb-Sr and K-Ar) determinations were made on some detrital smectites of Palaeocene and Cenomanian ages from DSDP. drillings in the Atlantic Ocean. These minerals are not inert in their depositional environment; authigenic laths grow on detrital sheets with sharp borders. This authigenesis could occur slightly after deposition in a closed system, for some of these smectites. It has been tentatively quantified by the Rb-Sr and K-Ar isotopic methods, which seem also well suited to evaluate the chemical extent of this authigenesis. At least, no preferential loss of 40Ar vs. 87Sr could be detected in the minerals, even in those which are smaller than 0.2 ?m.

Nd and Sr isotope compositions, REE concentrations and Al/Ca ratios of different phases of surface sediments in the South Pacific

The radiogenic isotope composition of neodymium (Nd) and strontium (Sr) are useful tools to investigate present and past oceanic circulation or input of terrigenous material. We present Nd and Sr isotope compositions extracted from different sedimentary phases, including early diagenetic Fe-Mn coatings, "unclean" foraminiferal shells, fossil fish teeth, and detritus of marine surface sediments (core-tops) covering the entire midlatitude South Pacific. Comparison of detrital Nd isotope compositions to deep water values from the same locations suggests that "boundary exchange" has little influence on the Nd isotope composition of western South Pacific seawater. Concentrations of Rare Earth Elements (REE) and Al/Ca ratios of "unclean" planktonic foraminifera suggest that this phase is a reliable recorder of seawater Nd isotope composition. The signatures obtained from fish teeth and "nondecarbonated" leachates of bulk sediment Fe-Mn oxyhydroxide coatings also agree with "unclean" foraminifera. Direct comparison of Nd isotope compositions extracted using these methods with seawater Nd isotope compositions is complicated by the low accumulation rates yielding radiocarbon ages of up to 24 kyr, thus mixing the signal of different ocean circulation modes. This suggests that different past seawater Nd isotope compositions have been integrated in authigenic sediments from regions with low sedimentation rates. Combined detrital Nd and Sr isotope signatures indicate a dominant role of the Westerly winds transporting lithogenic material from South New Zealand and Southeastern Australia to the open South Pacific. The proportion of this material decreases toward the east, where supply from the Andes increases and contributions from Antarctica cannot be ruled out.

Sedimentary facies, clay mineralogy and geochemistry of the Paleogene sediments of ODP Site 105-647

Claystones immediately overlying the early Eocene age ocean-floor basalt, cored at Ocean Drilling Program (ODP) Site 647, underwent hydrothermal and thermal alterations originating from the basalt, which resulted in changes in both the mineralogical and chemical composition of the sediments. Chlorites and higher magnesium and iron concentrations were found in the lowermost sediment sequence. Upcore, changes in the bulk chemical composition of the sediments become smaller, when compensated for variations in the carbonate content originating from biogenic and authigenic components. Chlorite disappears upcore, but still only part of the swelling clay minerals have survived the thermal influence. Thirty meters above the basalt, the clay mineralogy and chemical composition become uniform throughout the Paleogene section. Iron-rich smectites (i.e., nontronitic types), totally dominate the clay mineral assemblage. Biogenic components, responsible for the dominant part of the calcite and cristobalite contents, vary in amount in the upper part, and so do the authigenic carbonate and sulfide contents. Detrital components, such as kaolinite, illite, quartz, and feldspars, make up a very small proportion of the sediment record. The nontronitic smectites are believed to be authigenic, formed by a supply of iron from the continuous formation of ocean-floor basalt in the ridge area that reacted with the detrital and biogenic silicates and alumina silicates.

(Table 1) Gypsum, pyrite and glauconite at DSDP Hole 36-329

Authigenic gypsum, pyrite, and glauconite are disseminated throughout an unusually long (346 m) Miocene section of mixed biogenic carbonate and diatomaceous ooze drilled on the Falkland Plateau at DSDP Site 329 (water depth, 1519 m). The present organic carbon content of the sediment is low, ranging between 0.1 and 0.7%. Gypsum occurs as euhedral single or twinned crystals of selenite up to 5 mm in diameter, sometimes in the form of gypsum rosettes. These crystals are intact and unabraded, comprising up to 4% of the washed sample. The authigenic nature of the gypsum is demonstrated by the presence of diatoms and radiolarians embedded within the gypsum crystals. The gypsum co-occurs with pyrite and glauconite in these samples. The pyrite occurs as framboids, foraminiferal infillings, rods, and granular sheetlike masses composed of pyrite octahedra. The glauconite occurs as foraminiferal infillings and as free grains. The gypsum and pyrite were identified by energy-dispersive X-ray analysis and scanning electron micrographs. Some of the gypsum has grown on pyrite, indicating that it precipitated after the pyrite, perhaps in response to a change in pH conditions. The formation of the mineral suite can be explained by current models of in situ sulfide and sulfate precipitation coincident with diagenesis and oxidation of much of the original organic carbon.

Mineral composition, rare earth elements, and trace elements from samples of the northern Congo deep-sea fan

Authigenic carbonates were collected from methane seeps at Hydrate Hole at 3113 m water depth and Diapir Field at 2417 m water depth on the northern Congo deep-sea fan during RV Meteor cruise M56. The carbonate samples analyzed here are nodules, mainly composed of aragonite and high-Mg calcite. Abundant putative microbial carbonate rods and associated pyrite framboids were recognized within the carbonate matrix. The d13C values of the Hydrate Hole carbonates range from -62.5 permil to -46.3 permil PDB, while the d13C values of the Diapir Field carbonate are somewhat higher, ranging from -40.7 permil to -30.7 permil PDB, indicating that methane is the predominant carbon source at both locations. Relative enrichment of 18O (d18O values as high as 5.2 permil PDB) are probably related to localized destabilization of gas hydrate. The total content of rare earth elements (REE) of 5% HNO3-treated solutions derived from carbonate samples varies from 1.6 ppm to 42.5 ppm. The shale-normalized REE patterns all display positive Ce anomalies (Ce/Ce* > 1.3), revealing that the carbonates precipitated under anoxic conditions. A sample from Hydrate Hole shows a concentric lamination, corresponding to fluctuations in d13C values as well as trace elements contents. These fluctuations are presumed to reflect changes of seepage flux.