Geochemistry of ODP Hole 135-839B basalts

Experimental phase relations were used to assess the role of volatiles and crustal level fractional crystallization in the petrogenesis of lavas from Hole 839B in the central Lau Basin. Melting experiments were performed on Sample 135-839B-15R-2, 63-67 cm, at 1 atm, anhydrous, and 2 kbar, H2O-saturated (~6 wt% H2O in the melt) to determine the influence of variable pressure and H2O content on phase appearances, mineral chemistry, and liquid line of descent followed during crystallization. The effects of H2O are to depress the liquidus by ~100°C, and to suppress crystallization of plagioclase and orthopyroxene relative to olivine and high-Ca clinopyroxene. At 1 atm, anhydrous, olivine and plagioclase coexist near the liquidus, whereas orthopyroxene and then clinopyroxene appear with decreasing temperature. Crystallization of 50 wt% produces a residual liquid that is rich in FeO* (10.8 wt%) and poor in Al2O3 (13.6 wt%). At 2 kbar, H2O-saturated, the liquidus phases are olivine and chromian spinel, with high-Ca clinopyroxene appearing after ~10% crystallization. Plagioclase saturation is suppressed until ~20% crystallization has occurred. The residual liquid from 35 wt% crystallization is rich in AI2O3 (17.4 wt%), and poor in MgO (4.82 wt%); it contains moderate FeO* (8.2 wt%), and resembles the low-MgO andesites recovered from Hole 839B. On the basis of these experiments we conclude that the primitive lavas recovered from Hole 839B have experienced crystallization along the Ol + Cpx saturation boundary, under hydrous conditions (an ankaramitic liquid line of descent), and variable amounts of olivine and chromian spinel accumulation. The low-MgO andesites from Hole 839B are the products of hydrous fractional crystallization, at crustal pressures, of a parent magma similar to basaltic andesite Sample 135-839B-15R-2, 63-67 cm.

Stable isotope record of foraminifera from the subarctic Pacific

Benthic (Uvigerina spp., Cibicidoides spp., Gyroidinoides spp.) and planktonic (N. pachyderma sinistral, G. bulloides) stable isotope records from three core sites in the central Gulf of Alaska are used to infer mixed-layer and deepwater properties of the late glacial Subarctic Pacific. Glacial-interglacial amplitudes of the planktonic delta18O records are 1.1-1.3 per mil, less than half the amplitude observed at core sites at similar latitudes in the North Atlantic; these data imply that a strong, negative deltaw anomaly existed in the glacial Subarctic mixed layer during the summer, which points to a much stronger low-salinity anomaly than exists today. If true, the upper water column in the North Pacific would have been statically more stable than today, thus suppressing convection even more efficiently. This scenario is further supported by vertical (i.e., planktic versus benthic) delta18O and delta13C gradients of >1 per mil, which suggest that a thermohaline link between Pacific deep waters and the Subarctic Pacific mixed layer did not exist during the late glacial. Epibenthic delta13C in the Subarctic Pacific is more negative than at tropical-subtropical Pacific sites but similar to that recorded at Southern Ocean sites, suggesting ventilation of the deep central Pacific from mid-latitude sources, e.g., from the Sea of Japan and Sea of Okhotsk. Still, convection to intermediate depths could have occurred in the Subarctic during the winter months when heat loss to the atmosphere, sea ice formation, and wind-driven upwelling of saline deep waters would have been most intense. This would be beyond the grasp of our planktonic records which only document mixed-layer temperature-salinity fields extant during the warmer seasons. Also we do not have benthic isotope records from true intermediate water depths of the Subarctic Pacific.

Neodymium isotope record of late Paleocene to eraly Eocene fish teeth

High-resolution, fish tooth Nd isotopic records for eight Deep Sea Drilling Project and Ocean Drilling Program sites were used to reconstruct the nature of late Paleocene-early Eocene deep-water circulation. The goal of this reconstruction was to test the hypothesis that a change in thermohaline circulation patterns caused the abrupt 4-5°C warming of deep and bottom waters at the Paleocene/Eocene boundary - the Paleocene-Eocene thermal maximum (PETM) event. The combined set of records indicates a deep-water mass common to the North and South Atlantic, Southern and Indian oceans characterized by mean epsilon-Nd values of ~-8.7, and different water masses found in the central Pacific Ocean (epsilon-Nd ~-4.3) and Caribbean Sea (epsilon-Nd ~1.2). The geographic pattern of Nd isotopic values before and during the PETM suggests a Southern Ocean deep-water formation site for deep and bottom waters in the Atlantic and Indian ocean basins. The Nd data do not contain evidence for a change in the composition of deep waters prior to the onset of the PETM. This finding is consistent with the pattern of warming established by recently published stable isotope records, suggesting that deep- and bottom-water warming during the PETM was gradual and the consequence of surface-water warming in regions of downwelling.

Chemical composition of East Pacific Rise basal sediments

Analyses by atomic absorption spectrophotometry and spark-source mass spectrography of 25 basal metalliferous sediment units from widely spaced locations on the western flank of the East Pacific Rise show that the deposits are enriched relative to normal pelagic sediment in Fe, Mn, Ni, Cu, Pb, Zn, and many trace elements. The elements are partitioned differently between the various mineralogic constituents of the sediment, with Fe and Mn largely in separate phases and many of the remaining elements primarily associated with reducible ferromanganese oxide minerals but also with iron minerals and other phases. Most of the iron in the deposits is probably of volcanic origin, and much of the manganese and minor elements is derived from sea water. The bulk composition of the deposits varies with age; this is thought to be due to variations in the incidence of volcanic activity at the East Pacific Rise crest where the deposits were formed.

(Table 4) Estimated surface water phosphate concentrations for the past 200 kyr at the location of sediment core GeoB1016-3

We compared ocean atlas values of surface water [PO4]3- and [CO2(aq)] against the carbon isotopic fractionation (ep) of alkenones obtained from surface sediments of the South Atlantic and the central Pacific (Pacific data are from Pagani et al. 2002, doi:10.1029/2002PA000756). We observed a positive correlation between ep and 1/[CO2(aq)], which is opposite of what would be expected if the concentration of CO2(aq) were the major factor controlling the carbon isotopic fractionation of C37:2 alkenones. Instead, we found inverse relationships between ep and [PO4]3- for the two ocean basins (for the Atlantic, ep = -4.6*[PO4]3- + 15.1, R = 0.76; for the Pacific, ep = -4.1*[PO4]3- + 13.7, R = 0.64), suggesting that ep is predominantly controlled by growth rate, which in turn is related to nutrient concentration. The similarity of the slopes implies that a general relationship between both parameters may exist. Using the relationship obtained from the South Atlantic, we estimated surface water nutrient concentrations for the past 200,000 years from a deep-sea sediment core recovered off Angola. Low ep values, indicating high nutrient concentrations, coincide with high contents of total organic carbon and C37 alkenones, low surface water temperatures, and decreased bulk d15N values, suggesting an increased upwelling of nutrient-rich cool subsurface waters as the main cause for the observed ep decrease.

Benthic foraminifera of the central Indian Ocean

Late Oligocene to late Pliocene vertical water-mass stratification along depth traverses in the northern Indian Ocean is depicted in this paper by benthic foraminifer index faunas. During most of this time, benthic faunas indicate well-oxygenated, bottom-water conditions at all depths except under the southern Indian upwelling and in the Pliocene in the southern Arabian Sea. Faunas suggest the initiation of lower oxygen conditions at intermediate depths in the northern Indian Ocean beginning in Oligocene Zone P21a. Lower oxygen conditions intensified during primary productivity pulses, possibly related to increased upwelling vigor, in the latest Oligocene and throughout most of the late middle through late Miocene. During times of elevated primary production, there may be more oxygen flux into sedimentary pore waters and the shallow infaunal habitat may become more oxygenated. One criterion for locating the source of "new" water masses is vertical homogeneity of benthic foraminifer indexes for well-oxygenated water masses from intermediate through abyssal depths. In the northern Mascarene Basin, this type of faunal homogeneity with depth corroborates the proposal that the northern Indian Ocean was an area of sinking well-oxygenated waters through most of the Miocene before Zone N17. Oxygenated, possibly "new" intermediate-water masses in the low- to middle-latitude Mascarene and Central Indian basins first developed in the late Oligocene. These well-oxygenated waters were probably more fertile than the Antarctic Intermediate Waters (AAIW) that cover intermediate depths in these areas today. Production of intermediate waters more similar to modern AAIW is indicated by the sparse benthic population of epifaunal rotaloid species in the northern Mascarene Basin during middle Miocene Zone N9 and from early through late Pliocene time. Deep-water characteristics are more difficult to interpret because of the extensive redeposition at the deeper sites. Redeposited intermediate, rather than shallow, water fossils and erosion from north to south in the Mascarene Basin are incompatible with the sluggish circulation from south to north through the western Indian Ocean basins today. Such erosion could result from the vigorous sinking of an intermediate-depth water mass of northern origin. Before late Oligocene Zone P22, benthic faunas indicate a twofold subdivision of the troposphere, with the boundary between upper and lower well-oxygenated water masses located from 2500-3000 mbsl. No characteristic bottom-water fauna developed before the end of late Oligocene Zone P22. Deep and abyssal benthic indexes suggest the development of water masses similar to those of the present day in the latest Miocene. Faunas containing deep-water benthic indexes, including the uvigerinids, suggestive of a water mass similar to modern Indian Deep Water (IDW), appeared during the late Miocene in the northern Mascarene and Central Indian basins. In the early Pliocene, this deep-water fauna was found only in the Central Indian Basin, whereas a fauna typical of modern Antarctic Bottom Water (AABW) spread through deep waters at 2800 mbsl in the Mascarene Basin. By late Pliocene Zone N21, however, deep-water faunas similar to their modern analogs were developed in both the eastern and western basins. Abyssal faunas, studied only in the Mascarene Basin, show more or less similarity to those under modern AABW. Bottom-water faunas containing Nuttallides umbonifera or Epistominella exiguua were first differentiated at the end of Zone P22, then appeared episodically during the early Miocene. These AABW-type faunas reappeared and migrated updepth into deep waters during the glacial episodes at the end of the Miocene and at the beginning of the Pliocene. By late Pliocene Zone N21, however, a bottom-water fauna similar to that under eastern Indian Bottom Water (IBW) developed in the Mascarene Basin. Modern bottom-water characteristics of the Mascarene Basin must have developed after ZoneN21.

Accumulation rates and grain sizes of eolian dust in sediments of the Intertropical Convergence Zone in the northeast Pacific

A 328 cm-long piston core (KODOS 02-01-02) collected from the northeast equatorial Pacific at 16°12'N, 125°59'W was investigated for eolian mass fluxes and grain sizes to test these proxies as a tool for the paleo-position of the Intertropical Convergence Zone (ITCZ). The eolian mass fluxes of the lower interval below 250 cm (15.5-7.6 Ma) are very uniform at 5 +/- 1 mg/cm**2/kyr, while those of the upper interval above 250 cm (from 7.6 Ma) are over 2 times higher than the lower interval at 12 +/- 1 mg/cm**2/kyr. The median grain size of the eolian dusts in the lower interval increases from 8.4 Phi to 8.0 Phi downward, while that of the upper interval varies in a narrow range from 8.8 Phi to 8.6 Phi. The determined values compare well in magnitude to those of central Pacific sediments for the upper interval and equatorial and southeast Pacific sediments for the lower interval. This result suggests a possibility that the study site had been under the influence of southeast trade winds at its earlier depositional period due to the northerly position of the ITCZ, and subsequently of the northeast trade winds for a later period when the upper sediments were deposited. This interpretation is consistent with a mineralogical and geochemical study published elsewhere that assigned the provenance of the study core dust to Central/South America for the lower interval and to Asia for the upper interval. This study suggests that the distinct differences in eolian mass flux and grain size observed across the ITCZ can be used to trace the paleo-latitude of the ITCZ.

Uranium and radioactive isotopes in bottom sediments and Fe-Mn nodules and crusts of seas and oceans

The main stages of the sedimentary cycle of uranium in modern marine basins are under consideration in the book. Annually about 18 thousand tons of dissolved and suspended uranium enters the ocean with river runoff. Depending on a type of a marine basin uranium accumulated either in sediments of deep-sea basins, or in sediments of continental shelves and slopes. In the surface layer of marine sediments hydrogenic uranium is predominantly bound with organic matter, and in ocean sediments also with iron, manganese and phosphorus. In diagenetic processes there occurs partial redistribution of uranium in sediments, as well as its concentration in iron-manganese, phosphate and carbonate nodules and biogenic phosphate detritus. Concentration of uranium in marine sediments of various types depending on their composition, as well as on forms of its entering, degree of differentiation and of sedimentation rates, on hydrochemical regime and water circulation, and on intensity of diagenetic processes.

Age determinations and analyses of organic matter of sediments from the Argentine Basin

The first radiocarbon chronology for sediments of the Argentine basin has been determined using accelerator mass spectrometer (AMS) analyses of 54 total organic carbon samples from four box and two piston cores collected from the downstream and upstream sides of two central Argentine Basin mudwaves. Throughout the Holocene, sediment from the geomorphically defined upstream side of each wave accumulated at rates of 30 to 105 cm/1000 years. Sediments from the downstream side of each wave accumulated at rates of 2 to 10 cm/1000 years in the late and early Holocene, while the mid Holocene is characterized by sedimentation rates less than 1.0 cm/1000 years. During the mid-Holocene, increased aridity reduced chemical weathering and the flow of the rivers draining to the continental shelf, causing a concomitant decrease in fine-grained terrigenous input to the basin as evidenced by decreased sedimentation rates, lower N/C ratios, and depleted delta13Corg values. It is estimated that all of the organic carbon deposited in the central basin during the mid-Holocene was of a marine origin. During the late and early Holocene, however, approximately 35% of the organic carbon deposited was of terrestrial origin. Bottom water flow speeds in the late Holocene were estimated using a lee-wave model and found to average 14 cm/s. This estimate is comparable to 10 cm/s mean and 15-20 cm/s maximum flow speeds measured by current meters deployed within the basin. Flow speeds in the Argentine Basin were 10% higher than today from 8000 to 2000 B.P., and are consistent with a general invigoration of thermohaline circulation that began between 9000 and 8000 B.P. It is proposed that the introduction of warm, salty Indian Ocean water into the northern North Atlantic at 9000 B.P. was the mechanism that provided the excess salt needed to stabilize the North Atlantic Deep Water thermohaline circulation system in its present mode.

Concentrations and accumulation rates of platinum-group elements and rhenium in Pacific and Atlantic sediments, DSDP and ODP data

The nature of Re-platinum-group element (PGE; Pt, Pd, Ir, Os, Ru) transport in the marine environment was investigated by means of marine sediments at and across the Cretaceous-Tertiary boundary (KTB) at two hemipelagic sites in Europe and two pelagic sites in the North and South Pacific. A traverse across the KTB in the South Pacific pelagic clay core found elevated levels of Re, Pt, Ir, Os, and Ru, each of which is approximately symmetrically distributed over a distance of ~1.8 m across the KTB. The Re-PGE abundance patterns are fractionated from chondritic relative abundances: Ru, Pt, Pd, and Re contents are slightly subchondritic relative to Ir, and Os is depleted by ~95% relative to chondritic Ir proportions. A similar depletion in Os (~90%) was found in a sample of the pelagic KTB in the North Pacific, but it is enriched in Ru, Pt, Pd, and Re relative to Ir. The two hemipelagic KTB clays have near-chondritic abundance patterns. The ~1.8-m-wide Re-PGE peak in the pelagic South Pacific section cannot be reconciled with the fallout of a single impactor, indicating that postdepositional redistribution has occurred. The elemental profiles appear to fit diffusion profiles, although bioturbation could have also played a role. If diffusion had occurred over ~65 Ma, the effective diffusivities are ~10**?13 cm**2/s, much smaller than that of soluble cations in pore waters (~10**?6 cm**2/s). The coupling of Re and the PGEs during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances. If redistribution was caused by diffusion, then the effective diffusivities are the same. Fractionation of Os from Ir during the KTB interval must therefore have occurred during aqueous transport in the marine environment. Distinctly subchondritic Os/Ir ratios throughout the Cenozoic in the South Pacific core further suggest that fractionation of Os from Ir in the marine environment is a general process throughout geologic time because most of the inputs of Os and Ir into the ocean have Os/Ir ratios >/=1. Mass balance calculations show that Os and Re burial fluxes in pelagic sediments account for only a small fraction of the riverine Os (<10%) and Re (<0.1%) inputs into the oceans. In contrast, burial of Ir in pelagic sediments is similar to the riverine Ir input, indicating that pelagic sediments are a much larger repository for Ir than for Os and Re. If all of the missing Os and Re is assumed to reside in anoxic sediments in oceanic margins, the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes. However, putting all of the missing Os and Re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of Os at pelagic KTB sites, where at most ~25% of the K-T impactor's Os could have passed through estuaries. If Os is preferentially sequestered in anoxic marine environments, it follows that the Os/Ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition. There is thus a clear fractionation of Os and Re from Ir in precipitation out of sea water in pelagic sections. Accordingly, it is inferred here that Re and Os are removed from sea water in anoxic marine depositional regimes.