Lithologic, mineral, chemical and isotopic compositions of altered rocks with increasing distance across the eastern flank of the Juan de Fuca Ridge, ODP Leg 168 data

During ODP Leg 168, 10 sites were drilled across the eastern flank of the Juan de Fuca Ridge (JdFR), to examine the conditions of fluid-rock interaction in three distinct hydrothermal regimes (referred to as the Hydrothermal Transition (HT), Buried Basement (BB) and Rough Basement (RB) transects), extending over a ~120 km linear transect perpendicular to the spreading ridge. This was carried out in an attempt to constrain the conditions and processes that control the location, style and magnitude of low temperature (<150°C) fluid-rock interaction within this setting. This paper presents new data on the petrology, mineral chemistry and whole rock strontium and oxygen isotopic compositions of basalts from the eastern flank of the JdFR, in order to investigate the extent, style and sequence of low-temperature hydrothermal alteration and to establish how the hydrothermal regime evolved with time. Throughout the flank, a progressive sequence of low-temperature hydrothermal alteration has been identified, marked by changes in the dominant secondary mineral assemblage, changing from: chlorite+chlorite/smectite; to iron oyxhydroxide+celadonite; to saponite+/-pyrite; culminating at present with Ca- to CaMg(+/-Fe,Mn)-carbonate. The changes in secondary mineralogy have been used to infer a series of systematic shifts in the conditions of alteration that occurred as the basement moved off-axis and was progressively buried by sediment. In general, hydrothermal alteration of the uppermost oceanic crust commenced under open, oxidative conditions, with interaction between unmodified to slightly modified seawater and basaltic crust, to a regime in which circulation of a strongly modified seawater-derived fluid was more restricted, and alteration occurred under non-oxidative conditions. Across the flank, petrological observations and microprobe analyses indicate that the observed ranges in secondary mineral composition are directly related to changes in the geochemical and textural characteristics of the basement, as well as to interaction between fluids and phases from the four stages of alteration. This is suggestive of an increase in fluid-rock increased with time. Whole rock 87Sr/86Sr and d18O analyses of basalts from across the eastern flank of the JdFR reinforce petrological observations, with 87Sr/86Sr and d18O values slightly elevated above accepted pristine MORB values for this region. These results are consistent with an increase in the amount of fluid-rock interaction with time. Across the flank, enrichment in the 87Sr/86Sr and d18O relative to MORB, is influenced by a number of factors, including: local and regional variations in the crustal lithology and structure; the age of the crust; the extent of bulk rock alteration; and theoretically, the relative abundance of different isotopically-enriched secondary mineral phases in the crust.

(Table T1) Trace element concentration and isotopic composition of pore waters from ODP Leg 168 sites

Pore water was collected from each of 10 sites during Ocean Drilling Program (ODP) Leg 168 on the eastern flank of the Juan de Fuca Ridge. These ten sites delineate a transect perpendicular to the present ridge axis and span a crustal age of 0.86-3.59 Ma. At nine of the ten sites the entire sediment section, which ranged from 41.3 to 613.8 m thick, was cored and attempts were made to recover at least one whole round of sediment per section of core for extraction of pore water. Several (2-5) whole-round sediment samples were taken from the uppermost and lowermost cores to constrain the chemical gradient near the sediment/water and sediment/basalt interfaces, respectively. Pore water was extracted from whole-round sediment core sections by squeezing only the most pristine sediment in a titanium squeezer designed by Manheim and Sayles (1974). Two additional water samples were collected in situ using the water-sampler temperature probe (WSTP; Barnes, 1988, doi:10.2973/odp.proc.ir.110.104.1988). Both of these samples were collected in the cased section of the open borehole from ODP Hole 1026B. Formation fluids were flowing up the cased hole into the overlying deep seawater (Fisher et al., 1997, doi:10.1029/97GL01286). Detailed descriptions of the sampling methods that were used to collect fluids are given by the Shipboard Scientific Party (Davis, Fisher, Firth, et al., 1997, doi:10.2973/odp.proc.ir.168.1997).

Stable isotopic composition and carbonate concentrations of sediments of the Cretaceous/Paleogene boundary in the Antarctic

Stable isotopic records across the Cretaceous/Paleogene (K/P) boundary in Maud Rise Holes 689B and 690C indicate that significant climatic changes occurred during the latest Cretaceous, beginning approximately 500 k.y. prior to the mass extinction event and the enrichment of iridium at the K/P boundary (66.4 Ma). An oxygen isotopic decrease of ~0.7 per mil - ~1.0 per mil is recorded in the Late Cretaceous planktonic and benthic foraminifers between 66.9 and 66.6 Ma. The negative isotope excursion was followed by a positive excursion of similar magnitude between 66.6 Ma (latest Cretaceous) and ~66.3 Ma (earliest Paleocene). No other isotopic excursions of this magnitude are recorded in the planktonic and benthic microfossil records 1.0 m.y prior to, and for 2.0 m.y following the mass extinction event at the K/P boundary. The magnitude and duration of these isotopic excursions were similar to those at the Paleocene/Eocene and Eocene/Oligocene boundaries. A major d13C excursion occurred 200 k.y. prior to the boundary, involving a positive shift in planktonic and benthic d13C of ~0.5 per mil - 0.75 per mil. Similar changes observed in other deep-sea sequences indicate that this reflected a global change in d13C of the oceanic total dissolved carbon (TDC) reservoir. The magnitude of this inferred carbon reservoir change and its association with high latitude surface-water temperature changes recorded in the d18O records implies that it was linked to global climate change through feedback loops in the carbon cycle. At the K/P boundary, the surface-to-deep water d13C gradient is reduced by approximately 0.6 per mil - ~0.2 per mil. However, unlike sequences elsewhere, the planktonic-benthic d13C gradient (Delta d13C) was not eliminated in the Antarctic. The surface-to-deep water gradient was re-established gradually during the 400 k.y. following the mass extinction. Full recovery of the Delta d13C occurred by ~60.0 Ma. In addition to the reduced vertical d13C gradient across the K/P boundary, there was a negative excursion in both planktonic and benthic d13C beginning approximately 100 k.y. after the boundary (66.3 Ma). This excursion resulted in benthic d13C values in the early Paleogene that were similar to those in the pre-K/P boundary intervals. This negative shift appears to reflect a change in the d13C of the oceanic TDC reservoir shift that may have resulted from reduced carbon burial and/or increased carbon flux to the oceans. Any model that attempts to explain the demise of the oceanic plankton at the end of the Cretaceous should consider the oceanic environmental changes that were occurring prior to the massive extinction event.

(Table 1) Age determination and isotopic signature of last glacial Patagonian caly/silt

Ice cores provide a record of changes in dust flux to Antarctica, which is thought to reflect changes in atmospheric circulation and environmental conditions in dust source areas (Forster et al., 2007; Diekmann et al. 2000, doi:10.1016/S0031-0182(00)00138-3; Winckler et al., 2008, doi:10.1126/science.1150595; Reader et al., 1999, doi:10.1029/1999JD900033; Mahowald et al., 1999, doi:10.1029/1999JD900084; Petit et al., 1999, doi:10.1038/20859; 1990, doi:10.1038/343056a0 Delmonte et al., 2009, doi:10.1029/2008GL033382; Lambert et al., 2008, doi:10.1038/nature06763). Isotopic tracers suggest that South America is the dominant source of the dust (Grousset et al., 1992, doi:10.1016/0012-821X(92)90177-W; Basile et al., 1997, doi:10.1016/S0012-821X(96)00255-5; Gaiero et al., 2007, doi:10.1016/j.chemgeo.2006.11.003), but it is unclear what led to the variable deposition of dust at concentrations 20-50 times higher than present in glacial-aged ice (Petit et al., 1990, doi:10.1038/343056a0; Lambert et al., 2008, doi:10.1038/nature06763). Here we characterize the age and composition of Patagonian glacial outwash sediments, to assess the relationship between the Antarctic dust record from Dome C (refs Lambert et al., 2008, doi:10.1038/nature06763; Wolff et al., 2006, doi:10.1038/nature04614) and Patagonian glacial fluctuations (Sugden et al., 2005; McCulloch et al., 2005, doi:10.1111/j.0435-3676.2005.00260.x; Kaplan et al., 2008, doi:10.1016/j.quascirev.2007.09.013) for the past 80,000 years. We show that dust peaks in Antarctica coincide with periods in Patagonia when rivers of glacial meltwater deposited sediment directly onto easily mobilized outwash plains. No dust peaks were noted when the glaciers instead terminated directly into pro-glacial lakes. We thus propose that the variable sediment supply resulting from Patagonian glacial fluctuations may have acted as an on/off switch for Antarctic dust deposition. At the last glacial termination, Patagonian glaciers quickly retreated into lakes, which may help explain why the deglacial decline in Antarctic dust concentrations preceded the main phase of warming, sea-level rise and reduction in Southern Hemisphere sea-ice extent (Wolff et al., 2006, doi:10.1038/nature04614).

Concentrations and isotopic compositions of low molecular weight hydrocarbons in hydrothermal fluids from ODP Field, Middle Valley, northern Juan de Fuca Ridge

The presence of sedimentary organic matter blanketing midocean ridge crests has a potentially strong impact on metal transport in hydrothermal vent fluids. To constrain the role of organic matter in metal mobility during hydrothermal sediment alteration, we reacted organic-rich diatomaceous ooze from Guaymas Basin, Gulf of California, and organic-poor hemipelagic mud from Middle Valley, northern Juan de Fuca Ridge, with seawater and a Na-Ca-K-Cl fluid of seawater chlorinity, at 275° to 400°C, 350 to 500 bars, and initial fluid: sediment mass ratios ranging from 1.6 to 9.8. Reaction of these fluids with both sediment types released CO2 and high concentrations of ore-forming metals (Fe, Mn, Zn, Pb) to solution. Relatively low concentrations of Cu were observed in solution and likely reflect the reducing conditions that resulted from the presence of sedimentary organic matter. Both the concentrations of CO2 and dissolved metals were lower in fluids reacted with Middle Valley sediment compared with aqueous concentrations in fluids reacted with Guaymas Basin sediment. During alteration of both sediment types, metal concentrations varied strongly as a function of temperature, increasing by up to an order of magnitude over the 75°C range of each experiment. Major element fluid chemistry and observed alteration assemblages suggest that during hydrothermal alteration of organic-lean sediment from Middle Valley a feldspar-quartz-illite mineral assemblage buffered in situ pH. In contrast, data from the experimental alteration of organic-rich Guaymas Basin sediment suggest that a calcite-plagioclase-quartz assemblage regulated in situ pH. Fluid speciation calculations suggest that in situ pH during Guaymas Basin sediment alteration was lower than during alteration of Middle Valley sediment and accounts for the substantially greater metal mobility at a given temperature and pressure during the former experiment. Comparison of our results with the results of basalt alteration experiments indicate that except for Cu, hydrothermal sediment alteration results in equal or greater concentrations of ore-forming metals at a given temperature and pressure. Accordingly, the presence of ore-forming metals in fluids currently venting from sediment-covered hydrothermal systems at concentrations substantially lower than in fluids from bare-rock systems may reflect chemical reequilibration during subsurface cooling within the sediment pile.

Neodymium isotopic record of planktonic foraminifera in sediment core MD81-LC21

A well developed sapropel (S5) was deposited in the eastern Mediterranean during the Last Interglacial (Eemian), 124-119 ka. Freshwater contributions to the basin at this time can be traced using the isotopic composition of Nd in planktonic foraminifera. This enables differentiation between radiogenic sources to the south, under the influence of the African monsoon, and unradiogenic sources to the north, relating to the mid-latitude westerlies. Here we compare new Nd data, from a core in the southeast Aegean Sea, with published data from the Ionian and Levantine Seas. Shifts towards more radiogenic Nd in the lower and middle parts of sapropel S5 are most pronounced in the Ionian Sea record, with epsioln-Nd and d18O G. ruber co-varying more closely here than in the Levantine and Aegean Seas. This is consistent with a freshwater source proximal to the Ionian Sea site, likely indicating a substantial reactivation of rivers flowing northward from the central Saharan watershed. The lack, during S5 deposition, of a noticeable shift towards more unradiogenic Nd in the Aegean record would exclude a large influx of water from the northern borders of the eastern Mediterranean during sapropel deposition. These findings support a scenario whereby the Last Interglacial eastern Mediterranean was influenced strongly by the remote effects of an intensified African monsoon, with more local precipitation in the northern borders contributing relatively little to the sea surface composition.

(Table) Carbon, nitrogen and n-alkane content and isotopic composition of bottom sediments obtained in the East Siberian Sea

The chemical composition of organic matter (Corg, Norg, d13C, d1SN, and n-alkanes) was studied in the top layer of bottom sediments of the East Siberian Sea. Possible ways were proposed to estimate the amount of the terrigenous component in their organic matter (OM). The fraction of terrigenous OM estimated by the combined use of genetic indicators varied from 15% in the eastern part of the sea, near the Long Strait, to 95% in the estuaries of the Indigirka and Kolyma rivers, averaging 62% over the sea area.

(Table 3) Sulfur and oxygen isotopic composition of cold water soluble sulfate, and strontium content and isotopic composition of upper Miocene evaporite samples recovered from DSDP Site 23-227

The origin of three Red Sea submarine brine pools was investigated by analysis of the S and O isotope ratios of dissolved sulfate and Sr isotope ratios of dissolved Sr in the brines. Sulfur and O isotope ratios of sulfate and Sr isotope ratios of evaporitic source rocks for the brines were measured for comparison. The S, O and Sr isotope ratios of evaporites recovered from DSDP site 227 are consistent with an upper Miocene evaporites age. The Valdivia Deep brine formed by karstic dissolution of Miocene evaporites by overlying seawater and shows no signs of hydrothermal input. The Suakin Deep brines are derived from, or have isotopically exchanged with Miocene or older evaporites. There has been only minor dilution of the brine by overlying seawater. Strontium isotope ratios of Suakin brine may indicate addition of a minor (15%) amount of volcanic Sr to the brine, but there is no evidence of high temperature brine-rock interaction. The sulfate in the Atlantis II brine was apparently derived from seawater. The O isotope ratio of sulfate in the present Atlantis II brine could reflect isotopic exchange between seawater sulfate and the brine at approximately 255°C. Approximately 30% of the Sr in the Atlantis II brine is derived from the underlying basalt, probably by hydrothermal leaching. Atlantis II brine is the only known example from the Red Sea which has a significant high-temperature hydrothermal history.

Comparison of the stable carbon and nitrogen isotopic values of gill and white muscle tissue of fish

The potential use of stable carbon and nitrogen isotope ratios (d13C, d15N) of fish gills for studies on fish feeding ecology was evaluated by comparing the d13C and d15N of gill tissue with the more commonly used white muscle tissue. To account for the effect of lipid content on the d13C signatures, a study-specific lipid correction model based on C:N ratios was developed and applied to the bulk d13C data. For the majority of species in the study, we found no significant difference in d13C values between gill and muscle tissue after correction, but several species showed a small (0.3-1.4 per mil) depletion in 13C in white muscle compared to gill tissue. The average species difference in d15N between muscle and gill tissue ranged from -0.2 to 1.6 per mil for the different fish species with muscle tissue generally more enriched in 15N. The d13C values of muscle and gill were strongly linearly correlated (R**2 = 0.85) over a large isotopic range (13 per mil), suggesting that both tissues can be used to determine long-term feeding or migratory habits of fish. Muscle and gill tissue bulk d15N values were also strongly positively correlated (R**2= 0.76) but with a small difference between muscle and gill tissue. This difference indicates that the bulk d15N of the two tissue types may be influenced by different isotopic turnover rates or a different composition of amino acids.

Geochemical and isotopic composition of Bransfield Basin Lavas collected during SONNE cruise SO155

Bransfield Basin is an actively extending marginal basin separating the inactive South Shetland arc from the northern Antarctic Peninsula. Rift-related volcanism is widespread throughout the central Bransfield Basin, but the wider eastern Bransfield Basin was previously unsampled. Lavas recovered from the eastern subbasin form three distinct groups: (1) Bransfield Group has moderate large-ion lithophile element (LILE) enrichment relative to normal mid-ocean ridge basalt (NMORB), (2) Gibbs Group has strong LILE enrichment and is restricted to a relic seamount interpreted as part of the South Shetland arc, and (3) fresh alkali basalt was recovered from the NE part of the basin near Spanish Rise. The subduction-related component in Bransfield and Gibbs Group lavas is a LILE-rich fluid with radiogenic Sr, Nd, and Pb isotope compositions derived predominantly from subducting sediment. These lavas can be modeled as melts from Pacific MORB source mantle contaminated by up to 5% of the subduction-related component. They further reveal that Pacific mantle, rather than South Atlantic mantle, has underlain Bransfield Basin since 3 Ma. Magma productivity decreases abruptly east of Bridgeman Rise, and lavas with the least subduction component outcrop at that end. Both the eastward decrease in subduction component and occurrence of young alkali basalts require that subduction-modified mantle generated during the lifetime of the South Shetland arc has been progressively removed from NE to SW. This is inconsistent with previous models suggesting continued slow subduction at the South Shetland Trench but instead favors models in which the South Scotia Ridge fault has propagated westward since 3 Ma generating transtension across the basin.