Geochemical traces across the Paleocene-Eocene thermal maximum on Walvis Ridge

An understanding of sediment redox conditions across the Paleocene-Eocene thermal maximum (PETM) (?55 Ma) is essential for evaluating changes in processes that control deep-sea oxygenation, as well as identifying the mechanisms responsible for driving the benthic foraminifera extinction. Sites cored on the flanks of Walvis Ridge (Ocean Drilling Program Leg 208, Sites 1262, 1266, and 1263) allow us to examine changes in bottom and pore water redox conditions across a ~2 km depth transect of deep-sea sediments of PETM age recovered from the South Atlantic. Here we present measurements of the concentrations of redox-sensitive trace metals manganese (Mn) and uranium (U) in bulk sediment as proxies for redox chemistry at the sediment-water interface and below. All three Walvis Ridge sites exhibit bulk Mn enrichment factors (EF) ranging between 4 and 12 prior to the warming, values at crustal averages (Mn EF = 1) during the warming interval, and a return to pre-event values during the recovery period. U enrichment factors across the PETM remains at crustal averages (U EF = 1) at Site 1262 (deep) and Site 1266 (intermediate depth). U enrichment factors at Site 1263 (shallow) peaked at 5 immediately prior to the PETM and dropped to values near crustal averages during and after the event. All sites were lower in dissolved oxygen content during the PETM. Before and after the PETM, the deep and intermediate sites were oxygenated, while the shallow site was suboxic. Our geochemical results indicate that oxygen concentrations did indeed drop during the PETM but not sufficiently to cause massive extinction of benthic foraminifera.

Geochemistry of subducted volcaniclastic sediments from Mariana arc magmas

The phase relations of natural volcaniclastic sediments from the west Pacific Ocean were investigated experimentally at conditions of 3-6 GPa and 800-900 °C with 10 wt.% added H2O (in addition to ~ 10 wt.% structurally-bound H2O) to induce hydrous melting. Volcaniclastic sediments are shown to produce a sub-solidus assemblage of garnet, clinopyroxene, biotite, quartz/coesite and the accessory phases rutile ± Fe-Ti oxide ± apatite ± monazite ± zircon. Hydrous melt appears at temperatures exceeding 800-850 °C, irrespective of pressure. The melt-producing reaction consumes clinopyroxene, biotite and quartz/coesite and produces orthopyroxene. These phase relations differ from those of pelagic clays and K-bearing mid ocean ridge basalts (e.g. altered oceanic crust) that contain phengite, rather than biotite, as a sub-solidus phase. Despite their relatively high melt productivity, the wet solidus for volcaniclastic sediments is found to be higher (825-850 °C) than other marine sediments (700-750 °C) at 3 GPa. This trend is reversed at high-pressure conditions (6 GPa) where the biotite melting reaction occurs at lower temperatures (800-850 °C) than the phengite melting reaction (900-1000 °C). Trace element data was obtained from the 3 GPa run products, showing that partial melts are depleted in heavy rare earth elements (REE) and high field strength elements (HFSE), due to the presence of residual garnet and rutile, and are enriched in large ion lithophile elements (LILE), except for Sr and Ba. This is in contrast to previous experimental studies on pelagic sediments at sub-arc depths, where Sr and Ba are among the most enriched trace elements in glasses. This behavior can be partly attributed to the presence of residual apatite, which also host some light REE in our supra-solidus residues. Our new experimental results account for a wide range of trace element and U-series geochemical features of the sedimentary component of the Mariana arc magmas, including imparting a substantial Nb anomaly to melts from an anomaly-free protolith.

Trace element geochemistry of zircons frim in situ ocean lithoshere

We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a TiO2 ~ 0.7, a SiO2 ~ 1.0, pressure ~ 2 kbar); intra-sample variation is typically ~60-15°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680-740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La)N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.

Chemical composition of bottom sediments from the Baltic Sea

According to geochemical analyses carbonaceous sediments from deep basins of the Baltic Sea containing 3-5% of organic carbon are enriched in some metals such as Cu, Mo, Ni, Pb, Zn, V, and U relative to shallow-water facies of the Bay of Finland. These metals also enrich (relative to background values in clayey rocks) ancient carbonaceous shales, where the average Cu and V contents are slightly higher and that of Mo, Pb, and Zn lower than in deep-sea carbonaceous sediments of the Baltic Sea. In addition, the deep-sea carbonaceous sediments of the Baltic Sea are enriched (but less notably than ancient shales) in Ag, As, Bi, and Cd. These data confirm previous assumptions that carbonaceous sediments accumulating now in seas and oceans can be considered as recent analogs of ancient metalliferous shales.

Geochemistry of Central American Volcanic Arc basement samples

The Central American Volcanic Arc (CAVA) has been the subject of intensive research over the past few years, leading to a variety of distinct models for the origin of CAVA lavas with various source components. We present a new model for the NW Central American Volcanic Arc based on a comprehensive new geochemical data set (major and trace element and Sr-Nd-Pb-Hf-O isotope ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples from NW Nicaragua, Honduras, El Salvador and Guatemala. Additionally we present data on subducting Cocos Plate sediments (from DSDP Leg 67 Sites 495 and 499) and igneous oceanic crust (from DSDP Leg 67 Site 495), and Guatemalan (Chortis Block) granitic and metamorphic continental basement. We observe systematic variations in trace element and isotopic compositions both along and across the arc. The data require at least three different endmembers for the volcanism in NW Central America. (1) The NW Nicaragua VF lavas require an endmember with very high Ba/(La, Th) and U/Th, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb and low d18O, reflecting a largely serpentinite-derived fluid/hydrous melt flux from the subducting slab into a depleted N-MORB type of mantle wedge. (2) The Guatemala VF and BVF mafic lavas require an enriched endmember with low Ba/(La, Th), U/Th, high d18O and radiogenic Sr and Pb but unradiogenic Nd and Hf isotope ratios. Correlations of Hf with both Nd and Pb isotopic compositions are not consistent with this endmember being subducted sediments. Granitic samples from the Chiquimula Plutonic Complex in Guatemala have the appropriate isotopic composition to serve as this endmember, but the large amounts of assimilation required to explain the isotope data are not consistent with the basaltic compositions of the volcanic rocks. In addition, mixing regressions on Nd vs. Hf and the Sr and O isotope plots do not go through the data. Therefore, we propose that this endmember could represent pyroxenites in the lithosphere (mantle and possibly lower crust), derived from parental magmas for the plutonic rocks. (3) The Honduras and Caribbean BA lavas define an isotopically depleted endmember (with unradiogenic Sr but radiogenic Nd, Hf and Pb isotope ratios), having OIB-like major and trace element compositions (e.g. low Ba/(La, Th) and U/Th, high La/Yb). This endmember is possibly derived from melting of young, recycled oceanic crust in the asthenosphere upwelling in the back-arc. Mixing between these three endmember types of magmas can explain the observed systematic geochemical variations along and across the NW Central American Arc.

Benthic foraminifera in Tertiary sediments of DSDP Leg 90 holes

Eocene through Pliocene benthic foraminifers were examined from seven sites located at middle and lower bathyal depths on the Lord Howe Rise in the Tasman Sea, from another site at lower bathyal depths in the Coral Sea, and from a site in the intermediate-depth, hemipelagic province of the Chatham Rise, east of southern New Zealand. Age-related, depth-related, and bioprovincial faunal variations are documented in this chapter. One new species, Rectuvigerina tasmana, is named. The paleoecologic indications of several key groups, including the miliolids, uvigerinids, nuttallitids, and cibicidids, are combined with sedimentologic and stable isotopic tracers to interpret paleoceanographic changes in the Tasman Sea. Because the total stratigraphic ranges of many bathyal benthic foraminifers are not yet known, most endpoints in the Tasman Sea are considered ecologically controlled events. The disappearances of Uvigerina rippensis and Cibicidoidesparki and the first appearances of U. pigmaea, Sphaeroidina bulloides, and Rotaliatina sulcigera at the Eocene/Oligocene boundary can be considered evolutionary events, as also can the first appearance of Cibicides wuellerstorfi in Zone NN5. Species which are restricted to the lower bathyal zone except during discrete pulses, most of which are related to the development of glacial conditions, include Melonis pompilioides, M. sphaeroides, Pullenia quinqueloba, Nuttallides umbonifera, and U. hispido-costata. Middle bathyal indigenes include U. spinulosa, U. gemmaeformis, Ehrenbergina marwicki, R. sulcigera, and all rectuvigerinids except Rectuvigerina spinea. Although the miliolids first occurred at lower bathyal depths, they were more common in the middle bathyal zone. Although the Neogene hispido-costate uvigerinids first developed at lower bathyal depths and at higher middle latitude sites, in the later Neogene this group migrated to shallower depths and became predominant also in the middle bathyal zone. Despite the relatively similar sedimentologic settings at the six middle bathyal Tasman sites, there was extensive intrageneric and intraspecific geographic variation. Mililiolids, strongly ornamented brizalinids, bolivinitids, Bulimina aculeata, Osangularia culter, and strongly porous morphotypes were more common at higher latitudes. Osangularia bengalensis, striate brizalinids such as Brizalina subaenariensis, Gaudryina solida, osangularids in general, and finely porous morphotypes were more common in the subtropics. There was strong covariance between faunas at lower middle latitude, lower bathyal Site 591, and higher middle latitude, middle bathyal Site 593. The following oceanographic history of the Tasman Sea is proposed; using the stable isotopic record as evidence for glacials and examining the ecologic correlations between (1) miliolids and carbonate saturation, (2) nuttallitids and undersaturated, cooled, or "new" water masses, (3) uvigerinids with high organic carbon in the sediment and high rates of sediment accumulation, and (4) cibicidids and terrestrial organic carbon. The glacial located near the Eocene/Oligocene boundary is characterized by the penetration of cooler, more corrosive waters at intermediate depths in high southern latitudes. This may have caused overturn, upwelling pulses, in other Tasman areas. The development of Neogenelike conditions began in the late Oligocene (Zone NP24/NP25) with the evolution of several common Neogene species. A large number of Paleogene benthics disappeared gradually through the course of the early Miocene, which was not well preserved at any Tasman site. Corrosive conditions shallowed into the middle bathyal zone in several pulses during the early Miocene. The development of glacial conditions in the middle Miocene was accompanied by major changes throughout the Tasman Sea. Sediment accumulation rates increased and high-productivity faunas and corrosive conditions developed at all but the lowest-latitude Site 588. This increase in productivity and accumulation rate is attributed to the eutrophication of Antarctic water masses feeding Tasman current systems, as well as to invigorated circulation in general. It overlaps with the beginning of the Pacific High-productivity Episode (10-5 Ma). During the latest Miocene glacial episode, corrosive conditions developed at lower bathyal depths, while cooler water and lower nutrient levels shallowed to middle bathyal depths. Lower input of terrestrial organic carbon may be related to the lower nutrient levels of this time and to the termination of the Pacific High-productivity Episode. The moderate glacial episode during the mid-Pliocene (Zone NN15/NN16, ~3.2 Ma) corresponds to a decline in sediment accumulation rates and a reorganization of faunas unlike that of all other times. New genera proliferate and indices for cool, noncorrosive conditions and high organic carbon expand throughout the middle bathyal zone coeval with the sedimentation rate decreases. By the latest Pliocene (about 2.5 Ma), however, during another glacial episode, faunal patterns typical of this and later glacials develop throughout the Tasman Sea. Benthic foraminiferal patterns suggest increased input of terrestrial organic matter to Tasman Sea sediments during this episode and during later glacials.

Trace element contents in zircons from oceanic crust

We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the field defined by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fingerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid field, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB.

Geochemistry of basement carbonates from ODP Site 801 in the western Pacific Ocean

Many studies argue, based partly on Pb isotopic evidence, that recycled, subducted slabs reside in the mantle source of ocean island basalts (OIB) (Hofmann and White, 1982, doi:10.1016/0012-821X(82)90161-3; Weaver, 1991 doi:10.1016/0012-821X(91)90217-6; Lassiter, and Hauri, 1998, doi:10.1016/S0012-821X(98)00240-4). Such models, however, have remained largely untested against actual subduction zone inputs, due to the scarcity of comprehensive measurements of both radioactive parents (Th and U) and radiogenic daughter (Pb) in altered oceanic crust (AOC). Here, we discuss new, comprehensive measurements of U, Th, and Pb concentrations in the oldest AOC, ODP Site 801, and consider the effect of subducting this crust on the long-term Pb isotope evolution of the mantle. The upper 500 m of AOC at Site 801 shows >4-fold enrichment in U over pristine glass during seafloor alteration, but no net change to Pb or Th. Without subduction zone processing, ancient AOC would evolve to low 208Pb/206Pb compositions unobserved in the modern mantle (Hart and Staudigel, 1989 [Isotopic characterization and identification of recycled components, in: Crust/Mantle Recycling at Convergence Zones, Eds. S.R. Hart, L. Gqlen, NATO ASI Series. Series C: Mathematical and Physical Sciences 258, pp. 15-28, D. Reidel Publishing Company, Dordrecht-Boston, 1989]). Subduction, however, drives U-Th-Pb fractionation as AOC dehydrates in the earth's interior. Pacific arcs define mixing trends requiring 8-fold enrichment in Pb over U in AOC-derived fluid. A mass balance across the Mariana subduction zone shows that 44-75% of Pb but <10% of U is lost from AOC to the arc, and a further 10-23% of Pb and 19-40% of U is lost to the back-arc. Pb is lost shallow and U deep from subducted AOC, which may be a consequence of the stability of phases binding these elements during seafloor alteration: U in carbonate and Pb in sulfides. The upper end of these recycling estimates, which reflect maximum arc and back-arc growth rates, remove enough Pb and U from the slab to enable it to evolve rapidly (<<0.5 Ga) to sources suitable to explain the 208Pb/206Pb isotopic array of OIB, although these conditions fail to simultaneously satisfy the 207Pb/206Pb system. Lower growth rates would require additional U loss (29%) at depths beyond the zones of arc and back-arc magmagenesis, which would decrease upper mantle kappa (232Th/238U) over time, consistent with one solution to the "kappa conundrum" (Elliott et al., 1999, doi:10.1016/S0012-821X(99)00077-1). The net effects of alteration (doubling of l [238U/204Pb]) and subduction (doubling of omega [232Th/204Pb]) are sufficient to create the Pb isotopic signatures of oceanic basalts.