Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge

Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion. According to the widely discussed "gabbro glacier'' model, the formation of the lower oceanic crust requires efficient cooling of the axial melt lens, leading to partial crystallization and crystal-melt mush subsiding down to lower crust. These processes are believed to be controlled by periodical magma replenishment and hydrothermal circulation above the melt lens. Here we quantify the cooling rate above melt lens using chemical zoning of plagioclase from hornfelsic recrystallized sheeted dikes drilled from the East Pacific at the Integrated Ocean Drilling Program Hole 1256D. Weestimate the cooling rate using a forward modelling approach based on CaAl-NaSi interdiffusion in plagioclase. The results show that cooling from the peak thermal overprint at 1000-10506 degrees C to 6006 degrees C are yielded within about 10-30 years as a result of hydrothermal circulation above melt lens during magma starvation. The estimated rapid hydrothermal cooling explains how the effective heat extraction from melt lens is achieved at fast-spreading mid-ocean ridges.

The Southeast Indian Ridge between 88°E and 118°E: Gravity anomalies and crustal accretion at intermediate spreading rates

Although slow spreading ridges characterized by a deep axial valley and fast spreading ridges characterized by an axial bathymetric high have been extensively studied, the transition between these two modes of axial morphology is not well understood. We conducted a geophysical-survey of the intermediate spreading rate Southeast Indian Ridge between 88 degrees E and 118 degrees E, a 2300-km-long section of the ridge located between the Amsterdam hot spot and the Australian-Antarctic Discordance where satellite gravity data suggest that the Southeast Indian Ridge (SEIR) undergoes a change from an axial high in the west to an axial valley in the east. A basic change in axial morphology is found near 103 degrees 30'E in the shipboard data; the axis to the west is marked by an axial high, while a valley is found to the east. Although a well-developed axial high, characteristic of the East Pacific Rise (EPR), is occasionally present, the more common observation is a rifted high that is lower and pervasively faulted, sometimes with significant (> 50 m throw) faults within a kilometer of the axis. A shallow axial valley (< 700 m deep) is observed from 104 degrees E to 114 degrees E with a sudden change to a deep (>1200 m deep) valley across a transform at 114 degrees E. The changes in axial morphology along the SEIR are accompanied by a 500 m increase in near-axis ridge flank depth from 2800 m near 88 degrees E to 3300 m near 114 degrees E and by a 50 mGal increase in the regional level of mantle Bouguer gravity anomalies over the same distance, The regional changes in depth and mantle Bouguer anomaly (MBA) gravity can be both explained by a 1.7-2.4 km change in crustal thickness or by a mantle temperature change of 50 degrees C-90 degrees C. In reality, melt supply (crustal thickness) and mantle temperature are linked, so that changes in both may occur simultaneously and these estimates serve as upper bounds. The along-axis MBA gradient is not uniform. Pronounced steps in the regional level of the MBA gravity occur at 103 degrees 30'E-104 degrees E and at 114 degrees E-116 degrees E and correspond to the changes in the nature of the axial morphology and in the amplitude of abyssal hill morphology suggesting that the different forms of morphology do not grade into each other but rather represent distinctly different forms of axial (s)tructure and tectonics with a sharp transition between them. The change from an axial high to an axial valley requires a threshold effect in which the strength of the lithosphere changes quickly. The presence or absence of a quasi-steady state magma chamber may provide such a mechanism. The different forms of axial morphology are also associated with different intrasegment MBA gravity patterns. Segments with an axial high have an MBA low located at a depth minimum near the center of the segment, At EPR-like segments, the MBA low is about 10 mGal with along-axis gradients of 0.15-0.25 mGal/km, similar to those observed at the EPR, Rifted highs have a shallower low and lower gradients suggesting an attenuated composite magma chamber and a reduced and perhaps episodic melt supply. Segments with a shallow axial valley have very flat along-axis MBA profiles with little correspondence between axial depth and axial MBA gravity.

The Southeast Indian Ridge between 88°E and 118°E: Variations in crustal accretion at constant spreading rate

The temperature of the mantle and the rate of melt production are parameters which play important roles in controlling the style of crustal accretion along mid-ocean ridges. To investigate the variability in crustal accretion that develops in response to variations in mantle temperature, we have conducted a geophysical investigation of the Southeast Indian Ridge (SEIR) between the Amsterdam hotspot and the Australian-Antarctic Discordance (88 degrees E-118 degrees E). The spreading center deepens by 2100 m from west to east within the study area. Despite a uniform, intermediate spreading rate (69-75 mm yr-l), the SEIR exhibits the range in axial morphology displayed by the East Pacific Rise and the Mid-Atlantic Ridge (MAR) and usually associated with variations in spreading rate. The spreading center is characterized by an axial high west of 102 degrees 45'E, whereas an axial valley is prevalent east of this longitude. Both the deepening of the ridge axis and the general evolution of axial morphology from an axial high to a rift valley are not uniform. A region of intermediate morphology separates axial highs and MAR-like rift valleys. Local transitions in axial morphology occur in three areas along the ridge axis. The increase in axial depth toward the Australian-Antarctic Discordance may be explained by the thinning of the oceanic crust by similar to 4 km and the change in axial topography. The long-wavelength changes observed along the SEIR can be attributed to a gradient in mantle temperature between regions influenced by the Amsterdam and Kerguelen hot spots and the Australian-Antarctic Discordance. However, local processes, perhaps associated with an heterogeneous mantle or along-axis asthenospheric flow, may give rise to local transitions in axial topography and depth anomalies.

Molecular identification and localization of filamentous symbiotic bacteria associated with the hydrothermal vent annelid Alvinella pompejana

Alvinella pompejana is a polychaetous annelid that inhabits high temperature environments associated with active deep-sea hydrothermal vents along the East Pacific Rise. A unique and diverse epibiotic microflora with a prominent filamentous morphotype is found associated with the worm's dorsal integument. A previous study established the taxonomic positions of two epsilon proteobacterial phylotypes, 13B and 5A, which dominated a clone library of 16S rRNA genes amplified by PCR from the epibiotic microbial community of an A. pompejana specimen. In the present study deoxyoligonucleotide PCR primers specific for phylotypes 13B and 5A were used to demonstrate that these phylotypes are regular features of the bacterial community associated with A. pompejana. Assaying of other surfaces around colonies of A. pompejana revealed that phylotypes 13B and 5A are not restricted to A. pompejana. Phylotype 13B occurs on the exterior surfaces of other invertebrate genera and rock surfaces, and phylotype 5A occurs on a congener, Alvinella caudata. The 13B and 5A phylotypes were identified and localized on A. pompejana by in situ hybridization, demonstrating that these two phylotypes are, in fact, the prominent filamentous bacteria on the dorsal integument of A. pompejana. These findings indicate that the filamentous bacterial symbionts of A. pompejana are epsilon Proteobacteria which do not have an obligate requirement for A. pompejana.

Iron Transformation Pathways and Redox Micro-Environments in Seafloor Sulfide-Mineral Deposits: Spatially Resolved Fe XAS and delta Fe-57/54 Observations

Hydrothermal sulfide chimneys located along the global system of oceanic spreading centers are habitats for microbial life during active venting. Hydrothermally extinct, or inactive, sulfide deposits also host microbial communities at globally distributed sites. The main goal of this study is to describe Fe transformation pathways, through precipitation and oxidation-reduction (redox) reactions, and examine transformation products for signatures of biological activity using Fe mineralogy and stable isotope approaches. The study includes active and inactive sulfides from the East Pacific Rise 9 degrees 50'N vent field. First, the mineralogy of Fe(III)-bearing precipitates is investigated using microprobe X-ray absorption spectroscopy (RXAS) and X-ray diffraction (mu XRD). Second, laser-ablation (LA) and micro-drilling (MD) are used to obtain spatially-resolved Fe stable isotope analysis by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS). Eight Fe -bearing minerals representing three mineralogical classes are present in the samples: oxyhydroxides, secondary phyllosilicates, and sulfides. For Fe oxyhydroxides within chimney walls and layers of Si-rich material, enrichments in both heavy and light Fe isotopes relative to pyrite are observed, yielding a range of delta Fe-57 values up to 6 parts per thousand. Overall, several pathways for Fe transformation are observed. Pathway 1 is characterized by precipitation of primary sulfide minerals from Fe(II)aq-rich fluids in zones of mixing between vent fluids and seawater. Pathway 2 is also consistent with zones of mixing but involves precipitation of sulfide minerals from Fe(II)aq generated by Fe(III) reduction. Pathway 3 is direct oxidation of Fe(II) aq from hydrothermal fluids to form Fe(III) precipitates. Finally, Pathway 4 involves oxidative alteration of pre-existing sulfide minerals to form Fe(III). The Fe mineralogy and isotope data do not support or refute a unique biological role in sulfide alteration. The findings reveal a dynamic range of Fe transformation pathways consistent with a continuum of micro-environments having variable redox conditions. These micro-environments likely support redox cycling of Fe and S and are consistent with culture-dependent and -independent assessments of microbial physiology and genetic diversity of hydrothermal sulfide deposits.

Deep water formation in the North Pacific and deglacial CO2 rise

Deep water formation in the North Atlantic and Southern Ocean is widely thought to influence deglacial CO2 rise and climate change; here we suggest that deep water formation in the North Pacific may also play an important role. We present paired radiocarbon and boron isotope data from foraminifera from sediment core MD02-2489 at 3640 m in the North East Pacific. These show a pronounced excursion during Heinrich Stadial 1, with benthic-planktic radiocarbon offsets dropping to ~350 years, accompanied by a decrease in benthic d11B. We suggest this is driven by the onset of deep convection in the North Pacific, which mixes young shallow waters to depth, old deep waters to the surface, and low-pH water from intermediate depths into the deep ocean. This deep water formation event was likely driven by an increase in surface salinity, due to subdued atmospheric/monsoonal freshwater flux during Heinrich Stadial 1. The ability of North Pacific Deep Water (NPDW) formation to explain the excursions seen in our data is demonstrated in a series of experiments with an intermediate complexity Earth system model. These experiments also show that breakdown of stratification in the North Pacific leads to a rapid ~30 ppm increase in atmospheric CO2, along with decreases in atmospheric d13C and D14C, consistent with observations of the early deglaciation. Our inference of deep water formation is based mainly on results from a single sediment core, and our boron isotope data are unavoidably sparse in the key HS1 interval, so this hypothesis merits further testing. However we note that there is independent support for breakdown of stratification in shallower waters during this period, including a minimum in d15N, younging in intermediate water 14C, and regional warming. We also re-evaluate deglacial changes in North Pacific productivity and carbonate preservation in light of our new data, and suggest that the regional pulse of export production observed during the Bølling-Allerød is promoted by relatively stratified conditions, with increased light availability and a shallow, potent nutricline. Overall, our work highlights the potential of NPDW formation to play a significant and hitherto unrealized role in deglacial climate change and CO2 rise.

Stable isotope and Magnesium/Calcium ratios of 4 profiles from the equatorial Pacific/Caribbean

This thesis examines the closure history of the Central American Seaway (CAS) and its effect on changes in ocean circulation and climate during the time interval from ~6 - 2.5 Ma. It was accomplished within the DFG Research Unit "Impact of Gateways on Ocean Circulation, Climate and Evolution" at the University of Kiel. Proxy records from Ocean Drilling Program (ODP) Sites 999 and 1000 (Caribbean), and from ODP Sites 1237, 1239 and 1241 (low-latitude east Pacific) are developed and examined. In addition, previously established proxy data from Atlantic Sites 925/926 (Ceara Rise) and 1006 (western Great Bahama Bank) and from two east Pacific sites (851, 1236) are included for interpretations. The main objectives of this study are (1) to acquire a consistent stratigraphic framework for all sites, (2) to reconstruct Pliocene changes in Caribbean and tropical east Pacific upper ocean water masses (i.e. temperature, salinity, thermocline depth), and (3) to identify potential underlying forcing mechanisms.