Geo-referenced photo-mosaics of the Menez Gwen Hydrothermal Vent (Mid-Atlantic Ridge)

The Menez Gwen hydrothermal vents, located on the flanks of a small young volcanic structure in the axial valley of the Menez Gwen seamount, are the shallowest known vent systems on the Mid-Atlantic Ridge that host chemosynthetic communities. Although visited several times by research cruises, very few images have been published of the active sites, and their spatial dimensions and morphologies remain difficult to comprehend. We visited the vents on the eastern flank of the small Menez Gwen volcano during cruises with RV Poseidon (POS402, 2010) and RV Meteor (M82/3, 2010), and used new bathymetry and imagery data to provide first detailed information on the extents, surface morphologies, spatial patterns of the hydrothermal discharge and the distribution of dominant megafauna of five active sites. The investigated sites were mostly covered by soft sediments and abundant white precipitates, and bordered by basaltic pillows. The hydrothermally-influenced areas of the sites ranged from 59 to 200 m**2. Geo-referenced photomosaics and video data revealed that the symbiotic mussel Bathymodiolus azoricus was the dominant species and present at all sites. Using literature data on average body sizes and biomasses of Menez Gwen B. azoricus, we estimated that the B. azoricus populations inhabiting the eastern flank sites of the small volcano range between 28,640 and 50,120 individuals with a total biomass of 50 to 380 kg wet weight. Based on modeled rates of chemical consumption by the symbionts, the annual methane and sulfide consumption by B. azoricus could reach 1760 mol CH4 yr**-1 and 11,060 mol H2S yr**-1. We propose that the chemical consumption by B. azoricus over at the Menez Gwen sites is low compared to the natural release of methane and sulfide via venting fluids.

Geochemistry of Fe-Mn crusts in sediments of the Central Indian Ridge

Layered Fe-Mn crusts from the off-axis region of the first segment of the Central Indian Ridge north of the Rodrigues Triple Junction were studied geochemically and mineralogically. Vernadite (delta-MnO2) is the main mineral oxide phase. 230Thxs and Co concentrations suggest high growth rates of up to 29 mm/Myr and a maximum age of the basal crust layer of 1 Ma. Whereas most of the major and minor elements show concentrations which are typical of hydrogenetic formation, Co, Pb, Ni and Ti concentrations are strikingly lower. Concentrations and distribution of the strictly trivalent rare-earths and yttrium (REY) are typical of hydrogenetic ferromanganese oxide precipitates, but in marked contrast, the crusts are characterized by negative CeSN (shale normalized) anomalies and (Ce/Pr)SN ratios less than unity. Profiles through the crusts reveal only minor variations of the REY distribution and (Ce/Pr)SN ratios range from 0.45 to 0.68 (compared to ratios of up to 2 for typical hydrogenetic crusts from the Central Indian Basin). The apparent bulk partition coefficients between the crusts and seawater suggest that for the strictly trivalent REY the adsorption-desorption equilibrium has been reached. Positive Ce anomalies in the partition coefficient patterns reveal preferential uptake of Ce, but to a lesser extent than in normal hydrogenetic crusts. A new parameter (excess Ce, Cexs) to quantify the degree of decoupling of Ce from REY(III) is established on the basis of partition coefficients. Cexs/Cebulk ratios suggest that the CIR crusts formed by precipitation of Fe-Mn oxides from a hydrothermal plume and that in hydrothermal plumes and normal seawater the enrichment of Ce results from the same oxidative sorption process. The growth rates, calculated with 230Thxs data as well as with the Co formula, are inversely related to Cexs.

Geochemical processes and rare earth element analysis in cold-seep pore waters of Hydrate Ridge, northeast Pacific Ocean

The concentrations of rare earth elements (REEs), sulphate, hydrogen sulphide, total alkalinity, calcium, magnesium and phosphate were measured in shallow (<12 cm below seafloor) pore waters from cold-seep sediments on the northern and southern summits of Hydrate Ridge, offshore Oregon. Downward-decreasing sulphate and coevally increasing sulphide concentrations reveal sulphate reductionas dominant early diagenetic process from ~2 cm depth downwards. A strong increase of total dissolved REE concentrations is evident immediately below the sediment-water interface, which can be related to early diagenetic release of REEs into pore water resulting from the remineralization of particulate organic matter. The highest pore water REE concentrations were measured close to the sediment-water interface at ~2 cm depth. Distinct shale normalized REE patterns point to particulate organic matter and iron oxides as main REE sources in the upper ~2-cm depth interval. In general, the pore waters have shalenormalized patterns reflecting heavy REE (HREE) enrichment, which suggests preferential complexation of HREEs with carbonate ions. Below ~2 cm depth, a downward decrease in REE correlates with a decrease in pore water calcium concentrations. At this depth, the anaerobic oxidation of methane (AOM) coupled to sulphate reduction increases carbonate alkalinity through the production of bicarbonate, which results in the precipitation of carbonate minerals. It seems therefore likely that the REEs and calcium are consumed during vast AOM-induced precipitation of carbonate in shallow Hydrate Ridge sediments. The analysis of pore waters from Hydrate Ridge shed new light on early diagenetic processes at cold seeps, corroborating the great potential of REEs to identify geochemical processes and to constrain environmental conditions.

Geochemistry of Walvis Ridge basalts

The proposed origins for the Enriched Mantle I component are many and various and some require an arbitrary addition of an exotic component, be it pure sediment or an enriched melt from the subcontinental lithosphere. With Pitcairn, Walvis Ridge is the 'type-locality' for the Enriched Mantle I (EMI) component. We analyzed basalts from DSDP Site 525A, Site 527 and Site 528 on the Walvis Ridge with the aim to constrain the history of its source. The isotopic compositions we measured for the three sites overlap with the values obtained by Richardson et al. (1982a) and extend towards less radiogenic Sr and more radiogenic Pb and Nd isotopic compositions. We used our new trace element and radiogenic isotope (Hf, Nd, Pb and Sr) characterization in combination with the literature data to produce the simplest possible model that satisfies the trace element and isotopic constraints. Although the elevated 207Pb/204Pb with respect to 206Pb/204Pb predicts an ancient origin for EMI, none of the proposed origins had modeled it as such. The data is consistent with the EMI composition being formed by the addition of a melt to a mantle with bulk Earth-like composition followed by melt extraction of a low degree melt. The timing of these two events is such that the metasomatism has to have taken place prior to 4 Ga and the subsequent melt removal before 3.5 Ga. This confirms the expectation of an ancient character for the EMI component. The Walvis Ridge data shows two distinct two component mixing trends: one formed by the less enriched Site 527 and Site 528 basalts and one formed by the Site 525A basalts. The two trends have the EMI endmember in common. The less depleted end of the Site 527-Site 528 basalts is FOZO-like and can be explained by the addition of a recycled component (basaltic oceanic crust plus sediment). This recycled component was altered during subduction. The sense and magnitude of the chemical fractionation resulting from the subduction alteration are in agreement with dehydration experiments on basalts and sediment. Compared to other EMI like basalts the Walvis Ridge basalts have flatter REE patterns and show less fractionation between large ion lithophile and heavy REE elements. Using the isotopic compositions as constrains for the parent-daughter ratios we were able to model the trace element patterns of the basalts as melting between 5 and 10% for Site 525A and between 10 and 15% for the depleted end of the Site 528-Site 527 array. In all cases a significant portion of melting takes place in the garnet stability field.

Precious metals and volatiles in rocks of the Mid-Atlantic Ridge near 15°20'N

Distribution patterns of gold, platinum group metals (PGE), and volatile components are studied in the main rock types of the Mid-Atlantic Ridge lithosphere (ultrabasites, gabbro, and basalts) from the transform fracture zone at 15°20'N. It is ascertained that PGE content depends on the reduction degree of fluids, on rock types, and on their formation conditions. It is noted that concentrations of refractory elements (Os, Ir, Rh) decrease, while those of fusible elements (Pt, Pd, Au) increase with depth. The chondrite type of distribution is only noted in the ultrabasite rocks. Increase in water and CO2 contents and in oxidation degree of fluids occurs with transition from basalts to ultrabasites, as well as from normal magmatic systems to fluid-enriched anomalous systems.

Paleocene and Eocene nannofossil datums from Shatsky Rise and Walvis Ridge

The precision of late Paleocene to middle Eocene nannofossil datums is investigated by means of quantitative methods and correlated to the magnetic polarity stratigraphy, using sequences from the Northwest Pacific, Southeast Atlantic and Italy. It is the rule rather than the exception to find tails of very reduced abundances prior to, or after, a range of consistent and higher abundances. The absolutely first or final occurrence of a species, therefore, seldom provides a synchronous datum when material from different geographic areas are compared. On the other band, synchroneity is often confirmed when the initial sharp rise or the final sharp decline in abundance is used as datum level. The use of datums not employed in the two principal existing nannofossil zonal schemes can substantially improve the biostratigraphic resolution. Two established zonal markers show abundance patterns making them unsuitable as datums: the first occurrences of Ellipsolithus macellus (base NP4, diachronous) and Tribrachiatus nunnii (base NP10 and Paleocene/Eocene boundary, too rare and too short range in open ocean sections). The first occurrence of either Fasciculithus spp. or Sphenolithus spp. is a better marker near the base of NP4. The first occurrence of Discoaster diastypus at 56.6 Ma represents a suitable replacement for recognition of the Paleocene/Eocene boundary.

Geochemistry of highly depleted peridotites of the Mid-Atlantic Ridge

During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge (MAR) was drilled in the vicinity of the Fifteen-Twenty Fracture Zone (FZ) that offsets one of the slowest portions of the spreading ridge. We present here the results of a bulk rock multi-elemental study of 27 peridotites drilled at Sites 1272 and 1274 (to the south and the north of the FZ, respectively). The peridotites comprise mainly of harzburgites with minor dunites. Clinopyroxene (Cpx), which is interstitial and interpreted as secondary, is observed in Site 1274 peridotites. Sites 1272 and 1274 peridotites have low Al2O3 contents (<1 anhydrous wt.%), high Mg# (>91.5), and bulk rock trace elements compositions mostly below 0.1X primitive mantle (PM). These peridotites, and in particular Site 1272 peridotites, represent the most depleted peridotites yet sampled at a slow spreading ridge. Their compositions indicate high degrees of partial melting and melt extraction. A single open-system melting event (melting plus percolation of melts produced within upwelling mantle) can explain their highly depleted yet linear chondrite-normalized REE patterns, characterized by a steady depletion from HREE to LREE. Late melt-rock reactions and precipitation of Cpx explains the slightly less depleted compositions of Site 1274 peridotites. Hence, the differences in composition between Sites 1272 and 1274 peridotites do not provide evidence for regional variations in the degrees of partial melting from the south to the north of the FZ. The occurrence of highly refractory peridotites in the Fifteen-Twenty area suggests we sampled a more actively convecting mantle than generally supposed below slow spreading centers.

Microfossils and chemical elements in bottom sediments of the Ashadze-1 Hydrothermal Field (tropical Mid-Atlantic Ridge)

The first thorough analysis of microfossils from ore-bearing sediments of the Ashadze-1 Hydrothermal Field in the Mid-Atlantic Ridge sampled during Cruise 26 of R/V Professor Logachev in 2005 revealed substantial influence of hydrothermal processes on preservation of planktonic calcareous organisms as well as on preservation and composition of benthic foraminifera. From lateral and vertical distribution patterns and secondary alterations of microfossils it is inferred that the main phase of hydrothermal mineralization occurred in Holocene. Heavy metals (Cu, Co, Cr, and Ag) were accumulated by foraminiferal tests and in their enveloping Fe-Mn crusts. Distribution of authigenic minerals replacing foraminiferal tests demonstrates local zoning related to hydrothermal activity. There are three mineral-geochemical zones defined: sulfide zone, zone with elevated Mg content, and zone of Fe-Mn crusts.

Geochemistry of Mid-Atlantic Ridge sediments

Geochemical compositions and Sr and Nd isotopes were measured in two cores collected ~2 and 5 km from the Rainbow hydrothermal vent site on the Mid-Atlantic Ridge. Overall, the cores record enrichments in Fe and other metals from hydrothermal fallout, but sequential dissolution of the sediments allows discrimination between a leach phase (easily leachable) and a residue phase (refractory). The oxy-anion and transition metal distribution combined with rare earth element (REE) patterns suggest that (1) the leach fraction is a mixture of biogenic carbonate and hydrothermal Fe-Mn oxy-hydroxide with no significant contribution from detrital material and (2) >99.5% of the REE content of the leach fraction is of seawater origin. In addition, the leach fraction has an average 87Sr/86Sr ratio indistinguishable from modern seawater at 0.70916. Although we lack the epsilon-Nd value of present-day deep water at the Rainbow vent site, we believe that the REE budget of the leach fraction is predominantly of seawater origin. We suggest therefore that the leach fraction provides a record of local seawater epsilon-Nd values. Nd isotope data from these cores span the period of 4-14 ka (14C ages) and yield epsilon-Nd values for North East Atlantic Deep Water (NEADW) that are higher (-9.3 to -11.1) than those observed in the nearby Madeira Abyssal Plain from the same depth (-12.4 ± 0.9). This observation suggests that either the Iceland-Scotland Overflow Water (ISOW) and Lower Deep Water contributions to the formation of NEADW are higher along the Mid-Atlantic Ridge than in the surrounding basins or that the relative proportion of ISOW was higher during this period than is observed today. This study indicates that hydrothermal sediments have the potential to provide a higher-resolution record of deep water epsilon-Nd values, and hence deepwater circulation patterns in the oceans, than is possible from other types of sediments.

Geochemistry of rare earth elements in basalts from Walvis Ridge

Selected basalts from a suite of dredged and drilled samples (IPOD sites 525, 527, 528 and 530) from the Walvis Ridge have been analysed to determine their rare earth element (REE) contents in order to investigate the origin and evolution of this major structural feature in the South Atlantic Ocean. All of the samples show a high degree of light rare earth element (LREE) enrichment, quite unlike the flat or depleted patterns normally observed for normal mid-ocean ridge basalts (MORBs). Basalts from Sites 527, 528 and 530 show REE patterns characterised by an arcuate shape and relatively low (Ce/Yb)N ratios (1.46-5.22), and the ratios show a positive linear relationship to Nb content. A different trend is exhibited by the dredged basalts and the basalts from Site 525, and their REE patterns have a fairly constant slope, and higher (Ce/Yb)N ratios (4.31-8.50). These differences are further reflected in the ratios of incompatible trace elements, which also indicate considerable variations within the groups. Mixing hyperbolae for these ratios suggest that simple magma mixing between a 'hot spot' type of magma, similar to present-day volcanics of Tristan da Cunha, and a depleted source, possibly similar to that for magmas being erupted at the Mid-Atlantic Ridge, was an important process in the origin of parts of the Walvis Ridge, as exemplified by Sites 527, 528 and 530. Site 525 and dredged basalts cannot be explained by this mixing process, and their incompatible element ratios suggest either a mantle source of a different composition or some complexity to the mixing process. In addition, the occurrence of different types of basalt at the same location suggests there is vertical zonation within the volcanic pile, with the later erupted basalts becoming more alkaline arid more enriched in incompatible elements. The model proposed for the origin and evolution of the Walvis Ridge involves an initial stage of eruption in which the magma was essentially a mixture of enriched and depleted end-member sources, with the N-MORB component being small. The dredged basalts and Site 525, which represent either later-stage eruptives or those close to the hot spot plume, probably result from mixing of the enriched mantle source with variable amounts and variable low degrees of partial melting of the depleted mantle source. As the volcano leaves the hot spot, these late-stage eruptives continue for some time. The change from tholeiitic to alkalic volcanism is probably related either to evolution in the plumbing system and magma chamber of the individual volcano, or to changes in the depth of origin of the enriched mantle source melt, similar to processes in Hawaiian volcanoes.

Rare earth and trace elements in igneous and metamorphic minerals of oceanic gabbros from the Mid-Atlantic Ridge

SIMS analyses have been carried out on clinopyroxenes, plagioclases and amphiboles of six gabbroic samples from Holes 921-924 of the Ocean Drilling Program Leg 153 sited in the MARK area of the Mid-Atlantic Ridge at the ridge-transform intersection, to investigate the rare earth, trace and volatile element distribution in the lower ocean crust during igneous crystallization and higher grade metamorphic conditions. The metagabbros underwent granulite to subgreenschist facies conditions through three main tectono-metamorphic phases: (1) ductile regime (750 < T < 1000 °C and P = 0.3 GPa); (2) transitional regime (600 < T < 700 °C and P = 0.2 GPa); (3) brittle regime (350 < T < 600 °C and P < 0.2 GPa). Igneous clinopyroxenes show Cl-chondrite normalized patterns depleted in LREE, and nearly flat for HREE. The rare earth and trace element distributions in igneous clinopyroxenes and plagioclases indicate that these minerals act as REE reservoirs, and comprise the main contribution to the overall rock content. The abundances in igneous minerals reflect the degree of fractionation of the parent liquids. In metamorphic clinopyroxenes recrystallized in anhydrous assemblages, the REE and trace elements patterns mimic those of the primary ones. Conversely, clinopyroxerie re-equilibrated in amphibolebearing assemblages shows a significant increase in REE, Ti, Zr, Y and V, a negative Eu anomaly, and slight decreases in Sr and Ba. An overall increase of REE and some trace elements is evident in hydrous assemblages, with preferential partitioning in the amphibole. It shows high Ti (18196-22844 ppm), LREE depleted patterns and LaN/SmN = 0.10-0.33, LaN/YbN = 0.10-0.30. Amphiboles from granoblastic assemblages show homogeneous patterns with no or a positive anomaly for TiN and negative anomalies for SrN and ZrN. Volatiles in amphibole are low, with Cl/F < 1; H2O% is significantly lower than the stoichiometric ratio (1.33-1.53%). The composition of the clinopyroxene and amphibole recrystallized in low-strain domains records evidence of incomplete re-equilibration, and element diffusion and partitioning is in part controlled by the textural site. The possible origins of the fluids involved in the metamorphic recrystallization are discussed: (1) remobilization from igneous amphibole; (2) exsolution from evolved melts; (3) introduction of seawater-derived fluids modified in rock-dominated systems; (4) injection of highly evolved hydrous melts during the metamorphic process.

Geochemical analysis of two sediment cores from the southern Mendeleev Ridge

We present results of an inorganic geochemical pore water and sediment study conducted on Quaternary sediments from the western Arctic Ocean. The sediment cores were recovered in 2008 from the southern Mendeleev Ridge during RV Polarstern Expedition ARK-XXIII/3. With respect to sediment sources and depositional processes, peaks in Ca/Al, Mg/Al, Sr/Al and Sr/Mg indicate enhanced input of both ice-rafted (mainly dolomite) and biogenic carbonate during deglacial warming phases. Distinct and repetitive brown layers enriched in Mn (oxyhydr)oxides occur mostly in association with these carbonate-rich intervals. For the first time, we show that the brown layers are also consistently enriched in scavenged trace metals Co, Cu, Mo and Ni. The bioturbation patterns of the brown layers, specifically well-defined brown burrows into the underlying sediments, support formation close to the sediment-water interface. The Mn and trace metal enrichments were probably initiated under warmer climate conditions. Both river runoff and melting sea ice delivered trace metals to the Arctic Ocean, but also enhanced seasonal productivity and organic matter export to the sea floor. As Mn (oxyhydr)oxides and scavenged trace metals were deposited at the sea floor, a co-occurring organic matter "pulse" triggered intense diagenetic Mn cycling at the sediment-water interface. These processes resulted in the formation of Mn and trace metal enrichments, but almost complete organic matter degradation. As warmer conditions ceased, reduced riverine runoff and/or a solid sea ice cover terminated the input of riverine trace metal and fresh organic matter, and greyish-yellowish sediments poor in Mn and trace metals were deposited. Oxygen depletion of Arctic bottom waters as potential cause for the lack of Mn enrichments during glacial intervals is highly improbable. While the original composition and texture of the brown layers resulted from specific climatic conditions (including transient Mn redox cycling at the sediment-water interface), pore water data show that early diagenetic Mn redistribution is still affecting the organic-poor sediments in several meters depth. Given persistent steady state diagenetic conditions, purely authigenic Mn-rich brown layers may form, while others may completely vanish. The degree of diagenetic Mn redistribution largely depends on the depositional environment within the Arctic Ocean, the availability of Mn and organic matter, and seems to be recorded by the Co/Mo ratios of single Mn-rich layers. We conclude that brown Arctic sediment layers are not necessarily synchronous features, and correlating them across different parts of the Arctic Ocean without additional age control is not recommended.