Chemistry of submarine hydrothermal solutions at Guaymas Basin,Gulf of California

Reconnaisance ALVIN dives in the sediment-filled southern trough of the Guaymas Basin found active hot springs with temperatures ranging up to 315°C. High temperature activity is generally restricted to the crests of large mounds that rise out of the flat-lying basin sediments. The chemistry of the hydrothermal waters is distinctly different from that characteristic of sediment-starved, open ocean ridge axes in that the solutions are alkaline, contain ammonium as a major ion and are strongly depleted in the “ore-forming” metals. These compositions are interpreted as the result of reaction of a primary solution, similar in composition to those as 21°N, EPR, with the biogenous sediments overlying the intrusion zone. The pH of this fluid is raised both by the dissolution of carbonate and the addition of ammonium from thermocatalytic cracking of immature planktonic carbon. Metal sulfides are consequently precipitated at depth in the sediment column. The Guaymas Basin is thus the site of active formation of a sediment-hosted massive sulfide mineral deposit; the exiting waters are the “spent” ore-forming fluid. The ammonium data demonstrate that organic carbon (black shale) is, by itself, a sufficient source of alkalinity to induce the precipitation of sulfides from ascending solutions. Since ammonium does not participate directly in these reactions but does form secondary aluminosilicate minerals these latter should constitute a valuable exploration tool in the search for shale hosted deposits.     

Phenols in hydrothermal petroleums and sediment bitumen from Guaymas Basin, Gulf of California

The aliphatic, aromatic and polar (NSO) fractions of seabed petroleums and sediment bitumen extracts from the Guaymas Basin hydrothermal system have been analyzed by gas chromatography and gas chromatography-mass spectrometry (free and silylated). The oils were collected from the interiors and exteriors of high temperature hydrothermal vents and represent hydrothermal pyrolyzates that have migrated to the seafloor by hydrothermal fluid circulation. The downcore sediments are representative of both thermally unaltered and thermally altered sediments. The survey has revealed the presence of oxygenated compounds in samples with a high degree of thermal maturity. Phenols are one class of oxygenated compounds found in these samples. A group of methyl-, dimethyl- and trimethyl-isoprenoidyl phenols (C27-C29) is present in all of the seabed NSO fractions, with the methyl- and dimethyl-isoprenoidyl phenols occurring as major components, and a trimethyl-isoprenoidyl phenol as a minor component. A homologous series of n-alkylphenols (C13-C33) has also been found in the seabed petroleums. These phenols are most likely derived from the hydrothermal alteration of sedimentary organic matter. The n-alkylphenols are probably synthesized under hydrothermal conditions, but the isoprenoidyl phenols are probably hydrothermal alteration products of natural product precursors. The suites of phenols do not appear to be useful tracers of high temperature hydrothermal processes.     

Late Quaternary climate change record from two long sediment cores from Guaymas Basin,Gulf of California

Modern Guaymas Basin (Gulf of California, Mexico) is a region of high diatom productivity where exceptional preservation factors maintain biannually alternating sediment deposition as annual varves. New sediment cores from Guaymas Basin (MD02‐2512 and MD02‐2515) present the opportunity to construct climate records from below the last glacial period. A low‐resolution age model has been constructed from oxygen isotope analysis, correlation with other dated short piston cores from Guaymas Basin and an estimate of sedimentation rate. MD02‐2512 from eastern Guaymas Basin has an age range from the Holocene to late marine isotope stage 6 (MIS 6); MD02‐2515 from western Guaymas Basin has an age range from ～8000 to 40 000 yr. Shipboard analyses of colour reflectance, magnetic susceptibility and sediment density are combined with continuous X‐ray fluorescence scans to reconstruct a picture of glacial climate in the Gulf of California. Eastern Guaymas Basin is affected by glacial sea level fall, which results in a drastic change in productivity rates and sediment type. The laminated record of MIS 5 allows comparison with the Holocene, showing a similarity of sedimentation patterns during deglaciation and a series of very rapid variations just prior to the last glaciation. In western Guaymas Basin there are a series of Younger Dryas‐like events during the glacial, typified by low productivity and high terrigenous input. Long‐term climate and productivity changes appear to be caused by the southward displacement of the Subtropical High pressure zone. Copyright © 2005 John Wiley & Sons, Ltd.     

Hydrothermal alterations of the chemical composition in grain size fractions of sediments in the Guaymas Basin,Gulf of California

The paper presents data on the contents of macro- and microelements (rare earth elements included) determined in grain size fractions of the Upper Pleistocene hydrothermally altered and unaltered sediments from the Guaymas Basin (Gulf of California). Sediments subjected to high-temperature hydrothermal alteration were recovered by DSDP Hole 477A. In the finely dispersed fractions, which are mainly composed of clay minerals, alteration of the chemical composition was provoked by the hydrothermal transformation of terrigenous clay minerals. The concentration of microelements in these fractions takes place primarily at the cost of the hydrothermal finely dispersed ore minerals. Alteration of the chemical composition of the coarse-grained fractions is related to the replacement of clastogenic minerals by the secondary varieties and the formation of new minerals (including ore minerals and native metals) from the solutions. Hydrothermal alterations of the chemical composition of bulk samples depend on the degree of chemical element concentration in fractions and their content in samples.     

Geochemistry of shallow sediments in Guaymas Basin, gulf of California: hydrothermal gas and oil migration and effects of mineralogy

Offset of prograde hydrothermal alterations by retrograde reactions is evident in sediments from a piston core (PC6, 8 m long) from the Guaymas Basin, Gulf of California. Geochemical and mineralogical analyses of sediments and pore fluids show superimposed geochemical signals that indicate currently active reactions are modifying previously formed secondary solids. Hydrothermal barite was formed in a previously active flow channel between 150 and 250 cm depth. Hydrothermal gypsum is presently dissolving at depths > 780 cm, below which hydrothermal fluid flow is channeled by a secondary dolomite layer. Thermal stress of organic matter generates hydrothermal gas and petroleum having wide ranges of compositions and maturities. A significant amount of hydrothermally generated oil has been transported laterally through a porous debris flow (approx. 240 cm subbottom), overwhelming the indigenous bituminous matter. Water soluble petroleum constituents have disseminated throughout the cored sediments. From 400 to 700 cm depth the immature organic matter continues to experience thermally-enhanced diagenesis, and an even higher thermal stress has occurred at the bottom of the core. Enhanced bacterial degradation of organic matter has led to depletion of pore water sulfate and enrichment of ammonium (NH₄⁺) in both pore fluid and sediment, resulting in formation of NH₄⁺ bearing secondary clay minerals at depths below 780 cm.     

Influence of a turbidite deposit on the extent of pyritization of iron,manganese and trace metals in sediments from the Guaymas Basin,Gulf of California (Mexico)

A core collected in the Guaymas Basin contained an organic-poor, Mn oxide-rich and (relatively) Fe oxide-rich turbidite layer that affected the distribution of Fe, Mn, C, S and trace metals. Results indicate that sediments not influenced by the turbidite layer achieved a 100% degree of pyritization and, by extension, that pyrite production is Fe-limited in these sediments. In contrast, the mud slide layer apparently supplied enough reactive Fe to transfer essentially 98% of the total S present at the base of the turbidite (17–19 cm) to the pyrite reservoir. C/S ratios showed rapid decreases with depth, from a high of 38 close to the sediment-water interface, to minimum values of 2.8 at the lower limit of the turbidite layer, a ratio equal to the average C/S value of normal marine modern sediments, where concentrations of organic C and pyrite supposedly have attained quasi-steady values. A significant part of the reactive Mn was associated with carbonates (41±12%) and, to a much lower degree, with pyrite (2.7±1.2%). The turbidite layer is currently showing a depletion of Mn relative to the host sediment. It is possible that Mn, a major metal constituent in these sediments, was initially present in high concentrations in the mud slide, but was eventually mobilized and transferred either to the water column or to the sediments immediately below the turbidite layer. Metals associated with this element probably followed the same path, affecting their incorporation into pyrite. The turbidite layer apparently affected the distribution of most of the trace metals associated with pyrite, except maybe Cd, Pb and, to a certain, extent Cr. However, Cu, Cr, Zn, Ni and Co were all found to be highly pyritized (>80%) in the sediments of the Guaymas Basin.     

Clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin (Gulf of California)

The results of the study of clay mineral alterations in Upper Pleistocene sediments of the southern trough in the Guaymas Basin (Gulf of California) due to the influence of hydrothermal solutions and heat produced by sill intrusions are discussed. Core samples from DSDP Holes 477 and 477A were taken for the analysis of clay minerals. Application of the method of modeling X-ray diffraction patterns of oriented specimens of the finely dispersed particles made it possible to establish the phase composition of clay minerals, determine their structural parameters, and obtain reliable quantitative estimates of their contents in natural mixtures. The modeling data allowed us to characterize reliably the transformation of clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin. In Upper Pleistocene sandy–clayey sediments of the southern trough, changes in the composition of clay minerals occurred under the influence of a long-living hydrothermal system. Its lower part (interval 170.0–257.5 m) with maximum temperatures (~300°C) was marked by the formation of chlorite. Terrigenous clay minerals are not preserved here. Saponite appears at a depth of 248 m in the chlorite formation zone. Higher in the sedimentary section, the interval 146–170 m is also barren of terrigenous clay minerals. Sediments of this interval yielded two newly formed clay minerals (chlorite and illite), which were formed at lower temperatures (above 180°C and below 300°C, approximately up to ~250°C), while the relatively low-temperature upper part (110–146 m) of the hydrothermal system (from ~140°C to ~180°C) includes the mixture of terrigenous and newly formed clay minerals. Terrigenous illite is preserved here. Illitization of the mixed-layer illite–smectite was subjected to illitization. The terrigenous montmorillonite disappeared, and chlorite–smectite with 5–10% of smectite layers were formed. In the upper interval (down to approximately 110 mbsf), the composition of terrigenous clay minerals remains unchanged. They are composed of the predominant mixed-layer illite–smectite and montmorillonite, the subordinate illite, mixed-layer chlorite–smectite with 5% of smectite layers, mixed-layer kaolinite–smectite with 30% of smectite layers, and kaolinite. This composition of clay minerals changed under the influence of sill intrusions into the sedimentary cover at 58–105 m in the section of Hole 477. The most significant changes are noted in the 8-m-thick member above the sill at 50–58 m. The upper part of this interval is barren of the terrigenous mixed-layer illite–smectite, which is replaced by the newly formed trioctahedral smectite (saponite). At the same time, the terrigenous dioctahedral smectite (montmorillonite) is preserved. The composition of terrigenous clay minerals remains unchanged at the top of the unit underlying the sill base.     

Sedimentary evidence of deglacial megafloods in the northern Gulf of Mexico (Pigmy Basin)

Cored sediments from the Pigmy Basin, northern Gulf of Mexico, were analyzed in order to better constrain late deglacial and early Holocene paleoenvironmental and sedimentary changes in response to North American climate evolution. Mineralogical and geochemical proxies indicate the succession of two sedimentary regimes: dominantly detrital during the deglaciation (15–12.9 cal ka BP) whereas biogenic contribution relatively increased later on during the Younger Dryas and early Holocene (12.9 and 10 cal ka BP). Geochemical data reveal that the deglacial record mainly reflects variations of terrigenous supply via the Mississippi River rather than modifications of redox conditions in the basin. Specific variations of almost all the parameters measured in this paper are synchronous with the main deglacial meltwater episode (Meltwater Spike) described or modeled in previous marine or continental studies. During this episode, most parameters display “stair-step-like” – pattern variations highlighting three successive steps within the main meltwater flow. Variations in grain-size and clay mineral assemblage recorded in the Pigmy Basin indicate that the erosional regime was very strong on land during the first part of the Meltwater Spike, and then milder, inducing more subtle modifications in the sedimentary regime in this part of the Gulf. Specific geochemical and mineralogical signatures (notably, clay minerals and trace metal geochemistry) pinpoint a dominant origin from NW North America for detrital particles reflecting meltwater outflow from the south-western Laurentide Ice Sheet (LIS) margin during the most intense freshwater discharge. The observed decrease of the sedimentation rate from about 200 to 25 cm/ka at ca 12.9 ka evidenced a drastic decrease of erosional processes during late phase of discharge, consistently with the hypotheses of major reduction of meltwater flow. The major modification at 12.9 cal ka BP is interpreted to result from both modifications of the main Mississippi fluvial regime due to eastward and northward rerouting of meltwater flow at the onset of the Younger Dryas, and the increase of sea-surface temperature linked to insolation. Finally, slight grain-size modifications suggest that some freshwater discharges may have episodically reached the Gulf of Mexico after the Younger Dryas reflecting possible small adjustments of the postglacial hydrological regime.     

Bacterial incorporation of relict carbon in the hydrothermal environment of Guaymas Basin

Radiocarbon analyses of bulk carbon and individual organic compounds are presented for the hydrothermal environment of the Rebecca’s Roost vent in the southern trough of the Guaymas Basin hydrothermal field. The Δ¹⁴C values of CO₂ and CH₄in the hottest hydrothermal fluids (317°C) are nearly “radiocarbon dead” (−944‰ and −923‰, respectively). In contrast, the Δ¹⁴C values of sediments and individual fatty acids (−418‰ to −227‰) obtained from a bacterial mat located south of the vent site are similar to values previously reported for hydrothermal petroleum in this environment and are more depleted in ¹⁴C than overlying waters. Hydrothermal fluids moving through the sediments appear to supply ¹⁴C of intermediate age to the bacteria. This carbon may take the form of, or may be supplied by processes similar to, the generation of hydrothermal petroleum. Although the bacterial mat visibly was dominated by Beggiatoa spp., such mats are known to include numerous other species. Individual compound data show that preaged carbon is being consumed by the integrated bacterial assemblage. Values of δ¹³C and Δ¹⁴C indicate that petroleum-derived carbon is incorporated directly into fresh bacterial biomass. Subsequently, some of this newly synthesized material also is consumed by heterotrophs, as eukaryotic sterols from the same sample also have ¹⁴C-depleted values (Δ¹⁴C = −136‰ to −110‰). Therefore, the entire system may operate as a complex consortium to transform relict carbon back into biomass. Bacterial consumption of relict carbon occurs despite the ample supply of fresh carbon delivered from the productive, overlying water column.     

Mineralogy,geochemistry, and radiocarbon ages of deep sea sediments from the Gulf of Mexico,Mexico

The mineralogy, geochemistry, and radiocarbon ages of two sediment cores (GMX1 and GMX2) collected from the deep sea area of the Southwestern Gulf of Mexico (∼876–1752 m water depth) were studied to infer the sedimentation rate, provenance, heavy metal contamination, and depositional environment. The sediments are dominated by silt and clay fractions. The mineralogy determined by X-Ray diffractometry for the sediment cores reveals that montmorillonite and muscovite are the dominant clay minerals. The sections between 100 and 210 cm of the sediment cores GMX1 and GMX2, respectively, are characterized by the G. menardii group and G. Inflata planktonic foraminiferal species, which represent the Holocene and Pleistocene, respectively. The radiocarbon-age measurements of mixed planktonic foraminifera varied from ∼268 to 45,738 cal. years B.P and ∼104 to 25,705 cal. years B.P, for the sediment cores GMX1 and GMX2, respectively. The variation in age between the two sediment cores is due to a change in sediment accumulation rate, which was lowest at the location GMX1 (0.006 cm/yr) and highest at the location GMX2 (0.017 cm/yr).The chemical index of alteration (CIA), chemical index of weathering (CIW), and index of chemical maturity (ICV) values indicated a moderate intensity of weathering in the source area. The total rare earth element concentrations (∑REE) in the cores GMX1 and GMX2 vary from ∼94 to 171 and ∼78 to 151, respectively. The North American Shale Composite (NASC) normalized REE patterns showed flat low REE (LREE), heavy REE (HREE) depletion with low negative to positive Eu anomalies, which suggested that the sediments were likely derived from intermediate source rocks.The enrichment factor of heavy metals indicated that the Cd and Zn concentrations in the sediment cores were impacted by an anthropogenic source. The redox-proxy trace element ratios such as V/Cr, Ni/Co, Cu/Zn, (Cu + Mo)/Zn, and Ce/Ce* indicated that the sediments were deposited under an oxic depositional environment. The similarity in major element concentrations, REE content, and the NASC normalised REE patterns between the cores GMX1 and GMX2 revealed that the provenance of sediments remained relatively uniform or constant during deposition for ∼4.5 Ma. The major and trace element based multidimensional discrimination diagrams showed a rift setting for the core sediments, which is consistent with the geology of the Gulf of Mexico.     

Variation of the chemical composition of sediments under middle- and low-temperature hydrothermal conditions in the southern trough of Guaymas Basin,Gulf of California: Communication 1. results of the study of bulk sediment samples

The paper presents the results of the study of variations in the content of macro- and microelements (rare earth elements included) in the Upper Pleistocene sediments from DSDP Hole 477. The variations took place at the depth interval 110–191 m in the course of temperature drop to about 100°C in the upper part of the hydrothermal system. Variations in the content of most chemical elements in sediments from DSDP Hole 477 are manifested more weakly than in the underlying sediments of the interval 191–267 m of DSDP Hole 477A that were altered at a temperature of about 300°C. For many chemical elements, influence of the sedimentary cover on the compositional transformation of solutions during their interaction with sediments attenuates with the cooling of its upper part to the point of its complete termination.     

Variation of the chemical composition of sediments under middle- and low-temperature hydrothermal conditions in the southern trough of Guaymas Basin,Gulf of California: Communication 2. Results of the study of grain size fractions of sediments

The chemical composition of grain size fractions of the Upper Pleistocene sediments in the southern trough of Guaymas Basin (Gulf of California) is studied based on materials from DSDP Hole 477 (191 m). The sediments are located in the upper part of the main long-lived hydrothermal system. Therefore, they were subjected to long-term reworking by the middle- and low-temperature solutions (100?300°C) and short-term hydrothermal impact during the intrusion of basalt sills. The upper part of the main hydrothermal system is divided into three middle- to low-temperature hydrothermal alteration zones: lower (III), middle (II), and upper (I). In zone III (250?300°C), hydrothermal alterations of sediments are most significant. The coarsegrained fractions are enriched in Fe, S, Ni, Cu, Zn, Mo, Co, Se, Cd, Bi, Pb, and Ag due to the formation of sulfides and precipitation of native metals. The silt fractions are enriched in Be, Sc, Nb, Ta, W, Th, U, Y, and REE. In contrast, the pelite fractions are depleted in most of these elements. The fine-dispersed (<0.001 mm) pelite fractions are appreciably enriched in Mg (due to the formation of the authigenic Mg-chlorite), as well as MnO, Cr, V, Ga, Pb, and Zr. Virtually all grain size fractions are depleted in K₂O, Li, Rb, Cs, and Tl due to the dissolution of the K-bearing terrigenous minerals. In zones II (146?170 m) and I (110?146 m), where the temperature of hydrothermal solutions dropped successively from 250 to 180?195 and 100°C, the sedimentary environment was unfavorable for the concentration of Fe, S, and a large group of ore elements that are typical for sediments in zone III. Intrusion of basalt sills and a short-term hydrothermal impact changed concentrations of several components in the sediments overlying the sill complex: CaO, K₂O, Li, Rb, Cs, and Tl acquired a trend typical of the high-temperature lower zone III, whereas Fe, MgO, P₂O₅, TiO₂, V, Sc, and Y distribution pattern became typical of the low-temperature upper zone I. Significant compositional variations in the grain size fractions of sediments are lacking beneath the sill base.     

Jurassic Reconstruction of the Gulf of Mexico Basin

Octahedral, tetrahedral, and cubic forms of graphite, interpreted here as pseudomorphs after diamond, have been discovered in situ in crustal metamorphic rocks from central Macedonia, northern Greece. Several types of rocks, mainly of sedimentary origin, including eclogite, phyllite, quartzite, schist, and amphibolite, have been identified as hosts to inferred diamonds. All assemblages are invariably graphitic and retrograded under greenschist-facies metamorphism. The graphitized diamonds themselves occur as inclusions in garnet, quartz, amphibole, and graphite, and range in size from approximately 2 to 300 μm. In marked contrast with previously published Raman spectra of graphitized diamonds from crustal metamorphic rocks, the Raman spectra of the Greek specimens indicate very poor carbon crystallinity. This probably resulted from a rapid phase transition induced by high contact compressive stress (i.e., non-hydrostatic pressure) at ultradeep shear zones and subsequent rapid pressure release. The presence of former microdiamonds invalidates previous models on the geotectonic evolution of the Internal Hellenide zones, and demarcates a new ultrahigh-pressure zone, the width of which is currently uncertain, and which probably represents a Late Paleozoic suture marking the collision of individual continental blocks of unknown provenance.     

Gulf of Mexico Basin: Newest coal province

Post World War II oil came to replace coal as the dominant energy source in the U.S.A. Recently, coal in the form of lignite has again become a significant energy resource in the Gulf Coast region, where it occurs in Eocene and Oligocene rocks. Efficient exploitation requires a proper understanding of the facies in which it occurs. Studies of surface outcrops are providing the basis for predicting the nature of the reserves at depth.     

Climate and ocean variability during the middle and late Holocene recorded in laminated sediments from Alfonso Basin, Gulf of California, Mexico

A laminated sequence (core BAP96-CP 24°38.12′N, 110°33.24′W; 390 m depth) from the Alfonso Basin in Bay of La Paz, southern Gulf of California, contains a record of paleoceanographic and paleoclimatic changes of the past 7900 yr. Radiolarian assemblages and magnetic susceptibility are used as proxies of oceanographic and climatic variability. The records provide a regional scenario of the middle and late Holocene, suggesting two major climatic regimes and several millennial-scale events. Conditions relatively warmer and drier than today occurred from 7700 to 2500 cal yr BP, promoting the intensification of evaporation processes and the prevalence of the Gulf of California water in the Basin. These conditions correlate with strong droughts in the middle Holocene of North America and with minimal incursion of tropical waters into the Gulf of California. Proxies indicate a warm scenario and the dominance of the Equatorial Surface Water in the Alfonso Basin from 2400 to 700 cal yr BP, suggesting the intensification of ENSO cycles. A climatic signal between 1038 and 963 cal yr BP may be correlated with global signal of the “Medieval Warm Period.” Several cooling events are recognized at 5730, 3360, 2700, 1280 and 820 cal yr BP and are associated with intensification of northwest winds leading to upwellings and enhanced productivity in the Basin.     

Tectonic framework of Tiburon Basin,Gulf of California,from seismic reflection evidence

Tiburon Basin is characterized by a thick sedimentary fill that records the evolution of one of the rift segments of the East Pacific Rise. Its structure corresponds to an echelon pull-apart basin bounded by two dextral-oblique faults. Unlike basins in the southern Gulf of California that are underlain by oceanic crust, rift basins in the northern Gulf of California contain sedimentary thickness (up to 6 km) that masks the structure of the crust. To study the architecture of the Tiburon Basin, two-dimensional, multichannel seismic reflection data collected by Petróleos Mexicanos (PEMEX) in the early 1980s were used. The data base is a grid of lines, 5–20 km apart, with 6 s of record in 48 channels. Additional seismic data of the Ulloa 99 project were also interpreted. Our results indicate that the general structural pattern of the Tiburon Basin is controlled by two dextral-oblique faults: De Mar and Tiburon. De Mar lies to the east and ends in elevated basement transferring the stress to the Desemboque fault. The latter borders the incoming basement from the Sonora and Tiburon faults to the west, ending to the north in an antiform. Four structural domains are recognized: (1) the northern Tiburon domain is a high basement that divides the Delfin Basin to the northeast and exhibits extensional folds with their axes parallel to the basement and its flanks; (2) the Libertad domain is a sheared basement high along the margin of Sonora and forms the right step of the Tepoca Basin to the north; (3) the Tiburon central domain defines a broad sag cut by a dense NE-striking pattern of normal faults with opposed dips in the depocentre and abruptly ends to the west against the Tiburon fault; and (4) the southern Tiburon domain forms a basement ramp offshore Isla Tiburon and is controlled by a pattern of NNE-striking normal faults on the south that likely connect at an oblique angle (?60°) to the De Mar fault. We propose a rhombochasm basin model with more than 6 s of sedimentary record in the depocentre, in which the basement is not recorded. The NW-trending faults in the Libertad domain possibly continue towards the Sonora coastal plain. The principal NW-trending dextral faults and the secondary NNE-striking pattern of normal faults cut the shallow strata of this domain.     

Enzymatic microbial Mn(II) oxidation and Mn biooxide production in the Guaymas Basin deep-sea hydrothermal plume

Microorganisms play important roles in mediating biogeochemical reactions in deep-sea hydrothermal plumes, but little is known regarding the mechanisms that underpin these transformations. At Guaymas Basin (GB) in the Gulf of California, hydrothermal vents inject fluids laden with dissolved Mn(II) (dMn) into the deep waters of the basin where it is oxidized and precipitated as particulate Mn(III/IV) oxides, forming turbid hydrothermal “clouds”. Previous studies have predicted extremely short residence times for dMn at GB and suggested they are the result of microbially-mediated Mn(II) oxidation and precipitation. Here we present biogeochemical results that support a central role for microorganisms in driving Mn(II) oxidation in the GB hydrothermal plume, with enzymes being the primary catalytic agent. dMn removal rates at GB are remarkably fast for a deep-sea hydrothermal plume (up to 2 nM/h). These rapid rates were only observed within the plume, not in background deep-sea water above the GB plume or at GB plume depths (∼1750-2000 m) in the neighboring Carmen Basin, where there is no known venting. dMn removal is dramatically inhibited under anoxic conditions and by the presence of the biological poison, sodium azide. A conspicuous temperature optimum of dMn removal rates (∼40 °C) and a saturation-like (i.e. Michaelis-Menten) response to O₂ concentration were observed, indicating an enzymatic mechanism. dMn removal was resistant to heat treatment used to select for spore-forming organisms, but very sensitive to low concentrations of added Cu, a cofactor required by the putative Mn(II)-oxidizing enzyme. Extended X-ray absorption fine structure spectroscopy (EXAFS) and synchrotron radiation-based X-ray diffraction (SR-XRD) revealed the Mn oxides to have a hexagonal birnessite or δ-MnO₂-like mineral structure, indicating that these freshly formed deep-sea Mn oxides are strikingly similar to primary biogenic Mn oxides produced by laboratory cultures of bacteria. Overall, these results reveal a vigorous Mn biogeochemical cycle in the GB hydrothermal plume, where a distinct microbial community enzymatically catalyzes rapid Mn(II) oxidation and the production of Mn biooxides.     

Carbonates calibrated against oceanographic parameters along a latitudinal transect in the Gulf of California, Mexico

Trophic resources are an important control governing carbonate production. Though this importance has long been recognized, no calibration exists to quantitatively compare biogenic assemblages within trophic resource fields. This study presents a field calibration of carbonate producers in a range of settings against high‐resolution in situ measurements of nutrients, temperature and salinity. With its latitudinal extent from 30° to 23° N, the Gulf of California, Mexico, spans the warm‐temperate realm and encompasses nutrient regimes from oligo‐mesotrophic in the south to eutrophic in the north. Accordingly, from south to north carbonates are characterized by: (i) coral‐dominated shallow carbonate factories (5–20 m water depth) with average sea‐surface temperatures of 25 °C (min. 18 °C, max. 31 °C), average salinities of 35·06‰ and average chlorophyll a levels, which are a proxy for nutrients, of 0·25 mg Chl a m^?3 (max. 0·48, min. 0·1). (ii) Red algal‐dominated subtidal to inner‐shelf carbonate formation (10–25 m) in the central Gulf of California exhibiting average temperatures of 23 °C (min. 18 °C, max. 30 °C), average salinities of 35·25‰, and average Chl a levels of 0·71 Chl a m^?3 (max. 5·62, min. 0). (iii) Molluskan bryozoan‐rich inner to outer shelf factories in the northern Gulf of California (20–50 m) with average sea surface temperatures of only 20 °C (min. 13 °C, max 29 °C), average salinities of 35·01‰, and average contents of 2·2 mg Chl a m^?3 (max. 8·38, min. 0). By calibrating sedimentological data with in situ measured oceanographic information in different environments, the response of carbonate producers to environmental parameters was established and extrapolated to carbonates on a global scale. The results demonstrate the importance of recognizing and quantifying trophic resources as a dominant control determining the biogenic composition and facies character of both modern and fossil carbonates.