Non-conservative behavior of dissolved molybdenum in hypersaline waters of the Guerrero Negro saltern, Mexico (original) (raw)
Related papers
2014
Analysis of modern microbial mats from the Guerrero Negro (Mexico) hypersaline region showed substantial Mo enrichment factors (EF Mo = (Mo/Al) sample /(Mo/Al) background ) ranging from 63 to 558, with an average molar ratio value of 297 ± 135. These enrichments are caused by authigenic biogeochemical processes, which are independent of the local abundance of Mo. Our calculations indicate that approximately a half of the Mo enrichment in the microbial mats may be explained by geochemical processes (e.g., coprecipitation with authigenic sulfides such as pyrite), while the remaining half are included in our operationally defined residual fraction. Generally, it is accepted that the elevated concentrations of Mo found in ancient anoxic sediments are a consequence of high biological production. Our calculations of EF Mo based on concentrations of Mo and Al taken from the literature for different ancient environments (74 to 155 Ma) suggest that some of these elevated enrichments (EF Mo of up to 1261) could have been the result of benthic microbial mats colonizing such environments. Considering that modern microbial mats may be used as proxies to those which have existed on Earth 3.5 billion years ago, the results obtained in this work suggest that these consortia of microorganisms may have represented an important sink for Mo during periods in Earth's history when microbial mats were more abundant. This characteristic leads us to propose high EF Mo values, slight Fe impoverishments, low reactive Fe availability, and intermediate Degree of Pyritization (12 to 50%) as a new geochemical tool to indicate conditions associated with the presence of microbial mats in ancient hypersaline environments.
Geochimica et Cosmochimica Acta, 2007
Non-conservative behaviour of dissolved Mo was observed during specific time periods in the water column of the Wadden Sea of NW Germany. In July 2005 dissolved Mo declined within 36 h from a level only slightly below seawater (82 nM) to a minimum value of 30 nM, whereas in August 2002 dissolved Mo revealed a tidal cyclicity with maximum values up to 158 nM at low tide. In contrast, cruises in August 2003 and 2004 displayed an almost conservative behaviour of Mo. The decrease in dissolved Mo during July 2005 and elevated values in August 2002 were accompanied by Mo enrichments on aggregates in the water column of the Wadden Sea. Along with Mo, dissolved Mn showed unusual concentration patterns in July 2005
Applied Geochemistry, 2008
To understand the biogeochemical cycles of trace metals (Cd, Cu, Fe, Mn, Ni and Zn) in a hypersaline subtropical marsh, geochemical studies of both interstitial and solid phases were conducted on sediment cores from Chiricahueto marsh, SE Gulf of California. The sequential extraction procedure proposed by Tessier was used to estimate the percentages of the metals present in each geochemical phase of the sediment. Metal concentrations in the solid phase were found to be enriched in the upper layers and mainly associated with reactive fractions such as organic matter, Fe-Mn oxyhydroxides and carbonates (46-74% of Ni, Mn and Cd, and 11-19% of Cu and Zn). Principal factor analysis (PFA) and Spearman correlation analysis revealed a strong positive association of metals and their reactive phases with OC (the diagenetic component), and a negative or non-association with the mud content, Al, Fe and Li (the lithogenic component). Diagenetically released metals are mainly mobilized within hypersaline sediments by buoyancy transport (>90% of total flux) in response to an extreme salinity gradient by input of fresh groundwater (3-6 psu cm À1 ). The molecular diffusion due to the gradient of metals in porewater (maximum and higher levels at 5-7 and below 20 cm depth, respectively) is significantly less important to the advective transport. Most of the metals mobilized by diffusion-advection processes are re-precipitated in the sediments by authigenic minerals, only <10% of most metals are extruded out to the overlying water column. Authigenic accumulation rates were estimated as 1.42-7.09 mg m À2 a À1 for Cd; 58.8-378 for Cu; 6922-17,985 for Fe; 38.2-345 for Mn; 20.8-263 for Ni; and 282-2956 mg m À2 a À1 for Zn. The Mn-Fe oxyhydroxides (40-85% of reactive metals) in the upper oxic-suboxic layers (<5 cm below surface) and sulfide minerals (75-97%) in anoxic sediment layers (7-18 cm) constitute the main scavengers for metals.
Environmental analysis of paleoceanographic systems based on molybdenum���uranium covariation
2009
Modern low-oxygen marine systems exhibit patterns of molybdenum-uranium covariation that can be linked to specific attributes and processes of the depositional system, including (1) variation in benthic redox conditions, (2) the operation of particulate shuttles within the water column, and (3) the evolution of watermass chemistry. The importance of these factors in each depositional system can be assessed both from the degree of enrichment of authigenic molybdenum (Mo auth) and uranium (U auth) and from the (Mo/U) auth ratio of the sediment relative to the seawater Mo/U molar ratio of ∼ 7.5-7.9. In open-ocean systems with suboxic bottomwaters, U auth enrichment tends to exceed that of Mo auth owing to onset of U auth accumulation at the Fe(II)-Fe(III) redox boundary, resulting in sediment (Mo/U) auth ratios less than that of seawater. As bottomwaters become increasingly reducing and at least occasionally sulfidic, the rate of accumulation of Mo auth increases relative to that of U auth , and sediment (Mo/U) auth ratios equal or exceed that of seawater. In restricted marine systems with permanently sulfidic deepwaters, the relative enrichment of Mo auth and U auth depends on additional factors. In the Cariaco Basin, which has an aqueous Mo/U ratio similar to that of seawater, the operation of a particulate Mn-Fe-oxyhydroxide shuttle serves to accelerate the transfer of Mo to the seafloor, leading to strong enrichments in Mo auth relative to U auth. In the Black Sea, the chemistry of the deep watermass has evolved to the point where its aqueous Mo/U ratio is only ∼ 0.04 that of seawater, as a consequence of which sediments deposited under deepwater influence are depleted in Mo auth relative to U auth. These Mo-U covariation patterns can be used to gain a better understanding of the watermass attributes and processes of ancient low-oxygen marine systems. Analysis of anoxic facies from two North American paleomarine systems, the Late Pennsylvanian Midcontinent Sea (LPMS) and the Late Devonian Seaway (LDS), reveals authigenic Mo-U relationships similar to those of the modern marine environments above, implying similar redox and hydrographic controls. The observed patterns are consistent with laterally unconfined circulation and strong watermass exchange within the LPMS, and with markedly restricted deepwater circulation in silled basins of the LDS. Patterns of authigenic Mo-U covariation may prove useful in analysis of other paleoceanographic systems to reveal aspects of watermass composition and environmental dynamics.
Aquatic Geochemistry
Total trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn), Al, and pyrite- and reactive-associated metals were measured for the first time in a microbial mat and its underlying anoxic-sulfidic sediment collected in the saltern of Guerrero Negro (GN), Baja California Sur, Mexico. It is postulated that the formation of acid volatile sulfide (AVS) and pyrite in the area of GN could be limited by the availability of reactive Fe, as suggested by its limited abundance (mat and sediment combined average value of only 19 ± 10 μmol g−1; n = 22) as well as the low pyrite (0.89–7.9 μmol g−1) and AVS (0.19–21 μmol g−1) concentrations (for anoxic-sulfidic sediments), intermediate degrees of pyritization (12–50%), high degrees of sulfidization (14–100%), generally low degrees of trace metal pyritization, and slight impoverishment in total Fe. This is a surprising result considering the large potential reservoir of available Fe in the surrounding desert. Our findings suggest that pyrite formation in the cycling of trace metals in the saltern of GN is not very important and that other sedimentary phases (e.g., organic matter, carbonates) may be more important reservoirs of trace elements. Enrichment factors [EFMe = (Me/Al)sample/(Me/Al)background] of Co, Pb, and Cd were high in the mat (EFMe = 2.2 ± 0.4, 2.8 ± 1.6 and 34.5 ± 9.8, respectively) and even higher in the underlying sediment (EFMe = 4.7 ± 1.5, 14.5 ± 6.2 and 89 ± 27, respectively), but Fe was slightly impoverished (average EFFe of 0.49 ± 0.13 and 0.50 ± 0.27 in both mat and sediment). Organic carbon to pyrite-sulfur (C/S) molar ratios measured in the mat (2.9 × 102–27 × 102) and sediment (0.81 × 102–6.6 × 102) were, on average, approximately 77 times higher than those typically found in marine sediments (7.5 ± 2.1). These results may indicate that ancient evaporation basins or hypersaline sedimentary environments could be identified on the basis of extremely high C/S ratios (e.g., >100) and low reactive Fe.
Molybdenum isotope records as a potential new proxy for paleoceanography
New high-precision isotope ratios of dissolved Mo in seawater from different ocean basins and depths show a homogeneous isotope composition ('mean ocean water 98 Mo/ 95 Mo' (MOMO)), as expected from its long ocean residence time (800 kyr). This composition appears to have been constant for the past 60 Myr at a 1^3 Myr time resolution as indicated from thick sections of Fe^Mn crusts from the Atlantic and Pacific. These records yield a constant offset from MOMO (average of 33.1 and 32.9x). They are similar to our new data on recent oxic Mo sinks: pelagic sediments and six Fe^Mn crust surface layers range from 32.7 to 32.9x and 32.7 to 33.1x, respectively. Recent suboxic Mo sinks from open ocean basins display heavier and more variable isotope ratios (30.7 to 31.6x relative to MOMO). Crustal Mo sources were characterized by measuring two granites (and a mild acid leach of one granite), seven volcanic rocks and two clastic sediments. All show a narrow range of compositions (32.0 to 32.3x). These data indicate that isotope fractionation by chemical weathering and magmatic processes is insignificant on a global scale. They therefore represent good estimates of the composition of dissolved Mo input to the oceans and that of the average continental crust. Thus, the Mo input into the oceans appears to be distributed into lighter oxic sinks and heavier reducing sinks. This is consistent with steady-state conditions in the modern ocean. The constant isotope offset between oxic sediments and seawater suggests that the relative amounts of oxic and reducing Mo removal fluxes have not varied by more than 10% over the last 60 Myr. An equilibrium fractionation process is proposed assuming that Mo isotope fractionation occurs between (dominant) MoO 23 4 and (minor) Mo(OH) 6 species in solution, of which the latter is preferentially scavenged. ß
Molybdenum isotope signatures in continental margin marine sediments
We present molybdenum isotope data for four sediment profiles from continental margin settings. Each profile has a distinctive average isotope composition ranging from d 98 / 95 Mo À 0.5x to 1.3x (relative to J and M laboratory standard). This range lies between the modern ocean water value (2.3x) and the values typical of Mo adsorbed onto Mn oxides (À 0.7x F 0.1x). An important finding of this study is the apparent co-variation between the Mo isotope composition and the accumulation rate of authigenic Mo under reducing conditions. This relationship suggests that the chemical processes responsible for Mo accumulation under reducing conditions produce an isotope signature in marine sediments. In addition to the relationship between Mo accumulation and the Mo isotope signature there is also a relationship between these parameters and the rate of organic carbon oxidation and burial. These relationships suggest that the Mo isotope signature of reducing sediments may serve as a tracer for the cycling of organic carbon in continental margin sediments; however, additional data will be required to refine any such relationships.
Geochimica et Cosmochimica Acta, 2000
Pore water and sediment Mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the Santa Barbara Basin to examine the connection between authigenic Mo formation and pore water sulfide concentration. Only at the deepest site (580 m), where pore water sulfide concentrations rise to Ͼ0.1 M right below the sediment water interface, was there active authigenic Mo formation. At shallower sites (550, 430, and 340 m), where pore water sulfide concentrations were consistently Ͻ0.05 M, Mo precipitation was not occurring at the time of sampling. A sulfide concentration of ϳ0.1 M appears to be a threshold for the onset of Mo-Fe-S co-precipitation. A second threshold sulfide concentration of ϳ100 M is required for Mo precipitation without Fe, possibly as Mo-S or as particle-bound Mo.