Boron Isotope Fractionation in Bell Pepper

Various plant compartments of a single bell pepper plant were studied to verify the variability of boron isotope composition in plants and to identify possible intra-plant isotope fractionation. Boron mass fractions varied from 9.8 mg/kg in the fruits to 70.0 mg/kg in the leaves. Boron (B) isotope ratios reported as δ11B ranged from -11.0‰ to +16.0‰ (U ≤ 1.9‰, k=2) and showed a distinct trend to heavier δ11B values the higher the plant compartments were located in the plant. A fractionation of Δ11Bleaf-roots = 27‰ existed in the studied bell pepper plant, which represents about 1/3 of the overall natural boron isotope variation (ca. 80‰). Two simultaneous operating processes are a possible explanation for the observed systematic intra-plant δ11B variation: 1) B is fixed in cell walls in its tetrahedral form (borate), which preferentially incorporates the light B isotope and the remaining xylem sap gets enriched in the heavy B isotope and 2) certain transporter preferentially transport the trigonal 11B-enriched boric acid molecule and thereby the heavy 11B towards young plant compartments which were situated distal of the roots and typically high in the plant. Consequently, an enrichment of the heavy 11B isotope in the upper young plant parts located at the top of the plant could explain the observed isotope systematic. The identification and understanding of the processes generating systematic intra-plant δ11B variations will potentially enable the use of B isotope for plant metabolism studies

Chemical Alteration of Riverine Particles in Seawater and Marine Sediments: Effects on Seawater Composition and Atmospheric CO2

Numerous studies have shown that riverine particles react with seawater. Reactions include dissolution of reactive silicate minerals (e.g., feldspars) and formation of authigenic clays and carbonates. Previous studies have either focused on mineral dissolution (marine silicate weathering) or authigenic phase formation (reverse weathering). A comprehensive study that assesses all processes affecting the marine alteration of riverine particle has -to our knowledge- not yet been conducted. Our contribution aims to fill this gap. We first quantify cation exchange between seawater and riverine particles that occurs when particles enter the marine realm and show that significant global cation fluxes are induced by this process (-1.3 Tmol Na yr-1, -0.2 Tmol K yr-1, -0.4 Tmol Mg yr-1, +1.2 Tmol Ca yr-1) where the positive sign indicates cation release into seawater while the negative sign denotes uptake on particles. We then use thermodynamic and kinetic modeling to investigate how much of the suspended particle load dissolves in contact with seawater and estimate corresponding global release rates for dissolved cations and silica assuming congruent dissolution (+0.06 Tmol Na yr-1, +0.15 Tmol Ca yr-1, +2.8 Tmol Si yr-1). Subsequently, we investigate rates of mineral dissolution and authigenic clay and carbonate formation in marine sediments applying reactive transport modeling, porewater data and mass balance calculations. Our best estimates for net fluxes across the sediment/water interface (dissolution–mineral formation) result as +1.5 Tmol Na yr-1, -2.5 Tmol K yr-1, -2.0 Tmol Mg yr-1, +2.5 Tmol Ca yr-1, and +1.9 Tmol Si yr-1 where most of the Na and Ca release is induced by plagioclase dissolution, K is taken up in authigenic clays and Mg is removed from solution by authigenic clay and carbonate formation. We conclude that the alkalinity of seawater is not significantly affected by marine silicate alteration since cation release fluxes (Na, Ca) are as high as cation uptake fluxes (K, Mg) on equivalent basis. Moreover, marine silicate weathering and reverse weathering are closely coupled since Al required for clay formation is mostly provided by feldspar dissolution while Al removal in authigenic clay promotes and maintains feldspar dissolution in marine sediments. Authigenic carbonate formation in anoxic subsurface sediments sequesters significant amounts of carbon (2.5 Tmol C yr-1) according to our estimates where most of the Ca and alkalinity required for carbonate formation are provided by the dissolution of Ca-bearing silicate minerals. This hidden sedimentary cycle provides a sink for dissolved inorganic carbon that may drive a slow draw-down of atmospheric CO2 on geological timescales. Marine silicate alteration has an even stronger effect on the geochemical evolution of seawater by generating large fluxes of dissolved K, Mg, Ca and Si

Controls on Dissolved Silicon Isotopes along the US GEOTRACES Eastern Pacific Zonal Transect (GP16)

The distribution of dissolved silicon isotopes (δ30Si) was examined along the US GEOTRACES East Pacific Zonal Transect (GP16) extending from Peru to Tahiti (10°S and 15°S latitude). Surface waters in the subtropical gyre displayed high δ30Si due to strong utilization of silicic acid (DSi). In contrast, surface waters close to the Peruvian coast where upwelling prevailed were less depleted and only moderately fractionated. δ30Si of water masses along the transect were compared with the results of an Optimum Multiparameter Analysis that quantified the fractional contributions of endmember water masses in each sample. Strong admixture of intermediate waters obscured the expected heavy isotopic signatures of Subantarctic Mode Water and Antarctic Intermediate Water. Isotope values were nearly homogenous below 2000 m (Average: +1.3 ± 0.1 ‰, 1 s.d.) despite the 25 μmol kg‐1 range in the DSi content among water masses. This homogeneity confirms prior observations and model results that predict nearly constant δ30Si values of +1.0 to +1.2 ‰ for Pacific deep waters with [DSi] > 100 μmol kg‐1. Waters above the East Pacific Rise (EPR) influenced by hydrothermal activity showed a small increase in [DSi] together with dissolved iron, but overall stations close to the EPR were slightly depleted in [DSi] (3 to 6 μmol kg‐1) with no significant shift in δ30Si compared to adjacent waters. Hydrothermal [DSi] appears to precipitate within the conduit of the EPR or upon contact with cold seawater resulting in a negligible influence of hydrothermal fluids on δ30Si in deep water. Key Points Surface waters have a large range in dissolved silicon isotopes covering nutrient‐rich coastal upwelling to oligotrophic waters Deep water masses with DSi concentrations > 100 μmol kg‐1 show homogenous silicon isotope signatures despite up to 25 μmol kg‐1 differences in [DSi] Hydrothermal fluids have a negligible effect on Si isotope distributions in the deep Pacifi

Shelf-to-basin iron shuttle in the Guaymas Basin, Gulf of California

Enrichments of highly reactive iron (Fe) (sum of Fe (oxyhydr)oxide, carbonate and sulfide minerals) in marine sediments and sedimentary rocks are commonly interpreted as an indication of anoxic conditions in the bottom water at the time of deposition. The model system for this proxy rationale is the semi-restricted Black Sea, where sediments underneath the anoxic and sulfidic (i.e., euxinic) deep-water are enriched in reactive Fe, which was mobilized from the surrounding shelf areas. To test whether such a shelf-to-basin Fe shuttle can operate in semi-restricted basins without euxinic deep water, we investigated sedimentary Fe speciation and Fe isotope compositions in sediments of the Guaymas Basin, Gulf of California. Sediments on the slope underneath the eastern equatorial Pacific oxygen minimum zone and sediments within the oxic deep basin are both enriched in reactive Fe, with reactive Fe making up 45 ± 11 % of the total Fe pool. The following mechanisms may contribute to these Fe enrichments: (1) Release of dissolved Fe from anoxic shelf and slope sediments followed by lateral transport of dissolved and/or particulate Fe in the water column; (2) preferential transport of fine-grained, terrigenous particles with a high reactive Fe content into the basin; (3) microbially mediated conversion of non-reactive silicate minerals to reactive Fe minerals during transport; (4) hydrothermal venting and lateral Fe transport within the deep water. The first process can explain reactive Fe enrichments in slope sediments, whereas all processes may contribute to sedimentary Fe enrichments in the deeper basin. The δ56Fe value of sediments increases from shelf to slope and decreases from the slope into the basin. This lateral pattern of δ56Fe, as well as the pattern of Fe enrichment, is similar to that observed in other marine systems with a Fe shuttle. However, the size of the Fe enrichment, and the range in δ56Fe (-0.06 to +0.16‰) is smaller. This difference is due to higher terrigenous sedimentation rates in the Guaymas Basin and, therefore, more intense dilution of shuttle-derived reactive Fe. We argue that, depending on the extent of bathymetric restriction and terrigenous background sedimentation, reactive Fe enrichments can form under a broad range of redox conditions and in diverse sedimentary environments. The concepts applied in this study can be used to identify those circumstances in the paleo-record

Alkaline mineral addition to anoxic to hypoxic Baltic Sea sediments as a potentially efficient CO2-removal technique

Recent studies have begun to explore the potential of enhanced benthic weathering (EBW) in the Baltic Sea as a measure for climate change mitigation. To augment the understanding of EBW under seasonally changing conditions, this study aims to investigate weathering processes under anoxia to hypoxia in corrosive bottom waters, which reflect late summer conditions in the Baltic Sea. Dunite and calcite were added to sediment cores retrieved from Eckernförde Bay (Western Baltic Sea) with a constant flow-through of deoxygenated, CO2-enriched Baltic Sea bottom water. The addition of both materials increased benthic alkalinity release by 2.94 μmol cm−2 d−1 (calcite) and 1.12 μmol cm−2 d−1 (dunite), compared to the unamended control experiment. These excess fluxes are significantly higher than those obtained under winter conditions. The comparison with bottom water oxygen concentrations emphasizes that highest fluxes of alkalinity were associated with anoxic phases of the experiment. An increase in Ca and Si fluxes showed that the enhanced alkalinity fluxes could be attributed to calcite and dunite weathering. First order rate constants calculated based on these data were close to rates published in previous studies conducted under different conditions. This highlights the suitability of these proxies for mineral dissolution and justifies the use of these rate constants in modeling studies investigating EBW in the Baltic Sea and areas with similar chemical conditions. Generally stable pH profiles over the course of the experiment, together with the fact that the added minerals remained on the sediment surface, suggest that corrosive bottom waters were the main driving factor for the dissolution of the added minerals. These factors have important implications for the choice of mineral and timing for EBW as a possible marine carbon dioxide removal method in seasonally hypoxic to anoxic regions of the Baltic Sea

Mesocosm experiments in ocean alkalinity enhancement research

An essential prerequisite for the implementation of ocean alkalinity enhancement (OAE) applications is their environmental safety. Only if it can be ensured that ecosystem health and ecosystem services are not at risk will the implementation of OAE move forward. Public opinion on OAE strategies will depend first and foremost on reliable evidence that no harm will be done to marine ecosystems, and licensing authorities will demand measurable criteria against which environmental sustainability can be determined. In this context mesocosm experiments represent a highly valuable tool in determining the safe operating space of OAE applications. By combining biological complexity with controllability and replication, they provide an ideal OAE test bed and a critical stepping stone towards field applications. Mesocosm approaches can also be helpful in testing the efficacy, efficiency and permanence of OAE applications. This chapter outlines strengths and weaknesses of mesocosm approaches, illustrates mesocosm facilities and suitable experimental designs presently employed in OAE research, describes critical steps in mesocosm operation, and discusses possible approaches for alkalinity manipulation and monitoring. Building on a general treatise on each of these aspects, the chapter describes pelagic and benthic mesocosm approaches separately, given their inherent differences. The chapter concludes with recommendations for best practices in OAE-related mesocosm research

Impact of ambient conditions on the Si isotope fractionation in marine pore fluids during early diagenesis

Benthic fluxes of dissolved silica (Si) from sediments into the water column are driven by the dissolution of biogenic silica (bSiO2) and terrigenous Si minerals and modulated by the precipitation of authigenic Si phases. Each of these processes has a specific effect on the isotopic composition of silica dissolved in sediment pore waters such that the determination of pore water δ30Si values can help to decipher the complex Si cycle in surface sediments. In this study, the δ30Si signatures of pore fluids and bSiO2 in the Guaymas Basin (Gulf of California) were analyzed, which is characterized by high bSiO2 accumulation and hydrothermal activity. The δ30Si signatures were investigated in the deep basin, in the vicinity of a hydrothermal vent field, and at an anoxic site located within the pronounced oxygen minimum zone (OMZ). The pore fluid δ30Sipf signatures differ significantly depending on the ambient conditions. Within the basin, δ30Sipf is essentially uniform averaging +1.2 ± 0.1 ‰ (1SD). Pore fluid δ30Sipf values from within the OMZ are significantly lower (0.0 ± 0.5 ‰, 1SD), while pore fluids close to the hydrothermal vent field are higher (+2.0 ± 0.2 ‰, 1SD). Reactive transport modelling results show that the δ30Sipf is mainly controlled by silica dissolution (bSiO2 and terrigenous phases) and Si precipitation (authigenic aluminosilicates). Precipitation processes cause a shift to high pore fluid δ30Sipf signatures, most pronounced at the hydrothermal site. Within the OMZ however, additional dissolution of isotopically depleted Si minerals (e.g. clays) facilitated by high mass accumulation rates of terrigenous material (MARterr) is required to promote the low δ30Sipf signatures while precipitation of authigenic aluminosilicates seems to be hampered by high water / rock ratios. Guaymas OMZ δ30Sipf values are markedly different from those of the Peruvian OMZ, the only other marine setting where Si isotopes have been investigated to constrain early diagenetic processes. These differences highlight the fact that δ30Sipf signals in OMZs worldwide are not alike and each setting can result in a range of δ30Sipf values as a function of the environmental conditions. We conclude that the benthic silica cycle is more complex than previously thought and that additional Si isotope studies are needed to decipher the controls on Si turnover in marine sediment and the role of sediments in the marine silica cycle

Controls on the Silicon Isotope Composition of Diatoms in the Peruvian Upwelling

The upwelling area off Peru is characterized by exceptionally high rates of primary productivity, mainly dominated by diatoms, which require dissolved silicic acid (dSi) to construct their frustules. The silicon isotope compositions of dissolved silicic acid (δ 30 Si dSi ) and biogenic silica (δ 30 Si bSi ) in the ocean carry information about dSi utilization, dissolution, and water mass mixing. Diatoms are preserved in the underlying sediments and can serve as archives for past nutrient conditions. However, the factors influencing the Si isotope fractionation between diatoms and seawater are not fully understood. More δ 30 Si bSi data in today’s ocean are required to validate and improve the understanding of paleo records. Here, we present the first δ 30 Si bSi data (together with δ 30 Si dSi ) from the water column in the Peruvian Upwelling region. Samples were taken under strong upwelling conditions and the bSi collected from seawater consisted of more than 98% diatoms. The δ 30 Si dSi signatures in the surface waters were higher (+1.7‰ to +3.0‰) than δ 30 Si bSi (+1.0‰ to +2‰) with offsets between diatoms and seawater (Δ 30 Si) ranging from −0.4‰ to −1.0‰. In contrast, δ 30 Si dSi and δ 30 Si bSi signatures were similar in the subsurface waters of the oxygen minimum zone (OMZ) as a consequence of a decrease in δ 30 Si dSi . A strong relationship between δ 30 Si bSi and [dSi] in surface water samples supports that dSi utilization of the available pool (70 and 98%) is the main driver controlling δ 30 Si bSi . A comparison of δ 30 Si bSi samples from the water column and from underlying core-top sediments (δ 30 Si bSi_ sed. ) in the central upwelling region off Peru (10°S and 15°S) showed good agreement (δ 30 Si bSi_ sed. = +0.9‰ to +1.7‰), although we observed small differences in δ 30 Si bSi depending on the diatom size fraction and diatom assemblage. A detailed analysis of the diatom assemblages highlights apparent variability in fractionation among taxa that has to be taken into account when using δ 30 Si bSi data as a paleo proxy for the reconstruction of dSi utilization in the region