Diatom Opal Research Articles

Variations of diatom opal Ge/Si in Prydz Bay, East Antarctica

Diatom frustules record the variations of seawater Ge/Si, a ratio related to continental weathering, early diagenesis, and biological productivity. To investigate the geochemistry of germanium and the possibility of using opal Ge/Si as a proxy of changes in Ge and Si budgets in high latitude Antarctic marginal sea, nine surface sediment samples and one sediment core were collected from Prydz Bay during Chinese Antarctic Expeditions from 2005 to 2009. Diatom opal was separated and purified from sediment samples, and opal Ge/Si was determined. The results showed that (Ge/Si)opal ranged from 0.48 to 0.78 μmol mol−1 (0.60 ± 0.09 μmol mol−1) in Prydz Bay. (Ge/Si)opal from sites in the open sea and at the edge of the Amery Ice Shelf are comparable to modern oceanic seawater, while it is relatively low on the continental shelf. The variations in (Ge/Si)opal were not significantly influenced by biological fractionation during diatom uptake, but were related to changes in external input or authigenic loss of Ge. An authigenic sink of Ge was detected at all sites and accounted for 20% ~ 52% of Ge in bulk sediments. Authigenic precipitation of Ge is generally controlled by precipitation of Fe-rich minerals, but is also influenced by biogenic opal delivery in the centre of the bay where siliceous productivity is extremely high. The higher (Ge/Si)opal relative to those on the continental shelf is suggested to be caused by Ge supplementation from oceanic water in the open sea area and by high input of glacial debris from ice-melting at the edge of the Amery Ice Shelf.

The pH dependency of the boron isotopic composition of diatom opal (<i>Thalassiosira weissflogii</i>)

Abstract. The high-latitude oceans are key areas of carbon and heat exchange between the atmosphere and the ocean. As such, they are a focus of both modern oceanographic and palaeoclimate research. However, most palaeoclimate proxies that could provide a long-term perspective are based on calcareous organisms, such as foraminifera, that are scarce or entirely absent in deep-sea sediments south of 50∘ S in the Southern Ocean and north of 40∘ N in the North Pacific. As a result, proxies need to be developed for the opal-based organisms (e.g. diatoms) found at these high latitudes, which dominate the biogenic sediments recovered from these regions. Here we present a method for the analysis of the boron (B) content and isotopic composition (δ11B) of diatom opal. We apply it for the first time to evaluate the relationship between seawater pH, δ11B and B concentration ([B]) in the frustules of the diatom Thalassiosira weissflogii, cultured across a range of carbon dioxide partial pressure (pCO2) and pH values. In agreement with existing data, we find that the [B] of the cultured diatom frustules increases with increasing pH (Mejía et al., 2013). δ11B shows a relatively well defined negative trend with increasing pH, completely distinct from any other biomineral previously measured. This relationship not only has implications for the magnitude of the isotopic fractionation that occurs during boron incorporation into opal, but also allows us to explore the potential of the boron-based proxies for palaeo-pH and palaeo-CO2 reconstruction in high-latitude marine sediments that have, up until now, eluded study due to the lack of suitable carbonate material.

220 Year Diatom δ18O Reconstruction of the Guaymas Basin Thermocline Using Microfluorination

AbstractThe removal of exchangeable oxygen from diatom opal prior to δ18Odiatom analysis is a crucial first step before analyzing frustule oxygen isotopes for paleoceanographic applications. We present the results of experiments that quantify the temperature‐dependent reactivity of biogenic silica with water under laboratory conditions. We demonstrate that controlled exchange between rinse water and diatom opal at room temperature results in predictable alteration of δ18Odiatom, after vacuum dehydroxylation. Diatom samples equilibrated with an 18O‐enriched δ18Oequil. water solution of +94.4‰ at ~21 °C for 70 hr prior to dehydroxylation yield δ18Odiatom data that can be directly interpreted with existing empirical δ18Odiatom versus temperature relationships. We compare sediment trap‐based δ18Odiatom temperature data with modern δ18Oforam temperatures. Finally, we present an ~220 year record of δ18Odiatom data from an eastern Pacific Guaymas Basin boxcore, analyzed using the microfluorination technique, that indicates diatom chlorophyll maximum (Chlmax) temperatures have shifted from a period during the late eighteenth and nineteenth centuries that was 8 °C cooler during the fall bloom relative to mean annual alkenone sea surface temperature, to the late twentieth century when we observe no difference between alkenone sea surface temperature and diatom Chlmax. These data suggest a twentieth century seasonal shift in the timing of fall upwelling and Guaymas Basin stratification breakdown that could be due to reduced upwelling efficiency and/or increased summer mixed layer thermal stratification.

The relationship between zinc, its isotopes, and the major nutrients in the North-East Pacific

Trace metal micronutrients play key roles in photosynthesis by oceanic phytoplankton. Though they are required in much smaller amounts than the major nutrients (P, N, Si), their bioavailable forms are also present in the seawater solution at much lower levels. Relationships between the dissolved chemistry of the nutrient-type trace metals, their stable isotope variations, as well those of the major nutrients, have highlighted the importance of biological and physical processes in the Southern Ocean in controlling their oceanic biogeochemistry. However, the first-order Southern Ocean processes are overprinted by vertical cycling in other parts of the ocean, particularly upwelling regions remote from the Southern Ocean, with the North Pacific standing out in particular. Here we present new zinc (Zn) concentration and isotope, as well as major nutrient data for the NE Pacific, and couple these new data with a compilation of published data from across the region, with the objective of better understanding the impact of this important region on oceanic biogeochemical cycles.The new Zn isotope data for two stations along Line P (P04 and P26) show a large range in δ66Zn in the upper ocean (−0.4‰ up to >1‰), associated with a very small isotope fractionation but extreme depletion of the dissolved pool during photic zone biological uptake, and the regeneration of this cellular Zn at very shallow depths (50 m). Beneath this, the two profiles approach the δ66Zn value of +0.5‰, seen throughout the deep ocean, by about 500 m. The minimum δ66Zn resulting from regeneration is associated with very high Zn concentrations, particularly at the marginal P04 station where diatoms dominate the phytoplankton ecology. Combining the new data with published Zn and major nutrient concentrations from across the North Pacific emphasises the role of vertical biological cycling in controlling regional biogeochemistry in the North Pacific, resulting in the partial overprinting of biogeochemical signatures transported out of the Southern Ocean by the ocean circulation. Zinc isotope data document the uptake of this metal into diatoms and the co-regeneration of Zn with phosphate in the upper water column. Silica in contrast is regenerated at greater depth, resulting in a decoupling of the Zn–Si correlation that is set in the Southern Ocean and that dominates the Atlantic. Previous work has suggested that the decoupling of Zn and Si in the subarctic North Pacific results from removal of Zn (and other metals) to water column particulate sulphide. In our dataset, and in the compilation of data documenting relationships between Zn and the major nutrients across the North Pacific, this decoupling is clearly due to the different lengthscales of regeneration for organic matter (Zn and P) and diatom opal (Si).

Zinc and nickel isotopes in seawater from the Indian Sector of the Southern Ocean: The impact of natural iron fertilization versus Southern Ocean hydrography and biogeochemistry

The Southern Ocean is the site of 20% of global ocean carbon export, and the origin of water masses that significantly impact the chemistry of the global ocean. Much of the Southern Ocean is a high nutrient low chlorophyll (HNLC) region. Iron is the primary limiting nutrient for phytoplankton growth, and may also be an important control on the biogeochemistry of the water masses emanating from the Southern Ocean. Here we investigate the response of Zn and Ni and their isotopes to natural Fe fertilization from Southern Ocean islands, using samples collected during the KEOPS-2 (Oct–Nov 2011) program in the early austral spring near the Kerguelen plateau, Indian Sector of the Southern Ocean.Zinc and Ni concentrations show depth distributions and relationships to major nutrients that are very similar to published data from the Atlantic sector of the Southern Ocean, away from island fertilization. As in the Atlantic sector, and other regions outside the Southern Ocean, there is a slight minimum in δ66Zn at about 100–150 m depth and a slightly heavier value at the surface. At the KEOPS-2 sites, the light sub-surface Zn isotopes occur at the depth level where measured chl-a has declined to near zero. The heaviest Zn isotope values in either the Atlantic or Indian sectors occur in bottom waters, are associated with samples that show high dissolved Si relative to Zn. These latter represent possible evidence for a heavy input directly to deep water or from sediment, and the Zn-Si-δ66Zn systematics are consistent with a source in diatom opal. Nickel concentrations show much less variation than those of Zn, with the most depleted surface value only 15–20% lower than those in upwelled deep water in the Southern Ocean. These small variations in Ni concentration are associated with no significant variability in Ni isotopes, as also previously observed in the Atlantic sector.Overall, the key feature of the dataset is the apparent absence of any significant impact of natural Fe fertilization on the distribution of Zn and Ni, with the dominant control being the station location relative to the Polar Front in both the Atlantic and Indian sectors. Zinc and Ni isotopes are invariant in surface waters, whether inside or outside the fertilized zone and whether south or north of the Polar Front. This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Modelling silicon supply during the Last Interglacial (MIS 5e) at Lake Baikal

Limnological reconstructions of primary productivity have demonstrated its response over Quaternary timescales to drivers such as climate change, landscape evolution and lake ontogeny. In particular, sediments from Lake Baikal, Siberia, provide a valuable uninterrupted and continuous sequence of biogenic silica (BSi) records, which document orbital and sub-orbital frequencies of regional climate change. We here extend these records via the application of stable isotope analysis of silica in diatom opal (δ30Sidiatom) from sediments covering the Last Interglacial cycle (Marine Isotope Stage [MIS] 5e; c. 130 to 115 ka BP) as a means to test the hypothesis that it was more productive than the Holocene. δ30Sidiatom data for the Last Interglacial range between +1.29 and +1.78‰, with highest values between c. 127 to 124 ka BP (+1.57 to +1.78‰). Results show that diatom dissolved silicon (DSi) utilisation, was significantly higher (p = 0.001) during MIS 5e than the current interglacial, which reflects increased diatom productivity over this time (concomitant with high diatom biovolume accumulation rates [BVAR] and warmer pollen-inferred vegetation reconstructions). Diatom BVAR are used, in tandem with δ30Sidiatom data, to model DSi supply to Lake Baikal surface waters, which shows that highest delivery was between c. 123 to 120 ka BP (reaching peak supply at c. 120 ka BP). When constrained by sedimentary mineralogical archives of catchment weathering indices (e.g. the Hydrolysis Index), data highlight the small degree of weathering intensity and therefore representation that catchment-weathering DSi sources had, over the duration of MIS 5e. Changes to DSi supply are therefore attributed to variations in within-lake conditions (e.g. turbulent mixing) over the period, where periods of both high productivity and modelled-DSi supply (e.g. strong convective mixing) account for the decreasing trend in δ30Sidiatom compositions (after c. 124 ka BP).

Iron Availability Influences Silicon Isotope Fractionation in Two Southern Ocean Diatoms (Proboscia inermis and Eucampia antarctica) and a Coastal Diatom (Thalassiosira pseudonana)

The fractionation of silicon (Si) isotopes was measured in two Southern Ocean diatoms (Proboscia inermis and Eucampia Antarctica) and a coastal diatom (Thalassiosira pseudonana) that were grown under varying iron (Fe) concentrations. Varying Fe concentrations had no effect on the Si isotope enrichment factor (e) in T. pseudonana, whilst E. antarctica and P. inermis exhibited significant variations in the value of e between Fe-replete and Fe-limited conditions. Mean e values in P. inermis and E. antarctica decreased from (± 1SD) -1.11 ± 0.15‰ and -1.42 ± 0.41‰ (respectively) under Fe-replete conditions, to -1.38 ± 0.27‰ and -1.57 ± 0.5‰ (respectively) under Fe-limiting conditions. These variations likely arise from adaptations in diatoms arising from the nutrient status of their environment. T. pseudonana is a coastal clone typically accustomed to low Si but high Fe conditions whereas E. antarctica and P. inermis are typically accustomed to High Si, High nitrate low Fe conditions. Growth induced variations in silicic acid (Si(OH)4) uptake arising from Fe-limitation is the likely mechanism leading to Si-isotope variability in E. antarctica and P. inermis. The multiplicative effects of species diversity and resource limitation (e.g. Fe) on Si-isotope fractionation in diatoms can potentially alter the Si-isotope composition of diatom opal in diatamaceous sediments and sea surface Si(OH)4. This work highlights the need for further in-vitro studies into intracellular mechanisms involved in Si(OH)4 uptake, and the associated pathways for Si-isotope fractionation in diatoms.

Silicon and zinc biogeochemical cycles coupled through the Southern Ocean

Zinc is vital for the physiology of oceanic phytoplankton. The striking similarity of the depth profiles of zinc to those of silicate suggests that the uptake of both elements into the opaline frustules of diatoms, and their regeneration from these frustules, should be coupled. However, the zinc content of diatom opal is negligible, and zinc is taken up into and regenerated from the organic parts of diatom cells. Thus, since opaline frustules dissolve deep in the water column while organic material is regenerated in the shallow subsurface ocean, there is little reason to expect the observed close similarity between zinc and silicate, and the dissimilarity between zinc and phosphate. Here we combine observations with simulations using a three-dimensional model of ocean circulation and biogeochemistry to show that the coupled distribution of zinc and silicate, as well as the decoupling of zinc and phosphate, can arise in the absence of mechanistic links between the uptake of zinc and silicate, and despite contrasting regeneration length scales. Our simulations indicate that the oceanic zinc distribution is, in fact, a natural result of the interaction between ocean biogeochemistry and the physical circulation through the Southern Ocean hub. Our analysis demonstrates the importance of uptake stoichiometry in controlling ocean biogeochemistry, and the utility of global-scale elemental covariation in the ocean in understanding these controls. Zinc and silicon distributions co-vary in much of the global oceans. Observations and numerical modelling suggest that this co-variation can arise in the absence of mechanistic links between the uptake of zinc and silicate.

Zn/Si records in diatom opal from Prydz Bay, East Antarctica

The Zn/Si records of diatom opal from Prydz Bay were investigated. The results show that the Zn/Si ratios range from 8 to 53μmolmol−1. The ratio from the continental slope is higher than those from the shelf. The difference in Zn/Si records could be attributed to the change of bioavailable Zn via the upwelling of Zn2+-rich deep water or the influence of oceanic water on the slope. However, phytoplankton productivity influences Zn/Si records on the continental shelf. Increased productivity leads to elevated Zn-pumping, and eventually to higher Zn/Si ratio in diatom opal. In this case, it is suggested that Zn/Si proxy could be a signal of marine productivity on the continental shelf of Prydz Bay. The down core profile of Zn/Si records shows an increase in marine productivity over the last century.

What controls silicon isotope fractionation during dissolution of diatom opal?

The silicon isotope composition of opal frustules from photosynthesising diatoms is a promising tool for studying past changes in the marine silicon cycle, and indirectly that of carbon. Dissolution of this opal may be accompanied by silicon isotope fractionation that could disturb the pristine silicon isotope composition of diatom opal acquired in the surface ocean. It has previously been shown that dissolution of fresh and sediment trap diatom opal in seawater does fractionate silicon isotopes. However, as the mechanism of silicon isotope fractionation remained elusive, it is uncertain whether opal dissolution in general is associated with silicon isotope fractionation considering that opal chemistry and surface properties are spatially and temporally (i.e. opal of different age) diverse.In this study we dissolved sediment core diatom opal in 5mM NaOH and found that this process is not associated with significant silicon isotope fractionation. Since no variability of the isotope effect was observed over a wide range of dissolution rates, we can rule out the suggestion that back-reactions had a significant influence on the net isotope effect. Similarly, we did not observe an impact of temperature, specific surface area, or degree of undersaturation on silicon isotope partitioning during dissolution, such that these can most likely also be ruled out as controlling factors. We discuss the potential impacts of the chemical composition of the dissolution medium and age of diatom opal on silicon isotope fractionation during dissolution. It appears most likely that the controlling mechanism of silicon isotope fractionation during dissolution is related to the reactivity, or potentially, aluminium content of the opal. Such a dependency would imply that silicon isotope fractionation during dissolution of diatom opal is spatially and temporally variable. However, since the isotope effects during dissolution are small, the silicon isotope composition of diatom opal appears to be robust against dissolution in the deep sea sedimentary environment.

Diatoms spread a high ^|^epsilon;Nd-signature in the North Pacific Ocean

Copyright © 2014 by The Geochemical Society of Japan. columns in the North Pacific Ocean (Akagi et al., 2011). The surface concentrations of dissolved REEs, expressed as “leftover”, in any oceans have been well explained by the diatom-incorporation theory (Akagi, 2013). Briefly, in this theory, a surface-to-deep concentration ratio is smaller in the Pacific Ocean than in the Atlantic Ocean, because the portion of REEs complexed with silicic acids is greater in the silica-rich Pacific Ocean, and thus more effectively incorporated in diatom frustules together with dissolved silica. This theory also derives the equation of distribution coefficients of REEs in diatom opal to predict the REEs concentration in diatom frustules in any ocean. The surface-to-deep concentration ratio explained by the theory implies that diatom frustules should be the dominant vertical carrier of REEs in all oceans except the Arctic Ocean, where no/small vertical gradient in the REE profiles is seen, resulting from the absence of dissolved silica throughout the water columns. The study also implied the presence of unknown input of REEs to diatom frustules to satisfy the steady-state condition, indicating the presence of unknown sources of REEs in surface water (Akagi, 2013). The conclusion of model studies (Arsouze et al., 2009; Rempfer et al., 2011) is challenged by the idea of input in surface water by Akagi (2013) and Nd isotope dynamics of diatom silica frustules needs to be assessed. Diatoms spread a high eNd-signature in the North Pacific Ocean

Biogeochemical cycling of zinc and its isotopes in the Southern Ocean

We report Zn concentration and isotope data for seawater samples from the Atlantic sector of the Southern Ocean, collected during the IPY/GEOTRACES ANT-XXIV/III cruise along the Greenwich Zero Meridian. Data are reported for the full depth range of the water column at three stations, as well as a transect of surface samples, using a new analytical approach that is presented in detail here.Zn concentrations increase with depth, though due to proximity to upwelling sites, surface concentrations are not as low as in some parts of the ocean such as further northward into the Sub-Antarctic Zone. For two depth profiles south of the Polar Front Zone, the physical stratification of the upper water column is reflected in sudden near-surface changes in Zn concentration with depth. In contrast, beneath 100–300m Zn concentrations barely change with depth. Zn isotopic data beneath 1000m, for the Southern Ocean data presented here as well as published data from the North Atlantic and North Pacific, are strikingly homogeneous, with an average δ66Zn=+0.53±0.14‰ (2SD, 2SE=0.03, n=21). The surface Southern Ocean is more variable, with δ66Zn ranging from 0.07‰ to 0.80‰. Between the two is a thin horizon at 40–80m which, in the Southern Ocean as well as the North Atlantic and North Pacific, is characterised by distinctly light isotopic signatures, with δ66Zn about 0.3‰ lower than surface waters.Strong correlations between Si and Zn concentrations seen here and elsewhere, coupled to the lack of any systematic relationship between Si and Zn isotopes in the Southern Ocean, suggest that the removal of Zn associated with diatom opal involves little or no isotopic fractionation. Regeneration of this Zn also explains the homogeneous Zn isotopic composition of the global deep ocean so far sampled. However, the low Zn content of opal requires that deep ocean Zn does not directly come from the opal phase itself, but rather from associated organic material external to the diatom frustule during growth. Experimental data are consistent with little or no fractionation during incorporation of Zn into this material. On the other hand, the light zinc at 40–80m is most consistent with the regeneration of an intra-cellular pool that both culturing experiments and field data suggest will be isotopically light. The data thus imply two processes by which Zn is taken up in the surface ocean, that these pools have very different regeneration lengthscales, and that physical mixing of the oceans cannot eradicate their isotopic signatures. Finally, the deep δ66Zn ocean value is significantly higher than the current best estimate of the input to the oceans. The most obvious candidate for the required light sink is the survival of some of the cellular Zn to be buried in sediment.

Size-specific opal-bound nitrogen isotope measurements in North Pacific sediments

The nitrogen isotope composition of diatom opal (δ15Ndb) is a valuable recorder of nitrate utilization in the polar surface oceans and a measure of the efficiency of the biological pump. Past down-core records of δ15Ndb involved the measurement of the biogenic opal fraction up to 150μm in size, which should represent the bulk of the preserved diatoms but may also include non-diatom opal such as radiolaria and sponge spicules. In this study, the opal from subarctic Pacific and Bering Sea sediments from the Holocene back to the last glacial was separated into different size fractions to measure their individual δ15N. We found a general trend of decreasing δ15N with increasing size at all sites and through time. Microscopic investigation of smear slides and image area analysis of microphotographs of the analyzed opal revealed that the larger size fractions contained greater proportions of sponge spicules and radiolaria. Manual isolation and measurement of the sponge spicules showed that they have a very low δ15N (∼ −11‰). Ultrasonication during sample preparation caused greater spicule and radiolaria contamination due to fragmentation of these relatively large fossils, leading to a dramatic δ15Ndb decrease with increasing size and lower δ15Ndb across all size fractions in sonicated versus non-sonicated samples. Nevertheless, these contaminants were also present albeit less abundant in the various size fractions of samples separated without sonication, and these samples also showed a δ15Ndb decrease with increasing size. Simple isotope mass-balance calculations of Holocene Bering Sea sediments indicate that most of the δ15Ndb variations among the larger size fractions can be explained by the relative abundance of low-δ15N sponge spicules in each fraction. However, some of the size fraction δ15N differences in the downcore records require a different explanation. Both diatom inter- or intra-species effects are evident and indicate lower δ15Ndb among the larger (centric) versus smaller (pennate) diatom species and a δ15Ndb decrease with increasing size of centric diatom frustules.Contamination of N by non-diatomaceous opal should not normally compromise total-diatom-bound δ15N (0–150μm) because the non-diatom opal typically contributes less than ∼5% to the total opal. However, the early deglacial (Heinrich Stadial 1-correlative) period in the subarctic North Pacific is an important possible exception: a substantial fraction of its low concentration of opal appears to be sponge spicules and radiolaria, such that the reconstructed total-diatom δ15Ndb decrease at this time may be an artifact. While the glacial-age sediments are also vulnerable to non-diatom contamination, this should work to lower δ15Ndb, such that the observed high glacial δ15Ndb in North Pacific sediments cannot be explained by contamination. Thus, the previous interpretation of enhanced nutrient consumption in the North Pacific regions during the last ice age remains valid.

Diagenetic aluminum uptake into diatom frustules and the preservation of diatom-bound organic nitrogen

The chemistry and isotopic composition of siliceous diatom frustules and the organic matter bound within them have been used to reconstruct past changes in the environment. However, the alteration of biogenic silica in the sediments and its influence on the chemical properties of the diatom opal and diatom-bound organic matter has not been broadly studied. We use inductively coupled plasma atomic emission spectrometry and energy dispersive X-ray analysis to investigate the concentrations of various cations, especially aluminum, in cleaned diatom frustules extracted from sediments at three sites from the Bering Sea, where the opal content in the sediments and preservation conditions vary both spatially and over the last glacial cycle. Relative to cultured diatoms, cleaned diatom frustules from the sediments are found to have a high Al/Si ratio, most likely by homogenous Al incorporation but possibly also by formation of an aluminosilicate coating of significant thickness relative to the diatom walls. Variations in the Al/Si ratio of the cleaned diatoms among samples are strongly correlated with the aluminosilicate to opal ratio of the bulk sediment, suggesting that sedimentary clays are the primary source of the aluminum taken up by the diatoms. The aluminum and nitrogen contents of the diatom frustules are negatively correlated, which is best explained by the loss of intrinsic N during the chemical changes associated with Al incorporation. The lack of a consistent relationship between nitrogen isotopic composition and the aluminum content of the cleaned opal suggests that the diagenetic organic N loss does not fractionate N isotopes, which is as expected given that this loss process relates to the geochemistry of the opal, not the organic matter. Our findings help to explain the previous observations that opal from clay-rich sediments (1) requires harsher chemical cleaning to remove diagenetic N and (2) has diatom-native N that is better protected from alteration by excessively harsh chemical cleaning. The diagenetic alteration of the opal may have led to partial protection of the external N from the surrounding sediments. At the same time, the diatom frustules have been chemically ‘hardened’ by the diagenesis associated with Al uptake, reducing the chemical accessibility of the diatom-native N that remains.

B content and Si/C ratios from cultured diatoms (Thalassiosira pseudonana and Thalassiosira weissflogii): Relationship to seawater pH and diatom carbon acquisition

Despite the importance of diatoms in regulating climate and the existence of large opal-containing sediments in key air-ocean exchange areas, most geochemical proxy records are based on carbonates. Among them, Boron (B) content and isotopic composition have been widely used to reconstruct pH from foraminifera and coral fossils. We assessed the possibility of a pH/CO2 seawater concentration control on B content in diatom opal to determine whether or not frustule B concentrations could be used as a pH proxy or to clarify algae physiological responses to acidifying pH. We cultured two well-studied diatom species, Thalassiosira pseudonana and Thalassiosira weissflogii at varying pH conditions and determined Si and C quotas. Frustule B content was measured by both laser-ablation inductively coupled mass spectrometry (LA-ICPMS) and secondary ion mass spectrometry (SIMS/ion probe). For both species, frustules grown at higher pH have higher B contents and higher Si requirements per fixed C. If this trend is representative of diatom silicification in a future more acidic ocean, it could contribute to changes in the efficiency of diatom ballasting and C export, as well as changes in the contribution of diatoms relative to other phytoplankton groups in Si-limited regions. If B enters the cell through the same transporter employed for HCO3− uptake, an increased HCO3− requirement with decreasing CO2 concentrations (higher pH), and higher B(OH)4/HCO3− ratios would explain the observed increase in frustule B content with increasing pH. The mechanism of B transport from the site of uptake to the site of silica deposition is unknown, but may occur via silicon transport vesicles, in which B(OH)4− may be imported for B detoxification and/or as part of a pH regulation strategy either though Na-dependent B(OH)4−/Cl− antiport or B(OH)4−/H+ antiport. B deposition in the silica matrix may occur via substitution of a B(OH)4− for a negatively charged SiO− formed during silicification. With the current analytical precision, B content of frustules is unlikely to resolve ocean pH with a precision of paleoceanographic interest. However, if frustule B content was controlled mainly by HCO3− uptake for photosynthesis, which appears to show a threshold behavior, then measurements of B content might reveal the varying importance of active HCO3− acquisition mechanisms of diatoms in the past.

Rare earth element (REE)–silicic acid complexes in seawater to explain the incorporation of REEs in opal and the “leftover” REEs in surface water: New interpretation of dissolved REE distribution profiles

In light of a new finding that diatom opal carries rare earth elements (REEs) at an amount exceeding that which would explain the REEs/Si ratio in open ocean columns, the complex formation of REEs with silicic acid and subsequent incorporation of the complex into diatom opal has been explored. Reported complex formation constants of EuH3SiO42+ and an empirical linear relationship of the constants with hydrolysis constants were employed in the species calculation. The calculation reveals that REE–silicic acid complex is the dominant species of dissolved REEs especially in deep layers of the Pacific and the Atlantic Oceans. The degree of complex formation varies depending on dissolved silica concentration, pH, and pCO2. Assuming that only REEs in the form of REEH3SiO42+, which is diffused/advected from deep water to surface water together with dissolved silica, are incorporated into diatom opal, a REE incorporation theory has been developed. The theory links the intake of REEs and silica by diatoms in surface water and their concentrations in deep water and presents two distribution coefficients (Ds) of REEs in diatom opal, conventional D against surface water and columnar D against deep water as functions of pH and dissolved silica concentration. The theory successfully explains the significant REE concentration level (“leftover REEs”) in the surface water of the oceans and reproduces a concentration of REEs in diatom opal that is consistent with that observed in diatom opal in the North Pacific Ocean. The REE composition of oceanic deep water is largely explained by two fractionation processes: REE incorporation into opal and carbonate/oxide scavenging of REEs.

Cleaning methods for the isotopic determination of diatombound nitrogen in non‐fossil diatom frustules

Ground‐truthing the stable isotope ratio of diatom frustule‐bound organic nitrogen (N) as a paleoceanographic proxy of phytoplankton nutrient consumption calls for studies of modern diatoms from cultures and the field. This work has been hindered by the lack of a method to prepare fresh diatom material, which has significant geochemical differences from sedimentary fossil diatom, for analysis. We tested and compared the performance of five oxidative treatment protocols in cleaning fresh diatom frustules of external organic material for isotopic analysis of diatom frustule‐bound N. We applied each treatment to diatom material from a T. weiss‐flogii culture and measured the diatom‐bound N content and isotopic composition (δ15N) of samples. A favored treatment is described that includes a multi‐step, progressive, oxidative cleaning. The results suggest that the application of H2O2 to fresh biogenic opal for more than 20 min increased the accessibility of the N within the opal matrix to subsequent chemical treatment, an effect not observed for diatom frustules from deep ocean sediments. We attribute this effect to structural alteration and dissolution of the diatom opal by the H2O2, consistent with previous observations regarding the high reactivity of fresh diatom opal and its subsequent chemical hardening in the sedimentary environment. In addition, the presence of high levels of organic matter upon treatment with H2O2 may exacerbate the chemical alteration of frustules. We find that the δ15N of diatom‐bound N is ~3 ‰ higher than that of the bulk culture biomass, consistent with expectations from surface sediment and down‐core measurements.

Species-dependent silicon isotope fractionation by marine diatoms

Fractionation of silicon (Si) isotopes was measured in seven species (nine strains) of polar and sub-polar marine diatoms grown in semi-continuous unialgal cultures under optimal irradiance and temperature for each diatom strain. Results from this work provide the first evidence that Si isotope fractionation by diatoms is species-dependent. The greatest difference in the Si isotope fractionation factor (ε) was observed between two Southern Ocean diatoms, Fragilariopsis kerguelensis (−0.54‰, average for two strains) and Chaetoceros brevis (−2.09‰). The ε for the other species, both polar and sub-polar, ranged from −0.72‰ to −1.21‰. The two remaining polar diatoms had ε values of −0.74±0.05‰ for Thalassiosira antarctica, and −1.21±0.04‰ for Thalassiosira nordenskioeldii, while the sub-polar species had ε values of −0.72±0.04‰ for Thalassiosira weissflogii, −0.88±0.06‰ for Thalassiosira pseudonana (CCCM58), −0.97±0.14‰ for Thalassiosira pseudonana (CCMP1014), and −1.15±0.03‰ for Porosira glacialis. The range in ε for the diatoms evaluated in this study may be large enough to significantly impact the Si isotope composition measured in diatom opal (δ30Si-bSiO2) from marine sediments and its subsequent interpretation. To test the influence of diatom taxonomic composition on δ30Si-bSiO2, we developed a model that considered the relative abundance of diatom species and the ε values (from this study) for each species present within the sediment core (i.e. weighted-average ε). The model was applied to records from a Southern Ocean sediment core (TN057-13) where both diatom abundance and δ30Si-bSiO2 data were available. The analysis indicated that 67% of the variation in δ30Si-bSiO2 could be explained by species-dependent Si isotope fractionation. We suggest that future work should assess phytoplankton taxonomic composition when using δ30Si-bSiO2 as a proxy for Si utilization.

The Zn abundance and isotopic composition of diatom frustules, a proxy for Zn availability in ocean surface seawater

We have developed cleaning methods for extracting diatomopal from bulk marine sediment samples, for measurement of both zinc (Zn) abundance and isotope composition. This cleaning technique was then applied to a set of Holocene core-top samples from the Southern Ocean. The measured δ66Zn (reported relative to the JMCLyon standard) and Zn/Si ratios from the Southern Ocean diatomopal samples range from 0.7 to 1.5‰, and from 14 to 0.9μmol/mol, respectively. The Zn abundance and isotope composition data show a clear correlation with opal burial rates and other oceanographic parameters. In common with previous work, we interpret the systematic changes in the Zn/Si ratio to be linked to the variability in the concentrations of bioavailable Zn in the ambient surface seawater where the diatom opal is formed. This variability is likely to be primarily controlled by the degree to which Zn is taken up into phytoplankton biomass. The observed systematic pattern in the δ66Zn compositions of the diatomopal core-top samples is, similarly, likely to reflect changes in the δ66Zn composition of the ambient Zn in the surface waters above the core-top sites, which is progressively driven towards isotopically heavier values by preferential incorporation of the lighter isotopes into phytoplankton organic material. Thus, the systematic relationship between Zn isotopes and abundance observed in the core-top diatomopal samples suggests a potential tool for investigating the biogeochemical cycling of Zn in the past surface ocean for down-core diatomopal material. In this respect, it may be possible to test hypotheses that attribute variations in atmospheric CO2 on glacial–interglacial timescales to the degree to which trace metals limited primary productivity in HNLC zones.

Fractionation of silicon isotopes during biogenic silica dissolution

Silicon isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization, primarily by diatoms, in surface waters of the ocean. Interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate silicon pools in the upper ocean, as driven by biological fractionation during the production of biogenic silica (bSiO 2, or opal) by diatoms. However, other factors which may influence the silicon isotope composition of diatom opal, particularly post-formational aging and maturation processes, remain largely uninvestigated. Here, we report a consistent fractionation of silicon isotopes during the physicochemical dissolution of diatom bSiO 2 suspended in seawater under closed conditions. This fractionation acts counter to that occurring during bSiO 2 production and at about half its absolute magnitude, with dissolution discriminating against the release of the heavier isotopes of silicon at an enrichment factor ε DSi–BSi of −0.55‰, corresponding to a fractionation factor α 30/28 of 0.99945. The enrichment factor did not vary with source material, indicating the lack of a significant species effect, or with temperature from 3 to 20 °C. Thus, the dissolution of bSiO 2 produces dissolved silicon with a δ 30Si value that is 0.55‰ more negative than its parent bSiO 2, an effect that must be accounted for when interpreting oceanic δ 30Si distributions. The δ 30Si values of both the dissolved and particulate silicon pools increased linearly as dissolution progressed, implying a measurable (±0.1‰) change in the relative δ 30Si of opal samples whenever the difference in preservation efficiency between them is >20%. This effect could account for ∼10–30% of the difference in diatom δ 30Si values observed between glacial and interglacial conditions. It is unlikely, however, that the inferred maximum possible change in δb 30SiO 2 of +0.55‰ would be manifested in situ, as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage.