164 research outputs found
Skeletal P/Ca tracks upwelling in Gulf of Panama coral: Evidence for a new seawater phosphate proxy
No full textThe supply of limiting nutrients to the low latitude ocean is controlled by physical processes linked to climate variations, but methods for reconstructing past nutrient concentrations in the surface ocean are few and indirect. Here, we present laser ablation mass spectrometry results that reveal annual cycles of P/Ca in a 4-year record from the scleractinian coral Pavona gigantea (mean P/Ca = 118 mmol mol?1). The P/Ca cycles track variations in past seawater phosphate concentration synchronously with skeletal Sr/Ca-derived temperature variations associated with seasonal upwelling in the Gulf of Panama´. Skeletal P/Ca varies seasonally by 2–3 fold, reflecting the timing and magnitude of dissolved phosphate variations. Solution cleaning experiments on drilled coral powders show that over 60% of skeletal P occurs in intracrystalline organic phases. Coral skeleton P/Ca holds promise as a proxy record of nutrient availability on time scales of decades to millennia
Seawater nutrient and carbonate ion concentrations recorded as P/Ca, Ba/Ca, and U/Ca in the deep-sea coral D. dianthus
No full textAs paleoceanographic archives, deep sea coral skeletons offer the potential for high temporal resolution and precise absolute dating, but have not been fully investigated for geochemical reconstructions of past ocean conditions. Here we assess the utility of skeletal P/Ca, Ba/Ca and U/Ca in the deep sea coral D. dianthus as proxies of dissolved phosphate (remineralized at shallow depths), dissolved barium (trace element with silicate-type distribution) and carbonate ion concentrations, respectively. Measurements of these proxies in globally distributed D. dianthus specimens show clear dependence on corresponding seawater properties. Linear regression fits of mean coral Element/Ca ratios against seawater properties yield the equations: P/Cacoral (lmol/mol) = (0.6 ± 0.1) P/Casw(lmol/mol) – (23 ± 18), R2 = 0.6, n = 16 and Ba/Cacoral(lmol/mol) = (1.4 ± 0.3) Ba/Casw(lmol/mol) + (0 ± 2), R2 = 0.6, n = 17; no significant relationship is observed between the residuals of each regression and seawater temperature, salinity, pressure, pH or carbonate ion concentrations, suggesting that these variables were not significant secondary dependencies of these proxies. Four D. dianthus specimens growing at locations withOarag 6 0.6 displayed markedly depleted P/Ca compared to the regression based on the remaining samples, a behavior attributed to an undersaturation effect. These corals were excluded from the calibration. Coral U/Ca correlates with seawater carbonate ion: U/Cacoral(lmol/mol) = (?0.016 ± 0.003) ½CO2? 3 ? (lmol/kg) + (3.2 ± 0.3), R2 = 0.6, n = 17. The residuals of the U/Ca calibration are not significantly related to temperature, salinity, or pressure. Scatter about the linear calibration lines is attributed to imperfect spatialtemporal matches between the selected globally distributed specimens and available water column chemical data, and potentially to unresolved additional effects. The uncertainties of these initial proxy calibration regressions predict that dissolved phosphate could be reconstructed to ±0.4 lmol/kg (for 1.3–1.9 lmol/kg phosphate), and dissolved Ba to ±19 nmol/kg (for 41–82 nmol/kg Basw). Carbonate ion concentration derived from U/Ca has an uncertainty of ±31lmol/kg (for 60–120 lmol=kg CO2? 3 ). The effect of microskeletal variability on P/Ca, Ba/Ca, and U/Ca was also assessed, with emphasis on centers of calcification, Fe–Mn phases, and external contaminants. Overall, the results show strong potential for reconstructing aspects of water mass mixing and biogeochemical processes in intermediate and deep waters using fossil deep-sea corals
Biogeochemical cycling of dissolved zinc in the Western Arctic (Arctic GEOTRACES GN01)
No full textThe biogeochemical cycling of dissolved zinc (dZn) was investigated in the Western Arctic along the U.S. GEOTRACES GN01 section. Vertical profiles of dZn in the Arctic are strikingly different than the classic “nutrient‐type” profile commonly seen in the Atlantic and Pacific Oceans, instead exhibiting higher surface concentrations (~1.1 nmol/kg), a shallow subsurface absolute maximum (~4–6 nmol/kg) at 200 m coincident with a macronutrient maximum, and low deep water concentrations (~1.3 nmol/kg) that are homogeneous (sp.) with depth. In contrast to other ocean basins, typical inputs such as rivers, atmospheric inputs, and especially deep remineralization are insignificant in the Arctic. Instead, we demonstrate that dZn distributions in the Arctic are controlled primarily by (1) shelf fluxes following the sediment remineralization of high Zn:C and Zn:Si cells and the seaward advection of those fluxes and (2) mixing of dZn from source waters such as the Atlantic and Pacific Oceans rather than vertical biological regeneration of dZn. This results in both the unique profile shapes and the largely decoupled relationship between dZn and Si found in the Arctic. We found a weak dZn:Si regression in the full water column (0.077 nmol/μmol, r2 = 0.58) that is higher than the global slope (0.059 nmol/μmol, r2 = 0.94) because of the shelf‐derived halocline dZn enrichments. We hypothesize that the decoupling of Zn:Si in Western Arctic deep waters results primarily from a past ventilation event with unique preformed Zn:Si stoichiometries
Coral skeleton P/Ca proxy for seawater phosphate: Multi-colony calibration with a contemporaneous seawater phosphate record
No full textA geochemical proxy for surface ocean nutrient concentrations recorded in coral skeleton could provide new insight into the connections between sub-seasonal to centennial scale nutrient dynamics, ocean physics, and primary production in the past. Previous work showed that coralline P/Ca, a novel seawater phosphate proxy, varies synchronously with annual upwelling-driven cycles in surface water phosphate concentration. However, paired contemporaneous seawater phosphate time-series data, needed for rigorous calibration of the new proxy, were lacking. Here we present further development of the P/Ca proxy in Porites lutea and Montastrea sp. corals, showing that skeletal P/Ca in colonies from geographically distinct oceanic nutrient regimes is a linear function of seawater phosphate (PO4 SW) concentration. Further, high-resolution P/Ca records in multiple colonies of Pavona gigantea and Porites lobata corals grown at the same upwelling location in the Gulf of Panama were strongly correlated to a contemporaneous time-series record of surface water PO4 SW at this site (r2 = 0.7–0.9). This study supports application of the following multi-colony calibration equations to down-core records from comparable upwelling sites, resulting in ±0.2 and ±0.1 lmol/kg uncertainties in PO4 SW reconstructions from P. lobata and P. gigantea, respectively.P/Ca Porites lobata (lmol/mol) = (21.1 ? 2.4)PO4 SW (lmol/kg) + (14.3 ? 3.8)P/Ca Pavona gigantea (lmol/mol) = (29.2 ? 1.4)PO4 SW (lmol/kg) + (33.4 ? 2.7)Inter-colony agreement in P/Ca response to PO4 SW was good (±5–12% about mean calibration slope), suggesting that species-specific calibration slopes can be applied to new coral P/Ca records to reconstruct past changes in surface ocean phosphate. However, offsets in the y-intercepts of calibration regressions among co-located individuals and taxa suggest that biologically-regulated “vital effects” and/or skeletal extension rate may also affect skeletal P incorporation. Quantification of the effect of skeletal extension rate on P/Ca could lead to corrected calibration equations and improved inter-colony P/Ca agreement. Nevertheless, the efficacy of the P/Ca proxy is thus supported by both broad scale correlation to mean surface water phosphate and regional calibration against documented local seawater phosphate variations
Increasing stoichiometric imbalance in North America’s largest lake: Nitrification in Lake Superior
No full textLake Superior has exhibited a continuous, century-long increase in nitrate whereas phosphate remains at very low levels. Increasing nitrate and low phosphate has led to a present-day severe stoichiometric imbalance; Lake Superior’s deepwater NO3 :PO43 molar ratio is 10,000, more than 600 times the mean requirement ratio for primary producers. We examine the rate of [NO3?] increase relative to budgets for NO3 and fixed N. Nitrate in Lake Superior has continued to rise since 1980, though possibly at a reduced rate. We constructed whole-lake NO3 and N budgets and found that NO3 must be generated in the lake at significant rates. Stable O isotope results indicate that most NO3 in the lake originated by in-lake oxidation. Nitrate in the lake is responding not just to NO3 loading but also to oxidation of reduced forms of nitrogen delivered to the lake. The increasing [NO3]:[PO43] stoichiometric imbalance in this large lake is largely determined by these in-situ processes
Carbon isotope ratios of organic compound fractions in oceanic suspended particles
No full textAuthor Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 33 (2006): L23610, doi:10.1029/2006GL027928.To study cycling of organic fractions in the ocean, relative abundances and radio- and stable-carbon isotope measurements of total lipid extract, acid-soluble, and acid-insoluble fractions of suspended particulate organic carbon (POC) were made. Changes in relative abundances occurred mostly in the upper 1000 m of the water column, with a decrease in total lipid extract and the acid-soluble fraction and an increase in the acid-insoluble fraction with increasing depth. We found lower Δ14C values for total lipid extract and the acid-insoluble fraction than for the acid-soluble fraction, which is consistent with the previous suggestion of incorporation of dissolved organic carbon and/or resuspended sediment to POC (Druffel and Williams, 1990; Sherrell et al., 1998). The Δ14C values of these fractions in a given organic carbon pool must be understood in terms of acquisition of 14C-depleted carbon from other carbon pools in addition to aging within the reservoir.This research was supported by NSF
chemical oceanography program and Petroleum Research Fund of ACS
Antarctic glaciers export carbon-stabilised iron(II)-rich particles to the surface Southern Ocean
No full textIron is an essential micronutrient for phytoplankton and plays an integral role in the marine carbon cycle. The supply and bioavailability of iron are therefore important modulators of climate over glacial-interglacial cycles. Inputs of iron from the Antarctic continental shelf alleviate iron limitation in the Southern Ocean, driving hotspots of productivity. Glacial meltwater fluxes can deliver high volumes of particulate iron. Here, we show that glacier meltwater provides particles rich in iron(II) to the Antarctic shelf surface ocean. Particulate iron(II) is understood to be more bioavailable to phytoplankton, but less stable in oxic seawater, than iron(III). Using x-ray microscopy, we demonstrate co-occurrence of iron and organic carbon-rich phases, suggesting that organic carbon retards the oxidation of potentially-bioavailable iron(II) in oxic seawater. Accelerating meltwater fluxes may provide an increasingly important source of bioavailable iron(II)-rich particles to the Antarctic surface ocean, with implications for the Southern Ocean carbon pump and ecosystem productivity
Efficient Decoupled Electrolytic Water Splitting in Acid through Pseudocapacitive TiO2
No full textThe authors acknowledge support from the Latvian national research program Project No. VPP-EM-FOTONIKA-2022/1-0001 Smart Materials, Photonics, Technologies and Engineering Ecosystems. P.C.S. acknowledges support from RMIT University through the RMIT Vice\u2010Chancellor's Research Fellowship scheme (2023). The European Union’s Horizon 2020 Framework Program H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2.Water electrolysis remains a key component in the societal transition to green energy. Membrane electrolyzers are the state-of-the-art technology for water electrolysis, relying on 80 °C operation in highly alkaline electrolytes, which is undesirable for many of the myriad end-use cases for electrolytic water splitting. Herein, an alternative water electrolysis process, decoupled electrolysis, is described which performed in mild acidic conditions with excellent efficiencies. Decoupled electrolysis sequentially performs the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), at the same catalyst. Here, H+ ions generated from the OER are stored through pseudocapacitive (redox) charge storage, and released to drive the HER. Here, decoupled electrolysis is demonstrated using cheap, abundant, TiO2 for the first time. To achieve decoupled acid electrolysis, ultra-small anatase TiO2 particles (4.5 nm diameter) are prepared. These ultra-small TiO2 particles supported on a carbon felt electrode show a highly electrochemical surface area with a capacitance of 375 F g−1. When these electrodes are tested for decoupled water splitting an overall energy efficiency of 52.4% is observed, with excellent stability over 3000 cycles of testing. This technology can provide a viable alternative to membrane electrolyzers—eliminating the need for highly alkaline electrolytes and elevated temperatures. © 2024 The Authors. Advanced Science published by Wiley-VCH GmbH. --//-- This is an open-access article M. Iesalnieks, M. Vanags, L. L. Alsiņa, R. Eglītis, L. Grīnberga, P. C. Sherrell, A. Šutka, Efficient Decoupled Electrolytic Water Splitting in Acid through Pseudocapacitive TiO2. Adv. Sci. 2024, 11, 2401261. https://doi.org/10.1002/advs.202401261 published under the CC BY licence.Latvian National Research Program Project No. VPP-EM-FOTONIKA-2022/1-0001; European Union’s Horizon 2020 Framework Program H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2
Characterization of aerosol trace elements over the polar regions
No full textAtmospheric deposition of dust is an important pathway supplying nutrient trace elements to the surface water of remote oceans. Recent warming in the sensitive polar regions might potentially enhance the regional dust emission. Bulk and size-segregated aerosol samples were collected in the western Antarctic Peninsula and the Arctic Ocean to characterize atmospheric trace elements and to evaluate the importance of aeolian dust input to the high latitude marine ecosystems. The objectives of this study are to (1) characterize atmospheric trace elements in aerosols and identify their major sources, (2) quantify the atmospheric dust deposition, and (3) assess the aerosol iron bioavailability through characterizing the aerosol Fe mineralogy and oxidation states. Sampling of both size-segregated and bulk aerosol particles was carried out at Palmer Station in the western Antarctic Peninsula and during a cruise in the Arctic Ocean. Results from the western Antarctic Peninsula showed that trace elements in aerosols over this region are primarily derived from (1) regional crustal emissions, (2) long-range transport, and (3) sea salt aerosols. Elements derived from crustal sources (Al, P, Ti, V, Mn, Ce) with crustal enrichment factors (EFcrust) 1.8 µm) and peaked around 4.4 µm in diameter. Other elements including Ca, Ni, Cu, Zn, and Pb showed EFcrust > 10. The particle size distribution of aerosol Pb was dominated by fine particles and peaked at 0.14–0.25 µm, suggesting an anthropogenic contribution through long-range transport. The estimated dry deposition fluxes of mineral dust during the 2016-2017 austral summer in the Antarctic Peninsula ranged from 0.65 to 28 mg m−2 yr−1 with a mean of 5.5±5.0 mg m−2 yr−1, which were lower than most fluxes reported previously in coastal Antarctica. The Fe minerals in the dust particles over the Antarctic Peninsula were predominantly hematite and biotite with a minor fraction of pyrite and ilmenite. The aerosol Fe oxidation state was higher during the austral summer than the winter due to a higher fraction of biotite. Multivariate linear models involving meteorological data indicated that the wind speed, relative humidity, and solar irradiance were the factors that significantly controlled the percentage of Fe(II) in the austral summer. In addition, the snow depth was significantly (p < 0.05) correlated with the aerosol Fe concentrations, suggesting the effects of snow/ice cover on the regional dust emissions. Samples from the Arctic cruise showed aerosol Fe mineralogy could be dominated by biotite, ferrihydrite, and hematite, varying spatially. Results from the Arctic cruise showed that aerosol particles collected near coastal Alaska were heavily affected by the biotite-enriched glacial flour, whereas ferrihydrite was the most enriched Fe-containing mineral in the remote Arctic Ocean. A few samples impacted by air masses from the inland North American and Eurasian contained a high fraction of hematite with minor Fe(III) sulfate, biotite, and ferrihydrite. The aerosol Fe fractional solubility demonstrated a positive linear relationship with the Fe oxidation state, suggesting that the atmospheric aging processes (e.g., acidic reactions) could modify the Fe bioavailability during long-rang transport.
Although the dust deposition fluxes over polar regions are small, the dust derived from high latitude could potentially be more bioavailable than the tropical and subtropical dust. That can be attributed to the high fraction of Fe(II) minerals, mostly biotite from glacial flour. The atmospheric processes such as photoreduction and acidic reactions can further improve the Fe solubility. Therefore, dust sources in high latitudes are expected to be increasingly important under the rapid warming conditions.Ph.D.Includes bibliographical reference
Coralline P/CA: development and application of a novel surface ocean phosphate proxy
No full textThe supply of limiting nutrients to the low latitude ocean is controlled by physical processes linked to climate variations, but methods for reconstructing past nutrient concentrations in the surface ocean are few and indirect. This thesis presents the first calibration and downcore application of a new surface ocean phosphate (PO4 SW) proxy derived from coral skeleton P/Ca ratios. Skeletal P/Ca cycles tracked variations in past PO4 SW synchronously with temperature variations associated with seasonal upwelling in a 4-year Gulf of Panamá Pavona gigantea coral. P/Ca records in co-located Gulf of Panamá Pavona gigantea and Porites lobata colonies were strongly correlated to a contemporaneous time-series record of PO4 SW, supporting application of multi-colony calibration equations to down-core records. The P/Ca proxy was applied downcore to reconstruct an 18-year history of central equatorial Pacific PO4 SW. The PO4 SW record decreased during El Niño warm phases, resulting from decreased upwelling. In addition, a large 6-fold decrease in PO4 SW occurred from ~1982-1990, distinct from sea surface temperature changes in the central Equatorial Pacific. The decadal scale nutrient regime shift likely occurred in response to the observed decreases in wind stress and equatorial upwelling with possible linkages to the 1976/77 climate shift and changes in mid-latitude upwelling source water. The reliability of P/Ca was tested in low nutrient environments (Gulf of Eilat, Israel and Florida Keys, USA). Downcore P/Ca variability is low on centennial timescales in the Florida Keys record, implying that other possible influences on P incorporation (i.e. biological vital effects) are minimal and that the P/Ca proxy is not subject to major diagenetic alteration. With further development, the coralline P/Ca proxy could be applied to reconstruct PO4 SW on millennial timescales and be used to place modern tropical and subtropical nutrient distributions and dynamics in a paleoceanographic perspective.Ph.D.Includes abstractVitaIncludes bibliographical referencesby Michèle Gloria LaVign
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