Rickaby
Rosalind E. M.
Rickaby
Rosalind E. M.
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PreprintSilicon isotopes in Antarctic sponges : an interlaboratory comparison( 2010-06-08) Hendry, Katharine R. ; Leng, Melanie J. ; Robinson, Laura F. ; Sloane, Hilary J. ; Blusztajn, Jerzy S. ; Rickaby, Rosalind E. M. ; Georg, R. Bastian ; Halliday, Alex N.Cycling of deep-water silicon (Si) within the Southern Ocean, and its transport into other ocean basins, may be an important player in the uptake of atmospheric carbon, and global climate. Recent work has shown that the Si isotope (denoted by δ29Si or δ30Si) composition of deep-sea sponges reflects the availability of dissolved Si during growth, and is a potential proxy for past deep and intermediate water silicic acid concentrations. As with any geochemical tool, it is essential to ensure analytical precision and accuracy, and consistency between methodologies and laboratories. Analytical bias may exist between laboratories, and sponge material may have matrix effects leading to offsets between samples and standards. Here, we report an interlaboratory evaluation of Si isotopes in Antarctic and subAntarctic sponges. We review independent methods for measuring Si isotopes in sponge spicules. Our results show that separate subsamples of non-homogenised sponges measured by three methods yield isotopic values within analytical error for over 80% of specimens. The relationship between δ29Si and δ30Si in sponges is consistent with kinetic fractionation during biomineralisation. Sponge Si isotope analyses show potential as palaeoceaongraphic archives, and we suggest Southern Ocean sponge material would form a useful additional reference standard for future spicule analyses.
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PreprintConstraints on the vital effect in coccolithophore and dinoflagellate calcite by oxygen isotopic modification of seawater( 2014-05) Hermoso, Michael ; Horner, Tristan J. ; Minoletti, Fabrice ; Rickaby, Rosalind E. M.In this study, we show that there are independent controls of 18O/16O and 13C/12C fractionation in coccolithophore and dinoflagellate calcite due to the contrasting kinetics of each isotope system. We demonstrate that the direction and magnitude of the oxygen isotope fractionation with respect to equilibrium is related to the balance between calcification rate and the replenishment of the internal pool of dissolved inorganic carbon (DIC). As such, in fast growing cells, such as those of Emiliania huxleyi and Gephyrocapsa oceanica (forming the so-called “heavy group”), calcification of the internal carbon pool occurs faster than complete isotopic re-adjustment of the internal DIC pool with H2O molecules. Hence, coccoliths reflect the heavy oxygen isotope signature of the CO2 overprinting the whole DIC pool. Conversely, in large and slow growing cells, such as Coccolithus pelagicus ssp. braarudii, complete re-equilibration is achieved due to limited influx of CO2 leading to coccoliths that are precipitated in conditions close to isotopic equilibrium (“equilibrium group”). Species exhibiting the most negative oxygen isotope composition, such as Calcidiscus leptoporus (“light group”), precipitate coccolith under increased pH in the coccolith vesicle, as previously documented by the “carbonate ion effect”. We suggest that, for the carbon isotope system, any observed deviation from isotopic equilibrium is only “apparent”, as the carbon isotopic composition in coccolith calcite is controlled by a Rayleigh fractionation originating from preferential incorporation of 12C into organic matter. Therefore, species with low PIC/POC ratios as E. huxleyi and G. oceanica are shifted towards positive carbon isotope values as a result of predominant carbon fixation into the organic matter. By contrast, cells with higher PIC/POC as C. braarudii and C. leptoporus maintain, to some extent, the original negative isotopic composition of the CO2. The calcareous dinoflagellate Thoracosphaera heimii exhibits different behaviour for both isotopic systems, in particular with respect to its very negative carbon isotope composition, owing to coeval intra and extracellular biomineralisation in this group. In this study, we also investigate the sensitivity of 18O/16O fractionation to varying ambient oxygen isotope composition of the medium for inorganic, coccolithophore, and dinoflagellate calcite precipitated under controlled laboratory conditions. The varying responses of different taxa to increased oxygen isotope composition of the growth medium may point to a potential bias in sea surface temperature reconstructions that are based on the oxygen isotopic compositions of sedimentary calcite, especially during times of changing seawater oxygen isotopic composition. Overall, this study represent an important step towards establishing a mechanistic understanding of the “vital effect” in coccolith and dinoflagellate calcite, and provides valuable information for interpreting the geochemistry of the calcareous nannofossils in the sedimentary record, at both monospecific and interspecies levels.
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PreprintErratum to “Deep ocean nutrients during the Last Glacial Maximum deduced from sponge silicon isotopic compositions” [Earth Planet. Sci. Lett. 292 (2010) 290–300]( 2010-12) Hendry, Katharine R. ; Georg, R. Bastian ; Rickaby, Rosalind E. M. ; Robinson, Laura F. ; Halliday, Alex N.The relative importance of biological and physical processes within the Southern Ocean for the storage of carbon and atmospheric pCO2 on glacial-interglacial timescales remains uncertain. Understanding the impact of surface biological production on carbon export in the past relies on the reconstruction of the nutrient supply from upwelling deep-waters. In particular, the upwelling of silicic acid (Si(OH)4) is tightly coupled to carbon export in the Southern Ocean via diatom productivity. Here, we address how changes in deep-water Si(OH)4 concentrations can be reconstructed using the silicon isotopic composition of deep-sea sponges. We report δ30Si of modern deep-sea sponge spicules and show that they reflect seawater Si(OH)4 concentration. The fractionation factor of sponge δ30Si compared to seawater δ30Si shows a positive relationship with Si(OH)4, which may be a growth rate effect. Application of this proxy in two down-core records from the Scotia Sea reveals that Si(OH)4 concentrations in the deep Southern Ocean during the Last Glacial Maximum (LGM) were no different than today. Our result does not support a coupling of carbon and nutrient build up in an isolated deep-ocean reservoir during the LGM. Our data, combined with records of stable isotopes from diatoms, are only consistent with enhanced LGM Southern Ocean nutrient utilization if there was also a concurrent reduction in diatom silicification or a shift from siliceous to organic-walled phytoplankton.
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ArticleChanges in micronutrient supply to the surface Southern Ocean (Atlantic sector) across the glacial termination(American Geophysical Union, 2011-09-16) Hendry, Katharine R. ; Rickaby, Rosalind E. M. ; Allen, Claire S.Major deepwater masses upwell and reach the surface in the Southern Ocean, forming an important conduit supplying nutrients and micronutrients to the surface and playing a key role in the regulation of global climate through ocean-atmosphere gas exchange. Here, we reconstruct changes in micronutrient distribution in this region in response to past changes in upwelling, oceanic mixing, and sea-ice seasonality. We present two downcore (Zn/Si)opal records from the Scotia Sea and Drake Passage region, which we interpret in the context of micronutrient distribution in the Atlantic sector of the Southern Ocean over the last glacial termination. Previous work shows that micronutrient availability in the surface waters in the South Atlantic appear to be controlled dominantly by upwelling and mixing of micronutrient rich deepwaters, which are additionally fuelled by the terrestrial sediment sources of the Scotia Arc and South Georgia. This is supported by our reconstructions, which show micronutrient availability to the west of the Scotia Arc and South Georgia are consistently lower than to the east over the last glacial termination due to downstream transport and mixing into surface waters of continentally derived material in the Antarctic Circumpolar Current. Micronutrient availability in this region was at a minimum from 20 to 25 ky BP, coinciding with maximum sea-ice coverage, and increased due to an expansion of the seasonal sea-ice zone and increased mixing of subsurface waters. Our findings are consistent with largely diminished upwelling of micronutrients during the maximum glacial extent, and reduced mixing due to the presence of persistent sea-ice. During the deglacial there was an increase in micronutrient availability, as well as other nutrients and inorganic carbon, within the Antarctic Circumpolar Current as a result of an increase in deep oceanic upwelling, mixing and strengthened zonal transport.
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PreprintThe role of sea ice formation in cycling of aluminium in northern Marguerite Bay, Antarctica( 2009-11-17) Hendry, Katharine R. ; Meredith, Michael P. ; Measures, Christopher I. ; Carson, Damien S. ; Rickaby, Rosalind E. M.The use of dissolved Al as a tracer for oceanic water masses and atmospheric dust deposition of biologically important elements, such as iron, requires the quantitative assessment of its sources and sinks in seawater. Here, we address the relative importance of oceanic versus atmospheric inputs of Al, and the relationship with nutrient cycling, in a region of high biological productivity in coastal Antarctica. We investigate the concentrations of dissolved Al in seawater, sea ice, meteoric water and sediments collected from northern Marguerite Bay, off the West Antarctic Peninsula, from 2005-2006. Dissolved Al concentrations at 15 m water depth varied between 2 and 27 nM, showing a peak between two phytoplankton blooms. We find that, in this coastal setting, upwelling and incorporation of waters from below the surface mixed layer are responsible for this peak in dissolved Al as well as renewal of nutrients. This means that changes in the intensity and frequency of upwelling events may result in changes in biological production and carbon uptake. The waters below the mixed layer are most likely enriched in Al as a result of sea ice formation, either causing the injection of Al-rich brines or the resuspension of sediments and entrainment of pore fluids by brine cascades. Glacial, snow and sea ice melt contributes secondarily to the supply of Al to surface waters. Total particulate Al ranges from 93 to 2057 μg/g, and increases with meteoric water input towards the end of the summer, indicating glacial runoff is an important source of particulate Al. The (Al/Si)opal of sediment core top material is considerably higher than water column opal collected by sediment traps, indicative of a diagenetic overprint and incorporation of Al at the sediment-water interface. Opal that remains buried in the sediment could represent a significant sink of Al from seawater.
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PreprintControls on stable isotope and trace metal uptake in Neogloboquadrina pachyderma (sinistral) from an Antarctic sea-ice environment( 2008-11) Hendry, Katharine R. ; Rickaby, Rosalind E. M. ; Meredith, Michael P. ; Elderfield, HenryThe polar foraminifera Neogloboquadrina pachyderma (sinistral) dominates assemblages from the high latitude Southern Ocean, which is a key region for paleoclimate studies. Here, we use N. pachyderma (s.) harvested from sediment traps off the West Antarctic Peninsula to construct a seasonal time series for the calibration of calcite proxies in a high latitude seasonal sea-ice environment where temperature is decoupled from other environmental parameters. We have used a combination of δ18OCaCO3 and δ13CCaCO3 to decipher the calcification temperature and salinity, which reflect that N. pachyderma (s.) live in surface waters throughout the year, and at the ice-water interface in austral winter. Further, our results demonstrate that, during winter, the uptake of trace metals into N. pachyderma (s.) calcite is influenced by secondary environmental conditions in addition to temperature during periods of sea-ice. We suggest an elevated carbonate ion concentration at the ice-water interface resulting from biological utilisation CO2 could influence calcification in foraminifera. We demonstrate that for N. pachyderma (s.) Mg/Ca and Sr/Ca ratios are linear functions of calcification temperature and [CO32-]. N. pachyderma (s.) Mg/Ca ratios exhibit temperature sensitivity similar to previous studies (~ 10 % per °C) and a sensitivity to [CO32-] of ~ 1 % per μmol kg-1). Sr/Ca ratios are less sensitive to environmental parameters, exhibiting < 1% increase per °C and per 10 μmol kg-1. We show how a multi-proxy approach could be used to constrain past high latitude surface water temperature and [CO32-].
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ArticleOpal (Zn/Si) ratios as a nearshore geochemical proxy in coastal Antarctica(American Geophysical Union, 2008-06-18) Hendry, Katharine R. ; Rickaby, Rosalind E. M.During the last 50 years, the Antarctic Peninsula has experienced rapid warming with associated retreat of 87% of marine and tidewater glacier fronts. Accelerated glacial retreat and iceberg calving may have a significant impact on the freshwater and nutrient supply to the phytoplankton communities of the highly productive coastal regions. However, commonly used biogenic carbonate proxies for nutrient and salinity conditions are not preserved in sediments from coastal Antarctica. Here we describe a method for the measurement of zinc to silicon ratios in diatom opal, (Zn/Si)opal, which is a potential archive in Antarctic marine sediments. A core top calibration from the West Antarctic Peninsula shows (Zn/Si)opal is a proxy for mixed layer salinity. We present down-core (Zn/Si)opal paleosalinity records from two rapidly accumulating sites taken from nearshore environments off the West Antarctic Peninsula which show an increase in meltwater input in recent decades. Our records show that the recent melting in this region is unprecedented for over 120 years.
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PreprintCadmium and phosphate in coastal Antarctic seawater : implications for Southern Ocean nutrient cycling( 2008-09) Hendry, Katharine R. ; Rickaby, Rosalind E. M. ; de Hoog, Jan C. M. ; Weston, Keith ; Rehkamper, MarkCadmium is a biologically important trace metal that co-varies with phosphate (PO43- or Dissolved Inorganic Phosphate, DIP) in seawater. However, the exact nature of Cd uptake mechanisms and the relationship with phosphate and other nutrients in global oceans remains elusive. Here, we present a time series study of Cd and PO43- from coastal Antarctic seawater, showing that Cd co-varies with macronutrients during times of high biological activity even under nutrient and trace metal replete conditions. Our data imply that Cd/PO43- in coastal surface Antarctic seawater is higher than open ocean areas. Furthermore, the sinking of some proportion of this high Cd/PO43- water into Antarctic Bottom Water, followed by mixing into Circumpolar Deep Water, impacts Southern Ocean preformed nutrient and trace metal composition. A simple model of endmember water mass mixing with a particle fractionation of Cd/P (αCd-P) determined by the local environment can be used to account for the Cd/PO43- relationship in different parts of the ocean. The high Cd/PO43- of the coastal water is a consequence of two factors: the high input from terrestrial and continental shelf sediments and changes in biological fractionation with respect to P during uptake of Cd in regions of high Fe and Zn. This implies that the Cd/PO43- ratio of the Southern Ocean will vary on glacial-interglacial timescales as the proportion of deep water originating on the continental shelves of the Weddell Sea is reduced during glaciations because the ice shelf is pinned at the edge of the continental shelf. There could also be variations in biological fractionation of Cd/P in the surface waters of the Southern Ocean on these timescales as a result of changes in atmospheric inputs of trace metals. Further variations in the relationship between Cd and PO43- in seawater arise from changes in population structure and community requirements for macro- and micronutrients.
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PreprintDeep ocean nutrients during the Last Glacial Maximum deduced from sponge silicon isotopic compositions( 2010-02) Hendry, Katharine R. ; Georg, R. Bastian ; Rickaby, Rosalind E. M. ; Robinson, Laura F. ; Halliday, Alex N.The relative importance of biological and physical processes within the Southern Ocean for the storage of carbon and atmospheric pCO2 on glacial-interglacial timescales remains uncertain. Understanding the impact of surface biological production on carbon export in the past relies on the reconstruction of the nutrient supply from upwelling deep-waters. In particular, the upwelling of silicic acid (Si(OH)4) is tightly coupled to carbon export in the Southern Ocean via diatom productivity. Here, we address how changes in deep-water Si(OH)4 concentrations can be reconstructed using the silicon isotopic composition of deep-sea sponges. We report δ30Si of modern deep-sea sponge spicules and show that they reflect seawater Si(OH)4 concentration. The fractionation factor of sponge δ30Si compared to seawater δ30Si shows a positive relationship with Si(OH)4, which may be a growth rate effect. Application of this proxy in two down-core records from the Scotia Sea reveals that Si(OH)4 concentrations in the deep Southern Ocean during the Last Glacial Maximum (LGM) were no different than today. Our result does not support a coupling of carbon and nutrient build up in an isolated deep-ocean reservoir during the LGM. Our data, combined with records of stable isotopes from diatoms, are only consistent with enhanced LGM Southern Ocean nutrient utilization if there was also a concur rent reduction in diatom silicification or a shift from siliceous to organic walled phytoplankton.
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ArticleDiatom silicon isotopes as a proxy for silicic acid utilisation : a Southern Ocean core top calibration(Elsevier, 2012-08-11) Egan, Katherine E. ; Rickaby, Rosalind E. M. ; Leng, Melanie J. ; Hendry, Katharine R. ; Hermoso, Michael ; Sloane, Hilary J. ; Bostock, Helen ; Halliday, Alex N.Despite a growing body of work that uses diatom δ30Si to reconstruct past changes in silicic acid utilisation, few studies have focused on calibrating core top data with modern oceanographic conditions. In this study, a microfiltration technique is used to divide Southern Ocean core top silica into narrow size ranges, separating components such as radiolaria, sponge spicules and clay minerals from diatoms. Silicon isotope analysis of these components demonstrates that inclusion of small amounts of non-diatom material can significantly offset the measured from the true diatom δ30Si. Once the correct size fraction is selected (generally 2–20 μm), diatom δ30Si shows a strong negative correlation with surface water silicic acid concentration (R2 = 0.92), highly supportive of the qualitative use of diatom δ30Si as a proxy for silicic acid utilisation. The core top diatom δ30Si matches well with mixed layer filtered diatom δ30Si from published in situ studies, suggesting little to no effect of either dissolution on export through the water column, or early diagenesis, on diatom δ30Si in sediments from the Southern Ocean. However, the core top diatom δ30Si shows a poor fit to simple Rayleigh or steady state models of the Southern Ocean when a single source term is used. The data can instead be described by these models only when variations in the initial conditions of upwelled silicic acid concentration and δ30Si are taken into account, a caveat which may introduce some error into quantitative reconstructions of past silicic acid utilisation from diatom δ30Si.
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PreprintReply to Morel : cadmium as a micronutrient and macrotoxin in the oceans( 2013-04) Horner, Tristan J. ; Lee, Renee B. Y. ; Henderson, Gideon M. ; Rickaby, Rosalind E. M.We thank François Morel for his interest in our study. Morel states that our conclusions are based on the approximate match between the Cd-isotope composition of cultured bacteria and the fractionation of Cd isotopes seen in seawater (1). This match is only a minor component of our argument, and we welcome the opportunity to reiterate our case.
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ArticleCommentary: Planktic foraminifera iodine/calcium ratios from plankton tows(Frontiers Media, 2023-07-11) Lu, Zunli ; Thomas, Ellen ; Rickaby, Rosalind E. M. ; Lu, Wanyi ; Prow, Ashley N.I/Ca in planktic foraminiferal shells is one of the few proxies proposed for reconstructing past upper ocean oxygenation. Foraminiferal I/Ca values have been measured in globally distributed core-top samples, with low I/Ca values generally indicative of low-oxygen water (Lu et al., 2020a; Lu et al., 2020b). Winkelbauer et al. (2023) observed that “I/Ca values of plankton tow foraminifera from environments with well oxygenated subsurface waters, however, are an order of magnitude lower compared to core-tops from similarly well-oxygenated regions,” and concluded that “planktic foraminifera gain iodine post-mortem, either when sinking through the water column, or during burial.” These tow-derived low I/Ca values add to the information needed to mechanistically understand the planktic I/Ca proxy. However, we highlight the fact that tow-specimens do not represent the same populations as those present in core tops, and we think that further evidence is needed before reinterpreting the core-top and down-core planktic I/Ca records.
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ArticleNatural analogs to ocean alkalinity enhancement(Copernicus Publications, 2023-11-27) Subhas, Adam V. ; Lehmann, Nadine ; Rickaby, Rosalind E. M.Ocean alkalinity enhancement (OAE) research can be supplemented by studying the natural alkalinity cycle. In this chapter, we introduce the concept of natural analogs to ocean alkalinity enhancement. We describe Earth system processes relevant to OAE deployment and its measurement, reporting, and verification. We then describe some suitable natural analog locations that could serve as study sites to understand how these processes may interact with OAE. Approaches to examining the geological record are also considered. Practical considerations for establishing a natural analog study are discussed, including geochemical mass balance, choosing a site, establishing a control, choosing a measurement suite and platform, and coordinating with ocean models. We identify rivers and their plumes, glacial fjords, whiting events, and basinal seas with elevated alkalinity as promising candidates for initial natural analog studies. This chapter is not meant to be prescriptive but instead is written to inspire researchers to creatively explore the power of natural analogs to advance our understanding of OAE. Key recommendations include considering appropriate spatial and temporal scales of the study and associated measurement criteria and designing the study with applicable outcomes to OAE research, including implications for deployment and/or monitoring.