Katharine R. Hendry

Ice sheets as a missing source of silica to the polar oceans

Ice sheets play a more important role in the global silicon cycle than previously appreciated. Input of dissolved and amorphous particulate silica into natural waters stimulates the growth of diatoms. Here we measure dissolved and amorphous silica in Greenland Ice Sheet meltwaters and icebergs, demonstrating the potential for high ice sheet export. Our dissolved and amorphous silica flux is 0.20 (0.06–0.79)u2009Tmolu2009year−1, ∼50% of the input from Arctic rivers. Amorphous silica comprises >95% of this flux and is highly soluble in sea water, as indicated by a significant increase in dissolved silica across a fjord salinity gradient. Retreating palaeo ice sheets were therefore likely responsible for high dissolved and amorphous silica fluxes into the ocean during the last deglaciation, reaching values of ∼5.5u2009Tmol year−1, similar to the estimated export from palaeo rivers. These elevated silica fluxes may explain high diatom productivity observed during the last glacial–interglacial period.

Changes in micronutrient supply to the surface Southern Ocean (Atlantic sector) across the glacial termination

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.

Enrichment of dissolved silica in the deep equatorial Pacific during the Eocene-Oligocene

Silicon isotope ratios (expressed as δ30Si) in marine microfossils can provide insights into silica cycling over geologic time. Here we used δ30Si of sponge spicules and radiolarian tests from the Paleogene Equatorial Transect (Ocean Drilling Program Leg 199) spanning the Eocene and Oligocene (~50–23xa0Ma) to reconstruct dissolved silica (DSi) concentrations in deep waters and to examine upper ocean δ30Si. The δ30Si values range from −3.16 to +0.18‰ and from −0.07 to +1.42‰ for the sponge and radiolarian records, respectively. Both records show a transition toward lower δ30Si values around 37xa0Ma. The shift in radiolarian δ30Si is interpreted as a consequence of changes in the δ30Si of source DSi to the region. The decrease in sponge δ30Si is interpreted as a transition from low DSi concentrations to higher DSi concentrations, most likely related to the shift toward a solely Southern Ocean source of deep water in the Pacific during the Paleogene that has been suggested by results from paleoceanographic tracers such as neodymium and carbon isotopes. Sponge δ30Si provides relatively direct information about the nutrient content of deep water and is a useful complement to other tracers of deep water circulation in the oceans of the past.

A review of the stable isotope bio-geochemistry of the global silicon cycle and its associated trace elements

Silicon (Si) is the second most abundant element in the Earth’s crust and is an important nutrient in the ocean. The global Si cycle plays a critical role in regulating primary productivity and carbon cycling on the continents and in the oceans. Development of the analytical tools used to study the sources, sinks, and fluxes of the global Si cycle (e.g., elemental and stable isotope ratio data for Ge, Si, Zn, etc.) have recently led to major advances in our understanding of the mechanisms and processes that constrain the cycling of Si in the modern environment and in the past. Here, we provide background on the geochemical tools that are available for studying the Si cycle and highlight our current understanding of the marine, freshwater and terrestrial systems. We place emphasis on the geochemistry (e.g., Al/Si, Ge/Si, Zn/Si, δ13 C, δ15 N, δ18 O, δ30 Si) of dissolved and biogenic Si, present case studies, such as the Silicic Acid Leakage Hypothesis, and discuss challenges associated with the development of these environmental proxies for the global Si cycle. We also discuss how each system within the global Si cycle might change over time (i.e., sources, sinks, and processes) and the potential technical and conceptual limitations that need to be considered for future studies.

Competition between Silicifiers and Non-silicifiers in the Past and Present Ocean and Its Evolutionary Impacts

Competition is a central part of the evolutionary process, and silicification is no exception: between biomineralized and non-biomineralized organisms, between siliceous and non-siliceous biomineralizing organisms, and between different silicifying groups. Here we discuss evolutionary competition at various scales, and how this has affected biogeochemical cycles of silicon, carbon, and other nutrients. Across geological time we examine how fossils, sediments, and isotopic geochemistry can provide evidence for the emergence and expansion of silica biomineralization in the ocean, and competition between silicifying organisms for silicic acid. Metagenomic data from marine environments can be used to illustrate evolutionary competition between groups of silicifying and non-silicifying marine organisms. Modern ecosystems also provide examples of arms races between silicifiers as predators and prey, and how silicification can be used to provide a competitive advantage for obtaining resources. Through studying the molecular biology of silicifying and non-silicifying species we can relate how they have responded to the competitive interactions that are observed, and how solutions have evolved through convergent evolutionary dynamics.

Biosilicification drives a decline of dissolved si in the oceans through geologic time

Biosilicification has driven variation in the global Si cycle over geologic time. The evolution of different eukaryotic lineages that convert dissolved Si (DSi) into mineralized structures (higher plants, siliceous sponges, radiolarians, and diatoms) has driven a secular decrease in DSi in the global ocean leading to the low DSi concentrations seen today. Recent studies, however, have questioned the timing previously proposed for the DSi decreases and the concentration changes through deep time, which would have major implications for the cycling of carbon and other key nutrients in the ocean. Here, we combine relevant genomic data with geological data and present new hypotheses regarding the impact of the evolution of biosilicifying organisms on the DSi inventory of the oceans throughout deep time. Although there is no fossil evidence for true silica biomineralization until the late Precambrian, the timing of the evolution of silica transporter genes suggests that bacterial silicon-related metabolism has been present in the oceans since the Archean with eukaryotic silicon metabolism already occurring in the Neoproterozoic. We hypothesize that biological processes have influenced oceanic DSi concentrations since the beginning of oxygenic photosynthesis. (Less)

Carnivorous sponges (Porifera, Demospongiae, Poecilosclerida, Cladorhizidae) from the Drake Passage (Southern Ocean) with a description of eight new species and a review of the family Cladorhizidae in the Southern Ocean.

Abstract. n This study reviews the taxonomy and biogeography of carnivorous sponges (family Cladorhizidae) in the Southern Ocean. Specimens were collected from seamounts in the Drake Passage by dredging and trawling and biogeographical information from other sources was compiled and reviewed. Eight new species of carnivorous sponges are described: Abyssocladia leverhulmei, sp. nov., Asbestopluma (Asbestopluma) sarsensis, sp. nov., A. (A.) gemmae, sp. nov., A. (A.) rhaphidiophorus, sp. nov., Asbestopluma (Helophloeina) keraia, sp. nov., Chondrocladia (Chondrocladia) saffronae, sp. nov., Cladorhiza scanlonae, sp. nov. and Lycopodina drakensis, sp. nov. Specimens of three previously described species, L. callithrix, L. calyx and A. (A.) bitrichela, were also found. These new records increase the number of known carnivorous sponge species in the Southern Ocean by more than a third. We demonstrate that the Cladorhizidae is the second most species-rich family of Demospongiae in the Southern Ocean and many of its species are highly endemic, with 70% found only in this region. Southern Ocean species represent close to 20% of all known carnivorous sponges. This study highlights the importance of seamount and bathyal benthic habitats for supporting the rich and endemic carnivorous sponge fauna of the Southern Ocean.

The marine system of the West Antarctic Peninsula: status and strategy for progress

The West Antarctic Peninsula (WAP; figure 1) is one of the most climatically sensitive regions on Earth and one of the most variable. The strong climatic variability gives us the opportunity to study and understand how the ocean responds to—and gives feedback on—climate change, and hence to learn about the key mechanisms that are at work, which might apply around the Southern Ocean as a whole. Data coverage is still inadequate across the Southern Ocean (because of remoteness and harsh conditions) and, despite being better observed than many other regions around Antarctica, the nature of oceanographic and atmospheric change on the WAP is poorly constrained. This theme issue addresses some of the most important and pressing questions surrounding marine system variability at the WAP. How has the WAP changed and how will it change in future? Whats driving these changes? And why is there such an extraordinary degree of spatial and temporal variability in the region? nnnnFigure 1. nA map of the Antarctic Peninsula, showing the main islands and ice shelves (grey areas), with research locations mentioned in the text marked with triangles. The main map is an enlarged view of the blue box and was made using etopo1 bathymetry (for bedrock and ice surface files, see https://www.ngdc.noaa.gov/docucomp/page?xml=NOAA/NESDIS/NGDC/MGG/DEM/iso/xml/316.xml&view=getDataView&header=none).nnnnThese questions were addressed in two interlinked meetings held in 2017. A 2 day meeting was hosted in May at the British Antarctic Survey (BAS) in Cambridge, UK (co-sponsored by the Southern Ocean Observing System, the Scientific Committee for Antarctic Research and the Scientific Committee for Ocean Research), aimed at gathering and critically assessing a broad view from the international community on the gaps and challenges in WAP oceanographic research. A second meeting was held at the Kavli International Centre at Chicheley Hall, UK (funded by the Royal Society), specifically to identify the key …

The silicon cycle impacted by past ice sheets

Globally averaged riverine silicon (Si) concentrations and isotope composition (δ30Si) may be affected by the expansion and retreat of large ice sheets during glacial−interglacial cycles. Here we provide evidence of this based on the δ30Si composition of meltwater runoff from a Greenland Ice Sheet catchment. Glacier runoff has the lightest δ30Si measured in running waters (−0.25u2009±u20090.12‰), significantly lower than nonglacial rivers (1.25u2009±u20090.68‰), such that the overall decline in glacial runoff since the Last Glacial Maximum (LGM) may explain 0.06–0.17‰ of the observed ocean δ30Si rise (0.5–1.0‰). A marine sediment core proximal to Iceland provides further evidence for transient, low-δ30Si meltwater pulses during glacial termination. Diatom Si uptake during the LGM was likely similar to present day due to an expanded Si inventory, which raises the possibility of a feedback between ice sheet expansion, enhanced Si export to the ocean and reduced CO2 concentration in the atmosphere, because of the importance of diatoms in the biological carbon pump.The role ice sheets play in the silica cycle over glacial−interglacial timescales remains unclear. Here, based on the measurement of silica isotopes in Greenland meltwater and a nearby marine sediment core, the authors suggest expanding ice sheets considerably increased isotopically light silica in the oceans.

Spatiotemporal Variability of Barium in Arctic Sea‐Ice and Seawater

Freshwater export from the Arctic is critical in determining the density of water at sites of North Atlantic deep water formation, which in turn influences the global flux of oceanic heat and nutrients. We need geochemical tracers and high-resolution observations to refine our freshwater budgets and constrain models for future change. The use of seawater barium concentrations in the Arctic Ocean as a freshwater tracer relies on the conservative behavior of barium in seawater; while this has been shown to be an unreliable assumption in Arctic summers, there are a lack of studies observing seasonal progressions. Here, we present barium concentrations from seawater and sea-ice collected during the Norwegian Young Sea ICE expedition from boreal winter into summer. We use other tracers (salinity, oxygen isotopes, and alkalinity) to reconstruct freshwater inputs and calculate a barium ‘‘deficit’’ that can be attributed to nonconservative processes. We locate a deficit in winter when biological production is low, which we attribute to uptake by barite formation associated with old organic matter or by internal sea-ice processes. We also find a significant barium deficit during the early spring bloom, consistent with uptake into organic-matter associated microenvironments. However, in summer, there no strong barium deficit near the surface, despite high biological production and organic carbon standing stocks, perhaps reflecting phytoplankton assemblage changes, and/or rapid internal cycling. Our findings challenge the assumptions surrounding the use of barium as an Arctic freshwater tracer, and highlight the need to improve our understanding of barium in sea-ice environments.