1.
Alvarez, Belinda, et al.
(författare)
Assessing the potential of sponges (Porifera) as indicators of ocean dissolved Si concentrations
2017
Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 4
Tidskriftsartikel (refereegranskat) abstract
We explore the distribution of sponges along dissolved silica (dSi) concentration gradients to test whether sponge assemblages are related to dSi and to assess the validity of fossil sponges as a palaeoecological tool for inferring dSi concentrations of the past oceans. We extracted sponge records from the publically available Global Biodiversity Information Facility (GBIF) database and linked these records with ocean physiochemical data to evaluate if there is any correspondence between dSi concentrations of the waters sponges inhabit and their distribution. Over 320,000 records of Porifera were available, of which 62,360 met strict quality control criteria. Our analyses was limited to the taxonomic levels of family, order and class. Because dSi concentration is correlated with depth in the modern ocean, we also explored sponge taxa distributions as a function of depth. We observe that while some sponge taxa appear to have dSi preferences (e.g., class Hexactinellida occurs mostly at high dSi), the overall distribution of sponge orders and families along dSi gradients is not sufficiently differentiated to unambiguously relate dSi concentrations to sponge taxa assemblages. We also observe that sponge taxa tend to be similarly distributed along a depth gradient. In other words, both dSi and/or another variable that depth is a surrogate for, may play a role in controlling sponge spatial distribution and the challenge is to distinguish between the two. We conclude that inferences about palaeo-dSi concentrations drawn from the abundance of sponges in the stratigraphic records must be treated cautiously as these animals are adapted to a great range of dSi conditions and likely other underlying variables that are related to depth. Our analysis provides a quantification of the dSi ranges of common sponge taxa, expands on previous knowledge related to their bathymetry preferences and suggest that sponge taxa assemblages are not related to particular dSi conditions.
2.
Conley, Daniel J., et al.
(författare)
Biosilicification drives a decline of dissolved si in the oceans through geologic time
2017
Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 4:DEC
Forskningsöversikt (refereegranskat) abstract
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.
3.
Fontorbe, Guillaume, et al.
(författare)
Enrichment of dissolved silica in the deep equatorial Pacific during the Eocene-Oligocene
2017
Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 32, s. 848-863
Tidskriftsartikel (refereegranskat) abstract
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 (~50–23 Ma) 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 37 Ma. 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.
4.
Mosimane, Keotshephile, et al.
(författare)
Variability in chemistry of surface and soil waters of an evapotranspiration-dominated flood-pulsed wetland : Solute processing in the okavango delta, Botswana
2017
Ingår i: Water S.A.. - : Academy of Science of South Africa. - 1816-7950 .- 0378-4738. ; 43:1, s. 104-115
Tidskriftsartikel (refereegranskat) abstract
Water chemistry is important for the maintenance of wetland structure and function. Interpreting ecological patterns in a wetland system therefore requires an in-depth understanding of the water chemistry of that system. We investigated the spatial distribution of chemical solutes both in soil pore water and surface water, along island-floodplain-channel hydrological gradients in seasonally and permanently inundated habitats between major regions in the Okavango Delta, Botswana. Our results show that major cations (Ca, Na, Mg, and K), dissolved silica (DSi), dissolved boron (B), dissolved organic matter (DOC) and electrical conductivity increased significantly, at p ≤ 0.05, from the inlet of the Delta (the Panhandle) to the distal downstream reaches, suggesting the influence of evapoconcentration. Concentrations of dissolved Fe, Al, Zn, Cu, and Mn significantly decreased, at p ≤ 0.05, from the inflow of the Delta to the distal reaches. Only Na, Mn, Fe, Al, and DOC showed significant differences, at p ≤ 0.05, along the local floodplain-channel hydrological gradients, with higher solute concentrations in the floodplains than the channels. Solute concentrations in soil water exhibited similar distribution patterns to those in surface water, but concentrations were higher in soil water. Based on the results, we hypothesise that floodplain emergent vegetation and the channel-fringing vegetation in the Panhandle (a fault-bounded entry trough to the Delta) and the permanently inundated eco-region together influence the cycling of solutes that enter the Delta through uptake.
5.
Robertson, Elizabeth, 1987, et al.
(författare)
DATA SET: Benthic nitrate reduction process rates from around the Baltic Sea (May 2016).
2021
Ingår i: PANGAEA. - Germany : PANGAEA.
Annan publikation (övrigt vetenskapligt/konstnärligt) abstract
Nitrate reduction process rates (denitrification, dissimilatory nitrate reduction to ammonium rate - DNRA, anammox) were collected during a research cruise to the Baltic Sea during May-June 2016 as part of the Vici Project “Response of the Iron Biogeochemical Cycle on Continental Shelves to Seawater Deoxygenation” and the BONUS-COCOA project. Sediment sampling and incubation experiments were carried out onboard the RV Pelagia (NIOZ, Netherlands, cruise 64PE411). Whole cores were incubated on board at in situ bottom water temperature and oxygen conditions in the dark. Stable isotopically labelled nitrate (15N-NO3-) was added to overlying water and cores were capped according to Nielsen et al. (1992). Three cores were 'sacrificed' over time (2, 4h, 6h, 8h) after 15-NO3- addition by slurrying the surface and sampling for dissolved gas and solutes. Slurry experiments were also carried out in parallel to core incubations at each site (Risgaard-Petersen et al., 2003; Song et al., 2013). Analysis of 15N compounds was carried out at the University of Southern Denmark (Dalsgaard et al., 2013). Process rates were calculated according to Song et al. (2016). In addition to the calculated process rates, the dataset includes the following in situ parameters: bottom water salinity, temperature and oxygen (obtained from CTD sensors), nitrate (measured spectrophotometrically according to Schnetget and Lehners, 2014) and total organic carbon data (derived from Hermans et al., 2019).
6.
Schoelynck, Jonas, et al.
(författare)
The trapping of organic matter within plant patches in the channels of the Okavango Delta : a matter of quality
2017
Ingår i: Aquatic Sciences. - : Springer Science and Business Media LLC. - 1015-1621 .- 1420-9055. ; 79:3, s. 661-674
Tidskriftsartikel (refereegranskat) abstract
The role of in-stream aquatic vegetation as ecosystem engineers in the distribution of organic matter was investigated in the Okavango Delta, one of the world’s largest oligotrophic wetlands. The Okavango channel beds are covered up to 50% with submerged macrophyte patches. By accumulating and concentrating organic matter in the sediments below the patches, macrophytes are likely able to locally forestall a deficiency of nutrients. Up to 21 times more N, 18 times more C, 13 times more P and 6 times more Si can be found in vegetated sediments compared to non-vegetated sediments. Nutrient specific accumulation relates to its relative scarcity in the overlaying water. There is a depletion of dissolved N relative to P, whereas Si is relatively abundant. The Okavango Delta water can generally be characterised as oligotrophic based on plant species composition (e.g. presence of carnivorous plants and absence of floating plants), low plant N:P ratios, and low nutrient- and element-concentrations. Local mineralization and intensified nutrient cycling in the sediments is hypothesized to be crucial for the macrophytes’ survival because it provides a key source of the essential nutrients which plants otherwise cannot obtain in sufficient quantities from the nutrient poor water. By engineering the ecosystem as such, channel vegetation also retards the loss of elements and nutrients to island groundwater flow, contributing to one of the key processes driving the high productivity of the Okavango Delta, making it unique among its kind.
7.
Sutton, Jill N., et al.
(författare)
A review of the stable isotope bio-geochemistry of the global silicon cycle and its associated trace elements
2018
Ingår i: Frontiers in Earth Science. - : Frontiers Media SA. - 2296-6463. ; 5
Forskningsöversikt (refereegranskat) abstract
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.
8.
Robertson, Elizabeth, 1987, et al.
(författare)
Application of the isotope pairing technique in sediments: use, challenges and new directions.
2019
Ingår i: Limnology and Oceanography : Methods. - : Wiley. - 1541-5856. ; 17:2, s. 112-136
Tidskriftsartikel (refereegranskat) abstract
Determining accurate rates of benthic nitrogen (N) removal and retention pathways from diverse environments is critical to our understanding of process distribution and constructing reliable N budgets and models. The whole‐core 15N isotope pairing technique (IPT) is one of the most widely used methods to determine rates of benthic nitrate‐reducing processes and has provided valuable information on processes and factors controlling N removal and retention in aquatic systems. While the whole core IPT has been employed in a range of environments, a number of methodological and environmental factors may lead to the generation of inaccurate data and are important to acknowledge for those applying the method. In this review, we summarize the current state of the whole core IPT and highlight some of the important steps and considerations when employing the technique. We discuss environmental parameters which can pose issues to the application of the IPT and may lead to experimental artifacts, several of which are of particular importance in environments heavily impacted by eutrophication. Finally, we highlight the advances in the use of the whole‐core IPT in combination with other methods, discuss new potential areas of consideration and encourage careful and considered use of the whole‐core IPT. With the recognition of potential issues and proper use, the whole‐core IPT will undoubtedly continue to develop, improve our understanding of benthic N cycling and allow more reliable budgets and predictions to be made.
9.
Zhang, Zhouling, et al.
(författare)
Impact of human disturbance on the biogeochemical silicon cycle in a coastal sea revealed by silicon isotopes
2020
Ingår i: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 65:3, s. 515-528
Tidskriftsartikel (refereegranskat) abstract
Biogeochemical silicon (Si) cycling in coastal systems is highly influenced by anthropogenic perturbations in recent decades. Here, we present a systematic study on the distribution of stable Si isotopes of dissolved silicate (delta Si-30(DSi)) in a highly eutrophic coastal system, the Baltic Sea. Besides the well-known processes, diatom production and dissolution regulating delta Si-30(DSi) values in the water column, we combined field data with a box model to examine the role of human disturbances on Si cycling in the Baltic Sea. Results reveal that (1) damming led to increased delta Si-30(DSi) values in water but had little impacts on their vertical distribution; (2) decrease in saltwater inflow due to enhanced thermal stratification had negligible impacts on the delta Si-30(DSi) distribution. An atypical vertical distribution of delta Si-30(DSi) with higher values in deep water (1.57-1.95 parts per thousand) relative to those in surface water (1.24-1.68 parts per thousand) was observed in the central basin. Model results suggest the role of enhanced biogenic silica (BSi) deposition and subsequently regenerated dissolved silicate (DSi) flux from sediments. Specifically, eutrophication enhances diatom production, resulting in elevated exports of highly fractionated BSi to deep water and sediments. In situ sedimentary geochemical processes, such as authigenic clay formation, further fractionate Si isotopes and increase pore-water delta Si-30(DSi) values, which then leads to pore-water DSi flux carrying higher delta Si-30(DSi) compositions into deep water. Our findings provide new quantitative information on how the isotope-based Si cycle responds to human perturbations in coastal seas and shed lights on shifts of Si export to open ocean.
10.
Fontorbe, Guillaume, et al.
(författare)
A silicon depleted North Atlantic since the Palaeogene : Evidence from sponge and radiolarian silicon isotopes
2016
Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X. ; 453, s. 67-77
Tidskriftsartikel (refereegranskat) abstract
Despite being one of Earth's major geochemical cycles, the evolution of the silicon cycle has received little attention and changes in oceanic dissolved silica (DSi) concentration through geologic time remain poorly constrained. Silicon isotope ratios (expressed as δ30Si) in marine microfossils are becoming increasingly recognised for their ability to provide insight into silicon cycling. In particular, the δ30Si of siliceous sponge spicules has been demonstrated to be a useful proxy for past DSi concentrations. We analysed δ30Si in radiolarian tests and sponge spicules from the Blake Nose Palaeoceanographic Transect (ODP Leg 171B) spanning the Palaeocene–Eocene (ca. 60–30 Ma). Our δ30Si results range from +0.32 to +1.67‰ and −0.48 to +0.63‰ for the radiolarian and sponge records, respectively. Using an established relationship between ambient dissolved Si (DSi) concentrations and the magnitude of silicon isotope fractionation in siliceous sponges, we demonstrate that the Western North Atlantic was DSi deplete during the Palaeocene–Eocene throughout the water column, a conclusion that is robust to a range of assumptions and uncertainties. These data can constitute constraints on reconstructions of past-ocean circulation. Previous work has suggested ocean DSi concentrations were higher than modern ocean concentrations prior to the Cenozoic and has posited a drawdown during the Early Palaeogene due to the evolutionary expansion of diatoms. Our results challenge such an interpretation. We suggest here that if such a global decrease in oceanic DSi concentrations occurred, it must predate 60 Ma.
