Stable isotope composition of mineral proxies as a record of fluid evolution in fractured bedrock, the Olkiluoto site, Finland

Fluid evolution in fractured bedrock at the Olkiluoto site in Finland was investigated by examining the stable oxygen and carbon isotope composition of secondary calcite and sulfur isotope composition of secondary sulfide minerals. The study site, located on the western coast of Finland, is the planned site for the geological disposal of spent nuclear fuel. Therefore, hydrogeochemical conditions in the deep bedrock are of special interest. The aim of this study was to characterize the latest fracture fillings and use the isotope composition of these fillings as a proxy of past geochemical conditions. In addition to traditional stable isotope analyses, sulfur and carbon isotope variations in the samples were analysed in situ using secondary ion mass spectrometry (SIMS). The results demonstrated a high variability in isotopic composition of mineral grains within a few tens of micrometers. The study material consists of drill core sections hosting fracture infills, obtained from water conducting fractures in hydrologically active zones in the bedrock. Fracture filling calcites were grouped into five groups according to calcite morphology, mineral associations and their relative position in the fracture sequence. The latest fracture fillings were identified from the surfaces of open fractures as clear, platy or euhedral fillings (Group 1) which occasionally contained minor amount of silicate inclusions (Group 2). Massive, older calcite fillings were separated into three groups (Groups 3-5) representing high temperature fluid circulation. Group 3 fillings preceded the late-stage calcite fillings and are interpreted to be associated with a Paleozoic (541-252 Ma) thermal event. Group 5 and Group 4 fillings represent the earliest hydrothermal events in the bedrock, likely related to the rapakivi granite and diabase intrusions of Mesoproterozoic (1600-1000 Ma) age. A characteristic feature of the late-stage fillings (Group 1, 2) is small-scale variations in stable isotope compositions, indicating the influence of microbial processes on groundwater conditions. Furthermore, a comparison with the groundwater data showed that the late-stage fillings are generally not in isotopic equilibrium with the current groundwaters. Based on the results obtained in this work, the following characteristics of the paleogroundwaters representing Group 1-2 mineralizations can be identified: Transitions between geochemically distinct environments can be identified in the shallow bedrock, at depths < 100 m. The upper ca. 30 m were characterized by bacterial sulfate reduction (BSR) and depths < 54 m characterized by methanogenetic activity. A narrow zone existed at the depths of ~34-54 m, where BSR was accompanied by minor methanotrophic activity. After the precipitation of calcite in Groups 1-2, the methanic environment in the depth range of ~60-300 m has been replaced by sulfidic conditions. Changes to this environment were most likely associated with the infiltration events related to deglaciation events and glacioisostatic land movements, which have occurred repeatedly during the Quaternary period (< 2.58 Ma).Tämän tutkimuksen tarkoituksena oli selvittää Suomen länsirannikolla sijaitsevan Olkiluodon saaren kalliopohjavesien geokemiallista kehitystä. Tutkimuskohteelle on rakenteilla ydinjätteiden loppusijoitustilat, joihin on suunniteltu sijoitettavan nyt käytössä olevien ydinvoimalaitosten polttoaineesta muodostuva jäte. Nyt tehty tutkimus antaa arvokasta lisätietoa alueen syvien kalliopohjavesien kehityksestä, jonka perusteella voidaan paremmin ennakoida kallioperän olosuhteiden muutoksia myös tulevaisuudessa. Tutkimuksessa tarkasteltiin muinaisten kalliopohjavesien geokemiallista kehitystä käyttäen hyväksi kallion rakojen pinnoille kiteytyneiden kalsiitin ja sulfidimineraalien stabiilien happi-, hiili- ja rikki-isotooppien suhteita. Stabiilien isotooppien määräsuhteiden vaihtelu mineraaleissa on seurausta niistä olosuhteista, jotka vallitsivat mineraalien kiteytyessä. Isotooppikoostumusten vaihtelun perusteella on mahdollista tunnistaa esimerkiksi pohjaveden liuenneiden ainesten lähteitä ja niissä tapahtuneita muutoksia, sekä erilaisia biogeokemiallisia prosesseja. Tämän lähtökohdan pohjalta tutkimuksessa keskityttiin karakterisoimaan rakopintojen viimeisimmät kiteymät ja käyttäen hyväksi niiden isotooppisuhteiden vaihteluita rakentamaan kuva muinaisesta matalien lämpötilojen pohjavesiympäristöstä. Tutkimuksessa havaittiin rakopintojen viimeisten mineralisaatioiden koostuvan pienistä kalsiitti ja pyriittikiteymistä. Ne ovat osittain kiteytyneet uudelleen aktivoituneisiin rakovyöhykkeisiin, joissa on ennestään ollut massiivisia, hydrotermiseen kiertoon liittyviä raontäytteitä. Tutkimuksessa havaittiin, että myöhäisimmät rakomineralisaatiot eivät ole kiteytyneet nykyisistä kalliopohjavesistä, vaan edustavat aiempaa pohjavesien kiertoa kallioperän raoissa. Myöhäisimpien rakomineralisaatioiden erityispiirre on niiden stabiilien hiili- ja rikki-isotooppikoostumusten voimakas vaihtelu. Tämä vaihtelu on osoitus biogeenisesta toiminnasta, jota on karakterisoinut mikrobien välittämä sulfaatin pelkistys, metaanin tuotto ja pienissä määrin myös metaanin anaerobinen hapetus. Muutos sulfaattia pelkistävästä ympäristöstä metaania tuottavaan on tapahtunut noin 50-60 metrin syvyydellä, mutta merkkejä sulfaatin pelkistyksestä on havaittu vielä 111 metrin syvyydellä. Tulokset osoittavat, että pohjaveden ja sen kuljettamien aineiden kierto kallioperässä on ollut monimutkainen prosessi, joka on pitkälti riippuvainen maan pinnalta kallioperään suotautuvien vesien alkuperästä ja koostumuksesta. Yleisesti voidaan sanoa että mineraaliaineksesta saatu informaation osoittaa biogeenisten prosessien merkityksen kalliopohjavesien olosuhteiden muokkaajina, ja että nämä prosessit ovat aktiivisia erityisesti kallion pintaosiss

Biogenic processes in crystalline bedrock fractures indicated by carbon isotope signatures of secondary calcite

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Estimating intraseasonal intrinsic water-use efficiency from high-resolution tree-ring delta C-13 data in boreal Scots pine forests

Intrinsic water-use efficiency (iWUE), a key index for carbon and water balance, has been widely estimated from tree-ring delta C-13 at annual resolution, but rarely at high-resolution intraseasonal scale. We estimated high-resolution iWUE from laser-ablation delta C-13 analysis of tree-rings (iWUE(iso)) and compared it with iWUE derived from gas exchange (iWUE(gas)) and eddy covariance (iWUE(EC)) data for two Pinus sylvestris forests from 2002 to 2019. By carefully timing iWUE(iso) via modeled tree-ring growth, iWUE(iso) aligned well with iWUE(gas) and iWUE(EC) at intraseasonal scale. However, year-to-year patterns of iWUE(gas), iWUE(iso), and iWUE(EC) were different, possibly due to distinct environmental drivers on iWUE across leaf, tree, and ecosystem scales. We quantified the modification of iWUE(iso) by postphotosynthetic delta C-13 enrichment from leaf sucrose to tree rings and by nonexplicit inclusion of mesophyll and photorespiration terms in photosynthetic discrimination model, which resulted in overestimation of iWUE(iso) by up to 11% and 14%, respectively. We thus extended the application of tree-ring delta C-13 for iWUE estimates to high-resolution intraseasonal scale. The comparison of iWUE(gas), iWUE(iso), and iWUE(EC) provides important insights into physiological acclimation of trees across leaf, tree, and ecosystem scales under climate change and improves the upscaling of ecological models.Peer reviewe

Estimating intra-seasonal photosynthetic discrimination and water use efficiency using delta C-13 of leaf sucrose in Scots pine

Comparison of assimilate delta C-13 values estimated from different leaf carbon pools, Picarro measurements, and modelled data demonstrates the potential for misinterpretation arising from bulk organic matter delta C-13 analysis.Sucrose has a unique role in recording environmental and physiological signals during photosynthesis in its carbon isotope composition (delta C-13) and transport of the signal to tree rings. Yet, instead of sucrose, total organic matter (TOM) or water-soluble carbohydrates (WSC) are typically analysed in studies that follow delta C-13 signals within trees. To study how the choice of organic material may bias the interpretation of delta C-13 records, we used mature field-grown Scots pine (Pinus sylvestris) to compare for the first time delta C-13 of different leaf carbon pools with delta C-13 of assimilates estimated by a chamber-Picarro system (delta C-13(A_Picarro)), and a photosynthetic discrimination model (delta C-13(A_model)). Compared with sucrose, the other tested carbon pools, such as TOM and WSC, poorly recorded the seasonal trends or absolute values of delta C-13(A_Picarro) and delta C-13(A_model). Consequently, in comparison with the other carbon pools, sucrose delta C-13 was superior for reconstructing changes in intrinsic water use efficiency (iWUE), agreeing in both absolute values and intra-seasonal variations with iWUE estimated from gas exchange. Thus, deriving iWUE and environmental signals from delta C-13 of bulk organic matter can lead to misinterpretation. Our findings underscore the advantage of using sucrose delta C-13 to understand plant physiological responses in depth.Peer reviewe

Explicitly accounting for needle sugar pool size crucial for predicting intra-seasonal dynamics of needle carbohydrates delta O-18 and delta C-13

We explore needle sugar isotopic compositions (delta O-18 and delta C-13) in boreal Scots pine (Pinus sylvestris) over two growing seasons. A leaf-level dynamic model driven by environmental conditions and based on current understanding of isotope fractionation processes was built to predict delta O-18 and delta C-13 of two hierarchical needle carbohydrate pools, accounting for the needle sugar pool size and the presence of an invariant pinitol pool. Model results agreed well with observed needle water delta O-18, delta O-18 and delta C-13 of needle water-soluble carbohydrates (sugars + pinitol), and needle sugar delta C-13 (R-2 = 0.95, 0.84, 0.60, 0.73, respectively). Relative humidity (RH) and intercellular to ambient CO2 concentration ratio (C-i/C-a) were the dominant drivers of delta O-18 and delta C-13 variability, respectively. However, the variability of needle sugar delta O-18 and delta C-13 was reduced on diel and intra-seasonal timescales, compared to predictions based on instantaneous RH and C-i/C-a, due to the large needle sugar pool, which caused the signal formation period to vary seasonally from 2 d to more than 5 d. Furthermore, accounting for a temperature-sensitive biochemical O-18-fractionation factor and mesophyll resistance in C-13-discrimination were critical. Interpreting leaf-level isotopic signals requires understanding on time integration caused by mixing in the needle sugar pool.Peer reviewe

Tree organ growth and carbon allocation dynamics impact the magnitude and δ13C signal of stem and soil CO2 fluxes

Incomplete knowledge of carbon (C) allocation dynamics in trees hinders accurate modeling and future predictions of tree growth. We studied C allocation dynamics in a mature Pinus sylvestris L. dominated forest with a novel analytical approach, allowing the first comparison of: (i) magnitude and delta C-13 of shoot, stem and soil CO2 fluxes (A(shoot), R-stem and R-soil), (ii) concentration and delta C-13 of compound-specific and/or bulk non-structural carbohydrates (NSCs) in phloem and roots and (iii) growth of stem and fine roots. Results showed a significant effect of phloem NSC concentrations on tracheid growth, and both variables significantly impacted R-stem. Also, concentrations of root NSCs, especially starch, had a significant effect on fine root growth, although no effect of root NSC concentrations or root growth was detected on R-soil. Time series analysis between delta C-13 of A(shoot) and delta C-13 of R-stem or delta C-13 of R-soil revealed strengthened C allocation to stem or roots under high C demands. Furthermore, we detected a significant correlation between delta C-13 of R-stem and delta C-13 of phloem sucrose and glucose, but not for starch or water-soluble carbohydrates. Our results indicate the need to include C allocation dynamics into tree growth models. We recommend using compound-specific concentration and delta C-13 analysis to reveal C allocation processes that may not be detected by the conventional approach that utilizes bulk organic matter.Peer reviewe

Fast recovery of suppressed Norway spruce trees after selection harvesting on a drained peatland forest site

Continuous cover forestry (CCF) has been promoted as an environmentally sustainable option for drained peatlands. The CCF management has been challenged due to potentially lower tree growth compared to tradi-tional even-aged management, especially with suppressed trees that are released during a selection harvesting under CCF management. Our objective was to quantify the time lag of stem diameter growth response of suppressed Norway spruce trees (Picea abies Karst.) after a selection harvesting compared to that of dominant trees. We also tested if the carbon assimilation of the trees increased immediately after selection harvesting. We used radial increment cores from suppressed Norway spruce trees to estimate the impact of selection harvesting on the diameter growth and intrinsic water use efficiency (iWUE). We measured carbon isotope composition (delta 13C) of wood, to quantify how the reduced competition between trees altered iWUE and its components, the photosynthetic rate (A) and sto-matal conductance (g). The study was conducted in the Lettosuo experimental site on fertile forestry drained peatland area in southern Finland. Approximately 70 % of the initial stand area (18.5 ha) was harvested according to CCF principles by applying selection harvesting, and the rest of the area was divided to intact control area and to clear-cut area. In the study site, by selection harvest, trees were removed from multiple age classes, but especially mature trees individually or in a small groups were taken away to maintain uneven-aged structure of the forest. All the target trees grew in the similar competitive position before selection harvesting. Our results show that there was a delay with the diameter growth of the suppressed trees to selection har-vesting, whereas the most significant growth-enhancing effect occurred three-four years after selection har-vesting. In contrast to the delay in the increment, the photosynthetic rate relative to stomatal conductance increased immediately after selection harvesting, as shown by the instant 2.5 parts per thousand increase in delta 13C to a post-harvest level. Our results show that carbon uptake increased immediately for suppressed Norway spruce trees after selection harvesting, but the harvest did not induce a clear increase in stem diameter growth during the first years after the harvest.Peer reviewe

Sequential extraction of the phosphate and collagen fractions of small bone samples for analysis of multiple isotope systems (δ18OPO4, δ13C, δ15N)

Rationale Stable isotope analyses are used on precious archeological and paleontological materials despite their destructive nature, because the information gained by these methods on, for example, feeding habits, migration and health of individuals cannot otherwise be obtained. We approached this issue by devising a new sequential extraction scheme aimed at producing multiple (O, C, N) isotope proxies from small amounts of sample. Methods The new extraction scheme includes dissolution of the bone in dilute HNO(3)followed by separate treatments of the collagenous and phosphate fractions. The collagen fraction is treated further adopting the methods presented in the literature for collagen extraction, modified to accommodate small sample sizes. The phosphate-containing fraction is purified from organic contaminants by H(2)O(2)and the phosphate is precipitated as Ag(3)PO(4)following methods presented in the literature. The use of HF as demineralization agent is also tested. Results A starting amount ofca2 mg produced enough material for meaurement by isotope ratio mass spectrometry of the collagen C and N isotope compositions and bone phosphate O isotope composition. We show that the isotopic data obtained from the sequential extraction scheme are comparable with the isotopic composition measured following conventional methodologies that are usually based on 100-500 mg sample sizes. Conclusions The new sequential extraction scheme combines the preparation for stable isotope analysis of bone mineral and organic phases, thus minimizing the amounts of sample needed and damage caused on a sample piece. The method may allow analysis of skeletal samples previously excluded from isotope analysis due to material limitations.Peer reviewe

Bone Collagen Measurements

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Tree Ring Measurements of δ13C using Laser Ablation LA-IRMS

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