Mixing Event Research Articles

Influence of episodic wind events on thermal stratification and bottom water hypoxia in a Great Lakes estuary

Hypoxia formation and breakdown were tracked during 2015 in Muskegon Lake estuary at multiple locations, and five years (2011–2015) of time-series buoy observatory data were evaluated for the effect of episodic wind-events on lake mixing. Bi-weekly water temperature and dissolved oxygen (DO) profiles at four locations revealed that hypoxia occurred at all sites and persisted for 2–3 months during summer 2015. On one date in late-summer, up to 24% of the lake’s volume was estimated to be mildly hypoxic (DO < 4 mg L−1) as defined by lake sturgeon requirements. Patterns of wind speed and water column stability in late spring indicated that high winds and low stability delayed the onset of hypoxia while in late summer low winds and high stability delayed degradation of hypoxia. Wind speeds appear to play a great role in the interannual variability of stratification and subsequent hypoxia. Water temperature and DO profiles taken before and after one mid-summer mixing event (wind speed >7.7 m s−1 for 10 h), indicated that while the wind was unable to completely mix the entire water column, it deepened the epilimnion by ∼1.5 m and sheared a thin layer from the upper hypolimnion. By entraining internally loaded nutrients, such episodic wind-events may initiate and sustain algal blooms in nutrient limited surface waters. Quantifying the variable role of wind and mixing events will be key to integrating limnological processes into climate models of the future.

Storm-Induced Predator-Prey Decoupling Promotes Springtime Accumulation of North Atlantic Phytoplankton

During NAAMES May 2016 field campaign, we examined the response of plankton population dynamics to rapid re-stratification of surface waters following a deep mixing event in the subpolar North Atlantic. Over the 4-day occupation of a station, we measured phytoplankton growth and grazer/virus-induced mortality rates in dilution assays conducted across a range of light intensities. Rates were estimated from changes in chlorophyll a and the abundance of three phytoplankton groups identified by flow cytometry. Initially, biological and physical water column properties were homogeneous down to >200 m, followed by rapid shoaling of the mixed layer to 20-30 m. Initial in situ chlorophyll a concentration was 0.4 µg L-1, and phytoplankton biomass accumulated at an average rate of 0.4 d-1 over the next two days. When mixed layer depth was maximal, there were no mortality losses and phytoplankton growth rates increased with increased light intensity. After shoaling, grazing rates increased, but never matched the magnitude of phytoplankton growth rates. When the mixed layer was shallowest, growth rates exceeded >1 d-1 at all non-dark light intensities. Chlorophyll a based grazing rates were consistent across light levels (~0.3 d-1) and were highest on Synechococcus (0.3-0.6 d-1) and lowest on pico-eukaryotes (~0.2 d-1). The delay with which grazing resumed resulted in growth exceeding losses and consumption of an average of 30% of primary production. Virus-induced mortality rates were minimal across all mixing profiles and light intensities. Overall, both chlorophyll a and group-specific phytoplankton accumulation rates predicted from the shipboard experiments matched those in situ, suggesting that incubation conditions faithfully captured the growth and loss processes governing in situ population dynamics. The observations made here indicate that transient deepening of the mixed layer followed by rapid stratification provided conditions under which phytoplankton escape protistan grazing and accumulate biomass. During the physically dynamic springtime in the North Atlantic, frequent ephemeral fluctuations in the depth of mixing may represent an important mechanism governing the formation and the magnitude of the North Atlantic spring bloom.

Formation of rapakivi feldspar in the Deer Isle Granite Complex, coastal Maine: in situ lead isotope and trace-element analysis

Rapakivi and alkali feldspar phenocrysts from the Devonian Deer Isle Granite Complex, Maine were investigated to see if magma mixing or isothermal decompression was responsible for their formation. Pb-isotope and trace-element analyses of alkali feldspar ovoids and their plagioclase inclusions were obtained by LA-ICP-MS to determine the characteristics of the initial magma; these were compared with the plagioclase mantles. Rapakivi grains show a decrease in Pb-isotopic values and an increase in Ba, La, and Ce concentrations from the ovoids and inclusions to the innermost plagioclase mantles adjacent to the ovoids. These variations, along with CL imagery of quartz phenocrysts, indicate an open system with compositional changes in the magma chamber occurring during rapakivi feldspar growth. Repeated episodes of magma mixing/replenishment by Ba and LREE enriched magmas with lower Pb-isotopic ratios (along with hybridized variations with the host magma) created localized dissolution of alkali feldspar and quartz. Alkali feldspar phenocrysts proximal to these zones of thermal perturbation were first resorbed and then mantled by plagioclase that records the more primitive signature of the more mafic melt. Pb-isotopic values and Ba, La, and Ce concentrations trend back to the initial magma composition at the mantle rims as the effects of the mixing event dissipated. For grains that were somewhat more distal to these zones of mixing, resorption of the alkali feldspar crystals did not occur, yet the fresh supply of Ba resulted in Ba-rich alkali feldspar mantles over Ba-poor alkali feldspar cores. Other alkali feldspar crystals were too far from the site of mixing; hence, they lack any evidence of open system crystallization. As crystallization of the chamber continued along solidification fronts, batches of cooler crystal-rich magmas settled en masse to the floor. Disaggregation of these batches during settling, and subsequent accumulation on the reservoir floor, brought grains with disparate crystallization histories together. Filter pressing of the cumulate pile flushed highly evolved melts/fluids out from interstitial pores to the high silica, upper portions of the chamber. The crystallization, transportation, and juxtaposition of disparate types of feldspar phenocrysts suggest that a relatively large and active magma chamber, periodically recharged by localized batches of melt, must have existed. In this case, it is clear that rapakivi mantled feldspars are the result of magma mixing; such interpretations may apply to many other occurrences as well.

Atmospheric Control of Deep Chlorophyll Maximum Development

The evolution of the near-surface phytoplankton bloom towards a Deep Chlorophyll Maximum (DCM) in mid-latitudes and subpolar regions of the global ocean is a well-known biological feature. However, our knowledge about the exact mechanism that determines the end of the bloom and its irreversible evolution towards a DCM is still limited. In this work, combining satellite and in-situ oceanographic data together with reanalysis data, we investigate why and when this transition between the near-surface phytoplankton bloom and the development of a DCM occurs. For this aim, we investigate the links between changes in air-sea heat exchanges, the near-surface signature of phytoplankton bloom, and the water column vertical structure by calculating the mixed layer depth (MLD) and depth of the DCM on hydrographic and chlorophyll profiles. We find that the occurrence of the last convective mixing event (heat loss by the ocean surface) at the end of the spring which is able to reach the base of the MLD and inject new nutrients into the mixed layer marks the end of the near-surface bloom and its transition towards a DCM. Identified in this way, the spring bloom duration and the start of the transition towards a DCM can be systematically and objectively determined, providing sensitive indexes of climate and ecosystem variability.

Worldwide alteration of lake mixing regimes in response to climate change

Lakes hold much of Earth’s accessible liquid freshwater, support biodiversity and provide key ecosystem services to people around the world. However, they are vulnerable to climate change, for example through shorter durations of ice cover, or through rising lake surface temperatures. Here we use a one-dimensional numerical lake model to assess climate change impacts on mixing regimes in 635 lakes worldwide. We run the lake model with input data from four state-of-the-art model projections of twenty-first-century climate under two emissions scenarios. Under the scenario with higher emissions (Representative Concentration Pathway 6.0), many lakes are projected to have reduced ice cover; about one-quarter of seasonally ice-covered lakes are projected to be permanently ice-free by 2080–2100. Surface waters are projected to warm, with a median warming across lakes of about 2.5 °C, and the most extreme warming about 5.5 °C. Our simulations suggest that around 100 of the studied lakes are projected to undergo changes in their mixing regimes. About one-quarter of these 100 lakes are currently classified as monomictic—undergoing one mixing event in most years— and will become permanently stratified systems. About one-sixth of these are currently dimictic—mixing twice per year—and will become monomictic. We conclude that many lakes will mix less frequently in response to climate change. Many lakes that currently mix once or twice a year may become permanently stratified or mix only once in a warming climate, suggest numerical simulations of lake mixing regimes. Mixing regimes are most affected by ice-cover duration and surface temperatures.

On Energy and Turbulent Mixing in the Thermocline

AbstractA new method for computing the rate at which turbulent mixing builds potential energy in the ocean is described. The traditional approach has focused on the rate of change of the background potential energy associated with an adiabatically leveled state. We argue that when examining mixing events, so‐called “Thorpe” sorting yields a useful and local measure of diabatically generated potential energy and exhibits some advantages relative to adiabatic leveling. Among these, the open question about the leveling domain is avoided, the fate of kinetic energy during a mixing event is clearly defined, and the computational load associated with the leveling is relieved. The resultant kinetic energy equation leads to a natural definition of mixing efficiency and turbulent diffusivity in terms of sign definite viscous and diffusive contributions. Applications to 2‐D Kelvin Helmholtz instability demonstrate the utility of the procedure. We find an integrated efficiency of ≈ 0.15 for a Prandtl number of 1, and of ≈ 0.08 for a Prandtl number of 10. The larger is comparable to the classical value of 0.2 used frequently by the mixing community and smaller than that found in some recent simulations.

Whole-Ecosystem Experiments Reveal Varying Responses of Phytoplankton Functional Groups to Epilimnetic Mixing in a Eutrophic Reservoir

Water column mixing can influence community composition of pelagic phytoplankton in lakes and reservoirs. Previous studies suggest that low mixing favors cyanobacteria, while increased mixing favors green algae and diatoms. However, this shift in community dominance is not consistently achieved when epilimnetic mixers are activated at the whole-ecosystem scale, possibly because phytoplankton community responses are mediated by mixing effects on other ecosystem processes. We conducted two epilimnetic mixing experiments in a small drinking water reservoir using a bubble-plume diffuser system. We measured physical, chemical, and biological variables before, during, and after mixing and compared the results to an unmixed reference reservoir. We observed significant increases in the biomass of cyanobacteria (from 0.8 ± 0.2 to 2.4 ± 1.1 μg L−1, p = 0.008), cryptophytes (from 0.7 ± 0.1 to 1.9 ± 0.6 μg L−1, p = 0.003), and green algae (from 3.8 to 4.4 μg L−1, p = 0.15) after our first mixing event, likely due to increased total phosphorus from entrainment of upstream sediments. After the second mixing event, phytoplankton biomass did not change but phytoplankton community composition shifted from taxa with filamentous morphology to smaller, rounder taxa. Our results suggest that whole-ecosystem dynamics and phytoplankton morphological traits should be considered when predicting phytoplankton community responses to epilimnetic mixing.

Effect of familiarity and mixing method on gestating sow welfare and productivity in large dynamic groups.

The objectives of this study were twofold: first, to investigate the effects of sow familiarity prior to mixing into a larger dynamic group of gestating sows and, second, to examine the impact of how the sows entered the pen during this mixing event. The cohort of sows was either familiar with each other because of premixing (PMIX) or unfamiliar (UMIX). This PMIX or containment of sows in a stall (UMIX), occurred from weaning until the sows were mixed into the large gestation pen 8 d later. The cohort of incoming sows was introduced either as a batched unit (BAT) or new sows were introduced into the dynamic group singly (IND) though the electronic sow feeder. Lesion severity and quantity score, lameness, and body condition score (BCS) were tracked throughout the entire gestation period for 213 sows. Overall, there was little effect of the treatment but a strong impact of parity on the outcomes. Younger animals had significantly higher risk for lesions (P < 0.001) and higher risk for more severe lesions (P < 0.001) than higher parity animals. Lower parity had an association with the risk of lameness (P < 0.05), but it had no significant effect on BCS (P > 0.05). The risk of lameness increased on days 15 and 62 compared to weaning (P < 0.001). Risk of low BCS decreased on days 62 and 113 relative to scores at weaning (P < 0.001). A degree of familiarity by day interaction was present for lesion quantity (P < 0.001) and lesion severity (P < 0.001). The risk of more lesions was higher in the premixed groups before going into the dynamic group, but equilibrated with the unmixed group after day 11, 3 d in the large dynamic pen. The highest risk for the greatest quantity of lesions peaked at day 11 then declined, but never reached the lowest level again which was measured at weaning. Despite the variability in the welfare measures, there was no significant impact of treatment or parity on sow productivity. In conclusion, our data demonstrate that a period of premixing sows and varying the method of entering sows into the pen did not have a long-term impact on the welfare of the sows or on their productivity.

Physical and chemical impacts of a major storm on a temperate lake: a taste of things to come?

Extreme weather can have a substantial influence on lakes and is expected to become more frequent with climate change. We explored the influence of one particular extreme event, Storm Ophelia, on the physical and chemical environment of England’s largest lake, Windermere. We found that the substantial influence of Ophelia on meteorological conditions at Windermere, in particular wind speed, resulted in a 25-fold increase (relative to the study-period average) in the wind energy flux at the lake-air interface. Following Ophelia, there was a short-lived mixing event in which the Schmidt stability decreased by over 100 Jm−2 and the thermocline deepened by over 10 m during a 12-h period. As a result of changes to the strength of stratification, Ophelia also changed the internal seiche regime of Windermere with the dominant seiche period increasing from ~ 17 h pre-storm to ~ 21 h post-storm. Following Ophelia, there was an upwelling of cold and low-oxygenated waters at the southern-end of the lake. This had a substantial influence on the main outflow of Windermere, the River Leven, where dissolved oxygen concentrations decreased by ~ 48%, from 9.3 to 4.8 mg L−1, while at the mid-lake monitoring station in Windermere, it decreased by only ~ 3%. This study illustrates that the response of a lake to extreme weather can cause important effects downstream, the influence of which may not be evident at the lake surface. To understand the impact of future extreme events fully, the whole lake and downstream-river system need to be studied together.

Campur Kode Bahasa Indonesia Dan Bahasa Kutai Dialeg Tenggarong Kota Di Kantor Desa Separi Kabupaten Kutai Kartanegara

This research was conducted to develop the Kutai language in the Tenggarong Kota dialect and at the same time as a contribution of knowledge related to the mixing of the codes for the Kutai language in the Tenggarong Dialect. Thus, local languages ​​can continue to be preserved so that speakers of regional languages ​​can use regional languages ​​according to their place and situation. The research objective was to determine the code mixing event that occurred in the Separi Village Office, Kutai Kartanegara Regency and to describe the insertion of language elements that was more dominant in the code mixing event. The method used in this qualitative research is observation, the observation method with basic techniques, namely tapping techniques and note taking techniques. The results showed that code mixing occurred in the Separi Village Office, Kutai Kartanegara Regency, between Indonesian and Kutai languages. The code mixing that occurs is indicated by the insertion of linguistic elements in the form of words, phrases and clauses. The most dominant insertion of linguistic elements in the code mixing process at the Separi Village Office, Kutai Kartanegara Regency, is the insertion of linguistic elements in the form of phrases.

Increased CO2 exacerbates the stress of ultraviolet radiation on photosystem II function in the diatom Thalassiosira weissflogii

Diatoms usually dominate phytoplankton community in coastal waters and experience rapid changes of underwater light. However, little is known regarding how increased CO2 would affect diatoms’ capacity in dealing with changing photosynthetically active radiation (PAR) and ultraviolet radiation (UVR). Here, we cultured a globally abundant diatom Thalassiosira weissflogii under two levels of CO2 (400, 1000 ppmv), and then analysed its PSII function during an increase in PAR and UVR to mimic an upward mixing event. UVR noticeably reduced photosystem II (PSII) activity (FV/FM) during the high light exposure, which was more significant for cells grown at the higher CO2 condition. The PsbA removal rate (KPsbA) was synergistically increased by high CO2 and UVR, while the PsbD removal rate (KPsbD) was decreased under higher CO2. Both CO2 and UVR had an inducible effect on sustained phase of nonphotochemical quenching (NPQs). The higher CO2 decreased the ratio of Rubisco large subunit (RbcL) to PsbA regardless of the radiation treatments. It seems that the increased NPQs and turnover of PsbA induced by higher CO2 were not enough to offset the stressful effect it brought about, particularly when higher CO2 was combined with UVR. These findings indicate that increased CO2 may exacerbate the harmful effect of UVR on PSII function in the T. weissflogii through reducing PsbD removal rate and the ratio of RbcL to PsbA during UVR exposure, and thus would affect its abundance and distribution in future ocean environment.

Growth of, and diffusion in, olivine in ultra-fast ascending basalt magmas from Shiveluch volcano

Complex core-rim zoning of Mg-Fe-Ni-Ca-Cr-Al-P in high-Mg olivine crystals from a tuff ring of Shiveluch volcano, Kamchatka, enables reconstruction of the entire olivine crystallization history from mantle conditions to eruption. Bell-shaped Fo86–92 and Ni profiles in crystal cores were formed by diffusion after mixing with evolved magma. Diffusion proceeded to the centres of crystals and completely equilibrated Fo and Ni in some crystals. Diffusion times extracted from Fo and Ni core profiles range from 100 to 2000 days. During subsequent mixing with mafic mantle-equilibrated melt, the cores were partially dissolved and overgrown by Fo90 olivine. Times extracted from Fo and Ni diffusion profiles across the resorption interface between the core and its overgrowth range within 1–10 days, which corresponds to the time of magma ascent to the surface. The overgrowth shows identical smooth Fo-Ni decreasing zoning patterns for all crystals towards the margin, indicating that all crystals shared the same growth history after last mixing event prior to eruption. At the same time, Ca, and to an even greater extent Cr, Al, and P have oscillatory growth patterns in the crystals overgrowth. Our data show that magma ascent can be extremely short during maar/tuff ring eruption.

Magma plumbing and hybrid magma formation at an active back-arc basin volcano: North Su, eastern Manus basin

North Su is the central volcanic edifice of the SuSu Knolls hydrothermal district, located in an extensional transform zone of the eastern Manus back-arc basin. The submarine volcano hosts a vigorous hydrothermal system with black and white smokers, diffuse fluid discharge, and accumulation of native sulfur. Among its dacitic to andesitic lavas are remarkable hybrid rocks, containing highly magnesian olivine (Fo81–94) phenocrysts with thin orthopyroxene reaction rims in a dacitic matrix. We investigated these rocks in order to understand their formation, to constrain temperature and depth of magma storage, and to identify links between magmatic processes and hydrothermal activity. The compositions, zonations and reaction rims of phenocrysts suggest that the hybrid lavas formed by mixing when dacitic to rhyodacitic magma was invaded by Mg-rich andesite to basaltic andesite with boninite affinities. The resulting melts show strong enrichment of large ion lithophile elements, moderate enrichment of light rare earth elements and depletion of Nb, Ta and Ti relative to N-MORB, characterizing them as back-arc basin magmas. Compositional zonations at the outermost rims of olivine phenocrysts suggest a period of no more than a few days between the inferred mixing event and eruption. Mineral-melt thermometers indicate a temperature of ∼1010 °C for the hybrid magma. Mineral-melt barometers give a wide range of pressures with a mean of 19 ± 217 MPa, reflecting equilibration at shallow depths as well as limited barometer performance at low pressures. In addition, many clinopyroxene phenocrysts lack equilibrium between their strongly zoned rims and the host melt. The H2O (0.9–5.3 wt%) and CO2 (70–4730 ppm) contents of melt inclusions in clinopyroxene and plagioclase phenocrysts provide better constraints on the pressure range for pre-eruptive magma storage. The data suggest that dacite phenocryst crystallization and magma mixing occurred at 40–140 MPa pressure, about 1–5 km beneath the summit of North Su. The sulfur–H2O relationship in melt inclusions reflect a degassing trend that is consistent with formation of an aqueous-sulfurous magmatic fluid, inferred to produce highly acidic vent fluids at North Su (Seewald et al., 2015). The melt inclusion data also indicate significant fractionation between F, Cl and H2O after magma degassing, possibly reflecting formation of a high-salinity brine at depth and fluorine fixation in minerals formed in the subsurface. Our results suggest that the dynamic magmatic system, with magma mixing occurring shortly before ascent and eruption, causes rapid changes in composition and quantity of exsolved fluids. Due to the short distance to the seafloor, such fluid fluctuations are rapidly transferred to the hydrothermal systems. This may be the main cause for the observed temporal, spatial and compositional variations in hydrothermal activity at North Su, and possibly at other vent fields of the Manus basin.

Photoacclimation Responses in Subarctic Atlantic Phytoplankton Following a Natural Mixing-Restratification Event

Strong physical disturbances in the surface ocean vertically redistribute plankton and lead to new relationships between organisms and their growth environment. Entrainment of phytoplankton to deeper depths decreases light exposure while stratification exposes cells in the surface mixing layer to higher light. Phytoplankton can rapidly respond to alterations in growth irradiance using photoacclimation processes such as nonphotochemical quenching and alterations in pigments and other macromolecules. While it is common to observe photophysiological responses to shifts in growth irradiance in the laboratory it is rare that a study is able to document these processes in the field in a lagrangian framework. We were able to follow a natural phytoplankton community for four days and observe rapid photophysiological responses and net accumulation patterns of biomass down to 200 m following a deep mixing and restratification event in the subarctic Atlantic ocean. Applying flow cytometry and fast rate repetition fluorometry (FRRf) we report on the photoacclimation responses of multiple taxonomic groups in relation to the rapidly evolving physical environment. Significant stratification occurred following the deep entrainment event, isolating cells within an actively mixing and shoaling surface layer, the lower euphotic zone, and below the photic zone. Distinct differences in patterns of accumulation and photoacclimation are apparent between these depths of disparate light histories. Photoacclimation patterns are generally consistent between Synechococcus, picoeukaryote, and nanoeukaryote groups yet diel patterns and variability in the range of photoacclimation suggest divergent physiology. Results from continuous FRRf measurements collected on bulk seawater show a decrease in the number photosystem II reaction centers and a concurrent increase in the functional cross section with implications for maintaining photosynthetic rates throughout the mixed layer. Understanding group specific differences as well as bulk community acclimation strategies will be important for informing models relying on photoacclimation parameters to determine growth and accumulation rates and net primary production. More so, the rates at which cells acclimate within the context of the frequency and spatial dynamics of storm events, such as the one recorded here, directly impact interpretations and applications of remote retrievals of phytoplankton chlorophyll, carbon, and their ratio in the context of unbalanced growth.

Automatic water mixing event identification in the Koljö fjord observatory data

This study addresses the task of automatically identifying water mixing events in the multivariate time series of salinity, temperature and dissolved oxygen provided by the Koljo fjord observatory. The observatory is used to test new underwater sensory technology and to monitor water quality with respect to hypoxia and oxygenation in the fjord and has been collecting data since April 2011. The fjord water properties change, manifesting as peaks or drops of dissolved oxygen, salinity and temperature, when affected by inflows of new water originating from the open sea or by rivers connected to the fjord system. An acute state of oxygen depletion can harm wildlife and the ecosystem permanently. The major challenge for the analysis is that the water property changes are marked by highly varying peak strength and correlation between the signals. The proposed data-driven analysis method extends existing univariate outlier detection approaches, based on clustering techniques, to identify the water mixing events. It incorporates three major steps: 1. smoothing of the input data, to counter noise, 2. individual outlier detection within the separate variables, 3. clustering of the results using the DBSCAN clustering algorithm to determine the anomalous events. The proposed approach is able to detect the water mixing events with a $$F{\textit{1}}$$ -measure of 0.885, a precision of 0.931—that is 93.1% of all events have been correctly detected—and a recall of 0.843–84.3% of events that should have been found actually also have been. Using the proposed method, the oceanographers can be informed automatically about the status of the fjord without manual interaction or physical presence at the experiment site.

Evidence for Subsoil Specialization in Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal (AM) fungal communities are now known to vary with depth in arable land. Here we use two previously published high-throughput Illumina sequencing data sets, and compare a 52 year long chronosequence of recultivated agriculture fields after a topsoil and subsoil mixing event, with a set of undisturbed topsoil and subsoil samples from a similar field. We show that AM fungal taxa identified as subsoil indicators are exclusively present in early stages of the chronosequence, whereas topsoil indicator taxa can be found across the chronosequence. Also, community-level similarities from the chronosequence fields to the subsoil decrease with time. Our results provide evidence of ecological specialization of certain AM fungal taxa to deep soil layers.

Sulfur isotope signatures of eucrites and diogenites

High precision sulfur isotope data on eight eucrites and two diogenites are reported to explore the provenance of sulfur and processes that affected the sulfur isotope composition and sulfur abundance within the HED (Howardite, Eucrite, Diogenite) parent body (Asteroid 4 Vesta) or parent bodies for eucrite-like meteorites. Sulfur isotope values for a subset of six of the eucrites (EET 90020, PCA 82502, BTN 00300, MIL 11290, QUE 97014, and Sioux County) and two diogenites (Johnstown and Shalka) converge on a singular sulfur isotope composition, yielding a mean δ34S of 0.26 ± 0.17‰ (2 SD), Δ33S of 0.010 ± 0.007‰ (2 SD), and Δ36S of −0.22 ± 0.21‰ (2 SD). The variances on these mean values can be entirely accounted for by analytical uncertainties. The homogenous positive Δ33S for these samples suggests that sulfur in Asteroid 4 Vesta has a Δ33S of 0.010 ± 0.002‰ (2 SE) which is consistent with the occurrence of a process that allowed for promoted-isotopic-exchange within the parent body to homogenize Δ33S. This process was previously recognized in the homogenous Δ17O composition that has been linked to large-scale silicate melting and mixing. The positive δ34S of this group of samples is attributed to fractionations associated with sulfur loss, and the homogeneity of sulfur isotopes suggests that this loss predates the mixing event. The sulfur isotope data alone do not differentiate whether this loss occurred prior to or post core formation, or even prior to accretion.Two eucrites (GRA 98098 and CMS 04049) yield isotopic compositions that are distinct from this mean for δ34S, and in the case of CMS 04049, for Δ33S as well. GRA 98098 yields δ34S of 0.81 ± 0.18‰ (2 σest, estimated external precision), Δ33S of 0.010 ± 0.008‰ (2 σest) and Δ36S of 0.01 ± 0.34‰ (2 σest). CMS 04049 yields δ34S of −0.22 ± 0.18‰ (2 σest), Δ33S of 0.024 ± 0.008‰ (2 σest), and Δ36S of −0.17 ± 0.34‰ (2 σest). The more positive δ34S of GRA98098 suggests fractionation of sulfur (34S/32S) by a combination of parent body processes that may be linked to evaporation. The more positive Δ33S and the negative δ34S of CMS 04049 suggests that this meteorite may be from another ‘Vesta-like’ parent body.

Stable isotope records across the Cretaceous-Paleogene transition, Stevns Klint, Denmark: New insights from the chromium isotope system

Late Cretaceous to Early Paleogene time represents a critical transitional period in Earth history. Global events include short-term instability in climate and ocean circulation, large igneous province emplacement, and catastrophic extinction of marine and terrestrial fauna due at least in part to a bolide impact event. The response of the ocean system to Cretaceous-Paleogene (K-Pg) global events has been the subject of much research, yet fundamental questions remain regarding carbon cycling, climate, ocean mixing, and redox conditions. To help elucidate paleoceanographic changes across the K-Pg transition, we revisit the classic Maastrichtian to Danian chalk-limestone succession at Stevns Klint, Denmark with a suite of new geochemical data, including insights from the redox-sensitive chromium isotope system. We use carbon and oxygen isotope trends to form a well-constrained framework for the succession, before presenting strontium isotope, chromium isotope, and rare-earth element data for the units deposited both before and after the K-Pg event. Carbon isotopes show only minor perturbation across the K-Pg boundary and oxygen isotopes record a prominent positive excursion, with both of these trends standing in contrast to many K-Pg boundary sections worldwide. Strontium isotopes record a spike towards more radiogenic values, which is consistent with a globally-observed, short-term weathering pulse. The chromium isotope profile of Maastrichtian chalk begins with strongly positive values followed by a sharp negative excursion associated with a facies shift and initiation of bottom currents in the Chalk Sea. δ53Cr values then recover, before falling steadily across the K-Pg boundary and into the Danian limestone. Chromium isotope trends do not correspond to changes in hydrothermal input or basin redox conditions—instead, we propose that mixing of water masses with distinct chromium isotope signatures controlled temporal chromium isotope variability. The local mixing event inferred from our Maastrichtian chromium isotope data could be related to a temporary inflow of Tethyan bottom waters, or the shift from end-Maastrichtian greenhouse warming (associated with Deccan volcanism) to the worldwide cooling phase that immediately preceded the K-Pg boundary. No immediate change in chromium isotopes is recorded across the K-Pg boundary, indicating that the bolide impactor did not substantially perturb the marine chromium cycle, at least at the scale resolvable by our data. Instead, chromium isotope trends continued to be controlled by local oceanographic processes, highlighting the potential of the chromium isotope system to record isotopic heterogeneity and mixing events in ancient oceans.

The physiological cost of diazotrophy for Trichodesmium erythraeum IMS101.

Trichodesmium plays a significant role in the oligotrophic oceans, fixing nitrogen in an area corresponding to half of the Earth’s surface, representing up to 50% of new production in some oligotrophic tropical and subtropical oceans. Whilst Trichodesmium blooms at the surface exhibit a strong dependence on diazotrophy, colonies at depth or at the surface after a mixing event could be utilising additional N-sources. We conducted experiments to establish how acclimation to varying N-sources affects the growth, elemental composition, light absorption coefficient, N2 fixation, PSII electron transport rate and the relationship between net and gross photosynthetic O2 exchange in T. erythraeum IMS101. To do this, cultures were acclimated to growth medium containing NH4+ and NO3- (replete concentrations) or N2 only (diazotrophic control). The light dependencies of O2 evolution and O2 uptake were measured using membrane inlet mass spectrometry (MIMS), while PSII electron transport rates were measured from fluorescence light curves (FLCs). We found that at a saturating light intensity, Trichodesmium growth was ~ 10% and 13% lower when grown on N2 than with NH4+ and NO3-, respectively. Oxygen uptake increased linearly with net photosynthesis across all light intensities ranging from darkness to 1100 μmol photons m-2 s-1. The maximum rates and initial slopes of light response curves for C-specific gross and net photosynthesis and the slope of the relationship between gross and net photosynthesis increased significantly under non-diazotrophic conditions. We attribute these observations to a reduced expenditure of reductant and ATP for nitrogenase activity under non-diazotrophic conditions which allows NADPH and ATP to be re-directed to CO2 fixation and/or biosynthesis. The energy and reductant conserved through utilising additional N-sources could enhance Trichodesmium’s productivity and growth and have major implications for its role in ocean C and N cycles.

Mathematical Modelling as a Tool for the Assessment of Impact of Thermodynamics on the Algal Growth in Dam Reservoirs – Case Study of the Goczalkowice Reservoir

Abstract Depending on climate conditions, bathymetry, basin morphology, management practices and other factors, some reservoirs are more or less prone to stratification, which can strongly affect both chemical and biological status of water. In the Goczalkowice Reservoir (the biggest dam reservoir in the south of Poland), water quality parameters, such as chlorophyll a concentration, change very dynamically. In the presented study, the Estuary, Lake and Coastal Ocean Model (ELCOM) was applied to simulate water thermodynamics in a period of summer and autumn when the highest concentrations of chlorophyll a were observed, based on the continuous water monitoring in the ZiZOZap project (Łaszczyca et al., 2011). The purpose of the application of ELCOM was to simulate the thermal stratification in the reservoir and to prepare a basis for analyses of the relationship between water thermodynamics and quality. Simulations allowed the identification of several short water mixing events in summer and the final mixing event at the end of summer. The study also included the first application of model results to analyse the relation of changes in water temperature with observed concentration of chlorophyll a (ChlA). Analysis confirmed that each water mixing event was followed by a significant increase in ChlA concentration.