An experiment to assess the effects of diatom dissolution on oxygen isotope ratios

Rationale: Current studies which use the oxygen isotope composition from diatom silica (δ18Odiatom) as a palaeoclimate proxy assume that the δ18Odiatom value reflects the isotopic composition of the water in which the diatom formed. However, diatoms dissolve post mortem, preferentially losing less silicified structures in the water column and during/after burial into sediments. The impact of dissolution on δ18Odiatom values and potential misinterpretation of the palaeoclimate record are evaluated. Methods: Diatom frustules covering a range of ages (6 samples from the Miocene to the Holocene), environments and species were exposed to a weak alkaline solution for 48 days at two temperatures (20 °C and 4 °C), mimicking natural dissolution post mucilage removal. Following treatment, dissolution was assessed using scanning electron microscope images and a qualitative diatom dissolution index. The diatoms were subsequently analysed for their δ18O values using step-wise fluorination and isotope ratio mass spectrometry. Results: Variable levels of diatom dissolution were observed between the six samples; in all cases higher temperatures resulted in more frustule degradation. Dissolution was most evident in younger samples, probably as a result of the more porous nature of the silica. The degree of diatom dissolution does not directly equate to changes in the isotope ratios; the δ18Odiatom value was, however, lower after dissolution, but in only half the samples was this reduction outside the analytical error (2σ analytical error = 0.46‰). Conclusions: We have shown that dissolution can have a small negative impact on δ18Odiatom values, causing reductions of up to 0.59‰ beyond analytical error (0.46‰) at natural environmental temperatures. These findings need to be considered in palaeoenvironmental reconstructions using δ18Odiatom values, especially when interpreting variations in these values of <1‰

Lake Baikal isotope records of Holocene Central Asian precipitation

Climate models currently provide conflicting predictions of future climate change across Central Asia. With concern over the potential for a change in water availability to impact communities and ecosystems across the region, an understanding of historical trends in precipitation is required to aid model development and assess the vulnerability of the region to future changes in the hydroclimate. Here we present a record from Lake Baikal, located in the southern Siberian region of central Asia close to the Mongolian border, which demonstrates a relationship between the oxygen isotope composition of diatom silica (δ18Odiatom) and precipitation to the region over the 20th and 21st Century. From this, we suggest that annual rates of precipitation in recent times are at their lowest for the past 10,000 years and identify significant long-term variations in precipitation throughout the early to late Holocene interval. Based on comparisons to other regional records, these trends are suggested to reflect conditions across the wider Central Asian region around Lake Baikal and highlight the potential for further changes in precipitation with future climate change

Adjunctive rifampicin for Staphylococcus aureus bacteraemia (ARREST): a multicentre, randomised, double-blind, placebo-controlled trial.

BACKGROUND: Staphylococcus aureus bacteraemia is a common cause of severe community-acquired and hospital-acquired infection worldwide. We tested the hypothesis that adjunctive rifampicin would reduce bacteriologically confirmed treatment failure or disease recurrence, or death, by enhancing early S aureus killing, sterilising infected foci and blood faster, and reducing risks of dissemination and metastatic infection. METHODS: In this multicentre, randomised, double-blind, placebo-controlled trial, adults (≥18 years) with S aureus bacteraemia who had received ≤96 h of active antibiotic therapy were recruited from 29 UK hospitals. Patients were randomly assigned (1:1) via a computer-generated sequential randomisation list to receive 2 weeks of adjunctive rifampicin (600 mg or 900 mg per day according to weight, oral or intravenous) versus identical placebo, together with standard antibiotic therapy. Randomisation was stratified by centre. Patients, investigators, and those caring for the patients were masked to group allocation. The primary outcome was time to bacteriologically confirmed treatment failure or disease recurrence, or death (all-cause), from randomisation to 12 weeks, adjudicated by an independent review committee masked to the treatment. Analysis was intention to treat. This trial was registered, number ISRCTN37666216, and is closed to new participants. FINDINGS: Between Dec 10, 2012, and Oct 25, 2016, 758 eligible participants were randomly assigned: 370 to rifampicin and 388 to placebo. 485 (64%) participants had community-acquired S aureus infections, and 132 (17%) had nosocomial S aureus infections. 47 (6%) had meticillin-resistant infections. 301 (40%) participants had an initial deep infection focus. Standard antibiotics were given for 29 (IQR 18-45) days; 619 (82%) participants received flucloxacillin. By week 12, 62 (17%) of participants who received rifampicin versus 71 (18%) who received placebo experienced treatment failure or disease recurrence, or died (absolute risk difference -1·4%, 95% CI -7·0 to 4·3; hazard ratio 0·96, 0·68-1·35, p=0·81). From randomisation to 12 weeks, no evidence of differences in serious (p=0·17) or grade 3-4 (p=0·36) adverse events were observed; however, 63 (17%) participants in the rifampicin group versus 39 (10%) in the placebo group had antibiotic or trial drug-modifying adverse events (p=0·004), and 24 (6%) versus six (2%) had drug interactions (p=0·0005). INTERPRETATION: Adjunctive rifampicin provided no overall benefit over standard antibiotic therapy in adults with S aureus bacteraemia. FUNDING: UK National Institute for Health Research Health Technology Assessment

Orbital forcing of glacial/interglacial variations in chemical weathering and silicon cycling within the upper White Nile basin, East Africa: Stable-isotope and biomarker evidence from Lakes Victoria and Edward

On Quaternary time scales, the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae (notably diatoms, for which Si is an essential nutrient). Large tropical river systems dominate the export of Si from the continents to the oceans. Here, we investigate variations in Si cycling in the upper White Nile basin over the last 15 ka, using sediment cores from Lakes Victoria and Edward. Coupled measurements of stable O and Si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and Si cycling, while the abundances of lipid biomarkers characteristic of terrestrial/emergent higher plants, submerged/floating aquatic macrophytes and freshwater algae document past ecosystem changes. During the late-glacial to mid-Holocene, 15–5.5 ka BP, orbital forcing greatly enhanced monsoon rainfall, forest cover and chemical weathering. Riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower Si-isotope values. Since 5.5 ka BP, increasingly dry climates and more open vegetation, reinforced by the spread of agricultural cropland over the last 3–4 ka, have reduced dissolved silica inputs into the lakes. Centennial-to millennial-scale dry episodes are also evident in the isotopic records and merit further investigation

Isotope offsets in marine diatom [delta]18O over the last 200 ka

Previous work has suggested that a species effect may be present in diatom oxygen isotope ratios (18Odiatom). While predominantly attributed to be a size-related species effect, currently the precise mechanism remains unknown. Here, two size fractions of diatoms (38-75 µm and >100 µm) covering the last 200 ka are analysed for 18O from ODP Site 882 in the North-West Pacific Ocean. Synchronous variations of up to 13 occur in both size fractions. However, large isotope offsets (mean = 2.02) exist between the two fractions with no relationship between their magnitude and the overlying palaeoenvironmental conditions. In contrast to earlier work from the same site, no one size fraction is constantly higher or lower in 18O relative to the other. As such, the dominant mechanism is most likely separate to the size effect previously detected. In addition, no relationship exists between the magnitude of the offsets and the species composition of the two size fractions. The presence of these isotope offsets imposes significant constraints upon the future use of 18Odiatom in palaeoceanographic reconstructions and reiterates the need to extract and analyse only species- and size-specific diatom samples

A combined oxygen and silicon diatom isotope record of Late Quaternary change in Lake El’gygytgyn, North East Siberia

Determining the response of sites within the Arctic Circle to long-term climatic change remains an essential pre-requisite for assessing the susceptibility of these regions to future global warming and Arctic amplification. Here we present diatom δ18O and δ30Si data from Lake El'gygytgyn, Russia, and suggest environmental changes that would have impacted across West Beringia from the last glacial maximum to the modern day. In combination with other records, the results raise the potential for climatic teleconnections to exist between the region and sites in the North Atlantic. The presence of a series of 2-3‰ decreases in δ18Odiatom during both the last glacial and the Holocene indicates the sensitivity of the region to perturbations in the global climate system. Evidence of an unusually long Holocene thermal maximum from 11.4-7.6 ka BP is followed by a cooling trend through the remainder of the Holocene in response to changes in solar insolation. This is culminated over the last 900 years by a significant decrease in δ18Odiatom of 2.3‰, which may be related to a strengthening and easterly shift of the Aleutian Low in addition to possible changes in precipitation seasonality

Diatom oxygen isotopes : evidence of a species effect in the sediment record

Diatom oxygen isotope measurements are commonly made on bulk mixed species assemblages due to the difficulty in purifying and separating individual taxa. As such, it is essential to understand processes in diatoms which may lead to isotope offsets both between and within individual species. Existing studies have suggested that mechanisms which may lead to isotopes offset in diatoms, such as vital effects, are either nonexistent or negligible. Here, we present a suite of diatom oxygen isotope data from the onset of major Northern Hemisphere Glaciation at ODP site 882 in the northwest Pacific Ocean which display large offsets (mean = 1.23‰, max = 3.51‰, error = 0.84‰) between two different size fractions (75–150 μm and >150 μm) that are dominated by only two species: Coscinodiscus marginatus and Coscinodiscus radiatus. These offsets are most likely size related, although additional interspecies and intraspecies effects may also be important in determining the exact magnitude of the offsets. Consequently, considerable care is needed when interpreting bulk diatom oxygen isotope data in relation to paleoenvironmental change, especially when the amount of stratigraphical change within the isotopes is small

Diatom δ18O evidence for the development of the modern halocline system in the subarctic northwest Pacific at the onset of major Northern Hemisphere glaciation

Establishing a time frame for the development of the modern halocline and stratified water column in the subarctic North Pacific has significant paleoclimatic implications. Here we present a δ18O(diatom) record consisting of only two species that represents autumn/winter conditions in the region across the onset of major Northern Hemisphere glaciation boundary. At circa 2.73 Ma a decrease in δ18O(diatom) of 4.6‰ occurs, whereas previously published δ18O(foram) results show a 2.6‰ increase. The δ18O(diatom) and U37 k sea surface reconstructions indicate both a significant freshening of 2–4 practical salinity units and an increase in surface temperatures in the summer to early winter period from circa 2.73 Ma onward. In contrast, the concomitant increase in δ18O(foram) is likely to be reflective of conditions beneath the mesothermal structure and/or spring conditions when warmer sea surface temperatures are not present in the region. These results are consistent with the development of the modern halocline system at 2.73 Ma with year-round stratification of the water column and a strengthened seasonal thermocline during the summer to early winter period, resulting in one of the largest summer to winter temperature gradients in the open ocean. The onset of stratification would also have led to a warm pool of surface water from circa 2.73 Ma, which may have provided a potential source of extra moisture needed to supply the growing North American ice sheets at this time