Measurements of rhenium isotopic composition in low-abundance samples

Rhenium (Re) is a trace element whose redox chemistry makes it an ideal candidate to trace a range of geochemical processes. In particular, fractionation of its isotopes 187Re (62.6% abundance) and 185Re (37.4%) may be used to improve our understanding of redox reactions during weathering, both in the modern day and in geological archives. Published methods for measurement of Re isotopic composition are limited by the requirements of Re mass to reach a desirable precision, making the analysis of many geological materials unfeasible at present. Here we develop new methods which allow us to measure Re isotope ratios (reported as δ187Re) with improved precision: ±0.10‰ (2σ) for a mass of Re of ∼1 ng to ±0.03‰ (2σ) for a mass of Re of >10 ng. This is possible due to the combination of a modified column chemistry procedure and the use of 1013 Ω amplifiers for measurement via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). For river water samples (with Re concentrations typically ∼10−12 g g−1) we design a field-based pre-concentration of Re that can be used with large volumes of filtered water (5–20 L) shortly after sample collection to provide abundant Re for isotope analysis. As a result of these developments we provide new measurements of δ187Re in standards reference materials (δ187Re values range from −0.06 ± 0.07‰ to +0.19 ± 0.05‰) and a seawater standard (δ187Re = +0.10 ± 0.04‰), providing impetus for further exploration of the Re isotope system

Validating the rhenium proxy for rock organic carbon oxidation using weathering profiles

Chemical weathering over geological timescales acts as a source or sink of atmospheric carbon dioxide (CO2), while influencing long-term redox cycling and atmospheric oxygen (O2) at Earth's surface. There is a growing recognition that the oxidative weathering of rock organic carbon (OCpetro) can release more CO2 than is locally drawn down by silicate weathering, and may vary due to changes in erosion and climate. The element rhenium (Re) has emerged as a proxy to track the oxidative weathering of OCpetro, yet uncertainties in its application remain namely that we lack a systematic assessment of the comparative mobility of Re and OCpetro during sedimentary rock weathering. Here we measure Re and OCpetro loss across gradients in rock weathering at 9 global sites, spanning a range of initial OCpetro values from ∼0.2 % to >10 %. We use titanium to account for volume changes during weathering and assess Re and OCpetro loss alongside major elements that reflect silicate (Na, Mg), carbonate (Ca, Mg) and sulfide (S) weathering. Across the dataset, Re loss is correlated with OCpetro loss but not with loss of any other major element. Across the weathering profiles, the average molar ratio of OCpetro to Re loss was 0.84 ± 0.15, with 8 out of 9 sites having a ratio >0.74. At one site (Marcellus Shale), the average ratio was lower at 0.58 ± 0.11. The excess loss of Re matches expectations that, typically, between ∼0 and 20 % of the Re liberated by sedimentary rock weathering derives from silicate or sulfide phases, while some OCpetro may be physically or chemically protected from weathering. Overall, our measurements provide validation for the Re proxy of OCpetro oxidation and allow future work to further improve our knowledge of regional and global-scale rates of this important source of CO2 in the geochemical carbon cycle

Temperature control on CO2 emissions from the weathering of sedimentary rocks

Sedimentary rocks can release carbon dioxide (CO2) during the weathering of rock organic carbon and sulfide minerals. This sedimentary carbon could act as a feedback on Earth’s climate over millennial to geological timescales, yet the environmental controls on the CO2 release from rocks are poorly constrained. Here, we directly measure CO2 flux from weathering of sedimentary rocks over 2.5 years at the Draix-Bléone Critical Zone Observatory, France. Total CO2 fluxes approached values reported for soil respiration, with radiocarbon analysis confirming the CO2 source from rock organic carbon and carbonate. The measured CO2 fluxes varied seasonally, with summer fluxes five times larger than winter fluxes, and were positively correlated with temperature. The CO2 release from rock organic carbon oxidation increased by a factor of 2.2 when temperature increased by 10 °C. This temperature sensitivity is similar to that of degradation of recent-plant-derived organic matter in soils. Our flux measurements identify sedimentary-rock weathering as a positive feedback to warming, which may have operated throughout Earth’s history to force the surface carbon cycle

Evolution of the alpine Critical Zone since the Last Glacial Period using Li isotopes from lake sediments

Comprehending and predicting the way humans affect the Earth's Critical Zone remains a challenge. An understanding of the past changes resulting from human and non-human influences in the dynamics of the Critical Zone is crucial. Here, we use a retrospective approach to address this question based on a new lithium (Li) isotope record from the Late Glacial Period to the present from a pre-Alpine lake sediment sequence (Lake La Thuile, France). Coupled with the lake sediment archive, the investigation of present-day soils in the lake catchment suggests that lake sediments are not necessarily recording the erosoin of topsoil in the catchment. Our findings indicate that soil particles can be sorted during transportation to the lake, with finer particles being preferentially mobilized, highlighting the influence of fine particle transport on the Li isotope signature of soils and lake sediments. Characterized by low Li isotope signatures, changes in weathering signatures in lake sediments can be amplified by the combined effect of soil development and selective transport. In the La Thuile catchment, soil development was limited during the Late Glacial Period, whereas it became a dominant process during the Holocene climatic optimum together with enhanced selective transport of fine particles. Human activities since 3,000–4,000 yr cal BP induced a strong perturbation hindering both soil formation and selective transport by reinforcing erosion rates. After a period of topsoil destruction caused by intense deforestation and agriculture, lake Li isotopes record the evolution of soil profiles associated with changes in agricultural practices

Macroevolution of the plant–hummingbird pollination system

ABSTRACTPlant–hummingbird interactions are considered a classic example of coevolution, a process in which mutually dependent species influence each other's evolution. Plants depend on hummingbirds for pollination, whereas hummingbirds rely on nectar for food. As a step towards understanding coevolution, this review focuses on the macroevolutionary consequences of plant–hummingbird interactions, a relatively underexplored area in the current literature. We synthesize prior studies, illustrating the origins and dynamics of hummingbird pollination across different angiosperm clades previously pollinated by insects (mostly bees), bats, and passerine birds. In some cases, the crown age of hummingbirds pre‐dates the plants they pollinate. In other cases, plant groups transitioned to hummingbird pollination early in the establishment of this bird group in the Americas, with the build‐up of both diversities coinciding temporally, and hence suggesting co‐diversification. Determining what triggers shifts to and away from hummingbird pollination remains a major open challenge. The impact of hummingbirds on plant diversification is complex, with many tropical plant lineages experiencing increased diversification after acquiring flowers that attract hummingbirds, and others experiencing no change or even a decrease in diversification rates. This mixed evidence suggests that other extrinsic or intrinsic factors, such as local climate and isolation, are important covariables driving the diversification of plants adapted to hummingbird pollination. To guide future studies, we discuss the mechanisms and contexts under which hummingbirds, as a clade and as individual species (e.g. traits, foraging behaviour, degree of specialization), could influence plant evolution. We conclude by commenting on how macroevolutionary signals of the mutualism could relate to coevolution, highlighting the unbalanced focus on the plant side of the interaction, and advocating for the use of species‐level interaction data in macroevolutionary studies

Effects of resuscitation with crystalloid fluids on cardiac function in patients with severe sepsis

<p>Abstract</p> <p>Background</p> <p>The use of hypertonic crystalloid solutions, including sodium chloride and bicarbonate, for treating severe sepsis has been much debated in previous investigations. We have investigated the effects of three crystalloid solutions on fluid resuscitation in severe sepsis patients with hypotension.</p> <p>Methods</p> <p>Ninety-four severe sepsis patients with hypotension were randomly assigned to three groups. The patients received the following injections within 15 min at initial treatment: Ns group (n = 32), 5 ml/kg normal saline; Hs group (n = 30), with 5 ml/kg 3.5% sodium chloride; and Sb group (n = 32), 5 ml/kg 5% sodium bicarbonate. Cardiac output (CO), systolic blood pressure, mean arterial pressure (MAP), body temperature, heart rate, respiratory rate and blood gases were measured.</p> <p>Results</p> <p>There were no differences among the three groups in CO, MAP, heart rate or respiratory rate during the 120 min trial or the 8 hour follow-up, and no significant differences in observed mortality rate after 28 days. However, improvement of MAP and CO started earlier in the Sb group than in the Ns and Hs groups. Sodium bicarbonate increased the base excess but did not alter blood pH, lactic acid or [HCO₃]^-values; and neither 3.5% hypertonic saline nor 5% sodium bicarbonate altered the Na⁺, K⁺, Ca²⁺or Cl^-levels.</p> <p>Conclusion</p> <p>All three crystalloid solutions may be used for initial volume loading in severe sepsis, and sodium bicarbonate confers a limited benefit on humans with severe sepsis.</p> <p>Trial registration</p> <p>ISRCTN36748319.</p

Surviving Sepsis Campaign: International guidelines for management of severe sepsis and septic shock: 2008

SCOPUS: ar.jinfo:eu-repo/semantics/publishe