Investigating the 8.2 ka event in northwestern Madagascar: Insight from data–model comparisons

The 8.2 ka event is a well-known cooling event in the Northern Hemisphere, but is poorly understood in Madagascar. Here, we compare paleoclimate data and outputs from paleoclimate simulations to better understand it. Records from Madagascar suggest two distinct sub-events (8.3 ka and 8.2 ka), that seem to correlate with records from northern high latitude. This could indicate causal relationships via changes in the Atlantic Meridional Overturning Circulation (AMOC) with changes in moisture source's δ18O, and changes in the mean position of the Inter-Tropical Convergence Zone (ITCZ), as climate modelling suggests. These two sub-events are also apparent in other terrestrial records, but the climatic signals are different. The prominent 8.2 ka sub-event records a clear antiphase relationship between the northern and southern hemisphere monsoons, whereas such relationship is less evident during the first 8.3 ka sub-event. Data–model comparison have also shown a mismatch between the paleoclimate data and the model outputs, the causes of which are more or less understood and may lie in the proxies, in the model, or in both data and model. Knowing that paleoclimate proxies and climate models produce different sets of variables, further research is needed to improve the data–model comparison approach, so that both paleoclimate data and paleoclimate models will better predict the likely climate status of a region during a specified time in the past with minimal uncertainties

Contrasting genesis and environmental significance of aragonite inferred from minor and trace element variation in speleothems

This study compares minor and trace element variation in two speleothems from two caves of southern Sardinia, Italy. Samples have been analysed by XRD and Laser Ablation-ICP-MS. The first sample (SPD) is a drapery from the Spaghetto Cave (Santadi), hosted in dolostones without sulphide mineralization. SPD consists of a layer of primary calcite between two layers of primary aragonite. The second sample (SDF) is a flowstone from a natural cave (Sesta Sorella) intercepted by a gallery of the mixed sulphide mine of Sa Duchessa (Domusnovas). SDF consists of a layer of primary calcite underlying a layer of primary aragonite. In the calcite layer of SPD, Mg concentration is high just above the underlying aragonite, decreases to a minimum in the middle of the calcite, and increases to a maximum of 5 mol% MgCO3 just below the overlying aragonite. Magnesium is commonly believed to inhibit calcite precipitation, and greater concentration of Mg in calcite is commonly attributed to upstream precipitation of CaCO3 and resultant increase of Mg/Ca ratio in the residual solution (because the partition coefficient for Mg in both calcite and aragonite is < 1). The Mg pattern in SPD suggests that the initial Mg/Ca ratio in concretioning water was sufficiently high to inhibit precipitation of calcite and favor deposition of the lower aragonite layer, but then smaller Mg/Ca ratio allowed calcite to form the middle layer. Finally, Mg/Ca ratio increased so that calcite precipitation ceased and aragonite was again deposited. This depositional sequence suggests that climatic conditions evolved from dry to wetter for the transition aragonite-calcite, and then drier for the transition calcite-aragonite. This hypothesis is supported by inverse correlation of P with Mg in the calcite, with maximum concentration of P in the middle of the calcite layer. In fact, concentration of P is known to decrease in drier periods, when Mg increases. In SDF, the inhibitory effect of Mg on calcite deposition cannot explain the appearance of aragonite, because Mg concentration is small in the calcite layer and even decreases in the upper part, nearest the overlying aragonite. However, the inferences regarding Mg in SPD apply very well to Zn in SDF. Zinc in the calcite layer increases abruptly toward the aragonite layer, reaching its maximum (2 mol% ZnCO3) just below the aragonite. Like Mg, Zn is known to inhibit calcite precipitation, although its role is less clear than Mg. However, the partition coefficient of Zn is > 1, so that an increase of Zn/Ca ratio in calcite can be simply explained by an increase of dissolved Zn in the fresh (not residual) feeding water, due to greater oxidation of sulphides in wetter periods. This hypothesis is supported by positive correlation of Zn with P in the calcite, as well as with Pb, Cu and Cd. Thus, contrary to SPD from a natural cave, the presence of aragonite in SDF from a mine cave seems to be controlled by the Zn/Ca ratio rather than the Mg/Ca ratio, and to reflect wetter conditions rather than drier ones

Stable isotope data as constraints on models for the origin of coralloid and massive speleothems: The interplay of substrate, water supply, degassing, and evaporation

Many speleothems can be assigned to one of two morphological groups: massive speleothems, which consist of compact bulks of material, and coralloids, which are domal to digitate in form. Faster growth on protrusions of the substrate occurs in the typical growth layers of coralloids (where those layers are termed “coralloid accretions”), but it is not observed in the typical layers of massive speleothems, which in contrast tend to smoothen the speleothem surface (and can therefore be defined as “smoothing accretions”). The different growth rates on different areas of the substrate are explainable by various mechanisms of CaCO3 deposition (e.g., differential aerosol deposition, differential CO2 and/or H2O loss fromacapillary filmof solution, deposition in subaqueous environments). To identify the causes of formation of coralloids rather than massive speleothems, this article provides data about δ13C and δ18O at coeval points of both smoothing and coralloid accretions, examining the relationship between isotopic composition and the substratemorphology. In subaerial speleothems, data showenrichment in heavy isotopes both along the direction ofwater flow and toward the protrusions. The first effect is due toH2OevaporationandCO2 degassing during a gravity-driven flowof water (gravity stage) and is observed in smoothing accretions; the second effect is due to evaporation and degassing duringwatermovement by capillary action from recesses to prominences (capillary stage) and is observed in subaerial coralloids. Both effects coexist in smoothing accretions interspersed among coralloid ones (intermediate stage). Thus this study supports the origin of subaerial coralloids from dominantly capillary water and disproves their origin by deposition of aerosol fromthe cave air. On the other hand, subaqueous coralloids seem to form by a differential mass-transfer from a still bulk of water toward different zones of the substrate along diffusion flux vectors of nutrients perpendicular to the iso-depleted surfaces. Finally, this isotopic method has proved useful to investigate the controls on speleothem morphology and to obtain additional insights on the evolution of aqueous solutions inside caves

First continuous shipboard δ18O and δD measurements in sea water by diffusion sampling: cavity ring-down spectrometry

Combined measurements of salinity and the oxygen/hydrogen stable isotope composition of marine waters can characterise processes such as freshwater mixing, evaporation, precipitation and sea-ice formation. However, stable isotope data with high spatial and temporal resolution are necessary for a detailed understanding of mixed water bodies with multiple inputs. So far analysis of δ18O and δD values in water has been a relatively expensive, laboratory-based technique requiring collection of discrete samples. This has greatly limited the scope and scale of field research that can be undertaken using stable isotope analysis. Here, we report the first continuous shipboard measurements of δ18O and δD values in water by diffusion sampling-cavity ring-down spectrometry. Combined with continuous salinity recordings, a data set of nearly 6,000 measurements was made at 30-s intervals during a 3-day voyage through the Great Barrier Reef Lagoon. Our results show that continuous shipboard measurement of δ18O/δD values provides additional discriminatory power for assessing water mass formation processes and histories. Precise identification of river plumes within the Great Barrier Reef Lagoon was only possible because unique δ18O/δD–salinity relationships of individual plumes were measured at high spatial and temporal resolution. The main advantage of this new technique is the ability to collect continuous, real-time isotope data at a small fraction of the cost of traditional isotope analysis of discrete samples. Water δ18O and δD values measured by diffusion sampling-cavity ring-down spectrometry and laboratory-based isotope ratio mass spectrometry have similar accuracy and precision

The potential for the use of agent-based models in ecotoxicology

This chapter introduces ABMs, their construction, and the pros and cons of their use. Although relatively new, agent-basedmodels (ABMs) have great potential for use in ecotoxicological research – their primary advantage being the realistic simulations that can be constructed and particularly their explicit handling of space and time in simulations. Examples are provided of their use in ecotoxicology primarily exemplified by different implementations of the ALMaSS system. These examples presented demonstrate how multiple stressors, landscape structure, details regarding toxicology, animal behavior, and socioeconomic effects can and should be taken into account when constructing simulations for risk assessment. Like ecological systems, in ABMs the behavior at the system level is not simply the mean of the component responses, but the sum of the often nonlinear interactions between components in the system; hence this modeling approach opens the door to implementing and testing much more realistic and holistic ecotoxicological models than are currently used

Testing the annual nature of speleothem banding

Speleothem laminae have been postulated to form annually and this lamina-chronology is widely applied to high-resolution modern and past climate reconstructions. However, this argument has not been directly supported by high resolution dating methods. Here we present contemporary single-lamina 230Th dating techniques with 2σ precision as good as ±0.5 yr on a laminated stalagmite with density couplets from Xianren Cave, China, that covers the last 300 years. We find that the layers do not always deposit annually. Annual bands can be under- or over-counted by several years during different multi-decadal intervals. The irregular formation of missing and false bands in this example indicates that the assumption of annual speleothem laminae in a climate reconstruction should be approached carefully without a robust absolute-dated chronology

Evidence for decoupling of atmospheric CO2 and global climate during the Phanerozoic eon

Atmospheric carbon dioxide concentrations are believed to drive climate changes from glacial to interglacial modes', although geological(1-3) and astronomical(4-6) mechanisms have been invoked as ultimate causes. Additionally, it is unclear(7,8) whether the changes between cold and warm modes should be regarded as a global phenomenon, affecting tropical and high-latitude temperatures alike(9-13), or if they are better described as an expansion and contraction of the latitudinal climate zones, keeping equatorial temperatures approximately constant(14-16). Here we present a reconstruction of tropical sea surface temperatures throughout the phanerozoic eon (the past similar to 550 Myr) from our database(17) of oxygen isotopes in calcite and aragonite shells. The data indicate large oscillations of tropical sea surface temperatures in phase with the cold-warm cycles, thus favouring the idea of climate variability as a global phenomenon. But our data conflict with a temperature reconstruction using an energy balance model that is forced by reconstructed atmospheric carbon dioxide concentrations(18). The results can be reconciled if atmospheric carbon dioxide concentrations were not the principal driver of climate variability on geological timescales for at least one-third of the Phanerozoic eon, or if the reconstructed carbon dioxide concentrations are not reliable