Late Quaternary highstands at Lake Chilwa,Malawi: Frequency,timing and possible forcing mechanisms in the last 44 ka

The extensive shoreline deposits of Lake Chilwa, southern Malawi, a shallow water body today covering 600 km² of a basin of 7500 km², are investigated for their record of late Quaternary highstands. OSL dating, applied to 36 samples from five sediment cores from the northern and western marginal sand ridges, reveal a highstand record spanning 44 ka. Using two different grouping methods, highstand phases are identified at 43.7–33.3 ka, 26.2–21.0 ka and 17.9–12.0 ka (total error method) or 38.4–35.5 ka, 24.3–22.3 ka, 16.2–15.1 ka and 13.5–12.7 ka (Finite Mixture Model age components) with two further discrete events recorded at 11.01 ± 0.76 ka and 8.52 ± 0.56 ka. Highstands are comparable to the timing of wet phases from other basins in East and southern Africa, demonstrating wet conditions in the region before the LGM, which was dry, and a wet Lateglacial, which commenced earlier in the southern compared to northern hemisphere in East Africa. We find no evidence that wet phases are insolation driven, but analysis of the dataset and GCM modelling experiments suggest that Heinrich events may be associated with enhanced monsoon activity in East Africa in both timing and as a possible causal mechanism.     

The impact of climate change on tribal communities in the US: displacement, relocation, and human rights

Tribal communities in the United States, particularly in coastal areas, are being forced to relocate due to accelerated rates of sea level rise, land erosion, and/or permafrost thaw brought on by climate change. Forced relocation and inadequate governance mechanisms and budgets to address climate change and support adaptation strategies may cause loss of community and culture, health impacts, and economic decline, further exacerbating tribal impoverishment and injustice. Sovereign tribal communities around the US, however, are using creative strategies to counter these losses. Taking a human rights approach, this article looks at communities’ advocacy efforts and strategies in dealing with climate change, displacement, and relocation. Case studies of Coastal Alaska and Louisiana are included to consider how communities are shaping their own relocation efforts in line with their cultural practices and values. The article concludes with recommendations on steps for moving forward toward community-led and government-supported resettlement programs.     

Remelting of refractory inclusions in the chondrule‐forming regions: Evidence from chondrule‐bearing type C calcium‐aluminum‐rich inclusions from Allende

Abstract— We describe the mineralogy, petrology, oxygen, and magnesium isotope compositions of three coarse‐grained, igneous, anorthite‐rich (type C) Ca‐Al‐rich inclusions (CAIs) (ABC, TS26, and 93) that are associated with ferromagnesian chondrule‐like silicate materials from the CV carbonaceous chondrite Allende. The CAIs consist of lath‐shaped anorthite (An₉₉), Cr‐bearing Al‐Ti‐diopside (Al and Ti contents are highly variable), spinel, and highly åkermanitic and Na‐rich melilite (Åk_63–74, 0.4–0.6 wt% Na₂O). TS26 and 93 lack Wark‐Lovering rim layers; ABC is a CAI fragment missing the outermost part. The peripheral portions of TS26 and ABC are enriched in SiO₂ and depleted in TiO₂ and Al₂O₃ compared to their cores and contain relict ferromagnesian chondrule fragments composed of forsteritic olivine (Fa_6–8) and low‐Ca pyroxene/pigeonite (Fs₁Wo_1–9). The relict grains are corroded by Al‐Ti‐diopside of the host CAIs and surrounded by haloes of augite (Fs_0.5Wo_30–42). The outer portion of CAI 93 enriched in spinel is overgrown by coarse‐grained pigeonite (Fs_0.5–2Wo_5–17), augite (Fs_0.5Wo_38–42), and anorthitic plagioclase (An₈₄). Relict olivine and low‐Ca pyroxene/pigeonite in ABC and TS26, and the pigeonite‐augite rim around 93 are ¹⁶O‐poor (Δ17O ～ ?1‰ to ?8‰). Spinel and Al‐Ti‐diopside in cores of CAIs ABC, TS26, and 93 are ¹⁶O‐enriched (Δ17O down to ?20‰), whereas Al‐Ti‐diopside in the outer zones, as well as melilite and anorthite, are ¹⁶O‐depleted to various degrees (Δ17O = ?11‰ to 2‰). In contrast to typical Allende CAIs that have the canonical initial ²⁶Al/²⁷Al ratio of ～5 × 10^?5 ABC, 93, and TS26 are ²⁶Al‐poor with (²⁶Al/²⁷Al)₀ ratios of (4.7 ± 1.4) × 10^?6 (1.5 ± 1.8) × 10^?6 <1.2 × 10^?6 respectively. We conclude that ABC, TS26, and 93 experienced remelting with addition of ferromagnesian chondrule silicates and incomplete oxygen isotopic exchange in an ¹⁶O‐poor gaseous reservoir, probably in the chondrule‐forming region. This melting episode could have reset the ²⁶Al‐²⁶Mg systematics of the host CAIs, suggesting it occurred ～2 Myr after formation of most CAIs. These observations and the common presence of relict CAIs inside chondrules suggest that CAIs predated formation of chondrules.     

Laboratory simulation of salt weathering under moderate ageing conditions: Implications for the deterioration of sandstone heritage in temperate climates

Salt weathering is a significant process affecting the deterioration and conservation of stone-built heritage in many locations and environments. While much research has focused on the impact of salt weathering under arid or coastal conditions with characteristic climatic conditions and salt types, many sites found to be experiencing salt-induced deterioration, such as sandstone rock-hewn cave temples in Gansu Province, China and sandstone buildings in the northern UK, experience high humidities, moderate temperature ranges, and different salt types. To evaluate the impact of salt weathering on sandstone-built heritage under such mild humid environmental conditions, a lab simulation experiment was designed. The experiment was carried out on three types of sandstone (used in the northern UK and Gansu Province, China) and utilized a realistic diurnal humidity and temperature cycle (85% RH/16°C + 60% RH/22°C), and three widespread damaging salts, that is, Na₂SO₄, MgSO₄, and the mixture of Na₂SO₄–MgSO₄. The nature and extent of deterioration was monitored by photography, weight loss, and the changes in petrophysical properties measured using hardness, ultrasonic pulse velocity (P-wave velocity), water absorption coefficient by capillarity, open porosity, and apparent density. All three sandstones were found to be susceptible to MgSO₄ and the mixture of Na₂SO₄–MgSO₄, but weakly affected by Na₂SO₄ under mild humid environmental conditions.     

Preserved‐amplitude angle domain migration by shot‐receiver wavefield continuation

We present preserved‐amplitude downward continuation migration formulas in the aperture angle domain. Our approach is based on shot‐receiver wavefield continuation. Since source and receiver points are close to the image point, a local homogeneous reference velocity can be approximated after redatuming. We analyse this approach in the framework of linearized inversion of Kirchhoff and Born approximations. From our analysis, preserved‐amplitude Kirchhoff and Born inverse formulas can be derived for the 2D case. They involve slant stacks of filtered subsurface offset domain common image gathers followed by the application of the appropriate weighting factors. For the numerical implementation of these formulas, we develop an algorithm based on the true amplitude version of the one‐way paraxial approximation. Finally, we demonstrate the relevance of our approach with a set of applications on synthetic datasets and compare our results with those obtained on the Marmousi model by multi‐arrival ray‐based preserved‐amplitude migration. While results are similar, we observe that our results are less affected by artefacts.     

Coping with climate variability and climate change in La Ceiba,Honduras

La Ceiba, Honduras, a city of about 200,000 people, lies along the Caribbean Sea, nestled against a mountain range and the Rio Cangrejal. The city faces three flooding risks: routine flooding of city streets due to the lack of a stormwater drainage system; occasional major flooding of the Rio Cangrejal, which flows through the city; and flooding from heavy rainfall events and storm surges associated with tropical cyclones. In this study, we applied a method developed for the U.S. Agency for International Development and then worked with stakeholders in La Ceiba to understand climate change risks and evaluate adaptation alternatives. We estimated the impacts of climate change on the current flooding risks and on efforts to mitigate the flooding problems. The climate change scenarios, which addressed sea level rise and flooding, were based on the Intergovernmental Panel on Climate Change estimates of sea level rise (Houghton et al. 2001) and published literature linking changes in temperature to more intense precipitation (Trenberth et al., Bull Am Meteorol Soc, 84:1205–1217, 2003) and hurricanes (Knutson and Tuleya, J Clim, 17:3477–3495, 2004). Using information from Trenberth et al., Bull Am Meteorol Soc, 84:1205–1217, (2003) and Knutson and Tuleya, J Clim, 17:3477–3495, 2004, we scaled intense precipitation and hurricane wind speed based on projected temperature increases. We estimated the volume of precipitation in intense events to increase by 2 to 4% in 2025 and by 6 to 14% by 2050. A 13% increase in intense precipitation, the high scenario for 2050, could increase peak 5-year flood flows by about 60%. Building an enhanced urban drainage system that could cope with the estimated increased flooding would cost one-third more than building a system to handle current climate conditions, but would avoid costlier reconstruction in the future. The flow of the Rio Cangrejal would increase by one-third from more intense hurricanes. The costs of raising levees to protect the population from increased risks from climate change would be about $1 million. The coast west of downtown La Ceiba is the most vulnerable to sea level rise and storm surges. It is relatively undeveloped, but is projected to have rapid development. Setbacks on coastal construction in that area may limit risks. The downtown coastline is also at risk and may need to be protected with groins and sand pumping. Stakeholders in La Ceiba concluded that addressing problems of urban drainage should be a top priority. They emphasized improved management of the Rio Cangrejal watershed and improved storm warnings to cope with risks from extreme precipitation and cyclones. Adoption of risk management principles and effective land use management could also help reduce risks from current climate and climate change.     

Mechanisms controlling the intra-annual mesoscale variability of SST and SPM in the southern North Sea

Thermal and optical remote sensing data were used to investigate the spatial and temporal distribution of sea surface temperature (SST) and of suspended particulate matter (SPM) in the southern North Sea. Monthly SST composites showed pronounced seasonal warming of the southern North Sea and delineated the English coastal and continental coastal waters. The East-Anglia Plume is the dominant feature of the English coastal waters in the winter and autumn SPM composites, and the Rhine region of freshwater influence (ROFI), including the Flemish Banks, is the dominant feature of the continental waters. These mesoscale spatial structures are also influenced by the evolution of fronts, such as the seasonal front separating well-mixed water in the southern Bight, from the seasonally stratified central North Sea waters. A harmonic analysis of the SST and SPM images showed pronounced seasonal variability, as well as spring-neap variations in the level of tidal mixing in the East Anglia Plume, the Rhine ROFI and central North Sea. The harmonic analysis indicates the important role played by the local meteorology and tides in governing the SST and near-surface SPM concentrations in the southern North Sea. In the summer, thermal stratification affects the visibility of SPM to satellite sensors in the waters to the north of the Flamborough and Frisian Fronts. Haline stratification plays an important role in the visibility of SPM in the Rhine ROFI throughout the year. When stratified, both regions typically exhibit low surface SPM values. A numerical model study, together with the harmonic analysis, highlights the importance of tides and waves in controlling the stratification in the southern North Sea and hence the visibility of SPM.     

Quantification of the ferric/ferrous iron ratio in silicates by scanning transmission X-ray microscopy at the Fe L2,3 edges

Estimation of Fe³⁺/ΣFe ratios in materials at the submicrometre scale has been a long-standing challenge in the Earth and environmental sciences because of the usefulness of this ratio in estimating redox conditions as well as for geothermometry. To date, few quantitative methods with submicrometric resolution have been developed for this purpose, and most of them have used electron energy-loss spectroscopy carried out in the ultra-high vacuum environment of a transmission electron microscope (TEM). Scanning transmission X-ray microscopy (STXM) is a relatively new technique complementary to TEM and is increasingly being used in the Earth sciences. Here, we detail an analytical procedure to quantify the Fe³⁺/ΣFe ratio in silicates using Fe L_2,3-edge X-ray absorption near edge structure (XANES) spectra obtained by STXM, and we discuss its advantages and limitations. Two different methods for retrieving Fe³⁺/ΣFe ratios from XANES spectra are calibrated using reference samples with known Fe³⁺ content by independent approaches. The first method uses the intensity ratio of the two major peaks at the L₃-edge. This method allows mapping of Fe³⁺/ΣFe ratios at a spatial scale better than 50 nm by the acquisition of 5 images only. The second method employs a 2-eV-wide integration window centred on the L₂ maximum for Fe³⁺, which is compared to the total integral intensity of the Fe L₂-edge. These two approaches are applied to metapelites from the Glarus massif (Switzerland), containing micrometre-sized chlorite and illite grains and prepared as ultrathin foils by focused ion beam milling. Nanometre-scale mapping of iron redox in these samples is presented and shows evidence of compositional zonation. The existence of such zonation has crucial implications for geothermometry and illustrates the importance of being able to measure Fe³⁺/ΣFe ratios at the submicrometre scale in geological samples.     

Estimating phytolith influx in lake sediments

So far, no phytolith extraction protocols have been tested for accuracy and repeatability. Here we aim to display a phytolith extraction method combining the strengths of two widely used protocols, supplemented with silica microspheres as exogenous markers for quantifying phytolith concentrations. Phytolith concentrations were estimated for samples from two sedimentary sequences in which numerical age–depth models make it possible to calculate phytolith influxes (phytolith numbers per cm²per yr). Analysis of replicates confirmed the statistical robustness, the repeatability and the very few biases of our extraction technique for small phytoliths, since the relationship between grass silica short cells and microspheres was kept stable. Furthermore, we demonstrated that silica microspheres are robust exogenous markers for estimating phytolith concentrations. The minimum number of items (i.e., phytoliths plus silica microspheres) that must be counted to estimate phytolith concentrations and thus influxes depends on the ratio of phytoliths to microspheres (R) and is minimized when R = 1. Nevertheless, we recommend using ratios R ≤ 1 in order to avoid having the counting process become excessively time-consuming, because microspheres are easier to identify and count than phytoliths.