Ostracods: The ultimate survivors

Ostracods are tiny crustacean arthropods just a few millimetres long, with a bivalved carapace made of calcium carbonate that covers the whole body, and into which the animal can retreat from the world outside. Because of their diminutive size they are largely overlooked as fossils, but they have a fascinating history. Silent witnesses to life in the seas since the time of trilobites, they have a fossil record extending back to the Early Ordovician, and possibly the Cambrian. Ostracods have survived nearly 500 million years of Earth history including the ‘big five’ mass extinctions of the Phanerozoic Eon; they are true survivors. They are almost perfectly adapted for the aquatic environments in which they live, and can be found from the ocean abyssal plains to damp leaf litter. The ostracod carapace is a triumph of biological engineering that has been re‐configured into myriad different morphologies according to environment. Streamlined and agile species plough through the ocean water column, sometimes reaching a ‘giant’ size of a centimetre in length, whilst their tinier sea bottom cousins make elaborately ornamented carapaces to withstand the pressures of living at the seabed, or shape their carapaces into forms that facilitate burrowing into sediment. Ostracods are key components of aquatic ecosystems. As primary consumers they are food for larger animals both in seabed and planktonic habitats, and they recycle much of the organic detritus produced by larger animals and plants. Delve into the history of ostracods and it is possible to find pioneers who triumphed in the plankton, early colonisers of terrestrial aquatic ecosystems, and ostracods that literally conquered the land. And in more recent times, ostracods have even hitched rides on rockets into space.     

A detailed energy budget analysis of river supercooling and the importance of accurately quantifying net radiation to predict ice formation

River supercooling and ice formation is a regular occurrence throughout the winter in northern countries. The resulting frazil ice production can obstruct the flow through intakes along the river, causing major problems for hydropower and water treatment facilities, among others. Therefore, river ice modellers attempt to calculate the river energy budget and predict when supercooling will occur in order to anticipate and mitigate the effects of potential intake blockages. Despite this, very few energy budget studies have taken place during freeze-up, and none have specifically analysed individual supercooling events. To improve our understanding of the freeze-up energy budget detailed measurements of air temperature, relative humidity, barometric pressure, wind speed and direction, short- and longwave radiation, and water temperature were made on the Dauphin River in Manitoba. During the river freeze-up period of late October to early November 2019, a total of six supercooling events were recorded. Analysis of the energy budget throughout the supercooling period revealed that the most significant heat source was net shortwave radiation, reaching up to 298 W/m², while the most significant heat loss was net longwave radiation, accounting for losses of up to 135 W/m². Longwave radiation was also the most significant heat flux overall during the individual supercooling events, accounting for up to 84% of the total heat flux irrespective of flux direction, highlighting the importance of properly quantifying this flux during energy budget calculations. Five different sensible (Q_h) and latent (Q_e) heat flux calculations were also compared, using the bulk aerodynamic method as the baseline. It was found that the Priestley and Taylor method most-closely matched the bulk aerodynamic method on a daily timescale with an average offset of 8.5 W/m² for Q_h and 10.1 W/m² for Q_e, while a Dalton-type equation provided by Webb and Zhang was the most similar on a sub-daily timescale with average offsets of 20.0 and 14.7 W/m² for Q_h and Q_e, respectively.     

<Superscript>18</Superscript>O/<Superscript>16</Superscript>O and V/Cr ratios in gem tsavorites from the Neoproterozoic Mozambique metamorphic belt: a clue towards their origins?

The combination of oxygen isotope composition with V–Cr–Mn trace element concentrations of V-bearing garnets (tsavorites) originating from the main deposits of the Neoproterozoic Mozambique Metamorphic Belt is reported for the first time. The database enables the identification of the geological and geographical sources of the main productive areas from northern and southern Tanzania, Kenya, and Madagascar. Three consistent sets of δ¹⁸O values between 9.5‰ and 11.0‰, 11.6‰ and 14.5‰, and 15.5‰ and 21.1‰ have been recognized for primary deposits hosted in graphitic gneisses related to the Neoproterozic metasedimentary series. The δ¹⁸O value of tsavorite is a good tracer of the environment of its formation; the δ¹⁸O of the fluid in equilibrium with tsavorite was buffered by the host rock during metamorphism and fluid-rock interaction. This study is a first step in characterizing the geochemistry of gem tsavorite from most of the deposits and occurrences worldwide.     

Effects of short-term ecosystem experimental warming on water-extractable organic matter in an ombrotrophic Sphagnum peatland (Le Forbonnet, France)

In a future warmer world, peatlands may change from a carbon sink function to a carbon source function. This study tracks changes in water-extractable organic matter (WEOM) after 1 year of in situ experimental warming using open top chambers (OTCs). WEOM was studied in the upper peat layers (0–10 cm) through analysis of water-extractable organic carbon (WEOC), stable C isotopic composition (δ¹³C), specific UV absorbance at 280 nm and sugar composition of cores taken from an open bog (DRY sites) and a transitional poor fen (WET sites). At the DRY sites, the impact of OTCs was weak with respect to WEOM parameters, whereas at the WET sites, the air warming treatment led to a decrease in peat water content, suggesting that the supply of heat by OTCs was used mainly for evapotranspiration. OTCs at the WET sites also induced a relative enrichment at the surface (0–5 cm depth) of aliphatic and/or aromatic compounds with concomitant decrease in WEOC, as a result of decomposition. On the contrary, WEOC and sugar content increased in the deeper peat layer (7.5–10 cm depth) probably as a result of increased leaching of phenolic compounds by roots, which then inhibits microbial activity. The different response to experimental warming at DRY and WET sites suggests that the spatial variability of moisture is critical for understanding of the impact of global warming on the fate of OM and the carbon cycle in peatlands.     

The sensitivity of the Late Saalian (140?ka) and LGM (21?ka) Eurasian ice sheets to sea surface conditions

This work focuses on the Late Saalian (140?ka) Eurasian ice sheets?? surface mass balance (SMB) sensitivity to changes in sea surface temperatures (SST). An Atmospheric General Circulation Model (AGCM), forced with two preexisting Last Glacial Maximum (LGM, 21?ka) SST reconstructions, is used to compute climate at 140 and 21?ka (reference glaciation). Contrary to the LGM, the ablation almost stopped at 140?ka due to the climatic cooling effect from the large ice sheet topography. Late Saalian SST are simulated using an AGCM coupled with a mixed layer ocean. Compared to the LGM, these 140?ka SST show an inter-hemispheric asymmetry caused by the larger ice-albedo feedback, cooling climate. The resulting Late Saalian ice sheet SMB is smaller due to the extensive simulated sea ice reducing the precipitation. In conclusion, SST are important for the stability and growth of the Late Saalian Eurasian ice sheet.     

High-resolution modelling of the Antarctic surface mass balance, application for the twentieth, twenty first and twenty second centuries

About 75 % of the Antarctic surface mass gain occurs over areas below 2,000 m asl, which cover 40 % of the grounded ice-sheet. As the topography is complex in many of these regions, surface mass balance modelling is highly dependent on horizontal resolution, and studying the impact of Antarctica on the future rise in sea level requires physical approaches. We have developed a computationally efficient, physical downscaling model for high-resolution (15 km) long-term surface mass balance (SMB) projections. Here, we present results of this model, called SMHiL (surface mass balance high-resolution downscaling), which was forced with the LMDZ4 atmospheric general circulation model to assess Antarctic SMB variability in the twenty first and the twenty second centuries under two different scenarios. The higher resolution of SMHiL better reproduces the geographical patterns of SMB and increase significantly the averaged SMB over the grounded ice-sheet for the end of the twentieth century. A comparison with more than 3200 quality-controlled field data shows that LMDZ4 and SMHiL reproduce the observed values equally well. Nevertheless, field data below 2,000 m asl are too scarce to efficiently show the added value of SMHiL and measuring the SMB in these undocumented areas should be a future scientific priority. Our results suggest that running LMDZ4 at a finer resolution (15 km) may give a future increase in SMB in Antarctica that is about 30 % higher than by using its standard resolution (60 km) due to the higher increase in precipitation in coastal areas at 15 km. However, a part (～15 %) of these discrepancies could be an artefact from SMHiL since it neglects the foehn effect and likely overestimates the precipitation increase. Future changes in the Antarctic SMB at low elevations will result from the competition between higher snow accumulation and runoff. For this reason, developing downscaling models is crucial to represent processes in sufficient detail and correctly model the SMB in coastal areas.     

Influence of tropical cyclones on sea surface temperature seasonal cycle and ocean heat transport

Recent studies suggested that tropical cyclones (TCs) contribute significantly to the meridional oceanic heat transport by injecting heat into the subsurface through mixing. Here, we estimate the long-term oceanic impact of TCs by inserting realistic wind vortices along observed TCs tracks in a 1/2° resolution ocean general circulation model over the 1978–2007 period. Warming of TCs’ cold wakes results in a positive heat flux into the ocean (oceanic heat uptake; OHU) of ~480 TW, consistent with most recent estimates. However, ~2/5 of this OHU only compensates the heat extraction by the TCs winds during their passage. Another ~2/5 of this OHU is injected in the seasonal thermocline and hence released back to the atmosphere during the following winter. Because of zonal compensations and equatorward transport, only one-tenth of the OHU is actually exported poleward (46 TW), resulting in a marginal maximum contribution of TCs to the poleward ocean heat transport. Other usually neglected TC-related processes however impact the ocean mean state. The residual Ekman pumping associated with TCs results in a sea-level drop (rise) in the core (northern and southern flanks) of TC-basins that expand westward into the whole basin as a result of planetary wave propagation. More importantly, TC-induced mixing and air-sea fluxes cool the surface in TC-basins during summer, while the re-emergence of subsurface warm anomalies warms it during winter. This leads to a ~10 % reduction of the sea surface temperature seasonal cycle within TCs basins, which may impact the climate system.     

Observed Changes in One-in-20 Year Extremes of Canadian Surface Air Temperatures

Weather and climate extremes are often associated with substantial adverse impacts on society and the environment. Assessment of changes in extremes is of great and broad interest. This study first homogenizes daily minimum and maximum surface air temperatures recorded at 146 stations in Canada. In order to assess changes in one-in-20 year extremes (i.e., extremes with a 20-year return period) in temperature, annual maxima and minima of both daily minimum temperatures and daily maximum temperatures are derived from the homogenized daily temperature series and analyzed with a recently developed extreme value analysis approach based on a tree of generalized extreme value distributions (including stationary and non-stationary cases). The procedure is applied to estimate the changes over the period 1911 to 2010 at 115 stations, located mainly in southern Canada, and over the period 1961 to 2010 at 146 stations across Canada (including 37 stations in the North). The results show that warming is strongest for extreme low temperature and weakest for extreme high temperature and is much stronger in the Canadian Arctic than in southern Canada. Warming is stronger in winter than in summer and stronger during nighttime than daytime of the same season.     

The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part B: Results and Process Understanding

We describe and analyze the results of the third global energy and water cycle experiment atmospheric boundary layer Study intercomparison and evaluation study for single-column models. Each of the nineteen participating models was operated with its own physics package, including land-surface, radiation and turbulent mixing schemes, for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer (SBL), such as the onset of evening momentum decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morning transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of processes in the SBL; the diagrams include the results of a number of sensitivity runs performed with one of the models. The models are characterized in terms of thermal coupling to the soil, longwave radiation and turbulent mixing. It is shown that differences in longwave radiation schemes among the models have only a small effect on the simulations; however, there are significant variations in downward radiation due to different boundary-layer profiles of temperature and humidity. The differences in modelled thermal coupling to the land surface are large and explain most of the variations in 2-m air temperature and longwave incoming radiation among models. Models with strong turbulent mixing overestimate the boundary-layer height, underestimate the wind speed at 200 m, and give a relatively large downward sensible heat flux. The result is that 2-m air temperature is relatively insensitive to turbulent mixing intensity. Evening transition times spread 1.5 h around the observed time of transition, with later transitions for models with coarse resolution. Time of onset in the morning transition spreads 2 h around the observed transition time. With this case, the morning transition appeared to be difficult to study, no relation could be found between the studied processes, and the variation in the time of the morning transition among the models.