Cross-basin differences in particulate organic carbon export and flux attenuation in the subtropical North Atlantic gyre

We compared in-situ and satellite-derived measures of the biological carbon pump efficiency at the two seemingly similar subtropical North Atlantic gyre time series sites, the Bermuda time series (BATS, Bermuda Atlantic time-series study and OFP, ocean flux program) in the western gyre and the ESTOC time series (European station for time-series in the ocean, Canary Islands) in the eastern gyre. Satellite-derived surface chlorophyll a was slightly lower at Bermuda compared to ESTOC (annual average of 0.10±0.04 vs. 0.14±0.05-mg-m^?3), as was satellite-derived primary production (annual average of 380±77 vs. 440±80-mg C-m^?2 d^?1). However, export production normalized to primary production (export ratio) was higher at Bermuda by a factor of 2–3 when estimated using mesopelagic traps moored at 500-m depth and by a factor of 3–4 when estimated using surface-tethered drifting traps. When averaged seasonally, flux at BATS was highest in spring (March, April, May) at all depths followed by summer (June, July, August) and decreasing towards fall, but this seasonality was less visible at ESTOC. Seasonal comparison showed the fastest flux attenuation at Bermuda in winter and spring, coinciding with the highest POC flux. POC/PIC ratios derived from the moored traps were significantly higher at BATS than at ESTOC in fall and winter, but this difference was not significant in spring (p>0.05). This study shows that while the western and eastern Atlantic subtropical gyres have similar rates of primary production, the biological carbon pump differs between the two provinces. Higher new nutrient input observed at Bermuda compared to ESTOC might explain part of the difference in export ratio but alone is insufficient. Greater winter mixed-layer depths and higher mesoscale eddy activity at Bermuda resulting in pulsed production events of labile organic matter might explain both the higher export flux and export ratios found at Bermuda.     

Using very long-range terrestrial laser scanner to analyze the temporal consistency of the snowpack distribution in a high mountain environment

This study demonstrated the usefulness of very long-range terrestrial laser scanning (TLS) for analysis of the spatial distribution of a snowpack, to distances up to 3000 m, one of the longest measurement range reported to date. Snow depth data were collected using a terrestrial laser scanner during 11 periods of snow accumulation and melting, over three snow seasons on a Pyrenean hillslope characterized by a large elevational gradient, steep slopes, and avalanche occurrence. The maximum and mean absolute snow depth error found was 0.5-0.6 and 0.2-0.3 m respectively, which may result problematic for areas with a shallow snowpack, but it is sufficiently accurate to determine snow distribution patterns in areas characterized by a thick snowpack. The results indicated that in most cases there was temporal consistency in the spatial distribution of the snowpack, even in different years. The spatial patterns were particularly similar amongst the surveys conducted during the period dominated by snow accumulation (generally until end of April), or amongst those conducted during the period dominated by melting processes (generally after mid of April or early May). Simple linear correlation analyses for the 11 survey dates, and the application of Random Forests analysis to two days representative of snow accumulation and melting periods indicated the importance of topography to the snow distribution. The results also highlight that elevation and the Topographic Position index (TPI) were the main variables explaining the snow distribution, especially during periods dominated by melting. The intra- and inter-annual spatial consistency of the snowpack distribution suggests that the geomorphological processes linked to presence/absence of snow cover act in a similar way in the long term, and that these spatial patterns can be easily identified through several years of adequate monitoring.     

The contrasted evolution of high and low flows and precipitation indices in the Duero basin (Spain)

Abstract

Trends in high and low flows are valuable indicators of hydrological change because they highlight changes in various parts of the frequency distribution of streamflow series. This enables improved assessment of water availability in regions with high seasonal and inter-annual variability. There has been a substantial reduction in water resources in the Duero basin (Iberian Peninsula, Spain) and other areas of the Mediterranean region during the last 50 years, and this is likely to continue because of climate change. In this study, we investigated the evolution and trends in high and low flows in the Spanish part of the Duero basin, and in equivalent or closely-related precipitation indices for the period 1961–2005. The results showed a general trend of decrease in the frequency and magnitude of high flows throughout most of the basin. Moreover, the number of days with low flows significantly increased over this period. No clear relationship was evident between the evolution of high/low flows and changes in the distribution frequencies of the precipitation series. In contrast to what was expected, the number of days with heavy precipitation and the mean annual precipitation did not show significant trends across the basin, and the number of days without rainfall decreased slightly. The divergence between precipitation and runoff evolution was more accentuated in spring and summer. In the absence of trends in precipitation, it is possible that reforestation processes in the region, and increasing temperatures in recent decades, could be related to the decreasing frequency of high flows and the increasing frequency of low flows.

Editor Z.W. Kundzewicz; Associate editor S. Grimaldi

Citation Morán-Tejeda, E., López-Moreno, J.I., Vicente-Serrano, S.M., Lorenzo-Lacruz, J. and Ceballos-Barbancho, A., 2012. The contrasted evolution of high and low flows and precipitation indices in the Duero basin (Spain). Hydrological Sciences Journal, 57 (4), 591–611.     

The exhumed Eocene Sultepec-Goleta Volcanic Center of southern Mexico: record of partial collapse and ignimbritic volcanism fed by wide pyroclastic dike complexes

The Sultepec-Goleta area in southern Mexico hosts one of the major Eocene silicic volcanic centers that make up the Cenozoic volcanic record of the north-central part of the Sierra Madre del Sur. This center is represented by a partially exhumed NNE-SSW trending ignimbritic field that covers an area of ∼400 km² with a preserved volume of ∼200 km³. A remarkable feature of volcanic center is the exposure of pyroclastic dike complexes up to 1 km wide that extend almost continuously along the western and southern flanks of the volcanic field. The ignimbritic record comprises four units with different textural features as well as variable proportions of the main components (i.e. phenocrysts, pumice and lithic clasts). The most extensive unit is the basal Goleta Ignimbrite, of which the middle and upper members are richer in phenocrysts than the lower member. It has a thickness that ranges from ∼200 m in the north to at least 600 m in the south of the volcanic center. The overlying ignimbrite units (Cienega, Lobera and Potrero ignimbrites) are phenocryst-poor and are distributed in the northern sector of the volcanic field. The semi-curvilinear trend of the pyroclastic dikes along the southern flank of the Goleta Range, coupled with the greater thickness of the Goleta Ignimbrite encircled by the dikes, is indicative of the development of a partial-collapse caldera. Based on spatial relationships as well as analogies in the nature and abundances of the components, it has been recognized that the central and southern dike complexes fed the relatively phenocryst-rich Goleta Ignimbrite, whereas the phenocryst-poor units of the northern sector were extruded through the pyroclastic conduits distributed around Sultepec. We suggest that the stirring produced by the partial collapse of the magma chamber roof beneath the southern sector of the study area triggered the tapping of phenocryst-rich portions of the zoned magma chamber, resulting in an increase of phenocryst content in the upper members of the Goleta Ignimbrite. The presence of breccias and lithic-rich facies dominated by wallrock fragments at the borders of the pyroclastic dikes, including those of the northern extra-caldera sector, indicate that erosion of the conduit walls above the fragmentation level contributed to widening of the conduits and the construction of wide dike complexes.     

L'or dans la métallogenie du massif de Bohême

Les minéralisations aurifères du massif de Bohême sont représentées par plusieurs types morphologiques et paragénétiques, dont l'origine est expliquée par trois modèles genétiques: I — un modèle volcanogénique pour les minéralisations sulfurés stratiformes, II — un modèle hydrothermal-métamorphique pour les minéralisations de quartz aurifères synmétamorphiques, de morphologie très variable et caractérisées par la présence de minéraux de Bi, Te, Mo et W (scheelite), III — un modèle hydrothermal de shear zones aurifères, pour les minéralisations postmétamorphiques, caractérisées par des teneurs en argent plus élevées (electrum) ou par une minéralisation complexe à stibine et or.Les complexes volcanosédimentaires, principalement protérozoiques, sont considérés comme les sources de l'or pour les minéralisations plus tardives, liées à la cratonisation du massif de Bohême. Au course de l'évolution géotectonique et métallogenique du massif de Bohême les conditions de mis en place des minéralisations aurifères ont évolué d'un contrôle initialement lithologique vers un contrôle lithologique et structural pour aboutir à un contrôle exclusivement structural. Ce développement a été accompagné par une augmentation de la taille des particules d'or et par leur pépitisation dans les minéralisations tradives.     

Stable isotope analysis of consumer food webs indicates ecosystem recovery following prolonged drought in a subtropical estuarine lake

Assessing changes in food-web structure provides a useful monitoring tool for gauging the resilience of ecosystems in the face of climatic impacts. We consider the ecological resilience of a large estuarine lake (St Lucia Estuary, South Africa) in the wake of an extreme climatic event (prolonged drought). Using carbon and nitrogen stable isotopes, food-web structure was assessed at five sites across the estuary during the winter and spring of 2013. Sampling occurred approximately three years after heavy rains flooded the system and returned it to a relatively diluted state following an almost decade-long drought that decimated food webs in the upper parts of the estuary due to hypersalinity effects. Comparisons of niche width and variance of consumer food webs among sites revealed a general homogenisation of food webs across the entire system, contrasting with the spatial differentiation of food webs documented during the drought phase. Our results indicate that the estuary is able to maintain ecological resilience at the whole-system level in the face of an extreme drought. This is likely facilitated by source pools of species residing in the relatively stable lower estuary, which are able to rapidly recolonise areas denuded by drought in the upper estuary.     

Geographic and hydromorphologic controls on interactions between hyporheic flow and discharging deep groundwater

Hyporheic exchange flow (HEF) at the streambed–water interface (SWI) has been shown to impact the pattern and rate of discharging groundwater flow (GWF) and the consequential transport of heat, solutes and contaminants from the subsurface into streams. However, the control of geographic and hydromorphological catchment characteristics on GWF–HEF interactions is still not fully understood. Here, the spatial variability in flow characteristics in discharge zones was investigated and averaged over three spatial scales in five geographically different catchments in Sweden. Specifically, the deep GWF discharge velocity at the SWI was estimated using steady-state numerical models, accounting for the real multiscale topography and heterogeneous geology, while an analytical model, based on power spectral analysis of the streambed topography and statistical assessments of the stream hydraulics, was used to estimate the HEF. The modeling resulted in large variability in deep GWF and HEF velocities, both within and between catchments, and a regression analysis was performed to explain this observed variability by using a set of independent variables representing catchment topography and geology as well as local stream hydromorphology. Moreover, the HEF velocity was approximately two orders of magnitude larger than the deep GWF velocity in most of the investigated stream reaches, indicating significant potential to accelerate the deep GWF velocity and reduce the discharge areas. The greatest impact occurred in catchments with low average slope and in reaches close to the catchment outlet, where the deep GWF discharge velocity was generally low.

     

New indices and calibrations derived from the distribution of crenarchaeal isoprenoid tetraether lipids: Implications for past sea surface temperature reconstructions

Several studies have shown that there is a strong relationship between the distribution of crenarchaeotal isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs) and sea surface temperature (SST). Based on this, a ratio of certain GDGTs, called TEX₈₆ (TetraEther indeX of tetraethers consisting of 86 carbon atoms), was developed as a SST proxy. In this study, we determined the distribution of crenarchaeotal isoprenoid GDGTs in 116 core-top sediments mostly from (sub)polar oceans and combined these data with previously published core-top data. Using this extended global core-top dataset (n = 426), we re-assessed the relationship of crenarchaeal isoprenoid GDGTs with SST. We excluded data from the Red Sea from the global core-top dataset to define new indices and calibration models, as the Red Sea with its elevated salinity appeared to behave differently compared to other parts of the oceans. We tested our new indices and calibration models on three different paleo datasets, representing different temperature ranges. Our results indicate that the crenarchaeol regio-isomer plays a more important role for temperature adaptation in (sub)tropical oceans than in (sub)polar oceans, suggesting that there may be differences in membrane adaptation of the resident crenarchaeotal communities at different temperatures. We, therefore, suggest to apply two different calibration models. For the whole calibration temperature range (−3 to 30 °C), a modified version of TEX₈₆ with a logarithmic function which does not include the crenarchaeol regio-isomer, called , is shown to correlate best with SST: (r² = 0.86, n=396, p <0.0001). Application of on sediments from the subpolar Southern Ocean results in realistic absolute SST estimates and a similar SST trend compared to a diatom SST record from the same core. , which is defined as the logarithmic function of TEX₈₆, yields the best correlation with SST, when the data from the (sub)polar oceans are removed: (r² = 00.87, n = 255, p < 0.0001). Furthermore, gives the best correlation for mescosm data with temperatures ranging between 10 and 46 °C. For Quaternary sediments from the tropical Arabian Sea, both and yield similar trends and SST estimates. However, the extrapolation of calibration on a sediment record from a greenhouse world ocean predicts more reliable absolute SST estimates and relative SST changes in agreement with estimates based on the δ¹⁸O of planktonic foraminifera. Based on the comparison of and derived SSTs using the core top data, we recommend applying above 15 °C and below 15 °C. In cases where paleorecords encompass temperatures both below and above 15 °C, we suggest to use .     

Gamma ray burst studies with THESEUS

Gamma-ray Bursts (GRBs) are the most powerful transients in the Universe, over–shining for a few seconds all other γ-ray sky sources. Their emission is produced within narrowly collimated relativistic jets launched after the core–collapse of massive stars or the merger of compact binaries. THESEUS will open a new window for the use of GRBs as cosmological tools by securing a statistically significant sample of high-z GRBs, as well as by providing a large number of GRBs at low–intermediate redshifts extending the current samples to low luminosities. The wide energy band and unprecedented sensitivity of the Soft X-ray Imager (SXI) and X-Gamma rays Imaging Spectrometer (XGIS) instruments provide us a new route to unveil the nature of the prompt emission. For the first time, a full characterisation of the prompt emission spectrum from 0.3 keV to 10 MeV with unprecedented large count statistics will be possible revealing the signatures of synchrotron emission. SXI spectra, extending down to 0.3 keV, will constrain the local metal absorption and, for the brightest events, the progenitors’ ejecta composition. Investigation of the nature of the internal energy dissipation mechanisms will be obtained through the systematic study with XGIS of the sub-second variability unexplored so far over such a wide energy range. THESEUS will follow the spectral evolution of the prompt emission down to the soft X–ray band during the early steep decay and through the plateau phase with the unique ability of extending above 10 keV the spectral study of these early afterglow emission phases.

     

The effect of slope aspect on the response of snowpack to climate warming in the Pyrenees

The aim of this study was to analyse the effect of slope aspect on the response of snowpack to climate warming in the Pyrenees. For this purpose, data available from five automatic weather stations were used to simulate the energy and mass balance of snowpack, assuming different magnitudes of an idealized climate warming (upward shifting of 1, 2 and 3 °C the temperature series). Snow energy and mass balance were simulated using the Cold Regions Hydrological Modelling platform (CRHM). CRHM was used to create a model that enabled correction of the all-wave incoming radiation fluxes from the observation sites for various slope aspects (N, NE, E, SE, S, SW,W,NW and flat areas), which enabled assessment of the differential impact of climate warming on snow processes on mountain slopes. The results showed that slope aspect was responsible for substantial variability in snow accumulation and the duration of the snowpack. Simulated variability markedly increased with warmer temperature conditions. Annual maximum snow accumulation (MSA) and annual snowpack duration (ASD) showed marked sensitivity to a warming of 1 °C. Thus, the sensitivity of the MSA in flat areas ranged from 11 to 17 % per degree C amongst the weather stations, and the ASD ranged from 11 to 20 days per degree C. There was a clear increase in the sensitivity of the snowpack to climate warming on those slopes that received intense solar radiation (S, SE and SW slopes) compared with those slopes where the incident radiation was more limited (N, NE and NW slopes). The sensitivity of the MSA and the ASD increased as the temperature increased, particularly on the most irradiated slopes. Large interannual variability was also observed. Thus, with more snow accumulation and longer duration the sensitivity of the snowpack to temperature decreased, especially on south-facing slopes.