Carbon biogeochemistry of the Betsiboka estuary (north-western Madagascar)

Madagascar’s largest estuary (Betsiboka) was sampled along the salinity gradient during the dry season to document the distribution and sources of particulate and dissolved organic carbon (POC, DOC) as well as dissolved inorganic carbon (DIC). The Betsiboka was characterized by a relatively high suspended matter load, and in line with this, low DOC/POC ratios (0.4–2.5). The partial pressure of CO₂ (pCO₂) was generally above atmospheric equilibrium (270–1530 ppm), but relatively low in comparison to other tropical and subtropical estuaries, resulting in low average CO₂ emission to the atmosphere (9.1 ± 14.2 mmol m⁻² d⁻¹). Despite the fact that C4 vegetation is reported to cover >80% of the catchment area, stable isotope data on DOC and POC suggest that C4 derived material comprises only 30% of both pools in the freshwater zone, increasing to 60–70% and 50–60%, respectively, in the oligohaline zone due to additional lateral inputs. Sediments from intertidal mangroves in the estuary showed low organic carbon concentrations (<1%) and δ¹³C values (average −19.8‰) consistent with important inputs of riverine imported C4 material. This contribution was reflected in δ¹³C signatures of bacterial phospholipid derived fatty acids (i + a15:0), suggesting the potential importance of terrestrial organic matter sources for mineralization and secondary production in coastal ecosystems.     

Sublittoral seaweed communities on natural and artificial substrata in a high-latitude coral community in South Africa

Coral mortality may result in macroalgal proliferation or a phase shift into an alga-dominated state. Subtidal, high-latitude western Indian Ocean coral communities at Sodwana Bay on the KwaZulu-Natal coast, South Africa, have experienced some mortality because of warm-water anomalies, storms and other causes, but the response of the macroalgae is unknown. We investigated the abundance and diversity of benthic algae on different hard natural substrata (dead digitate, brain and plate corals and beach rock) on Two-Mile Reef, Sodwana Bay. We also compared algal communities colonising ceramic, marble and pretreated ceramic tiles placed on the reef for six months. We identified 95 algae (14 Chlorophyta, 11 Phaeophyceae, 69 Rhodophyta and one cyanobacterium). Assemblages on natural and artificial substrata were dominated by the brown alga Lobophora variegata (Lamouroux) Womersley ex Oliveira and non-geniculate corallines (Rhodophyta, Corallinaceae). Cluster and ordination analyses revealed that the algae showed no affinity for particular substrata, whether natural or artificial. Algal cover was occasionally higher on rougher tiles and crustose corallines were significantly more abundant on marble than ceramic tiles. Two-Mile Reef had 23.1% dead and 48.4% live scleractinian coral cover, where dead corals were colonised indiscriminately by many small algal species, but there was no evidence of algal proliferation. The results provide a baseline for monitoring this high-latitude reef system.     

Dissolved organic nitrogen dynamics in the North Sea: A time series analysis (1995–2005)

Dissolved organic nitrogen (DON) dynamics in the North Sea was explored by means of long-term time series of nitrogen parameters from the Dutch national monitoring program. Generally, the data quality was good with little missing data points. Different imputation methods were used to verify the robustness of the patterns against these missing data. No long-term trends in DON concentrations were found over the sampling period (1995–2005). Inter-annual variability in the different time series showed both common and station-specific behavior. The stations could be divided into two regions, based on absolute concentrations and the dominant times scales of variability. Average DON concentrations were 11 μmol l⁻¹ in the coastal region and 5 μmol l⁻¹ in the open sea. Organic fractions of total dissolved nitrogen (TDN) averaged 38 and 71% in the coastal zone and open sea, respectively, but increased over time due to decreasing dissolved inorganic nitrogen (DIN) concentrations. In both regions intra-annual variability dominated over inter-annual variability, but DON variation in the open sea was markedly shifted towards shorter time scales relative to coastal stations. In the coastal zone a consistent seasonal DON cycle existed with high values in spring–summer and low values in autumn–winter. In the open sea seasonality was weak. A marked shift in the seasonality was found at the Dogger Bank, with DON accumulation towards summer and low values in winter prior to 1999, and accumulation in spring and decline throughout summer after 1999. This study clearly shows that DON is a dynamic actor in the North Sea and should be monitored systematically to enable us to understand fully the functioning of this ecosystem.     

辉钼矿Re-Os同位素定年方法的改进与应用

公认的Re-Os同位素定年代表物辉钼矿,目前已在金属硫化物矿床年代学研究领域获得了广泛的应用。本研究采用浓HNO₃分解辉钼矿样品,大大地简化了Re和Os的化学制备过程,并根据辉钼矿中正常Os含量水平相对放射成因¹⁸⁷Os可以忽略的特点,以正常Os标准为稀释剂,实现了仪器测量过程中Os同位素质量分馏的在线校正,改善了分析数据的质量。该方法经辉钼矿国家标准物质进行验证,获得了满意的Re、Os含量及Re-Os年龄数据,并且在南岭地区与连阳复式岩体相关的姓坪夕卡岩型钼矿床成矿年龄研究中获得了成功应用。在实际应用中,为了获得有意义的能反映真实地质事件的年龄数据,辉钼矿样品不仅要有足够的取样量,而且还要保证粒度细、混合均匀。     

Using an unbaited stationary video system to investigate the behaviour and interactions of bull sharks Carcharhinus leucas under an aquaculture farm

Bull sharks Carcharhinus leucas are common along the coast of Reunion Island (South-West Indian Ocean) and were suspected to aggregate in the vicinity of an aquaculture farm in Saint-Paul Bay on the west coast. In order to understand the behaviour and interaction of bull sharks near aquaculture cages at Saint-Paul Bay, we deployed an experimental unbaited stationary video camera. From 175 hours of recording during daylight hours from March to April 2012, eight individual female bull sharks (seven adults and one immature) were identified based on their natural markings. These sharks were resighted between 3 and 45 times. Residency analysis revealed site attachment under the aquaculture cages for at least three individuals over the course of the study. Recorded behaviours included intraspecific social interactions such as synchronised swimming. Social interactions and relatively strong paired associations for two pairs of females suggest some level of sociality among bull sharks around Reunion Island. Overall, our results demonstrate the utility of unbaited video systems to monitor the behaviour of adult coastal sharks.     

Possible diagenetic mobilization of barium in sapropelic sediment from the eastern Mediterranean

In the last few years it has frequently been suggested that Ba is a useful indicator of paleoproductivity. The formation of some sapropels in the Eastern Mediterranean is considered to be related to, or to coincide with, periods of enhanced productivity. A high-resolution sampling study has been undertaken in order to investigate whether the Ba distribution in sapropels reflects a primary input signal or whether it has been altered by diagenetic processes.

On the basis of our results we suggest that three diagenetic stages determine the distribution of Ba. During deposition of the sapropel (stage 1) Ba is mobilized as anoxic conditions prograde. After deposition of the sapropel (stage 2), a progressive oxidation front develops. This front induces the formation of Mn and Fe enrichments and barite precipitation at the oxic/anoxic boundary. Barite precipitation is believed to be caused mainly by a rise in the porewater sulphate concentration after sulphides have been oxidized by the front.

Upon burial (stage 3), suboxic conditions develop as the oxygen becomes exhausted again. In contrast to Fe- and Mn-oxyhydroxides which dissolve and reprecipitate at higher levels, barite is preserved because dissolved sulphate is not depleted.

The interpretation of the Ba distribution in organic-rich sediment is not straightforward. Diagenetic reallocation of a primary Ba signal will possibly disturb the relationship between Ba and organic production. Consequently, one must be very cautious when invoking Ba as a paleoproductivity indicator.     

Geochemistry of eastern Mediterranean sediments: Primary sediment composition and diagenetic alterations

Two cores recovered in the eastern Mediterranean were analysed for major, minor and trace elements. The primary chemical composition of the sediment is different at each location, probably because the lithological sources and the relative biogenic contributions differ.

Carbonates are important for the concentration of Ca, Mg and Sr, whereas aluminosilicates determine the concentration of Si, Al, K, Li, Y and Be, and to a lesser extent that of Fe, Cr, Ti, Mg, Zn and Zr. In sapropels, organic carbon and sulphur seem to be closely related. Bromine, Mo, P, Fe, V, Cu, Zn, Co, Ni and Cr are closely associated with organic and sulphidic compounds. The concentration versus depth profile for organic carbon in two sapropels points to a rapid establishment of conditions that gave rise to sapropel formation, followed by a gradual transition back to “normal” conditions.

The primary composition is overprinted by diagenetic processes. Sulphate-reducing conditions occurred during and just after sapropel deposition. A progressive oxidation front mechanism, which became active after sapropel deposition, is responsible for additional major geochemical changes. This diagenetic phenomenon has strong implications for the chemistry of Fe, Mn, Ni, Co, Zn, Cu, Cr, V, U, As and Sb.     

豫西沙沟脉状Ag-Pb-Zn矿床地质特征和成矿流体研究

豫西沙沟薄脉状Ag-Pb-Zn硫化物矿床位于华北陆块南缘熊耳山地区,主要由多金属硫化物-石英-碳酸盐脉型和石英-碳酸盐-绢云母-多金属硫化物蚀变岩型两种矿化类型组成。主要矿脉的矿物共生序列可以分为成矿前的石英-黄铁矿阶段(Ⅰ)、闪锌矿-石英-方铅矿-少量银矿物阶段(Ⅱ1)、方铅矿-石英-闪锌矿-含铁白云石-银矿物阶段(Ⅱ2)和成矿后的方解石-(石英)阶段(Ⅲ)。对不同阶段的成矿流体研究表明,石英-黄铁矿阶段(Ⅰ)中的含氯化钠子晶三相(LVH)包裹体(Ⅰ1)可能是直接从饱和水的结晶岩浆熔体中出溶形成或是由岩浆流体的减压沸腾形成,显示该区很可能存在岩浆流体端元。多金属硫化物阶段(Ⅱ1Ⅱ2)捕获富液相包裹体(LV型)和个别CO2包裹体(C型),这两个阶段流体包裹体反映了主成矿阶段流体的基本特征,结合包裹体气相和液相成分色谱分析以及包裹体初融温度,认为成矿流体应该为中-低温低盐度含CO2的H2O-NaCl体系。其中,阶段(Ⅱ2)的均一温度(145～288℃,平均为194℃)比阶段(Ⅱ1)的均一温度(185～357℃,平均240℃)低46℃;同时,阶段(Ⅱ2)的盐度(1.91%～10.86%,平均6.38%)较阶段(Ⅱ1)盐度(4.65%～10.11%,平均7.77%)略低。对这一温度和盐度的总体下降趋势的合理解释是大气水的逐渐混入。多金属硫化物阶段(Ⅱ1Ⅱ2)之后的方解石-(石英)阶段普遍为富液相包裹体(LV型),该阶段显著降低的温度(129～208℃,平均165℃)和盐度(1.40%～4.03%,平均2.50%),进一步佐证大气水的不断混入。而且,流体混合可能在引起矿石矿物从热液中沉淀方面起到重要作用。     

Sulphate-related equilibria in the hypersaline brines of the Tyro and Bannock Basins, eastern Mediterranean

The Tyro and Bannock Basins, which are depressions in the eastern Mediterranean, contain hypersaline anoxic brines. These brines are of different composition: Tyro brine is primarily an early-stage halite (NaCl) brine, whereas Bannock brine includes the more soluble ions of late-stage evaporite minerals. Accordingly, the Bannock brine contains a much greater sulphate concentration than the Tyro Brine. This difference in sulphate concentration is reflected in the concentrations of ions such as Ca, Sr and Ba, which form sparingly soluble sulphate minerals.Equilibrium calculations using the Pitzer specific ion interaction model indicate that the brines in both basins are saturated with respect to gypsum (CaSO₄-2H₂O) and supersaturated to saturated with respect to dolomite (CaMg(CO₃)₂). The degree of saturation with respect to dolomite is greater in the Bannock Basin than it is in the Tyro Basin. Correspondingly, recent gypsum crystals and dolomite hardgrounds have been found in the Bannock Basin but not in the Tyro Basin.The Tyro brine is homogeneous in composition, whereas the Bannock brine demonstrates a clear two-layer brine structure. At the interface of the upper and the lower brine distinct positive anomalies occur in the total alkalinity and the concentration of phosphate, and negative anomalies occur in the concentrations of Mn²⁺ and the rare earth elements (REE). These anomalies and the observed association of gypsum/dolomite in the sediments are all consistent with a recent precipitation of dolomite and gypsum in the Bannock Basin. The brines in both basins are also saturated with respect to barite (BaSO₄).The ⁸⁷Sr/⁸⁶Sr and δ³⁴S ratios of the Bannock brines are amazingly consistent but differ dramatically from the values for modern or Messinian-age seawater. The Sr concentration and Sr and S isotope ratios in the gypsum crystals indicate that most of these crystals have resulted from precipitation/recrystallization from the brine and not from seawater. The observed variations between crystals are thought to reflect the recrystallization of (sub-) outcropping Messinian gypsum with a low ⁸⁷Sr/⁸⁶Sr ratio in the presence of seawater or brine fluids and with different extents of diagenesis.     

Ocean robotics in support of fisheries research and management

South Africa’s small-pelagic fishery is a socio-economically important component of the country’s commercial fisheries sector, second in value only to the demersal trawl fishery. Management of this sector relies on infrequent hydro-acoustic surveys, which provide measures of anchovy Engraulis encrasicolus and sardine Sardinops sagax biomass used in the assessments of stock status and in the development of management plans for the sustainable utilisation of these resources. We demonstrate how technological capabilities in ocean robotics at the Council for Scientific and Industrial Research (CSIR) could augment the current resource-intensive hydro-acoustic ship-based survey programme and create opportunities for expanding its spatial and temporal resolution. We successfully implement and demonstrate an autonomous wave glider, fitted with a hydro-acoustic sensor and compare the data to a collocated ‘traditional’ ship-based acoustics survey. In the future these autonomous systems approaches could be seen as a means to lessen the cost burden of the ship-based survey, while at the same time with the added advantage of continuous collection over much wider spatial and temporal domains. This could enable a more reflexive stock management approach taking into account the seasonal characteristics of the fishery and its ecosystem. Gliders thus have potential to increase dramatically the quantity of information available to fisheries managers, thereby reducing uncertainty and contributing to improved management of valuable fish resources. They are likely to contribute to improved knowledge of the ecology of small pelagic fish species off the coast of South Africa in a changing climate and should potentially also permit the collection of biomass data for other marine resources currently not routinely monitored.