Martin G. Wiesner

Fallout of volcanic ash to the deep South China Sea induced by the 1991 eruption of Mount Pinatubo (Philippines)

An ongoing sediment trap experiment in the central South China Sea provided the opportunity to investigate directly the mode and rates of a medial fallout of volcanic ash following the June 15, 1991, eruption of Mount Pinatubo (Philippines). Within less than three days after the release of the major eruption plume, ash accumulation (9 kg/m 2 ) was simultaneously recorded by traps positioned in 1190 and 3730 m water depth at a distance of 586 km to the west of the volcano. A numerical simulation of the fallout reveals that the vertical trajectories of the pyroclasts in both the atmosphere and ocean were controlled by particle aggregation. The aggregation process caused a premature subaerial fallout of fine-grained ash. After crossing the air-sea interface, vertical settling of the ash clusters was enhanced by absorption of water leading to settling rates of more than 1670 m/d. Aggregates were observed at all depths, and their rapid settling is reflected in identical pyroclast spectra in intermediate and deep water. This implies that particle sorting must have been complete in the upper water column and that the fallout was not perturbed by oceanic currents. The tephra blankets an area of 37 × 10 4 km 2 in a fan-shaped westward distribution, which reflects the prevailing direction of the upper-level winds. Our data show that atmospheric tephra dispersal patterns are recorded almost instantaneously on the deep-sea floor and corroborate the key role of deep-sea ash layers as highly reliable chronostratigraphic markers, and as a geological tool to infer ancient wind fields.

Estimations of primary production and export production in the South China Sea based on sediment trap experiments

The results of time series sediment trap experiments in the South China Sea show that particulate organic carbon (POC) fluxes are influenced by the monsoons. The increase of productivity in the northern South China Sea is mainly due to northeast monsoon while in the central South China Sea the influence of southwest monsoon becomes more prominent. The annual primary production and export production calculated based on POC fluxes are 53.0–63.4 and 10.32–12.93 gC m-2a-1, respectively. The enhancement of POC flux during monsoon period suggest that higher palaeoproductivity or organic carbon accumulation rate in glacial age in the South Chma Sea might be the result of strengthening of the monsoons.

The impact of the 1991 Mount Pinatubo tephra fallout on the geochemical environment of the deep-sea sediments in the South China Sea

Abstract Seven years after the 1991 Mount Pinatubo eruption, the current geochemical environment of ash-covered deep-sea sediments in the South China Sea was investigated. The depth distributions of O2, NO3−, Mn2+, Corg and porosity at six representative sites covered with varying thicknesses of tephra are reported. The shapes of the depth profiles reveal that O2 is totally exhausted within ash layers thicker than 3 cm, while thinner layers are penetrated by oxygen followed by linear downcore profiles with depletion between 8 and 12 cm. Hence, the zone between ash layer and total depletion of oxygen represents a zone of negligible oxygen consumption. Various multi-layer models were developed to explain the observed oxygen concentration profiles. These mathematical simulations also serve to assess both the development since the surface sediments were sealed by the ash fallout as well as the future evolution. Whereas thick tephra layers provoke a nearly stationary situation, i.e. are capable of preventing oxygen from penetrating into the underlying sediments, oxygenation of sediments below thin layers increases rapidly. In these sediments, the effect of oxygen diffusion velocity is drastically reduced by the reaction with dissolved manganese, and therefore explains the linear profiles. Besides, this study is suitable as a case study to assess the effects of human interventions in the deep-sea ecosystem, such as mining and waste dumping.

Vertical changes of POC flux and indicators of early degradation of organic matter in the South China Sea

Tie-series sediment trap materials at different water depths and surface sediments in northern and central South China Sea (SCS) were analyzed for organic carbon, amino acids, amino sugars and carbohydrates. Results show that particulate organic carbon (POC) is mainly derived from marine plankton, only 1.4%–1.6% of primary production sinks into deep SCS water column and less than 0.22 % of primary production ultimately reaches the sediments. The ranineralization and dissolution of organic matter as well as the compositional alterations of organic matter mixtures may mainly take place in the upper few hundred meters of water column, deep carbonate (opal) lysocline zones, and interface layers between sediments and water column, rather than in mid-waters. The organic geochemical parameters such as (Taa+Tsug)OC%, AA/AS, Gluam/Galam, Arom. AA/non-prot. AA, ASP/b-ALA, Glu/g-ABA decrease from living marine plankton (or planktonic shells), to settling particulate matter and to sediments suggesting that they appear to be gad early degraded indicators of organic matter.

Pyrolysis of recent marine sediments—I. Biopolymers

Abstract Analytical conditions for temperature controlled pyrolysis of recent sediments were optimised using 1:10 mixtures of a Skagerrak sediment with various biopolymers. These included polysaccharides, proteins, lignins, pollen, an acylglyceride, ribonucleic acid and a marine humic acid. Lowering the S1 temperature to 180°C and increasing the S2 heating rate to 50 K min −1 was found necessary to give C ORG values comparable with those from other techniques. Recent sediments give complex pyrograms potentially useful in elucidation of source and bulk composition.

Physicochemical analyses of surface sediments from the Lonar Lake, central India - implications for palaeoenvironmental reconstruction

We report the results of our investigations on the catchment area, surface sediments, and hydrology of the monsoonal Lonar Lake, central India. Our results indicate that the lake is currently stratified with an anoxic bottom layer, and there is a spatial heterogeneity in the sensitivity of sediment parameters to different environmental processes. In the shallow (0-5 m) near shore oxic-suboxic environments the lithogenic and terrestrial organic content is high and spatially variable, and the organics show degradation in the oxic part. Due to aerial exposure resulting from lake level changes of at least 3m, the evaporitic carbonates are not completely preserved. In the deep water (>5 m) anoxic environment the lithogenics are uniformly distributed and the delta C-13 is an indicator not only for aquatic vs. terrestrial plants but also of lake pH and salinity. The isotopic composition of the evaporites is dependent not only on the isotopic composition of source water (monsoon rainfall and stream inflow) and evaporation, but is also influenced by proximity to the isotopically depleted stream inflow. We conclude that in the deep water environment lithogenic content, and isotopic composition of organic matter can be used for palaeoenvironmental reconstruction.

Upper Pleistocene cold-water corals from the Inner Sea of the Maldives: taphonomy and environment

Cold-water corals of the Late Pleistocene (21,400–22,500 BP) are recorded from the sea-bottom of two inter-atoll channels (Kardiva Channel at 457-m depth and Malé Vaadhoo Channel at 443-m depth) of the eastern row of the Maldives archipelago. Coral assemblages are composed mainly by Lophelia pertusa and secondarily by Madrepora oculata and Enallopsammia rostrata. These cold-water coral patches are places where the benthic life, mainly sessile, is concentrated, which is clearly absent off-rubble patches. The main epibionts are tube-dwelling polychaetes (mainly Spirorbis and Serpula), bryozoans, siliceous sponges, barnacles, gorgonids, solitary corals, encrusting foraminifera, and microbial mats. The analysis of epibionts assemblages shows different biocoenoses between both studied sites as well as a dependency of the epibiont coverage with regard to the coral genus. Some living benthic organisms such as brachiopods, bivalves, gastropods, barnacles, and ophiuroids find refuge among coral branches. The common record of juvenile specimens of vagile organisms such as small ophiuroids, is probably related to the nursery function of the cold-water corals in spite they are fossils. Environmental requirements of Recent cold-water corals (Lophelia, Madrepora and Enallopsammia) differ of conditions at both sampling sites with sensibly lower oxygenation degree and density of waters than needed for cold-water corals. Therefore, it is proposed that the present-day oxygen and density conditions are the factors which inhibit modern cold-water coral growth in the inter-atoll channels.