Geochemistry and age determination of sediments from teh Rainbow hydrothermal plume

A geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the Rainbow hydrothermal field. As well as a large biogenic component (>80% CaCO3) these sediments record clear enrichments of the elements Fe, Cu, Mn, V, P, and As from hydrothermal plume fallout but only minor detrital background material. Systematic variations in the abundances of "hydrothermal" elements are observed at increasing distance from the vent site, consistent with chemical evolution of the dispersing plume. Further, pronounced Ni and Cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near-field locations. Radiocarbon dating reveals mean Holocene accumulation rates for all four cores of 2.7 to 3.7 cm.kyr?1, with surface mixed layers 7 to 10+ cm thick, from which a history of deposition from the Rainbow hydrothermal plume can be deduced. Deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced (e.g., Fe, Mn, Cu) or scavenged from seawater via the hydrothermal plume (e.g., V, P, As). Holocene fluxes into to the cores' surface mixed layers are presented which, typically, are an order of magnitude greater than "background" authigenic fluxes from the open North Atlantic. One core, collected closest to the vent site, indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 14C kyr ago; the preferred explanation is that this variation reflects the initiation/intensification of hydrothermal venting at the Rainbow hydrothermal field at this time - perhaps linked to some specific tectonic event in this fault-controlled hydrothermal setting.

Biogenic silica in the coastal plume of the Mississippi River

The surface distributions of dissolved silicic acid, chlorophyll and diatom abundance were measured in the plume of the Mississippi River and adjacent waters during spring (late April and early May 1993) and summer (July 1992). In spring, the time of maximum river flow, there was an intense diatom bloom with a mean diatom abundance of 1.5 x 10**7 cells/l, more than an order of magnitude higher than in summer. Mixing curves of silicic acid concentration ([Si(OH)4]) versus salinity indicate that biological uptake within the river plume removed >99% of the Si(OH)4 supplied by the river in spring and 80 to 95% in summer. In spring [Si(OH)4] was occasionally depleted to <0.2 µM-among the lowest values ever reported from the ocean-with extensive depletion to >=0.5 µM over the shelf. In summer [Si(OH)4] was less severely depleted; the lowest measured was 0.93 µM and all others were >=2.4 µM. 30Si kinetic experiments were performed during both spring and summer to measure the degree to which the rate of Si uptake by the natural diatom assemblages was limited in situ by substrate availability. In spring the dependence of the specific uptake rate (V) on extracellular [Si(OH)4] conformed much more closely to the Michaelis-Menten saturation function than has been observed in past studies. Strong dependence of V on [Si(OH)4] was observed throughout the most Si(OH)4-depleted (<0.5 µM) region, where V was limited to 12 to 45% of the diatom assemblages' maximum uptake rate (Vmax). Half-saturation concentrations for Si uptake (Ks) averaged 0.85 uM (range = 0.48 to 1.71; n = 7) in spring, with the lowest values equal to the lowest previously reported for natural diatom assemblages. There was only 1 station in summer where V was limited by [Si(OH)4], and at that station Ks was 5.3 µM-quite high in comparison with previous studies. At stations where V was limited by [Si(OH)4], in both spring and summer, Chaetoceros spp. were numerically dominant; where there was no Si limitation other diatoms, usually Skeletonema costatum, dominated. The data thus indicate strong Si limitation in spring, with diatom assemblages well adapted to low [Si(OH)4], but little or no Si limitation in summer. Historical data suggest that coastal Si(OH)4 depletion and Si limitation may be recent phenomena in the northern Gulf of Mexico, resulting from increasing [NO3-] and decreasing [Si(OH)4] in the Mississippi River during the past 30 to 50 yr.

(Table 1) Turbidity, temperature, and salinity in the hydrothermal plume and background waters above the northern part of the Mohn's Ridge at Station AMK36-3573-5

In July 1995 geological and biological studies in the axial zone of the northern part of the Mohn's Ridge (72°20'N) were carried out during Cruise 36 of R/V Akademik Mstislav Keldysh. Slopes of the neovolcanic zone, as well as a caldera on its crest were investigated with use of deep-sea manned submersibles Mir, geological and biological samples were also collected. Use of the Rosette sounding complex provided recognition of several major hydrothermal plumes. Bottom sediments of the marginal depression are enriched in metals characteristic for hydrothermal metalliferous sediments. Thus, a new unknown hydrothermal field was found.

Plume-top altitude time-series during May 2011 volcanic eruption of Grímsvötn, Iceland

Plume-top altitude time series of the volcanic plume during the eruption of Grímsvötn in Iceland 21-28 May 2011. The altitude was estimated from weather radar echo top data from two weather radars, Keflavik and Klaustur. Keflavik radar is a fixed position C-band weather radar close to Keflavik International Airport, at 64°01.583'N, 22°38.150'W. The height of the antenna is 47 m a.s.l. and the distance to Grímsvötn volcano is 257 km. Klaustur radar is a mobile X-band weather radar located close to Kirkjubaejarklaustur, at 63°46.500'N, 17°57.817'W. The height of the antenna is also 47 m a.s.l. and the distance to Grímsvötn volcano is 75 km.

Plume-top altitude time-series during 2010 volcanic eruption of Eyjafjallajökull

The eruption of Eyjafjallajökull volcano in 2010 lasted for 39 days, 14 April-23 May. The eruption had two explosive phases separated by a phase with lava formation and reduced explosive activity. The height of the plume was monitored every 5 min with a C-band weather radar located in Keflavík International Airport, 155 km distance from the volcano. Furthermore, several web cameras were mounted with a view of the volcano, and their images saved every five seconds. Time series of the plume-top altitude were constructed from the radar observations and images from a web camera located in the village Hvolsvöllur at 34 km distance from the volcano. This paper presents the independent radar and web camera time series and performs cross validation.