OS42A-01
Microbial Communities at Non-Volcanic and Volcanic Sites of the Gakkel Ridge
The Gakkel Ridge in the eastern Arctic Ocean is the slowest spreading, deepest, and most isolated portion of the global mid-ocean ridge system and therefore predestined for comparative investigations on deep-sea vent communities. However, the perennial cover of thick sea ice has made this area largely inaccessible to science. The Arctic Gakkel Vents Expedition (AGAVE) utilized the icebreaker ODEN and newly developed vehicles for exploration and sampling in connection with a CTD/rosette equipped with different sensors and a high-resolution multi-beam bathymetry system. We focused our studies on the peridotite-hosted region at 85°N, 7°E and on the basaltic volcanism area at 85°N, 85°E. Water, sediment, and rock samples were taken to describe the microbial communities in different zones of these two sites. Sampling was guided by anomalies of backscattering, temperature, Eh, as well as by high-resolution seafloor imagery. Samples were preserved or processed on board immediately after sampling. Molecular analyses, cultural methods, total bacterial counts, and activity measurements were employed to describe the structure of the microbial communities, their phylogeny, potential adaptations, and possible role in biogeochemical cycles. The first molecular biological results of the bacterial communities of the 85°E site indicated atypical of deep- sea venting communities. These preliminary results were supported by the images of the under-ice vehicle "Camper" which showed thick yellow "fluffy" mats (often > 5cm thick) and orange "pebbly" material without any smell of H2S markedly different than the white, consolidated Beggiatoa mats often observable at deep venting sites. Foraminifera occurred regularly on top of basalt rocks as well as within the bacterial mats.
OS42A-02
Characteristic Microearthquakes of the Active Submarine Volcanic Complex at 85°E, Gakkel Ridge
New oceanic crust is formed at mid-ocean ridges by magmatic intrusion, volcanic eruptions, tectonic extension and hydrothermal cooling. These active processes give rise to earthquakes, which are usually too small to be recorded on land, but can be recorded locally with ocean bottom seismometers (OBS). OBS studies on slow and fast spreading ridges have greatly contributed to our understanding of active seafloor spreading. However, at ultraslow spreading rates, predictions of amagmatic spreading proved incorrect and the processes of crustal generation are still poorly understood, partly because of the remote location of the main ultraslow ridges like the Arctic ridge system. No comprehensive microearthquake studies of ultraslow-spreading ridges have yet been undertaken. During the Arctic Mid-Ocean Ridge Expedition(AMORE2001), we made a first attempt at observing microearthquakes at Gakkel Ridge in the ice-covered Arctic Ocean using ice floes as platforms for small seismological arrays consisting of four conventional short-period seismometers. We successfully recorded microearthquakes including a swarm of 200 explosive sounds at the volcanic complex near 85°E which was active in 1999. We interpreted these sounds as signs of an ongoing eruption. The presence of a hydrothermal event plume in the water column supported this interpretation. Following this pilot study, we now started a comprehensive and comparative study of the seismicity of ultraslow- spreading ridges. As part of this project, we conducted a microseismicity survey during the Arctic Gakkel Vents Expedition (AGAVE2007). We deployed a network of three seismological arrays in the rift valley at the 85°E volcanic complex greatly improving the detection threshold and location capabilities compared to AMORE2001. The arrays had triangular shape with a central station. Each array measured about 1 km in size and was located on one ice floe. The inter-array distance was 15 km. The arrays recorded continuously at a sampling rate of 100 Hz for 16 days. We present the results of a first screening of the microearthquake data of AGAVE2007 showing typical seismic events, their frequency and waveform characteristics. We compare the seismicity patterns to those of the pilot study AMORE2001, especially to the explosive volcanic activity of 2001 which appears to have ceased by now.
OS42A-03
Mapping of Hydrothermal Plumes on the Gakkel Ridge During AGAVE 2007
During the Arctic Gakkel Vents Expedition in July and August, 2007, hydrothermal plumes were located and mapped in two distinct regions of the Gakkel Ridge, using both a CTD-rosette and the AUV PUMA, deployed from the icebreaker Oden and equipped with optical (backscatter and transmission) and redox (Eh) sensors in addition to standard CTD instrumentation. CTD casts were conducted in two modes, standard vertical casts and "drift-yo's", which are analogous to tow-yos but whose speed and direction are determined by the ice drift rather than purposeful movement of the ship. At 7.5 degrees east, two MAPR profiles separated by about 10 km in 2001 showed sharp anomalies in temperature and optical backscatter at about 2800 m water depth. We conducted 16 CTD casts in this region, successfully relocating the plume at 2800 m and finding it to be confined to a narrow (approximately 800 m wide in the across-axis direction), along-axis flow. While the amplitude and smoothness of the temperature and backscatter profiles varied with location indicating relative proximity to the source of the plume, no Eh anomalies were observed nor was a seafloor source located. At the volcanically active 85 degrees E site, a total of 20 CTD casts and drifts, and 3 PUMA dives identified at least 6 different plumes, that can be differentiated based on their depths, spatial variability, and/or the strength and nature of the various signals obtained, but again no seafloor source was localized.
OS42A-04
Physical properties and constraints of hydrothermal plumes on the Gakkel Ridge during AGAVE 2007
The unique hydrographic characteristics of the Arctic Ocean have important implications for the dynamical behavior of hydrothermal plumes. Some of the main issues include the weak density stratification of the deep bottom layer, topographical effects from a deep axial valley, and high-latitude tides. We address these issues using analytical and numerical models, and comparing the results to hydrographic water column plume data acquired during the Arctic Gakkel Vents Expedition (AGAVE) from July 1 to August 10, 2007. A total of 36 CTD casts were conducted from the icebreaker Oden at two main sites (85N 7E and 85N 85E), where different modes of hydrothermal circulation appear to generate different kinds of water column plumes. Several plume signals of varying thickness and rise height above the bottom were observed, which implies that several seafloor sources with distinct discharge characteristics were active during the surveys. We use our models to constrain the character of the seafloor sources, and discuss observational strategies for future field work aimed at locating and mapping hydrothermal sources in the deep Arctic.