ALBERT

Description: EGU2010-10518 Seafloor compliance is the transfer function between pressure and vertical displacement at the seafloor Infragravity waves in the oceanic layer have long periods in the range of 30 – 500 s and obey a simple frequencywavenumber relation. Seafloor compliance from infragravity waves can be analyzed with single station recordings to determinate sub-seafloor shear wave velocities. Previous studies in the Pacific Ocean have demonstrated that reliable near-surface shear wave profiles can be derived from infragravity wave compliance. However, these studies indicate that, beside the water depth the compliance measurements are limited by instrument sensitivity, calibration uncertainties and possibly other effects. In this work seafloor compliance and infragravity waves are observed at two different locations in the Atlantic Ocean: the Logatchev hydrothermal field at the Mid Atlantic Ridge and the Azores (Sao Miguel Island). The data was acquired with the broadband ocean compliance station developed at the University of Hamburg as well as ocean station from the German instrument pool for amphibian seismology (DEPAS) equipped with broadband seismometers and pressure sensors. Vertical velocity and pressure data were used to calculate power spectral densities and normalized compliance along two profiles (one in each location). Power spectral densities show a dominant peak at low frequencies (0.01-0.035Hz) limited by the expected cut-off frequency, which is dependent on the water depth at each station. The peak has been interpreted as a strong infragravity wave with values between 10-14 and 10-11 (m/s2)2/Hz and 104 and 106 (Pa2)2/Hz for acceleration and pressure respectively. The results show compliance values between 10-10 and 10-8 1/Pa and its estimations take into account the coherence between seismic and pressure signals in order to confirm that the seismic signals in the infragravity waves are caused by pressure sources. Shear wave velocity models, with depth resolution from 200 to 2500 m for the deep water stations, were derived from compliance. Preliminary results indicate shear wave velocity increasing from 200 to 3500 m/s.

Description: EGU2010-9841 Active mud volcanoes, where changing salinities of pore fluids, large temperature gradients and occurrences of free gas are frequently observed, should potentially exhibit significant variability in their internal resistivity structure. This is due to the fact that the bulk resistivity is mainly determined by the porosity of sediments and the electrical resistivity of the pore filling contained therein. The resistivity variations may be derived from controlled source electromagnetic (CSEM) measurements. CSEM systems consist of an electric dipole transmitter producing a time varying source field and electric dipole receivers, which measure the earth´s response to this signal. For a RWE Dea funded investigation of fluid and gas leakages at the North Alex Mud Volcano (NAMV) - a comparatively small target with an area of about 1km2 - we have developed a new high resolution CSEM system. The system consists of several autonomous electric dipole receivers and a lightweight electric dipole transmitter, which can be mounted on a small remotely operated underwater vehicle (ROV). The use of a ROV allows for a precise placement of the transmitter, which is a necessary prerequisite for the investigation of such a small target. Furthermore, electromagnetic signals may be transmitted from different directions with respect to the stationary receivers, allowing for a 3D-style tomographic experiment. In this experiment, ten receivers were deployed over the surface of NAMV at a total of 16 receiver locations. During three successful dives with a Cherokee ROV (Ghent University, Belgium), the transmitter was deployed at a total of 80 locations. Here we present first quantitative results consisting of apparent resistivity estimations from the CSEM time domain data for each transmitter-receiver pair. The apparent resistivity map shows that the NAMV indeed has a heterogeneous resistivity structure with apparent resistivities varying by at least a factor of two: low apparent resistivities (~ 0.8Ωm) are found towards the center of the MV, whereas higher apparent resistivities (~ 1.6Ωm) prevail away from the center. In a second step, we interpret the time-domain data based on 1D inversions. Good data fits can be achieved by models containing 2-3 layers. Generally, the models indicate low resistivities at the surface, which can be associated with penetrating salt water and/or high temperatures. Toward greater depths, increasing resistivities presumably are due to a combination of compaction of sediments (i.e. reduced pore space), an increased presence of fresh water and possible occurrences of free gas. For some 1D models, the increase in resistivity exceeds a factor of 10 or more and layer interfaces are indicated down to depths of up to 70m. The derived resistivity variations observed at the NAMV will be interpreted in conjunction with temperature (Feseker, this session), fluid flow (Brückmann et al., this session) and seismic data (Bialas et al., this session) acquired. Temperature variations measured in the upper few meters are related to fluid flow, where high temperatures are indicative of upwelling fluids of low salinity and low temperature of either a downward flow of saline fluids or no flow activity. This type of surface measurement constitutes an integrative fluid flow gauge, which we can resolve vertically with our resistivity models. Seismic data yield a background structure to our resistivity model. New analysis of seismic data shows that seismic activity may also be linked to fluid flow activity, which we aim to match with resistivity variations and oscillations, which were observed in the electric and magnetic fields (Lefeldt et al., this session).

Description: Perturbations in stratospheric aerosol due to explosive volcanic eruptions are a primary contributor to natural climate variability. Observations of stratospheric aerosol are available for the past decades, and information from ice cores has been used to derive estimates of stratospheric sulfur injections and aerosol optical depth over the Holocene (approximately 10 000 BP to present) and into the last glacial period, extending back to 60 000 BP. Tephra records of past volcanism, compared to ice cores, are less complete but extend much further into the past. To support model studies of the potential impacts of explosive volcanism on climate variability across timescales, we present here an ensemble reconstruction of volcanic stratospheric sulfur injection (VSSI) over the last 140 000 years that is based primarily on terrestrial and marine tephra records. VSSI values are computed as a simple function of eruption magnitude based on VSSI estimates from ice cores and satellite observations for identified eruptions. To correct for the incompleteness of the tephra record, we include stochastically generated synthetic eruptions assuming a constant background eruption frequency from the ice core Holocene record. While the reconstruction often differs from ice core estimates for specific eruptions due to uncertainties in the data used and reconstruction method, it shows good agreement with an ice-core-based VSSI reconstruction in terms of millennial-scale cumulative VSSI variations over the Holocene. The PalVol reconstruction provides a new basis to test the contributions of forced vs. unforced natural variability to the spectrum of climate and the mechanisms leading to abrupt transitions in the palaeoclimate record with low- to high-complexity climate models. The PalVol volcanic forcing reconstruction is available at https://doi.org/10.26050/WDCC/PalVolv1 (Toohey and Schindlbeck-Belo, 2023).

Description: The Tristan da Cunha (TDC) is a volcanic island located above a prominent hotspot in the Atlantic Ocean. Many geological and geochemical evidences support a deep origin of the mantle material feeding the hotspot. However, the existence of a plume has not been confirmed as an anomalous structure in the mantle resolved by geophysical data because of lack of the observations in the area. Marine magnetotelluric and seismological observations were conducted in 2012–2013 to examine the upper mantle structure adjacent to TDC. The electrical conductivity structure of the upper mantle beneath the area was investigated in this study. Three-dimensional inversion analysis depicted a high conductive layer at ~ 120 km depth but no distinct plume-like vertical structure. The conductive layer is mostly flat independently on seafloor age and bulges upward beneath the lithospheric segment where the TDC islands are located compared to younger segment south of the TDC Fracture Zone, while the bathymetry is rather deeper than prediction for the northern segment. The apparent inconsistency between the absence of vertical structure in this study and geochemical evidences on deep origin materials suggests that either the upwelling is too small and/or weak to be resolved by the current data set or that the upwelling takes place elsewhere outside of the study area. Other observations suggest that 1) the conductivity of the upper mantle can be explained by the fact that the mantle above the high conductivity layer is depleted in volatiles as the result of partial melting beneath the spreading ridge, 2) the potential temperature of the segments north of the TDC Fracture Zone is lower than that of the southern segment at least during the past ~ 30 Myr.

Description: The active volcanic island Tristan da Cunha, located at the southwestern and youngest end of the Walvis Ridge – Tristan/Gough hotspot track, is believed to be the surface expression of a huge thermal mantle anomaly. While several criteria for the diagnosis of a classical hotspot track are met, the Tristan region also shows some peculiarities. Consequently it is vigorously debated if the active volcanism in this region is the expression of a deep mantle plume, or if it is caused by shallow plate tectonics and the interaction with the nearby Mid-Atlantic Ridge. Because of a lack of geophysical data in the study area, no model or assumption has been completely confirmed. We present the first amphibian P-wave finite-frequency travel time tomography of the Tristan da Cunha region, based on cross-correlated travel time residuals of teleseismic earthquakes recorded by 24 ocean-bottom seismometers. The data can be used to image a low velocity structure southwest of the island. The feature is cylindrical with a radius of ∼100km down to a depth of 250km. We relate this structure to the origin of Tristan da Cunha and name it the Tristan conduit. Below 250km the low velocity structure ramifies into narrow veins, each with a radius of ∼50km. Furthermore, we imaged a linkage between young seamounts southeast of Tristan da Cunha and the Tristan conduit.

Description: The most prominent hotspot in the South Atlantic is Tristan da Cunha, which is widely considered to be underlain by a mantle plume. But the existence, location and size of this mantle plume have not been established due to the lack of regional geophysical observations. A passive seismic experiment using ocean bottom seismometers aims to investigate the lithosphere and upper mantle structure beneath the hotspot. Using the Ps receiver function method we calculate a thickness of 5 to 8 km for the oceanic crust at 17 ocean-bottom stations deployed around the islands. Within the errors of the method the thickness of the oceanic crust is very close to the global mean. The Tristan hotspot seems to have contributed little additional magmatic material or heat to the melting zone at the mid-oceanic ridge, which could be detected as thickened oceanic crust. Magmatic activity on the archipelago and surrounding seamounts seems to have only effected the crustal thickness locally. Furthermore, we imaged the mantle transition zone discontinuities by analysing receiver functions at the permanent seismological station TRIS and surrounding OBS stations. Our observations provide evidence for a thickened (cold) mantle transition zone west and northwest of the islands, which excludes the presence of a deep-reaching mantle plume. We have some indications of a thinned, hot mantle transition zone south of Tristan da Cunha inferred from sparse and noisy observations, which might indicate the location of a Tristan mantle plume at mid-mantle depths. Sp receiver functions image the base of lithosphere at about 60 to 75 km beneath the islands, which argues for a compositionally controlled seismological lithosphere-asthenosphere boundary beneath the study area.