Description: The thermal state of the lithosphere and related geothermal heat flow (GHF) is a crucial parameter to understand a variety of processes related to cryospheric, geospheric, and/or biospheric interactions. Indirect estimates of GHF in polar regions from magnetic, seismological, or petrological data often show large discrepancies when compared to thermal in situ observations. Here, the lack of in situ data represents a fundamental limitation for both investigating thermal processes of the lithosphere and validating indirect heat flow estimates. During RV Polarstern expeditions PS86 and PS118, we obtained in situ thermal measurements and present the derived GHF in key regions, such as the Antarctic Peninsula and the Gakkel Ridge in the Arctic. By comparison with indirect models, our results indicate (1) elevated geothermal heat flow (75 ± 5 mW m−2 to 139 ± 26 mW m−2) to the west of the Antarctic Peninsula, which should be considered for future investigations of ice-sheet dynamics and the visco-elastic behavior of the crust. (2) The thermal signature of the Powell Basin characteristic for oceanic crust of an age between 32 and 18 Ma. Further, we propose (3) that at different heat sources at the slow-spreading Gakkel Ridge in the Aurora Vent Field region might explain the geothermal heat flow distribution. We conclude that in situ observations are urgently required to ground-truth and fine-tune existing models and that a multidisciplinary approach is of high importance for the scientific community’s understanding of this parameter

Description: Magmatic sill intrusions into organic-rich sediments cause the release of thermogenic CH4 and CO2. Pore fluids from the Guaymas Basin (Gulf of California), a sedimentary basin with recent magmatic activity, were investigated to constrain the link between sill intrusions and fluid seepage as well as the timing of sill-induced hydrothermal activity. Sampling sites were close to a hydrothermal vent field at the northern rift axis and at cold seeps located up to 30km away from the rift. Pore fluids close to the active hydrothermal vent field showed a slight imprint by hydrothermal fluids and indicated a shallow circulation system transporting seawater to the hydrothermal catchment area. Geochemical data of pore fluids at cold seeps showed a mainly ambient diagenetic fluid composition without any imprint related to high temperature processes at greater depth. Seep communities at the seafloor were mainly sustained by microbial methane, which rose along pathways formed earlier by hydrothermal activity, driving the anaerobic oxidation of methane (AOM) and the formation of authigenic carbonates. Overall, our data from the cold seep sites suggest that at present, sill-induced hydrothermalism is not active away from the ridge axis, and the vigorous venting of hydrothermal fluids is restricted to the ridge axis. Using the sediment thickness above extinct conduits and carbonate dating, we calculated that deep fluid and thermogenic gas flow ceased 28 to 7kyr ago. These findings imply a short lifetime of hydrothermal systems, limiting the time of unhindered carbon release as suggested in previous modeling studies. Consequently, activation and deactivation mechanisms of these systems need to be better constrained for the use in climate modeling approaches.

Description: SO241 set out to test the hypothesis that rift-related magmatism is able to increase carbon emissions from sedimentary basins to the extent that they can actively force climate. To this end we investigated a study area in the Guaymas Basin in the Gulf of California which is one of very few geological settings where rift-related magmatism presently leads to magmatic intrusions into a sediment basin. During the cruise we collected 1100 km of 2D seismic lines to image the extent and volume of magmatic intrusions as well as the extent of metamorphic overprinting of the surrounding sediments and associated subsurface sediment mobilization. We selected three typical seep sites above magmatic intrusions for detailed geochemical studies using gravity corers, multicorers and TV grab. With these samples we will be able to determine the pore water composition to assess the amount and composition of hydrocarbon compounds that are released from these systems. Detailed ocean bottom seismometer measurements at a seep site in the center of the Guaymas Basin will provide further insights into effects of magmatic intrusions on carbon release and diagenetic overprinting of the sediments. It will be possible to reconstruct its long-term seepage history from big carbonate blocks that we have collected with a TV-grab. The northeastern margin of the Guaymas Basin is known for the presence of gas hydrates. During the cruise we collected several seismic lines, which show a clear but unusually shallow BSR indicating high heat flow in the region. Using the seismic data we discovered a previously unknown geological structure on the flank of the northern rift segment: a large mound that seems to consist entirely of black smoker deposits. It seems to be the result of a recent intrusion into the underlying sediments and changes the view how such systems function. The structure was investigated with a comprehensive geochemical, geothermal, and video surveying program which revealed at least seven vents that are active simultaneously. These vents inject methane and helium-rich vent fluids several hundred meters up into the water column. These findings suggest that large-scale magmatism, for example during the opening of an ocean basin under the influence of a hot spot, can be an effective way of liberating large amounts of carbon high up into the water column. The data collected during SO241 will allow us to constrain the amount of carbon that can escape into the atmosphere during LIP emplacement and their relevance on a global scale can be assessed. In addition to reaching the main objectives of the project we discovered a large landslide complex that was probably associated with a tsunami.

Description: Seafloor heat flow provides information about the thermal evolution of the lithosphere, the magnitude and timing of volcanic activity, and hydrothermal circulation patterns. In the central Gulf of California, the Guaymas Basin is part of a young marginal spreading rift system that experiences high sedimentation (1–5 km/Myr) and widespread magmatic intrusions in the axial troughs and the off-axis regions. Heat flow variations record magmatic and sedimentary processes affecting the thermal evolution of the basin. Here, we present new seismic evidence of a widespread bottom-simulating reflection (BSR) in the northwestern Guaymas Basin. Using the BSR depths and thermal conductivity measurements, we determine geothermal gradient and surface heat flow variations. The BSR-derived heat flow values are less than the conductive lithospheric heat flow predictions for mid-oceanic ridges. They suggest that high sedimentation (0.3–1 km/Myr) suppresses the lithospheric heat flow. In the central and southeastern regions of the basin, the BSR-derived geothermal gradient increases as the intruded magmatic units reach shallower subsurface depths. Thermal modeling shows that recent (〈5000 years) igneous intrusions (〈500 m below the seafloor) and associated fluid flow elevate the surface heat flow up to five times. BSR-derived geothermal gradients correlate little with the depth of the shallowest magmatic emplacements to the north, where the intrusions have already cooled for some time, and the associated hydrothermal activity is about to shut down. Key Points - A regional bottom-simulating reflection (BSR) in the Guaymas Basin indicates a widespread occurrence of gas hydrates - The BSR derived thermal gradients show wavy patterns farther away from the spreading centre, indicating strong lateral heat flow variations - High sedimentation suppresses heat flow, while recent magmatic intrusion and fluid advection increase heat flow

Description: Temperature and bioavailable energy control the distribution of life on Earth, and interact with each other due to the dependency of biological energy requirements on temperature. Here we analyze how temperature-energy interactions structure sediment microbial communities in two hydrothermally active areas of Guaymas Basin. Sites from one area experience advective input of thermogenically produced electron donors by seepage from deeper layers, whereas sites from the other area are diffusion-dominated and electron donor-depleted. In both locations, Archaea dominate at temperatures 〉45 °C and Bacteria at temperatures 〈10 °C. Yet, at the phylum level and below, there are clear differences. Hot seep sites have high proportions of typical hydrothermal vent and hot spring taxa. By contrast, high-temperature sites without seepage harbor mainly novel taxa belonging to phyla that are widespread in cold subseafloor sediment. Our results suggest that in hydrothermal sediments temperature determines domain-level dominance, whereas temperature-energy interactions structure microbial communities at the phylum-level and below.

Description: Here we present a dataset of pore water chloride and sulfate concentrations of gravity core GeoB21363-1 (GC21) recovered during R/V Poseidon cruise POS499 from the summit of Sartori Mud Volcano (Calabrian Arc, Mediterranean Sea). The gravity core was taken in a plastic foil. Pore water was extracted ex-situ from selected sediment depths on whole round plastic foil using disposable syringes and rhizon samplers. The sampling interval was set to 10 cm. The untreated pore water samples of filtered and unacidified aliquots were stored at 4°C until analysis. Concentrations of chloride and sulfate were analyzed by ion chromatography (IC) using a Metrohm 882 Compact Ion Chromatograph with an 858 Professional Sample Processor at MARUM. Uploaded chloride and sulfate concentrations are given in in Millimol/Liter (mmol/L). Sediment depth is given in meter (m).

Description: Here we present a dataset of in-situ sediment temperatures of gravity core GeoB21363-1 (GC21) recovered during R/V Poseidon cruise POS499 from the summit of Sartori Mud Volcano (Calabrian Arc, Mediterranean Sea). In-situ temperature was measured using Miniaturized Temperature Data Loggers which were mounted on the core barrel of the gravity corer. In total, five loggers were used and measured temperatures in an interval of 0.5 up to 1 m. The gravity corer remained in the seabed for at least 10 minutes to obtain undisturbed temperature conditions of the seabed. Logger specific serial numbers are listed, penetration depth within the seafloor is given in meter (m) and in-situ temperature is given in Celsius (°C).

Description: Here we present pore water chloride and sulfate concentrations of several gravity and multi cores measured during R/V Sonne cruise SO278 at Sartori Mud Volcano (Calabrian Arc, Mediterranean Sea) to investigate the mud volcanic activity in more detail. Pore water was extracted ex-situ from selected sediment depths on whole round plastic liners using disposable syringes and rhizon samplers. Sampling interval for pore water measurements of chloride and sulfate ranges from 5 cm (MUC cores) to 10 cm (GC cores). The untreated pore water samples of filtered and unacidified aliquots were stored at 4°C until analysis. Concentrations of chloride and sulfate were analyzed by ion chromatography (IC) using a Metrohm 882 Compact Ion Chromatograph with an 858 Professional Sample Processor at MARUM.

Description: Here we present pore water chloride and sulfate concentrations of several gravity and multi cores measured during R/V Sonne cruise SO278 at Sartori Mud Volcano (Calabrian Arc, Mediterranean Sea) to investigate the mud volcanic activity in more detail. Pore water was extracted ex-situ from selected sediment depths on whole round plastic liners using disposable syringes and rhizon samplers. Sampling interval for pore water measurements of chloride and sulfate ranges from 5 cm (MUC cores) to 10 cm (GC cores). The untreated pore water samples of filtered and unacidified aliquots were stored at 4°C until analysis. Concentrations of chloride and sulfate were analyzed by ion chromatography (IC) using a Metrohm 882 Compact Ion Chromatograph with an 858 Professional Sample Processor at MARUM.

Description: Here we present pore water chloride and sulfate concentrations of several gravity and multi cores measured during R/V Sonne cruise SO278 at Sartori Mud Volcano (Calabrian Arc, Mediterranean Sea) to investigate the mud volcanic activity in more detail. Pore water was extracted ex-situ from selected sediment depths on whole round plastic liners using disposable syringes and rhizon samplers. Sampling interval for pore water measurements of chloride and sulfate ranges from 5 cm (MUC cores) to 10 cm (GC cores). The untreated pore water samples of filtered and unacidified aliquots were stored at 4°C until analysis. Concentrations of chloride and sulfate were analyzed by ion chromatography (IC) using a Metrohm 882 Compact Ion Chromatograph with an 858 Professional Sample Processor at MARUM.