Drilling the Bottom of the Sea

There are continuing expeditions to drill into the seafloor. Last month, the ocean drilling ship, JOIDES Resolution, was the first to ever drill into the Mantle. Drilling into the Mantle was done at the Atlantis Massif. There is a hydrothermal field called the Lost City on the ocean floor. Drilling in and under the seafloor gives us the history of life and vent fields could be similar to the moons of Saturn and Jupiter.

Above is a sample from the seafloor. The chalk on top is lots and lots of dead nannofossils that rained down from above.

The Atlantis Massif is an oceanic core complex at 30°N on the Mid-Atlantic Ridge on the north side of the Atlantis I transform fault, which offsets the ridge by ~60 kilometers. The massif is capped by a corrugated fault zone, and is composed of a large gabbro intrusion into serpentinized mantle rocks to the south. These altered mantle rocks host the Lost City hydrothermal field, which is famous for carbonate chimneys the height of a house, venting alkaline fluids rich in hydrogen and methane. The hydrogen is formed by the reaction between seawater and mantle mineral oliv- ine, and is a powerful source of energy that may have fueled the formation of the first building blocks of life on Earth. Before life could begin, small organic molecules must have formed abiotically. Scientists have suggested that vent fields such as Lost City may be an analog of systems where these prebiotic reactions occurred, leading to the early development of life. Similar systems may be present on “icy worlds” such as Enceladus, which is one of the moons of Saturn, and capable of supporting life.

The oceanic core complex comprising Atlantis Massif was formed within the past 1.5-2 Million years ago at the intersection of the Mid-Atlantic Ridge, 30°N, and the Atlantis Transform Fault. The corrugated, striated central dome prominently displays morphologic and geophysical characteristics representative of an ultramafic core complex exposed via long-lived detachment faulting. Sparse volcanic features on the massif’s central dome indicate that minor volcanics have penetrated the inferred footwall, which geophysical data indicates is composed predominantly of variably serpentinized peridotite. In contrast, the hanging wall to the east of the central dome is comprised of volcanic rock. The southern part of the massif has experienced the greatest uplift, shoaling to less than 700 m below sea level, and the coarsely striated surface there extends eastward to the top of the median valley wall. Steep landslide embayments along the south face of the massif expose cross sections through the core complex. Almost all of the submersible and dredge samples from this area are deformed, altered peridotite and lesser gabbro. Intense serpentinization within the south wall has likely contributed to the uplift of the southern ridge and promoted the development of the Lost City Hydrothermal Field near the summit. Differences in the distribution with depth of brittle deformation observed in microstructural analyses of outcrop samples suggest that low-temperature strain, such as would be associated with a major detachment fault, is concentrated within several tens of meters of the domal surface. However, submersible and camera imagery show that deformation is widespread along the southern face of the massif, indicating that a series of faults, rather than a single detachment, accommodated the uplift and evolution of this oceanic core complex.

Life Cycle of Oceanic Crust

All rocks in Earth’s crust are constantly being recycled through the rock cycle. The rock cycle is the transition of rocks among three different rock types over millions of years of geologic time. Igneous rock is formed by the cooling and crystallization of molten magma at volcanoes and mid-ocean ridges, where new crust is generated. Examples of igneous rock are basalt, granite, and andesite. Over time, igneous rocks may experience weathering and erosion from exposure to water and the atmosphere to produce sediments. The deposition and hardening of these sediments forms sedimentary rocks. Both igneous and sedimentary rock types can transform physically and chemically into a third rock type. Metamorphic rocks are formed when igneous or sedimentary rocks are exposed to conditions of high heat and pressure. Examples of metamorphic rock include marble, slate, schist, and gneiss. Metamorphic rocks can also transform to sedimentary rocks through weathering, erosion, and sediment deposition.

Seafloor Volcanoes and Hydrothermal Vents
Mid-ocean ridges and spreading zones are home to hydrothermal vents. Hydrothermal vents in the ocean are analogous to geysers and hot springs on continents where groundwater percolates up to 2 km below the surface to areas that are very hot. The resulting boiling water and steam rush to the surface. At hydrothermal vents, cool seawater percolates down in fissures and cracks created by the spreading seafloor. As water moves down, it is heated from geothermal sources, reaching temperatures as high as 400 °C. Throughout this process, minerals like copper, zinc, iron, and sulfur dissolve in the water. Although the water is very hot, it does not boil due to the high hydrostatic pressure. When the super heated water rises out through the vents because it is buoyant, it meets relatively cold and oxygen rich ocean water and many of the dissolved minerals precipitate out as particles. If the majority of precipitates are sulfides and have a black color, the vents are known as black smokers due to their dark billowing appearance. White smokers emit minerals with lighter hues. In some cases these particles combine to form chimney structures around the vents. In 2000 scientists discovered a field of chimneys in the Atlantic ocean basin that had reached 55 meters tall. Hydrothermal vents are found in spreading regions on the seafloor.

Depth of Drilling Terms and Units

The measurement of depth is a central concept for IODP activities. Use of the units meters below sea floor (mbsf) and meters below rig floor (mbrf) in ODP/IODP is inadequate because innovations in the methods by which depth is measured or calculated have progressed.

Sample notes from Some Days of Seafloor Drilling

Daily science report for 3 June 2023
Location: Hole U1309D (30°10.1195′N, 42°7.1131′W; water depth 1644.9 m)

Science Update: After reentry into Hole U1309D at the end of the previous day, the drill string was positioned at 32 mbsf at 0045 h. Two Kuster FTSes were made up together with the Elevated Temperature Boreholes Sonde (ETBS) and the Conductivity, Temperature, Depth (CTD) tool. The first run on the coring line was deployed at 0100 h and was back on the rig floor at 0315 h. The borehole water samples were taken at 200 mbsf and 400 mbsf (meters below seafloor). The tool string was retrieved and disassembled, the water samples and data were retrieved, and the tools were reassembled for the second run. The CTD was not included anymore because its temperature rating would have been exceeded at the deeper sampling stations. The second sampling run from 0400 h to 0645 h triggered the Kuster FTS at 550 mbsf and 736 mbsf. Ample time was available to redress the tools and conduct a third run from 0736 h to 1100 h. During this final run, the tools were not able to pass an obstruction at 1024 mbsf and the samples were taken at 923 mbsf and 1110 mbsf meters below seafloor) instead of a deeper planned station. After conclusion of the sampling runs, the drill string was tripped to 1421 mbsf and washed down to the bottom of the hole at 1498 mbsf. Next, the hole was flushed with seawater seven times the borehole volume to leave behind as clean as possible a hole for potential future water sampling and temperature measurement operations. Drill string retrieval began at 1945 h and was still in progress at midnight.

Daily Science Report for 30 April 2023
Location: Hole U1309D (30°10.1195′N, 42°7.1131′W; water depth 1644.9 m)

Science Update: Today we recovered Cores U1309D-305R through 309R from 1454.6 mbsf to 1478.6 mbsf. We recovered a total of 9.52 m from the 24.0 m cored interval, with core recoveries ranging from 27% to 66% (average of 40% recovery). 30 bbl mud sweeps were pumped every ~5 m.

Cores U1309D-300R through 305R consist predominantly of slightly to moderately altered, coarse to medium-grained olivine-bearing gabbro, with intervals of sparsely olivine-bearing gabbro, orthopyroxene-bearing gabbro, aphyric and sparsely olivine plagioclase phyric diabase, and leucocratic diorite.