Expeditions

Development and Field Testing of the Low-Cost DIC Sensor

Significant progress has been made in the development of the low-cost sensor for the autonomous measurement of dissolved inorganic carbon (DIC). The sensor has undergone a series of field deployments in the Baltic and North Sea. These deployments have been an essential part of the development process as they allowed the performance of the prototype to be evaluated under realistic conditions and provided valuable information for further improvements. The shallow-water DIC sensor, assembled at the end of Phase I, was first tested during the Littorina cruise L25-01 in the Baltic Sea in January 2025. The measurements were scattered, with temperature effects identified as an important factor for the sensor response. Following this deployment, laboratory experiments were conducted to characterise the temperature dependence of the baseline and CO₂ response. The sensor was immersed into a water bath over a temperature range of 5–40 °C. This work allowed the formulation of a temperature compensation algorithm for more consistent DIC measurements under different environmental conditions.

Sensor immersed into a temperature-controlled water bath at the GEOMAR laboratory

A second deployment took place during the Heincke cruise HE673/2 in the North Sea in November 2025. A blockage in the fluidic lines was found during the deployment. Modifications of the fluidic pathway and hardware were therefore undertaken to improve sample flow and reduce the risk of blockage with the addition of an acid flush. Further field tests were conducted during the Littorina cruise L26-07 in the North Sea in May 2026, followed by a deployment aboard the Alkor during cruise AL656 in July 2026. These deployments provided an opportunity to evaluate the successive hardware, fluidic and software modifications under field conditions. Each cruise has provided new information that has been used to refine the system and prepare it for the next deployment, progressively improving the robustness and reliability of the current DIC sensor.

DIC sensor deployed into a water tank continuously supplied with underway water on the Littorina deck in May 2026.

The third RIFLOR cruise, expedition M213 on RV Meteor, had the primary task to recover and replace the MeBo borehole observatories set two years earlier on expedition MSM119. The exchange was successful at Squid Pond, our key location, but strong currents at the second location closer to the ridge axis prevented the use of the MARUM ROV. We also ran multi-channel seismic measurements along 366 nautical miles and gained valuable insights regarding the thickness of the sediment layer above the basaltic oceanic crust. High-resolution mapping of the seafloor morphology with the EM122 multibeam echo sounder was also significantly expanded by 1300 km². The AIMS³ seafloor lander was deployed, a total of 7 sediment cores were obtained with the gravity corer, at core lengths from 3.8 m to 15.5 m. Heat flow measurements were complemented with a further 37 successful deployments of the 6 m- Heat Flow Probe.

The tracer injection-CORK (above) and the Osmosampler-CORK (below) about to be installed in the boreholes.

Composite picture of the Milos deployments: mobile platform and SPIRULA prior to deployment (upper left), below which sensor data of the SPIRULA vehicle are found (1. Panel = temperature & conductivity, 2. Panel =  turbidity and fluorescence of organic matter, 3. Panel depth). Righthandside shows examples of seafloor images with ripples, bacterial mats and gas escape structures.

We had three days of Littorina ship time and the prototypes of the DIC and pCO₂ sensors.

Picture shows pCO2 sensor, developed at Fraunhofer IPM in Freiburg, prior to deployment.

In fall 2023, another RV Littorina cruise in the Baltic Sea allowed us to refine the DIC sensor and also test the newly developed optical pCO2 sensor (joint development of Fraunhofer IPM Freiburg and Sea & Sun Technology) for the first time under marine conditions. All the experiments were promising and it was for the first time that representatives from all 4 partner institutions in AIMS3 were at sea together.

In fall 2022, the first of our annual test cruises in Baltic Sea shallow waters took place on RV Littorina. Eric Achterberg coordinated the work and as a main achievement, the newly developed DIC sensor was tested on board for the first time. Results were very promising and defined the path forward in the development, which is jointly done by GEOMAR and Sea & Sun Technology.

In summer 2022, the first large expedition on the Reykjanes segment of the Mid-Atlantic Ridge took place on RV Meteor. Cruise M183 was led by Wolfgang Bach and served mostly to geophysically characterize the area and find suitable locations for later drilling and in situ experiments as part of AIMS3. Besides hydroacoustic surveys to map the bathymetry and get Parasound profiles, we used a sub-bottom profiler on MARUM ROV Squid to identify sediment ponds with thin sedimentary fills on young ocean crust. Heat flow measurements, gravity coring, and deployment of a seafloor lander complemented the survey.

In spring 2022, RV Heincke cruise HE590 was led by Achim Kopf and Ralf Bachmayer to carry out shallow dives with the IMGAM AUV in the Baltic Sea. IMGAM is an autonomous platform with a depth rating of 2000m that can detect and sample gas seeps. In that respect, it may be a powerful tool for monitoring CCS sites in the future. We plan to use the AUV in future AIMS3 and GEOSTOR cruises.