<p>This article examines the complexities of marine carbon observations by exploring how non-humans and humans, including: scientists, floats, and geopolitics, (de-)stabilize these processes. Drawing on ethnographic fieldwork in Brazil and Germany, the study uses Karen Barad’s (2007) concept of agential realism to understand how these diverse actors are mutually co-producing knowledge on marine carbon. Instead of viewing entities as separate, intra-action emphasizes their co-constitution. Through theme-based coding, the analysis identifies both stabilizing and destabilizing forces in marine carbon observations. Stabilizing forces include the dedication of scientists, two-way communication between floats and humans, and the global accessibility of data on marine carbon observations. In contrast, destabilizing forces involve climate change’s impact on data collection and quality, funding shortages, and national borders. The research highlights how geopolitical and scientific practices are deeply dynamic and often overlooked in discussions of marine carbon observations. By following non-humans and humans and incorporating diverse perspectives from the sea and land, the study provides new insights into the (un-)becoming of marine carbon observations, emphasizing the importance of the more-than-human in shaping knowledge production practices. This work underscores the value of thinking with Science and Technology Studies and new materialism about marine environments.</p>