A long-standing mystery
For decades, scientists have debated the origin of the drastic environmental changes that led to a major deoxygenation event of the deep Eastern Mediterranean during the Early Holocene (approximately 11,000–6,000 years ago. These conditions resulted in the deposition of a widespread organic-rich sediment layer on the seafloor, known as Sapropel 1.
The prevailing explanation has centered on increased freshwater input associated with enhanced river discharge from North Africa, particularly the Nile, together with increased precipitation over the northern Mediterranean borderlands. These processes are thought to have strengthened water-column stratification and ultimately suppressing deep-water formation. Yet another potential source of freshwater has remained comparatively overlooked: the outflow from the Black Sea into the northern Aegean during the deglaciation and Early Holocene. Although previous studies have suggested that this freshwater source may have contributed to suppressing Aegean deep-water formation, its magnitude and significance have remained uncertain.
This led us to a simple but fundamental question: How important was Black Sea outflow in driving the collapse of Aegean deep-water circulation and the formation of Sapropel S1?
Back to the Aegean Sea (Greece)
Investigating this question also gave me the opportunity to reconnect with my home country. During my postdoctoral research at the Universitat de Barcelona, I established a collaboration between the Universitat de Barcelona, the Department of Marine Sciences at the University of the Aegean where I completed my undergraduate studies, and the Hellenic Centre for Marine Research (HCMR). This collaboration gave me access to a unique sediment archive from the Aegean Sea and brought together an outstanding team of co-authors – Sara Campderrós, Sergio Trias-Navarro, Cristina Garcia Briña, Eduardo Paredes Paredes, Leopoldo Pena, Jaime Frigola, Thomas Hasiotis, Grigoris Rousakis, Dimitris Sakellariou and Isabel Cacho – whose complementary expertise and contributions made this study possible.
Our research focused on sediment core KC-13, recovered from the Chios Basin in the central Aegean Sea. This location is ideally situated to record hydroclimatic changes originating from both the northern and southern margins of the Eastern Mediterranean and lies close to one of the region's principal deep-water formation areas. Even more remarkably, the core preserves over 40,000 years of environmental history, making it an exceptional archive for investigating the processes that led to Sapropel S1 formation.
Piecing together the evidence
The core was shipped from the HCMR core repository to the Universitat de Barcelona, where we carried out a suite of high-resolution multiproxy analyses. One of the greatest challenges was distinguishing the influence of Black Sea outflow from other freshwater sources reaching the Eastern Mediterranean. No single proxy could answer this question. Instead, we combined grain-size analysis and X-ray fluorescence (XRF) core scanning with radiogenic (Sr and Nd) and stable isotope analyses to reconstruct changes in sediment provenance, water masses and hydrographic conditions. As each dataset was added, the pieces gradually came together, revealing a coherent picture of the environmental evolution of the Aegean Sea during the Early Holocene.
The most exciting moment came when these independent records converged on the same conclusion. They showed that Black Sea outflow intensified during the Early Holocene and coincided with a major reorganisation of the Aegean Sea. Our results indicate that Black Sea outflow played a much more significant role in enhancing water-column stratification and suppressing deep-water formation than previously recognised.
Beyond identifying the importance of this freshwater source, our records also shed light on the mechanisms driving its evolution. We infer that Black Sea outflow was initially triggered by meltwater released from the retreating Fennoscandian Ice Sheet and outburst floods from glacial lakes in central Asia and was subsequently sustained and strengthened by enhanced precipitation over the extensive Black Sea catchment during the Early Holocene.
Rethinking Sapropel S1
These findings challenge a long-standing interpretation of Sapropel S1 formation. They do not diminish the importance of North African river discharge or regional precipitation. Rather, they demonstrate that the Black Sea should also be considered as a fundamental component of the system. The development of Sapropel S1 was likely the result of multiple interacting processes, with Black Sea outflow playing a much more prominent role than traditionally assumed.
As climate change continues to alter precipitation patterns, freshwater fluxes and ocean circulation around the world, understanding how these processes interacted in the past is more relevant than ever. The past cannot predict the future, but it provides one of our best laboratories for understanding how complex marine systems respond to large-scale climate change.
You can read our article here: https://www.nature.com/articles/s43247-026-03730-6