Seafloor manganese speciation and fluxes in a rapidly deoxygenated coastal sea: The case of Marmara Sea deep-waters and sediments

The Marmara Sea has been highly affected by the Black Sea inflow and anthropogenic activities such as overfishing, urban effluents, industrial and agricultural run-off, and shipping. Recent large-scale mucilage outbreaks have accelerated eutrophication and reduced deep-water oxygen levels. Despite this trend, there are only a few studies to understand the consequences of eutrophication and deoxygenation on sediment biogeochemistry, particularly for redox-sensitive elements such as manganese (Mn). Here, we present the most recent findings on manganese in the Marmara Sea's deep-waters and sediments, based on data collected by R/V Bilim-2 in July 2024 and August 2025. Understanding the vertical distribution of Mn, as well as other trace metals, in the Marmara Sea is crucial, as they play essential catalytic roles in many biogeochemical processes. Samples were collected from six stations distributed across the western, central, and eastern sub-regions of the Marmara basin. Mn(II) and Mn(III) concentrations were measured on board using a spectroscopic porphine method. Based on water measurements from the 2024 cruise, Mn(II) ranged from 1.07 μM to 3 μM, while Mn(III) ranged from 0.22 μM to 0.44 μM at two stations. However, during the 2025 cruise, manganese was detected at only one station (45C), where both Mn(II) and Mn(III) were measured at 1.32 μM. According to the manganese sediment profiles from the 2025 cruise, Mn(II) concentrations increased in the upper sediment layers due to the reduction of Mn(III) to Mn(II) and then decreased at greater depths where redox conditions became more stable. At several stations, including Gemlik, West, and the Southern Shelf, Mn(II) concentrations increased again in deeper layers, likely due to the dissolution of Mn-bearing minerals from the sediment. Mn(III) was detected at all six stations. Among all sites, Station 45C exhibited the highest manganese levels, with maximum concentrations of approximately 105 μM Mn(II) and 6 μM Mn(III). Station İzmit Deep followed with 80 μM Mn(II) and 3 μM Mn(III). The other stations showed lower peak values: Gemlik (~43 μM Mn(II), 6 μM Mn(III)), West (~22 μM Mn(II), 6 μM Mn(III)), Northern Shelf (~6 μM Mn(II), 6 μM Mn(III)), and Southern Shelf (~9 μM Mn(II), 6 μM Mn(III)). Our results provide an updated perspective on the vertical distribution of manganese in the Marmara Sea, along with associated flux estimates. They also emphasize the links between manganese dynamics, oxygen availability, and the system’s response to anthropogenic stressors.