Pore water geochemistry at two seismogenic areas in the Sea of Marmara

Abstract

Within the Sea of Marmara, the highly active North Anatolian Fault (NAF) is responsible for major earthquakes (Mw 7), and acts as a pathway for fluid migration from deep sources to the seafloor. This work reports on pore water geochemistry from three sediment cores collected in the Gulfs of Izmit and Gemlik, along the Northern and the Middle strands of the NAF, respectively. The resulting data set shows that anaerobic oxidation of methane (AOM) is the major process responsible for sulfate depletion in the shallow sediment. In the Gulf of Gemlik, depth concentration profiles of both sulfate and alkalinity exhibit a kink-type profile. The Sulfate Methane Transition Zone (SMTZ) is located at moderate depth in the area. In the Gulf of Izmit, the low concentrations observed near the seawater-sediment interface for sulfate, calcium, strontium, and magnesium result from rapid geochemical processes, AOM, and carbonate precipitation, occurring in the uppermost part of the sedimentary column and sustained by free methane accumulation. Barite dissolution and carbonate recrystallization have also been identified at deeper depth at the easternmost basin of the Gulf of Izmit. This is supported by the profile of the strontium isotope ratios ( 87 Sr/ 86 Sr) as a function of depth which exhibits negative anomalies compared to the modern seawater value. The strontium isotopic signature also shows that these carbonates had precipitated during the reconnection of the Sea of Marmara with the Mediterranean Sea. Finally, a first attempt to interpret the sulfate profiles observed in the light of the seismic activity at both sites is presented. We propose the hypothesis that seismic activity in the areas is responsible for the transient sulfate profile, and that the very shallow SMTZ depths observed in the Gulf of Izmit is likely due to episodic release of significant amount of methane.

In situ Raman Quantitative Detection of the Cold Seep Vents and Fluids in the Chemosynthetic Communities in the South China Sea

“…Geochemistry, Geophysics, Geosystems 2.51 ± 0.007 mmol/L, respectively. The concentration of the SO 4 2À in ambient seawater was 29.52 ± 1.09 mmol/L, very similar to that of standard seawater (approximately 28 mmol/L, tested by Ruffine et al, 2015). The Raman signals of dissolved SO 4 2À and gaseous CH 4 were found in the spectra acquired by the RiP-Cs probe without filter and tip.…”

Evidence and age estimation of mass wasting at the distal lobe of the Congo deep-sea fan

“…Our uppermost pore-water sample was collected at 62 cmbsf. Such low values close to the sediment-water interface are often encountered at sites characterized by methane emissions or shallow gas hydrates (Chuang et al, 2013;Ruffine et al, 2015;Tryon et al, 2010;Wilson et al, 2014), and usually results from the anaerobic oxidation of methane (AOM) coupled with sulfate reduction near this interface. However, we have demonstrated in the previous section that the shallowest SMTZ is located at around 400 cmbsf, therefore quite far from the sediment-water interface.…”

Fluids and processes at the seismically active fault zone in the Sea of Marmara

“…The recovered samples were the most propane-enriched hydrates ever collected (Bourry et al, 2009;Ruffine et al, 2012). The pore fluids were very diverse and complex in composition, reflecting mixing of fluids from different sources (Ruffine et al, 2015;Tryon et al, 2010;Zitter et al, 2008) as well as variations with time (Tryon et al, 2012). Such fluid emissions favored the development of microbial chemosynthetic communities (Ritt et al, 2010); especially those involved in the anaerobic oxidation of methane (AOM) and other heavier hydrocarbons.…”