Quantification of methane fluxes and authigenic carbonate formation at cold seeps along the continental margin offshore Costa Rica: A numerical modeling approach

The expulsion of liquids, gases and fluids at continental margins covers a wide range of processes including not only mud volcanism, mud diapirism and gas flares, but also continuously seeping methane-rich fluids leading to cold vent sites and even outbursts of over-pressured gases. Seepage of fluids frequently leads to precipitation of authigenic carbonates that modify sedimentary processes along the margin, and finally, the fluids constitute the energy source for a number of diverse and complex ecosystems. During the last two decades, researchers gained significant knowledge about the impact of fluid seepage on local ecosystems and the biogeochemical processes that result in carbonate formation via the anaerobic oxidation of methane (AOM). However, all the knowledge was gained in different areas and geological settings of the world ocean but left a regional gap in our knowledge about seeps at the continental margin offshore Costa Rica. Those processes including authigenic carbonate precipitation, quantification of the impact of fluid seepage and methane budgets are documented in the present thesis. Chapter II presents comprehensive results from five cold seep structures at the Costa Rican continental margin addressing the relationship between fluid advection, dissolved calcium concentrations in upward migrating fluids, and authigenic calcium carbonate precipitation. A numerical transport-reaction model was used to determine rates of AOM, CaCO3 precipitation, and benthic fluxes of solutes. Production of carbonate alkalinity and formation of authigenic carbonates is most intense at intermediate flow rates (3-40 cm a-1) and reduced under low and high flux conditions (0.1 and 200 cm a-1). Systematic model runs showed that high Ca concentrations in ascending fluids enhance the rate of authigenic carbonate production at moderate flow rates leading to an almost quantitative fixation of deeply derived Ca in authigenic carbonates. Hence, CaCO3 precipitation is not only controlled by Ca diffusing into the sediment from bottom water, but also by the Ca concentration in ascending fluids. Based on average precipitation rates deduced from the systematic model runs the global Ca-fixation at cold seeps (~2·1010 mol Ca a-1) suggesting that cold seeps are most likely not of key importance with respect to Ca cycling in the ocean. Chapter 3 comprises the quantitative estimates of dissolved methane discharge from wellstudied mud mounds (Mound 11 and Mound 12) at the submarine section of the Costa Rica fore-arc combining geochemical and geoacoustic data. The study is supported by 75 kHz sidescan sonar data, pore-water analysis and visual sea-floor observations by remotely operated vehicle (ROV). A numerical transport reaction model was applied to determine dissolved methane fluxes considering AOM and upward fluid flow. Model results reveal that a significant portion of methane from greater depth is discharged into the bottom water only at high fluid flow velocities that are not sufficiently moderate to allow for AOM. The overall amount of dissolved methane released from the entire mud mounds into the water column was moderate with a discharge of 0.36·106 mol a-1 at Mound 11 whereas it was calculated as 0.58·106 mol a-1 at Mound 12. Compared to other active cold seeps (mainly mud volcanoes), mud mounds at the submarine section of the Costa Rica fore arc do not represent a pathway for significant methane discharge from the seafloor. Sea floor methane emissions from bacterial mat sites of a submarine slide at the Costa Rica continental margin are presented in Chapter IV. The estimates of methane fluxes into the water column are based on (i) detailed mapping in order to determine the abundance of seeps, and thus the spatial validity of the flux measurements; and (ii) application of numerical model to estimate the amount of methane that is transported into the bottom water. Model results illustrate that the majority of the studied seeps transport rather limited amount of methane into the water column due to medium to low advection rates (average 10 cm a-1 ) allowing high methane consumption by AOM (average 45%) and limiting the methane discharge into the water column. Depth-integrated AOM rates (56-1538 μmol CH4 cm-2 a-1) are comparable with the values reported at other very active vents sites, suggesting that the Quepos Slide should be regarded as one of the most active sites at the seafloor. The overall amount of dissolved methane released from the entire bacterial mat sites into the water column is determined as 0.56·106 mol a-1. This conservative estimate, relying on rather accurate determinations of sea floor methane fluxes out of bacterial mats emphasizes the importance of submarine slides as sites of natural methane seepage.