Carbonate authigenesis and tube worm mineralization - biogeochemical and geobiological processes at methane seeps on the Congo deep-sea fan

Abstract

This thesis represents an approach to describe biogeochemical and geobiological processes during carbonate authigenesis at the Kouilou methane seeps on the northern Congo Fan off southwest Africa. The work comprises the analyses of methane derived limestones, the formation of carbonates modified by macroorganisms, and the diagenetic mineralization of vestimentiferan worm tubes. In high agreement with other methane seeps, it could be shown that carbonate formation is initially induced by microbial activity of anaerobic methane oxidizing (AOM) consortia. Aragonite and high-Mg-calcite are the carbonate minerals of these carbonate structures. The principle factors in respect of carbonate formation and mineralogy are i.) methane flux intensity ii.) sulfate concentration and iii.) the aggregation of specific AOM consortia. High methane flux is identified to displace carbonate formation towards the seafloor, and favor, due to chemical and kinetic effects, the formation of aragonite. Highly negative carbon isotopic compositions, nearly seawater equivalent oxygen isotopic compositions and the dominance of ANME -2 biomarkers are characteristics of these carbonates. High-Mg-calcite carbonates, in contrast, are characterized by more positive carbon isotopic compositions and the dominance of ANME-1 biomarkers, both a result of moderate methane flux. The positive oxygen isotopic anomalies of the high-Mg-calcites point to reduced seawater access in higher sediment depths, and are very likely a result of a high proximity to decomposing gas hydrates. The shape of the carbonate structures is directly linked to methane migration and related pathway formation, as well as the subsequent aggregation of AOM consortia. Structures formed within the sediments are of nodular, tubular, or irregularly-complex shape, from which the latter varies highly in size. Structures close to the seafloor are of platy shape, and display, in contrast, rough surfaces and a higher porosity. Modification of carbonate structure formation is observed by benthic bivalves and holothurians. Burrowing activities within the sediments cause seawater infiltration in higher sediment depths and the creation of small-scale niches of highly diverse carbonate chemistry. The assemblage of specific microorganisms and those activities is probably responsible for the local occurrence of protodolomite within some molds.The colonization of chemosymbiotic vestimentiferan tube worms affects carbonate formation conditions in a comprehensive way. Carbonate precipitates occur close to the posterior worm tube which is used as an important metabolism device within the sediment. Tube worms change the chemistry by i.) the uptake of sulfide off the sediment, ii.) the release of sulfate into the sediment and iii.) the release of hydrogen ions. The uptake of sulfide maintains AOM induced carbonate formation and prevents carbonate solution by balancing the pH. Whereas sulfate release, enhances carbonate formation by stimulating the turn over of AOM consortia. Additionally, increasing sulfate concentrations are most likely responsible for the predominant formation of aragonite after tube worm colonization. The elimination of hydrogen ions through the posterior tube prevents carbonate formation directly on the tube surface and avoids a reduction of the respiratory area. This is responsible for the co-occurrence of organic tubes (live), partly (dead) and totally lithified posterior tubes (degraded) within some complex tube worm/carbonate aggregates, formed by tube worm colonies. Nodular high-Mg-calcite carbonates, incorporated within the structures are formed prior to tube worm colonization, and are supposably used for tube fixation. Diagenetic mineralization of posterior vestimentiferan worm tubes finally forms two characteristic mineralization patterns by the carbonate mineral aragonite. The mineralization process starts on the exterior tube and proceeds successively to the interior tube wall. The resulting tube configuration depends on the size and fabric of aragonite crystals. Microcrystalline aragonite crystals do not change the tube wall diameter or its laminar configuration. Whereas tube wall mineralization by inwards growing large-scale aragonite splays results in laminae separation, lateral spreading of the wall and the reduction of the internal tube diameter. Some of the tube walls are rapidly affected by recrystallisation processes. Microcrystalline aragonite crystals get overgrown by radial fibrous aragonite crystals, responsible for the delamination of still unaltered organic laminae and the formation of bulges. Exactly these features were observed in Paleozoic tube fossils, and make it more likely, that these fossil tubes are vestimentiferans