Unique authigenic mineral assemblages reveal different diagenetic histories in two neighbouring cold‐water coral mounds on Pen Duick Escarpment, Gulf of Cadiz

Abstract

Alpha Mound and Beta Mound are two cold-water coral mounds, located on the Pen Duick Escarpment in the Gulf of Cadiz amidst the El Arraiche mud volcano field where focused fluid seepage occurs. Despite the proximity of Alpha Mound and Beta Mound, both mounds differ in their assemblage of authigenic minerals. Alpha Mound features dolomite, framboidal pyrite and gypsum, whereas Beta Mound contains a barite layer and predominantly euhedral pyrite. The diagenetic alteration of the sedimentary record of both mounds is strongly influenced by biogeochemical processes occurring at shallow sulphate methane transition zones. The combined sedimentological, petrographic and isotopic analyses of early diagenetic features in gravity cores from Alpha Mound and Beta Mound indicate that the contrast in mineral assemblages between these mounds is caused by differences in fluid and methane fluxes. Alpha Mound appears to be affected by strong fluctuations in the fluid flow, causing shifts in redox boundaries, whereas Beta Mound seems to be a less dynamic system. To a large extent, the diagenetic regimes within cold-water coral mounds on the Pen Duick Escarpment appear to be controlled by fluid and methane fluxes deriving from layers underlying the mounds and forcings like pressure gradients caused by bottom current. However, it also becomes evident that authigenic mineral assemblages are not only very sensitive recorders of the diagenetic history of specific cold-water coral mounds, but also affect diagenetic processes in turn. Dissolution of aragonite, lithification by precipitation of authigenic minerals and subsequent brecciation of these lithified layers may also exert a control on the advective and diffusive fluid flow within these mounds, providing a feedback mechanism on subsequent diagenetic processes.

Constraining the formation of authigenic carbonates in a seepage‐affected cold‐water coral mound by lipid biomarkers

“…Fluctuation of methane flux at Alpha Mound has already been discussed in earlier studies (Pirlet et al, ; Wehrmann et al, ). Increasing methane flux is commonly accompanied by brecciation of semi‐lithified sediment (Figure e).…”

“…Interestingly, the presence of superimposed geochemical signals has already been suggested after measurement of δ 13 C carbonate values of Alpha Mound bulk sediments (Pirlet et al, ), which yielded values between −21.5‰ and −1.9‰. Pirlet et al (2012) suggested that these values might reflect a combination of dissolved inorganic carbon (DIC) produced during AOM and organoclastic sulphate reduction, indicating mixing of DIC derived from different sources. Our new biomarker data agree with the inference of Pirlet et al (2012) that AOM played a prominent role in mound biogeochemistry in the past.…”

“…Pirlet et al (2012) suggested that these values might reflect a combination of dissolved inorganic carbon (DIC) produced during AOM and organoclastic sulphate reduction, indicating mixing of DIC derived from different sources. Our new biomarker data agree with the inference of Pirlet et al (2012) that AOM played a prominent role in mound biogeochemistry in the past. Our study also confirms that lipid biomarkers are commonly more suitable to discriminate between early diagenetic processes than stable isotopes of carbonate fractions, although a combination of organic and inorganic geochemical approaches will always be advantageous.…”

“…Today, methane gas has been measured in the pore waters of Alpha Mound, but no active bubbling of gas has been observed at its surface (Van Rooij et al, ; Wehrmann et al, ). Previous studies have used Alpha Mound as a model for investigating the impact of methane seepage on CWC mound sediments (Maignien et al, ; Pirlet et al, ; Templer, Wehrmann, Zhang, Vasconcelos, & McKenzie, ; Wehrmann et al, ). Various proxies (e.g.…”

Carbonate mounds: From paradox to World Heritage

“…water coral dissolution and carbonate precipitation Foubert and Henriet, 2009). The Alpha Mound on Pen Duick Escarpment, featuring a shallow zone of anoxic oxidation of methane (AOM) (Maignien et al, 2010), comprises up to 40% of microbial dolomite Wehrmann et al, 2011;Pirlet et al, 2012). …”