Shallow water methane-derived authigenic carbonate mounds at the Codling Fault Zone, western Irish Sea

Abstract

24 Methane-derived authigenic carbonate (MDAC) mound features at the Codling Fault 25Zone (CFZ), located in shallow waters (50 to 120 m) of the western Irish Sea were 26 investigated and provide a comparison to deep sea MDAC settings. Carbonates 27 conststed of aragonite as the major mineral phase, with 13 C values as low as -50‰. 28These isotope signatures, together with the co-precipitation of framboidal pyrite 29 confirm that anaerobic oxidation of methane (AOM) is an important process 30 mediating methane release to the water column and the atmosphere in this region. The 31 13 C depletion of bulk carbonate and sampled gas (-70‰) suggests a biogenic source, 32 but significant mixing of thermogenic gas and depletion of the original isotope 33signature cannot be ruled out. Active seepage was recorded from one mound and 34 together with extensive areas of reduced sediment, confirms that seepage is ongoing. 35The mounds appear to be composed of stacked pavements that are largely covered by 36 sand and extensively eroded. The CFZ mounds are colonized by abundant Sabellaria 37 polychaetes and possible Nemertesia hydroids, which benefit indirectly from available 38 hard substrate. In contrast to deep sea MDAC settings where seep-related macrofauna 39 are common reported, seep-specialist fauna appear to be lacking at the CFZ. In 40 addition, unlike MDAC in deep waters where organic carbon input from 41 photosynthesis is limited, lipid biomarkers and isotope signatures related to marine 42 planktonic production (e.g. sterols, alkanols) were most abundant. Evidence for 43 microbes involved in AOM was limited from samples taken; possibly due to this 44 dilution effect from organic matter derived from the photic zone, and will require 45 further investigation. 46 47 3

Orbital Signals in Carbon Isotopes: Phase Distortion as a Signature of the Carbon Cycle

“…The role of changing isotopic composition and mass of authigenic carbonates (Schrag et al, ) is not explicitly modeled. The isotopic composition of authigenic carbonate can be highly variable depending on the sources of alkalinity (e.g., O'Reilly et al, ; Schrag et al, ), and its potential link to astronomical‐scale climate change has not been established.…”

Axial obliquity control on the greenhouse carbon budget through middle‐ to high‐latitude reservoirs

“…The role of changing isotopic composition and mass of authigenic carbonates [ Schrag et al ., ] is not explicitly modeled. The fractionation factors of this carbon sink are highly variable depending on the sources of alkalinity [e.g., Anderson et al ., ; Roberts and Aharon , ; Reimers et al ., ; Meister et al ., ; Naehr et al ., ; O ' Reilly et al ., ]. Authigenic carbonate formation could be responsible for local or regional differences in the δ 13 C carb signatures and might have particularly affected δ 13 C 0 during intervals of widespread anoxia (e.g., OAE II; cf.…”

The Croker Carbonate Slabs: extensive methane-derived authigenic carbonate in the Irish Sea—nature, origin, longevity and environmental significance

“…& In the Irish sector of the Irish Sea, there are 23 elongate MDAC mounds along the Codling Fault Zone. Typically, these are > 250-m long and 80-m wide, occupy areas of > 20,000 m 2 and stand 5-10 m above the general seabed (O' Reilly et al 2014). These features lie within the Codling Fault Zone SAC (NPWS 2015).…”

“…However, a significant number lie outwith the predicted ranges, suggesting that the porewater in which they precipitated was influenced by water of a different composition to seawater, and/or temperature conditions varied from those of the present day. Enrichment of porewater by 18 O may be caused by waters rising from the gas source (O' Reilly et al 2014), or from dissociating gas hydrates (Crémière et al 2016).…”