Diagenetic formation of gypsum and dolomite in a cold-water coral mound in the Porcupine Seabight, off Ireland

Abstract

Authigenic gypsum was found in a gravity core, retrieved from the top of Mound Perseverance, a giant cold-water coral mound in the Porcupine Basin, off Ireland. The occurrence of gypsum in such an environment is intriguing, because gypsum, a classic evaporitic mineral, is undersaturated with respect to sea water. Sedimentological, petrographic and isotopic evidence point to diagenetic formation of the gypsum, tied to oxidation of sedimentary sulphide minerals (i.e. pyrite). This oxidation is attributed to a phase of increased bottom currents which caused erosion and enhanced inflow of oxidizing fluids into the mound sediments. The oxidation of pyrite produced acidity, causing carbonate dissolution and subsequently leading to pore-water oversaturation with respect to gypsum and dolomite. Calculations based on the isotopic compositions of gypsum and pyrite reveal that between 21·6% and 28·6% of the sulphate incorporated into the gypsum derived from pyrite oxidation. The dissolution of carbonate increased the porosity in the affected sediment layer but promoted lithification of the sediments at the sediment-water interface. Thus, authigenic gypsum can serve as a signature for diagenetic oxidation events in carbonate-rich sediments. These observations demonstrate that fluid flow, steered by environmental factors, has an important effect on the diagenesis of coral mounds

A new CT scan methodology to characterize a small aggregation gravel clast contained in a soft sediment matrix

“…A recent review of computed tomography (CT) scans in the geosciences (Cnudde and Boone, 2013) demonstrates the growing application possibilities of X-ray technology as well as the limits of the technique. Improvements in CT scanning allowed exploration of complex sedimentary structures through 3-D reconstructions, leading to improvement compared to classic 2-D imaging (Pirlet et al, 2010;Bendle et al, 2015). The method is based on the relative density of each voxel (i.e., volumetric pixel) constituting the chosen sample.…”

Carbonate mounds in shallow and deep time

“…Enhanced flow conditions led to erosion intervals, during which early diagenetic processes could overprint geochemical signals. The dissolution of coral skeletons and the formation of gypsum, pyrite and dolomite in those basal intervals display a striking similarity with the processes described by Pirlet et al (2010) on top of Mound Perseverance, further west in Porcupine Seabight.…”

Carbonate mounds: From paradox to World Heritage

“…In mounds, such transport is an important factor of early diagenesis. Pirlet et al (2010) demonstrated that a bottom current-induced convective flow of oxidizing fluids in the shallow subsurface of a mound led to the formation of authigenic gypsum and to an early lithification of sediments close to the sediment-water interface. Modeling of full-mound flushing by Depreiter (2009) can explain the observed removal of a significant carbonate fraction, mainly aragonite, from recent mounds.…”

“…An interesting experiment would be to drill a sequence of mounds from exposed ones to recently buried ones, to track in particular the diagenetic expression of the switch from open diagenetic system (the "mound flushing" stage of exposed mounds) to encapsulation, both in a proven pathway of advecting fluids (Melilla mound province, Alboran Sea, Morocco, Comas et al, 2009) and in a setting, possibly not affected by migrating fluids (Magellan -Perseverance mound suite, Porcupine Seabight, Huvenne et al, 2007;Pirlet et al, 2010).…”