Data report: dolomite in Neogene sediments of the Belgica carbonate mound province, Porcupine Seabight, North Atlantic

Abstract

Dolomite was observed, using petrography, X-ray diffraction (XRD), and scanning electron microscopy (SEM), in cored Neogene sediments from the Porcupine Seabight in the north Atlantic Ocean, Integrated Ocean Drilling Program Expedition 307 Sites U1316, U1317, and U1318, ~100 km off the west coast of Ireland. Trace amounts (<10% of total carbonate component) of dolomite were observed in Challenger Mound, a deep coldwater mixed silicaclastic-carbonate mud mound located in the Porcupine Seabight. Significant dolomitization (>50% of total carbonate component) was observed primarily in Miocene strata in submound and off-mound sediments.Petrographic examination of smear slides indicates that dolomite is composed of micrometer-to decimicron-sized, limpid, rhombic crystals. XRD analysis indicates that the dolomite is calcium rich and poorly ordered. SEM confirms light petrographic analysis of size and morphology of dolomite crystals, and electron microprobe analysis indicates that the dolomite is ferroan and displays distinct compositional zoning. Microbes and associated microbial filamentous structures were observed associated with dolomite crystals.Saturation indexes of carbonate minerals calculated for interstitial waters collected from the three sites are consistent with the petrographic and XRD observation of dolomite.

Dissolved silica-catalyzed disordered dolomite precipitation

“…The difficulty in understanding sedimentary dolomite formation comes from the scarcity of modern examples and the inability to precipitate dolomite from seawater at surface temperature. Although modern seawater is supersaturated with respect to dolomite, primary dolomite has mostly been reported in limited settings such as alkaline lakes (Peterson et al 1963;Rosen et al 1988;Last 1990), deep marine carbonate pavements associated with methane seeps (Lumsden 1988;Gregg and Frank 2009;Xu 2010;Lu et al 2018) and shallow marine environments with microbial mats (Curtis et al 1963;Kendall and Skipwith 1968;Vasconcelos and McKenzie 1997;Meister et al 2007;Zhang et al 2015). Land (1998) concluded after a 32year experiment with a 1000-fold oversaturated solution at room temperature which failed to precipitate dolomite that the "dolomite problem" results from the kinetic inhibition of dolomite crystallization.…”

Linking sedimentary sulfur and iron biogeochemistry to growth patterns of a cold‐water coral mound in the Porcupine Basin, S.W. Ireland (IODP Expedition 307)

“…). The occurrence of intensively dolomitized sediment layers (>50% of total carbonate component) in the submound sediments supports this hypothesis (Gregg & Frank, ).…”

“…A general trend to higher carbonate contents in shallower sections of M2 is observed. In this unit, dolomite displays near ideal stoichiometry, typical of Paleozoic dolomites, indicating a likely detrital origin (Gregg & Frank, ). Cyclic variations in carbonate content and color lightness of the sediment are less apparent (Titschack et al ., ).…”

“…The remaining 34 wt.% are calcite, and a minor amount is dolomite (<5 wt.%; Titschack et al, 2009). Onboard XRD analysis indicates that the dolomite in the deeper sections of Challenger Mound is calcium-rich and poorly ordered (M1a; Gregg & Frank, 2009). Cyclic fluctuations of 10-20 m thickness are observed in sediment color and lightness of color reflectance and are visible in variations in the background carbonate content, but not always in the coral content (Titschack et al, 2009).…”

“…Challenger Mound is rooted on a firmground consisting of unlithified glauconitic sandy, partially clayey silt (Ferdelman et al ., ). This early to late Miocene unit exhibits carbonate contents of 26–52 weight % (wt.%), much of which is dolomite (Gregg & Frank, ).…”