Diagenetic Dolomite, Calcite, Rhodochrosite, Magnesite, and Lansfordite from Site 799, Japan Sea-Implications for Depositional Environments and the Diagenesis of Organic-Rich Sediments

Abstract

Abundant and various diagenetic carbonates were recovered from a 1084-m-thick, Quaternary to lower Miocene section at ODP Site 799 in the Japan Sea. Petrographic, XRD, SEM, EDS-chemical, and isotopic analyses revealed wide variations in occurrence and textural relations and complex mineralogy and chemistry. Diagenetic carbonates include calcite, calcium-rich rhodochrosite, iron-and manganese-rich magnesite, iron-and manganese-rich dolomite and ankerite, and iron-and manganeserich lansfordite (hydrous Mg-carbonate).Rhodochrosite commonly occurs as small, solid nodules and semi-indurated, thin layers in bioturbated, mottled sediments of Units I and II (late Miocene to Quaternary). Lansfordite occurs as unindurated nodules and layers in Unit II (late Miocene and Pliocene), whereas magnesite forms indurated beds a few centimeters thick in slightly bioturbated-to-faintly laminated sediments of Unit III (middle and late Miocene). Some rhodochrosite nodules have dark-colored, pyritic cores, and some pyrite-rhodochrosite nodules are overgrown by and included within magnesite beds. Dolomite and ankerite tend to form thick beds (>IO cm) in bedded to laminated sediments of Units III, IV, and V (early to late Miocene). Calcite occurs sporadically throughout the Site 799 sediments. The δ 18 θ values of carbonates and the interstitial waters, and the measured geothermal gradient indicate that almost all of the Site 799 carbonates are not in isotopic equilibrium with the ambient waters, but were precipitated in the past when the sediments were at shallower depths. Depths of precipitation obtained from the δ 18 θ of carbonates span from 310 to 510 mbsf for magnesite and from 60 to 580 mbsf for dolomite-ankerite. Rhodochrosite and calcite are estimated to have formed within sediments at depths shallower than 80 mbsf.Diagenetic history in the Site 799 sediments have been determined primarily by the environment of deposition; in particular, by the oxidation-reduction state of the bottom waters and the alkalinity level of the interstitial waters. Under the well-oxygenated bottom-water conditions in the late Miocene and Pliocene, manganese initially accumulated on the seafloor as hydrogenous oxides and subsequently was mobilized and reprecipitated as rhodochrosite within the shallow sulfate-reduction, sub-oxic zone. Precipitation of lansfordite occurred in the near-surface sediments with abundant organic carbon and an extremely high alkalinity during the latest Miocene and Pliocene. The lansfordite was transformed to magnesite upon burial in the depth interval 310 to 510 mbsf. Dolomite first precipitated at shallow depths in Mn-poor, anoxic, moderately biocalcareous sediments of early to late Miocene. With increasing temperature and depth, the dolomite recrystallized and reequilibrated with ambient waters at depths below about 400 mbsf.

Site U1422

“…In some cases, this may occur because Fe can form a series of authigenic minerals, including oxides (magnetite), sulfides (greigite and pyrite), carbonates (ankerite and ferroan dolomite), and Fe-rich clay (glauconite). Horizons of pyrite, ferroan dolomite, and glauconite have been documented in the upper few hundred meters of sediment at several sites in the marginal sea Matsumoto, 1992). The complex Fe profile at Site U1422 may indicate that various Fe-rich minerals are precipitating and dissolving across multiple depth intervals in the sediment column.…”

“…For Mn and at sites with high alkalinity, a common phase is rhodochrosite (MnCO 3 ). Rhodochrosite has been documented between 50 and 700 mbsf at ODP Site 799 (Matsumoto, 1992). Another possibility is Mn-rich calcite, as perhaps indicated by the low dissolved Ca concentrations at this depth ( Fig.…”

“…However, both HCO 3 -and PO 4 3-can precipitate into (or onto) authigenic minerals, so their concentrations decrease with depth. Beyond the formation of authigenic carbonate phases (Matsumoto, 1992), previous work on deep-sea sediment sequences from the marginal sea has documented significant formation of francolite, a phosphate mineral . Manganese oxides and hydroxides in the upper 1 m below the seafloor may also concentrate P, which could be released to interstitial water as they dissolve with burial (Cha et al, 2005).…”

Methods

“…This includes carbonates and silicates, which stem in part from dissolution of microfossils (Matsumoto, 1992;Nobes et al, 1992), as well as sulfides, which can react with oxygen during transport and storage. Sufficiently resolved interstitial water profiles are desirable to un-derstand the location of authigenic mineral formation because mineral diagenesis impacts the sedimentary record and paleoceanographic objectives.…”

Manganese mineralogy and diagenesis in the sedimentary rock record

“…To examine the petrogenesis of manganese in this deposit, we acquired several samples from the ODP Site 799 core drilled into organic carbon-rich sediments deposited in a failed rift basin in the Japan Sea (Matsumoto, 1992). Three samples (196 m, 326 m, 502 m, Pliocene to late Miocene in age) were unlithified, but the deepest sample at 533 m (from the Late-Middle Miocene boundary, approximately 11 Ma) was sufficiently consolidated to make a thin section that preserved textures.…”