Dissolution Of Biogenic Carbonates Research Articles

Hyperthermal events recorded in the Palaeogene carbonate sequence of southern Gulf of Mexico—Santa Elena borehole, Yucatan Peninsula

Based on geochemical and magnetic susceptibility analyses, maximum warming events (hyperthermal) in the Palaeogene are recognized in the carbonate rocks of the Santa Elena borehole (SEB) in the Yucatan Peninsula, the Palaeocene‐Eocene Thermal Maximum (PETM) and the Eocene Thermal Maximum 2 (ETM‐2). The site records the continental shelf marine response during these global events. Major and trace element records (Al, Ba, Ca, Fe, K, Si, and Ti), Ca/Fe, Si/Al ratios, and magnetic susceptibility are used as proxies of terrigenous input, and Ba/Al ratio as a proxy of palaeoproductivity. The hyperthermal events are characterized by the dilution and/or dissolution of biogenic carbonates. The high input of terrigenous materials is linked to extreme precipitation, common during these warming events. Our records suggest a decrease in palaeoproductivity associated with a nutrients gradient in a shallow ecosystem, with deeper thermocline and stratified column water. The PETM is characterized by high eustatic sea‐level conditions, with a high contribution of detrital material, indicating sedimentary condensation and marked increase in precipitation, calcite dilution and/or dissolution, and low productivity. The ETM‐2 event is less extreme than the PETM, with high precipitation, although evaporation could also play an important role, as evidenced by the presence of evaporites in this interval. These changes might affect the higher trophic levels of the shelf sea ecosystem, declining productivity. The study contributes to our understanding of the global and regional effects of these past warming events and the future climate change.

Ferruginous groundwaters as a source of P, Fe, and DIC for coastal waters of the southern Baltic Sea: (Isotope) hydrobiogeochemistry and the role of an iron curtain

We report first results from a study on water and element exchange across the land-ocean boundary at the southern Baltic Sea. The focus is set on ferruginous fresh ground waters escaping at the shore line, flowing in air contact before entering a subterranean mixing zone with brackish Baltic Sea water. The present study combines the results from multiple sampling campaigns that investigated the composition of several springs as well as the surface and subsurface development of fresh waters on the way to the Baltic Sea. This is achieved by a combination of hydroand solid-phase geochemical and stable isotope measurements with ground water dating. Results are compared to the composition of groundwaters recovered from wells in the catchment area and the local isotope meteoric water line developed for Warnemünde. The spring water is shown to be impacted by the dissolution of biogenic carbon dioxide and marine carbonate as well as the oxidation of pyrite in glacial sediments. Dating yields a surprisingly high diversity between some closely associated springs with average ages of about 25 to 32 years, but different mixing proportions with older tritium-free ground-water.

Controls on ferromanganese crust composition and reconnaissance resource potential, Ninetyeast Ridge, Indian Ocean

A reconnaissance survey of Fe–Mn crusts from the 5000km long (~31°S to 10°N) Ninetyeast Ridge (NER) in the Indian Ocean shows their widespread occurrence along the ridge as well as with water depth on the ridge flanks. The crusts are hydrogenetic based in growth rates and discrimination plots. Twenty samples from 12 crusts from 9 locations along the ridge were analyzed for chemical and mineralogical compositions, growth rates, and statistical relationships (Q-mode factor analysis, correlation coefficients) were calculated. The crusts collected are relatively thin (maximum 40mm), and those analyzed varied from 4mm to 32mm. However, crusts as thick as 80mm can be expected to occur based on the age of rocks that comprise the NER and the growth rates calculated here. Growth rates of the crusts increase to the north along the NER and with water depth. The increase to the north resulted from an increased supply of Mn from the oxygen minimum zone (OMZ) to depths below the OMZ combined with an increased supply of Fe at depth from the dissolution of biogenic carbonate and from deep-sourced hydrothermal Fe. These increased supplies of Fe increased growth rates of the deeper-water crusts along the entire NER. Because of the huge terrigenous (rivers, eolian, pyroclastic) and hydrothermal (three spreading centers) inputs to the Indian Ocean, and the history of primary productivity, Fe–Mn crust compositions vary from those analyzed from open-ocean locations in the Pacific.The sources of detrital material in the crusts change along the NER and reflect, from north to south, the decreasing influence of the Ganga River system and volcanic arcs located to the east, with increasing influence of sediment derived from Australia to the south. In addition, weathering of NER basalt likely contributed to the aluminosilicate fraction of the crusts. The southernmost sample has a relatively large detrital component compared to other southern NER crust samples, which was probably derived predominantly from weathering of local volcanic outcrops.Fe–Mn crusts from a dredge haul at 3412m water depth, 2°S latitude, are pervasively phosphatized along with the substrate rocks (site D7). Phosphatization took place through replacement of carbonate, preferential replacement of Fe oxyhydroxide relative to Mn oxide in the crusts, preferential replacement of silica-rich phases relative to Al-rich phases in the crusts, and precipitation of carbonate fluorapatite in pore space. The preferentially replaced silica may have been Si adsorbed on the Fe oxyhydroxide. The enrichment of Ni, Zn, and Cu in the phosphatized crust reflects preferential adsorption into the tunnel structure of todorokite. The rare earth element plus yttrium (REY) patterns indicate a lower oxidation potential during phosphatization of the NER crusts compared to Pacific phosphatized crusts. NER phosphatization occurred in a deeper-water environment than typical for phosphatization of Pacific crusts, occurred post-middle Miocene, a younger age than phosphatization the Pacific crusts, and had in part a different set of chemical changes produced by the phosphatization than did the Pacific crusts.The southern third of NER has Fe–Mn crusts with the highest Co (0.91%), Ni (0.43%), ΣREY (0.33%), Cu (0.22%), Te (146ppm), Pt (1.5ppm), Ru (52ppb), and Rh (99ppb) contents. These are among the highest Pt, Ru, and Rh concentrations measured in marine Fe–Mn deposits. Because of these high metal concentrations, exploration is warranted for the southern sector of the NER, especially at shallower-water sites where the platinum group elements (PGE) and Co are likely to be even more enriched.

Stable carbon isotope cycling in mobile coastal muds of Amapá, Brazil

Approximately 10% of the sediment delivered by the Amazon River moves northwest along the coast of Amapá, Brazil, initiating the Guianas mobile mud belt. Amapá coastal muds generally have a two-layer transport structure and are characterized by highly non-steady-state sedimentation. Isotopic compositions of pore water ∑CO 2 and solid phase C org demonstrate that remineralization in the surficial mobile zone (∼0.3–1 m thick) is dominated by terrestrial sources at sites in proximity to the mangrove fringe (∼1–2 m water depth), and marine (plus possible carbonate dissolution) sources further offshore (∼21 km, ∼7 m depth). The net δ 13C of ∑CO 2 produced and C org remineralized is ∼−26‰ and −25.9‰, respectively inshore, and ∼−14‰ and −18.6‰, respectively offshore (compared to average terrestrial and marine C org end members of −28‰ and −20‰). Efficient remineralization in the suboxic mobile zone lowers particle surface loading of C org from ∼0.35 mg C m −2 in the Amazon delta topset to ∼0.13–0.16 along Amapá. Sequential, temperature-dependent extractions were used to operationally fractionate inorganic C pools. Authigenic carbonates, mostly siderite and mixed Ca, Mg, Fe, Mn-carbonates, dominate sediment inorganic C (∼50–200 μmol g −1). The mass weighted δ 13C of carbonates, ∼−15‰ to −19‰, is relatively restricted in range compared to pore water ∑CO 2, implying most precipitation in the reactive mobile surface sediments. Periodic mixing with bottom seawater and/or dissolution of biogenic carbonates in the surficial layer shift δ 13C values of pore water to heavier values than C org reactant sources. At one offshore site (∼7 m), about 22% of pore water ∑CO 2 has undergone reduction during methanogenesis below the mobile surface zone, extracting ∑CO 2 with δ 13C∼−90‰ and leaving a residual δ 13C∼−0.37‰. Diagenetic processes in the suboxic mobile mud zone dominate C remineralization and storage along the coast of Amapá.

Dissolution of biogenic carbonates: effects of skeletal structure: Henrich, Rüdiger and Gerold Wefer, 1986. Mar. Geol., 71(3–4):341–362

Dissolution of biogenic carbonates: effects of skeletal structure: Henrich, Rüdiger and Gerold Wefer, 1986. Mar. Geol., 71(3–4):341–362