1/7/2023 0 Comments Tide graph van geldern coveA subsequent decrease in carbonate primary productivity and an increase in siliciclastic supply are associated with the negative δ 13C trend in the overlying unit, at the base of the La Providencia Group. Samples of the LNF fabric-retentive micrite preserve an unusually narrow range of oxygen isotope compositions, which is consistent with potential climatic variation, considering the multi-proxies retained. Our sedimentological and geochemical study of the LNF sections shows extraordinarily consistent carbon and oxygen isotope trends that may be useful for detailed intrabasinal correlations. The LNF limestones studied, obtained from five sections near the Olavarría and Barker areas, reveal constant curves of positive δ 13C values (∼+2 to +4%), similar to those recorded in other middle-Ediacaran marine proxies worldwide. This extended carbonate platform (>100 km) is divided into two sedimentary facies: one related to the basal section, in which lime mud originated by suspension fall-out in an open-marine ramp (reddish or greenish limestones), and the other facies represented by black to dark grey limestones from the middle and upper sections. The age of the Loma Negra Formation (LNF), the uppermost unit of the Sierras Bayas Group, has been debated, but it should be older than 560 Ma (age of the overlying succession) and younger than 600 Ma (age of the underlying iron-rich mudstone of the Colombo Formation). This unit is exposed in the Olavarría and Barker areas within the Tandilia Basin, and composed entirely of lime mudstones (∼40 m thick). These findings suggest strong effects of freshwater-saltwater mixing on dissolved carbon dynamics, which should be taken into account in carbon processing and budgeting in the world's estuarine systems.The Ediacaran Loma Negra Formation may be deposited in a platform environment related to the Clymene Ocean. DIC concentration increased six times from freshwater (0.24 mM) to saltwater (1.64 mM), while DOC showed an opposing trend, but to a lesser degree (from 1.13 to 0.56 mM). For the entire study period, the river's freshwater discharged 0.25 x 10⁹ mol dissolved inorganic carbon (DIC) and 1.77 x 10⁹ mol dissolved organic carbon (DOC) into the mixing zone. In the freshwater-saltwater mixing zone with wide channels and river lakes, however, a much larger amount of carbon (3.04 x 10⁸ kg) was emitted to the atmosphere during the same period. In the short freshwater river reach before a saltwater barrier, 0.079 × 10⁸ kg carbon was emitted to the atmosphere during the study year. The average CO₂ outgassing fluxes at site 1 through site 6 were 162, 177, 165, 218, 126, and 15 mol m⁻² year⁻¹, respectively, with a mean o f 140 mol m⁻² year⁻¹ for the entire river reach. We found that throughout the sampling period, all six sites exhibited CO₂ supersaturation with respect to the atmospheric CO₂ pressure during most of the sampling trips. From November 2013 to December 2014, we investigated freshwater-saltwater mixing effects on dissolved carbon concentrations and CO₂ outgassing at six locations along an 88-km-long estuarine river entering the Northern Gulf of Mexico with salinity increasing from 0.02 at site 1 to 29.50 at site 6 near the river's mouth. The delivery of dissolved carbon from rivers to coastal oceans is an important component of the global carbon budget.
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