Carbonate and organic matter sedimentation and isotopic signatures in Lake Chungará, Chilean Altiplano, during the last 12.3kyr

Abstract

Sediments in lakes in the Andean volcanic setting are often made up of diatomaceous ooze together with volcaniclastics and small amounts of carbonates. Despite their scarcity, carbonates along with organic matter provide significant paleoenvironmental information about lake systems. This study focuses on the carbonates in Lake Chungará, their morphologies, distribution and origin deduced from the isotopic markers. These markers reflected changes in the water and the biomass between the onset of the Holocene and around 9.6 cal kyr BP. These changes are marked by general increases in TOC, TN, and TN-δ 15 N AIR , and by fluctuating values of TOC-δ 13 C VPDB in its sediments and are probably related to major shifts in the lake surface/volume associated with rises in lake level. An increase in salinity around 10 cal kyr BP is thought to be linked to a short dry period, giving rise to the onset of carbonate production. The mid-Holocene arid period between 7.3 and 3.5 cal ka BP, with a maximum of aridity around 6.0 cal kyr BP, was deduced from δ 18 O VPDB values in the endogenic carbonates. These results match the reconstructions in Lake Titicaca based on benthic diatoms and paleoshore levels. Offshore sediments mainly consist of a diatomaceous ooze, laminated in the lower half (Unit 1), and bandedmassive with tephra layers in the upper half of the sequence (Unit 2). TOC-δ 13 C VPDB and the C/N ratio confirm that phytoplankton was the main source of organic matter in these sediments. Shallower sediments (units 3 to 5) developed in platform and littoral settings, providing evidence of subaqueous macrophytes and, to a lesser extent, land plants. Carbonate content ranges between 0.1 and 6 wt.% in offshore settings (30 to 40 m water depth) and reaches the maximum values in the lower part of Unit 2. Carbonate minerals (low magnesium calcite and minor amounts of high magnesium calcite and aragonite) are scarce and are arranged in mm-thick layers, commonly forming cm-thick levels or bioclasts. Carbonate layers are made up of euhedral-to-subhedral spindle-shaped calcite crystals and, to a lesser extent, aragonite needles, all in the μm range. Aragonite spheroids coexist in littoral sediments with other carbonate shapes and charophyte remains, where carbonate reaches locally up to 20 wt.%. CO 2 photosynthetic depletions related to seasonal phytoplankton blooms were responsible for the high frequency deposition of mm-thick carbonate layers.The average values for δ 13 C VPDB in lake water, plankton and sediments of Lake Chungará (as organic matter or as carbonate) are around 15‰ higher than commonly reported values in other lakes. This 13 C enrichment is attributed to carbon assimilation from a DIC affected by methanogenesis, in which HCO 3 − is the dominant species. The δ 13 C VPDB and δ 18O VPDB variations and their covariation in endogenic carbonates suggest that lake water volume and lake level increased along the Holocene.

Sedimentology and stable isotopes from a lacustrine-to-palustrine limestone deposited in an arid setting, climatic and tectonic factors: Miocene–Pliocene Opache Formation, Atacama Desert, Chile

“…Grande rift (Mack et al, 2000), and East African rifts (Mount and Cohen, 1984;Ashley and Hay, 2002;Ashley et al 2013 (Grosjean et al, 1995;Valero Garcés et al, 2008;Pueyo et al, 2011).…”

“…The decrease in 87 Sr/ 86 Sr from carbonate-charged water as it equilibrates with the atmosphere (Grosjean, 1994;Valero Garcés et al, 1999, Rech et al, 2010Pueyo et al, 2011).…”

Going nano: A new step toward understanding the processes governing freshwater ooid formation

“…These comparatively lower δ 13 C values in the cortices are clearly related to microbial activity; the preferential uptake of 12 C-enriched compounds during microbial mineralization leads to the precipitated carbonate being more depleted in 13 C than the source material (e.g., Lee et al, 1987). The cortex δ 13 C values suggest incorporation of organically derived carbonate ions (Luzon et al, 2009), but they are too high to be related to sulfate-reduction (commonly δ 13 C < -12‰) and too low to be related to fermentation or methanogenesis (commonly δ 13 C > +5‰; Irwin et al, 1977;Pueyo et al, 2011). Similar δ 13 C values (-7‰ to -10‰) have been obtained from a calcifi ed freshwater stromatolite (Souza-Egipsy et al, 2006) and a laminated biofi lm (Shiraishi et al, 2008), where photosynthesis driven by cyanobacteria has been identifi ed as the main process inducing calcite precipitation.…”

Paleolimnology of a coastal lake on the Southern Cape coast of South Africa: Sediment geochemistry and diatom distribution

“…Carbon isotopes (d 13 C) are generally used to differentiate between sources of organic matter (Meyer, 2003;Pueyo et al, 2011). In aquatic environments, carbon is mainly available as bicarbonate (HCO À 3 ) and dissolved CO 2 (inorganic carbon) and thus aquatic organisms have isotopic values close to such a source (Degens et al, 1968;Falkowski, 1991).…”