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

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-δ15NAIR, and by fluctuating values of TOC-δ13CVPDB 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 δ18OVPDB 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 banded-massive with tephra layers in the upper half of the sequence (Unit 2). TOC-δ13CVPDB 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%. CO2 photosynthetic depletions related to seasonal phytoplankton blooms were responsible for the high frequency deposition of mm-thick carbonate layers.

The average values for δ13CVPDB 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 13C enrichment is attributed to carbon assimilation from a DIC affected by methanogenesis, in which HCO3- is the dominant species. The δ13CVPDB and δ18OVPDB variations and their covariation in endogenic carbonates suggest that lake water volume and lake level increased along the Holocene.