Quantitative evaluation of the sedimentary organic matter in Laguna Mar Chiquita, Argentina

VARANDAS DA SILVA, L., PIOVANO, E., AZEVEDO Y F. AQUINO NETO

ORGANIC GEOCHEMISTRY

Año: 2008 p. 450 - 464

0146-6380

Laguna Mar Chiquita (Central Argentina) is a highly variable saline lake particularly suitable for palaeolimnological studies. The most recent (20th century) and past (several 1000s of years) regional hydrological balance variability in southeastern South America has produced dramatic lake level fluctuation, making Laguna Mar Chiquita a sensitive climatic indicator. These fluctuations in the hydrological cycle additionally control the chemistry and biology of the water column and a variety of sedimentary processes, which are distinctively recorded in the composition of the sedimentary organic matter (OM). Lipid extracts, after fractionation, were analysed using gas chromatography/mass spectrometry (GC/MS) and gas chromatography/flame ionization detection (GC/FID). The n-alkanes range from n-C15 to n-C35, with total hydrocarbon concentration in the range 8.49?133 lg g1, and carbon preference index (CPI) values for n-C15?35 between 0.55 and 1.19. C/N values are in the range 4.6?12.5, indicative of mainly algal, bacterial and non-vascular plant origins. The d13C values of individual n-alkanes in the C15?33 range vary between ?22.4? and ?34.1?, suggesting a mixed OM input, i.e., C3 and C4 plants, and lacustrine algae. The presence of C27, C28 and C29 sterols (C27 dominant) and their diagenetic products reflects a major algal contribution. Evaluation of all the data together points to a mainly lacustrine algal input to the OM. Samples for which a high lake level occurred showed a clear predominance in water column production of OM. On the other hand, samples for which an increase in vascular plant (C3 and C4) input occurred are in accord with a low lake level. The data, combined with sedimentology, geochemical and isotopic information allow the development of a robust model for reconstructing past lake level fluctuation. Furthermore, the results are useful for interpreting similar saline lacustrine records on wider geographical and temporal scales. 2008 Elsevier Ltd. All rights reserved.n-alkanes range from n-C15 to n-C35, with total hydrocarbon concentration in the range 8.49?133 lg g1, and carbon preference index (CPI) values for n-C15?35 between 0.55 and 1.19. C/N values are in the range 4.6?12.5, indicative of mainly algal, bacterial and non-vascular plant origins. The d13C values of individual n-alkanes in the C15?33 range vary between ?22.4? and ?34.1?, suggesting a mixed OM input, i.e., C3 and C4 plants, and lacustrine algae. The presence of C27, C28 and C29 sterols (C27 dominant) and their diagenetic products reflects a major algal contribution. Evaluation of all the data together points to a mainly lacustrine algal input to the OM. Samples for which a high lake level occurred showed a clear predominance in water column production of OM. On the other hand, samples for which an increase in vascular plant (C3 and C4) input occurred are in accord with a low lake level. The data, combined with sedimentology, geochemical and isotopic information allow the development of a robust model for reconstructing past lake level fluctuation. Furthermore, the results are useful for interpreting similar saline lacustrine records on wider geographical and temporal scales. 2008 Elsevier Ltd. All rights reserved.