Influence of bottom water anoxia on nitrogen isotopic ratios and amino acid contributions of recent sediments from small eutrophic Lonar Lake, central India

Abstract

Lonar Lake is a eutrophic, saline soda lake with permanently anoxic deep water. The high pH and deoxygenation result in very elevated d 15 N of suspended particulate matter (SPM) and sediments due to denitrification and pH-related loss of gaseous ammonium. SPM and sinking particles are predominantly aquatic in origin, whereas surface sediments are of mixed terrestrial plant and planktonic source. An indicator of degradation intensity was derived from a principal component analysis of the spectral distribution of amino acids and named Lonar degradation index (LI). A ratio of individual amino acids (Ox : Anox ratio) was additionally used to determine the relative degree of aerobic vs. anaerobic degradation. These two biogeochemical indicators can be used to detect changes in degradation intensity and redox conditions in the geological history, and thus the paleoclimatic interpretation of Lonar sediments. Surface sediments can be divided into three zones: (1) a nearshore, oxic zone of predominantly aquatic organic matter, in which oxidation leads to a strong diagenetic increase of d 15 N; (2) an alluvial zone with a predominance of isotopically depleted land plant and soil organic matter degraded under oxic conditions; and (3) an anoxic, deep zone, which receives aquatic organic matter and land plant-derived material transported near the bottom and in which organic matter is well preserved due to anoxic diagenetic conditions. Lonar Lake is a crater lake with small streams and surface runoff as major water sources and no outflow. It is saline, alkaline, eutrophic, and below , 4 m water depth permanently sub-to anoxic, which is typical for closedbasin saline lakes (Eugster and Hardie 1978). The laminated sediments in the deep, permanently anoxic part of the lake provide one of the very scarce climate records in Central India (S. Prasad unpubl.). Due to their high resolution, these sediments provide an excellent archive of recent climate change. We tested common productivity and degradation proxies that are based on nitrogen isotopic ratios and amino acid assemblages for their applicability in Lonar Lake. For the current investigation terrestrial plants and soils from the inner Lonar crater as well as suspended particulate matter (SPM) were sampled. Sediment traps were installed to sample sinking particles in seasonal resolution and to study the effect of early degradation on sinking organic matter (OM). Surface sediments were analyzed to determine the state of degradation and to understand the differences in effect of early degradation under oxic and sub-to anoxic conditions. Furthermore, the ratio of the two stable carbon isotopes was used to determine the percentage of terrestrial and aquatic OM in the surface sediments and the pathways of CO 2 uptake of different plants.The nitrogen cycle in aquatic systems is often investigated using the ratio of nitrogen isotopes 15 N : 14 N of OM. This ratio, expressed as d 15 N, can serve as an indicator of the inorganic nitrogen source of aquatic plankton, and it refl...

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