A 100-year record of changes in organic matter characteristics and productivity in Lake Bhimtal in the Kumaon Himalaya, NW India

Abstract

Sediment variables total organic carbon (TOC), total nitrogen (TN), total sulfur (TS), as well as their accumulation rates and atomic ratios (C/N and C/S), were studied along with stable isotopes (δ 13 C, δ 15 N, and δ 34 S), and specific biomarkers (n-alkanes and pigments) in a 35-cm-long sediment core from Lake Bhimtal, NW India. The average sedimentation rate is 3.6 mm yr -1 , and the core represents a provisional record of 100 years of sedimentation history. Bulk elemental records and their ratios indicate that sediment organic matter (OM) is derived primarily from algae. In-lake productivity increased sharply over the last two decades, consistent with paleoproductivity reconstructions from other lakes in the area. An up-core decrease in δ 13 C values, despite other evidence for an increase in lake productivity, implies that multiple biogeochemical processes (e.g. reflect the presence of N-fixing cyanobacteria, and an increase in lake productivity. The  34 S profile shows enrichment of up to 5.6‰, and suggests that sulfate reduction occurred in these anoxic sediments. Increases in total n-alkane concentrations and their specific ratios, such as the Carbon Preference Index (CPI) and Terrestrial Aquatic Ratio (TAR), imply in-lake algal production. Likewise, pigments indicate an up-core increase in total concentration and dominance of cyanobacteria over other phytoplankton.Geochemical trends indicate a recent increase in the lake's trophic state as a result of human-induced changes in the catchment. The study highlights the vulnerability of mountain lakes in the Himalayan region to both natural and anthropogenic processes, and the difficulties associated with reversing trophic state and ecological changes.

Recent Contributions in the Field of Sediment Geochemistry

“…The study of Choudhary et al (2013) found an average sedimentation rate of 3.6 mm/year for a sediment core (35 cm) from the Lake Bhimtal, NW India with a provisional 100 years of sedimentation history. The data on total organic carbon (TOC), total nitrogen (TN), total sulfur (TS), stable isotopes (δ 13 C, δ 15 N and δ 34 S), and specific biomarkers (n-alkanes and pigments) indicated an increase in primary productivity as a result of the change in trophic state correlating temporally with the increase in anthropogenic activities.…”

“…Total organic carbon (TOC) and total nitrogen (TN) have been used to understand the source of organic carbon (Choudhary et al, 2013;Agrawal et al, 2015). Source of carbon can be identified using 14 C, 12 C, 13 C isotopes.…”

“…Radiocarbon ( 14 C) dating provides the age of the carbon, which is useful in dating sediment deposits (Kotlia et al, 2010;Raj et al, 2015). 13 C/ 12 C ratios are required to calculate δ 13 C values to identify C3 and C4 photosynthesis pathways and type of vegetation in the provenance (see Agrawal et al, 2015;Choudhary et al, 2013;Das et al, 2010;Kotlia et al, 2010). In addition to δ 13 C, δ 15 N values (calculated using 15 N/ 14 N ratios) is also used as an important proxy to the vegetation in the palaeoclimate studies (Choudhary et al, 2013;Agrawal et al, 2015).…”

“…13 C/ 12 C ratios are required to calculate δ 13 C values to identify C3 and C4 photosynthesis pathways and type of vegetation in the provenance (see Agrawal et al, 2015;Choudhary et al, 2013;Das et al, 2010;Kotlia et al, 2010). In addition to δ 13 C, δ 15 N values (calculated using 15 N/ 14 N ratios) is also used as an important proxy to the vegetation in the palaeoclimate studies (Choudhary et al, 2013;Agrawal et al, 2015). Phosphorus fractions (organic, inorganic and iron bound) extracted from sediments have also been used as an important proxy for the biological productivity and weathering conditions in the provenance .…”

“…We document here important research work on unconsolidated sediment deposits of the Quaternary period,where the geochemical proxy is an important proxy among others in the paleoclimate study. Studies in this regard include lake deposits of the Himalaya (Choudhary et al, 2013;Srivastava et al, 2013;Kotlia et al, 2010;Agrawal et al, 2015;Das et al, 2010), the Ganga plain (Saxena et al, 2015) and the peninsular region (Chauhan et al, 2013a;Chauhan et al, 2013b;Prasad et al, 2014;Raj et al 2015). Studies on fluvial deposits of the Narmada valley (Sridhar et al, 2015) and Cauvery delta in the peninsular region have also yielded good results.…”