BIOGEOGHEMICAL ANALYSIS OF BIOMARKERS FROM BIRCH LAKE, ALASKA

Katey M. Walter
Department of Geology and Geography
Mount Holyoke College

Organic matter in the form of proteins, lipids, carbohydrates, and other biochemicals from the tissues of organisms that populate a lake and its watershed, make up an important component of lake sediments. Primary sources of organic matter in lake systems include the particulate detritus of aquatic organisms, mainly algae and zooplankton; and the residues of terrestrial biota. Different types of biota living in the lake environment have distinct biochemical compositions; and changes in community structure of these organisms can induce variations in the amounts and types of organic matter deposited in the sediments at different times in the lakes history . In this way, organic compounds can serve as biomarkers to create the paleolimnological record of a lake.

This study looks at the biogeochemistry of Birch Lake in central Alaska through analysis of biomarkers extracted from extant terrestrial and aquatic organisms, a modern lake sediment sample, and lake core samples. The goal of this work is to test the relationship between the modern organims and lake lipids found in modern and core sediments to see how closely molecular signals match. The five core samples were selected based on variability in sedimentology and total organic carbon content. The approximate ages of the core samples range in 14C ages from 4810 yr B.P. to 13,330 yr B.P.

Study of the lacustrine deposits of Birch Lake is advantageous because the lake lies in the interior corridor of central Alaska, a region that remained unglaciated throughout the Holocene. These lacustrine deposits have preserved continuous records of ecosystem response to dynamic climate changes that include both vegetational shifts (e.g. the transitions from shrub tundra at the end of the Wisconsin glaciation through scrub forest successions of Populus, Picea, and Alnus to the boreal forests that were established 6000 yr B.P ) and shifts in lake ecosystem.

Lipids were extracted from the plant and algae tissues, and from the sediments by processes of sonication with (2/1) chloroform/methanol solvents, saponification, and acid hydrolysis. Samples were then separated into distinct compound classes, derivitized, and then analyzed using gas chromatography and molecular mass spectrometry. By comparing the results of this experiment to on-going sedimentological, geochemical, and pollen studies, it should be possible to determine how closely the changes observed in biomarkers represent substantive changes in ecosystem community structure over time in Birch Lake.