Org. Geochem. Vol. 5, No. 3, pp. 131 142, 1983 0146-6380/83 $3.00+0.00 Printed in Great Britain. All rights reserved Copyright (6 1983 Pergamon Press Ltd Characterization of sedimentary humic matter by alkaline hydrolysis RICHARD A. BOURBONNIERE National Water Research Institute, Canada Centre for Inland Waters, P.O. Box 5050, Burlington, Ontario, Canada L7R 4A6 and PHILIP A. MEYERS Oceanography Program, Department of Atmospheric and Oceanic Science, The University of Michigan Ann Arbor, MI 48109, U.S.A. (Received 7 April 1983; accepted 16 August 1983) Abstract--Humic matter fractions from modern sediments of Lake Huron and Lake Michigan have been compared. Large yields of saccharinic acids from alkaline hydrolysis suggest that these fractions contain large portions of carbohydrate materials. Evidence for contributions of aquatic lipid (C-16 fatty acids) and of liginin (phenolic acids) to these sediments is also present in the hydrolysis products. Qualitative differences among fulvic acid, humic acid and humin from the same lake are minor, suggesting common (or similar) organic sources for these fractions. The lability of sedimentary humic matter to alkaline hydrolysis is inversely related to its degree of exposure to oxidative weathering. Lability may also be related to diagenetic state as fulvic acids generally yield greater quantities of hydrolysis components than humic acids which in turn yield more than humin. INTRODUCTION Humic matter comprises the bulk of organic matter in natural systems. Although much research effort has gone into the study of soil humic matter, com- paratively few studies have focused on sedimentary humic materials. Among these, elemental and spec- troscopic characteristics of the humic materials iso- lated from freshwater sediments have been reported (Otsuki and Hanya, 1967; Ishiwatari, 1970, 1973; Kemp, 1973; Kemp and Mudrochova, 1973). In general, these studies have shown that sedimentary humic matter is composed of material of both aquatic and terrestrial origin. Recent studies of humic matter of various origin by NMR spectroscopy have shown varying proportions of aliphatic and aromatic struc- tures (Stuermer and Payne, 1976; Hatcher et al., 1980a,b; Dereppe et al., 1980; Hatcher, et al., 1981a,b; Wershaw et al., 1981). Chemical degradation has been used to character- ize soil humic matter (Schnitzer and Khan, 1972) and has also been applied to freshwater studies. Ishiwatari (1975) oxidized humic acid from Lake Haruna sedi- ment with alkaline KMnO4 and released a variety of aliphatic and aromatic acids. Kemp and Mudrochova (1973) studied amino acids released by acid hydro- lysis of fulvic and humic acids from a Lake Ontario sediment sample. Alkaline hydrolysis has been used successfully on soil humic matter by Jakab et al. (1963), Neyroud and Schnitzer (1975) and Tsutsuki and Kuwatsuka (1979). Here we report chemical characteristics of humic matter from modern sediments of Lakes Huron and Michigan. The focus of this paper is on products released by alkaline hydrolysis of humic matter frac- tions. These are compared to hydrolysis products from a number of locally derived materials which are potential sources of, or precursors to, the sedimen- tary humic matter. EXPERIMENTAL Sediment samples The sediment samples analyzed were obtained with a 6.7cm I.D. Benthos :~: gravity corer. Physical disturbance during the coring operation was minimized by using a butterfly type valve and leaving the penetration end open, without a core catcher. Cores were extruded vertically and sectioned within one hour after collection, frozen immedi- ately and stored at -20°C in glass containers until analysis. One core, designated SLH75-18, was taken in the Goderich Basin of southern Lake Huron (44°00'N., 81°59'W.; water depth 63 m) in September, 1975. The other core designated SLM77-26, came from southeastern Lake Michigan (42c'20'N., 86~:50'W.; water depth 118 m) and was collected in July, 1977. The Lake Huron site is characterized by a present-day sedimentation rate of 0.11 + 0.01 cm/yr. This was deter- mined by 2~°Pb geochronology measured on a companion core by Robbins et al. (1977) at their station 18. Vertical mixing caused mostly by bioturbation extends to a depth of 3 cm at this location (Robbins et al., 1977; Krezoski et al., 1978). The present day sedimentation rate at the Lake Michigan site is 0.10 cm/yr (Edgington and Robbins, 1975, their station 26). This was determined from ~37Cs data as was the vertical mixing depth of 2 cm. Fractionation of humic matter and chemical analyses were done on 1 cm sections for the upper parts of both cores. For the purposes of this report, data for the top 4 cm of each core were combined and weighed according to the proportion that the dry sediment weight of each 1 em section is to the total dry weight of the 0-4 cm interval. 131