CHEMICAL AND ISOTOPIC EVIDENCE FOR THE IN SITU ORIGIN OF MARINE HUMIC SUBSTANCES1,2

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Abstract
Humic and fulvic acids were extracted from marine and nonmarine Recent sediments and from soils. These acids are shown to be major components of the organic matter from marine and nonmarine sediments—some marine sediments may contain 70% of their organic carbon in the humic and fulvic acid fraction. Marine and terrestrial humic acids have similar carbon and hydrogen content, but the former generally contain more sulfur and nitrogen. δ34S values of marine humic acid indicate that the sulfur is introduced into the organic matter as hydrogen sulfide produced by sulfate reduction. Marine humates have a rather constant δ13C value of −20 to −22%, whereas the δ13C of soil humic acid is related to its plant source material and usually ranges around −25 to −26%. The evidence shows that marine humic acids can be formed in situ from degradation products of plankton and are not necessarily transported from the continent. The suggested pathway of marine humic acid formation and transformation in the sediment is (1) degraded cellular material→ (2) water‐soluble complex containing amino acids and carbohydrates→ (3) fulvic acids→ (4) humic acids→ (5) kerogen.

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