VARIATIONS IN I-129 I-127 RATIOS IN RECENT MARINE-SEDIMENTS - EVIDENCE FOR A FOSSIL ORGANIC-COMPONENT/

The value of the pre-bomb I-129/I-127 ratio in the hydrosphere is impo rtant for the use of this isotope system for dating and tracing purpos es. In order to determine this value and to examine spatial variabilit y, I-129/I-127 ratios were measured in 25 sediment samples from five c ores taken at geographically distinct coastal areas. The results indic ate a pre-anthropogenic I-129/I-127 ratio of 1500 x 10(-15), in good a greement with values derived from previous sediment and groundwater st udies. Results from shallow sediments show that the layer of bioturbat ion is dominated by the addition of anthropogenic I-129. The good agre ement between results for this layer from different cores supports the understanding that marine iodine is isotopically homogeneous and asso ciated with organic material which is actively remineralized at the se diment-water interface. Lower ratios (down to 354 X 10(-15), correspon ding to an age of 32.5 Ma) and larger variations than expected were fo und in sections of the cores below the layer of bioturbation. These ob servations can be explained by the presence of refractory fossil organ ic material, transported by rivers to the sediments from uplifted, con tinental shales. The results suggest that marine iodine is present in two forms, a labile component which is isotopically homogeneous, and a refractory component which is associated with kerogen (or other fossi l organic compounds) whose I-129/I-127 ratio reflects the age of these compounds. I-129/I-127 ratios can be used to identify and date fossil organic material in Recent sediments, the presence of which can alter the assumptions underlying models of the global carbon cycle. (C) 199 8 Elsevier Science B.V. All rights reserved.