Isotopic Biogeochemical Records of Large Closed-Basin Lakes Compared to the Ocean
Lisa M. Pratt
Geological remnants of large closed-basin lakes are important archives of high-resolution paleoclimatic records. Several studies have recently demonstrated the sensitivity and rapid response of lacustrine ecosystems to changes in water column stability driven by windiness, aridity, or seasonal temperature differences.
Carbon and nitrogen isotopic compositions of organic matter in the Paleogene Green River Formation and Quaternary Lake Bosumtwi, Ghana show large isotopic variation (more than 10 per mil) with maximum 15N and13C enrichments occurring both in phase and out of phase. Nitrogen isotope effects result primarily from ammonia volatilization and the coupling of nitrification and denitrification. Carbon isotope effects result primarily from the presence of anaerobic photosynthetic bacteria and the recycling of methane. Such biogeochemical processes often dominate for long periods in closed-basin lakes but generally are minor or ephemeral processes in the ocean.
Carbon isotopic compositions of organic matter and sulfur isotopic compositions of pyrite, acid-volatile sulfide, and organic matter have been studied in a number of closed-basin lakes and oceans ranging in age from Precambrian to Quaternary. These data show wide isotopic variation in many lacustrine deposits with less frequent and gradual variation in oceanic deposits. Abundance crossplots of reduced sulfur versus organic carbon show broad scatter for many saline lacustrine deposits, in contrast to strong positive covariance for most oceanic deposits.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California