FROM UGLY STEPSISTER TO UGLY DUCKLING – d13C VALUES OF FRESHWATER MOLLUSK SHELLS – A SWAN’S STORY

The d¹³C values of freshwater mollusk shells have long been considered the ugly stepsister of the paleo-climate world. Carbon isotope data collected and never interpreted -- ignored as useful proxies of ambient water conditions. The reason for this appears to be concern about ‘vital effects' that could obscure accurate interpretation of these data. While biological fractionation is problematic, researchers should nonetheless attempt to interpret these data. Several datasets from modern and ancient freshwater mollusks indicate that d¹³C values can provide important information regarding seasonal changes in primary productivity, stratification, and sources of dissolved inorganic carbon in surface waters. Case studies of the utility of molluscan d¹³C values will be presented from a variety of latitudes and ages. These include: 1) a Late Cretaceous-Early Paleocene river in North Dakota, 2) the Mackenzie River in the Canadian Arctic, 3) a modern lake in New Jersey, and 4) modern and Pleistocene rivers in Indonesia. Among these case studies is the seasonal variation in primary productivity observed in proglacial river systems that is out of phase with temperature and insolation due to variation in discharge and turbidity. Another case study describes a lacustrine system that records springtime increases in productivity, summer stratification, a fall overturn, and wintertime oxidation of plant organic matter. A third case study examines tropical mollusks that faithfully record local vegetation changes (C4 to C3 variation) in their overall d¹³C values. Carbon isotope data interpreted in conjunction with oxygen isotope data and in the context of regional geography and stratigraphy can yield important paleo-climate information. While problematic, attempts should be made at interpreting these very useful and interesting data.