SEMINAR: The Sources and Cycling of Carbon in Lake Superior: Insights from Delta14C

Organic carbon (OC) in aquatic ecosystems occupies a biogeochemical crossroads where carbon has the potential to become sequestered in sedimentary organic matter or recycled and contributed to the atmospheric pool via microbial respiration. The ultimate fate of OC is largely dependent upon its source, physical transport mechanisms, and biogeochemical transformations that occur within the water column. The relative importance of these processes varies among aquatic systems. In the oceans, it is difficult to assess the biogeochemical importance of old OC versus modern autotrophy with in situ measurements because the relative lack of mixing between surface and deep waters and the dominance of thermohaline circulation patterns help to maintain distinct age and chemical characteristics of these OC pools. However, unraveling the source, age, and relative reactivity of OC within the water column is a problem of critical importance that must be addressed in order to resolve key issues in global biogeochemical carbon cycles and the apparent net heterotrophy of most aquatic environments. We have addressed this problem by studying OC dynamics in Lake Superior, the world’s largest freshwater lake (by area). Lake Superior’s biogeochemistry is very similar to that of the world ocean but its physics varies in one key aspect that makes it an ideal natural laboratory to study the relative importance of biogeochemical processes and water column mixing on C cycling: it is dimictic, so there is complete water column mixing twice annually.

In this study, water-column depth profiles of Delta14C within dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and particulate organic carbon (POC) were used to investigate the sources and cycling of organic matter in Lake Superior. Radiocarbon data was obtained from western Lake Superior during periods of water-column stratification and mixing. The bulk DeltaDI14C (~86 to 74 permil) of the stratified water column indicates DIC is in exchange with atmospheric CO2. During stratification, the Delta-Delta14C (DIC-DOC) for surface and deep waters (10 permil and 151permil respectively) indicates that there may be a deep-water source of ‘old’ DOC. The Delta-Delta14C (DIC-POC) of 22 permil, 27 permil and 57 permil respectively for surface, mid and deep waters suggests that part of the POC pool consists of pre-aged material. The well-mixed water column reflects a modern (post-bomb) radiocarbon signal for DOC, DIC and POC across the entire sampling depth suggesting a stronger role for semi-reactive DOC in Lake Superior than in most oceans. Our Delta14C data shows that organic carbon in Lake Superior consists of both labile modern and old refractory portions, and that ‘old’ carbon can be important to lake biogeochemistry.