Surface sediments from coastal Lake J (unofficial name, 76° 11.4’ N, 90° 46.4’ W), Colin Archer Peninsula, Devon Island, Canada, exhibit a complex arrangement of clastic and organic laminae (Fig. 1). Detailed sedimentary examinations, including thin section and electron microscope imagery, suggest that the laminae are microbially induced sedimentary structures (MISS). The 37.5 cm core contained 2102 individual laminae, ranging in thickness from 3 µm to 8.5 mm. Each structure was categorized as either a clastic (Type-1), cyanobacteria (Type-2), or biofilm (Type-3) unit. Clastic units were characterized by normally graded sediment particles, often containing detrital organic material. Cyanobacteria units contained sheet-like deposits of filamentous material, while biofilm units were composed of relatively featureless material and often contained isolated lithic grains.

Analysis of the sediment showed significant first- and second-order Markov properties (c2 test, a = 0.05). A typical sequence consisted of a basal clastic unit, a laminated cyanobacteria unit, and a mucus-like biofilm unit. Less common sequences include one missing laminae (of all types), and sequences that exhibited unusual laminae contacts. Based on the strong seasonal hydroclimatic regime in this setting, a hypothetical deposition model was devised in order to develop an annual chronology. Starting in late spring, nival melt drives a brief period of sediment transport which enters the lake and accumulates on the lake bottom. Due to their phototactic behaviour, cyanobacteria from the lake bottom move upwards through the accumulating sediment to maintain a mat structure at the sediment-water interface. As sediment accumulation slows through the summer, the cyanobacterial mat develops further, and a secondary community of extracellular polymeric substances (biofilm) accumulates on the mat surface. This structure persists through the winter until the next nival melt period and resumption of sediment deposition.

Based on this annual accumulation model, annual structures were identified in the stratigraphic record. Although clastic-cyanobacteria-biofilm sequences dominated the record, other lesser sequences occurred, and necessitated heuristic rules to objectively identify the annual sequence (based on sediment type, where 1 = clastic, 2 = cyanobacteria, 3 = biofilm):
1. 1-2-3 is the typical annual sequence (61% of annual rhythmites); 2. 1-3 is an annual sequence that is missing the cyanobacteria unit (17% of annual rhythmites); 3. 1-2 is an annual sequence that is missing the biofilm unit (11% of annual rhythmites); 4. 2-3 is a rare annual sequence that is missing the clastic unit (4% of annual rhythmites); 5. 2 is a rare annual sequence that is missing both the clastic unit and the biofilm unit (7% of annual rhythmites); 6. Multiple sequences of 1-3 rhythmites that exhibited folds and other structural variations that suggest dynamic accumulation and were removed from the chronology (6 sequences in core, 189 laminae).

Application of these heuristics identified 747 annual rhythmites. Current and future work will examine the distribution of thickness for the three sedimentary components to explore biophysically plausible environmental analogs.