Variability in the length of the sea ice season in the Middle Eocene Arctic

Finely laminated Middle Eocene sediments from the central Arctic contain high abundances of the delicate, sea ice-dwelling fossil diatoms Synedropsis spp. and sea ice-rafted debris (sea ice-IRD), establishing an offshore seasonal sea ice regime ca. 47 Ma. Synedropsis spp. co-occur with other diatom taxa and microfossils requiring open water. This strongly indicates seasonality; nonetheless, seasonal reconstruction of the flux cycle cannot be resolved by standard bulk-sediment analysis, which destroys sedimentary fabrics and averages data within samples. Here we resolve and reconstruct seasonal-scale flux events from these sediments using backscattered electron imagery (BSEI) of resin-embedded sediment, a nondestructive technique that preserves the integrity of sedimentary microfabrics, thus revealing discrete productivity-flux events at ultrahigh (e.g., <30 mu m) resolution. Seasonality is expressed at the submillimeter scale by successions of discrete mono-specific laminae and micro-lenses of Synedropsis spp., terrigenous material (sea ice-IRD), and open-water taxa, indicating that first-year ice existed in the central Arctic. Further, BSEI reveals millimeter-scale alternation of bundles of laminae and microlenses of two distinct types: one characterized by Synedropsis spp. and terrigenous material, the other by mainly open-water taxa and little terrigenous material. The sedimentation rate and preliminary assessment of annual cycles indicate suborbital variability on the order of multi-decadal to centennial duration; we argue that this reflects variations in the sea ice-season length.