Abstract:
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.