Sea ice has disappeared completely during the summer. Intermittent winter ice is all that remains. In response to stratification of the water column, diatom populations have shifted, complicating productivity with possibly profound implications for the ocean’s biological carbon pump in its role in sequestering atmospheric CO2.
While these words might be written about our not so distant future, they actually describe a new window into the Arctic of ~ 70 Mya. Davies et al. tackle the subject in their paper (or here) “Late Cretaceous seasonal ocean variability from the Arctic” in the current issue of Nature. The climate of the Arctic for this period has been the subject of healthy debate, with previous studies suggesting an average temperature of some 15°C- implying an Arctic devoid of sea ice. The Davies team has produced the region’s first seasonally resolved “paleosediment trap”, which shows evidence of intermittent sea ice and indicates a maximum summer temperature of around 15°C, agreeing with other evidence suggesting winter temperatures would have been much lower.
Their cores display in their own words “superbly preserved diatoms”, and I’d call that an understatement. The image below should click through to a much larger version, so you can see for yourself:
Click to embiggen- it’s worth it.
The seasonality is inferred from a series of layers indicating autumnal (as the thermocline falls off during the transition to the Arctic winter/polar night) and vernal (nutrient limit induced) “dumps” of diatoms, which are clearly visible in the cores. Interspersed layers of poorly sorted terrigenous material (i.e. clays, silts, and fine sands) indicate sporadic sea ice formations which incorporate suspended, fine seds as they form (as frazil and then turbid ice) [corrected and edited for clarity].
In addition to its contribution to the Arctic temperature question for the Late Cretaceous, the Davies paper is notable in that it provides good evidence for a highly stratified Arctic Ocean and associated diatom populations during a known “greenhouse” period. A concern for our current and future greenhouse warming is increased stratification reducing the effectiveness of the oceanic carbon pump, exacerbating the atmospheric CO2 problem- another feedback to worry about. And with the current state of Arctic warming, worry we should.