Climate Fact: Ocean Stratification and Diatoms
Upwellings of water from the ocean depths to the surface are a key nutrient source for the algae that live on the ocean surface. Because these algae ultimately feed all other marine life, the characteristics and variability of such upwellings are important factors for understanding ocean ecosystems. During warmer periods of Earth’s past, such upwellings were rare and the vast majority of the ocean was considered “stratified,” meaning that a pervasive, warm surface layer prevented cool and nutrient rich deep water from reaching the surface. During the much warmer late Cretaceous epoch (65-99 million years ago), the Arctic Ocean, though not nearly as frigid as it is today, was one of the few places on Earth with waters cool enough for a seasonal upwelling to occur. This was an annual event that happened during the rapid transition from the sunless winter to the nightless summer, which takes about one month. The Arctic Ocean was likely where many of the ancestors of the seasonal upwelling-adapted diatoms, which are common throughout the world’s oceans today, originated. The rest of the world’s ocean surface was dominated by diatom species adapted to stratified conditions. Seasonal upwellings have become more common over the past 65 million years as the Earth has cooled and diatoms adapted to stratified conditions have become less dominant. For example, one such genus had over 40 species during the late-Cretaceous, but today only four still exist.
Seasons: Winter, Spring, Summer, Fall
Source: Davies, et al. “Late Cretaceous seasonal ocean variability from the Arctic.” Nature 460 (2009): 254-257.
