| Abstract: | Transient climate events often lead to profound changes in ocean circulation. This modified circulation has in turn led to increase or decrease in the supply of nutrients to surface waters, both delivered from the continents via runoff and upwelled from the deep ocean. Paleoceanographers have often painted a black and white picture of productivity change during transient climate events. For example, Cretaceous Oceanic Anoxic Events (OAEs) have been thought of as periods of high oceanic productivity as indicated by the abundance of siliceous microfossils in particular black shale horizons and by planktic foraminiferal carbon isotope data. We present nannoplankton assemblage data illustrating that it is impossible to generalize about productivity changes on an ocean-wide basis for transient climate events.
Nannoplankton assemblage data from sections in Kansas in the Western Interior Seaway and the New Jersey coastal plain indicate reduced productivity at the onset of OAE2 at the Cenomanian/Turonian Boundary. A marked decrease in the relative abundance of Biscutum constans, a species that is widely thought to be an indicator of high-fertility environments, is observed at the onset of the event. Conversely, taxa that are interpreted as indicative of oligotrophic environments increase in abundance. In both sections, productivity appears to increase in the later part of the event. Similar trends have been observed by Paul et al. (1999) and Gale et al. (2000) for contemporaneous shelf sections from northwest Europe. Combined with other microfossil assemblage and geochemical data, nannoplankton assemblage data suggest sequestration of nutrients in open ocean environments and starvation of the shelf and epicontinental seaways during OAE2. This is in marked contrast to current hypotheses regarding the origin of anoxia and organic matter accumulation.
In summary, transient climate events should not be thought of us resulting in ocean-wide increases or decreases in productivity. Instead, these events appear to have resulted in profound changes in the distribution of nutrients in the ocean. These changes had significant effects on marine biotas and may be responsible for at least part of the turnover that took place during short-lived climate events such as OAE2. |