The last million years of Earth's history was dominated by the Ice Ages, the repeated advance and retreat of ice sheets that covered large portions of North America, Europe, and Asia.  While it has long been known that the Ice Ages were paced by changes in the distribution of sunlight over the Earth's surface, the mechanisms responsible for amplifying small variations in sunlight into continent-scale ice sheets remains unclear.  One of the most likely amplifiers is atmospheric carbon dioxide (CO₂), the levels of which vary depending on Earth's climate state.  During glacial intervals when the Earth is cold and ice sheets are large, CO₂ levels are low.  During interglacial intervals when the Earth is warm and ice sheets are small, CO₂ levels are high.  The correspondence between temperature, ice sheet size, and CO₂ during the Ice Ages is one of the primary lines of evidence showing Earth's climate depends strongly on atmospheric CO₂levels.  

Given that the deep ocean holds ~50 times more carbon than the atmosphere, ocean-atmosphere carbon exchange likely controls CO₂ levels during the Ice Ages.  Changes in marine biological productivity, sea ice coverage, and the position of ocean fronts are all thought to play important roles in regulating atmospheric CO₂.  The goal of this NSF-funded project was to develop new proxies for sea ice and frontal positions in the Southern Ocean, where deep waters upwell to the surface and release CO₂ to the atmosphere.  Sea ice can physically block the release of carbon while frontal positions reflect prevailing winds and upwelling of carbon-rich deep water to the surface.  It is therefore essential to have reliable reconstructions of sea ice and frontal positions to determine the ocean's role in regulating atmospheric CO₂.

For the sea ice component of the project, we used a latitudinal transect of cores from the Southern Ocean that straddles the modern sea ice edge to determine whether the oxygen isotopic composition of plankton shells (N. pachyderma) that recently accumulated on the sea floor reflect sea ice cover in the overlying water.  Using published data, we demonstrated that the oxygen isotope method works well for recent sediments, largely tracking the sea ice edge in the Atlantic sector of the Southern Ocean.  We also generated new data based on analyses of individual N. pachyderma shells to better assess natural variability in polar surface ocean conditions.  While some of the records are compromised by mixing of shells in the sediment (a process known as bioturbation), the new results support the idea that the oxygen isotope composition of N. pachyderma can be used to infer sea ice extent.  For the frontal component of the project, we used a different species of plankton (G. bulloides) whose habitat spans a broader latitude range.  The oxygen isotopic composition and the magnesium to calcium ratio (Mg/Ca) of G. bulloides were used to infer the temperature at which the plankton grew.  Our findings show that this species grows near the ocean surface (0-200 m water depth) at the Sub-Antarctic Front but deeper in the water column (200-500 m) near the Subtropical Front.  Given that fronts are defined using spatial gradients in near surface properties, our findings imply that G. bulloides will likely yield unreliable estimates of frontal position in the geologic past. Our results also indicate that paleoceanographic records based on G. bulloides should be interpreted with caution given that its depth habitat changes depending on ambient ocean conditions.  Our findings will help ensure reliable reconstructions of sea ice extent and frontal position in the Southern Ocean, both of which are key to understanding the ocean's role in the carbon cycle.

Last Modified: 09/17/2024
Modified by: David C Lund