Evaluation of Paleoproductivity Proxies in the Southern Ocean

Driven by human perturbations of the global carbon cycle, attention has focused recently on removing CO2 from the atmosphere via the biological pump and subsequent carbon burial in marine sediments. However, our knowledge of each component of the carbon cycle through time, including the role of primary production, remains incomplete. Several paleoproductivity proxies already exist, but all have problems that make them unsuitable under certain conditions: biogenic silica is affected by saturation state, while others become unreliable under anoxic bottom waters. In particular, biogenic silica is an important indicator of primary productivity because of its association with diatoms that grow in high nutrient, low productivity waters. Given the right stimulus, these ocean regions have the potential to increase productivity. We therefore require a new proxy for paleoproductivity that functions where others do not. Silver shows promise as a new proxy because it is known to be concentrated by diatoms; furthermore, water column profiles of Ag and silica appear to be correlated. An ideal location to study carbon burial is the Southern Ocean because it comprises a large portion of total export production to the deep ocean, and because it is known as a major sink for biogenic silica. Before we can evaluate Ag as a proxy, it is necessary to understand the modern behavior of other productivity proxies in the Southern Ocean and how they track one another. In this study we analyzed a suite of core-top sediments for excess Ba, biogenic silica, and organic carbon concentrations from sites in the Southern Ocean within both the Atlantic and Pacific basins. Barium and biogenic silica are well correlated with one another and show expected patterns of enrichment. An increase in both Ba and biogenic silica concentrations are observed ~62° S , consistent with a band of “opal ooze” that occurs in a zone north of major terrigenous input and within a region that experiences a large upwelling of dissolved Si. Finally, excess Ba is only observed in sediments deeper than 2.2 km water depth due to concentration of Ba by sinking organic debris.