Not all calcite ballast is created equal: differing effects of foraminiferan and coccolith calcite on the formation and sinking of aggregates

摘要

Correlation between particulate organic carbon (POC) and calcium carbonatesinking through the deep ocean has led to the idea that ballast provided bycalcium carbonate is important for the export of POC from the surface ocean.While this idea is certainly to some extent true, it is worth considering inmore nuance, for example, examining the different effects on the aggregationand sinking of POC of small, non-sinking calcite particles like coccolithsand large, rapidly sinking calcite like planktonic foraminiferan tests. Wehave done that here in a simple experiment carried out in roller tanks thatallow particles to sink continuously without being impeded by containerwalls. Coccoliths were efficiently incorporated into aggregates that formedduring the experiment, increasing their sinking speed compared to similarlysized aggregates lacking added calcite ballast. The foraminiferan tests,which sank as fast as 700 m d?1, became associated with only veryminor amounts of POC. In addition, when they collided with other, larger,foram-less aggregates, they fragmented them into two smaller, more slowlysinking aggregates. While these effects were certainly exaggerated within theconfines of the rolling tanks, they clearly demonstrate that calciumcarbonate ballast is not just calcium carbonate ballast – different forms ofcalcium carbonate ballast have notably different effects on POC aggregation,sinking, and export.