Friday, May 29, 2015

Radiocarbon dating of organic material dissolved in seawater: How old is it?

The ocean contains far more than salts and living organisms. The next time you take a swim think about the many thousands of diverse dissolved organic molecules (DOM) you are swimming through. DOM is operationally defined as smaller than a bacterial cell (0.1µm). It includes viruses and large (such as DNA) to very small molecules (amino acids and sugars).  The world's ocean mixes every ~1500 years, however the age of DOM is much older (4-6,000 yrs) suggesting many of these molecules are not efficiently removed by bacteria and persist for multiple ocean mixing cycles. In this way, the large DOM reservoir (662 GtC) stores carbon and mediates Earth's climate on immediate to millennial timescales. 
Our group's goal on this cruise is to understand the relative cycling rates of these molecules by determining their radiocarbon (14C) ages . We are particularly interested in determining the sources and cycling of highly-aged DOM components with human-induced inputs, such as combustion products, which we call Black Carbon (BC). We collect filtered seawater from the Niskin bottles at many depths and freeze them on board until we can analyze them back on land at the UC Irvine, Keck Carbon Cycle Accelerator Mass Spectrometer (AMS) Lab.  

Freezer samples on board
Mass Spectrometer back on land

by Brett Walker