The ocean stores a vast amount of carbon in the form of free dissolved organic molecules (DOM) that are smaller than a bacterial cell or viral particle. From radiocarbon (14C) dating of these molecules, it is known that some DOM have persisted in the ocean for over 10,000 years. This is quite the conundrum given that: 1) the ocean fully mixes every 1500 years and 2) DOM represent a major store of potential energy for marine organisms to use. However, most DOM is not bioavailable — a fact that has puzzled oceanographers for the past 40 years.
Following recent technological advancements in ultrafiltration of seawater DOM, Prof. Walker recently collaborated with research groups from UC Santa Cruz and Lawrence Livermore National labs to use a new technique coupling ultrafiltration and solid-phase extraction to selectively purify the oldest and smallest DOM in the ocean. Prof. Walker and his collaborators measured properties of these small DOM molecules and found that the chemical structure and composition of DOM was fairly homogeneous throughout the global ocean. They measured the stable (δ13C) and radiocarbon (Δ14C) isotopic signatures, and DOM composition (C/N ratio, and 13C solid-state NMR) properties of these small DOM molecules to determine that DOM is quite homogeneous throughout the global ocean with respect to chemical structure and composition. Using radiocarbon dating, they confirmed that some of this material is removed through deep microbial respiration as deep water transits from the North Atlantic along the “deep ocean conveyor belt” to North Pacific Ocean over 1500 years.
Since the ocean’s DOM reservoir contains roughly the same amount of carbon as atmospheric CO2, DOM acts as a large buffer for Earth’s carbon cycle – mitigating climate change on timescales of years to several millennia. Despite the importance of this carbon reservoir to Earth’s climate system, until very recently we knew little about its sources and cycling in the ocean. This study will help inform the next generation of Earth System Climate Models and our ability to understand and predict future global climate change.
The University of Ottawa is home to the only accelerator mass spectrometer in Canada and has been very successful in providing Canadian researchers such as Professor Walker with valuable services including radiocarbon dating. The Walker lab is currently developing novel techniques to specifically determine the age of DOM using radiocarbon dating and assess its cycling in rapidly changing Canadian Arctic ecosystems.
