Deglacial Warming – Oceanic Denitrification Acceleration

Australia: The Land Where Time Began

Deglacial Warming – Oceanic Denitrification Acceleration

Productivity over much of the surface of the ocean is limited by a scarcity of bioavailable nitrogen. It is suggested by the records of sedimentary δ¹⁵N that span the last deglaciation that were marked shifts at this time, though the complexity of the nitrogen cycling has hindered the quantification of these changes. In this paper a database of δ¹⁵N in sediments from throughout the oceans of the world, including 2,329 samples from the modern seafloor and 76 time series that span the past 30,000 years are presented. These demonstrate that sediments from the modern seafloor contain δ¹⁵N values that are consistent with values that have been predicted by knowledge of nitrogen cycling in the water column. The δ¹⁵N values of sinking organic matter were similar during the Last Glacial Maximum (LGM) and the Early Holocene, which were averaged globally, in spite of many deglacial changes. Considering the global isotopic mass balance, Galbraith et al. explain these observations with the following deglacial history of nitrogen inventory processes. The nitrogen cycle was near steady state during the LGM, and then denitrification in the pelagic water column accelerated during the deglaciation. Denitrification at the seafloor increased subsequent to the flooding of the continental shelves, denitrification again reaching near steady-state in the Early Holocene. A recent parameterisation of denitrification of the seafloor was used to estimate a 30-120 % increase in benthic denitrification between 15,000 and 8,000 years ago. In this paper it is inferred that pelagic denitrification must have increased by a similar amount between the 2 steady states, based on the similarity of globally averages δ¹⁵N values during the LGM and the Early Holocene.