Award: OCE-1537485

The ocean affects climate as it is a major transporter of heat via its circulation and highly influences the amount of atmospheric CO2, a greenhouse gas, much of it in the deep ocean.  It is societally important to understand the relationship between the ocean’s overturning circulation, climate and CO2 as these are all changing due to the human caused release of CO2. Specifically we developed and applied methods to help reconstruct past ocean circulation during the Last Glacial Maximum (LGM) and to test hypotheses of that explain the variation of CO2 between the LGM and the present by collecting and chemically analyzing old water from the LGM. These waters were extracted from seafloor sediment after we collected long sediment cores.

Additionally, we used the core samples to further out understanding of biological nitrogen transformations in the seafloor. Nitrogen is a key nutrient influencing biological activity in the seafloor and limits biological productivity in much of the ocean.

The data collection, synthesis and interpretation proposed here will inform the broader climate change community, including climate modelers who advance scientific understanding of what controls the oceanic response to climate change. The results of their advances will ultimately guide the societal response to anthropogenic CO2.

 

In summary, we accomplished the following:

1. We developed a high precision, density-based method for determining sedimentary porewater salinity that can be performed shipboard on small volume samples with greater efficiency than the currently available technique. We applied this method to porewater samples extracted from adjacent long cores collected from the deep western North Atlantic. This method will allow for the reconstruction of past ocean circulation with much greater precision than was previously possible.

 

2. We compare the high precision chloride concentration profiles determined using our method to profiles determined from chloride titrations of parallel samples. Salinity change at our site between the LGM and pre-industrial is 3.07 ± 0.03 % and 3.65 ± 0.06 % when determined from density, consistent with  nearby deep Atlantic paleosalinity data and global sea-level-change determined salinity change.

 

3.We use the abundance of nitrate and oxygen to infer that the ratio of  deep ocean waters forming in northern to southern latitudes that mix in the southern ocean and feed the deep waters of the Pacific did not significantly change between the LGM to the present. This implies that despite the expansion of the southern sourced water mass and shoaling of the northern sourced water mass in the Atlantic during the LGM, their relative fluxes did not change and that the heat flux in the South Atlantic was northward, during the late glacial, as it is today.

 

4.Our data does not support the hypotheses that lower PCO2 during the LGM was simply due to a more efficient biological utilization of nutrients. We argue that increased biological production in the southern. Instead we argue that as a result of the shoaling of the northern sourced water and extension of the southern sourced waters, there was an increase in nutrients in the deeper cell of the overturning circulation and a decrease in the shallower cell (a result of the AABW becoming a more efficient nutrient trap). This, combined with Fe fertilization led to greater productivity in the Southern Ocean but no significant change in pre-formed nutrients (since total nutrients were higher). Higher Southern Ocean productivity is expected to lead to the sequestering of carbon in the deep ocean via gas exchange disequilibrium.  In summary, we argue, that the lower glacial PCO2 resulting from increased productivity in the Southern Ocean was the sequestration of disequilibrium CO2 with the proximal cause being Fe-fertilization but that this did not increase the overall whole ocean efficiency of nutrient use.

 

5. Based on stable N and O isotope measurements of nitrate and nitrite to constrain rates of nitrogen cycling processes, we conclude that there are exceptionally high rates of nitrite oxidation and nitrate reduction near the top of the anoxic zone We posit that chemoautotrophic nitrite-oxidizing bacteria persist in these organic-lean environments when carbon is limiting to heterotrophic denitrifying bacteria.

Last Modified: 12/17/2019
Modified by: Arthur J Spivack