| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | August 25, 2015 |
| Latest Amendment Date: | August 25, 2015 |
| Award Number: | 1537485 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Candace Major
OCE Division Of Ocean Sciences GEO Directorate for Geosciences |
| Start Date: | September 1, 2015 |
| End Date: | August 31, 2019 (Estimated) |
| Total Intended Award Amount: | $319,978.00 |
| Total Awarded Amount to Date: | $319,978.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
75 LOWER COLLEGE RD RM 103 KINGSTON RI US 02881-1974 (401)874-2635 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
215 South Ferry Rd Narragansett RI US 02882-1127 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Marine Geology and Geophysics |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Increasing atmospheric CO2 is recognized as one of the world's most pressing environmental challenges. Many details of this challenge require further scientific understanding, including the feedbacks between ocean circulation, atmospheric CO2 and climate. It is generally accepted that CO2 transfer between the ocean and atmosphere has driven the co-variation of CO2 and climate over the past 800,000 years. However, there is no consensus on what drives these transfers principally because there is a lack of data to test proposed mechanisms. The proposed project is directly aimed at understanding the relationship between ocean circulation and climate, particularly atmospheric CO2, during the Last Glacial Maximum (LGM, ~18,000 years ago). The improved understanding of the relationship between ocean circulation and climate is societally important because future changes in the circulation are likely to have large effects, such as changes in the climate of northwest Europe, the position of the Intertropical Convergence Zone (ITCZ) and the uptake of anthropogenic CO2 into the ocean. Scientific understanding of the controls of ocean response to climate change will inform the societal response to anthropogenic CO2. Additionally, this project will support full participation and leadership development of a female Ph.D. student in science.
More specifically, the work focuses on laboratory analysis of sedimentary pore fluids collected during a recent research expedition in the North Atlantic, and interpretation of the data. This analytical and interpretative work is aimed at addressing the following hypotheses:
- During the Last Glacial Maximum (LGM), deep-ocean density stratification in the North Atlantic was dominated by salinity variation rather than temperature variation.
- During the LGM, North Atlantic deep water was a mixture of water originating in the North Atlantic and water originating in the Southern Ocean.
- The present relationship between d18O and water density existed during the LGM, enabling use of benthic carbonate d18O to infer density.
- The relative balance of the key nitrate (NO3-) removal processes was similar to that of today, as was the average NO3- concentration in deep water.
- Deep water in the LGM North Atlantic was dominated by low preformed nutrient water.
- A significant fraction of atmospheric pCO2 reduction during the LGM was due to the low pre- formed nutrients in the Atlantic.
The analytical work includes, high precision determination of chloride concentrations by titration, the development of a new density based method of paleo-salinity determination and the isotopic analysis (O and N isotopes) of nitrate. This data will be used to constrain inverse diffusion models to infer bottom water properties of the LGM western North Atlantic.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
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
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