Atlantic Meridional Overturning Circulation (AMOC) is a vital process that transfers heat and nutrients throughout the world's oceans, helping to regulate global climate and support marine ecosystems. The timing and nature of the shift to modern AMOC, and especially to deep-water formation in the North Atlantic, has been a topic of ongoing study, with the Eocene-Oligocene Transition (EOT, ~34 Ma) as a potential focal point of this shift. However, the role played by abrupt EOT cooling and Antarctic glaciation in North Atlantic circulation remains unclear. Improved constraints on Paleogene circulation will provide insight into the sensitivity of AMOC to perturbations in global climate, which is particularly relevant in light of contemporary climate change.
To examine deep North Atlantic circulation response to the EOT we obtained grain-size data from the terrigenous fraction of the mud-dominated sediments of the Southeast Newfoundland Ridge contourite drift complex at IODP Site U1411, which is interpreted to have formed under the influence of the Deep Western Boundary Current. We analyzed 195 samples that span 150 m of stratigraphy from 36-26 Ma. The main objective was to use the 'sortable silt' fraction (10-63 µm) to generate a record of relative change in bottom-current intensity. These data are complemented with a record of the abundance and size of lithogenic sand (>63 µm).
Here we present the first physical proxy record of abyssal current intensity in the North Atlantic, from late Eocene to mid Oligocene. Invigoration of North Atlantic deep circulation occurred gradually (over Myr timescales), with no significant changes linked temporally to the EOT. We infer that deep circulation in the North Atlantic was not sensitive to the abrupt global cooling and Antarctic glaciation associated with the EOT. Rather, our data suggest that changes in North Atlantic circulation were likely governed by longer-term processes related to the opening of key tectonic gateways, such as the Greenland-Scotland Ridge in the North Atlantic, and the Drake and Tasman Passages in the Southern Ocean. Additionally, we identify a significant mid-Oligocene invigoration of North Atlantic abyssal circulation, which climaxes around 27.9 Ma, and is coeval with a decrease in atmospheric CO2. / Master of Science / Atlantic Meridional Overturning Circulation (AMOC) is a vital process that transfers heat and nutrients throughout the world’s oceans, helping to regulate global climate and support marine ecosystems. However, AMOC has not always existed as we know it, and different modes of ocean circulation have operated in Earth’s past. The timing and nature of the shift to the modern form of AMOC/global ocean circulation has been a topic of ongoing study. The Eocene-Oligocene Transition (EOT, ~34 million years ago), a period of intense global cooling and Antarctic ice sheet growth, is thought to be a potential focal point of this shift to modern ocean circulation. However, the role played by abrupt EOT cooling and Antarctic glaciation in the evolution of ocean circulation remains unclear, especially in the North Atlantic. Understanding how and why ocean circulation has changed in more recent geologic time (within the past 65 million years), and ultimately evolved into its modern state, will help us understand the processes which led to, and now maintain, Earth’s modern climate state, as well as provide insight into the sensitivity of AMOC to perturbations in global climate, which is particularly relevant in light of contemporary climate change.
To examine the response of ocean circulation in the North Atlantic to climate changes at the EOT, we obtained grain-size data from the mud-dominated sediments of the Southeast Newfoundland Ridge contourite drift complex at IODP Site U1411, which is interpreted to have formed under the influence of the Deep Western Boundary Current. Contourite drifts are massive sediment build-ups on the ocean floor, which are formed due to bottom-currents depositing large amounts of sediment in one area. Therefore, contourite drift deposits can hold valuable records of bottom-current activity over millions of years. We analyzed 195 samples that span 150 m of stratigraphy from 36-26 Ma (Ma = millions of years ago). The main objective was to use the ‘sortable silt’ fraction (10-63 µm) to generate a record of relative change in bottom-current intensity. These data are complemented with a record of the abundance and size of sand (>63 µm).
Here we present the first physical proxy record of abyssal current intensity in the North Atlantic, from late Eocene to mid Oligocene. We find that invigoration of North Atlantic deep circulation occurred gradually (over million-year timescales), with no significant changes linked temporally to the EOT. We infer that deep circulation in the North Atlantic was not sensitive to the abrupt global cooling and Antarctic glaciation associated with the EOT. Rather, our data suggest that changes in North Atlantic circulation were likely governed by longer-term processes related to the opening of key tectonic gateways, such as the Greenland-Scotland Ridge in the North Atlantic, and the Drake and Tasman Passages in the Southern Ocean. Additionally, we identify a significant midOligocene invigoration of North Atlantic abyssal circulation, which climaxes around 27.9 Ma, and coincides with a decrease in atmospheric CO<sub>2</sub>.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/83539 |
| Date | 20 December 2016 |
| Creators | Chilton, Kristin Danielle |
| Contributors | Geosciences, Romans, Brian W., Eriksson, Kenneth A., Gill, Benjamin C. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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