The waters of the Indian Ocean southeast of Africa are a crucial junction for surface and deep ocean processes that serve as vital controls on Earth’s climate system. At the surface, the Agulhas Current, its retroflection, and Agulhas Leakage transfer water from the Indian and South Atlantic. The addition of this heat and salt to the Atlantic Basin helps drive the Meridional Overturning Circulation and the formation of deep water in the North Atlantic Ocean. On the timescales of centuries, the Meridional Overturning Circulation ultimately returns this water back to the Indian Ocean in the form of North Atlantic Deep Water. Proxy reconstructions show that the vigor of ocean overturning is immensely important to the global climate system, driving changes in atmospheric CO₂ concentrations and temperature and precipitation patterns across the planet.
I use x-ray fluorescence core scanning, sediment provenance techniques, and core images from International Ocean Discovery Program Site U1474, located in the Natal Valley of the southwest Indian Ocean, to investigate past changes in the Agulhas Current and North Atlantic Deep Water. 40K/40Ar provenance ages measured on the clay fraction of sediment from Site U1474 indicate that, despite its great distance from the core site, the Zambezi is the most important factor influencing the deposition of terrigenous sediment in the Natal Valley. We present these results in a quantitative way, reinforcing the conclusions of previous studies. However, a comparison to newly available proxy records influenced by current speed and hydroclimate suggests that the strength of the Agulhas does not have a major influence on terrigenous sediment sources, at least at the headwaters of the Agulhas Current.
Instead, I suggest that low-latitude hydrologic processes driven by zonal and meridional temperature gradients in conjunction with sea level are responsible for sediment source variability. In core photos, I found evidence for deep water variability in the Natal Valley in the form of millimeter-to-centimeter scale layers of olive-green sediment. To an overwhelming extent, these layers are formed during glacial periods, especially at their termination. I complement observations at Site U1474 with published proxy data for oxygen concentrations and measurements of total organic carbon percent in the Natal Valley and by extending our search for these green layers to core sites around the world.
With these data, it is possible to confidently connect these layers to organic carbon concentrations in the sediment, reduction-oxidation processes in sediments following burial, and the local concentration of dissolved oxygen in the deep water. There are comparable fluctuations in the abundances of green layers in core sites in the path of North Atlantic Deep Water during glacial cycles, where more frequent and more intense green layer formation is driven by higher bottom water oxygen concentrations. Peaks in the abundance of green layers approximately 250 ka and 900 thousand years ago coincide with global scale excursions toward isotopically light benthic carbon isotopes. Connecting the green layers to the release of isotopically light organic carbon from sediments leads me to propose that long-observed fluctuations in the carbon cycle may be attributable to deep ocean oxygenation.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/dh7t-c025 |
| Date | January 2023 |
| Creators | Babin, Daniel Paul |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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