Orbital-scale controls on biogenic silica accumulation in the Indian Sector of the Southern Ocean over the past 600,000 years
Date
2020
Authors
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Publisher
University of Delaware
Abstract
Variations in Earth’s orbital geometry and relative location to the Sun have influenced climate throughout geologic time. These cycles include orbital eccentricity, obliquity, and precession. Combined, these parameters influence the distribution of radiative forcing received by each latitude on Earth (insolation), which has been linked to climate warming and cooling. Records of climate, such as oxygen isotopes, display variations in concert with the amount of radiative forcing received by Earth. Another factor that plays into glacial/interglacial climate is the amount of greenhouse gases in the atmosphere. Previous studies, utilizing ice cores, have revealed that the concentrations of greenhouse gas, such as carbon dioxide, in the atmosphere vary over glacial/interglacial timescales. One hypothesis to explain the difference between low glacial CO2 and high interglacial CO2 relies on stratification within the Southern Ocean (e.g., Sigman & Boyle, 2000). In this study, I hypothesize that atmospheric CO2 levels are regulated by changes in Southern Ocean stratification. For example, interglacial climates have high atmospheric CO2 levels due to a well-ventilated Southern Ocean. Within the modern Antarctic Zone of the Southern Ocean, nutrient- rich deep waters are pulled to the surface via Ekman pumping (upwelling), fueling one of the most biologically productive regions of the world’s oceans. These deepwater masses that are upwelled also contain dissolved CO2. On glacial/interglacial timescales, Southern Ocean stratification has been linked to global climate change as a mechanism that regulates atmospheric CO2 levels. ☐ Downcore variations in opal production are generally used as a proxy for upwelling throughout geologic time. Typically, these records are restricted to the past glacial/interglacial cycle due to the vast ability of sedimentological records spanning recent geologic time. In this study, I expand understanding of Southern Ocean stratification, evidenced by changes in upwelling, over the past 600 kyr years by utilizing biologic production proxies. To test my hypothesis, I constructed a record of opal mass accumulation rates in the Antarctic Zone of the Southern Ocean over the past 600,000 years, spanning eight glacial/interglacial cycles. During this time, the intensity of interglacial climate (e.g., decrease in global ice volume and, by extension, climatic warmth), as evidenced by δ18O values, increase without any apparent changes in orbital forcing. This climatic shift is called the mid-Brunhes climate event. By applying this technique of recording changes in biologic production to Southern Ocean sediments, we can further understand the relationship between water column stratification and the mid-Brunhes climate event. Thus, I hypothesize that there was an increase in Southern Ocean upwelling during interglacial periods following the MBE, as compared to interglacial climates before. ☐ Instead of showcasing large glacial/interglacial variations in stratification, as expected, my opal mass accumulation record reveals a tight connection between biological production and orbital forcing. This novel record is interpreted to reflect precession driven changes in open ocean upwelling driven by westerly winds. These westerly winds, which are largely controlled by variations in radiative forcing, control biologic production on precession timescales. While most of the opal record displays clear responses to orbital precession (in both amplitude and timing), MIS 12 stands out as a glacial interval with high productivity. This unique production maximum during an extreme glacial climate may be caused by increased nutrient content of the waters being upwelled and/or increased silicic acid as a result of chemical weathering from Antarctica.
Description
Keywords
Biogenic silica, Orbital-scale, Paleoceanography, Pleistocene, Southern Ocean, Stratification