Theses Doctoral

Reconstruction of the North Atlantic Deep Water end-member of the Atlantic Meridional Overturning Circulation through the Pleistocene

Kim, Joohee

The global ocean overturning circulation is a major means of distributing heat around the Earth, and an important trigger or amplifier of climate change. The formation of North Atlantic Deep Water (NADW) is a major driver of the critical Atlantic arm of the global system, the Atlantic Meridional Overturning Circulation (AMOC). Paleoceanographic studies have suggested considerable reorganizations of AMOC under varying climate conditions, through changes in the southward export of past analogs of NADW (referred to in this thesis as “North Atlantic-sourced waters” or NASW). Here, through measurements of Nd isotope ratios of Fe-Mn oxide encrusted foraminifera from Deep Sea Drilling Project (DSDP) Site 607, located in the present-day core of NADW, new constraints on the composition of the North Atlantic AMOC end-member through time are provided. Moreover, this dissertation reconstructs bottom water masses in the deep North Atlantic, and by inference changes in AMOC, under interglacial and glacial conditions, and across important shifts in the climate system such as the Mid-Pleistocene Transition (MPT; ~1250-650 ka) and the intensification of Northern Hemisphere glaciation (iNHG; ~3.6-2.4 Ma).

Chapter 1 focuses on constraining the Nd isotope composition of NASW and changes in its southward export to Site 607 over the last 1.5 Myr. The results suggest increased incursions of southern-sourced waters (SSW) in the deep North Atlantic between ~960 and 860 ka, suggesting that a major AMOC disruption previously observed in the South Atlantic over the same time interval was a basin-wide event. Excluding this interval of AMOC disruption and a 200-kyr-long prolonged interval of weak AMOC that followed, interglacials over the last 1.5 Myr were characterized by the prevalence of water masses similar to the present-day NADW in the deep Atlantic, indicating consistent interglacial AMOC dynamics and North Atlantic end-member. In contrast, glacials were marked by SSW incursions, which further intensified during and after the MPT, suggesting a fundamental change in glacial AMOC.

Chapter 2 investigates heightened terrigenous signals right before the onset of the MPT-AMOC disruption seen in the 1.5-Myr-long authigenic Nd isotope record from the first chapter, by further increasing the temporal resolution of the record. Results indicate pronounced inputs of old continental material in the North Atlantic across ~1250-950 ka, perhaps through intense glacial erosion of the cratonic shields in the region. These lend support for the regolith hypothesis for the MPT, whereby removal of regolith via ice-sheet erosion exposed high-friction crystalline bedrock that facilitated thicker and stable ice sheets, fundamentally changing their response to orbital forcing.

Chapter 3 provides terrestrial evidence for the erosional events explored in the second chapter, through measurements of major and trace element concentrations, Nd-Sr-Pb isotope ratios, and K-Ar ages on detritus fractions of the Site 607 sediment core. While the detritus deposited at Site 607 during glacials across ~1250-960 ka are more chemically weathered and resemble the average shale that originated from older North Atlantic continental sources such as the late Archean and Proterozoic rocks of the Superior and Churchill Province, and West Greenland, less chemically weathered detritus resembling the average upper continental crust from continental sources such as the Grenville Province and the Appalachians comprise the glacial terrigenous sediments at Site 607 after ~960 ka. These findings further support the regolith hypothesis for the MPT and underline the impact of the Northern Hemisphere erosional events suggested in Chapter 2.

Chapter 4 focuses on reconstructing changes in the AMOC and NASW exports in the North Atlantic during the Late Pliocene-Early Pleistocene, by combining a newly generated Nd isotope data at Site 607 covering ~1.5-2.4 Ma with a published record at the same site covering ~2.4-3.3 Ma during the iNHG. The composite record suggests an overall strong NASW export in the North Atlantic during both interglacials and glacials. However, the Nd isotope composition of NASW inferred from the record indicates that the proportions of deep waters from the Greenland-Iceland-Norwegian seas (GINS) in NASW were greater during the Late Pliocene and the first ~500-600 kyr into the Early Pleistocene, relative to the rest of the Pleistocene and today. A gradual increase of sea ice cover in the GINS during glacials across ~2.5-2 Ma may have eventually led to a perennial ice cover in the region and significantly reduced exports of overflow waters to the North Atlantic. By ~2 Ma, the modern Nd isotope composition of NADW was obtained.

Chapter 5 aims to qualitatively assess the effectiveness of Nd isotope ratios as a paleoceanographic tracer, through a compilation of Nd isotope records from multiple sites located throughout the Atlantic Ocean. The Nd isotope ratios observed at the sites are consistent with NASW-SSW gradients expected from the AMOC structure and reflect even the local water mass structure and bathymetric settings, confirming the quasi-conservative nature of seawater Nd isotope ratios. An additional comparison of published Atlantic Nd isotope records for the Last Glacial Maximum to Holocene also demonstrates that authigenic Nd isotope ratios reflect past changes in water mass mixing in the Atlantic. These observations reaffirm that Nd isotopes are an effective paleoceanographic tool.

Geographic Areas

Files

This item is currently under embargo. It will be available starting 2027-01-15.

More About This Work

Academic Units
Earth and Environmental Sciences
Thesis Advisors
Goldstein, Steven L.
Degree
Ph.D., Columbia University
Published Here
January 15, 2025