2020 Theses Doctoral

Geochemical studies of marine sediments from the Atlantic Ocean: Implications for past transitions in ocean circulation and dust deposition

Yehudai, Maayan

In recent decades geochemical-paleoclimatic data allowed for the discovery of the persistent past occurrence of abrupt changes and transitions within the climate system. This highlights the need to develop a detailed understanding of the dynamics by which the various components of the climate system interact. Using a range of geochemical tools applied to marine sediments from the Atlantic Ocean, this dissertation explores a number of paleoclimatic transitions, in the context of two major climatic components: ocean circulation and dust supply.

The first chapter focuses on the onset of the shift from ~41-to-100-kyr interglacial-glacial cyclicity between ~1250-700 ka (also termed ‘the mid-Pleistocene transition’ or MPT), which is a subject of intense controversy, as this fundamental change occurred without substantial changes in the astronomical climate forcings (e.g.: Clark et al., 2006) and the timing, locations, and identities of its drivers remain unresolved. Recent studies disagree whether the transition occurred gradually over several interglacial-glacial cycles (Ford et al., 2016) or abruptly at around 900 ka (Elderfield et al., 2012), as well as whether it was driven by events in the Northern (Detlef et al., 2018; Ford et al., 2016; Kender et al., 2018; Pena and Goldstein, 2014; Sosdian and Rosenthal, 2009) or the Southern (Elderfield et al., 2012; Farmer et al., 2019; Hasenfratz et al., 2019; Lear et al., 2016) Hemisphere. In chapter one, we address these questions using a new north-to-south reconstruction, based on sedimentary neodymium isotope ratios, of the Atlantic meridional overturning circulation (AMOC; Broecker, 1991), a primary means for distributing heat around the globe. Our results reveal an abrupt glacial erosional event in the cratonic shields surrounding the North Atlantic and a breakdown of the ice-sheet at ~965-950 ka (Marine Isotope Stage or MIS-26 (Lisiecki and Raymo, 2005)). This event directly preceded a major weakening of the ocean conveyor circulation (Pena and Goldstein, 2014), which our Atlantic transect shows had global impact, and which coincided with the first ~100-kyr-long interglacial-glacial cycle. Moreover, between ~1250-965 ka, leading up to the ‘MIS-26 erosional event’, increasing cratonic weathering contributions to the deep North Atlantic during glacial periods point to increasingly effective continental ice-sheet erosional activity over this critical time-interval. The evidence thus implicates both long-term ice-sheet processes and an abrupt event that occurred just prior to the cyclicity shift. This new global view reveals evidence of a Northern Hemisphere-sourced initiation for the MPT, induced through regolith loss (Clark et al., 2006), leading to more stable ice-sheets facilitated by increased friction. These processes enabled enhanced ice sheet growth and thickening characteristic of the 100-kyr interglacial-glacial cycles.

In the second chapter we further explore the changes in the AMOC, which is widely believed to weaken during glacials and strengthen during interglacials, and expand the temporal scale of Nd-isotopes profiles, presenting data from the North to the Equatorial Atlantic over the past ~1.5 Ma. We have identified five modes of the AMOC. The most common ones are (1) the “interglacial norm”, where the Northern Sourced Water (NSW) signal remains strong into the South Atlantic, and (2) the “glacial norm”, indicating a weaker AMOC, with southern source water (SSW) extending into the deep North Atlantic. Less common are the (3) “weak AMOC” mode, typical of Heinrich events, the AMOC-crisis event (MIS 24-22) during the MPT, and MIS 10 and 16, where even the deep North Atlantic shows a strong SSW signal, and its counterpart the (4) “ultra-strong AMOC”, in MIS 9, 11, 19, 21 and 25, when the NSW signal is unusually strong south of the equator, similar to the present-day. (5) “AMOC-Divergence” (MISs 41, 27, 26 and 14), possibly indicating a decoupling between the northern and southern AMOC cells. The AMOC time-slice profiles provide a useful new framework to relate climate past changes directly to concurrent ocean circulation through time.

The third chapter reports a first attempt to address changes in dust deposition due to the formation of the Sahara Desert, as documented at the nutrient-poor region, of the Great-Bahama-Bank carbonate platform. The dust deposition records produced here span over ~14 million years, thus providing a direct, coarse resolution examination, of the co-evolution of the two systems, the Great-Bahama-Bank and the Saharan Desert dust source, since the mid-Miocene till the late Pleistocene. Here, I utilized a suite of newly applied methods across the hypothesized time of Sahara Desert formation on sediments from two core sites at the Great Bahama Bank. Results from the Great Bahama Bank marine sediments show changes in the accumulation of terrigenous material through time with a possible notable increase occurring around ~11 Ma and ~4 Ma. The geochemical evidences point to at least two possible sources: Northwest Africa (namely, the northern Sahel region) and the northern part of South America (namely, the Orinoco drainage basin), which is more evident in samples younger than 4 Ma. The emergence of the northern-South-American isotopic signature suggests that the closure of the Panama Isthmus may not only have played a crucial role in the evolution of the Great-Bahama-Bank, as suggested in earlier studies (Reijmer et al., 2002), but also led to the transport of weathering products all the way to the northern part of the Gulf of Mexico. Taken together, these first results highlight the potential of the Great Bahama Bank sediments as archives of Saharan dust deposition and global changes.