Atlantic meridional overturning circulation variability since the last glaciation: Insights from a novel multiproxy approach
Valley, Shannon Gabrielle
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Atlantic Meridional Overturning Circulation (AMOC) transports warm surface water northward across the equator, carrying heat from the Southern to the Northern Hemisphere. AMOC plays a central role in the global redistribution of heat and precipitation during both abrupt and longer-term climate shifts. Over the next century, AMOC is projected to weaken due to greenhouse gas warming, though projecting the variability of its future behavior is dependent on a better understanding of how AMOC changes are forced, including the evolving states of its constituent water masses. To address this, I analyzed several water mass tracer records from Florida Straits intermediate water- a water mass that forms part of AMOC’s upper branch. To investigate AMOC variability during the Younger Dryas (YD) and Heinrich Stadial 1 (HS1)- climatological periods associated with ice sheet melt- I generated a new, high-resolution record of benthic seawater Cd (Cdw) from a Florida Straits sediment core at 546 m water depth. This record provides additional evidence for lower Cdw relative to modern during both the YD and HS1. Lower Cdw values are interpreted as AMOC weakening, reflecting a decreased northward transport of southern-sourced higher nutrient intermediate waters by the surface return flow of AMOC. Comparison of this new Cdw record with previously published neodymium isotope and δ18O records from the same core shows synchronous transitions, further illustrating the connection between Cdw levels and AMOC strength in the Florida Straits. An increase in Cdw near 16 ka bolsters existing evidence for a resumption of upper branch AMOC strength approximately midway through Heinrich Stadial 1. The Cdw record was extended to ∼35,000 yr before present, including all of Marine Isotope Stages (MIS) 2 and part of MIS 3, and temperature and oxygen isotopic composition of seawater were also reconstructed from the same core in order to provide a fuller picture of water mass properties and circulation at this location. During the Last Glacial Maximum (LGM, 19-23 kyr before present), Cdw levels were generally low. A novel Mg/Li-derived temperature record reveals persistently cold glacial temperatures (∼3.6 ℃, two to three degrees colder than during the Holocene or MIS3). My published study is one of the first to make use of this promising new temperature proxy. In contrast to the YD and HS1, there is no indication of AMOC variability over Heinrich Stadials 2 and 3, consistent with previous studies that conclude the AMOC is less sensitive to freshwater forcing during glaciations than during the last deglaciation. While there is some inconsistency between proxies, Cdw increases over some MIS3 Dansgaard-Oeschger interstadials, providing qualified support for a strengthening in AMOC. This study highlights the distinct nature of water masses and circulation during the LGM, relative to the stadial (weak AMOC) periods of the deglaciation and Late MIS3. Seeking to resolve an apparent contradiction of AMOC trends from paleorecords of the more recent past, I applied the Cdw Florida Straits transport characterization to infer upper AMOC change over the last ~1,000 years. In combined core records from intermediate water depth with high resolution over the Common Era, there is little evidence of AMOC reduction over the Little Ice Age relative to that during the YD stadial. Limited data since 1850 CE prohibits comparison of this record to other AMOC proxies in the modern era. However, from the Medieval Warm Period through the Little Ice Age, the newly generated Cdw and Mg/Li-derived temperature records are consistent with other indicators of AMOC variability that were reconstructed from further north in the Atlantic. This agreement supports evidence for a meridionally consistent AMOC on decadal and greater timescales.