Atlantic Meridional Overturning Circulation Variability Since the Last Glaciation: Insights from a Novel Multiproxy Approach

By: 
Shannon Valley
Time: 
Wednesday, October 9, 2019 - 1:00pm
Place: 
Ford ES&T, Rm. L1114
Committee: 
Dr. Jean Lynch-Stieglitz (Advisor), Dr. Kim Cobb, Dr. Taka Ito, Dr. Annalisa Bracco, and Dr. Tom Marchitto
Summary: 

The 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 longerterm climate shifts. Over the next century, AMOC is projected to weaken due to greenhouse gas warming, though the skill of these projections 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 values relative to modern during both the YD and HS1. Lower Cdw values are interpreted as AMOC weakening, reflecting a decreased northward transport of southernsourced 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. An increase in Cdw near 16 ka bolsters existing evidence for strengthening in the upper branch of AMOC midway through Heinrich Stadial 1.

A novel Mg/Li-derived temperature record reveals persistently cold glacial temperatures (∼3.6℃). This 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. This study highlights the distinct nature of water masses and circulation during the Last Glacial Maximum, relative to the stadial periods of the deglaciation and Late MIS 3.

Seeking to resolve 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. From the Medieval Warm Period through the Little Ice Age, the newly generated Cdw and Mg/Li-derived temperature records are consistent with other records 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.