An Atlantic Warm Pool Trigger for the Younger Dryas Climate Event
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N. A. Abdul1; R. A. Mortlock1; J. D. Wright1; R. G. Fairbanks2, 1 ; L. Teneva31. Earth and Planetary Sciences, Rutgers University, Piscataway, NJ; 2. Columbia University, Palisades, NY; 3. Stanford University, Stanford CA.
The Younger Dryas (YD) (12,900 to 11,600 calendar years before present) is the “poster child” example for rapid climate change. Well documented in the scientific literature as a rapid return to glacial-like conditions concentrated in the high latitudes of the Northern Hemisphere and Europe (Fig. 1), the triggers for the YD cooling range from comet impacts to a catastrophic melt-water discharge that grossly hindered deep water formation and consequently the pole-ward transfer of heat and moisture from the equator.
There is growing evidence that variability in the size and heat content of the equatorial Atlantic Warm Pool (AWP), defined as a body of water that develops during the boreal summer with sea surface temperatures (SSTs) of in excess of 28.5 ⁰C, impacts circum-North Atlantic climate via the Atlantic Multi-decadal Oscillation Mode (Wang et al., 2008). The AWP spans the Gulf of Mexico, Caribbean Sea and the western tropical North Atlantic and is linked tropospherically to the eastern North Pacific Ocean (Fig. 2). Coupled ocean models indicate that Barbados remains in the tropical AWP under varying wind stress simulations (Fig. 3). Hence, Barbados is well positioned to reconstruct AWP SST variations during the YD.
For the LGM and YD, it is hypothesized that North Atlantic sea ice expanded and coupled with stronger cooler trade winds, depressed the ITCZ southward deep into the Southern Hemisphere (Fig. 4). Based on the modern seasonal distribution of SSTs and their relationship to the position of the ITCZ it follows that prior to the YD successive small AWPs (relatively cooler tropical SSTs) may have persisted to promote a sustained AMO mode that later tipped the climate state into the YD stadial.
We measured δ18O values in samples of A. palmata to reconstruct SSTs in the Equatorial North Atlantic. Samples were taken from the Barbados offshore collection and span the time interval from 14.0 to 10 kyrBP. This interval records a sea level rise from 71.4 to 61.1 meters below present and includes the YD (Fig. 5). Our paleo-SST record is then used to evaluate the role of AWP temperature variability and its role as a driver of high latitude climate change.
Bathymetric map of the south coast of Barbados showing the drill sites. All drilling was conducted on submerged reef ridges. Red arrows show locations drilled in 1988 while blue arrows represent those drilled in 2007.
Fig 1: (Right)
The Younger Dryas Chronozone, as identified in the NGRIP ice core records indicates a shift towards colder temperatures at 12.9 kyrBP.
Fig 2: (Left)
Atlantic Warm Pool SSTs define small and large modes (Wang et al., 2011). The blue star represents the study area
AWP formation begins in the in the Pacific Ocean (March). By June, heat and moisture from the Pacific via the troposphere, across the narrow Isthmus of Panama into the Gulf of Mexico, the Caribbean Sea, and eventually extends into the Atlantic Ocean(Wang and Enfield, 2002).
(a) Small AWP SST
(Wang et al., 2011)
Fig. 9: (Above)
AWP SST estimates from Barbados show a rapid 3⁰C cooling that begins ~300 years before the classically defined YD Chronozone. Cooler SSTs persists throughout the YD after which gradual warming to near modern temperatures occurs. Temperature error includes combined uncertainty of δ18Omeasured, ice volume correction, and the uncertainty in the temperature equation.
(a) Large AWP SST
Fig. 10: (Below)
Fig. 3: (Right)
(A) Inter-annual variability of SST anomalies show an extensive warm pool focused in the Atlantic Ocean and centered north of the equator. There is a very positive correlation between the warming in the tropical Atlantic and the warming seen in the Sub-Polar North Atlantic on an interannual basis.
(B) Multidecadal SSTs shows that on longer time scales the warm pool’s influence moderates whole ocean SSTs including the North Atlantic. Warm pool heat is transferred around the ocean via the major ocean current systems leaving areas outside of these pathways much cooler.
Regression of Global SSTA on AWP Index
(Wang et al., 2008)
Changes in the δ18Owater of surface water at Barbados may have contributed to the observed δ18Ocoral recorded by fossil A. palmata. Today, variability in δ18Owater are primarily driven by seasonal changes in Amazon River discharge (Fig. 10 left). Seasonal equilibrium aragonite based on temperature and δ18Owatervary by 0.8‰ (right). The observed increase in δ18O of A. palmata prior to the onset of the YD is 1.2 ‰ and would require a significant change in one or both of the seawater and Amazon end-members. Thus, we conclude that the increase in δ18Oof A. palmata is largely driven by a decrease in SSTs.
Fig. 7: (Above)
A. palmata δ18O values displaya monotonic increase from -2.94 to -1.88 ‰ (change in δ18O = ~1.1‰) between 13.2 and 12.7 kyr BP as sea level rose by 6 m. Half of the δ18O increase predates the onset of the YD by 300 years as defined by the Greenland Ice Core Chronology (12.8 kyrBP). 1 sigma error bars are plotted on δ18O values.
Fig. 4: (Left)
Seasonal variations in the mean position of the Inter-Tropical Convergence Zone (ITCZ) over the Caribbean region and South America, illustrated for typical boreal summer (top) and winter (bottom) conditions. These variations control the pattern and timing of regional rainfall. Numbers and colors reflect SSTs in ⁰C. Reference points Barbados and the Cariaco Basin are labeled. Figure and legend modified from Haug et al., 2003.
Fig. 5: (Right)
Sea level reconstruction for the period 14000 to 11000 years before present(Abdul et al., 2010). Record is constructed solely of the reef crest coral species Acropora palmata which is ecologically restricted to a maximum depth of 5 m (Lighty et al., 1982). Depth uncertainty is + 6 m (+5 m habitat range and ± 1 m for drill depth correction).
(Left) 1) Ice Volume Correction
We removed the “whole ocean” ice volume effect by applying the following correction (Fairbanks and Mathews 1978):
0.011‰ change in δ18O per 1 m change in SL
(Right) 2) Conversion of δ18O coralline aragonite to Temperature
We applied the paleo-temperature equation (based on cultured A. palmata by Reynaud et al., 2007)
δ18O (‰) = - 0.34‰ * T (⁰C) + 6.35
Note: the temperature sensitivity from the cultured A. palmata differs from that typically prescribed for tropical coral (-0.22 ‰/⁰C)
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Captain and Crew RV Knorr 189-2 ; L. Cao (Pacific Northwest National Labs), J. Mey (Lamont-Doherty Earth Observatory); National Science Foundation; School of Arts and Sciences, Rutgers University, New Brunswick NJ.; A. Cotet (Rutgers University)