Meltwater flooding events in the Gulf of Mexico revisited: Implications for rapid climate changes during the last deglaciation

Abstract

North American freshwater runoff records have been used to support the case that climate flickers were caused by shutdowns of the ocean thermohaline circulation (THC) resulting from reversals of meltwater discharges. Inconsistencies in the documentation of these meltwater switches, however, continue to fuel the debate on the cause/s of the oscillatory nature of the deglacial climate. New oxygen and carbon isotope records from the northern Gulf of Mexico depict in exceptional detail the succession of meltwater floods and pauses through the southern routing during the interval 16 to 8.9 ka (14C years BP; ka, kiloannum). The records underscore the bimodal role played by the Gulf of Mexico as a destination of meltwater discharges from the receding Laurentide Ice Sheet. The evidence indicates that the Gulf of Mexico acted as the principal source of superfloods at 13.4, 12.6, and 11.9 ka that reached the North Atlantic and contributed significantly to density stratification, disruption of ocean ventilation, and cold reversals. Gulf of Mexico lapsed into a “relief valve” position in post‐Younger Dryas time, when meltwater discharges were rerouted south at 9.9, 9.7, 9.4, and 9.1 ka, thus temporarily interrupting North Atlantic‐bound freshwater discharges from Lake Agassiz. The history of meltwater events in the Gulf of Mexico contradicts the model that meltwater flow via the eastern outlets into the North Atlantic disrupted the ocean THC, causing cooling, while diversions to the Gulf of Mexico via the Mississippi River enhanced THC and warming.

Climate change in lowland Central America during the late deglacial and early Holocene

“…The oldest event (PBE-1) coincides with the Preboreal Oscillation (Björck et al, 1997;van der Plicht et al, 2004). The three younger events appear to correspond with episodic diversions of glacial meltwater to the Gulf of Mexico (Table 4; Aharon, 2003) that were related to changes in glacial boundary conditions (Clark et al, 2001). A possible explanation for this phenomenon comes from modeling studies that impose a 6ºC cooling of the Gulf of Mexico related to the delivery of cold, fresh glacial waters to the basin (Overpeck et al, 1989;Oglesby et al, 1989;Maasch and Oglesby, 1990).…”

“…A recent study, however, suggests that episodic meltwater floods to the Gulf of Mexico continued after the Younger-Dryas period. Aharon (2003) identified four events during the Preboreal, when meltwater was rerouted south. These are centered at 9,900, 9,700, 9,400, and 9,100 14 C yr BP.…”

The Holocene history of the North American Monsoon: ‘known knowns’ and ‘known unknowns’ in understanding its spatial and temporal complexity

“…In the late glacial and early Holocene, the presence of the Laurentide Ice sheet (LIS) and the impact of pulses of glacial meltwater entering the Gulf of Mexico (and its wider effect on the Atlantic Table 2 and Figures 5 and 6 reflect the generally cooler SSTs in the GoM in the latest glacial and in some cases into the early Holocene, Aharon (2003) reports a series of meltwater pulses (termed MWF's) entering the GoM via the Mississippi between 16,000 and 9000 cal yr BP. As the GoM is one of the major source regions of monsoonal moisture, the impact of this must be considered.…”

Oxygen isotope geochemistry of Laurentide ice-sheet meltwater across Termination I

“…Oxygen isotope analyses of marine sedimentary records proximal to LIS meltwater outflow offer the most direct evidence of the isotopic evolution of the LIS, and help to constrain the location and timing of routing of meltwater (e.g., Aharon, 2003Aharon, , 2006Broecker et al, 1989;Carlson, 2009;Carlson et al, 2007;Flower et al, 2004;Hill et al, 2006;Hoffman et al, 2012;Keigwin et al, 1991;Williams et al, 2012). However, mixing models of meltwater contributions to the ocean typically assume a constant d 18 O w value for the LIS (À25 or À35‰; Aharon, 2006;Carlson, 2009;Carlson et al, 2007;Hill et al, 2006;Obbink et al, 2010), and do not attempt to incorporate the complex spatial heterogeneity in d 18 O w observed in modern continental ice sheets (Masson-Delmotte et al, 2008;Vinther et al, 2009).…”