Synchronizing a sea-level jump, final Lake Agassiz drainage, and abrupt cooling 8200 years ago

doi:10.1016/j.epsl.2011.05.034

Earth and Planetary Science Letters

Volumes 315–316, 15 January 2012, Pages 41-50

https://doi.org/10.1016/j.epsl.2011.05.034 Get rights and content

Abstract

Freshwater pulses draining into the North Atlantic Ocean are commonly hypothesized to have perturbed the Atlantic meridional overturning circulation (MOC), triggering abrupt climate changes such as Heinrich events, the Younger Dryas, and the 8.2 ka event. However, dating uncertainties have prevented causal links between freshwater pulses and climate events from being firmly established. Here we report a high-resolution relative sea-level record from the Mississippi Delta that documents a sea-level jump that occurred within the 8.18 to 8.31 ka (2σ) time window and is attributed to the final drainage of proglacial Lake Agassiz–Ojibway (LAO). This age is indistinguishable from the onset of the 8.2 ka climate event, consistent with a nearly immediate ocean–atmosphere response to the freshwater perturbation. This constitutes a rare currently available example of a major abrupt climate cooling that can be directly linked to a well-documented freshwater source with a temporal resolution on the order of a century. The total inferred eustatic sea-level rise associated with the very final stage of LAO drainage at 8.2 ka ranges from 0.8 to 2.2 m, considerably higher than previous estimates. These new constraints on the timing and amount of final LAO drainage permit significantly improved quantitative analysis of the sensitivity of MOC to freshwater perturbation, a crucial step toward understanding abrupt climate change.

Research highlights

► A sea-level jump of 0.8 to 2.2 m occurred within the 8.18–8.31 ka time window. ► This sea-level jump is coeval with the onset of the 8.2 ka abrupt climate event. ► The sea-level jump most likely resulted from the final drainage of Lake Agassiz. ► The tightest control on the timing and magnitude of final drainage of Lake Agassiz. ► The only major abrupt climate event with tightly linked cause and climate response.

Introduction

Abrupt climate change has received extensive interest for a wide range of reasons, including its potential role in a future warming world (Alley et al., 2003). Over the past few decades, the connection between freshwater forcing and abrupt climate change due to perturbation of the Atlantic meridional overturning circulation (MOC) has enjoyed widespread popularity, since it offers a potential mechanism to explain phenomena such as Heinrich events (Heinrich, 1988), the Younger Dryas (Broecker et al., 1989), and the 8.2 ka event (Barber et al., 1999). However, the past few years have seen this hypothesis becoming increasingly challenged (e.g., Broecker et al., 2010, Fisher et al., 2008, Lowell et al., 2009), in part reflecting the fact that very few abrupt climate events have been unequivocally linked to a well-mapped and well-dated freshwater source (cf. Clement and Peterson, 2008).

The 8.2 ka cold event is the most prominent abrupt North Atlantic climate change of the Holocene and is increasingly recognized in many other parts of the world (Alley and Ágústsdóttir, 2005, Cheng et al., 2009). This event is often believed to have resulted from the final outburst of proglacial Lake Agassiz–Ojibway (LAO) when an ice dam over Hudson Bay collapsed (Barber et al., 1999, Lajeunesse and St-Onge, 2008) and the rapid drainage flooded the North Atlantic Ocean with freshwater and perturbed the Atlantic MOC (Ellison et al., 2006, Kleiven et al., 2008), leading to widespread cooling. In addition, the rerouting of western Canadian Plains runoff following the collapse of the ice dam over Hudson Bay may have contributed to the 8.2 ka climate event (Carlson et al., 2009). Despite the popularity of a causal link between the final LAO drainage and the 8.2 ka climate event, this relationship has yet to be firmly demonstrated because the catastrophic LAO drainage remains poorly constrained in terms of its timing and amount. The only available direct dating of the final LAO drainage yields an age range of 8.16 to 8.74 ka at the 1σ level (Barber et al., 1999). This large age uncertainty precludes an unequivocal connection between LAO drainage and the 8.2 ka event and allows for alternative hypotheses such as a role for solar forcing around this time interval (Muscheler et al., 2004, Rohling and Pälike, 2005). Also, the amount of LAO drainage is not well known as reflected by highly variable estimates (e.g., Barber et al., 1999, Hijma and Cohen, 2010, Leverington et al., 2002, Törnqvist et al., 2004a), inhibiting our understanding of the sensitivity of MOC to freshwater perturbation.

This study seeks to refine previous work (Törnqvist et al., 2004a) that provided the first evidence for a sea-level jump around 8.2 ka based on stratigraphic data from the Mississippi Delta, Louisiana, USA. We present a high-resolution relative sea-level (RSL) record around this time interval using basal peat to track sea-level change. The rationale of this approach is that rising seas drown the coastal landscape and transform it into a peat-forming wetland that accumulates over a consolidated, compaction-free Pleistocene basement. Therefore, intertidal basal peats can be used to determine past sea levels with high accuracy via precise measurements of their age and elevation. The robustness of this approach has been demonstrated in a variety of coastal settings (e.g., Donnelly et al., 2004, Jelgersma, 1961).

Section snippets

Study area

Coastal plains worldwide (e.g., the US Atlantic Coast) rarely capture the age/depth range necessary to sample early Holocene sea-level records that are more likely found in large, prograding deltas. However, not all deltas contain basal peat and even fewer also occur in microtidal settings which are particularly favorable for high-resolution sea-level studies. Our sampling sites are located in the Bayou Sale area in the western part of the Mississippi Delta (Fig. 1). The US Gulf Coast is

Methods

We collected cores with a Geoprobe system (model 6610 DT). The early stage of coring aimed at mapping the stratigraphy along a ~ 6-km-long transect (Fig. 1), exhibiting a transgressive surface associated with the Pleistocene–Holocene transition. Subsequent efforts were focused on coring at key locations for detailed sampling to improve the precision of depth measurements of this transgressive surface.

Cores were initially described in the field and then transported to Tulane University for cold

Stratigraphy

We drilled 37 sites along the ~ 6-km-long transect to map the stratigraphy in the Bayou Sale area (Fig. 1); key stratigraphic information for all core sites is summarized in Table 1. Multiple cores that capture the Pleistocene–Holocene transition were drilled at the majority of the sites.

The transgressive succession at the stratigraphically deeper sites (V, 32, VII, and VI) is characterized by a basal-peat bed that caps the dark gray paleosol described above and is abruptly overlain by

Identifying a sea-level jump

While the entire data set (Table 1) exhibits evidence of transgression and RSL rise, only one portion of the record (including sites 25 and 29; Fig. 2, Fig. 3) features open-water lagoonal muds that conformably onlap the paleosol with no basal peat. Collectively, sites VI, 25, 29, and IV record an abrupt flooding event that is unlike anything seen elsewhere in our record (Table 1). The sharp transition from basal peat to lagoonal mud at the deeper elevation of site VI marks the onset of this

Conclusions

We present a high-resolution early Holocene sea-level record from the Mississippi Delta that documents a distinct sea-level jump, marked by a characteristic stratigraphic succession that is corroborated by paleoenvironmental reconstruction. The 0.20–0.56 m local sea-level jump occurred within the 8.18 to 8.31 ka (2σ) time window and is attributed to the final drainage of proglacial Lake Agassiz–Ojibway (LAO). Since the timing of the sea-level jump is indistinguishable from the onset of the 8.2 ka

Acknowledgements

Mike Blum (Louisiana State University) kindly made his Geoprobe drilling system available for this study. Zhixiong Shen, Shiyong Yu, Juan González, and Floyd DeMers are thanked for field assistance and land owners Debi Lauret and Antoine Luke for providing access to their property. We are grateful to John Southon and his staff for radiocarbon dating, to Brad Rosenheim for help with the stable carbon isotope analysis, and to Hans Renssen, Marc Hijma, Sergio Fagherazzi, Irv Mendelssohn, and

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¹: Present address: School of Earth Sciences, Stanford University, Stanford, CA 94305, USA.

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Synchronizing a sea-level jump, final Lake Agassiz drainage, and abrupt cooling 8200 years ago

Abstract

Research highlights

Introduction

Section snippets

Study area

Methods

Stratigraphy

Identifying a sea-level jump

Conclusions

Acknowledgements

Quat. Sci. Rev.

Quat. Sci. Rev.

Quat. Sci. Rev.

Quat. Sci. Rev.

Quat. Sci. Rev.

Quat. Res.

Quat. Sci. Rev.

Quat. Res.

Quat. Sci. Rev.

Quat. Sci. Rev.

Estuarine Coastal Shelf Sci.

Quat. Sci. Rev.

Quat. Sci. Rev.

Abrupt climate change

Science

Quaternary geology of the Lower Mississippi Valley

Forcing of the cold event 8200 yr ago by catastrophic drainage of laurentide lakes

Nature

Routing of meltwater from Laurentide Ice Sheet during the Younger Dryas cold episode

Nature

Radiocarbon calibration and analysis of stratigraphy: the OxCal program

Radiocarbon

Geochemical proxies of North American freshwater routing during the Younger Dryas cold event

Proc. Natl. Acad. Sci.

Routing of western Canadian Plains runoff during the 8.2 ka cold event

Geophys. Res. Lett.

Timing and structure of the 8.2 kyr event inferred from 18O records of stalagmites from China, Oman, and Brazil

Geology

An inventory of 13C abundances in coastal wetlands of Louisiana, USA: vegetation and sediments

Oecologia

Freshwater discharge, sediment transport, and modeled climate impacts of the final drainage of glacial lake Agassiz

J. Clim.

Mechanisms of abrupt climate change in the last glacial period

Rev. Geophys.

Oxygen isotope precipitation anomaly in the North Atlantic region during the 8.2 ka event

Geology

Timing and structure of the 8.2 kyr event inferred from ¹⁸O records of stalagmites from China, Oman, and Brazil

An inventory of ¹³C abundances in coastal wetlands of Louisiana, USA: vegetation and sediments