Geochemical constraints on the Laurentide Ice Sheet contribution to Meltwater Pulse 1A

Abstract

Planktonic and benthic δ¹⁸O records adjacent to the runoff outlets of the Laurentide Ice Sheet (LIS) indicate that the LIS contributed to the abrupt ∼20 m rise in sea level ∼14.6 ka, Meltwater Pulse 1A (MWP-1A). However, the magnitude of the LIS contribution still remains unresolved. Here, I use a freshwater runoff–ocean mixing model to calculate the LIS meltwater required to explain the decreases in planktonic and benthic δ¹⁸O observed during MWP-1A at the southern, eastern and northern runoff outlets of the LIS. Maximum LIS contributions in equivalent sea level rise for a 500-year long MWP-1A are 2.7 m discharged into the Gulf of Mexico as a combined hyperpycnal and hypopycnal flow, 2.1 m discharged into the North Atlantic, and 0.5 m into the Arctic Ocean, for a total LIS contribution of ≤5.3 m. A LIS contribution of <30% to MWP-1A supports the hypothesis that a significant component of this MWP was sourced from the Antarctic Ice Sheet.

Introduction

Meltwater Pulse 1A (MWP-1A) occurred ∼14.6 ka and is an ∼20 m rise in sea level in <500 years (Fairbanks, 1989, Bard et al., 1990, Edwards et al., 1993; Hanebuth et al., 2000). Due to its large size, the Laurentide Ice Sheet (LIS) was originally assumed to be the sole source of MWP-1A (Fairbanks, 1989, Peltier, 1994). However, LIS margin reconstructions show relatively small margin retreat during MWP-1A (Fig. 1) and steady state ice sheet models suggest that only a fraction of this meltwater pulse was sourced from the LIS (Clark et al., 1996, Licciardi et al., 1998, Licciardi et al., 1999, Dyke, 2004), which implies a large contribution from the Antarctic Ice Sheet (Clark et al., 1996). Sea-level fingerprinting and earth model studies suggest that an Antarctic source could account for ∼75% of the total sea level rise (Clark et al., 2002a, Bassett et al., 2005), although some LIS contribution cannot be excluded (Bassett et al., 2005).

Supporting a significant LIS contribution, one ice sheet modeling study reconstructed 8–10 m of MWP-1A coming from the LIS (Tarasov and Peltier, 2005, Tarasov and Peltier, 2006), but still less than the LIS contribution of ∼16.5 m in the ICE-5G model (Peltier, 2004, Peltier, 2005). This large ICE-5G LIS contribution would cause a 0.36–0.38 Sverdrup (Sv, 10⁶ m³ s⁻¹) increase in LIS meltwater discharge during the course of the event. During MWP-1A, LIS meltwater was routed to the ocean via three outlets, the southern outlet (Mississippi River) to the Gulf of Mexico, the eastern outlet (Hudson and St. Lawrence Rivers) to the North Atlantic, and the northern outlet (Mackenzie River) to the Arctic Ocean (Fig. 1) (Licciardi et al., 1999, Tarasov and Peltier, 2005). Because a 0.36–0.38 Sv freshwater flux would force a reduction in Atlantic meridional overturning circulation (AMOC) strength (Stouffer et al., 2006) and AMOC increased during MWP-1A (Boyle and Keigwin, 1987, Weaver et al., 2003, McManus et al., 2004, Robinson et al., 2005), Tarasov and Peltier, 2005, Tarasov and Peltier, 2006) hypothesized that the meltwater discharged through the southern and eastern outlets (Fig. 1) entered the ocean as a dense, sediment laden, hyperpycnal flow along the ocean floor, which would presumably not affect AMOC. Note, however, that general circulation model simulations suggest that the southern outlet can only accommodate less than 6 m of sea level rise equivalent discharged as a hyperpycnal flow without reducing AMOC strength (Roche et al., 2007).

Reworked nannofossil records suggest increased southern outlet discharge during MWP-1A (Marchitto and Wei, 1995). Similarly, benthic foraminifera records from the Gulf of Mexico show a 0.9–2.3‰ decrease in δ¹⁸O during MWP-1A (Fig. 2e), reflecting the input of ¹⁸O-depleted terrestrial runoff and meltwater, and indicating that some portion of the LIS contribution entered the ocean as a hyperpycnal flow (Aharon, 2006). In contrast, grain size and sedimentation rate records from the eastern outlet indicate decreased sediment discharge, arguing against a hyperpycnal flow through the eastern outlet during MWP-1A (Keigwin and Jones, 1995). Nevertheless, there is a 0.69‰ light planktonic δ¹⁸O anomaly observed adjacent to the eastern outlet at ∼14.5 ka, suggesting increased freshwater discharge or warming during MWP-1A (Fig. 2c) (Keigwin et al., 2005). A 0.75‰ decrease in planktonic δ¹⁸O in the Arctic Ocean has been correlated with MWP-1A and indicates increased discharge through the northern outlet (Poore et al., 1999, Hall and Chan, 2004) (Fig. 2b). In contrast, iceberg discharge from the LIS into the Labrador Sea decreased and planktonic δ¹⁸O increased during MWP-1A implying that the northeastern LIS did not contribute significantly to this MWP (Andrews and Tedesco, 1992, Clark et al., 1996, Hillaire-Marcel and Bilodeau, 2000).

Here, I use a freshwater runoff-ocean mixing model to determine the amount of LIS meltwater and thus the LIS contribution to MWP-1A through the southern, eastern and northern outlets recorded by these light planktonic and benthic δ¹⁸O anomalies (Fig. 1, Fig. 2). Assuming that no temperature adjustments are necessary (see Section 3), these records suggest increased runoff from the LIS during part or all of MWP-1A through the main outlets. This analysis indicates, however, that the contribution from the LIS was only a small fraction (<30%) of the total sea level rise during MWP-1A, suggesting that the LIS was not the primary source of this MWP.