An early Pleistocene Mg/Ca-δ18O record from the Gulf of Mexico: Evaluating ice sheet size and pacing in the 41-kyr world

PUBLICATIONS Paleoceanography RESEARCH ARTICLE 10.1002/2016PA002956 Key Points: • Six ice sheet meltwater events identiﬁed in Gulf of Mexico seawater δ O record from 2.55-1.70 Ma • Events are typically long, occur late in benthic δ O deglaciations, and line up with summer insolation • This challenges view of early Pleistocene marine δ O as simply recording obliquity-driven Northern Hemisphere ice volume Supporting Information: • Supporting Information S1 • Supporting Information S2 Correspondence to: J. D. Shakun, jeremy.shakun@bc.edu Citation: Shakun, J. D., M. E. Raymo, and D. W. Lea (2016), An early Pleistocene Mg/Ca-δ O record from the Gulf of Mexico: Evaluating ice sheet size and pacing in the 41-kyr world, Paleoceanography, 31, 1011–1027, doi:10.1002/2016PA002956. Received 25 MAR 2016 Accepted 5 JUL 2016 Accepted article online 11 JUL 2016 Published online 25 JUL 2016 An early Pleistocene Mg/Ca-δ 18 O record from the Gulf of Mexico: Evaluating ice sheet size and pacing in the 41-kyr world Jeremy D. Shakun 1 , Maureen E. Raymo 2 , and David W. Lea 3 Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA, USA, 2 Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA, 3 Department of Earth Science, University of California, Santa Barbara, CA, USA Early Pleistocene glacial cycles in marine δ 18 O exhibit strong obliquity pacing, but there is a perplexing lack of precession variability despite its important inﬂuence on summer insolation intensity – the presumed forcing of ice sheet growth and decay according to the Milankovitch hypothesis. This puzzle has been explained in two ways: Northern Hemisphere ice sheets instead respond to insolation integrated over the summer, which is mostly controlled by obliquity, or anti-phased precession-driven variability in ice volume between the hemispheres cancels out in global δ 18 O, leaving the in-phase obliquity signal to dominate. We evaluated these ideas by reconstructing Laurentide Ice Sheet (LIS) meltwater discharge to the Gulf of Mexico from 2.55-1.70 Ma using foraminiferal Mg/Ca and δ 18 O. Our δ 18 O sw record displays six prominent anomalies, which likely reﬂect meltwater pulses, and they have several remarkable characteristics: (1) their presence suggests that the LIS expanded into the mid-latitudes numerous times; (2) they tend to occur or extend into interglacials in benthic δ 18 O; (3) they generally correlate with summer insolation intensity better than integrated insolation forcing; and (4) they are perhaps smaller in amplitude but longer in duration than their late Pleistocene counterparts, suggesting comparable total meltwater ﬂuxes. Overall, these observations suggest that the LIS was large, sensitive to precession, and decoupled from marine δ 18 O numerous times during the early Pleistocene – observations difﬁcult to reconcile with a straightforward interpretation of the early Pleistocene marine δ 18 O record as a proxy for Northern Hemisphere ice sheet size driven by obliquity forcing at high latitudes. Abstract 1. Introduction The Milankovitch hypothesis holds that ice sheets are sensitive to the intensity of summer insolation, which depends on both the tilt of the earth – which varies with the 41-kyr obliquity cycle – and the seasonal distance to the sun – which varies with the 23-kyr precession cycle. The Milankovitch model has had consid- erable success in explaining late Pleistocene ice volume variations of the past one million years, cycles that are concentrated at eccentricity, precession and obliquity frequencies as well as their multiples [Huybers, 2011; Imbrie and Imbrie, 1980; Raymo, 1997]. Nonetheless, the marine δ 18 O record suggests that the immedi- ately preceding glacial cycles of the late Pliocene-early Pleistocene (3–1 Ma) occurred at the almost purely 41-kyr pacing (Figure 1b) [Huybers, 2007; Pisias and Moore, 1981; Raymo and Nisancioglu, 2003; Ruddiman et al., 1989]. This 41-kyr world is difﬁcult to reconcile with the Milankovitch hypothesis – why is precession variabil- ity absent in the early Pleistocene if summer insolation intensity controls ice sheet mass balance? ©2016. American Geophysical Union. All Rights Reserved. SHAKUN ET AL. Two hypotheses have been suggested to rectify the apparent conﬂict between the ice volume changes pre- dicted by Milankovitch forcing and those actually observed in the marine δ 18 O record. The Integrated Insolation hypothesis points out that the most intense summers are also the shortest, since the Earth orbits faster when closer to the sun [Huybers, 2006]. Since these competing precession-driven effects, intensity ver- sus duration, nearly cancel out when integrated over the course of the summer, one might not expect to see a strong precession signal in ice volume variability. The Antiphase hypothesis instead argues that ice sheets are driven by both obliquity and precession (as expressed in summer insolation intensity), but while obliquity is in phase between the hemispheres (i.e., increased axial tilt causes stronger summers in both hemispheres), precession forcing is anti-phased (i.e., when one hemisphere’s summer occurs closest to the sun, the other’s summer occurs farthest from the sun six months later) [Raymo et al., 2006]. Therefore, if a record of global ice volume, such as marine δ 18 O or sea level, was recording ice volume changes in both hemispheres, it would EARLY PLEISTOCENE MELTWATER EVENTS