Assessing changes in temperature, ice volume and ice sheet dynamics across the Middle Pleistocene Transition

Lead Research Organisation: University of Cambridge
Department Name: Earth Sciences


Earth's climate for the last 2.5 million years has been characterized by the waxing and waning of large continental ice sheets in the Northern Hemisphere. Prior to about a million years ago, the glacial and interglacial cycles marched to the beat of the tilt of Earth's rotational axis (i.e., obliquity), which varied with a cycle of 41,000 years. About a million years ago, the length of glacial-interglacial cycles became longer, averaging about 100,000 years. This change, known as the Middle Pleistocene Transition (MPT), represents an important unsolved problem in paleoclimatology. Why did the frequency of glacial-interglacial cycles suddenly shift in the middle Pleistocene from 41- to 100-kyrs? The size of the ice sheets became larger across the MPT, thereby allowing individual domes of the North American ice sheet to coalesce to form a massive ice sheet. The great volume and thickness of this ice made it less sensitive to changes in boreal summer insolation, thereby lengthening the glacial cycle as the ice sheets survived through insolation maxima and deglaciation began skipping precessional (~21 kyr) and/or obliquity (~41 kyr) cycles. One of the hindrances to furthering our understanding of the MPT is the lack of a quantitative estimate of the change in ice volume that occurred across the MPT. This shortcoming results largely because the benthic oxygen isotope record reflects both temperature and ice volume changes. Using Mg/Ca of infaunal benthic species, we aim to deconvolve the benthic d18O signal and provide a quantitative estimate of changes in the d18O of seawater that is related to global ice volume. As the North American Ice Sheet increased in volume and thickness following the MPT, the dynamical behavior of the ice sheet may have changed. During the last glacial period, Laurentide Ice Sheet dynamics was dramatically expressed by so-called Heinrich layers, which consist of sediment rich in ice rafted debris (IRD) deposited in the North Atlantic stretching from Labrador to Portugal. Heinrich layers represent massive discharges of icebergs from surging of the Laurentide Ice Sheet in the region of Hudson Strait in northern Canada. Although Heinrich events have been studied for the last glacial cycle, little is known about their occurrence in older glaciations of the Pleistocene. At Site U1308 in the central North Atlantic, we found that the first Heinrich layer occurred at 650 ka coinciding with the onset of the 100-kyr cycle. It is not known, however, whether this represented the initiation of surging of the Laurentide Ice Sheet via the Hudson Strait Ice Stream or was it the first time icebergs produced by this process survived the transport to Site U1308. The former interpretation implies a fundamental change in Laurentide ice sheet dynamics before and after the MPT, whereas the latter interpretation indicates a change in atmospheric and/oror surface water conditions that changed the location of iceberg melting. Were Heinrich events limited to the 100-kyr world or did they also occur prior to the MPT? We will address this question by examining Site U1304 located to the north and west of Site U1308. Site U1304 should record Heinrich layers if they were produced prior to the MPT because the IRD belt was probably displaced to higher latitude when ice sheets were smaller and North Atlantic SST was warmer than late Pleistocene glaciations.


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Description Earth's climate underwent a fundamental change between 1250 and 700 thousand years ago, the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large
effects of changes in deep-ocean temperature. We sought to address two questions regarding the MPT:

1.) How much did ice volume increase and/or deep water cool in glacial periods across the Middle Pleistocene transition?

We estimated changes in ice volume and temperature across the MPT by paired measurements of Mg/Ca and d18O of infaunal benthic foraminifera. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum.

2.) Did ice sheet dynamics change across the MPT? We will examine if an increase in ice volume across the MPT resulted in a fundamental change in the dynamics of the Laurentide Ice Sheet by examining ice rafted detritus (IRD) in North Atlantic sediment cores.

We studied Sites U1308 (49° 52.7'N, 24° 14.3'W; 3871 m) and U1304 (53° 3.4'N, 33° 31.8'W; 3024 m) in the North Atlantic to determine how ice sheet dynamics and millennial-scale climate variability evolved as glacial boundary conditions changed across the MPT.
We found the frequency of ice-rafted detritus (IRD) in the North Atlantic was greater during glacial stages prior to 650 ka (MIS 16), reflecting more frequent crossing of an ice volume threshold when the climate system spent more time in the "intermediate ice volume" window, resulting in persistent millennial scale variability. The rarity of Heinrich Events containing detrital carbonate and more frequent occurrence of IRD events prior to 650 ka may indicate the presence of "low-slung, slippery ice sheets" that flowed more readily than their post-MPT counterparts. Ice volume surpassed a critical threshold across the MPT that permitted ice sheets to survive boreal summer insolation maxima, thereby increasing ice volume and thickness, lengthening glacial cycles, and activating the dynamical processes responsible for Laurentide Ice Sheet instability in the region of Hudson Strait (i.e., Heinrich events). The excess ice volume during post-MPT glacial maxima provided a large, unstable reservoir of freshwater to be released to the North Atlantic during glacial terminations with the potential to perturb Atlantic Meridional Overtunring Circulation. We suggest that orbital- and millennial-scale variability co-evolved across the MPT and the interaction of processes on orbital and suborbital time scales gave rise to the changing patterns of glacial-interglacial cycles through the Quaternary.
Exploitation Route Our results have resulted in a fundamental change in the manner in which scientists view temperature and ice volume changes during the Pleistocene.
Sectors Environment

Description The deep-ocean sediment oxygen isotopic record is commonly considered to be a record of global ice volume and thus a chronicle of ice ages, but it is also affected by deep-water temperature variability. The contributions of ice volume and temperature to the oxygen isotope record have been debated for more than 40 years, and definition of the trajectories that eachfollowed during the mid-Pleistocene transition (MPT) has been elusive. Our results have deconvolved this signal over the MPT which has changed the way paleoclimatogists view the transition.
First Year Of Impact 2012
Sector Environment