Global climate evolution during the last deglaciation

Deciphering the evolution of global climate from the end of the Last Glacial Maximum approximately 19 ka to the early Holocene 11 ka presents an outstanding opportunity for understanding the transient response of Earth’s climate system to external and internal forcings. During this interval of global warming, the decay of ice sheets caused global mean sea level to rise by approximately 80 m; terrestrial and marine ecosystems experienced large disturbances and range shifts; perturbations to the carbon cycle resulted in a net release of the greenhouse gases CO_2 and CH_4 to the atmosphere; and changes in atmosphere and ocean circulation affected the global distribution and fluxes of water and heat. Here we summarize a major effort by the paleoclimate research community to characterize these changes through the development of well-dated, high-resolution records of the deep and intermediate ocean as well as surface climate. Our synthesis indicates that the superposition of two modes explains much of the variability in regional and global climate during the last deglaciation, with a strong association between the first mode and variations in greenhouse gases, and between the second mode and variations in the Atlantic meridional overturning circulation

Sedimentological Observations from the Tiskilwa Till, Illinois, and Sky Pilot Till, Manitoba

We present sedimentological observations from the Tiskilwa Till in northern Illinois, and the Sky Pilot Till in northern Manitoba, that indicate deposition of these tills by subglacial deformation. These generally homogenous tills grade downward into more heterogeneous tills that incorporate underlying sediment into their matrix, indicating entrainment of older sediments by sediment deformation. Deformed sand inclusions within these tills imply deformation of the tills and inclusions prior to deposition. The Tiskilwa Till has relatively high fabric strength throughout its thickness, whereas fabric strength in the Sky Pilot Till generally increases up-section in 2 to 3 m thick increments. Fabric orientations in both tills rotate up-section, possibly due to changes in ice-flow direction associated with the thickening and thinning of ice, and changes in ice-flow divide location. In both the Tiskilwa and Sky Pilot Tills, the change in fabric orientation occurs over intervals of ~1 m, suggesting that the maximum depth of deformation was ≤1 m insofar as any greater depth of deformation would have reoriented till fabric during maximum ice extent and retreat. In the case of the Sky Pilot Till, the up-section increase in macrofabric strength indicates that strain increased up-section. These data suggest that these tills were deposited in a time transgressive manner as strain migrated upwards with the delivery of new till either released from the ice base or advected from up-ice.Les observations sédimentologiques des tills de Tiskilwa, Illinois, et de Sky Pilot, Manitoba, indiquent que ces tills sont issus d’une déformation sous-glaciaire. Ces tills, généralement homogènes, deviennent hétérogènes vers leur base et ils incorporent du matériel sous-jacent dans leur matrice, ce qui indique un déplacement des sédiments plus âgés par déformation. La présence d’inclusions de sable dans ces tills impliquent leur déformation avant leur dépôt. Le till de Tiskilwa présente une matrice très cohérente sur toute son épaisseur tandis que celle du till de Sky Pilot augmente vers le haut tous les 2 ou 3 mètres. La rotation de l’orientation des matrices de ces deux tills est probablement associée aux changements de l’écoulement glaciaire liés à l’épaisseur de la glace et à la migration de la ligne de partage des marges glaciaires. Pour ces tills, le changement d’orientation du matériel se produit sur des intervalles d’environ 1 m, où la profondeur maximale de déformation devrait réorienter le matériel du till durant le maximum glaciaire et le retrait des glaces. Dans le cas du till de Sky Pilot, la section supérieure montre une augmentation dans la force de cohésion du matériel. Ces données indiquent que ces tills se sont déposés de manière diachronique, où la force de tension a migré vers le haut, entraînant le dépôt de matériel basal frais à partir de la base de la glace ou par advection depuis la glace

Evolution of a Coupled Marine Ice Sheet–Sea Level Model

We investigate the stability of marine ice sheets by coupling a gravitationally self-consistent sea level model valid for a self-gravitating, viscoelastically deforming Earth to a 1-D marine ice sheet-shelf model. The evolution of the coupled model is explored for a suite of simulations in which we vary the bed slope and the forcing that initiates retreat. We find that the sea level fall at the grounding line associated with a retreating ice sheet acts to slow the retreat; in simulations with shallow reversed bed slopes and/or small external forcing, the drop in sea level can be sufficient to halt the retreat. The rate of sea level change at the grounding line has an elastic component due to ongoing changes in ice sheet geometry, and a viscous component due to past ice and ocean load changes. When the ice sheet model is forced from steady state, on short timescales (<∼500 years), viscous effects may be ignored and grounding-line migration at a given time will depend on the local bedrock topography and on contemporaneous sea level changes driven by ongoing ice sheet mass flux. On longer timescales, an accurate assessment of the present stability of a marine ice sheet requires knowledge of its past evolution.Earth and Planetary Science

Assessing population exposure to coastal flooding due to sea level rise

The exposure of populations to sea-level rise (SLR) is a leading indicator assessing the impact of future climate change on coastal regions. SLR exposes coastal populations to a spectrum of impacts with broad spatial and temporal heterogeneity, but exposure assessments often narrowly define the spatial zone of flooding. Here we show how choice of zone results in differential exposure estimates across space and time. Further, we apply a spatio-temporal flood-modeling approach that integrates across these spatial zones to assess the annual probability of population exposure. We apply our model to the coastal United States to demonstrate a more robust assessment of population exposure to flooding from SLR in any given year. Our results suggest that more explicit decisions regarding spatial zone (and associated temporal implication) will improve adaptation planning and policies by indicating the relative chance and magnitude of coastal populations to be affected by future SLR.PRIFPRI3; ISIDSG

Millennial-Scale Instability in the Geomagnetic Field Prior to the Matuyama-Brunhes Reversal

Changes in the Earth's magnetic field have global significance that reach from the outer core extending out to the uppermost atmosphere. Paleomagnetic records derived from sedimentary and volcanic sequences provide important insights into the geodynamo processes that govern the largest geomagnetic changes (polarity reversals), but dating uncertainties have hindered progress in this understanding. Here, we report a paleomagnetic record from multiple lava flows on Tahiti that bracket the Matuyama‐Brunhes (M‐B) polarity reversal ∼771,000 years ago. Our high‐precision ^(40)Ar/^(39)Ar ages constrain several rapid and short‐lived changes in field orientation up to 33,000 years prior to the M‐B reversal. These changes are similar to ones identified in other less well‐dated lava flows in Maui, Chile, and La Palma that occurred during an extended period of reduced field strength recorded in sediments. We use a simple stochastic model to show that these rapid polarity changes are highly attenuated in sediment records with low sedimentation rates. This prolonged 33,000 year period of reduced field strength and increased geomagnetic instability supports models that show frequent centennial‐to‐millennial‐scale polarity changes in the presence of a strongly weakened dipole field

Be-10 age constraints on latest Pleistocene and Holocene cirque glaciation across the western United States

Paleoclimate: A rocky reworking of Holocene glaciology New dating of glacially-deposited rocks substantially revises our understanding of the waxing and waning of ice since the last glacial maximum. Glaciologists have long thought that moraines throughout the western United States represent ‘neoglacial’ advances about 6,000 years ago. Now, a multi-institution team led by Shaun Marcott at the University of Wisconsin-Madison has found — using cosmogenic isotopes — that these terminal deposits left by advancing glaciers are instead 9,000 to 15,000 years old. The research advances prior work by using absolute, not relative ages, and documents that glaciers retreated after the last glacial maximum ~ 21,000 years ago, fluctuated locally throughout much of the Holocene, and re-advanced during the Little Ice Age of a few hundred years ago. Glacial advances that might have occurred during the neoglacial were wiped away by the more extensive glaciations of the Little Ice Age