Paul J. Valdes

Deglacial rapid sea level rises caused by ice-sheet saddle collapses

The last deglaciation (21 to 7 thousand years ago) was punctuated by several abrupt meltwater pulses, which sometimes caused noticeable climate change. Around 14 thousand years ago, meltwater pulse 1A (MWP-1A), the largest of these events, produced a sea level rise of 14–18u2009metres over 350u2009years. Although this enormous surge of water certainly originated from retreating ice sheets, there is no consensus on the geographical source or underlying physical mechanisms governing the rapid sea level rise. Here we present an ice-sheet modelling simulation in which the separation of the Laurentide and Cordilleran ice sheets in North America produces a meltwater pulse corresponding to MWP-1A. Another meltwater pulse is produced when the Labrador and Baffin ice domes around Hudson Bay separate, which could be associated with the ‘8,200-year’ event, the most pronounced abrupt climate event of the past nine thousand years. For both modelled pulses, the saddle between the two ice domes becomes subject to surface melting because of a general surface lowering caused by climate warming. The melting then rapidly accelerates as the saddle between the two domes gets lower, producing nine metres of sea level rise over 500 years. This mechanism of an ice ‘saddle collapse’ probably explains MWP-1A and the 8,200-year event and sheds light on the consequences of these events on climate.

Late Pleistocene climate change and the global expansion of anatomically modern humans

The extent to which past climate change has dictated the pattern and timing of the out-of-Africa expansion by anatomically modern humans is currently unclear [Stewart JR, Stringer CB (2012) Science 335:1317–1321]. In particular, the incompleteness of the fossil record makes it difficult to quantify the effect of climate. Here, we take a different approach to this problem; rather than relying on the appearance of fossils or archaeological evidence to determine arrival times in different parts of the world, we use patterns of genetic variation in modern human populations to determine the plausibility of past demographic parameters. We develop a spatially explicit model of the expansion of anatomically modern humans and use climate reconstructions over the past 120 ky based on the Hadley Centre global climate model HadCM3 to quantify the possible effects of climate on human demography. The combinations of demographic parameters compatible with the current genetic makeup of worldwide populations indicate a clear effect of climate on past population densities. Our estimates of this effect, based on population genetics, capture the observed relationship between current climate and population density in modern hunter–gatherers worldwide, providing supporting evidence for the realism of our approach. Furthermore, although we did not use any archaeological and anthropological data to inform the model, the arrival times in different continents predicted by our model are also broadly consistent with the fossil and archaeological records. Our framework provides the most accurate spatiotemporal reconstruction of human demographic history available at present and will allow for a greater integration of genetic and archaeological evidence.

Sea-surface temperature records of Termination 1 in the Gulf of California: Challenges for seasonal and interannual analogues of tropical Pacific climate change

[1]xa0Centennial-scale records of sea-surface temperature and opal composition spanning the Last Glacial Maximum and Termination 1 (circa 25–6xa0ka) are presented here from Guaymas Basin in the Gulf of California. Through the application of two organic geochemistry proxies, the U37K′ index and the TEX86H index, we present evidence for rapid, stepped changes in temperatures during deglaciation. These occur in both temperature proxies at 13xa0ka (∼3°C increase in 270xa0years), 10.0xa0ka (∼2°C decrease over ∼250xa0years) and at 8.2xa0ka (3°C increase in <200xa0years). An additional rapid warming step is also observed in TEX86H at 11.5xa0ka. In comparing the two temperature proxies and opal content, we consider the potential for upwelling intensity to be recorded and link this millennial-scale variability to shifting Intertropical Convergence Zone position and variations in the strength of the Subtropical High. The onset of the deglacial warming from 17 to 18xa0ka is comparable to a “southern hemisphere” signal, although the opal record mimics the ice-rafting events of the north Atlantic (Heinrich events). Neither the modern seasonal cycle nor El Nino/Southern Oscillation patterns provide valid analogues for the trends we observe in comparison with other regional records. Fully coupled climate model simulations confirm this result, and in combination we question whether the seasonal or interannual climate variations of the modern climate are valid analogues for the glacial and deglacial tropical Pacific.

Evaluating the effects of terrestrial ecosystems, climate and carbon dioxide on weathering over geological time: a global-scale process-based approach

Global weathering of calcium and magnesium silicate rocks provides the long-term sink for atmospheric carbon dioxide (CO2) on a timescale of millions of years by causing precipitation of calcium carbonates on the seafloor. Catchment-scale field studies consistently indicate that vegetation increases silicate rock weathering, but incorporating the effects of trees and fungal symbionts into geochemical carbon cycle models has relied upon simple empirical scaling functions. Here, we describe the development and application of a process-based approach to deriving quantitative estimates of weathering by plant roots, associated symbiotic mycorrhizal fungi and climate. Our approach accounts for the influence of terrestrial primary productivity via nutrient uptake on soil chemistry and mineral weathering, driven by simulations using a dynamic global vegetation model coupled to an ocean–atmosphere general circulation model of the Earths climate. The strategy is successfully validated against observations of weathering in watersheds around the world, indicating that it may have some utility when extrapolated into the past. When applied to a suite of six global simulations from 215 to 50 Ma, we find significantly larger effects over the past 220 Myr relative to the present day. Vegetation and mycorrhizal fungi enhanced climate-driven weathering by a factor of up to 2. Overall, we demonstrate a more realistic process-based treatment of plant fungal–geosphere interactions at the global scale, which constitutes a first step towards developing ‘next-generation’ geochemical models.

Controls on the tropospheric oxidizing capacity during an idealized Dansgaard‐Oeschger event, and their implications for the rapid rises in atmospheric methane during the last glacial period

The ice core record reveals large variations in the concentration of atmospheric methane, CH4, over the last 800 kyr. Amongst the most striking natural features are the large, rapid rises in CH4, of 100-200 ppbv, on timescales of less than 100 years, at the beginning of Dansgaard-Oeschger (D-O) events during the last glacial period (21-110 kyr before present). Despite the potential insight they could offer into the likelihood of future rapid rises in CH4, the relative roles of changes in methane sources and sinks during D-O events have been little explored. Here, we use a global atmospheric chemistry-transport model to explore-for the first time, in a process-based fashion-controls on the oxidizing capacity during an idealized D-O event that features a characteristically rapid rise in CH4. We find that the two controls previously identified in the literature as having had significant (though opposing) influences on the oxidizing capacity between glacial and interglacial periods-changes in air temperature and emissions of non-methane volatile organic compounds from vegetation-offset one another between idealized Heinrich stadial and Greenland interstadial states. The result is, the net change in oxidizing capacity is very small, implying the rapid rises in CH4 at the beginning of D-O events were almost entirely source-driven. This poses a challenge to earth-system models-to generate a sufficiently large increase in methane emissions in response to a simulated D-O event, via a more realistic freshwater forcing impacting the strength of the Atlantic meridional overturning circulation or, possibly, other climate-change mechanisms. Citation: Levine, J. G., E. W. Wolff, P. O. Hopcroft, and P. J. Valdes (2012), Controls on the tropospheric oxidizing capacity during an idealized Dansgaard-Oeschger event, and their implications for the rapid rises in atmospheric methane during the last glacial period, Geophys. Res. Lett., 39, L12805, doi:10.1029/2012GL051866.

Predictive petroleum play fairway mapping using plate tectonic, palaeogeographic and palaeo-Earth systems modelling - Exa: Tools for Configuring, Building and Running Models

Collected articles in this series are dedicated to the development and use of software for earth system modelling and aims at bridging the gap between IT solutions and climate science. The particular topic covered in this volume addresses the process of configuring, building, and running earth system models. Earth system models are typically a collection of interacting computer codes (often called components) which together simulate the earth system. Each code component is written to model some physical process which forms part of the earth system (such as the Ocean). This book is concerned with the source code version control of these code components, the configuration of these components into earth system models, the creation of executable(s) from the component source code and related libraries and the running and monitoring of the resultant executables on the available hardware.

Predictive petroleum play fairway mapping using plate tectonic, palaeogeographic and palaeo-Earth systems modelling - Exa

For frontier basins, the development of petroleum play concepts and the construction of play fairway maps are difficult. In particular, the presence and potential extent of source rocks and reservoirs represents some of the main uncertainties. To provide an objective, process based, predictive methodology focused on these problems, global plate reconstructions were used as the basis for palaeogeographic mapping. These maps are underpinned by data and were coupled with state-of-the-art palaeo-Earth systems models (HadCM3 palaeoclimate model). Detailed palaeotectonics and palaeoenvironments maps were prepared and a new method relating topography and bathymetry to plate tectonic environments was used as the basis for palaeo digital elevation models (DEMs). These were gridded in GIS and used to provide the topographic and bathymetric boundary conditions for coupled ocean-atmosphere general circulation models. The compilation of the base maps was based on a global database of palaeoenvironmental and lithofacies data. These data include climate proxies that were used to test the veracity of the modelling results. This work is used to provide an understanding of the geohistory of the North Atlantic. It also provides key petroleum play concepts and unique, innovative methods to produce improved play fairway maps for North Atlantic margin basins.