Stable Carbon Cycle–Climate Relationship During the Late Pleistocene

Abstract

extended-H[ckel approach to electronic coupling calculation is certainly dependent on its parameterization.The existence of multiple tunneling regimes also provides insight into several recent (and otherwise puzzling) experimental and theoretical observations in biological ET reaction kinetics. Winkler, Gray, and co-workers found that ET across protein-protein interfaces in protein crystals mediated by three water molecules is nearly as rapid as unimolecular ET is over the same distance (7). Canters and co-workers showed that water dimers between covalently cross-linked azurin complexes could substantially enhance the intermolecular ET kinetics (15). Similarly, Klinman and coworkers investigated the copper-to-copper ET over about 7 ) in the hydroxylating domain of peptidylglycine a-amidylating monooxygenase and found an unusually large electronic coupling mediated, apparently, by water rather than by the protein or substrate (17). Using Pathways-level analysis, Onuchic and co-workers found that water molecules mediate the dominant ET coupling routes between cytochrome c 2 and the photosynthetic reaction center (18). Cave and co-workers showed that water molecules between model D and A pairs substantially enhance intermolecular ET rates as well (19). All of these recent observations support our conclusion that a small number of structured water molecules interposed between the donor and the acceptor cofactors can substantially enhance ET rates.The influence of aqueous tunneling pathways on interprotein ET kinetics has remained a key open issue in biological ET for some time. Single-exponential decay models fail to describe water-mediated ET reactions properly. The existence of multiple tunneling mediating regimes identified above is evinced by a body of recent experimental and theoretical observations. Most importantly, the structured water coupling regime may provide an important mechanism to facilitate ET reactions in the critical near-contact distance range relevant to biological ET kinetics. We hypothesize that water may be a particularly strong tunneling mediator when it occupies a sterically constrained space between redox cofactors with strong organizing forces that favor constructively interfering coupling pathways. It will be particularly interesting to use both theory and experiment to explore how the watermediated coupling between proteins varies with protein-protein shape complementarity, surface charge and polarity, and dynamical fluctuations of the proteins and of the organized water at the interface. Chem. 88, 1321Chem. 88, (1984 . 106, 6647 (1997). 23. M. D. Newton, J. Electroanal. Chem. 438, 3 (1997). 24. S. Larsson, J. Phys.

The middle Pleistocene transition: characteristics, mechanisms, and implications for long-term changes in atmospheric pCO2

“…This explanation for the MPT tacitly assumes a known climate sensitivity to atmospheric pCO 2 in order to induce a response at precisely the time of the MPT. Moreover, the EPICA ice core, which provides the longest available record of atmospheric pCO 2 (Siegenthaler et al, 2005), indicates that mean atmospheric pCO 2 was lower at 720 ka ($230 ppmV) than today ($240 ppmV). Accordingly, if a secular decrease in atmospheric pCO 2 ultimately caused the MPT, that trend had reversed by at least 720 ka.…”

Thermodynamic Forecasts of the Mediterranean Sea Acidification

“…Ocean acidification is one of the consequences of approximately 79 million tons of carbon dioxide (CO 2 ) released into the atmosphere every day from fossil fuel burning, deforestation and cement production (IPCC, 2007;IPCC,2013). As a result of human activities, today's atmospheric CO 2 concentration is rising at a rate of ∼ 0.5% year −1 (Forster et al, 2007), which is ∼100 times faster than any change during the past 650 000 years (Royal Society, 2005;Siegenthaler et al, 2005). The atmospheric increase of CO 2 from 2012 to 2013 was 2.9 microatmosphere (matm), which is the largest annual increase for the period 1984(WMO, 2014.…”

Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?

“…Actually, considering the great importance of the lower stratosphere/upper troposphere (LS/UT) for the radiation balance, chemistry, and dynamics of the atmosphere, its research should anyhow be intensified. For instance, it is not well known how much of the large quantities of anthropogenic SO 2 emitted at ground level reaches the LS/UT to produce sulfate particles, what regulates temperatures, water vapour concentrations and cirrus cloud formation in the LS/UT region, and how these factors may change in response to growing CO 2 concentrations, which are already 30-40% higher than ever experienced during the past 650,000 years (Siegenthaler et al, 2005). Progress in the understanding of the complicated earth climate system is generally slow.…”