Publisher Correction: Ice sheets matter for the global carbon cycle.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Ice sheets matter for the global carbon cycle.

The cycling of carbon on Earth exerts a fundamental influence upon the greenhouse gas content of the atmosphere, and hence global climate over millennia. Until recently, ice sheets were viewed as inert components of this cycle and largely disregarded in global models. Research in the past decade has transformed this view, demonstrating the existence of uniquely adapted microbial communities, high rates of biogeochemical/physical weathering in ice sheets and storage and cycling of organic carbon (>104 Pg C) and nutrients. Here we assess the active role of ice sheets in the global carbon cycle and potential ramifications of enhanced melt and ice discharge in a warming world.

Strong and deep Atlantic meridional overturning circulation during the last glacial cycle.

Extreme, abrupt Northern Hemisphere climate oscillations during the last glacial cycle (140,000 years ago to present) were modulated by changes in ocean circulation and atmospheric forcing. However, the variability of the Atlantic meridional overturning circulation (AMOC), which has a role in controlling heat transport from low to high latitudes and in ocean CO2 storage, is still poorly constrained beyond the Last Glacial Maximum. Here we show that a deep and vigorous overturning circulation mode has persisted for most of the last glacial cycle, dominating ocean circulation in the Atlantic, whereas a shallower glacial mode with southern-sourced waters filling the deep western North Atlantic prevailed during glacial maxima. Our results are based on a reconstruction of both the strength and the direction of the AMOC during the last glacial cycle from a highly resolved marine sedimentary record in the deep western North Atlantic. Parallel measurements of two independent chemical water tracers (the isotope ratios of (231)Pa/(230)Th and (143)Nd/(144)Nd), which are not directly affected by changes in the global cycle, reveal consistent responses of the AMOC during the last two glacial terminations. Any significant deviations from this configuration, resulting in slowdowns of the AMOC, were restricted to centennial-scale excursions during catastrophic iceberg discharges of the Heinrich stadials. Severe and multicentennial weakening of North Atlantic Deep Water formation occurred only during Heinrich stadials close to glacial maxima with increased ice coverage, probably as a result of increased fresh-water input. In contrast, the AMOC was relatively insensitive to submillennial meltwater pulses during warmer climate states, and an active AMOC prevailed during Dansgaard-Oeschger interstadials (Greenland warm periods).

The serotonin transporter gene polymorphism is not associated with smoking behavior.

Nicotine increases serotonin release in the brain and symptoms of nicotine withdrawal may be modulated by diminished serotonergic neurotransmission. The promoter region of the serotonin transporter gene, solute carrier family neurotransmitter transporter member 4 (SLC6A4), contains a functional tandem repeat polymorphism. The long (L) variant is more actively transcribed than the short (S) variant and is associated with a higher serotonin uptake. To investigate the potential role of this polymorphism for smoking behavior, SLC6A4 genotypes were determined in two different studies, the SMOKING GENES case-control study (470 current smokers and 419 subjects who had never smoked) and the cross-sectional Ludwigshafen risk and cardiovascular health (LURIC) study (777 current smokers and 1178 subjects who had never smoked). In the SMOKING GENES case-control study, SLC6A4 genotype frequencies were not statistically different between smokers (LL: 30.9%; LS: 46.8%; SS: 16.4%) and non-smokers (LL: 36.3%; LS: 41.8%; SS: 14.3%; P=0.13). Similar results were obtained in the cross-sectional LURIC study (smokers: LL, 36.5%, LS, 45.6%, SS, 17.9%; non-smokers: LL, 33.6%, LS, 48.9%, SS, 17.6%; P=0.33). SLC6A4 genotypes were furthermore not associated with Fagerstrom Tolerance Questionnaire score, packyears, number of cigarettes smoked per day or previous attempts to quit smoking. We conclude that the SLC6A4 promoter polymorphism is not a major determinant of smoking behavior in Caucasian.

Endogenous brain-derived neurotrophic factor and neurotrophin-3 antagonistically regulate survival of axotomized corticospinal neurons in vivo.

Neuronal growth factors regulate the survival of neurons by their survival and death-promoting activity on distinct populations of neurons. The neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) promote neuronal survival via tyrosine kinase (Trk) receptors, whereas NGF and BDNF can also induce apoptosis in developing neurons through p75(NTR) receptors in the absence of their respective Trk receptors. Using mutant mice and inactivation of neurotrophins and their receptors with antibodies in rats, we show that endogenous NT-3 induces death of adult BDNF-dependent, axotomized corticospinal neurons (CSNs). When NT-3 is neutralized, the neurons survive even without BDNF, suggesting complete antagonism. Whereas virtually all unlesioned and axotomized CSNs express both trkB and trkC mRNA, p75 is barely detectable in unlesioned CSNs but strongly upregulated in axotomized CSNs by day 3 after lesion, the time point when cell death occurs. Blocking either cortical TrkC or p75(NTR) receptors alone prevents death, indicating that the opposing actions of NT-3 and BDNF require their respective Trk receptors, but induction of death depends on p75(NTR) cosignaling. The results show that neuronal survival can be regulated antagonistically by neurotrophins and that neurotrophins can induce neuronal death in the adult mammalian CNS. We further present evidence that signaling of tyrosine kinase receptors of the trk family can be crucially involved in the promotion of neuronal death in vivo.