Pore pressure measurements on IODP Exp308 sites

Overpressures measured with pore pressure penetrometers during Integrated Ocean Drilling Program (IODP) Expedition 308 reach 70% and 60% of the hydrostatic effective stress (View the MathML source) in the first 200 meters below sea floor (mbsf) at Sites U1322 and U1324, respectively, in the deepwater Gulf of Mexico, offshore Louisiana. High overpressures are present within low permeability mudstones where there have been multiple, very large, submarine landslides during the Pleistocene. Beneath 200 mbsf at Site U1324, pore pressures drop significantly: there are no submarine landslides in this mixture of mudstone, siltstone, and sandstone. The penetrometer measurements did not reach the in situ pressure at the end of the deployment. We used a soil model to determine that an extrapolation approach based on the inverse of square route of time (View the MathML source) requires much less decay time to achieve a desirable accuracy than an inverse time (1/t) extrapolation. Expedition 308 examined how rapid and asymmetric sedimentation above a permeable aquifer drives lateral fluid flow, extreme pore pressures, and submarine landslides. We interpret that the high overpressures observed are driven by rapid sedimentation of low permeability material from the ancestral Mississippi River. Reduced overpressure at depth at Site U1324 suggests lateral flow (drainage) whereas high overpressure at Site U1322 requires inflow from below: lateral flow in the underlying permeable aquifer provides one mechanism for these observations. High overpressure near the seafloor reduces slope stability and provides a mechanism for the large submarine landslides and low regional gradient (2°) offshore from the Mississippi delta.

Benthic foraminiferal stable isotope stratigraphy of ODP Site 138-846 in the tropical Pacific Ocean

A stable-isotope stratigraphy at Site 846 (tropical Pacific, 3°06'S, 90°49'W, 3307 m water depth), based on the benthic foraminifers Cibicides wuellerstorfi and Uvigerina peregrina, yields a high-resolution record of deep-sea delta18O and delta13C over the past 1.8 Ma, with an average sampling interval of 3 k.y. Variance in the delta18O and delta13C records is concentrated in the well-known orbital periods of 100, 41, and 23 k.y. In the 100-k.y. band, both isotopic signals grow from relatively low amplitudes prior to 1.2 Ma, to high amplitudes in the late Quaternary since 0.7 Ma. The amplitude of delta18O and especially of delta13C decreases in the 41-k.y. band as it grows in the 100-k.y. band, consistent with a transfer of energy into an orbitally-paced internal oscillation. A weak 30-k.y. rhythm, present in both delta18O and delta13C, may reflect nonlinear interaction between the 41-k.y. and 100-k.y. bands in the evolving climate system. In the 23-k.y. and 19-k.y. bands associated with orbital precession, delta18O and delta13C are not coherent with each other on long time scales, and do not evolve like the 100-k.y. and 41-k.y. bands. This suggests that the source of the growing 100-k.y. oscillation is not a nonlinear response to precession, in contrast to predictions of some climate models. Sedimentation rates at this site also vary with a strong 100-k.y. cycle. Unlike the isotope records, the amplitude of 100-k.y. variations in sedimentation rate is relatively constant over the past 1.8 Ma, ranging from about 15 to 70 m/m.y. Prior to 0.9 Ma, sedimentation rates co-vary with orbital eccentricity, rather than with global climate as reflected by delta18O or delta13C. A source of this 100-k.y. cycle of sedimentation rate in the absence of similar ice volume fluctuations may be precessional heating of equatorial land masses, which in an energy balance climate model drives variations of monsoonal climates with a 100-k.y. rhythm. For the interval younger than 0.9 Ma, high sedimentation rates in the 100-k.y. band are consistently associated with glacial stages. This change of pattern suggests that when the amplitude of glacial cycles become large enough, their global effects overpower a local monsoon-driven variation in sedimentation rate at Site 846.

(Table 1) Number of alleles per locus and heterozygosity of Arctic charr (Salvelinus alpinus) populations in Iceland

The ecological theory of adaptive radiation predicts that the evolution of phenotypic diversity within species is generated by divergent natural selection arising from different environments and competition between species. Genetic connectivity among populations is likely also to have an important role in both the origin and maintenance of adaptive genetic diversity. Our goal was to evaluate the potential roles of genetic connectivity and natural selection in the maintenance of adaptive phenotypic differences among morphs of Arctic charr, Salvelinus alpinus, in Iceland. At a large spatial scale, we tested the predictive power of geographic structure and phenotypic variation for patterns of neutral genetic variation among populations throughout Iceland. At a smaller scale, we evaluated the genetic differentiation between two morphs in Lake Thingvallavatn relative to historically explicit, coalescent-based null models of the evolutionary history of these lineages. At the large spatial scale, populations are highly differentiated, but weakly structured, both geographically and with respect to patterns of phenotypic variation. At the intralacustrine scale, we observe modest genetic differentiation between two morphs, but this level of differentiation is nonetheless consistent with strong reproductive isolation throughout the Holocene. Rather than a result of the homogenizing effect of gene flow in a system at migration-drift equilibrium, the modest level of genetic differentiation could equally be a result of slow neutral divergence by drift in large populations. We conclude that contemporary and recent patterns of restricted gene flow have been highly conducive to the evolution and maintenance of adaptive genetic variation in Icelandic Arctic charr.

Regulation of intracellular pH in cnidarians: response to acidosis in Anemonia viridis

The regulation of intracellular pH (pHi) is a fundamental aspect of cell physiology that has received little attention in studies of the phylum Cnidaria, which includes ecologically important sea anemones and reef-building corals. Like all organisms, cnidarians must maintain pH homeostasis to counterbalance reductions in pHi, which can arise because of changes in either intrinsic or extrinsic parameters. Corals and sea anemones face natural daily changes in internal fluids, where the extracellular pH can range from 8.9 during the day to 7.4 at night. Furthermore, cnidarians are likely to experience future CO2-driven declines in seawater pH, a process known as ocean acidification. Here, we carried out the first mechanistic investigation to determine how cnidarian pHi regulation responds to decreases in extracellular and intracellular pH. Using the anemone Anemonia viridis, we employed confocal live cell imaging and a pH-sensitive dye to track the dynamics of pHi after intracellular acidosis induced by acute exposure to decreases in seawater pH and NH4Cl prepulses. The investigation was conducted on cells that contained intracellular symbiotic algae (Symbiodinium sp.) and on symbiont-free endoderm cells. Experiments using inhibitors and Na-free seawater indicate a potential role of Na/H plasma membrane exchangers (NHEs) in mediating pHi recovery following intracellular acidosis in both cell types. We also measured the buffering capacity of cells, and obtained values between 20.8 and 43.8 mM per pH unit, which are comparable to those in other invertebrates. Our findings provide the first steps towards a better understanding of acid-base regulation in these basal metazoans, for which information on cell physiology is extremely limited.