Detecting foraminiferal photosymbionts in the fossil record: a combined micropalaeontological and geochemical approach

Poster from International Conference on Paleoceanography 12 (ICP 12). See the above link for more information on the conference

Relation of sortable silt grain-size to deep-sea current speeds: Calibration of the ‘Mud Current Meter’

Fine grain-size parameters have been used for inference of palaeoflow speeds of near-bottom currents in the deep-sea. The basic idea stems from observations of varying sediment size parameters on a continental margin with a gradient from slower flow speeds at shallower depths to faster at deeper. In the deep-sea, size-sorting occurs during deposition after benthic storm resuspension events. At flow speeds below 10–15 cm s−1 mean grain-size in the terrigenous non-cohesive ‘sortable silt’ range (denoted by View the MathML source, mean of 10–63 µm) is controlled by selective deposition, whereas above that range removal of finer material by winnowing is also argued to play a role. A calibration of the View the MathML source grain-size flow speed proxy based on sediment samples taken adjacent to sites of long-term current meters set within ~100 m of the sea bed for more than a year is presented here. Grain-size has been measured by either Sedigraph or Coulter Counter, in some cases both, between which there is an excellent correlation for View the MathML source (r = 0.96). Size-speed data indicate calibration relationships with an overall sensitivity of 1.36 ± 0.19 cm s−1/μm. A calibration line comprising 12 points including 9 from the Iceland overflow region is well defined, but at least two other smaller groups (Weddell/Scotia Sea and NW Atlantic continental rise/Rockall Trough) are fitted by sub-parallel lines with a smaller constant. This suggests a possible influence of the calibre of material supplied to the site of deposition (not the initial source supply) which, if depleted in very coarse silt (31–63 µm), would limit View the MathML source to smaller values for a given speed than with a broader size-spectrum supply. Local calibrations, or a core-top grain-size and local flow speed, are thus necessary to infer absolute speeds from grain-size. The trend of the calibrations diverges markedly from the slope of experimental critical erosion and deposition flow speeds versus grain-size, making it unlikely that the View the MathML source (or any deposit size for that matter) is simply predicted by the deposition threshold. A more probable control is the rate of deposition of the different size fractions under changing flows over several tens of years (the typical averaging period of a centimetre of deposited sediment). This suggestion is supported by a simple depositional model for which the deposited View the MathML source is calculated from measured currents with a size-varying depositional threshold. More surficial sediment samples taken near long-term current meter sites are needed to make calibrations more robust and explore regional differences

Evidence of Silica Leakage to the Tropical Atlantic via Antarctic Intermediate Water during Marine Isotope Stage 4

Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are the main conduits for the supply of dissolved silica (silicic acid) from the deep Southern Ocean to the low latitude surface ocean, and therefore have an important control on low latitude diatom productivity. Enhanced supply of silicic acid by AAIW (and SAMW) during glacial periods may have enabled tropical diatoms to outcompete carbonate-producing phytoplankton, decreasing the relative export of inorganic to organic carbon to the deep ocean and lowering atmospheric CO2. This mechanism is known as the 'Silicic Acid Leakage Hypothesis' (SALH). Here we present records of neodymium and silicon isotopes from the western tropical Atlantic that provide the first direct evidence of increased silicic acid leakage from the Southern Ocean to the tropical Atlantic within AAIW during glacial Marine Isotope Stage (MIS) 4 (~60-80 ka). This leakage is coeval with enhanced diatom export in the NW Atlantic and across the eastern equatorial Atlantic and provides support for the SALH as a contributor to CO2 drawdown during full glacial development

More efficient North Atlantic carbon pump during the Last Glacial Maximum

During the Last Glacial Maximum (LGM; ~20,000 years ago), the global ocean sequestered a large amount of carbon lost from the atmosphere and terrestrial biosphere. Suppressed CO2 outgassing from the Southern Ocean is the prevailing explanation for this carbon sequestration. By contrast, the North Atlantic Ocean—a major conduit for atmospheric CO2 transport to the ocean interior via the overturning circulation—has received much less attention. Here we demonstrate that North Atlantic carbon pump efficiency during the LGM was almost doubled relative to the Holocene. This is based on a novel proxy approach to estimate air–sea CO2 exchange signals using combined carbonate ion and nutrient reconstructions for multiple sediment cores from the North Atlantic. Our data indicate that in tandem with Southern Ocean processes, enhanced North Atlantic CO2 absorption contributed to lowering ice-age atmospheric CO2

Long-term variations in Iceland–Scotland overflow strength during the Holocene

The overflow of deep water from the Nordic seas into the North Atlantic plays a critical role in global ocean circulation and climate. Approximately half of this overflow occurs via the Iceland–Scotland (I–S) overflow, yet the history of its strength throughout the Holocene (~0–11 700 yr ago, ka) is poorly constrained, with previous studies presenting apparently contradictory evidence regarding its long-term variability. Here, we provide a comprehensive reconstruction of I–S overflow strength throughout the Holocene using sediment grain size data from a depth transect of 13 cores from the Iceland Basin. Our data are consistent with the hypothesis that the main axis of the I–S overflow on the Iceland slope was shallower during the early Holocene, deepening to its present depth by ~7 ka. Our results also reveal weaker I–S overflow during the early and late Holocene, with maximum overflow strength occurring at ~7 ka, the time of a regional climate thermal maximum. Climate model simulations suggest a shoaling of deep convection in the Nordic seas during the early and late Holocene, consistent with our evidence for weaker I–S overflow during these intervals. Whereas the reduction in I–S overflow strength during the early Holocene likely resulted from melting remnant glacial ice sheets, the decline throughout the last 7000 yr was caused by an orbitally induced increase in the amount of Arctic sea ice entering the Nordic seas. Although the flux of Arctic sea ice to the Nordic seas is expected to decrease throughout the next century, model simulations predict that under high emissions scenarios, competing effects, such as warmer sea surface temperatures in the Nordic seas, will result in reduced deep convection, likely driving a weaker I–S overflow

Reconstructing extreme AMOC events through nudging of the ocean surface: a perfect model approach

While the Atlantic Meridional Overturning Circulation (AMOC) is thought to be a crucial component of the North Atlantic climate, past changes in its strength are challenging to quantify, and only limited information is available. In this study, we use a perfect model approach with the IPSL-CM5A-LR model to assess the performance of several surface nudging techniques in reconstructing the variability of the AMOC. Special attention is given to the reproducibility of an extreme positive AMOC peak from a preindustrial control simulation. Nudging includes standard relaxation techniques towards the sea surface temperature and salinity anomalies of this target control simulation, and/or the prescription of the wind-stress fields. Surface nudging approaches using standard fixed restoring terms succeed in reproducing most of the target AMOC variability, including the timing of the extreme event, but systematically underestimate its amplitude. A detailed analysis of the AMOC variability mechanisms reveals that the underestimation of the extreme AMOC maximum comes from a deficit in the formation of the dense water masses in the main convection region, located south of Iceland in the model. This issue is largely corrected after introducing a novel surface nudging approach, which uses a varying restoring coefficient that is proportional to the simulated mixed layer depth, which, in essence, keeps the restoring time scale constant. This new technique substantially improves water mass transformation in the regions of convection, and in particular, the formation of the densest waters, which are key for the representation of the AMOC extreme. It is therefore a promising strategy that may help to better constrain the AMOC variability and other ocean features in the models. As this restoring technique only uses surface data, for which better and longer observations are available, it opens up opportunities for improved reconstructions of the AMOC over the last few decades

Energy metabolism, altered proteins, sirtuins and ageing: converging mechanisms?

The predominant molecular symptom of ageing is the accumulation of altered gene products. Nutritional studies show that ageing in animals can be significantly influenced by dietary restriction. Genetics has revealed that ageing may be controlled by changes in intracellular NAD/NADH ratio regulating sirtuin activity. Physiological and other approaches indicate that mitochondria may also regulate ageing. A mechanism is proposed which links diet, exercise and mitochondria-dependent changes in NAD/NADH ratio to intracellular generation of altered proteins. It is suggested that ad libitum feeding conditions decrease NAD availability which also decreases metabolism of the triose phosphate glycolytic intermediates, glyceraldehyde-3-phosphate and dihydroxyacetone-phosphate, which can spontaneously decompose into methylglyoxal (MG). MG is a highly toxic glycating agent and a major source of protein advanced-glycosylation end-products (AGEs). MG and AGEs can induce mitochondrial dysfunction and formation of reactive oxygen species (ROS), as well as affect gene expression and intracellular signalling. In dietary restriction–induced fasting, NADH would be oxidised and NAD regenerated via mitochondrial action. This would not only activate sirtuins and extend lifespan but also suppress MG formation. This proposal can also explain the apparent paradox whereby increased aerobic activity suppresses formation of glycoxidized proteins and extends lifespan. Variation in mitochondrial DNA composition and consequent mutation rate, arising from dietary-controlled differences in DNA precursor ratios, could also contribute to tissue differences in age-related mitochondrial dysfunction

Advanced glycoxidation and lipoxidation end products (AGEs and ALEs): an overview of their mechanisms of formation

Advanced lipoxidation end products (ALEs) and advanced glycation end products (AGEs) have a pathogenetic role in the development and progression of different oxidative-based diseases including diabetes, atherosclerosis, and neurological disorders. AGEs and ALEs represent a quite complex class of compounds that are formed by different mechanisms, by heterogeneous precursors and that can be formed either exogenously or endogenously. There is a wide interest in AGEs and ALEs involving different aspects of research which are essentially focused on set-up and application of analytical strategies (1) to identify, characterize, and quantify AGEs and ALEs in different pathophysiological conditions ; (2) to elucidate the molecular basis of their biological effects ; and (3) to discover compounds able to inhibit AGEs/ALEs damaging effects not only as biological tools aimed at validating AGEs/ALEs as drug target, but also as promising drugs. All the above-mentioned research stages require a clear picture of the chemical formation of AGEs/ALEs but this is not simple, due to the complex and heterogeneous pathways, involving different precursors and mechanisms. In view of this intricate scenario, the aim of the present review is to group the main AGEs and ALEs and to describe, for each of them, the precursors and mechanisms of formation