The Mid-Pleistocene Transition: a delayed response to an increasing positive feedback?

Glacial-interglacial cycles constitute large natural variations in Earth's climate. The Mid-Pleistocene Transition (MPT) marks a shift of the dominant periodicity of these climate cycles from ∼40 to ∼100 kyr. Recently, it has been suggested that this shift resulted from a gradual increase in the internal period (or equivalently, a decrease in the natural frequency) of the system. As a result, the system would then have locked to ever higher multiples of the external forcing period. We find that the internal period is sensitive to the strength of positive feedbacks in the climate system. Using a carbon cycle model in which feedbacks between calcifier populations and ocean alkalinity mediate atmospheric CO2, we simulate stepwise periodicity changes similar to the MPT through such a mechanism. Due to the internal dynamics of the system, the periodicity shift occurs up to millions of years after the change in the feedback strength is imposed. This suggests that the cause for the MPT may have occurred a significant time before the observed periodicity shift.

Quantifying nutrient throughput and DOM production by algae in continuous culture.

Freshwater and marine algae can balance nutrient demand and availability by regulating uptake, accumulation and exudation. To obtain insight into these processes under nitrogen (N) and phosphorus (P) limitation, we reanalyze published data from continuous cultures of the chlorophyte Selenastrum minutum. Based on mass budgets, we argue that much of the non-limiting N and P had passed through the organisms and was present as dissolved organic phosphorus or nitrogen (DOP or DON). We construct a model that describes the production of biomass and dissolved organic matter (DOM) as a function of the growth rate. A fit of this model against the chemostat data suggests a high turnover of the non-limiting N and P: at the highest growth rates, N and P atoms spent on average only about 3 h inside an organism, before they were exuded as DON and DOP, respectively. This DOM exudation can explain the observed trends in the algal stoichiometric ratios as a function of the dilution rate. We discuss independent evidence from isotope experiments for this apparently wasteful behavior and we suggest experiments to quantify and characterize DON and DOP exudation further.

Global relationship between phytoplankton diversity and productivity in the ocean.

The shape of the productivity-diversity relationship (PDR) for marine phytoplankton has been suggested to be unimodal, that is, diversity peaking at intermediate levels of productivity. However, there are few observations and there has been little attempt to understand the mechanisms that would lead to such a shape for planktonic organisms. Here we use a marine ecosystem model together with the community assembly theory to explain the shape of the unimodal PDR we obtain at the global scale. The positive slope from low to intermediate productivity is due to grazer control with selective feeding, which leads to the predator-mediated coexistence of prey. The negative slope at high productivity is due to seasonal blooms of opportunist species that occur before they are regulated by grazers. The negative side is only unveiled when the temporal scale of the observation captures the transient dynamics, which are especially relevant at highly seasonal latitudes. Thus selective predation explains the positive side while transient competitive exclusion explains the negative side of the unimodal PDR curve. The phytoplankton community composition of the positive and negative sides is mostly dominated by slow-growing nutrient specialists and fast-growing nutrient opportunist species, respectively.

Mesoscale iron enrichment experiments 1993-2005: synthesis and future directions.

Since the mid-1980s, our understanding of nutrient limitation of oceanic primary production has radically changed. Mesoscale iron addition experiments (FeAXs) have unequivocally shown that iron supply limits production in one-third of the world ocean, where surface macronutrient concentrations are perennially high. The findings of these 12 FeAXs also reveal that iron supply exerts controls on the dynamics of plankton blooms, which in turn affect the biogeochemical cycles of carbon, nitrogen, silicon, and sulfur and ultimately influence the Earth climate system. However, extrapolation of the key results of FeAXs to regional and seasonal scales in some cases is limited because of differing modes of iron supply in FeAXs and in the modern and paleo-oceans. New research directions include quantification of the coupling of oceanic iron and carbon biogeochemistry.

Gastro-oesophageal reflux is more prevalent in Western dyspeptics: a prospective comparison of British and South-East Asian patients with dyspepsia.

BACKGROUND: There is a paucity of data directly comparing dyspepsia in Western and Eastern populations. AIM: To compare clinical symptoms, epidemiological factors and endoscopic diagnoses in two sample populations with dyspepsia from the United Kingdom and South-East Asia in a cross-sectional study. METHODS: Patients with uncomplicated dyspepsia attending endoscopy units in Leeds, UK, and Kuala Lumpur, Malaysia, were prospectively interviewed and underwent subsequent endoscopy. RESULTS: A total of 1003 Malaysian patients (January 2002 to August 2003) and 597 Caucasian British patients (January 2000 to October 2002) were studied. The mean age was 48.7 +/- 15.8 and 47.5 +/- 13.8 years for the Malaysian and British patients respectively (P = NS). There was a higher proportion of cigarette smoking (35.7% vs. 12.4%, P < 0.0001) and alcohol consumption (34.4% vs. 2.0%, P < 0.0001) amongst British patients, but no difference in non-steroidal anti-inflammatory drug use nor having Helicobacter pylori infection. Gastro-oesophageal reflux disease (GERD) symptoms were more common in British compared with South-East Asian patients [heartburn (72% vs. 41%), regurgitation (66% vs. 29.8%) and dysphagia (21.1% vs. 7.3%), P < 0.0001]. This correlated with an increased endoscopic finding of oesophagitis (26.8% vs. 5.8%) and columnar-lined oesophagus (4.4% vs. 0.9%) amongst British patients (P < 0.001). A logistic regression model revealed that British Caucasian race (OR 9.7; 95% CI = 5.0-18.8), male gender (OR 2.0; 95% CI = 1.4-2.9) and not having H. pylori infection (OR 0.5; 95% CI = 0.3-0.7) were independent predictors for oesophagitis. CONCLUSION: GERD is more common in British compared with South-East Asian dyspeptic patients suggesting that race and/or western lifestyle are important risk factors.

Open-access endoscopy: are age-based guidelines justified? An audit of experience of 1000 open-access endoscopies at a district general hospital.

BACKGROUND AND STUDY AIMS: Open-access endoscopy has been developed to meet the increasing demand for endoscopy, and guidelines have been produced for selecting suitable patients. This study was designed to assess the appropriateness of these guidelines in patients referred to open-access endoscopy services at a district general hospital. PATIENTS AND METHODS: A series of 1000 consecutive patients referred for open-access endoscopy was audited. The waiting time was 18 weeks and patients continued to take any acid-suppressing medication prescribed by their general practitioner. The endoscopy findings were compared in those who met and did not meet currently accepted guidelines. In addition the outcome for all patients in whom gastro-oesophageal malignancy was detected was reviewed. RESULTS: 301 patients (30.1 %) did not meet accepted guidelines for open-access endoscopy. No cases of malignancy would have been missed if the guidelines had been implemented. Gastro-oesophageal malignancy was detected in 17 patients, all of whom died. When malignancy was excluded, there were no statistically significant differences between the detection rates for pathology in the two groups. The commonest finding, in 32.3 % of patients, was of a normal endoscopic appearance. CONCLUSIONS: If current guidelines are applied, all cases of malignancy may be picked up, but identification of patients with curable disease is poor. Use of the guidelines did not select patients with other upper gastrointestinal disease, although many patients were receiving acid-suppression therapy at the time of their endoscopy. Implementation of test-and-treat strategies would result in a significant reduction in the workload of open-access endoscopy.