̽��ֱ�� of Cambridge - oxygen

1.5 million years of climate history revealed after scientists solve mystery of the deep

bjb42 — Fri, 10 Aug 2012 02:51:00 +0000

Scientists have announced a major breakthrough in understanding the Earth’s climate machine by reconstructing highly accurate records of changes in ice volume and deep-ocean temperatures over the last 1.5 million years.

̽��ֱ��study, which is reported in the journal Science, offers new insights into a decades-long debate about how the shifts in the Earth’s orbit relative to the sun have taken the Earth into and out of an ice-age climate.

Being able to reconstruct ancient climate change is a critical part of understanding why the climate behaves the way it does. It also helps us to predict how the planet might respond to man-made changes, such as the injection of large quantities of carbon dioxide into the atmosphere, in the future.

Unfortunately, scientists trying to construct an accurate picture of how such changes caused past climatic shifts have been thwarted by the fact that the most readily available marine geological record of ice-ages – changes in the ratio of oxygen isotopes (Oxygen 18 to Oxygen 16) preserved in tiny calcareous deep sea fossils called foraminifera – is compromised.

This is because the isotope record shows the combined effects of both deep sea temperature changes, and changes in the amount of ice volume. Separating these has in the past proven difficult or impossible, so researchers have been unable to tell whether changes in the Earth’s orbit were affecting the temperature of the ocean more than the amount of ice at the Poles, or vice-versa.

̽��ֱ��new study, which was carried out by researchers in the ̽��ֱ�� of Cambridge Department of Earth Sciences, appears to have resolved this problem by introducing a new set of temperature-sensitive data. This allowed them to identify changes in ocean temperatures alone, subtract that from the original isotopic data set, and then build what they describe as an unprecedented picture of climatic change over the last 1.5 million years – a record of changes in both oceanic temperature and global ice volume.

Included in this is a much fuller representation of what happened during the “Mid-Pleistocene Transition” (MPT) - a major change in the Earth’s climate system which took place sometime between 1.25 million and 600 thousand years ago. Before the MPT, the alternation between glacial periods of extreme cold, and warmer interglacials, happened at intervals of approximately 41,000 years. After the MPT, the major cycles became much longer, regularly taking 100,000 years. ̽��ֱ��second pattern of climate cycles is the one we are in now. Interestingly, this change occurred with little or no orbital forcing.

“Previously, we didn’t really know what happened during this transition, or on either side of it,” Professor Harry Elderfield, who led the research team, said. “Before you separate the ice volume and temperature signals, you don’t know whether you’re seeing a climate record in which ice volume changed dramatically, the oceans warmed or cooled substantially, or both.”

“Now, for the first time, we have been able to separate these two components, which means that we stand a much better chance of understanding the mechanisms involved. One of the reasons why that is important, is because we are making changes to the factors that influence the climate now. ̽��ֱ��only way we can work out what the likely effects of that will be in detail is by finding analogues in the geological past, but that depends on having an accurate picture of the past behaviour of the climate system.”

Researchers have developed more than 30 different models for how these features of the climate might have changed in the past, in the course of a debate which has endured for more than 60 years since pioneering work by Nobel Laureate Harold Urey in 1946. ̽��ֱ��new study helps resolve these problems by introducing a new dataset to the picture - the ratio of magnesium (Mg) to calcium (Ca) in foraminifera. Because it is easier for magnesium to be incorporated at higher temperatures, larger quantities of magnesium in the tiny marine fossils imply that the deep sea temperature was higher at that point in geological time.

̽��ֱ��Mg/Ca dataset was taken from the fossil record contained in cores drilled on the Chatham Rise, an area of ocean east of New Zealand. It allowed the Cambridge team to map ocean temperature change over time. Once this had been done, they were able to subtract that information from the oxygen isotopic record. “ ̽��ֱ��calculation tells us the difference between what water temperature was doing and what the ice sheets were doing across a 1.5 million year period,” Professor Elderfield explained.

̽��ֱ��resulting picture shows that ice volume has changed much more dramatically than ocean temperatures in response to changes in orbital geometry. Glacial periods during the 100,000-year cycles have been characterised by a very slow build-up of ice which took thousands of years, the result of ice volume responding to orbital change far more slowly than the ocean temperatures reacted. Ocean temperature change, however, reached a lower limit, probably because the freezing point of sea water put a restriction on how cold the deep ocean could get.

In addition, the record shows that the transition from 41,000-year cycles to 100,000-year cycles, the characteristic changeover of the MPT, was not as gradual as previously thought. In fact, the build-up of larger ice sheets, associated with longer glacials, appears to have begun quite suddenly, around 900,000 years ago. ̽��ֱ��pattern of the Earth’s response to orbital forcing changed dramatically during this “900,000 year event”, as the paper puts it.

̽��ֱ��research team now plan to apply their method to the study of deep-sea temperatures elsewhere to investigate how orbital changes affected the climate in different parts of the world.

“Any uncertainty about the Earth’s climate system fuels the sense that we don’t really know how the climate is behaving, either in response to natural effects or those which are man-made,” Professor Elderfield added. “If we can understand how earlier changes were initiated and what the impacts were, we stand a much better chance of being able to predict and prepare for changes in the future.”

Study successfully reconstructed temperature from the deep sea to reveal how global ice volume has varied over the glacial-interglacial cycles of the past 1.5 million years.

̽��ֱ��only way we can work out what the likely effects of the changes we are making to the climate will be is by finding analogues in the geological past. That depends on having an accurate picture of the past behaviour of the climate system.

Harry Elderfield

Julian Dowdeswell.

Tabular iceberg. ̽��ֱ��production of tabular icebergs is a major mechanism of mass loss from the Antarctic Ice Sheet.

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Prenatal origins of heart disease

bjb42 — Sun, 04 Jan 2009 15:27:10 +0000

Heart disease is the greatest killer in the UK today, accounting for four in every 10 deaths and imposing a substantial burden on the nation’s health and wealth. ̽��ֱ��concept is familiar to us all that traditional risk factors, such as smoking and obesity, and genetic makeup increase the risk of heart disease. However, it is now becoming apparent that a third factor is at play – a developmental programming that is predetermined before birth, not only by our genes but also by their interaction with the quality of our prenatal environment.

A biological trade-off

Pregnancies that are complicated by adverse conditions in the womb, such as happens during pre-eclampsia or placental insufficiency, enforce physiological adaptations in the unborn child and placenta. While these adaptations are necessary to maintain viable pregnancy and sustain life before birth, the adaptations come at a cost, claiming reduced growth as a biological trade-off. In fact it’s more than just growth that is affected – we now know that the trade-off extends to the development of key organs and systems such as the heart and circulation, which increases the risk of cardiovascular disease in adult life. Overwhelming evidence in more than a dozen countries has linked development under sub-optimal intrauterine conditions leading to low birth weight with increased rates in adulthood of coronary heart disease and its major risk factors – hypertension, atherosclerosis and diabetes.

̽��ֱ��idea that a fetus’ susceptibility to disease in later life could be programmed by the conditions in the womb has been taken up vigorously by the international research community, with considerable efforts focusing on nutrient supply across the placenta as a risk factor. But nutrient supply is just part of the story. How much oxygen is available to the fetus is also a determinant of growth and the risk of adult disease. Dr Dino Giussani’s research group in the Department of Physiology, Development and Neuroscience is asking what effect reduced oxygen has on fetal development by studying populations at high altitude.

Lessons from high altitude

Bolivia lies at the heart of South America, split by the Andean Cordillera into areas of very high altitude to the west and areas at sea-level to the east, as the country extends into the Amazon Basin. At 400 m and almost 4000 m above sea-level, respectively, the Bolivian cities of Santa Cruz and La Paz are striking examples of this difference.

Pregnancies at high altitude are subjected to a lower partial pressure of oxygen in the atmosphere compared with those at sea-level. Women living at high altitude in La Paz are more likely to give birth to underweight babies than women living in Santa Cruz. But is this a result of reduced oxygen in the womb or poorer nutritional status?

̽��ֱ��research team studied birth weight records from healthy term pregnancies in La Paz and Santa Cruz, especially from obstetric hospitals and clinics selectively attended by women from either high- or low-income backgrounds. High-altitude babies showed a pronounced reduction in birth weight compared with low-altitude babies, even in cases of high maternal nutritional status. Babies born to low-income mothers at sea-level also showed a reduction in birth weight, but the effect of under-nutrition was not as pronounced as the effect of high altitude on birth weight; clearly, fetal oxygenation was a more important determinant of fetal growth within these communities

Remarkably, although one might assume that babies born to low-income mothers at high altitude would show the greatest reduction in birth weight, these babies were actually heavier than babies born to high-income mothers at high altitude. It turns out that the difference lies in ancestry. ̽��ֱ��lower socio-economic groups of La Paz are almost entirely made up of Aymara Indians, an ancient ethnic group with a history in the Bolivian highlands spanning two millennia. On the other hand, individuals of higher socio-economic status in Bolivia represent a largely European and North American admixture, relative newcomers to high altitude. It seems therefore that an ancestry linked to prolonged high-altitude residence confers protection against reduced atmospheric oxygen.

Do these early influences of oxygenation feed through to increased risk of cardiovascular disease? A large-scale, five-year study to determine this has been initiated in the two cities that will link birth weight data with measurements of cardiovascular health and disease in Bolivian high- and lowlanders. But an early indication has been supplied by a somewhat unlikely source: Bolivian hen eggs.

Mountain chicks

Dr Giussani’s group has discovered that they can replicate the results found in Andean pregnancies in eggs: fertilised eggs from Bolivian hens native to sea-level show growth restriction when incubated at high altitude, whereas eggs from hens that are native to high altitude show a smaller growth restriction. Using hen eggs has allowed the researchers to accomplish something that would take generations of migration in human populations to demonstrate: moving fertilised eggs from hens native to high altitude down to sea-level. This not only restored growth, but the embryos were actually larger than sea-level embryos incubated at sea-level. And, importantly, when looking for early markers of cardiovascular disease, it was discovered that growth restriction at high altitude was indeed linked with cardiovascular defects – shown by an increase in the thickness of the walls of the chick heart and aorta.

Bringing the Andes to Cambridge

To have hopes of clinical intervention, we need to understand why reduced oxygen should be a trigger for a prenatal origin of heart disease. Towards this goal, the group’s most recent data studying rat pregnancy complicated with reduced fetal oxygenation have indicated that the adverse effects on cardiovascular development may be secondary to a disturbance known as oxidative stress. ̽��ֱ��body normally produces by-products of oxygen called free radicals and, unless these are neutralised by antioxidants, they cause damage to cells. If oxidative stress is the underlying cause of cardiovascular defects, this offers the highly interesting possibility of using antioxidants to treat pregnancies complicated by reduced oxygen delivery to the fetus, be it at sea-level or high altitude. This may halt the development of heart disease at its very origin, bringing preventive medicine back into the womb.

For more information, please contact the author Dr Dino Giussani (dag26@cam.ac.uk) at the Department of Physiology, Development and Neuroscience. Research described here was sponsored by the British Heart Foundation, Biotechnology and Biological Sciences Research Council (BBSRC), Lister Institute of Preventive Medicine, Royal Society and Wellcome Trust. Dr Giussani is a member of the Centre for Trophoblast Research.

Studies in La Paz, the highest city in the world, are helping to uncover a link between prenatal conditions and heart disease in later life.

High-altitude babies showed a pronounced reduction in birth weight compared with low-altitude babies, even in cases of high maternal nutritional status.

Kristin Gussani

La Paz, Chile

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