Understanding How Climatic Relationships Led To Major Ice Age Cycles
April Flowers for redOrbit.com – Your Universe Online
A new study has developed crucial new information about how the ice ages came about based on newly discovered relationships between deep-sea temperature and ice-volume changes. The international team of researchers, led by the University of Southampton, developed a new method for determining sea-level and deep-see temperature variability over the past 5.3 million years. Their findings were published in a recent issue of Nature.
The new method provides unprecedented insight into the climatic relationships that caused the development of major ice-age cycles during the past two million years.
The research team, consisting of scientists from the University of Southampton, the National Oceanography Center and the Australian National University, found that the long-trends in cooling and continental ice-volume cycles over the past 5.3 million years were not the same. They also found that the major step toward the ice ages that have characterized the past two to three million years in terms of temperature was a cooling event 2.7 million years ago. In contrast, for ice-volume the crucial step occurred around 2.15 million years go with the development of the first intense ice age. Prior to this study, scientists believed these two events occurred together at around 2.5 million years ago.
Dr Gavin Foster, from Ocean and Earth Science at the University of Southampton, commented, “Our work focused on the discovery of new relationships within the natural Earth system. In that sense, the observed decoupling of temperature and ice-volume changes provides crucial new information for our understanding of how the ice ages developed.”
“However, there are wider implications too. For example, a more refined sea-level record over millions of years is commercially interesting because it allows a better understanding of coastal sediment sequences that are relevant to the petroleum industry. Our record is also of interest to climate policy developments, because it opens the door to detailed comparisons between past atmospheric CO2 concentrations, global temperatures, and sea levels, which has enormous value to long-term future climate projections.”
Microscopic plankton fossils recovered from the Mediterranean Sea provided records of oxygen isotope ratios—which reveal a record of ancient water temperature—that span over the last 5.3 million years.
During the ice ages, the continental ice sheets grew and blocked the flow through the Strait of Gibraltar. This caused measurable increases in the oxygen isotope O-18 (8 protons and 10 neutrons) relative to O-16 (8 protons and 8 neutrons) in Mediterranean waters. This isotope became preserved in the shells of ancient plankton. Prior studies have analyzed such microfossil-based oxygen isotope records from carefully dated sequences obtained from long drill cores and uplifted sections of sea-floor sediments.
The current team of researchers added a numerical model for calculating water exchange through the Strait of Gibraltar as a function of sea-level change. This allowed the team to use the microfossil records as a sensitive recorder of global sea-level changes. This new data was then combined with existing deep-sea oxygen isotope records from the open ocean to demonstrate deep-sea temperature changes.
Professor Eelco Rohling of Australian National University, says, “This is the first step for reconstructions from the Mediterranean records. Our previous work has developed and refined this technique for Red Sea records, but in that location it is restricted to the last half a million years because there are no longer drill cores. In the Mediterranean, we could take it down all the way to 5.3 million years ago. There are uncertainties involved, so we included wide-ranging assessments of these, as well as pointers to the most promising avenues for improvement. This work lays the foundation for a concentrated effort toward refining and improving the new sea-level record.”
Dr Mark Tamisiea, of the National Oceanography Centre, Southampton, noted the importance of the Strait of Gibraltar to the analysis.
“Flow through the Strait will depend not only on the ocean’s volume, but also on how the land in the region moves up and down in response to the changing water levels. We use a global model of changes in the ocean and the ice sheets to estimate the deformation and gravity changes in the region, and how that will affect our estimate of global sea-level change,” he says.