Past Climate and Expedition 363.
Yair Rosenthal and Ann Holbourn are the co-chief scientists for Expedition 363. It was a privilege to be able to discuss the expedition with Yair. He has spent many years researching Earth Sciences and is well qualified for this role. He started his academic career at university in Israel. Initially he was enrolled on a course in engineering but quickly realized this was not for him. For seven years he enjoyed the opportunity to travel and work in Africa and Asia as a tour guide. In East Africa he watched successive droughts gradually take hold of the land. It is here that his interest in climate started. He returned to Israel to undertake a degree then a Master?s degree at The Hebrew University of Jerusalem. Shortly after completion he moved to the USA where he both worked then studied for a PhD at Massachusetts Institute of Technology. Currently, he holds the position of Distinguished Professor at Rutgers University.
On Expedition 363 we are looking to find out more about the climate, the ocean water temperatures and the currents of the last 20 million years. The Western Pacific Warm Pool is the largest body of warm seawater on the planet. Water has a high heat capacity, which means it takes a great deal of energy to change the temperature of water. This energy then drives major global climate systems including the Asian Monsoon, which brings much needed water to a highly populated region, and the Northern Australian Monsoon. It is also linked to the El Nino Southern Oscillation. El Nino events have global consequences in terms of climate patterns. Small changes to the sea surface temperature influence the strength of the convection cells in the atmosphere (the Hadley and Walker circulations). Currently we have incomplete understanding of how changes to the Western Pacific Warm Pool will affect global climate. The aim of this expedition is to determine past climate in this region and link that with past sea water temperature, density and salinity ? examining the surface water, intermediate water and bottom water, and the extent and strength of past currents. Understanding how this region has changed in the past will help us increase our understanding of the modern climate and it will improve predictions regarding future climate.
Yair has recently published a paper with Braddock Lindsey and Delia Oppo entitled Pacific Heat Content During the Past 10,000 Years. Here, they explain that the ocean can be divided into vertical layers where the layers of water have originated from different sources. The paper examined these layers of water in the Pacific and found that at times during the past 10,000 years both climate and water temperature has been warmer. They also discovered that the rate of increase of water temperature is much more rapid in the present time than it has ever been before. Yair thinks the ability of the oceans to store heat may help to slow the immediate impacts of climate forcing due to increased atmospheric carbon dioxide. However, it will not stop it.
For this expedition Yair wants to extend further back into the past 450,000 years. This will include 3 periods when the climate was warm. During these times the tectonics and land masses of the area were similar to today so we think the climate system and water temperatures will respond in a similar way to the way they did in the more recent past. Understanding how the warm pool is linked with weather systems is highly relevant to understanding what will happen as the Earth?s temperatures increase.
With the cores taken, Yair and the team of scientists should obtain the high resolution data to extend their previous research and to compare changes in the Pacific with those in the Atlantic over the same time period.
Planning the expedition
The planning for the expedition has taken 6 years. Proposals were written and funding requests made. Subsequently site surveys were made and health and safety issues considered. Risks include poor weather, piracy, drilling into oil and gas deposits and other political issues. Permission from various countries was also needed. Six years is actually a relatively short time to plan and recruit for an expedition of this nature. Yair was disappointed not to be able to drill in a site near the Philippines which was ruled out due to the piracy risk, but has been happy with the cores recovered during the expedition from the other sites.
How drilling the ocean floor help us understand past climate conditions.
At the sites a long thin core of sediment is removed from the ocean floor. As soon as the core is bought ?on deck? and preliminary shipboard data are gathered. Physical properties are measured first to allow different cores from adjacent holes to be correlated. Paleontologists also quickly sample the core and use microfossils to date the sediment. The magnetic signature of the sediment is measured and the concentration of ions in the pore water (water between the grains of sediment) is analyzed. This preliminary data starts to inform the scientists about the environmental conditions when the sediment was laid down.
Scientists cannot go back in time to measure temperature and other data directly so they measure things that change in relation to the things they want to know. These data are called proxies. Some of the proxies used are explained below. Scientists will use samples to gain different data which when put together will allow us to understand better the past climate in this area.
Proxies used to infer past ocean conditions
Proxies for temperature
Many organisms make their shells from calcium carbonate. In these shells a small amount of magnesium carbonate is made and incorporated alongside the calcium carbonate. The amount of magnesium incorporated changes relative to temperature. Scientists can analyze the preserved shells of these organisms found in the sediment and determine the water temperature in which they lived.
Proxies for current. Smaller particles are transported further than larger particles. Faster moving currents can transport larger particles. Currents also moves sediments into piles in the same way the wind blows snow into drifts. Looking at the shape, size and location of these piles of sediment can also give us clues about the direction and strength of past currents. We can use seismic surveys to identify interesting locations and use the core samples to check or ?ground truth? what we believe the seismic survey to be showing us.
Proxies for sea level. Some forams live on the sea floor. Different forams live on the sea floor in deeper water. The benthic forams found in the sediment helps scientist determine the depth of the sea when they were alive.
In locations on the edge of the continental shelf we see a change in the material deposited as the sea level changes. When sea level is low, land mass increases and locations near the edge of the continental shelf are closer to land so they receive more sediment from the land. When sea level is high (during interglacial periods) the same location is far from the coast. The sediment received by the ocean floor during these times is predominantly biogenic or from things that were alive in the water column. Little is from weathered rock from the land.
Proxies for rainfall. In material formed near the land there is a lot of material from weathered rocks. The weathering rate is an indication of the rainfall of the time. How quickly sediment accumulates can be determined.
Paleomagnetism records. Sediment that is magnetic will align with the Earth?s magnetic fields as it settles on the ocean floor. It then becomes trapped in this sediment. Periodically the Earth?s magnetic field reverses. These reversals are recorded in the sediment and can be used to age the sediment. They can also be useful to determine the rate of sedimentation.
Stable Oxygen isotopes. Oxygen atoms can have different masses. As a result of these isotopes water molecules will have different masses. Water with the heavier mass requires slightly more energy to evaporate and consequently water vapor in clouds has a slightly higher proportion of the lower mass water molecules. In an ice age these lower mass water molecules remain at the poles as solid ice leaving the sea level lower and containing a higher concentration of the higher mass water molecules. The relative numbers of the two oxygen isotopes found in the calcium carbonate shells of the preserved fossils of forams from the past can allow us to determine if they came from a glacial or interglacial period. This also allows us to determine an estimate for the salinity of the seawater at this time.
Science has already established patterns of past cycles of climate change. These changes are regular and their signature can be seen in many long-term climate records. Essentially the Earth?s orbit gradually changes in three ways and these impact global climate. First the orbit around the sun changes from a more elliptical to more circular orbit and back. Second the Earth?s tilt changes making seasons more or less extreme (northern hemisphere summer occurs when the northern hemisphere is tilted towards the sun). Finally the Earth also wobbles so that it is not always tilting in the same direction. 14000 years ago the northern hemisphere was tilted towards the sun when the Earth was also closest to the sun in its orbit. Now the northern hemisphere is tilted towards the sun when the Earth is furthest from the sun. All of these oscillations have their own period and affect climate differently. Volcanoes emitting particulates and carbon dioxide, the movement of continents, changes of rock weathering, and solar flares are also been implicated in climate changes. Scientists will be looking for evidence of these cycles in the data.
After the expedition.
As the expedition draws to a close the real work is about to begin. Almost 7000 m of core was collected from 8 different sites. Over 40,000 samples are due to be taken and these will be taken back to the home institutions of the 32 scientists aboard. Analyses will be undertaken, and research findings will be discussed, shared and published. The coming months will be a busy time for all those involved and we await with interest to hear their findings.